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

Abstract

MnV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 MnVO is better represented as MnIV–O•. While recent CASPT2 studies confirmed that the MnIV–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 MnVO and MnIV–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.

Discovery of Open Cubane Core Structures for Biomimetic LnCo3(OR)4 Water Oxidation Catalysts

“…The stronger radical character should potentially enhance the reactivity of the ligand and thereby facilitate the oxygen–oxygen bond formation . However, for a conceptually similar Mn–oxo system, the radical character alone was found not to be crucial for a WNA …”

Computational Assessment of MLCT versus MC Stabilities in First‐to‐Third‐Row d6 Pseudo‐Octahedral Transition Metal Complexes

“…The standard procedure is to compute MS‐CASPT2 states at the GS DFT‐optimized structure. However, previous studies have shown that the use of GS DFT structures may not be appropriate and thus, some coordinates have been reoptimized with multiconfigurational ab initio methods for a particular purpose . Herein, we evaluate the variability of the MS‐CASPT2 vertical excited states with respect to the choice of the DFT‐optimized geometry for first‐to‐third‐row d 6 pseudo‐octahedral TMCs.…”

Insights into artificial water oxidation—A computational perspective

“…For those species one might draw the structure as {M --O} ← − → {M ---O}, where the formal oxidation states are M +(n -I) + O +I . Here, the +I oxidation state of the ligand highlights its electrophilic character, in the case of neutral counting 23 .…”