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Copper and cadmium coordination polymers of di-3-pyridinylmethanone: Subtle anion effect on the dimensionality of Cd(II) frameworks
Inorganic Chemistry Communications 8 (2005) 766–768 www.elsevier.com/locate/inoche
Copper and cadmium coordination polymers of di-3-pyridinylmethanone: Subtle anion effect on the dimensionality of Cd(II) frameworks Xu-Dong Chen a, Jian-Hua Guo b, Miao Du b, Thomas C.W. Mak a
a,*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, P.R. China b College of Chemistry and Life Science, Tianjin Normal University, Tianjin, P.R. China Received 8 April 2005; accepted 3 June 2005 Available online 7 July 2005
Abstract Syntheses and characterization of two pairs of Cu(II) and Cd(II) coordination polymers with di-3-pyridinylmethanone revealed that variation the counter anion from ClO 4 to BF4 does not change the structures of Cu(II) complexes, but it lowers the dimensionality of the Cd(II) framework. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Di-3-pyridinylmethanone; Coordination polymer; Anion effect; Dimensionality
In the design of coordination polymers with diverse dimensionalities [1–5], the counter anion has been shown to play a dominant role [6–12]. Under normal circumstances, coordination compounds constructed from metal tetrafluoroborates and perchlorates are similar in structure because these two tetrahedral monoanions have only minor differences in their coordinating capability [13,14]. A wide variety of complexes of di-2-pyridinylmethanone (di-2-pyridyl ketone) have been explored during the past decade, which may be ascribed to the diverse ligand behavior of its neat keto form, doubly and singly deprotonated gem-diol forms, and monoanionic hemiacetal form in its metal complexes [15–19]. For the positional isomer di-3-pyridinylmethanone (L) [20], free rotation about the C–C(py) single bonds enables it to function as an angular bridging ligand with flexible conformation. We report here a series of four Cu(II) and Cd(II) coordination polymers, namely *
Corresponding author. Tel.: +852 2609 6279; fax: +852 2603 5057. E-mail address: [email protected] (T.C.W. Mak).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2005.06.001
[(CuL2)(ClO4)2]1 (1), [(CuL2)(BF4)2]1 (2), [CdL2 (ClO4)2]1 (3) and [CdL2(BF4)2]1 (4), which exhibit two- and one-dimensional architectures according to the extent to which different counter anions interact with the metal center. Ligand L was prepared according to our previously reported procedure [20]. Complexes 1 and 2 were synthesized by reacting L with Cu(ClO4)2 Æ 6H2O and Cu(BF4)2 Æ xH2O, respectively, in methanol followed by addition of water. They constitute an isomorphous pair, comprising the same two-dimensional framework as illustrated in Fig. 1. The Cu(II) atoms each adopts square-planar coordination geometry, acting as nodes that are linked by exo-bidentate bridging ligand L to form a ½CuL2þ 2 1 (4,4) network. The carbonyl O atom lies outside the coordination sphere, having no interaction with both the metal center and any hydrogen atom. The presence of different counter anions does not lead to obvious structural difference in these two coordination ˚ polymers. The Cu–N distances are 2.032(2)/2.021(2) A ˚ for 2, and similar dihedral for 1 and 2.031(2)/2.016(2) A angles of the two pyridyl rings within an individual L
X.-D. Chen et al. / Inorganic Chemistry Communications 8 (2005) 766–768
767
Table 1 ˚ ) and bond angles (°) of complexes 1–4 Selected bond lengths (A Complex 1 Cu(1)–N(2)#1 Cu(1)–N(1)
2.021(2) 2.032(2)
N(2)#1–Cu(1)–N(1)
90.68(7)
Complex 2 Cu(1)–N(1) Cu(1)–N(2)#2 N(1)–Cu(1)–N(2)#2
Fig. 1. The ½CuL2þ 2 1 (4,4) network in complexes 1 and 2. Hydrogen atoms are omitted for clarity.
