A Gas-Phase Ca n Mn 4-n O 4 + Cluster Model for the Oxygen-Evolving Complex of Photosystem II

One of the fundamental processes in nature, the oxidation of water, is catalyzed by a small CaMn O ⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Ca Mn O (n=0-4) cluster ions is reported, which are employed as structural models for the catalytically active site of PS II. Gas-phase reactivity experiments with D O and H O in an ion trap reveal the facile deprotonation of multiple water molecules via hydroxylation of the cluster oxo bridges for all investigated clusters. However, only the mono-calcium cluster CaMn O is observed to oxidize water via elimination of hydrogen peroxide. First-principles density functional theory (DFT) calculations elucidate mechanistic details of the deprotonation and oxidation reactions mediated by CaMn O as well as the role of calcium.