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

Cascade Catalytic Systems for Converting CO2 into C2+ Products

The excessive emission and continuous accumulation of CO2 have precipitated serious social and environmental issues. However, CO2 can also serve as an abundant, inexpensive, and non-toxic renewable C1 carbon source for synthetic reactions. To achieve carbon neutrality and recycling, it is crucial to convert CO2 into value-added products through chemical pathways. Multi-carbon (C2+) products, compared to C1 products, offer a broader range of applications and higher economic returns. Despite this, converting CO2 into C2+ products is difficult due to its stability and the high energy required for C-C coupling. Cascade catalytic reactions offer a solution by coordinating active components, promoting intermediate transfers, and facilitating further transformations. This method lowers energy consumption. Recent advancements in cascade catalytic systems have allowed for significant progress in synthesizing C2+ products from CO2. This review highlights the features and advantages of cascade catalysis strategies, explores the synergistic effects among active sites, and examines the mechanisms within these systems. It also outlines future prospects for CO2 cascade catalytic synthesis, offering a framework for efficient CO2 utilization and the development of next-generation catalytic systems.

Heterobimetallic NiFe Complex for Photocatalytic CO2 Reduction: United Efforts of NiFe Dual Sites

Catalytic CO2 reduction poses a significant challenge for the conversion of CO2 into chemicals and fuels. Ni-Fe carbon monoxide dehydrogenase ([NiFe]-CODH) effectively mediates the reversible conversion of CO2 and CO at a nearly thermodynamic equilibrium potential, highlighting the heterobimetallic cooperation for the design of CO2 reduction catalysts. However, numerous NiFe biomimetic model complexes have realized little success in CO2 reduction catalysis, which underscores the crucial role of precise bimetallic configuration and functionality. Herein, we presented a heterobimetallic NiFe complex for the photocatalytic reduction of CO2 to CO, demonstrating significantly enhanced catalytic performance compared to the homonuclear NiNi catalyst. Photocatalytic and mechanistic investigations revealed that with the assistance of a redox-active phenanthroline ligand, NiFe achieves dual-site activation of CO2 through a pivotal intermediate, NiII(μ-CO22--κC:κO)FeII, where the Lewis acidity of the FeII site plays an important role, as corroborated in the homonuclear FeFe system. This study introduces the first heteronuclear NiFe molecular catalyst capable of efficiently catalyzing the reduction of CO2 to CO, deepening insights into heterobimetallic cooperation and offering a novel strategy for designing highly active and selective CO2 reduction catalysts.

Exploring the Impact of Water Content in Solvent Systems on Photochemical CO2 Reduction Catalyzed by Ruthenium Complexes

To achieve artificial photosynthesis, it is crucial to develop a catalytic system for CO2 reduction using water as the electron source. However, photochemical CO2 reduction by homogeneous molecular catalysts has predominantly been conducted in organic solvents. This study investigates the impact of water content on catalytic activity in photochemical CO2 reduction in N,N-dimethylacetamide (DMA), using [Ru(bpy)3]2+ (bpy: 2,2'-bipyridine) as a photosensitizer, 1-benzyl-1,4-dihydronicotinamide (BNAH) as an electron donor, and two ruthenium diimine carbonyl complexes, [Ru(bpy)2(CO)2]2+ and trans(Cl)-[Ru(Ac-5Bpy-NHMe)(CO)2Cl2] (5Bpy: 5'-amino-2,2'-bipyridine-5-carboxylic acid), as catalysts. Increasing water content significantly decreased CO and formic acid production. The similar rates of decrease for both catalysts suggest that water primarily affects the formation efficiency of free one-electron-reduced [Ru(bpy)3]2+, rather than the intrinsic catalytic activity. The reduction in cage-escape efficiency with higher water content underscores the challenges in replacing organic solvents with water in photochemical CO2 reduction.

Dicarbonyl[10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene]ruthenium(II): discovery of the first ruthenium tetrapyrrole cis-dicarbonyl complex by X-ray and electron diffraction

Dicarbonyl[10,10-dimethyl-5,15-bis(pentafluorophenyl)biladiene]ruthenium(II), [Ru(C33H16F10N4)(CO)2] or Ru(CO)2[DMBil1], is the first reported ruthenium(II) cis-dicarbonyl tetrapyrrole complex. The neutral complex sports two carbonyls and an oligotetrapyrrolic biladiene ligand. Notably, the biladiene adopts a coordination geometry that is well distorted from square planar and much more closely approximates a seesaw arrangement. Accordingly, Ru(CO)2[DMBil1] is not only the first ruthenium cis-dicarbonyl with a tetrapyrrole ligand, but also the first metal biladiene complex in which the tetrapyrrole does not adopt a (pseudo-)square-planar coordination geometry. Ru(CO)2[DMBil1] is weakly luminescent, displaying λem = 552 nm upon excitation at λex = 500 nm, supports two reversible 1 e- reductions at -1.45 and -1.73 V (versus Fc+/Fc), and has significant absorption features at 481 and 531 nm, suggesting suitability for photocatalytic and photosensitization applications. While the structure of Ru(CO)2[DMBil1] was initially determined by X-ray diffraction, a traditionally acceptable quality structure could not be obtained (despite multiple attempts) because of consistently poor crystal quality. An independent structure obtained from electron diffraction experiments corroborates the structure of this unusual biladiene complex.

