CaMn O Cubane Models of the Oxygen-Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation.

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn O and YMn O complexes as structural and spectroscopic models of the cuboidal subunit of the oxygen-evolving complex (OEC). The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on the spin-state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn O complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo-C -symmetric Ca(Y)Mn O core leads to a lower S=5/2 spin ground state. The magnitude of the magnetic exchange coupling is attenuated upon oxo protonation, and an S=3/2 spin ground state is observed in CaMn O (OH). Our studies complement the observation that the interconversion between the low-spin and high-spin forms of the S state is pH-dependent, suggesting that the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin-state changes.