Electrocatalytic reduction of CO2 with palladium bis-N-heterocyclic carbene pincer complexes

Bioinspired Binickel Catalyst for Carbon Dioxide Reduction: The Importance of Metal-ligand Cooperation

Catalyst design for the efficient CO2 reduction reaction (CO2RR) remains a crucial challenge for the conversion of CO2 to fuels. Natural Ni-Fe carbon monoxide dehydrogenase (NiFe-CODH) achieves reversible conversion of CO2 and CO at nearly thermodynamic equilibrium potential, which provides a template for developing CO2RR catalysts. However, compared with the natural enzyme, most biomimetic synthetic Ni-Fe complexes exhibit negligible CO2RR catalytic activities, which emphasizes the significance of effective bimetallic cooperation for CO2 activation. Enlightened by bimetallic synergy, we herein report a dinickel complex, NiIINiII(bphpp)(AcO)2 (where NiNi(bphpp) is derived from H2bphpp = 2,9-bis(5-tert-butyl-2-hydroxy-3-pyridylphenyl)-1,10-phenanthroline) for electrocatalytic reduction of CO2 to CO, which exhibits a remarkable reactivity approximately 5 times higher than that of the mononuclear Ni catalyst. Electrochemical and computational studies have revealed that the redox-active phenanthroline moiety effectively modulates the electron injection and transfer akin to the [Fe3S4] cluster in NiFe-CODH, and the secondary Ni site facilitates the C-O bond activation and cleavage through electron mediation and Lewis acid characteristics. Our work underscores the significant role of bimetallic cooperation in CO2 reduction catalysis and provides valuable guidance for the rational design of CO2RR catalysts.

Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction

The conversion of CO2 into CO as a substitute for processing fossil fuels to produce hydrocarbons is a sustainable, carbon neutral energy technology. However, the electrochemical reduction of CO2 into a synthesis gas (CO and H2) at a commercial scale requires an efficient electrocatalyst. In this perspective, a series of six new palladium complexes with the general formula [Pd(L)(Y)]Y, where L is a donor-flexible PYA, N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, N2,N6-bis(1-butylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, or N2,N6-bis(1-benzylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, and Y = OAc or Cl-, were utilized as active electrocatalysts for the conversion of CO2 into a synthesis gas. These palladium(ii) pincer complexes were synthesized from their respective H-PYA proligands using 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) or sodium acetate as a base. All the compounds were successfully characterized by various physical methods of analysis, such as proton and carbon NMR, FTIR, CHN, and single-crystal XRD. The redox chemistry of palladium complexes toward carbon dioxide activation suggested an evident CO2 interaction with each Pd(ii) catalyst. [Pd(N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide)(Cl)]Cl showed the best electrocatalytic activity for CO2 reduction into a synthesis gas under the acidic condition of trifluoracetic acid (TFA) with a minimum overpotential of 0.40 V, a maximum turnover frequency (TOF) of 101 s-1, and 58% FE of CO. This pincer scaffold could be stereochemically tuned with the exploration of earth abundant first row transition metals for further improvements in the CO2 reduction chemistry.

Insights into the Mechanism of CO2 Electroreduction by Molecular Palladium-Pyridinophane Complexes

Herein, we report the synthesis, characterization, and electrocatalytic CO2 reduction activity of a series of Pd(II) complexes supported by tetradentate pyridinophane ligands. In particular, we focus on the electrocatalytic CO2 reduction activity of a Pd(II) complex supported by the mixed hard--soft donor ligand 2,11-dithia[3.3](2,6)pyridinophane (N2S2). We also provide spectroscopic evidence of a CO-induced decomposition pathway for the same catalyst, which provides insights into catalyst poisoning for molecular Pd CO2 reduction electrocatalysts.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Neto A. cis-[6-(Pyridin-2-yl)-1,3,5-triazine-2,4-diamine](dichloride) Palladium(II)-Based Electrolyte Membrane Reactors for Partial Oxidation Methane to Methanol

