Tandem copper hydride–Lewis pair catalysed reduction of carbon dioxide into formate with dihydrogen

Trapping of soluble, KCl-stabilized Cu(I) hydrides with CO2 gives crystalline formates

Cu(I)-Hydrido complexes supported by dibenzo[b,f]azepinyl P-alkene hybrid ligands and stabilized by electrostatic interactions in a Cu-H⋯KCl⋯BR3 arrangement can be trapped with CO2 at low temperature to afford Cu(I)-formates. The complexes are isolable with and without a pendant BEt3 group and show strong Cu-O and weak B-O interactions.

CuH-Catalyzed Selective N-Methylation of Amines Using Paraformaldehyde as a C1 Source

Copper hydride (CuH) complexes have been proposed as key intermediates in synthesis and catalysis. Herein, we developed a highly efficient strategy for CuH-catalyzed N-methylation of aromatic and aliphatic amines using paraformaldehyde and polymethylhydrosiloxane (PMHS) under mild reaction conditions. The reaction proceeded smoothly without additives to furnish the corresponding N-methylated products using cyclic(alkyl)(amino)carbene (CAAC)CuH as a reaction intermediate, which results from a reaction between PMHS and (CAAC)CuCl.

Synthesis, Structural Characterization, and CO2 Reactivity of a Constitutionally Analogous Series of Tricopper Mono-, Di-, and Trihydrides

The formation of hydrides at heterogeneous copper surfaces results in dramatic structural and reactivity changes, yet the morphologies of these materials and their respective roles in catalysis are not well understood. Of particular interest is the reactivity of heterogeneous copper hydrides with carbon dioxide (CO₂), an early mechanistic branching point in the CO₂ reduction reaction. Herein, we report the synthesis, characterization, and reactivity of tricopper compounds supported by a facially biased, chelating tris(carbene) ligand scaffold. This sterically bulky environment affords access to an isolable series of tricopper hydrides: [LCu₃H]²⁺ (4), [LCu₃H₂]⁺ (3), and LCu₃H₃ (6). Single-crystal X-ray diffraction and solution NMR spectroscopy studies reveal both geometric flexibility within the Cu₃ core and fluxionality of hydride ligands across the Cu₃ cluster, providing both atomically precise experimental analogues of static surface species and emulating dynamic ligand behavior proposed for surfaces. Electronic structure calculations serve as a predictor of hydricity, which was likewise benchmarked experimentally via both protonolysis and hydride abstraction reactions. Increased hydride number (and commensurately lower cluster charge) results in more hydridic complexes, with a thermodynamic hydricity range spanning >30 kcal/mol. These thermochemical studies serve as an accurate predictor of CO₂ reactivity. Together, this Cu₃H_x series exhibits the structure/reactivity relationships proposed for catalytically active copper surfaces, validating the application of carefully designed molecular clusters toward elucidating mechanisms in surface science.

Reaction and separation system for CO2 hydrogenation to formic acid catalyzed by iridium immobilized on solid phosphines under base-free condition

Hydrogenation of carbon dioxide (CO₂) to produce formic acid (HCOOH) in base-free condition can avoid waste producing and simplify product separation process. However, it remains a big challenge because of the unfavorable energy in both thermodynamics and dynamics. Herein, we report the selective and efficient hydrogenation of CO₂ to HCOOH under neutral conditions with imidazolium chloride ionic liquid as the solvent, catalyzed by a heterogeneous Ir/PPh₃ compound. The heterogeneous catalyst is more effective than the homogeneous one because it is inert in catalyzing the decomposition of product. A turnover number (TON) of 12700 can be achieved, and HCOOH with a purity of 99.5% can be isolated by distillation because of the non-volatility of the solvent. Both the catalyst and imidazolium chloride can be recycled at least 5 times with stable reactivity.

