Electrons, Photons, Protons and Earth-Abundant Metal Complexes for Molecular Catalysis of CO2 Reduction

No competitive inhibition of bicarbonate or carbonate for formate dehydrogenase from Candida boidinii -catalyzed CO2 reduction

Formate dehydrogenase from Candida boidinii (CbFDH) reversibly catalyzes the formate to CO2 with the redox coupling NAD+/NADH. While many studies on CbFDH-catalyzed formate oxidation in the presence of NAD+ are...

Electronic structure analysis of electrochemical CO2 reduction by iron-porphyrins reveals basic requirements to design catalysts bearing non-innocent ligands

Electrocatalytic CO2 reduction is a possible solution to the increasing CO2 concentration in the earth atmosphere, because it enables storage of energy while using the harmful CO2 feedstock as starting...

Influences of the precursor molar ratio in synthesis on the structures and visible-light driven CO2 reduction into solar fuel of CdS catalyst

Synthesizing highly efficient photocatalysts for photoreduction of CO2 into solar fuel is of great significance for solving the energy shortage and environmental pollution. Here a series of CdS photocatalysts are...

In Silico Design of Metal-Free Hydrophosphate Catalysts for Hydrogenation of CO2 to Formate

CO2 reduction by H2 using metal-free catalysts is highly challenging. Frustrated Lewis pairs (FLPs) have been considered potential metal-free catalysts for this reaction. However, most FLPs are unstable, which limits...

Promoting photocatalytic CO2 reduction through facile electronic modification of N-annulated perylene diimide rhenium bipyridine dyads

The development of CO₂ conversion catalysts has become paramount in the effort to close the carbon loop. Herein, we report the synthesis, characterization, and photocatalytic CO₂ reduction performance for a series of N-annulated perylene diimide (NPDI) tethered Re(bpy) supramolecular dyads [Re(bpy-C2-NPDI-R)], where R = –H, –Br, –CN, –NO₂, –OPh, –NH₂, or pyrrolidine (–NR₂). The optoelectronic properties of these Re(bpy-C2-NPDI-R) dyads were heavily influenced by the nature of the R-group, resulting in significant differences in photocatalytic CO₂ reduction performance. Although some R-groups (i.e. –Br and –OPh) did not influence the performance of CO₂ photocatalysis (relative to –H; TON_co ∼60), the use of an electron-withdrawing –CN was found to completely deactivate the catalyst (TON_co < 1) while the use of an electron-donating –NH₂ improved CO₂ photocatalysis four-fold (TON_co = 234). Despite being the strongest EWG, the –NO₂ derivative exhibited good photocatalytic CO₂ reduction abilities (TON_co = 137). Using a combination of CV and UV-vis-nIR SEC, it was elucidated that the –NO₂ derivative undergoes an in situ transformation to –NH₂ under reducing conditions, thereby generating a more active catalyst that would account for the unexpected activity. A photocatalytic CO₂ mechanism was proposed for these Re(bpy-C2-NPDI-R) dyads (based on molecular orbital descriptions), where it is rationalized that the photoexcitation pathway, as well as the electronic driving-force for NPDI²⁻ to Re(bpy) electron-transfer both significantly influence photocatalytic CO₂ reduction. These results help provide rational design principles for the future development of related supramolecular dyads.

Seven N-annulated perylene diimide tethered rhenium (2,2′-bipyridine) supramolecular dyads are evaluated as photocatalysts for the reduction for carbon dioxide, highlighting the importance of photoexcitation pathway and electronic driving-force.

An NAD+-type earth-abundant metal complex enabling photo-driven alcohol oxidation

For the first time, an NAD⁺-type earth-abundant metal complex [Zn(pbn)₂(H₂O)](ClO₄)₂ (1) was found to exhibit photo-induced oxidizing ability to convert various primary and secondary alcohols to the corresponding aldehyde and ketone compounds. In addition, a two-electron-reduced Zn(II) complex [Zn(pbnH-pbnH)(ClO₄)₂] (1red) comprising the novel C-C coupling ligand, obtained by the photo-induced oxidation of alcohols by 1, was successfully isolated and completely characterized. We clarified that the photochemical oxidation of alcohols by 1 to produce 1red proceeds via an electron transfer followed by proton transfer mechanism as elucidated by kinetic analysis on the basis of absorption spectroscopic measurements.

