1 - Repositories

EFFECTS OF STRUCTURAL VARIATION WITHIN

POLYETHER AND CALIX[4]ARENE LIGANDS AND

MATRIX VARIATION ON METAL ION COMPLEXATION

by

SANGKICHUN, B.S., M.S.

A DISSERTATION

IN

CHEMISTRY

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

DOCTOR OF PHILOSOPHY

Approved

Chairperson of the Committee

' )'-r

Accepted

Dean of the Graduate School

May, 2002

ACKNOWLEDGEMENTS

"Theory Guides, Experiment Decides"

I. M. Kolthoff

Like climbing a huge mountain, everyday I hope to improve my understanding of

chemistry. There remains but one sensible disposition after reaching the mountaintop of

a Ph. D., an attitude of humility and gratitude. Thus, I would like to express my deepest

appreciation to those people who were my mentors and who supervised the development

those educational tools such as the ability to think, reason and creativity which are

primary elements to a doctoral education. Most notable on this list is Dr. Richard A.

Bartsch. Through his encouragement and guidance, my research has flourished. To the

other members of my graduate research committee, Drs. Carol Korzeniewski and Edward

L. Quitevis, I wish to express my gratitude for their efforts in overseeing my doctoral

program and the completion of the dissertation requirement. 1 would also like to express

my gratitude to Dr. Pumendu K. Dasgupta. His practical advice and teaching strengthen

my knowledge in analytical chemistry. I am also indebted to those mentors who taught

and guided me throughout my earlier academic career. I would like to express my special

thanks to Drs. Soo-un Kim and Hasuck Kim at Seoul National University and Dr. Il-Woo

Yang in Korea Military Academy for their helping me lighten up a bit.

I am also indebted to those individuals who worked with me, my fellow graduate

and postdoctoral coworkers. I thank them for helping me with procedures and the

synthesis of chemicals. I thank them for their friendship and encouragement. I wish to

express my gratitude to Dr. Galina G. Talanova for her suggestions and comments which

resulted in a more accurate and readable document.

All the way, my family has been most important in my life. I would like to

express my deepest appreciation to my parents, Mr. Donghyun Chun and Mrs. Jongsoon

Kim who have been fundamental to my success. I wish to thank them for their

encouraging words and for their profound love. And, I am gratefiil to the other members

of my immediate family and my adopted family (my in-laws) for their quiet but strong

support of my pursuits. I wish to express my gratitude to my wife in the very deepest

fashion, Mrs. Eunkyung Kwon, who has made the greatest sacrifice toward my success.

Her love, support and excellent care of our family were the main things that kept me

going through my academic career. I wish to thank my twin sons, Joonyoung and

Jaeyoung, for the happiness they brought to me after work each day.

i l l

TABLE OF CONTENTS

ACKNOWLEDGMENTS ii

LIST OF TABLES x

LIST OF FIGURES xiv

CHAPTER

L INTRODUCTION 1

1.1. Historical Overview 1

1.2. Cro'wn Ethers and Analogous Compounds 2

1.2.1. History 2

1.2.2. Nomenclature 4

1.3. Calixarenes 6

1.3.1. History 6


1.4. Room-Temperature Ionic Liquids 9

1.4.1. History 9


1.5. Cyclophosphazene-based Crown Ethers 10

1.5.1. History 10


1.6. Cation Complexation by Multidentate Ligands 13

1.6.1. Complexation Efficiency 14

1.6.2. Complexation Selectivity 14

1.7. Solvent Extraction: Background and Theory 17

1.8. Solvent Polymeric Membrane Electrodes: Background and Theory 21

1.9. Research Objectives 23

1.10.References 25

II. SOLVENT EXTRACTION OF METAL CATIONS WITH PROTON-IONIZABLE ACYCLIC POLYETHERS 30

IV

2.1. Introduction 30

2.2. Results and Discussion 33

2.2.1. Competitive Solvent Extraction of Alkaline Earth Metal

Cations and Solvent Variation 33

2.2.1.1. Effect of Bridge Variation 34

2.2.1.2. Effect of X Group Variation 39

2.2.1.3. Comparison of Solvent Extraction and

Polymeric Inclusion Membrane Transport Results 40

2.2.1.4. Solvent Variation in Solvent Extraction 44

2.2.1.5. Metal Ion Binding Site in a Ligand 47

2.2.2. Solvent Extraction of Pb^^ 48

2.2.2.1. Stoichiometry for Pb "" Complexation 48

2.2.2.2. Pb^^ Extraction 49

2.2.3. Solvent Extraction of Hg "" 52

2.2.3.1. Stoichiometry for Hg " Complexation 52

2.2.3.2. Hg^* Extraction 53

2.3. Chapter Summary 56

2.4. Experimental 56

2.4.1. Competitive Solvent Extraction of Alkaline Earth Metal Cations and Solvent Variation 56 2.4.1.1. Reagents 56

2.4.1.2. Apparatus and Instrumentation 57

2.4.1.3. Extraction Procedure 57

2.4.2. Solvent Extraction of Pb "" 61

2.4.2.1. Reagents 61



2.4.3. Solvent Extraction of Hg " 63

2.4.3.1. Reagents 63



2.5. References 66

III. SOLVENT EXTRACTION OF METAL CATIONS WITH

PROTON-IONIZABLE CALIX[4]ARENES 67



3.2.1. Competitive Solvent Extraction of Alkali Metal Cations 71

3.2.1.1. Calix[4]arenecarboxylic Acids 71

3.2.1.2. N-(X)sulfonyl Calix[4]arenecarboxamides 75

3.2.1.2.1. Effect of Conformation Variation 77

3.2.1.2.2. Effect of X Group Variation 80

3.2.2. Competitive Solvent Extraction of

Alkaline Earth metal Cations 84


3.2.2.2. N-(X)sulfonylCalix[4]arenecarboxamides 86





3.2.3.2. N-(X)sulfonyl Calix[4]arenecarboxamides 93



3.2.4. Solvent Extraction of Hg "" 97


3.2.4.2. N-(X)sulfonylCalix[4]arenecarboxamides 98 3.2.4.2.1. Effect of Conformation Variation 98


100 3.3. Chapter Summary

3.4. Experimental

3.4.1. Competitive Solvent Exttaction of Alkali Metal Cations 102

VI

3.4.1.1. Reagents 102



3.4.2. Competitive Solvent Extraction of

Alkaline Earth Metal Cations 104

3.4.2.1. Reagents 104




3.4.4. Solvent Extraction of Hg^ 106

3,5. References 107

IV. SOLVENT EXTRACTION OF METAL CATIONS WITH DC 18C6

IN ROOM-TEMPERATURE IONIC LIQUIDS 109



4.2.1. Physical Properties I l l

4.2.1.1. Denshy I l l

4.2.1.2. Water Solubility 112

4.2.2. Competitive Solvent Extraction of Alkali Metal Cations 113

4.2.2.1. Molecular Solvents 113 4.2.2.2. 1 -Alkyl-3-methylimidazolium

Hexafluorophosphates 113 4.2.2.3. l-Alkyl-3-methylimidazolium

Bis[(trifluoromethyl)sulfonyl]imides 116

4.2.2.4. Selectivity Ratio in [Cn-mim]PF6 and [Cn-mim]NTf2 . 119

4.2.3. Competitive Solvent Extraction of Alkaline Earth Metal Cations 120

4.2.3.1. Molecular Solvents 120

4.2.3.2. l-Alkyl-3-methylimidazolium Hexafluorophosphates 121


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4.2.4.1. Molecular Solvents 124

4.2.4.2. l-Alkyl-3-methylimidazolium Hexafluorophosphates 124

4.2.4.3. l-Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imides 126



4.4.1. Reagents, Apparatus and Instrumentation 128

4.4.2. Procedures 130

4.4.2.1. Density Measurement 130

4.4.2.2. Water Solubility 130

4.4.2.3. Competitive Solvent Extraction of AlkaH Metal Salts into Molecular Solvents 130

4.4.2.4. Competitive Solvent Extraction of AlkaH Metal Salts into RTILs 131

4.4.2.5. Competitive Solvent Extraction of Alkaline Earth Metal Salts into Molecular Solvents.... 132

4.4.2.6. Competitive Solvent Extraction of Alkaline Earth Metal Salts into RTILs 132

4.4.2.7. Single Species Solvent Extraction of Pb " into Molecular Solvents 132

4.4.2.8. Single Species Solvent Extraction of

Pb^* into RTILs 132

4.5. References 134

V. SOLVENT POLYMERIC MEMBRANE ELECTRODES

BASED ON CROWN ETHERS 135



5.2.1. Calculations 138

5.2.2. Crown-Based Cyclophosphazenes 139

5.2.2.1. Potentiometric Study of Ion Selectivities of Mono-Substituted lonophores 140

Vlll

5.2.2.2. Potentiometric Study of Ion Selectivities of Tetraaryloxy-Substituted lonophores 142

5.2.2.3. Effect of Variation of the Ring Size on lonophore Selectivity 145

5.2.2.4. Effect of Variation of the Number of Substituents on Selectivity 147

5.2.2.5. Potentiometric Study of Ion Selectivities of Amino-Substituted lonophores 148

5.2.2.6. Effect of Chain Length between Two lonophores 151

5.2.3. Dinaphtho-16-Crown-4 153

5.2.4. Diazacrown Ethers 155



5.4.1. Reagents 157

5.4.2. Electrode Construction 158

5.4.3. Potentiometric Measurements 158

5.4.3.1. Cyclophosphazene-Containing PNP-crown

and Dinaphtho-16-crown-4 Macrocycles 158

5.4.3.2. Diazacrown Ether lonophores 160

5.5. References 161

APPENDIX A. ALKALI METAL CATIONS 163

APPENDIX B. ALKALINE EARTH METAL CATIONS 166

APPENDIX C. LEAD 172

APPENDIX D. MERCURY 178

APPENDIX E. ROOM-TEMPERATURE IONIC LIQUIDS 184 APPENDIX F. ION-SELECTIVE ELECTRODES 194

IX

LIST OF TABLES

1.1. Cavity Diameters for Various Crown Ethers and Diameters of Alkali and Alkaline Earth Metal Cations 15

2.1. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by Proton-Ionizable Acyclic Polyether Carboxylic Acids 20-22 34

2.2. Efficiency and Selectivity Ratio in Competitive Solvent Extraction of Aqueous Solutions Containing Alkaline Earth Metal Cations (10.0 mM in Each) with 5.0 mM Chloroform Solutions of the Proton-ionizable Acyclic Polyether N-(X)Sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 38

2.3. Hansen and Beerbower's Dispersion, Polar and Hydrogen Bonding Cohesion Vectors for Various Solvent 47

2.4. Aqueous Phase Formulations for Competitive Solvent Extraction of Alkaline Earth Metal Cations by Proton-Ionizable Acyclic Polyethers 20-34 58

2.5. Additional Aqueous Phase Formulations for Competitive Solvent Extraction of Alkaline Earth Metal Cations by Proton-Ionizable Acyclic Polyether N-(CF3)sulfonyl Carboxamides 32-34 60

2.6. Compositions of the Organic Phase Solutions Used in the Pb " Extraction Experiments for the Slope Analysis Study 62

2.7. Compositions of the Organic Phase Solutions Used in the Hg " Extraction Experiments for the Slope Analysis Study 64

2.8. Aqueous Phase Formulations for Compethive Solvent Extraction of Hg " by Proton-Ionizable Acyclic Polyether Carboxylic Acids 20-22 65

2.9. Aqueous Phase Formulations for Competitive Solvent Extraction of Hg " by Proton-Ionizable Acyclic Polyethers N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 65

3.1. Efficiency and Selectivity of Competitive Solvent Extraction of Alkali Metal Cations into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenedicarboxylic Acids 37-41 72

3.2. Efficiency and Selectivity of Competitive Solvent Extraction of Alkali Metal Cations into Chloroform by Conformationally Restticted N-(X)Sulfonyl Calix[4]arenecarboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3 at pH 9.5 79

3.3. Efficiency and Selectivity of Competitive Solvent Extraction of Alkali Metal Cations into Chloroform by cone,paco-hutyl-wp,paco-acid-up and 1,3-alternate N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 83

3.4. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenecarboxylic Acids 37-41 86

3.5. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by Conformationally Restricted cone, paco-h\xty\-\xp, paco-acid-np and 1,3-alternate N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3 88

3.6. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 andCF3 91

3.7. Efficiency and Half Extraction pH of Single Species Solvent Extraction of Pb " into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenedicarboxylic Acids 37-41 93

3.8. Aqueous Phase Formulations for Competitive Solvent Extraction of Alkali Metal Cations by Proton-Ionizable Calix[4]arenecarboxylic Acids 37-41 103

3.9. Aqueous Phase Formulations for Competitive Solvent Extraction of Alkali Metal Cations by Proton-Ionizable N-(X)sulfonyl Calix[4]arene-carboxamides 42-53 with X = CH3, Ph and 4-NO2C6H4 103

3.10. Aqueous Phase Formulations for Competitive Solvent Extraction of Alkali Metal Cations by N-(CF3)sulfonyl Calix[4]arenecarboxamides 54-57 104

3.11 Aqueous Phase Formulations for Competitive Solvent Extraction of Alkaline Earth Metal Cations by Proton-Ionizable Calix[4]arenes 37-57 105

4.1. Percent Loading for Competitive Solvent Extraction of Alkali Metal Chlorides from Aqueous Solutions by DC18C6 in Molecular Solvents 113

XI

4.2. Effect of Anion on Percent Loading for Competitive Solvent Extraction of Alkali Metal Salts from Aqueous Solution by DC18C6 in 1 -Octyl-3-methylimidazolium Hexafluorophosphate 116

4.3. Effects of Anion on Percent Loading for Competitive Solvent Extraction of Alkali Metal Salts from Aqueous Solution by DC18C6 in l-Nonyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imides 118

4.4. Percent Loading for Competitive Solvent Extraction of Alkaline Earth Metal Chlorides from Aqueous Solutions by DC18C6 in Molecular Solvents 120

4.5. Metal Loading for Solvent Extraction of Lead Nitrate from Aqueous Solutions by DC 18C6 in Molecular Solvents 124

5.1. Backgroimd Concentrations of Interfering Ions for Cyclophosphazene Containing PNP-Crown Macrocycles 160

5.2. Background Concentrations of Interfering Ions

for Dinaphtho-16-Crown-4 Macrocycles 160

5.3. Background Concentrations of Interfering Ions for Diazacrown Ethers 160

E.l. Physical Properties (Density and Water Solubility) 185

E.2. Percent Loading for Competitive Solvent Extraction of Alkali Metal Cations from Aqueous Phase into RTIL( [Cn-mim]PF6, n=4-9) 186

E.3. Percent Loading for Competitive Solvent Extraction of Alkali Metal Cations from Aqueous Phase into RTIL([Cn-mim]NTf2, n=3-10) 187

E.4. Selectivity Ratios of K /Rb^ and K /Cs^ 188

E.5. Percent Loading for Competitive Solvent Extraction of Alkaline Earth Metal Cations from Aqueous Phase into RTIL([Cn-mim]PF6, n=4-9) 189

E.6. Percent Loading for Competitive Solvent Extraction of Alkaline Earth Metal Cations from Aqueous Phase by DC18C6 into RTIL ([Cn-mim]PF6, n=4-9) 190

E.7. Selectivity Ratios of Ba /Sr ^ 191

E. 8. Percent Loading for Single Species Solvent Extraction of Pb from Aqueous Phase into RTIL ([C„-mim]PF6, n=4-8) 192

E.9. Percent Loading for Single Species Solvent Extraction of Pb from Aqueous Phase into RTIL ([Cn-mim]NTf2, n=3-10) 193

F. 1. Potentiometrically Measured Selectivity (-log K " N,M) of Crown-Based Cyclophosphazene 195

Xll

F.2. Potentiometrically Measured Selectivity (-log K^ 'N.M) of Dinaphtho-16-Crown-4 198

F.3. Potentiometrically Measured Selectivity (-log K''°'N,M) of Diazacrown Ether 198

XllI

LIST OF FIGURES

1.1 Structures of Acyclic Polyether Ligands 4

1.2. Nomenclature for Some Crown Ethers and an Acyclic Polyether 5

1.3. Representation of Calix[4]arene and Designation of the Faces 7

1.4. Structures of Representative Calixarenes 8

1.5. Limiting Conformations of Calix[4]arenes 8

1.6 Representative Room-Temperature Ionic Liquids 10

1.7 Structural of Isomers from Disubstitution of N3P3CI6 12

1.8 Structural Isomers N3P3CI6 Reacted with a Bifunctional Reagent 12

1.9 Representative Cyclophosphazene-Based Crown Ethers 13

1.10 Different Complexation Modes for Polyether Ligands and Cations 16

1.11 Schematic Diagram for the Extraction of a Metal Cation by Proton-Ionizable Ligand 19

1.12. Solvent Extraction by (a) a Neutral Polyether and (b) a Proton-ionizable Polyether 20

1.13. Metal Ion Stripping from a Proton-Ionizable Polyether Complex Using Acid 20

1.14. Determination of the Nicolskii Coefficients According to the Fixed Interference Method (FIM) as Proposed by the lUPAC Commission 22

2.1. Design Concept of Proton-Ionizable Acyclic Polyether Ligand for Solvent Extraction. (Y = SO2X with X = CH3, CF3, Ph and 4-NO2C6H4) 32

2.2. Bridge, Terminal Groups and Pseudo-Cavity in a Proton-Ionizable Acyclic Polyether Ligand 32

2.3. Structures of Di-ionizable Acyclic Polyether Carboxylic Acids 20-22 34

2.4. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether Carboxylic Acids 20-22 in Chloroform 35

2.5. Structures of Proton-ionizable Acyclic Polyether N-(X)Sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 36

XIV

2.6. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(CH3)sulfonyl Carboxamides 26-28 in Chloroform 37

2.7. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(X)sulfonyl Carboxamides 25 (X=Ph), 28 (X=CH3), 31 (X=4-N02C6H4) and 34 (X=CF3) in Chloroform 40

2.8. Effect of Bridge Variation on (a) the pH for Half Extraction and (b) the Ba " Selectivity of the Acyclic Polyether N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 in Competitive Solvent Extraction of Alkaline Earth Metal Cations 43

2.9. Effect of Bridge Variation on (a) the Total Metal Ion Flow and (b) the Ba " Selectivity of the Acyclic Polyethers N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 in Competitive Transport of Alkaline Earth Metal Cations across CTA-o-Nitrophenyl Pentyl Ether Polymer Inclusion Membranes 44

2.10. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(4-N02C6H4)sulfonyl Carboxamide 29 in Different Diluents 45

2.11. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(4-N02C6H4)sulfonyl Carboxamide 31 in Different Diluents 46

2.12. A Suggested Chemical Structure for the Ba " Complex with Ionized 29 47

2.13. Plots of (a) log D versus pH and (b) log D versus log [H2L] at pH 9.5 for Solvent Extraction of Pb " from Aqueous Solution into Chloroform by Acyclic Polyether N-(CF3)sulfonyI Carboxamides 32-34 49

2.14. Effect of Bridge Variation on Pb "" Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solutions of Lead Nitrate with 0.5 mM Chloroform Solutions of Acyclic Polyethers 20-34 51

2.15. Effect of X Group Variation on Pb "" Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of Acyclic Polyethers 20-34 52

XV

2.16. Plot of (a) log D versus pH and (b) log D versus log[H2L] at pH 1 for Solvent Exttaction of Hg "" from Aqueous Solution into Chloroform by Acyclic Polyether N-(CH3)sulfonyl Carboxamides 26-28 53

2.17. Effect of Bridge Variation on Hg "" Exttaction versus the Equilibrium pH of the Aqueous Phase for Extractions of 0.25 mM Solutions of Mercuric Nitrate with 0.25 mM Chloroform Solutions of Acyclic Polyethers 20-34... 54

2.18. Effect of X Group Variation on Hg "" Extraction versus the Equilibrium pH of the Aqueous Phase for Extractions of 0.25 mM Solutions of Mercuric Nitrate with 0.25 mM Chloroform Solutions of Acyclic Polyethers 20-34... 55

2.19. General Procedure for Competitive Solvent Extraction 59

3.1. Two Probable Interconversion Modes of Calix[4]arene 68

3.2. Four Limiting Conformational Isomers of Calix[4]arenes 68

3.3. Matching of Charges on the Metal Ion and Ionized Ligand to Provide Efficient Extraction 69

3.4. Structures of N-(X)sulfonyl/>-?ert-Butylcalix[4]arenecarboxamides 70

3.5. Sti^ctures of Flexible and Restricted Calix[4]arenecarboxylic Acids 37-41 71

3.6. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in each) Solutions of Alkali Metal Cations with 1.0 mM 1,2-Dichloroethane Solutions of calix[4]arenecarboxylic Acids 37-41 73

3.7. Structures of Conformationally Restricted N-(X)sulfonyl Calix[4]arene-carboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3 75

3.8. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10 mM (in Each) Alkali Metal Cations with 1.0 mM Chloroform Solutions of 54 (a) before and (b) after washing with 1.0 N HCl. Before washing, [Na^] was 8.0 mM 76

3.9. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkali Metal Cations with 1.0 mM Chloroform Solutions of N-(Ph)sulfonyl Calix[4]arenecarboxamides 46 (cone), 47 (paco-butyl-up), 48 (paco-acid-up), and 49 {1,3-alternate) 77

3.10. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkali Metal Cations with 1.0 mM Chloroform Solutions of cone N-(X)sulfonyl Calix[4]arenecarboxamides 42 (X=CH3), 46 (X=Ph), 50 (4-NO2C6H4) and 54 (X=CF3) 81

XVI

3.11. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM 1,2-Dichloroethane Solutions of Calix[4]arenecarboxylic Acids 37-41 85

3.12. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM Chloroform Solutions of N-(Ph)sulfonyl Calix[4]arenecarboxamides 46 (cone), 47 (paco-butyl-up), 48 (paco-acid-up), and 49 {1,3-alternate) 87

3.13. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM Chloroform Solutions of cone N-(X)sulfonyl Calix[4]arenecarboxamides 42 (X=CH3), 46 (X=Ph), 50 (4-NO2C6H4) and 54 (X=CF3) 90

3.14. Pb Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM 1,2-Dichloroethane of Calix[4]arenecarboxylic Acids 37-41 92

3.15. Effect of Conformation Variation on Pb " Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) CH3, (b) Ph, (c) 4-NO2C6H4 and(d)CF3 95

3.16. Effect of X Group Variation on Pb " Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) cone, (h) paco-hutyl-up and (c) 1,3-alternate • 96

3.17. Hg " Extraction versus the Equilibrium pH of the Acidic Aqueous Phase for Extractions of 0.25 mM Solution of Mercuric Nitrate with 0.25 mM 1,2-Dichloroethane Solutions of Calix[4]arenecarboxylic Acids 37-41 97

3.18. Effect of Conformation Variation on Hg " Extraction versus the Equilibrium pH of the Acidic Aqueous Phase for Extractions of 0.25 mM Solution of Mercuric Nitrate with 0.25 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) CH3, (b) Ph, (c) 4-NO2C6H4 and (d) CF3 98

3.19. Effect of X Group Variation on Hg "" Exttaction versus the Equilibrium pH of the Acidic Aqueous Phase for Extractions of 0.25 mM Solution of Mercuric Nitrate with 0.25 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) cone, (b)/7flco-butyl-up and (c) 1,3-alternate 100

xvu

4.1. Structures of Dicyclohexano-18-crown-6 (DC18C6), l-Alkyl-3-methylimidazolium Hexafluorophosphates ([Cn-mimjPFe) and l-Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imides ([Cn-mim]NTf2). 110

