Journal of Molecular Structure 1075 (2014) 286–291

Contents lists available at ScienceDirect

Journal of Molecular Structure

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

Synthesis of 2D polymeric dicyanamide bridged hexa-coordinated Cu(II)complex: Structural characterization, spectral studies and TDDFTcalculation

http://dx.doi.org/10.1016/j.molstruc.2014.06.0800022-2860/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9804734352.E-mail address: saugata.konar@gmail.com (S. Konar).

Saugata Konar a,⇑, Urmila Saha a, Malay Dolai a, Sudipta Chatterjee b

a Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, Indiab Department of Chemistry, Serampore College, Serampore, Hooghly 712 201, India

h i g h l i g h t s

� 2D polymeric dicyanamide bridged hexa-coordinated Cu(II) complex has been synthesized.� X-ray crystallographic structure of Cu(II) complex has been performed.� DFT and TDDFT calculations have been performed to compare the experimental values.� EPR and CV studies have done on complex 1.

a r t i c l e i n f o

Article history:Received 29 April 2014Received in revised form 11 June 2014Accepted 25 June 2014Available online 3 July 2014

Keywords:Pyrazine based ligandCopper(II) complexDicyanamide bridged 2D polymerX-ray crystal structureDFT and TDDFT studies

a b s t r a c t

A rare 2D polymeric dicyanamide bridged hexa-coordinated copper(II) complex [Cu(L01)(l1,5-dca)2]n (1)(L01 = 2-carboxypyrazine) has been synthesized from the reaction of Cu(NO3)2�6H2O, 2-pyrazinecarbonit-rile (L1) and sodium dicyanamide (Nadca) in methanolic medium. Single crystal X-ray analysis revealsthat the complex has a 2D infinite zigzag chain structure in which copper(II) ions are bridged by singledicyanamide ligand in an end-to-end fashion. Such 2-carboxypyrazine can be obtained on the way ofmetal-assisted nitrile hydrolysis which well connected with Cu(NO3)2�6H2O and dicyanamide (dca) togive rare 2D Cu(II) polymeric complex due to the flexibility in the coordination ability of the copper(II)ions within the polymeric chain. The geometry of the asymmetric unit of the complex was optimizedin singlet state by DFT method with multilayer ONIOM model at doublet spin state accordance withrepeating asymmetric unit only. The electronic spectrum of the complex is explained using TDDFTcalculation.

� 2014 Elsevier B.V. All rights reserved.

Introduction tions such as nucleophilic or electrophilic additions to their car-

Diazines, the important nitrogen containing heterocycles aresix-membered aromatic rings with two nitrogen atoms. Pyrazinederivatives have been intensively studied because the 1,4-diazinecore is found in many natural and man-made compounds [1,2].Moreover numerous pyrazine derivatives exhibit a large range ofbiological activities and are used for pharmaceuticals or phytosan-itary applications [3–5]. Besides their medicinal uses, pyrazinederivatives have been extensively found in some technical applica-tions as dyes, electroluminescent materials and organic semi-con-ductors and as suitable ligands in coordination chemistry. Suchpyrazine based organonitriles play an important role in syntheticchemistry as versatile reactants that undergo various transforma-

bon–nitrogen triple bond. This property makes them attractivesubstrates in organic chemistry where they can be employed forthe novel creation of CAC, CAN, CAO, CAP or CAS bonds [6]. Inthe present study, the addition to organonitriles is activated bytheir coordination to a metal center. Such metal-mediatedreactions have recently been highlighted in general reviews onreactivity of RCN ligands [7,8]. On the example of pyrazinecarbo-nitrile (pz-CN) with Cu(II) in MeOH solution gave solid complexescontaining O-methylpyrazinecarboximidate (pz-C(NH)OMe) as aligand [9], these investigations have shown that the compoundswith bound 2-pyrazinecarboxylate can be obtained on the way ofmetal-assisted nitrile hydrolysis [10]. On the other hand, dicyan-amide (N(CN)2

�) (dca) is well known building block for producing2D framework owing to its flexibility and versatility incoordination modes [11]. The common bidentate bridging coordi-nation modes of dicyanamido ligand are the l1,5-N(CN)2 via two

Table 1Experimental data for crystallographic analysis of 1.

