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CO2 reduction mechanisms by (PoCoP)Ir and (PeXeP)Co pincer catalysts for production of formate and CO

Johnson, Samantha I. and Nielsen, Robert J. and Shaffer, David W. and Yang, Jenny Y. and Goddard, William A. (2015) CO2 reduction mechanisms by (PoCoP)Ir and (PeXeP)Co pincer catalysts for production of formate and CO. In: 250th American Chemical Society National Meeting & Exposition, August 16-20, 2015, Boston, MA.

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Exptl., (PoCoP)Ir has been shown to reduce CO_2 (CO2RR) to formate at high Faradaic efficiencies both as a homogeneous catalyst and immobilized on an electrode. We use d. functional theory (B3LYP and M06 including solvation) to elucidate the mechanism for CO_2 redn. The derived mechanism agrees with the exptl. redn. potentials (irreversible redn. of (PoCoP)Ir(H)(NCMe)2+ ca. -1.4 V vs NHE; -1.37 V vs NHE calcd. for oneelectron redn. of IrIII(H)(NCMe)2+) and the surprising selectivity in this catalyst. We show that there are several thermodynamically favorable pathways for the hydrogen evolution reaction (HER); however, most are kinetically limited with calcd. barriers above 25 kcal/mol in neat water. These hydrogen transfer barriers can be lowered by participation of carboxylates. Addnl., the preference of (PoCoP)Ir for dihydride formation over formation of the dihydrogen adduct contributes to the selectivity. Theor. pathways also elucidate the role of acetonitrile, which promotes CO_2RR by enforcing an octahedral geometry, with the hydrides trans to one another. The trans effect of the opposite hydride creates the best possible environment for hydride transfer. Substituting cobalt for iridium, we find that the Ir-derived mechanism is not viable, as HER is preferred to CO_2RR and a larger overpotential would be required for turnover. However, related (PENEP)Co complexes are obsd. to reduce CO_2 to a CO stoichiometrically at a low potential. HER and CO_2RR in (PENEP)Co, (PoCoP)Co and (PoCoP)Ir suggests how to consider the effects of systematic ligand modification on functionality, providing a blueprint for future ligand design.

Item Type:Conference or Workshop Item (Paper)
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URLURL TypeDescription Website
Nielsen, Robert J.0000-0002-7962-0186
Goddard, William A.0000-0003-0097-5716
Additional Information:© 2015 American Chemical Society.
Record Number:CaltechAUTHORS:20151021-132151821
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:61387
Deposited By: Tony Diaz
Deposited On:21 Oct 2015 20:45
Last Modified:03 Oct 2019 09:07

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