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6-1976

The preparation and kinetic studies of two stericallyhindered gold (III) complexesJames Forrest StevensUnion College - Schenectady, NY

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Recommended CitationStevens, James Forrest, "The preparation and kinetic studies of two sterically hindered gold (III) complexes" (1976). Honors Theses.2253.https://digitalworks.union.edu/theses/2253

THE PREPARATION AND KINETIC STUDIES - OF TWO STERICALLY HINDERED

GOLD(III) COMPLEXES

by JAMES F~VENS ft}, 5, t'1J fo Ill 1

This thesis is submitted in partial fulfillment of

the requirements for the degree of Master of Science in

Chemistry.

UNION COLLEGE

Schenectady, New York

July, 1975

Approved. _ _,.__('_~ L~uL~~~I_:_. -..;::;.U~~:....::..:::...'-=----- Approved~~.::....::;-=' _..._,CY)?'--+'~T-~~e-s~~=s~(/}1~A=~-v~i-s-o-r~~~~~~~~~~

Committee:ldn Graduate Studies

Date __ -7fg~fA/?1L~---+--'/ 1-+-Z-=-l _

This thesis is gratefully dedicated to my wonderful

wife, Rosemary, whose continual encouragement and under-

standing made this work possible.

ii

ACKNOWLEDGMENT

I heartfully thank my Research Advisor, Professor

Charles F. Weick, for his suggesting of this topic and

for his many evening and Saturday hours given in guidance,

assistance and encouragement towards its completion.

iii

TABLE OF CONTENTS

Acknowledgment

List of Figures

List of Tables

List of Abbreviations

Abstract

Introduction

Experimental

Materials

Analyses

Preparation of Complexes

Kinetic Studies

Results and Discussions

References and Bibliography

iii

v

vi

vii

ix

1

5

5

5

7

10

23

31

iv

LIST OF FIGURES

Figure Page

1.

2.

Absorbance of Auc14 vs Au Concentration at 313 nm

Absorption ~ectra of the ~romo and Chloro Species of4LAu(Me5dien)~ +at Concentrations of 4 x 10- M 13

6

3. Absoprtion S;>ectra of the Bromo and Chloro Species o~4LAu(MeEt4dien)~ 2+ at Concentrations of 4 x 10 M 14

4. A Typical Single Wavelength Scan of the Reaction of [Au(Me5dien)cg 2+ with 0.1 M Br

A Typical Scan for2ihe Reaction of C Au(MeEt4dien) c:g with 0. 05 M Br-

Graph of kb for t2~ Reaction of [ Au(MeEt4H:t~n)Cij with 0. 05 M Br-

Observed Rate Constant for [Au(Me5dien) qJ 2+

versus Bromide Concentration@ T - 25oc

Observed Rate Constant for [Au(MeEt4dien)C~ versus Bromide Concentration@ T = 25oc 21

16

5. 18

6. 19

7. 20

8. 2+

9. Observed Rate Constant for [Au(MeEt4dien)C1] 2+

versus Bromide Concentration@ T = 25°C and at wavelength 380 nm 22

10. R~action of [Au(M~Et4dien)CU 2+ with 0.05 M Br with respect to time 29

v

LIST OF TABLES

Table

I Pseudo First Order Rate Constants at 25°C

II First Order and Second Order Rate Constants

Page

2L~

25

vi

LIST OF ABBREVIATIONS

The following abbreviations will be used throughout

the text of this paper:

Abbreviation Name & Formula

amine any <lien or substituted <lien

amine-H the conjugate base of amine

dien Diethylenetriamine

dien-H

NH2CH2CH2NHCH2CH2NH2

the conjugate base of dien

NH2CH2CH2NCH2cH2NH2

1,1' ,7,7' - tetraethyldiethylene-

triamine

Et2dien

(C2H5)2NcH2cH2NHCH2cH2N(C2H5)2

the conjugate base of Et4dien

( C2H 5) 2NCH2CH2NCH2CH2N ( c2H5) 2 1,1'-diethydiethylenetriamine

Me4dien

(C2H5)2NCH2cH2NHCH2cH2NH2

the conjugate base of Et2dien

(C2H5) 2NCH2CH2NCH2CH2NH2-

l, l' , 7, 7' - tetramethyldiethylene-

triamine

Ma4dien-R

(CH3)2NcH2cH2NHCH2CH2N(CH3)2

the conjugate hase of Me4die.n

(CH) NCH0CH0NCH0CH N(CH3)0 3 2 e: Z.. L 2 IL

vii

Me2dien 1,1'-dimethyldiethylenetriamine

Me5dien

(CH3)2NCH2CH2NHCH2CH2NH2

the conjugate base of Me2dien

(CH3)zNCH2CH2NCHzCH2NH2

1,1' ,4,7,7'-pentamethyldiethylene-

triamine

(CH3)2NCH2CH2NCH3CH2CH2N(CH3)2

4-methyl-1,1' ,7,7'-tetraethyl-

diethylenetriamine

viii

ABSTRACT

Previous studies indicate substitution of bromide ion

for chloride ion in [Au(Et4dien~H)c.i] +occurs at a rate

almost independent of the bromide ion concentration, while

similar reactions with [Au(dien-H)Cl]+, [Au(Me2dien-H)c!]+

and [~u(Me4dien-H)c~+ show rates dependent on bromide ion

concentration as usually encountered for square planar

complexes. There is retardation of the reaction by steric

and electronic effects with increased N-alkyl substitution

of the triamine. It has also been postulated that substitu­

tion of [Au(Et2dien-H)cJ]+ proceeds via a ring-opening

mechanism.

