Spectroscopic Properties of a Biologically Relevant [Fe₂(-O)₂] Diamond Core Motif with a Short Iron-Iron Distance

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 angstrom were attributed to "open" or "closed" cores with bridging or terminal oxido groups. We report the crystallographic and spectroscopic characterization of a Fe-2(III)(mu-O)(2) complex (2) with tetrahedral (4C) centres and short Fe-Fe distance (2.52 angstrom), persisting in organic solutions. 2 shows a large Fe K-pre-edge intensity, which is caused by the pronounced asymmetry at the T-D Fe-III centres due to the short Fe-mu-O bonds. A approximate to 2.5 angstrom Fe-Fe distance is unlikely for six-coordinate sites in Q or X, but for a Fe-2(mu-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 O-2 activation in biology.