Vanadium(V) hydroxylamido complexes: Solid state and solution properties

Abstract

A novel series of vanadium(V) hydroxylamido complexes with weak ligands 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 characterized both in solution and in the solid state. All complexes were prepared in aqueous solution at neutral pH at ambient temperature and 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 hydroxylamido nitrogen cis to the organic ligand in the equatorial plane. In complex 4, the hydroxylamido groups are coordinated side-on with the hydroxylamido nitrogen trans to the imidazole ligand in the equatorial plane. The UV/vis spectra of these complexes were also examined, and the absorbance peaks show similarities between the properties of the vanadium(V) hydroxylamido complexes and known side-on peroxovanadium complexes. Multinuclear NMR studies were conducted to characterize the solution structure and properties of compounds 1-4. A particularly detailed study of compound 3 was carried out since the analogous vanadium(V) peroxo complex could also be prepared. Complex 4 was less labile and more stable than the corresponding diperoxovanadium(V)-imidazole complex, H[VO(O2)2(imidazole)] (5). In solution the inherent asymmetry of the hydroxylamido ligand has facilitated an in-depth study of ligand exchange. Upon dissolution, compound 4 forms three isomeric complexes, all of which have one of the original two-coordinated imidazole groups in the complex dissociated. 1D and 2D EXSY and multinuclear NMR spectroscopies 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 dihydroxylamine-vanadium(V)-imidazole involving both the coordinated imidazole and the coordinated hydroxylamido groups. The intramolecular exchange of the coordinated imidazole in 5 was compared to the exchange in the hydroxylamido complex, and the hydroxylamido compounds were found to have some properties that may be advantageous over those of the diperoxovanadium(V) complexes. In summary, evidence was generated to support 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 organic ligands.