Journal of Molecular Structure 935 (2009) 1–7

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Journal of Molecular Structure

journal homepage: www.elsevier .com/ locate /molst ruc

Novel structures based on 1-(4-cyanophenyl)-imidazole resulting from weakbonding interactions

Ashish Kumar Singh a, Mahendra Yadav a, Prashant Kumar a, Sanjay Kumar Singh a, Sailaja Sunkari b,Daya Shankar Pandey a,*

a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, Indiab Mahila Mahavidyalaya, Banaras Hindu University, Varanasi 221 005, India

a r t i c l e i n f o

Article history:Received 5 June 2009Received in revised form 19 June 2009Accepted 24 June 2009Available online 28 June 2009

Keywords:Weak interaction1-(4-Cyanophenyl)-imidazole3d metal complexesLuminescence

0022-2860/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.molstruc.2009.06.031

* Corresponding author. Tel.: +91 542 2307321x10E-mail address: dspbhu@bhu.ac.in (D.S. Pandey).

a b s t r a c t

Mono-nuclear complexes [M(CPI)6](X)2 (M = Co, X = NO3� 1; M = Ni, X = NO3

� 2; M = Cu, X = ClO4� 3) and

[Cu(CPI)4(H2O)2](NO3)2 4, based on novel bridging ligand 1-(4-cyanophenyl)-imidazole (CPI) arereported. The complexes have been characterized by elemental analyses, spectral studies and theirmolecular structures have been authenticated by single crystal X-ray diffraction analyses. In molecularspecies 1–3 metal centers are coordinated octahedrally by six CPI ligands wherein cyanophenyl ringsof six ligands about the complex cations are involved in face-to-face p–p interaction with six co-planarmolecules to afford unprecedented 2D sheet. Along ‘c’-axis it gives a beautiful rim like motif. In complex4, coordination about metal center Cu is distorted octahedral with four CPI ligands in the equatorial posi-tion and two water molecules occupying axial positions. In this complex hydrogen bonding interactionsbetween the coordinated water molecules and nitrate anions results in a 1D straight chain along ‘a’-axisand 2D sheets having hexagonal cavity filled by nitrate ions. The complexes upon excitation at 275–285 nm exhibit luminescences with emission maxima centered at 438–445 nm attributable to the ligandCPI at room temperature.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Self-organization of metal complexes through non-covalentinteractions including electrostatic and hydrogen bonding hasgreat potential in creating supramolecular architectures withwell-defined shape and functions [1–10]. During past couple ofdecades a large number of metal–organic frameworks have beensuccessfully designed and synthesized through judicious choiceof a metal and ligand. Main strategy utilized in the constructionof novel multi-functional materials is based on self-assemblymethods which employs polydentate organic ligands containingN- and/or O-donors as building blocks [11–17]. The self-assemblyof metal–organic frameworks is highly influenced by factors suchas solvent system, pH of the solution, geometric requirements ofthe metal ions, and counter ions [18–23]. Further, several typesof forces such as coordination bonding, hydrogen bonding, p�pstacking and electrostatic interactions have been recognized andused in the construction of extended inorganic networks. Amongthese coordination bonding and hydrogen bonding interactionsare most important. Metal ion and counter ions(anions) presentin the system which behave as Lewis acid and Lewis base, respec-

ll rights reserved.

5.

tively, strongly affect the strength of inter-molecular interactionsand ordering of the crystal lattice owing to their ability to influenceelectron density of the functional groups present in the neighbor-hood of metal and tendency to accept hydrogen bonds [21–26].

In this context, imidazole and ligands based on it have drawnspecial attention owing to their good coordination ability, diversecoordination modes and ability to act as linker in the constructionof extended solids [27–31]. Cyanide is another ligand that havelong been studied by inorganic chemists and like imidazole havebeen employed in the formation of extended structures [32–34].So far, only one ligand containing both the imidazole and cyanidegroup have been used as a building block in the construction offunctional MOFs [35]. 1-(4-Cyanophenyl)-imidazole (CPI) whichcontains both the imidazole and cyanide groups is an attractivemolecule in this regard [36–38]. The adoption of an unsymmetricalbridging ligand like CPI is expected to give rise a broader palette ofMOF’s than can be achieved with symmetrical ligands. Further-more, the CN group of CPI may undergo hydrolysis to form 4-(imidazolyl)-benzoic acid, another unsymmetrical ligand that havebeen used in the construction of interesting networks [39–41].

