Coordination sphere flexibility of active-site models for Fe-only hydrogenase: Studies in intra- and intermolecular diatomic ligand exchange

A series of dinuclear complexes, (mu -SRS)Fe-2(Co)(6) (R = -CH2CH2-, -CH2CH2CH2-, -CH2- C6H4-CH2-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu -pdt)Fe-2(CO)(6) while in the (mu -oxyldt)Fe-2(CO)(6) compound, the bridge is fixed. Temperature-dependent C-13 NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu -SRS unit. Kinetic studies of intermolecular CO/CN- ligand-exchange reactions establish associative or I-a mechanisms in sequential steps to form the dicyano dianion, (mu -SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN-/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H-2 activation/production process in the enzyme.