4.2. Linear Dependence of the Density of RTILs on the Types of Cations (1 -Alkyl Group Chain Length) and Anions (PFe" and NTf2") I l l

4.3. Exponential Decrease of Water Solubility of RTILs on the Type of Cations (I -Alkyl Group Chain Length) and Anions (PFe" and NTf2") 112

4.4. Influence of 1 -Alkyl Group Variation on the Efficiency of Competitive Alkali Metal Cation Extraction from Aqueous Solution into 1 -Alkyl-3-methylimidazolium Hexafluorophosphates (a) without and (b) with DC 18C6 114

4.5. Influence of 1 -Alkyl Group Variation on the Efficiency of Competitive Alkali Metal Cation Extraction from Aqueous Solutions into 1 -Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imides (a) without and (b) with DC 18C6 117

4.6. Influence of the 1 -Alkyl Group and Anion Variations on the KVCS"" and K^Rb" Selectivities in Competitive Alkali Metal Cation Extraction from Aqueous Solutions by DC18C6 in (a) l-Alkyl-3-methylimidazolium Hexafluorophosphates and (b) l-Alkyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imides 119

4.7. Influence of 1 -Alkyl Group and Anions Variation ((a) CI", (b) NO3" and (c) CIO4") on Efficiency of Competitive Alkaline Earth Metal Cation Extraction from Aqueous Solutions into l-Alkyl-3-methylimidazolium Hexafluorophosphates in the Absence of DC18C6 121

4.8. Influence of 1-Alkyl Group and Anion Variations ((a) CI", (b) NO3" and (c) CIO4") on Efficiency in Competitive Alkaline Earth Metal Cation Extraction from Aqueous Solutions by DC18C6 into 1 -Alkyl-3 -methylimidazolium Hexafluorophosphates 122

4.9. Influence of the 1-Alkyl Group and Anion Variation on Ba ' /Sr " Selectivity in Competitive Alkaline Earth Metal Cation Extraction from Aqueous Solutions by DC18C6 into 1-Alkyl-3-methylimidazolium Hexafluorophosphates 123

4.10. Influence of 1 -Alkyl Group and Anion Variation (CI", NO3" and CIO4") on the Efficiency of Pb " Extraction from Aqueous Solution (a) without and (b) with DC18C6 into I-AIkyl-3-methylimimdazolium Hexafluorophosphates ([Cn-mimJPFe) 125

xviu

4.11. Influence of 1 -Alkyl Group and Anion Variations (CI", NO3" and CIO4") on the Efficiency of Pb ^ Extraction from Aqueous Solution (a) without and (b) with DC18C6 into l-Alkyl-3-methylimimdazolium Bis[(trifluoromethyl)sulfonyl]imides ([Cn-mim]NTf2) 127

4.12. General Procedure for Competitive Solvent Extraction in RTIL 131

5.1. Calculated Electrode Responses as a Function of the Primary Ion (log ai) for a Sample Containing a Constant Interfering Ion Background (log aj) 139

5.2. Structures of Mono-Substituted PNP-Crown lonophores 58-65 141

5.3. Selectivity Coefficients, log K''°Vb, M, for Solvent Polymeric Membranes Containing Monoaryloxy-Substituted lonophores 58-65 141

5.4. Structures of Tetra-Substituted PNP-Crown lonophores 66-72 142

5.5. Selectivity Coefficients, log K''°Vb, M, for Solvent Polymeric Membranes Containing Tetra-Substituted lonophores 66-72 143

5.6. Structures of Tetra-Substituted PNP-Crown lonophores 66, 73 and 74 144

5.7. Selectivity Coefficients, log K''°Vb, M, for Solvent Polymeric Membranes Containing Tetra-Substituted lonophores 66, 73 and 74 145

5.8. Structures of Tetra-Substituted PNP-Crown lonophores 66, 67, 75 and 76.. 146

5.9. Selectivity Coefficients, log K ^ b , M, for Solvent Polymeric Membranes Containing Tetra-Substituted lonophores 66, 67, 75 and 76 146

5.10. Selectivity Coefficients, log K''°Vb, M, for Solvent Polymeric Membranes Containing lonophores 58 versus 66, 60 versus 68, 64 versus 67, 61 versus 69, 62 versus 70, and 63 versus 71 147

5.11. Structures of Monoamino-Substituted PNP-Crown Macrocycles 77-80 148

5.12. Selectivity Coefficients, log K^bccs), M, for Solvent Polymeric Membranes Containing Mono-Substituted lonophores 77-80 149

5.13. Structures of Mixed with Different Substituents Amino-Aryloxy PNP-Crown Macrocycles 81-84 150

5.14. Selectivity Coefficients, log KP Vbccs), M, for Solvent Polymeric Membranes Containing with Amino-Substituted lonophores 64 and 80-84 151

5.15. Structures of Bis-Lariat PNP-Crown Macrocycles 85-90 152

5.15. Selectivity Coefficients, log K^\b, u, for Solvent Polymeric Membranes Containing lonophores 67 and 85-90 152

5.17. Structures of PNP-Crown Macrocycles 91-97 153

xix

5.18. Structures of Dinaphtho-16-Crown-4 Macrocyles 98 and 99 154

5.19. Selectivity Coefficients, log K''° cs(Li), M, for Solvent Polymeric Membranes Containing lonophores 98 and 99 154

5.20. Structures of Diazacrown Macrocyles 100-104 155

5.21. Selectivity Coefficients, log K''°'pb, M, for Solvent Polymeric Membranes Containing lonophores 100-104 156

5.22. Schematic Diagram of a Membrane Electrode Measuring Circuit

and Cell Asssembly 159

A. 1. Alkali Metal Loading versus the Equilibrium pH of Aqueous Phase 164

B. 1. Alkaline Earth Metal Loading versus the Equilibrium pH

of Aqueous Phase 167

C. 1. Pb"* Loading versus the Equilibrium pH of Aqueous Phase 173

D. 1. Hg " Extraction versus the Equilibrium pH of Aqueous Phase 179

XX

CHAPTER I

INTRODUCTION

1.1. Historical Overview

"Molecular recognition" has been defined as the formation of molecular

complexes in biological reactions caused by the combination of several kinds of

molecular interactions between a host and a guest molecule like the initial stage of an

enzymatic reaction.' It has been regarded as one of the most fundamental differences

between a biological and an artificial chemical reaction. The discovery of crown ethers

and their characteristics introduced the concept of molecular recognition into non-

biological, artificial chemical reactions.

The first public announcement of the discovery of crown ethers and their unusual

complexes with alkali and alkaline earth metal cations was made by C. J. Pedersen at the

lO" International Conference on Coordination Chemistry on September 15, 1967.

Subsequently, a short communication^ and a subsequent full paper were published in the

Journal of the American Chemical Society by the end of 1967.' By 1971, Pedersen

reported the discovery of crystalline complexes and additional novel crown compounds,

including macrocyclic polyether sulfides containing sulfur atoms in the ring structure.'*'

Pedersen named the macrocyclic polyethers "crown ethers" because of their chemical

structure as well as the shape of the complexes that resembled a crown on a metal ion.

This discovery had a great impact on many chemists in the next several decades.

Since then, many crown ethers and analogous compounds have been synthesized. Their

specific characteristics have been investigated leading to the terminologies of "host-guest

chemistry" and "supramolecular chemistry."

D. J. Cram defined "host-guest chemistry" as the field of chemistry consisting of

syntheses and applications of highly structural molecular complexes formed by

recognition and incorporation of the matched guest by the host molecule having a

designed cavity.^ He laid the foundation of "host-guest chemistry" through systematic

studies on asymmetric recognition, including optical resolution, optically selective

ttansport, and asymmettic reaction by use of series of chiral crown ethers with optically

active binaphtyl groups.^

As an extension of host-guest chemistry, J.-M. Lehn introduced "supramolecular

chemistry" that including the structures and functions of supramolecules that result from

binding substrates to molecular receptors as well as higher-order molecular aggregates

which are formed by molecular interaction between two or more molecules. Thus

supramolecular chemistry extends beyond the concept of the molecule.'"'"

Currently, molecular recogmtion is defined by the energy and the information

involved in the binding and selection of substrate(s)/guest(s) by a given receptor/host

molecule. Thus new conceptual species have been added to organic chemistry, which

was previously comprised of molecules formed by strong covalent bonds.'^ This is an

epoch in the history of chemistry. For their pioneering work, C. J. Pedersen, D. J. Cram

and J.-M. Lehn were jointly awarded the Nobel Prize in Chemistry in 1987.

1.2. Crown Ethers and Analogous Compounds

1.2.1. History

In 1937, the first macrocyclic polyethers were reported by Liittringhaus as part of

an investigation of medium- and large-sized rings.''* However, it was C. J. Pedersen who

published the discovery of crown ethers and their unusual complexes with alkali and

alkaline earth metal salts. His appreciation of the importance of the discovery followed

by energetic research in the area established the foundation for the present status of

crown ethers in chemistry.

Following the initial work by Pedersen, crown ethers were utilized in ion-

selective electrodes,'^ heavy metal separations from wastes' and ion-exchange

chromatography.' "^" The intense interest in crown ethers and their use in separation

science and technology led to the publication of several books during last decade.

The original definition of a crown ether was a compound with multiple ether

oxygen atoms incorporated into a monocyclic backbone. Presently, crown ether

analogues that include other heteroatoms, such as sulfur and nitrogen, are known as

thiacrownethers and azacrown ethers, respectively.

The attachment of one or more side arms with potential metal ion coordination

site(s) to a crovra ether framework produces a complexing agent known as a "lariat

ether." ' The side arm contains one or more heteroatoms (O, S, N and/or P) which due

to their lone electron pairs are Lewis bases and may function as complexation sites.

New acyclic crown compounds have been designed in which the basic structures

was ahered by changing the topology by variation of the terminal groups or by insertion

of a fianctionalized segment into the middle of the ether chain.^' The key point in their

design were to wrap the guest cation completely by a linear molecule. In 1979, Vogtie

and Weber suggested that open-chain analogues, "podands," such as 1 in Figure 1.1 could

serve as substitutes for crown ethers.^' Synthetic acyclic podands generally form

complexes of lower stability than those of corresponding crown ethers and cryptands

(steric cage-type bicyclic crown compounds). Typical orders of magnitude for the

stability constants for the complementary guest cations (K, M"' in methanol) are lO' -lO^

for acyclic polyethers, 10'*-10 for crown ethers, and 10 -10^ for cryptands. However,

several naturally occurring lonophores of acyclic podand type effectively accommodate

guest cations within pseudo-cavities.^' Thus, monensin (2) has a higher stability constant

for Na" (log K = 4.9 in methanol) than those of synthetic 15-crown-5 and 18-crown-6

(log K = 3.5 and 4.4, respectively). Monensin has a characteristic pseudo-cyclic

conformation, which is stabilized by head-to-tail hydrogen bonding between the terminal

groups. Thus, it effectively traps Na^ in the pseudo-cavity. Most synthetic acyclic

polyethers do not have preformed intramolecular cavities for guest bindings, but are able

to build up a different type of "pseudo-cavity" in which guest cations can nest.

Kuboniwa and his coworkers demonstrated that acyclic polyether 3 could form a pseudo-

cyclic complex with K* in a similar fashion to naturally occurring lonophores. The

backbone formed a 31-membered pseudo-ring by head-to-tail hydrogen bonding and bent

to wrap the K" ion completely. Hiratani and his coworkers also designed several acyclic

crown compounds, such as 4, with carboxylate terminal groups, which exhibited high

selectivity for Li' . ''

Figure 1.1. Structures of Acyclic Polyether Ligands.

1.2.2. Nomenclature

According to lUPAC nomenclature, dibenzo-18-crown-6 should be named

2,5,8,15,18,21 -hexaoxa-tricyclo[20.4.0.0'^"*]hexacosa-l (22),8,11.13.23.24-hexaene,

which is neither convenient nor easily understandable. The trivial nomenclature for

crown ethers^ proposed by C. J. Pedersen is more popular than the more complicated and

lengthy lUPAC names.

The trivial name for a crown ether contains in order: (i) the number, name and

positioning of the substituents on the crown ether ring, if any; (ii) the total number of

atoms, regardless of their kind, which make up the crown ether ring; (iii) the word

"crown" which indicates the class of the compound; and (iv) the number of oxygens that

are part of the crown ether ring. This nomenclature system has made communication

about crown ether simpler and easier.

The structure of the molecule known as 15-crown-5 is shown as 5 in Figure 1.2.

The word "crown" refers to the class of compound - a cyclic polyether. The first number,

"15," designates the total number of atoms, both carbon and oxygen, within the polyether

ring. The second number, "5," specifies the number of oxygens in the polyether ring.

O C

5 15-crown-5

0 0.

O O

14-crown-4

NH HN

O 7

1,7-diaza-18-crown-6

O O

O 8

dicyclohexano-18-crown-6

O

O

O. 9

dibenzo-18-crown-6

0 o

o o 10

2,3,8,9-dibenzo-18-crown-6

/ O O '

11

l,2-bis[2-(2'-carboxyalkyloxy)phenoxy]ethane

Figure 1.2. Nomenclature for Some Crown Ethers and an Acyclic Polyether.

Molecular stability requires that there be at least a two-carbon bridge between the

ether oxygens. For simple structures, the placement of oxygens within the ring need not

be specified. However when more than one three-carbon bridge is present, it may be

necessary to specify the placement of the ether oxygens. A 14-crown-4 molecule is

shown in 6. The compound is identified as 14-crown-4 or l,4,8,12-14-crown-4. The

isomer with the highest symmetry is given the simple name. If none of the isomers have

greater symmetry, then numbers are used to identify the placement of the ether oxygens

for all isomers.

Crown ether compounds, which contain heteroatoms such as N, S, P, etc., are

given trivial names that indicate the number, placement and type of such atoms. For

example, 7 is called l,7-diaza-18-crown-6.

There are trivial names for crown ethers containing cyclohexano (e.g., in 8) or

benzo (e.g., in 9 and 10) units. In this case, the placement of the substituent is not

specified if the molecule is a single or the most symmetric isomer.

For acyclic polyether ligands, there is no trivial nomenclature and lUPAC names

are used. The systematic name for 11 is given in Figure 1.2.

1.3. Calixarenes

1.3.1. History

Calixarenes are a group of phenolic macrocycles that are metacyclophanes

armulated by single methylene groups. Although they have become significant in

supramolecular chemistry only during the past two decades, their origins can be traced

back to the von Baeyer's discovery of phenol-formaldehyde resins in 1872. " The cyclic

products remained uncharacterized for almost 70 years until the cyclic tetramer structure

was first assigned by Zinke in 1942.

In spite of its long history, calixarene chemistry practically started in the 1970's

and rapidly developed during the decade of 1980's. Gutsche was the first to draw

attention to their potential as molecular receptors or enzyme mimics. He proposed

"calixarene" (Greek calyx for chalice or cup) in 1978 as the name of this homologous

series of macrocyclic phenol-formaldehyde condensates, whose shape is similar to that of

a Greek crater vase.''^ The term "calix" was chosen to describe the shape of cyclic

tetramers when they assume what is now called the ''cone" conformation and the

extension "arene" designates the aromatic nature of the six-membered rings. Their

constitution and structure has been the subject of much investigation in recent years and

calixarenes are regarded as "third-generation" supramolecules following cyclodextrins

and crown ethers.^^ They may become another milestone in receptor chemistry although

they have more in common with spherands (compounds having a spherical cavity) and

podands than with crown ethers or cryptands. ^ It is their rigidity and amenability to

chemical modification that distinguish calixarenes from other synthetic macrocyclic

compounds.

1.3.2. Nomenclature

Since the term "calixarene" was originally chosen to describe the shape of the

phenol-derived cyclic tetramer in the conformation in which all four aryl group are

oriented in the same direction (now called the "cone'" conformation), calixarene

structures are depicted with the aryl carbon usually carrying an oxygen function between

the methylene groups pointing downward and the aryl para carbon pointing upward.

Accordingly, the face bearing the endo hydroxyl groups (Figure 1.3) is designated as the

"lower rim" and the face bearing the para substituents is designated as the "upper rim."

Upper Rim

OH I I HO •* Lower Rim OH OH

Figure 1.3. Representation of Calix[4]arene and Designation of the Faces.

To accommodate the name to the subsequently discovered oligomers containing

more than four aryl groups, a bracketed number is inserted between calix and arene. For

more systematic application of the calixarene nomenclature, the basic name

"calix[n]arene" is retained, and the identities of all of the substituents are indicated and

their positions specified by numbers. For instance, the lUPAC name for compound 12

(Figure 1.4) is pentacyclo[19.3.1.1^'M^''ll'^-'']octacosa-l(25),3,5,7(28),9,ll,13(27),15,-

17,19(26),21,23 dodecaene. Using alternate nomenclature suggested by C. D.

Gutsche, '^''^^ macrocyclic ligand 13 is named 25,26,27,28-tetrahydroxycalix[4]arene.

The calix[4]arenes were first recognized as being capable of assuming four

limiting conformations with different numbers of aryl groups projecting upward or

downward relative to a mean plane defined by the bridging methylene groups. Later

Gutsche designated these as ''cone,'" "partial cone,'" "1,3-alternate,'" and " 1.1-alternate"

conformations (Figure l.S).''^

Figure 1.4. Structures of Representative Calixarenes.

R R R R

OY I I YO OY OY

Cone

R R

R R

1,3-Alternate

R Partial Cone

R

1,2-Alternate

Figure 1.5. Limiting Conformations of Calix[4]arenes.

1.4. Room-Temperature Ionic Liquids'*^

1.4.1. History

Much of our understanding of chemistry is based upon the behavior of molecules

in solution. Although any liquid can be used as a solvent, only a few are in general use.

Since most solvents employed by chemists are organic compounds that are volatile and

often produce noxious vapors, they are difficult to dispose of or recycle without

generating flammable and environmentally damaging pollutants. Furthermore, they are

used in huge amounts in industry.

Recently, room-temperature ionic liquids have emerged as a novel replacement

for volatile organic solvents, especially as solvents and catalysts for a wide variety of

organic reactions.'*''"'' They have some useful properties: they dissolve many

compounds, are non-volatile, non-flammable and generally non-toxic. They are solvents

that are often liquids at room temperature and consist entirely of ionic species. Since

ionic liquids are made up of two components that can be varied (the cation and anion),

the solvents can be designed for a particular use or for particular properties. Hence, the

term "Designer Solvents" has come into common use.'* The first room-temperature ionic

liquid [EtNH3][N03] (m.p. 12 °C) was discovered in 1914, * ' ' but interest did not

develop until the discovery of binary ionic liquids made from mixtures of aluminum(III)

chloride and N-alkylpyridinium or 1,3-dialkylimidazolium chloride. In 1990, another

kind of ionic liquids, [emim][BF4] (m.p. 12 °C), were prepared via metathesis of [emimjl

(where [emim]" is the l-ethyl-3-methylimidazolium cation) with AgBF4 in methanol. ^

Easy preparation along with stability to moisture and immiscibility with a number of

organic solvents has led to its increasing use in biphasic catalysis.

Ionic liquids consist of a sah where one or both of the ions are large, and have low

symmetry. Due to this, ionic liquids have low crystal lattice energy and low melting

points.

The properties (melting point and viscosity) of ionic liquids can be adjusted by

simple changes in the structure of the ions. The solvent properties of ionic liquids have

been investigated using chromatographic techniques.""^^ Generally, ionic liquids are

polar phases being largely determined by the ability of the salt to act as a hydrogen-bond

donor and/or acceptor and the degree of localization of the charge on the anion. Another

important property is the miscibility of water in these ionic liquids. It was found that

increasing the chain length of alkyl substituents on both the cation and the anion leads to

greater lipophilicity of the ionic liquid. ' This property can be an important advantage

when carrying out the solvent extraction or product separations, because the relative

solubilities of the ionic liquid phase can be adjusted to make the separation as easy as

possible. 61-67

1.4.2. Nomenclature

The nomenclature for ionic liquids follows lUPAC recommendations. The

suffix, -ium, is added to indicate the electropositive constituent in binary type names and

-ate or -ide for polyatomic anions in ionic liquids. Structure of l-alkyl-3-

methylimidazolium hexafluorophosphate ([amimjPFe, 14), 1-alkyl-3-methylimidazolium

bis[(ttifluoromethyl)sulfonyl]imide ([amim]NTf2, 15) and 1-alkylpyridinium

tetrafluoroborate (16) are shown in Figure 1.6.

\^ PF, N[CF3S0 '2J2

14 15

Figure 1.6. Representative Room-Temperature Ionic Liquids.

BF.

1.5. Cvclophosphazene-based Crown Ethers

1.5.1. History

There is a very extensive chemistry of compounds with P-N and P=N bonds. The

PNP cation is only one special example of a group of compounds called phosphazenes.^^

Phophazenenes are inorganic heterocyclic or linear compounds containing alternate

phosphorous and nittogen atoms. In such compounds, the phosphorous atoms are

pentavalent and tettacoordinate while nittogen is dicoordinate and ttivalent.

Monophosphazenes (R3P=NR'), diphosphazenes (R3P=N-PR2') and polyphosphazenes

(R3P=N-(PR2=N)n-PR2) are known. However, it is the cyclophosphazenes^" that

currently attract the greatest attention. Most important is the six-membered

cyclophosphazene. The most common is N3P3CI6, which is commercially available. This

compound can be thermally polymerized to obtain a series of industrially important

polymers. The six ring atoms are neariy planar. The P-N distances are in the range of

1.55 to 1.61 A, somewhat shorter than an expected single-bond length of-1.75 to 1.80 A.

Considerable attention has been paid to the nature of the P-N n bonding, which the P-N

distance indicates is appreciable, but the matter is still subject to controversy. Due to the

large number of orbitals potentially involved and to the general lack of ring planarity, it is

impossible to assign a and n character rigorously to the individual orbitals.

In cyclic phosphazene (N3P3CI6), the presence of three phosphorous atoms allows

easy access to compounds with two or three azido groups per molecules. Furthermore,

the properties of phosphazenes can be tailored by the organic substituent groups that are

connected to the phosphazene ring. Amines, alcohols and thiols react with N3P3CI6 in a

stepwise marmer. The reaction of one equivalent of nucleophile gives rise to a

monosubstituted product. However, attachment of a second nucleophile yields three

isomeric disubstituted derivatives that differ by the spatial arrangement of the organic

group. In the geminal derivative, the incoming nucleophiles substitute the chlorines of

the same phosphorous atom. In the non-geminal derivatives, where the chlorine atoms of

the two neighboring phosphorous atoms are displaced, both cis and trans isomers are

possible. These possibiUties are illustrated in Figure 1.7.

Cl-

Cl-

N

4 CI

,CI

^^^ X I 1:1 CI-

• - N ^ ' CI

i CI

< . ,CI

. .^ CI

CI .R

1:2 cu

I N I CI CI

Geminal

,ci + cu P- , .^P

.v\R + cu

^ P \

CI CI

Non-Geminal (cis and trans) CI R

.-v^^\' CI

Figure 1.7. Structural of Isomers from Disubstitution of N3P3CI6.

The reaction of difunctional nucleophiles (such as diamines, amino alcohol, diols,

etc.) can lead to a series of derivatives. When one equivalent of bifunctional reagent is

treated with N3P3CI6, four products are possible. The possible isomers are illustrated in

the Figure 1.8.

CI + CI

-Fr~-^«^

CI + cu X I P

CI CI Bino or Bridge

l~-NÎ CI CI

Dangling or Fly-over

Figure 1.8. Structural Isomers N3P3CI6 Reacted with a Bifunctional Reagent.

