From CO2 activation to catalytic reduction: a metal-free approach

Homogeneous catalytic hydrogenation of CO2 - amino acid-based capture and utilization

In this review, we provide an overview of research efforts to integrate carbon dioxide capture specifically using amino acid-based sorbents with its thermocatalytic hydrogenation promoted by homogeneous metal complexes. Carbon capture and utilization (CCU) is a promising strategy for the production of fuels, chemicals and materials using CO2 scrubbed from point sources and the atmosphere as a C1 feedstock while mitigating CO2 emissions. Compared to established (alkanol)amines, amino acids offer some advantages as CO2 capture agents due to their lower volatility, higher oxygen stability and lower regeneration energies. We report how the structural diversity of amino acids and the possibility of combining them with cations in salts and ionic liquids have been exploited in the design of absorbers for improved absorption kinetics and capacity. Furthermore, we discuss selected examples from the literature illustrating the use of 1°/2° (poly)amines, since the 1°/2° amino groups are mainly responsible for CO2 chemisorption in amino acid-based capture media, the nature of the corresponding adducts, and the most promising catalysts capable of converting the latter to formate and methanol while regenerating the scrubber. General trends regarding the influence of catalyst structure and reaction parameters on the efficiency, productivity, and selectivity of such processes will be highlighted. We will detail how this knowledge has informed the design of novel processes in which CO2 is chemisorbed by amino acid-based solvents and hydrogenated in situ to formate and methanol, or alternatively used as a fuel to implement a "hydrogen battery" where, after metal-catalyzed H2 release from formate, CO2 is retained by the amino acid-based solvent in the "spent battery" which can then be recharged by hydrogenation of the retained CO2 promoted by the same catalyst. The topic is still in its infancy, and several issues have emerged that will be critically discussed in the final section of this review. These issues need to be addressed in order to improve performance and provide a playground for researchers whose interest we hope to have aroused with this review.

A Biocompatible Cinchonine-Based Catalyst for the CO2 Valorization into Oxazolidin-2-ones Under Ambient Conditions

A metal-free, biocompatible catalyst for the cycloaddition of CO2 to N-alkyl aziridines was easily obtained by protonating the natural and nontoxic alkaloid (+)-cinchonine. This bifunctional catalytic system promoted the synthesis of the desired products under very mild experimental conditions (room temperature and atmospheric CO2 pressure) and without the aid of any cocatalyst. No specific equipment is required, making the procedure practical for application in any laboratory. The high synthetic value of this methodology can be attributed to the combination of excellent regioselectivity in oxazolidinone synthesis and the remarkable chemical stability of the catalyst, which can be recycled and reused for at least three consecutive cycles without any significant loss of activity.

Metal-Free Catalytic N-Methylation of NH-Sulfoximines Using CO2

This study reports the catalytic N-methylation of NH-sulfoximines using carbon dioxide (CO2) under metal-free conditions. A mesoionic N-heterocyclic olefin (mNHO) was used as a catalyst for the N-methylation of NH-sulfoximines in the presence of 9-borabicyclo[3.3.1]nonane (9-BBN) under mild conditions. This process was used to convert various NH-sulfoximines into N-methylsulfoximines. This protocol was also applicable for the synthesis of 13C-labeled N-methylsulfoximines using 13CO2. A mechanistic cycle was proposed by performing a series of control reactions and successfully characterizing active catalytic intermediates by either single-crystal X-ray or NMR spectroscopic studies.

Integrated Capture and Electrocatalytic Conversion of CO2: A Molecular Electrocatalysts Perspective

The ever-increasing concentration of atmospheric CO2, primarily driven by anthropogenic activities, has raised urgent environmental concerns, spurring the development of carbon capture and utilization (CCU) technologies. This review focuses on the integrated capture and electrochemical conversion of CO2 (ICECC), a promising approach that combines carbon capture with its direct electroreduction into value-added products. By eliminating energy-intensive steps such as CO2 release, compression, and transportation, ICECC offers a more energy-efficient and cost-effective alternative to conventional CCU methods. In this review, particular attention is given to molecular electrocatalysts, which offer high tunability and selectivity in electrochemical CO2 reduction reaction (eCO2RR). The role of capturing agents, including both external and dual-functional molecular systems, is critically examined to understand their influence on CO2 binding and catalytic efficiency. Whereas ICECC has significant potential, research in this area remains underexplored compared to conventional CO2 reduction methods. The review discusses the mechanistic insights into ICECC processes, highlighting key challenges and potential future research directions for improving catalyst design, enhancing capture efficiency, and scaling up ICECC technologies. These developments can make ICECC a critical component in achieving carbon neutrality and addressing climate change.

