ELECTROCATALYTIC CO2 OR CO REDUCTION

An electrochemical cell for converting carbon dioxide to a value-added product includes a vessel and a liquid electrolyte composed of an organic solvent and an organic soluble salt disposed in the vessel. A cathode and anode are at least partially immersed in the electrolyte, and they may or may not be separated by a semipermeable membrane. In a first version, a carbon dioxide source provides carbon dioxide to the electrolyte. In a second version, a carbon monoxide source provides carbon monoxide to the electrolyte. A power supply biases the cathode relative to the anode. An organometallic complex is dispersed in the electrolyte and/or deposited onto the cathode. The organometallic complex is design to convert carbon dioxide or carbon monoxide to a value added product.

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Description
CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser. No. 63/352,965 filed Jun. 16, 2022, the disclosure of which is hereby incorporated in its entirety by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under Contract No. CHE-2018740 awarded by the National Science Foundation, and Contract No. DE-SC0019236 awarded by the U.S. Department of Energy (DOE). The Government has certain rights to the invention.

TECHNICAL FIELD

In at least one aspect, the present invention is related to the catalytic reduction of carbon dioxide and/or carbon monoxide.

BACKGROUND

Due to the inherently unsustainable nature of fossil fuels, the development of renewable energy alternatives, which can sustain the rising global energy demands without detrimental environmental impacts, is paramount.[1,2] Unfortunately, large-scale application of these sources such as solar and wind, has been deficient due to their intermittent nature, with peak supply often asynchronous with matching demand.[3] To combat this issue, the storage of renewable energy into chemical bonds during peak supply has been proposed to mitigate this spatio-temporal demand mismatch.[4-9] In one strategy, naturally abundant small molecules, such as H2O and CO2, are electrochemically converted into value-added products, such as H2 in the hydrogen evolution reaction (HER) and C1 products in the CO2 reduction reaction (CO2RR).[7,10-16] Additionally, the CO2RR provides a renewable means to produce CO as a C1 feedstock for essential industrial processes that produce methanol and diesel that would otherwise rely on fossil fuel-sourced substrates.[17-19] Current heterogeneous catalysts for the CO2RR are often limited due to their low selectivity, with their ill-defined surface mechanisms inhibitive towards rational iterative improvement. [7,20] On the other hand, homogeneous catalysts can provide modular platforms that can be synthetically tuned to increase rates and selectivities.[7,21,22] As a result, fundamental research of homogeneous catalytic systems can provide insights into new chemical strategies to further optimize and increase the efficiencies of future catalytic systems.

One possible route toward small molecule-based energy storage is the reduction of CO2 to formate/formic acid (Equation 1), due to its unique opportunities for energy storage and conversion. While other CO2RR products potentially need additional chemical steps to yield substrates amenable to energy extraction, such as in the case of CO, formate/formic acid can be directly and effectively employed in fuel cells.[23,24] Additionally, formic acid can be effectively utilized as an easily transportable and non-toxic “liquid H2 carrier” which can release H2 via oxidation back to CO2.[24-26] One strategy of reducing CO2 to formate is through hydrogenation with gaseous H2 (Equation 2).[11,27] Drawbacks of this approach are the necessary high pressure and temperatures needed to drive these reactions.[11,27] As a result, decoupling the addition of protons and electrons through electrochemical methods has been a proposed solution to circumvent this issue (Equation 1).

Amongst homogeneous electrocatalysts active in the CO2RR, production of formate is rare[7,27,28], with purely selective formate-producing electrocatalysts even more scarce due to the competitive HER.[24,29] Some electrocatalysts of note that are highly selective towards formate production include iridium pincer complexes that display faradaic efficiency (FE) of 93-97%.[30,31] Platinum phosphine complexes were also reported to electrocatalyze the reversible conversion between CO2 and HCO2 with high selectivity and low overpotential.[32-35] Additionally, catalysts based on non-precious elements, such as the Fe-carbonyl cluster containing an interstitial N atom have been reported to perform the CO2RR selectively with FE as high as 96% in aqueous media.[36]Notably, upon exchanging the nitrogen atom for carbon in the Fe-carbonyl clusters, the selectively shifts toward the HER, indicating the hydricity of the cluster can be tuned and it drastically affects reactivity.[37]A similarly selective cobalt-pentadienyl complex incorporating a disphosphine ligand amended with two pendant amines was reported to selectively produce formate with 90% FE and at high turnover frequency (TOF>1000 s−1).[38] Lastly, an iron tetradentate phosphine complex was reported to display exceptionally high formate selectivity as large as 97% FE, and was shown to form methanol in the presence of an amine cocatalyst.[39]

Notably, these catalysts have been identified to produce formate via a hydride transfer mechanism where CO2 inserts into the metal-hydride bond (Scheme 2a). It should be noted that while this is the more commonly reported mechanism, a separate mechanism involving protonation of the carbon atom of the metal-CO2 adduct is a possible pathway (Scheme 2b), albeit less commonly reported.[40-42] Though these previously reported catalysts provide useful knowledge in understanding CO2RR formate selectivity, the scarcity of these reports in the literature necessitates additional research into new catalytic platforms that can provide additional knowledge in controlling the selectivity of these systems in the CO2RR.

While the current state of artificial CO2RR and HER catalysts is limited, evolution has provided highly efficient catalytic systems for these reactions in biological settings. Enzymes such as hydrogenase can reversibly catalyze the HER and the hydrogen oxidation reaction, while CO dehydrogenase and Formate dehydrogenase can selectively and reversibly convert CO2 to CO or formate, respectively, near the thermodynamic potential.[11,18,43-45] While some research has explored the reactivity of these enzymes directly as electroactive catalysts,[45-47] synthetic chemists have studied metal complexes with common structural motifs located in the active sites of these enzymes for insights into their catalytic performance. One such common motif is the extensive presence of thiolate moieties, which have been subsequently incorporated into reported catalysts for both the HER and CO2RR.[7, 11,18,48,49] In one set of catalysts, thiolates and their heavier chalcogen congeners were employed as ligands in cobalt bis(dithiolene) and bis(diselenolene) complexes.[50,51] Both catalytic systems were reported to exhibit high activity towards the HER with catalytic turnovers (TONs), and TOFs as high as 9,000 and 3,400 h−1, respectively. These systems suggest that chalcogenide-based ligands are an activating ancillary ligand in biologically inspired systems, due to its propensity to act as a proton relay.[50,52,53] Based on the success of these homogeneous species, the metal dithiolene motifs were incorporated into a heterogeneous metal-organic frameworks and polymers, displaying exceptionally high activity and stability under aqueous acidic electrocatalytic conditions.[54,55]

Catalytic systems featuring metal thiolates moieties have also been studied for activity towards the CO2RR. Cobalt pyridyl thiolates incorporating diphosphine ancillary ligands have exemplified significant activity and selectivity towards electrocatalytic CO production with FE>92%, and low overpotentials accessible due to the proton shuttling of the activating ligand.[56,57]A similar pyridyl cobalt thiolate complex incorporating bipyridine ligands displayed markedly low overpotentials, modest TOFs, selectivity towards formate production as high as 64%, but suffer from catalyst deactivation due to CO poisoning.[58]A cobalt complex incorporating the non-innocent phosphinobenzenethiolate ligand has been reported to produce variable CO:H2 ratios as a function of acid pKa with faradaic efficiencies>99%.[59]A structurally-derived formate dehydrogenase-based catalyst has also been synthesized, comprising of a Ni bis(dithiolene) metal center with the dithiolene ligands structurally similar to the molybdopterin motif found in the active center of the enzyme.[60,61] This catalyst was notably selective towards formate production, though a prior irreversible reduction of the ligand is necessary to produce the active catalyst.[60,61] Similarly, a biologically inspired bimetallic oxo Mo—Cu benzenedithiolate complex derived from the active site of Mo—Cu CO dehydrogenase was reported to display formate selectivity of 74%, with experimental data indicating that oxo transfer to CO2 to form carbonate is necessary before the active catalyst could be generated.[62] Though these prior studies have demonstrated positive results of the use of metal complexes with metal-thiolate motifs towards small molecule reduction, there still exists a scarcity of catalytic reports on sulfur-based metal complexes towards these catalytic processes, necessitating the continual study in this area.

Also of interest is the reduction of CO. Reduction of CO2 beyond CO is difficult for homogeneous systems due to the diffusion of the catalyst from the electrode, which makes multi-electron and multi-proton processes kinetically unfavorable. Based on the Sabatier principle the binding energy of CO, EB(CO), is used as a descriptor to understand the catalytic selectivity for CO2RR catalysts. If EB(CO) is too positive, CO binds too weakly and is the dominant CO2RR product. On the other hand, if EB(CO) is too negative, CO binds too strongly and requires high overpotentials for further reduction, which results in a selectivity switch from CO2RR to the dominant HER.5,10,11 Additionally, most CO2RR catalysts that are efficient at converting CO2 into CO have been shown to cause catalyst deactivation.12,13,14 To enable reduction of CO, a moderate EB(CO) is required for CO to remain bound to the catalytic site, while also a reduced reaction energy barrier.

Accordingly, there is a need for improved methods for forming value-added products from carbon dioxide and/or carbon monoxide.

SUMMARY

In at least one aspect, an electrochemical cell for converting carbon dioxide to a value-added product is provided. The electrochemical cell includes a vessel and a liquid electrolyte composed of an organic solvent and/or water and an organic soluble salt disposed in the vessel. A cathode and anode are at least partially immersed in the electrolyte, and they may or may not be separated by a semipermeable membrane. A carbon dioxide source provides carbon dioxide to the electrolyte. A power supply biases the cathode relative to the anode. A compound that includes an organometallic complex having formula 1 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal; n(+ or −) is the formal charge of the organometallic complex; n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;

Z is S, O, Se, Te, or NH;

L is an optional ligand or an optional donor group from another ligand attached to M; and
R1, R2, R3, R4, R5, R6, R7 and R8 are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl with the proviso that any 2 of R1, R2, R3, R4 can be bonded together to form a multidentate ligand.