ligand are observed at 72.3(1)° and 72.9(1)° for 1 and 2, respectively. The anions have weak interactions with the ˚ Cu(II) center at slightly different distances of 2.745(2) A ˚ for Cu–O in 1 and 2.677(2) A for Cu–F in 2. The reaction of Cd(ClO4)2 Æ 6H2O and Cd(BF4)2 Æ xH2O with L in methanol under the same conditions yielded the pair of coordination polymers 3 and 4. X-ray analysis revealed that 3 exhibits a (4,4) network similar to that in 1 and 2, with ligand L taking the same linkage mode, as shown in Fig. 2(a). The difference lies in the fact that the ClO 4 anion in 3 coordinates to ˚ , while the Cd(II) center at a bond length of 2.408(2) A the anions in 1 and 2 have only weak interactions with the Cu(II) center. However, complex 4 adopts a doubly bridged infinite chain structure, as illustrated in Fig. 2(b), rather than the expected (4,4) network despite the fact the Cd(II) atom exhibits the same octahedral coordination geometry as in 3, albeit a little more distorted. Further comparison of the two crystal structures reveals that the BF 4 anion is bound to the Cd(II) center ˚. at a significantly shorter distance at Cd–F = 2.295(2) A The Cd–N bond lengths are correspondingly longer in 4, ˚ , as compared to 2.367(2)/ being 2.401(2)/2.374(2) A ˚ 2.316(2) A in 3. Since both ClO 4 and BF4 are weakly coordinating tetrahedral anions of approximately the same size, they commonly play similar roles in the construction of coordination complexes [13,14]. Hence it is
Complex 3 Cd(1)–N(2)#3 Cd(1)–N(1) Cd(1)–O(2) N(1)–Cd(1)–O(2) N(2)#3–Cd(1)–O(2) N(2)#3–Cd(1)–N(1) Complex 4 Cd(1)–F(1) Cd(1)–N(1) Cd(1)–N(2)#4 N(1)–Cd(1)–N(2)#4 F(1)–Cd(1)–N(1) F(1)–Cd(1)–N(2)#4
2.016(2) 2.031(2) 90.70(8)
2.316(2) 2.367(2) 2.408(2) 91.97(8) 84.33(8) 90.07(7)
2.296(2) 2.374(2) 2.401(2) 92.22(7) 96.58(6) 92.83(6)
Symmetry codes: #1 x; y 112; z þ 12; #2 x; y þ 12; z þ 12; #3 x þ 12; y þ 12; z þ 12; #4 x, y, z 1.
noteworthy that in 3 and 4 the subtle anion effect on the Cd(II) metal center accounts for different manifestation of the solid-state architecture. Selected bond lengths and angles of complexes 1–4 are listed in Table 1. The dihedral angle of the two pyridyl rings within an individual ligand L is 47.0(1)° for 3 and 63.8(1)° for 4, being much smaller than those in the range of 72–73° for 1 and 2. The difference between the conformations adopted by ligand L in complexes 1–4 can also be gauged by comparing the torsion angles listed in Table 2. Interestingly, the crystal packing patterns of the four coordination polymers exhibit distinct characteristics owing to the presence of weak intermolecular interactions
Fig. 2. (a) The [CdL2(ClO4)2]1 (4,4) network in complex 3. (b) Doubly bridged [CdL2(BF4)2]1 infinite chain in complex 4. Hydrogen atoms are omitted for clarity.
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X.-D. Chen et al. / Inorganic Chemistry Communications 8 (2005) 766–768
Table 2 Atom labeling of di-3-pyridinylmethanone (L) in complexes 1–4, together with the torsion angle of the pyridyl rings with respect to the carbonyl group O 2 6' 3N 5' 1 1'
4
6
2'
5 Complex
1 2 3 4
N 3'
4'
Torsion angle (°) C2–C1–C@O
C2 0 –C1 0 –C@O
29.4(3) 133.6(3) 17.5(4) 140.7(2)
132.9(2) 31.7(4) 148.2(3) 29.2(3)
(Fig. S1). There is significant interlayer C–H F hydro˚ , C–H F 172.0°) [21] that gen bonding (H F 2.546 A links the (4,4) sheets in 2 into a three-dimensional network. However, such interlayer hydrogen bonding is non-existent in 1. The sheets in 3 are linked by interlayer ˚, C–H O (perchlorate) hydrogen bonds (H Cl 2.438 A C–H O 123.2°) [22] to form a three-dimensional network. The infinite chains in complex 4 are crosslinked along the other two dimensions by C–H F ˚ , C–H F 143.9°; H hydrogen bonds (H F 2.390 A ˚ F 2.515 A, C–H F 154.4°) to form a three-dimensional network. In contrast to our previous finding that the vibrational spectrum of L is little affected by coordination to silver(I) [20], more obvious differences are observed in its metal(II) coordination polymers. As to the m(C@O) mode, L exhibits a sharp and strong peak at 1651 cm1, while its metal(II) complexes 1–4 show the same peak at 1684, 1682, 1666 and 1670 cm1, respectively. In conclusion, the present study has demonstrated that the monoanions ClO 4 and BF4 , which normally play similar roles in coordination network construction, interact differently with the Cd(II) center to generate two- and one-dimensional coordination polymers, respectively, with di-3-pyridinylmethanone as a bridging ligand. This kind of selective anion effect on the dimensionality may have potential applications in supramolecular chemistry and crystal engineering. Studies on other metal complexes of di-3-pyridinylmethanone are in progress.
