Sulfur oxygenates of biomimetics of the diiron subsite of the [FeFe] hydrogenase active site: Structures, reactivity and oxygen damage repair possibilities
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This study explores the site specificity (sulfur vs the Fe-Fe bond) of oxygenation of diiron (FeIFeI and FeIIFe II) organometallics that model the 2-iron subsite in the active site of [FeFe]-hydrogenase: (-pdt)[Fe(CO)2L][Fe(CO) 2L] (L = L = CO (1); L = PPh3, L = CO (2); L = L = PMe3 (4)) and (-pdt)(-H)[Fe(CO) 2PMe3]2 (5). DFT computations find that the Fe-Fe bond in the FeIFeI diiron models is thermodynamically favored to produce the -oxo or oxidative addition product, FeII-O-FeII; nevertheless, the sulfur-based HOMO-1 accounts for the experimentally observed mono- and bis-O-atom adducts at sulfur, i.e., (-pst)[Fe(CO)2L][Fe(CO)2L] (pst = -S(CH2)3S(O)-, 1,3-propanesulfenatothiolate; L = L = CO (1-O); L = PPh3, L = CO (2-O); L = L = PMe 3 (4-O)) and (-pds)[Fe(CO)2L][Fe(CO) 2L] (pds = -(O)S(CH2)3S(O)-, 1,3-propanedisulfenato; L = PPh3, L = CO (2-O2)). The Fe II(-H)FeII diiron model (5), for which the HOMO is largely of sulfur character, exclusively yields S-oxygenation. The depressing effect of such bridging ligand modification on the dynamic NMR properties arising from rotation of the Fe(CO)3 correlates with higher barriers to the CO/PMe3 exchange of (-pst)[Fe(CO)3]2 as compared to (-pdt)[Fe(CO)3]2. Five molecular structures are confirmed by X-ray diffraction: 1-O, 2-O, 2-O2, 4-O, and 6. Deoxygenation with reclamation of the -pdt parent complex occurs in a proton/electron-coupled process. The possible biological relevance of oxygenation and deoxygenation studies is discussed. 2009 American Chemical Society.