Second Sphere Effects Promote Formic Acid Dehydrogenation by a Single-Atom Gold Catalyst Supported on Amino-Substituted Graphdiyne

Regulating the second sphere of homogeneous molecular catalysts is a common and effective method to boost their catalytic activities, while the second sphere effects have rarely been investigated for heterogeneous single-atom catalysts primarily due to the synthetic challenge for installing functional groups in their second spheres. Benefiting from the well-defined and readily tailorable structure of graphdiyne (GDY), an Au single-atom catalyst on amino-substituted GDY is constructed, where the amino group is located in the second sphere of the Au center. The Au atoms on amino-decorated GDY displayed superior activity for formic acid dehydrogenation compared with those on unfunctionalized GDY. The experimental studies, particularly the proton inventory studies, and theoretical calculations revealed that the amino groups adjacent to an Au atom could serve as proton relays and thus facilitate the protonation of an intermediate Au-H to generate H₂ . Our study paves the way to precisely constructing the functional second sphere on single-atom catalysts.

Use of 5,10-Disubstituted Dibenzoazaborines and Dibenzophosphaborines as Cyclic Supports of Frustrated Lewis Pairs for the Capture of CO2

The reactivity of 5,10-disubstituted dibenzoazaborines and dibenzophosphaborines towards carbon dioxide was studied at the DFT, M06-2X/def2-TZVP, computational level. The profile of this reaction comprises of three stationary points: the pre-reactive complex and adduct minima and the transition state(TS) linking both minima. Initial results show that dibenzoazaborines derivatives are less suitable to form adducts with CO₂ than dibenzophosphaborine systems. The influence of the basicity on the P atom and the acidity on the B center of the dibenzophosphaborine in the reaction with CO₂ was also explored. Thus, an equation was developed relating the properties (acidity, basicity and boron hybridization) of the isolated dibenzophosphaborine derivatives with the adduct energy. We found that modulation of the boron acidity allows to obtain more stable adducts than the pre-reactive complexes and isolated monomers.

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid

We disclose a 1,4,7-triazacyclononane (TACN) ligand featuring an appended boron Lewis acid. Metalation with Cu(I) affords a series of tetrahedral complexes including a boron-capped cuprous hydride. We demonstrate distinct reactivity modes as a function of chemical oxidation: hydride transfer to CO2 in the copper(I) state and oxidant-induced H2 evolution as well as alkyne reduction.

Recent developments in first-row transition metal complex-catalyzed CO2 hydrogenation

Our modern civilization is currently standing at a crossroads due to excessive emission of anthropogenic CO₂ leading to adverse climate change effects. Hence, a proper CO₂ management strategy, including appropriate CO₂ capture, utilization, and storage (CCUS), has become a prime concern globally. On the other hand, C₁ chemicals such as methanol (CH₃OH) and formic acid (HCOOH) have emerged as leading materials for a wide range of applications in various industries, including chemical, biochemical, pharmaceutical, agrochemical, and even energy sectors. Hence, there is a concerted effort to bridge the gap between CO₂ management and methanol/formic acid production by employing CO₂ as a C₁-synthon. CO₂ hydrogenation to methanol and formic acid has emerged as one of the primary routes for directly converting CO₂ to a copious amount of methanol and formate, which is typically catalyzed by transition metal complexes. In this frontier article, we have primarily discussed the abundant first-row transition metal-driven hydrogenation reaction that has exhibited a significant surge in activity over the past few years. We have also highlighted the potential future direction of the research while incorporating a comparative analysis for the competitive second and third-row transition metal-based hydrogenation.

Ferrocene tethered boramidinate frustrated Lewis pairs: stepwise capture of CO2 and CO

The synthesis and reactivity of novel ferrocene tethered boramidinate frustrated Lewis pairs (FLPs), capable of the sequential capture of small molecules, is reported. Reactions of 1,1'-dicarbodiimidoferrocenes with different boranes provides access to metallocene tethered FLPs. The reactivity of the boramidinate moieties can be tuned by the nature of the carbodiimido substituents (alkyl vs. aryl) and the borane used in the reduction (9-borabicyclo[3.3.1]nonane [(C₈H₁₄)₂BH]₂vs. bis-pentafluorophenyl borane [(C₆F₅)₂BH]₂). The boramidinate FLP arms do not engage in intramolecular reactions, allowing for independent small molecule capture by each FLP. By careful synthetic control, sequential capture of different gaseous small molecules (CO₂ and CO or CO₂ and CNtBu) by the same bis(boramidinate)ferrocene molecule has been demonstrated.

