Polyhedron 29 (2010) 3074–3080

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Polyhedron

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Bis(2-(1H-imidazol-2-yl)-pyridine)copper(II): Effects of counteranionson the molecular and supramolecular structures

Tulika Ghosh, Sunirban Das, Samudranil Pal ⇑School of Chemistry, University of Hyderabad, Hyderabad 500 046, India

a r t i c l e i n f o

Article history:Received 12 July 2010Accepted 12 August 2010Available online 29 September 2010

Keywords:Copper(II) complexesCounteranionsCrystal structuresCoordination geometrySelf-assemblySupramolecular structures

0277-5387/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.poly.2010.08.034

⇑ Corresponding author. Tel.: +91 40 2313 4829; faE-mail address: spsc@uohyd.ernet.in (S. Pal).

a b s t r a c t

The synthesis and physical properties of bis(2-(1H-imidazol-2-yl)-pyridine)copper(II) with chloride,nitrate and perchlorate as counteranions have been described. Microanalysis, magnetic susceptibility,conductivity and various spectroscopic measurements have been used for the characterization of thecomplexes. The crystal structures of all three complexes have been determined. Intermolecular hydro-gen-bonding interactions and the resulting self-assembly patterns for each of the species have been scru-tinized. The chloride containing complex crystallizes as a trihydrate, where the metal ion is in atetragonally elongated cis-N4Cl2 coordination sphere. This complex provides a three-dimensional honey-comb-like structure through N–H� � �Cl, O–H� � �Cl and O–H� � �O hydrogen bonds. In the nitrate containingspecies, one of the two counteranions coordinates to the metal centre to provide an irregular N4O2 coor-dination sphere, while the other counteranion, with the help of a lattice water molecule, assembles a lad-der-like structure via N–H� � �O and bifurcated O–H� � �O,O hydrogen bonds. A one-dimensional polymericspecies has been formed when perchlorate is the counteranion. Here one of the two perchlorates acts as abridge between the metal centres that are in tetragonally elongated trans-N4O2 coordination spheres.This polymeric chain, together with the second perchlorate and a water molecule, form a ribbon-likestructure due to N–H� � �O and O–H� � �O hydrogen bonds.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Extended networks of transition metal complexes having fasci-nating architectures are of considerable contemporary interest dueto their potential applications in a vast range of research areas suchas catalysis, separation science, gas and energy storage, drug deliv-ery and molecule based optical and magnetic materials [1–12].Self-assembly of small and simple complex molecules via variousintermolecular non-covalent interactions involving suitable func-tionalities on the metal coordinated organic ligands and solventmolecules trapped in the crystal lattice is one of the powerful strat-egies for the design and preparation of such materials. We havebeen working on such species over the past several years[13–28]. Herein, we report three hydrated copper(II) complexeswith the N,N-donor 2-(1H-imidazol-2-yl)-pyridine (impy) and dif-ferent counteranions. The counteranions used are chloride, nitrateand perchlorate. The reason for the choice of this ligand is the N–Hfunctionality on the imidazolyl fragment which can participate as adonor in strong hydrogen bonds with the counteranions. Theanions are varied from monoatomic to triangular and tetrahedralspecies, having different metal coordinating and hydrogen bondacceptor abilities. Thus both the molecular as well as the hydro-

ll rights reserved.

x: +91 40 2301 2460.

gen-bonding assisted supramolecular structures of these speciesare anticipated to differ significantly. The synthesis and character-ization of these complexes and their molecular and supramolecularstructures, determined by X-ray crystallography, are described be-low.

HN

N N(impy)

2. Experimental

2.1. Materials

The ligand 2-(1H-imidazol-2-yl)-pyridine (impy) was preparedby following a reported procedure [29]. All other chemicals andsolvents used in this work were of analytical grade, available com-mercially and were used without further purification.

2.2. Physical measurements

Elemental (C, H, N) analysis data were obtained with the helpof a Thermo Finnigan Flash EA1112 series elemental analyzer.

