A Gas-Phase Can Mn4-n O4 + 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 CaMn3 O4 ⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand-free tetrameric Can Mn4-n O4 + (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 D2 O and H2 18 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 CaMn3 O4 + 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 CaMn3 O4 + as well as the role of calcium.