Synthesis, structure and catalytic olefin epoxidation activity of a dinuclear oxo-bridged oxodiperoxomolybdenum(VI) complex containing coordinated 4,4′-bipyridinium

Abstract

A solution obtained by dissolving MoO3 in aqueous hydrogen peroxide reacts with 4,4′-bipyridine (4,4′-bipy) to give the dinuclear oxodiperoxo complex [{MoO(O2)2(4,4′-Hbipy)}2(μ-O)] (1). The unique structural feature of 1 with respect to other related complexes is the completion of the coordination sphere of the Mo atoms by a monoprotonated [4,4′-Hbipy]+ molecule, which is bound to the metal center through the unprotonated pyridine ring. The catalytic performance of 1 has been investigated for the epoxidation of the benchmark substrate cis-cyclooctene and three representative bio-olefins, namely limonene, methyl oleate and methyl linoleate, using either aq. H2O2 or tert-butylhydroperoxide (TBHP) oxidants. For the epoxidation of cis-cyclooctene, catalytic activity at 55°C with TBHP was comparable with that at 70°C with H2O2 (90–95% conversion at 24h; turnover number in the range 90–95molCymolMo−1; 100% selectivity towards cyclooctene epoxide). The catalytic reactions are homogeneously catalysed and compounds were isolated from the reaction mixtures in order to ascertain the nature of the active species present in solution. With H2O2 as oxidant, the liquid-phase conversion of 1 to β-octamolybdate anions, β-[Mo8O26]4−, takes place during the 24h catalytic batch run. The same transformation occurs when the molybdenum oxide-bipyridine hybrid compound [MoO3(4,4′-bipy)0.5] (2) is used as the (pre)catalyst instead of 1. With TBHP as oxidant, no species other than 1 were isolated from the recovered reaction solution. For the reactions with the bio-olefins (1, TBHP, 70°C, 24h), fairly high conversions of 68–92% were reached, with very high selectivity towards epoxide products (90–100%). Monoepoxides were the major products formed from limonene and methyl linoleate.