Mechanistic Insights Into Iron(II) Bis(pyridyl)amine-Bipyridine Skeleton for Selective CO2 Photoreduction

A bis(pyridyl)amine-bipyridine-iron(II) framework (Fe(BPAbipy)) of complexes 1-3 is reported to shed light on the multistep nature of CO₂ reduction. Herein, photocatalytic conversion of CO₂ to CO even at low CO₂ concentration (1 %), together with detailed mechanistic study and DFT calculations, reveal that 1 first undergoes two sequential one-electron transfer affording an intermediate with electron density on both Fe and ligand for CO₂ binding over proton. The following 2 H⁺ -assisted Fe-CO formation is rate-determining for selective CO₂ -to-CO reduction. A pendant, proton-shuttling α-OH group (2) initiates PCET for predominant H₂ evolution, while an α-OMe group (3) cancels the selectivity control for either CO or H₂ . The near-unity selectivity of 1 and 2 enables self-sorting syngas production at flexible CO/H₂ ratios. The unprecedented results from one kind of molecular catalyst skeleton encourage insight into the beauty of advanced multi-electron and multi-proton transfer processes for robust CO₂ RR by photocatalysis.

On the Roles of Electron Transfer in Catalysis by Nanoclusters and Nanoparticles

Electron transfer plays a major role in chemical reactions and processes, and this is particularly true of catalysis by nanomaterials. The advent of metal nanoparticle (NP) catalysts, recently including atomically precise nanoclusters (NCs) as parts of nanocatalyst devices has brought increased control of the relationship between NP and NC structures and their catalytic functions. Consequently, the molecular definition of these new nanocatalysts has allowed a better understanding and management of various kinds of electron transfer involved in the catalytic processes. This Minireview brings a chemist's view of several major aspects of electron-transfer functions concerning NPs and NCs in catalytic processes. Particular focus concerns the role of NPs and NCs as electron reservoirs and light-induced antenna in catalytic processes from H₂ generation to more complex reactions and sustainable energy production.

Prolonging the Triplet State Lifetimes of Rhenium Complexes with Imidazole-Pyridine Framework for Efficient CO2 Photoreduction

The photocatalytic reduction of CO₂ into fuels offers the prospect for creating a new CO₂ economy. Harnessing visible light-driven CO₂ -to-CO reduction mediated by the long-lived triplet excited state of rhenium(I) tricarbonyl complexes is a challenging approach. We here develop a series of new mononuclear rhenium(I) tricarbonyl complexes (Re-1-Re-4) based on the imidazole-pyridine skeleton for photo-driven CO₂ reduction. These catalysts are featured by combining pyridyl-imidazole with the aromatic ring and different pendant organic groups onto the N1 position of 1,3-imidazole unit, which display phosphorescence under Ar-saturated solution even at ambient conditions. By contrast, {Re[9-(pyren-1-yl)-10-(pyridin-2-yl)-9H-pyreno[4,5-d]imidazole)](CO)₃ Cl} (Re-4) by introducing pyrene ring at the N1 position of pyrene-fused imidazole unit exhibits superior catalytic performance with a higher turnover number for CO (TON_CO =124) and >99.9 % selectivity, primarily ascribed to the strong visible light-harvesting ability, long-lived triplet lifetimes (164.2 μs) and large reductive quenching constant. Moreover, the rhenium(I) tricarbonyl complexes derived from π-extended pyrene chromophore exhibit a long lifetime corresponding to its ligand-localized triplet state (³ IL) evidenced from spectroscopic investigations and DFT calculations.

