DFT/TDDFT Exploration of the Potential Energy Surfaces of the Ground State and Excited States of Fe2(S2C3H6)(CO)6: A Simple Functional Model of the [FeFe] Hydrogenase Active Site

Fe2(S2C3H6)(CO)6 (a) is a simple model of the [FeFe] hydrogenase catalytic site. The topology of the potential energy surface (PES) of this complex, of its cationic and anionic species (a+ and a-), and of its lowest triplet state was studied using density functional theory (DFT) with BP86 and B3LYP functionals, while selected low- and high-lying singlet excited states were studied with the time-dependent density functional theory (TDDFT). The global minima of a and a- PESs are characterized by an all-terminal CO ligand arrangement, while the two rotated forms are transition states (TS). On the contrary, for the a+ and lowest triplet state PES, the three forms considered are local minima, and the syn rotated form is the global minimum. The relative stability of the rotated forms and the all-terminal CO form on the a, a+, and a- PESs is discussed in light of the Quantum Theory of Atoms in Molecules (QTAIM) analysis of the electron density. By comparing the Fe-Fe bond features of the three forms for each PES, we found that the global minimum structure is characterized by the shortest Fe-Fe bond distance and highest electron density at the Fe-Fe critical point. This approach gave evidence that in the a rotated forms, the weak Fe-Cµ interaction between the Fe atom of the unrotated Fe(CO)3 and the C atom of the semibridged CO is formed to the detriment of the Fe-Fe bond interaction. These results suggest that the stabilization of the rotated forms on the cationic PES might be due to the formation of the weak Fe-Cµ interaction minimizing the weakening of the Fe-Fe bond. The low-lying and lowest triplet excited-state PES investigated are characterized by the stabilization of the rotated forms over the all-terminal CO ligand arrangement. On the first singlet 11A′′ excited-state PES, an Fe(CO)3 semirotated structure is the lowest-energy stationary point, while the exploration of the 11A′ and 21A′′ singlet excited PESs evidences the stabilization of the rotated over the all-terminal CO forms. Singlet excited-state optimized geometry results are compared with excited-state nuclear distortions recently obtained from resonance Raman excitation profiles. Finally, the results of the exploration of the 6 1A′ and 9 1A′ high-lying excited PESs are discussed in light of the recent ultraviolet photolysis experiments on a.