Oxidative and Reductive Manipulation of C1 Resources by Bio-Inspired Molecular Catalysts to Produce Value-Added Chemicals

ConspectusTo tackle the energy and environmental concerns the world faces, much attention is given to catalytic reactions converting methane (CH4) and carbon dioxide (CO2) as abundant C1 resources into value-added chemicals with high efficiency and selectivity. In the oxidative conversion of CH4 to methanol, it is necessary to solve the requirement of strong oxidants due to the large bond-dissociation energy (BDE) of the C-H bonds in methane and achieve suppression of overoxidation due to the smaller BDE of the C-H bond in methanol as the product. On the other hand, to efficiently perform CO2 reduction, proton-coupled electron transfer (PCET) processes are required since the reduction potential of CO2 becomes positive by using proton-coupled processes; however, under the acidic conditions required for PCET, hydrogen evolution by the reduction of protons becomes competitive with CO2 reduction. Thus, it is indispensable to develop efficient catalysts for selective CO2 reduction. Recently, we have developed efficient catalytic reactions toward the alleviation of the concerns mentioned above. Concerning CH4 oxidation, inspired by metalloenzymes that oxidize hydrophobic organic substrates, a hydrophobic second coordination sphere (SCS) was introduced to an FeII complex bearing a pentadentate N-heterocyclic carbene ligand, and the FeII complex was used as a catalyst for CH4 oxidation in aqueous media. Consequently, CH4 was efficiently and selectively oxidized to methanol with 83% selectivity and a turnover number of 500. In contrast, when methanol was used as a substrate for catalytic oxidation by the FeII complex, oxidation products were obtained in a negligible yield, which was comparable to that of the control experiment without the catalyst. Therefore, the hydrophobic SCS of the FeII complex can capture only hydrophobic substrates such as CH4 and release hydrophilic products such as methanol to the aqueous medium for suppressing overoxidation ("catch-and-release" mechanism). On the other hand, for photocatalytic CO2 reduction, we have developed NiII complexes with N2S2-chelating ligands as catalysts, which have been inspired by carbon monoxide dehydrogenase, and have also introduced a binding site of Lewis-acidic metal ions to the SCS of the Ni complex. When Mg2+ was applied as a moderate Lewis acid, a Mg2+-bound Ni catalyst allowed us to achieve remarkable enhancement of the photocatalytic CO2 reduction to afford CO as the product with over 99% selectivity and a quantum yield of 11.4%. Divalent metal ions besides Mg2+ also showed similar positive impacts on photocatalytic CO2 reduction, whereas monovalent metal ions exhibited almost no effects and trivalent metal ions exclusively promoted hydrogen evolution. In this Account, we highlight our recent progress in the catalytic manipulations of CH4 and CO2 as C1 resources.

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Photocatalytic Conversion of CO2 to Formate/CO by an (η6-para-Cymene)Ru(II) Half-Metallocene Catalyst: Influence of Additives and TiO2 Immobilization on the Catalytic Mechanism and Product Selectivity

The catalytic efficacy of the monobipyridyl (η6-para-Cymene)Ru(II) half-metallocene, [(p-Cym)Ru(bpy)Cl]+ was evaluated in both mixed homogeneous (dye + catalyst) and heterogeneous hybrid systems (dye/TiO2/Catalyst) for photochemical CO2 reduction. A series of homogeneous photolysis experiments revealed that the (p-Cym)Ru(II) catalyst engages in two competitive routes for CO2 reduction (CO2 to formate conversion via RuII-hydride vs CO2 to CO conversion through a RuII-COOH intermediate). The conversion activity and product selectivity were notably impacted by the pKa value and the concentration of the proton source added. When a more acidic TEOA additive was introduced, the half-metallocene Ru(II) catalyst leaned toward producing formate through the RuII-H mechanism, with a formate selectivity of 86%. On the other hand, in homogeneous catalysis with TFE additive, the CO2-to-formate conversion through RuII-H was less effective, yielding a more efficient CO2-to-CO conversion with a selectivity of >80% (TONformate of 140 and TONCO of 626 over 48 h). The preference between the two pathways was elucidated through an electrochemical mechanistic study, monitoring the fate of the metal-hydride intermediate. Compared to the homogeneous system, the TiO2-heterogenized (p-Cym)Ru(II) catalyst demonstrated enhanced and enduring performance, attaining TONs of 1000 for CO2-to-CO and 665 for CO2-to-formate.

Electrocatalytic Reduction of CO2 to CO by Molecular Cobalt-Polypyridine Diamine Complexes

Cobalt complexes have previously been reported to exhibit high faradaic efficiency in reducing CO2 to CO. Herein, we synthesized capsule-like cobalt-polypyridine diamine complexes [Co(L1)](BF4)2 (1) and [Co(L2) (CH3CN)](BF4)2 (2) as catalysts for the electrocatalytic reduction of CO2. Under catalytic conditions, complexes 1 and 2 demonstrated the electrocatalytic reduction of CO2 to CO in the presence or absence of CH3OH as a proton source. Experimental and computational studies revealed that complexes 1 and 2 undergo two consecutive reversible one-electron reductions on the cobalt core, followed by the addition of CO2 to form a metallocarboxylate intermediate [CoII(L)-CO22-]0. This crucial reaction intermediate, which governs the catalytic cycle, was successfully detected using high resolution mass spectrometry (HRMS). In situ Fourier-transform infrared spectrometer (FTIR) analysis showed that methanol can enhance the rate of carbon-oxygen bond cleavage of the metallocarboxylate intermediate. DFT studies on [CoII(L)-CO22-]0 have suggested that the doubly reduced species attacks CO2 on the C atom through the dz2 orbital, while the interaction with CO2 is further stabilized by the π interaction between the metal dxz or dxz orbital with p orbitals on the O atoms. Further reductions generate a metal carbonyl intermediate [CoI(L)-CO]+, which ultimately releases CO.

