Oxidative Electrochemistry of Molecular Catalysts Public Deposited

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  • March 20, 2019
  • Walden, Andrew
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • The tris(2-pyridyl)phosphine oxide (Py3PO) complex [Ru(Py3PO)(bpy)(OH2)]2+ (bpy is 2,2'-bipyridine) is a pH-dependent water oxidation electrocatalyst that accelerates dramatically with increasing pH — up to 780 s−1 at pH 10 (~1 V overpotential). Despite retaining the pentakis(pyridine) ligand arrangement common to previously reported catalysts, the tripodal Py3PO ligand framework supports much faster electrocatalysis. The early stages of the catalytic cycle are proposed to follow the typical pattern of single-site ruthenium catalysts, with two sequential 1H+/1e– proton-coupled electron transfer (PCET) oxidations, but the pH-dependent onset of catalysis and rapid rates are distinguishing features of the present system. With a view towards replacing sacrificial hydrogen acceptors in alkane dehydrogenation catalysis, electrochemical methods for oxidative activation of a pincer-ligated iridium hydride intermediate were explored. A 1H+/2e– oxidation process was observed in THF solvent, with net hydride loss leading to a reactive cationic intermediate that can be trapped by chloride. Analogous reactivity was observed with the concerted hydride transfer reagent Ph3C+, and with chemical oxidants and bases, connecting chemical and electrochemical hydride loss pathways. Coordination by pyridine was shown to stabilize the cationic hydride intermediate generated from 1H+/2e– oxidation of iridium hydrides complexes. A series of cationic hydride complexes incorporating pyridine, 2,6-lutidine, 2-ethylpyridine, or 2-phenylpyridine was synthesized and characterized. The deprotonation of these cationic hydrides resulted in solvent activation reactions as well as formation of cyclometallated derivatives of 2-phenylpyridine and added biphenyl. Activation of strong C-H bonds upon deprotonation strongly suggests the generation of the 14e– Ir(I) species that is implicated in alkane dehydrogenation mechanisms. This reactivity coupled with dihydride oxidation at an electrode represents net removal of 2H+/2e– from an iridium dihydride complex, an important step toward realizing electrochemical alkane dehydrogenation cycles.
Date of publication
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Rights statement
  • In Copyright
  • Templeton, Joseph
  • Leibfarth, Frank
  • Meek, Simon
  • Miller, Alexander
  • Brookhart, Maurice
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2016

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