Crystal structure of a synthetic high-valent complex with an Fe2(μ-O)2 diamond core.: Implications for the core structures of methane monooxygenase intermediate Q and ribonucleotide reductase intermediate X

In our efforts to model high-valent intermediates in the oxygen activation cycles of nonheme diiron enzymes such as methane monooxygenase (MMOH-Q) and ribonucleotide reductase (RNR R2-X), we have synthesized and spectroscopically characterized a series of bis(mu-oxo)diiron(III,IV) complexes, [Fe-2(mu-O)(2)(L)(2)](ClO4)(3), where L is tris(2-pyridylmethyl)amine (TPA) or its ring-alkylated derivatives. We now report the crystal structure of [Fe-2(mu-O)(2)(5-Et-3-TPA)(2)](ClO4)(3) (2), the first example of a structurally characterized reactive iron(IV)-oxo species, which provides accurate metrical parameters for the diamond core structure proposed for this series of complexes. Complex 2 has Fe-mu-O distances of 1.805(3) Angstrom and 1.860(3) Angstrom, an Fe-Fe distance of 2.683(1) Angstrom, and an Fe-mu-O-Fe angle of 94.1(1)degrees. The EXAFS spectrum of 2 can be fit well with a combination of four shells: 1 O at 1.82 Angstrom, 2-3 N at 2.03 Angstrom, 1 Fe at 2.66 Angstrom, and 7 C at 2.87 Angstrom. The distances obtained are in very good agreement with the crystal structure data for 2, though the coordination numbers for the first coordination sphere are underestimated. The EXAFS spectra of MMOH-Q and RNR R2-X contain features that match well with those of 2 (except for the multi-carbon shell at 2.87 Angstrom arising from pyridyl carbons which are absent in the enzymes), suggesting that an Fe-2(mu-O)(2) core may be a good candidate for the core structures of the enzyme intermediates. The implications of these studies are discussed.