Water oxidation catalysis in natural and artificial photosynthesis

Alternative Mechanism for O2 Formation in Natural Photosynthesis via Nucleophilic Oxo–Oxo Coupling

“…The exact mechanism of O–O bond formation in the S 4 state has become a long-term and central scientific debate as it has so far been inaccessible to direct experimental investigation. Among numerous hypotheses, − the open-cubane oxo-oxyl radical coupling (RC) mechanism ,,− has been increasingly preferred in recent years, especially over the water nucleophilic attack (WNA) mechanism. ,,,,,− The main reasons include the identification of O5 as substrate water, ,,− the favorable energetics, ,, as well as the evidence from the X-ray free electron laser (XFEL) experiments (on cyanobacterial PSII) ,− and advanced electron paramagnetic resonance (EPR) spectroscopy supporting the key starting structure of the RC mechanism, that is the S 3 state (S 3 A,W ) containing an extra oxygen ligand (Ox or O6) on Mn1 (Figure c). Thus, presently the active species responsible for O 2 production in the OEC is generally regarded as a Mn­(IV)-oxyl entity, which couples with a μ 3 -oxo for O–O bond formation, a reaction that is, however, unprecedented in Mn model systems outside PSII. , …”