Available online at www.sciencedirect.com

www.elsevier.com/locate/poly

Polyhedron 27 (2008) 1248–1255

Design and synthesis of coordination networks containingamide, pyridine and carboxylate functionalities

Lalit Rajput, Kumar Biradha *

Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India

Received 23 October 2007; accepted 16 December 2007Available online 20 February 2008

Abstract

The reactions of bis(pyridinecarboxamido)alkane with copper(II) in the presence of mono or dicarboxylic acids resulted in discretespecies, one-dimensional and two-dimensional networks. The carboxylates considered for this study include m-nitrobenzoic acid, iso-phthalic acid and succinic acid. In the presence of m-nitrobenzoic acid the ligands with Cu(II) form 1D coordination networks whichinclude m-nitrobenzoic acid and water molecules as guests. The use of isophthalic acid resulted in discrete species while the use of succinicacid resulted in a two-dimensional layer containing rectangular grids of dimension 9.7 � 16.5 A2. The 2D layers in this complex exhibitinclined 2-fold interpenetration. Further, all these coordination networks are assembled via hydrogen bonding interactions between theamides and water molecules. The Cu(II) centre exhibits a unique octahedral coordination geometry, for the complexes reported here, as itcoordinates with two each of pyridine moieties, water molecules and carboxylates.� 2008 Elsevier Ltd. All rights reserved.

Keywords: Coordination polymers; One-dimensional networks; Two-dimensional networks

1. Introduction

The design and synthesis of metal-organic framework(MOF) materials is an attractive area of research owingto their potential functional properties and interestingstructural features [1]. The majority of MOFs have beenprepared by utilizing the coordinative ability of pyridineor carboxylate functional groups with transition metalatoms. Both these functional groups exhibit versatility incoordination modes which results in various networks withexotic topologies for a given ligand and transition metalatom [2]. Currently, the linking of metal ions by two ormore types of ligands within the structure has been awell-employed strategy to generate functional MOFs.Accordingly, there are several studies available in designingnetworks with mixed ligands containing pyridine and–COOH functional groups [3]. Recently, we have shownthat bis(pyridinecarboxamido)alkane ligands are capable

0277-5387/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2007.12.027

* Corresponding author. Tel.: +91 3222 283346; fax: +91 3222 282252.E-mail address: Kbiradha@chem.iitkgp.ernet.in (K. Biradha).

of forming various coordination networks with Cu(II)metal salts [4]. The amide groups of these ligands are foundto assemble the coordination networks into higher aggre-gates via amide-to-amide hydrogen bonds (Scheme 1). Inthe present work, we report the coordination networks ofCu(II) complexes containing bis(3-pyridinecarboxami-do)alkanes (L1, L2 and L3) and carboxylate anions. Thecarboxylic acids used for this purpose are a mono carbox-ylic acid (3-nitrobenzoic acid, HNBA) and dicarboxylicacids (succinic acid, H2SA and isophthalic acid, H2IPA).A database search on pyridine containing coordinationpolymers shows that there has not been a single MOFreported thus far containing the NBA moiety [5]. On theother hand, both the diacids are well-known linkers andhave been extensively used together with 4,40-bipyridineto prepare MOFs containing mixed ligands [6].

2. Results and discussion

The treatment of L1, L2 or L3 in the presence of acids(HNBA/H2IPA/H2SA) with Cu(II) resulted in complexes

N

O

N

H

XN

H

O

N

L1: X= -(CH2)2-; L2: X= -(CH2)4-; L3: X= -(CH 2)6-

O

N

H

XN

H

O

O

N

H

XN

H

O

β-sheet

Scheme 1. Exo bidentate ligands with a bis(amido)alkane spacer; b-sheet.

