In situ hydrothermal syntheses, structures and photoluminescent properties of three coordination polymers based on 3-amidecarbonylpyrazine-2-carboxylic acid

Inorganica Chimica Acta 387 (2012) 52–57

Contents lists available at SciVerse ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

In situ hydrothermal syntheses, structures and photoluminescent properties of three coordination polymers based on 3-amidecarbonylpyrazine-2-carboxylic acid Sheng Zhang, Qing Wei ⇑, Gang Xie, Qi Yang, Sanping Chen ⇑ Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710069, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 22 November 2011 Received in revised form 24 December 2011 Accepted 26 December 2011 Available online 8 January 2012 Keywords: In situ hydrothermal synthesis 3-Amidecarbonylpyrazine-2-carboxylic acid Coordination polymer Photoluminescent

a b s t r a c t Three coordination polymers based on 3-amidecarbonylpyrazine-2-carboxylic acid (HPyzca), [Cd(Pzdc)(H2O)]n(H2O)n, [Pb3(Pzdc)3(H2O)]n, [Cu0.5(HPzdc)]n (H2Pzdc = 2,3-pyrazinedicarboxylic acid) have been in situ hydrothermally synthesized and characterized by single crystal X-ray diffraction analysis. The structural analysis revealed that HPyzca underwent hydrolysis to H2Pzdc under hydrothermal conditions. In compound 1, the Pzdc2 ligands link Cd(II) ions into a 2D architecture with the l4-g1:g1:g2:g2 coordination mode. In compound 2, the ligands Pzdc2 adopt l5-g1:g1:g2:g2:g2 and l4-g1:g2:g2:g2 coordination modes connecting metal centers to form a new (3,4,5)-connected 6-nodal net with {426}{4462}{43628}{456382}2{4862} topology symbol. In compound 3, the adjacent Cu(II) centers are connected by HPzdc to furnish polymeric chains along the [0 1 0] direction. These 1D arrays are further interlinked through Pzdc2 spacers to a 2D (4,4) coordination layer along the bc plane. Furthermore, thermal stabilities and photoluminescent properties of the compounds were also studied in the solid state. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In coordination and supramolecular chemical fields, metal– organic frameworks (MOFs) are receiving increasing attention owing to their potential applications in catalysis, gas absorption, non-linear optics, ion-exchange, luminescence, magnetism and other fields [1–5]. However, it is still a great challenge to rationally select ligands to construct coordination polymers with predicted configuration. Pyrazine, as a bridging ligand, is widely used in the design and construction of coordination polymers and low-dimensional materials [6–13]. Carboxylates have been also considered as an excellent coordination group, leading to the formation of mononuclear, dinuclear, polymeric or network compounds [14]. It is thus that the rigid ligands of multifunctional carboxylic acid with hetero atoms are becoming a better choice for preparing novel frameworks with good stability. To our knowledge, pyrazine carboxylic acid is a multifunction ligand with N/O-donors. This type of ligand could bind metal atoms in various coordination modes. During the past decades, 2,3-pyrazinedicarboxylic acid was employed to build numerous coordination compounds, and 13 kinds of coordination modes of ligand H2Pzdc were reported [3,15–29] (Scheme 1). However, the ligand amidecarbonylpyrazine-2-carboxylic acid has been scarcely studied so far [4,5]. ⇑ Corresponding authors. E-mail addresses: [email protected] (Q. Wei), [email protected] (S. Chen). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.12.051

In this paper, we reported three new coordination compounds, namely [Cd(Pzdc)(H2O)]n(H2O)n, [Pb3(Pzdc)3(H2O)]n and [Cu0.5(HPzdc)]n (H2Pzdc = 2,3-pyrazinedicarboxylic acid), which were in situ hydrothermally synthesized in the presence of 3-amidecarbonylpyrazine-2-carboxylic acid (HPyzca = 3-amidecarbonylpyrazine-2-carboxylic acid) and structurally determined by X-ray analysis. 2. Experimental 2.1. Materials and physical measurements All reagents were purchased commercially and used without further purification. Elemental analyses were carried out with an Elementar Vario EL III analyzer. IR spectra were recorded on a Bruker FTTR instrument with KBr pellets (4000–400 cm1). Thermogravimetric measurements were performed with a Netzsch STA449C apparatus under a static air atmosphere with a heating rate of 10 °C min1 from 30 to 900 °C. Photoluminescent spectra were measured using a Hitachi F-4500 Fluorescence Spectrometer for the solid powder samples under room temperature. 2.2. Synthesis 2.2.1. Preparation of [Cd(Pzdc)(H2O)]n(H2O)n (1) A mixture of CdCl22.5H2O (0.0228 g, 0.1 mmol), HPyzca (0.0167 g, 0.1 mmol) and H2O (6 mL) was sealed in a 10 mL

53

S. Zhang et al. / Inorganica Chimica Acta 387 (2012) 52–57

Scheme 1. The reported coordination modes of Pzdc2.

