Chemical studies of diiron complexes related to iron-only hydrogenase.

摘要

The focus of my research is the study of the fundamental properties of diiron organometallic complexes of the type (μ-SRS)[Fe(CO)₃] ₂ as models of the [Fe]H₂ase active site. An interesting feature of these complexes is their reaction with CN− to readily substitute CO and give dicyanide derivatives with one CN − on each Fe atom. A series of (μ-SRS)[Fe(CO)₃] ₂ complexes with different bridges were employed in kinetic studies of CN−/CO exchange in order to determine the effect of the bridge on the rate and the mechanism of the substitution. The results of these studies showed a complicated R-dependent reactivity pattern for the first CN− addition.; The explanation of the kinetic results came in combination with the fluxional mobility of the Fe(CO)₃ units in these complexes that results in basal/apical CO site exchange. The relationship of this fluxionality and the reactivity of the molecule towards CN− was substantiated by DFT calculations that modeled these intramolecular processes and found that the semi-bridging μ-CO that results from the Fe(CO)₃ unit rotation is also formed upon CN− attack. Thus, the assumption that the Fe(CO)₃ rotational barrier is an important contributor to the overall activation energy of CN− attack, explains the experimental observation that generally the second CN− addition finds a lower Fe(CO)₃ rotational barrier due to the presence of the already coordinated CN− ligand.; From the above FeIFeI complexes, homovalent FeIIFeII complexes can be obtained through binuclear oxidative addition of electrophiles such as H+ or SMe + to yield FeII(μ-H)FeII or Fe II(μ-SMe)FeII respectively. These complexes serve as functional models of [Fe]H₂ase based on hydrogenase activity test reactions that typically involve H/D exchange reactivity in H₂/D ₂O or H₂/D₂/H₂O mixtures. Activity studies using the FeIIFeII complexes {lcub}(μ-H)(μ-pdt)[Fe(CO) ₂(PMe₃)]₂)+ and {lcub}(μ-SMe)(μ-pdt)[Fe(CO) ₂(PMe₃)]₂{rcub}+ indicated that both demonstrate H₂ (or D₂) uptake and heterolytic cleavage by D₂O (or H₂O) under photolytic conditions. The singular requirement for these processes is an open site on an FeII center. Studies using the nitrogen-containing bridged complex (μ-SCH ₂N(Me)CH₂S)[Fe(CO)₃]₂ suggest that the lone pair on the nitrogen induces instability to the complex and it is responsible for decomposition of the bridge or even split of the dinuclear to a mononuclear complex. This is a phenomenon that has not been observed in any of the hydrocarbon bridged compounds.