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Inner and outer sphere ligand effects on palladium pincer complexes as electrocatalysts for CO₂ reduction

University of British Columbia

A series of novel palladium pincer complexes have been investigated for activity towards the electrochemical conversion of CO₂ to CO. Controlled variation of the inner and outer ligand sphere has provided mechanistic insight and valuable information towards rational ligand design for this variety of catalyst. Stabilizing interactions between charged electrocatalytic intermediates and a host of cationic residues were explored through the synthesis and characterization of six isoelectronic palladium bis(N-heterocyclic carbene) (NHC) complexes bearing unique onium functionalities. The presence of a positively charged, pendant substituent was found to mediate electrode kinetics and facilitate CO₂ coordination to the catalytic center in a systematic fashion. Ultimately, the introduction of cationic moieties onto this system was shown to enhance catalytic selectivity for the conversion of CO₂ to CO by as much as 5 times that of an alkyl-bearing analog. A combination of electrochemical experiments and computational analysis demonstrate that catalyst performance is most benefited by a bulky onium unit tethered to the catalyst through a flexible linker. This behavior was interpreted as the preference for a wide, hydrophobic reaction pocket that allows for the unhindered formation of catalytic intermediates and mediated interaction with the solution. With the aim of gaining better insight towards the influence of inner-sphere ligands on selectivity in palladium bis(N-heterocyclic carbene) CO₂ reduction catalysts, a novel, mixed phosphine-NHC complex was synthesized with variation in the fourth coordinate ligand and screened for activity towards the transformation of CO₂ to CO. The newly created structures were demonstrated to catalyze the reduction of CO₂ to CO in the presence of acetic acid with no deactivation detected after several hours of electrolysis. In addition to exhibiting versatility in the palladium bis-(NHC) pincer motif, this marks the first report of a palladium pincer catalyst capable of sustained turnover of CO₂ to CO with the assistance of a weak acid.

Text

2018-09-06

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/67124

Chemistry

University of British Columbia

UBCV

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