Table 1Selected crystallographic data for [Cu(impy)2Cl2]�3H2O (1), [Cu(impy)2(NO3)]NO3�H2O(2) and [Cu(impy)2(ClO4)]ClO4�H2O (3).

Complex 1 2 3

Empirical formula CuC16H20N6O3Cl2 CuC16H16N8O7 CuC16H16N6O9Cl2

Formula weight(g mol�1)

478.82 495.91 570.79

Crystal system monoclinic triclinic monoclinicSpace group Cc P1 P21/c

a (Å) 17.975(3) 8.098(4) 7.233(3)b (Å) 12.166(2) 11.188(7) 26.683(7)

T. Ghosh et al. / Polyhedron 29 (2010) 3074–3080 3075

Solution electrical conductivities were measured using a DigisunDI-909 conductivity meter. A Sherwood scientific balance was usedto measure the magnetic susceptibilities with powdered samplesof the complexes. Diamagnetic corrections calculated from Pascal’sconstants [30] were used to obtain the molar paramagnetic suscep-tibilities. Infrared spectra were collected by using KBr pellets on aJasco-5300 FT-IR spectrophotometer. A Cary 100 Bio UV/Visspectrophotometer was used to record the electronic spectra. X-band EPR measurements were performed on a Jeol JES-FA200spectrometer.

c (Å) 9.4124(18) 12.059(7) 11.579(4)a (�) 90 102.53(3) 90b (�) 102.837(3) 95.675(15) 105.214(6)c (�) 90 110.75(4) 90V (Å3) 2006.9(7) 978.6(9) 2156.3(1)Z 4 2 4q (g cm�3) 1.585 1.683 1.758l (mm�1) 1.385 1.177 1.325Measured

reflections8827 5700 12,909

Unique reflections 3379 5700 3448Reflections

(I P 2r(I))2792 4350 2090

Parameters 271 295 313R1, wR2 (I P 2r(I)) 0.0564, 0.1409 0.0492,

0.13130.0757, 0.1782

R1, wR2 (all data) 0.0669, 0.1480 0.0674,0.1439

0.1282, 0.2072

Goodness-of-fit(GOF) on F2

1.011 1.041 0.963

Dqmax, Dqmin

(e �3)1.316, �0.507 0.676, �0.833 0.892, �0.363

2.3. Synthesis of [Cu(impy)2Cl2]�3H2O (1)

A methanol solution (15 ml) of CuCl2�2H2O (60 mg, 0.35 mmol)was added to a methanol solution (15 ml) of 2-(1H-imidazol-2-yl)-pyridine (impy) (100 mg, 0.70 mmol). The mixture was stirred atroom temperature in air for 4 h. The resulting green solution wasslowly evaporated to about 5 ml. The crystalline green complex,deposited in about 3–4 days, was collected by filtration. Yield130 mg (78%). Anal. Calc. for CuC16H20N6O3Cl2: C, 40.13; H, 4.21;N, 17.55. Found: C, 39.85; H, 4.03; N, 17.32%. leff. (lB) at 298 K:1.81. KM (ohm�1 cm2 mol�1) in CH3CN at 298 K: 23. UV–Vis inCH3CN: k (nm) (� (M�1 cm�1)) = 745 (65), 300 (19 800), 264(18 900), 218 (13 600). X-band EPR in C2H5OH–CH3OH (1:1) at130 K: g|| = 2.31, A|| = 133 � 10�4 cm�1 and g\ = 2.11.

2.4. [Cu(impy)2(NO3)]NO3�H2O (2)

This complex was synthesized in 70% yield by following thesame procedure as described for 1 using Cu(NO3)2�3H2O and2-(1H-imidazol-2-yl)-pyridine (impy) in a 1:2 mol ratio. Anal. Calc.for CuC16H16N8O7: C, 38.75; H, 3.25; N, 22.60. Found: C, 38.59; H,3.10; N, 22.44%. leff. (lB): 1.96. KM (ohm�1 cm2 mol�1) in CH3CNat 298 K: 224. UV–Vis in CH3CN: k (nm) (� (M�1 cm�1)) = 715(80), 302 (20 500), 260 (17 700), 222 (18 700). X-band EPR inC2H5OH–CH3OH (1:1) at 130 K: g|| = 2.29, A|| = 137 � 10�4 cm�1

and g\ = 2.09.