Table 2Bond lengths around the Cu(II) centre in the crystal structures of 1–5

Complex 1 2 3 4 5

Cu–O(carboxylate) 1.961(3) 1.974(2) 1.954(3) 1.980(3) 1.960(3)Cu–N(pyridine) 2.043(5) 2.030(2) 2.020(4) 2.043(3) 2.052(4)Cu–O(water) 2.680(5) 2.466(2) 2.676(5) 2.494(4) 2.519(4)

Table 3Hydrogen bonds formed between water and amide functionalities in thecrystal structures of 1–5

D–H� � �A D� � �A (A) D–H� � �A (�)

1 N(21)–H(21)� � �O(1W)a 2.997(3) 165

O(1W)� � �O(2W)b 2.700(4)O(2W)� � �O(16)c 2.810(3)O(2W)� � �O(16)c 2.861(3)

2 O(1W)a� � �O(16)b 2.686(3)

3 O(1W)–H(1A)� � �O(12A) 2.754(8) 107O(1W)–H(1B)� � �O(16)c 2.684(7) 174O(2W)–H(2A)� � �O(1W)b 2.748(7) 167N(21)–H(21)� � �O(2W)a 2.939(7) 163

4 O(1W)–H(1A)� � �O(2W)b 2.882(5) 169(5)O(2W)–H(2A)� � �O(26)c 2.720(5) 171(4)O(2W)–H(2B)� � �O(3W)b 2.789(6) 175(5)O(3W)–H(3A)� � �O(41) 2.786(5) 163(6)O(3W)–H(3B)� � �O(16)c 2.787(5) 173(5)N(31)–H(31)� � �O(3W)b 2.979(5) 156N(32)–H(32)� � �O(2W)b 2.880(5) 157O(42)–H(42O)� � �N(11)py 2.609(5) 173(6)

5 N(12)–H(12)� � �O(2W)a 2.846(5) 155O(2W)� � �O(1W)b 2.817(5)O(16)� � �O(1W)b 2.746(5)

a Coordinated water.b Free water.c Amide O-atom.

L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255 1249

1–5. The crystal structures of these complexes reveal that1–3 exhibit a 1D-coordination network, 4 is a 0D-complexand 5 exhibits a doubly interpenetrated 2D-network con-taining 4,4-topology. The crystallographic parameters aregiven in Table 1, bond lengths around Cu(II) in complexes1–5 are given in Table 2 and parameters for hydrogenbonding are given in Table 3.

Cu(L1)(NBA)2(H2O)2 � (HNBA) � 2H2O (1)Cu(L2)(NBA)2(H2O)2 � (HNBA) � EtOH (2)Cu(L3)(NBA)2(H2O)2 � 2H2O (3)Cu(L1)2(HIPA)2 � 6H2O (4)Cu(L1)(SA)(H2O)2 � H2O (5)

2.1. Coordination geometry around the Cu(II) ions

In all the complexes the Cu(II) ions have an octahedralcoordination geometry with two pyridine N atoms of theligands, two water molecules and two O atoms of carbox-ylates, and it also sits on an inversion center. The Cu–O(carboxylate) distances are somewhat shorter (1.954(3)–1.980(3) A) than the Cu–N(pyridine) distances (2.020(4)–2.052(4) A). The Cu–O(aqua) distances (2.466(2)–2.680(5)A) are much longer than those of Cu–O(carboxylate) andCu–N(pyridine). Therefore, the coordination geometryaround Cu(II) can be best described as a ‘‘Jahn–Teller” dis-

Table 1Crystallographic parameters for the crystal structures of complexes 1–5

Compound 1 2

Formula C35H35N7O18Cu C39H41N7O17CuMolecular weight 905.24 943.33T (K) 293(2) 293(2)System triclinic triclinicSpace group P�1 P�1a (A) 8.6530(2) 8.7310(2)b (A) 11.355(2) 11.137(2)c (A) 11.945(2) 12.603(3)a (�) 111.36(3) 106.51(3)b (�) 102.79(3) 90.29(3)c (�) 100.55(3) 98.08(3)V (A3) 1020.0(4) 1162.0(4)Z 1 1Dcalc (Mg/m3) 1.474 1.348R1 (I > 2r(I)) 0.0757 0.0454wR2 (on F2, all data) 0.2006 0.1298

torted octahedron with elongated axial distances. Only oneO-atom of the carboxylates coordinates to Cu(II) in amonodentate fashion while the other uncoordinated O-atom of the carboxylates is involve in intramolecular

3 4 5

C32H38N6O14Cu C44H50N8O18Cu C18H24N4O9Cu794.22 1042.47 503.95293(2) 293(2) 293(2)triclinic triclinic monoclinicP�1 P�1 C2/c8.6297(2) 7.0130(1) 10.196(2)8.9671(2) 10.660(2) 16.435(3)12.467(3) 16.628(3) 12.705(3)93.51(3) 98.21(3) 90.00106.47(3) 92.96(3) 97.14(3)96.74(3) 108.14(3) 90.00914.2(4) 1163.1(5) 2112.5(8)1 1 41.443 1.488 1.5850.0648 0.0497 0.04970.2011 0.1544 0.1672

O

CuO O

O

O

R

O

R

HH

H H

L

L

Scheme 2. Coordination geometry around Cu(II) and intramolecularhydrogen bonding observed in the crystal structures of 1–5.

