Sulfur Oxygenates of Biomimetics of the Diiron Subsite of the [FeFe]-Hydrogenase Active Site: Properties and Oxygen Damage Repair Possibilities

This study explores the site specificity (sulfur vs the Fe-Fe bond) of oxygenation of diiron ((FeFeI)-Fe-I and (FeFeII)-Fe-II) organometallics that model the 2-iron subsite in the active site of [FeFe]-hydrogenase: (mu-pdt)[Fe(CO)(2)L][Fe(CO)(2)L'] (L = L' = CO (1); L = PPh3, L' = CO (2); L = L, = PMe3 (4)) and (mu-pdt)(mu-H)[Fe(CO)(2)PMe3](2) (5). DFT computations find that the Fe-Fe bond in the (FeFeI)-Fe-I diiron models is thermodynamically favored to produce the mu-oxo or oxidative addition product, Fe-II-O-Fe-II; nevertheless, the sulfur-based HOMO-1 accounts for the experimentally observed mono- and bis-O-atom adducts at sulfur, i.e., (mu-pst)[Fe(CO)(2)L][Fe(CO)(2)L'] (pst = -S(CH2)(3)S(O)-, 1,3-propanesulfenatothiolate; L = L' = CO (1-O); L = PPh3, L' = CO (2-O); L = L' PMe3 (4-O)) and (mu-pds)[Fe(CO)(2)L][Fe(CO)(2)L'] (pds = -(O)S(CH2)(3)S(O)-, 1,3-propanedisulfenato; L = PPh3, L' = CO (2-O-2)). The Fe-II(mu-H)Fe-II 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 (mu-pst)[Fe(CO)(3)](2) as compared to (mu-pdt)[Fe(CO)(3)](2). Five molecular structures are confirmed by X-ray diffraction: 1-O, 2-O, 2-O-2, 4-O, and 6. Deoxygenation with reclamation of the mu-pdt parent complex occurs in a proton/electron-coupled process. The possible biological relevance of oxygenation and deoxygenation studies is discussed.