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Synthesis and crystal structure of [Ni{bis(2,5-dihydroxysalicylidene)ethylenediaminato}]: a hydrogen bonded assembly of Ni(II)–salen complex
Inorganic Chemistry Communications 6 (2003) 154–156 www.elsevier.com/locate/inoche
Synthesis and crystal structure of [Ni{bis(2,5-dihydroxysalicylidene)ethylenediaminato}]: a hydrogen bonded assembly of Ni(II)–salen complex Mitsuru Kondo a,*, Keiko Nabari a, Tomonori Horiba a, Yasuhiko Irie a, Md. Khayrul Kabir a, Ram Prosad Sarker b, Emi Shimizu a, Yusuke Shimizu a, Yumiko Fuwa a a b
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan Dhaka Women College, House #4, Road #26, Sector #7, Uttara Model Town, Dhaka 1230, Bangladesh Received 30 July 2002; accepted 28 October 2002
Abstract The hydrogen bonded assembly of salen type complexes was studied. The nickel(II) complex with N,N0 -bis(2,5-dihydroxysalicylidene)ethylenediaminato was synthesized, and structurally characterized. The compound has two hydroxy groups at the both terminals of the tetradentate ligand. One of the two hydroxy groups of the nickel(II) complex forms a hydrogen bond with a coordinating oxygen donor of the adjacent molecule to yield a hydrogen bonded one-dimensional chain. These chains are also associated by p–p interactions with interlocking fashion to produce a two-dimensional structure. The guest ethanol molecules are incorporated between the two-dimensional layers. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystal structure; Salen complex; Hydrogen bonded assembly; Coordination network
1. Introduction Coordination polymers have attracted intense attentions because of their unique applications such as heterogeneous catalysis, molecular adsorptions, and ion exchanges [1]. In most cases, these compounds have been constructed by direct reactions between inorganic metal ion sources and organic bridging ligands. On the other hand, metal complexes with macroyclic chelates have also been utilized as the useful building units, because these components often reveal unique functions such as homogeneous catalysis, electron transfers, and chemical sensors [2]. For example, a number of assembled metalloporhpyrins have been synthesized toward the developments of new functional solids [2]. Although
*
Corresponding author. Tel.: +81-54-238-4763; fax: +81-54-2373384. E-mail address: [email protected] (M. Kondo).
metal complexes with N,N0 -disalicylideneethylenediaminato (salen) are also known as unique functional materials [3], their assembled compounds are still unexplored. We have succeeded in the synthesis and structural characterization of a new hydrogen bonded assembly of metal complexes with salen type ligand that has two hydroxy groups in the framework.
2. Results and discussion The N,N0 -bis(2,5-dihydroxysalicylidene)ethylenediamine (H2 bhsal) was prepared according to the literature [4]. The reaction of bhsal with NiðCH3 COOÞ2 4H2 O afforded [Ni(bhsal)] EtOH (1) as a microcrystalline product. The crystals suitable for X-ray crystal analysis
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 ( 0 2 ) 0 0 7 0 8 - 6
M. Kondo et al. / Inorganic Chemistry Communications 6 (2003) 154–156
