Inorganica Chimica Acta 366 (2011) 184–190

Contents lists available at ScienceDirect

Inorganica Chimica Acta

journal homepage: www.elsevier .com/locate / ica

One-dimensional ladder like and two-dimensional polymorphsof heterometallic thiocyanate bridged copper(II) and mercury(II) coordinationpolymer: Syntheses, structural, vibration, luminescence and EPR studies

Ali Akbar Khandar a,⇑, Axel Klein b, Akbar Bakhtiari a,b, Ali Reza Mahjoub c, Roland W.H. Pohl b

a Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, 5166614766 Tabriz, Iranb Institut für Anorganische Chemie, Universität zu Köln, Greinstraße 6, 50939 Köln, Germanyc Department of Chemistry, School of Science, Tarbiat Modares University, P.O. Box 14155-4838 Tehran, Iran

a r t i c l e i n f o

Article history:Received 21 July 2010Received in revised form 18 October 2010Accepted 26 October 2010Available online 11 November 2010

Keywords:Coordination polymersPolymorphsCopper(II)Mercury(II)ThiocyanateLuminescence

0020-1693/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.ica.2010.10.028

⇑ Corresponding author. Tel.: +98 411 3393128; faxE-mail address: akhandar@yahoo.com (A.A. Khand

a b s t r a c t

The 1D triclinic {CuHg(en)(l-NCS–N,S)4}n (1) and 2D monoclinic {CuHg(en)(l-NCS–N,S)3(SCN)}n (2)(en = ethylenediamine) heterometallic coordination polymers, as the two polymorphs of the{CuHg(en)(NCS)4}n, were synthesized at room temperature by the reaction of mercuric thiocyanate,potassium thiocyanate, copper(II) malonate and ethylenediamine using different reagent ratios. XRDon single crystals shows that the compound 1 consists of 1D ladder like zigzag ribbons extended alongthe b axis, whereas compound 2 shows a 2D wavy polymeric structure running parallel to the ab plane.In the crystal packing of the both polymorphs, the polymeric structures are further interlinked to eachother via weak interactions and hydrogen bonding to afford a 3D network. Diagnostic ligand and metal–ligand bands in the IR, far-IR and Raman spectra are assigned for the studied compounds. While com-pound 1 shows no significant emission upon excitation at any wavelength in the UV–Vis region,compound 2 exhibits intense emission at around 410 nm. Moreover, the room temperature X-bandEPR spectrum of a powdered sample of 1, shows a signal of rhombic symmetry with g1 = 2.2637,g2 = 2.0765 and g3 = 2.0483. In contrast to this, 2 reveals an axial signal with g\ = 2.0742; however,the g|| is unresolved.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Supramolecular isomerism [1,2], which is in close relation withcrystal polymorphism, is an important subject matter in the area ofsupramolecular chemistry and crystal engineering [3–6]. Its impor-tance becomes obvious considering the fact that bulk properties ofsolids are collective properties, depend on the convolution ofmolecular/ionic properties with lattice periodicity and thereforeon their supramolecular (crystal) structures [7]. However, theexamples of true supramolecular isomers are less and most ofthe time, pseudo-supramolecular isomers are obtained, in whichsolvent molecules are included in the crystal lattice of the coordi-nation polymers to satisfy the packing [8,9]. Examples of both trueand pseudo-supramolecular isomers have been reported by vari-ous groups [7,10–14]. Furthermore, with regard to the possiblecorrelation between structure and function [15,16], coordinationpolymers have became promising and attractive as the materialsof the future [17] with specifically tailored, useful properties, such

ll rights reserved.

: +98 411 3340191.ar).

as magnetism [18], electrical conductivity [19], non-linear opticalbehavior [20], luminescence [21], drug delivery [22], porosity[23] and gas storage [24].

At the same time, a boost on the study of transition metal poly-mers supported by pseudo-halides NCO [25,26], NCS [27,28], NCSe[29,30] and N3 [31–34] bridges has been given by the discovery ofa wide variety of multidimensional crystalline phases. This interestis based on not only the architectonical diversity and extreme ver-satility of their bridging modes, but also on their encouraging prop-erties and exceptional potential applications. In this context, thethiocyanate ligand with the ambidentate character of its versatilebridging coordination modes, is expected to afford a number ofhomo- and heterometallic discrete, one-, two and three-dimen-sional structural assemblies with specific structural features andoptical and magnetic properties [35–44]. Simultaneous presenceof two different metals might give rise to interesting physico-chemical properties and lead to attractive novel topologies andintriguing frameworks.

From the point of view of structural consistency, Cu(II) displaysa great flexibility of its coordination sphere, which makes difficul-ties in its incorporation into extended assemblies with predetermi-nated connectivity and topologies. Anions and counteranions have

A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190 185

been shown to have key effects on the structure of the finalproduct [45,46]. Malonates and mono protonated malonates inassociation with late 3d transition metals, have been demonstratedto provide a wealth of coordination modes to the M(II) ions andmany coordination polymers containing malonate ligands havebeen reported (see for example [47,48]). However, in the currentresearch work, we demonstrate that the malonate (as di-anion ofpropane dicarboxylic acid) is a potential candidate for crystalgrowth without entering the final product.