In a follow-up of our studies on binding properties of 1-(4-cya-nophenyl)-imidazole with d-block metals we have carried outreactions of CPI with metal salts and have isolated self-assembledstructures arising from weak bonding interactions [42,43]. In this

2 A.K. Singh et al. / Journal of Molecular Structure 935 (2009) 1–7

paper we report properties and structures of some novel molecularspecies [M(CPI)6](X)2 (M = Co, X = NO3

� 1; M = Ni, X = NO3� 2;

M = Cu, X = ClO4� 3) and [Cu(CPI)4(H2O)2](NO3)2 4.

2. Experimental

2.1. Materials

The solvents and reagents employed in the reactions were usedas received without further purifications. 1-(4-Cyanophenyl)imid-azole (CPI) was prepared and purified following the literature pro-cedure [36]. Carbon, hydrogen and nitrogen analyses on thesamples were acquired on a CE-440 CHN Analyzer. IR and elec-tronic absorption spectra were recorded on a Shimadzu-8201PCand Shimadzu-UV-1601 spectrophotometers, respectively. 1HNMR spectra were obtained on a JEOL 300 MHz machine usingTMS as an internal reference. Emission spectra were recorded onPerkin-Elmer LS-45 luminescence spectrophotometer at roomtemperature and thermal analyses on the complexes were per-formed on a Shimadzu DTG-50 Thermobalance.

Syntheses of the complexes. Following general procedure wasadopted for the synthesis of 1–4. In a typical reaction, metal salt(1.0 mmol) and CPI (2.0 mmol) in 10 ml water–ethanol mixture(4:1) were heated in a sealed tube in an oil-bath at 120 �C for72 h. After cooling to room temperature diffraction quality crystalswere obtained directly from the reaction mixture. These were sep-arated washed with ethanol, diethyl ether and dried under vac-uum. Selected data of the complexes are given below:

Table 1Distances as assigned in Figs. 2 and 3.

Intra-layer(a)

Inter-layer(b)

Phenyl ring centerto center separation(inter/intra-layer)(c/d)

Imidazole ringcenter tocenterseparation (e)

Complex 1 3.425 3.587 3.625/3.918 3.878Complex 2 3.424 3.587 3.623/3.921 3.880Complex 3 3.334 3.612 3.664/3.876 3.847

Table 2Crystallographic data for complexes 1–4.

1 2

Chemical formula C60H42CoN20O6 C60H42N20

fw 1198.07 1197.85Color, habit Light pink, blocks White, bloSpace group P�3c1 P�3c1Cryst. syst. Rhombohedral Rhombohea (Å) 15.2440(3) 15.1847(1b (Å) 15.2440(3) 15.1847(1c (Å) 14.0238(3) 14.0210(1a (�) 90.00 90.00b (�) 90.00 90.00c (�) 120.00 120.00V (Å3) 2822.24(10) 2799.8(4)Z 2 2Dcalc (g cm�3) 1.410 1.421l (mm�1) 0.377 0.420T (K) 150(2) 293(2)No. of reflections 3330 2331No. of parameters 132 133R factor all 0.0993 0.1070R factor [I > 2r(I)] 0.0733 0.0898wR2 0.1906 0.2664wR2 [I > 2r(I)] 0.1793 0.2517GOF 1.195 1.949

Synthesis of [Co(CPI)6](NO3)2 1. Yield (based on ligand) 63%(0.251 g). Anal. Calcd (%) for 1 (C60H42N20O6Co): C, 60.10; H,3.50; N, 23.37. Found (%): C, 59.62; H, 3.12; N, 22.78. IR (KBrcm�1); 3443(vbr), 3145(m), 2940(br), 2230(vs), 1679(w),1612(vs), 1520(s), 1425(br), 1391(vs), 1310(vs), 1265(vs),1185(w), 1120(vs), 1066(vs), 965(w), 842(s), 771(s), 661(w),622(vs), 565(w). kmax (nm) {kex (nm)} = 443 {280}.

