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

Fe-mediated biological nitrogen fixation is thought to proceed via either 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 N-to-NH conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-NH 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-NH, 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 N fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N and Fe-CN species at the terminal N atom (e.g., Fe═N-NH, Fe-C≡NH, Fe≡C-NH). These same Fe-N and Fe-CN systems are functionally active for N-to-NH and CN-to-CH/NH 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 (BDE) via redox and acidity titrations. Very low N-H bond dissociation enthalpies, ranging from 65 (Fe-C≡NH) to ≤37 kcal/mol (Fe-N═NH), are determined. The collective data presented herein provide insight into the facile reactivity profiles of early stage protonated Fe-N and Fe-CN species.