Electrochemical Reduction of CO₂ Catalyzed by Re(pyridine-oxazoline)(CO)₃Cl Complexes

Abstract

A series of rhenium tricarbonyl complexes coordinated by asymmetric diimine ligands containing a pyridine moiety bound to an oxazoline ring were synthesized, structurally and electrochemically characterized, and screened for CO reduction ability. The reported complexes are of the type Re(N-N)(CO)Cl, with N-N = 2-(pyridin-2-yl)-4,5-dihydrooxazole (1), 5-methyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (2), and 5-phenyl-2-(pyridin-2-yl)-4,5-dihydrooxazole (3). The electrocatalytic reduction of CO by these complexes was observed in a variety of solvents and proceeds more quickly in acetonitrile than in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The analysis of the catalytic cycle for electrochemical CO reduction by 1 in acetonitrile using density functional theory (DFT) supports the C-O bond cleavage step being the rate-determining step (RDS) (ΔG = 27.2 kcal mol). The dependency of the turnover frequencies (TOFs) on the donor number (DN) of the solvent also supports that C-O bond cleavage is the rate-determining step. Moreover, the calculations using explicit solvent molecules indicate that the solvent dependence likely arises from a protonation-first mechanism. Unlike other complexes derived from fac-Re(bpy)(CO)Cl (I; bpy = 2,2'-bipyridine), in which one of the pyridyl moieties in the bpy ligand is replaced by another imine, no catalytic enhancement occurs during the first reduction potential. Remarkably, catalysts 1 and 2 display relative turnover frequencies, (i/i), up to 7 times larger than that of I.

Solvent and Ligand Substitution Effects on the Electrocatalytic Reduction of CO2 with [Mo(CO)4(x,x′‐dimethyl‐2,2′‐bipyridine)] (x=4–6) Enhanced at a Gold Cathodic Surface

“…Despite this, continuing research efforts to produce commercially viable functionalization of CO 2 remains some distance away . Currently, much of the literature regarding molecular electrochemical activation of CO 2 still continues to explore the catalytic properties of the heavier members of Group‐7 (Re), Group‐8 (Ru and Os) and Group‐9 (Rh, and Ir) triads. These “first‐generation” electrocatalysts must be phased out; encouragingly, cheaper alternatives exist within the same groups – Mn substituting Re, Fe substituting Ru and Os, and Co substituting Rh and Ir .…”

Rhenium – A Tuneable Player in Tailored Hydrogenation Catalysis

“…They demonstrated that the catalysts presented superior relative turnover frequencies to the Re(bpy)(CO) 3 Cl catalyst due to the presence of the oxazoline ligand, which is a better σ donor than pyridine. Also, they found out that the reaction of these complexes proceeded more quickly in acetonitrile than in DMF and dimethyl sulfoxide (DMSO) due to a protonation‐first mechanism [168] . Following modifications of the rhenium bpy‐based catalyst using benzylic pendant amines, the complex Re(dEAbpy)(CO) 3 Cl (dEAbpy=N,N′‐(([2,2′‐bipyridine]‐6,6′‐diylbis(2,1‐phenylene))bis(methylene))bis(N‐ethylethanamine)) was prepared, showing selective production of CO, with 11.8±3.5 % HCOOH formation.…”

Assessing the Electrocatalytic Properties of the {Cp*RhIII}2+‐Polyoxometalate Derivative [H2PW11O39{RhIIICp*(OH2)}]3– towards CO2 Reduction

“…In an example closely related to this study, complexes [Cp*Ir(bpy‐R,R′)Cl] + prepared with various unsymmetrically substituted bipyridine ligands were found to have a moderate electrocatalytic activity in CO 2 reduction but with no direct correlation between the redox potentials and the catalytic activity. Regarding the replacement of the bpy ligand by another redox non‐innocent ligand, rationalization is still missing: none of the [Re(N ^ N)(CO) 3 X] complexes with N ^ N being 2‐pyridyl‐1,2,3‐triazole ligand was found to outperform the benchmark catalyst [Re(bpy)(CO) 3 X], whereas a Re‐pyridyl‐NHC species displays a slightly higher faradic efficiency and some Re‐pyridine‐oxazolines higher TOF but decreased faradic efficiency; [Mn(N ^ N)(CO) 3 X] where N ^ N stands for iminopyridines or α‐diimines do catalyze the conversion of CO 2 to CO and CO 3 2– but at the cost of an increased overpotential. These examples underline that more structure‐activity relationships studies are needed to improve our understanding of the relevant parameters and to find a good balance between decreasing the overpotential and keeping sufficient reducing power in the redox non‐innocent ligand .…”