The Synthesis of Protein-Encapsulated Ceria Nanorods for Visible-Light Driven Hydrogen Production and Carbon Dioxide Reduction

1D rare earth-based nanomaterials have attracted significant attention due to their excellent photo/electro-catalytic performance. The corresponding challenge is how to synthesize shape and size-controlled nanostructures in an easy scale-up way. Herein, the authors present a facile one-step strategy to design 1D multifunctional protein-encapsulated cerium oxide nanorods (PCNRs) by utilizing bovine serum albumin as an efficient biotemplate. Remarkably, the PCNRs exhibit high chemical and interfacial adhesion stability with intriguing properties, resulting in an exceptionally high activity towards H₂ evolution and CO₂ reduction. The photocatalytic activity of PCNRs to produce H₂ is about 10 times higher than conventional CeO₂ nanorods. The incorporation of rhodamine B into the PCNRs brings unprecedentedly high photocatalytic H₂ evolution rate being 123 times higher than that of conventional CeO₂ nanorods. Further the presence of the -NH₂ groups on the PCNRs facilitated the adsorption and activation of CO₂ and efficiently suppressed the proton reduction, and as a result, the PCNRs photocatalyst is highly active in converting CO₂ to CO and CH₄ , with the evolution rates being 50 and 83 times higher than those of conventional CeO₂ nanorods, respectively. Achieving such efficient photocatalyst is a critical step toward practical production of high-value renewable fuels using solar energy.

Development of a panchromatic photosensitizer and its application to photocatalytic CO2 reduction

We designed and synthesized a heteroleptic osmium(ii) complex with two different tridentate ligands, Os. Os can absorb the full wavelength range of visible light owing to S–T transitions, and this was supported by TD-DFT calculations. Excitation of Os using visible light of any wavelength generates the same lowest triplet metal-to-ligand charge-transfer excited state, the lifetime of which is relatively long (τ_em = 40 ns). Since excited Os could be reductively quenched by 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole, Os displays high potential as a panchromatic photosensitizer. Using a combination of Os and a ruthenium(ii) catalyst, CO₂ was photocatalytically reduced to HCOOH via irradiation with 725 nm light, and the turnover number reached 81; irradiation with light at λ_ex > 770 nm also photocatalytically induced HCOOH formation. These results clearly indicate that Os can function as a panchromatic redox photosensitizer.

The osmium(ii) complex functioned as a panchromatic photosensitizer and drove CO₂ reduction.

Click-Derived Triazoles and Triazolylidenes of Manganese for Electrocatalytic Reduction of CO2

A series of new fac-[Mn(L)(CO)₃Br] complexes where L is a bidentate chelating ligand containing mixed mesoionic triazolylidene-pyridine (MIC^py, 1), triazolylidene-triazole (MIC^trz, 2), and triazole-pyridine (trz^py, 3) ligands have been prepared and fully characterized, including the single crystal X-ray diffraction studies of 1 and 2. The abilities of 1–3 and complex fac-[Mn(MIC^MIC)(CO)₃Br] (4) to catalyze the electroreduction of CO₂ has been assessed for the first time. It was found that all complexes displayed a current increase under CO₂ atmosphere, being 3 and 4 the most active complexes. Complex 3, bearing a N^N-based ligand exhibited a good efficiency and an excellent selectivity for reducing CO₂ to CO in the presence of 1.0 M of water, at low overpotential. Interestingly, complex 4 containing the strongly electron donating di-imidazolylidene ligand exhibited comparable activity to 3, when the experiments were performed in neat acetonitrile at slightly higher overpotential (−1.86 vs. −2.14 V).

Sensitized Photocatalytic CO2 Reduction With Earth Abundant 3d Metal Complexes Possessing Dipicolyl-Triazacyclononane Derivatives

Complexes based on nitrogen and sulfur containing ligands involving 3d metal centers are known for the electrocatalytic reduction of CO_2. However, photocatalytical activation has rarely been investigated. We herein present results on the light-driven CO₂ reduction using either Ir(dFppy)₃ [Ir, dFppy = 2-(4,6-difluorophenyl)pyridine] or [Cu(xant)(bcp)]⁺, (Cu, xant = xantphos, bcp = bathocuproine) as photosensitizer in combination with TEA (triethylamine) as sacrificial electron donor. The 3d metal catalysts have either dptacn (dipicolyl-triazacyclononane, L^N3) or dpdatcn (dipicolyl-diazathiocyclononane, L^N2S) as ligand framework and Fe³⁺, Co³⁺ or Ni²⁺ as central metal ion. It turned out that the choice of ligand, metal center and solvent composition influences the selectivity for product formation, which means that the gaseous reduction products can be solely CO or H₂ or a mixture of both. The ratio between these two products can be controlled by the right choice of reaction conditions. With using Cu as photosensitizer, we could introduce an intermolecular system that is based solely on 3d metal compounds being able to reduce CO₂.