1250 L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255

hydrogen bonds with coordinated water molecules to forma cyclic ring containing Cu(II) (Scheme 2). In these sixmembered rings the O� � �O distances of intramolecularhydrogen bonds are 2.676(4), 2.694(3), 2.705(7), 2.688(5)and 2.614(6) A in the structures of 1–5, respectively.

2.2. One-dimensional networks in complexes 1–3

In the crystal structures of 1–3, the ligands L1, L2 and L3

exhibit the lengths of 12.850, 14.146 and 16.629 A, respec-tively.1 However, the separation of the Cu(II) atoms by theligand in the crystal structures does not follow a similartrend; the separation distances are 16.227, 15.839 and19.153 A in 1–3, respectively (Fig. 1). In all the three struc-tures, the interplanar angles between the amide and pyri-dine are quite similar (22–25�). However, the interplanarangles between the pyridine and alkyl chain (3.4�) andthe amide and alkyl chain (19.7�) of 2 differ significantlyfrom those of 1 (81.3� and 86�) and 3 (97.8� and 58.2�).

Moreover, complexes 1 and 2 include HNBA and wateror EtOH as guest molecules. The available guest volume inthe structures of 1 and 2 accounts for 31.5% and 39.5% ofthe crystal volume respectively. In contrast, complex 3

includes water but not HNBA. In all the three structuresthe 1D-chains are assembled into two-dimensional layers.However, the compositions of the layers differ significantlyfrom each other (Fig. 2). In 1, hydrogen bonding betweencoordinated water molecules and N–H groups and alsobetween CO groups and uncoordinated water moleculesjoin the chains. Two water molecules indeed form an eightmembered ring with amide CO groups, which comprises offour O-atoms and four H-atoms. The shortest interchainseparation of Cu(II) atoms in the layer is 8.653 A. Thenitro benzoate groups are positioned above and belowthe layer in a perpendicular fashion.

In complex 2 the chains are joined together by hydrogenbonding between coordinated water and CO groups. TheNBA moieties are positioned above and below the layerwith some inclination towards plane of the layer. Theshortest interchain separation of the Cu(II) atoms in thelayers is 8.731 A. Within the layer there exist rectangular

1 The length of the ligand has been considered as the distance betweenthe two pyridine N-atoms of the ligand.

cavities which are occupied by EtOH molecules (notlocated).

The 1D-chains in complex 3 also form a layer via O–H� � �O hydrogen bonds between water molecules and ben-zoates. Further, the joining of these chains occurs throughthe formation of a cyclic hydrogen bond aggregate com-prising of six O-atoms and six H-atoms. The amide CO isalso involved in O–H� � �O hydrogen bonding with a freewater molecule.

In complexes 1 and 2, the layers pack on each other suchthat channels are formed parallel to the layers. These chan-nels are occupied by HNBA molecules which have a disor-der in both the complexes. In both cases each moiety ofHNBA (guest) is surrounded by the six coordinated NBAmoieties (Fig. 3). In short, these structures can be describedas bilayer architectures in which the layer of metal ion andligand alternates with the layer of coordinated and uncoor-dinated NBA moieties.

