Robust and efficient amide-based nonheme manganese(III) hydrocarbon oxidation catalysts: Substrate and solvent effects on involvement and partition of multiple active oxidants

Two new mononuclear nonheme manganese(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Mn(bpc)Cl(H₂O)] (1) and [Mn(Me₂bpb)Cl(H₂O)]·CH₃OH (2), were prepared and characterized. Complex 2 has also been characterized by X-ray crystallography. Magnetic measurements revealed that the complexes are high spin (S=5/2) Mn^III species with typical magnetic moments of 4.76 and 4.95 μ_B, respectively. These nonheme Mn^III complexes efficiently catalyzed olefin epoxidation and alcohol oxidation upon treatment with MCPBA under mild experimental conditions. Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants Mn^V=O, Mn^IV=O, and Mn^III-OO(O)CR. Evidence for this approach was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, H₂¹⁸O-exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. In addition, it has been proposed that the participation of Mn^V=O, Mn^IV=O, and Mn^III-OOR could be controlled by changing the substrate concentration, and that partitioning between heterolysis and homolysis of the O-O bond of a Mn-acylperoxo intermediate (Mn-OOC(O)R) might be significantly affected by the nature of solvent, and that the O-O bond of the Mn-OOC(O)R might proceed predominantly by heterolytic cleavage in protic solvent. Therefore, a discrete Mn^V=O intermediate appeared to be the dominant reactive species in protic solvents. Furthermore, we have observed close similarities between these nonheme Mn^III complex systems and Mn(saloph) catalysts previously reported, suggesting that this simultaneous operation of the three active oxidants might prevail in all the manganese-catalyzed olefin epoxidations, including Mn(salen), Mn(nonheme), and even Mn(porphyrin) complexes. This mechanism provides the greatest congruity with related oxidation reactions by using certain Mn complexes as catalysts.