2.5. [Cu(impy)2(ClO4)]ClO4�H2O (3)

This complex was synthesized in 80% yield using the sameprocedure as described for 1 and 2 from Cu(ClO4)2�6H2O and2-(1H-imidazol-2-yl)-pyridine (impy) in a 1:2 mol ratio. Anal. Calc.for CuC16H16N6O9Cl2: C, 33.67; H, 2.83; N, 14.72. Found: C, 33.41;H, 2.57; N, 14.48%. leff. (lB): 1.82. KM (ohm�1 cm2 mol�1) in CH3CNat 298 K: 259. UV–Vis in CH3CN: k (nm) (� (M�1 cm�1)) = 730 (95),305 (21 300), 259 (19 200), 223 (17 900). X-band EPR in C2H5OH–CH3OH (1:1) at 130 K: g|| = 2.30, A|| = 135 � 10�4 cm�1 and g\ = 2.09.

2.6. X-ray crystallography

Single crystals of 1–3 were collected directly from the productsobtained during their syntheses. A Bruker-Nonius SMART APEXCCD single crystal diffractometer, equipped with a graphite mono-chromator and a Mo Ka fine-focus sealed tube (k = 0.71073 Å)operated at 2.0 kW was used to determine the unit cell parametersand to collect the intensity data at 298 K for both 1 and 3. Thedetector was placed at a distance of 6.0 cm from the crystal. Datawere collected at 298 K with a scan width of 0.3� in x and an expo-sure time of 10 s/frame. SMART software was used for data acqui-sition and SAINT-Plus software was used for data extraction [31].The absorption corrections were performed with the help of theSADABS program [32]. On the other hand, determination of the unitcell parameters and the intensity data collection for 2 were per-formed on an Enraf-Nonius Mach-3 single crystal diffractometer

using graphite monochromated Mo Ka radiation (k = 0.71073 Å)by the x-scan method at 298 K. Intensities of three check reflec-tions were measured after every 1.5 h during the data collectionto monitor the crystal stability. No decay was observed in the60 h of exposure to X-ray. An empirical absorption correctionwas performed based on w-scans [33] of selected reflections. Thecalculations for data reduction and absorption corrections weredone using the WinGX package [34]. The structures of all threecomplexes were solved by direct method and refined on F2 byfull-matrix least-squares procedures. The non-hydrogen atomswere refined using anisotropic thermal parameters. The hydrogenatoms of the lattice water molecules were located in the differencemap and refined with Uiso(H) = 1.5Uiso(O) and geometric restraintsto place them at idealized positions [35]. All the other hydrogenatoms were included in the structure factor calculations at ideal-ized positions by using a riding model. The SHELX-97 programs[36] available in the WinGX package [34] were used for structuresolution and refinement. The ORTEX6a [37] and Platon [38] pack-ages were used for molecular graphics. Selected crystal and refine-ment data are summarized in Table 1.

3. Results and discussion

3.1. Syntheses and characterization

The hydrated complexes [Cu(impy)2Cl2]�3H2O (1), [Cu(impy)2

(NO3)]NO3�H2O (2) and [Cu(impy)2(ClO4)]ClO4�H2O (3) have beensynthesized in 70–80% yields from two mole equivalents of 2-(1H-imidazol-2-yl)-pyridine (impy) and one mole equivalent ofthe corresponding hydrated copper(II) salts in methanol medium.The elemental (C, H, N) analyses data for the three species supporttheir molecular formulae. The effective magnetic moments of thecomplexes at 298 K in the powder phase are within 1.81–1.96 lB.These moments indicate the presence of one unpaired electron atthe metal centre, as expected for uncoupled copper(II) species.The electrical conductivities of the three complexes have been

