C-13 NMR SPECTROSCOPIC STUDIES ON THE REACTIONS OF VANADIUM(V) WITH C-13(2)-LABELED OXALATE IN AQUEOUS AND 50-PERCENT V/V AQUEOUS ACETIC-ACID SOLUTIONS

C-13 n.m.r. spectroscopy has been used to study the reactions of vanadate, H2VO4-, with C-13(2)-labelled oxalate in aqueous and 50% v/v aqueous acetic acid media. The results of the aqueous system corroborate the natural abundance oxalate C-13 n.m.r, studies of Ehde and coworkers (Ehde, P. M., Petterson, L., and Glaser, J., Acta Chem. Scand., 1991, 45, 998) who report the formation of both mono(oxalato)-, [VO2(ox)(OH2)](-), and bis (oxalato)-vanadium(v), cis-[VO2(ox)(2)](3-), complexes in solution. At 270 K, pH 3 . 3, and an n.m.r. operating frequency of 100 . 16 MHz, the former complex appears as a singlet (167 . 3 ppm), while the latter resolves into an AB coupling pattern (169 . 6, 168 . 8, 166 . 6, 165 . 8 ppm; J(c-c) 85 Hz) owing to the inequivalence of carbons trans and cis to the oxo ligands. In 50% v/v aqueous acetic acid both the mono(oxalato) singlet (166 . 8 ppm) and the bis(oxalato) AB coupling pattern (169 . 2, 168 . 5, 166 . 2, 165 . 5 ppm; J(c-c) approximate to 75 Hz) are present but are somewhat broadened and shifted downfield, which is indicative of other species being present. There is also some resolution of new peaks within the broadened signals. These are believed to be due to the presence of the additional complexes, mainly [VO2(ox)(OAc)](2-) and cis-[VO2(ox) (OAc)(2)](3-), generated in the acetic acid medium. At 298 K, the bis(oxalato) complex exhibits a single C-13 peak for the inequivalent carbons at 167 . 6 ppm because of fluxional behaviour. The mono(oxalato) complex is in rapid equilibrium with free oxalate, giving a second peak at lower value of 165 . 7 ppm that is pH-dependent. The mechanisms of the fluxionality of the different complexes are discussed.