1.5.2. Nomenclature

Compound 17 (Figure 1.9) is named hexachlorocyclottiphosphazatriene. A new

class of fimctionalized crown compound 18 was developed by combination of the

12

versatile reactivity of N3P3CI and the complexing properties of a crown ether ligand.'"'''

Compound 18 is named [oxy(tetraethylenoxy)]-tetrachlorocyclotriphosphazatriene.

CI. .CI Cl , ^Cl c u CI

CI CI Ck II I ^C C k II I n I n y A N'' K A N'' P-NH(CH,) ,HN—P. , \ ' ^ '

Ck^ i^ I c,

CI ' CI ^ 0 o ^ ^ o o ^ ^ o o ^

17 18 19

Figure 1.9. Representative Cyclophosphazene-Based Crown Ethers.

Similar to other macrocyclic ligands, a trivial nomenclature system is applied to

these compounds. Since the structure of 18 is based on a crown ether backbone, it is

designated a PNP-crown. Using the prefix of the structural isomers (Figures 1.7 and 1.8),

compound 19 is named 1,2-diaminoethane-bino-ansa-PNP-crown.

1.6. Cation Complexation by Multidentate Ligands

Complexation of a metal ion by a multidentate ligand in solution can be pictured

as successive replacement of the solvent molecules from the first coordination sphere of

the cation, in a fiilly stepwise manner, by the donor atoms of the ligand. ' '* This process

is usually accompanied by conformational changes within the ligand. However,

solvation and complexation may compete with each other since both involve the same

forces (i.e., hydrogen bonding, ion pairing, metal-ion ligation, donor-acceptor ion-dipole,

and van der Waals interaction).

Recently, several studies have revealed the importance of interactions of the metal

ion with an aromatic substituent in the ligand.^^"" Complexation was found to be

influenced by n-cation interaction not only with d-block elements, but also with some of

alkali and alkaline earth metal cations.

13

1.6.1. Complexation Efficiency

Acyclic polyether ligands usually, but not always, exhibit lower complex stability

constant (Ks) than do the corresponding cyclic polyether ligands. ^ This increased

binding strength for the cyclic ligands is described as the "macrocyclic effect." Most

crown ether compounds bind metal cations with increased strength and selectivity

compared to their open-chain analogues (i.e., podands). ^ A metal ion is held in the

crown ether cavity by a combination of electrostatic and ion-dipole or donor-acceptor

interactions between the charged cation and dipoles created by the nonbonding electrons

of donor atoms. For crown ethers, the macrocyclic effect is believed to be of enthalpic

origin which is different from the chelating effect. The latter describes the additional

binding strength shown by multidentate ligands over their monodentate analogous and is

mainly of entropic origin. ^ It has been suggested that preorganization of the ligand

binding site is a major contributor to the macrocyclic effect. For a preorganized ligeind,

binding a particular metal ion requires only minimal adjustment of the ligand

conformation. Preorganization of the ligand can also cause increased dipole-dipole

repulsion in the cavity of the ligand and reduced ligand solvation. Since solvation

usually tends to relieve ligand strain energy caused by dipole-dipole repulsion, a less

solvated macrocyclic ligand possesses high strain energy that could be significantly

reduced by complexing a metal cation.

One of the most powerful strategies in creating more preorganized ligands is to

rigidify the macrocycle backbone. Carefully designed, structurally rigid ligands have

been synthesized. The most prominent examples are cryptands, spherands and

calixarenes.*" Another ttend towards preorganized ligands is the addition of pendent

arms to the macrocycle that provide additional ligation sites. Lariat ethers with a side 0 1

arm containing one or more donor groups can provide three-dimensional ligation.

1.6.2. Complexation Selectivity

A key feature of macrocyclic compounds is that they exhibit a significant

selectivity toward metal ions compared to open-chain chelators.^" When the metal ion

14

diameter matches the ligand cavity (Table 1.1), the complex is usually more stable than

the complexes with other metal ions. Even for calixarenes, which are structurally

preorganized, their selectivities toward cations are a function of the cavity dimensions

and the nature of the binding groups.*^

Table 1.1. Cavity Diameters for Various Crown Ethers and Diameters of Alkali and Alkaline Earth Metal Cations.

Ligand

12-Crown-4

14-Crown-4

15-Crovra-5

16-Crown-5

18-Crovm-6

21-Crown-7

24-Crown-8

Cavity Size

1.2

1.2

1.8

1.9

2.7

3.3-3.5

4.1-4.3

(A) Metal Cation

Li"

Na"

K"

Rb"

Cs"

Mg^"

Ca^"

Sr "

Ba^"

Ionic Diameter (A)''

1.48

2.02

2.76

2.98

3.70

1.44

2.00

2.36

2.70

: Values were estimated from CPK space filling molecular models. The values predicted from CPK models are typically equivalent to the lower limit of ranges estimated from available crystallographic results.

'': Ionic diameters were taken from X-ray crystallographic results for a coordination number of six.

The prediction of ligand selectivity is more complicated than a simple size-match

concept. Many other factors involving the ligand, ionic guest, and solvent must be

considered. In addition, crown ethers can form different types of complexes with metal

ions both in solution and in the solid-state.* ' ^ A nesting complex forms when there is a

close match between the polyether cavity size and the cationic diameter of the guest

(Figure 1.10.a).*^ If the cation is larger than the polyether cavity, then a perching

complex may be formed (Figure 1.10.b). In such cases, the cation perches on the atoms

of the macrocyclic ligand. The sandwich-type complex is a different form of perching

15

complex in which two polyether ligands form perching complexes with one cation

(Figure l.lO.c).**'*^ This results in a 1:2 ratio (i.e., two ligands per cation) for

complexation. There are cases where three ligands form perching complexes with two

guests (Figure l.lO.d) to form club-sandwich-type complexes. Finally, if the polyether

cavity is too large for one cation, it may accommodate two cations at the same time

(Figure 1.10.e). These cations may be the same or different.^"'^'

(a) Nesting Complex (b) Perching Complex (c) Sandwich-Type Complex

(d) Club-Sandwich-Type Coplex

(e) Two Guests Complexed by One Host

Figure 1.10. Different Complexation Modes for Polyether Ligands and Cations.

Hancock found that medium-to-large metal ions generally form more stable

complexes with five-membered than six-membered chelate rings. He also suggested that

adding neuttal oxygen donor atoms to acyclic polyethers leads to an increase in

selectivity for large metal ions over smaller ones.^^ The type of donor atoms is also an

important factor determining the selectivity of a ligand.^^ According to the hard-soft

acid-base (HSAB) theory,^'' crown ethers with oxygen donor atoms should exhibit high

affinity toward hard metal ions, such as alkali and alkaline earth cations.^^ However,

some crown ethers display a significant binding sttength for heavy metal ions, such as

16

Pb , which is considered to be an intermediate acid. This is related to the closed-shell

electronic configuration of the cation that makes it spherical. In addition, unlike most of

heavy transition metal, Pb^" does not have strong geometric preference in coordination

bond arrangement.

1.7. Solvent Extraction: Background and Theory

Liquid-liquid extraction, commonly known as solvent extraction, has been used as

an effective tool by separation scientists and analytical chemists for decades. ^ One of

the main advantages that made this method so attractive is that it is applicable to trace as

well as macroquantities of compounds. Other advantages, which contributed to the

popularity of solvent extraction as an effective analytical tool are its technical simplicity

and relatively short contact time.

Di-ionizable acyclic polyethers and calix[4]arenes can be represented as a weak

acid, H2L, which loses two protons when it binds a divalent metal cation.

H2L(aq) ^ 2H"(aq) + L''(aq) Ka = [H"]'aq [L'"]aq / [H2L]aq (1.1)

L'"(aq) + M'"(aq) ^ ML(aq) (3 = [ML]aq / [M "] aq [L'"]aq (1.2)

As can be seen, the efficiency is controlled by the pH and p controls the selectivity.

Let's derive an equation for the distribution coefficient of a divalent metal cation

between two phases when essentially all of the metal in the aqueous phase (aq) is in the

form M^" and all of the metal in the organic phase (org) is in the form ML (Figure 1.11).

The partition coefficient for ligand and complex can be defined as follows:

H2L(aq) ^ H2L(org) KL = [H2L]org / [HzLjaq (1-3)

ML(aq) ? ML(org) KM = [MLjorg / [MLJaq (1-4)

The distribution coefficient is:

D = [total metaljorg / [total metal]aq = [ML]org / [M "]aq (1 -5)

From Equations 1.2 and 1.4 followed by Equation 1.1, [ML]org can be written as:

[ML]org = K M [MLjaq = K M P [ M ' " ] aq [L'"]aq Ka [HzLjaq / [ H " ] \ q (1.6)

Putting [MLjorg into Equation 1.5, D can be written as

17

D = KM P [L'"]aq Ka [H2L]aq / [H^^q (1 J )

Because most of the H2L is in the organic phase, KL = [H2L]org /[UiLU can be substituted

to produce the most useful expression for the distribution coefficient:

Distribution of metal-ligand , , , ^ D = KM P [L'"]aq Ka [H2L]org / KL [H"]\q (1.8)

complex between phases

Equation 1.8 shows the distribution coefficient for metal ion extraction depends on

the control of variables, such as pH, ligand concentration and choice of solvent.^ ' ^ A

large "D" value means that most of the metal cation has been transferred into the organic

phase, resulting in a nearly complete extraction. In dealing with proton-ionizable ligands,

pH plays a very important role in liquid-liquid extraction.

Compared to neutral ligands (non-ionizable), proton-ionizable ligands have the

advantage that transport of metal cations into organic diluent does not require

concomitant transport of anions from the aqueous phase. This factor is important to the

potential practical applications of proton-ionizable ligands in which the aqueous phase

anions of chloride, nitrate and sulfate will be involved. These anions are poorly extracted

into the organic phase due to their high solvation energies. An additional advantage of

proton-ionizable ligands over neutral ligands is a buih-in mechanism for the stripping of

the cations in liquid-liquid extraction (Figure 1.12). Following the extraction step, the

separated organic phase containing metal-ligand complex is brought into contact with

strong acidic solution (Figure 1.13). This results in regeneration of the neutral form of

the extractant and release of the metal cation into the contacting aqueous solution. The

acidity of the aqueous phase must be high enough to protonate the ligand-metal ion

complex and release the metal cation. This factor depends on the pKa of the proton-

ionizable ligand. To determine the concentration of metal cations that was extracted by

proton-ionizable ligands, ion chromatography and atomic absorption specttophotometry

are routinely used.

Figure 1.11. Schematic Diagram for the Extraction of a Metal Cation by Proton-Ionizable Ligand.

19

Before extraction After extraction

Aq. Org.

c^

0

©

0 ' 1

©

1 (a) Neutral Polyether

Before extraction

0

© Aq. Org

After extraction

0 H"

0 «

fe fe

©H" \

© i 0 j

i

1 1

(b) Proton-ionizable Polyether

Figure 1.12. Solvent Extraction by (a) a Neuttal Polyether and (b) a Proton-ionizable Polyether.

+ HX + fu'j X"

Figure 1.13. Metal Ion Stripping from a Proton-Ionizable Polyether Complex Using Acid.

20

1.8. Solvent Polymeric Membrane Electrodes: Background and Theory

Over the past 30 years, the application of carrier-based, ion-selective electrodes

(ISEs) has evolved into a well-established analytical technique. Potentiometry with ISEs

based on neutral carriers is utilized to characterize the complexing ability of these

ligands. "' ' The essential part of a carrier-based ISE is the ion-sensitive solvent

polymeric membrane, physically a water-immiscible liquid of high viscosity that is

commonly placed between two aqueous phases, the sample and the internal electrolyte

solution. It contains various constituents, commonly an ionophore (ion carrier) and a

lipophilic salt as ion exchanger. Its selectivity is related to the equilibrium constant of the

exchange reaction of target and interfering ions between the organic and aqueous phases.

It also strongly depends on the ratio of complex formation constants of these ions with

the ionophore in the membrane phase.'"^'"'^ A unique feature of the ISEs is that the

potential obtained depends on the activity of the target ion. It assumes that the local

thermodynamic equilibrium at the sample-membrane interface, which results in a direct

dependence of the interfacial potential on the sample activity.

Under ideal conditions, the electrode response fiinction follows the Nemst

equation.

E = E°i + RT/ZiF In (ai(I)) (1-10)

where ai(I) is the primary ion activity in the sample without interference from other

sample ions. The constant potential contributions are unique for every ion measured and

included in E°i. According to the Nicolskii-Eisenman equation, the activity term in the

Nemst equation is replaced by a sum of selectivity-weighted activhies.

E = E", + RT/z,F In (a,(IJ) + K'% aj(IJ)^""') (1 • 11)

where ai(IJ) and aj(IJ) are the activities of I and J in the mixed sample. The activity ai(I)

can be related to ai(IJ) of the mixed sample that gives the same potential E by combining

the equations.

a,(I) = ai(IJ) + K%aj(IJ)^''^ (1-12)

For extremely selective electtodes, the Nicolskii coefficient K^ 'u is very small and a[(IJ)

approaches a,(I). If interference is observed, a lower activity ai(IJ) of the mixed sample

21

will give the same response as the activity ai(I) of a solution containing no interference

ions. The Nicolskii coefficient is often determined by the so-called fixed interference

method (FIM), in which calibration curves for the primary ion are determined in a

constant background of interfering ions.

In the FIM, a calibration curve as shown in Figure 1.14 is measured for the

primary ion in a constant interfering ion background (aj(BG)). The linear (i.e., Nemstian)

response curve of the electrode as a function of the primary ion activity (ai(IJ) » K''°'ij

aj(IJ)^^) is extrapolated until, at the lower detection limit ai(DL), it intersects with the

observed potential for the backgroimd alone (ai(IJ) « K^°\} aj(IJ)''''^). The Nicolskii

coefficient is calculated from these extrapolated linear segments of the calibration curve,

each relating the analytical response of the ISE to one respective ion only.

For convenience in experiment, the lUPAC commission proposed that the

detection limit can be obtained from the extrapolated Nemstian line as the point at which

the potential difference between the experimental curve and the extrapolated EMF value

(E) equals 18.0 mV/zi.

UJ

K''"' =a,(DL)/a,(BG)""' [J 1 J

18mV/Z, /

^ (?? )___^

t a,(DL)

/©a i

log a,

Figure 1.14. Determination of the Nicolskii Coefficients According to the Fixed Interference Method (FIM) as Proposed by the lUPAC Commission.

22

1.9. Research Objectives

The overall objective of this research endeavor is to investigate the effects of the

variation of proton-ionizable and neuttal ligands and solvents on the efficiency and/or

selectivity of metal ion complexation and separation.

The investigation involves the use of two different methods. The first is liquid-

liquid extraction, commonly called solvent extraction, and the second is the solvent

polymeric membrane electrode, also called an ion-selective electrode.

The results of this research are presented in four chapters. Each chapter describes

a separate project.

1. The effects of structural variation (i.e., the size of the pseudo cavity and the identity of

the proton-ionizable groups) within proton-ionizable acyclic polyether ligands on the

efficiency and/or selectivity of metal ion separation will be investigated by means of

liquid-liquid extraction of different metal cations into an organic phase. Furthermore,

the effects of solvent variation, the effects of the metal ion variation including Pb^"

and Hg ", and its stoichiometry upon extraction efficiency and/or selectivity of the

proton-ionizable acyclic polyethers will be studied. The results for this part of the

research project are discussed in Chapter II.

2. The effects of structural variation within proton-ionizable calix[4]arene ligands (i.e.,

conformational variation and the identity of the proton-ionizable group) on the

efficiency and/or selectivity of metal ion separation (i.e., alkali metal cations, alkaline

earth metal cations, Pb " and Hg ") will be investigated by means of liquid-liquid

extraction of different metal cations into an organic phase. The results for this part of

the research project are dealt with in Chapter III.

3. The effects of matrix variation with dicyclohexano-18-crown-6 (DC18C6) ligand (i.e.,

the variation of 1-alkyl group chain length and the identity of the anion for room-

temperature ionic liquids) on the efficiency and/or selectivity of metal ion separation

(i.e., alkali metal cations, alkaline earth metal cations and Pb^") will be investigated

by means of liquid-liquid extraction of different metal cations into room-temperature

23

ionic liquid phase. The results for this part of the research project are contained in

Chapter IV.

4. Cyclophosphazene-based crown ethers behave differently than cyclic polyethers in

solvent polymeric membrane electrode. The effects of structural variation within

different series of cyclophosphazene-based crown ethers upon the selectivity of alkali

metal cations in solvent polymeric membrane electrode will be probed. In the final

Chapter V, the results of this research project are discussed.

24

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10. Lehn, J.-M. Science 1985, 227, 849.

11. Lehn, J.-M. Angew. Chem. Int Ed. Engl. 1988, 27, 90.

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18. Warshwsky, A.; Kalir, R.; Deshe, A.; Berkovitz, H.; Patchorink, A. J. Am. Chem. Soc. 1979, 707, 4249.

19. Grossman, P.; Simon, W. J. Chromatogr. 1982, 235, 351.

20. Nakajima, M.; Kimura, K.; Hayata, E.; Shono, T. J Liq. Chromatogr. 1984, 7, 2115.

21. Lindoy, L. F. The Chemistry of Macrocyclic Ligand Complexes; Cambridge University: New York, 1989.

22. Cation Binding by Macrocycles. Complexation of Cationic Species by Crown Ethers; Inoue, Y.; Gokel, G. W., Eds.; Marcel Dekker: New York, 1990.

23. Crown Ethers and Analogous Compounds; Hiraoka, M., Ed.; Elsevier: New York, 1992.

25

24. Crown Compounds. Toward Future Applications; Cooper, S R Ed - VCH- New York, 1992. , , .

25. Macrocyclic Compounds in Analytical Chemistry; Zolotov, Yu A Ed • Wiley New York, 1997. ' ''

26. Constable, E. C. Coordination Chemistry of Macrocyclic Compounds; Oxford University: New York, 1999; pp 6-31.

27. Molecular Recognition: Receptors for Cationic Guests; Gokel, G. W., Ed.; Comprehensive Suparmolecular Chemistry Vol. 1.; Elsevier: New York, 1996.

28. Bartsch, R. A. In Metal-Ion Separation and Preconcentration: progress and opportunity; Bond, A. H.; Dietz, M. L.; Rogers, R. D., Eds; ACS symposium series 716; American Chemical Society: Washington, DC, 1999; Chapter 9.

29. Walkowiak, W.; Charewicz, W. A.; Kang, S. I.; Yang, I.-W.; Pugia, M. J.; Bartsch, R. A. Anal. Chem. 1990, 62, 2018.

30. Bartsch, R. A. Solvent Ext. Ion Exch. 1989, 7, 829.

31. Vogtie, F.; Weber, E. Angew, Chem. Int. Engl, 1979,18, 753.

32. Kuboniwa, H.; Yamaguchi, K.; Nakahama, K.; Hori, K.; Ohashi, Y. Chem. Lett. 1988, 932.

33. Hiratani, K.; Taguchi, K.; Sugihara, H.; fio, K. Bull. Chem. Soc. Jpn, 1984, 57, 1976.

34. Calixarenes: A Versatile Class of Macrocyclic Compounds; Vicens, J.; Bohmer, V. Eds.; Kluwer: Dordrecht, 1991

35. Zinke, A.; Ziegler, E. Ber. 1944, 77, 264.

36. Gutsche, C. D.; Muthukrishman, R. J Org Chem. 1978, 43, 4905.

37. Gutsche, C. D. Calixarenes In Monographs in Supramolecular Chemistry; Stoddard, J. F., Eds; Royal Society of Chemistry: Cambridge, 1989.

38. Bohmer, V. Angew. Chem. Int. Ed Engl. 1995, 34, 713.

39. Gutsche, C D . Calixarenes Revisited In Monographs in supramolecular chemistry.; Stoddard, J. F., Eds; Royal Society of Chemistry: Cambridge, 1998; Chapter I.

40. Vogtie, F.; Weber, E. Host Guest Complex Chemistry Macrocycles; Springer-Verlag: Berlin, 1985; p 375.

41. Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D. Inclusion Compounds: Key Organic Host Systems; Oxford University: New York, 1991, pp 27-63.

42. Gutsche, C. D.; Dhawan, B.; Levine, J. A.; No, K. H.; Bauer, L. J. Tetrahedron 1983, 39, 409.

26

43. Note: room-temperature ionic liquid, nonaqueous ionic liquid, molten salt, liquid organic salt, and fused salt have all been used describe salts in the liquid phase. The term "ionic liquid" implies the salt has low melting point.

44. Carlin, R. T.; Wilkes, J. S., In Advances in Nonaqueous Chemistry; Mamantov, G., Popov, A., Eds.; VCH: New York, 1994.

45. Chauvin, Y.; Olivier-Bourbigou, H. CHEMTECH1995, 25, 26.

46. Seddon, K. R. KineL Catal. 1996, 37, 693.

47. Olivier-Bourbigou, H. In Aqueous-Phase Organometallic Catalysis: Concepts and Applications; Comils, B.; Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, 1998.

48. Welton, T. Chem. Rev. 1999, 99, 2071.

49. Freemantle, M. Chem. Eng News, 1998, 7^(30* march), 32.

50. Walden, P. Bull Acad Imper. Sci. (St Petersburg) 1914, 1800.

51. Sugden, S.; Wilkins, H. J Chem. Soc. 1929, 1291.

52. Wilkes, J. S.; Zaworotko, M. J. J. Chem. Soc, Chem. Commun. 1990, 965.

53. Codden, M. E.; Furton, K. G.; Poole, C. F. J Chromatogr. 1985, 356, 59.

54. Furton, K. G.; Poole, S. K.; Poole, C. F. Anal Chim. Acta, 1987,192, 49.

55. Furton, K. G.; Poole, C. F. Anal Chem. 1987, 59, 1170.

56. Furton, K. G.; Poole, C. F. J Chromatogr. 1987, 399, 47.

57. Pomaville, R. M.; Poole, S. K.; Davis, L. J.; Poole, C. F. J Chromatogr. 1988, 438, 1.

58. Pomaville, R. M.; Poole, C. Y.Anal. Chem. 1988, 60, 1103.

59. Shetty, P. H.; Youngberg, P. J.; Kersten, B. R.; Poole, C. F. J Chromatogr. 1987, ^77,61.

60. Furton, K. G.; Morales, R. Anal. Chim. Acta 1991, 246, 171.

61. Huddleston, J. G.; WiUauer, H. D.; Swatloski, R. P.; Wisser, A. E.; Rogers, R. D. J. Chem. Soc, Chem. Commun, 1998, 1765.

62. Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature, 1999, 399, 28.

63. Armstrong, D. W.; He, L.; Liu, Y.-S. Anal Chem. 1999, 77, 3873.

64. Dai, S.; Ju, Y. H.; Barnes, C. E. J Chem. Soc. Dalton Trans 1999, 1201.

65. Visser, A. E.; Swatioski, R. P.; Rogers, R. D. Green Chem. 2000, 7, 1.

66. Visser, A. E.; Swatioski, R. P.; Reichert, W. M.; Griffin, S. T.; Rogers, R. D. Ind Eng Chem. Res. 2000, 39, 3596.

27

67. Visser, A. E.; Swatioski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H., Jr.; Rogers, R. D. J. Chem. Soc, Chem. Commun, 2001, 135.

68. Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry: Principles of Structure and Reactivity, 4" ed.; Harper Collins College: New York, 1993; pp A46-A77.

69. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6"" ed.; John Wiley & Sons: New York, 1999; pp 404-409.