In another aspect, a method for reducing carbon dioxide includes a step of electrolytically reducing carbon dioxide in the presence of a compound that includes an organometallic complex having formula 1 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero:

In another aspect, an electrochemical cell for converting carbon monoxide to a value-added product is provided. The electrochemical cell includes a vessel and a liquid electrolyte composed of an organic solvent and an organic soluble salt disposed in the vessel. A cathode and anode are at least partially immersed in the electrolyte, and they may or may not be separated by a semipermeable membrane. A carbon monoxide source provides carbon monoxide to the electrolyte. A power supply biases the cathode relative to the anode. An organometallic complex having formula 4 is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

o is an integer from 0 to 4; and
R9, R10, are each independently C1-10 alkyl or C6-14 aryl; and
R11 is C1-6 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

In another aspect, a method for reducing carbon monoxide includes a step of electrolytically reducing carbon monoxide in the presence of a compound that includes an organometallic complex having formula 4.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature, objects, and advantages of the present disclosure, reference should be had to the following detailed description, read in conjunction with the following drawings, wherein like reference numerals denote like elements and wherein:

FIG. 1. Schematic illustration of an electrochemical cell for reducing carbon dioxide.

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, and 2H. Examples of organometallic complexes used in the electrochemical cell of FIG. 1. A) is cobalt triphosphine-thiolate complex ([Co(triphos)(bdt)]+).

FIG. 3. CVs of 0.5 mM of [Co(triphos)(bdt)]− in a MeCN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2. Scan rate is 100 mV/s.

FIG. 4. Solid-state structure of [Co(triphos)(bdt)]−. Aryl and aliphatic protons, counterions, and solvent molecules are omitted for clarity.

FIG. 5. Variable temperature overlay of: a) 600 MHz 31P-{1H}NMR spectra of [Co(triphos)(bdt)]− in acetonitrile-d3; 600 MHz 1H NMR spectra of [Co(triphos)(bdt)]− in b) aromatic and c) aliphatic region in acetonitrile-d3. Temperature varied between 26 and −35° C.

FIG. 6. CVs of 0.45 mM of [Co(triphos)(bdt)]+ in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of N2 (black), CO2 (red), and under CO2 in the presence of 0.3 M H2O (blue) or 0.3 M TFE (green). Scan rate is 100 mV/s.

FIG. 7. Summary of the controlled potential electrolysis a) in the presence of 0.3 M of TFE, H2O, or no exogenous proton source (N/A) at −2.15 V vs Fc/Fc, b) in the presence of 0.3 M H2O at potentials of −2.15 or −2.60 V vs Fc/Fc+, and c) in the presence of 0.3 or 0.6 M H2O at −2.15 V vs Fc/Fc+. All electrolyses were performed with 0.45 mM of [Co(triphos)(bdt)]+ in a CH3CN solution containing 0.1 M [nBu4N][PF6] under an atmosphere of CO2.

FIG. 8. Scheme 3: Proposed mechanism for electrocatalytic CO2RR to HCOO− employing [Co(triphos)(bdt)]+.

FIG. 9A. Scheme 4: Addition of 1 atm of CO to a solution of cobalt 2-(diisopropylphosphanyl)benzenethiolate complex (CoPS) in acetonitrile results in the formation of the corresponding CO adduct, labeled as CoPS(CO).

FIG. 9B. Solid state structure of CoPS(CO) with the following color designations for atoms: Co=purple, S=yellow, P=orange, O=red, C=gray, H=pink.

FIG. 10. Cyclic voltammograms of 1 mM CoPS in 0.1 M [nBu4N][PF6]acetonitrile solutions under an atmosphere of N2 or CO (scan rate=100 mV s−1).

FIGS. 11A and 11B. A) SEC-UV-Vis of 1 mM CoPS with 0.25 M [nBu4N][PF6] in acetonitrile solution under 1 atm of CO. UV-Vis spectra under CPE at 0.11 V (black), −0.11 V (red), −0.22 V (orange), −0.33 V (green), −1.21 V (blue), −1.43 V (purple), and −2.09 V (brown) vs Ag/Ag+. B) SEC-FTIR of 10 mM CoPS with 0.25 M [nBu4N][PF6] in DCM solution under 1 atm of CO. FT-IR spectra before CPE (black) and after CPE at −1.5 V vs Fc−/0 (blue).

FIG. 12. Cyclic voltammograms of 1 mM CoPS in an acetonitrile solution with 0.1 M [nBu4N][PF6] electrolyte with scan rate of 100 mV/s under CO atmosphere (black) with increasing concentrations of PhOH, such as 0.10 M (red), 0.50 M (green), 1.00 M (purple), and 2.00 M (brown).

FIGS. 13A, 13B, 13C and 13D. A) CV of a 0.1 M [nBu4N][PF6]acetonitrile solution under 1 atm of CO and 0.5 M PhOH in the presence of 1 mM CoPS at a scan rate of 100 mV s−1 (purple). B) 1 h CPE traces at potentials of −2.4 V, −2.1 V, and −1.6 V. C) Product distribution generated at different potentials. D) Amounts of products generated at different potentials.

 DETAILED DESCRIPTION

Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: all R groups (e.g. Ri where i is an integer) include hydrogen, alkyl, lower alkyl, C1-6 alkyl, C6-10 aryl, C6-10 cycloalkyl (e.g. cyclohexyl), C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation; single letters (e.g., “n” or “o”) are 1, 2, 3, 4, or 5; in the compounds disclosed herein including compounds described by formula or by name, a CH bond can be substituted with alkyl, lower alkyl, C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation; percent, “parts of,” and ratio values are by weight; the term “polymer” includes “oligomer,” “copolymer,” “terpolymer,” and the like; molecular weights provided for any polymers refers to weight average molecular weight unless otherwise indicated; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It is understood that any charged chemical moieties are charged balanced with a sufficient number of counterions to achieve charge neutrality. For example, positively charged moieties are balanced by negatively charged counter ions such as halide (e.g., Cl−, Br−, etc.), SO42-, NO3−, PO43-, CH3COO−, tosylate, and the like. Similarly, negatively charged moieties are balanced by positively charged counter ions such as Na+, K+, Li+, and the like.

The term “alkyl” refers to C1-20 inclusive, linear (i.e., “straight-chain”), branched, saturated or at least partially and in some cases fully unsaturated (i.e., alkenyl and alkynyl) hydrocarbon chains, including for example, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, neopentyl, hexyl, octyl, ethenyl, propenyl, butenyl, pentenyl, hexenyl, octenyl, butadienyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, and allenyl groups. “Branched” refers to an alkyl group in which a lower alkyl group, such as methyl, ethyl or propyl, is attached to a linear alkyl chain. “Lower alkyl” refers to an alkyl group having 1 to about 8 carbon atoms (i.e., a C1-8 alkyl), e.g., 1, 2, 3, 4, 5, 6, 7, or 8 carbon atoms. “Higher alkyl” refers to an alkyl group having about 10 to about 20 carbon atoms, e.g., 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 carbon atoms.

It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.

The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

It should also be appreciated that integer ranges explicitly include all intervening integers. For example, the integer range 1-10 explicitly includes 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Similarly, the range 1 to 100 includes 1, 2, 3, 4 . . . 97, 98, 99, 100. Similarly, when any range is called for, intervening numbers that are increments of the difference between the upper limit and the lower limit divided by 10 can be taken as alternative upper or lower limits. For example, if the range is 1.1. to 2.1 the following numbers 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, and 2.0 can be selected as lower or upper limits. In the specific examples set forth herein, concentrations, temperature, and reaction conditions (e.g. pressure, pH, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to three significant figures. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to three significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pH, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to three significant figures of the value provided in the examples.

In the examples set forth herein, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 50 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In a refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 30 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples. In another refinement, concentrations, temperature, and reaction conditions (e.g., pressure, pH, flow rates, etc.) can be practiced with plus or minus 10 percent of the values indicated rounded to or truncated to two significant figures of the value provided in the examples.

The term “transition metal” means an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell. Examples of transition metals includes scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, and gold.

It should be appreciated that in the organometallic complex described herein, S can be replaced by O, Se, Te, or NH. Moreover, any aromatic groups are optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. Similarly, the cobalt in any organometallic complex can be substituted by Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. Finally, any specific alkyl substitute can be replaced by Cio alkyl, and in particular, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, isopentyl, neopentyl, or hexyl.

Throughout this application, where publications are referenced, the disclosures of these publications in their entireties are hereby incorporated by reference into this application to more fully describe the state of the art to which this invention pertains.

Abbreviations

    • “BDT” means 1,2-benzodithiol.
    • “GC” means gas chromatography.
    • “CPE” means controlled potential electrolysis.
    • “CoPS” means cobalt 2-(diisopropylphosphanyl)benzenethiolate complex.
    • “DFT” means density functional theory.
    • “FE” means faradaic efficiency.
    • “KIE” means kinetic isotope effect.
    • “ph” means phenyl.
    • “TFE” means 2,2,2-trifluoroethanol.
    • “TONs” means Turnover numbers.
    • “XPS” means X-ray photoelectron spectroscopy.

With reference to FIG. 1, a schematic of an electrochemical cell for converting carbon dioxide to a value-added product. Electrochemical cell 10 includes vessel 12 and a liquid electrolyte 14 disposed in the vessel. Electrochemical cell 10 also includes cathode 16 at least partially immersed in the electrolyte. In a refinement, the liquid electrolyte includes an organic solvent and an organic soluble salt, water or a C1-4 alcohol. Anode 18 is at least partially immersed in the electrolyte. Cathode 16 may or may not be separated from the anode 18 by a semipermeable membrane. Source 20 can be a carbon dioxide source or a carbon monoxide source. Organometallic complexes having formulae 1 to 10 can be used to reduce carbon dioxide and/or a carbon monoxide.

In a first variation, source 20 is a carbon dioxide source that provides carbon dioxide to the electrolyte. For example, carbon dioxide can be bubbled through tube 22 into electrolyte 14. In a second variation, source 20 is a carbon monoxide source that provides carbon monoxide to the electrolyte. For example, carbon monoxide can be bubbled through tube 22 into electrolyte 14. Power supply 26 biases the cathode relative to the anode. Typically, cathode 16 is biased negatively with respect to anode 18 (e.g., 1 to 3 volts).

In the first variation that used carbon dioxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 1 and one or more counterions Xn(− or −) and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero are dispersed in the electrolyte and/or deposited onto the cathode:

wherein:

    • M is a transition metal;
    • Z is S, O, Se, Te, or NH;
    • n(+ or −) is the formal charge of the organometallic complex;
    • n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex. For example, positively charged moieties are balanced by negatively charged counter ions such as halide (e.g., Cl−, Br−, etc.), SO42-, NO3−, PO43-, CH3COO−, tosylate, and the like. Similarly, negatively charged moieties are balanced by positively charged counter ions such as Na+, K+, Li+, and the like;
    • L is an optional ligand or an optional donor group from another ligand attached to M; and
    • R1, R2, R3, R4, R5, R6, R7 and R8 are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl with the proviso that any 2 of R1, R2, R3, R4 can be bonded together to form a multidentate ligand. In a refinement, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, the value-added product is a formate or oxalate, and in particular, a metal formate or a metal oxalate.