Synthesis of [Ni(CPI)6](NO3)2 2. Yield (based on ligand) 61%(0.243 g). Anal. Calcd (%) for 2 (C60H42N20O6Ni): C, 60.15; H, 3.51;N, 23.39. Found (%): C, 60.29; H, 3.21; N, 22.73. IR (KBr cm�1);3440(vbr), 3141(m), 2925(br), 2228(vs), 1681(w), 1610(vs),1525(s), 1422(br), 1388(vs), 1310(vs), 1268(vs), 1184(w),1125(vs), 1070(vs), 964(w), 839(s), 773(s), 656(w), 623(vs),560(w). kmax (nm) {kex (nm)} = 442 {281}.

Synthesis of [Cu(CPI)6](ClO4)2 3. Yield (based on ligand) 64%(0.272 g). Anal. Calcd (%) for 3 (C60H42N18O8Cl2Cu): C, 56.40; H,3.29; N, 19.74. Found (%): C, 56.06; H, 3.12; N, 19.40. IR (KBrcm�1); 3440(vbr), 2932(br), 2228(vs), 1685(w), 1619(vs), 1525(s),1425(br), 1385(vs), 1323(vs), 1274(vs), 1190(w), 1120(vs),1065(vs), 963(w), 835(s), 763(s), 650(w), 624(vs), 565(w). kmax

(nm) {kex (nm)} = 441 {282}.Synthesis of [Cu(CPI)4(H2O)2](NO3)2 4. Yield (based on ligand)

64% (0.183 g). Anal. Calcd (%) for 8 (C40H32N14O8Cu): C, 53.36; H,3.56; N, 18.68. Found (%): C, 53.62; H, 3.12; N, 19.40. IR (KBrcm�1); 3422(vbr), 3122(m), 2855(br), 2433(m), 2230(vs),1749(w), 1609(vs), 1521(s), 1393(br), 1326(vs), 1270(vs),1184(w), 1064(vs), 960(w), 840(s), 737(s), 655(w), 554(w). kmax

(nm) {kex (nm)} = 441 {281}.

2.2. Crystal structure determinations

Crystals suitable for single crystal X-ray diffraction analyses for1–4 were obtained directly from the reaction mixture. Crystal datafor the complexes 1 and 4 were collected on OXFORD DIFFRACTIONX CAUBER-S and for 2 and 3 on a BRUKER SMART APEX diffractom-eter using graphite monochromatized Mo Ka radiation and x scantechnique for all complexes. The unit cell parameters were ob-tained from 12379 reflections for 1, 3814 for 2, 4235 for 3 and4681 for 4 in the cell measurement angle range 3.0320–32.5962,3.05–26.37, 2.8315–29.8867, 3.1432–32.5711, respectively. Thestructures were solved by direct methods and refined by full ma-

3 4

NiO6 C60H42Cl2CuN18O8 C40H32CuN14O8

1277.56 900.34cks Light green, blocks Light blue, blocks

P�3c1 P2/cdral Rhombohedral Monoclinic

0) 15.3242(8) 17.6036(5)0) 15.3242(8) 8.4987(2)3) 13.8937(14) 14.4818(4)

90.00 90.0090.00 106.251(3)120.00 90.002825.6(4) 2080.02(10)2 21.502 1.4380.557 0.596293(2) 150(2)2260 7057135 2940.1150 0.06750.0580 0.03570.1819 0.09180.1264 0.08220.857 0.968

Table 3Selected bond parameters for complexes 1–4.

