A Thiourea Tether in the Second Coordination Sphere as a Binding Site for CO

Area of Science:

• Electrochemistry
• Catalysis
• Organometallic Chemistry

Background:

• Electrochemical reduction of carbon dioxide (CO2) is crucial for sustainable energy.
• Understanding catalytic mechanisms is key to developing efficient CO2 reduction catalysts.
• Biomimetic approaches, inspired by enzymes like carbon dioxide hydrogenase, offer insights into stabilizing intermediates.

Purpose of the Study:

• To design and synthesize a rhenium(I) complex with enhanced CO2 binding and stabilization capabilities.
• To investigate the catalytic activity and mechanism of the modified rhenium(I) complex for CO2 electroreduction.
• To explore the role of a second-sphere thiourea tether in the catalytic cycle.

Main Methods:

• Synthesis of a fac-Re(I)bipyridine(CO)3Cl complex modified with a thiourea tether.
• Electrochemical characterization to assess catalytic activity and selectivity for CO2 reduction.
• Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopy to identify intermediates and study reaction mechanisms.
• Density Functional Theory (DFT) calculations to elucidate the proton transfer pathway.

Main Results:

• The modified Re(I) catalyst demonstrated excellent electrocatalytic activity for CO2 to CO reduction with a turnover frequency of 3040 s-1.
• The thiourea tether was shown to bind CO2 and stabilize carboxylic acid intermediates through hydrogen bonding.
• The thiourea moiety acted as an intrinsic proton source, and external proton sources inhibited catalysis.
• DFT calculations revealed a direct proton transfer to CO2, explaining the absence of a kinetic isotope effect.

Conclusions:

• The thiourea tether in the second coordination sphere significantly enhances catalyst performance by facilitating CO2 binding, stabilizing intermediates, and providing a local proton source.
• This biomimetic design offers a promising strategy for developing highly active and selective electrocatalysts for CO2 reduction.
• The mechanistic insights gained provide a foundation for designing next-generation catalysts for CO2 conversion.