Lowering Electrocatalytic CO2 Reduction Overpotential Using N-Annulated Perylene Diimide Rhenium Bipyridine Dyads with Variable Tether Length

We report the design, synthesis, and characterization of four N-annulated perylene diimide (NPDI) functionalized rhenium bipyridine [Re(bpy)] supramolecular dyads. The Re(bpy) scaffold was connected to the NPDI chromophore either directly [Re(py-C0-NPDI)] or via an ethyl [Re(bpy-C2-NPDI)], butyl [Re(bpy-C4-NPDI)], or hexyl [Re(bpy-C6-NPDI)] alkyl-chain spacer. Upon electrochemical reduction in the presence of CO₂ and a proton source, Re(bpy-C2/4/6-NPDI) all exhibited significant current enhancement effects, while Re(py-C0-NPDI) did not. During controlled potential electrolysis (CPE) experiments at E_appl = -1.8 V vs Fc^+/0, Re(bpy-C2/4/6-NPDI) all achieved comparable activity (TON_co ∼ 25) and Faradaic efficiency (FE_co ∼ 94%). Under identical CPE conditions, the standard catalyst Re(dmbpy) was inactive for electrocatalytic CO₂ reduction; only at E_appl = -2.1 V vs Fc^+/0 could Re(dmbpy) achieve the same catalytic performance, representing a 300 mV lowering in overpotential for Re(bpy-C2/4/6-NPDI). At higher overpotentials, Re(bpy-C4/6-NPDI) both outperformed Re(bpy-C2-NPDI), indicating the possibility of coinciding electrocatalytic CO₂ reduction mechanisms that are dictated by tether-length and overpotential. Using UV-vis-nearIR spectroelectrochemistry (SEC), FTIR SEC, and chemical reduction experiments, it was shown that the NPDI-moiety served as an electron-reservoir for Re(bpy), thereby allowing catalytic activity at lower overpotentials. Density functional theory studies probing the optimized geometries and frontier molecular orbitals of various catalytic intermediates revealed that the geometric configuration of NPDI relative to the Re(bpy)-moiety plays a critical role in accessing electrons from the electron-reservoir. The improved performance of Re(bpy-C2/4/6-NPDI)dyads at lower overpotentials, relative to Re(dmbpy), highlights the utility of chromophore electron-reservoirs as a method for lowering the overpotential for CO₂ conversion.

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.

Electrocatalytic reduction of CO2 to CO and HCO2- with Zn(II) complexes displaying cooperative ligand reduction

Air-stable zinc(II) pyridyl phosphine complexes, [Zn(κ²-2,6-{Ph₂PNMe}₂(NC₅H₃))Br₂] (1) and [Zn(κ²-2-{Ph₂PNMe}(NC₅H₃))Br₂] (2) are reported and 1 was capable of electrocatalytic reduction of CO₂ at -2.3 V vs. Fc^+/0 to yield CO/HCO₂H in mixed water/acetonitrile solutions. DFT computations support a proposed mechanism involving electron transfer reactions from a species with the anionic PN³P ligand ("L^-/Zn(II)").

A Water-Soluble Sodium Pectate Complex with Copper as an Electrochemical Catalyst for Carbon Dioxide Reduction

A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO2 reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO2 to CH4 in water. Stability and selectivity of conversion of CO2 to CH4 as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO2 reduction reaction (CO2RR) proceeds at -1.5 V vs. Ag/AgCl at ~10 mA/cm2 current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO2). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.