were grown up from the ethanol solution. 1 Because the crystal structure of [Ni(salen)] (2) previously reported shows a poor quality [5], we re-measured the X-ray structure of 2 2 in order to estimate the effect of the hydroxy groups of 1 on the crystal structure. Fig. 1 shows the structure of monomeric unit of 1 and the guest ethanol molecule with the aspects of hydrogen bonding interactions around them. The nickel center is in the square planar environment with the tetradentate Schiff base ligand, in which any atoms of solvent molecules or adjacent units do not bind to the metal center from the axial sites. The Ni(1)–O(3) distance (1.827(3) ) is significantly shorter than the other Ni(1)–O(1) A ) of 1, and those of 2 (Ni–O ¼ 1.852(3) bond (1.842(3) A and 1.851(3) A). The shorter Ni(1)–O(3) bond found in 1 is due to the enhancement of the coordination abilities of the coordinating oxygen by the strong electron donating effects of the hydroxy group added in the salen framework, in which the longer Ni(1)–O(1) distance is likely the result of the hydrogen bonding interaction of hydroxy group O(4) to the O(1) atom, as mentioned below. On the other hand, the O(1)–C(1) and O(3)–C(8) , respecbond distances of 1 (1.331(5) and 1.324(5) A tively) are slightly longer than those of the correspond) of 2. The C–O ing distances (1.309(4) and 1.313(4) A bond distances formed by the coordinating oxygen at, O(3)–C(8) ¼ 1.324(5) A ) oms (O(1)–C(1) ¼ 1.331(5)A are significantly shorter than those of the C–O bonds at , O(4)–C(11) ¼ the terminalsites (O(2)–C(4) ¼ 1.388(6)A ). The one of the structural feature of the 1.394(5) A bhsal ligand is the p-quinone type framework that is known to afford various oxidation states by redox process. The oxidation state is estimated by the C–O distances [6]. That is, the typical C–O bond distances of , requinonate and semiquinonate are 1.34 and 1.29 A ) found in 1 spectively. The bond distances (1.32–1.39 A
1 For 1: C18 H20 N2 O5 Ni: M ¼ 403.07. Monoclinic, space group C 2/c , b ¼ 122:414ð5Þ°, (no. 15), a ¼ 26.28(1), b ¼ 7.889(4), c ¼ 20.32(1) A 3 , Z ¼ 8, l (MoKa) ¼ 1.123 mm1 , T ¼ 293 K, R ¼ 0.056, V ¼ 3557(2) A wR ¼ 0.061 for 2699 unique reflections (Rint ¼ 0.038) with I > 2rðIÞ 3 . The data and 235 parameters. Highest electron density 0.49 e A collection was performed on a Rigaku-CCD Mercury system. The structure was solved by direct methods using SIR-98. All nonhydrogen atoms were treated anisotoropically. Hydrogen atoms bound to carbon atoms were placed at the calculated positions, and were included but not refined. 2 For 2: C16 H14 N2 O2 Ni: M ¼ 325.00. Orthorhombic, space group , V ¼ 2705(1) Pbca (no. 61), a ¼ 7.484(3), b ¼ 13.822(6), c ¼ 26.15(1) A 3 , Z ¼ 8, l (MoKa) ¼ 1.439 mm1 , T ¼ 293 K, R ¼ 0.048, wR ¼ 0.048 A for 2052 unique reflections (Rint ¼ 0.051) with I > 2rðIÞ and 190 3 . The data collection parameters. Highest electron density 0.53 e A was performed on a Rigaku-CCD Mercury system. The structure was solved by direct methods using SIR-98. All non-hydrogen atoms were treated anisotoropically. Hydrogen atoms bound to carbon atoms were placed at the calculated positions, and were included but not refined.
155
Fig. 1. View of the molecular strucure and the aspect of intermolecular hydrogen bonds of [Ni(bhsal)]. Thermal elliposids for the non-hydrogen atoms are drawn at the 30% probability level. Selected bond lengths and angles at the nickel(II) atom. Ni(1)–O(1), 1.842(3); . Ni(1)–O(3), 1.827(3); Ni(1)–N(1), 1.839(4); Ni(1)–N(2), 1.834(4) A . O(4*)–O(1), 2.758(5); O(2)–O(5), 2.701(6); O(40 )–O(5), 2.793(6)A O(1)–Ni(1)–O(3), 83.2(1); O(1)–Ni(1)–N(1), 95.2(1); O(1)–Ni(1)–N(2), 175.4(1); O(3)–Ni(1)–N(1), 175.8(1); O(3)–Ni(1)–N(2), 95.1(1); N(1)– Ni(1)–N(2), 86.7(2)°. Symmetry operation, * ;1=2 x; 1=2 þ y; 1=2 z; 1=2 x; 1=2 y; z.