3076 T. Ghosh et al. / Polyhedron 29 (2010) 3074–3080

measured in acetonitrile. The low molar conductivity (KM) value(23 ohm�1 cm2 mol�1) of 1 indicates that both chlorides are fairlywell coordinated to the metal centre. On the other hand, the KM

values of 2 and 3 are comparatively much larger. These are 224and 259 ohm�1 cm2 mol�1 for 2 and 3, respectively. These valuesare consistent with 1:2 electrolytic behavior [39] and indicate thatin solution the counteranions are not coordinated to the metal cen-tre in either of these two complexes.

3.2. Spectroscopic features

The infrared spectra of 1–3 display a broad band centred in therange 3400–3500 cm�1 due to water molecules. A comparativelysharper band observed within 3070–3170 cm�1 is attributed tothe N–H stretching [20,21,40] of the imidazole fragment of theligand impy. The strong band observed at 1312 cm�1 in the caseof 2 is most likely associated with the nitrate counteranion [41].The typical perchlorate stretches for 3 are observed as a verystrong broad band and a sharp band at 1080 and 620 cm�1,respectively.

The electronic spectra have been collected using acetonitrilesolutions of 1–3. The spectral profiles are very similar. A weakbroad band observed in the range 715–745 nm is followed by threevery strong bands at �300, �260 and �220 nm. The origin of theweak absorption is assigned to a d–d transition [42–48]. The strong

C2C3 C1

N6

C12

C11

C16C4

C14

C13

C10

N1

C15

C5N5

N4

C9

CuC6

N3

Cl2

N2

C8Cl1

C7

O3

N7O2

O1

C15

C16

N5

N6

Cu

C1C2 N1

C3

C14

C5C4N2C6

C7N3

C8

N4

C13

C9

C12

C10

C11

Fig. 1. Structures of cis-[Cu(impy)2Cl2] (top) and [Cu(impy)2(NO3)]+ (bottom) withthe atom labelling schemes. In both structures, all non-hydrogen atoms arerepresented by their 30% probability thermal ellipsoids.

absorptions in the UV region are most likely due to metal-to-ligandcharge transfer and intra-ligand transitions [42,44,45].

The EPR spectra of 1–3 in a frozen acetonitrile–toluene (1:1)mixture display an asymmetric signal near g � 2.1. In none of thesespectra is the g|| signal clearly visible, except for the two shouldersat the high-field half of the asymmetric signal. However, in a frozenmethanol–ethanol (1:1) mixture each of the three complexes dis-plays a well resolved axial spectrum having g|| � 2.3, A|| � 135 �10�4 cm�1 and g\ � 2.1. Such axial spectra are typical for copper(II)species having a square-based or a tetragonally elongated octahe-dral coordination geometry, where the unpaired electron resides inthe dx2�y2 orbital [49,50].

3.3. Molecular structures

The structures of the mononuclear [Cu(impy)2Cl2] and[Cu(impy)2(NO3)]+ are depicted in Fig. 1 and that of the polymeric[Cu(impy)2(ClO4)]+ is shown in Fig. 2. In each complex, the ligandimpy binds the metal centre through the pyridine and the imidaz-ole imine N-atoms, forming a five-membered chelate ring. It maybe noted that the X-ray structures of the free ligand [51] and its ir-on(II) [52] and mixed-anion copper(II) [53] complexes are known.The intra-ligand bond parameters in 1–3 are unexceptional andsimilar to the values observed in the reported structures men-tioned above. The bond parameters associated with the metal ionsin [Cu(impy)2Cl2], [Cu(impy)2(NO3)]+ and [Cu(impy)2(ClO4)]+ arelisted in Table 2. The metal to coordinating atom bond lengths inthese species are comparable with the bond lengths reported for

C8 C7

C10

C9C11

N3 N2N4

C12

C6 C13Cu

C14

O3

C5

N5

N6

N1

O1

C4

C15

Cl1

C16

O2

C1

O4

C3 C2

Fig. 2. Polymeric structure of [Cu(impy)2(ClO4)]+ with the atom labelling scheme.All non-hydrogen atoms are represented by their 30% probability thermalellipsoids.