As a part of our ongoing studies in synthesis and structuralcharacterization of thiocyanate based transition metal coordina-tion polymers and also the crystal polymorphism, here we reportthe synthesis and crystal structures of (1) and (2), as the two poly-morphs of the {CuHg(en)(NCS)4}n. In both of the structures, assem-bly of the polymeric structure in the crystal packing via the weakinteractions results in 3D architectures. Moreover, the IR, far-IRand Raman, photoluminescence and EPR studies of 1 and 2 willbe discussed.

Table 1Crystal data and structure refinements for compounds 1 and 2.

1 2

Empirical formula C6H8CuHgN6S4 C6H8CuHgN6S4

M (g mol�1) 556.55 556.55T (K) 293(2) 293(2)Crystal system Triclinic MonoclinicSpace group P�1 P21/ca (Å) 7.2855(14) 7.0237(10)b (Å) 9.6695(17) 10.8513(9)c (Å) 11.881(2) 19.025(3)a (�) 104.269(14) 90.00b (�) 99.961(15) 96.792(11)c (�) 108.837(14) 90.00V (Å3) 738.0(2) 1439.8(3)Z 2 4Crystal size (mm3) 0.17 � 0.15 � 0.11 0.15 � 0.12 � 0.09F (0 0 0) 518 1036Dcalc (mg/m3) 2.504 2.567l (Mo Ka)/mm�1 12.384 12.696Reflections collected 7436 5994Independent reflections 4107 3207Number of parameters 160 166D(q) (e Å�3) 1.254 and �1.675 1.374 and �0.997Goodness-of-fit (GOF) 0.723 0.940Ra 0.0276, 0.0805b 0.0335, 0.0504b

wRa 0.0372, 0.0455b 0.0810, 0.0902b

a R = R||Fo| � |Fc||/R|Fo|, wR2 = [R(w(Fo2 � Fc

2)2/R(w(Fo2)2]1/2; [Fo > 4r(Fo)].

b Based on all data.

2. Experimental

2.1. Materials and methods

All chemicals were commercially purchased and used withoutfurther purification. Elemental analyses (C, H, N and S) were car-ried out on a Hekatech CHNS EuroEA 3000 Analyzer. Infrared (IR)spectra (4000–50 cm�1) were recorded using a Bruker IFS66mSspectrometer. Raman spectra were obtained from powder sampleswith a Bruker FRA106S spectrometer. Luminescence spectra for thesolid powder samples were recorded with a Spex FlouroMax-3 UV/Vis emission spectrophotometer. EPR spectra were recorded atroom temperature in the X band on a Bruker ELEXSYS 500E instru-ment. Single-crystal X-ray diffraction data were collected on a IPDSII (STOE and Cie.).

2.2. Syntheses

2.2.1. Syntheses of {CuHg(en)(l-NCS–N,S)4}n (1)To afford a solution of copper(II) malonate, a solution of cop-

per(II) acetate monohydrate (0.2995 g, 1.5 mmol) and malonic acid(0.1561 g, 1.5 mmol) in 40 ml of distilled water was heated whilestirring to ca. half volume. After the resultant solution was cooleddown to room temperature, ethylenediamine (0.10 ml, 1.5 mmol)was added while stirring. The obtained solution was added whilestirring at room temperature to a solution of mercuric thiocyanate(0.3168 g, 1 mmol) and potassium thiocyanate (0.0972 g, 1 mmol)in 40 ml of methanol/H2O (3:1, v/v). The resultant solution was fil-tered and left undisturbed at room temperature to give deep bluecrystals suitable for X-ray diffraction over days. (yield: ca. 0.388 g,93% based on Hg(SCN)2). Elemental analysis (%). Anal. Calc. forC6H8CuHgN6S4: C, 12.95; H, 1.45; N, 15.10; S, 23.04. Found: C,12.93; H, 1.32; N, 15.21; S, 23.30%.

2.2.2. Syntheses of {CuHg(en)(l-NCS–N,S)3(SCN)}n (2)To afford a solution of copper(II) malonate, a solution of cop-

per(II) acetate monohydrate (0.1997 g, 1 mmol) and malonic acid(0.1041 g, 1 mmol) in 40 ml of distilled water was heated whilestirring to ca. half volume. After the resultant solution was cooleddown to room temperature, ethylenediamine (0.07 ml, 1.04 mmol)was added while stirring. A solution of mercuric thiocyanate(0.3168 g, 1 mmol) and potassium thiocyanate (0.0972 g, 1 mmol)in 40 ml of methanol/H2O (3:1, v/v) was added to the obtainedsolution while stirring at room temperature. The solution becamecloudy by small amount of light blue precipitate, which was fil-tered and discarded. The resultant solution was left undisturbed

at room temperature which, result in blue crystals suitable for X-ray diffraction over days. (yield: ca. 0.326 g, 78% based onHg(SCN)2). Elemental analysis (%). Anal. Calc. for C6H8CuHgN6S4:C, 12.95; H, 1.45; N, 15.10; S, 23.04. Found: C, 12.95; H, 1.40; N,15.12; S, 22.97%.

