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Journal of Molecular Structure 826 (2007) 177–184

Second sphere coordination in oxoanion binding:Synthesis, spectroscopic characterisation and crystal structures of

trans-[bis(ethylenediamine)dinitrocobalt(III)] diclofenac and chlorate

Rajni Sharma a, Raj Pal Sharma a,*, Ritu Bala a, B.M. Kariuki b,*

a Department of Chemistry, Panjab University, Chandigarh-160014, Indiab School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom

Received 1 April 2006; received in revised form 27 April 2006; accepted 27 April 2006Available online 20 September 2006

Abstract

In the exploration of cationic cobaltammine [trans-Co(en)2(NO2)2]+ as an anion receptor, binding with oxoanions diclofenac andchlorate ions has been investigated. Yellow crystals of [trans-Co(en)2(NO2)2]C14H10Cl2NO2. 2H2O I, and [trans-Co(en)2(NO2)2]ClO3

II, have been obtained from a mixture of trans-[bis(ethylenediamine)dinitrocobalt(III)] nitrate solution with sodium diclofenac and sodi-um chlorate, respectively, in aqueous medium. The products were characterised by elemental analyses, IR, UV/vis, 1H and 13C NMRspectroscopy. Single crystal X-ray structure determinations revealed that electrostatic forces of attraction besides second sphere hydro-gen bonding interactions stabilize the crystal lattice. Oxygen atoms of the halate and carboxylate group in diclofenac ions act as hydro-gen bond acceptors thereby forming NAHen� � �O bonds. The results show that [trans-Co(en)2(NO2)2]+ is a promising anion receptor forthe weakly coordinating halate and diclofenac ions in aqueous medium. Solubility measurements indicate that the affinity of cationiccobaltammine [trans-Co(en)2(NO2)2]+ is greater for diclofenac than for the chlorate ion.� 2006 Elsevier B.V. All rights reserved.

Keywords: Cobalt(III); Second sphere coordination; Diclofenac; Chlorate; Hydrogen bonding; X-ray crystallography

1. Introduction

The coordination chemistry of anions [1] continues toattract increasing interest from the supramolecular chemis-try community due to the crucial role the ions play in bio-logical processes, medicine, catalysis and molecularassembly. In addition, various pollutant anions have dele-terious effects on the environment. Anions pose a greaterchallenge than isoelectronic cations because they tend tohave more varied shapes and sizes [2] (for example, Cl�,

0022-2860/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2006.04.045

* Corresponding authors. Tel.: +91 172 2534433; fax: +91 0172 2545074(R.P. Sharma); tel.: +44 0 121 414 7481; fax: +44 0 121 414 4403(B.M. Kariuki).

E-mail addresses: rpsharma@yahoo.co.in (R.P. Sharma), b.m.kariuki@bham.ac.uk (B.M. Kariuki).

Br�, and I� are spherical; CO32� and NO3

� are trigonalplanar; PO4

3� and SO42� are tetrahedral; [FeCN6]4� and

[CoCN6]3� are octahedral; DNA is a double helix; N3�

and SCN� are linear). An important aspect of modernsupramolecular chemistry [3] is the utilization of ‘‘hydro-gen bonding’’ in the development of receptors for the rec-ognition of anions. Compared to the relatively simpledesign principles for cation receptors (electronic interactionand sizes), there are more factors that can influence theeffectiveness of artificial anion receptors. Two fundamentalpoints which are needed to be considered in the design [4]of any architecture are the physical features of the unitsto be assembled and the means by which the units are tobe held together. A reliable synthon to act as anion recep-tor (binding agent) for molecular recognition in supramo-lecular chemistry involves molecular couples, typicallyfunctional groups or faces of molecules which have a high

178 R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184

degree of complementarity with respect to intermolecularinteractions.