Table 2Selected bond lengths (Å) and angles (�) for [Cu(impy)2Cl2]�3H2O (1), [Cu(impy)2

(NO3)]NO3�H2O (2) and [Cu(impy)2(ClO4)]ClO4�H2O (3).

[Cu(impy)2Cl2]�3H2O (1)Cu–N1 2.165(6) Cu–N2 1.963(4)Cu–N4 2.343(6) Cu–N5 1.983(5)Cu–Cl1 2.444(2) Cu–Cl2 2.718(2)N1–Cu–N2 79.0(2) N1–Cu–N4 90.62(19)N1–Cu–N5 93.0(2) N1–Cu–Cl1 171.56(14)N1–Cu–Cl2 86.88(15) N2–Cu–N4 95.5(2)N2–Cu–N5 168.9(3) N2–Cu–Cl1 92.65(17)N2–Cu–Cl2 94.89(16) N4–Cu–N5 76.81(19)N4–Cu–Cl1 91.52(14) N4–Cu–Cl2 168.67(13)N5–Cu–Cl1 95.47(14) N5–Cu–Cl2 92.28(15)Cl1–Cu–Cl2 92.56(6)

[Cu(impy)2(NO3)]NO3�H2O (2)Cu–N1 2.103(2) Cu–N2 1.965(2)Cu–N4 2.076(2) Cu–N5 1.945(2)Cu–O1 2.359(4) Cu–O2 2.474(4)N1–Cu–N2 81.00(9) N1–Cu–N4 131.41(9)N1–Cu–N5 97.56(9) N1–Cu–O1 129.26(9)N1–Cu–O2 79.66(11) N2–Cu–N4 100.11(9)N2–Cu–N5 178.51(8) N2–Cu–O1 88.58(10)N2–Cu–O2 87.65(11) N4–Cu–N5 81.11(9)N4–Cu–O1 99.25(9) N4–Cu–O2 148.64(10)N5–Cu–O1 92.06(11) N5–Cu–O2 91.73(11)O1–Cu–O2 50.24(11)

[Cu(impy)2(ClO4)]ClO4�H2O (3)Cu–N1 2.026(6) Cu–N2 1.963(5)Cu–N4 2.014(5) Cu–N5 1.949(5)Cu–O1 2.551(7) Cu–O4a 2.610(5)N1–Cu–N2 81.3(3) N1–Cu–N4 162.03(19)N1–Cu–N5 99.2(3) N1–Cu–O1 78.7(3)N1–Cu–O4a 86.86(19) N2–Cu–N4 101.6(2)N2–Cu–N5 162.42(19) N2–Cu–O1 95.8(3)N2–Cu–O4a 81.10(19) N4–Cu–N5 83.4(3)N4–Cu–O1 83.4(2) N4–Cu–O4a 111.10(18)N5–Cu–O1 101.5(3) N5–Cu–O4a 81.4(2)O1–Cu–O4a 165.5(2)

a Symmetry transformation used to generate equivalent atoms: x + 1, y, z.

T. Ghosh et al. / Polyhedron 29 (2010) 3074–3080 3077

copper(II) complexes having the same coordinating atoms [13–28,42–48,53,54].