In a refinement, R1, R2 are bonded together to form a first multidentate ligand and R3, R4 are bonded together to form a second multidentate ligand as depicted in Formula 2A, 2B, or 2C. In another variation, R1, R4 are bonded together to form a first multidentate ligand and R2, R3 are bonded together to form a second multidentate ligand.

In a variation, the organometallic complex has formula 2A or 2B:

wherein Z is S, 0, Se, Te, or NH; M is a transition metal;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. When M is Co in formula 2 the organometallic complex has a formal charge of 1+.

In another variation, the organometallic complex has formula 3:

wherein Z is S, O, Se, Te, or NH; M is a transition metal; and each phenyl is optional substituted with C1-6 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), C6-10 aryl, C6-10 cycloalkyl (e.g., cyclohexyl), C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. When M is Co in formula 3 the organometallic complex has a formal charge of −1.

In another aspect, a method for reducing carbon dioxide is provided. The method includes a step of electrolytically reducing carbon dioxide in the presence of a compound (e.g., a catalyst) that includes an organometallic complex having formula 1 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero:

wherein:

    • M is a transition metal;
    • Z is S, O, Se, Te, or NH;
    • n(+ or −) is the formal charge of the organometallic complex;
    • n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
    • L is an optional ligand or an optional donor group from another ligand attached to M; and
    • R1, R2, R3, R4, R5, R6, R7 and R8 are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl with the proviso that any 2 of R1, R2, R3, R4 can be bonded together to form a multidentate ligand. In a refinement, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, the value-added product is a formate or an oxalate, and in particular, a metal formate or a metal oxalate. Details for variations of the organometallic complex having formula 1 are set forth above.

In the second variation that used carbon monoxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 4 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:

    • M is a transition metal;
    • Z is S, O, Se, Te, or NH;
    • n(+ or −) is the formal charge of the organometallic complex;
    • n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
    • o is an integer from 0 to 4; and
    • R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl.
    • R11 is C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. Advantageously, the value-added product can be methanol. When M is Co in formula 4 the organometallic complex has a formal charge of 0.

In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co.

In a variation, R9, R10 are each independently methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, cyclohexyl, or phenyl.

In a refinement, the liquid electrolyte includes an organic solvent and an organic soluble salt, water or a C1-4 alcohol.

In a refinement, the organometallic complex has formula 5 and one or more counterions Xn(− or −) if the formal charge of the organometallic complex is not zero:

where Z is S, O, Se, Te, or NH and iPr is isopropyl. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. When M is Co in formula 5 the organometallic complex has a formal charge of 0.

In another embodiment, a method for reducing carbon monoxide using the electrochemical cell of FIG. 1 is provided. The method includes a step of electrolytically reducing carbon monoxide in the presence an organometallic complex having formula 4 and one or more counterions Xn(− or −) if the formal charge of the organometallic complex is not zero:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
o is an integer from 0 to 4;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and
R11 is C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R″″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. Advantageously, the value-added product can be methanol. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above.

In another variation that uses carbon dioxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 6A or 6B and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal;
n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;

Z is S, O, Se, Te, or NH; and

each aromatic ring (e.g., phenyl) is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′ + is a metal cation. In a variation, M is Co, Rh, Jr, Fe, Ru, Os, Ni, Pd. Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be a formate or an oxalate. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. When M is Co in formula 6 the organometallic complex has a formal charge of 3+.

In another variation that uses carbon dioxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 7 and one or more counterions Xn(− or +)if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal;
n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
R12, R13 are each independently C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, or CN;
Z is S, O, Se, Te, or NH; and each aromatic ring (e.g., phenyl) is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′−, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo, or W. In a refinement, M is Co. Advantageously, the value-added product can be a formate or an oxalate. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell.

In another variation that uses carbon monoxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 8 and one or more counterions Xn(− or +)if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. It should be appreciated that organometallic complex 8 is an extended network which can be repeated 2 to 100 times or more.

In another variation that uses carbon monoxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 9 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

o is an integer from 0 to 4;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above. When M is Co in formula 9 the organometallic complex has a formal charge of 0. It should be appreciated that organometallic complex 9 is an extended network which can be repeated 2 to 100 times or more.

In another variation that uses carbon monoxide, a compound (e.g., a catalyst) that includes an organometallic complex having formula 10 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero is dispersed in the electrolyte and/or deposited onto the cathode:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

o is an integer from 0 to 4;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above. It should be appreciated that organometallic complex 10 is an extended network which can be repeated 2 to 100 times or more.

In another aspect, a method for reducing carbon dioxide is provided. The method includes a step of electrolytically reducing carbon dioxide in the presence of a compound (e.g., a catalyst) that includes an organometallic complex having formula 6A or 6B and one or more counterions Xn(− or −) or if the formal charge of the organometallic complex is not zero:

wherein:

    • M is a transition metal;
    • Z is S, O, Se, Te, or NH;
    • n(+ or −) is the formal charge of the organometallic complex;
    • n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
    • n is an integer; and
    • each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a refinement, the value-added product is a formate or an oxalate, and in particular, a metal formate or a metal oxalate. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Details for variations of the organometallic complex having formula 6 are set forth above. When M is Co in formula 6 the organometallic complex has a formal charge of 3+.

In another aspect, a method for reducing carbon dioxide is provided. The method includes a step of electrolytically reducing carbon dioxide in the presence of a compound (e.g., a catalyst) that includes an organometallic complex having formula 7 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero:

wherein:

    • M is a transition metal;
    • Z is S, O, Se, Te, or NH;
    • n(+ or −) is the formal charge of the organometallic complex;
    • n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
    • R12, R13 are each independently C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, or CN; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a refinement, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, the value-added product is a formate or an oxalate, and in particular, a metal formate or a metal oxalate. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Details for variations of the organometallic complex having formula 6 are set forth above. When M is Co in formula 7 the organometallic complex has a formal charge of 1+.

In another embodiment, a method for reducing carbon monoxide using the electrochemical cell of FIG. 1 is provided. The method includes a step of electrolytically reducing carbon monoxide in the presence an organometallic complex having formula 8 and one or more counterions Xn(− or −) if the formal charge of the organometallic complex is not zero:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above.

In another embodiment, a method for reducing carbon monoxide using the electrochemical cell of FIG. 1 is provided. The method includes a step of electrolytically reducing carbon monoxide in the presence an organometallic complex having formula 9 and one or more counterions Xn(− or −) if the formal charge of the organometallic complex is not zero:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above.

In another embodiment, a method for reducing carbon monoxide using the electrochemical cell of FIG. 1 is provided. The method includes a step of electrolytically reducing carbon monoxide in the presence an organometallic complex having formula 10 and one or more counterions Xn(− or −) if the formal charge of the organometallic complex is not zero:

wherein:
M is a transition metal;

Z is S, O, Se, Te, or NH;

n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
R9, R10, are each independently C1-10 alkyl, C6-10 cycloalkyl (e.g., cyclohexyl), or C6-14 aryl; and each phenyl is optionally substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation. In a variation, M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W. In a refinement, M is Co. Advantageously, the value-added product can be methanol. In a refinement, the carbon monoxide is electrolytically reduced at the cathode of an electrochemical cell. Details for the variations and refinements of the organometallic complex having formula 4 are set forth above.

FIG. 2A provides examples of the organometallic complexes described above.

The following examples illustrate the various embodiments of the present invention. Those skilled in the art will recognize many variations that are within the spirit of the present invention and scope of the claims.

1. Electrocatalytic CO2 Reduction to Formate or Oxalate by a Cobalt Phosphino-Thiolate Complex

Electrochemical conversion of CO2 to value-added products serves as an attractive method to store renewable energy as energy-dense fuels. Selectivity in this type of conversion is limited, often leading to the formation of side products such as H2. In the experiments of this section, the activity of a cobalt phosphino-thiolate complex ([Co(triphos)(bdt)]+) towards the selective reduction of CO2 to formate is investigated. In the presence of H2O, we observe selective production of formate (as high as 90%) at overpotentials of 750 mV, displaying negligible current degradation over 8 hours. Chemical reduction studies of [Co(triphos)(bdt)]+ indicates deligation of the apical phosphine moiety is likely before catalysis. Computational and experimental results suggest a metal-hydride pathway, indicating an ECEC based mechanism.

Motivated by the success of aforementioned cobalt-based thiolate complexes towards the electrocatalytic reduction of small molecules, herein we report the reactivity of a cobalt based catalyst ([Co(triphos)(bdt)]+) incorporating 1,1,1,-tris(diphenylphosphinomethyl) ethane (triphos) and 1,2-benezenedithiolate as the ancillary ligands towards the electrocatalytic CO2RR (FIG. 2B). A multidentate phosphine donor ligand was chosen as an ancillary ligand due to its extensive use in catalysts active towards the chemical[11,63] and electrochemical[7,28,29] CO2RR. Moreover, triphos was selected as the phosphine ligand of choice based on reports of Co-triphos complexes active towards CO2 hydrogenation to methanol[64] and HER[65], in addition to a similarly constructed Fe(triphos)(bdt) complex reported as an active HER electrocatalyst.[66] Cyclic voltametric studies were performed to characterize the electrochemical behavior of [Co(triphos)(bdt)]+ under reducing conditions, and the reactivity of the complex in the presence of CO2 and Brønsted acids was investigated. Controlled potential electrolysis studies were employed to explore the selectivity of the catalyst under various conditions and the reaction intermediates were synthesized and characterized via NMR spectroscopy to determine the possible mechanistic pathways. Lastly, density functional theory (DFT) computational methods were utilized to help elucidate potential mechanistic pathways.

