Oxygen exchange and protonation of polyanions: a multinuclear magnetic resonance study of tetradecavanadophosphate(9–) and decavanadate(6–)

Unlike decavanadate(6–), tetradecavanadophosphate(9–) shows pH-dependent differential rates of exchange of oxygen with solvent water. If the anion's structure is described as a Keggin-type ‘sphere’ with polar ‘caps,’ then at pH > 5 the caps are kinetically stabilised with respect to the ‘equator,’ whereas at pH < 1 the caps are destabilised and only the four half-hemisphere three-octahedron units resist exchange. These observations are related to bulk distortions of the heteropolyanion which can be deduced from the variations of the 17O, 51V, and 31P chemical shifts with pH. The values pKa1= 4.7 and pKa2= 1.3 have been determined for the heteropolyanion. Decavanadate(6–) distorts similarly. The chemical shifts of singly bridging oxygen are shown to increase as the bond angle at oxygen decreases.