Methane is an abundant resource and the main constituent of natural gas. It can be converted into higher value-added products and as a subproduct of electricity co-generation. The application of polymer electrolyte reactors for the partial oxidation of methane to methanol to co-generate power and chemical products is a topic of great interest for gas and petroleum industries, especially with the use of materials with a lower amount of metals, such as palladium complex. In this study, we investigate the ideal relationship between cis-[6-(pyridin-2-yl)-1,3,5-triazine-2,4-diamine(dichloride)palladium(II)] (Pd-complex) nanostructure and carbon to obtain a stable, conductive, and functional reagent diffusion electrode. The physical and structural properties of the material were analyzed by Fourier transform infrared (FT-IR) and Raman spectroscopies, transmission electron microscopy (TEM), and X-ray powder diffraction (XRD) techniques. The electrocatalytic activity studies revealed that the most active proportion was 20% of Pd-complex supported on carbon (m/m), which was measured with lower values of open-circuit and power density but with higher efficiency in methanol production with reaction rates of r = 4.2 mol L^-1·h^-1 at 0.05 V.

CuBi electrocatalysts modulated to grow on derived copper foam for efficient CO2-to-formate conversion

Electrochemical reduction of CO₂ to fuels and chemicals is an effective way to reduce greenhouse gas emissions and alleviate the energy crisis, but the highly active catalysts necessary for this reaction under mild conditions are still rare. In this work, we grew CuBi bimetallic catalysts on derived copper foam substrates by co-electrodeposition, and then investigated the correlation between co-electrodeposition potential and electrochemical performance in CO₂-to-formate conversion. Results showed that the bimetallic catalyst formed at a low potential of - 0.6 V vs. AgCl/Ag electrode achieved the highest formate Faradaic efficiency (FE_formate) of 94.4% and a current density of 38.5 mA/cm² at a low potential of - 0.97 V vs. reversible hydrogen electrode (RHE). Moreover, a continuous-flow membrane electrode assembly reactor also enabled the catalyst to show better performance (a FE_formate of 98.3% at 56.6 mA/cm²) than a traditional H-type reaction cell. This work highlights the vital impact of co-electrodeposition potential on catalyst performance and provides a basis for the modulated growth of bimetallic catalysts on substrates. It also shows the possibility of preparing Bi-based catalysts with no obvious decrease in catalytic activity that have been partially replaced with more economic copper.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Redox-Active Manganese Pincers for Electrocatalytic CO2 Reduction

The decrease of total amount of atmospheric CO2 is an important societal challenge in which CO2 reduction has an important role to play. Electrocatalytic CO2 reduction with homogeneous catalysts is based on highly tunable catalyst design and exploits an abundant C1 source to make valuable products such as fuels and fuel precursors. These methods can also take advantage of renewable electricity as a green reductant. Mn-based catalysts offer these benefits while incorporating a relatively cheap and abundant first-row transition metal. Historically, interest in this field started with Mn(bpy-R)(CO)3X, whose performance matched that of its Re counterparts while achieving substantially lower overpotentials. This review examines an emerging class of homogeneous Mn-based electrocatalysts for CO2 reduction, Mn complexes with meridional tridentate coordination also known as Mn pincers, most of which contain redox-active ligands that enable multi-electron catalysis. Although there are relatively few examples in the literature thus far, these catalysts bring forth new catalytic mechanisms not observed for the well-established Mn(bpy-R)(CO)3X catalysts, and show promising reactivity for future studies.

Crystal structure and Hirshfeld surface analysis of 2-phenyl-1H-phenanthro[9,10-d]imidazol-3-ium benzoate

In the title compound, C21H15N2 +·C7H5O2 -, 2-phenyl-1H-phenanthro[9,10-d]imidazole and benzoic acid form an ion pair complex. The system is consolidated by hydrogen bonds along with π-π inter-actions and N-H⋯π inter-actions between the constituent units. For a better understanding of the crystal structure and inter-molecular inter-actions, a Hirshfeld surface analysis was performed.

Biomimetic heterobimetallic architecture of Ni(ii) and Fe(ii) for CO2 hydrogenation in aqueous media. A DFT study

In this work, density functional theory has been employed to design a heterobimetallic catalyst of Ni(ii) and Fe(ii) for the effective CO₂ hydrogenation to HCOOH. Based on computational results, our newly designed catalyst is found to be effective for such conversion reactions with free energy as low as 14.13 kcal mol⁻¹ for the rate determining step. Such a low value of free energy indicates that the NiFe heterobimetallic catalyst can prove to be very efficient for the above said conversion. Moreover, the effects of ligand substitutions at the active metal center and the effects due to various spin states are also explored, and can serve as a great tool for the rational design of NiFe catalyst for CO₂ hydrogenation.