1.2 Results and Discussion

Complex [Co(triphos)(bdt)]− was synthesized according to a reported literature procedure.[67,68] Cyclic voltammograms (CVs) of [Co(triphos)(bdt)]+ (0.45 mM) were obtained under an N2 atmosphere using a glassy carbon electrode (GCE) in acetonitrile (MeCN) solutions with 0.1 M tetrabutylammonium hexafluorophosphate ([nBu4N][PF6]) as the supporting electrolyte. All potentials are referenced versus Fc+/0 and all CVs were first scanned cathodically and subsequently returned anodically. CVs of [Co(triphos)(bdt)]+ reveal a reversible redox couple at −0.74 V. This reversible feature is attributed to a formal CoIII/II process based on previous reports and is assigned the [Co(triphos)(bdt)]+/0 couple.[67-69] Upon scanning further cathodically, CVs exhibit an irreversible feature at −2.08 V, and a quasi-reversible couple at −2.39 V, and are assigned to the [Co(triphos)(bdt)]0/− and [Co(triphos)(bdt)]−/−2 couples, respectively (FIG. 3). Scanning anodically from this potential, two irreversible redox events are observed at −1.51 V and −1.13 V. These oxidation features do not appear in the CVs if the potential is reversed before reaching [Co(triphos)(bdt)]0/−, suggesting that these oxidative events originate from the reduction of [Co(triphos)(bdt)]0. The irreversible nature of the redox feature of the [Co(triphos)(bdt)]0/− is likely due to a chemical/structural change to the complex in conjunction with the associated reduction of the complex (vide infra). CVs of variable scan rates performed on the [Co(triphos)(bdt)]0/− feature do not display any change in the reversibility of the feature, though some changes were observed in the return oxidation features. This may be a result of the chemical step occurring too rapid to be observed compared to the CV timescale.

To understand the electrochemical behavior observed at the [Co(triphos)(bdt)]0/− couple, [Co(triphos)(bdt)]0 was chemically reduced using excess KC8 and in the presence of 18-crown-6. X-ray quality crystals were grown via vapor diffusion of n-pentane into tetrahydrofuran solutions containing the reduced complex. Solid state structure of this species indicates a four-coordinate cobalt complex with the triphos ligand acting as a bidentate ligand in this structure, with one of the phosphine linkers disassociated from the metal center (FIG. 4). The angular structural parameter, τ, was calculated to be ~0.3, suggesting that the metal center adopts a distorted square planar geometry, with a torsion angle of 30.91°. Additionally, a single potassium cation chelated by 18-crown-6 is present as a counterion for each molecular unit in the lattice, suggesting this complex can be identified as [Co(triphos)(bdt)]−.

TABLE 1.1 Average selected bond lengths (Å) for [Co(triphos)(bdt)]x complexes, where x = 1, 0, −1. Bond Length (Å) Bond [Co(triphos)(bdt)]+ [Co(triphos)(bdt)]0 [Co(triphos)(bdt)]− Co—S 2.169(2) 2.223(1) 2.186(1) Co—Papical 2.183(2) 2.300(1) — Co—Pbasal 2.232(2) 2.212(1) 2.107(1) C—S 1.734(4) 1.750(4) 1.750(3) Ref. [68] [70] this work

Average bond lengths for [Co(triphos)(bdt)]− can be found in Table 1 in addition to those reported for [Co(triphos)(bdt)]+ and [Co(triphos)(bdt)]0. Although an elongation of the Co—S bond from 2.169(2) to 2.223(1) Å is reported upon reduction of [Co(triphos)(bdt)]+ to [Co(triphos)(bdt)]0, a subsequent contraction of the Co—S bond to 2.186(1) Å is observed upon further reduction to [Co(triphos)(bdt)]−. On the other hand, consecutive contractions of the basal Co—P bond (labeled as CO—Pbasal) are observed from 2.232(2) Å in [Co(triphos)(bdt)]+ to 2.212(1) Å in [Co(triphos)(bdt)]0, and then to 2.107(1) Å in [Co(triphos)(bdt)]−. Additionally, while the C—S bond length was observed to slightly elongate from 1.734(4) to 1.750(4) Å upon reduction from [Co(triphos)(bdt)]+ to [Co(triphos)(bdt)]0, no change to the bond length is observed upon generating [Co(triphos)(bdt)]− (C—S bond of 1.750(3) Å in [Co(triphos)(bdt)]−), indicating the innocent nature of the dithiolene ligand at the [Co(triphos)(bdt)]0/− couple.

To determine if the solution-state structure of [Co(triphos)(bdt)]− is comparable to that of its solid-state structure, 1H and 31P-{1H}nuclear magnetic resonance (NMR) spectrum of [Co(triphos)(bdt)]− were acquired at varying temperatures (FIG. 5). At 26° C., the 31P-{1H}NMR spectrum of [Co(triphos)(bdt)]− in MeCN-d6 displays two broad peaks at δ 46.67 and −30.96 ppm (FIG. 5a). At −35° C. (FIG. 5a), two sharp peaks are observed at δ 41.75 and −28.20 ppm in a 2:1 ratio, and are attributed to both the bound and un-bound phosphines, respectively. The 1H NMR spectrum of [Co(triphos)(bdt)]− in MeCN-d6 at 26° C. (FIGS. 5b and 5c) displays two broad aliphatic singlets at δ 2.23 (s) and 0.41 (s) ppm in a 6:3 ratio corresponding to the methylene and methyl moieties on the triphos ligand, respectively. Three aromatic signals appear at δ 6.43 (m), 7.21 (m), and 7.24 (m) ppm attributed to the aromatic protons on both the dithiolene and triphos ligand, in addition to a broad peak at δ 7.75 pm. At −35° C., the 1H NMR spectrum of [Co(triphos)(bdt)]− in MeCN-d6 (FIGS. 5b and 5c) displays three new features in the aliphatic region, including two doublets at δ 2.26 and 2.19 ppm and a singlet at δ 2.04 ppm in a ratio of 2:2:2 attributed to the individual methylene linkers on both the ligated phosphine and the unbound phosphines, respectively, and three new aromatic singlets at δ 8.10, 7.68, and 7.26 ppm, attributed to the aromatic protons of the unbound phosphine of the triphos ligand. The reversibility of the observed temperature-dependent solution-state changes was investigated by remeasuring the 1H and 31P NMR spectra of [Co(triphos)(bdt)]− at room temperature, and the observed spectra are identical to the ones observed previously. The presence of peak coalescence and broadening in the proton and phosphorus resonances of [Co(triphos)(bdt)]− at room temperatures is indicative of a fast exchange process on the NMR timescale. As the temperature is decreased the exchange can be limited. The 31P NMR spectrum at −35° C. is indicative of two different phosphine environments in the 2:1 ratio, corresponding to the bound and deligated phosphines, respectively. The 1H NMR spectrum of [Co(triphos)(bdt)]− at low temperatures also suggests that one of the phosphine in the triphos ligand is dissociated from the metal center. These results indicate that the low temperature solution-state structure is in agreement to the solid-state crystal structure. The 1H and 31P-{1H}NMR spectra of [Co(triphos)(bdt)]− at room temperature suggest that the deligated phosphine is interchanging with the bound phosphine moieties in a fast exchange process. Based on the variable temperature (VT) NMR data obtained of the bound methylene linkers [Co(triphos)(bdt)]−, the exchange rate (kc) and the free energy of activation (ΔG‡) at coalescence were determined to be 129 s−1 and 13.5(0.5) kcal/mol, respectively. Consequently, VT studies indicate similar low temperature solution state and solid-state crystal structures where one phosphine moiety is deligated from the metal center, whereas at room temperature a fast exchange is observed. This chemical step is likely the origin of the irreversibility of the [Co(triphos)(bdt)]0/− couple.

The catalytic behavior of [Co(triphos)(bdt)]+ was studied using CV experiments in the presence of CO2 and with variable proton sources. CVs of [Co(triphos)(bdt)]− under CO2 display enhanced current densities at potential corresponding to the irreversible [Co(triphos)(bdt)]0/− couple (FIG. 6). An anodic shift is observed at the onset of the [Co(triphos)(bdt)]0/− couple under CO2, indicative of an association of CO2 to the metal center upon reduction, suggesting an EC mechanism.[71-73] Upon scanning anodically, the oxidative features at −1.51 V and −1.13 V are not observed under CO2, suggesting that the faradaic process associated with the observed current response is catalytic in nature and is consuming the electrons that would otherwise be available for oxidation at these oxidation features. Addition of 0.3 M of a Bronstred acid, such as 2,2,2-trifluoroethanol (TFE), under a CO2 atmosphere leads to the formation of a characteristic catalytic plateau, with a 5-fold increase in the current density (FIG. 6). Increasing the concentration of TFE yields an increase in the catalytic current density and an anodic shift in the catalytic onset potential. Upon reaching 0.7 M TFE, a new catalytic feature appears at −2.54 V, which progressively increases in current density upon further titration of TFE. Addition of 0.3 M TFE under N2 display only mild increases in the current response at the [Co(triphos)(bdt)]0/− couple, which represents a 2.5-fold decrease in the current density displayed under CO2. This result indicates that the current responses observed under CO2 in the presence of TFE are likely not largely contributed by direct TFE/proton reduction, similar to what is observed for other active CO2RR catalysts.[74-77]

Addition of 0.3 M H2O as a proton source under a CO2 atmosphere yields a catalytic response that displays a trace-crossing event upon scanning anodically (FIG. 6). This phenomenon has been previously attributed to the formation and subsequent reduction of a newly formed species with a standard reduction potential more positive than that of [Co(triphos)(bdt)]0/−.[78-80] Notably, this trace crossing event is not observed in the CVs performed with scan rates of 0.5 and 1 V/s, suggesting that at fast scan rates the rate of formation of this species is too sluggish to be observed on the CV timescale. Titrations of H2O beyond the 0.3 M concentration yields CV traces that decrease in catalytic current, which is in contrary to what is expected for CO2RR dependent on proton concentration, indicating a separate chemical step is occurring in conjunction with the CO2RR. Titration of H2O at lower concentrations (20-100 M) yield CV traces that increase in current, suggesting this additional chemical step is disfavored at low acid concentrations. Titration of D2O were performed under a CO2 atmosphere at similarly low concentrations, giving rise to a H/D kinetic isotope effect (KIE) of 5.0(2), indicating protonation is integral to the rate limiting step (RLS). Addition of 0.3 M H2O under N2 displays only a minor increase in current at the [Co(triphos)(bdt)]0/− couple. The onset of the [Co(triphos)(bdt)]0/− couple shifts anodically as [H2O] increases, indicating association of a substrate upon initial reduction (EC mechanism), which in this case suggests the formation of a metal-hydride. Additionally, the oxidative features at −1.51 V and −1.13 V are still present at all [H2O] under N2, suggesting that the current response is not catalytic and no faradaic process is present to consume the electrons provided at the [Co(triphos)(bdt)]0/− couple which can be subsequently oxidized. This result implies that [Co(triphos)(bdt)]+ exhibits very little activity towards HER with H2O as a proton source, and the current response exhibited under CO2 can exclude HER as a dominant competitive faradaic process.

