Divanadium(V) and trapped valence linear tetravanadium(IV,V,V,IV) complexes

Abstract

In an acetonitrile/water mixture, reactions of the N, N′- bis(diacetyl)hydraz:ine (H2diah), bis(acetylacetonato) oxidovanadium(IV) [VO(acac)2] and monodentate N-coordinating heterocycles (he) in a 1:2:2 mol ratio provide yellow divanadium(V) complexes of formula [(hc)O2V(μ-diah)VO2(hc)] (1, hc = imidazole; 2, hc = pyrazole; 3, hc = 3, 5-dimethylpyrazole). On the other hand, in the same solvent mixture reactions of the same reagents in a 1:4:2 mol ratio produce green linear tetravanadium(IV,V,V,IV) complexes of formula [(acac) 2OV(μ-O)VO(hc)(μ-diah)(hc)OV(μ-O)VO(acac)2] (4, hc = imidazole; 5, hc = pyrazole; 6, hc = 3, 5-dimethyl pyrazole). The complexes 1-6 have been characterized, by elemental analysis, magnetic susceptibility, and various spectroscopic and electrochemical measurements. The X-ray crystal structures of 1, 3 and 6 have been determined. In all three structures, the diazine ligand. diah2- is in trans configuration. Metal-centred bond parameters are consistent with the localized electronic structure of the two frans-bent {OV(μ-O)VO}3+ cores present in 6. The pentavalent metal centres in 1, 3 and 6 are in a distorted trigonal-bipyramidal N 2O3 coordination environment, while the terminal tetravalent metal centres in 6 are in a distorted octahedral O6 coordination sphere. The eight-line EPR spectra of the tetravanadium species (4-6) in dimethyl sulfoxide at ambient temperature indicate the rare valence localized electronic structure in the fluid, phase. All the complexes are redox active and display metal-centred electron transfer processes in dimethyl sulfoxide solution. A reduction within -0.78 to -0.94 V (vs. Ag/AgCl) is observed for the divanadium(V) species 1-3, while a reduction and an oxidation, are observed in the potential ranges -0.82 to -0.90 V and 0.96 to 1.12 V (vs. Ag/AgCl), respectively, for the tetravanadium species 4-6. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.