2.3. 0D and interpenetrated 2D networks containing

dicarboxylate moieties (4 and 5)

As the usage of monocarboxylic acid in 1–3 resulted in1D-chains, we considered dicarboxylic acids (H2IPA andH2SA) in anticipation of forming higher dimensional coor-dination networks. As anticipated, complex 5 formed a 2Dnetwork whereas complex 4 differed from our expectationsand formed a 0D species. The 0D complex results as onlyone carboxylate of HIPA and one pyridine moiety of L1

coordinates with Cu(II). The second –COOH of H2IPAdoes not lose a proton and exhibits C–O and C@O bondlengths of 1.309(5) and 1.216(5) A, respectively, while thedeprotonated –COOH has C–O bond lengths of 1.273(5)and 1.248(5) A.2

The –COOH of HIPA and the uncoordinated pyridinegroup of L1 interact with each other via an O–H� � �Nhydrogen bond (Fig. 4). Differing from all the structuresreported here, the metal to ligand ratio in this complex is1:2. The O–H� � �N hydrogen bond between the pyridineand COOH group assembles the aggregates into 1D-chainswhich are further linked into a 2D layer via hydrogenbonds between water molecules and amide functionalgroups.

In complex 5, the coordination environment is similar tothe above complex but both the carboxylates of SA andboth the pyridines of L1 coordinates to Cu(II) to form a(4,4)-layer containing rectangular cavities of the dimension9.67 � 16.46 A2 (see Fig. 5). The diagonal-to-diagonal dis-tances are 17.25 and 20.77 A, which confirm the rectangu-lar nature of the cavities. The layers are interpenetrated in a2-fold fashion with an inclined orientation in the diagonal-to-diagonal mode, the Cu(II) planes of these layers exhibitan interplanar angle of 48.8�. The Cu(OH2)2 moieties of

2 The deprotonation might have occurred by the ligand which is presentin excess or by the presence of counter anions of the Cu(II) salts ClO4

�.

Fig. 1. One-dimensional coordination networks exhibited by crystal structures: (a) 1, (b) 2; and (c) 3. Compare the network in 1 with those in 2 and 3.

L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255 1251

one network fit into the cavities of the other network by theformation of N–H� � �O hydrogen bonding between amideN–H and coordinated water molecules. Also the CO ofthe amide forms a hydrogen bond with a free watermolecule.

2.4. Thermogravometric analysis for complexes 1 and 2

The TGA of complexes 1 and 2 were analyzed, and bothcomplexes show differences in the loss of solvent molecules.Complex 1 loses all four water molecules (two loosely coor-dinated and two uncoordinated) at 78 �C (weight loss: expt.7.94% and calc.7.96%) and loses guest HNBA by 239 �C(weight loss: expt. 17.85% and calc.18.46%). Whereascomplex 2 loses EtOH at room temperature as the crystalswere taken out of the mother liquor and kept at roomtemperature for a week. Complex 2 without EtOH losescoordinated water molecules by 179 �C.

3. Conclusion

The ligands L1, L2 and L3 were shown to form coordina-tion networks in the presence of carboxylic acids HNBA,H2IPA and H2SA. The presence of HNBA resulted in 1Dnetworks with the inclusion of HNBA itself as guest mole-cules in complexes 1 and 2. Although H2IPA is a diacidunder the given conditions, it exhibits mono deprotonationand forms discrete species as one of the pyridine groups ofthe ligand is occupied by the –COOH group of HIPA. In

contrast, H2SA forms a dianion and acts as a spacerbetween Cu(II) ions to form a higher dimensional coordi-nation network. The resultant network contains rectangu-lar grids of dimension 9.7 � 16.5 A2 and doublyinterpenetrates in an inclined mode. In all the complexes,due to the presence of coordinated and free water mole-cules, amide-to-amide hydrogen bonds were not observed.

4. Experimental

4.1. Synthesis of complex 1

An ethanolic solution (10.0 mL) of Cu(ClO4)2 (370.15mg, 1.0 mmol) was added to a solution of 3-nitrobenzoicacid (668.0 mg, 4.0 mmol) in EtOH (10.0 mL). The resul-tant solution was added to a stirred solution of the ligandL1 (540 mg, 2.0 mmol) in 30 mL of EtOH, which resultedin the formation of a blue precipitate. The precipitatewas dissolved by adding 20.0 mL of water and the clearsolution was kept for slow evaporation. Complexes 2–5

were synthesized in a similar way by taking the correspond-ing ligand and carboxylic acids.