Although cationic organic anion receptors have beenextensively investigated [5], cationic metal complexes havenot received much attention [6]. In continuation of ourinterest in cobalt(III) complex salts [7], we have undertakenan extensive research programme to explore cationic cobal-tammines [Co(en)2X2]+ (X = Cl or N3) as anion receptors[8]. This is because these cationic metal complexes can besynthesized easily in excellent yields from readily availablematerials and can be stored for months without any notice-able decomposition. The cationic cobaltammine [trans-Co(en)2(NO2)2]+ fulfills the criterion [6a] for an anionreceptor; it has unit positive charge for electrostatic inter-action, eight NAH hydrogen bond donor groups and a sta-ble framework. We envisaged that the presence ofhydrogen bond donor groups on a positively charged cat-ion [trans-Co(en)2(NO2)2]+ would facilitate interactionwith suitably oriented negatively charged chlorate and dic-lofenac ions through the oxygen atoms to yield donor–acceptor complexes involving second sphere coordination.The result is highly likely to be an intricate network ofhydrogen bonds stabilizing the entire lattice in thesolid state. The understanding of such a network ofinteractions for judicially chosen cations and anions wouldbe rewarding as it could provide a means for constructingnovel molecular assemblies based on second-spherecoordination.

Recovery of halates is biologically and commerciallyimportant [9] due to the fact that their metal salts are usedas powerful oxidizing agents, as a component of matchheads and pyrotechnics, in the brightening of pulp, inprinting, dye auxiliary and hair permagent. Salts of diclofe-nac, 2-[2-(2,6-dichlorophenyl)aminophenyl]acetate, (shownin Scheme 1) are used as non-steroidal anti-inflammatoryand analgesic agents (NSAID) [10]. In the study oftrans-dinitrobis (ethylenediamine)cobalt(III) as a bindingagent, we have previously reported the crystal structuresof [trans-Co(en)2(NO2)2]2Cr2O7, [trans-Co(en)2(NO2)2]C7H4NSO3, [trans-Co(en)2(NO2)2]IO4 and the acidsalt [Co(en)2(NO2)(4-NO2C6H4CO2)](4-NO2C6H4CO2).4-NO2C6H4CO2H [11–14] and more recently reported thepotential of another cationic cobaltammine, hexaamminecobalt(III), as a binding agent for halates [15].

Scheme 1. Structural formula of the diclofenac ion.

2. Experimental

2.1. Materials

Analytical grade chemicals and solvents were purchasedcommercially and used without further purification. Dou-ble distilled water was used as the solvent and [trans-Co(en2)(NO2)2]NO3 was prepared according to literaturemethod [16].

2.2. Instrumentation

Cobalt was determined by the standard method [17]. C,H, N were estimated microanalytically using an automaticPerkin Elmer 2400 CHN elemental analyzer. The sampleswere dried in the desiccator before elemental analyses.Infrared spectra were recorded using a Perkin Elmer spec-trum RX FT-IR system and Nujol mull. 1H and 13C NMRwere recorded in appropriate deuterated solvents using aBruker AC 300 F(300 MHz) spectrometer with TMS asthe internal reference. UV/vis spectra were recorded usinga Hitachi 330 spectrometer in the appropriate solvent.

2.3. Synthesis of [trans-Co(en)2(NO2)2]

(C14H10Cl2 NO2)Æ2H2O (I)

One gram (0.003 mol) of bis(ethylenediamine)dinitroco-balt(III) nitrate was dissolved in 50 ml hot water and fil-tered. To this solution was added 0.9552 g (0.003 mol) ofsodium diclofenac dissolved in a minimum amount of water.The light yellow precipitate that appeared within 30 min wasfiltered, washed with water and air-dried. Single crystalswere obtained after two days from a solution in a 1:1 ace-tone–water mixture (yield = 65%). The complex is solublein water as well as DMSO and is stable in air but is insolublein chloroform. The newly formed complex salt, I, decom-poses at 180 �C. The elemental analysis is consistentwith the composition [trans-Co(en)2(NO2)2]C14H10Cl2NO2

(Found: (%) C, 38.2; H, 4.4; N, 17.4; Co, 10.3; calculatedC, 38.1; H, 4.5; N, 17.3; Co, 10.4). Solubility: 0.50 g/100 ml at 25 �C, Ksp = 7 · 10�5.