The hydrogenation of CO₂ by our newly designed [NiFe] heterobimetallic catalyst inspired by the active site of [NiFe] hydrogenase.

A look at periodic trends in d-block molecular electrocatalysts for CO2 reduction

Periodic trends in the electronic structure of the transition metal centers can be used to explain the observed CO₂ reduction activities in molecular electrocatalysts for CO₂ reductions. Research activities concerning both horizontal and vertical trends have been summarized with mononuclear complexes from Group 6 to Group 10.

A Highly Active N-Heterocyclic Carbene Manganese(I) Complex for Selective Electrocatalytic CO2 Reduction to CO

We report here the first purely organometallic fac-[MnI (CO)3 (bis-Me NHC)Br] complex with unprecedented activity for the selective electrocatalytic reduction of CO2 to CO, exceeding 100 turnovers with excellent faradaic yields (ηCO ≈95 %) in anhydrous CH3 CN. Under the same conditions, a maximum turnover frequency (TOFmax ) of 2100 s-1 was measured by cyclic voltammetry, which clearly exceeds the values reported for other manganese-based catalysts. Moreover, the addition of water leads to the highest TOFmax value (ca. 320 000 s-1 ) ever reported for a manganese-based catalyst. A MnI tetracarbonyl intermediate was detected under catalytic conditions for the first time.

Directing the Outcome of CO2 Reduction at Bismuth Cathodes Using Varied Ionic Liquid Promoters

Ionic liquids (ILs) have been established as effective promoters for the electrocatalytic upconversion of CO₂ to various commodity chemicals. Imidazolium ([Im]⁺) cathode combinations have been reported to selectively catalyze the 2e^-/2H⁺ reduction of CO₂ to CO. Recently our laboratory has reported energy-efficient systems for CO production featuring inexpensive bismuth-based cathode materials and ILs comprised of 1,3-dialkylimidazolium cations. As part of our ongoing efforts to understand the factors that drive CO₂ reduction at electrode interfaces, we sought to evaluate the catalytic performance of alternative ILs in combination with previously described Bi cathodes. In this work, we demonstrate that protic ionic liquids (PILs) derived from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) effectively promote the electrochemical reduction of CO₂ to formate (HCOO^-) with high selectivity. The use of PILs comprised of the conjugate acid of DBU, [DBU-H]⁺, efficiently catalyzed the reduction of CO₂ to HCOO^- (FE_FA ≈ 80%) with significant suppression of CO production (FE_CO ≈ 20%) in either MeCN or MeCN/H₂O (95/5) solution. When they were used in combination with [DBU-H]⁺-based PILs, Bi-based cathodes achieved current densities for CO₂ reduction (j _tot ≈ 25-45 mA/cm²) that are comparable to or greater than those reported with imidazolium ILs such as [BMIM]PF₆. As we demonstrate herein, the selectivity of the 2e^- reduction of CO₂ toward HCOO^- or CO can be dictated through the choice of the IL promoter present in the electrolysis solution, even in cases in which the same electrocatalyst material is studied. These findings highlight the tunability of bismuth/IL systems for the electrochemical reduction of CO₂ with high efficiency and rapid kinetics.

Synthesis and comparison of nickel, palladium, and platinum bis(N-heterocyclic carbene) pincer complexes for electrocatalytic CO2 reduction

A valence isoelectronic and isostructural series of charged bis(N-heterocyclic carbene) pincer complexes [M(bC^N^bC)X]OTf and [M(bC^N^bC)CH₃CN](OTf)₂ (where M = Ni, Pd, and Pt, bC^N^bC = 1,1'-(pyridine-2,6-diylbis(methylene))bis(3-butylbenzo[d]imidazol-2-ylidene)) were synthesized, characterized, modelled by density functional theory calculations, and compared for their electrochemical properties and reactivity with CO₂. Although the electrochemical response of each complex is altered by the presence of CO₂, controlled potential electrolysis experiments demonstrated the superior ability of [Pd] to reduce CO₂ to CO in faradaic efficiencies up to 58% in the presence of trifluoroacetic acid, compared to [Pt] and [Ni] which showed only marginal production of CO, giving the trend [Pd] ≫ [Pt] > [Ni] for this series.