70. Allen, C. W. Coord Chem. Rev. 1994,130, 137.

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72. Brandt, K.; Porwolik, I.; Kupka, T.; Olejnik, A.; Shaw, R. A.; Davies, D. B. J Org

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73. Marcus, Y. Ion Solvation; Wiley: New York, 1985; pp 265-276.

74. Schneider, H.; Cox, ZB. G. Pure Appl. Chem. 1990, 62, 2259.

75. Ma, J. C ; Dougherty, D. A. Chem. Rev. 1997, 97, 1303.

76. Ikeda, A.; Shinkai, S. Chem. Rev. 1997, 97, 1713.

77. Ikeda, A.; Shinkai, S. J Am. Chem. Soc. 1994, 77^, 3102.

78. Zolotov, YU. A. Macrocyclic Compounds in Analytical Chemistry; Wiley: New York, 1997; p 41.

79. Molecular Recognition: Receptors for Cationic Guests; Gokel, G. W., Ed.; Comprehensive Suparmolecular Chemistry Vol. 1.; Elsevier: New York, 1996; p 35.

80. Patai, S.; Rapport, Z. Crown Ethers and Analogs; Wiley: New York, 1989; p 207.

81. Gokel, G. W. Chem. Soc. Rev. 1992, 39.

82. Izatt, R. M.; Pawlak, K.; Bradshaw, J. S. Chem. Rev. 1995, 95, 2529.

83. Shanon, R. D. Acta Cryst. 1916, A32, 751.

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85. Lindoy, L. F. The Chemistry of Macrocyclic Ligand Complexes; Cambridge: New York, 1989; pp 90-135.

86. Poonia, N. S. J Am. Chem. Soc. 1974, 96, 1012.

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88. Mallinson, P. R.; Tmter, M. R. J Chem. Perkin Trans. 2,1972, 1818.

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28

90. Parsons, D. G.; Truter, M. R.; Wingfield, J. N. Inorg Chim. Acta. 1975,14, 45.

91. Hughes, D. L. J Chem. Soc. Dalton Trans. 1975, 2374.

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29

CHAPTER II

SOLVENT EXTRACTION OF METAL CATIONS WITH

PROTON-IONIZABLE ACYCLIC POLYETHERS

2.1. Introduction

In 1974, the Toei group noted that the chelate stability constant ordering for

alkaline earth metal cation complexation by H02CCH20(CH2CH20)nCH2C02H in water

varied with changes in the polyether chain length. This finding suggested that acyclic

polyether dicarboxylic acids with appropriate lipophilicity might be useful agents for the

extraction of alkaline earth metal cations into organic media.' Stability constants for

complexation with Mg^", Ca^", Sr " and Ba " were determined by potentiometry and

compared to binding constant for analogous diacids that did not contain ether linkages.

Their results revealed that these polyether lonophores bind alkaline earth metal cations

better than their non-ether counterparts (i.e., glutaric acid). In addition, there was some

discrimination based on cation size. Ca^" was bound the strongest by the molecules with

one or two ether oxygens; whereas molecules with three or four ether oxygens complexed

Mg^", Sr ", and Ba^" with about equal affinity. Those findings suggested that binding

occurred within a pseudocyclic cavity in which the carboxylic acid units and ether

oxygen atoms faced inward and coordinated the metal cation.

The use of podands in liquid membranes and organic solvents as phase transfer

agents requires that they possess adequate lipophilicity to remain in the organic phase '

and resist precipitation when the complex forms. Lipophilicity can be regulated by

attaching either aromatic residues or alkyl pendants to the podand. The Inokuma^ group

used membranes containing both a lipophilic organic acid and an acyclic polyether to

investigate their synergistic action in the transport of alkali and alkaline earth metal

cations across chloroform membranes. The ttansfer efficiency was found to peak and

then diminish as the alkyl chains attached to both the organic acids and acyclic polyethers

were lengthened. Their study suggested that podands with low lipophilicity tended to

remain at the phase boundary upon complexation because the complex was somewhat

30

hydrophilic. On the other hand, bulky, highly lipophilic molecules moved slowly

through the liquid membrane. The best phase transfer agents were molecules with

intermediate alkyl chain lengths. Although the lipophilic character of the ionophore

influences its organic phase solubility, several studies have revealed that such

lipophilicity is rarely related to the stability of the complex.' "''

For effective extraction of alkaline earth metal cations from aqueous media into

less polar organic diluents, attention should be given to certain design precepts in the

synthesis of host molecules. Transport of metal ions from water into chloroform by

proton-ionizable ligands can be performed efficiently by an ion-exchange mechanism.

When hosts that possess one or more acid functionalities are utilized, metal cations may

be transported across the phase interface without concomitant transport of an anion from

the aqueous phase. For neutral lonophores, concomitant transport of one or more

aqueous phase anions is required which is typically less efficient. ' Thus, the

incorporation of proton-ionizable terminal groups into acyclic polyethers enhances their

efficiency in solvent extraction.*'

Considerable effort has been devoted to the development of chelating ligands with

high selectivity towards toxic heavy metal ions.* Acyclic polyethers have been used in

the separation of several cationic species, such as lead, "'"* mercury,'^ bismith'* and

cadmium.'^

The lipophilic host molecules used in the present study have two proton-ionizable

N-(X)sulfonyl carboxamide sidearms. Variation of the electron-withdrawing group X

allows the acidity of the ligands to be "tuned." To improve the rigidity and the solubility

of these ligands in organic media and prevent their loss to the aqueous phase, two

benzene rings and large octyl pendant groups are introduced (Figure 2.1). The structural

variations to be investigated include varying the "bridge" that connects the two proton-

ionizable portions of the ligand (Figure 2.2). The bridge was changed from (CH2)2 to

(CH2)3 or CH2CH2OCH2CH2 by adding a methylene or an ethylene oxide group to the

(CH2)2-bridged podand, respectively. The bridge is the key factor in controlling the size

of the pseudo-cavity of the ligand. Also, the acidity of the ligands will be varied by

31

changing the terminal groups so the effect of acidity upon selectivity and/or efficiency of

the ligands in competitive solvent exttaction of alkaline earth metals, Pb "" and Hg " can

be investigated.

P o

-o^p-0 o

, -OHHO-i o o

o o hOHHO-<

o o

p o rvo

p o C8Hl7~( )—CsHiy

o I I o Figure 2.1. Design Concept of Proton-Ionizable Acyclic Polyether Ligand for Solvent Exttaction (Y = SO2X with X = CH3, CF3, Ph and 4-NO2C6H4).

Bridge

-Pseudo-Cavity

Oo=s=oo=s=oO

Terminal Groups

Figure 2.2. Bridge, Terminal Groups and Pseudo-Cavity in a Proton-Ionizable Acyclic Polyether Ligand.

32

2.2. Resuhs and Discussion

For the competitive alkaline earth metal extraction studies described in this

chapter, the total loading of a ligand is defined as a sum of the individual metal cation

percent loadings. The percent loading for a particular metal cation is calculated by

dividing the number of moles of that metal cation recovered in the stripping step of the

extraction experiment by the initial number of moles of the ligand present in the organic

phase. The quotient of this division is multiplied by one hundred to convert it into a

percentage. In this approach, a 1:1 ratio of di-ionizable ligand to alkaline earth metal

cation in the complex is assumed.

The selectivity ratio for a specific metal cation is calculated by dividing the

concentration of the best-extracted cation by that of the specific cation. Due to this, no

selectivity ratio is given for the best-extracted metal cation (i.e., the ratio is equal to one).

Based on this definition, the selectivity ratio cannot be less than one and the greater the

selectivity ratio for a specific cation is, the less selective is the ligand toward that cation.

This means that the larger is the selectivity ratio for a cation the less of that cation is

extracted.

The solvent extraction profile for a ligand is obtained by plotting the percent

loading for the metal cations involved against the equilibrium pH of the aqueous phase.

There are virtually no changes in the percent loadings of the metal cations at high pH

(i.e., a plateau for the percent loadings is reached). Therefore, the selectivity ratios for

the metal cations are calculated using concentrations of the metal ions in these high pH

regions.

2.2.1. Competitive Solvent Extraction of Alkaline Earth Metal Cations

and Solvent Variation

In a previous investigation of alkaline earth metal cations extraction by proton-

ionizable acyclic polyether carboxylic acids, it was found that selectivity and efficiency

for competitive extraction in multi-ion systems were quite different from expectations

based upon the results of single ion exttactions.^ Therefore, competitive solvent

extractions were used in this study.

33

2.2.1.1. Effect of Bridge Variation

To probe the role of the proton-ionizable sidearm, proton-ionizable acyclic

polyether dicarboxylic acids 20-22 (Figure 2.3) were studied also. Based on a 1:1

stoichiometry for the alkaline earth metal cations, the results for competitive solvent

extraction of alkaline earth metal cations into chloroform by proton-ionizable acyclic

polyether dicarboxylic acids 20-22 are reported in Table 2.1 and Figure 2.4. In this

series, the bridge is systematically varied from (CH2)2 to (CH2)3 to CH2CH2OCH2CH2.

All members of this series showed 100 % total loading. The variation of bridge has a

pronounced effect on the selectivity orders for 20-22: for podand 20, Sr "" > Ca "" > Ba "" >

Mg'^; for podand 21, Ca'^ > Sr'^ > Mg'^ > Ba'*; and, for podand 22, Ba'^ > Ca'^ > Sr'^

>Mg2^

Y

a Compound y

20 (CH2)2

_^0 0 C _ ^ 21 (CH2)3

/ I 22 (CH2CH2)20 O OH HO ^ O

Figure 2.3. Structures of Di-ionizable Acyclic Polyether Carboxylic Acids 20-22.

Table 2.1. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by Proton-ionizable Acyclic Polyether Carboxylic Acids 20-22.

Total Loading (%) Selectivity Order and Ratios

20 100

21 100

22 100

Sr >

Ca >

Ba >

Ca 1.1 Sr 1.7 Ca 1.1

>

>

>

Ba 1.4 Mg 2.3 Sr 3.3

>

>

>

Mg 7.9 Ba 2.6 Mg 7.1

34

50

40

C T3 30 (0 o O UJ 20 <

10

" T — I — I — r

20

A-

A

Sr

• : Ca Ba

^7

, E ? -

Mg

J L_

~ i — I — I — I — I — r 21

Ca

A'

;T'w

...%

Ba

-- fc i"s

_l_fi_L J L

22 "1 I I — \ — r

Ba

Ca

A A---

Sr -A-£i

D i4 n-i^' Mg

J I I I L

• D

4 5 6 7 8 9 10 4 5 6 7 8 9 10 4 5 6 7 8 9 10 11 pH

Figure 2.4. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether Carboxylic Acids 20-22 in Chloroform.

The next step was to study analogous ligands with N-(X)sulfonyl carboxamide

groups (Figure 2.5). The series of lipophilic di-ionizable acyclic polyether N-(X)sulfonyl

carboxamides 23-34 possessed sufficient solubility and lipophilicity in chloroform to be

utilized in the solvent extraction studies. The acidity of these ligands was varied by

changing the X group of the proton-ionizable sidearm. The results for competitive

solvent extraction of alkaline earth metal cations into chloroform by proton-ionizable

acyclic polyether N-(methyl)sulfonyl carboxamides 26-28 are presented in Figure 2.6. In

this series, the "bridge" between the ionizable groups was varied from (CH2)2 to (CH2)3

to CH2CH2OCH2CH2. Presumably, the two anionic carboxamide and four (26 and 27) or

35

five (28) ethereal oxygens coordinate the metal cation within a flexible pseudo-cyclic

cavity. All members of this series showed essentially 100 % total loading with Ba " as

the best-extracted cation. Variation of the bridge in going from 26 to 27 to 28 has a

dramatic impact on the selectivity ratios. Both 26 and 27 are very selective for Ba ^ with

almost equal Ba "'/Sr "' selectivhies of - 60. The selectivity ratio for Ba '*' over other

alkaline earth metal cations for this group of ligands varies as (CH2)2 ~ (CUi)^ >

CH2CH2OCH2CH2 and the pHi/2 varies (CH2)2 < CH2CH2OCH2CH2 « (CH2)3.

o o

CoH O

sni? o -CRH 8^17

O NH HN O

0=S-00=S=0 I I

X X

X\Y (CH2)2 (CH2)3 CH2CH2OCH2CH2

CeHs

CH3

4-NO2C6H4

CF3

23

26

29

32

24

27

30

33

25

28

31

34

Figure 2.5. Stmctures of Proton-ionizable Acyclic Polyether N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3.

By comparison of the results presented in Figures 2.4 and 2.6, the presence of a

N-(X)sulfonyl carboxamide instead of a carboxylic acidic group in the (CH2)2 and (CH2)3

series is seen to dramatically enhance the extraction selectivity for Ba .

36

100

80

60 c

T3 m o

_ j

O

LU 40 <

20

"I r 26

T r-• • Ba

^ CaSrMg

27 Ba

Y CaSrMg •I - . ivwaaa--- - :

28

T •

^ A i 7 8 9 10 5 7 8 9 10 5 6

pH

Ba r - ^

Ca

Sr - A -/A

Mgi • q

7 8 9 10 11

Figure 2.6. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N(CH3)-suIfonyI Carboxamides 26-28 in Chloroform.

As can be seen from the data in Table 2.2, the effect of varying the bridge from

(CH2)2 to (CH2)3 to CH2CH2OCH2CH2 on the selectivity ratio becomes even more

obvious from comparison of the series of ligands with four different X groups (i.e., Ph,

CH3, 4-NO2C6H4 and CF3). The pronounced decrease in the selectivity ratio upon

lengthening the bridge may be explained based on an enthalpy approach. This means that

it is less favorable to organize a ligand with a CH2CH2OCH2CH2 bridge than one with a

(CH2)3 and (CH2)2 bridge for complex formation. The pronounced decrease in the

selectivity for Ba " which occurs with the CH2CH2OCH2CH2 bridge may be due to the

fact that a large pseudo-cavity exhibits less discrimination in formation of complexes

37

with divalent metal ions. The same effect is observed for large crown ethers which

exhibh plateau selectivity (i.e., form complexes with less discrimination for different

cations). This is due to a three-dimensional, "vwap-around" coordination of the metal

cation and is similar to what was observed for podands.'*

Table 2.2. Efficiency and Selectivity Ratio in Competitive Solvent Extraction of Aqueous Solutions Containing Alkaline Earth Metal Cations (10.0 mM in Each) vwth 5.0 mM Chloroform Solutions of the Proton-Ionizable Acyclic Polyether N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3.

23

24

25

26

27

28

29

30

Total Loading (%)

97

96

97

99

98

100

100

100

pHi/2

6.49

7.37

7.19

6.13

7.24

7.11

5.63

6.43

Ba

Ba

Ca

Ba

Ba

Ba

Ba

Ba

Selectivity Order and Ratios^

> Sr 94.4

> Ca 53.8

> Ba 1.1

> Sr 66.7

> Sr 59.4

> Ca 1.3

> Sr 93.7

> Sr >100

>

>

>

>

>

>

>

>

Ca >100

Sr 84.1 Sr 3.1 Ca

>100 Ca

60.2 Sr 4.3 Ca

>100 Ca

>100

>

>

>

>

>

>

>

>

Mg >100 Mg

>100 Mg

>100 Mg

>100 Mg

>100 Mg

>100 Mg

>100 Mg

>100 b . „ . .^. Ba > Ca > Sr > Mg

31° 100 6.24 2.0 3.4 >100

32'' 100 2.71 Ba > Ca > Sr > Mg

95.0 >100 >100

33" 100 3.66

34 100 3.61

Ba > Ca > Sr > Mg 59,0 9Z8 >100

Ba > Sr > Ca > Mg 7.9 9.5 >100

: Selectivity ratio = [best-extracted metal ion]/[metal ion]. '': Calculated at pH 8.5 except for 31 at pH 7.07, 32 at pH 7.27 and 33 at pH 6.29.

38

2.2.1.2. Effect of X Group Variation

To probe the influence of varying the group X in the lipophilic acyclic di-

ionizable polyether N-(X)sulfonyl carboxamides, alkaline earth metal cation extractions

into chloroform by 25 (X=Ph), 28 (X=CH3), 31 (X=4-N02C6H4)and 34 (X=CF3) were

conducted. The results for competitive solvent extraction of alkaline earth metal cations

into chloroform by ligands 25, 28, 31 and 34 are shown in Figure 2.7 and Table 2.2. In

this series, the group X in the proton-ionizable sidearm is systematically varied from Ph

to CF3. From the viewpoint of physical organic chemistry, the electron-withdrawing

ability of X varies in the order: CH3 < Ph < 4-NO2C6H4 < CF3. Methyl is the least

electron-withdrawing of the four terminal groups and CF3 is the most electron-

withdrawing. This ordering correlates with acidity of the ligands.'^ Hence, the ligand

bearing CF3 is the most acidic in the series.

All members of this series showed approximately 100 % total loading. Variation

of X in the series of ligands with CH2CH2OCH2CH2 bridges in 25, 28, 31 and 34 has a

dramatic effect on the selectivity ratios for Ba' . Values of the pH for half-loading (pH/,)

in the extraction profiles are found to vary with the X group in the order (Table 2.2): CF3

(34) < 4-PhN02 (31) < CH3 (28) < Ph (25). This provides an effective acidity ordering

for the series.

Solvent extraction with complexing agents 23, 26, 29 and 32 was conducted to

investigate whether the more important factor in selectivity control is the bridge or the

identity of X. As shown by the data in Table 2.2, all of the ligands with (CH2)2 bridges

show same high selectivity for Ba "". In the (CH2)3 bridge case, the efficiency of Ba ""

extraction changes moderately as the X group is varied, but the selectivity for Ba

remains high. Therefore, the bridge is judged to be the dominant factor.

39

100

80

C

05 O

60

O

LU <

40

20

T—I—r 25

m,'

*

Ca

-w- -Ba

4

i Mg

T—I—I—\—I—r 28

T

Ba

Ca

f

f

ii Sr

-A-A

Ma

T — I — I — I — I — r 31

Ba+

¥

Ca

Sr-

Mg

1—I—r 34

Ba

; A:

jBBBdBBzL.

Sr ^#-

Mg Ca -A

> I t

5 6 7 8 9 1011 5 6 7 8 9 10115 6 7 8 9 10110 2 4 6 8 1012 pH

Figure 2.7. Alkaline Earth Metal Loading versus Equilibrium Aqueous Phase pH for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(X)sulfonyl Carboxamides 25 (X=Ph), 28 (X=CH3), 31 (X=4-N02C6H4) and 34 (X=CF3) in Chloroform.

2.2.1.3. Comparison of Solvent Extraction

and Polymer Inclusion Membrane Transport Results

It is also of interest to determine the more important factor in a polymer inclusion

membrane transport system between the pseudocavity size (bridge identity) and acidity

(pHi/2) of the carrier. Such transport was investigated previously for podands 23-34 by

another coworker.'^ Before discussing the results, the common and different features of

solvent extraction and polymer inclusion membrane transport should be mentioned.

In principle, solvent extraction is a thermodynamically controlled system. Each

loading percent data in the pH profile reflects the equilibrium for ligand-metal

40

complexation. As mentioned in Sectionl.7, the distribution coefficient (D) depends on

Ka, p, KL, KM and the pH. Thus, pHi/2 depends on the acid dissociation constant (Ka), the

formation constant (P) and the lipophilicity (KL, KM). If there is structural similarity for

two ligands with sufficient lipophilicity, the relative effects of KL and KM will have a

negligible influence on their pH profiles. In solvent extraction, there is contact of an

aqueous phase containing metal ions with an organic phase containing the ligand. After

the two phases are separated, the pH of the residual aqueous phase is measured. The

extracted metal ions are then removed (stripped) from the organic phase in a separate step

by shaking with aqueous acid.

In contrast, the polymer inclusion membrane transport is kinetically controlled.

This system consists of three components, the source, membrane and receiving phases.

There are three steps in the transport process when both source and receiving phases are

stirred: (i) complexation of the metal ion by the ligand at the source phase-membrane

interface; (ii) diffusion of the ligand-metal complex through the membrane layer; (iii)

decomplexation at the membrane-receiving phase interface by contact with the acidic

receiving phase. The rate-determining step in membrane transport mainly depends on the

identity of the carrier molecule, the kind of medium, and the solute being transported.

With the same medium and solute, only the identity of the carrier controls transport of the

solute. The complexation or decomplexation reactions or diffusion of the complex

through the membrane may be the rate-determining step for the flux. From the flux ratio

for the primary ion to other metal ions, the selectivity ratio can be obtained. Usually the

flux is mainly influenced by the complexation at the source phase-membrane interface

followed by diffusion due to the concenttation gradient of the ligand-metal complex.

Therefore, Ka and P of the ligand control complexation of the metal ion by the ligand and

generate the concentration gradient between the membrane's interfaces. Movement of a

large-sized ligand-metal complex through the membrane meets with resistance.

Therefore, pHi/2 and the size of the ligand are important factors for complexation of a

given metal ion and hs transport through the membrane, respectively. A ligand with

lower pHi/2 and smaller size will show higher flux.

41

To assess the relative importance to selectivity of pHi/2 and bridge variation on

competitive solvent extraction of alkaline earth metal cations and their competitive

ttansport across polymer inclusion membranes, one factor was varied while the other was

fixed.

For ligands with the same X group in solvent extraction, the pHi/2 (the pH for half

loading) follows the order (CH2)2 « CH2CH2OCH2CH2 - (CH2)3 (Figure 2.8.a).

Similarly, the total flux through the polymer inclusion membranes follows the order

(CH2)2 > (CH2)3 « CH2CH2OCH2CH2 (Figure 2.9.a). For a lower pHi/2 value, a higher

level of ligand deprotonation at the source phase-membrane interface is expected. The

greater apparent acidity of the (CH2)2-containing ligand makes the concentration gradient

of ligand-metal complex in the membrane much higher than an analog containing a

(CH2)3 or CH2CH2OCH2CH2 bridge. Therefore, the ligand containing the (CH2)2 bridge

has the highest flux due to greater acidity. The (CH2)2 bridge should also give a metal

ion-ionized ligand complex of the smallest size which would facilitate its diffiision

through the membrane. In the cases of podands containing (CH2)3 and

CH2CH2OCH2CH2 bridges, the latter has a slightly lower pHi/2 than the former in solvent

extraction. On the other hand, podands containing the CH2CH2OCH2CH2 bridge show

the lowest fluxes in polymer inclusion membrane transport which reveals a counteracting

size effect of the ligand upon the efficiency of membrane transport. In such evaluations,

h should be noted that the Y-axis in Figure 2.8.a for solvent extraction is a log scale (pH)

and that for polymer inclusion membrane transport is a linear scale (in Figure 2.9.a).

With the same bridge in the ligands, the evaluation can be made for the effect of

X group variation. The pHi/2 order for X group variation in solvent extraction (CF3 < 4-

NO2C6H4 < CH3 < Ph) is the same as the flux order of CF3 > 4-NO2C6H4 > CH3 > Ph in

polymer inclusion membrane transport. Thus, pHi/2 of the ligand influences the total flux

as a rate-determining step in the complexation process in polymer inclusion membrane

transport.

42

2 -

X Q. 5

7 -

a

-

-

-

1

T ,

A ,

• D, ^^

1

1 -• - - Q -

- - • ----A-

---•-

• -

'-._ A -

' " • • 8 -

1

1 -Ph

CH3 4-NO^C^H^.

