Surface modified copper foam with cobalt phthalocyanine carbon nanotube hybrids for tuning CO2 reduction reaction products

The CO2 reduction reaction (CO2RR) is a feasible way to convert this greenhouse gas into molecules of industrial interest. Herein we present the modification of the Cu foam cathode using molecular catalyst hybrid from cobalt phthalocyanine (CoPc) to increase selectivity and stability towards CO2RR products in a flow cell setup.

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.

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.

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.

Bimetallic Molecular Catalyst Design for Carbon Dioxide Reduction

The core challenge in developing cost-efficient catalysts for carbon dioxide (CO2 ) conversion mainly lies in controlling its complex reaction pathways. One such strategy exploits bimetallic cooperativity, which relies on the synergistic interaction between two metal centers to activate and convert the CO2 substrate. While this approach has seen an important trend in heterogeneous catalysis as a handle to control stabilities of surface intermediates, it has not often been utilized in molecular and heterogenized molecular catalytic systems. In this review, we gather general principles on how natural CO2 activating enzymes take advantage of bimetallic strategy and how phosphines, cyclams, polypyridyls, porphyrins, and cryptates-based homo- and hetero-bimetallic molecular catalysts can help understand the synergistic effect of two metal centers.

Regulating the d-band electrons of the Fe-N-C single-atom catalyst for high-efficiency CO2 electroreduction by electron-donating S-doping

Developing highly efficient electrocatalysts is crucially significant for the application of advanced energy conversion. The Fe-N-C single-atom catalyst is promising for CO₂ electroreduction reaction (CO₂RR) but suffers from insufficient intrinsic activity and inferior conductivity, which could be addressed by redistributing the electron density via heteroatom doping. Herein, we synthesized S-doped Fe-N-C (Fe-SN-C) as an advanced electrocatalyst for CO₂RR using a simple trapping-pyrolysis strategy. Density functional theory calculations and experimental results indicate that S doping increases the d-band electrons and conductivity of Fe-SN-C by electron donating, and thus boosts *CO desorption during the CO₂RR process and suppresses the competing hydrogen evolution reaction. Consequently, Fe-SN-C exhibits the maximum CO faradaic efficiency of 93% at -0.5 V and the highest partial current density of 10.1 mA cm^-2 at -0.8 V for 2e^- CO₂RR. This finding provides a feasible and controllable method to achieve advanced electrocatalysts for efficient energy conversion.

Bio-Inspired Bimetallic Cooperativity Through a Hydrogen Bonding Spacer in CO2 Reduction

At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO₂ and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO₂ reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue. Interestingly, quasi-identical catalytic performances are obtained when one of the two iron centers was replaced by a redox inactive Zn metal, questioning the cooperative action of the two metals. Snapshots of X-ray structures indicate that the two metalloporphyrin units tethered by a urea group is a good compromise between rigidity and flexibility to accommodate CO₂ capture, activation, and reduction.

Microporous Polymelamine Framework Functionalized with Re(I) Tricarbonyl Complexes for CO2 Absorption and Reduction

A mixture of polymeric complexes based on the reaction between Re(CO)₅Cl and the porous polymeric network coming from the coupling of melamine and benzene-1,3,5-tricarboxaldehyde was obtained and characterized by FTIR, NMR, SEM, XPS, ICP, XRD, and cyclic voltammetry (CV). The formed rhenium-based porous hybrid material reveals a noticeable capability of CO₂ absorption. The gas absorption amount measured at 295 K was close to 44 cm³/g at 1 atm. An interesting catalytic activity for CO₂ reduction reaction (CO₂RR) is observed, resulting in a turn over-number (TON) close to 6.3 under 80 min of test at -1.8 V vs. Ag/AgCl in a TBAPF₆ 0.1 M ACN solution. A possible use as filler in membranes or columns can be envisaged.

Potential- and Buffer-Dependent Selectivity for the Conversion of CO2 to CO by a Cobalt Porphyrin-Peptide Electrocatalyst in Water

A semisynthetic electrocatalyst for carbon dioxide reduction to carbon monoxide in water is reported. Cobalt microperoxidase-11 (CoMP11-Ac) is shown to reduce CO₂ to CO with a turnover number of up to 32,000 and a selectivity of up to 88:5 CO:H₂. Higher selectivity for CO production is favored by a less cathodic applied potential and use of a higher pK _a buffer. A mechanistic hypothesis is presented in which avoiding the formation and protonation of a formal Co(I) species favors CO production. These results demonstrate how tuning reaction conditions impact reactivity toward CO₂ reduction for a biocatalyst previously developed for H₂ production.

