On the Nature of the Mn-O Bond in Trigonal Bipyramidal High-Valent Manganese Complexes

Abstract

Water oxidation in natural photosynthesis is performed by the oxygen-evolving complex (OEC), an oxo-bridged Mn 4 CaO 5 cluster embedded in the photosystem II protein complex. The precise chemical identity of its catalytically active forms, and thus the mechanistic details of water oxidation, remain under investigation. A key question concerns the nature of the active species that initiates O-O bond formation, specifically its metal-oxo or oxyl character, which would in turn dictate the type of reactivity. This depends on the electronic structure and local coordination characteristics of the implicated Mn ions. Synthetic models are crucial for shedding light into such correlations. Here we focus on a series of five-coordinate Mn compounds with a terminal Mn-O group. They exist in a sequence of oxidation states, allowing us to probe changes in the nature of the terminal Mn-O group upon increasing the total oxidation level. Multireference calculations are used to distinguish between oxo and oxyl formulations and to describe the latter by analyzing specific electronic configurations. The results demonstrate that Mn sites of this type are unlikely to participate in radical-based chemistry. If five-coordinate Mn centers with terminal oxo groups are involved in O-O bond formation, they would instead be implicated in a nucleophilic attack (or acid-base) mechanism. By contrast, oxyl states are readily accessible in the case of six-coordinate high-valent Mn sites, in line with previously proposed oxo-oxyl coupling models for the OEC.