Synthesis and Structure of a Three-dimensional Organically TemplatedZinc Ethylenediphosphonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4]

Srinivasan Natarajan*

Bangalore /India, Framework Solids Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdvancedScientific Research

Received July 2nd, 2003.

Abstract. A hydrothermal reaction of a mixture of ZnO, HCl, ethyl-enediphosphonic acid, ethylenediamine, acetic acid in a water, THFmixture gave rise to a new three-dimensional zinc ethylenediphos-phonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4], I. The structure, de-termined by single crystal X-ray diffraction, (monoclinic, spacegroup � C2/c, a � 16.9948(14), b � 6.7383(6), c � 16.8886(14) A,β � 1113.568(1)°, V � 1772.7(3) A3, Z � 4, R1 � 0.0227, wR2 �

0.0601), consists of a network of strictly alternating ZnO4 andPO3C tetrahedral units linked through their vertices forming thethree-dimensional structure. The amine molecules occupy the

Synthese und Struktur eines dreidimensionalen Zinkethylendiphosphonat mitorganischem Templat, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4]

Inhaltsübersicht. Durch eine hydrothermale Reaktion einer Mi-schung von ZnO, HCl, Ethylendiphosphonsäure, Ethylendiamin,Essigsäure in einem Wasser/THF-Gemisch wurde der neue dreidi-mensionale Zinkethylendiphosphonat-Komplex, [NH3(CH2)2NH3]-[Zn3{O3P(CH2)2}4], I erhalten. Nach der Einkristall-Röntgenstruk-turanalyse (monoklin, Raumgruppe � C2/c, a � 16.9948(14), b �

6.7383(6), c � 16.8886(14) A, β � 1113.568(1)°, V � 1772.7(3) A3,Z � 4, R1 � 0.0227, wR2 � 0.0601) besteht die Verbindung auseinem Gerüst von streng alternierenden tetraedrischen Einheiten

Introduction

Research in the area of framework solids continues to beinteresting for their many novel structural features andproperties. Traditional framework structures are the alumi-nosilicates and the aluminophosphates [1]. The successfulsynthesis and structure determination of a large number ofmetal phosphates clearly indicate the growing trend in thesematerials [2]. The last few years witnessed the emergenceof a new class of compounds based on carboxylates [3],phosphonates [4], and carboxy-phosphonates [5]. In manyof these compounds, the variable coordination of the metalion along with the flexible carbon backbone of the func-

* Prof. Srinivasan NatarajanFramework Solids Laboratory, Chemistry and Physics of Materi-als UnitJawaharlal Nehru Centre for Advanced Scientific ResearchJakkur P.O.Bangalore 560 064 / Indiae-mail: raj@jncasr.ac.in

Z. Anorg. Allg. Chem. 2004, 630, 291�295 DOI: 10.1002/zaac.200300282 2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 291

middle of the 8-membered channels and interact with the frame-work through the hydrogen bonds. Unlike other zinc diphosphon-ates, I appear to have close similarity to zinc phosphate structuresreported in the literature. To our knowledge, this is the first three-dimensional zinc diphosphonate prepared in the presence of an or-ganic amine molecule.

Keywords: Zinc; Diphosphonate; Ethylenediamine; Crystal struc-ture

von ZnO4 und PO3C, die über ihre Spitzen zu einer dreidimensio-nalen Struktur verbunden sind. Die Aminmoleküle besetzen dieMitten der achtzähligen Kanäle und bilden Wasserstoffbrückenbin-dungen zum Gerüst. Im Gegensatz zu anderen Zinkdiphosphona-ten scheint I eine große Ähnlichkeit zu den in der Literatur be-schriebenen Zinkphosphatstrukturen zu haben. Unseres Wissens istdies das erste dreidimensionale Zinkdiphosphonat, das in Gegen-wart eines organischen Aminmoleküls präpariert wurde.

tional ligand has been utilized to give rise to the unusualstructures. Phosphonic acids [RPO(OH)2, where R is an or-ganic group] and diphosphonic acids [(HO)2OPRPO(OH)2]are good sources for the synthesis of organic-inorganic hy-brid solids. The diphosphonate groups are structurally con-nected with the organic backbone acting as a controllablespacer and the inorganic head groups act as the chelatingcenter with the metal ions. Most of these compounds, ingeneral, are prepared using hydrothermal methods.

