CO2 Adducts of N-Heterocyclic Carbenes: Thermal Stability and Catalytic Activity toward the Coupling of CO2 with Epoxides

Computational Design of an Electro-Organocatalyst for Conversion of CO2 into Formaldehyde

Density functional theory calculations employing a hybrid implicit/explicit solvation method were used to explore a new strategy for electrochemical conversion of CO2 using an electro-organocatalyst. A particular structural motif is identified that consists of an electron-rich vicinal enediamine (>N-C═C-N<) backbone, which is capable of activating CO2 by the formation of a C-C bond while subsequently facilitating the transfer of electrons from a chemically inert cathode to ultimately produce formaldehyde. Unlike transition metal-based electrocatalysts, the electro-organocatalyst is not constrained by scaling relations between the formation energies of activated CO2 and adsorbed CO, nor is it expected to be active for the competing hydrogen evolution reaction. The rate-limiting steps are found to occur during two proton-coupled electron transfer (PCET) sequences and are associated with the transfer of a proton from a proton transfer mediator to a carbon atom on the electro-organocatalyst. The difficulty of this step in the second PCET sequence necessitates an electrode potential of -0.85 V vs RHE to achieve the maximum turnover frequency. In addition, it is postulated that the electro-organocatalyst should also be capable of forming long-chain aldehydes by successively carrying out reductive aldol condensation to grow the alkyl chain one carbon at a time.

Switchable Organocatalysis from N-heterocyclic Carbene-Carbodiimide Adducts with Tunable Release Temperature

N-Heterocyclic carbenes (NHCs) are powerful organocatalysts, but practical applications often require in situ generation from stable precursors that "mask" the NHC reactivity via reversible binding. Previously established "masks" are often simple small molecules, such that the NHC structure is used to control both catalytic activity and activation temperature, leading to undesirable tradeoffs. Herein, we show that NHC-carbodiimide (CDI) adducts can be masked precursors for switchable organocatalysis and that the CDI substituents can control the reaction profile without changing the NHC structure. Large electronic variations on the CDI (e.g., alkyl versus aryl) drastically change the catalytically active temperature, whereas smaller perturbations (e.g., different para-substituted phenyls) tune the catalyst release within a narrower window. This control was demonstrated for three classic NHC-catalyzed reactions, each influencing the NHC-CDI equilibrium in different ways. Our results introduce a new paradigm for controlling NHC organocatalysis as well as present practical considerations for designing appropriate masks for various reactions.

3D Cyclophane for the Selective Conversion of Epoxide to Cyclic Carbonate

A novel three-dimensional (3D) cyclophane molecule 1 was synthesized and fully characterized. Cyclophane 1, which can form a N heterocyclic carbene, was tested for conversion of certain epoxides (3-6) [scheme 2] to cyclic carbonates in the presence of CO2. Propylene oxide (3) was found to have more reactivity with cyclophane 1 compared to the other epoxides. The theoretical calculations based on N,N'-disubstituted imidazol(in)ium-2-carboxylates derived from N,N' disubstituted imidazole as the source of N-heterocyclic carbene show lower activation energy in the case of the reactivity of epoxides 5 and 6 as compared to 3 and 4. However, cyclophane 1, which possesses a 3D geometry, can form the open intermediate with CO2 and propylene oxide more feasibly than the other three epoxides, which have larger sizes as compared to propylene oxide. Hence, the reaction of propylene oxide, CO2, and cyclophane 1 can follow the mechanistic path 1, whereas the epoxides 4-6 can follow a different mechanistic path 2. Cyclophane 1 is the first example of a cyclophane to act as an organocatalyst for the conversion of CO2 and epoxide to cyclic carbonate via the N heterocyclic carbene pathway.

Organocatalytic Polymers from Affordable and Readily Available Building Blocks for the Cycloaddition of CO2 to Epoxides

The catalytic cycloaddition of CO₂ to epoxides to afford cyclic carbonates as useful monomers, intermediates, solvents, and additives is a continuously growing field of investigation as a way to carry out the atom-economic conversion of CO₂ to value-added products. Metal-free organocatalytic compounds are attractive systems among various catalysts for such transformations because they are inexpensive, nontoxic, and readily available. Herein, we highlight and discuss key advances in the development of polymer-based organocatalytic materials that match these requirements of affordability and availability by considering their synthetic routes, the monomers, and the supports employed. The discussion is organized according to the number (monofunctional versus bifunctional materials) and type of catalytically active moieties, including both halide-based and halide-free systems. Two general synthetic approaches are identified based on the postsynthetic functionalization of polymeric supports or the copolymerization of monomers bearing catalytically active moieties. After a review of the material syntheses and catalytic activities, the chemical and structural features affecting catalytic performance are discussed. Based on such analysis, some strategies for the future design of affordable and readily available polymer-based organocatalysts with enhanced catalytic activity under mild conditions are considered.