4.2. Elemental analysis

1 Anal. Calc. for C35H35CuN7O18: C, 46.43; H, 3.87; N,10.83. Found: C, 45.95; H, 4.05; N, 11.24%.

2 Anal. Calc. for C39H41CuN7O17: C, 49.75; H, 4.36; N,10.42. Found: C, 49.86; H, 4.33; N, 10.07%.

Fig. 2. Assembling of one-dimensional networks into two-dimensional layers via hydrogen bonds in the crystal structures: (a) 1, (b) 2, and (c) 3. Notice thedifferences in the assembling.

1252 L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255

3 Anal. Calc. for C32H38CuN6O14: C, 48.51; H, 4.80; N,10.61. Found: C, 47.90; H, 4.52; N, 10.34%.

4 Anal. Calc. for C44H50CuN8O18: C, 50.69; H, 4.80; N,10.75. Found: C, 51.12; H, 4.64; N, 10.63%.

5 Anal. Calc. for C18H24CuN4O9: C, 42.90; H, 4.77; N,11.12. Found: C, 43.40; H, 4.88; N, 10.99%.

4.3. X-ray crystal structure determination

The single crystal data were collected on a Bruker-Nonius Mach3 CAD4 X-ray diffractometer using graph-ite monochromated Mo Ka radiation (l = 0.71073 A) bythe x-scan method. The structures were solved by directmethods and refined by least square methods on F2

using SHELX-97 [7]. Non-hydrogen atoms were refined

anisotropically and hydrogen atoms were fixed at calcu-lated positions and refined using a riding model. Thehydrogen atoms of water molecules were located andrefined with afix-2 instruction in the complexes of 3

and 4. PLATON was used for the calculation of availableguest volumes [8]. The HNBA moieties were disorderedin both complexes 1 and 2 and were modelled as shownin Scheme 3.

Acknowledgements

We gratefully acknowledge financial support from theCouncil of Scientific and Industrial Research, India (CSIR,01(2114)/07/EMR-II) and DST-FIST for the single crystalX-ray facility. L.R. thanks IIT (Kharagpur) for a researchfellowship.

Fig. 3. Encapsulation of HNBA molecules between hydrogen bonded layers in the crystal structures of: (a) 1 and (b) 2. Illustrations of the middle layerconstituted by coordinated NBA and guest HNBA in (c) 1 and (d) 2.

Fig. 4. Illustrations for the crystal structure 4: (a) side view of the hydrogen bonded 2D layer of the discrete complex and (b) illustration of hydrogenbonds between L1 and HIPA.

L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255 1253

Appendix A. Supplementary material

CCDC 664688, 664689, 664690, 664691 and 664692contain the supplementary crystallographic data for 1,2, 3, 4 and 5. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/retrieving.html, or

from the Cambridge Crystallographic Data Centre, 12Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-plementary data associated with this article can befound, in the online version, at doi:10.1016/j.poly.2007.12.027.

Fig. 5. Illustrations for the crystal structure of 5. Two-dimensional layer containing rectangular cavities: (a) cylinder mode and (b) space-filling mode;(c) 2-fold interpenetration of two-dimensional layers in an inclined fashion in the space-filling mode and (d) hydrogen bonds between coordinated watermolecules and amide N–H groups of interpenetrated layers.

N

N

O

O

O

O

O O

Scheme 3. The disorder of HNBA moieties in 1 and 2.

1254 L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255

References

[1] (a) G.R. Desiraju, Angew. Chem., Int. Ed. Engl. 34 (1995) 2311;(b) M. O’Keeffe, O.M. Yaghi, Acc. Chem. Res. 34 (2001) 319;(c) S.R. Batten, R. Robson, Angew. Chem., Int. Ed. 37 (1998)1460;(d) B. Moulton, M.J. Zaworotko, Chem. Rev. 101 (2001) 1629;(e) S. Kitagawa, R. Kitaura, S. Noro, Angew. Chem., Int. Ed. 43(2004) 2334;(f) W. Lin, O.R. Evans, Acc. Chem. Res. 35 (2002) 511;(g) K. Biradha, M. Fujita, Angew. Chem., Int. Ed. 41 (2002) 3392;(h) G.S. Papaefstathiou, L.R. MacGillivray, Coord. Chem. Rev.246 (2003) 169;(i) J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim,Nature 404 (2000) 982;(j) M. Lahav, R. Gabai, A.N. Shipway, I. Willner, Chem.Commun. (1999) 1937;(k) K. Biradha, Cryst. Eng. Commun. 5 (2003) 374;(l) O. Ohmori, M. Kawano, M. Fujita, J. Am. Chem. Soc. 126(2004) 6292;

(m) D. Sun, S. Ma, Y. Ke, D.J. Collins, H.-C. Zhou, J. Am.Chem. Soc. 128 (2006) 3896;(n) S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K.Mochizuki, Y. Kinoshita, S. Kitagawa, J. Am. Chem. Soc. 129(2007) 2607.