Continued research during the last few years resulted inthe synthesis and characterization of a large number of me-tal diphosphonates [4, 6�29]. Unlike the metal phosphates,the use of structure-directing agents for the synthesis of di-phosphonates appears to be uncommon. Currently, the di-phosphonates of vanadium [22], iron [24], nickel [25], cop-per [26], zinc [27], aluminum [17, 20], and molybdenum [28]have been prepared and characterized in the presence of anorganic amine. These compounds have zero-, one-, two- andthree-dimensional structures.

During the last few years, we have been investigating theformation of new materials in the presence of organic am-

S. Natarajan

ines, resulting in large number of new metal phosphates[30]. Now we are studying the formation of zinc diphos-phonates in the presence of simple organic amine molecules.An examination of the available literature shows that zincdiphosphonates of varying compositions have been pre-pared and their structures determined. Thus,Zn2[O3PRPO3)(H2O)2, R � ethyl and propyl] [9],Zn(HO3P(CH2)3PO3H) [10], Zn3[(HO3P(CH2)3PO3)2]·2H2O[10], Zn2[(O3PC6H4PO3)(H2O)2] [11], Zn2[(O3P(C6H4)2PO3)]·2H2O [13], and Zn[HO3P(C6H4)2PO3H] [11] are known. Inaddition, the zinc diphosphonates, Zn[O3PCH2PO(C6H5)2][16], Zn[(O3PCH2PO(CH3)(C6H5)]·0.67H2O [16],Zn(O3P(CH2)2NH2) [21], Zn3(O3P(CH2)2CO2)2 [29], andZn3(O3P(CH2)2CO2H)·1.5H2O [29], with functional groupshave also been prepared. Presently, we have discovered a newzinc diphosphonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4], I,with three-dimensional structure possessing one-dimen-sional channels. The doubly protonated ethylenediaminemolecule is located in the middle of these channels. In thispaper, we present the synthesis, structure and characteri-zation of I.

Experimental Section

Synthesis

The zinc diphosphonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4], I,was synthesized under mild hydrothermal conditions starting froma mixture containing ethylenediamine (en). In a typical synthesis,0.020 g of ZnO was dispersed in a mixture of 3 ml of water and4 ml of THF. To this, 0.41 ml of HCl (35 %), 1.425 g of ethylenedi-phosphonic acid, [(HO)2OP(CH2)2PO(OH)2], and 0.15 ml of aceticacid were added under continuous stirring. Finally 0.67 ml of enwas added and the mixture was homogenized for 30 min. atroom temperature. The final mixture with the composition,ZnO : 2HCl : 3[(HO)2OP(CH2)2PO(OH)2] : 4en : CH3COOH :(66.7H2O � 19.5THF), was sealed in a 23 ml PTFE-lined acid di-gestion bomb and heated successfully at 75 °C for 72 h, 150 °C for24 h and finally at 180 °C for 24 h under autogeneous pressure. Theresulting product contained some rod-like crystals along with whitepowder were filtered under vacuum and dried at ambient tempera-ture. The initial pH of the mixture was �6 and did not show ap-preciable change after the reaction. The yield of I was about �25 %based on Zn. The single crystals can be separated from the bulkunder optical microscope. The white powder could not be charac-terized as yet and it does not match with any of the starting com-pounds. An EDAX analysis on the single crystals gave a Zn : Pratio of 3 : 4 which agrees with the composition obtained from thesingle-crystal X-ray study. Our efforts to improve the quantity ofthe title compound by modifications on the synthesis conditionsdid not come to fruition. Similar behavior has been observed dur-ing many synthesis involving hydrothermal methods.

X-ray crystallography

A suitable single crystal of compound I (0.20 x 0.24 x 0.32 mm)was carefully selected under a polarizing microscope and glued toa thin glass fiber. Crystal structure determination by X-ray diffrac-tion was performed on a Siemens Smart-CCD diffractometerequipped with a normal focus, 2.4 kW sealed tube X-ray source

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 291�295292

Table 1 Crystal data and structure refinement parameters for I,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4].

