Benzene hydroxylation with hydrogen peroxide catalyzed by vanadium(V)-substituted polyoxomolybdates

Synthesis and P-31 and V-51 NMR characterization of Na-5[PMo10V2O40]. 14H(2)O 1 as a mixture of five alpha-Keggin and two beta-Keggin divanadium(V)-substituted polyoxomolybdates, recently reported by Pettersson's group, were reproduced. Benzene hydroxylation with hydrogen peroxide catalyzed by 1 was examined under varied conditions: amounts of catalyst precursor, amounts of 30% aqueous H2O2, amounts of CH3CN and different temperature. The turnover vs. time curve has shown a reduced induction period. The NMR characterization revealed that the reaction by 1 was catalyzed by the vanadium(V) species within the Keggin polyoxomolybdates. The traditional vanadium(V)-substituted polyoxomolybdates, "H-4[PMo11VO40]" 2, "H-5[PMo10V2O40]" 3 and "H-6[PMo9V3O40]" 4, prepared by the so-called ether-extraction method, consist of Keggin-type H-4[PMo11VO40] and many non-Keggin-type unstable polyoxo species, and do not contain any divanadium(V)-substituted Keggin species like 1. The reactions by 2-4 were catalyzed by a cationic oxovanadium species VO(O-2)(+), directly derived from the contaminated unstable non-Keggin-type species, and, also, indirectly from H-4[PMo11VO40]. (C) 2000 Elsevier Science B.V. All rights reserved.