Compound 1

Empirical formula C9 H4 Cu N8 O2

Formula weight 319.75Temperature (K) 150(2)Wavelength (Å) 0.71073Crystal system MonoclinicSpace group P 21/cUnit cell dimensions a = 10.2074(3) Å

b = 12.3670(3) Åc = 13.4551(3) Åa = 90�, b = 131.1690(10), c = 90�

Volume (Å3) 1278.58(6)Z 4Density (calc) (Mg/m3) 1.661Absorption coefficient (mm�1) 1.723F (000) 636.0h Range (�) for data collection 2.6–31.1Index ranges �13 6 h 6 14

S. Konar et al. / Journal of Molecular Structure 1075 (2014) 286–291 287

terminal nitrogen atoms [12] and the l1,3-N(CN)2 through one ter-minal and central nitrogen atoms [13]. In this context, variability ofthe coordination modes of the dca ligand is particularly interesting,which allows the preparation of compounds with various kinds ofarchitectures: mononuclear, polynuclear, as well as 1D, 2D and 3Dnetworks [14]. The copper(II) ion often adopts a six-coordinatedstructure due to plasticity of its coordination geometry [15]. Quitea few 1D compounds have been reported in which pseudohalogensconnected with adjacent metal ions along the chain due to the flex-ibility in the coordination ability of the copper(II) ions [16,17]. Inorder to synthesize rare 2D dicyanamide bridged Cu(II) polymer,the key is the design and selection of metal, ligand and also bridg-ing ligand. The cyano ligand and dicyanamide play an importantrole in the design of such 2D polymer [18]. Based on those, wereport here the synthesis, spectroscopic analysis, X-ray crystallog-raphy and density functional theory of dicyanamide bridgedcopper(II) complex with 2-pyrazinecarbonitrile (L1) ligand.

�17 6 k 6 13�19 6 k 6 19

Goodness-of-fit on F2 1.139Completeness to theta = 25.00� 99.9%Independent reflections [Rint] 4106 [0.039]Refinement method Full-matrix least squares on F2

Data/restraints/parameters 4106/1/185Reflections collected 16,863Final R indices [I > 2r (I)] R1 = 0.0471

wR2 = 0.1755Largest difference peak and hole (e�3) �0.75 and 0.78

Table 2Selected bond distance (Å) and angle (�) data for 1 and their theoretical values are intoparentheses.

Bond type Distances (Å) Angle type Bond angles (�)

1Cu1AO1 1.967(3) [2.035] O1ACu1AN1 80.57(12) [80.929]Cu1AN1 2.045(4) [1.926] O1ACu1AN4 175.63(13) [142.563]Cu1AN4 1.957(4) [1.846] O1ACu1AN7 91.15(14) [109.613]Cu1AN7 1.963(5) [1.857] O1ACu1AN6a 91.77(10) [93.258]Cu1AN6a 2.801(4) [2.735] O1ACu1AN9b 94.28(11) [104.353]Cu1AN9b 2.574(4) [2.497] N1ACu1AN4 96.73(15) [112.214]

N1ACu1AN7 171.72(13) [156.324]N1ACu1AN6a 83.67(12) [82.321]N1ACu1AN9b 84.20(12) [90.047]N4ACu1AN7 91.55(17) [93.210]N4ACu1AN6a 84.49(10) [79.654]N4ACu1AN9b 88.85(11) [74.233]N6aACu1AN7 96.92(12) [94.526]N7ACu1AN9a 96.26(12) [97.224]N6aACu1AN9b 165.38(12) [168.457]

Translation of symmetry code to equiv. pos: a = 1 � x, �y, �z; b = x, 1/2 � y,�1/2 + z.

