Spin-Gated Selectivity of the Water Oxidation Reaction Mediated by Free Pentameric CaMn_5-O₅⁺ Clusters

We report on the first preparation of isolated ligand-free CaMn₄O₅⁺ gas-phase clusters, as well as other pentameric CaMn_5-O₅⁺ (x = 0-4) clusters with varying Ca contents, which serve as molecular models of the natural CaMn₄O₅ inorganic cluster in photosystem II. Ion trap reactivity studies with D₂O and H₂¹⁸O reveal a pronounced cluster composition-dependent ability to mediate the oxidation of water to hydrogen peroxide. First-principles density functional theory simulations elucidate the mechanism of water oxidation, proceeding via formation of a terminal oxyl radical followed by oxyl/hydroxy (O/OH) coupling. The critical coupling reaction step entails a single electron transfer from the oxyl radical to the accommodating cluster core with a concurrent O/OH coupling forming an adsorbed OOH intermediate group. The spin-conserving electron transfer step takes place when the spin of the transferred electron is aligned with the spins of the d-electrons of the Mn atoms in the cuboidal high-spin cluster isomer. The d-electrons provide a ferromagnetically ordered environment that facilitates the spin-gated selective electron transfer process, resulting in parallel-spin-exchange stabilization and a lowered transition state barrier for the coupling reaction involving the frontier orbitals of the oxyl and hydroxy reactant intermediates.