Oxidoperoxidotungsten(VI) complexes with secondary hydroxamic acids:: Synthesis, structure and catalytic uses in highly efficient, selective and ecologically benign oxidation of olefins, alcohols, sulfides and amines with H2O2 as a terminal oxidant

The reaction of a solution of freshly precipitated WO3 in H2O2 separately with the secondary hydroxamic acids N-benzoyl-N-phenylhydroxamic acid (BPHAH), N-benzoyl-N-ortho-tolylhydroxamic acid (BOTHAH), N-benzoyl-N-meta-tolylhydroxamic acid (BMTHAH), N-benzoyl-N-para-tolylhydroxamic acid (BPTHAH) and N-cinnamyl-N-phenylhydroxamic acid (CPHAH) afforded [WO(O-2)(BPHA)(2)] (1), [WO(O-2)(BOTHA)(2)] (2), [WO(O-2)(BMTHA)(2)] (3), [WO(O-2)(BPTHA)(2)] (4) and [WO(O-2)(CPHA)(2)] (5), respectively. Aqueous tungstate solution, on reaction with all these hydroxamic acids, produced [W(O)(2)(hydroxamato)(2)] (6). The complexes show excellent catalytic functions in the oxidation of (a) olefins at room temperature in the presence of NaHCO3 as promoter, (b) alcohols, sulfides and amines, at reflux, with H2O2 as a terminal oxidant, yielding a high turnover number (TON), the highest being for olefin-to-epoxide conversion. An attempt to synthesize peroxide-rich complexes of the type PPh4[WO(O-2)2(hydroxamato)] (7), for example PPh4[WO(O-2)(2)BMTHA (7C), resulted in the isolation of PPh4[WO(O-2)(2)(C6H5COO)] (8), which was probably obtained by the hydrolysis of coordinated BMTHA. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).