Electrocatalytic reduction of CO2 by thiophene-substituted rhenium(i) complexes and by their polymerized films

In situ Electropolymerized 3D Microporous Cobalt-Porphyrin Nanofilm for Highly Effective Molecular Electrocatalytic Reduction of Carbon Dioxide

Electrocatalytic CO2 reduction reaction (CO2 RR) based on molecular catalysts, for example, cobalt porphyrin, is promising to enhance the carbon cycle and mitigate current climate crisis. However, the electrocatalytic performance and accurate evaluations remain problems because of either the low loading amount or the low utilization rate of the electroactive CoN4 sites. Herein a monomer is synthesized, cobalt(II)-5,10,15,20-tetrakis(3,5-di(thiophen-2-yl)phenyl)porphyrin (CoP), electropolymerized onto carbon nanotubes (CNTs) networks, affording a molecular electrocatalyst of 3D microporous nanofilm (EP-CoP, 2-3 nm thickness) with highly dispersed CoN4 sites. The new electrocatalyst shortens the electron transfer pathway, accelerates the redox kinetics of CoN4 sites, and improves the durability of the electrocatalytic CO2 RR. From the intrinsic redox behavior of CoN4 sites, the effective utilization rate is obtained as 13.1%, much higher than that of the monomer assembled electrode (5.8%), and the durability is also promoted dramatically (>40 h) in H-type cells. In commercial flow cells, EP-CoP can achieve a faradic efficiency for CO (FECO ) over 92% at an overpotential of 160 mV. At a higher overpotential of 620 mV, the working current density can reach 310 mA cm-2 with a high FECO of 98.6%, representing the best performance for electrodeposited molecular porphyrin electrocatalysts.

Photoelectrocatalytic Dioxygen Reduction Based on a Novel Thiophene-Functionalized Tricarbonylchloro(1,10-phenanthroline)rhenium(I)

A novel Re (I) complex of [Re(CO)3Cl(L)], {L = 2-([2,2’-bithiophen]-5-yl)-1-phenyl-1H-imidazo [4,5-f][1,10]phenanthroline}, was synthesized, and its optical (UV–Visible absorption and emission spectroscopy), cyclovoltammetric and photoelectrochemical oxygen reduction properties were studied. The geometric and electronic properties were also investigated by density functional theory calculations. It was found that the ITO electrode coated with drop-casted [Re(CO)3Cl(L)] film exhibited cathodic photocurrent generation characteristics. The illuminated film exhibited a maximum cathodic photocurrent up to 30.4 μA/cm2 with an illumination intensity of 100 mW/cm2 white light at a bias potential of −0.4 V vs. SCE in O2-saturated electrolyte solution, which was reduced by 5.1-fold when thoroughly deoxygenated electrolyte solution was used, signaling that the electrode performed well on the photoelectrochemical oxygen reduction. The photo-electrocatalytic hydrogen peroxide production was proved with a maximum H2O2 concentration of 6.39 μM during 5 h of the photoelectrocatalytic process. This work would guide the construction of more efficient rhenium-based photo(electro)catalytic molecular systems for O2 sensing, hydrogen peroxide production and other types of photoelectrochemical energy conversion and storage.

Cobalt Phthalocyanine Cross-Linked Polypyrrole for Efficient Electroreduction of Low Concentration CO2 To CO

Immobilizing cobalt phthalocyanine (CoPc) onto the electrode surface is a significant approach to performing efficient electrochemical CO₂ reduction reaction (ECO₂ RR). Herein, sulfylphenoxy decorated CoPc cross-linked polypyrrole is prepared by in situ polymerization on the surface of carbon cloth. The synthesized N-rich catalyst exhibits above 95 % Faradaic efficiency toward CO (FE_CO ) at -0.9 V versus reversible hydrogen electrode (RHE) at least for 10 h in aqueous solution and even enables direct electrolysis at low CO₂ concentrations, being potential for coupling ECO₂ RR with CO₂ capture. This facile in situ polymerization strategy would pave the way for developing efficient and practical electrocatalysis for ECO₂ RR.