The purpose of this research was to study further the

effect of steric hindrance on rates of substitution of two

additional gold(III) complexes. The complexes of Me5dien

and MeEt4dien with gold(III) were prepared and investigated,

As observed with other less methyl substituted gold(III)

dien complexes the substitution reaction of [Au(Me5dien)C:£j 2+

with bromide ion is dependent on the bromide ion concentra­

tion. The results of the reaction rate studies on

[Au(MeEt4dien)CY Z+ with bromide ion can most easily be

rationalized in terms of the previously postulated ring-

opening mechanism.

ix

1

INTRODUCTION

Extensive quantitative studies on square planar, low

8 spin d complexes, especially those of Pt(II) and Pd(II),

have shown that these complexes generally undergo substi- 1

tution at rates that are reagent dependent.

Square planar substitution reactions of the form:

MA Xn+ + Y 3 > MA yn+ + X

3

are believed to follow a b~molecular displacement mechanism

which can be represented as:

-x > fast

-S fast +Y

- - y A3M<: - -.x -x )

fast 4

where Sis the solvent and k1 represents the solvent path

rate constant, Y is the substituting nucleophile and k2 is

the substituting nucleophile path rate constant. This

mechanism follows the two term rate law:

2

which under pseudo first order conditions with excess y-

reduces to:

Rate = k r MA Xn+l obs LI 3 -1

1

However, the square planar complexes of Pt(II), Pd(II)

and Au(III) with certain N-alkyl-substituted diethylenetri-

amines undergo substitution at rates that are almost indepen-

d f h . f h . 1· d 2•3•4 I ent o t e concentration o t e entering igan . t

has been suggested that the alkyl groups which occupy the

regions above and below the plane ·of the ion sterically

hinder the attack of the entering ligand.1

Studies of a

[Au(amine-H) CU+

series of reactions of the type

+ Br --~) · [Au(amine-H)B:cl + + Cl

with the amines having various degrees of alkyl substitution

reveal that the variation in the experimental bromide ion rate

dependence as a function of the amine is d i.eu-R> Me2dien-H>

Et2dien-H>Me4dien-H>.> Et4dien-H. However, examination of space

filling molecular models indicates that alkyl-group shielding of

the central Au(III) ion increases in the order; dien-H <: Me2dien-H < Me4dien-H <Etzdien-H < <Et4dien-H. Thus, if only

steric effects are considered the experimental rate dependence

should have shown Me4dien-H > Et2dien-H. 3' 5 To explain this anomaly between the molecular model

indications and the observed kinetics for (.Au(Et2dien-H)C~+

3

vs [Au(Me4dien-H)C1] +it has been suggested that the

substitution reaction of bromide with the [Au(Et2dien-H)C1] +

complex proceeds by the following ring-opening mechanism.6

(NEt2 N-Au--Br \_J

A similar ring-opening mechanism has been suggested for

the reaction of [Au(Et4dien-H)Clj +with N3-, 3

and for

[Pd(Et4dien)SeC~ with Br-. 7

4

To.investigate further the effects of alkyl substitution,

I J 2+ the gold(III) complexes, Au(Me5dien)Cl and

r . ] 2+ LAu(MeEt4dien)Cl have been prepared and their bromide

substitution reactions studied.

5

EXPERIMENTAL

Materials: Fine gold powder, purity better than 99.99%,

and ammonium hexafluorophosphate were obtained from Alfa

Inorganics, Inc. The 1,1' ,4,7,7'-pentamethyldiethylenetriamine

and the 4-methyl-1,1' ,7,7'-tetraethyldiethylenetriamine were

purchased from Ames Laboratories, Inc. All other chemicals

used were reagent grade.

Analyses: The gold analyses of the complexes, prepared as

the hexafluorophosphate salts, were performed by dissolving a

small accurately weighed quantity of complex in aqua regia fol­

lowed by repeated evaporation with concentrated HCl to remove all

volatile nitrogen oxides. Throughout the evaporation the solutions

were not allowed to go to dryness, since dryness caused non-

reproducible results by volatilization of gold in the presence of

the hexafluorophosphate ion.3 The tetrachloroauric acid solutions

thus obtained were diluted with lM HCl to obtain solutions with

final concentrations in the range of 10-4 molar. The absorbance

of these solutions was measured at 314 nm using either a Perkin

Elmer Model 202.Recording Spectrophotometer or a Cary Recording

Spectrophotometer model 14MS .. The concentration of the gold was

then read from a standard curve of absorbance vs concentration

of HAuC14 prepared from pure gold. (Figure 1)

The chloride content of the complexes was determined by

the Mohr method.8

-N C'1")

N ,,....., N \.0 ,,....., Ji.< \.0 p...