Empirical Formula Zn3P4O12N2C6H18

Formula mass 630.27Crystal system monoclinicSpace group C2/c (no. 15)a /A 16.9948(14)b /A 6.7383(6)c /A 16.8886(14)β /° 113.568(1)V/ A3 1772.7(3)Z 4T(K) 293ρcalc /gcm�3 2.361µ /mm�1 4.452Reflections total, observed 3570, 1280R indexes [I >2σ(I)] R1 � 0.0227a), wR2 � 0.0601b)

a) R1 � Σ �F0�-�Fc� / Σ �F0�; b) wR2 � {Σ[w(F02 � Fc

2)2] / Σ[w(F02)2]}1/2. w �

1/[σ2(F0)2 � (aP)2 � bP], P � [max.(F02,0) � 2(Fc)2]/3, where a � 0.0399

and b � 0.0 for I

Table 2 Final Atomic coordinates (x 104) and equivalent isotropicdisplacement parameters (A2 x 103) for I, [NH3(CH2)2NH3]-[Zn3{O3P(CH2)2}4].

Atom x y z U(eq)

Zn(1) 0 0.16247(6) 3/4 0.0122(2)Zn(2) 0.21922(2) �0.34272(5) 0.61034(2) 0.0124(1)P(1) 0.16151(5) �0.13294(10) 0.74860(5) 0.0120(2)P(2) 0.11979(5) 0.40332(11) 0.91192(5) 0.0119(2)O(1) 0.09130(13) 0.0147(3) 0.73934(14) 0.0243(7)O(2) 0.03027(12) 0.3257(3) 0.85465(13) 0.0159(6)O(3) 0.31215(13) �0.2376(4) 0.58881(14) 0.0282(7)O(4) 0.25184(12) �0.5793(3) 0.68074(13) 0.0172(6)O(5) 0.16716(13) �0.1498(3) 0.66024(13) 0.0163(7)O(6) 0.12520(12) �0.4406(3) 0.50390(12) 0.0154(6)C(1) 0.1334(2) �0.3697(4) 0.7791(2) 0.0169(10)C(2) 0.1336(2) �0.3605(4) 0.8700(2) 0.0231(11)N(11) 0.07708(16) �0.1983(4) 1.06519(16) 0.0188(8)C(12) 0.02433(19) �0.0799(4) 0.98778(19) 0.0189(9)

U(eq) � one third of the trace of the orthogonalized Uij Tensor

(Mo Kα radiation, λ � 0.71073 A) operating at 40 kV and 40 mA.Pertinent details for the structure determinations are presented inTable 1. An empirical absorption correction based on symmetryequivalent reflections was applied using the SADABS program [31].The structure was solved and refined using SHELXTL-PLUS suiteof program [32]. The direct methods solution readily established allthe heavy atom positions (Zn and P) and facilitated the identifi-cation of most of the other fragments (O, C, N and H) from thedifference Fourier maps and the refinements to proceed to R <10 %. All the hydrogen atoms were initially located in the differenceFourier maps and for the final refinement the hydrogen atoms wereplaced structurally ideal positions and held in the riding mode. Fi-nal R values of R1 � 0.0227 and wR2 � 0.0601 were obtained forrefinements varying atomic positions for all the atoms, anisotropicthermal parameters for all non-hydrogen atoms and isotropic ther-mal parameters for all the hydrogen atoms. Full-matrix least-squares refinement against �F2� was carried out using theSHELXTL-PLUS [32] suite of programs. Details of the final refine-ments are given in Table 1. The selected bond distances and anglesfor I is listed in Tables 2 and 3.

Results and Discussion

Compound I crystallize in the centro-symmetric spacegroup C2/c. The asymmetric unit consists of two Zn atoms,

Three-dimensional Organically Templated Zinc Ethylenediphosphonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4]

Table 3 Selected bond distances and angles in I,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4].

Bond Distance, A Bond Distance, A

Zn(1) � O(1)#1 1.912(2) P(1) � O(1) 1.513(2)Zn(1) � O(1) 1.912(2) P(1) � O(4)#2 1.522(2)Zn(1) � O(2) 1.968(2) P(1) � O(5) 1.537(2)Zn(1) � O(2)#1 1.968(2) P(1) � C(1) 1.799(3)Zn(2) � O(3) 1.894(2) P(2) � O(3)#2 1.500(2)Zn(2) � O(4) 1.933(2) P(2) � O(2) 1.532(2)Zn(2) � O(5) 1.944(2) P(2) � O(6)#3 1.540(2)Zn(2) � O(6) 1.981(2) P(2) � C(2)#4 1.795(3)N(11) � C(12) 1.487(4) C(12) � C(12)#8 1.512(6)