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.

How to rationally design homogeneous catalysts for efficient CO2 electroreduction?

Electrified converting CO2 into valuable fuels and chemicals using a homogeneous electrochemical CO2 reduction (CO2ER) approach simplifies the operation, providing a potential option for decoupling energy harvesting and renewable chemical production. These merits benefit the scenarios where decentralization and intermittent power are key factors. This perspective aims to provide an overview of recent progress in homogeneous CO2ER. We introduce firstly the fundamentals chemistry of the homogeneous CO2ER, followed by a summary of the crucial factors and the important criteria broadly employed for evaluating the performance. We then highlight the recent advances in the most widely explored transition-metal coordinate complexes for the C1 and multicarbon (C2+) products from homogeneous CO2ER. Finally, we summarize the remaining challenges and opportunities for developing homogeneous electrocatalysts for efficient CO2ER. This perspective is expected to favor the rational design of efficient homogeneous electrocatalysts for selective CO2ER toward renewable fuels and feedstocks.

Heterogenized Molecular Electrocatalyst Based on a Hydroxo-Bridged Binuclear Copper(II) Phenanthroline Compound for Selective Reduction of CO2 to Ethylene

Molecular copper catalysts have emerged as promising candidates for the electrochemical reduction of CO2 . Notable features of such systems include the ability of Cu to generate C2+  products and the well-defined active sites that allow for targeted structural tuning. However, the frequently observed in situ formation of Cu nanoclusters has undermined the advantages of the molecular frameworks. It is therefore desirable to develop Cu-based catalysts that retain their molecular structures during electrolysis. In this context, a heterogenized binuclear hydroxo-bridged phenanthroline Cu(II) compound with a short Cu···Cu distance is reported as a simple yet efficient catalyst for electrogeneration of ethylene and other C2 products. In an aqueous electrolyte, the catalyst demonstrates remarkable performance, with excellent Faradaic efficiency for C2 products (62%) and minimal H2 evolution (8%). Furthermore, it exhibits high stability, manifested by no observable degradation during 15 h of continuous electrolysis. The preservation of the atomic distribution of the active sites throughout electrolysis is substantiated through comprehensive characterizations, including X-ray photoelectron and absorption spectroscopy, scanning and transmission electron microscopy, UV-vis spectroscopy, as well as control experiments. These findings establish a solid foundation for further investigations into targeted structural tuning, opening new avenues for enhancing the catalytic performance of Cu-based molecular electrocatalysts.

Schottky Junction and D-A1 -A2 System Dual Regulation of Covalent Triazine Frameworks for Highly Efficient CO2 Photoreduction

Covalent triazine frameworks (CTFs) are emerging as a promising molecular platform for photocatalysis. Nevertheless, the construction of highly effective charge transfer pathways in CTFs for oriented delivery of photoexcited electrons to enhance photocatalytic performance remains highly challenging. Herein, a molecular engineering strategy is presented to achieve highly efficient charge separation and transport in both the lateral and vertical directions for solar-to-formate conversion. Specifically, a large π-delocalized and π-stacked Schottky junction (Ru-Th-CTF/RGO) that synergistically knits a rebuilt extended π-delocalized network of the D-A1 -A2 system (multiple donor or acceptor units, Ru-Th-CTF) with reduced graphene oxide (RGO) is developed. It is verified that the single-site Ru units in Ru-Th-CTF/RGO act as effective secondary electron acceptors in the lateral direction for multistage charge separation/transport. Simultaneously, the π-stacked and covalently bonded graphene is regarded as a hole extraction layer, accelerating the separation/transport of the photogenerated charges in the vertical direction over the Ru-Th-CTF/RGO Schottky junction with full use of photogenerated electrons for the reduction reaction. Thus, the obtained photocatalyst has an excellent CO2 -to-formate conversion rate (≈11050 µmol g-1 h-1 ) and selectivity (≈99%), producing a state-of-the-art catalyst for the heterogeneous conversion of CO2 to formate without an extra photosensitizer.

Electrocatalytic Interconversions of CO2 and Formate on a Versatile Iron-Thiolate Platform

Exploring bidirectional CO2/HCO2- catalysis holds significant potential in constructing integrated (photo)electrochemical formate fuel cells for energy storage and applications. Herein, we report selective CO2/HCO2- electrochemical interconversion by exploiting the flexible coordination modes and rich redox properties of a versatile iron-thiolate platform, Cp*Fe(II)L (L = 1,2-Ph2PC6H4S-). Upon oxidation, this iron complex undergoes formate binding to generate a diferric formate complex, [(L-)2Fe(III)(μ-HCO2)Fe(III)]+, which exhibits remarkable electrocatalytic performance for the HCO2--to-CO2 transformation with a maximum turnover frequency (TOFmax) ∼103 s-1 and a Faraday efficiency (FE) ∼92(±4)%. Conversely, this iron system also allows for reduction at -1.85 V (vs Fc+/0) and exhibits an impressive FE ∼93 (±3)% for the CO2-to-HCO2- conversion. Mechanism studies revealed that the HCO2--to-CO2 electrocatalysis passes through dicationic [(L2)-•Fe(III)(μ-HCO2)Fe(III)]2+ generated by unconventional oxidation of the diferric formate species taking place at ligand L, while the CO2-to-HCO2- reduction involves a critical intermediate of [Fe(II)-H]- that was independently synthesized and structurally characterized.

