A V-51 NMR investigation of the interactions of aqueous vanadate with hydroxylamine

The aqueous reactions of hydroxylamine and N-methylhydroxylamine with vanadate have been studied. Ligand, vanadium, and proton stoichiometries of the products have been obtained as have their formation constants. At lower ligand concentrations the predominant complex contains a single hydroxamido ligand and this has been assigned a five-coordinate, tetrahedral-like structure where the hydroxamido group is coordinated in a side-on fashion. Four bishydroxamido products were observed. These four complexes consisted of two distinct groups of two isomeric complexes each and have been assigned six-coordinate (tetrahedral-like) and seven-coordinate (trigonal bipyramidal-like) geometries. For the N-methylhydroxamido ligand, 10 bisligand products in 2 groups of 5 isomers each were observed. The additional products over the hydroxylamine situation arise from the relative orientations of the ligand methyl groups. For both ligands, the monoligand products carried a single negative charge and did not have a pK(a) within the range of about 6-10. The bisligand products did not carry a charge under neutral conditions. The tetrahedral bisligand products could be protonated under acidic conditions (pK(a) similar to 6.6) but could not be deprotonated. In contrast, the trigonal bipyramidal products were not protonated under acidic conditions but could give up a proton under basic conditions (pK(a) similar to 7.4) to form anionic complexes. The relative orientations of the ligands and also the number of methyl groups and their relative orientations in the complex have a significant influence on the V-51 chemical shifts. One additional complex was observed to form and was assigned to an ester-like product formed by reaction at the hydroxyl group.