Discovery of Electronic Structure and Interfacial Interaction Features in Catalytic Activity

The selective transformation of inert bonds (C-H, C-O, C-C, C-F, etc.) via various catalysts is one of the most challenging areas, with applications in organic synthesis, materials science, and biological and pharmaceutical chemistry. The catalytic performance of homogeneous and heterogeneous catalysts can be rationally controlled in two ways: (i) electronic structure modulation of the active site, such as the metal center, ligands, and coordination modes, to improve the catalytic activity and stability and (ii) tuning intermolecular or interfacial interactions to promoting the reaction kinetics by accelerating the transmission of electrons between the catalyst and solvents or support. The rational design of catalysts based on adjustable features, such as metal (monometallic or bimetallic) active sites, crystal phase, ligands, solvents, and supports for inert bond activation under mild conditions remains a challenge. This Perspective summarizes the features of electronic structures, interfacial interactions, and their effects on molecular catalysis, metal-organic frameworks (MOFs), and natural mineral catalysis. The discovery of efficient catalysts could be promoted using machine-learning methods with high-performance descriptors. More attention should be paid to high-throughput quantum-chemical computations and experiments, automatic searches of chemical reaction pathways, and efficient machine-learning or deep-learning methods to accelerate catalyst design and synthesis in the future.

Formate Complexes of Tri- and Tetravalent Titanium Supported by a Tris(phenolato)amine Ligand

Titanium(III) and titanium(IV) formate complexes supported by the sterically encumbering tris(phenolato)amine ligand (H3(O3N) = tris(4,6-di-tert-butyl-2-hydroxybenzyl)amine) are described. Salt metathesis of the chlorido precursor [(O3N)TiCl] (1-Cl) with sodium formate in a...

Challenges and recent advancements in the transformation of CO2 into carboxylic acids: straightforward assembly with homogeneous 3d metals

Transformation of carbon dioxide (CO2) into valuable organic carboxylic acids is essential for maintaining sustainability. In this review, such CO2 thermo-, photo- and electrochemical transformations under 3d-transition metal catalysis are described from 2017 until 2022.

Transforming CO2 into Methanol with N-Heterocyclic Carbene-Stabilized Coinage Metal Hydrides Immobilized in a Metal-Organic Framework UiO-68

By synergizing the advantages of homogeneous and heterogeneous catalysis, single-site heterogeneous catalysis represents a highly promising opportunity for many catalytic processes. Particularly, the unprecedented designability and versatility of metal-organic frameworks (MOFs) promote them as salient platforms for designing single-site catalytic materials by introducing isolated, well-defined active sites into the frameworks. Herein, we design new MOF-supported single-site catalysts for CO₂ hydrogenation to methanol (CH₃OH), a reaction of great significance in CO₂ valorization. Specifically, N-heterocyclic carbene (NHC), a class of excellent modifiers and anchors, is used to anchor coinage metal hydrides M(I)-H (M = Cu, Ag, and Au) onto the organic linker of UiO-68. The strong metal-ligand interactions between NHC and M(I)-H verify the robustness and feasibility of our design strategy. On the tailor-made catalysts, a three-stage sequential transformation is proposed for CH₃OH synthesis with HCOOH and HCHO as the transit intermediates. A density functional theory-based comparative study suggests that UiO-68 decorated with NHC-Cu(I)-H performs best for CO₂ hydrogenation to HCOOH. This is further rationalized by three linear relationships for the Gibbs energy barrier of CO₂ hydrogenation to HCOO intermediate, the first with the NBO charge of the hydride in NHC-M(I)-H, the second with the electronegativity of M, and the third with the gap between the lowest unoccupied molecular orbital of CO₂ and the highest occupied molecular orbital of the catalyst. It is confirmed that the high efficiency of MOF-supported NHC-Cu(I)-H for CO₂ transformation to CH₃OH is via the proposed three-stage mechanism, and in each stage, the step involving heterolytic dissociation of H₂ together with product generation is the most energy-intensive. The rate-limiting step in the entire mechanism is identified to be H₂ dissociation accompanying with simultaneous HCHO and H₂O formation. Altogether, the tailor-made UiO-68 decorated with NHC-Cu(I)-H features well-defined active sites, enables precise manipulation of reaction paths, and demonstrates excellent reactivity for CO₂ hydrogenation to CH₃OH. It is also predicted to surpass a recently reported MOF-808 catalyst consisting of neighboring Zn²⁺-O-Zr⁴⁺ sites. The designed MOFs as well as the proposed strategy here establish a new paradigm and can be extended to other hydrogenation reactions.

