Kyle A. Grice

Manganese as a Substitute for Rhenium in CO2 Reduction Catalysts: The Importance of Acids

Electrocatalytic properties, X-ray crystallographic studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(bpy-tBu)(CO)3Br and [Mn(bpy-tBu)(CO)3(MeCN)](OTf) are reported. Addition of Brönsted acids to CO2-saturated solutions of these Mn complexes and subsequent reduction of the complexes lead to the stable and efficient production of CO from CO2. Unlike the analogous Re catalysts, these Mn catalysts require the addition of Brönsted acids for catalytic turnover. Current densities up to 30 mA/cm(2) were observed during bulk electrolysis using 5 mM Mn(bpy-tBu)(CO)3Br, 1 M 2,2,2-trifluoroethanol, and a glassy carbon working electrode. During bulk electrolysis at -2.2 V vs SCE, a TOF of 340 s(-1) was calculated for Mn(bpy-tBu)(CO)3Br with 1.4 M trifluoroethanol, corresponding to a Faradaic efficiency of 100 ± 15% for the formation of CO from CO2, with no observable production of H2. When compared to the analogous Re catalysts, the Mn catalysts operate at a lower overpotential and exhibit similar catalytic activities. X-ray crystallography of the reduced species, [Mn(bpy-tBu)(CO)3](-), shows a five-coordinate Mn center, similar to its rhenium analogue. Three distinct species were observed in the IR-SEC of Mn(bpy-tBu)(CO)3Br. These were of the parent Mn(bpy-tBu)(CO)3Br complex, the dimer [Mn(bpy-tBu)(CO)3]2, and the [Mn(bpy-tBu)(CO)3](-) anion.

Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis

With the goal of improving previously reported Mn bipyridine electrocatalysts in terms of increased activity and reduced overpotential, a bulky bipyridine ligand, 6,6-dimesityl-2,2-bipyridine (mesbpy), was utilized to eliminate dimerization in the catalytic cycle. Synthesis, electrocatalytic properties, X-ray diffraction (XRD) studies, and infrared spectroelectrochemistry (IR-SEC) of Mn(mesbpy)(CO)3Br and [Mn(mesbpy)(CO)3(MeCN)](OTf) are reported. Unlike previously reported Mn bipyridine catalysts, these Mn complexes exhibit a single, two-electron reduction wave under nitrogen, with no evidence of dimerization. The anionic complex, [Mn(mesbpy)(CO)3](-), is formed at 300 mV more positive potential than the corresponding state is formed in typical Mn bipyridine catalysts. IR-SEC experiments and chemical reductions with KC8 provide insights into the species leading up to the anionic state, specifically that both the singly reduced and doubly reduced Mn complexes form at the same potential. When formed, the anionic complex binds CO2 with H(+), but catalytic activity does not occur until a ~400 mV more negative potential is present. The Mn complexes show high activity and Faradaic efficiency for CO2 reduction to CO with the addition of weak Brønsted acids. IR-SEC experiments under CO2/H(+) indicate that reduction of a Mn(I)-CO2H catalytic intermediate may be the cause of this unusual over-reduction required to initiate catalysis.

Elucidation of the Selectivity of Proton-Dependent Electrocatalytic CO2 Reduction by fac-Re(bpy)(CO)3Cl

A complete mechanism for the proton-dependent electrocatalytic reduction of CO2 to CO by fac-Re(bpy)(CO)3Cl that is consistent with experimental observations has been developed using first principles quantum chemistry. Calculated one-electron reduction potentials, nonaqueous pKas, reaction free energies, and reaction barrier heights provide deep insight into the complex mechanism for CO2 reduction as well as the origin of selectivity for this catalyst. Protonation and then reduction of a metastable Re-CO2 intermediate anion precedes Brønsted-acid-catalyzed C-O cleavage and then rapid release of CO at negative applied potentials. Conceptually understanding the mechanism of this rapid catalytic process provides a useful blueprint for future work in artificial photosynthesis.

Kinetic and structural studies, origins of selectivity, and interfacial charge transfer in the artificial photosynthesis of CO

The effective design of an artificial photosynthetic system entails the optimization of several important interactions. Herein we report stopped-flow UV-visible (UV-vis) spectroscopy, X-ray crystallographic, density functional theory (DFT), and electrochemical kinetic studies of the Re(bipy-tBu)(CO)3(L) catalyst for the reduction of CO2 to CO. A remarkable selectivity for CO2 over H+ was observed by stopped-flow UV-vis spectroscopy of [Re(bipy-tBu)(CO)3]-1. The reaction with CO2 is about 25 times faster than the reaction with water or methanol at the same concentrations. X-ray crystallography and DFT studies of the doubly reduced anionic species suggest that the highest occupied molecular orbital (HOMO) has mixed metal-ligand character rather than being purely doubly occupied , which is believed to determine selectivity by favoring CO2 (σ + π) over H+ (σ only) binding. Electrocatalytic studies performed with the addition of Brönsted acids reveal a primary H/D kinetic isotope effect, indicating that transfer of protons to Re -CO2 is involved in the rate limiting step. Lastly, the effects of electrode surface modification on interfacial electron transfer between a semiconductor and catalyst were investigated and found to affect the observed current densities for catalysis more than threefold, indicating that the properties of the electrode surface need to be addressed when developing a homogeneous artificial photosynthetic system.

