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N-H Bond Dissociation Enthalpies and Facile H-atom Transfers for Early Intermediates of Fe-N_2 and Fe-CN Reductions

Rittle, Jonathan and Peters, Jonas C. (2017) N-H Bond Dissociation Enthalpies and Facile H-atom Transfers for Early Intermediates of Fe-N_2 and Fe-CN Reductions. Journal of the American Chemical Society, 139 (8). pp. 3161-3170. ISSN 0002-7863. http://resolver.caltech.edu/CaltechAUTHORS:20170201-092020177

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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 N_2-to-NH_3 conversion involves a distal pathway, an alternating pathway, or some hybrid of these limiting scenarios, Fe-N_xH_y 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_2, 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_2 fixation catalysts. We recently reported several reactive complexes derived from the direct protonation of Fe-N_2 and Fe-CN species at the terminal N-atom (e.g., Fe=N-NH_2, Fe-CNH, FeC-NH_2). These same Fe-N_2 and Fe-CN systems are functionally active for N_2-to-NH_3 and CN-to-CH_4/NH_3 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-N_2 and Fe-CN species.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/jacs.6b12861DOIArticle
http://pubs.acs.org/doi/abs/10.1021/jacs.6b12861PublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/jacs.6b12861PublisherSupporting Information
ORCID:
AuthorORCID
Peters, Jonas C.0000-0002-6610-4414
Additional Information:© 2017 American Chemical Society. Received: December 14, 2016; Published: January 31, 2017. This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation. JR was additionally supported by a fellowship from the Caltech Center for Environmental Microbial Interactions (CEMI).
Group:Caltech Center for Environmental Microbial Interactions (CEMI)
Funders:
Funding AgencyGrant Number
NIHGM-070757
Gordon and Betty Moore FoundationUNSPECIFIED
Caltech Center for Environmental Microbial Interactions (CEMI)UNSPECIFIED
Record Number:CaltechAUTHORS:20170201-092020177
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20170201-092020177
Official Citation:N–H Bond Dissociation Enthalpies and Facile H Atom Transfers for Early Intermediates of Fe–N2 and Fe–CN Reductions Jonathan Rittle and Jonas C. Peters Journal of the American Chemical Society 2017 139 (8), 3161-3170 DOI: 10.1021/jacs.6b12861
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:73916
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:01 Feb 2017 17:51
Last Modified:20 Oct 2017 23:44

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