Experimental section

Materials and physical methods

All reagents and chemicals (including Cu(NO3)2�6H2O, sodiumdicyanamide and 2-pyrazinecarbonitrile) were of AR grade andprocured from commercial sources (SD Fine Chemicals, India;and Aldrich) and used without further purification. Solvents likemethanol (Merck, India) were of reagent grade and dried beforeuse.

Elemental analyses (carbon, hydrogen and nitrogen) of themetal complex were determined with a Perkin–Elmer CHN ana-lyzer 2400 at the Indian Association for the Cultivation of Science,Kolkata. The electronic spectra of the complex in DMF solutionwere recorded on a Schimadzu U-1200 spectro-photometer. IRspectra (KBr pellet, 300–4000 cm�1) were recorded on a Perkin–Elmer model 883 infrared spectrophotometer. EPR spectra wererecorded for solid samples on an X-band EPR-Spectrometer(Model: JEOL, JES-FA 200). Cyclic voltammetric study was carriedout using a computer controlled AUTOLAB (model 263A VERSAS-TAT) electrochemical instrument with Pt-tip as working electrodeat 25 �C, and Ag/AgCl reference electrode in DMF solution underpure nitrogen atmosphere with 0.1 (M) tetrabutylammoniumperchlorate [TBAP] as supporting electrolyte.

Synthesis of the complex

Preparation of complex [C9H4CuN8O2] (1)2-Pyrazinecarbonitrile (L1) (0.052 g, 0.5 mmol) was added to a

stirring 20 mL methanolic solution of Cu(NO3)2�6H2O (0.1475 g,0.5 mmol) to give a light blue solution. After 30 min, sodium dicy-anamide (0.178 g, 2.0 mmol) was added to get deep blue solutionwhich was stirred for additional 20 min. The resulting solutionwas filtered, and the filtrate was kept at room temperature, undis-turbed for slow evaporation. After few days, blue block-shapedcrystals suitable for X-ray diffraction of 1 were isolated. (Yield:61.5%). Anal. Calc. for C9H4CuN8O2: C, 33.77; H, 1.25; N, 35.02.Found: C, 33.71; H, 1.20; N, 35.09. IR (KBr, cm�1): 2280 (ms

C„N + mas), 2175 (ms C„N). UV–Vis in DMF [kmax nm (e M�1

cm�1)]: 298 (4.2 � 103 M�1 cm�1), 468 (8.7 � 102 M�1 cm�1).

X-ray crystallography study

Selected crystal data for 1 is given in Table 1 and selected met-rical parameter of the complex is given in Table 2. For complex 1data collections were made using Bruker SMART APEX II CCD areadetector equipped with graphite monochromated Mo Ka radiation

(k = 0.71073 Å) source in u and x scan mode at 150(2) K. Cellparameters refinement and data reduction were carried out usingthe Bruker SMART APEX II. The structure of all the complexes weresolved by conventional direct methods and refined by full-matrixleast square methods using F2 data. SHELXS-97 and SHELXL-97programs [19] were used for structure of all the complexes solu-tion and refinement respectively. For complex 1, nonhydrogenatoms were refined anisotropically till the convergence is attained.All the hydrogen atoms were placed in their geometrically ideal-ized positions and constrained to ride on their parent atoms.

Computational method

DFT study has proven to be an important tool to obtainbetter insights into the geometry and electronic structure of thesesystems. Becke’s hybrid function [20] with the Lee–Yang–Parr

Scheme 1. Schematic representation of ligand (L1) and its complex 1.

Fig. 1. The molecular view along with coordination environment of asymmetricdimeric unit of 1.

Fig. 2. (A) The 2D polymeric structure along the crystallographic bc plane anddimeric orientation of A and B type in 2D network in 1. (B) The probable inter-metallic distance through dimeric orientation of A and B type in 2D network in 1. Allhydrogen atoms are omitted for clarity.