are in the range expected for the C–O single bond of the quinonate ligand. The introductions of two hydroxy groups in the frameworks successfully lead to the assembly of these monomeric units by intermolecular hydrogen bonds. As illustrated in Figs. 1 and 2, one of the terminal hydroxy oxygen atoms of the bhsal ligand is bound to one of the ), in coordinating oxygen atom (O(1)–O(4*) ¼ 2.758(5) A which the two monomers are not in the same plane but tilt about 57.3° to each other. As shown in Fig. 2, these interactions afford a one-dimensional network of ½NiðbhsalÞn along the b axis. These chains are connected by p–p interactions with interlocking fashion to afford a two-dimensional layer in the bc plane. The guest ethanol molecules are included between the layers. These molecules are hydrogen bonded to the two hydroxy groups of the two different [Ni(bhsal)] monomers that are in the adjacent chains in the same layer. In other words, each chain that is associated by the p–p interactions, is also connected by hydrogen bonds via the guest ethanol molecules. In contrast to the assembly of 1, monomeric units of 2 form dimmer structure associated by weak p–p interactions. The dimmers obtained are not significantly interacted to each other. These results demonstrate that the introduction of hydrogen bonding sites in the Salen ligand is an effective method for creation of a new functional solid. Quinone is well known as a redox active compound. The quinonate form affords a radical species when it is oxidized by one-electron process. Because the bhsal contains the quinonate type framework, the bhsal might be a redox active ligand. Although we preliminary measured the cyclic voltammograms of free H2 bhsal and [Ni(bhsal)] EtOH in an N,N-dimethylformaldehyde solution, any redox waves were not observed in the
156
M. Kondo et al. / Inorganic Chemistry Communications 6 (2003) 154–156
Fig. 2. The hydrgeon bonded assembled structures of 1. (a) The one-dimensional chain connected by intermolecular hydrogen bonds. (b) The twodimensional structure formed by p–p interactions and interlockings.
range from 1.5 to )2.0 V (vs. SCE). This seems to be the result of the stabilization effects by the coordination to the Ni(II) ion. Nevertheless, this ligand could be useful as the potential macrocyclic ligand containing organic radicals. We are still in progress of the synthetic studies of the metal complexes with the radical-chelates by chemical oxidation of 1.
[2]
Acknowledgements This research was supported by Yazaki Memorial Foundation for Science and Technology.
[3]
References [1] (a) For example P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 2638; (b) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474; (c) M.A. Withersby, A.J. Blake, N.R. Champness, P. Hubberstey, W.-S. Li, M. Schr€ oder, Angew. Chem. Int. Ed. 36 (1997) 2327; (d) M.J. Zaworotko, Angew. Chem. Int. Ed. 37 (1998) 1211; (e) M. Fujita, Y.J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Soc. 116 (1994) 1151;
[4] [5] [6]
(f) M. Kondo, M. Miyazawa, Y. Irie, R. Shinagawa, T. Horiba, A. Nakamura, T. Naito, K. Maeda, S. Utsuno, F. Uchida, Chem. Commun. (2002) 2156. (a) For example I. Goldberg, H. Krupitsky, Z. Stein, Y. Hsiou, C.E. Strouse, Supramolecular Chem. 4 (1995) 203; (b) K.-J. Lin, Angew. Chem. Int. Ed. 38 (1999) 2730; (c) Y.D. Posner, S. Dahal, I. Goldberg, Angew. Chem. Int. Ed. 39 (2000) 1288; (d) D. Hagrman, P.J. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 3165; (e) T. Lee, V.-D. Le, D. Lim, Y.-C. Lin, G.M. Morris, A.L. Wong, A.J. Olson, J.H. Elder, C.-H. Wong, J. Am. Chem. Soc. 121 (1999) 1145; (f) B.F. Abrahams, B.F. Hoskins, D.M. Michail, R. Robson, Nature 369 (1994) 727. (a) for example A.L. Gavrilova, C.J. Qin, R.D. Sommer, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 124 (2002) 1714; (b) D. Chen, A.E. Martell, Inorg. Chem. 26 (1987) 1926; (c) M.H. Wu, K.B. Hansen, E.N. Jaconsen, Angew. Chem. Int. Ed. 38 (1999) 2012; (d) C.J. Chang, J.A. Labinger, H.B. Gray, Inorg. Chem. 36 (1997) 5927. E. Lamour, S. Routier, J.-L. Bernier, J.-P. Catteau, C. Bailly, H. Vezin, J. Am. Chem. Soc. 121 (1999) 1862. L.M. ShkolÕnikova, E.M. Yutal, E.A. Shugam, V.A. Voblikova, Zh. Strukt. Khim. 11 (1970) 886. C.G. Pierpont, R.M. Buchanan, Coord. Chem. Rev. 38 (1981) 45.
