Vibrational Analysis of the Model Complex (μ-edt[Fe(CO)3]2 and Comparison to Iron-Only Hydrogenase: The Activation Scale of Hydrogenase Model Systems

Research on simple [FeFe] hydrogenase model systems of type (mu-S2R)[Fe(CO)(3)](2) (R = C2H4 (edt), C3H6 (pdt)) which have been shown to function as robust electrocatalysts for proton reduction, provides a reference to understand the electronic and vibrational properties of the active site of [FeFe] hydrogenases and of more sophisticated model systems. In this study, the solution and solid state Raman spectra of (mu-edt)[Fe(CO)(3)](2) and of the corresponding (CO)-C-13-labeled complex are presented and analyzed in detail, with focus on the v(C=O) and v(Fe-CO)/delta(Fe-C=0) vibrational regions. These regions are specifically important as vibrations involving CO ligands serve as probes for the "electron richness" of low-valent transition metal centers and the geometric structures of the complexes. The obtained vibrational spectra have been completely assigned in terms of the v(C=O), v(Fe-CO), and (5(Fe-C=0) modes, and the force constants of the important C=O and Fe-CO bonds have been determined using our Quantum Chemistry Centered Normal Coordinate Analysis (QCC-NCA). In the 400-650 cm(-1) region, fifteen mixed v(Fe-CO)/delta(Fe-C=0) modes have been identified. The most prominent Raman peaks at 454, 456, and 483 cm(-1) correspond to a combination of v(Fe-CO) stretching and delta(Fe-C=O) linear bending modes. The less intense peaks at 416 cm(-1) and 419 cm(-1) correspond to pure delta(Fe-C=0) linear bends. In the v(C=O) region, the v(C=O) normal modes at lower energy (1968 and 1964 cmT1) are almost pure equatorial (eq) v(C=O)eqstretching vibrations, whereas the remainin9 four v(C=0) normal modes show dominant (C=O)(eq) (2070 and 1961 cm=(-1)) and (C=O)ax (2005 and 1979 cm T'; ax = axial) contributions. Importantly, an inverse correlation between the f(C=O)(ax/eq) and f(Fe CO)(ax/eq) force constants is obtained, in agreement with the idea that the Fe(I) CO bond in these types of complexes is dominated by pi-backdonation. Compared to the reduced form of [FeFe] hydrogenase (H-red), the v(C=O) vibrational frequencies of (mu-edt)[Fe(CO)(3)](2) are higher in energy, indicating that the dinuclear iron core in (p-edt)[Fe(CO)(3)](2) is less electron rich compared to H-red in the actual enzyme. Finally, quantum yields for the photodecomposition of (mu-edt)[Fe(CO)(3)](2) have been determined.