Selective Organocatalytic Synthesis of 3-Methylindoles and Diindolylmethanes through Reductive Methylation of Indoles with CO2

The direct reductive methylation of indoles with CO2 remains a formidable challenge due to competing reduction pathways and precise selectivity control. Herein, cooperative catalytic systems comprising 2-aminopyridine-BH3 and DIC-BH3 are disclosed, which enable unprecedented chemoselective C3-methylation of indoles under mild conditions (1 atm CO2). This strategy delivers two distinct product classes through controlled reduction: 3-methylindoles (up to 81% yield) and diindolylmethanes (up to 82% yield) with excellent functional group tolerance. This protocol offers novel synthetic routes for the synthesis of indole-based bioactive molecules.

Three-Component Synthesis of Oxazolidinones via Phosphine-Catalyzed Fixation of Carbon Dioxide and Mechanistic Investigation in Mass Spectrometry

A phosphine-catalyzed three-component cyclization reaction between anilines, carbon dioxide, and chloroalkanes was developed for the synthesis of oxazolidinones. This strategy not only proceeds under ambient CO2 pressure and metal-free condition but also shows a broad substrate scope, including aromatic amines, aliphatic amines, chiral amino acid esters, and bioactive molecules, providing an efficient and environmentally benign route to synthesize pharmaceutically relevant N-aryl-oxazolidinones. Mechanistic investigations utilizing mass spectrometry (MS) indicate the involvement of multiple phosphine intermediates in this process, thereby elucidating the underlying mechanism. Moreover, the relationships between these phosphine intermediates and Tolman cone angles or the solvent effect of phosphines were examined through mass spectrometry.

Ce[N(SiMe3)2]3(THF)3-Catalyzed Hydroboration of CO2, Esters and Epoxides with Pinacolborane: Selective Synthesis of Methanol in Multigram Scale

In this work, we have reduced CO2 with HBpin to afford borylated methanol product selectively in ~99 % yield using Ce[N(SiMe3)2]3(THF)3 as a catalyst. This led to multigram scale isolation of methanol obtained from CO2 reduction via the hydrolysis of borylated methanol, this establishes the potential of Ce[N(SiMe3)2]3(THF)3 as an efficient homogeneous catalyst for the bulk scale methanol synthesis. A practical application of this catalytic system was also shown by reducing CO2-containing motorbike exhaust efficiently and selectively. Further, C-O bond activation of esters and epoxides using HBpin and 1-2 mol % of Ce[N(SiMe3)2]3(THF)3 at 60 °C afforded the borylated alcohols in good to excellent yields, which can easily be hydrolysed to the eco-friendly corresponding alcohol. The stoichiometric experiments were performed to prove the formation of in-situ generated cerium hydride [Ce]-H as an active catalyst.

Independent LUMO Reactivity in Mesoionic N-Heterocyclic Thiones: Synthesis of a Stable Radical Anion

Mesoionic compounds, with positive and negative charges, are expected to have dual-site highest occupied molecular orbital (HOMO, donor) and lowest unoccupied molecular orbital (LUMO, acceptor) reactivity. Herein, we report a novel class of air-stable mesoionic N-heterocyclic thiones (mNHTs) synthesized from abnormal N-heterocyclic carbenes (aNHCs). DFT studies revealed a highly polarized exocyclic thione moiety and computed Fukui function analysis suggests the dual-site HOMO/LUMO reactivity of mNHTs predicting donor property at the negatively charged 'S' center while acceptor property at the cationic imidazole ring. The independent LUMO reactivity of the mNHT was realized by its chemical reduction to an elusive radical anion, which was characterized by a single crystal X-raydiffraction study. Further, we explore the reactivity of radical anion for the activation of SO2 gas, C-Br bonds of aryl bromide and photocatalytic functionalization of C-X (X = F, Br) bonds. This work unlocks the independent LUMO reactivity of a mesoionic compound.