Molecular engineering for efficient and selective iron porphyrin catalysts for electrochemical reduction of CO2 to CO

Iron porphyrins Fe-pE, Fe-mE, and Fe-oE were synthesized and their electrochemical behavior for CO₂ reduction to CO has been investigated. The controlled potential electrolysis of Fe-mE gave exclusive 65% Faradaic efficiency (FE) whereas Fe-oE achieved quasi-quantitative 98% FE (2% H₂) for CO production.

Homogeneously Catalyzed Electroreduction of Carbon Dioxide-Methods, Mechanisms, and Catalysts

The utilization of CO₂ via electrochemical reduction constitutes a promising approach toward production of value-added chemicals or fuels using intermittent renewable energy sources. For this purpose, molecular electrocatalysts are frequently studied and the recent progress both in tuning of the catalytic properties and in mechanistic understanding is truly remarkable. While in earlier years research efforts were focused on complexes with rare metal centers such as Re, Ru, and Pd, the focus has recently shifted toward earth-abundant transition metals such as Mn, Fe, Co, and Ni. By application of appropriate ligands, these metals have been rendered more than competitive for CO₂ reduction compared to the heavier homologues. In addition, the important roles of the second and outer coordination spheres in the catalytic processes have become apparent, and metal-ligand cooperativity has recently become a well-established tool for further tuning of the catalytic behavior. Surprising advances have also been made with very simple organocatalysts, although the mechanisms behind their reactivity are not yet entirely understood. Herein, the developments of the last three decades in electrocatalytic CO₂ reduction with homogeneous catalysts are reviewed. A discussion of the underlying mechanistic principles is included along with a treatment of the experimental and computational techniques for mechanistic studies and catalyst benchmarking. Important catalyst families are discussed in detail with regard to mechanistic aspects, and recent advances in the field are highlighted.

A new triazine-based covalent organic polymer for efficient photodegradation of both acidic and basic dyes under visible light

COP-NT can be used as an efficient photocatalyst for the degradation of methyl orange (MO), rhodamine B (RhB) and methylene blue (MB).

Synthesis and structure of palladium(II) complexes supported by bis-NHC pincer ligands for the electrochemical activation of CO2

A series of bis-NHC pincer complexes of palladium(II) have been prepared and characterized. These pyridyl-spaced dicarbene complexes ([(PDC^R)Pd(MeCN)](PF₆)₂ ) were synthesized with substituents of varying steric bulk at the wingtip positions, which include R = methyl, ethyl, isopropyl, cyclohexyl, mesityl and 2,6-diisopropylphenyl. The synthesis of this library of complexes was accomplished either by direct metallation of the prerequisite pyridyl-spaced bis-imidazolium proligands with Pd(OAc)₂ or via treatment with Ag₂O to afford the corresponding silver carbenes, which were then transmetallated onto palladium. Solid-state structures for each of the [(PDC^R)Pd(MeCN)](PF₆)₂ derivatives were obtained via X-ray crystallography and allowed for the steric properties of each PDC^R ligand to be evaluated by two methods. These analyses, which included calculation of the percent buried volume (%V_Bur) and solid angles of the PDC^R ligands, served to characterize the steric environment around the palladium center in each of the complexes that was prepared. Finally, voltammetry and controlled potential electrolysis studies were performed to characterize the redox chemistry of the [(PDC^R)Pd(MeCN)](PF₆)₂ derivatives and assess if they could electrocatalyze the reduction of CO₂. The influence of the steric properties of the PDC^R ligand on the electrochemistry of the resulting complexes [(PDC^R)Pd(MeCN)](PF₆)₂ is also discussed.