[2] (a) D. Braga, J. Chem. Soc., Dalton. Trans. (2000) 3705;(b) S. Kitagawa, M. Kondo, Bull. Chem. Soc. Jpn. 71 (1998) 1739;(c) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc.Chem. Res. 31 (1998) 474;(d) R. Robson, J. Chem. Soc., Dalton. Trans. (2000) 3735;(e) D.-L. Long, A.J. Blake, N.R. Champness, C. Wilson, M. Schroder,Angew. Chem., Int. Ed. 113 (2001) 2510;(f) S.A. Boume, J. Lu, A. Mondal, B. Moulton, M.J. Zawarotko,Angew. Chem., Int. Ed. 113 (2001) 2169;(g) A.G. Wong-Foy, A.J. Matzger, O.M. Yaghi, J. Am. Chem. Soc.128 (2006) 3494;(h) Y.Q. Lan, X.L. Wang, S.L. Li, Z.M. Su, K.Z. Shao, E.B. Wang,Chem. Commun. (2007) 4863.

[3] (a) S. Horike, D. Tanaka, K. Nakagawa, S. Kitagawa, Chem.Commun. (2007) 3395;(b) L.R. MacGillivray, R.H. Groeneman, J.L. Atwood, J. Am. Chem.Soc. 120 (1998) 2676;(c) F. Luo, J.-m. Zheng, S.R. Batten, Chem. Commun. (2007) 3744;(d) K. Biradha, M. Sarkar, L. Rajput, Chem. Commun. (2006) 4169;(e) K. Biradha, C. Seward, M.J. Zaworotko, Angew. Chem., Int. Ed.38 (1999) 492;(f) Z. Lin, D.S. Wragg, J.E. Warren, R.E. Morris, J. Am. Chem. Soc.129 (2007) 10334.

[4] (a) M. Sarkar, K. Biradha, Cryst. Growth Des. 7 (2007) 1318;(b) M. Sarkar, K. Biradha, Cryst. Growth Des. 6 (2006) 1742;(c) M. Sarkar, K. Biradha, Chem. Commun. (2005) 2229;(d) M. Sarkar, K. Biradha, Eur. J. Inorg. Chem. (2006) 531.

L. Rajput, K. Biradha / Polyhedron 27 (2008) 1248–1255 1255

[5] F.H. Allen, O. Kennard, Chem. Des. Automat. News 8 (1993) 31.[6] (a) J. Tao, X. Yin, R. Huang, L. Zheng, Inorg. Chem. Commun. 5

(2002) 1000;(b) J. Tao, M.-L. Tong, J.-X. Shi, X.-M. Chen, S.W. Ng, Chem.Commun. (2000) 2043;(c) S.G. Ang, B.W. Sun, Cryst. Growth Des. 5 (2005) 383;(d) J. Zhang, Y. Kang, R.-B. Zhang, Z.-J. Li, J.-K. Chang, Cryst. Eng.Commun. 7 (2005) 177;(e) S.-M. Ying, J.-G. Mao, Y.-Q. Sun, H.-Y. Zeng, Z.-C. Dong,

Polyhedron 22 (2003) 3097;(f) L.-S. Long, Y.-R. wu, R.-B. Huang, L.-S. Zheng, Inorg. Chem. 43(2004) 3798;(g) M. Du, X.-J. Zhao, H. Cai, J. Ribas, Eur. J. Inorg. Chem. (2006)1245.

[7] G.M. Sheldrick, SHELX-97, Program for the Solution and Refinementof Crystal Structures, University of Gottingen, Germany, 1997.

[8] A.L. Spek, PLATON – A Multi Purpose Crystallographic Tool, UtrechtUniversity, Utrecht, The Netherlands, 2002.