SYNTHESIS, STRUCTURE AND METAL REDOX OF NEW VO3+ AND VO2+ COMPLEXES INCORPORATING MIXED TRIDENTATE BIDENTATE BINDING

The reaction of bis(acetylacetonato)oxovanadium(IV) with a mixture of a tridentate ONO-co-ordinating hydrazone H-2L1-H-2L4 (general abbreviation H-2L; 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(IV)O(L)(bipy)], [V(IV)O(L)(phen)] and [V(V)O(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 V=O distance is 1.592(3) angstrom and the atom lying trans to V=O is the quin nitrogen. There is a large decrease (almost-equal-to 700 mV) in the VO3+-VO2+ E1/2 values between [VO(L)(bipy)] for [VO(L)(phen)]} and [VO(L)(quin)]. The complex [V(IV)O(L)(quin)]- was electrogenerated in solution but spontaneously reoxidized to [V(V)O(L)(quin)] in air. For a given bidentate ligand, the VO3+-VO2+ E1/2 values follow the order L1 < L4 < L2 < L3. The ESR spectra of the VO2+ complexes correspond to an axially compressed d(xy)1 configuration. The V-51 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.