Equilibrating parent aminomercaptocarbene and CO2 with 2-amino-2-thioxoacetic acid via heavy-atom quantum tunneling

The search for methods to bind CO2 and use it synthetically as a C1-building block under mild conditions is an ongoing endeavor of great urgency. The formation of heterocyclic carbene-carbon dioxide adducts occurs rapidly when the carbene is generated in solution in the presence of CO2. Here we demonstrate the reversible formation of a complex of the hitherto unreported aminomercaptocarbene (H2N-C̈-SH) with CO2 isolated in solid argon by photolysis of 2-amino-2-thioxoacetic acid. Remarkably, the complex disappears in the dark as deduced by time-dependent matrix infrared measurements, and equilibrates back to the covalently bound starting material. This kinetically excluded process below ca. 8 K is made possible through heavy-atom quantum mechanical tunneling, as also evident from density functional theory and ab initio computations at the CCSD(T)/cc-pVTZ level of theory. Our results provide insight into CO2 activation using a carbene and emphasize the role of quantum mechanical tunneling in organic processes, even involving heavy atoms.

Evidence for the encounter complex in frustrated Lewis pair chemistry

Frustrated Lewis Pairs (FLPs) are combinations of bulky Lewis acids and bases that can carry out small-molecule activation and catalysis. Mechanistically, the reaction of the acid, base and substrate involves the collision of three distinct molecules, and so the pre-association of the acid and base to form an encounter complex has been proposed. This article will examine the evidence for the formation of this encounter complex, focusing on the archetypal main-group combinations P(^tBu)₃/B(C₆F₅)₃ and PMes₃/B(C₆F₅)₃ (Mes = mesityl), and includes quantum chemical calculations, molecular dynamics simulations, NMR spectroscopic measurements and neutron scattering. Furthermore, the recent discovery that the associated acid and base can absorb a photon to promote single-electron transfer has enabled the encounter complex to also be studied by UV-Vis spectroscopy, EPR spectroscopy, transient absorption spectroscopy, and resonance Raman spectroscopy. These data all support the notion that the encounter complex is only weakly held together and in low concentration in solution. The insights that these studies provide underpin the exciting transformations that can be promoted by FLPs. Finally, some observations and unanswered questions are provided to prompt further study in this field.

Sequestration of Carbon Dioxide with Frustrated Lewis Pairs Based on N-Heterocycles with Silane/Germane Groups

Frustrated Lewis pairs (FLPs) based on nitrogen heterocycles (pyridine, pyrazole, and imidazole) with a silane or germane group in the α-position of a nitrogen atom have been considered as potential molecules to sequestrate carbon dioxide. Three stationary points have been characterized in the reaction profile: a pre-reactive complex, an adduct minimum, and the transition state connecting them. The effect of external (solvent) or internal (hydroxyl group) electric fields in the reaction profile has been considered. In both cases, it is possible to improve the kinetics and thermodynamics of the complexation of CO2 by the FLP and favor the formation of adducts.

Competitive Reactivity of SO2 and NO2 with N-Heterocyclic Carbene: A Mechanistic Study

Recent DFT based molecular engineering to obtain stable oxathiirane S-oxide derivatives evokes the recommencement of the use of carbenes for the sequestering of SO₂, which has been kept separate so far. Carbene is one of the key chemicals for the sequestering of various premier greenhouse gases like CO₂, CO, N₂O, etc. In this respect, a comparative study of the reactivity of carbenes with variant greenhouse gases is highly demanding. The present investigation is engrossed in the comparative reactivity of SO₂ and NO₂ with carbenes. All three selected carbenes are highly susceptible to SO₂ and NO₂. Through an immaculate mechanistic study, we are able to corroborate that the end product of the carbene-SO₂ reaction is an adduct which has a preferable structure having a six-membered ring with hydrogen bonding instead of ketone and SO with higher thermodynamic stability than the corresponding oxathiirane S-oxide derivative. Carbene reacts with NO₂ to form a stable carbene N, N-dioxide derivative which forms vibrationally excited oxaziridine N-oxide which rapidly dissociates to form a ketone derivative. The formation of carbene S, S-dioxide and carbene N, N-dioxide is a barrierless process. The dissociation of oxaziridene N-oxide is also a barrierless process.