Ji.< .._, p... .._, q

-CO r;::::+ u N ,,....., u 0 ,,....._ Q,) 0 ·r-1 Q,) -e ·r-1 -cr -o .µ

l.J") w Q,) Q,)

@ ::<:: ::<:: .._, .._, -"" ;::l ::l C'1") ::l ~ ~

N

M ~ C'1") e 0 <l .µ ell

0 0 -r-i .µ -0 ell N H H .µ 0

Q,)

Q,) .µ

o •r-1

0 ..-=l M 0 u - w ::l r::i::< ;::l ;:::J ~ -'° ~ 0

~I ,....;

H Ji.<

eo s <r

M u ;::l ~ ~ _N 0 ,....;

Q,) o 0 qJ ,.D H 0 en ~ - co

6

0

M

co 0

<r 0

I N

0

aoueqz o sqv

0

7

~Preparation of Complexes: Tetrachloroauric acid was

prepared by dissolving a weighed quantity of fine gold powder

in a few ml of aqua regia followed by a repeated evaporation

with concentrated HCl to remove the volatile nitrogen oxides.

The solution was evaporated to a volume of 1-2 ml and allowed

to cool; crystals of tetrachloroauric acid then formed.

[Au(Me5dien)C1] (PF6)2: HAuC14, prepared as above,

equivalent to 0.5 g or 2.5 mmoles of gold was dissolved in

20 ml of cold H2o. The solution was cooled to 10°c. While

mechanically stirring the solution, 2.5 mmoles or approxi­

mately 0.7 ml of 1,1' ,4,7,7'-pentamethyldiethylenetriamine

was added. A yellow precipitate of [Au(Me5dien)cl] [Auc14]2

immediately formed. While maintaining the solution at a

temperature below 10°c and monitoring the pH of the solution

with a Fischer Accumet model 120 pH meter, 0.15 M sodium

hydroxide was added dropwise not allowing the pH to go above

7. With the addition of the sodium hydroxide the pH first

increased and then slowly decreased as the added OH- reacted.

The yellow precipitate dissolved. The addition of the sodium

hydroxide was continued until the solution remained at pH 5

for five minutes. The solution was filtered and a clear

orange filtrate was obtained. The filtrate was cooled to less

thau 5°C and 1 g of NH4PF6 was added. A peach colored pre­

cipitate formed which was then collected on filter paper and

washed with small portions of ice cold ethanol. This was

followed by a wash with cold ether and the product sucked

dry. The resulting cream colored crystals were then stored

8

over CaC12 in a refrigerated desiccator. A yield of 38%

was obtained (based on the weight of gold used).

Analysis: Calculated for [Au(Me5dien)C1] (PF6)2

Au, 28.3%; Cl, 5.11%. Found: Au, 28.1%; Cl, 5.2%.

[Au(MeEt4dien)C1] (PF6)2: HAuC14, prepared as stated

above, equivalent to 0.5 gor 2.5 mmoles of gold was dissolved

in 30 ml of H20 and cooled to 10°c. While mechanically stir­

ring the solution, 2.5 rnmoles or approximately 0.7 ml of

4-methyl-1,1', 7, 7'-tetraethyldiethylenetriamine was added.

A yellow precipitate [Au(MeEt4dien)c1_] lAuclJ 2] immediately

formed. This precipitate was slowly dissolved with a dropwise

addition of 0.15 M sodium hydroxide, while monitoring the pH

and maintaining it below pH 7. The addition of the sodium

hydroxide caused the solution to darken to an orange color

and then to a gray, ,presumably because of some decomposition

of the yellow precipitate. Sodtum hydroxide was added until

a pH of 5.5 was maintained for five minutes. The solution and

dark residue were filtered to obtain a yellow-orange filtrate.

The f il tr ate was cooled to about 5°C and 1 g of NH4PF 6

was

added. The off-white precipitate produced was collected on

filter paper and washed with cold H2o, cold ethanol and

finally cold ether and sucked dry. The compound was stored

over CaC12 in a refrigerated desiccator. A yield of 33%

\(based on the weight of gold used) was obtained.

Analysis: Calculated for [Au(MeEt4dien)C1J (PF6)2

Au, 26.2%; Cl, 4.72%. Found: Au, 26.3%; Cl, 4.70%

9

Klthough these compounds were refrigerated in the absence

of light, they slowly darkened and became gummy. Recrystalli­

zation was necessary after about one month. Recrystallization

was accomplished by dissolving the compound in a small quantity

of acetone and water. The mixture was then filtered and the

water-acetone filtrate collected. The acetone was removed by

vacuum. The resulting recrystallized compound-was collected

and washed with cold ethanol followed by cold ether and the

product sucked dry.

10

KINETIC STUDIES

Earlier studies of the aqueous solution chemistry of

Au(III) with various diens indicated that: (a) Au(III) dien-H

complexes undergo extensive hydrolysis in neutral aqueous

solutions; (b) an excess of halide ion represses hydrolysis

of these complexes in certain pH ranges; (c) the conjugate

acids of these complexes are stable in perchloric acid; and

(d) the conjugate acids of these complexes undergo decomposi­

tion in hydrohalic acids. 3,5,9

Since [Au(Me5dien)C1] 2+ and [Au(MeEt4dien)Cl] 2+ are

complexes of the Au(III) dien cat~gory, their aqueous solu­

tion chemistry was briefly examined to determine their

behavior at various pH's and to select suitable pH ranges

for kinetic study.