2.3. X-ray data collection, structure solutions and refinements

Suitable crystals of 1 and 2 were isolated as described aboveand mounted in sealed glass capillaries on a Stoe IPDS II singlecrystal diffractometer (T = 293(2) K, graphite-monochromatizedMo Ka radiation k = 7.1073 nm). For data collection and reductionthe STOE program package [49] was applied. The structures weresolved using WINGX suite [50] by direct methods (SHELXS97) [51]and refined using full-matrix least squares techniques usingSHELXL97 [51]. All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were included by using appropriate ridingmodels. A summary of crystal data and refinement results for 1and 2 are provided in Table 1.

3. Results and discussion

3.1. Description of crystal structures

The ORTEP diagram with atom numbering scheme of 1 and 2are shown in Figs. 1 and 2, respectively. Selected bond lengthsand angles in the crystal structure of 1 and 2 are given in Table 2.Also the weak interactions and hydrogen bonds of 1 and 2 are gi-ven in Table S1 in supplementary material. The asymmetric unitsof the crystals of compounds 1 and 2 contain one Cu(en)2+ and alsoHg(SCN)4

2� building blocks (Figs. 1 and 2) linked by l1,3-SCNbridges to afford 1D ladder like zigzag ribbons along the b axis in1 (Fig. 1) and 2D wavy polymeric arrays running parallel to abplane in 2 (Fig. 3).

Each Hg atom in these crystals is four coordinated, HgS4, in adistorted tetrahedral environment surrounded by four thiocya-nate-sulfur atoms. The Hg–S bond lengths vary in the range of2.4946(14)–2.6930(17) Å in 1 and from 2.446(2) to 2.646(2) Å in

Fig. 1. Fragment of the ladder like polymeric structure of 1 with the numbering scheme. Hydrogen atoms are omitted for clarity. Atom displacement ellipsoids are shown atthe 50% probability level. One of the [CuHg(l-NCS–N,S)2], first and second [Cu2Hg2(l-NCS–N,S)4] macrocycles in the ladder like structure of 1 are shown by A, B and C labels,respectively.

Fig. 2. The coordination environment of Hg and Cu atoms in the crystal structure of2 with the numbering scheme. Atom displacement ellipsoids are shown at the 50%probability level.

Table 2Selected bond lengths (Å) and angles (�) for 1 and 2.

Compound 1Bond lengthsHg1–S1 2.4946(14) Hg1–S2 2.5049(16)Hg1–S3 2.6930(17) Hg1–S4 2.5789(16)Cu1–N1 2.627(6) Cu1–N2 1.961(5)Cu1–N3 2.003(6) Cu1–N4 2.401(5)Cu1–N5 2.010(4) Cu1–N6 2.007(5)

Bond anglesS1–Hg1–S2 123.67(5) S1–Hg1–S4 109.62(5)S2–Hg1–S4 114.20(5) S1–Hg1–S3 104.80(5)S2–Hg1–S3 99.31(6) S4–Hg1–S3 101.59(5)N1–Cu1–N4 169.7(2) N2–Cu1–N5 173.0(2)N3–Cu1–N6 173.6(2) N2–Cu1–N3 90.7(2)N3–Cu1–N5 90.8(2) N5–Cu1–N6 83.9(2)N6–Cu1–N2 94.15(19) N1–Cu1–N2 92.6(2)N1–Cu1–N3 91.8(2) N1–Cu1–N5 80.5(2)N1–Cu1–N6 83.8(2) N4–Cu1–N2 93.56(19)N4–Cu1–N3 96.3(2) N4–Cu1–N5 93.09(18)N4–Cu1–N6 87.5(2)

Compound 2Bond lengthsHg1–S1 2.446(2) Hg1–S2 2.523(2)Hg1–S3 2.646(2) Hg1–S4 2.606(2)Cu1–N2 2.282(7) Cu1–N3 1.984(6)Cu1–N4 1.998(7) Cu1–N5 2.013(7)Cu1–N6 2.026(6)

Bond anglesS1–Hg1–S2 126.15(8) S1–Hg1–S4 113.35(7)S2–Hg1–S4 102.31(7) S1–Hg1–S3 108.15(8)S2–Hg1–S3 101.05(7) S4–Hg1–S3 103.07(6)N3–Cu1–N6 168.2(3) N4–Cu1–N5 165.8(3)N3–Cu1–N4 93.7(3) N3–Cu1–N5 91.3(3)N4–Cu1–N6 89.5(3) N5–Cu1–N6 83.1(3)N3–Cu1–N2 95.5(3) N4–Cu1–N2 93.6(3)N5–Cu1–N2 99.1(3) N6–Cu1–N2 95.7(3)

186 A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190

2. The S–Hg–S bond angles vary between 99.31(6)� and 123.67(5)�in 1 and between 101.05(7)� and 126.15(8)� in 2. All of the thiocy-anate groups around the Hg atoms are N-bonded to Cu atoms in 1.In contrast, one of the four thiocyanate groups on Hg atoms re-mains as the monodentate terminal group in 2.