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.

1,2-Azolylamidino ruthenium(II) complexes with DMSO ligands: electro- and photocatalysts for CO2 reduction

New 1,2-azolylamidino complexes fac-[RuCl(DMSO)3(NHC(R)az*-κ2N,N)]OTf [R = Me (2), Ph (3); az* = pz (pyrazolyl, a), indz (indazolyl, b)] are synthesized via chloride abstraction from their corresponding precursors cis,fac-[RuCl2(DMSO)3(az*H)] (1) after subsequent base-catalyzed coupling of the appropriate nitrile with the 1,2-azole previously coordinated. All the compounds are characterized by 1H NMR, 13C NMR and IR spectroscopy. Those derived from MeCN are also characterized by X-ray diffraction. Electrochemical studies showed several reduction waves in the range of -1.5 to -3 V. The electrochemical behavior in CO2 media is consistent with CO2 electrocatalytic reduction. The catalytic activity expressed as [icat(CO2)/ip(Ar)] ranged from 1.7 to 3.7 for the 1,2-azolylamidino complexes at voltages of ca. -2.7 to -3 V vs. ferrocene/ferrocenium. Controlled potential electrolysis showed rapid decomposition of the Ru catalysts. Photocatalytic CO2 reduction experiments using compounds 1b, 2b and 3b carried out in a CO2-saturated MeCN/TEOA (4 : 1 v/v) solution containing a mixture of the catalyst and [Ru(bipy)3]2+ as the photosensitizer under continuous irradiation (light intensity of 150 mW cm-2 at 25 °C, λ > 300 nm) show that compounds 1b, 2b and 3b allowed CO2 reduction catalysis, producing CO and trace amounts of formate. The combined turnover number for the production of formate and CO is ca. 100 after 8 h and follows the order 1b < 2b ≈ 3b.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Electrocatalytic properties of a novel ruthenium(II) terpyridine-based complex towards CO2 reduction

The electrocatalytic properties of Ru complexes are of great technological interest given their potential application in reactions such water splitting and CO₂ reduction. In this work, a novel terpyridine-based Ru(II) complex, [RuCl(trpy)(acpy)], trpy = 2,2':6',2''-terpyridine, acpy^- = 2-pyridylacetate was synthesized and its spectroscopic, electrochemical and catalytic properties were explored in detail. In dry acetonitrile, the complex exhibits two reduction peaks at -1.95 V and -2.20 V vs. Fc/Fc⁺, attributed to consecutive 1 e^- reduction. Under CO₂ atmosphere, a catalytic wave is observed (E_onset = 2.1 V vs. Fc/Fc⁺), with CO as the main reduction product. Bulk electrolysis reveals a turnover number (TON) of 12 (k_obs = 1.5 s^-1). In the presence of 1% water, an improvement in the catalytic activity is observed (TON_CO = 21 and k_obs = 2.0 s^-1) and, additionally, formate was also detected (TON_HCOO = 7). Spectroelectrochemical experiments allowed the identification of a metallocarboxylate (Ru-COO^-) intermediate under anhydrous conditions, while in water, the partial labilization of the acpy^- ligand was observed in the course of the catalytic cycle. The experimental data was combined with DFT calculations, allowing the proposal of a catalytic cycle. The results establish important relationships between selectivity, ligand structure and reaction conditions.

Mechanistic insights into CO2 conversion chemistry of copper bis-(terpyridine) molecular electrocatalyst using accessible operando spectrochemistry

The implementation of low-cost transition-metal complexes in CO₂ reduction reaction (CO₂RR) is hampered by poor mechanistic understanding. Herein, a carbon-supported copper bis-(terpyridine) complex enabling facile kilogram-scale production of the catalyst is developed. We directly observe an intriguing baton-relay-like mechanism of active sites transfer by employing a widely accessible operando Raman/Fourier-transform infrared spectroscopy analysis coupled with density functional theory computations. Our analyses reveal that the first protonation step involves Cu-N bond breakage before the *COOH intermediate forms exclusively at the central N site, followed by an N-to-Cu active site transfer. This unique active site transfer features energetically favorable *CO formation on Cu sites, low-barrier CO desorption and reversible catalyst regeneration, endowing the catalyst with a CO selectively of 99.5 %, 80 h stability, and a turn-over efficiency of 9.4 s⁻¹ at −0.6 V vs. the reversible hydrogen electrode in an H-type cell configuration. We expect that the approach and findings presented here may accelerate future mechanistic studies of next-generation CO₂RR electrocatalysts.

Transition metal complexes are potential low-cost electrocatalysts for CO₂ reduction. Here, using accessible spectrochemistry, the authors observe an intriguing baton relay-like mechanism of active site transfer during CO₂ reduction to CO on a carbon-supported copper bis-(terpyridine) complex.