Small portions of each complex were dissolved in H2o,

to a final concentration of 4 x. l0-4M. The solutions were

placed in a water jacketed beaker and maintained at 2s0c by

circulating water through the jacket from a P.M. Tamson

constant temperature bath. The pH of these solutions was

measured with a Fischer Accumet pH meter previously calibrated

using Bechman pH 4.00 and pH 6.86 buffers. The pH was

decreased in small increments using HC104 and increased in

small increments using NaOH. The spectrum of aliquots of

each of the solutions at the various pH's was obtained by

using a Perkin-Elmer 202 spectrophotometer and scanning from

11

250 nm to 390 nm.

It was also of interest to examine the stability of the

bromide substituted complexes at various pH's. The examina-

tion was similar to that stated above except that the chloro

species was allowed to react with a sufficient amount of

bromide ion at 25°C before decreasing or increasing the pH

at small increments with HGl04 or NaOH respectively.

Whereas with previously studied gold(III) <liens it was

possible to form their· conjugate bases and measure the dis-

sociation constants, there are no acid-conjugate base

relationships with [Au(Me5dien)Cl] 2+ and [Au(MeEt4dien)C1] 2+

because all amine sites are completely substituted.

The pH of a 4 x 10-4 M solution of [Au(Me5dien)C1] 2+

in H20 was 5.5. The pH of a similar concentration of

(Au(MeEt4dien)C1] 2+ in H2o was 5.8.

Examination of the scans at the various pH's revealed

that the [Au(Me5dien)C1] 2+ complex was stable with the

addition of HClO 4 to an acidic pH of 1. 0, at which decomposi-

tion occurred with the formation of Aucl4 With the addition

of NaOH the complex was stable to a pH of 6.3, then the

hydroxide ion noticeably replaced the chloride ion. The

pH studies of the bromide substituted complex, [ Au(Me5dien)B~ 2+,

indicated that at a pH less than 5 there was unwrapping of

the ligand and replacement with bromide at the amine sites,

and at a pH greater than 6.3 hydrolysis takes· place. The

pH studies of the [Au(MeEt4dien)C1J 2+ complex indicated that it was less stable than the [Au(Me5dien)C1] 2+ complex.

12

Decomvosition occurred below a pH of 2.3 and hydrolysis took

place above pH 5.8. The bromide substituted species,

[Au(MeEt4dien)Br] 2+, was stable between pH 5.2 and pH 6.2.

Based on the above information it was decided to study

the kinetics of both complexes in a pH 5.8 buffer. A

Na2HP04~NaH2Po4 buffer was used. Two solutions of each

complex were prepared in pH 5.8 buffer, one containing

0.10 M Cl- and the other containing 0.10 M Br-. The spectrum

of each solution was then obtained. The results for

[Au(Me5dien)X] 2+ are shown in Figure 2. Examination of

these spectra show a large difference in absorbance at a

wavelength of 330 nm. Therefore, ·the wavelength of 330 nm

was selected as optimum for the kinetic study for the Me5dien

complex.

A wavelength of 340 nm was selected for the kinetic

study for the MeEt4dien complex by a similar procedure.

Pertinent spectra for these complexes are shown in Figure 3.

Solutions of the chloro complexes were prepared by

dissolving exact amounts of the solid [Au(amine)Cl] (PF6)2

in pH 5.8 phosphate buffer. Kinetic runs were carried out

by allowing 2 ml of this solution to react with 1 ml of a

solution containing bromide ion at various concentrations.

The b~omide solutions were prepared by dissolving exact amounts

of NaBr and NaCl04 in pH 5.8 buffer to produce, after mixing,

the desired bromide ion concentration and a total ionic

strength of NaBr and NaCl04equal to 0.10 M. The concentration

4-l 0 U) ~ (]) . .., <r o I (]) 0 o, .-I Cf.)

~ 0 1-1 <r 0 0

I.I")

.-I 4-l ('")

,.£:: 0 u

U) + + "d i:: N N i:: 0 r;:::;i ~

....... ell . .., §

.µ u l::Q 0 ell ....... ....... '-"

N s 1-1 i:: i:: 0 .µ (]) (]) ,.£::

[l:l 1-1 i:: . .., . .., .µ

~ l::Q (]) "d -e 00

:::::> o I.I") I.I") i:: c..') (]) i:: (]) (])

(])

H ,.£:: 0 ~ ~ .-I µ., .µ u <;» <:» (])

;::l ;::l :>- 4-l .µ L'.!. ~ ell o <1l ~ ell + <t: l::Q 1-1 N .µ

~ u (]) ....... c, i:: U) (]) . .., 0

i:: "d 0 0 I.I")

('")

• r-i (]) .µ ~ c, <;»

1-1 ;::l 0 L.. U) .0 <t:

13

0 0 -cr

0 I.I")

0 co <o ;.N

-cr N 0 . . . . .-I 0 0 0 0 0

aotraq.ro sqv

l-1 l-1 4-1 Cl) Cl)

0 ~ 4-1 4-1 4-1 4-1

CJ) -.:t ;:! ;:! (!) I ,Ll .o ·rl 0 CJ .-i 00 00 Cl) p. ~· l-1 LI') LI') U) (!)

