Mechanistic elucidation of O2 production from tBuOOH in water using the Mn(ii) catalyst [Mn2(mcbpen)2(H2O)2]2+: a DFT study

Abstract

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 O₂ generation from ^tBuOOH, using H₂¹⁸O as the solvent, in the presence of the Mn(II) catalyst [Mn₂(mcbpen)₂(H₂O)₂]²⁺. 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⁻¹, ruling out the functioning of such a dimer as the active catalyst. Experimental evidence has shown that the oxidation of [Mn₂(mcbpen)₂(H₂O)₂]²⁺ by ^tBuOOH in H₂¹⁸O produces the Mn(IV) species [Mn(¹⁸O)(mcbpen)]⁺. Our investigations revealed a plausible mechanism for this observation in which [Mn (¹⁸O)(mcbpen)]⁺ acts as the active catalyst, generating the tert-butyl peroxyl radical (^tBuOO˙) through its reaction with ^tBuOOH. In this proposed mechanism, the O–O bond is formed through the interaction of ^tBuOO˙ with another [Mn(¹⁸O)(mcbpen)]⁺, finally leading to the formation of the ¹⁶O¹⁸O product. Our findings underscore the pivotal role of [Mn(¹⁸O)(mcbpen)]⁺ in both generating the active species ^tBuOO˙ and consuming it to produce ¹⁶O¹⁸O. With activation barriers as low as about 9 kcal mol⁻¹, these elementary steps highlight the feasibility of our proposed mechanism. Moreover, this mechanism elucidates why, experimentally, one of the oxygen atoms in the released O₂ comes from water, while the other originates from ^tBuOOH. 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.