Reversible CO2 insertion into the silicon-nitrogen σ-bond of an N-heterocyclic iminosilane

The reversible insertion of carbon dioxide into the silicon-nitrogen bond of an N-heterocyclic iminosilane is reported. Solution-phase thermodynamic investigations indicate that this process is thermoneutral and reversible, whereas in the solid-phase CO2 can be stored for extended periods and is only released upon heating to 133 °C.

Proton-Responsive Ligands Promote CO2 Capture and Accelerate Catalytic CO2/HCO2- Interconversion

The synthesis and investigation of [Rh(DHMPE)2][BF4] (1) are reported. 1 features proton-responsive 1,2-bis[(dihydroxymethyl)phosphino]ethane (DHMPE) ligands, which readily capture CO2 from atmospheric sources upon deprotonation. The protonation state of the DHMPE ligand was observed to have a significant impact on the catalytic reactivity of 1 with CO2. Deprotonation and CO2 binding to 1 result in a ∼10-fold rate enhancement in catalytic degenerate CO2 reduction with formate, monitored by 12C/13C isotope exchange between H12CO2- and 13CO2. Studies performed using a similar complex lacking the hydroxyl ligand functionality ([Rh(DEPE)2][BF4] where DEPE = 1,2-bis(diethylphosphino)ethane) do not show the same rate enhancements when base is added. Based upon the cation-dependent activity of the catalyst, Eyring analysis, and cation sequestration experiments, CO2 binding to 1 is proposed to facilitate preorganization of formate/CO2 in the transition state via ligand-based encapsulation of Na+ or K+ cations to lower the activation energy and increase the observed catalytic rate. Incorporation of proton-responsive DHMPE ligands provides a unique approach to accelerate the kinetics of catalytic CO2 reduction to formate.

Experimental and computational aspects of molecular frustrated Lewis pairs for CO2 hydrogenation: en route for heterogeneous systems?

Catalysis plays a crucial role in advancing sustainability. The unique reactivity of frustrated Lewis pairs (FLPs) is driving an ever-growing interest in the transition metal-free transformation of small molecules like CO2 into valuable products. In this area, there is a recent growing incentive to heterogenize molecular FLPs into porous solids, merging the benefits of homogeneous and heterogeneous catalysis - high activity, selectivity, and recyclability. Despite the progress, challenges remain in preventing deactivation, poisoning, and simplifying catalyst-product separation. This review explores the expanding field of FLPs in catalysis, covering existing molecular FLPs for CO2 hydrogenation and recent efforts to design heterogeneous porous systems from both experimental and theoretical perspectives. Section 2 discusses experimental examples of CO2 hydrogenation by molecular FLPs, starting with stoichiometric reactions and advancing to catalytic ones. It then examines attempts to immobilize FLPs in porous matrices, including siliceous solids, metal-organic frameworks (MOFs), covalent organic frameworks, and disordered polymers, highlighting current limitations and challenges. Section 3 then reviews computational studies on the mechanistic details of CO2 hydrogenation, focusing on H2 splitting and hydride/proton transfer steps, summarizing efforts to establish structure-activity relationships. It also covers the computational aspects on grafting FLPs inside MOFs. Finally, Section 4 summarizes the main design principles established so far, while addressing the complexities of translating computational approaches into the experimental realm, particularly in heterogeneous systems. This section underscores the need to strengthen the dialogue between theoretical and experimental approaches in this field.

Four-electron reduction of CO2: from formaldehyde and acetal synthesis to complex transformations

The expansive and dynamic field of the CO2 Reduction Reaction (CO2RR) seeks to harness CO2 as a sustainable carbon source or energy carrier. While significant progress has been made in two, six, and eight-electron reductions of CO2, the four-electron reduction remains understudied. This review fills this gap, comprehensively exploring CO2 reduction into formaldehyde (HCHO) or acetal-type compounds (EOCH2OE, with E = [Si], [B], [Zr], [U], [Y], [Nb], [Ta] or -R) using various CO2RR systems. These encompass (photo)electro-, bio-, and thermal reduction processes with diverse reductants. Formaldehyde, a versatile C1 product, is challenging to synthesize and isolate from the CO2RR. The review also discusses acetal compounds, emphasizing their significance as pathways to formaldehyde with distinct reactivity. Providing an overview of the state of four-electron CO2 reduction, this review highlights achievements, challenges, and the potential of the produced compounds - formaldehyde and acetals - as sustainable sources for valuable product synthesis, including chiral compounds.