In the structure of 1, each copper atom is six coordinated ina highly distorted 4+1+1 elongated octahedral environment(Fig. 1), coordinated by two N atoms from one bidentate ethylene-diamine ligand and two N atoms from thiocyanate bridging groupsin the equatorial positions. Also, two N atoms from thiocyanatebridging groups occupy the 5th and 6th coordination sites in theaxial positions. In the equatorial positions, the Cu–N bond lengthsare 2.007(5) and 2.010(4) Å for ethylenediamine-nitrogen donorset and are 1.961(5) and 2.003(6) Å for thiocyanate-nitrogenatoms. The axial Cu–N bond lengths are 2.401(5) and 2.627(6) Å.

In contrast to 1, in the structure of 2, each copper atom is fivecoordinated in a square pyramidal environment, coordinated totwo N atoms from ethylenediamine ligand in the basal positionsand to three N atoms from three thiocyanate bridging groups.The Cu–N bond lengths are 2.013(7) and 2.026(6) Å for ethylenedi-amine-nitrogens and 1.984(6) and 1.998(7) Å for thiocyanate-

nitrogen atoms in the basal positions and 2.282(7) Å for the apicalnitrogen atom. The apical Cu–N bond length is significantly shorterthan those found for axial Cu–N bonds in 1. The N–Cu–N bond an-gles around the copper atoms in 2 are in the range of 93.6(3)–99.1(3)� between the apical and basal bonds and in the range of83.1(3)–93.7(3)� and 165.8(3)–168.2(3)� between the cis and transbasal bonds, respectively.

The thiocyanate ligands are in an approximately linear modewith the S–C–N bond angles vary between 176.0(6)� and179.3(6)� in the compound 1 and between 176.1(8)� and177.6(7)� in the compound 2. The N–C and C–S bond lengths of

Fig. 3. (a) The 2D wavy polymeric layers in the crystal structure of 2. One of the [Cu3Hg3(l-NCS–N,S)6] macrocycles is presented in ball and stick style. (b) Projection of thecrystal packing down the a-axis showing the wavy polymeric layers. The ethylenediamine ligands, as well as the terminal thiocyanate groups are omitted for clarity.

A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190 187

thiocyanate in 1 and 2, respectively, are in the range of 1.140(7)–1.154(7) Å [and 1.146(11)–1.157(9) Å] and 1.639(6)–1.659(7) Å[and 1.642(8)–1.659(9) Å].

In the crystal structure of 1, as shown in Fig. 1, the overall lad-der like polymeric ribbon includes three types of macrocycles, con-sidered as tertiary building units (TBUs) [52]. Bridging of copperand mercury atoms by the S(1)C(1)N(1) and S(3)C(3)N(3) groups,results in [CuHg(l-NCS–N,S)2] macrocycles. The next class of mac-rocycles, is of chair-like [Cu2Hg2(l-NCS–N,S)4] formed by linking oftwo Cu and Hg atoms by a pair of S(2)C(2)N(2) and S(4)C(4)N(4)groups. In the resultant macrocycles, the atoms are related to eachother by the action of a center of symmetry. By sharing of the link-ing S(1)C(1)N(1) or S(3)C(3)N(3) and S(4)C(4)N(4) thiocyanategroups from the first and second macrocycles, the second[Cu2Hg2(l-NCS–N,S)4] type macrocycles are formed.

Looking at the crystal packing of 1, reveals further S� � �S weakinteractions (contacts longer than the sum of covalent radii andshorter than the sum of vdW radii) and S� � �H hydrogen bonds inthe structure (see Fig. S1 and Table S1 in supplementary material).Each S(3) is in weak interaction with S(1) atom from the adjacentribbon (S(1)� � �S(3) = 3.274(2) Å), resulting in a six membered{[Hg(S)2]2} ring that adopts a chair-like conformation. The corre-sponding Hg� � �Hg distance (4.510(1) Å) is comparable with thesum of the vdW radii of Hg atoms (2.23 Å) [53]. By viewing thecrystal packing along the b axis, linking of the 1D ladder like poly-mers by the mentioned S(1)� � �S(3) contacts leads to a 2D stair-likearray. Taking these 2D arrays into account, two additional TBUs as14 membered {[Cu(NCS)2]2} and also the 18 membered{[CuHg(S)(l-NCS–N,S)(NCS)]2} supramolecular macrocycles areidentified in the structure (see Fig. 4).

Each S(3) atom is further in short contact with one S(4) atom(S(3)� � �S(4) distance of 3.520(3) Å) from the neighboring 2D arrayto afford a 3D network structure. By introduction of S(3)� � �S(4)interactions, two additional supramolecular macrocycles of 14membered {[Cu(NCS)2]2} and 20 membered {[CuHg2(S)2(l-NCS–N,S)2][CuHg(S)(l-NCS–N,S)(NCS)]} are appeared in the structure,

which result in cage like motifs in the crystal packing (see Fig. 4cand d). Moreover, as shown in Fig. 4c, 8 membered {[Hg(S)2(S)]2}supramolecular macrocycles are generated by S(4)� � �S(3)� � �S(1)(angle of 66.75(5)�) weak interactions. As shown in Fig. S2 in sup-plementary material, the 1D polymeric ribbons are also linked to-gether via S(2)� � �H(6A) hydrogen bonds (S(2)� � �H(6A) distance of2.961(2) Å and N(6)–H(6A)� � �S(2) angle of 152.0(3)�) to afford dou-ble layered 2D sheets.