CF3

-

-

. - A

- : : : : ; : : §

1

1.0

(CH,)^ (CH^)3 {CH^CH^)^0

Bridge

0.8

c T3 CD O

0.6 TO * - < O

ro CO

0.4

0.2

--n:

- Q - Ph

• CH3

-A .4-NO^C^H^

T CF,

D

(CHJ, (CH,:

Bridge

Figure 2.8. Effect of Bridge Variation on (a) the pH for Half Loading and (b) the Ba' ' Selectivity of the Acyclic Polyether N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3 in Competitive Solvent Extraction of Alkaline Earth Metal Cations.

From the results for solvent extraction shown in Figure 2.8.b, ligands containing

(CH2)2 and (CH2)3 bridges are seen to have high selectivity for Ba " with all X groups.

Ligands containing a CH2CH2OCH2CH2 bridge exhibit lower Ba " selectivity that is

affected by variation of the group X. In this series, the greater ligand acidity gives higher

selectivity for Ba " . A similar result for X = CF3 and 4-NO2C6H4 groups in polymer

inclusion membrane transport is shown in Figure 2.9.b. This can be explained as a cavity

size effect or enthalpy effect. As explained earlier, the (CH2)2-bridged podand structure

can be related to 18C6 and would be expected to have similar cavity size as 18C6. The

cavity size of this podand is very suitable for binding Ba "" and high enthalpy will drive

43

complexation of Ba' ^ by the podand. With the addition of one carbon to the bridge, the

cavity size of a (CH2)3-bridged ligand is still similar to that of a (CH2)2-bridged podand.

The addition of CH2CH2O to the (CH2)2 bridge changes the cavity size from 18C6 to

21C7 and reduces the Ba " selectivity.

(CH^)^ (CH^)3 (CH^CH,),0

Bridge

(CH,), (CH,)3 (CH,CH,),0

Bridge

Figure 2.9. Effect of Bridge Variation on (a) the Total Metal Ion Flow and (b) the^Ba Selectivity of the Acyclic Polyether N-(X)sulfonyl Carboxamides 23-34 with X - Ph, CH3, 4-NO2C6H4 and CF3 in Competitive Transport of Alkaline Earth Metal Cations across CTA-o-Nitrophenyl Pentyl Ether Polymer Inclusion Membranes.

2.2.1.4. Solvent Variation in Solvent Extraction

To investigate the influence of the organic diluent, compethive solvent extraction

of alkaline earth metal ions by ligands 29-31 was performed in nitrobenzene, 1,2-

dichloroethane, chlorobenzene and toluene. For 29 (Figure 2.10) and 30, there was no

44

apparent change in selectivity and efficiency: both ligands showed very high selectivity

and efficiency for Ba ^ extraction.

Nitrobenzene CHCICKCI Chlorobenzene Toluene 1UU I I I ^ , , , ? , , . I I I

80

c 60 T3

O

UJ <

40

20 -

1 I—I—I—I—I—I—r

WW Ba

Ca Sr Mg

T—1—r-i—i—t \ I

- - T

w- • • Ba

' Ca Sr Mg <WAAH--t--A£^- I

Ba

- - T

Ca Sr Mg i.M»ykAi= = j . . A j

1 I I I I V X I

Ba

- - t

• Ca Sr

4 5 6 7 8 910114 5 6 7 8 910114 5 6 7 8 910114 5 6 7 8 91011

pH

Figure 2.10. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(4-N02C6H4)sulfonyl Carboxamide 29 in Different Diluents.

The results of competitive solvent extraction of alkaline earth metal cations into

the same series of diluents by podand 31 are presented in Figure 2.11 (Podand 31 did not

possess sufficient solubility in 1-octanol to allow incorporation of this solvent in the

solvent variation study). It is interesting that the polarity (5p) of the organic solvent

(Table 2.3)^" enhances the selectivity and efficiency for Ba " and reduces those values for

Ca " . However, there is no apparent change in extraction selectivity or efficiency for Sr "

45

as the polarity of the diluent is changed. Thus the bridge is shown to have a greater effect

on the extraction than variation of organic diluents. Interestingly, no dependence of pHi/2

on variation of the organic diluent is evident.

80

60

D) C

§ 40

o LU <

T — I — I — I — \ — r -Nitrobenzene

Ba

20 - Ca

• Sr

J ^ J T J J 1. L J_ J I

1—I—I—I—I—r CH^CICH^CI

5 fc^^lJ -I CT—

Ba

Ca

J L Mg

1—\—I—I—I—r-Chlorobenzene

T

i *

Ba

Ca-

Sr

Mg

I—I—I—I—r Toluene

Ca » • •

Ba • T T

- - * * Sr f ^-A-A--A-

I Mg

5 6 7 8 9 10115 6 7 8 9 10115 6 7 8 9 10115 6 7 8 9 1011 pH

Figure 2.11. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 5.0 mM Solutions of Acyclic Polyether N-(4-N02C6H4)sulfonyl Carboxamide 31 in Different Diluents.

46

Table 2.3. Hansen and Beerbower's Dispersion, Polar and Hydrogen Bonding Cohesion Vectors for Various Solvent.^"

Solvent

Nitrobenzene

1,2-Dichloroethane

Chlorobenzene

Toluene

Water

2.2.1.5. Metal Ion Binding

8d

20.0

19.0

19.0

18.0

15.6

Site in a

Cohesion Vectors (MPa)"^

5p

8.6

7.4

4.3

1.4

16.0

Ligand

8h

4.1

4.1

2.0

2.0

42.3

8T

22.2

20.9

19.6

18.2

47.8

To probe the metal ion binding site in the proton-ionizable sidearm, IR spectra of

podand 29 and its complex with Ba " were measured. The most obvious spectral change

was disappearance of the peak at 1727 cm"' for vc=o in 29 on going to the complex. The

suggested chemical stmcture of the complex that is be formed with podand 29 is

illustrated in Figure 2.12, in which the carbonyl group of podand 29 takes part in the

complexation.

N N

CsHiy

NO2 NO2

Figure 2.12. A Suggested Chemical Stmcture for the Ba ^ Complex with Ionized 29.

47

2.2.2. Solvent Extraction of Pb^^

2.2.2.1. Stoichiometry for Pb " Complexation

To determine the stoichiometry for exttaction of Pb "" by proton-ionizable acyclic

polyether N-(X)sulfonyl carboxamides, single species solvent extractions of aqueous Pb ""

into chloroform by podands 32-34 were performed.

When a metal ion (M " ) is extracted into an organic phase containing the ligand

(H2L) to form a n:l ligand-to-metal ion complex, the interaction is described by the

following equation:

M^ (a) + n H2L,o) ^ ML(H2L)„.i(o) + 2 H\a) (2.1)

Kex,H2L = [ML(H2L)n.l]o [H^]a'/ [H2L]o"[M'^]a (2.2)

where Kex and the subscripts "o" and "a" denote the overall extraction constant and the

organic and aqueous phases, respectively. By introducing the distribution ratio of the

metal ion between the organic and the aqueous phases (DH2L = [ML(H2L)n.i]o / [M^ ]a)

into Equation 2.2, the extraction equilibrium is logarithmically expressed as:

log DH2L = 2 pH + n log [HjLJo + log Kex,H2L (2.3)

Equation 2.3 indicates that a plot of log D against pH at constant [H2L]o should show a

straight line with a slope of 2 and an intercept of (n log [H2L]o + log Kex,H2L ). In this

case, it is expected from Equation 2.3 that the slope of a plot of log D against log [H2L]o

at constant pH should be n. This n is 1 for a 1:1 stoichiometry of metal ion to ligand.

For Pb " extraction by podands 32-34, the plots of the log DH2L versus pH are

shown in Figure 2.13.a. The slopes were analyzed by a linear regression least squares

method. The plots show linear relationships with slopes of 1.93, 2.13 and 1.97 for 32-34,

respectively, which demonstrates that ion-exchange extraction of Pb " with the ligands

takes place by liberation of two protons into the aqueous solution. The stoichiometry for

the extracted species (coordination number of ligands, n) can be determined from the plot

of log D versus log [HiLJo at constant pH. The resuhs are presented in Figure 2.13.b.

For ligands 32-34, the observed slopes are 1.00, 1.29 and 1.04, respectively. This

demonsttates that one ligand interacts with the Pb "" to form 1:1 (metal/ligand) complexes

in extraction. These results are also consistent with ion-exchange extraction by

48

replacement of the two protons in the ligands with forming ligand-metal complexes with

1:1 stoichiometry.

o

1.8

1.6

1.4

1.2

1.0

0.8

0,6

0.4

0.2

0.0

n 9

-a

-

" ^' •' 4

-'

-

_

1

a A

A'

D

• A

1

1 /

[j3 A

32 33 34

0

1 1

, • m

•-' •

•

1 1

_

_

-

-

,•

/ -

-

-

o

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

1 1 1

b -

-

-

-

/

•' ,'''' •' -'A-'

A '

••| 1 1

" 1 1

A

D/ , /

1 1

1 1 1

• / -

• • • / : ^ '

/ / A

-

• 32 • 33 A 34

1 1 1

.8 4.0 4.2 4.4 4.6 4.8

pH

-4.2-4.0-3.8-3.6-3.4-3.2-3.0-2.8

log [H L]

Figure 2.13. Plots of (a) log D versus pH and (b) log D versus log [H2L] at pH 9.5 for Solvent Extraction of Pb " from Aqueous Solution into Chloroform by Acyclic Polyether N-(CF3)sulfonyl Carboxamides 32-34.

2.2.2.2. Pb "" Extraction

Similar to solvent extraction of alkaline earth metal cations, the behavior of

polyether carboxylic acids 20-22 and N-(X)sulfonyl carboxamides 23-34 was evaluated

by solvent extraction of Pb " from aqueous solutions into chloroform. The empirical

stability order of many O and N type ligands for divalent metal ions as proposed by

Maley and Mellor is Cu ^ > Ni "" > Pb^^ > Co^ , Zn^ . ' In the present study, the

extraction of Pb "" by polyether N-(X)sulfonyl carboxamides was compared with that by

polyether carboxylic acids to probe the influence of the proton-ionizable group.

49

Previous results for the competitive solvent extraction of alkaline earth metal

cations showed sufficient lipophilicity of those ligands so that the extractant loss into the

aqueous phase for the effective pH region of Pb " extraction was expected to be

negligibly small. However, these ligands possess relatively weak extraction behavior

towards Pb " . To achieve a measurable Pb' ' extraction into chloroform under the

previously utilized conditions, high aqueous phase pH would be required. This is not

possible due to Pb " precipitation from basic aqueous solutions. To adjust the efficiency

of Pb " extraction by the polyether N-(X)sulfonyl carboxamides, the initial metal ion

concentration in the aqueous phase was increased to 1.0 mM, while the ligand

concentration in the organic phase was 0.5 mM. To determine the Pb " concentration in

the organic phase after extraction, solution was stripped with 0.1 N HNO3 and the Pb

concentration in the strippant was determined by atomic absorption spectrophotometry.

From extraction of aqueous lead nitrate solutions in which the pH was adjusted by

addition of TMAOH with chloroform solutions of the ligand, the data presented in Figure

2.14 were obtained. Based on a 1:1 stoichiometry, the Pb " loading was calculated for

each extraction system by determining the percentage of ligand filled. ResuUant loading

percentages of Pb " in the chloroform phase as a function of the equilibrium pH (after

extraction) are shown. To understand the role of the N-(X)sulfonyl carboxamide units in

the polyether stmcture, solvent extractions with carboxylic acids 20-22 were performed.

As shovm in Figure 2.14, the extraction efficiency of carboxylic acids varies in the order

(CH2)2 > CH2CH2OCH2CH2 > (CH2)3. In the cases of the polyether N(X)-sulfonyl

carboxamides with X = Ph, CH3 and 4-NO2C6H4, the loading did not reach 100% and the

extraction efficiency depends on the bridge in the order: CH2CH2OCH2CH2 > (CH2)2 >

(CH2)3. However, in case of X = CF3, the loading reaches 100 % and the exttaction

efficiency order changes to (CH2)2 > CH2CH2OCH2CH2 > (CH2)3.

50

80

».o ^ D>60

T3

o _1

^^ 40

20

n

1 1 1 D 20 a^ • 21 ^ ;• A 22^ f

^ : , ;' :: • ;' •' ''•

; : A :

°.' •' o i * ^ i [3i ;

- ^ : -

°;' • ;A •

îf\ 01 1

1 1 • 23 • 24 A 25

A

Q

A : A ;'

i ': ~ ^ g •

ô° A ^ •'

^ ^ a i » ,

1 1

• 26 • 27 Q A 2 8 .•••

A •

- ,''' r-

0 ;

f •-A: : A ; •

^ B * - A* • -

:& : £n *

1 1

• 29 • 30 A 31 A

/D _

A'' A ;

A ; ^ ; • A ; ; -

; D ! ' D '

; D ;

A

; C

3 ;'

# a

A;' .' .' D '

A '' • ' b -

A ; * ,' D

^'-00 ,

dA ;• 0

• '

1 • ci; ;

,' ; ; • " # 1 -

* • ' ;

1 :

1 ; *

- i t -c 32 > 33

• ^ . - ; , ^ ?4 5 4 6 4 6 4

pH

Figure 2.14. Effect of Bridge Variation on Pb " Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solutions of Lead Nitrate with 0.5 mM Chloroform Solutions of Acyclic Polyethers 20-34.

To probe the influence of varying the X group on Pb " extraction efficiency, the

same data are rearranged in Figure 2.15. For the same bridge, the ligand efficiency

changed as X was varied: CF3 > 4-NO2C6H4 > CH3 > Ph. The pHi/2 order is same as

previously observed for solvent extraction of alkaline earth metal cations (Section 2.2.3).

With X = 4-NO2C6H4, the exttaction profile showed di-protic acid behavior. For the

same polyether bridge, carboxylic acids gave higher pHi/2 values than the analogous CF3-

containing podands, but lower pHi/2 values than ligands containing other X groups.

51

100

pH

.2+ Figure 2.15. Effect of X Group Variation on Pb Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of Acyclic Polyethers 20-34.

,2+

2+

2.2.3. Solvent Extraction of Hg

2.2.3.1. Stoichiometry for Hg " Complexation

Based on the same analysis advanced in Section 2.2.2.1 for the Pb

stoichiometry, plots of log DH2L versus pH at constant log [H2L]o for Hg " are shown in

Figure 2.16.a for ligands 26-28. The plots show linear relationships with slopes of 1.92,

2.22 and 1.94, respectively, indicating that ion-exchange extraction of Hg " with the

ligands takes place by liberating two protons into the aqueous solution. The

stoichiometry for the extracted species (coordination number of ligands, n) was

determined from a plot of log D against log [H2L]o at constant pH. The results are

52

presented in Figure 2.16.b. For ligands 26-28, the observed slopes are 0.92, 1.17 and

1.08, respectively. This reveals that one ligand interacts with Hg ^ to form a 1:1

(metal/ligand) complex.

1.5

1.0 -

Q 0,0.5

0.0 -

-0.5

1

a

-

1

' I I I

A

A/'

9'

-A

,'P

,*

0

•''

1 1

• 26 • 27 A 28

1 1

-

0.6 0.8 1.0 1.2 1.4

pH

-6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0

log [H L]

Figure 2.16. Plot of (a) log D versus pH and (b) log D versus log[H2L] at pH 1 for Solvent Extraction of Hg " from Aqueous Solution into Chloroform by Acyclic Polyether N-(CH3)sulfonyl Carboxamides 26-28.

2.2.3.2. Hg^^ Extraction

From extraction of aqueous mercuric nitrate solutions in which the pH was

adjusted by addition of HNO3 with chloroform solutions of 20-34, the data presented in

Figure 2.17 were obtained. Based on 1:1 extraction stoichiometry for Hg " , the loading

was calculated for each extraction system by determining the percentage of ligand filled.

Resultant loading percentages of Hg " in the chloroform phase as a function of the

53

equilibrium pH (after extraction) are shown in Figures 2.17 and 2.18. To understand the

role of terminal group in the polyether stmcture, solvent exttaction with podands 20-22

containing carboxylic acids was performed also.

100

80

.2 ®°

X LU

40

20 -

— I — I — r D 20 • 21 A 22 ^

4

A

•r

I T - A "

D 23 / • 24 /« A 25 ATJ

A ^

A

•AP,'

;' * A ;;

A Y

;A •'

a,-A

A:

A • 26 • 27 A 28

^1^ a:

A / -' '*

A;'; '

/ R' A;'; ; : *

A ,•'

- " ,' •

- - • • 29 • 30 A 31

I

/ - A ' A

A •

A

- ^

• 32 • 33 A 34

j _

0 1 2 3 0 1 2

pH

1

Figure 2.17. Effect of Bridge Variation on Hg ^ Extraction versus the Equilibrium pH of the Aqueous Phase for Extractions of 0.25 mM Solutions of Mercuric Nitrate with 0.25 mM Chloroform Solutions of Acyclic Polyethers 20-34.

2+ As shown in Figures 2.17 and 2.18, the Hg loading did not reach 100 % and the

extraction efficiency order varied as the bridge was changed (CH2)2 ~ CH2CH2OCH2CH2

> (CH2)3. In the cases of X = Ph and 4-NO2C6H4, the loading reaches 100 % and the

ligand extraction efficiencies vary as the bridge was changed in the order

CH2CH2OCH2CH2 > (CH2)2 > (CH2)3. However, in the case of X = CH3, the loading

reaches 100 % and the extraction efficiency order changes to (CH2)2 > CH2CH2OCH2CH2

54

> (CH2)3. For X = CF3, the loading reaches 100 % and the extraction efficiency order

changes to (CH2)2 > (CH2)3 > CH2CH2OCH2CH2.

To illustrate the influence of varying the X group of the ligands, the same data are

rearranged as shown in Figure 2.18. The pHi/i order is: CF3 < CH3 < 4-NO2C6H4 < Ph

for ligands with the (CH2)2 bridge; CF3 < 4-NO2C6H4 < CH3 < Ph for the (CH2)3 bridge;

and, 4-NO2C6H4 ~ CF3 ~ CH3 < Ph for the CH2CH2OCH2CH2 bridge. For the same

bridge, carboxylic acids have higher pHi/2 values compared with the N-(X)sulfonyl

carboxamides. The low extraction efficiency of carboxylic acids may be due to the

hardness of oxygen donor atoms relative to the soft Hg^ .

100

80 -

O 03 —' X LU

60 -

40

X

20 -

1 ..0-^^

9 AT • A ^ •

- 6 4 p

0 f y T ','

• x^ / 0 4 ;';'

• / ' •

•A n ^/ 0 F

A % / ^

• ' P A

9'' •

...0- --'• 0 n" 1 J

1

•

20 23_ 26 29 32 1

0

9 T' A :

9 / ;' A 0 f I

' I

1

P ,'

9 I .• ^ ^ / p'

: A 0

,' : 0; •

- \ ^ ' ' / A

t\ n D' a---' 0 1 1

21 24. 27 30 33 1

79? f.'O :'

.0 •

9i / ' •

„• ; / 0 T .'

^ ^ ^ 6 / / .6 ;'••

;'• D

f ;'n 22 _ ^ ! • 25.

• 9 0 28 0 .' ». q T 31

.9-' 0 34 ^-a-9

3 0 3 0

pH

Figure 2 18 Effect of X Group Variation on Hg ^ Extraction versus the Equilibrium pH of the Aqueous Phase for Extractions of 0.25 mM Solutions of Mercuric Nitrate with 0.25 mM Chloroform Solutions of Acyclic Polyethers 20-34.

55

2.3. Chapter Summary

The efficiency and selectivity with which lipophilic acyclic polyether N-

(X)sulfonyl carboxamides extract alkaline earth metal cations from aqueous solutions

into chloroform are strongly influenced by the structure of the complexing agent. The

change of the proton-ionizable group from carboxylic acid to N-(X)sulfonyl

carboxamides shows dramatic change in the ligand selectivity. Within the N-(X)sulfonyl

carboxamides series, the bridge is more important in controlling selectivity than the X

groups in the proton-ionizable sidearm. With the same bridge, the effective acidity

ordering is CF3 > 4-NO2C6H4 > CH3 > Ph. Increasing the organic solvent polarity

generally enhances the extraction selectivity for Ba ^ and diminishes the selectivity for

Ca^^

The stoichiometry for Pb' ' extraction is 1:1 (metal to ligand). The efficiency is

controlled by the X group and the bridge of the ligand. In the cases of X = Ph, CH3 and

4-NO2C6H4, the extraction efficiency varies in the order CH2CH2OCH2CH2 > (CH2)2 >

(CH2)3. In the case of X = CF3, the extraction efficiency order is (CH2)2 > CH2CH2O-

CH2CH2 > (CH2)3.

The stoichiometry for Hg "" extraction is l:l(metal to ligand). The efficiency is

controlled by the proton-ionizable groups and the bridge of the ligand. In the cases of X

= Ph and 4-NO2C6H4, the extraction efficiency varies CH2CH2OCH2CH2 > (CH2)2 >

(CH2)3. In the case of X = CH3, the extraction efficiency order is (CH2)2 > CH2CH2O-

CH2CH2 > (CH2)3. In the case of X = CF3, the extraction efficiency order is (CH2)2 >

(CH2)3 > CH2CH2OCH2CH2.

2.4. Experimental

2.4.1. Competitive Solvent Extraction of Alkaline Earth Metal Cations and Solvent Variation

2.4.1.1. Reagents

Ligands 20-34 were prepared by Dr. Sadik Elshani. Analytical grade

MgCl2-6H20 (99%), CaCl2-2H20 (98%), SrCl2-6H20 (99%), BaCl2-6H20 (99%) were

purchased from Aldrich. LiOH was purchased from Fisher Scientific Company.

56

Analytical grade HCl (1.000 ± 0.002 N) was purchased from Tristar. Before use, the

chloroform was washed with deionized water to remove the ethanol stabilizer and

saturate the chloroform with water. Deionized water was prepared by passing distilled

water through three Barnstead Model D8922 combination ion-exchange cartridges in

series. Nitrobenzene was distilled under reduced pressure and then washed with

deionized water. The 1,2-dichloroethane and chlorobenzene were first washed with

deionized water, distilled and then saturated with deionized water. Toluene was distilled

then washed with deionized water.

2.4.1.2. Apparatus and Instrumentation

The 15-ml, metal-free, conical polypropylene centrifuge tubes with polypropylene

caps were purchased from Elkay. A Fisher Vortex Genie 2 mixer was used to agitate the

samples. Separation of the layers in water-chloroform mixtures was achieved by

centtifuging the tube in a Clay Adams Safety-Head centrifuge. Hamilton Gaslight

syringes were used to obtain and transfer exact amounts of liquids (organic or aqueous).

Measurements of the aqueous phase pH were performed with a Fisher Accumet Model

AR 25 pH meter using a Coming Model No. 476157 glass-body combination electrode.

A Dionex Model DX-120 ion chromatograph with a self-regenerating cation suppressor

(CSRS-II 4-mm) with an lonPac CS12A column was used to determine the alkaline earth

metal cation concentrations in the final aqueous phases after sttipping. The DX-120 ion

chromatograph was connected to and conttolled by a Packard Bell Multimedia D142

personal computer with Dionex Peak-Net Chromatography Workstation version 5.0

software.

2.4.1.3. Extraction Procedure

The procedure for competitive solvent exttaction of alkaline earth metal cations

by proton-ionizable acyclic polyethers consisted of three steps: (i) extraction; (ii)

stripping; and (iii) metal ion determination (Figure 2.19). For compethive solvent

exttaction, a 5.0-mM solution of the proton-ionizable acyclic polyether in water-washed

57

chloroform was prepared. For each experiment, eleven samples were used. To each of

the eleven 15-mL, metal-free, conical polypropylene centrifuge tubes with polypropylene

caps, 2.0 mL of the proton-ionizable acyclic polyether solution in chloroform was added.