Teflon-lined stainless autoclave and heated at 140 °C under autogenous pressure for 3 days, then cooled at a rate of 5 °C h1 to room temperature and colorless block crystals were obtained. Yield: 0.0102 g, 61.08% based on HPyzca. Anal. Calc. for CdC6H6N2O6 (314.54): C, 22.9; N, 8.9; O, 30.5. Found: C, 23.1; N, 9.1; O, 30.7%. IR (KBr pellet, cm1) for 1: 3444(vs), 1574(vs), 1470(w), 1397(s), 1368(vs), 1172(w), 1125(m), 1067(w), 900(w), 847(w), 751(w).

program of the SHELXTL-97 package and refined with SHELXL. Crystallographic details are summarized in Table S1, selected bond lengths and angles of the compounds are shown in Table S2.

2.2.2. Preparation of [Pb3(Pzdc)3(H2O)]n (2) A mixture of PbCl26H2O (0.0278 g, 0.1 mmol), HPyzca (0.0167 g, 0.1 mmol) and H2O (6 mL) was sealed in a 10 mL Teflon-lined stainless autoclave and heated at 130 °C under autogenous pressure for 3 days, then cooled at a rate of 5 °C h1 to room temperature and colorless block crystals were obtained. Yield: 0.0099 g, 59.28% based on HPyzca. Anal. Calc. for Pb3C18H8N6O13 (1137.90): C, 19.0; N, 7.4; O, 18.3. Found: C, 18.8; N, 7.3; O, 18.4%. IR (KBr pellet, cm1): 3492(m), 1621(s), 1558(s), 1448(s), 1366(s), 1120(s), 891(m), 845(m), 837(m), 449(m).

Three compounds [Cd(Pzdc)(H2O)]n(H2O)n (1), [Pb3(Pzdc)3(H2O)]n (2) and [Cu0.5(HPzdc)]n (3) were in situ synthesized with the ligand of HPyzca. 1 and 3 display interesting 2D supramolecular network, while 2 shows a 3D network. As reported, there exhibited 13 kinds of coordination modes in the known compounds with the ligand H2Pzdc (Scheme 1) [29]. Herein, four different coordination modes of Pzdc2 (Scheme 2) are shown in the title compounds when HPyzca is employed as the ligand. In our experiments, the various molar ratios of the metal salts and ligand (1:2/2:1/2:3/ 3:2/3:5/5:3), and the different pH (pH 4–10) of the reaction system have been attempted under hydrothermal conditions. Structural analyses reveal that there are no others rather than the targets. Apparently, HPyzca underwent hydrolysis to H2Pzdc, as shown in Eq. (1).

2.2.3. Preparation of [Cu0.5(HPzdc)]n (3) A mixture of Cu(NO3)23H2O (0.0241 g, 0.1 mmol), HPyzca (0.0167 g, 0.1 mmol) and H2O (6 mL) was sealed in a 10 mL Teflon-lined stainless autoclave and heated at 130 °C under autogenous pressure for 3 days, then cooled at a rate of 5 °C h1 to room temperature and blue block crystals were obtained. Yield: 0.0132 g, 79.04% based on HPyzca. Anal. Calc. for CuC12H6N4O8 (397.76): C, 48.3; N, 14.1; O, 32.2. Found: C, 48.2; N, 13.9; O, 32.1%. IR (KBr pellet, cm1): 3375(m), 1632(s), 1585(s), 1471(s), 1340(s), 1126(s), 888(m), 832(m), 450(m).

