Vanadium(V) oxyanions: Interactions of vanadate with 1,1,1-tris(hydroxymethyl)ethane and with the buffer tris(hydroxymethyl)aminomethane

51V NMR spectroscopy has been used to study the interactions of vanadate with 1,1,1-tris(hydroxymethyl)ethane (tris-eth) and with the buffer tris(hydroxymethyl)aminomethane. The reaction of vanadate with tris-eth was found to proceed readily at pH 7.5 to give a variety of products. From variation of vanadate and ligand concentration it was possible to obtain the ligand stoichiometry and formation constants for the various products. The products identified included the monoester of vanadate, the mono- and diesters of divanadate, and a binuclear product for which a bipyramidal coordination geometry about each vanadium nucleus was proposed. The reaction of Tris buffer with vanadate was similar to that of tris-eth. Studies at pH 7.6 and 9.0 showed the formation of vanadate esters of tetrahedral coordination, a bis(ligand) product, assigned an octahedral coordination, and, as well, two binuclear products, one with a single Tris ligand and the second with two ligands. The results were consistent with a bipyramidal/tetrahedral mixed anhydride in the first case and a symmetrical bipyramidal anhydride analogous to that formed with tris-eth in the second case. A rather surprising observation was that these two products exhibited a large chemical shift dependence on pH, presumably reflecting the protonation/deprotonation of the amino group of Tris. Investigation of vanadate in the presence of cis- or trans-1,2-cyclohexanedial and with or without Tris buffer revealed a strong synergistic reaction of tris with the products of the reaction of vanadate with either of the diols. This result suggests that Tris buffer should be used with extreme caution when vanadate/ligand interactions are studied. 14 refs., 6 figs