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Oxo-bridged dinuclearCrIII-TaV complex: structural and spectroscopic characterization

The oxo-bridged metal units are interesting intramolecular motifs appearing in several bioinorganic molecules such as metalloproteins (Fe–O–Fe core) and in various inorganic systems, especially as polyoxometallates, materials with outstanding properties and functions.[1, 2] Recently, unusual behaviour that causes the changes in material color and magnetic properties has been observed in structures with Cr–O–Cr bridges, referred as metal-to-metal-charge transfer (MMCT).[3] Even though there are several reports of light- and heat-induced MMCT, the occurrence of this intriguing phenomena is hard to predict and the relationship between electron transfer and molecular structure is not jet understood.[4] In the recent years our research group has been involved in intensive studies related to tris(oxalato)oxotantalate(V) anion, with the aim of obtaining new metal-organic coordination systems with specific (electrical, optical, catalytic and/or magnetic) properties. It was found that this type of anion is prone to form heterometallic complexes with different topologies and nuclearities.[5] Depending on reaction conditions and reagents, the oxo group in [TaO(C2O4)3]3– can be protonated or replaced with other ligand in the coordination sphere of tantalum.[6] Reaction of water solutions of complex [Cr(bpy)2(OH2)]3+ cations and [TaO(C2O4)3]3– anions yielded compound with new bonding topology of this anion – dinuclear oxo-bridged complex [Cr(bpy)2(OH2)(μ-O)Ta(C2O4)3]2·7H2O (1) (bpy = 2, 2´-bipyridine). Compound 1 was characterized by single-crystal X-ray diffraction, IR and UV/Vis spectroscopy. Figure 1 shows dinuclear [Cr(bpy)2(OH2)(μ-O)Ta(C2O4)3] unit, with chromium in octahedral geometry and tantalum in distorted pentagonal bipyramidal geometry. Single crystal measurements at room temperature and at 100 K have shown some differences in the unit cell parameters, as well as in the M–(O, N) bond lengths, which my indicate the change of oxidation state of metal centres. Further magnetic and MMCT studies of 1 are in progress.

oxo-bridged complex; tantalum; chromium; metal-to-metal charge transfer

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