Re(I) NHC Complexes for Electrocatalytic Conversion of CO2

Mechanistic insights of electrocatalytic CO2 reduction by Mn complexes: synergistic effects of the ligands

The electrocatalytic mechanisms of CO2 reduction catalyzed by pyridine-oxazoline (pyrox)-based Mn catalysts were investigated by DFT calculations. In-depth comparative analyses of pyrox-based and bipyridine-based Mn complexes were carried out. C-OH cleavage is the rate-determining step for both the protonation-first path and the reduction-first path. The free energy of CO2 activation (ΔG1) and the electrons donated by CO ligands in this step are effective descriptors in regulating the C-OH cleavage barrier. The reduction of carboxylate complex 6 (E6) is the potential-determining step for the reduction-first path. Meanwhile, for the protonation-first path, the initial generation (E2) or the regeneration (E8) of active catalyst might be potential-determining. Hirshfeld charge and orbital contribution analysis indicate that E6 is definitely based on the heterocyclic ligand and E2 is related to both the heterocyclic ligand and three CO ligands. Therefore, replacement of the CO ligand by a stronger electron donating ligand can effectively boost the catalytic activity of CO2 reduction without increasing the overpotential in the reduction-first path. This hypothesis is supported by the mechanism calculations of the Mn complex in which the axial CO ligand is replaced by a pyridine or PMe3.

Ruthenium(II) Complexes of a Xanthene-Spanned Dicarbene Ligand

Octahedral ruthenium(II) complexes of a xanthene-di(N-heterocyclic carbene) ancillary ligand (XdC) have been prepared and structurally characterized. Examples catalyze the transfer hydrogenation of ketones {[Ru(CO)I2(C,O,C'-XdC)] (1) and [Ru(CO)(MeCN)2(C,O,C'-XdC)]2+ (22+)} and the selective electrochemical reduction of CO2 to CO {[Ru(N,N'-bpy)(CO)(C,O,C'-XdC)]2+ (32+) at 0.40 V overpotential in MeCN-H2O (1 M)}. The reaction of 1 with KBEt3H afforded isomers of [(C,C'-XdC)Ru(μ-H)(H)]2 dimers, which are stable to reductive elimination of the XdC ligand, thereby suggesting similar (XdC)Rh(coligand)(H)x species may be viable intermediates in catalyses. The electrochemical reduction of CO2 involves a double reduction of 32+ to 3••, which has been characterized by IR-SEC and DFT calculations. The DFT calculations suggest the Ru-Oxanth bond breaks in 3••, opening a metal site for CO2 binding with selectivity over protons enabled by the diffuse nature of the HOMO delocalized over the metal and the bipyridine and carbonyl coligands. The results point to the promise of metal complexes of flexible and hemilabile xanthene-(NHC)2 ancillary ligands in catalysis.

Electrocatalytic reduction of CO2 to CO by a series of organometallic Re(I)-tpy complexes

A series of organometallic Re(I)(L)(CO)3Br complexes with 4'-substituted terpyridine ligands (L) has been synthesised as electrocatalysts for CO2 reduction. The complexes' spectroscopic characterisation and computationally optimised geometry demonstrate a facial geometry around Re(I) with three cis COs and the terpyridine ligand coordinating in a bidentate mode. The effect of substitution on the 4'-position of terpyridine (Re1-5) on CO2 electroreduction was investigated and compared with a known Lehn-type catalyst, Re(I)(bpy)(CO)3Br (Re7). All complexes catalyse CO evolution in homogeneous organic media at moderate overpotentials (0.75-0.95 V) with faradaic yields of 62-98%. The electrochemical catalytic activity was further evaluated in the presence of three Brønsted acids to demonstrate the influence of the pKa of the proton sources. The TDDFT and ultrafast transient absorption spectroscopy (TAS) studies showed combined charge transfer bands of ILCT and MLCT. Amongst the series, the Re-complex containing a ferrocenyl-substituted terpyridine ligand (Re5) shows an additional intra-ligand charge transfer band and was probed using UV-Vis spectroelectrochemistry.

