title: N-H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe-N2and Fe-CN Reductions.
abstract: 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 N2-to-NH3conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-NxHyintermediates 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 N2fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N2and Fe-CN species at the terminal N atom (e.g., Fe═N-NH2, Fe-C≡NH, Fe≡C-NH2). These same Fe-N2and Fe-CN systems are functionally active for N2-to-NH3and CN-to-CH4/NH3conversion, 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 (BDEN-H) 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-N2and Fe-CN species.
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech) , Pasadena, California 91125, United States.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Rittle
authors.affiliation: Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech) , Pasadena, California 91125, United States.
authors.forename: Jonas C
authors.identifier: 0000-0002-6610-4414
authors.initials: JC
authors.lastname: Peters
mesh_terms: D018360:Crystallography, X-Ray; D003486:Cyanides / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008958:Models, Molecular; D009584:Nitrogen / Q000737:chemistry*; D010084:Oxidation-Reduction; D013816:Thermodynamics
chemical_list: D003486:Cyanides; D006859:Hydrogen; D007501:Iron; D009584:Nitrogen