Efficient olefin epoxidation by robust Re4 cluster-supported Mn(III) complexes with peracids: evidence of simultaneous operation of multiple active oxidant species, Mn(V)=O, Mn(IV)=O, and Mn(III)-OOC(O)R

Abstract

Two new tetranuclear chalcocyanide cluster complexes, [{Mn(saloph)H2O}4Re4Q4(CN)12]⋅4 CH3OH⋅ 8 H2O (saloph = N,N'-o-phenylenebis(salicylidenaminato), Q = Se (1-Se), Te (2-Te)), have been synthesized by the diffusion of a methanolic solution of [PPh4]4[Re4Q4(CN)12] into a methanolic solution of [Mn(saloph)](+). The structure of 2-Te has been determined by X-ray crystallography. These rhenium cluster-supported [Mn(III)(saloph)] complexes have been found to efficiently catalyze a wide range of olefin epoxidations under mild experimental conditions in the presence of meta-chloroperbenzoic acid (mCPBA). Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn(V)=O, Mn(IV)=O, and Mn(III)-OOC(O)R. Evidence in support of this interpretation has been derived from reactivity and Hammett studies, H2(18)O-exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. Moreover, it has been observed that the participation of Mn(V)=O, Mn(IV)=O, and Mn(III)-OOC(O)R can be controlled by changing the substrate concentration. This mechanism provides the greatest congruity with related oxidation reactions that employ certain Mn complexes as catalysts.

Keywords: cluster compounds; epoxidation; manganese; rhenium.