Magnetic Circular Dichroism Spectroscopic Studies of Mononuclear Non-Heme Ferrous Model Complexes. Correlation of Excited- and Ground-State Electronic Structure with Geometry

Abstract

Mononuclear non-heme iron enzymes catalyze a variety of biologically important reactions involving dioxygen, and yet, the non-heme ferrous active sites have been difficult to study by most spectroscopic methods. A combination of near-infrared (NIR) magnetic circular dichroism (MCD) and variable-temperature, variable-field (VTVH) MCD spectroscopies has been applied to 24 structurally defined mononuclear non-heme ferrous model complexes to rigorously correlate spectral data with geometric and electronic structure. While general trends for the excited-state splittings have been predicted by ligand field theory, these predictions are now evaluated by systematically studying the NIR MCD spectra of a series of high-spin (S = 2) ferrous models with a wide range of coordination numbers and geometries. VTVH MCD spectroscopy is used to probe ground-state electronic structure, and a complete MCD intensity expression for non-Kramers systems that includes z-polarization, ℬ-terms, and excited states has been derived. This expression has been applied to these model complexes to determine signs of the zero-field splitting and to obtain ground-state spin-Hamiltonian parameters, which can be related to ground-state ligand field splittings. These experimental ground-state data are used to develop the information content available from VTVH MCD, in particular the ability to probe specific metal−ligand bonding interactions for different coordination environments. The excited-state ligand field data are used to construct a set of spectroscopic guidelines which, combined with the ground-state information, allow one to clearly determine the coordination number and geometry of an unknown ferrous center, with the exception of only a few ambiguous cases. Additionally, the MCD data provide insight into the origin of the MCD 𝒞-term intensities and signs for low-symmetry ferrous centers. The results obtained through these model studies now provide the basis for investigating ferrous active sites of non-heme iron enzymes to probe the geometric and electronic structure of a site with respect to oxygen reactivity and understanding how differences in structure correlate with differences in reactivity.