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The kinetics of the reaction of neutral [Mo(S2C6H4)3] with hydrogen sulfite to produce the anionic Mo(V) complex, [Mo(S2C6H4)3]-, and sulfate have been investigated. It has been shown that [Mo(S2C6H4)3] acts as the electron—proton sink in the oxygenation reaction of HSO3- by water. Reaction rates, monitored by UV/vis stopped-flow spectrometry, were studied in THF/water media as a function of the concentration of HSO3- and molybdenum complex, pH, ionic strength, and temperature. The reaction exhibits pH-dependent HSO3- saturaiton kinetics, and it is first-order in complex concentration. The kinetic data and MS-ESI spectra are consistent with the formation of [Mo O(S2C6H4)2(S2C6H5)]- (1) adduct as a crucial intermediate that transfers the oxygen atom to HSO3- yielding the Mo(V) species quantitatively. [ABSTRACT FROM AUTHOR]

Abstract: The heterogeneous oxidation of benzenethiol catalyzed by the dianionic bis(2-sulfanyl-2,2-diphenylethanoxycarbonyl) dioxomolybdate (VI) complex intercalated into a Zn(II)–Al(III) layered double hydroxide (LDH) host have been investigated under aerobic conditions. The kinetics of the system has been analysed in detail. In ethanol, the benzenethiol is cleanly oxidized to diphenyl disulfide in the acidic media provided by the protonic resin Amberlite IR-120(H). The reaction is second-order in benzenethiol, and the apparent rate coefficient has been found to be proportional to the catalyst weight and inversely proportional to the initial concentration of the substrate. A catalytic cycle consistent with the kinetic data has been proposed. The catalytic cycle starts with the acid–base interaction of benzenethiol with the hydroxide anions present in the LDH to yield benzenethiolate anions which coordinate the Mo(VI) centres of the catalyst. The Mo(VI)-coordinated-sulphur species undergo decomposition to form neutral Mo(V) centres and benzenethiol radicals. In excess dioxygen, the Mo(V) species forms a peroxo-Mo centre which regenerates the Mo(VI) active sites by reacting with more benzenethiolate and protons, and producing hydroperoxyl radicals. The apparent activation energy is equal to 40kJmol−1, which agrees with the reduction to Mo(V) of the sulphur-coordinate Mo(VI) centre as the rate limiting step. The proposed catalytic cycle completes and generalizes the study of a reaction pathway previously reported, in which the bis(2-sulfanyl-2,2-diphenylethanoxycarbonyl) dioxomolybdate (VI) complex catalyzes the reduction of nitrobenzene to aniline coupled to the oxidation of benzenethiol to diphenyl disulfide under anaerobic conditions. The relevance of this catalyst-system lies in its ability to prevent the undesirable comproportionation reactions of the diverse oxidation state molybdenum intermediates making possible multiple-turnover oxidations of benzenethiol to diphenyl disulfide. [Copyright &y& Elsevier]