Manganese N-Heterocyclic Carbene Pincers for the Electrocatalytic Reduction of Carbon Dioxide

Palladium-anchored donor-flexible pyridylidene amide (PYA) electrocatalysts for CO2 reduction

The conversion of CO2 into CO as a substitute for processing fossil fuels to produce hydrocarbons is a sustainable, carbon neutral energy technology. However, the electrochemical reduction of CO2 into a synthesis gas (CO and H2) at a commercial scale requires an efficient electrocatalyst. In this perspective, a series of six new palladium complexes with the general formula [Pd(L)(Y)]Y, where L is a donor-flexible PYA, N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, N2,N6-bis(1-butylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, or N2,N6-bis(1-benzylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide, and Y = OAc or Cl-, were utilized as active electrocatalysts for the conversion of CO2 into a synthesis gas. These palladium(ii) pincer complexes were synthesized from their respective H-PYA proligands using 1,8-diazobicyclo[5.4.0]undec-7-ene (DBU) or sodium acetate as a base. All the compounds were successfully characterized by various physical methods of analysis, such as proton and carbon NMR, FTIR, CHN, and single-crystal XRD. The redox chemistry of palladium complexes toward carbon dioxide activation suggested an evident CO2 interaction with each Pd(ii) catalyst. [Pd(N2,N6-bis(1-ethylpyridin-4(1H)-ylidene)pyridine-2,6-dicarboxamide)(Cl)]Cl showed the best electrocatalytic activity for CO2 reduction into a synthesis gas under the acidic condition of trifluoracetic acid (TFA) with a minimum overpotential of 0.40 V, a maximum turnover frequency (TOF) of 101 s-1, and 58% FE of CO. This pincer scaffold could be stereochemically tuned with the exploration of earth abundant first row transition metals for further improvements in the CO2 reduction chemistry.

CO2 Activation with Manganese Tricarbonyl Complexes through an H-Atom Responsive Benzimidazole Ligand

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Selective Transfer Hydrogenation of C=O and Conjugated C=C Bonds Using An NHC-Based Pincer (CNC)MnI Complex in Methanol

Base metal catalyzed transfer hydrogenation reactions using methanol is highly challenging. Employing a single N-heterocyclic carbene (NHC)-based pincer (CNC)MnI complex, chemoselective single and double transfer hydrogenation of α, β-unsaturated ketones to saturated ketones or alcohols by utilizing methanol as the hydrogen source is disclosed. The protocol was tolerant towards the selective transfer hydrogenation of C=C or C=O bonds in the presence of several other reducible functional groups and led to the synthesis of several biologically relevant molecules and natural products. Notably, this is the first report of a Mn-catalyzed transfer hydrogenation of carbonyl groups with methanol. Several control experiments, kinetic studies, Hammett studies, and density functional theory (DFT) calculations were carried out to understand the mechanistic details of this catalytic process.

Analyzing Structure-Activity Variations for Mn-Carbonyl Complexes in the Reduction of CO2 to CO

Contemporary electrocatalysts for the reduction of CO₂ often suffer from low stability, activity, and selectivity, or a combination thereof. Mn-carbonyl complexes represent a promising class of molecular electrocatalysts for the reduction of CO₂ to CO as they are able to promote this reaction at relatively mild overpotentials, whereby rare-earth metals are not required. The electronic and geometric structure of the reaction center of these molecular electrocatalysts is precisely known and can be tuned via ligand modifications. However, ligand characteristics that are required to achieve high catalytic turnover at minimal overpotential remain unclear. We consider 55 Mn-carbonyl complexes, which have previously been synthesized and characterized experimentally. Four intermediates were identified that are common across all catalytic mechanisms proposed for Mn-carbonyl complexes, and their structures were used to calculate descriptors for each of the 55 Mn-carbonyl complexes. These electronic-structure-based descriptors encompass the binding energies, the highest occupied and lowest unoccupied molecular orbitals, and partial charges. Trends in turnover frequency and overpotential with these descriptors were analyzed to afford meaningful physical insights into what ligand characteristics lead to good catalytic performance, and how this is affected by the reaction conditions. These insights can be expected to significantly contribute to the rational design of more active Mn-carbonyl electrocatalysts.

Photocatalytic Reduction of CO2 to CO in Aqueous Solution under Red-Light Irradiation by a Zn-Porphyrin-Sensitized Mn(I) Catalyst

This work demonstrates photocatalytic CO₂ reduction by a noble-metal-free photosensitizer-catalyst system in aqueous solution under red-light irradiation. A water-soluble Mn(I) tricarbonyl diimine complex, [MnBr(4,4′-{Et₂O₃PCH₂}₂-2,2′-bipyridyl)(CO)₃] (1), has been fully characterized, including single-crystal X-ray crystallography, and shown to reduce CO₂ to CO following photosensitization by tetra(N-methyl-4-pyridyl)porphyrin Zn(II) tetrachloride [Zn(TMPyP)]Cl₄ (2) under 625 nm irradiation. This is the first example of 2 employed as a photosensitizer for CO₂ reduction. The incorporation of −P(O)(OEt)₂ groups, decoupled from the core of the catalyst by a −CH₂– spacer, afforded water solubility without compromising the electronic properties of the catalyst. The photostability of the active Mn(I) catalyst over prolonged periods of irradiation with red light was confirmed by ¹H and ¹³C{¹H} NMR spectroscopy. This first report on Mn(I) species as a homogeneous photocatalyst, working in water and under red light, illustrates further future prospects of intrinsically photounstable Mn(I) complexes as solar-driven catalysts in an aqueous environment.