Homogeneous Electrocatalytic Reduction of CO2 by a CrN3O Complex: Electronic Coupling with a Redox-Active Terpyridine Fragment Favors Selectivity for CO

Electrocatalyst design and optimization strategies continue to be an active area of research interest for the applied use of renewable energy resources. The electrocatalytic conversion of carbon dioxide (CO₂) is an attractive approach in this context because of the added potential benefit of addressing its rising atmospheric concentrations. In previous experimental and computational studies, we have described the mechanism of the first molecular Cr complex capable of electrocatalytically reducing CO₂ to carbon monoxide (CO) in the presence of an added proton donor, which contained a redox-active 2,2'-bipyridine (bpy) fragment, CrN₂O₂. The high selectivity for CO in the bpy-based system was dependent on a delocalized Cr^II(bpy^•-) active state. Subsequently, we became interested in exploring how expanding the polypyridyl ligand core would impact the selectivity and activity during electrocatalytic CO₂ reduction. Here, we report a new CrN₃O catalyst, Cr(tpy^tbupho)Cl₂ (1), where 2-(2,2':6',2″-terpyridin-6-yl)-4,6-di-tert-butylphenolate = [tpy^tbupho]^-, which reduces CO₂ to CO with almost quantitative selectivity via a different mechanism than our previously reported Cr(^tbudhbpy)Cl(H₂O) catalyst. Computational analyses indicate that, although the stoichiometry of both reactions is identical, changes in the observed rate law are the combined result of a decrease in the intrinsic ligand charge (L₃X vs L₂X₂) and an increase in the ligand redox activity, which result in increased electronic coupling between the doubly reduced tpy fragment of the ligand and the Cr^II center. The strong electronic coupling enhances the rate of protonation and subsequent C-OH bond cleavage, resulting in CO₂ binding becoming the rate-determining step, which is an uncommon mechanism during protic CO₂ reduction.

Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents

The electrochemical reduction of CO₂ provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO₂ reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)₃Br] and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)₃Br] was first shown to be an active and selective catalyst for reducing CO₂ to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism.

This class of Mn catalysts only shows CO₂ reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO₂ electrolyzers.

Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO₂ binding mechanism, where for the acids widely studied, lower pK_a values actually favor CO₂ binding over Mn–H formation, a precursor to H₂ evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)₃Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO₂ binding operates in both the “dimer” and primary (via [Mn(bpy)(CO)₃]⁻) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO₂ reduction catalysts.

Strategies for Breaking Molecular Scaling Relationships for the Electrochemical CO2 Reduction Reaction

The electrocatalytic CO2 reduction reaction (CO2RR) is a promising strategy for converting CO2 to fuels and value-added chemicals using renewable energy sources. Molecular electrocatalysts show promise for the selective conversion...

Group 6 transition metal-based molecular complexes for sustainable catalytic CO2 activation

CO2 activation is one of the key steps towards CO2 mitigation. In this context, the group 6 transition metal-based molecular catalysts can lead the way.

Promoting Selective Generation of Formic Acid from CO2 Using Mn(bpy)(CO)3Br as Electrocatalyst and Triethylamine/Isopropanol as Additives

Urgent solutions are needed to efficiently convert the greenhouse gas CO₂ into higher-value products. In this work, fac-Mn(bpy)(CO)₃Br (bpy = 2,2'-bipyridine) is employed as electrocatalyst in reductive CO₂ conversion. It is shown that product selectivity can be shifted from CO toward HCOOH using appropriate additives, i.e., Et₃N along with iPrOH. A crucial aspect of the strategy is to outrun the dimer-generating parent-child reaction involving fac-Mn(bpy)(CO)₃Br and [Mn(bpy)(CO)₃]^- and instead produce the Mn hydride intermediate. Preferentially, this is done at the first reduction wave to enable formation of HCOOH at an overpotential as low as 260 mV and with faradaic efficiency of 59 ± 1%. The latter may be increased to 71 ± 3% at an overpotential of 560 mV, using 2 M concentrations of both Et₃N and iPrOH. The nature of the amine additive is crucial for product selectivity, as the faradaic efficiency for HCOOH formation decreases to 13 ± 4% if Et₃N is replaced with Et₂NH. The origin of this difference lies in the ability of Et₃N/iPrOH to establish an equilibrium solution of isopropyl carbonate and CO₂, while with Et₂NH/iPrOH, formation of the diethylcarbamic acid is favored. According to density-functional theory calculations, CO₂ in the former case can take part favorably in the catalytic cycle, while this is less opportune in the latter case because of the CO₂-to-carbamic acid conversion. This work presents a straightforward procedure for electrochemical reduction of CO₂ to HCOOH by combining an easily synthesized manganese catalyst with commercially available additives.