2.4. Synthesis of [trans-Co(en)2(NO2)2]ClO3 (II)

One gram (0.003 mol) of [trans-Co(en)2(NO2)2]NO3 wasdissolved in 50 ml of water. In another beaker 0.36 g(0.003 mol) of KClO3 was dissolved in 20 ml of water atroom temperature. The two solutions were mixed and yel-low crystals of [trans-Co(en)2(NO2)2]ClO3 appeared withinhalf an hour. They were collected by drawing off themother liquor and air-dried (yield 80%). The complex issoluble in water as well as DMSO and is stable in air butis insoluble in acetone and ethanol. The newly formed com-plex salt II decomposes at 175 �C. The elemental analysis isconsistent with the composition [trans-Co(en)2(NO2)2]ClO3

(found: (%) C, 13.6; H, 4.4; N,15.8; Co, 16.5; calculated C,

Table 1Crystal data and structure refinement for I and II

I II

Formula {Co[(C2H8N2)2](NO2)2}C14H10Cl2NO2Æ2(H2O)

{Co[(C2H8N2)2](NO2)2}ClO3

Formula weight 602.32 354.61Temperature (K) 296(2) 296(2)Wavelength (A) 1.54178 0.71069Crystal system Triclinic MonoclinicSpace group P1 P21/na (A) 8.1150(2) 6.4686(7)b (A) 13.0167(3) 19.484(2)c (A) 14.1622(4) 10.1291(10)a (�) 65.421(2) –b (�) 74.932(2) 100.295(6)c (�) 73.075(2) –Volume (A3) 1284.80(6) 1256.0(2)Z 2 4Dcal (Mg m�3) 1.557 1.875l (mm�1) 7.653 1.621Crystal size (mm3) 0.24 · 0.20 · 0.18 0.30 · 0.20 · 0.15Collected/

independentreflections

8262/4264 7073/2058

Rint 0.023 0.051Goodness-of-fit on F2 1.034 1.144R1/wR2 [I > 2r (I)] 0.042/0.107 0.039/0.089

R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184 179

13.5; H, 4.5; N,15.7; Co, 16.6). Solubility: 1.6 g/100 ml at25 �C, Ksp = 2 · 10�3.

2.5. X-ray crystallography

Single crystal X-ray diffraction data for I were recordedon a Bruker Smart 6000 diffractometer equipped with aCCD detector and a copper tube source. Data for II wererecorded on a Rigaku R-axis II diffractometer equippedwith a molybdenum rotating anode source and an imageplate detector system. Both structures were solved by directmethods and refined using SHELX97 [18]. Crystal andstructure refinement data are presented in Table 1.

3. Results and discussion

3.1. Synthesis

Trans-dinitrobis(ethylenediamine)cobalt(III) nitrate hasbeen known for a long time and has played a pivotal rolehistorically in the development of coordination chemistry.However, there are only a few reports of X-ray diffractionstudies of the bisethylenediaminedinitrocobalt(III) salts inthe literature [19–21]. It is worth mentioning here that thefollowing reactions have been reported and involvereplacement of nitro groups inside the coordination sphereby another group -chloro [22] or oxalate [23,24] ions asshown:

trans-½CoðenÞ2ðNO2Þ2�NO3 þ 2HCl!trans-½CoðenÞ2Cl2�NO3 þNO2 þNOþH2O

trans-½CoðenÞ2ðNO2Þ2�NO3þNa2C2O4 !H2O

trans-½CoðenÞ2ðNO2ÞðC2O4Þ� �2H2OþNaNO2þNaNO3

The new cobalt(III) complexes I and II were synthesizedfrom direct reaction of [trans-Co(en2)(NO2)2]NO3 withsodium salts of diclofenac and chlorate in water.

½trans-CoðenÞ2ðNO2Þ2�NO3ðyellowÞ

þNaC14H10Cl2NO2 !H2O

½trans-CoðenÞ2ðNO2Þ2�C14H10Cl2NO2 � 2H2OI ðyellowÞ

þNaNO3

½trans-CoðenÞ2ðNO2Þ2�NO3ðyellowÞ

þKClO3 !H2O

½trans-CoðenÞ2ðNO2Þ2�ClO3I ðyellowÞ

þKNO3

The chemical compositions of I and II were initially indi-cated by elemental analysis and we proposed the presenceof the ionic structures based on these results. The solidsare yellow in colour and are stable in air and light. Boththe complexes were obtained in quantitative yields andthe analytical data are consistent with the proposed formu-lation of the complexes. The complexes do not exhibitsharp melting points and decompose at around 180�.