Ligand-Structure Effects on N-Heterocyclic Carbene Rhenium Photo- and Electrocatalysts of CO2 Reduction

Three novel rhenium N-heterocyclic carbene complexes, [Re]-NHC-1-3 ([Re] = fac-Re(CO)₃Br), were synthesized and characterized using a range of spectroscopic techniques. Photophysical, electrochemical and spectroelectrochemical studies were carried out to probe the properties of these organometallic compounds. Re-NHC-1 and Re-NHC-2 bear a phenanthrene backbone on an imidazole (NHC) ring, coordinating to Re by both the carbene C and a pyridyl group attached to one of the imidazole nitrogen atoms. Re-NHC-2 differs from Re-NHC-1 by replacing N-H with an N-benzyl group as the second substituent on imidazole. The replacement of the phenanthrene backbone in Re-NHC-2 with the larger pyrene gives Re-NHC-3. The two-electron electrochemical reductions of Re-NHC-2 and Re-NHC-3 result in the formation of the five-coordinate anions that are capable of electrocatalytic CO₂ reduction. These catalysts are formed first at the initial cathodic wave R1, and then, ultimately, via the reduction of Re-Re bound dimer intermediates at the second cathodic wave R2. All three Re-NHC-1-3 complexes are active photocatalysts for the transformation of CO₂ to CO, with the most photostable complex, Re-NHC-3, being the most effective for this conversion. Re-NHC-1 and Re-NHC-2 afforded modest CO turnover numbers (TONs), following irradiation at 355 nm, but were inactive at the longer irradiation wavelength of 470 nm. In contrast, Re-NHC-3, when photoexcited at 470 nm, yielded the highest TON in this study, but remained inactive at 355 nm. The luminescence spectrum of Re-NHC-3 is red-shifted compared to those of Re-NHC-1 and Re-NHC-2, and previously reported similar [Re]-NHC complexes. This observation, together with TD-DFT calculations, suggests that the nature of the lowest-energy optical excitation for Re-NHC-3 has π→π*(NHC-pyrene) and d_π(Re)→π*(pyridine) (IL/MLCT) character. The stability and superior photocatalytic performance of Re-NHC-3 are attributed to the extended conjugation of the π-electron system, leading to the beneficial modulation of the strongly electron-donating tendency of the NHC group.

Base-free synthesis of benchtop stable Ru(III)-NHC complexes from RuCl3·3H2O and their use as precursors for Ru(II)-NHC complexes

A series of Ru(III)-NHC complexes, identified as [Ru^III(PyNHC^R)(Cl)₃(H₂O)] (1a-c), have been prepared, starting from RuCl₃·3H₂O following a base-free route. The Lewis acidic Ru(III) centre operates via a halide-assisted, electrophilic C-H activation for carbene generation. The best results were obtained with azolium salts having the I^- anion, while ligand precursors with Cl^-, BF₄^-, and PF₆^- gave no complex formation and those with Br^- gave a product with mixed halides. The structurally simple, air and moisture-stable complexes represent rare examples of paramagnetic Ru(III)-NHC complexes. Furthermore, these benchtop stable Ru(III)-NHC complexes were shown to be excellent metal precursors for the synthesis of new [Ru^II(PyNHC^R)(Cl)₂(PPh₃)₂] (2a-c) and [Ru^II(PyNHC^R)(CNC^Me)I]PF₆ (3a-c) complexes. All the complexes have been characterised using spectroscopic methods, and the structures of 1a, 1b, 2c, and 3a have been determined using the single-crystal X-ray diffraction technique. This work allows easy access to new Ru-NHC complexes for the study of new properties and novel applications.

Molecular-Modified Photocathodes for Applications in Artificial Photosynthesis and Solar-to-Fuel Technologies

Nature offers inspiration for developing technologies that integrate the capture, conversion, and storage of solar energy. In this review article, we highlight principles of natural photosynthesis and artificial photosynthesis, drawing comparisons between solar energy transduction in biology and emerging solar-to-fuel technologies. Key features of the biological approach include use of earth-abundant elements and molecular interfaces for driving photoinduced charge separation reactions that power chemical transformations at global scales. For the artificial systems described in this review, emphasis is placed on advancements involving hybrid photocathodes that power fuel-forming reactions using molecular catalysts interfaced with visible-light-absorbing semiconductors.

