The reaction of electrophiles with models of iron-iron hydrogenase: A switch in regioselectivity

Simple dithiolate-bridged dinuclear iron clusters of the form (μ-S(CH2)xS)[Fe(CO)2(PR3)]2 act as models of the active site of the enzyme iron-iron hydrogenase ([FeFe]H2ase). These complexes have been shown react with the electrophilic species, H+ and Et+(Et+=CH3CH2+) with differing regioselectivity; H+ reacts to form a 3c-2e− Fe-H-Fe bond, while Et+ reacts to form a new C-S bond. We have used density functional theory (DFT) calculations to examine the reaction of these two electrophilic species using the computational model (μ-SCH2CH2S)[Fe(CO)2(PH3)]2. In agreement with the experimental results, protonation of the Fe-Fe bond density is found to yield a much more stable complex than protonation of a bridging sulfur atom, while alkylation of a sulfur atom of the bridging thiolate is found to yield a much more stable complex than alkylation of the iron centers. Additional computations show that a mononuclear iron(II) complex with an Fe-E bond (E=H or Et) is significantly more stable than its constitutional isomer with iron(0) and an S-E bond. The instability of a bridging ethyl complex is attributed to the inability of the ethyl group, in contrast to a hydride, to form a stable 3c-2e− bond with the two iron centers.