A Mn(I) bipyridyl tricarbonyl complex, where the diimine ligand is functionalized with water-solubilizing phosphonate ester groups, has been prepared and is shown to catalytically convert CO₂ to CO in aqueous solution following photosensitization from a water-soluble Zn(II) porphyrin under red-light irradiation.

C-H Bond Functionalization under Electrochemical Flow Conditions

Electrochemical C-H functionalization is a rapidly growing area of interest in organic synthesis. To achieve maximum atom economy, the flow electrolysis process is more sustainable. This allows shorter reaction times, safer working environments, and better selectivities. Using this technology, the problem of overoxidation can be reduced and less emergence of side products or no side products are possible. Flow electro-reactors provide high surface-to-volume ratios and contain electrodes that are closely spaced where the diffusion layers overlap to give the desired product, electrochemical processes can now be managed without the need for a deliberately added supporting electrolyte. Considering the importance of flow electrochemical C-H functionalization, a comprehensive review is presented. Herein, we summarize flow electrolysis for the construction of C-C and C-X (X=O, N, S, and I) bonds formation. Also, benzylic oxidation and access to biologically active molecules are discussed.

Coupling of CO2 and epoxides catalysed by novel N-fused mesoionic carbene complexes of nickel(II)

We report the syntheses of two rigid mesoionic carbene (MIC) ligands with a carbazole backbone via an intramolecular Finkelstein-cyclisation cascade and investigate their coordination behavior towards nickel(II) acetate. Despite the nickel(II) carbene complexes 4a,b showing only minor differences in their chemical composition, they display curious differences in their chemical properties, e.g. solubility. Furthermore, the potential of these novel MIC complexes in the coupling of carbon dioxide and epoxides as well as the differences in reactivity compared to classical NHC-derived complexes are evaluated.

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.

Synthesis, characterization, and electrocatalytic activity of bis(pyridylimino)isoindoline Cu(ii) and Ni(ii) complexes

Similar structure observed between Cu(ii) and Ni(ii) based bis(pyridylimino)isoindole complexes, yet greatly different levels of catalytic activity.

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.

Homogeneous and heterogeneous molecular catalysts for electrochemical reduction of carbon dioxide

Carbon dioxide (CO2) is a greenhouse gas whose presence in the atmosphere significantly contributes to climate change. Developing sustainable, cost-effective pathways to convert CO2 into higher value chemicals is essential to curb its atmospheric presence. Electrochemical CO2 reduction to value-added chemicals using molecular catalysis currently attracts a lot of attention, since it provides an efficient and promising way to increase CO2 utilization. Introducing amino groups as substituents to molecular catalysts is a promising approach towards improving capture and reduction of CO2. This review explores recently developed state-of-the-art molecular catalysts with a focus on heterogeneous and homogeneous amine molecular catalysts for electroreduction of CO2. The relationship between the structural properties of the molecular catalysts and CO2 electroreduction will be highlighted in this review. We will also discuss recent advances in the heterogeneous field by examining different immobilization techniques and their relation with molecular structure and conductive effects.

Research Progress in Conversion of CO2 to Valuable Fuels

Rapid growth in the world's economy depends on a significant increase in energy consumption. As is known, most of the present energy supply comes from coal, oil, and natural gas. The overreliance on fossil energy brings serious environmental problems in addition to the scarcity of energy. One of the most concerning environmental problems is the large contribution to global warming because of the massive discharge of CO₂ in the burning of fossil fuels. Therefore, many efforts have been made to resolve such issues. Among them, the preparation of valuable fuels or chemicals from greenhouse gas (CO₂) has attracted great attention because it has made a promising step toward simultaneously resolving the environment and energy problems. This article reviews the current progress in CO₂ conversion via different strategies, including thermal catalysis, electrocatalysis, photocatalysis, and photoelectrocatalysis. Inspired by natural photosynthesis, light-capturing agents including macrocycles with conjugated structures similar to chlorophyll have attracted increasing attention. Using such macrocycles as photosensitizers, photocatalysis, photoelectrocatalysis, or coupling with enzymatic reactions were conducted to fulfill the conversion of CO₂ with high efficiency and specificity. Recent progress in enzyme coupled to photocatalysis and enzyme coupled to photoelectrocatalysis were specially reviewed in this review. Additionally, the characteristics, advantages, and disadvantages of different conversion methods were also presented. We wish to provide certain constructive ideas for new investigators and deep insights into the research of CO₂ conversion.