Then 1.0 mL of a 20-mM solution of MgCb, CaCb, SrCb and BaCb (20 mM in each)

was added. A 40-mM LiOH solution was added in the amounts shown in Table 2.4. The

progressively increasing amounts of added LiOH solution were used to obtain a pH

gradient over which the percent loading of the proton-ionizable acyclic polyethers could

be studied.

Table 2.4. Aqueous Phase Formulations for Competitive Solvent Extraction of Alkaline Earth Metal Cations by Proton-ionizable Acyclic Polyethers 20-34.

~ , LiOH H2O, AEMC (20 mM in each salt) Organic Phase sample (49 ^ M ) , mL mL (Mg, Ca, Sr, Ba), mL (5 mM), mL

1

2

3

4

5

6

7

8

9

10

11

0.00

0.04

0.08

0.12

0.25

0.40

0.50

0.55

0.60

0.70

1.00

1.00

0.96

0.92

0.88

0.75

0.60

0.50

0.45

0.40

0.30

0.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

1.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

2.00

In the cases of the N-(ttifluoromethyl)sulfonyl carboxamide acyclic polyethers

where appreciable loading was noted at the beginning of the normal exttaction curve

(usually more than 10 %) due to the high acidity of the exttactant, additional exttaction

solutions at lower pH values were also used to obtain complete curves for "percent metal

loading versus pH." The way in which the lower pH region was obtained is shown in

58

Table 2.5. Variation of the hydrochloric acid volume was used to obtain a pH gradient

over which the percent loading of the proton-ionizable acyclic polyethers could be

studied.

2.0mLof5.0mM Podand in CHCI3

2.0 mLoflOmM AEMC with 20.0 mM LiOH in H2O

Organic Phase

Vortex for 10 minutes Centrifuge for 10 minutes

Extraction

Determine the Equilibrium pH

1.5 mL of Organic Phase

3.0mLof 0.1 N HCl

Vortex for 10 minutes Centrifuge for 10 minutes

10 X dilution

Metal Ion Determination by Ion Chromatography

Figure 2.19. General Procedure for Competitive Solvent Extraction.

59

In the extraction step, the two-phase mixtures were shaken vigorously with a

vortex mixer for 10 minutes to ensure maximal loading and then centrifuged for 10

minutes to ensure complete phase separation. In the stripping step, 1.5 mL of the organic

phase was removed and transferred to a clean centtifuge tube. The equilibrium pH of the

aqueous phase was measured. Before determining the pH, all of the remaining organic

phase was removed and discarded to prevent its interference with the pH measurement.

To the centrifuge tube containing 1.5 mL of the organic phase, 3.0 mL of a 0.10 M

hydrochloric acid solution was added to strip the metal cations. The two-phase mixtures

were vigorously shaken with a vortex mixer for 10 minutes to ensure complete stripping

and then centtifuged for 10 minutes to ensure complete phase separation.

In the metal ion determination step, a 1.0 mL aliquot of the aqueous phase after

the stripping step was transferred to a 10.0-mL volumetric flask and diluted to the mark

wdth deionized water. This sample was analyzed for its alkaline earth metal cation

content by ion chromatography. The results were analyzed using the Microsoft® Excel

2000 spreadsheet program.

Table 2.5. Additional Aqueous Phase Formulations for Competitive Solvent Extraction of Alkaline Earth Metal Cations by Proton-Ionizable Acyclic Polyether N-(CF3)sulfonyl Carboxamides 32-34.

„ , TT i T TT/ T AEMC (20 mM in each) Organic Phase Sample HCl, mL H2O, mL /»^ /-- o r. N T /r » *N T

^ (Mg, Ca, Sr, Ba), mL (5 mM), mL

1 1.00 (IM) 0.00 1.00 2.00

2 0.30 (IM) 0.70 1.00 2.00

3 1.00 (0.1 M) 0.00 1.00 2.00

4 0.30 (0.1 M) 0.70 1.00 2.00

5 1.00 (0.01 M) 0.00 1.00 2.00

6 0.30 (0.01 M) 0.70 1.00 2.00

7 1.00 (0.001 M) 0.00 1.00 2.00

8 0.30 (0.001 M) 0.70 1.00 2.00

60

2.4.2. Solvent Extraction of Pb ""

2.4.2.1. Reagents

Reagent-grade Pb(N03)2 was obtained from Fisher Scientific. TMAOH (25 wt

%) was purchased from Sachem. The metal concentrations were determined by

comparison with 1014 ppm lead standard solution purchased from Aldrich. Other

inorganic and organic compounds were reagent-grade commercial products and were

used as received.

2.4.2.2. Apparatus and Instmmentation

Concentrations of Pb " in the aqueous phases were determined with a Perkin

Elmer 5000 Atomic Absorption spectrophotometer. Measurements of the aqueous phase

pH were performed with a Fisher Accumet Model AR 25 pH meter using a Coming

Model No. 476157 glass-body combination electrode.


For determination of the stoichiometry of the extraction at different pH values,

Pb extraction was performed with 2.0 ml of an aqueous solution of 0.20 mM lead

nitrate and TMAOH (for pH regulation) and 2.0 mL of a 2.0 mM CHCI3 solution of the

ligand in a 15-ml, metal-free plastic centrifuge tube. The mixture in the tube was agitated

for 10 min with a vortex mixer and centrifuged for 10 min to ensure complete phase

separation. The Pb"* concentration and the equilibrium pH of the aqueous phase were

measured (vide supra). The percent extraction (E) and the distribution ratio (D) were

calculated by use of the following equations:

E(%) = 100 ([M'^J, -[M'^lf) / [M' li (2.4)

D = E/(100-E) (2.5)

where [M ""]: and [M '']f are the initial and the final Pb "" concenttations in the aqueous

phase, respectively.

For determination of the stoichiometry of the extraction complex at different

values of [H2L]o, Pb "" extraction was performed with 2.0 ml of an aqueous solution of

61

lead nitrate (0.4 mM) and CAPSO buffer (pH 9.5) and 2.0 mL of CHCI3 solution of the

ligand (2.0 mM for 32 and 3.0 mM for 33 and 34) in a 15-ml, metal-free, plastic

centrifuge tube. The mixture in the tube was agitated for 10 min with a vortex mixer and

centrifuged for 10 min to ensure complete phase separation. lonophore concentrations in

the organic phase were varied according to the formulations given in Table 2.6. A 1.5

mL sample of organic phase was removed and shaken with 3.0 mL of 0.1 N HNO3 for 10

min and centrifiiged for 10 min to sttip Pb^^ from the organic phase into the aqueous

solution for analysis by atomic absorption spectrophotometry.

Pb ^ extraction was performed with 2.0 ml of an aqueous solution of lead nitrate

(2.0 mM) and TMAOH (for pH regulation) and 2.0 mL of a 0.5 mM CHCI3 solution of

the ligand in a 15-ml, metal-free, plastic centriftige tube (Tables 2.4 and 2.5). The

mixture in the tube was agitated for 10 min with a vortex mixer and centtifuged for 10

min to ensure complete phase separation. For pH regulation, TMAOH solution (4.0 mM)

was used since almost no extraction of TMA"" from aqueous solutions into CHCI3 by

acyclic polyether N-(X)sulfonyl carboxamides was expected due to its bulkiness. A 1.5-

mL sample of the organic phase was removed and shaken with 3.0 mL of 0.2 N HNO3 for

5 min and centrifuged for 5 min to strip the Pb * from the organic phase into an aqueous

solution for the analysis by atomic absorption spectrophotometry. After removing the

remaining organic phase, the equilibrium pH of the aqueous phase was measured.

Table 2.6. Compositions of the Organic Phase Solutions Used in the Pb' ' Extraction Experiments for the Slope Analysis Study.

Sample Podand Concentration, mM lonophore in CHCI3, mL CHCI3, mL

1.50 0.50

1.00 1.00

0.70 1.30

0.50 1.50

0.25 1.75

0.10 1.90

0.05 1.95

62

1

2

3

4

5

6

7

1.50(2.25) 1.00(1.50)

0.70(1.05)

0.50 (0.75)

0.25 (0.38)

0.10(0.15)

0.05 (0.08)

2.4.3. Solvent Extraction of Hg ""

2.4.3.1. Reagents

Reagent-grade Hg(N03)2 was obtained from Fisher. A stock aqueous solution of

0.5 mM mercury nittate was stored in a polyethylene bottle. Dithizone and citric acid

were purchased from Fisher. Other inorganic and organic compounds were reagent-grade

commercial products and were used as received.

2.4.3.2. Apparatus and Instrumentation

Concentrations of Hg "" in the aqueous phase were determined with a Shimadzu

UV-2401PC Ultraviolet-Visible spectrophotometer. Measurements of the aqueous phase

pH were performed with a Fisher Accumet Model AR 25 pH meter using a Corning

Model No. 476157 glass-body combination electrode.


To determine the stoichiometry of the extraction at different values of pH, Hg "

extraction was performed with 4.0 ml of an aqueous solution of mercuric nitrate (0.05

mM) and HNO3 (for pH regulation) and 4.0 mL of a 0.50 mM CHCI3 solution of the

ligand in a 15-ml, metal free, plastic centrifuge tube. The tube was agitated for 10 min

with a vortex mixer and centrifuged for 10 min to ensure complete phase separation. A

1.5 mL sample of aqueous phase was mixed with 1.5 mL of citric acid buffer (pH 3.15).

The Hg^^ concentration in the aqueous phase was determined spectrophotometrically

after extraction into CHCI3 containing 14.0 ppm of dithizone (Xmax, 495 nm). After

removing the remaining organic phase, the equilibrium pH of the aqueous phase was

measured. The percent extraction (E) and the distribution ratio (D) were calculated by

use of the same equation shown in Section 2.4.2.3.

For determination of the stoichiometry of the extraction complex at different

[H2L]o values, Hg^* extraction was performed with 2.0 ml of an aqueous solution of

mercuric nitrate (0.50 mM) in 0.1 N HNO3 and 2.0 mL of CHCI3 solution of the ligand in

a 15-ml, metal free, plastic centrifuge tube. The tube was agitated for 10 min with a

63

vortex mixer and centrifuged for 10 min to ensure complete phase separation. lonophore

concentrations in the organic phase were varied according to the formulations given in

Table 2.7. The Hg " concentration in the aqueous phase was measured

spectrophotometrically.

Table 2.7. Composhions of the Organic Phase Solutions Used in the Hg " Extraction Experiments for the Slope Analysis Study.

Sample lonophore Concentration, mM ^ , ' CHCI3, mL

1 1.50

2 1.00

3 0.70

4 0.50

5 0.25

6 0.10

7 0.05

1.50

1.00

0.70

0.50

0.25

0.10

0.05

0.50 1.00

1.30

1.50

1.75

1.90

1.95

The Hg " extraction was performed with 4.0 ml of an aqueous solution of

mercuric nittate (0.25 mM) and HNO3 (for pH regulation) and 4.0 mL of a 0.25 mM

CHCI3 solution of the ligand in a 15-ml, metal free, plastic centriftige tube as Table 2.8

for podands 20-22 and Table 2.9 for podands 23-34. The tube was agitated for 10 min

with a vortex mixer and centrifuged for 10 min to ensure complete phase separation. The

mercuric cation concenttation in the aqueous phase was determined

specttophotomettically after exttaction into CHCI3 containing 14.0 ppm of dithizone

(? max, 495 nm). After removing the remaining organic phase, the equilibrium pH of the

aqueous phase was measured.

64

Table 2.8. Aqueous Phase Formulations for Compethive Solvent Extraction of Hg ^ by Proton-Ionizable Acyclic Polyethers Carboxylic Acids 20-22.

Sample

1

2

3

4

5

6

7

8

9

10

H2O

0.00

1.00

1.50

0.00

1.00

1.50

0.00

1.00

1.50

0.00

HNO3

2.00 (IM)

1.00 (IM)

0.50 (IM)

2.00 (O.IM)

1.00 (O.IM)

0.50 (O.IM)

2.00(0.01M)

I.OO(O.OIM)

0.50(0.01M)

2.00(10"^M)

Hg^^ (0.5mM)

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

Ligand (0.25mM)

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

Table 2.9. Aqueous Phase Formulations for Competitive Solvent Extraction of Hg ^ by Proton-Ionizable Acyclic Polyethers N-(X)sulfonyl Carboxamides 23-34 with X = Ph, CH3, 4-NO2C6H4 and CF3.

Sample H2O HNO3 Hg^" (0.5mM) (Q^SM)

1

2

3

4

5

6

7

8

9

10

0.00

0.80

1.40

1.60

0.00

0.80

1.40

1.60

0.00

0.80

2.00 (IM)

1.20 (IM)

0.60 (IM)

0.40 (IM)

2.00 (O.IM)

1.20 (O.IM)

0.60 (O.IM)

0.40 (O.IM)

2.00 (O.OIM)

1.20 (0.0IM)

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

2.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

4.0

65

2.5. References

1. Mayazaki, M.; Shimoishi, Y.; Miyata, H.; Toei, K. J. Inorg Nucl. Chem. 1974 36 2033.

2. Strzelbick, J.; Bartsch, R. A. Anal Chem. 1981, 53, 1894.

3. Strzelbick, J.; Bartsch, R. A. Anal Chem. 1981, 53, 2251.

4. Inokuma, S.; Azechi, S.; Hayase, T.; Kuwamura, T. J. Chem. Soc Jpn,l9SS, 4, 662, 667.

5. Bartsch, R. A. Solv. Extr. Ion Exch. 1989, 7, 829.

6. Kang, S. I.; Czech, A.; Czech, B. P.; Stewart, L. E.; Bartsch, R. A. Anal Chem. 1985, 57,1713.

7. Hiratani, K.; Sugihara, H.; Taguchi, K.; lio, K. Chem. Lett. 1983,1657.

8. Hanck, R. D.; Martell, A. E. Chem. Rev. 1989, 89, 1875.

9. Hayashita, T.; Yamasaki, K.; Huang, X.; Bartsch, R. A. Chem. Lett. 1994, 1487.

10. Hayashita, T.; Fujimoto, T.; Morita, Y.; Bartsch, R. A. Chem. Lett 1994, 2385.

11. Hiratani, K.; Takashi, T.; Fujiwara, K.; Hayashita, T.; Bartsch, R. A., Anal. Chem. 1996, 69, 3002.

12. Hiratani, K.; Sugihara, H.; Kasuga, K.; Fujiwara, K.; Hayashha, T.; Bartsch, R. A. J. Chem. Soc, Chem. Commun. 1994, 319.

13. Hayashita, T.; Yamasaki, K.; Huang, X.; Bartsch, R. A. Supramol Chem. 1996, 6, 347.

14. Hayashita, T.; Sawano, H.; Higuch, T.; Indo, M.; Hiratani, K.; Zhang, Z.; Bartsch, R. A. Anal Chem. 1999, 77,791.

15. Rogers, R. D.; Bond, A. H.; Wolff, J. L. J Coord Chem. 1993, 29, 187.

16. Rogers, R. D.; Bond, A. H.; Aguinaga, S. J Am. Chem. Soc. 1992,114, 2960.

17. Rogers, R. D.; Bond, A. H.; Aguinaga, S.; Reyes, A. Inorg. Chim. Acta 1993, 212, 225.

18. Walkowiak, W.; Kang, S.-L; Stewart, L. E.; Ndip, G.; Bartsch, R. A. Anal. Chem. 1990, 62, 2022.

19. Amiri-Eliasi, B., Ph. D. Dissertation, Texas Tech University, 2000, pp.122-134.

20. Barton, A. F. M., CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, Boca Raton, FL, 1991.

21. Maley, L. E.; Mellor, D. P. Aust J Sci. Res, A2 1949, 92, 579.

66

CHAPTER III

SOLVENT EXTRACTION OF METAL CATIONS

WITH PROTON-IONIZABLE CALIX[4]ARENES

3.1. Introduction

Calixarenes are macrocyclic molecules, like crown ethers and cyclodextrins.'"^

After reliable and reproducible one-step syntheses of these compounds became

available, calixarenes have been utilized for a variety of applications.^ Calixarenes have

attracted increasing attention as molecular substmctures or platforms on which to

assemble preorganized ligands capable of ionic and/or molecular reception. The

calixarenes have several attractive features. They are easily synthesized in large scale.

Five ring sizes containing 4 to 8 phenolic units are available, although the pentamers and

heptamers are much less accessible than the other three. Also, the phenolic groups in the

calixarene substmcture offer points of attachment for other fimctional groups. Since the

p-tert-hutyl group can be removed, calixarenes may be functionalized also at the para

position of the phenolic units.

Calixarenes made up of phenol and methylene groups have several

conformational isomers because of two possible rotational modes of the phenol unh

(Figure 3.1): the "oxygen through the annulus rotation" and the "para-substituent through

the annulus rotation." The conformational isomers thus obtained possess unique cavities

with different sizes and shapes.^ All four aryl units of calix[4]arenes can rotate

independently in a "oxygen through the annulus rotation" mode, but not in a "para-

substituent through the annulus rotation" mode. In the latter, the bridging methylene and

aryl units bump into each other. The cone isomer can be converted into the partial cone

isomer by inverting a single ring (Figure 3.2). Subsequently, another inversion produces

either the 1,3-alternate or 1,2-alternate isomer depending upon the location of the second

aryl unit that is rotated. Attachment of groups larger than ethyl to the lower rim oxygens

restricts the inversion of the aryl units in calix[4]arenes. However, thermal isomerization

of tetra-O-ethylated calix[4]arene was observed at high temperature (above 100 °C).

67

Thus, propyl groups on the lower rim are the minimum requirement to restrict

isomerization in calix[4]arenes.

para substituent through the annulus rotation

.f* 0R?R2 0\ R2

oxygen through the annulus rotation

Figure 3.1. Two Probable Interconversion Modes of Calix[4]arene.

RiR R1R1

4° \X Cone

/ \ K2 R2 R2

<1 Partial Cone

^^^^

R2 R2

M Ri

1,3-Alternate

Figure 3.2. Four Limiting Conformational Isomers of Calix[4]arenes.

Ri 1,2-Alternate

68

In view of their efficient and selective binding of a variety of ionic and molecular

guest species, calixarenes are an important class of host molecules.''^•""'•' Calixarene

compounds have applications in the separations of organic and inorganic species, ion-

selective electrodes, phase transfer catalysis, chromatography, etc." Calixarenes with

pendent proton-ionizable groups (carboxylic acid, hydroxyamic acid, phophonic acid, and

phosphonic acid monoalkyl ester) are of special interest for the separations of polyvalent

metal ions by solvent extraction and membrane transport'"'"' or sorption when

immobilized on polymer matrices. 20-23 The nature of the proton-ionizable group.

particularly its acidity, controls the metal ion complexation properties of such

compounds. For example, calixarenecarboxylic acids were found to be much more

efficient interphase carriers for alkaline earth metal cations than related unfianctionalized

calixarenes with phenolic groups on the lower rim.''* The availability of new types of

calixarenes with a wider variety of proton-ionizable groups should facilitate the

development of new metal ion separation processes.

An important concept in the design of efficient and selective proton-ionizable

ligands for metal ion separation processes is "matching charge" within the metal ion-

ionized ligand complex. A match of the charge on the metal ion with an appropriate

number of proton-ionizable groups in the ligand produces a neutral complex which

facilitates metal-ion transfer into the organic phase in a solvent extraction process, as well

as transportation across an organic liquid membrane. This concept of charge matching is

illustrated in Figure 3.3.

o=c I

OH

2+

C=0 I OH

+ M

2X" o=c

I 0

0=0 + 7+ I

M^^ O"

2H^

2X"

Figure 3.3. Matching of Charges on the Metal Ion and Ionized Ligand to Provide Efficient Extraction.

69

An outstanding example of the use of a calixarene with two proton-ionizable

groups in a divalent metal ion separation is provided by Shinkai and his coworkers. ''

These researchers report exclusive extraction of Ca " from an aqueous solution

containing four alkaline earth metal nitrate species at pH = 5.3 into chloroform by a

calix[4]arene-dicarboxylic acid diamide.

As discussed in Chapter II, the N-(X)sulfonyl carboxamide function,

-C(0)NHS02X, was introduced as a novel pendant proton-ionizable group for carbon-

pivot lariat ethers and acyclic polyethers. By variation of the electronic properties of X,

the acidity of the macrocyclic ligands is "tunable." For complexation of divalent metal

cation, Bartsch and his coworkers have prepared recently p-/err-butylcalix[4]arenes with

pendant N-(X)sulfonyl carboxamide groups on the lower rim (Figure 3.4). Among the

four N-(X)sulfonyl carboxamide groups, N-(CF3)sulfonyl /2-rerr-butylcalix[4]arene-

carboxamide showed the highest extraction efficiency for Pb " at pH = 2.5. With X =

CH3, Ph and 4-NO2C6H4 only very low Pb " extraction efficiency was observed at pH =

2.5. In the case of Hg " extraction from aqueous nitrate solutions into chloroform at pH =

2.5, N-(X)sulfonyl p-rerr-butylcalix[4]arenecarboxamides showed higher extraction

efficiency (100 %) than the analogous j!7-êrr-butylcalix[4]arenecarboxylic acid (50 %).

Since ligands 35 in Figure 3.4 exhibited high efficiency for Hg "", a dansyl(5-

dimethylamino-1-naphthalensulfonyl) fluorophore group was inttoduced to give a

calixarene-based flouorogenic reagent 36 for selective Hg "" recognition. '

t-Bu | " ^ 1 o , , t-Bu R= CH2C{0)NHS02X

Ii') 35; X = CH3, CF3, Ph and C6H4-P-NO2

36; X= O I OMe 9 (' \VMcrM ^

/ OMe \ \J^N(CH3)2 R R

Figure 3.4. Structures of N-(X)sulfonyl;7-/err-Butylcalix[4]arenecarboxamides.

To probe the influence of the para substituent in conformationally restticted

calix[4]arenes upon the propensity for alkali metal cations, alkaline earth metal cations,

Pb^^ and Hg^ extraction, ;7-unsubstituted calix[4]arenecarboxylic acids and N-

70

(X)sulfonyl calix[4]arenecarboxamides were prepared by Dr. C. Park^^ in the cone,

partial cone-{caxhonyl-up),partial co«e-(butyl-up) and 1,3-alternate conformations.

By means of liquid-liquid extraction of different metal cations into organic phase,

the effects of stmctural variation within proton-ionizable calix[4]arene ligands (i.e., the

conformational variation and the identity of proton-ionizable group) on the efficiency

and/or selectivity of metal ion separation (i.e., alkali metal cations, alkaline earth metal

cations, Pb " and Hg " ) have been studied.

3.2. Results and Discussion

The same definitions for the total loading of a ligand, the selectivity ratio and

solvent extraction profile as defined in Chapter 2.2 are employed for the alkali metal and

alkaline earth metal cation extractions by calixarenes.

Before studying solvent extraction, it was necessary to test each calix [4]arene

ligand for alkali metal cation contamination. The impurity level for each alkali metal ion

was below 5 p.M. Since the minimal concentration of ligand in this experiment is 0.5

mM, this level of contaminant is not expected to produce errors > 1%.

