Mechanistic elucidation of O 2 production from t BuOOH in water using the Mn(II) catalyst [Mn 2 (mcbpen) 2 (H 2 O) 2 ] 2+ : 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 2 generation from t BuOOH, using H 2 18 O as the solvent, in the presence of the Mn(II) catalyst [Mn 2 (mcbpen) 2 (H 2 O) 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 [Mn 2 (mcbpen) 2 (H 2 O) 2 ] 2+ by t BuOOH in H 2 18 O produces the Mn(IV) species [Mn( 18 O)(mcbpen)] + . Our investigations revealed a plausible mechanism for this observation in which [Mn ( 18 O)(mcbpen)] + acts as the active catalyst, generating the tert -butyl peroxyl radical ( t BuOO˙) through its reaction with t BuOOH. In this proposed mechanism, the O-O bond is formed through the interaction of t BuOO˙ with another [Mn( 18 O)(mcbpen)] + , finally leading to the formation of the 16 O 18 O product. Our findings underscore the pivotal role of [Mn( 18 O)(mcbpen)] + in both generating the active species t BuOO˙ and consuming it to produce 16 O 18 O. 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 O 2 comes from water, while the other originates from t 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.