CO2 Electroreduction on Carbon-Based Electrodes Functionalized with Molecular Organometallic Complexes—A Mini Review

Heterogeneous electrochemical CO2 reduction has potential advantages with respect to the homogeneous counterpart due to the easier recovery of products and catalysts, the relatively small amounts of catalyst necessary for efficient electrolysis, the longer lifetime of the catalysts, and the elimination of solubility problems. Unfortunately, several disadvantages are also present, including the difficulty of designing the optimized and best-performing catalysts by the appropriate choice of the ligands as well as a larger heterogeneity in the nature of the catalytic site that introduces differences in the mechanistic pathway and in electrogenerated products. The advantages of homogeneous and heterogeneous systems can be preserved by anchoring intact organometallic molecules on the electrode surface with the aim of increasing the dispersion of active components at a molecular level and facilitating the electron transfer to the electrocatalyst. Electrode functionalization can be obtained by non-covalent or covalent interactions and by direct electropolymerization on the electrode surface. A critical overview covering the very recent literature on CO2 electroreduction by intact organometallic complexes attached to the electrode is summarized herein, and particular attention is given to their catalytic performances. We hope this mini review can provide new insights into the development of more efficient CO2 electrocatalysts for real-life applications.

Efficient Electrochemical Reduction of CO2 to Formate in Methanol Solutions by Mn-Functionalized Electrodes in the Presence of Amines

Carbon cloth electrode modified by covalently attaching a manganese organometallic catalyst is used as cathode for the electrochemical reduction of CO₂ in methanol solutions. Six different industrial amines are employed as co‐catalyst in millimolar concentrations to deliver a series of new reactive system. While such absorbents were so far believed to provide a CO₂ reservoir and act as sacrificial proton source, we herein demonstrate that this role can be played by methanol, and that the adduct formed between CO₂ and the amine can act as an effector or inhibitor toward the catalyst, thereby enhancing or reducing the production of formate. Pentamethyldiethylentriamine (PMDETA), identified as the best effector in our series, converts CO₂ in wet methanolic solution into bisammonium bicarbonate. Computational studies revealed that this adduct is responsible for a barrierless transformation of CO₂ to formate by the reduced form of the Mn catalyst covalently bonded to the electrode surface. As a consequence, selectivity can be switched on demand from CO to formate anion, and in the case of (PMDETA) an impressive TON_HCOO− of 2.8×10⁴ can be reached. This new valuable knowledge on an integrated capture and utilization system paves the way toward more efficient transformation of CO₂ into liquid fuel.

Thiophene-Based Trimers and Their Bioapplications: An Overview

Certainly, the success of polythiophenes is due in the first place to their outstanding electronic properties and superior processability. Nevertheless, there are additional reasons that contribute to arouse the scientific interest around these materials. Among these, the large variety of chemical modifications that is possible to perform on the thiophene ring is a precious aspect. In particular, a turning point was marked by the diffusion of synthetic strategies for the preparation of terthiophenes: the vast richness of approaches today available for the easy customization of these structures allows the finetuning of their chemical, physical, and optical properties. Therefore, terthiophene derivatives have become an extremely versatile class of compounds both for direct application or for the preparation of electronic functional polymers. Moreover, their biocompatibility and ease of functionalization make them appealing for biology and medical research, as it testifies to the blossoming of studies in these fields in which they are involved. It is thus with the willingness to guide the reader through all the possibilities offered by these structures that this review elucidates the synthetic methods and describes the full chemical variety of terthiophenes and their derivatives. In the final part, an in-depth presentation of their numerous bioapplications intends to provide a complete picture of the state of the art.

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.

Role of the ancillary ligand in controlling the lysozyme affinity and electronic properties of terpyridine fac-Re(CO)3 complexes

The lysozyme binding affinity and the electronic properties of [ReX(CO)3(terpy-κ2N1,N2)] (X = Br- and triazolateCOOCH2CH3,CF3) were reported. The triazolate complex was prepared in a [3 + 2] cycloaddition click reaction. The bromo compound reacted with lysozyme affording adducts with Re(CO)3+ fragments, while the triazolate compound persisted. A red shift of the MLCT band of the triazolate compound in progressively less polar solvents may be due to the negative solvatochromism.

Tricarbonyl triazolato Re(i) compounds of pyridylbenzimidazole ligands: spectroscopic and antimicrobial activity evaluation

Catalyst-free [3+2] cycloaddition coupling between [Re_n(N₃)_n(CO)_3nL] (n = 1, L = 1-ethyl-2-(pyridin-2-yl)benzimidazole (L¹) and n = 2, L = 1,1′-(hexane-1,6-diyl)bis[2-(pyridin-2-yl)-1H-benzimidazole] (L²)) and dimethyl acetylene dicarboxylate (DMAD) afforded mono- and binuclear triazolate complexes. Spectroscopic data presented unambiguous evidence for isomerization of the kinetically formed N(1) bound triazolate isomer into the N(2) analogue. The solvatochromism properties were assessed by UV/Vis spectroscopy with the aid of time dependent density functional theory calculations. The free ligands and their tricarbonyl triazolato Re(i) complexes were screened for their potential antimicrobial activity against different bacterial and fungal pathogens.