3.2. Solubility product

Solubility of ionic salts in water varies considerably,depending on the nature of salt and the solvent. On thebasis of solubility criterion, salts may be classified intothree categories (a) soluble (solubility > 0.1 M), (b) slightlysoluble (solubility between 0.01 and 0.1 M) and (c) sparing-ly soluble (solubility < 0.01 M). Solubility measurements atroom temperature show that both the salts I and II are sol-uble in water but the latter is more soluble. The solubilityproducts of I and II are 7.0 · 10�5 and 2.0 · 10�3, indicat-ing that the affinity or binding of cationic cobaltammine[trans-Co(en)2(NO2)2]+ is greater for the diclofenac ionthan for chlorate. The affinity for the chlorate is obviouslygreater than that for the nitrate and thus the order isC14H10Cl2NO2 > ClO3 > NO3.

3.3. Spectroscopy

Infrared spectra for the newly synthesized complex saltshave been recorded in the region 4000–400 cm�1 and tenta-tive assignments have been made on the basis of earlierreports in the literature [25]. The band in the range890 cm�1 is assigned to the rocking mode of the ethylene-diamine CH2 groups and a band at 1572 cm�1 is assignedto asymmetric deformation [26] of NH2 groups of ethylene-diamine in I. The corresponding bands appear at 890 and1601 in II. A single peak in these regions confirms the transgeometry of the complexes. In I, the band at 1441 cm�1 isassigned to m (CAC). The band at 1548 cm�1 is assigned tomas (CAO) and that at 1452 cm�1 is assigned for ms (CAO) in

Table 3Hydrogen bonding distances (A) and angles (�) for I and II

D-H H� � �A D� � �A \(DHA)

I

N(1)AH(1C)� � �O(8)a 0.90 1.99 2.860(4) 163.5N(1)AH(1D). . .O(3)b 0.90 2.08 2.963(3) 165.3N(2)AH(1C). . .O(1)a 0.90 2.26 2.740(4) 112.9N(2)AH(1D). . .O(7) 0.90 2.19 2.931(3) 139.4N(2)AH(1D). . .O(2) 0.90 2.22 2.763(4) 118.2N(4)AH(4C). . .O(3) 0.90 2.31 2.875(3) 120.8N(4)AH(4C). . .O(5)c 0.90 2.60 3.435(4) 155.3N(4)AH(4D)� � �O(5)d 0.90 2.13 3.007(3) 164.5N(5)AH(5A)� � �O(7) 0.90 2.37 3.053(3) 132.4N(5)AH(5B). . .O(6)e 0.90 2.18 3.009(3) 152.1N(7)AH(7A). . .O(6) 0.86 2.10 2.771(3) 134.4O(7)AH(7B). . .O(6)e 0.979(10) 1.778(11) 2.755(3) 175(4)

f

180 R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184

the complex. The strong absorption bands at 945 and604 cm�1 are assigned to Cl@O [27] in II. The infraredspectra of the sodium/potassium salts of the chlorate ionshow a strong sharp band in the 950–620 region due tothe Cl@O bond. The lowering in the stretching frequencyof Cl@O bond in I may be due to NAH� � �O hydrogenbonding which weakens the bond.

The electronic spectra of the newly synthesized complexsalts were recorded in H2O. The solution state UV/visabsorption spectra of the salts show strong absorptionsat 440 and 356 nm in I and at 438 and 348 in II. Theseabsorptions correspond to d–d transitions typical for octa-hedral low spin cobalt(III) [28]. These transitions are from1A1g ground state of cobalt(III) to singlet state 1T1g (lowenergy) and from 1A1g ground state to 1T2g (higher energy).

NMR spectra of the newly synthesized complex saltswere recorded in D2O. The chemical shift values areexpressed as d value (ppm) downfield from tetramethylsi-lane as internal standard. In 1H NMR, the signal at4.9 ppm is attributed to nitrogen protons of ethylenedia-mine while CH2 protons of ethylenediamine [29] groupare observed at 2.4 ppm in I. The corresponding signalsin II appear at 5.3 and 2.5. The 13C NMR spectrum showsthe characteristic signal at 45 ppm for carbons [30] of eth-ylenediamine groups in both complexes. In complex I, thesignal at 6.9 ppm is attributed to ANH protons of the ben-zene ring. The aromatic protons are observed between 6.3and 7.5 ppm. The signal at 3.9 ppm is attributed to the pro-ton of the CH2 group attached to the benzene ring. In the13C NMR spectrum, the carboxylate carbon appears at142.7 ppm, the carbon of the CH2 group attached to ben-zene ring appears at 137.0 ppm and the aromatic carbonsin the range 115.9–130.0 ppm.