-cr 4-1 ::i::: ::i::: 0 4-1 p. c, l-1 4-1 ;:! _o 0 0 ,Ll p p LI')

.-i •rl •rl M

.r:: CJ) 00 u p ~ ?!

0 LI') '\j •rl M M ,....,. p .µ ::i::: 0 0 ~ (1j (1j p.

l-1 0 0 <;»

0 .µ p v " M a p •rl .r:: 0 (!) I I .µ

~ l-1 CJ + l-1 l-1 00 pq p N pq pq p

t5 0 r;::;1 (!)

(!) u CJ) CJ) ..-! H .r:: u ;:! ;:! (!)

J:;<.. .µ .µ ,-, ..-! ..-! :> (1j p p. p. (1j

4-1 (lJ + + ::;:

0 + •rl N 'CJ 1 N N

(1j GZ1 .µ -.:t r;::jl r;:n l-1 .µ »<; ~ u u CJ p Q) ,-, ,-, Q) (!) ?! p Q p. •rl '-' Q) (lJ U) '\j

~ •rl •rl

-.:t '\j '"d 0 Q .µ L-1 -.:t '::!" -o 0 w .µ .µ M ·rl Q) <11 w w .µ ?! (!) (!) p. '-' ?! ;:.:: l-1 ~

'-' "-./

0 ;:! ;:! CJ) l--1 ~ i..::!J ,Ll

<11 pq u

14

00

0

I

'° I

-.:t I

N

0 .

0 .

0

0 _o -.:t

0 - LI')

0 N . 0

15

-4 of the complex after mixing was 4 x 10 M. The reaction

occurring for [Au(Me5dien)Cl] (PF6)2 is as follows:

L 2+ ~u(Me5dien) Cl J + slow ) [Au(Me5dien)Br]2+ +Cl

Rates were determined using a Cary Recording Spectrophotometer

model 14 MS with 1 cm quartz cells in conjunction with a

Temptrol 153 waterbath from Precision Scientific Company that

pumped water through the cell compartment of the spectrophoto-

meter to keep the system at a constant temperature. The

scans obtained showed the change in absorbance at the specific

wavelength as a function of time. A typical scan is shown in

Figure 4. All reactions were performed at a temperature of

z5°c ~ 0.1°C. Bromide concentrations of 0.005 M, 0.01 M,

0.03 M, 0.05 M, 0.08 Mand 0.1 M were used for both complexes.

Additionally, [Au(MeEt4dien)Cl] Z+ was studied at 0. 02 M and

0.04 M bromide ion concentration; also the kinetics of this

complex were studied at a wavelength of 380 nm. For each

complex, replicate runs were performed at the various bromide

concentration levels. Reproducibility was better than 1%

absorbance.

Two methods for determining the pseudo-first order rate

constant were used:

1. The log of the difference in the absorbance

value (obtained from the scan) at time "t" (At)

and at the completion of the reaction (A) was (X)

plotted as a function of time.

16

0 -.::!" <r .-1

0 -N

C'f') .--1

0 -0

I N 1-1 ..-l ~ ~ .--1 (1) 0 . a - O'.) 0 'M 0

.µ ~ .--1 ..c: .µ ..c: -.::!" 'M .µ I ::: 'M 0 ::: .-1 0

+ - \.0 N s ~ °' r,:::jl- -.::!" u 0 ,,-., C'f') Cll i:: C'f') 'M (1) 'M .µ + _o "d Cl) N <r

LI') r-;::n CX)

(1) (1) ~ CJ u ...._, i:: ,,-., ;:l Cl) i:: CJ

<r ~ .0 (1) (1)

1-1 'M _o (})

i::il 0 "CJ N ~ 4-1 Cll LI') ....... i:: c> 0 .0 (1) 'M 0 <rl ~ H i:: ...._, (1)

Ji.< 0 p ;:l s 'M 'M ~

'M .µ _o ~ CJ (1) 0 Cl) Cll 4-1 , \.0 (1) Cl) 0 ~ (1)

1-1 i:: (1) CJ 0 ..c: (1) 'M .µ i::i .µ _o

Cl) CX)

1-1 Cl) 1-1 -.::!" 0 .µ 4-1 Cll i::

Cl) (1) i:: CJ Cd "d i:: CJ (1) 0 _o (/) ::: u \.0

0 C'f')

.--1 .-1 Cl) .-1 CJ 0 'M Ji.< P< :>. 0 ~ <r

N <rl

0 N .-I

I

°' 0

I CX) N

0

.-1

0 .

0 0 .

0 .

0

0 . 0

17

11 2. The Guggenheim method: The log of a A vs time

where a A equals the difference in absorbance

for a constant time interval at. For most runs

A t = 2 min.

