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

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. T...