3. Results and discussion 3.1. Synthesis

OH

OH

hydrothermal conditions

N

O

+ NH 2

N O

N

O

H2O

OH

+

NH3

N O

ð1Þ

2.3. X-ray crystal structure determination All diffraction data of compounds were collected on a Bruker/ Siemens Smart Apex II CCD diffractometer with graphite monochromated Mo Ka radiation (k = 0.71073 Å) at 293(2) K. Cell parameters were retrieved using SMART [30] software and refined using SAINTPLUS [31,32] for all observed reflections. Data reduction and correction for Lp and decay were performed using the SAINTPLUS software. Absorption corrections were applied using SADABS [33]. All structures were solved by the direct methods using the SHELXS

3.1.1. Crystal structure of 1 X-ray single-crystal structure determination reveals that 1 consists of one Cd(II) ion, one pzdc2 ligand, one coordinated and one free water molecules in a crystallographic unit (Fig. 1). The geometry around the metal center is close to a distorted pentagonal bipyramid, in which five oxygen atoms from four carboxylic groups (Cd1–O2 2.254(4) Å, Cd1–O3 2.426(4) Å, Cd1–O4 2.386(4) Å) locate at the equatorial position, one nitrogen atom from pzdc2 ligand (Cd1–N1 2.350(5) Å) and one water molecule (Cd1–O5

54

S. Zhang et al. / Inorganica Chimica Acta 387 (2012) 52–57

Scheme 2. The new coordination modes of Pzdc2 observed in this work.

2.254(4) Å) lie at the axial position. The ligands are completely deprotonated and exhibit the same coordination fashion (Scheme 2a). The adjacent Cd(II) atoms are linked through pzdc2 with chelating/bridging polydentate mode to 1D chains, which are further linked by pzdc2 with a chelating/bridging polydentate mode to a 2D plane, as shown in Fig 2. The pzdc2 ligands adopt the coordination mode l4-g1:g1:g2:g2 in compound 1. Compared with 1, in the reported compound [Cd(Pzdc)(H2O)2]n [34] with the ligand H2Pzdc, Pzdc2 presents a l3-mode (one carboxylate group bridges two Cd(II) atoms, and another carboxylate group and adjacent pyrazine nitrogen atom bridge other Cd(II) atoms). Moreover, each Cd(II) atom is connected with three Pzdc2 ligands. The coordination mode of Pzdc2 is shown in Scheme 2c and different from that in compound 1. 3.1.2. Crystal structure of 2 Compound 2 is a 3D compound containing three crystallographically independent Pb ions, as shown in Fig. 3. Pb1(II) center displays a strongly distorted monozapped trigonal bipyramidal geometry via coordinating to six O atoms (Pb1–O6 2.747(13) Å, Pb1–O7 2.669(12) Å, Pb1–O8 2.559(10) Å, Pb1–O10 2.555(9) Å, Pb1–O11 2.546(10) Å) and one N atom (Pb1–N3 2.631(6) Å) belonging to five Pzdc2 ligands. Pb2(II) adopts a slightly triplzapped trigonal bipyramidal configuration with nine oxygen atoms (Pb2–O2 2.653(11) Å, Pb2–O3 2.647(10) Å, Pb2–O4 2.696(10) Å, Pb2–O9 2.794(10) Å, Pb2–O10 2.559(10) Å, Pb2–O11 2.546(10) Å, Pb2–O12 2.646(9) Å) from different chelating and bridging carboxylic groups. Pb3(II) ion is six-coordinated with a distorted trigonal bipyramidal configuration. Each Pb3(II) ion is coordinated to two oxygen atoms (O5, O13) (Pb3–O5 2.417(10) Å, Pb3–O13 2.795(13) Å) and two nitrogen atoms (N1, N2) (Pb3-N1 2.640(6) Å, Pb3–N2 2.633(6) Å) and one coordination water in the equatorial position, which come from different carboxylic groups and one coordinated water molecular. In addition, two oxygen atoms (O4, O7) (Pb3–O4 2.475(11) Å, Pb3–O7 2.669(12) Å) are from two pzdc2 ligands at the axial position. As shown in Fig. 4a and b, these trinuclear Pb(II) are further connected by oxygen atoms of carboxylate groups (O1, O2, O3, O5, O6, O7, O8, O9 from ligands) to form an infinite rod-shaped secondary building units

Fig. 1. Coordination environment of Cd2+ in compound 1 (H atoms have been omitted for clarity).

Fig. 2. The two-dimensional layer in compound 1.

Fig. 3. Coordination environments of metal centers in compound 2 (H atoms have been omitted for clarity).

