ja6b12861_si_001.cif (14.07 MB)
N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions
dataset
posted on 2017-01-31, 00:00 authored by Jonathan Rittle, Jonas C. PetersFe-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-NH3 conversion involves a distal pathway, an alternating
pathway, or some hybrid of these limiting scenarios, Fe–NxHy 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–NNH, FeN–NH2, Fe–NHNH). 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., FeN–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 (BDEN−H) via redox and acidity titrations.
Very low N–H bond dissociation enthalpies, ranging from 65
(Fe–CNH) to ≤37 kcal/mol (Fe–NNH),
are determined. The collective data presented herein provide insight
into the facile reactivity profiles of early stage protonated Fe–N2 and Fe–CN species.