Acknowledgements We thank the Hong Kong Research Grants Council (Ref. No. CUHK 402003) for financial support.
Appendix A. Supplementary material Supplementary crystal data are available from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk) on request, quoting the deposition numbers: CCDC 268219–268222 for 1–4, respectively. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2005.06.001.
References [1] O.M. Yaghi, H.L. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474. [2] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375. [3] D.B. Leznoff, B.-Y. Xue, B.O. Patrick, V. Sanchez, R.C. Thompson, Chem. Commun. (2001) 259. [4] J.Y. Lu, A.M. Babb, Chem. Commun. (2001) 821. [5] T. Kuroda-Sowa, T. Horino, M. Yamamoto, Y. Ohno, M. Maekawa, M. Munakata, Inorg. Chem. 36 (1997) 6382. [6] N. Schultheiss, D.R. Powell, E. Bosch, Inorg. Chem. 42 (2003) 8886. [7] S. Sailaja, M.V. Rajasekharan, Inorg. Chem. 42 (2003) 5675. [8] H.-P. Wu, C. Janiak, G. Rheinwald, H. Lang, J. Chem. Soc., Dalton Trans. (1999) 183. [9] C. Janiak, L. Uehlin, H.-P. Wu, P. Klu¨fers, H. Piotrowski, T.G. Scharmann, J. Chem. Soc., Dalton Trans. (1999) 3121. [10] N.-H. Hu, T. Norifusa, K. Aoki, Dalton Trans. (2003) 335. [11] S.J. Hong, J.Y. Ryu, J.Y. Lee, C. Kim, S.-J. Kim, Y. Kim, Dalton Trans. (2004) 2697. [12] M. Du, X.-H. Bu, Y.-M. Guo, H. Liu, S.R. Batten, J. Ribas, T.C.W. Mak, Inorg. Chem. 41 (2002) 4904. [13] S.L. Renard, C.A. Kilner, J. Fisher, M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2002) 4206. [14] Y.-P. Cai, H.-X. Zhang, A.-W. Xu, C.-Y. Su, C.-L. Chen, H.-Q. Liu, L. Zhang, B.-S. Kang, J. Chem. Soc., Dalton Trans. (2001) 2429. [15] G.S. Papaefstathiou, S.P. Perlepes, Comment Inorg. Chem. 23 (2002) 249, and references therein. [16] C. Dendrinou-Samara, M. Alexiou, C.M. Zaleski, J.W. Kampf, M.L. Kirk, D.P. Kessissoglou, V.L. Pecoraro, Angew. Chem. Int. Ed. 42 (2003) 3763. [17] C.J. Milios, E. Kefalloniti, C.P. Raptopoulou, A. Terzis, R. Vicente, N. Lalioti, A. Escuer, S.P. Perlepes, Chem. Commun. (2003) 819. [18] A.K. Boudalis, F. Dahan, A. Bousseksou, J.-P. Tuchagues, S.P. Perlepes, Dalton Trans. (2003) 3411. [19] K.N. Crowder, S.J. Garcia, R.L. Burr, J.M. North, M.H. Wilson, B.L. Conley, P.E. Fanwick, P.S. White, K.D. Sienerth, R.M. Granger, Inorg. Chem. 43 (2004) 72. [20] X.-D. Chen, T.C.W. Mak, J. Mol. Struct. 743 (2005) 1. [21] K. Reichenba¨cher, H.I. Su¨ss, J. Hulliger, Chem. Soc. Rev. 34 (2005) 22. [22] M. Du, Z.L. Shang, X.B. Leng, X.H. Bu, Polyhedron 20 (2001) 3065.
