VANADIUM(V) HYDROXYLAMIDO COMPLEXES - SOLID-STATE AND SOLUTION PROPERTIES

A novel series of vanadium(V) hydroxylamido complexes with weak ligand s including glycine, [VO(NH2O)(2)(Glycine)]. H2O (1); serine, [VO(NH2O )(2)(Serine)]. H2O (2); glycylglycine, [VO(NH2O)(2)(GlyGly)]. H2O (3); and imidazole, [VO(NH2O)(2)(imidazole)(2)]Cl (4) were prepared and ch aracterized both in solution and in the solid state. All complexes wer e prepared in aqueous solution at neutral pH at ambient temperature an d as crystalline solids. The vanadium atom in these four complexes is seven-coordinate with pentagonal bipyramidal geometry. In complexes 1- 3 the hydroxylamido groups are coordinated side-on with the hydroxylam ido nitrogen cia to the organic ligand in the equatorial plane. In com plex 4, the hydroxylamido groups are coordinated side-on with the hydr oxylamido nitrogen trans to the imidazole ligand in the equatorial pla ne. The UV/vis spectra of these complexes were also examined, and the absorbance peaks show similarities between the properties of the vanad ium(V) hydroxylamido complexes and known side-on peroxovanadium comple xes. Multinuclear NMR studies were conducted to characterize the solut ion structure and properties of compounds 1-4. A particularly detailed study of compound 4 was carried out since the analogous vanadium(V) p eroxo complex could-also be prepared. Complex 4 was less labile and mo re stable than the corresponding diperoxovanadium(V)-imidazole complex , H[VO(O-2)(2)(imidazole)] (5). In solution the inherent asymmetry of the hydroxylamido ligand has facilitated an in-depth study of ligand e xchange. Upon dissolution, compound 4 forms three isomeric complexes, all of which have one of the original two-coordinated imidazole groups in the complex dissociated. ID and 2D EXSY and multinuclear NMR spect roscopies were used to examine the stoichiometry of the isomers, their structures, and the dynamics of their ligand exchanges. Specifically; both inter- and intramolecular exchanges were observed for the dihydr oxylamine-vanadium(V)-imidazole involving both the coordinated imidazo le and the coordinated hydroxylamido groups. The intramolecular exchan ge of-the coordinated imidazole in 5 was compared to the exchange in t he hydroxylamido complex, and the hydroxylamido compounds were found t o have some properties that may be advantageous over those of the dipe roxovanadium(V) complexes. In summary, evidence was generated to suppo rt the existence of a novel and unprecedented asymmetric hydroxylamido -metal complex as well as the first isolation and characterization of a vanadium(V)-imidazole complex not enjoying stabilization by other or ganic ligands.