Table 3Hydrogen-bonding parameters (Å and �) for [Cu(impy)2Cl2]�3H2O (1), [

Complex D–H� � �A

[Cu(impy)2Cl2]�3H2O (1) N3–H3A� � �Cl2a

N6–H6A� � �Cl1b

O1–H1A� � �Cl1O1–H1B� � �O2c

O2–H2A� � �Cl2O2–H2B� � �O1d

O3–H3B� � �Cl2O3–H3C� � �Cl1d

[Cu(impy)2(NO3)]NO3�H2O (2) N3–H3A� � �O7N6–H6A� � �O5O7–H7A� � �O1e

O7–H7A� � �O3e

O7–H7B� � �O4f

O7–H7B� � �O5f

[Cu(impy)2(ClO4)]ClO4�H2O (3) N3–H3A� � �O4g

N6–H6A� � �O9O9–H9A� � �O5O9–H9B� � �O8h

a x, �y + 1, z � 1/2.b x, �y + 2, z � 1/2.c x + 1/2, y � 1/2, z.d x � 1/2, �y + 3/2, z � 1/2.e �x, �y + 1, �z + 2.f x, y + 1, z + 1.g �x, �y + 1, �z + 1.h x�1, y, z.

In the chloride containing complex (1), the metal centre is in adistorted octahedral N4Cl2 coordination sphere owing to coordina-tion of both counteranions in a mutually cis orientation (Fig. 1). Thedihedral angle (88.9(1)�) between the two chelate ring planes isvery close to the ideal value of 90�. The metal to coordinating atombond lengths (Table 2) clearly indicate that there is a tetragonalelongation due to the Jahn–Teller effect commonly observed foroctahedral copper(II) complexes [54,55]. One of the chlorides(Cl2) and one of the two pyridine-N-atoms (N4) occupy the elon-gated axial sites (Fig. 1).

For complex 2 only one nitrate is coordinated to the metal cen-tre in an unsymmetrical (with respect to the unequal Cu–O bondlengths) bidentate fashion, forming a four membered chelate ring(Fig. 1). Thus the metal centre is in an irregular N4O2 coordinationsphere. The dihedral angle (49.3(1)�) between the two five-membered chelate rings in [Cu(impy)2(NO3)]+ is significantlysmaller than that in [Cu(impy)2Cl2]. To have a better understand-ing of the coordination geometry around the metal centre in[Cu(impy)2(NO3)]+, the N-atom (N7) of the nitrate instead of itstwo O-atoms (O1 and O2) has been taken into consideration. TheN5 sphere can be best described as distorted trigonal-bipyramidal.The nitrate and the two pyridine-N-atoms (N7, N1 and N4, respec-tively) form the trigonal plane and the two imidazole-N-atoms (N2and N5) occupy the axial sites (Fig. 1). The extent of distortion ofthis trigonal-bipyramidal N5 sphere towards a square-pyramidalgeometry can be evaluated by the s-value, defined as (b–a)/60where a is the smallest and b is the largest trans bond angle[49]. The values of s are zero and one for ideal square-pyramidaland trigonal-bipyramidal geometries, respectively. Consideringthe two impy ligands for the trans bond angles, the s-value for[Cu(impy)2(NO3)]+ is calculated as 0.79.

As in complex 2, only one of the two counteranions is coordi-nated to the metal centre in complex 3. However, here the coordi-nated counteranion acts as a bridge and provides a polymericstructure for [Cu(impy)2(ClO4)]+ (Fig. 2). Consequently, the metalions are in a tetragonally elongated octahedral trans-N4O2 coordi-nation sphere (Table 2). The elongation occurs along the O–Cu–Obonds. In the N4 equatorial plane, each of the pyridine-N and the

Cu(impy)2(NO3)]NO3�H2O (2) and [Cu(impy)2(ClO4)]ClO4�H2O (3).

d (H� � �A) d (D� � �A) < (DHA)

2.26 3.116(6) 174.62.34 3.194(5) 176.32.41(2) 3.350(6) 163(5)1.80(1) 2.755(7) 165(5)2.29(2) 3.243(5) 165(5)1.81(1) 2.758(7) 163(4)2.29(2) 3.199(13) 155(5)2.68(2) 3.402(14) 131(15)

1.90 2.759(3) 1721.92 2.781(4) 1742.18(2) 3.025(4) 146(3)2.04(2) 2.849(5) 142(3)2.18(2) 3.041(4) 150(3)2.15(2) 3.001(4) 149(3)

2.15 2.979(8) 1621.89 2.74(1) 1712.08(2) 2.98(2) 153(5)1.93(2) 2.83(2) 155(6)

3078 T. Ghosh et al. / Polyhedron 29 (2010) 3074–3080

imidazole-N-atom pairs are mutually trans to each other. However,the equatorial N-atoms do not form a perfect square-plane due tothe non-planarity of the two chelate rings formed by the two impymoieties. The dihedral angle (28.3(1)�) between the two chelatering planes in [Cu(impy)2(ClO4)]+ is significantly smaller than thatin [Cu(impy)2Cl2] and [Cu(impy)2(NO3)]+.