The antimicrobial activity and solvatochromism properties of mono- and binuclear tricarbonyl triazolato Re(i) complexes of pyridylbenzimidazole, formed by catalyst-free [3+2] cycloaddition reaction, were examined.

Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol^–1. The 1e^– electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl^– from 1. 1-OPh, which exhibits an entropically favored intramolecular Re–O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1 and Mn-1OH display a selective reduction of CO₂ to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re–O bond and a preference for phenolate-based reactivity with CO₂ slightly inhibit the electrocatalytic reduction of CO₂ to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)₃Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO₂ has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified.

Organic-Inorganic Hybrid Nanomaterials for Electrocatalytic CO2 Reduction

Electrochemical CO₂ reduction (ECR) to value-added chemicals and fuels is regarded as an effective strategy to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. To achieve CO₂ conversion with high faradaic efficiency, low overpotential, and excellent product selectivity, rational design and synthesis of efficient electrocatalysts is of significant importance, which dominates the development of ECR field. Individual organic molecules or inorganic catalysts have encountered a bottleneck in performance improvement owing to their intrinsic shortcomings. Very recently, organic-inorganic hybrid nanomaterials as electrocatalysts have exhibited high performance and interesting reaction processes for ECR due to the integration of the advantages of both heterogeneous and homogeneous catalytic processes, attracting widespread interest. In this work, the recent advances in designing various organic-inorganic hybrid nanomaterials at the atomic and molecular level for ECR are systematically summarized. Particularly, the reaction mechanism and structure-performance relationship of organic-inorganic hybrid nanomaterials toward ECR are discussed in detail. Finally, the challenges and opportunities toward controlled synthesis of advanced electrocatalysts are proposed for paving the development of the ECR field.

Electropolymerizable N-heterocyclic carbene complexes of Rh and Ir with enantiotropic polymorphic phases

New Rh (1) and Ir (2) complexes of an N-heterocyclic carbene (NHC) featuring a terthiophene backbone were synthesized. Single crystal X-ray diffraction studies of 1 and 2 at 100 K and 298 K respectively, revealed two enantiotropic polymorphic phases with similar lattice parameters for each compound. The transition temperature between two crystalline forms for each compound was determined by measuring the percentage of reflections with a different space group ranging from 100 K to 298 K. Both 1 and 2 were also found to catalyze the hydrogen transfer reaction. The conducting metallopolymers poly-1 and poly-2 were synthesized from oxidative electropolymerization of 1 and 2, respectively, in CH₂Cl₂ electrolyte solution on indium tin oxide-(ITO) coated glass. The electrochromic properties of synthesized conducting metallopolymers were studied by UV-vis-NIR spectroelectrochemistry.

A dinuclear rhenium complex in the electrochemically driven homogeneous and heterogeneous H+/CO2-reduction

A dinculear Re(CO)3 complex with a proton responsive phenol unit and a pyrene anchor in the ligand backbone was investigated in the electrochemical CO2/H+ conversion in solution and adsorbed on multi walled carbon nanotubes (MWCNT) on an GC electrode surface. The pyrene group unit is introduced at the end of the ligand synthesis via a coupling reaction, which allows for a versatile ligand modification in order to tune the electronic properties or to introduce various anchor groups for heterogenisation at a late stage. The redox chemistry of the pyrene-α-diimine-Re(CO)3 complex, 1, was investigated in N,N-dimethylformamide (dmf), including IR-spectroelectrochemical (IR-SEC) characterisation of the short lived, reduced species. Subsequently, the electrochemical H+/CO2-reduction catalysis in dmf/water was investigated. The complex catalyses syngas formation yielding CO and H2 with similar rates, namely in Faraday yields of 45% and 35%, respectively. Since the similar complex without the pyrene anchor in the backbone, I, prefers CO2 over H+ reduction, the formation of syngas was rationalised by the small differences in the redox properties and pKa values of the phenol-pyrene unit in regard to phenol unit as in I. Subsequently, the complex was adsorbed on multi walled carbon nanotubes (MWCNT) on a GC electrode surface. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) confirmed coating of the electrode. The immobilised complex was utilised in the electrochemical CO2/H+ reduction in dmf/water, however, the complex quickly desorbed under reductive conditions, likely due to the good solubility of the reduced species. Water as a solvent prevents desorption as confirmed by XPS, however, then a preference for H2 formation over syngas formation was observed under electrocatalytic conditions. Thus, these experiments show, that the results obtained in aqueous organic solution are not easily transferable to the heterogeneous systems operating in water due to changes in the reaction rates for competing pathways.