Experimental and computational investigation on the formation pathway of [RuCl2(CO)2(ERR')2] (E = S, Se, Te; R, R' = Me, Ph) from [RuCl2(CO)3]2 and ERR'

The pathways to the formation of the series of [RuCl₂(CO)₂(ERR')₂] (E = S, Se, Te; R, R' = Me, Ph) complexes from [RuCl₂(CO)₃]₂ and ERR' have been explored experimentally in THF and CH₂Cl₂, and computationally by PBE0-D3/def2-TZVP calculations. The end-products and some reaction intermediates have been isolated and identified by NMR spectroscopy, and their crystal structures have been determined by X-ray diffraction. The relative stabilities of the [RuCl₂(CO)₂(ERR')₂] isomers follow the order cct > ccc > tcc > ttt ≈ ctc (the terms c/t refer to cis/trans arrangement of the ligands in the order of Cl, CO, and ERR'). The yields were rather similar in both solvents, but the reactions were significantly faster in THF than in CH₂Cl₂. The highest yields were observed for the telluroether complexes, and the yields decreased with lighter chalcogenoethers. PBE0-D3/def2-TZVP calculations indicated that the reaction path is independent of the nature of the solvent. The substitution of one CO ligand of the intermediate [RuCl₂(CO)₃(ERR')] by the second ERR' shows the highest activation barrier and is the rate-determining step in all reactions. The observed faster reaction rate in THF than in CH₂Cl₂ upon reflux can therefore be explained by the higher boiling point of THF. At room temperature the reactions in both solvents proceed equally slowly. When the reaction is carried out in THF, the formation of [RuCl₂(CO)₃(THF)] is also observed, and the reaction may proceed with the substitution of THF by ERR'. The formation of the THF complex, however, is not necessary for the dissociation of the [RuCl₂(CO)₃]₂. Thermal energy at room temperature is sufficient to cleave one of the bridging Ru-Cl bonds. The intermediate thus formed undergoes a facile reaction with ERR'. This mechanism is viable also in non-coordinating CH₂Cl₂.

Molecular Catalysts for the Reductive Homocoupling of CO2 towards C2+ Compounds

The conversion of CO2 into multicarbon (C2+ ) compounds by reductive homocoupling offers the possibility to transform renewable energy into chemical energy carriers and thereby create "carbon-neutral" fuels or other valuable products. Most available studies have employed heterogeneous metallic catalysts, but the use of molecular catalysts is still underexplored. However, several studies have already demonstrated the great potential of the molecular approach, namely, the possibility to gain a deep mechanistic understanding and a more precise control of the product selectivity. This Minireview summarizes recent progress in both the thermo- and electrochemical reductive homocoupling of CO2 toward C2+ products mediated by molecular catalysts. In addition, reductive CO homocoupling is discussed as a model for the further conversion of intermediates obtained from CO2 reduction, which may serve as a source of inspiration for developing novel molecular catalysts in the future.

Electrocatalytic CO2 Reduction by Molecular Ruthenium Complexes with Polypyridyl Ligands

Two series of ruthenium complexes with various polypyridyl ligands have been prepared. One series of complexes (5 examples) are featured with tetradentate polypyridyl ligands and two acetonitrile molecules at the axial positions of the coordination sphere; the other series (3 examples) include combinations of a tridentate polypyridyl ligand, one 2,2'-bipyridine (bpy) or two picolines, and one acetonitrile ligand. All these complexes were fully characterized by their NMR spectra as well as X-ray single crystal structures. Their electronic absorption and redox data were measured and reported. Of the 8 complexes, three candidates effectively catalyze electrochemical CO 2 reduction reaction (CO 2 RR) in wet acetonitrile medium, generating CO as the major product. All these three catalytically active complexes contain a 2,2':6',2″:6″,2‴-quaterpyridine (qpy) ligand scaffold. A maximum turnover frequency (TOF max ) of > 1000 s -1 was achieved for the electrocatalytic CO 2 reduction at a modest overpotential. On the basis of electrochemical and spectroelectrochemical evidences, the CO 2 substrate was proposed to bind with the ruthenium center at the two-electron reduced state of the complex and then enter the catalytic cycle.

HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes

Molybdenum(0) complexes with aliphatic aminophosphine pincer ligands have been prepared which are competent for the disproportionation of formic acid, thus representing the first example so far reported of non-noble metal species to catalytically promote such transformation. In general, formic acid disproportionation allows for an alternative access to methyl formate and methanol from renewable resources. MeOH selectivity up to 30% with a TON of 57 could be achieved while operating at atmospheric pressure. Selectivity (37%) and catalyst performance (TON = 69) could be further enhanced when the reaction was performed under hydrogen pressure (60 bars). A plausible mechanism based on experimental evidence is proposed.

Mo(0) complexes with aliphatic PNP-pincer ligands enable the first example of non-noble metal catalyzed formic acid disproportionation leading to methanol with a selectivity of up to 37% and a turnover number up to 69.

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation

The development of systems for photocatalytic CO2 reduction with water as a reductant and solar light as an energy source is one of the most important milestones on the way to artificial photosynthesis. Although such reduction can be performed using dye-sensitized molecular photocathodes comprising metal complexes as redox photosensitizers and catalyst units fixed on a p-type semiconductor electrode, the performance of the corresponding photoelectrochemical cells remains low, e.g., their highest incident photon-to-current conversion efficiency (IPCE) equals 1.2%. Herein, we report a novel dye-sensitized molecular photocathode for photocatalytic CO2 reduction in water featuring a polypyrrole layer, [Ru(diimine)3]2+ as a redox photosensitizer unit, and Ru(diimine)(CO)2Cl2 as the catalyst unit and reveal that the incorporation of the polypyrrole network significantly improves reactivity and durability relative to those of previously reported dye-sensitized molecular photocathodes. The irradiation of the novel photocathode with visible light under low applied bias stably induces the photocatalytic reduction of CO2 to CO and HCOOH with high faradaic efficiency and selectivity (even in aqueous solution), and the highest IPCE is determined as 4.7%. The novel photocathode is coupled with n-type semiconductor photoanodes (CoO x /BiVO4 and RhO x /TaON) to construct full cells that photocatalytically reduce CO2 using water as the reductant upon visible light irradiation as the only energy input at zero bias. The artificial Z-scheme photoelectrochemical cell with the dye-sensitized molecular photocathode achieves the highest energy conversion efficiency of 8.3 × 10-2% under the irradiation of both electrodes with visible light, while a solar to chemical conversion efficiency of 4.2 × 10-2% is achieved for a tandem-type cell using a solar light simulator (AM 1.5, 100 mW cm-2).

