Electrochemical CO 2 Reduction Catalyzed by Binuclear LRe 2 (CO) 6 Cl 2 and LMn 2 (CO) 6 Br 2 Complexes with an Internal Proton Source.

The synthesis of certain commodity chemicals, e.g., methanol and acetic acid, relies on CO, which is currently mainly produced by the combustion of carbon or natural gas. Photo- or electrochemical conversion of atmospheric CO 2 to CO represents an attractive alternative strategy as this approach is carbon-neutral. Such photo- or electrochemically formed CO can also be used in the Fischer-Tropsch process forming liquid hydrocarbons for energy storage applications. The multiple electroreduction of CO 2 is preferably coupled with proton transfer steps as this requires less energy than the single outer-sphere 1e - reduction of CO 2 .In 1984 and 2011, it was shown that [(L bpy )Re(CO) 3 Cl] ( 1 ) and [(L bpy )Mn(CO) 3 Br] ( 2 ), respectively, mediate the electrochemical 2e - /2H + reduction of CO 2 forming CO and water (L bpy = 2,2'-bipyridine). Since proton management is crucial for catalysis, recently the impact of internal proton sources close to the axial position in such complexes has been investigated. However, binuclear complexes have been used rarely as mediators although it has been shown very early for 1 that electron management is also important: the 2e - /2H + reduction pathway with 1 exhibits a higher reaction rate than going via the singly reduced species, though the pathway requires a higher overpotential. In this Account, we focus on recent developments of binuclear LMn 2 (CO) 6 and LRe 2 (CO) 6 mediators with an internal phenol group in the electroreduction of CO 2 . In contrast to mononuclear derivatives, for which the impact of the internal proton source on catalysis is very diverse, we always observed a higher reaction rate and for the Mn complexes also a lower overpotential with the binuclear complexes compared to the mononuclear variants. Spectroscopic, electrochemical, and computational studies on the mono- and binuclear complexes shed light on their reactivity under reductive conditions, elucidated the structure of reduced species, unraveled the kinetics for catalytically productive and unproductive (side) reactions, and allowed us to derive some hypothesis on the CO 2 reduction mechanism. Finally, I emphasize that the electrohydrogenation of the polar double bonds by the binuclear complex LMn 2 (CO) 6 with a central phenol unit is not restricted to CO 2 but is also applicable to organic compounds with C═O bonds.