Figure 5 shows a

l ;i 2+ Au(MeEt

4 <lien) ci]

typical scan for the reaction of

with 0.05 M Br-, followed as a decrease

in absorbance at 340 nm. Figure 6 shows a graph of kobs for

the reaction of [Au(MeEt dien)ci] 2+

with 0. 05 M Br-, c a I cu- 4

lated by the Guggenheim method.

From the linear plot obtained, the slope, or pseudo­

first order rate constant, was calculated for the specific

bromide concentration being studi~d. Plots of replicate

runs yielded slopes which agreed within 10%. A straight

line was obtained from a plot of the observed rate constant

as a function of the bromide concentration with an intercept

equal to k1, the solvent path rate constant, and a slope of

k2, the bromide path rate constant. (See Figures, 7, 8, and

9)

The Guggenheim method was used because the absorbance

at time infinity is not needed to determine the observed

rate constant. The absorbance at time infinity was questioned

because of the hydrolysis that occurred with time. However,

comp~rison of the two methods showed no significant difference

in the calculated observed rate constant.

18

0 - C"')

I r-l H r:Q

;:E1 0 -N

ll"l r-l 0 QJ

;::;::: s 0 .,.,

.µ -:t 0 ,..c: I - r-l .µ ,..c: 0 r-l .,., .µ r-l ~ .,.,

~ ::;: + <r 0 N ~ -o r= CJ) r-l

u 0 .,., r<; <r + p C"') QJ N -o .,., .µ o-, "d Cil ~ -:t .µ QJ u P<:i (.) ,,-.... QJ p i:: -o e Cil QJ 00 (.)

ll"l .D .,., QJ ::l H "d CJ)

P<:i ~ 0 -:t ~ CJ) .µ i:: :::::> ~

P<:i -o .,., c.!) 4-1 QJ " H o· ;::;::: QJ µ., i:: ..._,, s p .,., ::l .,.,

0 di H .,., QJ -o .µ CJ) <o (.) Cil 4-1 Cil QJ 0 QJ H p ~ o

QJ 0 -o QJ "d .,., LI") ,..c: .µ .µ Cil Cil

H H CJ) .µ 0 Cil i:: -o 4-1 QJ <r

"d (.) p QJ i:: Cil ::;: 0 (.) 0 u ti) r-l -o

r-l C"') r-l 0 Cil µ., (.) .,., c, -o :>-. N H <t1

-o r-l

.- ,- 0 I I I I I I I

°' 00 " '° Lil -:t C"') N r-l 0 . . . . . . . 0 0 0 0 0 0 0 0 0 0

a::m~q:wsqv

<11 9.3 <l ~ 0

ee 0 H

9.2

9.6

9.5

9.4

9.1-

9.0-

19

FIGURE 6

Graph of kobs for the reaction of [Au(MeEt4dien)c~2+

with O. 05 M Br

Calculated by Guggenheim Method

2 -1 k calculated as 2.4 x 10 sec obs

Time in sec.

6.0- ,....; I

CJ Cl) Cl)

C"'l s.o- + 0 ,....;

~ "Cl Cl) 4.0- :> !-< Cl) Cl) ..Q 0 ~

3.0-

20

FIGURE 7

Observed Rate Constant for ~u(Me5dien)ci] 2+

versus Bromide Concentration@ T = 2s0c

Data obtained from change in Absorbance at 330 nm

9.0

0.10

8.0-

7.0-

2.0-

1.0-

Q,Q=-- I --'-~

0.01 0.02 0.03 0.04 I

0.08 I

0.09 I I

0.06 0.07 I

0.05

Moles I Liter Br

21

FIGURE 8

< [ 1l 2+ Observed Rate Constant for Au(MeEt4dien)C~

versus Bromide Concentration@ T = 25°C

Data Obtained from change in Absorbance at 340 nm

4.0

3.5-

..-l 2.5- I (J Q) rJ)

N + 0 2.0- ..-l

:< -e Q) :> H 1. 5- Q) CJ) ..0 0

.!:<I

1. 0-

0.5-

0.0-1-~~~-r,~~~....,...,~~~-....,~~~-r,~~~-.....,~~~-.....,~~~-...~ 0.01 0.02 0.03 0.04 0.05 0.06 0.07

moles/ Liter Br

4.0-

3. 0-

'"""" I (.) Q) 2.5- U)

N + 0 '"""" ~ 2.0- "O Q) :> ~ Q) U) .D 1. 5- 0 ~

22

FIGURE 9

Observed Rate Constant for [Au(MeEt4dien)C~ 2+

versus Bromide Concentration@ T = 25°C

Data obtained from change in absorbance at 380 nm

3.5-

1.0-

0.5-

0.01 0.02 0.03 0.04 0.05 0.06

moles/ Liter Br

23

. RESULTS AND DISCUSSION

The pseudo-first order rate constants obtained for the

reaction:

[Au(amine)Cl] 2+ + Br- ~ [Au(amine)BrJ 2+ + Cl

(where amine= (Me5dien), studied at wavelength 330 nm, and

(MeEt4dien), studied at 340 and 380 nm) are given in Table I.