(SBUs). Moreover, these rod-shaped SBUs are linked into an extended 3D framework, as depicted in Fig. 4c and d. The coordination fashions of Pzdc2 are shown in Scheme 2b–d. In order to identify the complicated connectivity of the ligands and metal centers, the network topology of 2 is further analyzed. In 2, each Pb1 ion links five ligands and serves as a 5-connected node, and each Pb2 ion bridges five ligands and acts as a 5-connected node. While each Pb3 separately joins three ligands, which can be simplified as a 3-connected node. The topology of resulting 3D network 2 can be represented as a new (3,4,5)-connected 6-nodal net with {426}{4462}{43628}{456382}2{4862} topology symbol, as shown in Fig. 5.

3.1.3. Crystal structure of 3 The asymmetric unit of 3 shows one half of Cu(II) ion, one HPzdc ligand (Fig. 6). The half-occupied Cu(II) ion shows a similar

S. Zhang et al. / Inorganica Chimica Acta 387 (2012) 52–57

55

Fig. 4. (a) and (b) Ball-and-stick and polyhedral representation of triangular trinuclear Pb(II) unit. (c) The infinite rod-shaped inorganic SBU. (d) The polyhedral view of the 3D network. All the hydrogen atoms have been omitted for clarity. Color codes: blue for Pb1, Pb2 and Pb3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5. Schematic representation of the (3,4,5)-connected 6-nodal net of 2 with {426}{4462}{43628}{456382}2{4862} topology. Color codes: violet for 5-connected Pb1, purple for 5-connected Pb2, violet yellow for 3-connected Pb3. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

octahedral environment, which is attached to two HPzdc ligands via O1, N1 and O1A, N1A (Cu1–O1 1.958(3) Å, Cu1–O2 2.593(3) Å, Cu1–N1 1.961(4) Å) in the equatorial position and coordinated to two oxygen atoms (Cu1–O2 2.593(3) Å) from two HPzdc ligands at the axial position. The partly deprotonated Hpzdc employ a bidentate bridging/chelating coordination mode (Scheme 2e) to link the adjacent Cu(II) ions to form 1D chains, which are further interlinked through HPzdc spacers to generate a 2D (4,4) coordination layer along the bc plane, as shown in Fig. 7. However, in [Cu(HPzdc)Cl]n [35], the fully deprotonated Pzdc2 ligands employ the coordination mode in Scheme 2b. The N and O atoms of Pzdc2 ligand are coordinated to a copper atom while the

Fig. 6. Coordination environment of metal center in compound 3 (H atoms have

other N and O atoms of the same ligand are coordinated to an equivalent copper atom thus forming an infinite chain with the metal-ligand unit. There are two infinite chains in the compound that one propagates along the [0 1 1] axis, and another propagates  1] axis due to the symmetry. along the [0 1 3.2. Thermogravimetric analyses Thermogravimetric analyses for the compounds were performed with a NETZSCH STA 449C thermogravimetric instrument under a static air atmosphere with a heating rate of 10 °C min1. The TG profiles of compounds 1, 2 and 3 are shown in Fig. S1. The compound 1 displays three weight loss stages. The weight loss of 5.64% (calculated: 5.49%) of the first step (99.8–137.5 °C)

56

S. Zhang et al. / Inorganica Chimica Acta 387 (2012) 52–57

cent emission because similar emissions under the same conditions are observed for the free H2Pzdc at 463 nm [36]. It is clear that the coordination bonds significantly improve the luminescence intensity of H2Pzdc by enhancing the structural rigidity and reducing the fluorescence quenching effect. In addition, ligand-to-metal charge transfer may be partly attributed to the emission band at 400 nm. Therefore, the emission may be ascribed to the cooperative effects of intraligand emission and ligand emission and ligand-to-metal charge transfer (LMCT) [37,38]. Although more detailed theoretical and spectroscopic studies may be necessary for better understanding of the luminescent mechanism, the strong fluorescence emissions of these compounds endow them potentially useful photoactive materials. 4. Conclusion Fig. 7. The two-dimensional network in compound 3 along the bc plane.

