Synthesis, structure and metal redox of new VO 3+ and VO 2+ complexes incorporating mixed tridentate–bidentate binding

The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of a tridentate ONO-co-ordinating hydrazone H2L1–H2L4(general abbreviation H2L; condensates of benzoylhydrazine with benzoylacetone, salicylaldehyde, 2-hydroxy-1-naphthaldehyde and 2-hydroxybutyrophenone respectively) and a bidentate compound 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen) or quinolin-8-ol (Hquin) has afforded the complexes [VIVO(L)(bipy)]. [VIVO(L)(phen)] and [VVO(L)(quin)] in high yields. The crystal structure of [VO(L1)(quin)] has been determined, revealing the distorted-octahedral VO(ONO)(ON) co-ordination sphere with the L1 ligand spanning meridionally. The VO distance is 1.592(3)Å and the atom lying trans to VO is the quin nitrogen. There is a large decrease (≈ 700 mV) in the VO3+–VO2+E½ values between [VO(L)(bipy)]{or [VO(L)(phen)]} and [VO(L)(quin)]. The complex [VIVO(L)(quin)]– was electrogenerated in solution but spontaneously reoxidized to [VVO(L)(quin)] in air. For a given bidentate ligand, the VO3+–VO2+E½ values follow the order L1 < L4 < L2 < L3. The ESR spectra of the VO2+ complexes correspond to an axially compressed dxy1 configuration. The 51V hyperfine constants are slightly larger for [VO(L)(quin)]– than for [VO(L)-(bipy)] and [VO(L)(phen)]. The dxy→ dxz,dyz transition of the tetravalent complexes occurs in the region 700–950 nm.