Electrochemical Reduction of CO2: A Review of Cobalt Based Catalysts for Carbon Dioxide Conversion to Fuels

Electrochemical CO2 reduction reaction (CO2RR) provides a promising approach to curbing harmful emissions contributing to global warming. However, several challenges hinder the commercialization of this technology, including high overpotentials, electrode instability, and low Faradic efficiencies of desirable products. Several materials have been developed to overcome these challenges. This mini-review discusses the recent performance of various cobalt (Co) electrocatalysts, including Co-single atom, Co-multi metals, Co-complexes, Co-based metal-organic frameworks (MOFs), Co-based covalent organic frameworks (COFs), Co-nitrides, and Co-oxides. These materials are reviewed with respect to their stability of facilitating CO2 conversion to valuable products, and a summary of the current literature is highlighted, along with future perspectives for the development of efficient CO2RR.

Immobilising molecular Ru complexes on a protective ultrathin oxide layer of p-Si electrodes towards photoelectrochemical CO2 reduction

Photoelectrochemical CO2 reduction is a promising approach for renewable fuel generation and to reduce greenhouse gas emissions. Owing to their synthetic tunability, molecular catalysts for the CO2 reduction reaction can give rise to high product selectivity. In this context, a RuII complex [Ru(HO-tpy)(6-mbpy)(NCCH3)]2+ (HO-tpy = 4'-hydroxy-2,2':6',2''-terpyridine; 6-mbpy = 6-methyl-2,2'-bipyridine) was immobilised on a thin SiOx layer of a p-Si electrode that was decorated with a bromide-terminated molecular layer. Following the characterisation of the assembled photocathodes by X-ray photoelectron spectroscopy and ellipsometry, PEC experiments demonstrate electron transfer from the p-Si to the Ru complex through the native oxide layer under illumination and a cathodic bias. A state-of-the-art photovoltage of 570 mV was determined by comparison with an analogous n-type Si assembly. While the photovoltage of the modified photocathode is promising for future photoelectrochemical CO2 reduction and the p-Si/SiOx junction seems to be unchanged during the PEC experiments, a fast desorption of the molecular Ru complex was observed. An in-depth investigation of the cathode degradation by comparison with reference materials highlights the role of the hydroxyl functionality of the Ru complex to ensure its grafting on the substrate. In contrast, no essential role for the bromide function on the Si substrate designed to engage with the hydroxyl group of the Ru complex in an SN2-type reaction could be established.

Electrochemical reduction of CO2 to CO and HCOO- using metal-cyclam complex catalysts: predicting selectivity and limiting potential from DFT

Sustainable fuel production from CO₂ through electrocatalytic reduction is promising but challenging due to high overpotential and poor product selectivity. Herein, we computed the reaction free energies of electrocatalytic reduction of CO₂ to CO and HCOO^- using the density functional theory method and screened transition metal(M)-cyclam(L) complexes as molecular catalysts for CO₂ reduction. Our results showed that pK_a of the proton adduct formed by the protonation of the reduced metal center can be used as a descriptor to select the operating pH of the solution to steer the reaction toward either the CO or hydride cycle. Among the complexes, [LNi]²⁺ and [LPd]²⁺ catalyze the reactions by following the CO cycle and are the CO selective catalysts in the pH ranges 1.81-7.31 and 6.10 and higher, respectively. Among the complexes that catalyze the reactions by following the hydride cycle, [LMo]²⁺ and [LW]³⁺ are HCOO^- selective catalysts and have low limiting potentials of -1.33 V and -1.54 V, respectively. Other complexes, including [LRh]²⁺, [LIr]²⁺, [LW]²⁺, [LCo]²⁺, and [LTc]²⁺ catalyze the reactions resulting in either HCOO^- from CO₂ reduction or H₂ from proton reduction; however, HCOO^- formation is always thermodynamically more favorable. Notably, [LMo]²⁺, [LW]³⁺, [LW]²⁺ and [LCo]²⁺ have limiting potentials less negative than -1.6 V and are based on Earth-abundant elements, making them attractive for practical application.