Copper and cadmium coordination polymers of di-3-pyridinylmethanone: Subtle anion effect on the dimensionality of Cd(II) frameworks Xu-Dong Chen a, Jian-Hua Guo b, Miao Du b, Thomas C.W. Mak a
a,*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, P.R. China b College of Chemistry and Life Science, Tianjin Normal University, Tianjin, P.R. China Received 8 April 2005; accepted 3 June 2005 Available online 7 July 2005
Abstract Syntheses and characterization of two pairs of Cu(II) and Cd(II) coordination polymers with di-3-pyridinylmethanone revealed that variation the counter anion from ClO 4 to BF4 does not change the structures of Cu(II) complexes, but it lowers the dimensionality of the Cd(II) framework. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Di-3-pyridinylmethanone; Coordination polymer; Anion effect; Dimensionality
In the design of coordination polymers with diverse dimensionalities [1–5], the counter anion has been shown to play a dominant role [6–12]. Under normal circumstances, coordination compounds constructed from metal tetrafluoroborates and perchlorates are similar in structure because these two tetrahedral monoanions have only minor differences in their coordinating capability [13,14]. A wide variety of complexes of di-2-pyridinylmethanone (di-2-pyridyl ketone) have been explored during the past decade, which may be ascribed to the diverse ligand behavior of its neat keto form, doubly and singly deprotonated gem-diol forms, and monoanionic hemiacetal form in its metal complexes [15–19]. For the positional isomer di-3-pyridinylmethanone (L) [20], free rotation about the C–C(py) single bonds enables it to function as an angular bridging ligand with flexible conformation. We report here a series of four Cu(II) and Cd(II) coordination polymers, namely *
Corresponding author. Tel.: +852 2609 6279; fax: +852 2603 5057. E-mail address: [email protected] (T.C.W. Mak).
1387-7003/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2005.06.001
[(CuL2)(ClO4)2]1 (1), [(CuL2)(BF4)2]1 (2), [CdL2 (ClO4)2]1 (3) and [CdL2(BF4)2]1 (4), which exhibit two- and one-dimensional architectures according to the extent to which different counter anions interact with the metal center. Ligand L was prepared according to our previously reported procedure [20]. Complexes 1 and 2 were synthesized by reacting L with Cu(ClO4)2 Æ 6H2O and Cu(BF4)2 Æ xH2O, respectively, in methanol followed by addition of water. They constitute an isomorphous pair, comprising the same two-dimensional framework as illustrated in Fig. 1. The Cu(II) atoms each adopts square-planar coordination geometry, acting as nodes that are linked by exo-bidentate bridging ligand L to form a ½CuL2þ 2 1 (4,4) network. The carbonyl O atom lies outside the coordination sphere, having no interaction with both the metal center and any hydrogen atom. The presence of different counter anions does not lead to obvious structural difference in these two coordination ˚ polymers. The Cu–N distances are 2.032(2)/2.021(2) A ˚ for 2, and similar dihedral for 1 and 2.031(2)/2.016(2) A angles of the two pyridyl rings within an individual L
X.-D. Chen et al. / Inorganic Chemistry Communications 8 (2005) 766–768
767
Table 1 ˚ ) and bond angles (°) of complexes 1–4 Selected bond lengths (A Complex 1 Cu(1)–N(2)#1 Cu(1)–N(1)
2.021(2) 2.032(2)
N(2)#1–Cu(1)–N(1)
90.68(7)
Complex 2 Cu(1)–N(1) Cu(1)–N(2)#2 N(1)–Cu(1)–N(2)#2
Fig. 1. The ½CuL2þ 2 1 (4,4) network in complexes 1 and 2. Hydrogen atoms are omitted for clarity.