Electrocatalytic CO2 Reduction by [Re(CO)3Cl(3-(pyridin-2-yl)-5-phenyl-1,2,4-triazole)] and [Re(CO)3Cl(3-(2-pyridyl)-1,2,4-triazole)]

The exploration of novel electrocatalysts for CO₂ reduction is necessary to overcome global warming and the depletion of fossil fuels. In the current study, the electrocatalytic CO₂ reduction of [Re(CO)₃Cl(N-N)], where N-N represents 3-(2-pyridyl)-1,2,4-triazole (Hpy), 3-(pyridin-2-yl)-5-phenyl-l,2,4-triazole (Hph), and 2,2'-bipyridine-4,4' dicarboxylic acidic (bpy-COOH) ligands, was investigated. In CO₂-saturated electrolytes, cyclic voltammograms showed an enhancement of the current at the second reduction wave for all complexes. In the presence of triethanolamine (TEOA), the currents of Re(Hpy), Re(Hph), and Re(bpy-COOH) enhanced significantly by approximately 4-, 2-, and 5-fold at peak potentials of -1.60, -150, and -1.69 V_Ag/Ag+, respectively (in comparison to without TEOA). The reduction potential of Re(Hph) was less negative than those of Re(Hpy) and Re(COOH), which was suggested to cause its least efficiency for CO₂ reduction. Chronoamperometry measurements showed the stability of the cathodic current at the second reduction wave for at least 300 s, and Re(COOH) was the most stable in the CO₂-catalyzed reduction. The appearance and disappearance of the absorption band in the UV/vis spectra indicated the reaction of the catalyst with molecular CO₂ and its conversion to new species, which were proposed to be Re-DMF ⁺ and Re-TEOA and were supposed to react with CO₂ molecules. The CO₂ molecules were claimed to be captured and inserted into the oxygen bond of Re-TEOA, resulting in the enhancement of the CO₂ reduction efficiency. The results indicate a new way of using these complexes in electrocatalytic CO₂ reduction.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

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.

Methane Generation from CO2 with a Molecular Rhenium Catalyst

The atomic-level tunability of molecular structures is a compelling reason to develop homogeneous catalysts for challenging reactions such as the electrochemical reduction of carbon dioxide to valuable C₁-C_n products. Of particular interest is methane, the largest component of natural gas. Herein, we report a series of three isomeric rhenium tricarbonyl complexes coordinated by the asymmetric diimine ligands 2-(isoquinolin-1-yl)-4,5-dihydrooxazole (quin-1-oxa), 2-(quinolin-2-yl)-4,5-dihydrooxazole (quin-2-oxa), and 2-(isoquinolin-3-yl)-4,5-dihydrooxazole (quin-3-oxa) that catalyze the reduction of CO₂ to carbon monoxide and methane, albeit the latter with a low efficiency. To our knowledge, these complexes are the first examples of rhenium(I) catalysts capable of converting carbon dioxide into methane. Re(quin-1-oxa)(CO)₃Cl (1), Re(quin-2-oxa)(CO)₃Cl (2), and Re(quin-3-oxa)(CO)₃Cl (3) were characterized and studied using a variety of electrochemical and spectroscopic techniques. In bulk electrolysis experiments, the three complexes reduce CO₂ to CO and CH₄. When the controlled-potential electrolysis experiments are performed at -2.5 V (vs Fc^+/0) and in the presence of the Brønsted acid 2,2,2-trifluoroethanol, methane is produced with turnover numbers that range from 1.3 to 1.8. Isotope labeling experiments using ¹³CO₂ atmosphere produce ¹³CH₄ (m/z = 17) confirming that methane originates from CO₂ reduction. Theoretical calculations are performed to investigate the mechanistic aspects of the 8e^-/8H⁺ reduction of CO₂ to CH₄. A ligand-assisted pathway is proposed to be an efficient pathway in the formation of CH₄. Delocalization of the electron density on the (iso)quinoline moiety upon reduction stabilizes the key carbonyl intermediate leading to additional reactivity of this ligand. These results should aid the development of more robust catalytic systems that produce CH₄ from CO₂.

Enhanced Electrochemical CO2 Reduction by a Series of Molecular Rhenium Catalysts Decorated with Second-Sphere Hydrogen-Bond Donors

