Abstract
Heterogenization of homogenous catalysts on electrode surfaces provides a valuable approach for characterization of catalytic processes in operando conditions using surface selective spectroelectrochemistry methods. Ligand design plays a central role in the attachment mode and the resulting functionality of the heterogenized catalyst as determined by the orientation of the catalyst relative to the surface and the nature of specific interactions that modulate the redox properties under the heterogeneous electrode conditions. Here, we introduce new [Re(L)(CO)3Cl] catalysts for CO2 reduction with sulfur-based anchoring groups on a bipyridyl ligand, where L = 3,3′-disulfide-2,2′-bipyridine (SSbpy) and 3,3′-thio-2,2′-bipyridine (Sbpy). Spectroscopic and electrochemical analysis complemented by computational modeling at the density functional theory level identify the complex [Re(SSbpy)(CO)3Cl] as a multi-electron acceptor that combines the redox properties of both the rhenium tricarbonyl core and the disulfide functional group on the bipyridyl ligand. The first reduction at −0.85 V (vs. SCE) involves a two-electron process that breaks the disulfide bond, activating it for surface attachment. The heterogenized complex exhibits robust anchoring on gold surfaces, as probed by vibrational sum-frequency generation (SFG) spectroscopy. The binding configuration is normal to the surface, exposing the active site to the CO2 substrate in solution. The attachment mode is thus particularly suitable for electrocatalytic CO2 reduction.
Highlights
Electrochemical CO2 reduction powered by renewable energy sources, such as wind or solar light, is an attractive pathway for generation of fuels or chemical feedstock (Qiao et al, 2016)
Further characterization of the compounds by FTIR spectra in both KBr pellets and in CH3CN showed the three typical νC=O stretches of the fac-Re(CO)3 block found in similar compounds (Smieja and Kubiak, 2010), namely the higher energy a’ (Qiao et al, 2016) symmetric stretch, as well as the low energy symmetric a’ (Clark et al, 2018a) and antisymmetric a” stretches (Figure S10)
We have introduced two chloro-tricarbonyl rhenium complexes with thio-substituted bipyridine ligands for CO2 reduction, [Re(SSbpy)(CO)3Cl] (ReSS) and [Re(Sbpy)(CO)3Cl] (ReS) that were characterized as homogenous and heterogenized systems using spectroscopic, computational and electrochemical techniques
Summary
Electrochemical CO2 reduction powered by renewable energy sources, such as wind or solar light, is an attractive pathway for generation of fuels or chemical feedstock (Qiao et al, 2016). Rhenium complexes are effective homogeneous catalysts with high turnover frequencies for CO2 to CO conversion (Clark et al, 2018a). Surface-Attached Rhenium Complexes for CO2 Reduction understanding of structure/property relations of rhenium polypyridine complexes that can be exploited for the rational design and improvement of catalysts that could operate at lower overpotentials (Schneider et al, 2016). 2,2′-bipyridine (bpy) ligands have been extensively investigated in coordination chemistry. We explore the functionalization of bpy ligands with disulfide groups in order to develop rhenium complexes for CO2 reduction that strongly bind to gold electrode surfaces
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