CaMn3IVO4 Cubane Models of the Oxygen‐Evolving Complex: Spin Ground States S<9/2 and the Effect of Oxo Protonation

• Published in: Angewandte Chemie International Edition 2021-08, Vol.60 (32), p.17671-17679
• Main Authors: Lee, Heui Beom, Shiau, Angela A., Marchiori, David A., Oyala, Paul H., Yoo, Byung‐Kuk, Kaiser, Jens T., Rees, Douglas C., Britt, R. David, Agapie, Theodor
• Format: Article
• Language: English
• Subjects: Complement component C3; Computer applications; Crystal structure; Cubane; electronic structure; Evolution; Ground state; Magnetic permeability; Magnetic susceptibility; Mathematical models; model complexes; Oxygen; Oxygen atoms; oxygen-evolving complex; Protonation; Single crystals; spin states
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202105303

We report the single crystal XRD and MicroED structure, magnetic susceptibility, and EPR data of a series of CaMn3IVO4 and YMn3IVO4 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 CaMn3IVO4 complexes and variants with protonated oxo moieties need not be S=9/2. Desymmetrization of the pseudo‐C3‐symmetric Ca(Y)Mn3IVO4 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 CaMn3IVO3(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 the (de)protonation of bridging or terminal oxygen atoms in the OEC may be connected to spin‐state changes. To better describe the Sn‐state intermediates of Photosystem II, a ferromagnetically coupled CaMn3IVO4 subunit with an S=9/2 ground state has been proposed. This assignment has played a key role in the mechanism of water oxidation, but electronic‐structure studies of CaMn3IVO4 complexes remain rare. Through cluster desymmetrization or oxo protonation, lower spin ground states are found to be accessible, challenging prior models.