Synthesis and crystal structure of [Ni{bis(2,5-dihydroxysalicylidene)ethylenediaminato}]: a hydrogen bonded assembly of Ni(II)–salen complex Mitsuru Kondo a,*, Keiko Nabari a, Tomonori Horiba a, Yasuhiko Irie a, Md. Khayrul Kabir a, Ram Prosad Sarker b, Emi Shimizu a, Yusuke Shimizu a, Yumiko Fuwa a a b
Department of Chemistry, Faculty of Science, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan Dhaka Women College, House #4, Road #26, Sector #7, Uttara Model Town, Dhaka 1230, Bangladesh Received 30 July 2002; accepted 28 October 2002
Abstract The hydrogen bonded assembly of salen type complexes was studied. The nickel(II) complex with N,N0 -bis(2,5-dihydroxysalicylidene)ethylenediaminato was synthesized, and structurally characterized. The compound has two hydroxy groups at the both terminals of the tetradentate ligand. One of the two hydroxy groups of the nickel(II) complex forms a hydrogen bond with a coordinating oxygen donor of the adjacent molecule to yield a hydrogen bonded one-dimensional chain. These chains are also associated by p–p interactions with interlocking fashion to produce a two-dimensional structure. The guest ethanol molecules are incorporated between the two-dimensional layers. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystal structure; Salen complex; Hydrogen bonded assembly; Coordination network
1. Introduction Coordination polymers have attracted intense attentions because of their unique applications such as heterogeneous catalysis, molecular adsorptions, and ion exchanges [1]. In most cases, these compounds have been constructed by direct reactions between inorganic metal ion sources and organic bridging ligands. On the other hand, metal complexes with macroyclic chelates have also been utilized as the useful building units, because these components often reveal unique functions such as homogeneous catalysis, electron transfers, and chemical sensors [2]. For example, a number of assembled metalloporhpyrins have been synthesized toward the developments of new functional solids [2]. Although
*
Corresponding author. Tel.: +81-54-238-4763; fax: +81-54-2373384. E-mail address: [email protected] (M. Kondo).
metal complexes with N,N0 -disalicylideneethylenediaminato (salen) are also known as unique functional materials [3], their assembled compounds are still unexplored. We have succeeded in the synthesis and structural characterization of a new hydrogen bonded assembly of metal complexes with salen type ligand that has two hydroxy groups in the framework.
2. Results and discussion The N,N0 -bis(2,5-dihydroxysalicylidene)ethylenediamine (H2 bhsal) was prepared according to the literature [4]. The reaction of bhsal with NiðCH3 COOÞ2 4H2 O afforded [Ni(bhsal)] EtOH (1) as a microcrystalline product. The crystals suitable for X-ray crystal analysis
1387-7003/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 7 0 0 3 ( 0 2 ) 0 0 7 0 8 - 6
M. Kondo et al. / Inorganic Chemistry Communications 6 (2003) 154–156