In summary, we have synthesized three coordination polymers under in situ hydrothermal conditions and identified their single crystal structures. Compared with those in the previous described Cu(II), Cd(II) compounds constructed from H2Pzdc, in situ hydrothermal reaction involving HPyzca yields the distinct compounds 1 and 3 both in coordination modes and structures. Compound 2 presents a new (3,4,5)-connected 6-nodal net with {426}{4462}{43628}{456382}2{4862} topology symbol. The photoluminescent behaviors of 1 and 2 have been discussed. Acknowledgements The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 21173168) and the Natural Science Foundation of Shaanxi Provience (Nos. FF10091 and 11JS110). Fig. 8. Solid-state emission spectra of compounds 1 and 2 and H2Pzdc at room temperature. Red line for H2Pzdc, black line for 1 and blue line for 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Appendix A. Supplementary material

corresponds to the release of two guest water molecules per formula unit. The second step shows a weights loss of 5.64% within 137.5–225.7 °C, which closely matches the weight loss of 5.69% for removing one coordinated water molecule from the framework of 1. The decomposition of other residual components start beyond 287 °C with the total weight loss of 50.09% (calculated: 48.79%) at the third stage. The curve for compound 2 shows a mass loss of 60.15% (calculated: 58.71%). The curve for compound 3 shows a mass loss of 56.18% (the calculated is 55.70%). The remaining weight of compounds corresponds to the result of calculation for oxides. Obviously, thermodecomposition for these compounds would provide the very approaches to synthesis of oxide.

References

3.3. Fluorescent properties The solid-state fluorescent spectra of compounds 1 and 2 are depicted in Fig. 8. Upon excitation at ca. 330 nm for 1 and 2, these compounds exhibit intense fluorescence emission bands at ca. 395 and 490 nm for 1, and 415 and 463 nm for 2. In order to understand the nature of such emission bands, the luminescent properties of free ligand were also measured, upon excitation at ca. 330 nm for H2Pzdc, the free ligand H2Pzdc gives two broad emission bands from 338 to 433 nm with maximum at 400 nm, and from 433 to 481 nm with the maximum at 463 nm. Compound 1 exhibits an intense fluorescent emission peak at 463 nm. The emission can probably be assigned to intraligand (p–p⁄) fluores-

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.12.051.

[1] (a) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474; (b) D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375; (c) A.K. Cheetham, G. Ferey, T. Loiseau, Angew. Chem., Int. Ed. 38 (1999) 3268; (d) G.S. Papaefstathiou, L.R. MacGillivray, Coord. Chem. Rev. 246 (2003) 169; (e) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem., Int. Ed. 43 (2004) 2334; (f) D. Bradshaw, J.B. Claridge, E.J. Cussen, T.J. Prior, M.J. Rosseinsky, Acc. Chem. Res. 38 (2005) 273. [2] (a) W. Lin, O.R. Evans, R.G. Xiong, Z.J. Wang, Am. Chem. Soc. 120 (1998) 13272; (b) P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem., Int. Ed. 38 (1999) 2638; (c) K. Biradha, Y. Hongo, M. Fujita, Angew. Chem., Int. Ed. 41 (2002) 3395; (d) D.F. Sun, S.Q. Ma, Y.X. Ke, T.M. Petersen, H.C. Zhou, Chem. Commun. (2005) 2663. [3] F.-M. Wang, D. Xi’an, Northwest University, 2010. [4] R.H. Heyn, P.C.D. Ietzel, J. Coord. Chem. 60 (2007) 431. [5] U. Mukhopadhyay, R. Galian, I. Bernal, Inorg. Chim. Acta 362 (2009) 4237. [6] M. Inoue, M. Kubo, Coord. Chem. Rev. 21 (1976) 1. [7] J. Darriet, M.S. Haddad, E.N. Duesler, D.N. Hendrickson, Inorg. Chem. 18 (1979) 2679. [8] A.B. Blake, W.E. Hatfield, J. Chem. Soc., Dalton Trans. (1978) 868. [9] R.P. Eckberg, W.E. Hatfield, J. Chem. Soc., Dalton Trans. (1975) 616. [10] M. Inoue, M. Jubo, Coord. Chem. Rev. 21 (1976). [11] H.W. Richardson, W.E. Hatfield, J. Am. Chem. Soc. 98 (1976) 835. [12] H.W. Richardson, J.R. Wasson, W.E. Hatfield, Inorg. Chem. 16 (1977) 484. [13] A.B. Blake, W.E. Hatfield, J. Chem. Soc., Dalton Trans. (1978) 868. [14] G.B. Deacon, R.J. Phillips, Coord. Chem. Rev. 33 (1980) 251. [15] Y. Kubota, M. Takata, R. Matsuda, R. Kitaura, S. Kitagawa, T.C. Kobayashi, Angew. Chem., Int. Ed. 43 (2004) 2334. [16] O.Z. Yesilel, A. Mutlu, O. Buyukgungor, Polyhedron 27 (2008) 2471. [17] M.-J. Fang, M.-X. Li, X. He, J. Mol. Struct. 921 (2009) 137. [18] E.H. Hossein, N. Marek, A. Nafiseh, Acta Crystallogr., Sect. E 66 (2010) m1320.

