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Electrocatalytic CO2 reduction is of continued interest to sustainable energy research. Mononuclear transition metal complexes from Group 6 to Group 10 with a select subset of ligand frameworks have been demonstrated to be efficient electrochemical CO2 reduction catalysts. Here, we review the known mononuclear complexes from Group 6 to Group 10, examining trends in activity, electronic structure of catalytic intermediates, and product selectivity. The correlation between differences in electronic structure and CO2 reduction activity between these metal centers are discussed. [ABSTRACT FROM AUTHOR]

The use of hydrogen-bonding interactions to direct the non-covalent assembly of a heterobimetallic supramolecular system with Re and Mn bipyridine-based electrocatalysts is reported. Under catalytic conditions, the formation of hydrogen bonds generates a catalyst system which passes ∼10% more current than the individual current responses of the respective Re and Mn complexes for the reduction of CO2 to CO and H2O. Infrared spectroelectrochemical studies indicate that the Re and Mn metal centers interact during the reduction mechanism, even forming heterobimetallic bonds under reducing conditions in the absence of substrate. These findings demonstrate that non-covalent assembly is a powerful method for generating new co-catalyst systems with greater reactivity and efficiency for transformations of interest. [ABSTRACT FROM AUTHOR]

The use of hydrogen-bonding interactions to direct the noncovalent assembly of a Re-based bimetallic supramolecular electrocatalyst containing either tyrosine or phenylalanine residues is reported. Computational modeling and spectroelectrochemical characterization indicate that under catalytic conditions the phenol residues of tyrosine can act both as pendant proton sources and participate in the structural assembly of the bimetallic active species. As a result, an increased rate of catalysis is observed experimentally for the reductive disproportionation of CO2 to CO and CO33- by a tyrosine-modified complex in comparison to a control complex containing phenylalanine residues. These findings demonstrate that noncovalent assembly is a powerful method for generating new bimetallic electrocatalyst systems where the choice of substituent can be used to both control structural assembly and introduce cocatalytic moieties. [ABSTRACT FROM AUTHOR]

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl(CO)5 furnishes ReCl(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and ReCl(N-methyl-N'-2-pyrimidylbenzimidazol-2-ylidine)(CO)3. These Re(I) NHC complexes are shown to be capable of mediating the two-electron conversion of CO2 following one-electron reduction; the Faradaic efficiency for CO formation is observed to be >60% with minor H2 and HCO2H production. Data from cyclic voltammetry is presented and compared to well-studied ReCl(2,2'-bipyridine)(CO)3 and MnBr(2,2'-bipyridine)(CO)3 systems. Results from density functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed. [ABSTRACT FROM AUTHOR]

MnBr(2,2′-bipyridine)(CO)3 is an efficient and selective electrocatalyst for the conversion of CO2 to CO. Herein, substitution of the axial bromide for a pseudohalogen (CN) is investigated, yielding Mn(CN)(2,2′-bipyridine)(CO)3. This replacement shifts the first and second reductions to more negative potentials (−1.94 and −2.51 V vs Fc/Fc+, respectively), but imparts quasi-reversibility at the first feature. The two-electron, two-proton reduction of CO2 to CO and H2O is observed at the potential of the first reduction. Data from IR spectroelectrochemistry, cyclic voltammetry, and controlled potential electrolysis indicate that this behavior arises from the disproportionation of two one-electron-reduced species to generate the catalytically active species. Computations using density functional theory are also presented in support of this new mechanism. [ABSTRACT FROM AUTHOR]

The use of a bulky bipyridine ligand, 6,6'-dimesityl-2,2'-bipyridine (mesbpy), to enable the reduction of carbon dioxide by a Ru-based molecular electrocatalyst is reported. Under catalytic conditions, this compound exhibits turnover frequencies of 1300 s-1 and 95% Faradaic efficiency for the production of CO and H2O from CO2 in the presence of Brønsted acids. Mechanistic electrochemical and spectroelectrochemical studies, supplemented by the direct synthesis of relevant intermediates, indicate that this behavior is the result of the cooperative redox response of the bipyridine ligand and Ru metal center at negative potentials, as well as the inhibition of Ru-Ru bond formation through steric interactions. [ABSTRACT FROM AUTHOR]

The use of infrared spectroelectrochemistry(IR-SEC) as a characterization method for molecular electrocatalystsallows researchers to identify key intermediates and products duringthe course of an electrochemical reaction. Since such reactions aredriven by the application of potential, properly designed cells allowfor the examination of the stepwise formation of active species andtheir subsequent reactivity with substrate. These results can be comparedto and independently verified by the concomitant generation of similarspecies by chemical means. Recently we have used such an approachto characterize the catalytically relevant species and products forthe reduction of CO2by 2,2′-bipyridyl-based ReIand MnIfac-tricarbonyl electrocatalysts.This tutorial review summarizes the requirements of IR-SEC cells anddetails some examples of mechanistic questions addressed in our laboratoryusing these methods. IR-SEC is presented as a general method thatcould be adapted by other laboratories to answer questions of interestin the spectroscopic characterization of the full complement of redoxstates of organometallic complexes and their reactivities. [ABSTRACT FROM AUTHOR]

Review: 67 refs. [ABSTRACT FROM AUTHOR]

The use of CO 2 as a chemical feedstock has been the focus of much research in recent years due to the promise for carbon neutral fuel storage in chemical bonds. Specifically, electrocatalysts of the type Re(bpy)(CO) 3 Cl (bpy = 2,2′-bipyridine and analogues thereof) have been studied for their ability to reduce CO 2 to CO. These catalysts are among the most active, selective, and robust homogeneous catalysts for CO 2 reduction in the literature. Previous studies have involved understanding the mechanism for catalysis and the inductive effect of bipyridine functional groups on catalysis. In the study reported herein, we have assessed the CO 2 reduction capabilities of a series of Re( n , n ′-dimethyl-bpy)(CO) 3 Cl ( n = 3 and 5) catalysts with a bpy modified at the 3 and 5 positions with methyl substituents. While the 5,5′-positional substituents have the effect of increasing catalytic activity ( i cat / i p = 29.6) over the unsubstituted catalyst ( i cat / i p = 20.3); steric hindrance in the 3,3′-substituted bpy has the effect of decreasing catalytic activity ( i cat / i p = 17.0). This decreased catalytic current response can be explained by steric hindrance in the 3,3′-substituted catalyst disfavoring optimal charge transfer in the catalytic cycle. [ABSTRACT FROM AUTHOR]