Enhancement of electrocatalytic abilities toward CO2 reduction by tethering redox-active metal complexes to the active site

Tethering metal complexes, like [Ru(bpy)₂Cl₂] (bpy = 2,2'-bipyridine), which are redox-active at low reduction potentials and have the ability to transfer electrons to another complex, to a [Ni(cyclen)]²⁺ electrocatalyst enhanced the reduction of CO₂ to CO at low overpotentials. The [Ni(cyclen)]²⁺ electrocatalyst was modified by tethering redox-active metal complexes via 4-methylpyridyl linkers. The redox-active metal complexes were reduced after CO₂ bound to the active site. In controlled potential electrolysis (CPE) experiments in 95 : 5 (v/v) CH₃CN/H₂O, [{([Ru]pic)₄cyclen}NiCl]⁵⁺ ([Ru]⁺ = {Ru(bpy)₂Cl}⁺; pic = 4-methylpyridyl) could be used to reduce CO₂ into CO at a turnover frequency (TOF) of 708 s^-1 with a faradaic efficiency (FE) of 80% at an onset potential of -1.60 V vs. NHE. At the same time, this electrocatalyst was active at an onset potential of -1.25 V vs. NHE, which is the reduction potential of one of the bpy ligands of the [Ru]⁺ moieties, with FE = 84% and TOF = 178 s^-1. When the electrocatalysis was performed using [bn₄cyclenNiCl]Cl (bn = benzyl) without tethered redox-active metal complexes, the TOF value was determined to be 8 s^-1 with FE = 77% at an onset potential of -1.45 V vs. NHE. The results show that tethering redox-active metal complexes significantly improves the electrocatalytic activities by lowering the potential needed to reduce CO₂.

Secondary Coordination Effect on Monobipyridyl Ru(II) Catalysts in Photochemical CO2 Reduction: Effective Proton Shuttle of Pendant Brønsted Acid/Base Sites (OH and N(CH3)2) and Its Mechanistic Investigation

While the incorporation of pendant Brønsted acid/base sites in the secondary coordination sphere is a promising and effective strategy to increase the catalytic performance and product selectivity in organometallic catalysis for CO₂ reduction, the control of product selectivity still faces a great challenge. Herein, we report two new trans(Cl)-[Ru(6-X-bpy)(CO)₂Cl₂] complexes functionalized with a saturated ethylene-linked functional group (bpy = 2,2'-bipyridine; X = -(CH₂)₂-OH or -(CH₂)₂-N(CH₃)₂) at the ortho(6)-position of bpy ligand, which are named Ru-bpy^OH and Ru-bpy^diMeN, respectively. In the series of photolysis experiments, compared to nontethered case, the asymmetric attachment of tethering ligand to the bpy ligand led to less efficient but more selective formate production with inactivation of CO₂-to-CO conversion route during photoreaction. From a series of in situ FTIR analyses, it was found that the Ru-formate intermediates are stabilized by a highly probable hydrogen bonding between pendent proton donors (-diMeN⁺H or -OH) and the oxygen atom of metal-bound formate (Ru^I-OCHO···H-E-(CH₂)₂-, E = O or diMeN⁺). Under such conformation, the liberation of formate from the stabilized Ru^I-formate becomes less efficient compared to the nontethered case, consequently lowering the CO₂-to-formate conversion activities during photoreaction. At the same time, such stabilization of Ru-formate species prevents the dehydration reaction route (η¹-OCHO → η¹-COOH on Ru metal) which leads toward the generation of Ru-CO species (key intermediate for CO production), eventually leading to the reduction of CO₂-to-CO conversion activity.

6,6'-Ditriphenylamine-2,2'-bipyridine: Coordination Chemistry and Electrochemical and Photophysical Properties

A 2,2'-bipyridine with bulky triphenylamine substituents in the 6 and 6' positions of the ligand (6,6'-ditriphenylamine-2,2'-bipyridine, 6,6'-diTPAbpy) was generated. Despite the steric bulk, the ligand readily formed bis(homoleptic) complexes with copper(I) and silver(I) ions. Unfortunately, efforts to use the 6,6'-diTPAbpy system to generate heteroleptic [Cu(6,6'-diTPAbpy)(bpy)]⁺ complexes were unsuccessful with only the [Cu(6,6'-diTPAbpy)₂](PF₆) complex observed. The 6,6'-diTPAbpy ligand could also be reacted with 6-coordinate metal ions that featured small ancillary ligands, namely, the [Re(CO)₃Cl] and [Ru(CO)₂Cl₂] fragments. While the complexes could be formed in good yields, the steric bulk of the TPA units does alter the coordination geometry. This is most readily seen in the [(6,6'-diTPAbpy)Re(CO)₃Cl] complex where the Re(I) ion is forced to sit 23° out of the plane formed by the bpy unit. The electrochemical and photophysical properties of the family of compounds were also examined. 6,6'-diTPAbpy exhibits a strong ILCT absorption band (356 nm, 50 mM^-1 cm^-1) which displays a small increase in intensity for the homoleptic complexes ([Cu(6,6'-diTPAbpy)₂]⁺; 353 nm, 72 mM^-1 cm^-1, [Ag(6,6'-diTPAbpy)₂]⁺; 353 nm, 75 mM^-1 cm^-1), despite containing 2 equiv of the ligand, attributed to an increased dihedral angle between the TPA and bpy moieties. For the 6-coordinate complexes the ILCT band is further decreased in intensity and overlaps with MLCT bands, consistent with a further increased TPA-bpy dihedral angle. Emission from the ¹ILCT state is observed at 436 nm (τ = 4.4 ns) for 6,6'-diTPAbpy and does not shift for the Cu, Ag, and Re complexes, although an additional ³MLCT emission is observed for [Re(6,6'-diTPAbpy)(CO)₃Cl] (640 nm, τ = 13.8 ns). No emission was observed for [Ru(6,6'-diTPAbpy)(CO)₂Cl₂]. Transient absorption measurements revealed the population of a ³ILCT state for the Cu and Ag complexes (τ = 80 ns). All assignments were supported by TD-DFT calculations and resonance Raman spectroscopic measurements.

