Activating a Low Overpotential CO₂ Reduction Mechanism by a Strategic Ligand Modification on a Ruthenium Polypyridyl Catalyst

Abstract

The introduction of a simple methyl substituent on the bipyridine ligand of [Ru(tBu3 tpy)(bpy)(NCCH3 )](2+) (tBu3 tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; bpy=2,2'-bipyridine) gives rise to a highly active electrocatalyst for the reduction of CO2 to CO. The methyl group enables CO2 binding already at the one-electron reduced state of the complex to enter a previously not accessible catalytic cycle that operates at the potential of the first reduction. The complex turns over with a Faradaic efficiency close to unity and at an overpotential that is amongst the lowest ever reported for homogenous CO2 reduction catalysts.

Electrocatalytic CO2 Reduction with a Ruthenium Catalyst in Solution and on Nanocrystalline TiO2

“…[19,30] In the reaction, Ru(PO 3 Et 2 -ph-tpy)Cl 3 was allowed to react with 6-mbpy to give [Ru(PO 3 Et 2 -ph-tpy)(6mbpy)(Cl)] + and the chloride ligand was easily displaced by CH 3 CN upon reflux in mixed solutions of CH 3 CN/H 2 O( 3:1 v/v; 40 mL) to give Ru c P 2 + + .I nt he subsequenth ydrolysis step, the ester complex was dissolved in CH 3 CN and treated with TMSBr to give Ru c P 2 + + ',w hichw as used without further purification. [19,30] In the reaction, Ru(PO 3 Et 2 -ph-tpy)Cl 3 was allowed to react with 6-mbpy to give [Ru(PO 3 Et 2 -ph-tpy)(6mbpy)(Cl)] + and the chloride ligand was easily displaced by CH 3 CN upon reflux in mixed solutions of CH 3 CN/H 2 O( 3:1 v/v; 40 mL) to give Ru c P 2 + + .I nt he subsequenth ydrolysis step, the ester complex was dissolved in CH 3 CN and treated with TMSBr to give Ru c P 2 + + ',w hichw as used without further purification.…”

“…Synthesis of Ru c P 2 + + [19] In at hree-neck flask containing EtOH/H 2 O( 3:1 v/v;4 0mL), Ru(PO 3 Et 2 -ph-tpy)Cl 3 (220 mg, 0.33 mmol), 6-mbpy (54.4 mg, 0.33 mmol), TEA (0.10 mL, 0.72 mmol), and LiCl (43.6 mg 1.03 mmol) were added. The resulting homogeneous solution was purged with Ar for 30 min and then heated with stirring at reflux for 12 h. After cooling the reaction to room temperature, EtOH was removed by rotary evaporation.…”

“…(1), (2)]. [19] They concluded that following a1 e À tpy-based reduction, al igand exchange occurs by CH 3 CN dissociation and CO 2 bindingo ccurs to give a1 e À -reduced CO 2 adduct [Eq. The anodic and cathodic peak currents vary linearly with the square root of the scan rate, in accordance with ad iffusional-controlled reaction mechanism ( Figures S5 and S6).…”

“…For diffusionalc atalysts, af urther limitation arises from the relativelyh ighc oncentration level required for effective homogenous catalysis. [19] Surface immobilization eliminates catalystd iffusion. Electron delocalization over the nanographeneligand and the Re I -metal centers ignificantly decreases the energy re-quired for onseto fc atalytic reactivity.…”

Ruthenium Complexes in Homogeneous and Heterogeneous Catalysis for Electroreduction of CO2

“…In 2 , neocuproine replaces the phenanthroline co‐ligand. Ott and co‐workers have shown that the steric interactions caused by the flanking methyl groups (at the ortho positions to N in phenanthroline) to solvent (leaving group) bound at the ruthenium ion profoundly influence catalytic behaviour . We will first discuss the change in the homogeneous electrochemical behaviour of 1 and 2 (in acetonitrile), then the heterogeneous electroreduction of CO 2 by 1 and 2 immobilized on rGO/glassy carbon.…”