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.

(1,2-Azole)bis(bipyridyl)ruthenium(II) Complexes: Electrochemistry, Luminescent Properties, And Electro- And Photocatalysts for CO2 Reduction

New cis-(1,2-azole)-aquo bis(2,2'-bipyridyl)ruthenium(II) (1,2-azole (az*H) = pzH (pyrazole), dmpzH (3,5-dimethylpyrazole), and indzH (indazole)) complexes are synthesized via chlorido abstraction from cis-[Ru(bipy)₂Cl(az*H)]OTf. The latter are obtained from cis-[Ru(bipy)₂Cl₂] after the subsequent coordination of the 1,2-azole. All the compounds are characterized by ¹H, ¹³C, ¹⁵N NMR spectroscopy as well as IR spectroscopy. Two chlorido complexes (pzH and indzH) and two aquo complexes (indzH and dmpzH) are also characterized by X-ray diffraction. Photophysical and electrochemical studies were carried out on all the complexes. The photophysical data support the phosphorescence of the complexes. The electrochemical behavior of all the complexes in an Ar atmosphere indicate that the oxidation processes assigned to Ru(II) → Ru(III) occurs at higher potentials in the aquo complexes. The reduction processes under Ar lead to several waves, indicating that the complexes undergo successive electron-transfer reductions that are centered in the bipy ligands. The first electron reduction is reversible. The electrochemical behavior in CO₂ media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_p(Ar)] ranged from 2.9 to 10.8. Controlled potential electrolysis of the chlorido and aquo complexes affords CO and formic acid, with the latter as the major product after 2 h. Photocatalytic experiments in MeCN with [Ru(bipy)₃]Cl₂ as the photosensitizer and TEOA as the electron donor, which were irradiated with >300 nm light for 24 h, led to CO and HCOOH as the main reduction products, achieving a combined turnover number (TON_CO+HCOO^-) as high as 107 for 2c after 24 h of irradiation.

Computational study on the reactivity of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2 and Me4) for CO2-to-CO conversion over H2 formation

We have recently reported a series of imidazolium-functionalized manganese bipyridyl tricarbonyl electrocatalysts, [Mn[bpyMe(Im-R)](CO)3Br]+ (R = Me, Me2, and Me4), for CO2-to-CO conversion in the presence of H2O as the proton source [J. Am. Chem. Soc., 2019, 141, 6569]. These catalysts feature slightly acidic imidazolium moieties in the secondary coordination sphere and reduce CO2 at mild electrochemical potentials. Here, we employ density functional theory (DFT) calculations to understand the electronic structure and reactivity for the CO2 reduction reaction (CO2RR) over the competing hydrogen evolution reaction (HER) using [Mn[bpyMe(ImMe)](CO)3Br]+ (1+). Our work indicates that, in the absence of water, the imidazolium ligand stabilizes the Mn-CO2 adduct through hydrogen bonding-like interactions, similar to the activated CO2 molecule in the C-cluster of the Ni,Fe-carbon monoxide dehydrogenase II, and assists the protonation steps during CO2RR and HER. More significantly, based on the energy span model, we demonstrate that the selectivity for CO2 fixation over proton reduction results from a higher activation energy for yielding the manganese dihydrogen intermediate before H2 release, which is the TOF determining transition state (TDTS) under an applied potential of Φ = -1.82 V versus Fc0/+. The calculated TOF also reflects the selectivity for CO2RR, which is four orders of magnitude larger than for HER, consistent with the CPE experiments that show no hydrogen was obtained. In the case of CO2 reduction, the TOF determining intermediate (TDI) corresponds to the doubly reduced active catalyst, 1C2(red2), which features a manganese(0) center that couples ferromagnetically with one unpaired electron in the π* orbital of bipyridine. On the other hand, for HER, the metal-hydride intermediate, 1C2(I11-R), is the TDI. Finally, second-order perturbation analyses imply that the strongest hydrogen bonding-like interaction at the C2 position in 1+ contributes to the higher catalytic activity with respect to [Mn[bpyMe(ImMe2)](CO)3Br]+ (2+) and [Mn[bpyMe(ImMe4)](CO)3Br]+ (3+) for CO2 fixation, consistent with the experimental data.

