C-scorpionate Au(III) complexes as pre-catalysts for industrially significant toluene oxidation and benzaldehyde esterification reactions

Abstract

The new Au(III) complex [AuCl2(Tpms)] (1) and the previously reported [AuCl2(Tpm)]Cl (2), bearing the potentially tridentate ligands tris(1-pyrazolyl)methanesulfonate (SO3C(C3H3N2)3-, Tpms) or hydrotris(1-pyrazolyl)methane (HC(C3H3N2)3, Tpm), respectively, were synthesized in water at room temperature and characterized using NMR and IR spectroscopy. The molecular structure of 1 was authenticated by single crystal X-ray diffraction analysis. The catalytic performance of the Au(III) complexes was tested, for the first time, in toluene and benzyl alcohol oxidation reactions. The oxidative esterification of benzaldehyde, by-product of toluene oxidation, was further explored. In order to optimize the catalytic systems, the influence of parameters such as temperature, reaction time, amount of pre-catalyst and the presence of additives was evaluated. In the peroxidative (by H2O2 or t-BuOOH) oxidation reactions, a maximum total yield (benzylic alcohol and benzaldehyde) of 8% for toluene oxidation with pre-catalyst 1 (6 h, 80 °C, H2O2 30% aq. sol.) and a maximum total yield (benzaldehyde and benzoic acid) of 43% for benzyl alcohol oxidation with pre-catalyst 2 (24 h, 80 °C, t-BuOOH 70% aq. sol.) with a selectivity of 72% for benzaldehyde, were obtained. The esterification of benzaldehyde yielded, in the presence of 1, a maximum of 27% and 48% of methyl benzoate, at room temperature and 80 °C, respectively, and with a selectivity of 78% for methyl benzoate.