Crystal structure of a synthetic high-valent complex with an Fe-2(mu-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 spectroscopic ally characterized a series of bis(mu-oxo)diiron(III,IV) complexes, [Fe-2(m u-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(m u-O)(2)(5-Et-3-TPA)(2)](ClO4)(3) (2), the first example of a structurally c haracterized reactive iron(IV)-oxo species, which provides accurate metrica l parameters for the diamond core structure proposed for this series of com plexes. Complex 2 has Fe-mu-O distances of 1.805(3) Angstrom and 1.860(3) A ngstrom, 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 goo d agreement with the crystal structure data for 2, though the coordination numbers for the first coordination sphere are underestimated. The EXAFS spe ctra 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 intermed iates. The implications of these studies are discussed.