Compared to the 1D ribbons in 1, the 2D wavy polymeric struc-ture in 2 consists of 24 membered [Cu3Hg3(l-NCS–N,S)6] macrocy-clic tertiary building units (Fig. 3). In the resultant macrocycles, theshortest Cu� � �Hg distance (4.346(1) Å) is comparable with the sumof the vdW radii reported for Hg (2.23 Å) and Cu (1.96 Å) atoms[53]. In the 2D polymeric arrays, each S(4) atom from one macro-cyclic unit is in short contact with the N(4) atom from the macro-cycle in the vicinity (S(4)� � �N(4) distance: 3.328(7) Å). Moreover,S(1) atoms from the terminal thiocyanate groups are in short con-tact with C(4) atoms, with the distances of 3.456(8) Å within thesame layer.

The infinite 2D polymeric layers are further linked in a supra-molecular three-dimensional framework by hydrogen bondingand weak interactions (Fig. S3 in supplementary material). The ter-minal thiocyanate groups from adjacent layers are in short contactto each other. The respective S(1)� � �N(1) and C(1)� � �C(1) distancesare 3.276(8) and 3.25(1) Å respectively. Also, the N(1) and C(1)atoms are involved in hydrogen bonding interactions with the eth-ylenediamine ligands in the neighboring polymeric layer. Finally,weak S(2)� � �S(2) and S(2)� � �C(1) interactions between the layerscontribute to stabilize the weak 3D network. However, the weakinteractions in the crystal packing of 2, do not result in the supra-molecular macrocycles of the type observed in 1.

3.2. Vibrational spectroscopy

The far-IR and Raman spectra of 1 and 2 are given in Figs. S4 andS5 in supplementary material, respectively. The IR spectrum of 1

Fig. 4. The weak S� � �S interactions in the crystal packing of 1. (a) Projection down the b-axis showing the 2D stair-like array resultant by S(1)� � �S(3) weak interactions. (b) Oneof the supramolecular motifs of {[Hg(S)2]2}, {[Cu(NCS)2]2} and {[CuHg(S)(l-NCS–N,S)(NCS)]2} are shown in ball and stick style. (c) The {[Cu(NCS)2]2} and {[Hg(S)2(S)]2}supramolecular macrocycles. (d) Cage like structures in the crystal packing. One of the {[CuHg2(S)2(l-NCS–N,S)2][CuHg(S)(l-NCS–N,S)(NCS)]} macrocycles is represented inball and stick style. The hydrogen atoms in (a) and (b) and ethylenediamine ligands in (d) are omitted for clarity.

188 A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190

exhibits a strong peak at 2172 cm�1 and also very strong absorp-tion bands at 2132, 2122 and 2105 cm�1 which are in agreementwith the generally employed criterion that m(C„N) P 2100 cm�1

indicates a thiocyanate bridge with a 1,3-l or 1,1,3-l bridgingmode [54–56]. Compared to the 1, compound 2 also exhibits thesame bands (2171, 2123, 2104 cm�1) except the band at2132 cm�1. The compound 2, also exhibits a strong shoulder at2078 cm�1 due to the m(C„N) of the terminal thiocyanate groupsin the structure. In comparison, the respective C„N stretchingmodes are observed at 2170 and 2154 cm�1 for {Cu[Hg(SCN)4]}n

[57–59], at 2134 and 2124 cm�1 for K2[Hg(SCN)4] [57] and at2053 cm�1 for KSCN [57,60]. In the Raman spectrum of 1, theC„N stretching is observed as very strong peaks at 2166, 2133,2119 and 2103 cm�1, whereas it appears as a broad strong absorp-tion centered at 2134 cm�1 with shoulders at 2164 and 2121 cm�1

in the case of compound 2.The very weak absorption bands at 780 and 766 cm�1 and at

782 and 765 cm�1 in the IR spectrum of 1 and 2 respectively, areassigned to m(C–S), compared to those occurred at 795 cm�1 inthe case of {Cu[Hg(SCN)4]}n [57–59] and at 746 cm�1 for KSCN[57,60]. The bending vibration modes of the SCN groups of 1 and2 were observed as weak peaks at 473, 439 and 437 cm�1. The cor-responding 2d(SCN) occurred at 948 and 878 cm�1 for 1 and at 951and 878 cm�1 for 2. For comparison, the corresponding d(SCN)vibration modes of {Cu[Hg(SCN)4]}n are observed at 459 and437 cm�1 [57–59]. This is while, the d(SCN) bands are observedat 486 and 471 cm�1 in the case of KSCN [57,60].