Electron-rich pyridines with para-N-heterocyclic imine substituents: ligand properties and coordination to CO2, SO2, BCl3 and PdII complexes

Electron-rich pyridines with π donor groups at the para position play an important role as nucleophiles in organocatalysis, but their ligand properties and utilization in coordination chemistry have received little attention. Herein, we report the synthesis of two electron-rich pyridines 1 and 2 bearing N-heterocyclic imine groups at the para position and explore their coordination chemistry. Experimental and computational methods were used to assess the donor ability of the new pyridines showing that they are stronger donors than aminopyridines and guanidinyl pyridines, and that the nature of the N-heterocyclic backbone has a strong influence on the pyridine donor strength. Coordination compounds with Lewis acids including the CO2, SO2, BCl3 and PdII ions were synthesized and characterized. Despite the ambident character of the new pyridines, coordination preferentially occurs at the pyridine-N atom. Methyl transfer experiments reveal that 1 and 2 can act as demethylation reagents.

Skeletal Formation of Carbocycles with CO2: Selective Synthesis of Indolo[3,2-b]carbazoles or Cyclophanes from Indoles, CO2, and Phenylsilane

The catalytic reactions of indoles with CO2 and phenylsilane afforded indolo[3,2-b]carbazoles, where the fused benzene ring was constructed by forming two C-H bonds and four C-C bonds with two CO2 molecules via deoxygenative conversions. Nine-membered cyclophanes made up of three indoles and three CO2 molecules were also obtained, where the cyclophane framework was constructed by forming six C-H bonds and six C-C bonds. These multicomponent cascade reactions giving completely different carbocycles were switched simply by choosing the solvent, acetonitrile or ethyl acetate.

Borohydride Ionic Liquids as Reductants of CO2 in the Selective N-formylation of Amines

Borohydride imidazolium ionic liquids, [IL]BH4, used for the first time as reductants in the N-formylation of various amines with CO2, provided an excellent yield of formamides. Under the same conditions, 5 bar CO2 and 80 °C, NaBH4 produced a mixture of N-formylated and N-methylated products in a ratio of 1 : 2. An alternative approach, based on the addition of halide imidazolium salts ([IL]Cl or [IL]Br) to the reactions of amine with NaBH4 and CO2, resulted in a significant increase of selectivity to formamide. However, no effect was noted for [IL]BF4 and [IL]PF6. Monitoring the reaction course in time using 1H NMR brought about new insight into the role of BH3 in the reduction of CO2 and the functionalization of amines. The formation of N-methylaniline - borane intermediate was evidenced.

Controlling the Activation at NiII -CO2 2- Moieties through Lewis Acid Interactions in the Second Coordination Sphere

Nickel complexes with a two-electron reduced CO2 ligand (CO2 2- , "carbonite") are investigated with regard to the influence alkali metal (AM) ions have as Lewis acids on the activation of the CO2 entity. For this purpose complexes with NiII (CO2 )AM (AM=Li, Na, K) moieties were accessed via deprotonation of nickel-formate compounds with (AM)N(i Pr)2 . It was found that not only the nature of the AM ions in vicinity to CO2 affect the activation, but also the number and the ligation of a given AM. To this end the effects of added (AM)N(R)2 , THF, open and closed polyethers as well as cryptands were systematically studied. In 14 cases the products were characterized by X-ray diffraction and correlations with the situation in solution were made. The more the AM ions get detached from the carbonite ligand, the lower is the degree of aggregation. At the same time the extent of CO2 activation is decreased as indicated by the structural and spectroscopic analysis and reactivity studies. Accompanying DFT studies showed that the coordinating AM Lewis acidic fragment withdraws only a small amount of charge from the carbonite moiety, but it also affects the internal charge equilibration between the LtBu Ni and carbonite moieties.