To identify and quantify the products generated at the observed catalytic features, controlled potential electrolysis (CPE) experiments were performed in acetonitrile for 2 hours under 1 atm of CO2 with either TFE or H2O as the proton source. CPEs were performed with both TFE and H2O either at −2.15 V or at −2.60 V vs Fc+/0 and with various acid concentrations to determine if the product selectivity and total turnover changes as a function of these variables. At the end of the CPE experiment, gaseous products were sampled from the head space of the electrolysis cell, and quantification was determined by gas chromatography (GC) analysis. Products in the liquid phase were detected and quantified using 1H NMR spectroscopy or liquid chromatography. Results of these experiments are shown in FIG. 7 and Table 2. Turnover numbers (TONs) and faradaic efficiencies (FE %) were determined from the CPE studies, based on established equations (see SI for details). Electrolysis of [Co(triphos)(bdt)]+ in the presence of 0.3 M TFE at −2.15 V (Entry 1) yields formate as the primary CO2 reduction product with a faradaic efficiency (FE) of 59%. Oxalate is detected through liquid chromatography with a faradaic efficiency of 30%. Gaseous products such as H2 and CO were detected at FE of 6% and 2%, respectively, yielding a combined FE % of 97%.

TABLE 1.2 Controlled potential electrolysis results for [Co(triphos)(bdt)]+ in the presence of CO2 and a proton source. Electrolyses were performed with 0.45 mM of [Co(triphos)(bdt)]+ in a CH3CN solution containing 0.1M [nBu4N][PF6] under an atmosphere of CO2. Potential Charge H2 CO Time [Acid] (V) vs (C) FE %H2 TONH2 FE %CO TONCO Entry Acid (hours) (M) Fc/Fc+ (±4) (±3) (±0.3) (±2) (±0.10) 1 TFE 2 0.3 −2.15 11 6 0.2 2 0.04 2 H2O 0.3 −2.15 20 10 0.6 4 0.25 3 0.3 −2.60 38 4 0.5 1 0.06 4 0.6 −2.15 16 7 0.3 1 0.04 5 — — −2.15 1 Trace Trace Trace Trace 6 H2O 8 0.3 −2.15 91 9 2.4 2 5.30 HCOO− Total FE %HCOO− TONHCOO− FE % Entry (±3) (±1.5) (±6) 1 59 1.8  97[a] 2 84 4.9 98 3 83 9.2 88 4 80 3.7 88 5 3 0.01  3 6 90 23.7 >99  [a]oxalate is detected via liquid chromatography with a faradaic efficiency of 30%.

Upon employing H2O as a proton source (Entry 2), CPE results display a significant shift in selectivity towards the formation of formate at 84%, with a near unity total FE % and increase in total TON of CO2RR products. This high selectivity towards the CO2RR at the expense of HER is corroborated by substantially low current densities observed in the CVs of [Co(triphos)(bdt)]+ in the presence of H2O under N2 compared to CO2 (FIG. 6). An initial increase in the absolute value of the catalytic current is observed in the CPEs employing H2O as a proton source, which is in contrary to the initial decrease in current expected in electrolysis experiments due to the initial rapid consumption of substrate in the electrode's double layer. This preliminary induction period may indicate the formation of a more active form of the └Co(triphos)(bdt)┘ catalyst, which may also be associated with trace crossing events observed in the CVs experiments under CO2 and in the presence of H2O. After this initial increase in the absolute value of the catalytic current, stable currents are observed throughout the rest of the CPE. Performing CPE at a larger overpotential (−2.60 V) in the presence of 0.3 M H2O (Entry 3) yields higher HCOO− TONs, 9.2 compared to 4.9, at the expense of a loss of unity total FE % due to lower quantities of side products such as H2 (4%, 0.5 TON) and CO (1%, 0.4 TON) detected. Moreover, performing electrolysis with 0.6 M H2O at −2.15 V (Entry 4) results in a reduction in the HCOO− TONs compared to entry 2 from 4.9 to 3.7, along with a similar loss in FE % from 84 to 80, in addition to similar loss of total faradaic efficiency (from 98% to 88%). Notably, charge consumed throughout electrolysis is inversely proportional to acid concentration for 0.3 vs 0.6 M H2O conditions (entry 2 vs 4). A similar result was observed in the CVs obtained in the presence of [Co(triphos)(bdt)]+ and CO2 upon titration of H2O. Lastly, performing CPE at −2.15 V without an added exogenous Brønsted acid (Entry 5) yields only trace gaseous products and only minor amounts of formate detected with a TON of 0.01 and FE % of 3%, indicating the presence of a proton donor is necessary for significant product formation.

Due to high selectivity towards formate production in the presence of 0.3 M H2O at −2.15 V, additional control, long term stability, and degradation experiments were performed under these conditions. Control experiments in the absence of catalyst yield only trace amounts of CO2RR products, indicating the presence of [Co(triphos)(bdt)]+ is necessary to generate the products discussed above. Performing CPE experiments for 8 hours displays good current stability and a total formate TON of 23.7 (Table 1 entry 6). To determine if a catalytically active phase is depositing on the electrode during electrolysis, the working electrode was rinsed with clean CH3CN post-electrolysis and placed back in the working compartment with a CH3CN solution containing 0.1 M [nBu4N][PF6] and 0.3 M H2O under an atmosphere of CO2. CVs of the electrodes post-CPE display negligible current density, with currents comparable to those observed using a bare glassy carbon electrode. Performing a 2 hr-electrolysis with the rinsed post-CPE electrode produces only trace products, suggesting no catalytically active phase is deposited on the electrodes during CPE. The rinsed electrode was additionally analyzed using X-ray photoelectron spectroscopy (XPS) to determine if a cobalt-containing species is deposited on the working electrode during electrolysis. XPS spectra of the working electrode indicates trace amounts of cobalt and sulfur on the electrode surface. XPS spectra of a GCE immersed in a 0.45 mM CH3CN solution of [Co(triphos)(bdt)]+ exhibits similar Co 2p and S 2p features with the ones displayed in the spectra of the post-electrolysis electrode, suggesting that that these features originate from physiosorbed material, and that chemical deposition of cobalt-containing materials on the electrode surface during electrolysis is unlikely.

To evaluate the electrocatalytic activity of [Co(triphos)(bdt)]+, the CO2RR selectivity towards formate and over-potential are compared relative to the values reported for other molecular catalysts. Turnover frequency and the use of Tafel plot are avoided in this discussion due to the inherent coupled chemical steps corresponding to [Co(triphos)(bdt)]+ upon using H2O as a proton source, as illustrated by CV and CPE plots, in addition to the lack of an “S” shaped curve marking a purely kinetic regime, which makes these values not representative of the CO2RR catalytic kinetics. In that light, other factors such as relative overpotential and selectivity towards formate are used to help compare the activity of [Co(triphos)(bdt)]+ relative to that of other reported catalysts. The overpotential for the CO2RR to formate (I) is determined by taking the difference of the standard reduction potential of CO2/HCOOH relative to the applied overpotential and is considered at the applied CPE potential of −2.15 V vs Fc/Fc where high selectivity towards formate production was displayed. The standard reduction potential of CO2 to HCOOH was determined based on methods developed by Savéant and Artero (see SI for details).[38,81] Using this method, an overpotential of 750 mV was determined and is compared with other reported electrocatalysts for electrocatalytic CO2RR with their associated overpotentials and relative selectivities (Table S1). Selective electrocatalytic conversion of CO2 to formate>80% FE are quite rare amongst reported electrocatalysts active in the CO2RR, making [Co(triphos)(bdt)]+ an effective electrocatalyst towards selective conversion of CO2 to formate over other side reactions such as HER. On the other hand, [Co(triphos)(bdt)]+ displays relatively larger overpotentials compared to similarly selective catalysts such as Ir(POCOP) and HFeN(CO)12, though is comparable to other similarly comparable electrocatalysts such as CpCo(P2N2)2+ and Pt(dmpe)2.

Based on experimental data, a proposed mechanism for the electrochemical conversion of CO2 to HCOO− utilizing [Co(triphos)(bdt)]+ can be found in Scheme 3 (FIG. 8). To supplement experimental results, DFT calculation were employed to help elucidate potential intermediates and pathways that were not accessible via experimental methods. As formate was found to be the primary CO2RR product, mechanistic discussion will be limited to this product. Based on obtained electrochemical data, we only observe enhanced currents in the presence of CO2 and a proton source at the [Co(triphos)(bdt)]0/− couple, suggesting generation of [Co(triphos)(bdt)]− (III) is necessary before any catalytic activity is observed. Moreover, based on chemical reduction experiments, we can determine an additional chemical step in the form of apical phosphine delegation occurs concurrently upon reduction of [Co(triphos)(bdt)]0 (II). CVs of [Co(triphos)(bdt)]+ display an anodic shift at the [Co(triphos)(bdt)]0/− couple under an atmosphere of CO2 and in the presence of a proton source under N2, indicating the favorable binding of both CO2 and H*. As a result, DFT calculations were performed to model both Co—H (IV) or Co—CO2 (IV*) adducts to study their role in the reduction of CO2 to formate. Modelling of IV* yields a favored structure with CO2 bound apically in a position trans to the methyl moiety of the triphos ligand within a square pyramidal metal coordination environment favored by 4.8 kcal/mol compared to its associated isomer. Formation of IV* has a free energy change of −3.6 kcal/mol. This considerably low free energy change can be rationalized due to the large degree of ligand reorientation from that of a distorted square planar structure of III to that of a planar geometric orientation in IV* upon adduct formation. These results, in addition to CO being detected only as a negligible product, indicates that the CO2 bound adduct is unfavored and the metal hydride pathway will be considered in this discussion continuing forward. Additionally, a large KIE value of 5.0(2) supports a hydride based mechanism, with a RLS most likely involving a Co—H formation or transfer step, as reported in related electrocatalysts with similarly large KIE values.[38,58] Modelling of IV yields a similar result to IV*, with the optimized structure displaying a hydride adduct in a position trans to the methyl moiety of the triphos ligand, albeit with a smaller relative difference in energy of 1.7 kcal/mol, indicating that both of these isomers could be present in solution. Notably, the calculated structure of IV retains the distorted square planar structure of both phosphine and thiolate ligands in II, with the proton in the apical position. Protonation of the dithiolene was also explored due to previous reports of protonation as an initial step before turnover on similarly constructed complexes.[59,82] Due to both thiolate moieties being symmetrically inequivalent, four possible thiol permutations were calculated. Comparing the calculated IV structure to the lowest energy CoS—H state indicates the metal hydride is the thermodynamically favored product over the thiolate protonation product by 21.4 kcal/mol. Based on the optimized structure of IV and the calculated energy of a solvated free hydride, the hydricity (ΔGH−) of IV was calculated to be 58.7 kcal/mol. This ΔGH− value is larger compared to that reported for the hydricity of formate in acetonitrile (44 kcal/mol) suggesting that a formal hydride transfer from IV to CO2 is not thermodynamically favored. Similar reports on cobalt hydrides suggest an additional formal reduction of the generated Co(III)—H to Co(II)—H is necessary before a hydride transfer to the substrate can be thermodynamically driven.[38,58,65] As a result, the 1e- reduction of IV was considered and produces complex V. The optimized structure of V displays a distorted square pyramidal structure (τ=0.62) with the hydride ligand in the axial position. Calculating the hydricity of V yields a ΔGH− of 37.8 kcal/mol, indicating that reduction of IV to V is necessary to produce a Co—H hydridic enough to convert CO2 to HCOO−. Upon formation of formate, the formato-complex VI is generated, which is followed by deligation of formate from VI to regenerate II.