ligand are observed at 72.3(1)° and 72.9(1)° for 1 and 2, respectively. The anions have weak interactions with the ˚ Cu(II) center at slightly different distances of 2.745(2) A ˚ for Cu–O in 1 and 2.677(2) A for Cu–F in 2. The reaction of Cd(ClO4)2 Æ 6H2O and Cd(BF4)2 Æ xH2O with L in methanol under the same conditions yielded the pair of coordination polymers 3 and 4. X-ray analysis revealed that 3 exhibits a (4,4) network similar to that in 1 and 2, with ligand L taking the same linkage mode, as shown in Fig. 2(a). The difference lies in the fact that the ClO 4 anion in 3 coordinates to ˚ , while the Cd(II) center at a bond length of 2.408(2) A the anions in 1 and 2 have only weak interactions with the Cu(II) center. However, complex 4 adopts a doubly bridged infinite chain structure, as illustrated in Fig. 2(b), rather than the expected (4,4) network despite the fact the Cd(II) atom exhibits the same octahedral coordination geometry as in 3, albeit a little more distorted. Further comparison of the two crystal structures reveals that the BF 4 anion is bound to the Cd(II) center ˚. at a significantly shorter distance at Cd–F = 2.295(2) A The Cd–N bond lengths are correspondingly longer in 4, ˚ , as compared to 2.367(2)/ being 2.401(2)/2.374(2) A ˚ 2.316(2) A in 3. Since both ClO 4 and BF4 are weakly coordinating tetrahedral anions of approximately the same size, they commonly play similar roles in the construction of coordination complexes [13,14]. Hence it is
Complex 3 Cd(1)–N(2)#3 Cd(1)–N(1) Cd(1)–O(2) N(1)–Cd(1)–O(2) N(2)#3–Cd(1)–O(2) N(2)#3–Cd(1)–N(1) Complex 4 Cd(1)–F(1) Cd(1)–N(1) Cd(1)–N(2)#4 N(1)–Cd(1)–N(2)#4 F(1)–Cd(1)–N(1) F(1)–Cd(1)–N(2)#4
2.016(2) 2.031(2) 90.70(8)
2.316(2) 2.367(2) 2.408(2) 91.97(8) 84.33(8) 90.07(7)
2.296(2) 2.374(2) 2.401(2) 92.22(7) 96.58(6) 92.83(6)
Symmetry codes: #1 x; y 112; z þ 12; #2 x; y þ 12; z þ 12; #3 x þ 12; y þ 12; z þ 12; #4 x, y, z 1.
noteworthy that in 3 and 4 the subtle anion effect on the Cd(II) metal center accounts for different manifestation of the solid-state architecture. Selected bond lengths and angles of complexes 1–4 are listed in Table 1. The dihedral angle of the two pyridyl rings within an individual ligand L is 47.0(1)° for 3 and 63.8(1)° for 4, being much smaller than those in the range of 72–73° for 1 and 2. The difference between the conformations adopted by ligand L in complexes 1–4 can also be gauged by comparing the torsion angles listed in Table 2. Interestingly, the crystal packing patterns of the four coordination polymers exhibit distinct characteristics owing to the presence of weak intermolecular interactions
Fig. 2. (a) The [CdL2(ClO4)2]1 (4,4) network in complex 3. (b) Doubly bridged [CdL2(BF4)2]1 infinite chain in complex 4. Hydrogen atoms are omitted for clarity.
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X.-D. Chen et al. / Inorganic Chemistry Communications 8 (2005) 766–768
Table 2 Atom labeling of di-3-pyridinylmethanone (L) in complexes 1–4, together with the torsion angle of the pyridyl rings with respect to the carbonyl group O 2 6' 3N 5' 1 1'
4
6
2'
5 Complex
1 2 3 4
N 3'
4'
Torsion angle (°) C2–C1–C@O
C2 0 –C1 0 –C@O
29.4(3) 133.6(3) 17.5(4) 140.7(2)
132.9(2) 31.7(4) 148.2(3) 29.2(3)
(Fig. S1). There is significant interlayer C–H F hydro˚ , C–H F 172.0°) [21] that gen bonding (H F 2.546 A links the (4,4) sheets in 2 into a three-dimensional network. However, such interlayer hydrogen bonding is non-existent in 1. The sheets in 3 are linked by interlayer ˚, C–H O (perchlorate) hydrogen bonds (H Cl 2.438 A C–H O 123.2°) [22] to form a three-dimensional network. The infinite chains in complex 4 are crosslinked along the other two dimensions by C–H F ˚ , C–H F 143.9°; H hydrogen bonds (H F 2.390 A ˚ F 2.515 A, C–H F 154.4°) to form a three-dimensional network. In contrast to our previous finding that the vibrational spectrum of L is little affected by coordination to silver(I) [20], more obvious differences are observed in its metal(II) coordination polymers. As to the m(C@O) mode, L exhibits a sharp and strong peak at 1651 cm1, while its metal(II) complexes 1–4 show the same peak at 1684, 1682, 1666 and 1670 cm1, respectively. In conclusion, the present study has demonstrated that the monoanions ClO 4 and BF4 , which normally play similar roles in coordination network construction, interact differently with the Cd(II) center to generate two- and one-dimensional coordination polymers, respectively, with di-3-pyridinylmethanone as a bridging ligand. This kind of selective anion effect on the dimensionality may have potential applications in supramolecular chemistry and crystal engineering. Studies on other metal complexes of di-3-pyridinylmethanone are in progress.
