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Explorations and development of bis(N-heterocyclic carbene) pincer complexes for the electrocatalytic reduction of carbon dioxide

University of British Columbia

A variegated series of bis(N-heterocyclic carbene) pincer complexes have been tested for the ability to act as homogeneous CO₂ reduction electrocatalysts. Structure-activity relationships were investigated in order to enable the rational design of improved electrocatalysts and provide a platform for further development of mono- and bimetallic electrocatalysts within the pincer motif. Pyridine- and lutidine-linked bis-NHC palladium pincer complexes were screened for CO₂ reduction capability with trifluoroethanol, acetic acid, and trifluoroacetic acid as proton sources. The lutidine-linked pincer complexes were found to electrocatalytically reduce CO₂ to CO at potentials as low as -1.65 V vs. ferrocene in the presence of trifluoroacetic acid. The one-electron reduction of these complexes is shown to be chemically reversible, resulting in a monometallic species in solution. Computational models indicate charge transfer from a redox-active ligand upon interaction of the reduced species with CO₂, thus potentially addressing a source of deactivation in earlier pincer electrocatalysts. The presence of Lewis acids in solution was also investigated, assisting reactivity with CO₂. The NHC moieties of these lutidine-linked Pd pincer complexes were modified by phenanthro- and pyreno-annulation to investigate the effect of an extended NHC π-system on their electrochemical reactivity with CO₂. The polyannulated NHC groups are shown to be additional sites for redox-activity in the pincer ligand, enabling increased electron donation and activation of CO₂. Following this, modification of the pyridyl para position is reported (R = OMe, H, Br, and COOR), allowing the first reduction potential to be tuned over a 1 V range in relation to the substituent's Hammett σp constant, and labilizing the trans ligand in the case of electron-donating substituents, thus improving activity for one-electron reduced species. Finally, analogous Ni and Pt bis-NHC pincer complexes are synthesized, characterized, and compared to Pd, with the Pd bis(benzimidazol-2-ylidene) pincer complexes exhibiting the best performance with faradaic yields for CO production approaching 50% in the presence of trifluoroacetic acid. The remaining current resulted in the production of H₂, thus producing a CO-rich synthesis gas mixture as the overall product.

Text

2018-01-31

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/60151

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/