1.3 Conclusions.

This report focuses on the investigation of the clectrocatalytic activity of [Co(triphos)(bdt)]+ towards the CO2RR. In the presence of an exogenous proton source such as H2O, selective electrochemical conversion of CO2 to HCOO− is observed with faradic yields as high as 90% at an overpotential of 750 mV. The catalyst displays robust stability, with 8 hour CPE experiment displaying negligible reduction in current and no evidence of deposition on the electrode during electrolysis. Chemical reduction studies of [Co(triphos)(bdt)]+ indicate that deligation of the apical phosphine likely occurs before catalysis. A mechanism is proposed to occur through a hydride transfer pathway, and DFT calculation indicate an additional reduction of the [Co(triphos)(bdt)(H)]0 to [Co(triphos)(bdt)(H)]− is necessary for turnover, suggesting an overall ECEC mechanism. Ultimately, this study provides additional experimental evidence towards the beneficial role sulfur-based moieties play in molecular metal complexes as a method to increase their selectivity as electrocatalysts towards CO2RR. Further studies are underway to improve aspects of this catalyst, such as the relatively large overpotential, through functionalization of the ancillary ligands.

2. Selective Electrocatalytic Reduction of CO to Methanol by a Homogeneous Cobalt Complex 2.1 Results and Discussion 2.1.1 Synthesis and Characterization of Co(H)PS(CO)

Previous studies in our group have shown that CPE studies of CoPS at −2.18 V vs Fc+/0 in the presence of 0.5 M PhOH, TFE, or H2O and 1 atm of CO2 generated CO with 20, 8, and 29% Faradaic Efficiencies (FEs), respectively, while the rest of the FE corresponded to H2 in all three cases.34 Carbon monoxide is known to bind strongly to electron-rich, low valent metals as it is a σ-donor and π-acceptor, which allows for the existence of many carbonyl complexes.35 To investigate possible CO inhibition, NMR spectroscopy was employed. A J-Young NMR tube was charged with a solution of CoPS in MeCN-d3 under a N2 atmosphere, and its 1H-NMR spectrum was taken and was identical to the paramagnetic spectrum previously reported.34 The NMR tube underwent 3 cycles of freeze-pump-thaw, upon which was exposed to 1 atm of CO. An immediate color change from yellow to deep red occurred. The recorded 1H-NMR spectrum indicates the appearance of new resonances at δ 13.9, 9.9, 6.9, 1.3, 0.4, 0.3, and −0.2 ppm. Proton resonances corresponding to CoPS at δ 24, −16, and −23 ppm are absent and a new broad peak at 13.9 ppm appears under CO. Additionally, removal of CO from the NMR tube via 3 cycles of freeze-pump-thaw, followed by addition of 1 atm of N2, generates a 1H NMR spectrum that is identical to the one of the original CoPS complex. This behavior indicates that CoPS binds CO, and that this process is completely reversible. The change in spectra is not observed in air or under CO2, indicating a chemical reaction between CoPS and CO.34

UV-Vis spectroscopy was used to investigate possible CO binding in a solution of 0.2 mM CoPS in MeCN. Upon addition of CO, new adsorption features are observed at 320, 420, and 505 nm, which are red shifted relative to the ones in CoPS complex by 32, 32, and 25 nm, respectively. The weak absorbance observed in the CoPS complex at 830 nm is no longer present, but a new shoulder is observed at 675 nm. An N2 atmosphere was then introduced and during this time, a color change from deep red back to yellow was observed. The UV-Vis spectrum of the yellow solution contains absorption features that are identical to the ones observed for the CoPS complex, indicating reversible CO binding to CoPS.

FT-IR spectroscopy was employed to further analyze the CO binding to CoPS, leading to a possible metal carbonyl complex, which would provide more information about the degree of backbonding.13 A KBr pellet of CoPS was prepared in air and it was analyzed via FT-IR under N2. Significant stretches are observed at 3061 cm−1, 2957-2870 cm−1, and 1570-382 cm−1. A saturated solution of CoPS in dichloromethane was sparged with CO until all solvent evaporated which resulted in a red powder. A KBr pellet was prepared with the red powder and analyzed via FT-IR. Analogous stretches to those in the CoPS complex were observed along with a very intense stretch at 1967 cm−1, indicating the formation of a metal carbonyl complex. The IR stretching frequency of CO for terminal metal carbonyls range between 1850 and 2120 cm−1, depending on the nucleophilicity of the metal center, whereas free CO is observed at 2362 cm−1.36 Based on reported literature examples, the intense stretch at 1967 cm−1 is attributed to a terminal Co—CO, suggesting the formation of CoPS—CO adduct (Scheme 4, FIG. 9A). Solution phase FTIR was also performed with a 10 mM solution of CoPS in dichloromethane and similar diluted stretches were observed as in the solid state. CoPS solution was then sparged with CO and after solvent subtraction, two significant stretches are observed at 2362 cm−1 and 1968 cm−1, which are attributed to the stretches for the free CO and that of the cobalt carbonyl complex, respectively. The value of the cobalt carbonyl stretch is effectively the same both in solid state and in solution. The high frequency of the Co—CO stretch at 1967 cm−1 indicates a weak backdonation into the empty nπorbitals of CO. The weak CO backdonation character in the CoPS—CO adduct is consistent with the NMR and UV-Vis experiments, as de-ligation of CO can occur simply by purging a CO saturated CoPS solution with N2.

The red powder generated upon exposing CoPS to CO was dissolved in a CO-saturated benzene solution, and the resulting red solution was recrystallized via vapor diffusion with a CO-saturated pentane solution in a large Schlenk flask charged with CO at 0° C. Red prism crystals were afforded, and one of them was mounted on the X-ray diffractometer. The single crystal X-ray data was collected at 100 K under N2. After refinement, a unit cell containing 6 different versions of the complex was observed. All 6 versions represent a cobalt center coordinated by phosphinothiolate ligands and one CO molecule. The main difference between the 6 structures is the Co—C distance, ranging from 1.808 to 2.240 Å (the exact values are listed in Table S5). Thus, each complex represents a transient state of CO dissociation. Nonetheless, the CoPS(CO) complex with the shortest Co—C distance and least disorder was selected for investigation and is shown to adopt a distorted square pyramidal geometry with the carbonyl as the apical ligand verified by a τ5 of 0.13 (FIG. 9B). The C—O bond length in the carbonyl fragment is 1.154(6) Å, and this value is similar to that observed in free carbon monoxide (1.128 Å).37 The crystal structure for CoPS was previously reported and the reported average Co—S bond length was 2.165 Å,34 whereas the average Co—S bond in CoPS(CO) is elongated to 2.234 Å. These results are consistent with the formation of a carbonyl adduct, as electron density is diverted into the π-accepting CO ligand.35

2.2 Electrochemical Studies 2.2.1 Cyclic Voltammetry Studies Under N2 and CO

CoPS has been previously reported as a CO2RR and HER catalyst for CO2 to syngas conversion, however its electrochemical behavior under a CO atmosphere was not explored. Therefore, the electrochemistry of CoPS under a CO atmosphere is investigated here. Cyclic voltammetry (CV) experiments were performed in acetonitrile solutions with 0.1 M supporting electrolyte, [nBu4][PF6], and all CVs were first scanned anodically and subsequently returned cathodically. All the potentials are listed versus the ferrocene/ferrocenium couple. The electrochemistry of CoPS (1 mM) under N2 has been previously reported, but a summary of the reduction features and their assignments is listed in the Supporting Information (Table S1).34 CVs of a CO sparged solution of CoPS in acetonitrile displays a reversible one electron-oxidation with E1/2 at −0.12 V and assigned to [CoPS(CO)]0/+, and a reversible one-electron reduction with E1/2 at −1.63 V and assigned to [CoPS(CO)]0/−. In comparison to the previously reported CV of CoPS under N2, the disappearance of the quasi-reversible peak at −2.36 V is observed, along with anodic shifts of 100 and 450 mV for the [CoPS(CO)]0/+ and [CoPS(CO)]0/− couples, respectively. Upon changing the atmosphere of the electrochemical cell from CO to N2, and purging with N2 for 10 min, the color of the solution in the working compartment of the electrolysis cell changes from red to yellow. Performing a CV experiment, the reduction features observed are identical to those of the initial CoPS complex under N2, further suggesting complete regeneration of CoPS. This behavior indicates that CO binding to CoPS is reversible. From the 100 mV anodic shift in the one-electron oxidation (ΔE) under CO vs N2, the CO binding constant (KCO) for CoPS(CO) is determined using equation S2.28 A value of 3.2×102 M−1 is obtained, indicative of a moderate affinity of CO towards the cobalt center in CoPS, which is in agreement with the NMR, UV-Vis and FTIR spectroscopy studies that suggest full reversibility of CO binding. The 450 mV anodic shift observed for the [CoPS(CO)]0/− couple indicates a KCO of 4.1×108 M−1 in the one electron-reduced species, CoPS−. This larger value indicates a stronger binding of CO to the cobalt center in the one-electron reduced species, due to stronger back-bonding from the more electron rich metal center. The reported CO binding constant for [Ni(cyclam)]2+ was 2.8×105 M−1, indicating a stronger CO binding to the nickel(II) metal center relative to that observed for the CoPS complex.38 CVs of 1 mM CoPS under increasing concentrations of CO were conducted, starting with an acetonitrile solution of CoPS under N2 saturation (0% CO). At 10% CO saturated solution, reversible one-electron reduction with E1/2 at −1.63 V and reversible one-electron reduction with E1/2 at −0.12 V appear; however, reduction features attributed to CoPS under N2 are still present albeit with diminished current density. At 30% CO, complete saturation is observed as the CV recorded is identical to that displayed in the presence of 1 atm of CO (FIG. 10). These results indicate the effectiveness of CO binding to CoPS even at low concentrations of CO in the electrochemical solution.