Table 2Selected bond lengths (A) and angles (�) for I and II

I

C(18)AO(5) 1.245(4) C(2)AC(1)AN(1) 113.0(4)C(18)AO(6) 1.249(4) O(5)AC(18)AO(6) 123.1(3)N(1)ACo(1) 1.950(2) N(2)aACo(1)AN(1) 93.75(11)N(2)ACo(1) 1.939(2) N(2)ACo(1)AN(1) 86.25(11)N(3)ACo(1) 1.948(2) N(2)ACo(1)AN(1)a 93.75(11)N(4)ACo(2) 1.958(2) N(5)bACo(2)AN(4) 93.81(10)N(5)ACo(2) 1.948(2) N(5)ACo(2)AN(4) 86.19(10)

N(5)ACo(2)AN(4)b 93.81(10)

II

N(1)ACo(2) 1.953(2) O(2)AN(5)AO(1) 118.4(3)N(2)ACo(2) 1.944(2) O(3)AN(6)AO(4) 119.3(3)N(3)ACo(2) 1.956(2) O(6)ACl(1)AO(5) 108.5(2)N(4)ACo(2) 1.954(2) O(6)ACl(1)AO(7) 106.88(17)N(5)ACo(2) 1.942(3) O(5)ACl(1)AO(7) 105.84(15)N(6)ACo(2) 1.928(3) N(2)ACo(2)AN(1) 86.13(10)N(5)AO(2) 1.221(3) N(2)ACo(2)AN(4) 94.26(10)N(5)AO(1) 1.234(3) N(1)ACo(2)AN(3) 93.95(10)N(6)AO(3) 1.229(3) N(4)ACo(2)AN(3) 85.66(10)N(6)AO(4) 1.241(3)

Symmetry transformations used to generate equivalent atoms.a �x + 1, �y + 1, �z + 1.b �x + 1, �y, �z + 1.

3.4. Crystal structures

The crystal structures of the title complex salts havebeen unambiguously determined by single crystal X-raycrystallography. Crystal and structure refinement data arepresented in Table 1, selected bond lengths and angles inTable 2 and selected hydrogen bonding contacts in Table 3.

X-ray structure determination of I revealed the presenceof trans-[Co(en)2(NO2)2]+ and C14H10Cl2NO2 ions andwater molecules and of II revealed the presence of trans-[Co(en)2(NO2)2]+, and ClO3

� ions. ORTEP diagram of I

with the atom numbering scheme is shown in Fig. 1a andof II is shown in Fig. 2a. In the trans-[Co(en)2(NO2)2]+ cat-ion, the cobalt is surrounded by six nitrogen atoms origi-nating from two coordinated ethylenediamine ligands andtwo nitro ligands. The resulting geometry is distorted

O(7)AH(7C). . .O(5) 0.976(10) 1.884(14) 2.850(3) 170(4)O(8)AH(8A). . .O(1)g 0.969(10) 1.984(14) 2.936(4) 167(4)O(8)AH8B)� � �O(5)f 0.971(10) 1.868(16) 2.807(4) 162(4)

II

N(1)AH(1A). . .O(5) 0.90 2.13 2.942(3) 149.3N(1)AH(1B). . .O(1)h 0.90 2.26 3.039(3) 144.8N(2)AH(2A)� � �O(7)i 0.90 2.27 3.018(3) 140.7N(2)AH(2B)� � �O(7)j 0.90 2.24 3.021(4) 144.7N(2)AH(2B)� � �O(1) 0.90 2.24 2.830(3) 122.7N(3)AH(3A)� � �O(4) 0.90 2.27 2.847(3) 121.6N(3)AH(3A)� � �O(6)k 0.90 2.50 3.274(4) 144.2N(3)AH(3B). . .O(5) 0.90 2.35 3.132(4) 145.9N(4)AH(4A). . .O(4)l 0.90 2.20 3.004(3) 147.7N(4)AH(4B)� � �O(7)m 0.90 2.26 3.014(3) 141.3