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

Chapter Five – Recent Studies of Rhenium and Manganese Bipyridine Carbonyl Catalysts for the Electrochemical Reduction of CO2

Abstract The eventual deployment of large-scale systems for the electrochemical reduction of carbon dioxide (CO 2 ) to fuels and commodity chemicals depends on the development of stable, highly active, and selective catalysts. The fac -Re(bpy-R)(CO) 3 X system, originally reported three decades ago, is very efficient for CO 2 reduction to carbon monoxide (CO) even in the presence of proton sources. Recent studies from our group and others that have improved the catalyst activity and significantly expanded the understanding of these catalysts are highlighted in this report. The 4,4′- tert -butyl-substituted complexes fac -Re(bpy- t Bu)(CO) 3 X have been found to be more active than the parent 2,2′-bipyridine complexes. The presence of Bronsted acids increases the activity of these catalysts, with stronger acids leading to more rapid catalysis. The catalytically relevant [Re(bpy-R)(CO) 3 ] −xa01 anions have been isolated and studied in order to elucidate their structures and reactivities. X-ray crystallography, quantum chemical calculations, and synchrotron radiation experiments have shown that the electronic structures of the anions are best described as Re 0 (bpy-R) −xa01 states, with electron density delocalized over both the metal and the ligand. This delocalized ground state is thought to enable better overlap with CO 2 compared to protons, which explains the selectivity for CO 2 reduction with these types of catalysts even in the presence of acids. Recent reports have also shown that earth-abundant manganese can be substituted for rhenium to yield fac -Mn(bpy-R)(CO) 3 X catalysts that approach the CO 2 reduction activity of the analogous rhenium compounds. Indeed, the anionic [Mn(bpy- t Bu)(CO) 3 ] −xa01 species has recently been crystallized and studied, and it possesses a similar structure to the [Re(bpy- t Bu)(CO) 3 ] −xa01 anion. Future directions for the study of fac -M(bpy-R)(CO) 3 X catalysts are also discussed.

Electrocatalytic CO2 reduction by M(bpy-R)(CO)4 (M = Mo, W; R = H, tBu) complexes. Electrochemical, spectroscopic, and computational studies and comparison with group 7 catalysts

The tetracarbonyl molybdenum and tungsten complexes of 2,2′-bipyridine and 4,4′-di-tert-butyl-2,2′-bipyridine (M(bpy-R)(CO)4; R = H, M = Mo (1), W (2); R = tBu, M = Mo (3), W (4)) are found to be active electrocatalysts for the reduction of CO2. The crystal structures of M(bpy-tBu)(CO)4 (M = Mo (3), W (4)), the singly reduced complex [W(bpy)(CO)4][K(18-crown-6] (5) and the doubly reduced complex [W(bpy-tBu)(CO)3][K(18-crown-6)]2 (6) are reported. DFT calculations have been used to characterize the reduced species from the reduction of W(bpy-tBu)(CO)4 (4). These compounds represent rare examples of group 6 electrocatalysts for CO2 reduction, and comparisons are made with the related group 7 complexes that have been studied extensively for CO2 reduction.

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

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 CO2 reduction ability. The reported complexes are of the type Re(N-N)(CO)3Cl, 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 CO2 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 CO2 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-1). 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)3Cl (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, (icat/ip)2, up to 7 times larger than that of I.

Electrocatalytic Reduction of CO2 by Group 6 M(CO)6 Species without “Non-Innocent” Ligands

To understand the electrocatalytic CO2 reduction of metal carbonyl complexes without non-innocent ligands, the electrochemical responses of group 6 M(CO)6 (M = Cr, Mo, or W) and group 7 M2(CO)10 (M = Mn or Re) complexes were examined under Ar and CO2 at a glassy carbon electrode. All of the complexes showed changes in their cyclic voltammograms under CO2. The group 6 hexacarbonyl species show a significant increase in current under CO2 during metal-based reduction, corresponding to catalytic reduction of CO2. Bulk electrolysis experiments with Mo(CO)6 showed that CO was the primary product. The group 7 dimers showed very little change during metal-based reduction, but return oxidation responses disappeared, indicative of a chemical reaction after exposure to CO2 without catalysis. Addition of H2O, a proton source, to the solutions under CO2 decreased the catalytic current of the group 6 carbonyls and had no effect on the responses of the group 7 carbonyls. The group 6 M(CO)6 species are notable in that that they are effective catalysts without the need for an added non-innocent ligand such as 2,2-bipyridine.