were grown up from the ethanol solution. 1 Because the crystal structure of [Ni(salen)] (2) previously reported shows a poor quality [5], we re-measured the X-ray structure of 2 2 in order to estimate the effect of the hydroxy groups of 1 on the crystal structure. Fig. 1 shows the structure of monomeric unit of 1 and the guest ethanol molecule with the aspects of hydrogen bonding interactions around them. The nickel center is in the square planar environment with the tetradentate Schiff base ligand, in which any atoms of solvent molecules or adjacent units do not bind to the metal center from the axial sites. The Ni(1)–O(3) distance (1.827(3) ) is significantly shorter than the other Ni(1)–O(1) A ) of 1, and those of 2 (Ni–O ¼ 1.852(3) bond (1.842(3) A and 1.851(3) A). The shorter Ni(1)–O(3) bond found in 1 is due to the enhancement of the coordination abilities of the coordinating oxygen by the strong electron donating effects of the hydroxy group added in the salen framework, in which the longer Ni(1)–O(1) distance is likely the result of the hydrogen bonding interaction of hydroxy group O(4) to the O(1) atom, as mentioned below. On the other hand, the O(1)–C(1) and O(3)–C(8) , respecbond distances of 1 (1.331(5) and 1.324(5) A tively) are slightly longer than those of the correspond) of 2. The C–O ing distances (1.309(4) and 1.313(4) A bond distances formed by the coordinating oxygen at, O(3)–C(8) ¼ 1.324(5) A ) oms (O(1)–C(1) ¼ 1.331(5)A are significantly shorter than those of the C–O bonds at , O(4)–C(11) ¼ the terminalsites (O(2)–C(4) ¼ 1.388(6)A ). The one of the structural feature of the 1.394(5) A bhsal ligand is the p-quinone type framework that is known to afford various oxidation states by redox process. The oxidation state is estimated by the C–O distances [6]. That is, the typical C–O bond distances of , requinonate and semiquinonate are 1.34 and 1.29 A ) found in 1 spectively. The bond distances (1.32–1.39 A
1 For 1: C18 H20 N2 O5 Ni: M ¼ 403.07. Monoclinic, space group C 2/c , b ¼ 122:414ð5Þ°, (no. 15), a ¼ 26.28(1), b ¼ 7.889(4), c ¼ 20.32(1) A 3 , Z ¼ 8, l (MoKa) ¼ 1.123 mm1 , T ¼ 293 K, R ¼ 0.056, V ¼ 3557(2) A wR ¼ 0.061 for 2699 unique reflections (Rint ¼ 0.038) with I > 2rðIÞ 3 . The data and 235 parameters. Highest electron density 0.49 e A collection was performed on a Rigaku-CCD Mercury system. The structure was solved by direct methods using SIR-98. All nonhydrogen atoms were treated anisotoropically. Hydrogen atoms bound to carbon atoms were placed at the calculated positions, and were included but not refined. 2 For 2: C16 H14 N2 O2 Ni: M ¼ 325.00. Orthorhombic, space group , V ¼ 2705(1) Pbca (no. 61), a ¼ 7.484(3), b ¼ 13.822(6), c ¼ 26.15(1) A 3 , Z ¼ 8, l (MoKa) ¼ 1.439 mm1 , T ¼ 293 K, R ¼ 0.048, wR ¼ 0.048 A for 2052 unique reflections (Rint ¼ 0.051) with I > 2rðIÞ and 190 3 . The data collection parameters. Highest electron density 0.53 e A was performed on a Rigaku-CCD Mercury system. The structure was solved by direct methods using SIR-98. All non-hydrogen atoms were treated anisotoropically. Hydrogen atoms bound to carbon atoms were placed at the calculated positions, and were included but not refined.
155
Fig. 1. View of the molecular strucure and the aspect of intermolecular hydrogen bonds of [Ni(bhsal)]. Thermal elliposids for the non-hydrogen atoms are drawn at the 30% probability level. Selected bond lengths and angles at the nickel(II) atom. Ni(1)–O(1), 1.842(3); . Ni(1)–O(3), 1.827(3); Ni(1)–N(1), 1.839(4); Ni(1)–N(2), 1.834(4) A . O(4*)–O(1), 2.758(5); O(2)–O(5), 2.701(6); O(40 )–O(5), 2.793(6)A O(1)–Ni(1)–O(3), 83.2(1); O(1)–Ni(1)–N(1), 95.2(1); O(1)–Ni(1)–N(2), 175.4(1); O(3)–Ni(1)–N(1), 175.8(1); O(3)–Ni(1)–N(2), 95.1(1); N(1)– Ni(1)–N(2), 86.7(2)°. Symmetry operation, * ;1=2 x; 1=2 þ y; 1=2 z; 1=2 x; 1=2 y; z.