Fig. 3. The topology of 2D network of complex 1 is of uninodal 3-c net with Schläfli

288 S. Konar et al. / Journal of Molecular Structure 1075 (2014) 286–291

(LYP) correlation function [21] was used through the study. Thegeometry of the complex 1 was fully optimized with multilayer ONI-OM model at doublet spin state accordance with repeating asym-metric unit only. On the basis of the optimized ground stategeometry, the absorption spectral properties in dimethyl formamide(DMF) media were calculated by time-dependent density functionaltheory (TDDFT) [22] approach associated with the conductor-likepolarisable continuum model (CPCM) [23]. We computed the lowest40 singlet – singlet transition and results of the TD calculations werequalitatively very similar. Due to the presence of electronic correlationin the TDDFT (B3LYP) method it can yield more accurate electronicexcitation energies. Hence TDDFT had been shown to provide a reason-able spectral feature for our complex of investigation.

The effective core potential (ECP) approximation of Hay andWadt was used for describing the (1s22s22p6) core electron for cop-per whereas the associated ‘‘double-n’’ quality basis sets were usedfor the valence shell. For H atoms we used 6-31G basis set; for C, Nand O atoms we employed 6-31G as basis set for the optimizationof the ground state. All the calculations were performed with theGaussian 09W software package [24]. Gauss Sum 2.1 program[25] was used to calculate the molecular orbital contributions fromgroups or atoms.

symbol {63}.

Result and discussion

Synthesis

The complex 1 was synthesized in situ reaction of 2-pyrazine-carbonitrile (L1), Cu(NO3)2�6H2O and sodium dicyanamide in

methanol medium to afford [Cu(L01)(l1,5-dca)2]n (1) (L01 = 2-carb-oxypyrazine) (Scheme 1). The complex 1 was obtained by mixingthe ligand (L1) with Cu(NO3)2�6H2O and sodium dicyanamide,taken in a 1:1:4 M ratio in methanolic solution. X-ray qualitycrystals of 1 were obtained upon slow evaporation of the filtered

Table 3Main calculated optical transition for the complex 1 with composition in terms of molecular orbital contribution of the transition, vertical excitation energies, and oscillatorstrength in dimethyl formamide (DMF).

Electronic transition Composition Excitation energy Oscillator strength (f) CI kexp (nm)

S0 ? S11 HOMO ? LUMO+1 2.4352 eV (289 nm) 0.1465 0.2563 298HOMO�3 ? LUMO 0.1558HOMO�4 ? LUMO+1 0.1368HOMO�1 ? LUMO 0.2359HOMO�2 ? LUMO+1 0.6821

S0 ? S20 HOMO�1 ? LUMO+4 3.8526 eV (466 nm) 0.0642 0.2034 468HOMO�1 ? LUMO+1 0.6355HOMO�2 ? LUMO+2 0.3671

Fig. 4. Energy diagrams of HOMO and LUMO orbital’s of 1, calculated at the DFTlevel using a B3LYP/6-31G basis set.

S. Konar et al. / Journal of Molecular Structure 1075 (2014) 286–291 289

reaction mixtures. It has been characterized by elemental analysis,IR and UV–Vis spectroscopy and single-crystal X-ray diffractionstudies.

1 For interpretation of color in Fig. 5, the reader is referred to the web version othis article.

Structural description of complex 1

The single crystal X-ray diffraction study reveals that complex 1is a 2D dicyanamide bridged Cu(II) polymer crystallizes in mono-clinic system with P21/c space group. The unit cell of 1 comprisesof only four molecules. As shown in Fig. 1, the asymmetric dimericunit of 1 is composed of two CuII centers, two 2-carboxypyrazine(L01), four dicyanamide (l1,5-N(CN)2) as the bridging ligands.