3.2.1. Competitive Solvent Extraction of Alkali Metal Cations

3.2.1.1. Calix[41 arenecarboxyHc Acids

To probe the role of the proton-ionizable side arm, calix [4] arenecarboxyHc acids

37-41 (Figure 3.5) were studied.

umi Mi 'o'T T o o' I T o El o o °

H00CH2C'^g/° °^l^gCH2C00H H00CH2C''g^/° ^^g^cHsCOOH HOOCH2C' ' CHzCOOH

37 (flexible) 38 (cone) 39 (paco-butyl-up)

Bi/° °^Bu^^"'^°°" H00CH 2C'" %i^2C00H

40 (paco-acid-up) 41 (1,3-altemate)

Figure 3.5. Structures of Flexible and Restricted Calix[4]arenecarboxylic Acids 37-41.

71

Since ligands 37-41 were insoluble in chloroform, 1,2-dichloroethane was utilized as the

organic solvent. To dissolve 1 mM 40 and 41 in 1,2-dichloroethane completely,

somewhat longer time was required than for 37-39. To determine a suitable pH regulator,

independent single species solvent extractions of alkali metal hydroxides (LiOH, NaOH,

KOH and CsOH) and tetramethylammonium hydroxide (TMAOH) into 1,2-

dichloroethane by ligand 38 were performed. In all of these experiments, use of metal

hydroxides was observed to give heavy precipitation between the organic and aqueous

phases. Since tetrabutylammonium hydroxide (TBAOH) did not show precipitation, it

was used as the pH regulator. Resuhs for competitive extraction of alkali metal cations

(Li" , Na" , K"*", Rb" and Cs" ) into 1,2-dichloroethane by 37-41 are presented in Figure 3.6.

A 1:2 ligand-to-metal ion stoichiometry was assumed. The results are also summarized

in Table 3.1.

Table 3.1. Efficiency and Selectivity for Competitive Solvent Extraction of Alkali Metal Cations into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenedicarboxylic Acids 37-41.

37 (flexible)

38 (cone)

39 (paco-butyl-up)

40 (paco-acid-up)

41 {1,3-alternate)

Total Loading (%)

89

96

97

16

100

Na

Na

Na

Na

Na

Selectivity Order and Ratios

> Li 52

> Li >100

> K 62

> K

2.7

> K

1.2

>

>

>

>

>

K 88

Li 78

Cs 3.2

Rb 11

> Rb, Cs >100

K, Rb,Cs

> Rb, Cs >100

> Rb > Li

3.3 a

> Li > Cs 46 51

: No detectable amount of Li was extracted.

72

100

80

<3^

CO O

O

<

60

40 -

20

1 r 37

I I — I —

38 a • • • «

*

W-;Wa:W

39 — I — I —

# • • •

A : ^ ! [ : A ; A A

40

a

O

Li Na K

Cs

f^ffrst.:^.

" T — I — I — r

41

t /A

• - •

-AAA

A I

JC

4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10 4 6 8 10 12

pH

Figure 3.6. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkali Metal Cations with 1.0 mM 1,2-Dichloroethane Solutions of Calix[4]arenecarboxylic Acids 37-41.

In this series, the conformation of the calix[4]arenecarboxylic acid is

systematically varied from flexible to cone to paco-butyl-up to />aco-acid-up to 1,3-

alternate. For all of these ligands, the best-extracted cation is Na" . Although their total

loadings were expected to be 200 % based on the complexation stoichiometry, they were

found to be below 100 %.

To determine the reason, NMR spectroscopy was employed. The H NMR

spectral study of the extract obtained for ligand 37 showed that the ligand binds one Na""

and two TBA"" cations. Thus the extracted species is suggested to have the formula

[LM(TBA)2]C1. Based on this suggested complex and heavy precipitation observed with

73

other hydroxides, it can be concluded that the incorporation of TB A" in the complex may

prevent precipitation.

For flexible (37) and cone (38) conformational calix[4]arenes, the second best

extracted cation is Li" ; whereas for ligands 39-41, it is K" . Although the total loading is

somewhat different, the selectivity order and ratio for 38 is very similar to those of 37

(Table 3.1). Thus, flexible calix[4]arene 37 is postulated to adopt the cone conformation

during the extraction process. Comparison of data for the paco-butyl-up (39) and paco-

acid-up (40) extractants shows that the orientation of the proton-ionizable sidearms of the

calix[4]arene has a pronounced effect on the efficiency and selectivity of alkali metal

cation extraction. Ligand 40 has very low acidity compared to other ligands. In the IR-

spectmm of the extract obtained for ligand 40, both COOH and COO" bands are

observed. Presumably, cooperation between both proton-ionizable sidearms is needed to

make the ligand easily deprotonated for the complexation.

The cone 38 ligand exhibits a somewhat different selectivity order and ratios as

compared to the other conformationally restricted calix[4]arenecarboxylic acids 39-41.

In going from 38 to 40 and 41, the second-best extracted metal ion changes from Li"" to

K" and the selectivity ratio decreases as the number of oxygen donor atoms that may

interact with the metal ion simultaneously and the steric hindrance for the cation are

reduced. Ligand 41 has less steric hindrance for the metal cation; thus, its selectivity

ratio is decreased compared to 37, 38 and 39. Interestingly, at low pH, the exttaction

profiles for 39 and 41 (Figure 3.6) have the pronounced variations in the K"" and Rb

loading. Above pH = 6.7, loadings of K^ and Rb^ for ligand 39 decrease dramatically

and finally reach plateaus. Similar behavior is noted for Rb^ and Cs^ loading by ligand

41. Generally, 1,3-alteranate calixarenes favor Cs^ complexation due to Ti-interactions of

the phenyl rings with the soft Cs^ ion. Evidently, low propensities of 41 for Rb^ and Cs*

suggest incorporation of TBA* in the calixarene 41 during the extraction process.

74

3.2.1.2. N-(X)sulfonvl Calix[41arenecarboxamides

Solvent extraction study of calixarenes with N-(X)sulfonyl carboxamide groups

instead of carboxylic acid functions in the proton-ionizable sidearms (Figure 3.7) was

performed. Unlike calix[4]arenecarboxylic acids, these ligands were well-soluble in

CHCI3 and no preciphation was observed in the presence of LiOH. Therefore, CHCI3

and LiOH were used as organic medium and pH regulator, respectively. Precise

comparison of the extraction results for calix [4] arenecarboxyHc acids and N-(X)sulfonyl

calix[4]arenecarboxamides may not be done due to the different experimental conditions

(i.e., organic solvent and pH regulator) applied. Unfortunately, compounds 44, 52 and 56

could not be prepared due to separation problems in the synthesis. ^ Ligand 53 was

found to be slightly soluble in the aqueous phase.

Conformation \ X CH3 Ph 4-NO2C6H4 CF3

XOzSHNOCHjCgy/ ^BuCHzCONHSOjX

(cone)

42 46 50 54

43 47 51 55

XO2SHNOCH2C' "^CHzCfeNHSOzX

(paco-butyl-up) XO2SHNOCH2C

44^ 48 52' 56' P Ox ^CH2CONHS02X

Bu Bu ^

(paco-acid-up)

45 49 53 57

XO2SHNOCH2C CH2CONHSO2X

(1,3-alternate)

': Not prepared. Figure 3.7. Stmctures of Conformationally Restricted N-(X)sulfonyl Calix[4]arene-carboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3.

75

Since ligands 54, 55 and 57 were obtained as Na" salts, a control competitive

extraction of alkali metal cations into chloroform by N-(CF3)sulfonyl calix[4]arene-

carboxamide 54 before and after washing with 1.0 N HCl was performed. As shown in

Figure 3.8, the two pH profiles are very similar. Therefore, the Na-salts of 54, 55 and 57

were used in the alkali metal cations extraction, but the concentration of Na" in the

aqueous phase was corrected to 8 mM.

120

100 -

S5 80

_c T3

o 60 _ i

O

< 4 0 k

20 -

1 1 1 1 1

a •« • *

0

-

0 ^ ^ : :

- :f 54"Na'

1 1

: : : : : : . 1 ^

--I m

1 1 1 1 I f

b r 0

~

0

,06

• A

T 0

54

h —

Li Na K Rb CS

- - - 4

^ — V

---\-..n

10 0 pH

10 12

Figure 3.8. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10 mM (in Each) Alkali Metal Cations with 1.0 mM Chloroform Solutions of 54 (a) before and (b) after washing with 1.0 N HCl. Before washing, [Na""] was 8.0 mM.

76

3.2.1.2.1. Effect of Conformation Variation

To probe the effects of various calix[4]arene conformations, competitive solvent

extraction of alkali metal cations into chloroform by N-(Ph)sulfonyl calix[4]arene-

carboxamides was performed. The acidity of these ligands was varied by changing the X

group in the proton-ionizable sidearms. Based on a 1:2 ligand to metal stoichiometry, the

results for competitive extraction of alkali metal cations into chloroform by proton-

ionizable N-(Ph)sulfonyl calix[4]arenecarboxamides 46-49 are shown in Figure 3.9. The

results are also summarized in Table 3.2.

10 12

pH

Figure 3.9. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkali Metal Cations with 1.0 mM Chloroform Solutions of N-(Ph)sulfonyl Calix[4]arenecarboxamides 46 (cone), 47 (paco-butyl-up), 48 (paco-acid-up), and 49 (1,3-alternate).

11

In this series, the conformation of the calix[4]arene was varied from cone to paco-

butyl-up to paco-acid-up to 1,3-alternate. Through electrostatic, covalent and n-

interactions, the anionic carboxamides, ethereal oxygens and phenyl rings of the

calix[4]arene may coordinate the metal cation within the lower or upper rim of the

calix[4]arene. Except for 48, all ligands in the series showed almost 200 % total metals

loading with Na* as the best-exttacted cation. For the whole series of N-(Ph)sulfonyl

calix[4]arenecarboxamides, except for paco-acid-up 48, the same trend in selectivity ratio

and order is observed. The selectivity order for cone (46), paco-butyl-up (47) and 1,3-

alternate (49) is Na* > K* > Rb* > Cs* > Li*. The selectivity ratios of Na*/K*, Na*/Rb*,

Na*/Cs* decrease and Na*/Li* generally increases in going from cone (46) to jâco-butyl-

up (47) to 1,3-alternate (49). The pHi/2 values of the calixarenes changed in this order:

cone (5.8) <paco-butyl-up (6.4) < 1,3-alternate (6.1) <paco-acid-up (9.8).

Similar to the calix[4]arenecarboxyHc acids, the orientation of proton-ionizable

sidearms has a pronounced effect on the metal extraction selectivity, based on a

comparison of the paco-butyl-up (47) and paco-acid-up (48) ligands. In the case of

ligand 48, its best-extracted ion is Cs* and the pHi/2 is 9.8. Evidently, rotation of the

phenyl ring containing a proton-ionizable sidearm instead of a butoxy group prevents

cooperation of the two proton-ionizable sidearms, which is necessary for the

deprotonation process prior to the complexation. Therefore, the pHi/2 value is much

higher than that of ligand 47. Through covalent interaction with ethereal oxygen, %-

interaction with three phenyl rings, and ionic interaction with one proton-ionizable side

arm, the large cation Cs* complexes with the ligand in the upper rim because the upper

rim has less steric hindrance due to the repulsion of the three phenyl rings. Oppositely,

the small cation Na* is located in the lower rim through the interactions with three

ethereal oxygens and one proton-ionizable sidearm because the cavity size of the lower

rim is suitable for this ion.

As can be seen from the data in Table 3.2, the effect of conformational variation

of the calix[4]arene from cone to paco-butyl-up to 1,3-alternate on the selectivity ratio

becomes even more obvious from comparison of the series of ligands with four different

78

X groups (i.e., X = CH3, Ph, 4-NO2C6H4 and CF3). With the exception of X = CH3, the

pHi/2 values for the proton-ionizable calixarenes vary in the order cone <paco-butyl-up <

1,3-alternate, The general decrease in the Na*/K*, Na*/Rb* and Na*/Cs* selectivities

upon changing the conformation from cone to paco-butyl-up to 1,3-alternate can be

explained based on the Hard-Soft Base-Acid approach. '*

Table 3.2. Efficiency and Selectivity of Compethive Solvent Extraction of Alkali Metal Cations into Chloroform by Conformationally Restricted N-(X)Sulfonyl Calix[4]arene-carboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3 at pH 9.5.

Total Ligands Loading pHi/2 Selectivity Order and Ratios

^ (%)

51 (paco-butyl-up) X = 4-NO2C6H4

53 {1,3-alternate) X = 4-NO2C6H4

42 (cone) X = CH3

43 (paco-butyl-up) X = CH3

45 {1,3-alternate) X = CH3

46 (cone) X = Ph

47 (paco-butyl-up) X = Ph

48 (paco-acid-up) X = Ph

49 (1,3-alternate) X = Ph

50 (cone) X = 4-NO2C6H4

200

194

198

196

198

185

199

198

6.7

6.4

6.9

5.8

6.4

9.8

6.7

4.9

Na

Na

Na

Na

Na

Cs

Na

Na

>

>

>

>

>

>

>

>

K 3.5 K 3.4 K

2.2 K

4.0 K 3.9 Na 1.5 K

2.3 K 3.8

>

>

>

>

>

>

>

>

Rb 4.5 Rb 4.0 Rb 3.3 Rb 4.9 Rb 4.3 K

2.6 Rb 3.4

Rb 3.9

>

>

>

>

>

>

>

>

Cs 8.1 Cs 5.9 Cs 3.8 Cs 7.6 Cs 5.6 Rb 4.2 Cs 3.5 Cs 5.0

>

>

>

>

>

>

>

>

Li 10 Li 18 Li 15 Li 9.2 Li 19 Li 33 Li 17 Li 21

* 198

187

5.6

6.6

Na

Na

> Rb 3.7

> K 2.2

> K 3.8

> Rb 2.7

> Cs 4.0

> Cs 2.9

> Li 40

> Li 33

54(co«e) 200 1.6 Na > K = Cs > Rb > Li X = CF3 4.0 4.0 4.2 96

55 (paco-butyl-up) ^^^ , « Na > Cs = K > Rb > Li X = CF3 3.7 3.7 3.9 >100

51 {1,3-alternate) TZ Z\ Na > K > Rb > Cs > Li X = CF3 2.6 2.7 3.2 >100

79

In the cone conformation, two proton-ionizable sidearms and two ethereal

oxygens may participate in metal-ligand coordination. Compared to other conformations,

the lower rim of the cone calix[4]arene has a narrower cavity due to repulsions of the four

phenyl rings. A narrow cavity with hard donor atoms on the lower rim of the

calix[4]arene favors small alkali metal cations, Li* or Na*. Thus the cone conformation

favors Na* and Li* due to their size and hardness. For the paco-butyl-up conformation,

extraction levels of Rb* and Cs* relative to cone conformation increase, but the extraction

of K* is almost the same as the cone, and Li* is decreased. Probably, the rotation of a

phenyl ring containing the butoxy group from cone to paco-butyl-up conformation makes

the cavity size at the lower rim wider and reduces the hardness of the binding site. The

expected metal coordination sites are one ethereal oxygen, two proton-ionizable sidearms

and one phenyl ring. In the case of the 1,3-alternate conformation, the two proton-

ionizable sidearms and two phenyl rings may interact with the metal ion. Also steric

hindrance for metal ion complexation is lower than in the cone and paco-butyl-up

conformations. As a result, Na* and Li* loadings for the 1,3-alternate isomer decrease,

but the Cs*, Rb* and K* loadings increase compared to the other two conformations.

Thus, a decrease in the Na*/K*, Na*/Rb* and Na*/Cs* selectivities is observed through the

variation of conformation from cone to paco-butyl-up to 1,3-alternate.

In the case of N-(4-N02C6H4)sulfonyl carlix[4]arenecarboxamide 53, a small

distribution of ligand into the aqueous phase was evident because the aqueous phase

changed to a yellowish color after the extraction process. Therefore, the total loading of

53 is somewhat lower at 185%).

3.2.1.2.2. Effect of X Group Variation

To probe the influence of varying the group X in the lipophilic N-(X)sulfonyl

calix[4]arenecarboxamides, competitive solvent extraction of alkali metal cations into

chloroform by cone conformational N-(X)sulfonyl calix[4]arenecarboxamides 42, 46, 50

and 54 were conducted. The results for competitive solvent extraction of alkali metal

cations into chloroform by ligands 42, 46, 50 and 54 are shown in Figure 3.9 and Table

80

3.4. In this series, the group X in the proton-ionizable side arm is systematically varied

from CH3 to Ph to 4-NO2C6H4 to CF3. Their total loadings reached about 200 % with Na*

and K* as the best and second-best exttacted metal ions, respectively. The Na* extraction

efficiencies for this series of Hgands are 116, 115, 112 and 115 %, respectively. For

variation of tiie X group from CH3 to Ph, 4-NO2C6H4 and CF3, the Na*/Li* selectivity

ratio increases, Na*/Cs* decreases and the selectivity ratios for Na*/K*, Na*/Rb* and

Na*/Cs* converge as shown in Figure 3.10. The small changes in Na*/K* and Na*/Rb*

selectivities do not follow any trend and are well within the experimental error.

120

100

v^ 80 D) C

geo _ i

o < 40

20

42 < ' • "1 I r

46 ; • > • • T 1 r

50

-AAA

o-

3 - • • - • • - -_J L

- 0 0 0

54, 1 r

n • A

T 0

Li Na K Rb • Cs

- - ; ^ * ^ '

•<3-BB 4>

L rH,m:m-8 10 4 6 8 10 4 6 8 10 0

pH

^«

-jn

12

Figure 3.10. Alkali Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 10.0 mM (in Each) Solutions of Alkali Metal Cations with 1.0 mM Chloroform Solutions of cone N-(X)sulfonyl Calix[4]arenecarboxamides 42 (X=CH3), 46 (X=Ph), 50 (X=4-N02C6H4) and 54 (X=CF3).

81

The pHi/2 order is 54 (1.6) < 50 (4.9) < 46 (5.8) < 42 (6.7). As mentioned earlier,

the ordering of pHi/2 reflects the order of changing acidity of the ligands. Since the major

difference among ligands 42, 46, 50 and 54 is the acidity due to the X group variation, it

is concluded that fine-tuning of the acidity of the proton-ionizable side arm plays an

important role in determining the metal ion selectivities, similar to the case of acyclic

polyether ligands described in Chapter II.

Solvent extraction with 1,3-alternate N-(X)sulfonyl calix[4]arenecarboxamides

45, 49, 53 and 57 was conducted to investigate whether the more important factor in

selectivity conttol is the conformation or the identity of X. As shown in Table 3.3, all of

the ligands with 1,3-alternate conformations show a lower selectivity for Na over the

other alkali metal cations and lower extraction efficiencies for Na* (95, 96, 85 and 95 %>,

respectively) than do the other conformational calix[4]arene isomers. The lower

efficiency for Na* extraction by ligand 53 is due to the slight dissolution of ligand into

the aqueous phase. Therefore, the conformation is judged to be the dominant factor.

Results for the competitive solvent extraction of alkali metal cations into

chloroform by a series of calixarenes in other conformations are organized in Table 3.3.

The order of changing pHi/2 for these ligands is generally CH3 > Ph > 4-NO2C6H4 > CF3.

The same trend to increasing Na*/Li* and generally decreasing Na*/Cs* selectivities is

observed as noted for the cone conformation calix[4]arene series. Furthermore, the

general convergence of selectivity ratios of Na*/K*, Na*/Rb* and Na*/Cs* is observed

with variation of the X group from CH3 to Ph to 4-NO2C6H4 to CF3.

82

Table 3.3. Efficiency and Selectivity for Competitive Solvent Extraction of Alkali Metal Cations into Chloroform by cone, paco-butyl-up, paco-acid-up and 1,3-alternate N-(X)sulfonyl Calix[4]arenecarboxamides 42-57.

Ligands

42 (cone) X = CH3

46 (cone) X = Ph

50 (cone) X = 4-NO2C6H4

54 (cone) X = CF3



Total Loading

(%)

200

196

198

200

194

198

pHi/2

6.7

5.8

4.9

1.6

6.4

6.4

Na

Na

Na

Na

Na

Na

Selectivity Order and Ratios

>

>

>

>

>

>

K 3.5 K

4.0 K 3.8 K

4.0 K 3.4 K

3.9

>

>

>

>

>

>

Rb 4.5 Rb 4.9 Rb 3.9 Cs 4.0 Rb 4.0 Rb 4.3

>

>

>

>

>

>

Cs 8.1 Cs 7.6 Cs 5.0 Rb 4.2 Cs 5.9 Cs 5.6

>

>

>

>

>

>

Li 10 Li 9.2 Li 21 Li 96 Li 18 Li 19

51 (paco-butyl-up) .^^ . . Na > Rb > K > Cs > Li X = 4-N02C6H4 3.7 3.8 4.0 40

55 (paco-butyl-up) ^„„ , „ Na > Cs > K > Rb > Li X = CF3 3.7 3.7 3.9 >100

48 (paco-acid-up) 185 9 8 Cs > Na > K > Rb > Li X = Ph ' 1.5 2.6 4.2 33

45 {1,3-alternate) ^^g ^g Na > K > Rb > Cs > Li

X = CH3 2.2 3.3 3.8 15 49 {1,3-alternate) .^Q Na > K > Rb > CS > Li

X = Ph 2.3 3.4 3.5 17 53 (1,3-alternate) , „., . . Na > K > Rb > Cs > Li

X = 4-N02C6H4 ^^^ ^-^ 2.2 2.7 2.9 33 51 (1,3-alternate) ^^^ ^^ Na > K > Rb > Cs > Li

X = CF3 19' 2-^ 2.6 2.7 3.2 >100

83

3.2.2. Competitive Solvent Extraction of Alkaline Earth Metal Cations

3.2.2.1. Calix[41 arenecarboxyHc Acids

Based on a 1:1 ligand to metal stoichiometry, the results for the competitive

solvent extraction of aqueous alkaline earth metal cations into 1,2-dichloroethane by

proton-ionizable calix[4]arenecarboxyHc acids 37-41 are shown in Figure 3.11 and Table

3.4. As the pH regulator, Ba(0H)2 was utilized. A detailed preparation of the Ba(0H)2

solution is given in Section 3.4.2.3.

As shown in Figure 3.11 and Table 3.4, the extraction profile for

conformationally flexible ligand 37 is very similar to that of paco-butyl-up ligand 39.

This suggests that 37 adopts a paco-butyl-up conformation during the extraction of

alkaline earth metal cations. For cone conformation calix[4]arene 38, the best extracted

cation changes from Ca * (observed for 37 and 39) to Ba *. In general, the cavity of

calix[4]arenes in the cone conformation is known to match Na* and Ca *, which have

similar ionic diameters of 1.90 and 1.98 A, respectively. The Ba */Ca * selectivity of 2.7

in 38 suggests steric hindrance in the cavity on the lower rim. Two butyl and two

carboxylic acid groups could prevent the metal cations from binding into the

calix[4]arene pocket deeply and closely. Thus the metal cations are mostly extracted by

the two carbonyl oxygens, which are located below the four ethereal oxygens. This

would allow Ba' * to be better extracted than smaller Ca *.

84

100

80

DJ

T3 03 O

_ l

o LU <

60

40

20

37 38 39

5a

•

AA^

• •

T • • ,

A A A -

D QD I I

40

D Mg • Ca A Sr T Ba

41

A I ft

/ ' ^

6 8 104 6 8 10 » feiP^ng

8 104 6 8 104 8 10

pH

Figure 3.11. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM 1,2-Dichloroethane Solutions of Calix[4]arenecarboxylic Acids 37-41.

The conformation variation from cone calix[4]arene 38 to paco-butyl-up

calix[4]arene 39 changes the best extracted ion from Ba * to Ca * and reduces the Sr *

loading. Similar to the extraction of alkali metal cations, the paco-acid-up calix[4]arene

(40) shows very low extraction efficiency and is selective for Ca *. The pronounced drop

in the extraction profile of 40 is attributed to insolubility of the extraction complex. Also

heavy precipitation was observed above pH = 6.5 in the extraction profile for 1,3-

alternate calix [4] arene 41.

