Mechanistic elucidation of O production from BuOOH in water using the Mn(II) catalyst [Mn(mcbpen)(HO)]: a DFT study.

This study employs density functional theory at the SMD/B3LYP-D3/6-311+G(2d,p),def2-TZVPP//SMD/B3LYP-D3/6-31G(d),SDD level of theory to explore the mechanistic details of O generation from BuOOH, using HO as the solvent, in the presence of the Mn(II) catalyst [Mn(mcbpen)(HO)]. Since this chemistry was reported to occur through the reaction of Mn(III)(μ-O)Mn(IV)-O˙ with water, we first revaluated this proposal and found that it occurs with an activation barrier greater than 36 kcal mol, ruling out the functioning of such a dimer as the active catalyst. Experimental evidence has shown that the oxidation of [Mn(mcbpen)(HO)] by BuOOH in HO produces the Mn(IV) species [Mn(O)(mcbpen)]. Our investigations revealed a plausible mechanism for this observation in which [Mn (O)(mcbpen)] acts as the active catalyst, generating the -butyl peroxyl radical (BuOO˙) through its reaction with BuOOH. In this proposed mechanism, the O-O bond is formed through the interaction of BuOO˙ with another [Mn(O)(mcbpen)], finally leading to the formation of the OO product. Our findings underscore the pivotal role of [Mn(O)(mcbpen)] in both generating the active species BuOO˙ and consuming it to produce OO. With activation barriers as low as about 9 kcal mol, these elementary steps highlight the feasibility of our proposed mechanism. Moreover, this mechanism elucidates why, experimentally, one of the oxygen atoms in the released O comes from water, while the other originates from BuOOH. This research broadens our understanding of high oxidation state manganese chemistry, setting the stage for the development of more efficient Mn-based catalysts, aimed at improving processes in both renewable energy and synthetic chemistry.