Matthew D. Sampson

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.

Mechanistic Contrasts between Manganese and Rhenium Bipyridine Electrocatalysts for the Reduction of Carbon Dioxide

[Re(bpy)(CO)3](-) is a well-established homogeneous electrocatalyst for the reduction of CO2 to CO. Recently, substitution of the more abundant transition metal Mn for Re yielded a similarly active electrocatalyst, [Mn(bpy)(CO)3](-). Compared to the Re catalyst, this Mn catalyst operates at a lower applied reduction potential but requires the presence of a weak acid in the solution for catalytic activity. In this study, we employ quantum chemistry combined with continuum solvation and microkinetics to examine the mechanism of CO2 reduction by each catalyst. We use cyclic voltammetry experiments to determine the turnover frequencies of the Mn catalyst with phenol as the added weak acid. The computed turnover frequencies for both catalysts agree to within one order of magnitude of the experimental ones. The different operating potentials for these catalysts indicate that different reduction pathways may be favored during catalysis. We model two different pathways for both catalysts and find that, at their respective operating potentials, the Mn catalyst indeed is predicted to take a different reaction route than the Re catalyst. The Mn catalyst can access both catalytic pathways, depending on the applied potential, while the Re catalyst does not show this flexibility. Our microkinetics analysis predicts which intermediates should be observable during catalysis. These intermediates for the two catalyzed reactions have qualitatively different electronic configurations, depending on the applied potential. The observable intermediate at higher applied potentials possesses an unpaired electron and therefore should be EPR-active; however, the observable intermediate at lower applied potentials, accessible only for the Mn catalyst, is diamagnetic and therefore should be EPR-silent. The differences between both catalysts are rationalized on the basis of their electronic structure and different ligand binding affinities.

A porous proton-relaying metal-organic framework material that accelerates electrochemical hydrogen evolution

The availability of efficient hydrogen evolution reaction (HER) catalysts is of high importance for solar fuel technologies aimed at reducing future carbon emissions. Even though Pt electrodes are excellent HER electrocatalysts, commercialization of large-scale hydrogen production technology requires finding an equally efficient, low-cost, earth-abundant alternative. Here, high porosity, metal-organic framework (MOF) films have been used as scaffolds for the deposition of a Ni-S electrocatalyst. Compared with an MOF-free Ni-S, the resulting hybrid materials exhibit significantly enhanced performance for HER from aqueous acid, decreasing the kinetic overpotential by more than 200 mV at a benchmark current density of 10 mA cm−2. Although the initial aim was to improve electrocatalytic activity by greatly boosting the active area of the Ni-S catalyst, the performance enhancements instead were found to arise primarily from the ability of the proton-conductive MOF to favourably modify the immediate chemical environment of the sulfide-based catalyst.

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

Direct observation of the reduction of carbon dioxide by rhenium bipyridine catalysts

In order to further efforts in synthesis and catalysis, the mechanisms of catalysts must be completely understood. The Re(bpy)(CO)3Cl molecular catalysts are some of the most robust and well-characterized CO2 reduction catalysts known to date. Stopped-flow infrared spectroscopy is reported as a technique for studying the kinetics and mechanisms of the reactions of catalytically-relevant [Re(bpy-R)(CO)3]− anions (R = tBu or H) with CO2/H+. [Re(bpy-tBu)(CO)3]− reacts approximately ten times faster with CO2 than does [Re(bpy)(CO)3]−. These reactions occur via a direct two-electron oxidative addition of CO2 to the metal center and result in the formation of an intermediate CO2 reduction product, Re(bpy-R)(CO)3(CO2H). This is the first in situ identification of this key intermediate. Evidence for this Re–CO2H species includes isotopic labeling studies, stopped-flow experiments of the kinetics of its formation in the presence of proton sources, comparison with genuine Re(bpy)(CO)3(CO2H), and DFT calculations.

Electrocatalytic Dihydrogen Production by an Earth-Abundant Manganese Bipyridine Catalyst

Earth-abundant manganese bipyridine complexes have been extensively studied as homogeneous electrocatalysts for proton-coupled CO2 reduction. To date, these manganese complexes have not been examined as catalysts for the reduction of other small molecules. We report electrocatalytic H2 production by [Mn(mesbpy)(CO)3(MeCN)](OTf)]. In acetonitrile, this manganese electrocatalyst displays a turnover frequency (TOF) of 5500 s(-1) at an overpotential of 0.90 V (at Ecat/2) for the reduction of protons to H2 using trifluoroacetic acid (TFA) as the acid source. These findings show the flexibility of these manganese bipyridine complexes to serve as catalysts for a variety of small molecule reductions.

Synthesis and Structural Studies of Nickel(0) Tetracarbene Complexes with the Introduction of a New Four-Coordinate Geometric Index, τδ

The synthesis and characterization of two homoleptic chelating nickel(0) tetracarbene complexes are reported. These are the first group 10 M(0) (M = Ni, Pd, Pt) tetracarbene complexes. These species have geometries intermediate between C2v sawhorse and tetrahedral and show high UV-vis absorption in the 350-600 nm range, with extinction coefficients (ϵ) between 5600 and 9400 M(-1) cm(-1). Density functional theory analysis indicates that this high absorptivity is due to metal-to-ligand charge transfer. In order to better describe the unusual geometries encountered in these complexes, an adjustment to the popular τ4 index for four-coordinate geometries is introduced in order to better delineate between sawhorse and distorted tetrahedral geometries.

Correction to "A Molecular Ruthenium Electrocatalyst for the Reduction of Carbon Dioxide to CO and Formate".

page 8566, and page S14 of the Supporting Information. The turnover frequency (TOF) value for Ru(mesbpy)(CO)2Cl2 was calculated incorrectly. The value should be corrected to 320 s−1 from the old value of 1300 s−1 in the Abstract and Results; a corrected Supporting Information file is provided. The change does not impact the reported conclusions. ■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.5b07872. A selection of cyclic voltammograms, X-ray crystallographic information tables, IR-SEC plots, and chemical reductions (corrected) (PDF) Published: August 27, 2015 Addition/Correction