Now each Cu(II) is coordinated by one bi-dented ligand (L01),four dicyanamide in l2-g1:g5 (End to End) fashion and expand thisasymmetric unit into a 2D architecture along the crystallographicbc plane (Fig. 2A) via l2-g1:g5 (End to End) coordination modes.Each Cu-center is in octahedral geometry, coordinated by N1 andO1 from 2-carboxypyrazine, N4, N7 (l2-g1:g1) and N6, N9 (l2-g5:g5) from the four coordinated dicyanamide linkers. Hence,the basal planes are constituted by N1 and O1 from 2-carboxypyr-azine, N4, N7 of dca ligand and two axial positions are satisfied byN6, N9 of dca ligand. The central copper ion is in lie in basal plane.

The equatorial angles vary from 80.57� to 104.75� and thesemarked deviations from the ideal angle (90�) may be due to stericrequirements for adopting this geometry. The Cu(II)AN1/O1 bondlengths are fall in range 1.967(2)–2.045(2) Å along with CuAN(dicyanamide) distances fall in range 1.957(2)–2.801(2) Å. Nowthe Cu� � �Cu separation through the dicyanamide (EE) bridge is7.163 Å in dimer and 7.370 Å through (EE) bridging in the 2Dframework. In the 2D network, there are two types of dimeric unitswhich are designated by A and B. Now, two A and two B dimersconstitute a rhombus like framework where each corner of therhombus is being occupied by A and B alternatively. In this context,there are two different A to B distances namely 7.370 and 8.967 Å.Again, B to B and A to A Cu� � �Cu distances along the corners of therhombus are 11.241 and 11.987 Å respectively (Fig. 2B). The topol-ogy of 2D network suggests that 1 is of uninodal 3-c net withSchläfli symbol {63} (Fig. 3).

Geometry optimization of complex 1

The geometry of complex was optimized in the singlet groundstate. We have tabulated comparable experimental and theoreticalvalues of bond distances and bond angles in Table 3 and it is thegood agreement with experimental values. The energy differencebetween HOMO and LUMO is 1.564 eV (Fig. 4). The frontier molec-ular orbitals (FMOs) diagram with their respective positive andnegative regions of the optimization of 1 is shown in Fig. 5. Thepositive and negative phases are represented in blue1 and orangecolour, respectively.

UV–Vis spectrophotometric study

The electronic spectrum (Fig. 6) of 1 � 10�4 (M) solution ofcomplex 1 in dimethyl formamide (DMF), shows band at 298 nm(e = 4.2 � 103 M�1 cm�1) and 468 nm (e = 8.7 � 102 M�1 cm�1).The first one (298 nm) attributed to ligand to metal charge transfertransition (LMCT) and last one which of lower intensity bands at468 nm is due to d–d transition of Cu(II) center [26].

The experimentally observed absorption bands of the com-plexes have been explained with the help of TD-DFT calculations.The vertical excitation energies, oscillator strength and electronictransition and compositions of the transitions obtained at theTD-DFT level have been presented in Table 3. The electron densityat HOMO�2, HOMO�1, HOMO and LUMO+1 orbitals are mainlyreside on the dicyanamide moiety as well as metal center whilea considerable contribution comes from carboxylic moiety alongwith the contribution of pyrazine moiety in LUMO and LUMO+2orbitals. In complex 1, the HOMO�2, HOMO�1 and HOMO orbitalsare mainly originating from dicyanamide ligand p and p* orbitalcontribution while the LUMO+1 and LUMO+2 orbitals arises from

f

Fig. 5. Frontier molecular orbitals involved in the UV–Vis absorption of 1.

Fig. 6. Electronic spectra of 1 � 10�4 (M) of 1 in DMF.

Fig. 7. X-band EPR spectra of 1 at 77 K in the magnetic field range 100–200 mT insolid state.

290 S. Konar et al. / Journal of Molecular Structure 1075 (2014) 286–291

metal d orbital contribution along with pyrazine based ligand pand p* orbital contribution.

