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

Two new tetranuclear chalcocyanide cluster complexes, [(Mn(SaloPh)H ₂O}₄Re₄Q₄(CN)₁₂] ·4CH₃OH· 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^IIIOOC(O)R. Evidence in support of this interpretation has been derived from reactivity and Hammett studies, H ₂¹⁸Oexchange 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.