A series of rhenium(I) fac-tricarbonyl complexes containing pendent arylamine functionality in the second coordination sphere have been developed and studied as electrocatalysts for carbon dioxide (CO₂) reduction. Aniline moieties were appended at the 6 position of a 2,2'-bipyridine (bpy) donor in which the primary amine was positioned at the ortho- (1-Re), meta- (2-Re), and para- (3-Re) sites of the aniline substituent to generate a family of isomers. The relationship between the catalyst structure and activity was explored across the series, and the catalytic performance was compared to that of the benchmark catalyst Re(bpy)(CO)₃Cl (ReBpy). Catalysts 1-Re, 2-Re, and 3-Re outperform the benchmark catalyst both in anhydrous acetonitrile and with added trifluoroethanol (TFE) as an external proton source. In the presence of TFE, the aniline-substituted catalysts convert CO₂ to carbon monoxide (CO) with high Faradaic efficiencies (≥89%) and have superior turnover frequencies (TOFs) relative to ReBpy (72.9 s^-1), with 2-Re having the highest TOF of the series at 239 s^-1, a value that is twice that of the next most active catalyst. TOFs of 123 and 109 s^-1 were observed for the ortho- and para-substituted aniline complexes (1-Re and 3-Re), respectively. Indeed, catalytic activities vary widely across the series, showing a high sensitivity to the position of the amine functionality relative to the rhenium active site. IR and UV-vis spectroelectrochemical experiments were conducted on the aniline-substituted systems, revealing important differences between the catalysts and mechanistic insight.

Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic Path of [Re(3,3'-dihydroxy-2,2'-bipyridine)(CO)3Cl] and Catalytic Reduction of Carbon Dioxide

Herein, we present the cathodic paths of the Group-7 metal complex [Re(3,3'-DHBPY)(CO)₃Cl] (3,3'-DHBPY = 3,3'-dihydroxy-2,2'-bipyridine) producing a moderately active catalyst of electrochemical reduction of CO₂ to CO. The combined techniques of cyclic voltammetry and IR/UV-vis spectroelectrochemistry have revealed significant differences in the chemistry of the electrochemically reduced parent complex compared to the previously published Re/4,4'-DHBPY congener. The initial irreversible cathodic step in weakly coordinating THF is shifted toward much less negative electrode potentials, reflecting facile reductive deprotonation of one hydroxyl group and strong intramolecular hydrogen bonding, O-H···O^-. The latter process occurs spontaneously in basic dimethylformamide where Re/4,4'-DHBPY remains stable. The subsequent reduction of singly deprotonated [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- under ambient conditions occurs at a cathodic potential close to that of the Re/4,4'-DHBPY-H⁺ derivative. However, for the stabilized 3,3'-DHBPY-H⁺ ligand, the latter process at the second cathodic wave is more complex and involves an overall transfer of three electrons. Rapid potential step electrolysis induces 1e^--reductive cleavage of the second O-H bond, triggering dissociation of the Cl^- ligand from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2-. The ultimate product of the second cathodic step in THF was identified as 5-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3-, the equivalent of classical 2e^--reduced [Re(BPY)(CO)₃]^-. Each reductive deprotonation of the DHBPY ligand results in a redshift of the IR ν(CO) absorption of the tricarbonyl complexes by ca. 10 cm^-1, facilitating the product assignment based on comparison with the literature data for corresponding Re/BPY complexes. The Cl^- dissociation from [Re(3,3'-DHBPY-2H⁺)(CO)₃Cl]^2- was proven in strongly coordinating butyronitrile. The latter dianion is stable at 223 K, converting at 258 K to 6-coordinate [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3-. Useful reference data were obtained with substituted parent [Re(3,3'-DHBPY)(CO)₃(PrCN)]⁺ that also smoothly deprotonates by the initial reduction to [Re(3,3'-DHBPY-H⁺)(CO)₃(PrCN)]. The latter complex ultimately converts at the second cathodic wave to [Re(3,3'-DHBPY-2H⁺)(CO)₃(PrCN)]^3- via a counterintuitive ETC step generating the 1e^- radical of the parent complex, viz., [Re(3,3'-DHBPY)(CO)₃(PrCN)]. The same alternative reduction path is also followed by [Re(3,3'-DHBPY-H⁺)(CO)₃Cl]^- at the onset of the second cathodic wave, where the ETC step results in the intermediate [Re(3,3'-DHBPY)(CO)₃Cl]^•- further reducible to [Re(3,3'-DHBPY-2H⁺)(CO)₃]^3- as the CO₂ catalyst.

Rhenium Complexes of Pyridyl-Mesoionic Carbenes: Photochemical Properties and Electrocatalytic CO2 Reduction

Mesoionic carbenes have found wide use as components of homogeneous catalysts. Recent discoveries have, however, shown that metal complexes of such ligands also have huge potential in photochemical research and in the activation of small molecules. We present here three Re^I complexes with mesoionic pyridyl-carbene ligands. The complexes display reduction steps which were investigated via UV-vis-NIR-IR spectro-electrochemistry, and these results point toward an EC mechanism. The Re^I compounds emit in the visible range in solution at room temperature with excited state lifetimes that are dependent on the substituents of the mesoionic carbenes. These complexes are also potent electrocatalysts for the selective reduction of CO₂ to CO. Whereas the substituents on the carbenes have no influence on the reduction potentials, the electrocatalytic efficiency is strongly dependent on the substituents. This fact is likely a result of catalyst instability. The results presented here thus introduce mesoionic carbenes as new potent ligands for the generation of emissive Re^I complexes and for electrocatalytic CO₂ reduction.

