Reactions of vanadate with N,N-dimethylhydroxylamine: Aqueous equilibria and the crystal structure of the uncharged oxygen-bridged dimer of bis(N,N-dimethylhydroxamido)hydroxooxovanadate

摘要

V-51 nuclear magnetic resonance spectroscopy has been utilized in the investigation of the reactions of vanadate with N,N-dimethylhydroxylamine in aqueous medium. The major components of the reaction products were mono- and bisliganded mononuclear vanadate compounds with V-51 chemical shifts near -630 and -740 ppm, respectively. Variation of the concentration of the reactants enabled the determination of stoichiometry and formation constants of the products. The two major signals near -740 ppm were assigned to two stereoisomers of a bisligand product. The proton stoichiometries and pK(a) values of the major products were determined from pH variation studies. A crystalline product of the type [V(O)(ONMe2)(2)](2)O was isolated from the reaction of vanadate with dimethylhydroxylamine and its structure determined from X-ray diffraction studies. The compound possesses a dimeric ore-bridge structure with a six-coordinate vanadium core. The arrangement about each vanadium may be described as approximately tetrahedral considering the center of the N-O bond in each dimethylhydroxamide ligand as one vertex. Hydrolysis of the crystalline solid in D2O provided two isomers that corresponded to the two bisligand products. A variable temperature H-1 NMR study in D2O and 50% D2O/(CD3)(2)CO mixture revealed the existence of reasonably fast chemical exchange between the two predominant isomers. The nature of coordination of these and related compounds is discussed. Crystal structure of [V(O)(ONMe2)(2)](2)O: orthorhombic, space group P22(1)2(1); Z = 2; a = 7.0955(9) Angstrom; b = 10.2313(12) Angstrom; c = 11.5942(11) Angstrom; V = 841.69 Angstrom(3); T = 213 K; R-F = 0.021 for 1141 data (I-0 greater than or equal to 2.5 sigma(I-0)) and 137 variables.