Highly Selective Ortho-Directed Dicarboxylation of Cyclopentadiene by Methylcarbonates and CO2 or COS - First Insight into Co-ordination Chemistry of New Ambident Ligands

This research presents the highly regioselective syntheses of 1,2-dicarboxylated cyclopentadienide salts [Cat]2 [C5 H3 (CO2 )2 H] by reaction of a variety of organic cation methylcarbonate salts [Cat]OCO2 Me (Cat=NR4 + , PR4 + , Im+ ) with cyclopentadiene (CpH) or by simply reacting organic cation cyclopentadienides Cat[Cp] (Cat=NR4 + , PR4 + , Im+ ) with CO2 . One characteristic feature of these dianionic ligands is the acidic proton delocalized in an intramolecular hydrogen bridge (IHB) between the two carboxyl groups, as studied by 1 H NMR spectroscopy and XRD analyses. The reaction cannot be stopped after the first carboxylation. Therefore, we propose a Kolbe-Schmitt phenol-carboxylation related mechanism where the acidic proton of the monocarboxylic acid intermediate plays an ortho-directing and CO2 activating role for the second kinetically accelerated CO2 addition step exclusively in ortho position. The same and related thiocarboxylates [Cat]2 [C5 H3 (COS)2 H] are obtained by reaction of COS with Cat[Cp] (Cat=NR4 + , PR4 + , Im+ ). A preliminary study on [Cat]2 [C5 H3 (CO2 )2 H] reveals, that its soft and hard coordination sites can selectively be addressed by soft Lewis acids (Mo0 , Ru2+ ) and hard Lewis acids (Al3+ , La3+ ).

Precisely Constructed Silver Active Sites in Gold Nanoclusters for Chemical Fixation of CO2

Precise control of the composition and structure of active sites in an atom-by-atom fashion remains insuperable for heterogeneous catalysts. Here, we introduce tailor-made catalytic sites for the cycloaddition of CO₂ to epoxides achieved by implementing Ag atoms at different levels of liberation in atomically precise Au nanoclusters. Our results reveal that a single open Ag site on the Au₁₉ Ag₄ cluster improves the ring-opening of epoxides and sequent CO₂ insertion, while the partially exposed Ag site on the Au₂₀ Ag₁ cluster exhibits a weak affinity for epoxides and poor efficiency for CO₂ capture. Structural tunability imparted by the atom-by-atom tailoring and unusual atomic charges distributed on Au and Ag atoms of the three clusters seem to be crucial for promoting challenging bond cleavages and formations in the chemical utilization of CO₂ .

Soluble, crystalline, and thermally stable alkali CO2 - and carbonite (CO2 2-) clusters supported by cyclic(alkyl)(amino) carbenes

The mono- and dianions of CO2 (i.e., CO2 - and CO2 2-) have been studied for decades as both fundamentally important oxycarbanions (anions containing only C and O atoms) and as critical species in CO2 reduction and fixation chemistry. However, CO2 anions are highly unstable and difficult to study. As such, examples of stable compounds containing these ions are extremely limited; the unadulterated alkali salts of CO2 (i.e., MCO2, M2CO2, M = alkali metal) decompose rapidly above 15 K, for example. Herein we report the chemical reduction of a cyclic (alkyl)(amino) carbene (CAAC) adduct of CO2 at room temperature by alkali metals, which results in the formation of CAAC-stabilized alkali CO2 - and CO2 2- clusters. One-electron reduction of CAAC-CO2 adduct (1) with lithium, sodium or potassium metal yields stable monoanionic radicals [M(CAAC-CO2)] n (M = Li, Na, K, 2-4) analogous to the alkali CO2 - radical, and two-electron alkali metal reduction affords dianionic clusters of the general formula [M2(CAAC-CO2)] n (5-8) with reduced CO2 units which are structurally analogous to the carbonite anion CO2 2-. It is notable that crystalline clusters of these alkali-CO2 salts may also be isolated via the "one-pot" reaction of free CO2 with free CAAC followed by the addition of alkali metals - a process which does not occur in the absence of carbene. Each of the products 2-8 was investigated using a combination of experimental and theoretical methods.

