CaMn₃^(IV)O₄ Cubane Models of the Oxygen Evolving Complex: Spin Ground States S < 9/2 and the Effect of Oxo Protonation

We report single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn₃^(IV)O₄ and YMn₃^(IV)O₄ complexes as structural and spectroscopic models of the cuboidal subunit of the OEC. The effect of changes in heterometal identity, cluster geometry, and bridging oxo protonation on spin state structure was investigated. In contrast to previous computational models, we show that the spin ground state of CaMn₃^(IV)O₄ complexes and variants with protonated oxo moieties need not be S = 9/2. Desymmetrization of the pseudo-C3 symmetric Ca(Y)Mn₃^(IV)O₄ 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 CaMn₃^(IV)O₃(OH). Our studies complement the observation that the interconversion between the low spin and high spin forms of the S2 state is pH dependent, suggesting that (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin state changes