The complex shows two absorption bands at 298 and 468 nm inDMF solution at room temperature. The calculated absorptionbands are located at 289 and 466 nm for 1 which are in good agree-ment with experimental results of 298 and 468 nm (Table 3). Thisassignment was also supported by TDDFT calculations. These twoabsorption bands can be assigned to the S0–S11 and S0–S20 transi-tions, respectively.

EPR studies

The X-band EPR spectra of the copper(II) complex [Cu(L01)(l1,5-dca)2]n (1), as a representative one, were performed in solid state inliquid nitrogen atmosphere at 77 K. EPR spectrum of 1 in low tem-

perature is displayed in Fig. 7. The EPR spectrum in the solid stateat 77 K exhibits two signals one with g|| = 2.13, g\ = 1.85 and otheris a weak signal at g = 4.20 with DM � ±2.07 thus it informs thatthe asymmetric unit of the 2D network is Cu(II) monomer whichmay be then propagated to two dimensional frame.

Cyclic voltammetric study of complex 1

The electrochemical behavior of complex was studied by cyclicvoltammetry in the range +1.00 to �0.50 V at a scan rate100 mV s�1 in DMF at platinum electrode versus SCE using tetrabu-tyl-ammonium perchlorate (TBAP) as supporting electrolyte. Dicy-anamide ions being stronger electron donor [27] and behave as anoxidant thereby favour the oxidation of metal center in 1. Themetal center is oxidised first at Epa = 0.228 V and then reduced atEpc = 0.0418 V in a one electron redox system. The cyclicvoltammogram for Complex 1 was found (Fig. 8) to be quasi-

Fig. 8. CV of complex 1 in DMF solvent at 25 �C, [C] = 1.0 mM; [TBAP] = 0.10 M, scanrate = 100 mV s�1.

S. Konar et al. / Journal of Molecular Structure 1075 (2014) 286–291 291

reversible in nature with DE1/2 = 0.186 V as calculated from thebelow stated equation

DE1=2 ¼ jEpc � Epaj > 0:0591=n; where n ¼ 1

Conclusions

A rare dicyanamide bridge 2D polymeric hexa-coordinatedCu(II) complex has been synthesized and characterized by X-rayanalysis, UV–Vis, EPR and cyclic voltammetric studies. The topol-ogy of 2D network is found to be uninodal 3-c net [{63}]. The struc-tural and spectral parameters were further supported by DFT andTDDFT calculations which performed on the complex in gas phase.

Acknowledgements

S. Konar acknowledges the financial support provided byUniversity Grants Commission, India through the Dr. D.S. KothariPost Doctoral Fellowship (DSKPDF).

Appendix A. Supplementary data

CCDC 869807 contains the supplementary crystallographic datafor 1. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CambridgeCrystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.

References

[1] F. Buron, N. Plé, A. Turck, G. Queguiner, J. Org. Chem. 70 (2005) 2616–2621.[2] Y. Takahashi, Y. Iinuma, T. Kubota, M. Tsuda, M. Sekiguchi, Y. Mikami, Org. Lett.

13 (2011) 628–631.

[3] J.W. Corbett, M.R. Rauckhorst, F. Qian, R.L. Hoffman, C.S. Knauer, L.W.Fitzgerald, Bioorg. Med. Chem. Lett. 17 (2007) 6250–6256.

[4] M. Dolezal, P. Cmedlova, L. Palek, J. Vinsova, J. Kunes, V. Buchta, Eur. J. Med.Chem. 43 (2008) 1105–1113.

[5] P. Corona, A. Carta, M. Loriga, G. Vitale, G. Paglietti, Eur. J. Med. Chem. 44(2009) 1579–1591.

