N-H Bond Dissociation Enthalpies and Facile H-atom Transfers for Early Intermediates of Fe-N₂ and Fe-CN Reductions

Abstract

Fe-mediated biological nitrogen fixation is thought to proceed either via a sequence of proton and electron transfer steps, concerted H-atom transfer steps, or some combination thereof. Regardless of the specifics, and whether the intimate mechanism for N₂-to-NH₃ conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-NₓH_ᵧ intermediates are implicated that feature reactive N-H bonds. Thermodynamic knowledge of the N-H bond strengths of such species is scant, and is especially difficult to obtain for the most reactive early stage candidate intermediates (e.g., Fe-N=NH, Fe=N-NH₂, Fe-NH=NH). Such knowledge is essential to considering various mechanistic hypotheses for biological (and synthetic) nitrogen fixation, and to the rational design of improved synthetic N₂ fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N₂ and Fe-CN species at the terminal N-atom (e.g., Fe=N-NH₂, Fe-CNH, FeC-NH₂). These same Fe-N₂ and Fe-CN systems are functionally active for N₂-to-NH₃ and CN-to-CH₄/NH₃ conversion, respectively, when subjected to protons and electrons, and hence provide an excellent opportunity for obtaining meaningful N-H bond strength data. We report here a combined synthetic, structural, and spectroscopic/analytic study to estimate the N-H bond strengths of several species of interest. We assess the reactivity profiles of species featuring reactive N-H bonds, and estimate their homolytic N-H bond enthalpies via redox and acidity titrations. Very low N-H bond dissociation enthalpies (BDE_(N-H)), ranging from 65 (e.g., Fe-CNH) to ≤ 37 kcal/mol (Fe-N=NH), are determined. The collective data presented herein provides insight into the facile reactivity profiles of early stage protonated Fe-N₂ and Fe-CN species.