Chemical and Electrochemical Recycling of End-Use Poly(ethylene terephthalate) (PET) Plastics in Batch, Microwave and Electrochemical Reactors

This work describes new methods for the chemical recycling of end-use poly(ethylene terephthalate) (PET) in batch, microwave and electrochemical reactors. The reactions are based on basic hydrolysis of the ester moieties in the polymer framework and occur under mild reaction conditions with low-cost reagents. We report end-use PET depolymerization in refluxing methanol with added NaOH with 75% yield of terephthalic acid in batch after 12 h, while yields up to 65% can be observed after only 40 min under microwave irradiation at 85 °C. Using basic conditions produced in the electrochemical reduction of protic solvents, electrolytic experiments have been shown to produce 17% terephthalic acid after 1 h of electrolysis at −2.2 V vs. Ag/AgCl in 50% water/methanol mixtures with NaCl as a supporting electrolyte. The latter method avoids the use of caustic solutions containing high-concentration NaOH at the outset, thus proving the concept for a novel, environmentally benign method for the electrochemical recycling of end-use PET based on low-cost solvents (water and methanol) and reagents (NaCl and electricity).

Sustainable and Selective Alkylation of Deactivated Secondary Alcohols to Ketones by Non-bifunctional Pincer N-heterocyclic Carbene Manganese

A sustainable and green route to access diverse functionalized ketones via dehydrogenative-dehydrative cross-coupling of primary and secondary alcohols is demonstrated. This borrowing hydrogen approach employing a pincer N-heterocyclic carbene Mn complex displays high activity and selectivity. A variety of primary and secondary alcohols are well tolerant and result in satisfactory isolated yields. Mechanistic studies suggest that this reaction proceeds via a direct outer-sphere mechanism and the dehydrogenation of the secondary alcohol substrates plays a vital role in the rate-limiting step.

Ultrafast Spectroscopy of [Mn(CO)3] Complexes: Tuning the Kinetics of Light-Driven CO Release and Solvent Binding

Manganese tricarbonyl complexes are promising catalysts for CO₂ reduction, but complexes in this family are often photosensitive and decompose rapidly upon exposure to visible light. In this report, synthetic and photochemical studies probe the initial steps of light-driven speciation for Mn(CO)₃(^Rbpy)Br complexes bearing a range of 4,4'-disubstituted 2,2'-bipyridyl ligands (^Rbpy, where R = ^tBu, H, CF₃, NO₂). Transient absorption spectroscopy measurements for Mn(CO)₃(^Rbpy)Br coordination compounds with R = ^tBu, H, and CF₃ in acetonitrile reveal ultrafast loss of a CO ligand on the femtosecond time scale, followed by solvent coordination on the picosecond time scale. The Mn(CO)₃(^NO₂bpy)Br complex is unique among the four compounds in having a longer-lived excited state that does not undergo CO release or subsequent solvent coordination. The kinetics of photolysis and solvent coordination for light-sensitive complexes depend on the electronic properties of the disubstituted bipyridyl ligand. The results indicate that both metal-to-ligand charge-transfer (MLCT) and dissociative ligand-field (d-d) excited states play a role in the ultrafast photochemistry. Taken together, the findings suggest that more robust catalysts could be prepared with appropriately designed complexes that avoid crossing between the excited states that drive photochemical CO loss.

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.

CuI SNS triazole and imidazole pincers as electrocatalyst precursors for the production of solar fuels

This work reports the first example of mono-nuclear Cu pincers with SNS ligation acting as electrocatalyst precursors for the electrochemical conversion of carbon dioxide to CO and H₂ in protic organic media.

Synthesis of a Redox-Active NNP-Type Pincer Ligand and Its Application to Electrocatalytic CO2 Reduction With First-Row Transition Metal Complexes

We report the synthesis of a rigid phosphine-substituted, redox-active pincer ligand and its application to electrocatalytic CO₂ reduction with first-row transition metal complexes. The tridentate ligand was prepared by Stille coupling of 2,8-dibromoquinoline and 2-(tributylstannyl)pyridine, followed by a palladium-catalyzed cross-coupling with HPPh₂. Complexes were synthesized from a variety of metal precursors and characterized by NMR, high-resolution mass spectrometry, elemental analysis, and cyclic voltammetry. Formation of bis-chelated metal complexes, rather than mono-chelated complexes, was favored in all synthetic conditions explored. The complexes were assessed for their ability to mediate electrocatalytic CO₂ reduction, where the cobalt complex was found to have the best activity for CO₂-to-CO conversion in the presence of water as an added proton source.

Elucidating the origins of enhanced CO2 reduction in manganese electrocatalysts bearing pendant hydrogen-bond donors

A mechanistic analysis showing the critical importance of an intramolecular hydrogen bond for improved insight and understanding in CO₂ electroreduction.

A look at periodic trends in d-block molecular electrocatalysts for CO2 reduction

Periodic trends in the electronic structure of the transition metal centers can be used to explain the observed CO₂ reduction activities in molecular electrocatalysts for CO₂ reductions. Research activities concerning both horizontal and vertical trends have been summarized with mononuclear complexes from Group 6 to Group 10.