Electrocatalytic reduction of carbon dioxide by finely tuned molecular catalysts
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abstract
Electrocatalytic carbon dioxide (COv2) reduction is a promising approach to storing the intermittent solar electricity in the form of chemical bonds as well as fixing (over)abundant COv2 into value-added molecules. In particular, transition metal-based homogeneous electrocatalysts have been exhaustively studied as a principal class of electrocatalytic COv2 reduction system over the last four decades. This class of molecular catalysts feature highly tunable central bonding environment around the metal active site as well as periphery of the primary coordination environment, so called secondary coordination sphere, which enables systematic catalysts design modifications and subsequent comparative reactivity studies. Indeed, previous studies following the forgoing catalyst optimization process yielded highly reactive metal active sites for COv2 reduction; however, the secondary coordination sphere is much less explored and its effects on catalysis are poorly understood. As developing our interest in the secondary coordination sphere, we also identified the ubiquitous presence of imidazolium moieties as a key component in both biological and synthetic systems designed for COv2 activation. Most importantly, incorporation of imidazolium-based ionic liquids in electrocatalytic COv2 reduction systems as an electrolyte and/or solvent dramatically decreased overpotential and simultaneously increased catalytic activity. It was proposed that such significant catalytic improvements result from intermolecular interactions between the imidazolium cation and COv2 molecule. Nevertheless, discrete interactions between these chemical entities and clear involvement of the imidazolium moiety in each reaction stage are unknown. Therefore, we decided to chemically install the imidazolium moiety in close proximity to the metal active site on a molecular platform in order to i) investigate and exploit the effects of secondary coordination sphere on catalysis, ii) unravel discrete interactions of the imidazolium moiety with COv2 substrate as well as its mechanistic involvements, and ultimately, iii) optimize the catalytic metrics and establish highly efficient electrocatalytic COv2 reduction systems. In this dissertation, we will discuss our efforts to accomplish the foregoing goals with a series of noble Re and Mn bipyridyl tricarbonyl complexes bearing imidazolium moieties in the secondary coordination sphere. Beyond the mononuclear systems, we will also discuss unique dinuclear systems which bring two transition metal ions in close proximity via imidazolium spacers.
