Combined spectroscopic/computational study of binuclear Fe(I)-Fe(I) complexes: Implications for the fully-reduced active-site cluster of Fe-only hydrogenases

The Fe(I)-Fe(I) dimer complex [Fe-2(pdt)(CO)(4)(CN)(2)][Et4N](2) (2), where pdt = 1,3-propane dithiolate, serves as a model of the fully reduced [2Fe](H) component of the H cluster, which is the active site for catalysis in Fe-only hydrogenases (FeHases). Electronic absorption, magnetic circular dichroism (MCD), and resonance Raman (rR) spectroscopies have been employed to characterize both the ground and excited states of 2 as well as those of the related complex Fe2(pdt)(CO)6 (1). These results have been combined with density functional theory (DFT) computations to produce experimentally validated bonding descriptions of 1 and 2. It is shown that Fe(I)-S covalency is significantly reduced upon dicyano substitution (i.e., conversion of 1 -> 2), while the corresponding Fe(I)-CO/ CN pi-backbonding interactions are strengthened, results that are corroborated by normal-coordinate analyses of the vibrational data. Detailed assignments of the features observed in the electronic absorption spectra of 1 and 2 have been developed on the basis of time-dependent DFT (TD-DFT) calculations, which provide remarkably accurate simulations of the experimental data. For both complexes, all bands below 32 000 cm(-1) arise from transitions involving electronic excitation within the binuclear Fe-Fe core, with the most intense feature assigned to the Fe(sigma(b)) -> Fe(sigma(*)) transition. Analysis of the corresponding rR excitation profiles within the framework of time-dependent Heller theory reveals that in each case the Fe-Fe bond is elongated by similar to 0.3 angstrom in the Fe(sigma(b)) - Fe(sigma(*)) excited state. Finally, building upon the insights gained from the spectroscopic/computational studies of 1 and 2, our computational methodology has been extended to the reduced enzyme active site, providing insights into the electronic structure of the [2Fe](H) subduster in the H-red state and its relationship to catalysis.