[6] V.Y. Kukushkin, A.J.L. Pombeiro, Chem. Rev. 102 (2002) 1771–1802.[7] V.Y. Kukushkin, A.J.L. Pombeiro, Inorg. Chim. Acta 358 (2005) 1–21.[8] A.J.L. Pombeiro, V.Y. Kukushkin, in: J.A. McCleverty, T.J. Meyer (Eds.),

Comprehensive Coordination Chemistry II, vol. 1, Elsevier, Oxford, 2004, p.639 (Chapter 1.34).

[9] P. Segl’a, M. Palicová, M. Koman, T. Glowiak, J. Coord. Chem. 51 (2000) 101–113.

[10] D. Matoga, J. Szklarzewicz, K. Lewinski, Polyhedron 27 (2008)2643–2649.

[11] I. Dasna, S. Golhen, L. Ouahab, O. Pen, N. Daro, J.P. Sutter, C. R. Acad. Sci., Ser.IIc: Chim. 4 (2001) 125.

[12] A. Escuer, F.A. Mautner, N. Sanz, R. Vicente, Inorg. Chem. 39 (2000)1668–1673.

[13] W.F. Yeung, S. Gao, W.T. Wong, T.C. Lau, New J. Chem. 26 (2002)523–525.

[14] S.R. Batten, K.S. Murray, Coord. Chem. Rev. 246 (2003) 103–130.[15] S. Ray, S. Konar, A. Jana, A. Dhara, K. Das, S. Chatterjee, M.S.E. Fallah, J. Ribas,

S.K. Kar, Polyhedron 68 (2014) 212–221.[16] S. Saha, S. Koner, J.-P. Tuchagues, A.K. Boudalis, K.-I. Okamoto, S. Banerjee, D.

Mal, Inorg. Chem. 44 (2005) 6379–6385. and references therein.[17] L. Zhang, L.-F. Tang, Z.-H. Wang, M. Du, M. Julve, F. Lloret, J.-T. Wang, Inorg.

Chem. 40 (2001) 3619–3622.[18] (a) S. Wang, X.-H. Ding, Y.-H. Li, W. Huang, Coord. Chem. Rev. 256 (2012) 439–

464;(b) S. Biswas, C.J.G. García, J.M.C. Juan, S. Benmansour, A. Ghosh, Inorg. Chem.53 (2014) 2441–2449;(c) C.-C. Zhao, W.-W. Ni, J. Tao, A.-L. Cui, H.-Z. Kou, Cryst. Eng. Commun. 11(2009) 632–637;(d) Q.-J. Zhou, X.-J. Yao, L.-F. Hao, Y. Ouyang, J.-Y. Xu, C.-Z. Xie, J.-S. Lou, Z.Anorg. Allg. Chem. 636 (2010) 2487–2491.

[19] G.M. Sheldrick, SHELXS-97 and SHELXL-97, University of Göttingen, Germany,1997.

[20] A.D. Becke, J. Chem. Phys. 98 (1993) 5648–5652.[21] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1998) 785–789.[22] (a) M.E. Casida, C. Jamoroski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108

(1998) 4439–4449;(b) R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218–8224;(c) R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454–464.

[23] (a) V. Barone, M. Cossi, J. Phys. Chem. A 102 (1998) 1995–2001;(b) M. Cossi, V. Barone, J. Chem. Phys. 115 (2001) 4708–4717;(c) M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comp. Chem. 24 (2003) 669–681.

[24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato,X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M.Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y.Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J.E. Peralta, F. Ogliaro,M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J.Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M.Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo,J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C.Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth,P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman,J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian 09, (Revision A.1), Gaussian Inc.,Wallingford, CT, 2009.

[25] N.M. O’Boyle, A.L. Tenderholt, K.M. Langner, J. Comp. Chem. 29 (2008) 839–845.

[26] C.-H. Ge, H.-Z. Kou, Z.-H. Ni, Y.-B. Jiang, A.-L. Cui, Inorg. Chim. Acta 359 (2006)541–547.

[27] E. Th, V. Kouwe, D.J. Muller, Platinum Met. Rev. 21 (1983) 26–27.