Symmetry transformations used to generate equivalent atoms.a �x + 1, �y + 1, �z + 1.b x, y + 1, z.c �x, �y + 1, �z.d x, y � 1, z + 1.e �x + 1, �y + 1, �z.f x + 1, y � 1, z + 1.g x + 1, y, z.h x � 1, y, z.i �x + 3/2, y � 1/2, �z + 3/2.j x + 1/2, �y + 1/2, z + 1/2.

k x � 1/2, �y+ 1/2, z � 1/2.l x + 1, y, z.

m �x + 3/2, y � 1/2, �z + 3/2.

Fig. 1. (a) An ORTEP view of the asymmetric unit of I showing atom numbering, Thermal ellipsoids are drawn at the 40% probability level, (b) the crystalstructure with hydrogen atoms omitted for clarity, (c) cations in a stack with hydrogen bonding show as dashed lines and (d) segment of the structureshowing hydrogen bonding between cations, anions and water molecules.

R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184 181

octahedra as shown by the bite angle \NACoAN range[86.19(10) to 93.81(10) �] in I and \NACoAN range[85.66(10) to 94.26(10) �] in II. The endocyclic anglesformed by the ethylenediamine ligands are ca. 86 � whereasthe exocyclic angles are ca. 94 �. There are two types ofcrystallographically independent cations [Co(1) andCo(2)] and both sit on sites with twofold symmetry. Thedifference between the two types of sites is illustrated byconsideration of the CO2 groups. Selected interatomicparameters are given in Tables 2. For Co(1), the plane ofthe CO2 group lies close to the plane of trans-CACo bonds,whereas the plane of the CO2 group bisects the anglebetween cis CACoAC bonds for Co(2). The CoAN bondlengths are in the range of 1.939(2)–1.958(2) A for nitro-gens of the ethylenediamines coordinated to the cobalt inI and 1.944(2)–1.956(2) A in II. The bond lengths are1.937(2)–1.939(2) A for the nitrogen atoms of nitro group

in I and 1.928(3) and 1.942(2) A in II. The characteristicbond lengths and angles for I and II as well as values forsalts with similar anions or cations are shown in Table 4.Comparison shows general agreement with the literaturevalues for both cations and anions.

The packing in the crystal is shown in Fig. 1b. The cat-ions stack parallel to the a-axis and interact through hydro-gen bonding within the stack. A given pair of cations in thestack shares two CAH� � �O bonds, with each bondinvolving one ethylenediamine NAH donor and one NO2

oxygen acceptor (Fig. 1c). The stacks are arranged to formsheets of cations and water molecules, parallel to the ab

plane, which are separated by diclofenac molecules. Thestacks of cations are linked to the anions and water mole-cules through NAH� � �O hydrogen bonding and there is nodirect contact between the stacks. Hydrogen bondingparameters are given in Table 3.

Fig. 2. (a) An ORTEP view of the asymmetric unit of II showing atom numbering, Thermal ellipsoids are drawn at the 40% probability level, (b) thecrystal structure with hydrogen bonding shown as dashed lines, (c) cations in a stack with hydrogen bonding show as dashed lines.

Table 4Comparison of bond lengths (A) and bond angles [�) for the cation and anions in I and II

Cation, X = Co(en)2(NO2)2 CoAN1 CoAN2 CAN CAC NACoAN CoANAC CoANAO Ref.