A Hybrid Ru(II)/TiO2 Catalyst for Steadfast Photocatalytic CO2 to CO/Formate Conversion Following a Molecular Catalytic Route

Herein, we employed a molecular Ru(II) catalyst immobilized onto TiO₂ particulates of (4,4'-Y₂-bpy)Ru^II(CO)₂Cl₂ (RuP; Y = CH₂PO(OH)₂), as a hybrid catalyst system to secure the efficient and steady catalytic activity of a molecular bipyridyl Ru(II)-complex-based photocatalytic system for CO₂ reduction. From a series of operando FTIR spectrochemical analyses, it was found that the TiO₂-fixed molecular Ru(II) complex leads to efficient stabilization of the key monomeric intermediate, Ru^II-hydride (LRu^II(H)(CO)₂Cl), and suppresses the formation of polymeric Ru(II) complex (-(L(CO)₂Ru-Ru(CO)₂L)_n-), which is a major deactivation product produced during photoreaction via the Ru-Ru dimeric route. Active promotion of the monomeric catalytic route in a hetero-binary system (IrPS + TiO₂/RuP) that uses TiO₂-bound Ru(II) complex as reduction catalyst led to highly increased activity as well as durability of photocatalytic behavior with respect to the homogeneous catalysis of free Ru(II) catalyst (IrPS + Ru(II) catalyst). This catalytic strategy produced maximal turnover numbers (TONs) of >4816 and >2228, respectively, for CO and HCOO^- production in CO₂-saturated N,N-dimethylformamide (DMF)/TEOA (16.7 vol % TEOA) solution containing a 0.1 M sacrificial electron donor.

Modulation of Self-Assembly Enhances the Catalytic Activity of Iron Porphyrin for CO2 Reduction

Electrochemical reduction of CO₂ in aqueous media is an important reaction to produce value-added carbon products in an environmentally and economically friendly manner. Various molecule-based catalytic systems for the reaction have been reported thus far. The key features of state-of-the-art catalytic systems in this field can be summarized as follows: 1) an iron-porphyrin-based scaffold as a catalytic center, 2) a dinuclear active center for the efficient activation of a CO₂ molecule, and 3) a hydrophobic channel for the accumulation of CO₂ . This article reports a novel approach to construct a catalytic system for CO₂ reduction with the aforementioned three key substructures. The self-assembly of a newly designed iron-porphyrin complex bearing bulky substituents with noncovalent interaction ability forms a highly ordered crystalline solid with adjacent catalytically active sites and hydrophobic pores. The obtained crystalline solid serves as an electrocatalyst for CO₂ reduction in aqueous media. Note that a relevant iron-porphyrin complex without bulky substituents cannot form a porous structure with adjacent active sites, and the catalytic performance of the crystals of this relevant iron-porphyrin complex is substantially lower than that of the newly developed catalytic system. The present study provides a novel strategy for constructing porous crystalline solids for small-molecule conversions.

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.

Dirhodium complexes as electrocatalysts for CO2 reduction to HCOOH: role of steric hindrance on selectivity

A series of Rh₂(ii,ii) complexes were shown to electrocatalytically reduce CO₂ to HCOOH. Electrochemical and spectroelectrochemical studies reveal a correlation between catalytic selectivity and efficiency with the steric bulk at the axial sites afforded by the bridging ligands. Mechanistic studies point to the presence of a Rh₂(ii,i)-H hydride as a key intermediate in the catalytic cycle.

An amide-based second coordination sphere promotes the dimer pathway of Mn-catalyzed CO2-to-CO reduction at low overpotential

The [fac-Mn(bpy)(CO)3Br] complex is capable of catalyzing the electrochemical reduction of CO2 to CO with high selectivity, moderate activity and large overpotential. Several attempts have been made to lower the overpotential and to enhance the catalytic activity of this complex by manipulating the second-coordination sphere of manganese and using relatively stronger acids to promote the protonation-first pathway. We report herein that the complex [fac-Mn(bpy-CONHMe)(CO)3(MeCN)]+ ([1-MeCN]+; bpy-CONHMe = N-methyl-(2,2'-bipyridine)-6-carboxamide) as a pre-catalyst could catalyze the electrochemical reduction of CO2 to CO with low overpotential and high activity and selectivity. Combined experimental and computational studies reveal that the amide NH group not only decreases the overpotential of the Mn catalyst by promoting the dimer and protonation-first pathways in the presence of H2O but also enhances the CO2 electroreduction activity by facilitating C-OH bond cleavage, making [1-MeCN]+ an efficient CO2 reduction pre-catalyst at low overpotential.