85

Table 3.4. Efficiency and Selectivity for Competitive Solvent Extraction of Alkaline-Earth Metal Cations into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenecarboxylic Acids 37-41.

Ligands Total Loading

(%) pH 1/2 Selectivity Order and Ratios

37 (flexible)

38 (cone)

39 (paco-butyl-up)

40 (paco-acid-up)

41 {1,3-alternate)

89 (pH9.13)

91 (pH 10.02)

93 (pH 8.85)

10 (pH 10.02)

22 (pH8.18)

7.5

1.1

7.0

ND'

ND'

Ca

Ba

Ca

Ca

Ba

>

>

>

>

>

Ba 3.0 Ca 2.7 Ba 2.6 Mg 4.4 Ca 3.0

>

=

>

>

>

Sr 5.9 Sr

Sr 6.8 Ba 7.8 Sr 5.5

> M2 23

> Mg 40

> Mg 17

= Sr

Mg a

: Could not be determined due to precipitate formation

3.2.2.2. N-("X)sulfonvl Calix[41arenecarboxamides

Since ligands 54, 55 and 57 were obtained in the salt form, successive washing of

the chloroform solution of 54 in the sah form with aqueous 1.0 N HCl was performed to

replace Na* for H* on an analytical scale. Then the quantity of Na* in aqueous phase was

measured after washing. Three independent samples were prepared. At first washing,

99.5 % (± 1.23 n = 3) of Na* was replaced by H* and at second washing showed only

0.39 % (± 0.08, n = 3). Compounds 54, 55 and 57 were used for alkaline earth metal

extraction after washing with HCl. For Pb^* and Hg^* extraction, the Hgands were

washed with HNO3.

Ba(0H)2 was utilized as the pH regulator and chloroform was used as the organic

solvent instead of 1,2-dichloroethane which was used for calix[4]arenecarboxylic acids.

The identity of the organic solvent may affect the extraction efficiency and selectivity as

described in Section 2.2.1.4. Therefore, precise comparison of the alkaline earth metal

cations extraction by calix[4]arenecarboxylic acids and N-(X)sulfonyl carboxamides

studied under different experimental conditions (i.e., different solvents) is impossible.

86


Assuming a 1:1 ligand to metal ion stoichiometry, results for the competitive

solvent exttaction of alkaline earth metal cations into chloroform by N-(Ph)sulfonyl

calix[4]arenecarboxamides 46-49 are shown in Figure 3.12 and Table 3.5.

100

80

.E 60 T3 CO O

_ l

O

<

20

1 — r 46

A ^ A

• A A

A

T r 47

^ ^

"1 1 1 r 48

• Mg • Ca A Sr • Ba --

• I W I

1 1 1 r

49

T T

A A

J t Q H i a I i _ 6 7 8 9 10 6 7 8 9 10 6 7 8 9 10 6 7 8 9 10 11

pH

Figure 3.12. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM Chloroform Solutions of N-(Ph)sulfonyl Calix[4]arenecarboxamides 46 (cone), 47 (paco-butyl-up), 48 (paco-acid-up) and 49 (1,3-alternate).

87

Table 3.5. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by Conformationally Restricted N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3.

Ligands

42 (cone) X = CH3



46 (cone) X = Ph


Total Loading (%)

89 (pH 9.25)

93 (pH 9.42)

98 (pH 9.83)

80 (pH9.3n

95 (pH 9.57)

pHi/2

8.1

8.7

7.6

8.6

8.5

1

Ba

Ba

Ba

Ba

Ba

Selectivity Order

>

>

>

>

>

Sr 3.0 Sr 2.4 Ca 21 Sr 2.1 Ca 10

>

>

>

>

>

and Ratios

Ca 5.1 Ca 6.9 Sr 30 Ca 6.4 Sr 11

>

>

>

>

Me 10

Mg 9.7 Mg A

Ms 27 Mg 91


49 {1,3-alternate) X = Ph

2.9 (pH 10.14)

ND'' Ba > Ca

2.4 > Mg

3.7 98

(pH 9.28) 7.3 Ba > Ca

8.7 > Sr

13

50 {cone) X = 4-NO2C6H4

96 (pH 9.58)

8.0 Ba > Sr 1.8

> Ca 2.1


97 (pH 9.69)

7.5 Ba > Ca

2.9 > Sr

5.4


92 (pH 9.80)

7.4 Ba > Ca

12 > Sr

23

54 (cone) X = CF3

100 (pH 9.44)

5.8 Ba > Sr 1.1

> Ca 3.6

55 (paco-butyl-up) X = CF3

99 (pH 8.61)

4.6 Ba > Sr

6.1 > Ca

6.6

57 {1,3-alternate) X = CF3

100 (pH 9.36)

4.9 Ba > Sr

32 > Ca

63

': Could not be determined due to the low level of extraction.

>

>

>

Sr 9.6 Mg A

> Me 14

Mg 59 Mg A

Me 64 Mg

a Mg

a

The members of the series 46-49 showed 80, 95, 2.9 and 98 % total metals

loading, respectively, with Ba^* as the best-exttacted cation. In the case of 48, the

enhanced Mg'* loading at high pH may be due to colloidal Mg'* in the sttippant. The

very low exttaction efficiency of 48 reveals a poor arrangement of the sidearms for

divalent metal ion complexation. Variation of the conformation from cone in 46 to paco-

88

butyl-up in 47 to 1,3-alternate in 49 generally enhances the selectivity ratios. The same

explanation as in the exttaction of alkali metal cation can be applied. As mentioned in

section 3.2.1.2.1, the steric hindrance and number of oxygen donor atoms are reduced in

going from the cone to the paco-butyl-up to the 1,3-alternate conformation. Since Ba'*

is larger than the other alkaline earth metal cations, its extraction efficiency increases

along witii the reduced steric hindrance in going from cone to paco-butyl-up to 1,3-

alternate.

As can be seen from the data in Table 3.6, the effect of varying the conformation

from cone to paco-butyl-up to 1,3-alternate on the selectivity ratio becomes more

obvious from the comparison of the series of ligands with four different X groups (i.e,

CH3, Ph, 4-NO2C6H4 and CF3). All of the ligands with N-(X)sulfonyl carboxamide

groups favor Ba'* complexation. Generally, the selectivity for Ba'* over the other

alkaline earth metal cations increases with the conformational variation from cone to

paco-butyl-up to 1,3-alternate.


To probe the influence of varying the X group in the N-(X)sulfonyl calix[4]arene-

carboxamides, solvent extraction of alkaline earth metal cations into chloroform by 42

(X=CH3), 46 (X=Ph), 50 (X=4-N02C6H4) and 54 (X=CF3) in the cone conformation was

studied. The results are shown in Figure 3.13 and Table 3.6. The total metals loading of

the ligands ranged from 80 to 100 % with Ba'* and Sr'*as the best and second-best

extracted cations. For the variation of X from CH3 to Ph, 4-NO2C6H4 and CF3, the

Ba'*/Sr'* selectivity ratio decreases. However, this trend in selectivity ratio and order

does not extend to the other conformations.

In the case of the paco-butyl-up conformation, the selectivity order for CH3 (43)

and CF3 (55) is Ba'* > Ca'* > Sr'* > Mg'* and for Ph (47) and 4-NO2C6H4 (51) is Ba'*

>Sr'* > Ca'* > Mg'*. The selectivity ratio of Ba'* to other alkaline earth metal cations

depends on the group X as follow: Ph (47)> CF3 (55) > 4-NO2C6H4 (51) > CH3 (43). In

the case of the 1,3-alternate conformation, the selectivity order is Ba * > Ca * > Sr * >

89

,2+ Mg . The selectivity ratio of Ba * to other alkaline earth metal cations also depends on

the group X as follows: CF3 (57) > CH3 (45) > 4-NO2C6H4 (53) > Ph (49).

100

80 -

TO O

60

111 <

40

20

1 — 1 — l —

42

-

-

T

•

T

T

•

- "^ A

4-

1

T

A

•

P

1 1 I —1 '

46

T ^

T

•

" A A A

• A A

A

T • • • 8

" 1 1 1 - I —

50

• • T

•

• A A A A • •

1 1 1 1

54

• A

•

_ MX

•

l l * * M n l - , 1

1 1

Mg-Ca Sr Ba_

A A A

-

• • •

a

6 7 8 9 10 6 7 8 9 10 6 7 8 9 10 4 5 6 7 8 9 1 0 1 1

pH

Figure 3.13. Alkaline Earth Metal Loading versus the Equilibrium pH of the Aqueous Phase for Competitive Extractions of 2.0 mM (in Each) Solutions of Alkaline Earth Metal Cations with 1.0 mM Chloroform Solutions of cone N-(X)sulfonyl Calix[4]arenecarboxamides 42 (X=CH3), 46 (X=Ph), 50 (X=4-N02C6H4) and 54 (X=CF3).

90

Table 3.6. Efficiency and Selectivity of Competitive Solvent Extraction of Alkaline Earth Metal Cations into Chloroform by N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 with X = CH3, Ph, 4-NO2C6H4 and CF3.

Ligands

42 (cone) X = CH3

46 (cone) X = Ph

50 (cone) X = 4-NO2C6H4

54 (cone) X = CF3


Total Loading (%) P"'^^ Selectivity Order and Ratios

89 8.1 Ba > Sr 3.0

> Ca 5.1

80 8.6 Ba > Sr 2.1

> Ca 6.4

96 8.0 Ba > Sr 1.8

Ca 2.1

100 5.8 Ba > Sr 1.1

> Ca 3.6

93 8.7 Ba > Sr 2.4

> Ca 6.9

'*: Could not be determined ^: Could not be determined

due to the low level of extraction, due to precipitate formation.

> Me 10

> Mg 27

> Mg 14

> Mg 64

> Me 9.7



55 (paco-butyl-up) X = CF3



49 {1,3-alternate) X = Ph


57 {1,3-alternate) X = CF3

95

97

99

2.9

98

98

92

100

8.5

7.5

4.6

ND'

7.6

7.3

7.4

4.9

Ba

Ba

Ba

Ba

Ba

Ba

Ba

Ba

>

>

>

>

>

>

>

>

Ca 10 Ca 2.9 Sr 6.1 Ca 2.4

Ca 21 Ca 8.7 Ca 12 Sr 32

>

>

>

>

>

>

>

>

Sr 11 Sr 5.4 Ca 6.6

Me 3.7 Sr 30 Sr 13 Sr 23 Ca 63

Mg 91 Mg 59

> Mg b Sr 9.6

Me b

Mg b

Mg b

> Mg b

1.2+ 3.2.3. Solvent Extraction of Pb

3.2.3.1. Calix[41arenecarboxylic Acids

The experimental procedure utilized for this series was the same as that described

in Section 2.2.2.2. As the pH regulator, TBAOH was utilized. The Pb'* loading

percentage was measured by atomic absorption spectrophotometry.

91

Based on the assumption of 1:1 ligand to metal ion stoichiometry, results from

single species extraction of aqueous 1.00 mM Pb'* nitrate solution into 1,2-

dichloroethane by 0.50 mM calix[4]arenecarboxylic acids 37-41 are shown in Figure

3.14. The results are also summarized in Table 3.7.

3.0

100

80

o^

Load

ing

en

o

a. 40

20

n

- D

• A

- T

o

-

1

37 38 39 40 41

1

' 1 ' 1

/' /

f ^ •

9 I 1 a A

• A

Q - ' A ' ' 1 . • * - - |

-0,B--;.A

'' / /

4 !

•

,* •

•

,'

•

0

. - • • • - - '

1 1

1

,T

1

I

_

-

-

~

3.5 4.0 4.5 5.0

pH

5.5 6.0 6.5

Figure 3.14. Pb'* Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM 1,2-Dichloroethane solutions of Calix[4]arenecarboxylic Acids 37-41.

-2+ Except for 40, all members of this series reach 100 % Pb loading and their

extraction efficiency changes in the order: 1,3-alternate (41) > flexible (37) >paco-butyl-

up (39) > cone (38) > paco-acid-up (40). Unlike the alkali metal cation extractions (see

Section 3.2.1.1), the presence of TBA* does not diminish the metal ion extraction.

92

From the "charge match" concept, the stoichiometry for 40 may be changed to 1:2

metal ion-to-ligand because hs slope A(Pb'* loading %) / ApH in the pH range from 5.0

to 5.7 is about half that of the other Hgands.

For the cone, paco-butyl-up and 1,3-alternate stmctures, steric hindrance must

influence the pHi/2 and efflciency for the Pb'* extraction because of their common

orientation of proton-ionizable sidearms. It is interesting to note the similarity in pH

profile for flexible 37 with paco-butyl-up 39. ft should be recalled that the pH profile for

flexible conformational calix[4]arenecarboxylic acid 37 is similar to that for cone (38)

and paco-butyl-up (39) conformations for the extractions of alkali metal and alkaline

earth metal cations, respectively.

Table 3.7. Efficiency and Half Extraction pH of Single Species Solvent Extraction of Pb'* into 1,2-Dichloroethane by Conformationally Flexible and Restricted Calix[4]arenedicarboxyIic Acids 37-41.

37 (flexible)

38 {cone)

39 (paco-butyl-up)

40 (paco-acid-up)

41 {1,3-alternate)

pHi/2

4.37

5.24

4.51

5.77

3.96

Maximal Loading (%)

100

97

100

56

100

3.2.3.2. N-(X)sulfonyl Calix[41arenecarboxamides


Based on a 1:1 ligand to metal ion stoichiometry, results for the single species

extraction of aqueous Pb'* into chloroform by N-(Ph)sulfonyl calix[4]arenecarboxamides

42-57 are shown in Figure 3.15. Compared with their carboxylic acid analogs 37-41,

these proton-ionizable calix[4]arenes show dramatically lower Pb'* loading.

The results for extraction of Pb'* by the N-(CF3)sulfonyl calix[4]arene-

carboxamide series provide the clue for the role of TBA*. In the case of the N-

(CF3)sulfonyl calix[4]arenecarboxamide, additional exttaction solutions with lower pH

93

were also used to obtain complete curves of "percent Pb'* loading versus pH" (see

Sections 2.4.1.3 and 2.4.2.3). The choice of acidic solution depends on the anion of the

target metal salts. Since lead nitrate is used for the extraction, nitric acid was used to

make the acidic aqueous phase. Through the variation of the nitric acid volume, a pH

gradient for the acidic region of the first seven points was obtained. As shown in Figure

3.15.d, the Pb'* loading increases for these initial seven points. The 1,3-alternate 57 and

paco-butyl-up 55 conformation reach complete complexation with Pb around

equilibrium pH = 3, where no HNO3 or TBAOH is added to control the pH of the

aqueous phase. The Pb'* loading for cone conformation 54 reaches 38 % at pH 3.3.

With the addition of TBAOH as a pH regulator, the Pb'* loading shows a pronounced

decrease. Thus the TBA* is complexing with the calixarenes and competing with Pb *.

Therefore, complexation between TBA* and the proton-ionizable calix[4]arenes must be

considered.

The pH profiles for the N-(Ph)sulfonyl calix[4]arenecarboxamide series (Figure

3.15.b) show that the dominant factor for the Pb'* loading is the conformation. The Pb *

loading and pHi/2 order is 1,3-alternate > paco-butyl-up > cone > paco-acid-up. As was

established eariier in alkali and alkaline earth metal cations extractions (Sections 3.2.1.2

and 3.2.2.2), the orientation of the proton-ionizable sidearms is important among the

conformation effects for Pb'* exttaction. Unlike in the paco-acid-up calix[4]arene-

carboxylic acid 40, the Pb'* loading for the paco-acid-up N-(Ph)sulfonyl calix[4]arene-

carboxamide 48 is almost zero.

The general ttend shows that the addition of TBAOH reduces the Pb'* loading in

ligands which have more acidic terminal groups, such as 4-NO2C6H4 and CF3. The Pb

loading trend established in N-(Ph)sulfonyl calix[4]arenecarboxamide series is repeated

for all other terminal groups, i.e., 1,3-alternate > paco-hutyl-up » cone (> paco-acid-

up). However, the reason for the abmpt reduction in Pb'* loading for 51 above pH = 5 is

not clear now.

94

100

80

C3) C

o

60

40

20

•

0

A

• - D - 42 43 45

a -D-,

• -V-

46 47 48 49J-

"~i—r-w—r d fr-D-

ii*

-D- 50 51 53

i0 nv^ ln- imn- JLL

54 55 57

i.

• a

- -A ' h lii 7 0 1 2 3 4 5 6

pH

Figure 3.15. Effect of Conformation Variation on Pb'* Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) CH3, (b) Ph, (c) 4-NO2C6H4 and (d) CF3.

3.2.3.2.2. Effect of X group Variation

To probe the effect of X group within the same conformation, the data in Figure

3.15 is rearranged in Figure 3.16. The Figure 3.16.a for cone conformational N-

(X)sulfonyl calix[4]arenecarboxamides shows the dramatic conformation effect on the

extraction efficiency for Pb'* loading. It can be explained by the steric hindrance of the

lower rim and/or a favoring for TBA* instead of Pb * in the cone conformational series.

Thus there is no apparent X group effect in this cone conformation. In the case of paco-

hutyl-up, ligands with X = 4-NO2C6H4 and CF3 shows a dramatic variation of Pb'*

95

loading with the addition of TBAOH. However, there in no apparent effect of TBAOH

addition for the ligands with X = CH3 and Ph. In the case of 1.3-alternate conformation,

only the ligand with X=CF3 showed an obvious effect of TBAOH. Interestingly, the Pb'*

loading is increased above pH= 4 in spite of increasing the quantity of TBAOH. The

estimated pHi/2 order for those two series is CF3 « PhN02 ~ CH3 ~ Ph (see Figure

3.16.b and c).

100

80 -

^ 60

O

40

20

1 1 — 1

a n 42 « 46 A 50

• 54"

-

_ T

i :T

i_» mmmS<^^-'/\°

1 » 1 r b T ---D--

:'T - A -

T: -------

: •

; ; b / ; *

T ; 1 ; •

: ; 1 T • A« ;/

• ^

43 -47 51 55"" •

\ A ^

—-' Iw ,

1 T 1 I n c T

ft • • •

T

7 ^ '0

; f -4

'•• T O

i • :• • • ;;':

A; id :fa-- 45

; AJ-0--. 49 g - A - 53 ^ i - T - 57

-w 1 10 1 2 4 6 0

pH

.2+ Figure 3.16. Effect of X Group Variation on Pb Loading versus the Equilibrium pH of the Aqueous Phase for Extractions of 1.0 mM Solution of Lead Nitrate with 0.5 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) cone, (h) /»aco-butyl-up and (c) 1,3-alternate.

96

3.2.4. Solvent Extraction of Hg'*

3.2.4.1. Calix[4]arenecarboxyHc Acids

Assuming a 1:1 ligand to Hg'* stoichiometry, results from single species

extractions of 0.5 mM Hg'* solution with 0.25 mM 1,2-dichloroethane solutions of

calix[4]arenecarboxylic acids 37-41 are shown in Figure 3.17.

100

80

.2 60 o CD

•4—«

X LU

A ^ 4 0

20 -

"1 1 r 37

1 1 r

38 1 1 r

39

Q°0^0^ |0ôdnn^ lnn rÔ^ : :<? lo^ rSrA^ r^ l °OO^ InOnnnr^ rwv^L^^^^^nn . - Â

T 1 r

40 1 1 r

41

1 2

pH

Figure 3.17. Hg'* Extraction versus the Equilibrium pH of the Acidic Aqueous Phase for Extractions of 0.25 mM Solution of Mercuric Nitrate with 0.25 mM 1,2-Dichloroethane Solutions of Calix[4]arenecarboxylic Acids 37-41.

.2+ Although these calixarenes were found to be excellent for extraction of Pb from

aqueous solutions into 1,2-dichloroethane, none of them show appreciable extraction of

Hg'*. The results presented suggest a significant contribution of hardness of oxygen

97

donor atoms of the proton-ionizable side arm in 37-41 to the coordination of the soft Hg'*

rather than the contribution of the 7r-electron rich aromatic units.

3.2.4.2. N-(X)-sulfonyl Calix[4]arenecarboxamides


Based on the assumption of a 1:1 ligand to Hg'* stoichiometry, results for single

species extraction of acidic aqueous Hg'* solutions in which the pH was adjusted by

addition of HNO3 into chloroform by N-(X)sulfonyl calix[4]arenecarboxamides 42-57 are

shown in Figure 3.18.

100

80

O 60 o CO

. 1 — »

X LU

+ 40 CM

X

20

. • . - y

P T ^ «

- • •

h ° 42_^ • 43

' • 45

•b

D'

a

• » " c3

- - .-y

. • . .

S-A A

A -A.A-A--A

A

a 46, • 47 A 48 T 49

a ,' 0 ^ 0 ^

• •

n 50 __ • 51 • 53

1 4'^)'?

p i

•I

n 54J • 55 T 57

0

pH

Figure 3.18. Effect of Conformation Variation on Hg'* Extraction versus the Equilibrium pH of the Acidic Aqueous Phase for Extractions of 0.25 mM Solution of Mercuric Nitrate with 0.25 mM Chloroform Solutions of N-(X)sulfonyl Calix[4]arenecarboxamides 42-57 (a) CH3, (b) Ph, (c) 4-NO2C6H4 and (d) CF3.

98

Unlike the low levels of alkali metal, alkaline earth metal and Pb'* extraction, the

Hg extraction % for paco-acid-up calix[4]arene 48 (Figure 3.18.b) was about 38%

during the pH variation. This suggests a significant contribution of the n-electron-rich

aromatic unhs in the calix [4] arene moiety and the X group to the coordination of soft

Hg for ligand 48. Therefore, the phenomenon of n- cation interaction is obvious with

this ligand.

As shown in Figure 3.18.a (X = CH3), the Hg'* extraction efficiency of cone

conformation 42 is lower at below pH = 1 and higher at above pH = 1.5 than those of

other conformational calix[4]arenes 43 and 45. Similar pronounced enhancement of the

Hg * extraction efficiency of the cone conformation along with increasing pH is observed

for other X groups. Hg extraction efficiency by the cone conformation ligands reaches

100 %) with the exception of 46. In the cases of X = CH3, Ph and 4-NO2C6H4, the

extraction efficiency is in the order of cone < paco-butyl-up < 1,3-alternate in the low pH

region, but changes paco-butyl-up < 1,3-alternate < cone in the high pH region. In the

case of X = CF3, Hg'* extraction efficiency reaches 100 %. In spite of the variation of

the conformation in going from cone to paco-butyl-up to 1,3-alternate, the pH profiles

for those conformations are very similar. Thus, the effect of the conformation variation

can be concluded as unimportant.


To illustrate the influence of varying the X group of the ligand, the data in Figure

3.17 is rearranged in Figure 3.18. As shown in Figure 3.18.b, the general trends in the

pH profiles for paco-butyl-up calix[4]arenes 43 (X=CH3), 47 (X=Ph) and 51 (X=4-

NO2C6H4) are almost the same. The extraction efficiency is in the order CH3 s; Ph « 4-

NO2C6H4 < CF3 for the paco-butyl-up conformation. With the exception of X= CF3, the

effect of X group variation is very insignificant in the paco-butyl-up conformation.

However, variation of the X group influences the Hg'* exttaction efficiency in the other

cone and 1,3-alternate conformations. The extraction efficiency is in the order Ph < CH3

99

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