S. Zhang et al. / Inorganica Chimica Acta 387 (2012) 52–57 [19] A. Hossein, A.G. Jafar, P. Mahdieh, D. Zohreh, Acta Crystallogr., Sect. E 66 (2009) m83. [20] E.H. Hossein, A. Nafiseh, M. Masoud, Acta Crystallogr., Sect. E 67 (2011) m266. [21] Y. Ma, Z.-B. Han, Y.-K. He, J. Coord. Chem. 61 (2008) 563. [22] Y. Ma, Y.-K. He, L.-T. Zhang, J. Chem. Crystallogr. 38 (2008) 267. [23] X.-M. Li, Y.-L. Niu, B. Liu, Chinese J. Struct. Chem. 28 (2009) 29. [24] L.-F. Chen, Z.-J. Li, Y.-Y. Qin, J. Mol. Struct. 892 (2008) 278. [25] D. Xie, J. Ye, Z. Liu, Inorg. Chem. Commun. 12 (2009) 72. [26] Z.-L. Xu, X.-Y. Li, G.-B. Che, Chinese J. Struct. Chem. 27 (2008) 593. [27] Y.-L. Niu, X.-M. Li, B. Liu, Chinese J. Struct. Chem. 29 (2010) 712. [28] E.H. Hossein, H. Azam, A. Nafiseh, J. Coord. Chem. 63 (2010) 3175. [29] T.-L. Che, Q.-C. Gao, W.-P. Zhang, Z.-X. Nan, H.-X. Li, Y.-G. Cai, J.-S. Zhao, Russ. J. Coord. Chem. 35 (2009) 723. [30] Bruker AXS, SMART, Version 5.0, Bruker AXS, Madison, WI, USA, 1998. [31] Bruker AXS, SAINT-PLUS, Version 6.0, Bruker AXS, Madison, WI, USA, 1999. [32] R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995) 33. [33] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.

57

[34] L.-F. Chen, Z.-J. Li, Y.-Y. Qin, J.-K. Cheng, Y.-G. Yao, J. Mol. Struct. 892 (2008) 278. [35] J.O. Charles, L.K. Cheryl, J.M. Richard, M.T. Louis, Inorg. Chem. 21 (1982) 64. [36] (a) S.-L. Zheng, J.-H. Yang, X.-L. Yu, X.-M. Chen, W.-T. Wong, Inorg. Chem. 43 (2004) 830; (b) D.-R. Xiao, E.-B. Wang, H.-Y. An, Y.-G. Li, L. Xu, Cryst. Growth Des. 7 (2007) 506. [37] R.J. Butcher, J.W. Overman, E. Sinn, J. Am. Chem. Soc. 102 (1980) 3276. [38] (a) L.-Y. Wang, Y. Yang, K. Liu, ; B.-L. Li, Y. Zhang, Cryst. Growth Des. 8 (2008) 3902; (b) T.-L. Hu, R.-Q. Zou, J.-R. Li, X.-H. Bu, Dalton Trans. (2008) 1302; (c) W.-L. Zhang, Y.-Y. Liu, J.-F. Ma, H. Jiang, J. Yang, Polyhedron 27 (2008) 3351; (d) F. Luo, Y.-X. Che, J.-M. Zheng, J. Coord. Chem. 61 (2008) 2097; (e) F. Guo, W. Yu, X.-L. Zhang, Chinese J. Struct. Chem. 27 (2008) 1123; (f) X.-L. Wang, Y.-F. Bi, H.-Y. Lin, G.-C. Liu, Cryst. Growth Des. 7 (2007) 1086; (g) M. Du, X.-J. Jiang, X.-J. Zhao, Inorg. Chem. 46 (2007) 3984; (h) E. Suresh, K. Boopalan, R.V. Jasra, M.M. Bhadbhade, Inorg. Chem. 40 (2001) 4078.