Acknowledgements We thank the Hong Kong Research Grants Council (Ref. No. CUHK 402003) for financial support.
Appendix A. Supplementary material Supplementary crystal data are available from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or www: http://www.ccdc.cam.ac.uk) on request, quoting the deposition numbers: CCDC 268219–268222 for 1–4, respectively. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2005.06.001.
References [1] O.M. Yaghi, H.L. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474. [2] D. Braga, F. Grepioni, G.R. Desiraju, Chem. Rev. 98 (1998) 1375. [3] D.B. Leznoff, B.-Y. Xue, B.O. Patrick, V. Sanchez, R.C. Thompson, Chem. Commun. (2001) 259. [4] J.Y. Lu, A.M. Babb, Chem. Commun. (2001) 821. [5] T. Kuroda-Sowa, T. Horino, M. Yamamoto, Y. Ohno, M. Maekawa, M. Munakata, Inorg. Chem. 36 (1997) 6382. [6] N. Schultheiss, D.R. Powell, E. Bosch, Inorg. Chem. 42 (2003) 8886. [7] S. Sailaja, M.V. Rajasekharan, Inorg. Chem. 42 (2003) 5675. [8] H.-P. Wu, C. Janiak, G. Rheinwald, H. Lang, J. Chem. Soc., Dalton Trans. (1999) 183. [9] C. Janiak, L. Uehlin, H.-P. Wu, P. Klu¨fers, H. Piotrowski, T.G. Scharmann, J. Chem. Soc., Dalton Trans. (1999) 3121. [10] N.-H. Hu, T. Norifusa, K. Aoki, Dalton Trans. (2003) 335. [11] S.J. Hong, J.Y. Ryu, J.Y. Lee, C. Kim, S.-J. Kim, Y. Kim, Dalton Trans. (2004) 2697. [12] M. Du, X.-H. Bu, Y.-M. Guo, H. Liu, S.R. Batten, J. Ribas, T.C.W. Mak, Inorg. Chem. 41 (2002) 4904. [13] S.L. Renard, C.A. Kilner, J. Fisher, M.A. Halcrow, J. Chem. Soc., Dalton Trans. (2002) 4206. [14] Y.-P. Cai, H.-X. Zhang, A.-W. Xu, C.-Y. Su, C.-L. Chen, H.-Q. Liu, L. Zhang, B.-S. Kang, J. Chem. Soc., Dalton Trans. (2001) 2429. [15] G.S. Papaefstathiou, S.P. Perlepes, Comment Inorg. Chem. 23 (2002) 249, and references therein. [16] C. Dendrinou-Samara, M. Alexiou, C.M. Zaleski, J.W. Kampf, M.L. Kirk, D.P. Kessissoglou, V.L. Pecoraro, Angew. Chem. Int. Ed. 42 (2003) 3763. [17] C.J. Milios, E. Kefalloniti, C.P. Raptopoulou, A. Terzis, R. Vicente, N. Lalioti, A. Escuer, S.P. Perlepes, Chem. Commun. (2003) 819. [18] A.K. Boudalis, F. Dahan, A. Bousseksou, J.-P. Tuchagues, S.P. Perlepes, Dalton Trans. (2003) 3411. [19] K.N. Crowder, S.J. Garcia, R.L. Burr, J.M. North, M.H. Wilson, B.L. Conley, P.E. Fanwick, P.S. White, K.D. Sienerth, R.M. Granger, Inorg. Chem. 43 (2004) 72. [20] X.-D. Chen, T.C.W. Mak, J. Mol. Struct. 743 (2005) 1. [21] K. Reichenba¨cher, H.I. Su¨ss, J. Hulliger, Chem. Soc. Rev. 34 (2005) 22. [22] M. Du, Z.L. Shang, X.B. Leng, X.H. Bu, Polyhedron 20 (2001) 3065.