3.4. Supramolecular structures

Complexes 1–3 contain conventional hydrogen bond donors,such as water O–H and the impy N–H groups, and acceptors, suchas chloride (1) and O-atoms of the nitrate (2) and the perchlorate(3). In addition, the O-atoms of the water molecules can also actas hydrogen bond acceptors. We have therefore systematicallyinvestigated all the possible intermolecular hydrogen-bondinginteractions in 1–3 to reveal the resulting supramolecularstructures and their variations. Four different types of hydrogen-bonding are detected. These are N–H� � �Cl, N–H� � �O, O–H� � �Cl andO–H� � �O interactions. The structural parameters for these interac-tions are listed in Table 3 and the supramolecular structures thathave been formed by these hydrogen bonds are illustrated in Figs.3–5.

The imidazole N–H groups of both impy ligands in [Cu(impy)2

Cl2] are involved in hydrogen-bonding with the metal bound chlo-

Fig. 3. Two-dimensional ordering of cis-[Cu(impy)2Cl2] via N–H� � �Cl hydrogenbonds (top) and three-dimensional honeycomb-like structure of cis-[Cu(im-py)2Cl2]�3H2O (1) due to O–H� � �Cl and O–H� � �O hydrogen bonds between thetwo-dimensional layers (bottom).

Fig. 4. The ladder type structure of [Cu(impy)2(NO3)]NO3�H2O (2) assembled withthe help of N–H� � �O and bifurcated O–H� � �O,O hydrogen bonds.

rides of two adjacent molecules. These N–H� � �Cl interactions helpto form a two-dimensional layered structure (Fig. 3). Each of thethree water molecules present in the crystal lattice is involved intwo hydrogen bonds as a donor with the chlorides or with waterO-atoms (O1 and O2). Thus two of the three water moleculesparticipate in three hydrogen bonds (Table 3). These O–H� � �Cland O–H� � �O hydrogen bonds provide the links between the paral-lel layers of the N–H� � �Cl connected complex molecules andassemble a three-dimensional honeycomb-like structure (Fig. 3).

In the case of 2, the water molecule plays the most crucial roleto support and hold the supramolecular structure. Both H-atoms ofthe water molecule are involved in bifurcated hydrogen bonds(Table 3) with the two O-atoms of the uncoordinated nitrate (O4and O5) and that of the coordinated nitrate (O1 and O3). In addi-tion, the N–H groups of the two impy ligands are also hydrogenbonded to the O-atoms of the water molecule (O7) and the metaluncoordinated nitrate (O5). These N–H� � �O and bifurcated O–H� � �O,O hydrogen bonds lead to a one-dimensional ladder-likeassembly of 2 in the crystal lattice (Fig. 4). There is no other signif-icant interaction between the parallel ladders.

As in 2, only two types of hydrogen bonds are observed in thecase of 3. These are N–H� � �O and simple O–H� � �O bonds. We haveseen that the [Cu(impy)2(ClO4)]+ units form parallel polymericchains due to the bridging role of one of the two perchlorates in3 (Fig. 2). Two of these neighboring polymeric chains are connectedto each other by two complementary N–H� � �O hydrogen bondsinvolving one of the impy N–H groups and the metal coordinated

Fig. 5. The N–H� � �O and the O–H� � �O hydrogen bonds and the resulting ribbon-likestructure of [Cu(impy)2(ClO4)]ClO4�H2O (3).