Reduction of Carbon Dioxide with Ammonia-Borane under Ambient Conditions: Maneuvering a Catalytic Way

In the development of alternatives to the traditional catalytic hydrogenation of CO₂ with gaseous H₂, employing nongaseous H₂ storage compounds as potential reductants for catalytic transfer hydrogenation of CO₂ is promising. Ammonia-borane, due to its high hydrogen storage capacity (19.6 wt %), has been used for catalytic transfer hydrogenation of several organic unsaturated compounds. However, a similar protocol involving catalytic transfer hydrogenation of less reactive CO₂ with NH₃BH₃ is yet to be realized experimentally. Herein, we demonstrate the first catalytic CO₂ transfer hydrogenation process for generating formate salt with NH₃BH₃ under ambient conditions (1 atm and 30 °C) employing a cationic "Ir(III)-abnormal NHC" catalyst via an electrophilic NH₃BH₃ activation route. It exhibited an initial turnover frequency of 686 h^-1 and a high turnover number (TON) of ≈1300 in just 4 h. Most significantly, the catalyst was durable enough to maintain long-term activity, and upon only periodic recharging of NH₃BH₃, it furnished a total TON of >4200 in 10 h.

Recent Progress in (Photo-)-Electrochemical Conversion of CO2 With Metal Porphyrinoid-Systems

Since decades, the global community has been facing an environmental crisis, resulting in the need to switch from outdated to new, more efficient energy sources and a more effective way of tackling the rising carbon dioxide emissions. The activation of small molecules such as O₂, H⁺, and CO₂ in a cost—and energy-efficient way has become one of the key topics of catalysis research. The main issue concerning the activation of these molecules is the kinetic barrier that has to be overcome in order for the catalyzed reaction to take place. Nature has already provided many pathways in which small molecules are being activated and changed into compounds with higher energy levels. One of the most famous examples would be photosynthesis in which CO₂ is transformed into glucose and O₂ through sunlight, thus turning solar energy into chemical energy. For these transformations nature mostly uses enzymes that function as catalysts among which porphyrin and porphyrin-like structures can be found. Therefore, the research focus lies on the design of novel porphyrinoid systems (e.g. corroles, porphyrins and phthalocyanines) whose metal complexes can be used for the direct electrocatalytic reduction of CO₂ to valuable chemicals like carbon monoxide, formate, methanol, ethanol, methane, ethylene, or acetate. For example the cobalt(III)triphenylphosphine corrole complex has been used as a catalyst for the electroreduction of CO₂ to ethanol and methanol. The overall goal and emphasis of this research area is to develop a method for industrial use, raising the question of whether and how to incorporate the catalyst onto supportive materials. Graphene oxide, multi-walled carbon nanotubes, carbon black, and activated carbon, to name a few examples, have become researched options. These materials also have a beneficial effect on the catalysis through for instance preventing rival reactions such as the Hydrogen Evolution Reaction (HER) during CO₂ reduction. It is very apparent that the topic of small molecule activation offers many solutions for our current energy as well as environmental crises and is becoming a thoroughly investigated research objective. This review article aims to give an overview over recently gained knowledge and should provide a glimpse into upcoming challenges relating to this subject matter.

Rapid electron transfer via dynamic coordinative interaction boosts quantum efficiency for photocatalytic CO2 reduction

The fulfillment of a high quantum efficiency for photocatalytic CO₂ reduction presents a key challenge, which can be overcome by developing strategies for dynamic attachment between photosensitizer and catalyst. In this context, we exploit the use of coordinate bond to connect a pyridine-appended iridium photosensitizer and molecular catalysts for CO₂ reduction, which is systematically demonstrated by ¹H nuclear magnetic resonance titration, theoretical calculations, and spectroscopic measurements. The mechanistic investigations reveal that the coordinative interaction between the photosensitizer and an unmodified cobalt phthalocyanine significantly accelerates the electron transfer and thus realizes a remarkable quantum efficiency of 10.2% ± 0.5% at 450 nm for photocatalytic CO₂-to-CO conversion with a turn-over number of 391 ± 7 and nearly complete selectivity, over 4 times higher than a comparative system with no additional interaction (2.4%±0.2%). Moreover, the decoration of electron-donating amino groups on cobalt phthalocyanine can optimize the quantum efficiency up to 27.9% ± 0.8% at 425 nm, which is more attributable to the enhanced coordinative interaction rather than the intrinsic activity. The control experiments demonstrate that the dynamic feature of coordinative interaction is important to prevent the coordination occupancy of labile sites, also enabling the wide applicability on diverse non-noble-metal catalysts.

