Synthesis, structure and metal redox of new VO3+ and VO2+ complexes incorporating mixed tridentate–bidentate binding

Abstract

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 [V^IVO(L)(bipy)]. [V^IVO(L)(phen)] and [V^VO(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⁺E_½ values between [VO(L)(bipy)]{or [VO(L)(phen)]} and [VO(L)(quin)]. The complex [V^IVO(L)(quin)]^– was electrogenerated in solution but spontaneously reoxidized to [V^VO(L)(quin)] in air. For a given bidentate ligand, the VO³⁺–VO²⁺E_½ values follow the order L¹ < L⁴ < L² < L³. The ESR spectra of the VO²⁺ complexes correspond to an axially compressed d_xy¹ configuration. The ⁵¹V hyperfine constants are slightly larger for [VO(L)(quin)]^– than for [VO(L)-(bipy)] and [VO(L)(phen)]. The d_xy→ d_xz,d_yz transition of the tetravalent complexes occurs in the region 700–950 nm.