In the Raman spectrum of 1, the m(C–S) bands (782 cm�1 with ashoulder at 767 cm�1) show no significant shift compared to thoseobserved in the IR. The d(SCN) vibration mode is observed at473 cm�1, while the peaks of 439 and 437 cm�1 in the IR spectrumare disappeared. In contrast, the Raman spectrum of 2, exhibits asplit overlapping set of very weak m(C–S) absorption bands at780, 766, 755 and 742 cm�1. The corresponding bending vibrationmodes of SCN groups are observed at 473 cm�1 with shoulders at458, 442 and 430 cm�1.

In the far-IR spectra of 1 and 2, respectively, the two very weakbands at 311 and 281 cm�1 and at 312 and 283 cm�1 are assignedto the Cu–Nthiocyanate stretches, comparable to the bands observedat 318 and 282 cm�1 for {Cu[Hg(SCN)4]}n [57–59]. The band ob-served at 371 cm�1 in 1 and at 372 cm�1 in 2 is attributable tothe Cu–Nen vibration. Moreover, the broad weak bands centeredat 212 cm�1 in 1 and at 210 cm�1 in 2 are assigned to the Hg–Sstretching mode. The corresponding m(Hg–S) vibration mode in{Mn[Hg(SCN)4]}n, {Fe[Hg(SCN)4]}n, {Co[Hg(SCN)4]}n and{Zn[Hg(SCN)4]}n, are observed at 213, 216, 219 and 217 cm�1

respectively [59].In the Raman spectra of 1, the very weak m(Cu–Nthiocyanate),

m(Cu–Nen) and weak m(Hg–S) vibration modes were found to occurat 307, 373 and 220 cm�1 respectively. In the Raman spectra of 2,however, the m(Cu–Nthiocyanate) and m(Cu–Nen) are disappeared andthe corresponding m(Hg–S) stretching is observed as a weakabsorption at 221 cm�1.

A remarkable spectroscopic feature of the ethylenediamine li-gand in the IR spectra is the bending vibration mode of NH2, whichappears at 1573 cm�1 in 1 and at 1575 and 1539 cm�1 in 2. Theband at 1539 cm�1 is disappeared in the Raman spectrum of 2and a new band is observed at 1593 cm�1.

3.3. Optical properties and EPR data

Recently, Liu et al. have shown that coordination polymers con-taining halogeno and thiocyanato ligands and the d10 metal ionsAg(I) and Cu(I) exhibit efficient luminescence [38]. The origin ofthe luminescence behavior of the compounds was exploredthrough DFT-calculations and mainly ascribed to origin from an ex-cited metal-to-ligand charge transfer (MLCT) where the electronsare promoted from metal nd states (VBs) to the unoccupied p*orbital of thiocyanate groups (S-3p, Cthiocyanate-2p and Nthiocyanate-2p states, CBs). In the current study, compound 1 does not showsignificant emission upon excitation at any wavelength in theUV–Vis region. However, compound 2 exhibits very intense

Fig. 5. The excitation (left) and emission (right) spectra of 2.

Fig. 6. The EPR spectrum of 1 (top) and 2 (bottom).

A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190 189

emission at around 410 nm with discernible maxima at 403, 413and 423 nm, upon excitation at 302 nm. Fig. 5 shows the excitationand emission spectra of compound 2.

The existence of Cu(II) centers in 1 and 2 is also supported bythe EPR data. The room temperature X-band EPR spectrum of com-pounds 1 and 2 are shown in Fig. 6. The X-band EPR spectrum of

powdered 1, shows a signal of rhombic symmetry withg1 = 2.2637, g2 = 2.0765 and g3 = 2.0483. On the other hand, theX-band EPR spectrum of the powdered 2 reveals an axial signalwith anisotropic g\ = 2.0742, however, g|| is cannot be resolvedunequivocally. While the rhombic symmetry of the spectrum of 1indicates a distortion of the assumed tetragonal symmetry, the ax-ial spectrum of 2 is completely in line with a regular square pyra-midal geometry.

4. Conclusions

This paper reports on the polymorphic nature of the heterome-tallic coordination polymer {CuHg(en)(NCS)4}n. The reaction of the[Hg(SCN)3]� with the [Cu(mal)(en)]2+ species (mal = malonate)presumably generated in solution in different molar ratios, resultsin variation of the final (malonate-free) coordination polymers.Probably the intrinsic chelating behaviour of the malonate ionsto Cu(II) and possibly also to Hg(II) plays a key role in the forma-tion of the final structures.

X-ray crystal structures show that the triclinic {CuHg(en)(l-NCS–N,S)4}n polymorph (1) consist of a 1D ladder like zigzag ribbonextended along the b axis, whereas the monoclinic {CuHg(en)(l-NCS–N,S)3(SCN)}n polymorph (2) shows a 2D wavy polymericstructure running parallel to the ab plane. In the crystal packingof 1, the 1D polymeric structures are linked mainly via S� � �S weakinteractions leading to the 3D structure. This is while, the 3Dsupramolecular structure of 2, is mainly afforded by the packingof terminal thiocyanate groups which are in short contact to eachother.

Interestingly, compound 2 exhibits an intense and rather nar-row emission at room temperature in the UV–Vis region; whereassimilar optical property was not observed in the case of 1. More-over, X band EPR spectra recorded at ambient temperature onpowdered samples of 1 and 2 exhibit very different signal shape.While 1 reveals a rhombic spectrum indicative for a distortion ofthe tetragonal symmetry, 2 shows a weakly resolved axialspectrum.