Diverse Uses of the Reaction of Frustrated Lewis Pair (FLP) with Hydrogen

The articulation of the notion of "frustrated Lewis pairs" (FLPs) emerged from the discovery that H₂ can be reversibly activated by combinations of sterically encumbered main group Lewis acids and bases. This has prompted numerous studies focused on various perturbations of the Lewis acid/base combinations and the applications to organic reductions. This Perspective focuses on the new directions and developments that are emerging from this FLP chemistry involving hydrogen. Three areas are discussed including new applications and approaches to FLP reductions, the reductions of small molecules, and the advances in heterogeneous FLP systems. These foci serve to illustrate that despite having its roots in main group chemistry, this simple concept of FLPs is being applied across the discipline.

Selective Catalytic Frustrated Lewis Pair Hydrogenation of CO2 in the Presence of Silylhalides

The frustrated Lewis pair (FLP) derived from 2,6‐lutidine and B(C₆F₅)₃ is shown to mediate the catalytic hydrogenation of CO₂ using H₂ as the reductant and a silylhalide as an oxophile. The nature of the products can be controlled with the judicious selection of the silylhalide and the solvent. In this fashion, this metal‐free catalysis affords avenues to the selective formation of the disilylacetal (R₃SiOCH₂OSiR₃), methoxysilane (R₃SiOCH₃), methyliodide (CH₃I) and methane (CH₄) under mild conditions. DFT studies illuminate the complexities of the mechanism and account for the observed selectivity.

Aerobic oxidation of aldehydes to acids in water with cyclic (alkyl)(amino)carbene copper under mild conditions

In this work, cyclic (alkyl)(amino)carbene copper ((CAAC)Cu) catalyzed aerobic oxidation of aldehydes in water at room temperature has been reported. Good to excellent yields were obtained using different substrates. A possible reaction mechanism was proposed, in which (CAAC)Cu dioxygen activates the C-H bond of aldehyde with a low barrier of 10.6 kcal mol^-1.

Distorted Copper(II) Complex with Unusually Short CF···Cu Distances

We find a Cu(II)-(L-CF₃)₂ complex (L-CF₃ = 2,2,2-trifluoro-N-[2-(pyridin-2-yl)propan-2-yl]acetamide) with a distorted "seesaw" geometry. It has the shortest crystallographic CF···Cu distances yet reported, to the best of our knowledge (<2.6 Å), for which computational and experimental data indicate a secondary bonding interaction. A comparison with a CCl₃ version and one without ligand backbone gem-dimethyl groups suggests a steric origin for the distorted geometry, resulting from the specific ligand interactions.

