The Electronic States of Rhenium Bipyridyl Electrocatalysts for CO₂ Reduction as Revealed by X‐ray Absorption Spectroscopy and Computational Quantum Chemistry

Abstract

Industrial processes and fossil fuel combustion produce carbon dioxide (CO 2 ) unsustainably on the gigaton scale. Addressing this pressing issue has led to rapidly growing efforts to catalytically reduce CO 2 to liquid fuels. [1] Recycling CO 2 is a profoundly challenging problem that requires fundamental insights to guide advancements. Information regarding CO 2 transformations abound, [1,2] but no industrialscale process has capably reduced CO 2 to liquid fuels. Of the systems that electrocatalytically reduce CO 2 , the [Re-(bpy)(CO) 3 Cl] family of compounds (bpy = 2,2'-bipyridine) is one of the most robust and well-characterized systems known to date. [2a, 3] This system converts CO 2 into carbon monoxide (CO) with high rates and efficiencies; it suffers, however, from large overpotentials that are believed to arise from accessing the highly reduced, formally Re ÀI state in [Re(bpy)(CO) 3 ] À . This state has long been proposed as the active state of the electrocatalyst. [3a,b,d] Apart from this assumption, there is little known about the electronic structure of the catalyst in its reduced (active) state and its subsequent interaction with CO 2 .We recently reported stopped-flow kinetics studies showing the relative selectivities of the [Re(bpy-tBu)(CO) 3 ] À anion reacting with with CO 2 and proton sources. These studies revealed that reaction rates of the anion were about 35 times faster with CO 2 than with weak acid. [3b] The bpy ligand was proposed to play a non-innocent role by storing charge and preventing a doubly occupied d z 2 orbital at the Re center, which would be needed to form a metal hydride. Indeed, Xray diffraction (XRD) studies of both [Re(bpy)(CO) 3 ] À and [Re(bpy-tBu)(CO) 3 ] À show the bpy ligands exhibit bond length alternation and short C py ÀC py bonds (1.370 (15) , for bpy-tBu), indicating significant electron density on these ligands. The short inter-ring bonds suggest a doubly reduced bpy ligand, [4] more representative of a Re +I (bpy 2À ) state rather than a Re 0 (bpy À ) or Re ÀI (bpy 0 ) state. The redox activities of bpy ligands [4,5] as well as other non-innocent ligands have been extensively studied. [6] To fully confirm that the non-innocence of bpy contributes to this unique catalysis, we employed experimental spectroscopy and theoretical quantum chemistry to characterize this catalyst family. We compared the halide starting materials, [Re(bpy)(CO) 3 Cl] (1) and [Re(bpy-tBu)(CO) 3 Cl] (2), the one-electron reduced dimer [{Re(bpy)(CO) 3 } 2 ] (3), the twoelectron reduced anions [K([18]crown-6)][Re(bpy)(CO) 3 ] (4) and [K([18]crown-6)][Re(bpy-tBu)(CO) 3 ] (5), the commercially available standards, [Re(CO) 5 Cl] (6) and [Re 2 (CO) 10 ] (7), and a synthesized Re ÀI standard, [K([18]crown-6)][Re(CO) 5 ] (8). IR spectroscopy of the stretching frequencies of the carbonyl ligands characterizes the electronic states of these complexes. X-ray absorption spectroscopy (XAS) at the Re L 3 absorption edge using the strong "white-line" resonance arising from 2p!5d transitions probes ...

Synthesis, Spectroscopy, and Electrochemistry of (α-Diimine)M(CO)3Br, M = Mn, Re, Complexes: Ligands Isoelectronic to Bipyridyl Show Differences in CO2Reduction

“…Complex 2 is observed to undergo an initial reversible ligand-based reduction at −1.23 vs Fc/Fc + . 39 This is followed by an additional less chemically reversible redox feature at −1.77 V vs Fc/Fc + . 1 Under CO 2 -saturation conditions, complex 2 exhibited a modest increase in current as compared to voltammograms under N 2 .…”

Supramolecular Assembly Promotes the Electrocatalytic Reduction of Carbon Dioxide by Re(I) Bipyridine Catalysts at a Lower Overpotential

“…The influence of the methyl acetamidomethyl groups on the electrochemical behavior of complex 1 is evident by cyclic voltammetry under N 2 atmosphere, as four redox processes were observed in a potential range where the model compound 2 exhibits two features ( Figure 2A). 8,30,32 Starting from resting potential, an irreversible redox feature is observed at −1.77 V, a reversible one at −1.88 V, a quasi-reversible one at −2.14 V, and a final irreversible one at −2.34 V (versus Fc/Fc+). In comparison, complex 2 exhibits a reversible feature at −1.85 V and an irreversible one at −2.20 V (vs Fc/Fc+).…”

Local Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy–R)(CO)3Br] Complexes

“…In particular, Mn complexes bearing 4,4′‐disubstituted bipyridines with H, CH 3 and t Bu substituents (since all complexes herein mentioned are fac ‐ we will omit this label) are valid low‐cost alternatives to their Re I analogues in terms of stability, selectivity and efficiency . The combination of IR/UV/Vis spectroelectrochemistry (SEC) with non‐conventional spectroscopic techniques and computational methods, provided valuable complementary tools for elucidating the electrocatalytic mechanism of CO 2 reduction with Group VII metal catalysts. These studies suggest that doubly‐reduced pentacoordinated species [M(bpy–R)(CO) 3 ] − (M=Mn, Re) are responsible for the catalytic conversion of CO 2 into CO.…”