Reactivity of NHI-Stabilized Heavier Tetrylenes towards CO2 and N2 O

A heteroleptic amino(imino)stannylene (TMS2 N)(It BuN)Sn: (TMS=trimethylsilyl, It Bu=C[(N-t Bu)CH]2 ) as well as two homoleptic NHI-stabilized tetrylenes, (It BuN)2 E: (NHI=N-heterocyclic imine, E=Ge, Sn) are presented. VT-NMR investigations of (It BuN)2 Sn: (2) reveal an equilibrium between the monomeric stannylene at room temperature and the dimeric form at -80 °C as well as in the solid state. Upon reaction of the homoleptic tetrylenes with CO2 , both compounds insert two equivalents of CO2 , however differing bonding modes can be observed. (It BuN)2 Sn: (2) inserts one equivalent of CO2 into each Sn-N bond, giving carbamato groups coordinated κ2 O,O' to the metal center. With (It BuN)2 Ge: (3), the Ge-N bonds stay intact upon activation, being bridged by one molecule of CO2 respectively, forming 4-membered rings. Furthermore, the reactivity of 2 towards N2 O was investigated, resulting in partial oxidation to form stannylene dimer [((It BuN)3 SnO)(It BuN)Sn:]2 (6).

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

The rise of CO2 concentrations in the environment due to anthropogenic activities results in global warming and threatens the future of humanity and biodiversity. To address excessive CO2 emissions and its effects on climate change, efforts towards CO2 capture and conversion into value adduct products such as methane, methanol, acetic acid, and carbonates have grown. Frustrated Lewis pairs (FLPs) can activate small molecules, including CO2 and convert it into value added products. This review covers recent progress and mechanistic insights into intra- and inter-molecular FLPs comprised of varying Lewis acids and bases (from groups 13, 14, 15 of the periodic table as well as transition metals) that activate CO2 in stoichiometric and catalytic fashion towards reduced products.

Two-dimensional graphitic metal carbides: structure, stability and electronic properties

Via first-principles computational modeling and calculations, we propose a new class of two-dimensional (2D) atomically thin crystals that contain metal-C3(MC3) moieties periodically distributed in a graphenic lattice, which we refer to as 2D graphitic metal carbides (g-MCs). Most g-MCs are dynamically stable as verified by the calculated phonon spectra. Our detailed chemical bonding analyzes reveal that the high stability of g-MCs can be attributed to a unique bonding feature, which manifests as the carbon-backbone-mediated metal-metal interactions. These analyzes provide new insights for understanding the stability of 2D materials. It is found that the calculated electronic band gaps and magnetic moments (per unit cell) of g-MCs can range from 0 to 1.30 eV and 0 to 4.40μB, respectively. Highly tunable electronic properties imply great potential of 2D g-MCs in various applications. As an example, we show that 2D g-MnC can be an excellent electrocatalyst towards CO2reductive reaction for the formation of formic acid with an exceptionally high loading of Mn atoms (∼43 wt%). We expect this work to simulate new experiments for fabrication and applications of g-MCs.

Cu-catalyzed carboxylation of organoboronic acid pinacol esters with CO2

Chemical fixation of CO2 has received much attention. In particular, catalytic C-C bond formation with CO2 giving carboxylic acids is of great significance. Among the CO2 fixation methods, multiple carboxylation is one of the challenging subjects. Here we investigated the Cu-catalyzed carboxylation of a variety of boronic acid pinacol esters (C(sp2)-, C(sp3)-, and C(sp)-B compounds) with CO2, which efficiently provided the corresponding products, including aryl, alkenyl, alkyl, and alkynyl carboxylic acids. This carboxylation was also applicable to multiple CO2 fixation giving di- and tri-carboxylic acids under robust reaction conditions (totally 29 examples).

