PNNL

Abstract

Iron-sulfur clusters are crucial for biological electron transport and catalysis. Obtaining accurate geometries, energetics, the manifold of their excited electronic states, and reduction energies is important to understand their role in these processes. Using a [2Fe-2S] model complex in various oxidation states, [Fe2S2(SMe)4]2-,3-,4-, we benchmarked a variety of computational methodologies ranging from density functional theory (DFT) to post-Hartree-Fock methods, including complete active space self-consistent field (CASSCF), multireference configuration interaction plus single and double replacements (MRCISD), the second-order N-electron valence state perturbation theory (NEVPT2) and the linearized integrand approximation of adiabatic connection (AC0) approaches. Additionally, we studied three experimentally well-characterized complexes, [Fe2S2(SCys)4]2-, [Fe2S2(S-o-tol)4]2- and [Fe2S2(S-o-xyl)4]2- via DFT methods. We conclude that the dynamical electron correlation is important for accurately predicting the geometry of these complexes. Broken symmetry (BS) DFT correctly predicts experimental geometries of low spin multiplicity, while CASSCF does not. However, BS-DFT significantly overestimates the difference between the low and high spin electronic states for a given oxidation state, while CASSCF underestimates it but provides relative energies that are closer to the reference NEVPT2 results. Finally, AC0 provides energetics of NEVPT2 quality with the advantage of using large CASSCF sizes. NEVPT2 gives the best estimates of the FeIII/FeIII ® FeII/FIII (4.27 eV) and FeII/FIII ® FeII/FII (7.72 eV) reduction energies. Finally, the nature of the Fe-S chemical bond and the magnitude of the redox potentials of the complexes offer a physical rationale for the relative stabilization, structure, and speciation of these complexes.