Computational Study of Triphosphine-Ligated Cu(I) Catalysts for Hydrogenation of CO2 to Formate

The catalyzed hydrogenation of CO₂ to formate via a triphosphine-ligated Cu(I) was studied computationally at the density functional theory level in the presence of a self-consistent reaction field. Of the four functionals benchmarked, M06 was generally in the best agreement with the available experimentally estimated values. Two bases, DBU and TBD, were studied in the context of two proposed mechanisms in the MeCN solvent. Activation of H₂ was explored by using LCu(DBU)⁺ to form LCuH. Dissociation of a ligand arm results in higher barriers to form the key hydride complex, LCuH. The preferred mechanism passes through a transition state, where the H₂ has one H atom interacting with the copper center and the other H atom interacting with the N atom of the base, similar to H₂ insertion into a frustrated Lewis pair. There is no significant difference between the choice of a base, DBU or TBD, with respect to the proposed mechanisms. We propose that the experimentally observed differences between DBU and TBD reactivities for this mechanism are due to off-pathway changes.

Carbon Dioxide Hydrogenation to Formate Catalyzed by a Bench-Stable, Non-Pincer-Type Mn(I) Alkylcarbonyl Complex

The catalytic reduction of carbon dioxide is a process of growing interest for the use of this simple and abundant molecule as a renewable building block in C1-chemical synthesis and for hydrogen storage. The well-defined, bench-stable alkylcarbonyl Mn(I) bis(phosphine) complex fac-[Mn(CH₂CH₂CH₃)(dippe)(CO)₃] [dippe = 1,2-bis(diisopropylphosphino)ethane] was tested as an efficient and selective non-precious-metal precatalyst for the hydrogenation of CO₂ to formate under mild conditions (75 bar total pressure, 80 °C), in the presence of a Lewis acid co-catalyst (LiOTf) and a base (DBU). Mechanistic insight into the catalytic reaction is provided by means of density functional theory (DFT) calculations.

Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes

Two strategies were reported to prevent the deactivation of Frustrated Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh₃) with high stability was designed and synthesized. Excellent conversion (up to 99%) and selectivity can be achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh₃. This catalytic system works quite well for different substrates. In addition, the P-BPh₃ can be easily recycled.

Copper(I) Complexes Containing PCP Ligand Catalyzed Hydrogenation of Carbon Dioxide to Formate under Ambient Conditions

The five new copper(I) complexes [Cu₂(μ-Cl)₂(κ¹-PCP^t-Bu)] (1), [Cu₂(μ-Br)₂(κ¹-PCP^t-Bu)] (2), [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3), [Cu₂(μ-CN)₂(κ¹-PCP^t-Bu)] (4), and [Cu₄(μ₃-SCN)₄(κ¹-PCP^t-Bu)₂]·CH₂Cl₂ (5) bearing a 1,3-bis[(di-tert-butylphosphino)methyl]benzene ligand were synthesized and characterized spectroscopically, and the molecular structures of 1, 3, and 5 were determined by single-crystal X-ray diffraction techniques. Structural studies for 1 and 3 revealed their binuclear structures with Cu···Cu separations of 2.609(3) and 2.6359(19) Å, respectively. However, 5 has a tetranuclear cubane structure with an 18-electron configuration at each copper without any metal-metal bonds. The two copper centers in 1 and 3 are bonded to one bridging PCP^t-Bu ligand in a κ¹-manner and two bridging (pseudo)halido ligands in a μ₂-bonding mode to generate a nonplanar Cu₂(μ-X)₂ framework. The four copper centers in 5 are at the vertices of a tetrahedron. Each copper center has pseudo-tetrahedral coordination provided by two bridging PCP^t-Bu ligands in a κ¹-manner and the four bridging thiocyanate groups in a μ₃-manner. These complexes were used as catalysts for the hydrogenation of CO₂ to formate in the presence of DBU as a base to produce valuable energy-rich chemicals, and therefore it is a promising, safe, and simple strategy to conduct reactions under ambient pressure at room temperature. Among all of the five copper(I) complex based catalysts, 3 displayed the best catalytic performance with turnover number (TON) values of 38-8700 in 12-48 h of reaction at 25-80 °C. The outstanding catalytic performance of [Cu₂(μ-I)₂(κ¹-PCP^t-Bu)] (3) makes it a potential candidate for realizing the large-scale production of formate by CO₂ hydrogenation.