1 2 3 4

M1AN1 2.170(2) 2.125(3) 2.168(4) 2.0216(11)M1AN4 2.0193(12)M1AO1 2.3684(16)

2.3755(17)N1AM1AN1 180.0 180.00(12) 180.00(18) 179.07(6)N1AM1AN1 92.69(7) 92.37(10) 92.83(11) 90.42(5)N1AM1AN1 87.31(7) 87.63(10) 87.17(11) 89.55(5)N1AM1AO1 88.03(3)N4AM1AO1 89.54(3)O2AM1AO1 180.0N3AC10AC7 179.5(4) 178.2(4) 178.8(5) 176.7(2)N6AC18AC17 178.94(18)C3AN2AC4AC5 27.9(4) 29.7(5) 29.1(6) 32.29(19)C5AAC4AN2AC1 30.4(4) 28.9(5) 25.5(5) 35.8(2)C12AN5AC14AC15 20.6(2)C13AN5AC14AC20 19.6(2)

A.K. Singh et al. / Journal of Molecular Structure 935 (2009) 1–7 3

trix least squares on F2 using SHELX-97 [44]. The non-hydrogenatoms were refined with anisotropic thermal parameters [45]. Allthe hydrogen atoms were geometrically fixed and allowed to refineusing a riding model (Table 3). The NO3

� ions are highly disorderedalong the threefold axis. For complex 2, disordered O atom of theNO3

� is modeled by splitting into two halves with equal site occu-pation and refining the two split components anisotropically. Therefinement converged to a final R1 = 0.0740, wR2 = 0.2114 for 1;R1 = 0.0574, wR2 = 0.1393 for 2; R1 = 0.0532, wR2 = 0.1005 for 3;R1 = 0.0356, wR2 = 0.0797 for 4. Important crystal data were pre-sented in Table 2 for all the structures.

NN

N

M

N

NN

NN

NN

N

NN

N

N

NC

NC

CN

CN

CN

NC

[M(H2O)6]X2

(X)2

M = Co, X = NO3 (1)M = Ni, X= NO3 (2)M = Cu, X = ClO4 (3)

[Cu(H2

Scheme 1. Preparation

Fig. 1. General representation o

3. Results and discussion

3.1. Synthesis and characterization of 1–3

Three novel mononuclear species [M(CPI)6](X)2 (M = Co,X = NO3

� 1; M = Ni, X = NO3� 2; M = Cu, X = ClO4

� 3) have beensynthesized by reaction of the metal salts M(NO3)�xH2O orM(ClO4)�xH2O with CPI under analogous conditions (Scheme 1).These represent first examples of an octahedral metal center coor-dinated by six CPI ligands. There are only two reports in the liter-ature dealing with analogous molecular species based on 4-cyanoimidazolate and 4,40-bipyridine N,N0-dioxide [35,46,47]. Geo-metrical formulations of these molecular species have been estab-lished by elemental analyses and spectral studies. Infrared spectraof the complexes displayed characteristic bands associated withmC„N at �2230 cm�1. Vibrations corresponding to mC@N of thecoordinated imidazole moiety appeared at �1600 cm�1 [36]. Infra-red spectral data suggested coordination of the ligand CPI throughimidazole nitrogen. Characteristic bands corresponding to thecounter anions NO3

� and ClO4� were displayed at �1400 cm�1

and �1119 cm�1 in the respective complexes [48].

3.2. Synthesis and characterization of complexes 4

Reaction of the nitrate salt of copper with CPI surprisingly affor-ded [Cu(CPI)4(H2O)2](NO3)2 4 instead of [Cu(CPI)6](NO3)2. Further,it was observed that even in presence of an excess of CPI, the finalproduct was 4 not 3. Complex 4 has been characterized by IR, ele-mental analyses, TGA and finally its structure have been authenti-cated by single crystal X-ray diffraction studies. C, H and N analysesare consistent with the proposed formulation. IR spectrum of 4

CuN

N N

N

OH2

H2ON

N

N

NNC CN

CNNC

(4)

(NO3)2

O)3(NO3)]NO3

of complexes 1–4.

f the cationic part of 1–3.

