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Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution

Harshan, Aparna Karippara and Solis, Brian H. and Winkler, Jay R. and Gray, Harry B. and Hammes-Schiffer, Sharon (2016) Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution. Inorganic Chemistry, 55 (6). pp. 2934-2940. ISSN 0020-1669. https://resolver.caltech.edu/CaltechAUTHORS:20160316-073650099

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Abstract

The ability to tune the properties of hydrogen-evolving molecular electrocatalysts is important for developing alternative energy sources. Fluorinated diglyoxime-iron complexes have been shown to evolve hydrogen at moderate overpotentials. Herein two such complexes, [(dAr^FgBF_2)_2Fe(py)_2], denoted A, and [(dAr^Fg_2H-BF_2)Fe(py)_2], denoted B [dAr^Fg = bis(pentafluorophenyl-glyoximato); py = pyridine], are investigated with density functional theory calculations. B differs from A in that one BF_2 bridge is replaced by a proton bridge of the form O–H–O. According to the calculations, the catalytic pathway for A involves two consecutive reduction steps, followed by protonation of an Fe^0 species to generate the active Fe^(II)-hydride species. B is found to proceed via two parallel pathways, where one pathway is similar to that for A, and the additional pathway arises from protonation of the O–H–O bridge, followed by spontaneous reduction to an Fe^0 intermediate and intramolecular proton transfer from the ligand to the metal center or protonation by external acid to form the same active Fe^(II)-hydride species. Simulated cyclic voltammograms (CVs) based on these mechanisms are in qualitative agreement with experimental CVs. The two parallel pathways identified for B arise from an equilibrium between the protonated and unprotonated ligand and result in two catalytic peaks in the CVs. The calculations predict that the relative probabilities for the two pathways, and therefore the relative magnitudes of the catalytic peaks, could be tuned by altering the pK_a of the acid or the substituents on the ligands of the electrocatalyst. The ability to control the catalytic pathways through acid strength or ligand substituents is critical for designing more effective catalysts for energy conversion processes.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/acs.inorgchem.5b02857DOIArticle
http://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.5b02857PublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/acs.inorgchem.5b02857PublisherSupporting Information
ORCID:
AuthorORCID
Winkler, Jay R.0000-0002-4453-9716
Gray, Harry B.0000-0002-7937-7876
Additional Information:© 2016 American Chemical Society. Received: December 9, 2015; Publication Date (Web): March 4, 2016. This work was supported by the Center for Chemical Innovation of the National Science Foundation (Solar Fuels, Grant No. CHE-1305124). B.H.S. thanks the Alexander von Humboldt-Stiftung/Foundation for postdoctoral support during the writing of this paper. The authors declare no competing financial interest.
Group:CCI Solar Fuels
Funders:
Funding AgencyGrant Number
NSFCHE-1305124
Alexander von Humboldt FoundationUNSPECIFIED
Issue or Number:6
Record Number:CaltechAUTHORS:20160316-073650099
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20160316-073650099
Official Citation:Computational Study of Fluorinated Diglyoxime-Iron Complexes: Tuning the Electrocatalytic Pathways for Hydrogen Evolution Aparna Karippara Harshan, Brian H. Solis, Jay R. Winkler, Harry B. Gray, and Sharon Hammes-Schiffer Inorganic Chemistry 2016 55 (6), 2934-2940 DOI: 10.1021/acs.inorgchem.5b02857
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:65383
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:16 Mar 2016 20:25
Last Modified:22 Nov 2019 09:58

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