Organocatalytic Strategy for the Fixation of CO2 via Carboxylation of Terminal Alkynes

An organocatalytic strategy for the direct carboxylation of terminal alkynes with CO₂ has been developed. The combined use of a bifunctional organocatalyst and Cs₂CO₃ resulted in a robust catalytic system for the preparation of a range of propiolic acid derivatives in high yields with broad substrate scope using CO₂ at atmospheric pressure under mild temperatures (60 °C). This work has demonstrated that this organocatalytic method offers a competitive alternative to metal catalysis for the carboxylation of terminal alkynes and CO₂. In addition, this protocol was suitable for the three-component carboxylation of terminal alkynes, alkyl halides, and CO₂.

Building N-Heterocyclic Carbene into Triazine-Linked Polymer for Multiple CO2 Utilization

The development of new CO₂ detection technologies and CO₂ "capture-conversion" materials is of great significance due to the growing environmental crisis. Here, multifunctional triazine-linked polymers with built-in N-heterocyclic carbene (NHC) sites (designated as NHC-triazine@polymer) are presented for simultaneous CO₂ detection, capture, activation, and catalytic conversion. NHC-triazine@polymer were readily obtained through polymerization of cyanophenyl-substituted NHC. The obtained film-like polymers exhibited interesting CO₂ -triggered fluorescence "turn-on" response and CO₂ -sensitive reversible color change. Both NHC and triazine sites could act as efficient binding sites for CO₂ , and the CO₂ uptake of NHC and triazine reached 1.52 and 1.36 mmol g^-1 , respectively. Notably, after being captured by NHC, CO₂ was activated into a zwitterionic adduct NHC-CO₂ that could be easily transformed into cyclic carbonate in the presence of epoxides. Moreover, NHC-triazine@polymer were stable and active catalysts for the conversion of low-concentration CO₂ in a gas mixture (7 vol %) into cyclic carbonates as well as for hydrosilylation of CO₂ to formamides.

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

The use of iron in photoactive metal complexes has been investigated for decades. In this respect, the charge transfer (CT) states are of particular interest, since they are usually responsible for the photofunctionality of such compounds. However, only recently breakthroughs have been made in extending CT excited state lifetimes that are notoriously short-lived in classical polypyridine iron coordination compounds. This success is in large parts owed to the use of strongly σ-donating N-heterocyclic carbene (NHC) ligands that help manipulating the photophysical and photochemical properties of iron complexes. In this review we aim to map out the basic design principles for the generation of photofunctional iron NHC complexes, summarize the progress made so far and recapitulate on the synthetic methods used. Further, we want to highlight the challenges still existing and give inspiration for future generations of photoactive iron complexes.

Development of High-Capacity and Water-Lean CO2 Absorbents by a Concise Molecular Design Strategy through Viscosity Control

The exponentially increasing viscosity of water-lean CO₂ absorbents during carbon capture processes is a critical problem for practical application, owing to its strong correlation with systems' mass transfer properties, as well as convenience of transportation. In this work, a concise strategy based on structure-viscosity relationships is proposed and applied to construct a series of functionalized ethylenediamines as single-component absorbents for post-combustion CO₂ capture. These nonaqueous absorbents have outstanding viscosities (50-200 cP, 25 °C) at their maximal CO₂ capacities (up to 22 wt % or 4.92 mol kg^-1 , 1 bar), and are readily regenerated at low temperatures (50-80 °C) under ambient pressure. Additional capture of CO₂ through physisorption could also be achieved by operating at high pressures. The CO₂ capture and release process is systematically investigated by means of ¹³ C NMR spectroscopy, differential scanning calorimetry (DSC), in situ FTIR analysis, and density functional theory (DFT) calculations, which could provide sufficient spectroscopic details to reveal the ease of reversibility and enable rational interpretation of the absorption mechanism.

CO2 Capture and in situ Catalytic Transformation

The escalating rate of fossil fuel combustion contributes to excessive CO₂ emission and the resulting global climate change has drawn considerable attention. Therefore, tremendous efforts have been devoted to mitigate the CO₂ accumulation in the atmosphere. Carbon capture and storage (CCS) strategy has been regarded as one of the promising options for controlling CO₂ build-up. However, desorption and compression of CO₂ need extra energy input. To circumvent this energy issue, carbon capture and utilization (CCU) strategy has been proposed whereby CO₂ can be captured and in situ activated simultaneously to participate in the subsequent conversion under mild conditions, offering valuable compounds. As an alternative to CCS, the CCU has attracted much concern. Although various absorbents have been developed for the CCU strategy, the direct, in situ chemical conversion of the captured CO₂ into valuable chemicals remains in its infancies compared with the gaseous CO₂ conversion. This review summarizes the recent progress on CO₂ capture and in situ catalytic transformation. The contents are introduced according to the absorbent types, in which different reaction type is involved and the transformation mechanism of the captured CO₂ and the role of the absorbent in the conversion are especially elucidated. We hope this review can shed light on the transformation of the captured CO₂ and arouse broad concern on the CCU strategy.