4 A.K. Singh et al. / Journal of Molecular Structure 935 (2009) 1–7

displayed vibrations associated with coordinated CPI, water andnitrate anion [36,48].

Fig. 4. Complex 4 crystallizes in monoclinic space group with P2/c symmetry. Theasymmetric unit is shown in Fig. 5. Coordination geometry about Cu is distortedoctahedral with N4O2 arrangement about the metal center. Four CPI ligands arecoordinated through N of the imidazole group (Cu1AN1 2.0215(11), Cu1AN42.0186(11) [51,52] and two water molecules through oxygen atom at axialpositions [Cu1AO1 2.3687(16), Cu1AO2 2.3749(17)] (Table 3).

3.3. Crystallography

Important crystallographic data and selected geometricalparameters for 1–3 which crystallizes in rhombohedral systemwith P�3c1 space group are summarized in Tables 2 and 3, respec-tively. Interestingly, all these complexes are iso-structural. Generalpresentation of the cation of the complexes is depicted in Fig. 1. Acommon structural feature of 1–3 is that each of them are com-posed of discrete [M(CPI)6]2+ cations and counter ions (NO3

� orClO4

�). Further, the metal cation occupies an inversion center. Siteoccupancy of the counter ions are twice compared to that of themetal which suggested that corresponding to each metal centerthere are two counter ions consistent with the electroneutralityof compounds. Overall geometry about the metal centers are octa-hedral with MN6 environment wherein six N donors are derivedfrom the imidazole nitrogen of six different CPI ligands. The CPI li-gands are distributed alternately in two parallel planes about me-tal center so that the cation has approximately D3d symmetry(Fig. 2). The MAN bond lengths CoAN = 2.1708(19), 1;NiAN = 2.1260(17), 2; CuAN = 2.163(3), 3 fall in the normal rangefor respective MAN distances [49–52]. The cyanophenyl rings aretwisted with respect to imidazole ring within each of these ligands.The twist angles are recorded in Table 3.

Inter-molecular p–p interactions have been employed in theconstruction of coordination networks from discrete molecules(Figs. 2 and 3) [47]. The complexes 1–3 are unique in the sensethat, although these are mono-nuclear systems, overall layeredstructures have been observed for each of them. Inter-molecular

Fig. 2. Parallel arrangement of CPI ligands in two planes around the meta

Fig. 3. p–p stacking between p

face-to-face p–p interactions between discrete [M(CPI)6]2+ moie-ties involving the cyanophenyl rings of CPI results in the formationof a 2D sheet and a rim (molecular wheel) like arrangement along

l center and separate layers resulting from p–p stacking interactions.

henyl and imidazole rings.

Fig. 5. Molecular view of 4 (hydrogen atoms omitted for clarity) with 30% ellipsoid probability. The unlabeled part is generated by symmetry. Symmetry code = �x + 1, y,�z + 0.5.

A.K. Singh et al. / Journal of Molecular Structure 935 (2009) 1–7 5

‘c’-axis (Fig. 4). Layers are almost parallel to each other (Table 1,Figs. 2 and 3, S1 and S2). The sheets align in an eclipsed fashionalong ‘a’-axis through aromatic face-to-face p–p stacking interac-tions leading to AAA packing motif containing triangular channelsoccupied by counter ions (Figs. 4 and S2). Non-covalent interac-tions are analogous in all the cases with slight difference due tocounter ion nitrate and perchlorate. In [M(CPI)6](NO3)2 (M = Co,Ni) nitrate anions are disordered.

Fig. 6. S4 type of hydrogen bonding in 4 resulting in

Fig. 7. 2D layered structure in complex 4 along ‘a’-axis re

Weak interaction studies on the complex 4 revealed formationof S4 type of hydrogen bonding with the dimension of2.844 Å � 2.865 Å. In the present case nitrate ion acts as a buildingblock leading to the formation of a 1D chain (Fig. 6). Various typesof interactions resulted in mainly two types of arrangements (a) 1Dchain (Fig. 6) and (b) hexagon with a cavity occupied by nitrateions resulting from p–p stacking interactions between the phenylrings (Fig. 7).

one dimensional chain network along ‘a’-axis.

sulting in hexagonal cavity containing NO3� anions.