2.2.2 Synthesis and Characterization of [CoPS(CO)]−

CoPS was previously reduced by one electron upon addition of KCs to CoPS to generate [CoPS]−, and its characterization by X-ray crystallography, 1H-NMR, UV-Vis, and elemental analysis was reported.34 Addition of 1 atm of CO to a solution of [CoPS]− in benzene-d6 results in an immediate color change from green to light gold. The 1H-NMR spectrum of [CoPS]− under 1 atm of CO remains diamagnetic and has resonances at δ 8.0, 7.5, 7.4, 7.0, 2.4, and 1.05 ppm, with additional resonances being attributed to residual THF and pentane resonances. The 1H-resonance originally assigned to the methine protons on the isopropyl groups in the δ 2.0-2.5 ppm region has changed its splitting pattern under CO; however, other proton resonances remain unchanged. The 31P-NMR spectrum of [CoPS]− in the presence of CO contains only one phosphorus resonance at δ 10.5 ppm, which suggests that the two phosphine groups are symmetrical.

Spectroelectrochemical UV-Vis (SEC-UV-Vis) studies were employed to generate and observe [CoPS(CO)]− in-situ using a Pine-Avantes instrument. A 1 mM CoPS solution was prepared using 0.25 M [nBu4N][PF6] electrolyte in acetonitrile under 1 atm of N2. The UV-Vis spectrum before CPE is identical to that previously reported for CoPS.34 CPE at positive potentials and very negative potentials generate spectra attributed to [CoPS]+ and [CoPS]−, respectively. Performing a CPE at 0.11 V to −2.09 V generates spectra of the reduced states of CoPS (FIG. 11A). Charging the 1 mM CoPS solution with CO redshifts the absorbances to what has been reported herein. Stepwise CPE was employed which started at 0.11 V and climbed cathodically to −2.09 V (FIG. 11). As the potential decreases, the absorbances at 320, 420, and 505 nm decrease, and a new absorbance at 298 nm and a small shoulder at 390 nm appear, suggesting the formation of a new reduced species from CoPS(CO).

To further elucidate this new species, spectroelectrochemical FTIR (SEC FTIR) studies were employed to generate and observe [CoPS(CO)]− using a Pine-Bruker custom set up containing an Omni-Cell solution IR cell. A 10 mM CoPS solution was prepared using 0.25 M [nBu4N][PF6] electrolyte in dichloromethane (DCM) under 1 atm of CO to generate CoPS(CO). Dichloromethane was used to obtain a more concentrated solution of CoPS. A spectrum of the generated CoPS(CO) was taken and two very strong stretches are observed at 2307 cm−1 and 1967 cm−1, which correspond to the carbonyl stretch in the free CO and in the CoPS(CO) complex, respectively. After performing a 10 min CPE at −1.5 V, the stretch at 1967 cm−1 shows a decreased intensity, whereas two new stretches at 1869 cm−1 and 1713 cm−1 appear (FIG. 11B). The stretch at 1869 cm−1 likely corresponds to the formation of Co(I)—CO stretch as the degree of back-bonding has increased from a one-electron reduction and the strong stretch at 1713 cm−1 is characteristic of a ketone suggesting reduction of CO.

2.2.3 Cyclic Voltammetry Studies with Proton Donors and CO

The electrochemical behavior of CoPS in the presence of an exogenous proton donor in the form of PhOH, TFE, or H2O under CO vs N2 atmosphere was investigated using CV. The CV of 1 mM CoPS in the presence of 0.5 M PhOH and 1 atm of CO, results in loss of reversibility of the reductive feature at −1.63 V (FIG. 12), followed by a larger increase in the current density at more negative potentials, reaching −4.2 mA/cm2 at −2.18 V. In contrast, the CV of 1 mM CoPS in the presence of 0.5 M PhOH and 1 atm of N2 displays much larger current densities at more negative potentials, reaching −16.6 mA/cm2 at −2.18 V, and this behavior has been previously reported and attributed to hydrogen evolution.34 Comparing the CV of CoPS under N2 vs CO atmosphere it is observed that the shape of the features change from peak to plateau-like, with a concurrent 200 mV anodic shift in the onset of current from −1.98 V to −1.83 V. Moreover, a restoration of all redox features is observed when the CO-saturated solutions are purged with N2, in agreement with the previous electrochemical studies in the absence of exogeneous proton donors.

To get more information, CVs of 1 mM CoPS in the presence of increasing concentrations of PhOH and 1 atm of CO were performed. Upon addition of 0.1 M PhOH, a catalytic plateau is observed at −2.00 V. As the phenol concentration increases to 2.0 M, this reductive feature becomes more peak-like and the onset of catalysis shifts anodically from −1.87 to −1.74 V. Additionally, as the concentration of PhOH increases, the reversible feature at −1.63 V loses reversibility and the onset shifts anodically from −1.56 to −1.44 V, indicating a chemical step follows the [CoPS(CO)]0/− reduction, and that this event is favored with increasing the proton concentration (FIG. 12). Performing a similar CV study of 1 mM CoPS in the presence of increasing concentrations of PhOH and 1 atm of N2, however, indicates much larger current densities, compared to the ones under CO, and the onset of catalysis under N2 shifts anodically from −2.04 to −1.93 V.

The electrochemical behavior of CoPS under 1 atm of CO and in the presence of TFE and H2O as exogenous proton donors was investigated as well and compared to that in the presence of PhOH. The CVs in the presence of 0.5 M TFE or H2O do not result in the loss of reversibility of the reductive feature at −1.63 V, and smaller current densities are observed at more negative potentials reaching −0.4 mA/cm2 for TFE and −0.2 mA/cm2 for H2O at −2.18 V. Our previous studies indicate that the behavior under N2 is attributed to HER34. Comparing the CV of CoPS under N2 vs CO atmosphere it is also observed that the shape of the features change from peak to plateau-like, with a concurrent anodic shift in the onset of current from −1.98 to −1.87 V for TFE and from −1.98 to −1.88 V for H2O. A restoration of all redox features is observed when the CO-saturated solutions are purged with N2.

CVs of 1 mM CoPS in the presence of increasing concentrations of TFE and H2O under 1 atm of CO were performed. In contrast to PhOH, much lower current densities are observed at −2.00 V. As the TFE concentration increases to 3.0 M, the reductive feature at −2.00 V changes from plateau-to peak-like and the onset of catalysis shifts anodically from −1.91 to −1.78 V. Additionally, as the concentration of TFE increases, the reversible feature at −1.63 V loses some reversibility and the onset shifts anodically from −1.56 to −1.44 V, indicating a chemical step follows the [CoPS(CO)]0/− reduction, and that this event is favored with increasing the proton concentration. Performing a similar CV study of 1 mM CoPS in the presence of increasing concentrations of TFE and 1 atm of N2, however, indicates much larger current densities, compared to the ones under CO, and the onset of catalysis under N2 shifts anodically from −2.05 to −1.97 V. With increasing concentration of H2O, no current increase is observed at −2.00 V at any concentration. Additionally, the onset of the reversible feature at −1.63 V does not lose reversibility at any concentration but does experience a small anodic shift from −1.56 to −1.52 V. Although a small shift, this indicates a chemical step after the [CoPS(CO)]0/− reduction, and that this event is favored with increasing the proton concentration. Performing a similar CV study of 1 mM CoPS in the presence of increasing concentrations of H2O and 1 atm of N2 results in no current increase like that of under CO, and the onset of catalysis under N2 does not shift.

Furthermore, CVs of a solution of 0.5 M PhOH under 1 atm CO was titrated with CoPS from 0.2 mM to 1.0 mM. The reductive feature observed at −2.00 V has the same peak shape as current increases; however, the onset of catalysis shifts anodically from −2.02 to −1.84 V. The reversible redox feature at −1.63 V has the same peak shape as current increases and the onset of catalysis shifts anodically by 20 mV.

2.2.4 Controlled Potential Electrolysis for CH3OH and HER

To benchmark the CoPS catalyst as a viable homogeneous catalyst for the reduction of CO to CH3OH, it is important to determine the thermodynamic potential. To the best of our knowledge, the thermodynamic potential of this reaction is not reported in acetonitrile; however, the reduction of CO2 to CH3OH is reported in aqueous solvent vs SHE and the reduction of CO2 to CO is reported in CH3CN. These values can be converted into acetonitrile to calculate the thermodynamic potential based on literature precedent.39 The thermodynamic potential for E0(CO/CH3OH)=−0.28 V vs Fc/Fc+ with calculations found in the supporting information.

TABLE 1 Summary of the controlled potential electrolysis results and the conditions used for the electrolysis of CoPS in the presence of CO and 0.5M PhOH. Electrolyses were performed with 1 mM of CoPS in a CH3CN solution containing 0.1M [nBu4N][PF6] under an atmosphere of CO. Potential Charge FE % H2 FE % CH3OH FE % Trial (V) (C) (±4) (±3) (Total) 1 −2.40 (No 1.6 70% 0  70% catalyst) 2 −2.40 36.7 84% 16% 100% 3 −2.10 12.9 67% 34%  99% 4 −1.60 2.2 12% 89% 101%

To analyze and quantify the products generated, CPE of 1 mM CoPS in the presence of 0.5 M PhOH and 1 atm of CO were performed at −2.4, −2.1, and −1.6 V for 1 h (FIGS. 13b-d). At −2.4 V, a slight decrease in current is observed from 9.6 to 7.5 mA during the 1 h electrolysis. Analysis of the head space of the working compartment of the electrolysis cell via gas chromatography (GC) indicates production of H2 with a FE of 84%. Additionally, the liquid phase of the solution from the working compartment of the electrolysis cell was analyzed by 1H NMR spectroscopy. Resonances of δ 3.25 ppm were observed in the 1H NMR spectrum, suggesting the formation of methanol. The amount of methanol generated was quantified via gas chromatography using the procedure described in the SI. Thus, in addition to HER, the CPE studies at −2.4 V resulted in the production of methanol with a FE of 16%, generating a combined FE of 100%. No other C1 products, such as formaldehyde or methane, were detected.