Recent advances in nucleophile-triggered CO2-incorporated cyclization leading to heterocycles

CO₂, as a sustainable, feasible, abundant one-carbon synthon, has been utilized in carboxylative cyclization, carbonylative cyclization, and reductive cyclization.

Organocatalytic cycloaddition of carbonyl sulfide with propargylic alcohols to 1,3-oxathiolan-2-ones

Lewis base-COS adducts were firstly studied as organocatalysts for the cyclization of propargylic alcohols with carbonyl sulfide.

Selective and catalytic carbon dioxide and heteroallene activation mediated by cerium N-heterocyclic carbene complexes

A series of rare earth complexes of the form Ln(L^R)₃ supported by bidentate ortho-aryloxide-NHC ligands are reported (L^R = 2-O-3,5-^tBu₂-C₆H₂(1-C{N(CH)₂N(R)})); R = ⁱPr, ^tBu, Mes; Ln = Ce, Sm, Eu). The cerium complexes cleanly and quantitatively insert carbon dioxide exclusively into all three cerium carbene bonds, forming Ce(L^R·CO₂)₃. The insertion is reversible only for the mesityl-substituted complex Ce(L^Mes)₃. Analysis of the capacity of Ce(L^R)₃ to insert a range of heteroallenes that are isoelectronic with CO₂ reveals the solvent and ligand size dependence of the selectivity. This is important because only the complexes capable of reversible CO₂-insertion are competent catalysts for catalytic conversions of CO₂. Preliminary studies show that only Ce(L^Mes·CO₂)₃ catalyses the formation of propylene carbonate from propylene oxide under 1 atm of CO₂ pressure. The mono-ligand complexes can be isolated from reactions using LiCe(NⁱPr₂)₄ as a starting material; LiBr adducts [Ce(L^R)(NⁱPr₂)Br·LiBr(THF)]₂ (R = Me, ⁱPr) are reported, along with a hexanuclear N-heterocyclic dicarbene [Li₂Ce₃(OArC^Me-H)₃(NⁱPr₂)₅(THF)₂]₂ by-product. The analogous para-aryloxide-NHC proligand (p-L^Mes = 4-O-2,6-^tBu₂-C₆H₂(1-C{N(CH)₂NMes}))) has been made for comparison, but the rare earth tris-ligand complexes Ln(p-L^Mes)₃(THF)₂ (Ln = Y, Ce) are too reactive for straightforward Lewis pair separated chemistry to be usefully carried out.

B-Heterocyclic Carbene Arising from Charge Shift: A Computational Verification

1-Borabicyclo[1.1.0]but-2(3)-ene (1BB) is a singlet biradical with two single electrons that can form an ionic resonance structure through a charge shift. The ionic resonance structure is a B-heterocyclic carbene (BHC), which can act as a carbene, Lewis base, or L- and Z-type ligand, to give adducts and complexes. Through a range of quantum methods, four types of stable compounds (A-D) derived from 1BB have been designed. These compounds retain the unique features of 1BB. As a consequence, the structures, stability, and Wiberg bond indices of the Lewis adducts of A-D with Lewis acids (BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , and C₁₃ BH₉ ) and Cu^I , Ag^I , and Au^I complexes have been investigated. Results show that A-D can indeed react as carbenes. Interestingly, compounds A-D, as L-type ligands, can attach to BePh₂ , BH₃ , AlH₃ , AlCl₃ , C₅ BH₅ , C₁₃ BH₉ , and CuCl and form compounds with planar tetracoordinate carbon (ptC), whereas Z-type ligands A-D can bind to AgCl and AuCl to provide complexes with planar tetracoordinate boron (ptB). In addition, the binuclear complexes of ClX(1BB)CuCl (X=Ag, Au) have been studied and A-D behave as both L- and Z-type ligands, in which these complexes contain both ptC and ptB. Thus, a novel method for designing compounds with ptC and ptB is presented. These rationally designed compounds involve the elements of carbene, ptC, ptB, and L- and Z-type ligands, and are expected to be unique and useful in experimental chemistry once they are synthesized.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.