Fig. 8. Thermogram of 4.

6 A.K. Singh et al. / Journal of Molecular Structure 935 (2009) 1–7

During course of this study we came across several interestingobservations. It was observed that the metal salts employed inthe reaction plays an important role in the formation of final prod-uct. For example, hexahydrates salts M(NO3)2�6H2O (Co, Ni) orM(ClO4)2�6H2O (M = Cu) under analogous conditions affordedmolecular species [M(CPI)6](X)2 (X = NO3

�, ClO4�) 1–3, while reac-

tion of a tri-hydrate Cu(NO3)2�3H2O afforded [Cu(CPI)4(H2O)2](-NO3)2 4. It indicated that the counter anion is producing strongtemplate effect at the same time water of crystallization presentin the metal salt is also playing an important role. The formationof 4 over 3 may be attributed to Jahn–Teller distortion and waterof crystallization of the metal salt employed in the reaction. Fur-ther, in the synthesis of 4, it was observed that whether the reac-tion was carried out in 1:2 M ratios or in presence of an excessof ligand CPI, final product remains the same. In some of the reac-tions along with CPI, a good linker like 4,40-bipyridine was em-ployed. To our surprise, we could not get any extended networkcontaining the linker 4,40-bipyridine, it always afforded the molec-ular species [M(CPI)6](X)2 (X = NO3

�, ClO4�). Formation of 1–3 in

presence of a strong linker over formation of extended networksmay be attributed to higher stability of the molecular species[M(CPI)6](X)2 arising from face-to-face p–p interactions betweenco-planar molecules.

3.4. Thermogravimetric studies

Thermogram of 4 is depicted in argon atmosphere at the scanrate 10 �C/min (Fig. 8). It is observed that complex loses weightin three steps. In the first step weight loss of 4.17% takes place inthe temperature range of 130–150 �C corresponding to loss ofcoordinated water molecules. The observed value is slightly higherthan the calculated value (4.0%). Second and third steps in therange 220–300 and 520–800 �C associated with weight loses of52.3% (calculated 51.33%) and 32.45% (calculated �37.55%),respectively may be attributed to the loss of coordinated CPI andcounter anion in steps. These steps support formulation of thecomplex 4.

4. Conclusions

In this work we have presented some novel mono-nuclear com-plexes based on bridging ligand 1-(4-cyanophenyl)-imidazole

(CPI). It have been shown that the formation of final product isindependent on the presence of a linker like 4,40-bipyridine andit strongly depends on nature of the metal salt employed in thereaction. Molecular species [M(CPI)6]2+ represents first examplesin which respective octahedral metal centers are coordinated bysix 1-(4-cyanophenyl)-imidazole ligands.

Acknowledgments

Thanks are due to the Department of Science and Technology,New Delhi, India for providing financial assistance SR/S1/IC-15/2006. A.K.S. acknowledges CSIR for awarding JRF 09/013(0127)/2007, EMR-I. The authors are also grateful to Prof. V. Chandrase-khar, Department of Chemistry, IIT Kanpur for providing TG andEPR facilities, and the Head Department of Chemistry, Faculty ofScience, Banaras Hindu University, Varanasi (U.P.) for extendingfacilities. Special thanks are due to Prof. Pradeep Mathur, In-charge, National Single Crystal X-ray Diffraction Facility, IndianInstitute of Technology, Mumbai for providing single crystal X-ray data.

Appendix A. Supplementary data

CIF files containing X-ray crystallographic data for complexes1–4 (CCDC 702828, 702829, 702831 and 702832), matrices for in-ter-molecular interactions in 1–4 and supramolecular motifsformed in these crystalline substances. This material is availablefree of charge. Supplementary data associated with this articlecan be found, in the online version, at doi:10.1016/j.molstruc.2009.06.031.

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