Acknowledgments

This study has been supported by the Council of the Universityof Tabriz, Iran, and the University of Cologne, Germany. Theauthors thank Dr. Ingo Pantenburg and Mrs. Ingrid Müller for theirhelp with X-ray single crystal data collection, Katharina Butsch forher helps in crystal structure analysis, as well as Andreas Shürenfor EPR measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2010.10.028.

References

[1] T.L. Hennigar, D.C. MacQuarrie, P. Losier, R.D. Rogers, M.J. Zaworotko, Angew.Chem., Int. Ed. 36 (1997) 972.

[2] B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629.[3] J.-M. Lehn, Supramolecular Chemistry Concepts and Perspectives, VCH,

Weinheim, Germany, 1995.[4] F.S. Delgado, F. Lahoz, F. Lloret, M. Julve, C. Ruiz-Pérez, Cryst. Growth Des. 8

(2008) 3219.[5] C.B. Aakeröy, A.M. Beatty, in: J.A. McCleverty, T.J. Meyer, A.B.P. Lever (Eds.),

Comprehensive Coordination Chemistry-II: From Biology to Nanotechnology,vol. 1, Elsevier, Amsterdam, 2003, pp. 679–688.

[6] J. Chem. Soc., Dalton Trans. 37053998 (2000). Entire issue devoted to DaltonDiscussion 3: Inorganic Crystal Engineering.

[7] D.K. Kumar, A. Das, P. Dastidar, Cryst. Growth Des. 7 (2007) 2096.[8] J.-P. Zhang, Y.-Y. Lin, X.-C. Huang, X.-M. Chen, Dalton Trans. (2005) 3681.

190 A.A. Khandar et al. / Inorganica Chimica Acta 366 (2011) 184–190

[9] S. Masaoka, D. Tanaka, Y. Nakanishi, S. Kitagawa, Angew. Chem., Int. Ed. 43(2004) 2530.

[10] X.C. Huang, J.P. Zhang, X.M. Chen, J. Am. Chem. Soc. 126 (2004) 13218.[11] H. Abourahma, B. Moulton, V. Kravtsov, M.J. Zaworotko, J. Am. Chem. Soc. 124

(2002) 9990.[12] S.A. Barnett, A.J. Blake, N.R. Champness, C. Wilson, Dalton Trans. (2005)

3852.[13] A.M. Chippindale, S.M. Cheyne, S.J. Hibble, Angew. Chem., Int. Ed. 44 (2005)

7942.[14] A. Galet, M.C. Muñoz, A.B. Gaspar, J.A. Real, Inorg. Chem. 44 (2005) 8749.[15] T.K. Maji, R. Matsuda, S. Kitagawa, Nat. Mater. 6 (2007) 142.[16] D. Bradshaw, J.B. Claridge, E.J. Cussen, M.J. Rosseinsky, Acc. Chem. Res. 38

(2005) 273.[17] B.F. Abrahams, A. Hawley, M.G. Haywood, T.A. Hudson, R. Robson, D.A. Slizys, J.

Am. Chem. Soc. 126 (2004) 2894.[18] S. Hu, L. Yun, Y.-Z. Zheng, Y.-H. Lan, A.K. Powell, M.-L. Tong, Dalton Trans.

(2009) 1897.[19] M. Eddaoudi, J. Kim, D. Vodak, A. Sudik, J. Wachter, M. O’Keeffe, O.M. Yaghi,

PNAS 99 (2002) 4900.[20] L. Carlucci, G. Ciani, D. Proserpio, Coord. Chem. Rev. 246 (2003) 247.[21] Y. Ling, L. Zhang, J. Li, S.-S. Fan, M. Du, Cryst. Eng. Commun. 12 (2010) 604.[22] P. Horcajada, C. Serre, M. Vallet-Regi, M. Sebban, F. Taulelle, G. Ferey, Angew.

Chem., Int. Ed. 45 (2006) 5974.[23] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375.[24] S. Kitagawa, R. Matsuda, Coord. Chem. Rev. 251 (2007) 2490.[25] A.K. Boudalis, J.M. Clemente-Juan, F. Dahan, J.P. Tuchgues, Inorg. Chem. 43

(2004) 1574.[26] P. Talukder, A. Datta, S. Mitra, G. Rosair, M. Salah, M.S. El Fallah, J. Ribas, Dalton

Trans. (2004) 4161.[27] F.A. Mautner, R. Vicente, S.S. Massoud, Polyhedron 25 (2006) 1673.[28] K.-L. Zhang, W. Chen, Y. Xu, Z. Wang, Z.J. Zhong, X.-Z. You, Polyhedron 20

(2001) 2033.[29] A. Beheshti, W. Clegg, R. Hyvadi, H.F. Hekmat, Polyhedron 21 (2002) 1547.[30] R. Vicente, A. Escuer, J. Ribas, X. Solans, M. Font-Bardia, Inorg. Chem. 32 (1993)

6117.[31] J.-P. Zhao, B.-W. Hu, E.C. Sañudo, Q. Yang, Y.-F. Zeng, X.-H. Bu, Inorg. Chem. 48

(2009) 2482.[32] T.C. Stamatatos, G.S. Papaefstathiou, L.R. MacGillivray, A. Escuer, R. Vicente, E.