The values shown fork b are reproducible within 10% 0 s

and are the average of from four to eight replicate runs.

The estimated values of the first and second order rate

[ 2+ constants for Au(Me5dien)Clj and [Au(MeEt4dien)Cl] 2+

are listed in Table II, these represent rate constants for

the solvent and bromide paths, respectively. Also listed in

Table II are the data for the substitution reactions of Br-

for Cl in the complexes of [Au(dien)Cl] 2+, [Au(dien-H)Cl] +

[Au(Me2dien-H)C1]+, [Au(Me4dien-H)Cl] + [Au(Et2dien-H)Cl] +

and [Au(Et4dien-H)Cl] +

As has been stated previously, examination of molecular

models indicates that the shielding of the Au(III) by the

amine increases in the following order: dien-H < Me2dien-H < Me4dien-H<Et2dien-H <<Et4dien-H.

6 Examination of the

molecular models of Me5dien. and MeEt4dien indicates the

Me5dien shields the Au(III) slightly more than the Me4dien-H

and the MeEt4dien shields slightly more than the Et4dien-H.

However, it should be noted that Me5dien and J1eEt4dien com­

plexes are divalent positive ions. Thus the slight shielding

24

TABLE I

Pseudo First Order Rate Constants at ?:5° C for:

[Au(amine)Cl] 2+ + Br- ~~)~ [Au(amine)Br] 2+ +Cl-

amine= Me5dien

Br- concentration

0.10 M

0.08 M

0.05 M

0.03 M

0.01 M

0.005 M

kobs x 103 at 330 nm

8.3 sec-1

7.0

5.0

4.1

3.0

2.5

amine = MeEt4dien

Br - concentration kobsX 102 at 340 nm kohsx 102 at 380 nm

0.05 M 2.4 -1 2.4 sec-1 sec

0.04 M 2.2 2.2

0.03 M 1. 8 1. 8

0.02 M 1. 6 1. 6

0.01 M 1. 5 1. 5

0.005 M 1. 2 1. 2

25

TABLE II

First Order and Second Order

Rate Constants for the Reactions:

+ [Au(amine-H)Cl] + Br

~u(~mine)clJ 2+

+Br

Complex

~u(dien) CU 2+

~u(dien-H)Ctj +

~u(Me2dien-H)c1] +

~u(Me4 dien-H) c]J +

~u(Me5dien)C1] 2+

. ~(Et2dien-H)CLJ +

~u(Et4dien-H)cfj +

~u(MeEt4dien)c!} 2+

) [Au(amine-H)Br] + + Cl

and

) [Au (amine) Br] 2+ + Cl

k1 (sec-1) kz (M-lsec-1) Reference

0 380 5

0.6 190 5

0.20 44 6

0.062 0.76 6

0.002 0.058 this work

0.023 3.8 6

0.00012 0.0085 6

0.01 0.26 this work

The rate constants for the reactions were determined

at 25° "±" o.1°c.

26

effect" of the additional methyl group might be offset by

the increased charge of the Me5dien and the MeEt4dien

complexes.

The data in Table II shows that the bromide substitu­

tion of chloride in [Au(Me5dien)C1] 2+

proceeds via the

solvent path 30 times slower and via the bromide path 10

times slower than the same reactions for [Au(Me4dien-H)CJ] +

This decrease in rate can be attributed to the steric

blocking of the 5th methyl group. This decrease due to the

steric effect would presumably be larger but is offset by

the increased charge of the complex. Note that H 0 entry 2

is decreased by a larger factor than Br entry, because

ion-dipole interaction is less than ion-ion interaction.

However, the same phenomenon is not observed in the examina-

tion of the data in Table II for the MeEt4dien vs the

Et dien-H complexes. The br omi.de substitution of the 4 .

chloride in [Au(MeEt dien)ci] 2+ proceeds via the solvent 4

path approximately 100 times faster and via the bromide path

approximately 30 times faster than with [Au(Et4dien-H)C1J +

Also, by comparison [Au(MeEt 4 <lien) cl] 2+ reacts two times

slower via the solvent path and 15 times slower via the

bromide path than [Au(Et2dien-H)ci] +. While steric

hindrance can be invoked to rationalize the slower reaction

of [Au(MeEt4dien)Cl] Z+ over [Au(Et2dien-H)Cl] +, such a

rationalization is not consistent with the increase in rate

observed for ~u(MeEt4dien)C1] 2+ over [Au(Et4dien-H)Cl] +

27

. Furthermore, this latter increase cannot be explained in

terms of increased charge since this should cause increased

Br entry to a greater extent than H2o entry as was observed

for the methyl substituted complexes. This anomaly can be

rationalized in terms of the postulated ring-opening mechanism.

Evidence for ring-opening is apparent in the equilibrium

studies. The gold(III) complexes of Me2dien and Me4dien were

found stable in 1 Macidic chloride and bromide solutions.

A similar behavior was observed for the gold(III) Me5dien

complex. On the other hand, the complexes of gold(III)

containing Et2dien and Et4dien broke up in 1 M acidic chloride

solutions to yield Auc14- and in 1 M acidic bromide solutions - 6

to yield AuBr4. A similar behavior was also observed for

the gold(III) MeEt4dien complex. Therefore, in the acidic

solutions the ethyl groups on the terminal nitrogens of the

chelate exert a specific effect leading to ring-opening.

