MULTICOMPONENT POLYANIONS .45. A MULTINUCLEAR NMR-STUDY OF VANADATE(V) OXALATE COMPLEXES IN AQUEOUS-SOLUTION

The two complexes formed in the aqueous vanadooxalate system, V(Ox)- and V(Ox)2(3-), have been characterized using V-51, C-13 and O-17 NMR. For the V(Ox)2(3-) complex, two peaks are observed in C-13 NMR and four in O-17 NMR. This leads to the conclusion that each oxalate ligand has two different distances to the VO2 group. This fact, together with the peak integrals and the chemical shifts, indicates strongly that the hexacoordinate complex [VO2(C2O4)2]3- found in single-crystal X-ray structure determinations persists in aqueous solution. The dependence of the C-13 NMR linewidths upon temperature reveals two types of dynamic processes: (1) a rearrangement in which the two different V-O(ox) switch places and (2) an exchange of the oxalate ligands in the [VO2(C2O4)2]3- complex with free oxalate, probably through a dissociative process. Rate constants and activation parameters for the two dynamic processes involving [VO2(C2O4)2]3- have been calculated from the shape of the C-13 NMR signals. For the V(Ox)- complex, only one relatively narrow peak is obtained in C-13 NMR and three peaks in O-17. This fact, as well as the relative positions of these peaks, is in accordance with a pentacoordinate complex [VO2(C2O4)H2O]-, where the two V-O distances to the oxalate ligand are equal. We also show that, in the pH range 0.8-6.6, there is no protonation of the studied complexes, in agreement with previous potentiometric results.