are in the range expected for the C–O single bond of the quinonate ligand. The introductions of two hydroxy groups in the frameworks successfully lead to the assembly of these monomeric units by intermolecular hydrogen bonds. As illustrated in Figs. 1 and 2, one of the terminal hydroxy oxygen atoms of the bhsal ligand is bound to one of the ), in coordinating oxygen atom (O(1)–O(4*) ¼ 2.758(5) A which the two monomers are not in the same plane but tilt about 57.3° to each other. As shown in Fig. 2, these interactions afford a one-dimensional network of ½NiðbhsalÞn along the b axis. These chains are connected by p–p interactions with interlocking fashion to afford a two-dimensional layer in the bc plane. The guest ethanol molecules are included between the layers. These molecules are hydrogen bonded to the two hydroxy groups of the two different [Ni(bhsal)] monomers that are in the adjacent chains in the same layer. In other words, each chain that is associated by the p–p interactions, is also connected by hydrogen bonds via the guest ethanol molecules. In contrast to the assembly of 1, monomeric units of 2 form dimmer structure associated by weak p–p interactions. The dimmers obtained are not significantly interacted to each other. These results demonstrate that the introduction of hydrogen bonding sites in the Salen ligand is an effective method for creation of a new functional solid. Quinone is well known as a redox active compound. The quinonate form affords a radical species when it is oxidized by one-electron process. Because the bhsal contains the quinonate type framework, the bhsal might be a redox active ligand. Although we preliminary measured the cyclic voltammograms of free H2 bhsal and [Ni(bhsal)] EtOH in an N,N-dimethylformaldehyde solution, any redox waves were not observed in the
156
M. Kondo et al. / Inorganic Chemistry Communications 6 (2003) 154–156
Fig. 2. The hydrgeon bonded assembled structures of 1. (a) The one-dimensional chain connected by intermolecular hydrogen bonds. (b) The twodimensional structure formed by p–p interactions and interlockings.
range from 1.5 to )2.0 V (vs. SCE). This seems to be the result of the stabilization effects by the coordination to the Ni(II) ion. Nevertheless, this ligand could be useful as the potential macrocyclic ligand containing organic radicals. We are still in progress of the synthetic studies of the metal complexes with the radical-chelates by chemical oxidation of 1.
[2]
Acknowledgements This research was supported by Yazaki Memorial Foundation for Science and Technology.
[3]
References [1] (a) For example P.J. Hagrman, D. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 2638; (b) O.M. Yaghi, H. Li, C. Davis, D. Richardson, T.L. Groy, Acc. Chem. Res. 31 (1998) 474; (c) M.A. Withersby, A.J. Blake, N.R. Champness, P. Hubberstey, W.-S. Li, M. Schr€ oder, Angew. Chem. Int. Ed. 36 (1997) 2327; (d) M.J. Zaworotko, Angew. Chem. Int. Ed. 37 (1998) 1211; (e) M. Fujita, Y.J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Soc. 116 (1994) 1151;
[4] [5] [6]
(f) M. Kondo, M. Miyazawa, Y. Irie, R. Shinagawa, T. Horiba, A. Nakamura, T. Naito, K. Maeda, S. Utsuno, F. Uchida, Chem. Commun. (2002) 2156. (a) For example I. Goldberg, H. Krupitsky, Z. Stein, Y. Hsiou, C.E. Strouse, Supramolecular Chem. 4 (1995) 203; (b) K.-J. Lin, Angew. Chem. Int. Ed. 38 (1999) 2730; (c) Y.D. Posner, S. Dahal, I. Goldberg, Angew. Chem. Int. Ed. 39 (2000) 1288; (d) D. Hagrman, P.J. Hagrman, J. Zubieta, Angew. Chem. Int. Ed. 38 (1999) 3165; (e) T. Lee, V.-D. Le, D. Lim, Y.-C. Lin, G.M. Morris, A.L. Wong, A.J. Olson, J.H. Elder, C.-H. Wong, J. Am. Chem. Soc. 121 (1999) 1145; (f) B.F. Abrahams, B.F. Hoskins, D.M. Michail, R. Robson, Nature 369 (1994) 727. (a) for example A.L. Gavrilova, C.J. Qin, R.D. Sommer, A.L. Rheingold, B. Bosnich, J. Am. Chem. Soc. 124 (2002) 1714; (b) D. Chen, A.E. Martell, Inorg. Chem. 26 (1987) 1926; (c) M.H. Wu, K.B. Hansen, E.N. Jaconsen, Angew. Chem. Int. Ed. 38 (1999) 2012; (d) C.J. Chang, J.A. Labinger, H.B. Gray, Inorg. Chem. 36 (1997) 5927. E. Lamour, S. Routier, J.-L. Bernier, J.-P. Catteau, C. Bailly, H. Vezin, J. Am. Chem. Soc. 121 (1999) 1862. L.M. ShkolÕnikova, E.M. Yutal, E.A. Shugam, V.A. Voblikova, Zh. Strukt. Khim. 11 (1970) 886. C.G. Pierpont, R.M. Buchanan, Coord. Chem. Rev. 38 (1981) 45.