Positioning photosensitizer and catalyst complexes in photocatalytic systems is a promising method to direct desired electron transfers. Here, authors employ a dynamic coordinative interaction between molecular components to improve CO₂ photoreduction to CO with a high quantum efficiency of 27.9%.

Understanding entrapped molecular photosystem and metal-organic framework synergy for improved solar fuel production

Artificial photosystems assembled from molecular complexes, such as the photocatalyst fac-ReBr(CO)3(4,4'-dcbpy) (dcbpy = dicarboxy-2,2'-bipyridine) and the photosensitiser Ru(bpy)2(5,5'-dcbpy)Cl2 (bpy = 2,2'-bipyridine), are a wide-spread approach for solar fuel production. Recently metal-organic framework (MOF) entrapping of such complexes was demonstrated as a promising concept for catalyst stabilisation and reaction environment optimisation in colloidal-based CO2 reduction. Building on this strategy, here we examined the influence of MIL-101-NH2(Al) MOF particle size, the electron donor source, and the presence of an organic base on the photocatalytic CO2-to-CO reduction performance, and the differences to homogeneous systems. A linear relation between smaller scaffold particle size and higher photocatalytic activity, longer system lifetimes for benign electron donors, and increased turnover numbers (TONs) with certain additive organic bases, were determined. This enabled understanding of key molecular catalysis phenomena and synergies in the nanoreactor-like host-guest assembly, and yielded TONs of ∼4300 over 96 h of photocatalysis under optimised conditions, surpassing homogeneous TON values and lifetimes.

Photocatalytic deoxygenation of N-O bonds with rhenium complexes: from the reduction of nitrous oxide to pyridine N-oxides

The accumulation of nitrogen oxides in the environment calls for new pathways to interconvert the various oxidation states of nitrogen, and especially their reduction. However, the large spectrum of reduction potentials covered by nitrogen oxides makes it difficult to find general systems capable of efficiently reducing various N-oxides. Here, photocatalysis unlocks high energy species able both to circumvent the inherent low reactivity of the greenhouse gas and oxidant N₂O (E⁰(N₂O/N₂) = +1.77 V vs. SHE), and to reduce pyridine N-oxides (E_1/2(pyridine N-oxide/pyridine) = −1.04 V vs. SHE). The rhenium complex [Re(4,4′-tBu-bpy)(CO)₃Cl] proved to be efficient in performing both reactions under ambient conditions, enabling the deoxygenation of N₂O as well as synthetically relevant and functionalized pyridine N-oxides.

A rhenium-based photocatalyst enables the deoxygenation of several compounds containing N–O bonds, such as N₂O and pyridine N-oxides.

Electrocatalytic and Photocatalytic Reduction of Carbon Dioxide by Earth-abundant Bimetallic Molecular Catalysts

Converting CO₂ into useful resources by electrocatalysis and photocatalysis is a promising strategy for recycling of the gas and electrification of industries. Numerous studies have shown that multinuclear metal catalysts have higher selectivity and catalytic activity than monometallic catalysts due to the synergistic effects between the metal sites. In this review, we summarize some of the recent progress on the electrocatalytic and photocatalytic reduction of CO₂ by earth-abundant bimetallic molecular catalysts.

Ligand Radical Mediated Water Oxidation by a Family of Copper o-Phenylene Bis-oxamidate Complexes

Understanding the reactivity landscape for the activation of water until the formation of the O-O bond and O₂ release in molecular chemistry is a decisive step in guiding the elaboration of cost-effective catalysts for the oxygen-evolving reaction (OER). Copper(II) complexes have recently caught the attention of chemists as catalysts for the 4e^-/4H⁺ water oxidation process. While a copper(IV) intermediate has been proposed as the reactive intermediate species, no spectroscopic signature has been reported so far. Copper(III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper(II) o-phenylene bis-oxamidate complexes is a function of the substitution pattern on the periphery of the aromatic ring. In-situ EPR, FTIR, and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading toward the O₂ formation selectively at the copper center. EPR and FTIR spectroelectrochemistry suggests that ligand-centered oxidations are preferred over metal-centered oxidations. rR spectroelectrochemical study revealed the accumulation of a bis-imine bound copper(II) superoxide species, as the reactive intermediate, under catalytic turnover, which provides the evidence for the O-O bond formation during OER.