Angle Amplitude, ° Angle Amplitude, °

O(1)#1 � Zn(1) � O(1) 117.23(13) O(5) � P(1) � C(1) 109.81(13)O(1)#1 � Zn(1) � O(2) 97.88(9) x 2 O(3)#2 � P(2) � O(2) 110.68(12)O(1) � Zn(1) � O(2) 116.44(9) x 2 O(3)#2 � P(2) � O(6)#3 112.10(12)O(2) � Zn(1) � O(2)#1 112.01(12) O(2) � P(2) � O(6)#3 109.65(11)O(3) � Zn(2) � O(4) 111.75(9) O(3)#2 � P(2) � C(2)#4 109.58(15)O(3) � Zn(2) � O(5) 113.09(9) O(2) � P(2) � C(2)#4 107.86(15)O(4) � Zn(2) � O(5) 110.68(9) O(6)#4 � P(2) � C()#4 106.81(14)O(3) � Zn(2) � O(6) 112.48(9) P(1) � O(1) � Zn(1) 165.71(15)O(4) � Zn(2) � O(6) 102.27(8) P(2) � O(2) � Zn(1) 126.51(12)O(5) � Zn(2) � O(6) 105.91(8) P(2)#5 � O(3) � Zn(2) 160.57(16)O(1) � P(1) � O(4)#2 113.62(12) P(1)#5 � O(4) � Zn(2) 128.86(12)O(1) � P(1) � O(5) 107.53(12) P(1) � O(5) � Zn(2) 133.33(12)O(4)#2 � P(1) � O(5) 111.24(12) P(2)#6 � O(6) � Zn(2) 124.23(11)O(1) � P(1) � C(1) 108.90(14) C(2) � C(1) � P(1) 110.8(2)O(4)#2 � P(1) � C(1) 105.70(13) C(1) � C(2) � P(2)#7 114.2(2)N(11) � C(12) � C(12)#8 110.2(3)

Symmetry transformations used to generate equivalent atoms:#1 �x, y, �z�3/2; #2 �x�1/2, y�1/2, �z�3/2; #3 x, �y, z�1/2; #4 x, y�1, z; #5�x�1/2, y�1/2, �z�3/2; #6 x, �y, z�1/2; #7 x, y�1, z; #8 �x, �y, �z�2.

one ethylenediphosphonate group and one protonatedethylenediamine cation (Fig. 1). One of the Zn atoms,Zn(1), occupies a special position with a site occupancy(SOF) of 0.5, while the other atoms occupy general posi-tion. Both the Zn atoms have a tetrahedral coordinationwith respect to the oxygen atoms. The Zn�O distances arein the range 1.894(2) � 1.981(2) A (av. 1.939 A). TheO�Zn�O bond angles are in the range97.88(9) � 117.23(13)° (av. 110.13°). Both the Zn atoms areconnected to P atoms through the oxygen atoms. TheZn(1)�O(1)�P(1) and Zn(2)�O(3)�P(2) angles appear tobe uncommon with bond angles of 165.71(15) and160.57(16)°, respectively. One of the possible reasons for theobserved higher values could be attributed to the fact thatall the vertices of the phosphorus tetrahedra are not thesame (P atoms are coordinated by three O and one Catoms). To compensate and to maintain the tetrahedral nat-ure of the P atoms, it may be necessary to have one of thebonds to have unusually large bond angles with Zn atoms.The phosphorous atom has three P�O�Zn linkages andpossess one P�C bond. The average P�O bond distanceof 1.524 A, and P�C distance of 1.797 A result from suchbonding. The O�P�O, O�P�C and C�C�P bond anglesare within 2° deviation from their ideal values. The variousbond distances and angles, observed in I, are comparableto the values reported for similar compounds in the litera-ture [6�29].

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Fig. 1 ORTEP diagram showing the asymmetric unit in I,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4].Thermal ellipsoids are givenat 50 % probability.Asymmetric unit is labeled.

