A Gas‐Phase CanMn4−nO4+ 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 CaMn3O4⋅MnO cluster located in photosystem II (PS II). Now, the first successful preparation of a series of isolated ligand‐free tetrameric CanMn4−nO4+ (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 D2O and H218O 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 CaMn3O4+ 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 CaMn3O4+ as well as the role of calcium. Ca zählt! Einer der grundlegenden Prozesse in der Natur, die Oxidation von Wasser, wird von einem kleinen CaMn3O4⋅MnO‐Cluster im Photosystem II katalysiert. Die Verwendung freier CanMn4−nO4+‐Cluster (n=0–4) als Strukturmodelle gibt Einblicke in die Bedeutung eines Calciumatoms (d. h. n=1) für die Wasseroxidation. Ionenfallenreaktionen und Ab‐initio‐Rechnungen offenbaren Details der Deprotonierungs‐ und Oxidationsreaktionen von Wasser.