Metalloporphyrin Encapsulation for Enhanced Conversion of CO2 to C2H4

Electrochemical conversion of CO₂ into valuable products is a promising approach. Efficient electrocatalysts are highly desirable but remain to be developed. Here, we proposed a molecular encapsulation strategy to enrich intermediates for facilitating electrochemical conversion of CO₂ to C₂H₄. This strategy is combining M-TCPP [M = FeCl, Co, and Ni; TCPP = tetrakis(4-carboxyphenyl) porphyrin] with a Cu-based metal-organic framework (Cu-MOF) to create a series of metalloporphyrin-decorated Cu catalysts with a coral-like shape (named as M-TCPP@Cu). M-TCPP in the catalysts could supply more CO intermediates to the Cu sites, giving high selectivity for producing C₂H₄ and lowering overpotentials for CO₂ reduction. Meanwhile, the coral-like structure of the catalyst with abundant active sites is conducive to mass diffusion and benefits the conversion of CO₂. We realized a higher C₂H₄ Faradaic efficiency (FE) of 33.42% at -1.17 V versus reversible hydrogen electrode (RHE) on the Fe-TCPP@Cu electrode than that on the sole Cu electrode (16.85%, at -1.27 V vs RHE). Furthermore, due to the encapsulated structure resulted from one-pot reaction that ensures the dispersion of active centers in M-TCPP, metalloporphyrin-decorated Cu catalysts show better performance than the physical mixture of Cu-MOFs and M-TPPs (M = FeCl, Co, and Ni; TPP = 5,10,15,20-tetraphenylporphyrin). The results provide a new strategy for the design of high-performance Cu catalysts from Cu-MOFs for CO₂ conversion.

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.

Hierarchical Metal-Polymer Hybrids for Enhanced CO2 Electroreduction

The design of catalysts with high activity, selectivity, and stability is key to the electroreduction of CO₂ . Herein, we report the synthesis of 3D hierarchical metal/polymer-carbon paper (M/polymer-CP) electrodes by in situ electrosynthesis. The 3D polymer layer on CP (polymer-CP) was first prepared by in situ electropolymerization, then a 3D metal layer was decorated on the polymer-CP to produce the M/polymer-CP electrode. Electrodes with different metals (e.g. Cu, Pd, Zn, Sn) and various polymers could be prepared by this method. The electrodes could efficiently reduce CO₂ to desired products, such as C₂ H₄ , CO, and HCOOH, depending on the metal used. For example, C₂ H₄ could be formed with a Faradaic efficiency of 59.4 % and a current density of 30.2 mA cm^-2 by using a very stable Cu/PANI-CP electrode in an H-type cell. Control experiments and theoretical calculations showed that the 3D hierarchical structure of the metals and in situ formation of the electrodes are critical for the excellent performance.

Through-Space Electrostatic Interactions Surpass Classical Through-Bond Electronic Effects in Enhancing CO2 Reduction Performance of Iron Porphyrins

In his pioneering work to unravel the catalytic power of enzymes, Warshel has pertinently validated that electrostatic interactions play a major role in the activation of substrates. Implementing such chemical artifice in molecular catalysts may help improve their catalytic properties. In this study, a series of tetra-, di-, and mono-substituted iron porphyrins with cationic imidazolium groups were designed. Their presence in the second coordination sphere helped stabilize the [Fe-CO₂ ] intermediate through electrostatic interactions. It was found herein that the electrocatalytic overpotential is a function of the number of embarked imidazolium. Importantly, a gain of six orders of magnitude in turnover frequencies was observed going from a tetra- to a mono-substituted catalyst. Furthermore, the comparative study showed that catalytic performances trend of through-space electrostatic interaction, a first topological effect reported for iron porphyrins, outperforms the classical through-structure electronic effect.