Importance of the Ferryl Quintet State in Determining the Electronic Properties of P450 Compound I

We previously reported a selenolate-ligated P450 compound I intermediate (SeP450-I) to be more reactive toward C-H bonds than its thiolate-ligated counterpart. To gain insight into how the selenolate axial ligand influences the reactivity of compound I, we have investigated the electronic structure of the SeP450-I intermediate using variable temperature Mössbauer (VTM) spectroscopy. The VTM data indicate that electronic spin relaxation rates are significantly slower in SeP450-I than in P450-I. Analyses of these data provide Δ, the energy spacing between the two lowest electronic energy levels in compound I. This spacing is typically determined by the zero-field splitting of the ferryl moiety, <i>D</i>, and the exchange coupling, <i>J</i>, between the iron(IV)oxo unit and the ligand-based radical. However, the systems examined are antiferromagnetically coupled with |<i>J</i>/<i>D</i>| > 1. As a result, Δ ∼ (3/2) <i>J</i>, and measurements of Δ provide <i>J</i> (to within ∼5%). These measurements reveal that the sign and magnitude of <i>J</i> track with the reactivity of compound I toward C-H bonds. Efforts to analyze these and other data highlight the inadequacy of the standard ligand field model that is often used to explain the electronic properties of compound I. Additional analyses combining our data with state energies from a previous theoretical investigation support predictions of a low-lying quintet state within the iron(IV)oxo unit. We discuss these findings in light of computational studies that suggest that access to excited states, particularly those of a high-spin nature, can promote metal-oxo mediated C-H bond cleavage.