Study of the catalytic activity of non-heme manganese complexes toward oxidation of cyclooctene and cyclohexene

Abstract

The catalytic activity of two manganese(II) complexes, [MnL1(H2O)](ClO4)2 (1) and [MnL2]ClO4 (2) N,O-donor L1=N,N,N′-tris(2-methylpyridyl)-N′-hydroxyethyl-ethylenediamine and L2=N-(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-N′-hydroxyethyl-ethylenodiamine in the epoxidation of alkenes was investigated in acetonitrile:dichloromethane mixtures at room temperature, using iodosylbenzene as an oxygen source. The epoxidation of cyclooctene in 1h at the best reactants molar ratio (1:50:1000, catalyst:oxidant:substrate) showed similar yields for catalyst 2 (50%) and catalyst 1 (48%). When cyclohexene was the substrate, catalyst 1 showed best yield of 42% at 1:10:1000 and catalyst 2, 62% at 1:10:3000. As a hard Lewis base, the phenolato is expected to facilitate the production and the stabilization of catalytic intermediate species with higher oxidation state like Mn(III) or Mn(IV). The presence of the labile water molecule along with the tension imposed by the heptacoordination of L1 in (1) favors the formation of oxo or hydroxo Mn(II)-dimers as the catalytically active species. Conversely, the robust nature and the hexacoordination of L2 compared to L1 favors the stabilization of mononuclear active species. Oxidation reactions were monitored with Electron Paramagnetic Resonance (EPR) technique and plausible oxidation states for the manganese-catalysts are discussed as well.