XI 1.95 (1) 1.92 (1) 1.49 (2) 1.49 (3) 87.6 (6) 108.6 (1) 119.8 (1) [19]X SCN 1.95 (1) 1.92 (1) 1.48 (1) 1.50 (1) 90.0 (8) 108.6 (2) 121.0 (2) [19]XNO3 1.94 (2) 1.93 (2) 1.47 (5) 1.51 (5) 89.7 (1) 109.7 (2) 120.0 (2) [21]XClO4 1.95 (5) 1.94 (5) – – 89.6 (2) – 120.1 (4) [20]XC7H4NSO3 1.95 (3) 1.93 (4) 1.35 (6) 1.39 (6) 89.6 (6) 109.3 (2) 120.0 (3) [21]XC14H10Cl2NO2 1.948 (2) 1.944 (2) 1.467(5) 1.453(5) 90.4 (2) 109.4(2) 121.0(2) –*

XClO3 1.952 (1) 1.935 (2) 1.481(4) 1.505(4) 90.0 (1) 109.2(2) 120.6(2) –*

Anion Y = ClO3 ClAO OAClAO Ref. Y0 = C14H10Cl2NO2 CAO OACAO Ref.

NaY 1.485 (1) 106.9 (2) [31] [C6H16NO3]Y0 1.24 (3) 123.8 (2) [34]BaYH2O 1.485 (5) 106.3 (7) [32] [Mg(H2O)6] Y02Æ2H2O 1.25 (4) 123.4(3) [35]RbY 1.478 (5) 105.6 (3) [33] [C4H10N]Y 0ÆH2O 1.25(3) 123.8(2) [36][Co(en)2(NO2)2] Y 1.469(3) 107.1(14) –* [Co(en)2(NO2)2]Y0 1.247(4) 123.1 (3) –*

* This work.

182 R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184

The ionic character of the diclofenac anion is indicatedby the bond lengths [1.245(4) and 1.249(4) A] and bondangle (123.1(3) �) of the ACO2 group. An intramolecularNAH� � �O hydrogen bond is observed in the anion

(Fig. 1d). Hydrogen bonding between two water mole-cules and two diclofenac anions results in a 12-memberedring. A pair of these rings is located on opposite sides ofone type of cobalt site, Co(2), and they act as acceptors of

R. Sharma et al. / Journal of Molecular Structure 826 (2007) 177–184 183

several hydrogen bonds from the NAH groups of theethylenediamines. The ethylenediamine molecules coordi-nating to the other cation site [Co(1)] are not involvedin direct hydrogen bonding with the anions. This alternat-ing pattern of hydrogen bonds for the cobalt sites alongthe stack is a consequence of the need to accommodatethe bulky dichloro-phenyl group of the diclofenac anionin the structure.

The crystal structure is shown in Fig. 2b. Only one crys-tallographically distinct cobalt site is found in the structureof II. In a manner similar to the Co(2) site in I, the plane ofthe CO2 group bisects the angle between cis-CACoACbonds in II. The cations stack parallel to the a-axis in anarrangement similar to that found in I, with pairs ofNAH� � �O hydrogen bonds linking the cations (Fig. 2c).The ClO3

� ion slots perfectly between the stacks of cationsand accepts several NAH� � �O hydrogen bonds (Table 3)rom the cation.

4. Conclusions

Both the newly synthesized complex salts I and II formhydrogen bonded stacks of trans-[Co(en)2(NO2)2]+ cations.The stacking appears to be a robust arrangement as onlyslight modification is observed on changing the anion fromthe chlorate to the relatively large diclofenac. The stackingof cations is retained in I, although hydrogen-bonding issacrificed in order to accommodate the bulky diclofenacanion. In both I and II, the anions are accommodatedbetween stacks of cations and mediate between themthrough hydrogen bonding. In addition to coulombic inter-actions, the structures are stabilized by H-bonding interac-tions involving second sphere coordination. The formationof salts of definite composition suggests that this cationiccobaltammine is a promising anion binding agent for dic-lofenac and chlorate oxoanions. Solubility measurementsin aqueous medium showed that the affinity of cationiccobaltammine [trans-Co(en)2(NO2)2]+ is greater for dic-lofenac ion than for the chlorate ion.

5. Supplementary data

Crystallographic data for the structural analysis havebeen deposited at the Cambridge Crystallographic DataCenter, 12 Union Road, Cambridge, CB2 1EZ, UK, andare available free of charge from the Director on requestquoting the deposition numbers CCDC 286093 and286094 (Fax: 44-1223-336033, email: deposit@ccdc.cam.ac.uk or <http://www.ccdc.cam.ac.uk>).

Acknowledgment

The authors gratefully acknowledge the financialsupport of UGC vide Grant No. F.12-38/2003(SR) dated31-03-2003.

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