T. Ghosh et al. / Polyhedron 29 (2010) 3074–3080 3079

perchlorate-O atom (O4). These N–H� � �O interactions lead to a lad-der-like structure of the [Cu(impy)2(ClO4)]+ units (Fig. 5). Each ofthese ladders is flanked on both sides by two chains each consti-tuted by alternate water molecules and the uncoordinated perchlo-rates. Here each water molecule is connected to the two adjacentperchlorates by O–H� � �O hydrogen bonds. The connections be-tween the water–perchlorate chains and the ladder of [Cu(impy)2

(ClO4)]+ are provided by the water O-atoms (O9) through N–H� � �O interactions involving the second N–H groups directedoutwards on the two sides of the ladder-like assembly of the[Cu(impy)2(ClO4)]+ units. As a result, a ribbon type structure of 3is formed (Fig. 5). There is no other significant interaction betweenthe parallel ribbons.

4. Conclusion

To explore the effect of the counteranion on the molecular aswell as intermolecular hydrogen bond aided supramolecular struc-tures, three ternary complexes of copper(II) have been synthesized.In these complexes, the N,N-donor 2-(1H-imidazol-2-yl)-pyridine(impy) is the primary ligand and the counteranions, namely chlo-ride, nitrate and perchlorate, are the ancillary ligands. The com-plexes [Cu(impy)2Cl2]�3H2O (1), [Cu(impy)2(NO3)]NO3�H2O (2)and [Cu(impy)2(ClO4)]ClO4�H2O (3) have been characterized bymicroanalytical, magnetic susceptibility and spectroscopic(IR, UV–Vis and EPR) measurements. X-ray structure determina-tions of all three complexes have been performed. The complexcontaining chloride is a neutral hexacoordinated species due tothe coordination of both counteranions, while the remaining twocomplexes are cationic species due the coordination of only oneof the two counteranions. [Cu(impy)2(ClO4)]+ is polymeric whilethe other two complexes are monomeric. The coordination geom-etries are tetragonally elongated octahedral in both 1 and 3, whileit is distorted trigonal-bipyramidal in 2, when considering theN-atom instead of the two O-atoms of the bidentate chelating ni-

trate for the description. Complex 1 has the simplest countera-nions, chlorides, and both of them are coordinated to the metalion, while 2 and 3 contain triangular and tetrahedral countera-nions, respectively and only one counteranion is bound to the me-tal centre in both. Despite that, 1 has the most complexsupramolecular assembly compared to that of 2 and 3. One ofthe reasons for this is that 1 is a trihydrate, whilst both of the othertwo complexes are monohydrates. In the case of 1, a honeycomb-like structure through N–H� � �Cl, O–H� � �Cl and O–H� � �O hydrogenbonds is observed. A much simpler one-dimensional ladder-likestructure is assembled in the case of 2, due to N–H� � �O and bifur-cated O–H� � �O,O hydrogen bonds. Complex 3 is very similar tocomplex 2 with respect to composition, but here the cationic partis a linear polymer. Here also the final supramolecular assemblyvia N–H� � �O and simple O–H� � �O hydrogen bonds is one-dimen-sional, but in this case it is like a ribbon.

5. Supplementary material

CCDC 778449, 778450 and 778451 contains the supplementarycrystallographic data for 1–3. These data can be obtained free ofcharge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, orfrom the Cambridge Crystallographic Data Centre, 12 Union Road,Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail:deposit@ccdc.cam.ac.uk.

Acknowledgements

Financial assistance received from the Department of Scienceand Technology (DST), New Delhi (Grant No. SR/S1/IC-10/2007) isgratefully acknowledged. T. Ghosh and S. Das thank the UniversityGrants Commission (UGC) and the Council of Scientific and Indus-trial Research, respectively for research fellowships. The X-raystructures were determined at the National Single Crystal Diffrac-tometer Facility, School of Chemistry, University of Hyderabad(established by the DST). We thank the UGC for the facilities pro-vided under the UPE and the CAS programs.

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