Bicyclic (alkyl)(amino)carbene (BICAAC) in a dual role: activation of primary amides and CO2 towards catalytic N-methylation

Herein, we report the first catalytic methylation of primary amides using CO₂ as a C1 source. A bicyclic (alkyl)(amino)carbene (BICAAC) exhibits dual role by activating both primary amide and CO₂ to carry out this catalytic transformation which enables the formation of a new C-N bond in the presence of pinacolborane. This protocol was applicable to a wide range of substrate scopes, including aromatic, heteroaromatic, and aliphatic amides. We successfully used this procedure in the diversification of drug and bioactive molecules. Moreover, this method was explored for isotope labelling using ¹³CO₂ for a few biologically important molecules. A detailed study of the mechanism was carried out with the help of spectroscopic studies and DFT calculations.

Metal-free reduction of CO2 to formate using a photochemical organohydride-catalyst recycling strategy

Increasing levels of CO₂ in the atmosphere is a problem that must be urgently resolved if the rise in current global temperatures is to be slowed. Chemically reducing CO₂ into compounds that are useful as energy sources and carbon-based materials could be helpful in this regard. However, for the CO₂ reduction reaction (CO₂RR) to be operational on a global scale, the catalyst system must: use only renewable energy, be built from abundantly available elements and not require high-energy reactants. Although light is an attractive renewable energy source, most existing CO₂RR methods use electricity and many of the catalysts used are based on rare heavy metals. Here we present a transition-metal-free catalyst system that uses an organohydride catalyst based on benzimidazoline for the CO₂RR that can be regenerated using a carbazole photosensitizer and visible light. The system is capable of producing formate with a turnover number exceeding 8,000 and generates no other reduced products (such as H₂ and CO).

Direct Conversion of Low-Concentration CO2 into N-Aryl and N-Alkyl Carbamic Acid Esters Using Tetramethyl Orthosilicate with Amidines as a CO2 Capture Agent and a Catalyst

Herein, we report the direct conversion of low-concentration CO₂ (15 vol %), equivalent to the CO₂ concentration in the exhaust gas from a thermal power station, into carbamic acid esters (CAEs), which are precursors for pharmaceuticals, agrochemicals, and isocyanates. The reaction was performed using Si(OMe)₄ as a nonmetallic regenerable reagent and 1,8-diazabicyclo[5.4.0]undec-7-ene as a CO₂ capture agent and catalyst. This reaction system does not require the addition of metal complex catalysts or metal salt additives and is therefore simpler than our previously reported reaction system involving Ti(OR)₄ and a Zn(II) catalyst. A variety of N-aryl, N-alkyl, and bis CAEs (precursors of polyurethane raw materials) were obtained in moderate to high yields (45-77% for 6 examples, 84-89% for 7 examples). In addition, bis CAEs were successfully synthesized from simulated exhaust gas containing impurities such as SO₂, NO₂, and CO or on a gram scale. We believe that this method can eliminate the use of toxic phosgene as the raw material for isocyanate production and mitigate CO₂ emissions.

Efficiency in Carbon Dioxide Fixation into Cyclic Carbonates: Operating Bifunctional Polyhydroxylated Pyridinium Organocatalysts in Segmented Flow Conditions

Novel polyhydroxylated ammonium, imidazolium, and pyridinium salt organocatalysts were prepared through N-alkylation sequences using glycidol as the key precursor. The most active pyridinium iodide catalyst effectively promoted the carbonation of a set of terminal epoxides (80 to >95% yields) at a low catalyst loading (5 mol%), ambient pressure of CO₂, and moderate temperature (75 °C) in batch operations, also demonstrating high recyclability and simple downstream separation from the reaction mixture. Moving from batch to segmented flow conditions with the operation of thermostated (75 °C) and pressurized (8.5 atm) home-made reactors significantly reduced the process time (from hours to seconds), increasing the process productivity up to 20.1 mmol_(product) h^-1 mmol_(cat)^-1, a value ~17 times higher than that in batch mode.

Metal-Ligand Cooperativity in MnI -Catalysed N-Formylation of Secondary Amides and Lactams Using CO2 at Room Temperature

A Mn^I catalyst featuring redox-active tridentate phenalenyl (PLY) ligand has been used for catalytic N-formylation of secondary amides and lactams under 1 atm CO₂ as a C₁ source at room temperature for the first time. The protocol is applicable to a wide range of secondary amides including heterocycles, bio-active cinnamide derivatives and the diversification of therapeutic molecules. In-depth mechanistic investigations based on experimental outcomes and DFT calculations suggested an unconventional metal-ligand cooperation, where a ligand-centred radical plays a crucial role in initiating the reaction process.