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.

Electroreduction of CO2 to Formate with Low Overpotential using Cobalt Pyridine Thiolate Complexes

Electrocatalytic CO₂ reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Carrying out these transformations at reasonable energetic costs, for example, with low overpotential, remains a challenge. Molecular catalysts allow fine control of activity and selectivity via tuning of their coordination sphere and ligand set. Herein we investigate a series of cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO₂ reduction. The effect of the ligands and proton sources on activity was examined. We identified bipyridine bis(2-pyridinethiolato) cobalt(III) hexaflurophosphate as a highly selective catalyst for formate production operating at a low overpotential of 110 mV with a turnover frequency (TOF) of 10 s^-1 . Electrokinetic analysis coupled with density functional theory (DFT) computations established the mechanistic pathway, highlighting the role of metal hydride intermediates. The catalysts deactivate via the formation of stable cobalt carbonyl complexes, but the active species could be regenerated upon oxidation and release of coordinated CO ligands.

Highly Active Manganese-Based CO2 Reduction Catalysts with Bulky NHC Ligands: A Mechanistic Study

Because of the strong σ-donor and weak π-acceptor of the N-heterocyclic carbene (NHC), Mn-NHC complexes were found to be active for the reduction of CO₂ to CO with high activity. However, some NHC-based manganese complexes showed low catalytic activity and required very negative potentials. We report herein that complex fac-[Mn^I(bis-^MesNHC)(CO)₃Br] [1; bis-^MesNHC = 3,3-bis(2,4,6-trimethylphenyl)-(1,1'-diimidazolin-2,2'-diylidene)methane] could catalyze the electrochemical reduction of CO₂ to CO with high activity (TOF_max = 3180 ± 6 s^-1) at a less negative potential. Due to the introduction of the bulky Mes groups, a one-electron-reduced intermediate {[Mn⁰(bis-^MesNHC)(CO)₃]⁰ (2^•)} was isolated as a packed "dimer" and crystallographically characterized. Stopped-flow Fourier-transform infrared spectroscopy was used to prove the direct reaction between doubly reduced intermediate fac-[Mn(bis-^MesNHC)(CO)₃]^- and CO₂; the tetracarbonyl Mn complex [Mn⁺(bis-^MesNHC)(CO)₄]⁺ ([2-CO]⁺) was captured, and its further reduction proposed as the rate-limiting step.

Electrochemical CO2 Reduction in a Continuous Non-Aqueous Flow Cell with [Ni(cyclam)]2

A significant number of molecular catalysts have been developed for electrochemical CO₂ reduction with high efficiency and selectivity; however, testing of these electrocatalysts in an application-ready system is lacking. Here, we present an example of a nonaqueous flow cell electrolyzer with [Ni(cyclam)]²⁺ as the homogeneous electrocatalyst for CO₂ reduction. Using ferrocene as a sacrificial electron donor and ammonium salts as both electrolyte and proton donor, efficient catalytic CO₂ reduction is achieved. The nonaqueous design shows high selectivity for the reduction of CO₂ to CO (>80%) and achieves high current densities with a graphite felt working electrode (up to 50 mA·cm^-2 with 0.5 M proton donor in MeCN solution), producing >40 mL·h^-1 of CO. The choice of a molecular electrocatalyst, solvent, and proton donor are the key factors for achieving high activity with an efficient flow electrolyzer and the eventual development of a viable continuous process.

Coordination Chemistry of Ru(II) Complexes of an Asymmetric Bipyridine Analogue: Synergistic Effects of Supporting Ligand and Coordination Geometry on Reactivities

The reactivities of transition metal coordination compounds are often controlled by the environment around the coordination sphere. For ruthenium(II) complexes, differences in polypyridyl supporting ligands affect some types of reactivity despite identical coordination geometries. To evaluate the synergistic effects of (i) the supporting ligands, and (ii) the coordination geometry, a series of dicarbonyl-ruthenium(II) complexes that contain both asymmetric and symmetric bidentate polypyridyl ligands were synthesized. Molecular structures of the complexes were determined by X-ray crystallography to distinguish their steric configuration. Structural, computational, and electrochemical analysis revealed some differences between the isomers. Photo- and thermal reactions indicated that the reactivities of the complexes were significantly affected by both their structures and the ligands involved.

Molecular Catalysis for Utilizing CO2 in Fuel Electro-Generation and in Chemical Feedstock

Processes for the conversion of CO2 to valuable chemicals are highly desired as a result of the increasing CO2 levels in the atmosphere and the subsequent elevating global temperature. However, CO2 is thermodynamically and kinetically inert to transformation and, therefore, many efforts were made in the last few decades. Reformation/hydrogenation of CO2 is widely used as a means to access valuable products such as acetic acids, CH4, CH3OH, and CO. The electrochemical reduction of CO2 using hetero- and homogeneous catalysts recently attracted much attention. In particular, molecular CO2 reduction catalysts were widely studied using transition-metal complexes modified with various ligands to understand the relationship between various catalytic properties and the coordination spheres above the metal centers. Concurrently, the coupling of CO2 with various electrophiles under homogeneous conditions is also considered an important approach for recycling CO2 as a renewable C-1 substrate in the chemical industry. This review summarizes some recent advances in the conversion of CO2 into valuable chemicals with particular focus on the metal-catalyzed reductive conversion and functionalization of CO2.