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

Industrial processes and fossil fuel combustion produce carbon dioxide (CO2) unsustainably on the gigaton scale. Addressing this pressing issue has led to rapidly growing efforts to catalytically reduce CO2 to liquid fuels. [1] Recycling CO2 is a profoundly challenging problem that requires fundamental insights to guide advancements. Information regarding CO2 transformations abound, [1,2] but no industrialscale process has capably reduced CO2 to liquid fuels. Of the systems that electrocatalytically reduce CO2, the [Re(bpy)(CO)3Cl] family of compounds (bpy= 2,2’-bipyridine) is one of the most robust and well-characterized systems known to date. This system converts CO2 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 I state in [Re(bpy)(CO)3] . This state has long been proposed as the active state of the electrocatalyst. 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 CO2. We recently reported stopped-flow kinetics studies showing the relative selectivities of the [Re(bpy-tBu)(CO)3] anion reacting with with CO2 and proton sources. These studies revealed that reaction rates of the anion were about 35 times faster with CO2 than with weak acid. [3b] The bpy ligand was proposed to play a non-innocent role by storing charge and preventing a doubly occupied dz2 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] and [Re(bpy-tBu)(CO)3] show the bpy ligands exhibit bond length alternation and short Cpy Cpy 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, more representative of a Re(bpy ) state rather than a Re(bpy ) or Re (bpy) state. The redox activities of bpy ligands as well as other non-innocent ligands have been extensively studied. 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)3Cl] (1) and [Re(bpy-tBu)(CO)3Cl] (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)5Cl] (6) and [Re2(CO)10] (7), and a synthesized Re I 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 L3 absorption edge using the strong “white-line” resonance arising from 2p!5d transitions probes the Re5d unoccupied states. Kohn–Sham density functional theory (KS-DFT) calculations provide a first-principles description of electronic structures. Lastly, extended X-ray absorption fine structure (EXAFS) studies of frozen THF solutions of 1, 2, 4, and 5 confirm the monomeric nature of the catalysts and rule out solvent coordination to the Re centers in solution. Compounds 1–5 were prepared according to literature procedures. [K([18]crown-6)][Re(CO)5] (8) was prepared by the reduction of [Re2(CO)10] (7) in tetrahydrofuran (THF) by excess KC8 (potassium intercalated graphite) in the presence of [18]crown-6 (see the Supporting Information). The IR stretching frequencies of complexes 1–7 have been reported previously; however, we obtained frequencies for complexes 1–7 and the newly synthesized complex 8 under the same conditions for fair comparison (Table 1). The oneelectron reduction of the formally Re chloride species 2 results in formation of the one-electron reduced monomer, [*] Dr. E. E. Benson, M. D. Sampson, Dr. K. A. Grice, Dr. J. M. Smieja, J. D. Froehlich, Prof. Dr. C. P. Kubiak Department of Chemistry and Biochemistry, University of California, San Diego 9500 Gilman Drive,Code 0358, La Jolla, CA 92093-0358 (USA) E-mail: ckubiak@ucsd.edu