Optimal Icosahedral Copper-Based Bimetallic Clusters for the Selective Electrocatalytic CO2 Conversion to One Carbon Products

Electrochemical CO2 reduction reactions can lead to high value-added chemical and materials production while helping decrease anthropogenic CO2 emissions. Copper metal clusters can reduce CO2 to more than thirty different hydrocarbons and oxygenates yet they lack the required selectivity. We present a computational characterization of the role of nano-structuring and alloying in Cu-based catalysts on the activity and selectivity of CO2 reduction to generate the following one-carbon products: carbon monoxide (CO), formic acid (HCOOH), formaldehyde (H2C=O), methanol (CH3OH) and methane (CH4). The structures and energetics were determined for the adsorption, activation, and conversion of CO2 on monometallic and bimetallic (decorated and core@shell) 55-atom Cu-based clusters. The dopant metals considered were Ag, Cd, Pd, Pt, and Zn, located at different coordination sites. The relative binding strength of the intermediates were used to identify the optimal catalyst for the selective CO2 conversion to one-carbon products. It was discovered that single atom Cd or Zn doping is optimal for the conversion of CO2 to CO. The core@shell models with Ag, Pd and Pt provided higher selectivity for formic acid and formaldehyde. The Cu-Pt and Cu-Pd showed lowest overpotential for methane formation.

Mesoionic N-Heterocyclic Imines as Super Nucleophiles in Catalytic Couplings of Amides with CO2

An extended class of stable mesoionic N-heterocyclic imines (mNHIs), containing a highly polarized exocyclic imine moiety, were synthesized. The calculated proton affinities (PA) and experimentally determined Tolman electronic parameters (TEPs) reveal that these synthesized mNHIs have the highest basicity and donor ability among NHIs reported so far. The superior nucleophilicity of newly designed mNHIs was utilized in devising a strategy to incorporate CO₂ as a bridging unit under reductive conditions to couple inert primary amides. This strategy was further extended to hetero-couplings between amide and amine using CO₂ . These hitherto unknown catalytic transformations were introduced in the diversification of various biologically active drug molecules under metal-free conditions. The underlying mechanism was explored by performing a series of control experiments, characterizing key intermediates using spectroscopic and crystallographic techniques.

A Zintl Cluster for Transition Metal-Free Catalysis: C═O Bond Reductions

The first fully characterized boron-functionalized heptaphosphide Zintl cluster, [(BBN)P7]2- ([1]2-), is synthesized by dehydrocoupling [HP7]2-. Dehydrocoupling is a previously unprecedented reaction pathway to functionalize Zintl clusters. [Na(18-c-6)]2[1] was employed as a transition metal-free catalyst for the hydroboration of aldehydes and ketones. Moreover, the greenhouse gas carbon dioxide (CO2) was efficiently and selectively reduced to methoxyborane. This work represents the first examples of Zintl catalysis where the transformation is transition metal-free and where the cluster is noninnocent.

Hydroboration and Deoxygenation of CO2 Mediated by a Gallium(I) Cation

Hydroboration of CO₂ to formoxy borane occurs under ambient conditions in acetonitrile using pinacolborane HBpin in the presence of gallium(I) cation [(Me₄TACD)Ga][BAr₄] (1; Me₄TACD = N,N',N″,N'''-tetramethyl-1,4,7,10-tetraazacyclododecane; Ar = C₆H₃-3,5-Me₂). Slow turnover was accompanied by side reactions including ligand scrambling of HBpin to give BH₃(CH₃CN) and crystalline B₂pin₃. When 1 was reacted with CO₂ alone, the formation of the gallium(III) carbonato complex [(Me₄TACD)Ga(κ₂-O₂CO)][BAr₄] (3) along with CO was observed. This complex was assumed to form via the unstable oxido cation [(Me₄TACD)Ga=O]⁺ (4). Reaction of 1 with N₂O in the presence of BPh₃ confirmed the formation of the oxido cation, which was spectroscopically characterized as a triphenylborane adduct [(Me₄TACD)Ga=O(BPh₃)][BAr₄] (4·BPh₃). CO was also detected when CO₂ was reacted with 1 in the presence of HBpin, suggesting that compound 3 may also be formed in initial stages of catalysis. Compound 3 reacts with HBpin to give formoxy borane, borane redistribution products, and an unidentified Me₄TACD-containing species 5, which was also observed in "catalytic" runs starting from 1, HBpin, and CO₂. Hydroboration of CO₂ using HBpin with slow turnover and competitive ligand scrambling was also observed in the presence of gallium(III) hydride dication [(Me₄TACD)GaH][BAr₄]₂ (2), which is unreactive toward CO₂ in the absence of HBpin.