Red-emitting neutral rhenium(i) complexes bearing a pyridyl pyridoannelated N-heterocyclic carbene

Two novel neutral rhenium(i) tricarbonyl complexes bearing a pyridoannelated N-heterocyclic carbene are synthetized and fully characterized, which display red photoluminescence arising from a triplet excited state with ligand centered character.

Are Two Metal Ions Better than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with Proton-Responsive Ligands in CO2 Reduction Catalysis

Here, the reduction chemistry of mono- and binuclear α-diimine-Re(CO)₃ complexes with proton responsive ligands and their application in the electrochemically-driven CO₂ reduction catalysis are presented. The work was aimed to investigate the impact of 1) two metal ions in close proximity and 2) an internal proton source on catalysis. Therefore, three different Re complexes, a binuclear one with a central phenol unit, 3, and two mononuclear, one having a central phenol unit, 1, and one with a methoxy unit, 2, were utilised. All complexes are active in the CO₂ -to-CO conversion and CO is always the major product. The catalytic rate constant k_cat for all three complexes is much higher and the overpotential is lower in DMF/water mixtures than in pure DMF (DMF=N,N-dimethylformamide). Cyclic voltammetry (CV) studies in the absence of substrate revealed that this is due to an accelerated chloride ion loss after initial reduction in DMF/water mixtures in comparison to pure DMF. Chloride ion loss is necessary for subsequent CO₂ binding and this step is around ten times faster in the presence of water [2: k^Cl (DMF)≈1.7 s^-1 ; k^Cl (DMF/H₂ O)≈20 s^-1 ]. The binuclear complex 3 with a proton responsive phenol unit is more active than the mononuclear complexes. In the presence of water, the observed rate constant k_obs for 3 is four times higher than of 2, in the absence of water even ten times. Thus, the two metal centres are beneficial for catalysis. Lastly, the investigation showed that the phenol unit has no impact on the rate of the catalysis, it even slows down the CO₂ -to-CO conversion. This is due to an unproductive, competitive side reaction: After initial reduction, 1 and 3 loose either Cl^- or undergo a reductive OH deprotonation forming a phenolate unit. The phenolate could bind to the metal centre blocking the sixth coordination site for CO₂ activation. In DMF, O-H bond breaking and Cl^- ion loss have similar rate constants [1: k^Cl (DMF)≈2 s^-1 , k^OH ≈1.5 s^-1 ], in water/DMF Cl^- loss is much faster. Thus, the effect on the catalytic rate is more pronounced in DMF. However, the acidic protons lower the overpotential of the catalysis by about 150 mV.

A rational design of manganese electrocatalysts for Lewis acid-assisted carbon dioxide reduction

Herein, the mechanisms of Brønsted acid- and Lewis acid-assisted CO2 electroreduction by Mn(mesbpy)(CO)3Br (1) were investigated by density functional theory calculations. Our results indicate that for the Lewis acid-assisted cycle, an energy sink (13) is present owing to the interaction between Mg(OTf)2 and activated CO2, which is disadvantageous to the apparent activation energy (ΔG≠). Moreover, a series of substituted 13 counterparts were investigated to reduce the energy sink and decrease ΔG≠. Based on our study on the substituent effect, an excellent linear relationship was found between 2e reduction potentials and LUMO energies of substituted 1, and a moderate linear relationship was observed between ΔG of substituted 13 and the 2e reduction potential of substituted 1 counterparts. Moreover, for the CO2 reduction assisted by a Lewis acid, the formyl-substituted complex R8 has been predicted to be a more effective catalyst with lower overpotential and higher catalytic activity than its parent complex 1.