At −2.1 V, a more stable current of approximately 4 mA is observed during the 1 h electrolysis. Analysis of the gaseous and liquid phases indicate production of H2 and methanol with FEs of 67 and 34%, respectively. At −1.6 V, a current of approximately 1.4 mA is observed, along with the production of H2 and methanol with FEs of 12 and 89%, respectively. In all cases, no other C1 products, such as formaldehyde or methane, were detected, and the combined FE for H2 and methanol is >99%. A control study was performed in the absence of catalyst, and the analysis of the gaseous and liquid phases indicate negligible amounts of H2 with FE of 70% and no methanol was detected. Previous CPE studies of 1 mM CoPS in the presence of 0.5 M PhOH and 1 atm of N2 were performed at −2.18 V and were reported to result in the consumption of 96 C of charge, and the production of 534 μmol of H2 with >99% FE.34

2.3 Conclusions

Electrochemical reduction of CO into valuable C1 products such as methanol is key to solving the storage problem the renewable energy is currently facing, by storying the solar energy in chemical bonds. Molecular catalysts offer advantages of tunability and high selectivity in comparison to heterogeneous systems. Many homogeneous catalysts electrochemically reduce CO2 to CO or formic acid, but further reduction usually involves precious metals and high temperature and pressure. The development of a homogeneous system for the electrochemical production of MeOH remains a major challenge. We report herein that a cobalt 2-diisopropylphosphinothiolatebenzene (CoPS) complex has a moderate CO binding constant to allow the formation of long-lived CO adducts for further reduction. We report herein the first homogeneous electrocatalyst capable of reducing CO into CH3OH with high selectivity and stability at a moderate potential. Ongoing studies are optimizing the homogeneous system and exploring ways to improve the activity.

2.4 Experimental Methods 2.4.1 Materials and Synthesis

All manipulations of air- and moisture-sensitive materials were conducted under nitrogen atmosphere in a Vacuum Atmospheres glovebox or on a dual manifold Schlenk line with oven-dried glassware. Water was deionized with the Millipore Synergy system (18.2 MΩ·cm). All other solvents used were degassed with nitrogen, passed through activated alumina columns, and stored over 4 Å Linde-type molecular sieves. The desired ligand was synthesized according to the reported literature procedure.1 CoPS was synthesized using the reported procedure.2 All other chemical reagents were purchased from commercial vendors and used without further purification. Elemental analyses were performed by Robertson Microlit Laboratories, Ledgewood, New Jersey.

2.4.2 Synthesis of CoPS(CO) Complex

Under ambient conditions, solid CoPS (~5-10 mg) was added to a round bottom Schlenk flask equipped with a magnetic stir bar. Dichloromethane (20 mL) was added to the Schlenk flask to generate a yellow-colored solution. Switching the atmosphere from N2 to CO under continuous CO flow resulted in an immediate color change from yellow to dark red. The solution was then flushed under continuous CO flow, until the solvent evaporated and afforded a red powder. The red powder was dissolved in benzene. Recrystallization via vapor diffusion of pentane into the saturated benzene solution under CO atmosphere resulted in red crystals of X-ray quality.

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.

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Claims

1. An electrochemical cell for converting carbon dioxide to a value-added product, the electrochemical cell comprising: wherein:

a vessel;
a liquid electrolyte disposed in the vessel;
a cathode at least partially immersed in the electrolyte;
an anode at least partially immersed in the electrolyte;
a carbon dioxide source for providing carbon dioxide to the electrolyte;
a power supply for biasing the cathode relative to the anode; and
a compound that includes an organometallic complex having formula 1 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero dispersed in the electrolyte and/or deposited onto the cathode:
M is a transition metal;
Z is S, O, Se, Te, or NH;
n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
L is an optional ligand or an optional donor group from another ligand attached to M; and
R1, R2, R3, R4, R5, R6, R7 and R8 are each independently C1-10 alkyl, C6-10 cycloalkyl, or substituted or unsubstituted C6-14 aryl with the proviso that any 2 of R1, R2, R3, R4 can be bonded together to form a multidentate ligand.

2. The electrochemical cell of claim 1, wherein the value added product is a formate or an oxalate.

3. The electrochemical cell of claim 1, wherein R1, R2 are bonded together to form a first multidentate ligand and R3, R4 are bonded together to form a second multidentate ligand.

4.-5. (canceled)

6. The electrochemical cell of claim 1, wherein R1, R4 are bonded together to form a first multidentate ligand and R2, R3 are bonded together to form a second multidentate ligand.

7. The electrochemical cell of claim 1, wherein the liquid electrolyte includes an organic solvent and an organic soluble salt, water, a C1-4 alcohol or combinations thereof.

8. The electrochemical cell of claim 1, wherein the organometallic complex has formula 2A or 2B:

wherein:
Z is S, O, Se, Te, or NH;
R9, R10, are each independently C1-10 alkyl or C6-14 aryl; and
each phenyl is optional substituted with C1-6 alkyl, C6-10 cycloalkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

9. The electrochemical cell of claim 1, wherein the organometallic complex has formula 3:

wherein Z is S, O, Se, Te, or NH; and each phenyl is optional substituted with C1-6 alkyl, C6-10 cycloalkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3-M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

10. A method for reducing carbon dioxide comprising:

electrolytically reducing carbon dioxide in the presence of a compound that includes an organometallic complex having formula 1 and one or more counterions Xn(− or +)if the formal charge of the organometallic complex is not zero:
wherein: M is a transition metal; Z is S, O, Se, Te, or NH; n(+ or −) is the formal charge of the organometallic complex; n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex; and L is an optional ligand or an optional donor group from another ligand attached to M; and R1, R2, R3, R4, R5, R6, R7 and R8 are each independently C1-10 alkyl, C6-10 cycloalkyl, or C6-14 aryl with the proviso that any 2 of R1, R2, R3, R4 can be bonded together to form a multidentate ligand.

11. The method of claim 10, wherein the value added product is a formate or oxalate.

12. The method of claim 10, wherein carbon dioxides is electrolytically reduced at the cathode of an electrochemical cell.

13. The method of claim 10, wherein M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W.

14. (canceled)

15. The method of claim 10, wherein R1, R2 are bonded together to form a first multidentate ligand and R3, R4 are bonded together to form a second multidentate ligand. In another variation, R1, R4 are bonded together to form a first multidentate ligand and R2, R3 are bonded together to form a second multidentate ligand.

16. The method of claim 10, wherein the liquid electrolyte includes an organic solvent and an organic soluble salt, water or a C1-4 alcohol.

17. The method of claim 10, wherein the organometallic complex has formula 2:

wherein Z is S, O, Se, Te, or NH; and each phenyl is optional substituted with C1-6 alkyl, C6-10 cycloalkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

18. The method of claim 10, wherein the organometallic complex has formula 3:

wherein Z is S, O, Se, Te, or NH and each phenyl is optional substituted with C1-6 alkyl, C6-10 aryl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

19. An electrochemical cell for converting carbon monoxide to a value-added product, the electrochemical cell comprising:

a vessel;
a liquid electrolyte disposed in the vessel;
a cathode at least partially immersed in the electrolyte;
an anode at least partially immersed in the electrolyte;
a carbon monoxide source for providing carbon monoxide to the electrolyte;
a power supply for biasing the cathode relative to the anode; and
an organometallic complex having formula 4 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero:
wherein: M is a transition metal; Z is S, O, Se, Te, or NH; n(+ or −) is the formal charge of the organometallic complex; n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex; is an integer from 0 to 4; and R9, R10, are each independently C1-10 alkyl or C6-14 aryl; and R11 is C1-6 alkyl, C6-10 aryl, C6-10 cycloalkyl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

20. The electrochemical cell of claim 19, wherein the value added product is methanol.

21. The electrochemical cell of claim 19, wherein M is Co, Rh, Ir, Fe, Ru, Os, Ni, Pd, Pt, Cu, Cr, Mo or W.

22.-24. (canceled)

25. The electrochemical cell of claim 19, wherein the organometallic complex has formula 5:

where Z is S, O, Se, Te, or NH and iPr is isopropyl.

26. A method for reducing carbon monoxide comprising: wherein:

electrolytically reducing carbon monoxide in the presence of a compound that includes an organometallic complex having formula 4 and one or more counterions Xn(− or +) if the formal charge of the organometallic complex is not zero:
M is a transition metal;
Z is S, O, Se, Te, or NH;
n(+ or −) is the formal charge of the organometallic complex;
n(− or +) is the formal charge of the one or more counterions that balances the formal charge of the organometallic complex;
o is an integer from 0 to 4;
R9, R10, R11 are each independently C1-10 alkyl or C6-14 aryl; and
R11 is C1-6 alkyl, C6-10 aryl, C6-10 cycloalkyl, C6-10 heteroaryl, —NO2, —NH2, —N(R′R″), —N(R′R″R′″)+L−, Cl, F, Br, —CF3, —CCl3, —CN, —SO3H, —PO3H2, —COOH, —CO2R′, —COR′, —CHO, —OH, —OR′, —O−M′+, —SO3−M′+, —PO3−M′+, —COO−M′+, —CF2H, —CF2R′, —CFH2, and —CFR′R″ where R′, R″ and R′″ are C1-10 alkyl or C6-18 aryl groups, L− is a negatively charged counter ion, and M′+ is a metal cation.

27.-66. (canceled)

Patent History
Publication number: 20260258562
Type: Application
Filed: Jun 16, 2023
Publication Date: Sep 3, 2026
Applicant: UNIVERSITY OF SOUTHERN CALIFORNIA (Los Angeles, CA)
Inventors: Smaranda MARINESCU (Los Angeles, CA), Jeremy INTRATOR (Los Angeles, CA), David VELAZQUEZ (Los Angeles, CA)
Application Number: 18/875,343
Classifications
International Classification: C25B 3/26 (20210101); C25B 3/07 (20210101); C25B 11/085 (20210101);