Spectroscopic characterization of a new Re(i) tricarbonyl complex with a thiosemicarbazone derivative: towards sensing and electrocatalytic applications

A novel Re(i) complex with a thiosemicarbazone derivative is described and fully characterized. Its was further explored as CO₂ reduction electrocatalyst, being the first complex with a thiosemicarbazone derivative applied to this goal.

Pyridylbenzimidazole based Re(i)(CO)3 complexes: antimicrobial activity, spectroscopic and density functional theory calculations

fac-[ReBr(CO)3(L1,2)] L1 = 1-ethyl-2-(pyridin-2-yl)benzimidazole (1) and L2 = 1-[(pyridin-2-yl) benzimidazole]-propyl-sulfonic acid (2), fac-[Re2Br2(CO)6L3] (3) L3 = 1,1'-(hexane-1,6-diyl)bis[2-(pyridin-2-yl)1H-benzimidazole] and fac-[ReBr(CO)3(L4,5-κ2N1N2)] (L4 = 2,6-bis(benzimidazol-2'-yl)pyridine (4) and L5 = 2,6-bis(1-ethyl-benzimidazol-2'-yl)pyridine (5) were synthesized and fully characterized using different spectrocopic and analytical tools. The spectrocopic data showed coordination of L1-3 to fac-ReBr(CO)3 via the benzimidazole and pyridine N-atoms. For 4 and 5, the absence of a two-fold axis of symmetry for L4,5 in the 1H NMR spectra reflect the κ2N1,N2 mode of coordination. The electronic properties of 1-5 were investigated by time-dependent density functional theory calculations in the singlet and triplet states. The ligands and their Re(i) complexes were assessed for their potential antimicrobial activity. Compound 5 was screened against non-malignant cell line (noncancerous human embryonic kidney cell line (HEK293)) as well as evaluated for its blood compatibility.

Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO₂ at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Highly Photoactive Polythiophenes Obtained by Electrochemical Synthesis from Bipyridine-Containing Terthiophenes

According to numerous previous reports, a Z-scheme with two photon absorbers is the most promising strategy to achieve artificial photosynthesis, but in addition to two efficient catalysts — one for oxygen evolution, the other for CO2 reduction — two different and complementary semiconducting sensitizers are required. Here we present the synthesis of two bipyridine-functionalized terthiophenes, which can be electropolymerized to give photoactive p-type semiconductors the capability to perform as photocathode in photoelectrochemical cells for water photosplitting or artificial photosynthesis. Indeed the bipyridine moiety in their structure allows the binding of transition metal carbonyl complexes employed in CO2 reduction, and their band-gap is suitable for the coupling with wide band-gap semiconductors, which have already found application as photoanodes. Finally, they are characterized by photogenerated charge carrier density between 1.1 and 1.4 × 1019 cm−3, with first-order recombination constant of 0.7–1.8 × 10−2 s−1. These figures are of the same order of magnitude of their inorganic counterparts and would therefore guarantee photoconductivity of the device and the activation of the organometallic catalysts with which they should be coupled to function as photocathodes for CO2 reduction.

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.

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.

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

A broad review of homogeneous and heterogeneous catalytic approaches toward CO2 reduction using organic, organometallic, and bioorganic systems is provided. Electrochemical, bioelectrochemical and photoelectrochemical approaches are discussed in terms of their faradaic efficiencies, overpotentials and reaction mechanisms. Organometallic complexes as well as semiconductors and their homogeneous and heterogeneous catalytic activities are compared to enzymes. In both cases, their immobilization on electrodes is discussed and compared to homogeneous catalysts in solution.

Electrochemical CO2 Reduction at Glassy Carbon Electrodes Functionalized by MnI and ReI Organometallic Complexes

The catalytic activities towards electrochemical CO₂ reduction of two new rhenium and manganese complexes, namely fac-Mn(apbpy)(CO)₃ Br (1) and fac-Re(apbpy)(CO)₃ Cl (2) (apbpy=4-(4-aminophenyl)-2,2'-bipyridine), in both homogeneous and heterogeneous phases are compared. A glassy carbon electrode (GCE) surface has been functionalized with complexes 1 and 2 by two approaches: a) direct electrochemical oxidation of the amino group with formation of C-N bonds, and b) electrochemical reduction of the corresponding diazonium salts with formation of C-C bonds. The chemically modified GCEs show efficient conversion of CO₂ into CO, with turnover numbers (TONs) about 60 times higher than those of the corresponding catalysts in homogeneous solutions, and in a much shorter time.

Recent advances in catalytic CO2 reduction by organometal complexes anchored on modified electrodes

A critical overview of the very recent literature on heterogeneous electrochemical catalysis of CO₂ reduction with organic metal modified electrodes is summarized by taking into account different functionalization methods.