The Electronic Structure of [Mn(V)O]: What is the Connection between Oxyl Radical Character, Physical Oxidation State, and Reactivity?

Mn^VO functionalities are important in synthetic and bioinorganic chemistry, being relevant to both C–H activation and the O–O bond formation steps in enzymatic water oxidation, for example. The triplet and quintet spin states are believed to be active in these reactions, but they have only been sparingly characterized experimentally. Density functional theory (DFT) gives varying results, depending on the exchange-correlation functional employed, leading to ambiguity about whether the triplet Mn^VO is better represented as Mn^IV–O^•. While recent CASPT2 studies confirmed that the Mn^IV–O^• character is exaggerated by hybrid functionals, questions still remain about the nature of this bonding. Using high-level wave function methods, we investigated the fundamental relationship between the spin polarization, diradical character, and the physical oxidation state assignments. We conclude that, in terms of formal oxidation assignment, these species are best described as being between the Mn^VO and Mn^IV–O^• extremes. While the extent of the oxyl radical character is exaggerated in B3LYP, it is significantly underestimated by local functionals. We also exploited the DFT-functional dependence of the oxyl radical character to examine its effect on O–O bond formation barrier heights and concluded that, although, for radical combination reactions, the oxyl character is a significant effect, for nucleophilic water attack reactions, the effect is much smaller and is likely not a requisite feature