Catalytic and Mechanistic Approach to the Metal-Free N-Alkylation of 2-Aminopyridines with Diketones

N-alkylation of amines is an important catalytic reaction in synthetic chemistry. Herein, we report a simple strategy for the N-alkylation of 2-aminopyridines with 1,2-diketones using BF₃·OEt₂ as a catalyst. The reaction proceeds under aerobic conditions, leading to the formation of a diverse range of substituted secondary amines in good to excellent yields. A close inspection of the mechanistic pathway using various spectroscopic techniques and the computational study revealed that the reaction proceeds through the formation of an iminium-keto intermediate with the liberation of CO₂.

Mechanochemical vs Wet Approach for Directing CO2 Capture toward Various Carbonate and Bicarbonate Networks

The distinct research areas related to CO₂ capture and mechanochemistry are both highly attractive in the context of green chemistry. However, merger of these two areas, i.e., mechanochemical CO₂ capture, is still in an early stage of development. Here, the application of biguanidine as an active species for CO₂ capture is investigated using both solution-based and liquid-assisted mechanochemical approaches, which lead to a variety of biguanidinium carbonate and bicarbonate hydrogen-bonded networks. We demonstrate that in solution, the formation of the carbonate vs bicarbonate networks can be directed by the organic solvent, while, remarkably, in the liquid-assisted mechanochemical synthesis employing the same solvents as additives, the selectivity in network formation is inversed. In general, our findings support the view of mechanochemistry not only as a sustainable alternative but rather as a complementary strategy to solution-based synthesis.

Evidence for Borylene Carbonyl (LHB═C═O) and Base-Stabilized (LHB═O) and Base-Free Oxoborane (RB≡O) Intermediates in the Reactions of Diborenes with CO2

Doubly N-heterocyclic-carbene-stabilized diborenes undergo facile reactions with CO₂, initially providing dibora-β-lactones. These lactones convert over time to their 2,4-diboraoxetan-3-one isomers through a presumed dissociative pathway and hypovalent boron species borylene carbonyls (LHB═C═O) and base-stabilized oxoboranes (LHB═O). Repeating these reactions with doubly cyclic(alkyl)(amino)carbene-stabilized diborenes allowed the isolation of a borylene carbonyl intermediate, whereas a base-stabilized oxoborane could be inferred by the isolation of a boroxine from the reaction mixture. These results, supported by calculations, confirm the presumed mechanism of the diboralactone-to-diboraoxetanone isomerization while also establishing a surprising level of stability for three unknown or very rare hypovalent boron species: base-stabilized derivatives of the parent borylene carbonyl (LHB═C═O) and parent oxoborane (LHB═O) as well as base-free oxoboranes (RB≡O).

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pKa shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO2 .

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.

Catalyst-free CO2 Hydrogenation with BH3NH3 in Water: DFT Mechanistic Insights

Extensive DFT calculations show that BH3NH3 may transfer dihydrogen to CO2 rather than HCO3- in water over a barrier of 25.9 kcal/mol, followed by faster hydride transfer from borate anions...

Regioselective ring-opening of epoxides towards Markovnikov alcohols: A metal-free catalytic approach using abnormal N-heterocyclic carbene

Herein we report the first metal-free regioselective Markovnikov ring-opening of epoxides (selectivity up to 99%) using an abnormal N-heterocyclic carbene (aNHC) to yield secondary alcohols. DFT calculations and X-ray crystallography...