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

Abstract

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 pK_a1= 4.7 and pK_a2= 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.