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  Kass, D.; Yao, S.; Krause, K.B.; Corona, T.; Richter, L.; Braun, T.; Mebs, S.; Haumann, M.; Dau, H.; Lohmiller, T.; Limberg, C.; Drieß, M.; Ray, K.: Spectroscopic Properties of a Biologically Relevant [Fe₂(μ-O)₂] Diamond Core Motif with a Short Iron-Iron Distance. Angewandte Chemie - International Edition 62 (2023), p. e202209437/1-6

10.1002/anie.202209437
Open Access Version

Abstract:
Diiron cofactors in enzymes perform diverse challenging transformations. The structures of high valent intermediates (Q in methane monooxygenase and X in ribonucleotide reductase) are debated since Fe−Fe distances of 2.5–3.4 Å were attributed to “open” or “closed” cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a FeIII2(μ-O)2 complex (2) with tetrahedral (4C) centres and short Fe−Fe distance (2.52 Å), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the TD FeIII centres due to the short Fe−μ−O bonds. A ≈2.5 Å Fe−Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe2(μ-O)2 core containing four-coordinate (or by possible extension five-coordinate) iron centres there may be enough flexibility to accommodate a particularly short Fe−Fe separation with intense pre-edge transition. This finding may broaden the scope of models considered for the structure of high-valent diiron intermediates formed upon O2 activation in biology.