The Influence of para Substituents in Bis(N-Heterocyclic Carbene) Palladium Pincer Complexes for Electrocatalytic CO2 Reduction

The effect of modifying the pyridyl para position of lutidine-linked bis(N-heterocyclic carbene) Pd pincer complexes is studied both experimentally (R = OMe, H, Br, and COOR) and computationally, showing a strong effect on the first reduction potential of the complex and allowing the reduction potential to be tuned over a wide range in relation to the Hammett σ_p constant of the para substituent. The effect of the pyridyl para substituent on electron density of the metal center, frontier orbital energies, and dissociation energy of the trans ligand are also investigated in the context of reactivity with CO₂ through electrochemical characterization of the complexes under N₂ and CO₂ and controlled potential electrolysis experiments where CO₂ is reduced to CO.

Electrocatalytic Reduction of CO2 to CO With Re-Pyridyl-NHCs: Proton Source Influence on Rates and Product Selectivities

A series of four electron-deficient-substituted Re(I) pyridyl N-heterocyclic carbene (pyNHC) complexes have been synthesized, and their electrocatalytic reduction of CO2 has been evaluated by cyclic voltammetry and controlled potential electrolysis experiments. All of the catalysts were evaluated by cyclic voltammetry under inert atmosphere and under CO2 and compared to the known benchmark catalyst Re(bpy)(CO)3Br. Among the four Re-NHC catalysts, Re(pyNHC-PhCF3)(CO)3Br (2) demonstrated the highest catalytic rate (icat/ip)(2) at the first and second reduction events with a value of 4 at the second reduction potential (TOF = 0.8 s(-1)). The rate of catalysis was enhanced through the addition of proton sources (PhOH, TFE, and H2O; TOF up to 100 s(-1); (icat/ip)(2) = 700). Controlled potential electrolysis shows Faradaic efficiencies (FE) for CO production and accumulated charge for the Re(pyNHC-PhCF3)(CO)3Br catalyst exceed those of the benchmark catalyst in the presence of 2 M H2O (92%, 13 C at 1 h versus 61%, 3 C for the benchmark catalyst) under analogous experimental conditions. A peak FE of 100% was observed during electrolysis with Re(pyNHC-PhCF3)(CO)3Br.

Diiridium Bimetallic Complexes Function as a Redox Switch To Directly Split Carbonate into Carbon Monoxide and Oxygen

A pair of diiridium bimetallic complexes exhibit a special type of oxidation-reduction reaction that could directly split carbonate into carbon monoxide and molecular oxygen via a low-energy pathway needing no sacrificial reagent. One of the bimetallic complexes, Ir(III)(μ-Cl)2Ir(III), can catch carbonato group from carbonate and reduce it to CO. The second complex, the rare bimetallic complex Ir(IV)(μ-oxo)2Ir(IV), can react with chlorine to release O2 by the oxidation of oxygen ions with synergistic oxidative effect of iridium ions and chlorine atoms. The activation energy needed for the key reaction is quite low (∼20 kJ/mol), which is far less than the dissociation energy of the C═O bond in CO2 (∼750 kJ/mol). These diiridium bimetallic complexes could be applied as a redox switch to split carbonate or combined with well-known processes in the chemical industry to build up a catalytic system to directly split CO2 into CO and O2.

Polyannulated Bis(N-heterocyclic carbene)palladium Pincer Complexes for Electrocatalytic CO2 Reduction

Phenanthro- and pyreno-annulated N-heterocyclic carbenes (NHCs) have been incorporated into lutidine-linked bis-NHC Pd pincer complexes to investigate the effect of these polyannulated NHCs on the ability of the complexes to electrochemically reduce CO2 to CO in the presence of 2,2,2-trifluoroacetic acid and 2,2,2-trifluoroethanol as proton sources. These complexes are screened for their ability to reduce CO2 and modeled using density functional theory calculations, where the annulated phenanthrene and pyrene moieties are shown to be additional sites for redox activity in the pincer ligand, enabling increased electron donation. Electrochemical and computational studies are used to gain an understanding of the chemical significance of redox events for complexes of this type, highlighting the importance of anion binding and dissociation.

Electrochemical reduction of CO2 to HCOOH using zinc and cobalt oxide as electrocatalysts

HCOOH was produced electrochemically from CO₂ in the presence of Zn and Co₃O₄.

Preparation and structure of NHC Hg(ii) and Ag(i) macrometallocycles

A series of bis-azolium salts and their seven NHC metal (Hg(ii) and Ag(i)) complexes have been prepared and characterized.