Presumably, these complexes are in equilibrium with low con-

centrations of ions containing the triamine as a bidentate

ligand.

Additional evidence for ring-opening, can be seen in

the kinetic studies. Spectral changes during the bromide

substitution for chloride in [Au(MeEt4dien)cD z+ showed that for a concentration of bromide ion in the range 0.001 M

2+ to 0.03 M the substitution product was [Au(MeEt4dien)B~

Concentrations greater than 0.03 M Br- $howed unwrapping of

the ligand to form various substitution products presumably

28

l}uBT J-, [AuBr 3oig-, [AuBr 2 (OH) 2 J - , [ AuBr (OH3)J - and

[ Au(OH) 41-. A typical example of one spectrum obtained

is shown in Figure 3.

At 340 nm it was difficult to determine whether decom-

position had any effect on the kinetic results since the

absorbances of the [Au(MeEt4dien)BrJ 2+ and the decomposition products were essentially the same. Therefore, repetitive

scans from 320 nm to 390 nm were run on the reaction of

[Au(MeEt4dien)c1] 2+ with 0.05 M Br-. These are shown in

Figure 8. As can be seen, the Lscsbes.r Lc points at 323 nm

and 367 nm indicate that the formation of [Au(MeEt4dien)BrJ 2+

occurred before further unwrapping of the ligand took place.

Furthermore, when 0.1 M Br- was added dropwise to the product

of the reaction of [Au(MeEt4dien)C1] 2+ with 0.03 M Br­

spectral changes indicated that unwrapping of the ligand took

place.

To substantiate the kinetic rate values obtained from

the studies at 340 nm kinetic runs with the various concentra-

tions of bromide ion were also carried out at 380 nm. Since

the [Au(MeEt4dien)Br.J 2+ and the decomposition products have

different absorbances at 380 nm, the Guggenheim method was

used to calculate rate constants from these runs. The results

obtained at 380 nm were essentially the same as those obtained

at 340 nm.

Therefore, the kinetic results for the bromide substitu­

tions for chloride in [Au(MeEt dien)clJ z+ can be rationalized 4

I I-< 0 u P'.:l 0 P'.:l " 0\

i::; ("I)

?! 'M a l1") 0 r-f

0

,.i:;: P'.:l .µ 'M ~ (!) a + 'M

N .µ

r:;i 0 0 .-'I u II ,-.. ,-.. r>::i i::; <t: !§ ~ (!) t> 'M 0 <;»

0 "d l1") H <r (!) ("I) ,..c:: l't-< .µ a .µ

r>::i 'M en . (!) .µ i::; ?! (!) '-' 0 r-f :l .µ (!)

6 ::> .µ rd o ::SC (!)

4-l p.. 0 C/J

(!) i::; I-< 0 'M .c .µ .µ u 'M rd ~ (!) ~

29

0 0 ("I)

0

r-f

0 N

0 .

0 .

0

~30

. in t~rms of the following ring-opening mechanism:

(NEt2

MeN Au~~-Cl -. NEt2

-: MeN Au--Cl

~NEt 2

(I) (R)

(NEtz

MeN-- Au-- Br -. NEt2

The bidentate species (I) is much more susceptible to

bromide ion and solvent attack. Therefore, it is reason-

able that bromide ion substitution should occur at a rate

greater than observed for ~u(Me5dien)ci] 2+ The faster

rate observed for ~u(MeEt4dien)c.i] 2+ over_ ~u(Et4dien-H)cI]+

is presumably the result of the increased charge of the

former ion.

31

. REFERENCES AND BIBLIOGRAPHY

1. Fred Basolo and Ralph G. Pearson, "Mechanisms of

Inorganic Reactions," Second Edition, John Wiley &

Sons, Inc., New York, N. Y. (1967)

2. W. H. Baddley and F. Basolo, Journal of the American

Chemical Society, 88, 2944 (1966)

3. C. F. Weick and Fred Basolo, Inorganic Chemistry,

5, 576 (1966)

4. John R. Goddard and Fred Basolo, Inorganic Chemistry,

L. 936 (1968)

5. William H. Baddley and Fred Basolo, Inorganic

Chemistry, ~. 1089 (1964)

6. David L. Fant and C. F. Weick, Inorganic Chemistry,

12, 1964 (1973)

7. John L. Burmeister and John C. Lim, Chemical

Communications, 19, 1154. (1969)

8. William Reiman, III, Jacob D. Neuss, and Barnet Naiman,

Quantitative Analysis , McGraw-Hill Book Company, Inc.,

New York, N.Y. (1951)

9. W. H. Baddley, Fred Basolo, Harry B. Gray, Clarita

N8lting, and A. J. P8e, Inorganic Chemistry, 2, 923

(1963)

32

. 10. F~ed Basolo and Ronald Johnson, Coordination Chemistry,

W. A. Benjamin, Inc., New York, N. Y. (1964)

11. E. A. Guggenheim, Philadelphia Magazine, z, 538 (1926)