Reactivity of NHC/diphosphene-coordinated Au(I)-hydride

We report the reactivity of isolable Au(i)-hydride stabilized by an NHC-coordinated diphosphene towards substrates containing C-C and N-N multiple bonds (NHC = N-heterocyclcic carbene). Reactions with dimethyl acetylenedicarboxylate and azobenzene lead to a trans-addition of the Au(i)-H across the C-C triple bond and the N-N double bond, respectively. In contrast, the reaction with ethyl diazoacetate affords a gold(i)-hydrazonide as the 1,1-addition product to the terminal nitrogen atom. With phenyl acetylene, the corresponding Au(i)-alkynyl complex is obtained under the elimination of dihydrogen. Strikingly, diphosphene-containing Au(i)-hydride is more reactive - affording different products in some cases - than a related NHC-stabilized Au(i)-hydride without the mediating diphosphene moiety.

B(C6 F5 )3 -Catalyzed Tandem Friedel-Crafts and C-H/C-O Coupling Reactions of Dialkylanilines

Tandem Friedel-Crafts (FC) and C-H/C-O coupling reactions catalyzed by tris(pentafluorophenyl) borane (B(C₆ F₅ )₃ ) were achieved without using any other additive in the absence of solvent. This process can be used for the reactions between a series of dialkylanilines and vinyl ethers with good isolated yields of bis(4-dialkylaminophenyl) compounds. Based on combined theoretical and experimental studies, the possible reaction mechanism was proposed. B(C₆ F₅ )₃ can activate the C=C and C-O bond for FC and C-H/C-O coupling reactions respectively. The FC reaction is slow, which is followed by a fast C-H/C-O coupling.

UV/Vis Spectroscopy of Copper Formate Clusters: Insight into Metal-Ligand Photochemistry

The electronic structure and photochemistry of copper formate clusters, Cu^I₂(HCO₂)₃⁻ and Cu^II_n(HCO₂)_2n+1⁻, n≤8, are investigated in the gas phase by using UV/Vis spectroscopy in combination with quantum chemical calculations. A clear difference in the spectra of clusters with Cu^I and Cu^II copper ions is observed. For the Cu^I species, transitions between copper d and s/p orbitals are recorded. For stoichiometric Cu^II formate clusters, the spectra are dominated by copper d–d transitions and charge‐transfer excitations from formate to the vacant copper d orbital. Calculations reveal the existence of several energetically low‐lying isomers, and the energetic position of the electronic transitions depends strongly on the specific isomer. The oxidation state of the copper centers governs the photochemistry. In Cu^II(HCO₂)₃⁻, fast internal conversion into the electronic ground state is observed, leading to statistical dissociation; for charge‐transfer excitations, specific excited‐state reaction channels are observed in addition, such as formyloxyl radical loss. In Cu^I₂(HCO₂)₃⁻, the system relaxes to a local minimum on an excited‐state potential‐energy surface and might undergo fluorescence or reach a conical intersection to the ground state; in both cases, this provides substantial energy for statistical decomposition. Alternatively, a Cu^II(HCO₂)₃Cu⁰⁻ biradical structure is formed in the excited state, which gives rise to the photochemical loss of a neutral copper atom.

Experimental and theoretical study on the cyclic(alkyl)(amino)carbene-copper catalyzed Friedel-Crafts reaction of N,N-dialkylanilines with styrenes

The Friedel-Crafts reaction of alkalinous substrates is challenging because of the coordination between amines and traditional acid catalysts. Based on theoretical research, we designed a catalytic process for the Friedel-Crafts reactions of N,N-dialkylanilines in the presence of CAAC-CuCl [CAAC = cyclic(alkyl)(amino)carbene] and KB(C₆F₅)₄. Experimental results show that the catalytic system is suitable for a series of N,N-dialkylanilines and styrenes with good to excellent yields of para-selectivity products. The results are comparable to those obtained in carbene-gold catalyzed processes.