Ruiz, S.P. Perlepes, Inorg. Chem. 46 (2007) 8843.[33] A. Das, G.M. Rosair, Inorg. Chem. 45 (2006) 3301.[34] A.K. Ghosh, D. Ghoshal, E. Zangrando, J. Ribas, N. Ray Chaudhuri, Inorg. Chem.

44 (2005) 1786.[35] S. Paul, R. Clérac, N.G.R. Hearns, D. Ray, Cryst. Growth Des. 9 (2009) 4032.

[36] G.A. Bowmaker, C. Pakawatchai, S. Saithong, B.W. Skelton, A.H. White, DaltonTrans. (2009) 2588.

[37] Y.-Y. Niu, B.-L. Wu, X.-L. Guo, Y.-L. Song, X.-C. Liu, H.-Y. Zhang, H.-W. Hou, C.-Y.Niu, S.-W. Ng, Cryst. Growth Des. 8 (2008) 2393.

[38] X. Liu, K.-L. Huang, G.-M. Liang, M.-S. Wang, G.-C. Guo, Cryst. Eng. Commun. 11(2009) 1615.

[39] M. Fourmigué, P. Auban-Senzier, Inorg. Chem. 47 (2008) 9979.[40] H.-Y. Bie, J.-H. Yu, J.-Q. Xu, J. Lu, Y. Li, X.-B. Cui, X. Zhang, Y.-H. Sun, L.-Y. Pan, J.

Mol. Struct. 660 (2003) 107.[41] H. Zhang, D.E. Zelmon, G.E. Price, B.K. Teo, Inorg. Chem. 39 (2000) 1868.[42] M. Wriedt, S. Sellmer, C. Näther, Inorg. Chem. 48 (2009) 6896.[43] O.V. Nesterova, S.R. Petrusenko, V.N. Kokozay, B.W. Skelton, J. Jezierska, W.

Linert, A. Ozarowski, Dalton Trans. (2008) 1431.[44] J.-M. Shi, Y.-M. Sun, Z. Liu, L.-D. Liu, W. Shi, P. Cheng, Dalton Trans. (2006) 376.[45] Y.-Q. Huang, X.-Q. Zhao, W. Shi, W.-Y. Liu, Z.-L. Chen, P. Cheng, D.-Z. Liao, S.-P.

Yan, Cryst. Growth Des. 8 (2008) 3652.[46] D.-L. Long, R.J. Hill, A.J. Blake, N.R. Champness, P. Hubberstery, C. Wilson, M.

Schröder, Chem.-Eur. J. 11 (2005) 1384.[47] J. Pasán, F.S. Delgado, Y. Rodríguez-Martín, M. Hernández-Molina, C. Ruiz-

Pérez, J. Sanchiz, F. Lloret, M. Julve, Polyhedron 22 (2003) 2143.[48] C. Ruiz-Pérez, Y. Rodríguez-Martín, M. Hernández-Molina, F.S. Delgado, J.

Pasán, J. Sanchiz, F. Lloret, M. Julve, Polyhedron 22 (2003) 2111.[49] Stoe, IPDS manual, Stoe & Cie GmbH, Germany; X-Red 1.22, Stoe Data

Reduction Program, Stoe & Cie GmbH, Germany, 2001.[50] (a) WinGX Version 1.70.00, An Integrated System of Windows Programs for

the Solution, Refinement and Analysis of Single Crystal X-ray Diffraction Data,Department of Chemistry, University of Glasgow, 1997–2002.;(b) L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837.

[51] G.M. Sheldrick, SHELX-97, Programs for Crystal Structure Analysis, Institüt fürAnorganishe Chemie der Universität, Göttingen, Germany, 1998.

[52] M. Casarin, C. Corvaja, C. Di Nicola, D. Falcomer, L. Franco, M. Monari, L.Pandolfo, C. Pettinari, F. Piccinelli, Inorg. Chem. 44 (2005) 6265.

[53] S.-Z. Hu, Z.-H. Zhou, B.E. Robertson, Z. Kristallogr. 224 (2009) 375.[54] L. Shen, Y.-Z. Xu, J. Chem. Soc., Dalton Trans. (2001) 3413.[55] A. Golub, H. Kohler, V. Skopenko, Chemistry of Pseudohalides, Elsevier Science,

Amsterdam, 1986.[56] G.A.V. Albada, R.A.G. DeDraaff, G.A. Haasnoot, J. Reedjik, Inorg. Chem. 23

(1984) 1404.[57] R.A. Bailey, S.L. Kozak, T.W. Michelsen, W.N. Mills, Coord. Chem. Rev. 6 (1971)

407.[58] A. Tramer, J. Chem. Phys. 59 (1962) 232.[59] D. Forster, D.M.L. Goodgame, Inorg. Chem. 4 (1965) 715.[60] P.O. Kinell, B. Strandberg, Acta Chem. Scand. 13 (1959) 1607.