Role of 2nd sphere H-bonding residues in tuning the kinetics of CO2 reduction to CO by iron porphyrin complexes

Iron porphyrins are potential catalysts for the electrocatalytic and photocatalytic reduction of CO2. It has been recently established that the reduction of CO2 by an iron porphyrin complex with a hydrogen bonding distal pocket involves at least two intermediates: a Fe(ii)-CO22- and a Fe(ii)-COOH species. A distal hydrogen bonding interaction was found to be key in determining the stability of these intermediates and affecting both the selectivity and rate of CO2 reduction. In this report, a series of iron porphyrins that vary only in the distal H-bonding network are further investigated and these exhibit turnover frequencies (TOFs) ranging from 1.0 s-1 to 103 s-1. The experimental TOFs correlate with the H-bonding ability of the distal superstructure of these iron porphyrin complexes and analysis suggests that H-bonding alone can tune the rate of CO2 reduction by as much as 1000 fold. DFT calculations provide a detailed insight into how the, apparently weak, 2nd sphere interactions lead to efficient CO2 activation for reduction. The ability to tune CO2 reduction rates by changing the H-bonding residue instead of the acid source is a convenient way to tune CO2 reduction electrocatalysis without compromising selectivity by introducing competitive hydrogen evolution reaction or formate generation.

Electrocatalytic CO2 Reduction with Cis and Trans Conformers of a Rigid Dinuclear Rhenium Complex: Comparing the Monometallic and Cooperative Bimetallic Pathways

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic carbon dioxide (CO₂) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge, cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the cis conformer operates via cooperative bimetallic CO₂ activation and conversion and the trans conformer reduces CO₂ through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)₃Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s^-1) relative to the trans conformer (22.9 s^-1), with both outperforming Re(bpy)(CO)₃Cl (11.1 s^-1). Notably, at low applied potentials, the cis conformer does not catalyze the reductive disproportionation of CO₂ to CO and CO₃^2- in contrast to the trans conformer and mononuclear catalyst, demonstrating that the orientation of active sites and structure of the dinuclear cis complex dictate an alternative catalytic pathway. Further, UV-vis spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated Re-Re-bonded dimer. Indeed, the cis conformer also avoids intermolecular Re-Re bond formation.

A Robust Pyridyl-NHC-Ligated Rhenium Photocatalyst for CO2 Reduction in the Presence of Water and Oxygen

Re(pyNHC-PhCF3)(CO)3Br is a highly active photocatalyst for CO2 reduction. The PhCF3 derivative was previously empirically shown to be a robust catalyst. Here, the role of the PhCF3 group is probed computationally and the robust nature of this catalyst is analyzed with regard to the presence of water and oxygen introduced in controlled amounts during the photocatalytic reduction of CO2 to CO with visible light. This complex was found to work well from 0–1% water concentration reproducibly; however, trace amounts of water were required for benchmark Re(bpy)(CO)3Cl to give reproducible reactivity. When ambient air is added to the reaction mixture, the NHC complex was found to retain substantial performance (~50% of optimized reactivity) at up to 40% ambient atmosphere and 60% CO2 while the Re(bpy)(CO)3Cl complex was found to give a dramatically reduced CO2 reduction reactivity upon introduction of ambient atmosphere. Through the use of time-correlated single photon counting studies and prior electrochemical results, we reasoned that this enhanced catalyst resilience is due to a mechanistic difference between the NHC- and bpy-based catalysts. These results highlight an important feature of this NHC-ligated catalyst: substantially enhanced stability toward common reaction contaminates.

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.

Properties and prospects for rhenium(i) tricarbonyl N-heterocyclic carbene complexes

Rhenium tricarbonyl complexes bound to N-heterocyclic carbene ligands are emerging as a new class of complexes with promising applications in a wide variety of areas.

Electrolytic CO2 Reduction in Tandem with Oxidative Organic Chemistry

Electrochemical reduction of CO2 into carbon-based products using excess clean electricity is a compelling method for producing sustainable fuels while lowering CO2 emissions. Previous electrolytic CO2 reduction studies all involve dioxygen production at the anode, yet this anodic reaction requires a large overpotential and yields a product bearing no economic value. We report here that the cathodic reduction of CO2 to CO can occur in tandem with the anodic oxidation of organic substrates that bear higher economic value than dioxygen. This claim is demonstrated by 3 h of sustained electrolytic conversion of CO2 into CO at a copper-indium cathode with a current density of 3.7 mA cm-2 and Faradaic efficiency of >70%, and the concomitant oxidation of an alcohol at a platinum anode with >75% yield. These results were tested for four alcohols representing different classes of alcohols and demonstrate electrolytic reduction and oxidative chemistry that form higher-valued carbon-based products at both electrodes.