The structure of I consists of a network of strictly alter-nating ZnO4 and PO3C tetrahedral units linked throughtheir vertices to give rise to a three-dimensional structure.The connectivity between the ZnO4 and the (CH2)2(PO3)2

tetrahedral units are shown in Figure 2. As can be seen, thelinkages gives rise to apertures bound by 4- and 5-T atoms(T � Zn, P) with the two-fold axis going through themiddle. In the ac plane the connectivity can be viewed asone-dimensional corner-shared chains linked through theethylene backbone of the diphosphonate group, giving riseto a layer-like structure (Fig. 3). Within the layer, oneZn�O bond project above and below the plane of the layersin an alternating fashion facilitating further linkages. Theprojected Zn�O bond links with other (CH2)2(PO3)2 tetra-hedral units and give rise to the observed three-dimensionalstructure possessing one-dimensional channels bound by 8-T atoms (Fig. 4). The doubly protonated ethylenediaminemolecules occupy these channels. The position of the aminemolecules and its close proximity to the framework oxygenatoms has given rise to many hydrogen bond interactionsin I. Hydrogen bond interactions, generally, play a crucialrole in the stability of open-framework structures especiallyin lower-dimensional ones. In I, medium strength hydrogenbonds have been observed between the hydrogen atoms at-tached to the nitrogen and the carbon atoms of the aminemolecule and the framework oxygen atoms with donor andacceptor (D···A) distances in the range 2.806(3) �3.468(3) A and D�H···A angles >140°. The important hy-drogen bond interactions are given in Table 4.

The structure of I is different from other zinc diphos-phonates reported so far. One of the importance differencebeing that most of them are formed in the absence of or-

S. Natarajan

Fig. 2 The 4- and 6-membered rings formed by the linkages bet-ween Zn and ethylenediphosphonate.The direction of the 2-foldaxis is indicated.

Fig. 3 Structure of I, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4] in the bcplane. Note that the connectivity gives rise to a corner-shared 4-membered ring chain.

ganic amine molecules. In addition, in the earlier reportedstructures [9�11, 13], the compounds generally have a pil-lared layered structures, with the inter-layer separation larg-ely depending on the length of the carbon backbone. In thepresent compound, the ethylene backbone does not inter-fere with the connectivity of the three-dimensional struc-ture, probably due to the short C�C length. An ethylene-diamine templated Zn(hedpH2)2·2H2O (hedp � 1-hydroxy-

2004 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2004, 630, 291�295294

Fig. 4 Polyhedral view of the structure of I,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4] in the ac plane.Note that theamine molecule occupies the middle of an 8-membred one-dimen-sional channels.Dotted lines represent possible hydrogen bond in-teractions.

Table 4 Important hydrogen bond interactions in I,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4].

D�H···A D�H (A) H···A (A) D···A (A) D�H···A (°)

N(11) � H(10) ... O(6) 0.89 2.05 2.887(3) 158N(11) � H(11) ... O(2) 0.89 1.93 2.806(4) 168N(11) � H(12) ... O(5) 0.89 2.07 2.908(3) 156C(1) � H(1) ... O(2) 0.97 2.59 3.468(4) 151C(1) � H(2) ... O(4) 0.97 2.59 3.388(4) 140

ethylidenediphosphonate) with one-dimensional chainstructure has been reported recently [27]. This compoundhas Zn in distorted octahedral coordination and forms hy-drogen bonded structure involving the zinc diphosphonatechain and the amine molecule. The present compound hasZn in tetrahedral coordination and has a three-dimensionalstructure with channels. Compound I, on the other hand,appears to have close similarities to the ethylenediaminetemplated zinc phosphate, [NH3(CH2)2NH3][Zn6(PO4)4-(HPO4)]·2H2O [33], both structures having three-dimen-sional structure with 8-membered channels with ethylenedi-amine molecules occupying the middle. The T-atom (T �Zn, P) connectivity showing the 8-membred channels forboth the zinc phosphate and the title compound are shownin Figure 5.

In conclusion, a new zinc diphosphonate,[NH3(CH2)2NH3][Zn3{O3P(CH2)2}4], I, has been synthe-sized employing hydrothermal methods. To our knowledge,this is the first time a three-dimensional zinc diphosphonatehas been prepared in the presence of an organic amine. Ourcontinuing investigations reveal that it is possible to preparenewer zinc diphosphonates using other organic amine mol-ecules. Work on this theme is currently in progress.

Three-dimensional Organically Templated Zinc Ethylenediphosphonate, [NH3(CH2)2NH3][Zn3{O3P(CH2)2}4]

Fig. 5 (a) The T-atom (T � Zn, P) connectivity in I.(b) The T-atom (T � Zn, P) connectivity in the zinc phosphate,[NH3(CH2)2NH3][Zn6(PO4)4(HPO4)(H2O)]. Note the close resem-blance between the two structures (see text).

Acknowledgments. The author thanks the Council of Scientific andIndustrial Research (CSIR), Government of India for the award ofa research grant.

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