Enhanced Electrocatalytic Activity of a Zinc Porphyrin for CO2 Reduction: Cooperative Effects of Triazole Units in the Second Coordination Sphere

The control of the second coordination sphere in a coordination complex plays an important role in improving catalytic efficiency. Herein, we report a zinc porphyrin complex ZnPor8T with multiple flexible triazole units comprising the second coordination sphere, as an electrocatalyst for the highly selective electrochemical reduction of carbon dioxide (CO₂ ) to carbon monoxide (CO). This electrocatalyst converted CO₂ to CO with a Faradaic efficiency of 99 % and a current density of -6.2 mA cm^-2 at -2.4 V vs. Fc/Fc⁺ in N,N-dimethylformamide using water as the proton source. Structure-function relationship studies were carried out on ZnPor8T analogs containing different numbers of triazole units and distinct triazole geometries; these unveiled that the triazole units function cooperatively to stabilize the CO₂ -catalyst adduct in order to facilitate intramolecular proton transfer. Our findings demonstrate that incorporating triazole units that function in a cooperative manner is a versatile strategy to enhance the activity of electrocatalytic CO₂ conversion.

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.

Surprisingly big linker-dependence of activity and selectivity in CO2 reduction by an iridium(i) pincer complex

Here, we report the quantitative electroreduction of CO2 to CO by a PNP-pincer iridium(i) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker within the ligand. The complex 3-N is far superior to the analogues with methylene and oxygen linkers, showing higher activity and better selectivity for CO2 over proton reduction.

Luminescent Rhenium(I)tricarbonyl Complexes Containing Different Pyrazoles and Their Successive Deprotonation Products: CO2 Reduction Electrocatalysts

Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf (pz*H = pyrazole, pzH; 3,5-dimethylpyrazole, dmpzH; indazole, indzH; 3-(2-pyridyl)pyrazole, pypzH) were obtained from fac-[ReBr(CO)₃(pypzH)] by halide abstraction with AgOTf and subsequent addition of the corresponding pyrazole. Successive deprotonation with Na₂CO₃ and NaOH gave neutral fac-[Re(CO)₃(pz*H)(pypz)] and anionic Na{fac-[Re(CO)₃(pz*)(pypz)]} complexes, respectively. Cationic fac-[Re(CO)₃(pz*H)(pypzH)]OTf, neutral complexes fac-[Re(CO)₃(pz*H)(pypz)], and fac-[Re(CO)₃(pypz)₂Na] were subjected to photophysical and electrochemical studies. They exhibit phosphorescent decays from a prevalently ³MLCT excited state with quantum yields (Φ) in the range between 0.03 and 0.58 and long lifetimes (τ from 220 to 869 ns). The electrochemical behavior in Ar atmosphere of cationic and neutral complexes indicates that the oxidation processes assigned to Re^I → Re^II occurs at lower potentials for the neutral complex compared to cationic complex. The reduction processes occur at the ligands and do not depend on the charge of the complexes. The electrochemical behavior in CO₂ saturated media is consistent with CO₂ electrocatalyzed reduction, where the values of the catalytic activity [i_cat(CO₂)/i_cat(Ar)] ranged from 2.7 to 11.5 (compared to 8.1 for fac-[Re(CO)₃Cl(bipy)] studied as a reference). Controlled potential electrolysis for the pyrazole cationic (3a) and neutral (4a) complexes after 1 h affords CO in faraday yields of 61 and 89%, respectively. These values are higher for indazole complexes and may be related to the acidity of the coordinated pyrazole.

Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO2 reduction remains a highly creative approach.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

Enhanced Molecular CO2 Electroreduction Enabled by a Flexible Hydrophilic Channel for Relay Proton Shuttling

The effects of primary and second coordination spheres on molecular electrocatalysis have been extensively studied, yet investigations of third functional spheres are rarely reported. Here, an electrocatalyst (ZnPEG8T) was developed with a hydrophilic channel as a third functional sphere that facilitates relay proton shuttling to the primary and second coordination spheres for enhanced catalytic CO₂ reduction. Using foot-of-the-wave analysis, the ZnPEG8T catalyst displayed CO₂ -to-CO activity (TOF_max ) thirty times greater than that of the benchmark catalyst without a third functional sphere. A kinetic isotopic effect (KIE) study, in conjunction with voltammetry and UV/Vis spectroscopy, uncovered that the rate-limiting step was not the protonation step of the metallocarboxylate intermediate, as observed in many other molecular CO₂ reduction electrocatalysts, but rather the replenishment of protons in the proton-shuttling channel. Controlled-potential electrolysis using ZnPEG8T displayed a faradaic efficiency of 100 % for CO₂ -to-CO conversion at -2.4 V vs. Fc/Fc⁺ . A Tafel plot was also generated for a comparison to other reported molecular catalysts. This report validates a strategy for incorporating higher functional spheres for enhanced catalytic efficiency in proton-coupled electron-transfer reactions.

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.

Recent advances in metalloporphyrin-based catalyst design towards carbon dioxide reduction: from bio-inspired second coordination sphere modifications to hierarchical architectures

Research in the development of new molecular catalysts for the selective transformation of CO2 to reduced forms of carbon is attracting enormous interest from chemists. Molecular catalyst design hinges on the elaboration of ligand scaffolds to manipulate the electronic and structural properties for the fine tuning of the reactivity pattern. A cornucopia of ligand sets have been designed along this line and more and more are being reported. In this quest, the porphyrin molecular platform has been under intensive focus due to the unmatched catalytic properties of metalloporphyrins. There have been rapid advances in this particular field during the last few years wherein both electronic and structural aspects in the second coordination spheres have been addressed to shift the overpotential and improve the catalytic rates and product selectivity. Metalloporphyrins have also attracted much attention in terms of the elaboration of hybrid materials for heterogeneous catalysis. Here too, some promising activities have made metalloporphyrin derivatives serious candidates for technological implementation. This review collects the recent advances centred around the chemistry of metalloporphyrins for the reduction of CO2.

Ligand-Controlled Product Selectivity in Electrochemical Carbon Dioxide Reduction Using Manganese Bipyridine Catalysts

Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO₂ reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO₂ to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO₂ to formic acid, with a maximum turnover frequency of up to 5500 s^-1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn-hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, ¹H NMR, DFT calculations, and infrared spectroelectrochemistry.

Metalloradical intermediates in electrocatalytic reduction of CO2 to CO: Mn versus Re bis-N-heterocyclic carbene pincers

This work examines the relative reactivities of Re^I and Mn^I tricarbonyl pyridine-2,6-bis-N-heterocyclic carbene pincers M(CO)₃CNC^BnX (M = Re, Mn and X = Cl and Br) towards catalysis for the electrochemical conversion of CO₂ to CO. Unlike prior well-studied group VII catalysts, Mn(CO)₃CNC^BnX is extraordinarily active, while the new Re(CO)₃CNC^BnX complex surprisingly does not exhibit catalytic response. DFT calculations shed light on this puzzling behavior and show that the redox-active pyridine-2,6-bis-N-heterocyclic carbene ligand facilitates the reduction of the ground-state complexes; however, the extent of electronic delocalization in the reduced intermediates differs in the degree of metalloradical character. The highly-active Mn(CO)₃CNC^BnX complex proceeds through an intermediate with nucleophilic metalloradical character in which 66% of the unpaired electron spin resides on Mn. In contrast, Re(CO)₃CNC^BnX reduction proceeds through an intermediate with less metalloradical character in which only 38% of the unpaired spin is localized on Re with the remainder delocalized over the ligand. The energetic penalty of the electron delocalization of an electron on the ligand affects the M-CO bond strengths and related kinetic barriers. We discuss these observations in the context of turnover-enabling effects in CO₂ reductions mediated by group VII NHC pincer molecular electrocatalysts.

Transition metal-based catalysts for the electrochemical CO2 reduction: from atoms and molecules to nanostructured materials

An overview of the main strategies for the rational design of transition metal-based catalysts for the electrochemical conversion of CO₂, ranging from molecular systems to single-atom and nanostructured catalysts.