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The complexes [VO(L1)(hquin)] and [VO(L2)(hquin)] incorporating N-salicylidene-glycinate and -L-phenylalaninate respectively have been synthesised in excellent yields from [VIVO(L)(H2O)] and quinolin-8-ol (Hhquin) in air which acts as the oxidant. The crystal structure of [VO(L2)(hquin)] revealed tridentate ONO and bidentate ON binding by [L2]2– and hquin– respectively. The V–O (carboxylate) bond length is longer than V–O (phenolate) by ≈0.1 A. The CD spectrum of [VO(L2)(hquin)] in the visible region revealed the composite nature of the phenolato→vanadium charge-transfer band. The absolute configuration of [VO(L2)(hquin)] is CS both in the crystalline state and in solution. In CDCl3 solution the 1H NMR spectrum of the complex shows that the amino acid side chain has the same conformation as that in the crystal lattice. The complexes display the quasi-reversible one-electron couple [VO(L)(hquin)]–[VO(L)(hquin)]– near 0.0 V vs. saturated calomel electrode. Electrogenerated solutions of [VO(L)(hquin)]– are EPR-active corresponding to a dxy1 configuration.

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)A 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.

The reaction of [VIVO(L)(H2O)] with ethane-1,2-diol (H2ed) in methanol in air afforded complexes of type [VVO(L)(Hed)] in very good yields (H2L1=N-salicylideneglycine, H2L2=N-salicylidene-L-phenylalanine). The crystal structure of [VO(L2)(Hed)] has revealed the presence of tridentate [L2]2– and bidentate Hed–. In the latter the alcoholic oxygen lies trans to the oxo atom. In the VO5N co-ordination sphere the five vanadium–oxygen lengths follow the order: oxo < alkoxidic < phenoxidic < carboxylic < alcoholic. The absolute configuration of the complex is CS. The lattice consists of hydrogen-bonded (between alcoholic and carboxylic oxygen atoms) dimers having two-fold symmetry. The CD spectrum of [VO(L2)(Hed)] has revealed the composite nature of the O(alkoxide)→ V charge-transfer band in the visible region. In the 1H NMR spectra of the complexes the four methylene protons of co-ordinated Hed– are inequivalent. The [VVO(L)(Hed)] complexes represent authentic examples of the very rare mononuclear oxovanadium species incorporating ethane-1,2-diol as a ligand.

In the title complex [VOL1L2] derived from the tridentate 2-hydroxy-2′-carboxy-5-methylazobenzene (H2L1) and the bidentate quinolin-8-ol (HL2) the V–O(carboxylate) and V–N(azo) distances are respectively 1.892(4) and 2.098(5)A; frozen solution (77 K) EPR parameters of electrogenerated [VOL1L2]– lie close to those of reduced bromoperoxidase (low pH).

In [L(O)VOV(O)L]–, the one-electron reduced form of structurally characterised [L(O)VOV(O)L](L = azobenzene-2,2′-diolate), metal valence is delocalised at 300 K but is localised at one centre at 77 K with average hyperfine coupling constants of ≈50 and ≈100 G respectively; the estimated rate constant is ≈4 × 1010 s–1(300 K) and the activation energy is ≈3 kcal mol–1.