Selective Hydrosilation of CO2to a Bis(silylacetal) Using an Anilido Bipyridyl-Ligated Organoscandium Catalyst

Selective reductive conversion of CO2 to CH2-bridged compounds by using a Fe-functionalized graphene oxide-based catalyst

Anthropogenic climate change drastically affects our planet, with CO2 being the most critical gaseous driver. Despite the existing carbon dioxide capture and transformation, there is much need for innovative carbon dioxide hydrogenation catalysts with excellent selectivity. Here, we present a fast, effective, and sustainable route for coupling diverse alcohols, amines and amides with CO2 via heterogenization of a natural metal-based homogeneous catalyst through decorating on functionalized graphene oxide (GO). Combined synthetic, experimental, and theoretical studies unravel mechanistic routes to convergent 4‑electron reduction of CO2 under mild conditions. We successfully replace the toxic and expensive ruthenium species with inexpensive, ubiquitously available and recyclable iron. This iron-based functionalized graphene oxide (denoted as Fe@GO-EDA, where EDA represents ethylenediamine) functions as an efficient catalyst for the selective conversion of CO2 into a formaldehyde oxidation level, thus opening the door for interesting molecular structures using CO2 as a C1 source. Overall, this work describes an intriguing heterogeneous platform for the selective synthesis of valuable methylene-bridged compounds via 4‑electron reduction of CO2.

Challenges and Prospects in the Catalytic Conversion of Carbon Dioxide to Formaldehyde

Formaldehyde (HCHO) is a crucial C1 building block for daily-life commodities in a wide range of industrial processes. Industrial production of HCHO today is based on energy- and cost-intensive gas-phase catalytic oxidation of methanol, which calls for exploring other and more sustainable ways of carrying out this process. Utilization of carbon dioxide (CO2 ) as precursor presents a promising strategy to simultaneously mitigate the carbon footprint and alleviate environmental issues. This Minireview summarizes recent progress in CO2 -to-HCHO conversion using hydrogenation, hydroboration/hydrosilylation as well as photochemical, electrochemical, photoelectrochemical, and enzymatic approaches. The active species, reaction intermediates, and mechanistic pathways are discussed to deepen the understanding of HCHO selectivity issues. Finally, shortcomings and prospects of the various strategies for sustainable reduction of CO2 to HCHO are discussed.

Thermally stable zinc hydride catalyst for hydrosilylation of CO2 to silyl formate at atmospheric pressure

Neutral zinc complexes supported by H[PNNO], a diaminophenolate ligand bearing a pendant phosphine group, were synthesized and characterized. The phosphine arm adopts two different configurations in solution and prevents aggregation. The monomeric zinc hydride complex is stable at elevated temperatures up to 125 °C and reacts readily with CO₂ to afford a zinc formate complex. The zinc hydride is active for CO₂ hydrosilylation at atmospheric CO₂ pressure and is selective for CO₂ reduction to the silyl-formate product.

Catalytic Reductive Alcohol Etherifications with Carbonyl-Based Compounds or CO2 and Related Transformations for the Synthesis of Ether Derivatives

Ether derivatives have myriad applications in several areas of chemical industry and academia. Hence, the development of more effective and sustainable protocols for their production is highly desired. Among the different methodologies reported for ether synthesis, catalytic reductive alcohol etherifications with carbonyl-based moieties (aldehydes/ketones and carboxylic acid derivatives) have emerged in the last years as a potential tool. These processes constitute appealing routes for the selective production of both symmetrical and asymmetrical ethers (including O-heterocycles) with an increased molecular complexity. Likewise, ester-to-ether catalytic reductions and hydrogenative alcohol etherifications with CO2 to dialkoxymethanes and other acetals, albeit in less extent, have undergone important advances, too. In this Review, an update of the recent progresses in the area of catalytic reductive alcohol etherifications using carbonyl-based compounds and CO2 have been described with a special focus on organic synthetic applications and catalyst design. Complementarily, recent progress made in catalytic acetal/ketal-to-ether or ester-to-ether reductions and other related transformations have been also summarized.

More Than Just a Reagent: The Rise of Renewable Organohydrides for Catalytic Reduction of Carbon Dioxide

Stoichiometric carbon dioxide reduction to highly reduced C1 molecules, such as formic acid (2e^- ), formaldehyde (4e^- ), methanol (6e^- ) or even most-reduced methane (8e^- ), has been successfully achieved by using organosilanes, organoboranes, and frustrated Lewis Pairs (FLPs) in the presence of suitable catalyst. The development of renewable organohydride compounds could be the best alternative in this regard as they have shown promise for the transfer of hydride directly to CO₂ . Reduction of CO₂ by two electrons and two protons to afford formic acid by using renewable organohydride molecules has recently been investigated by various groups. However, catalytic CO₂ reduction to ≥2e^- -reduced products by using renewable organohydride-based molecules has rarely been explored. This Minireview summarizes important findings in this regard, encompassing both stoichiometric and catalytic CO₂ reduction.

Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2

A novel β-diketiminate stabilized gallium hydride, (^Dipp L)Ga(Ad)H (where (^Dipp L)={HC(MeCDippN)₂ }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ¹ -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆ F₅ )₃ , (^Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂ . Under stoichiometric conditions, the addition of one equivalent of B(C₆ F₅ )₃ to (^Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(^Dipp L)Ga(Ad)][HB(C₆ F₅ )₃ ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃ SiH (yielding H₂ C(OSiEt₃ )₂ ). When catalysis is undertaken in the presence of excess B(C₆ F₅ )₃ , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂ C(OSiEt₃ )₂ , CH₄ and O(SiEt₃ )₂ . While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^Dipp L)Ga(Ad){OC(H)OB(C₆ F₅ )₃ }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(^Dipp L)Ga(Ad)]⁺ cation (AN=69.8).

Effective and Reversible Carbon Dioxide Insertion into Cerium Pyrazolates

The homoleptic pyrazolate complexes [CeIII 4 (Me2 pz)12 ] and [CeIV (Me2 pz)4 ]2 quantitatively insert CO2 to give [CeIII 4 (Me2 pz⋅CO2 )12 ] and [CeIV (Me2 pz⋅CO2 )4 ], respectively (Me2 pz=3,5-dimethylpyrazolato). This process is reversible for both complexes, as observed by in situ IR and NMR spectroscopy in solution and by TGA in the solid state. By adjusting the molar ratio, one molecule of CO2 per [CeIV (Me2 pz)4 ] complex could be inserted to give trimetallic [Ce3 (Me2 pz)9 (Me2 pz⋅CO2 )3 (thf)]. Both the cerous and ceric insertion products catalyze the formation of cyclic carbonates from epoxides and CO2 under mild conditions. In the absence of epoxide, the ceric catalyst is prone to reduction by the co-catalyst tetra-n-butylammonium bromide (TBAB).

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.

Design of a catalyst through Fe doping of the boron cage B10H14 for CO2 hydrogenation and investigation of the catalytic character of iron hydride (Fe-H)

The innovative catalyst Fe@B₁₀H₁₄ is designed through Fe doping of the boron cage B₁₀H₁₄ and is employed to catalyze CO₂ hydrogenation using a quantum mechanical method. First, the structure of the Fe@B₁₀H₁₄ complex is characterized through calculated ¹¹B NMR chemical shifts and Raman spectra, and the interactions between Fe and the four H atoms of the opening in the cage are analyzed, which show that various iron hydride (Fe-H) characteristics exist. Subsequently, the potential of Fe@B₁₀H₁₄ as a catalyst for the hydrogenative reduction of CO₂ in the gas phase is computationally evaluated. We find that an equivalent of Fe@B₁₀H₁₄ can consecutively reduce double CO₂ to obtain the double product HCOOH through a two-step reduction, and Fe@B₁₀H₁₂ and Fe@B₁₀H₁₀ are successively obtained. The Fe presents single-atom character in the reduction of CO₂, which is different from the common iron(ii) catalyzed CO₂ reduction. The calculated total free energy barrier of the first CO₂ reduction is only 8.79 kcal mol^-1, and that of the second CO₂ reduction is 25.71 kcal mol^-1. Every reduction reaction undergoes two key transition states TSC-H and TSO-H. Moreover, the transition state of the C-H bond formation TSC-H is the rate-determining step, where the interaction between π_{C[double bond, length as m-dash]O}* and the weak σ_Fe-H bond plays an important role. Furthermore, the hydrogenations of Fe@B₁₀H₁₂ and Fe@B₁₀H₁₀ are investigated, which aim at determining the ability of Fe-H circulation in the Fe doped decaborane complex. We find that the hydrogenation of Fe@B₁₀H₁₀ undergoes a one-step H₂-adsorbed transition state TSH-adsorb with an energy barrier of 6.42 kcal mol^-1 from Fe@B₁₀H₁₂. Comparing with the hydrogenation of Fe@B₁₀H₁₀, it is slightly more difficult for the hydrogenation of Fe@B₁₀H₁₂, where the rate-determining step is the H₂-cleaved transition state TS2H-H with an energy barrier of 17.38 kcal mol^-1.

Cerium-Catalyzed Hydrosilylation of Acrylates to Give α-Silyl Esters

The homoleptic organocerium complex Ce{C(SiHMe₂ )₃ }₃ (1) reacts with B(C₆ F₅ )₃ to produce the zwitterionic bis(alkyl) hydridoborato Ce{C(SiHMe₂ )₃ }₂ HB(C₆ F₅ )₃ (2). NMR and IR spectroscopy and X-ray crystallography indicate that each alkyl ligand contains two bridging Ce↼H-Si interactions in both 1 and 2. Compound 2 serves as a precatalyst for the hydrosilylation of acrylates to give α-silyl esters at room temperature with a turnover number of 2200.

Steric and Electronic Effects of Bidentate Phosphine Ligands on Ruthenium(II)-Catalyzed Hydrogenation of Carbon Dioxide

The reactivity difference between the hydrogenation of CO2 catalyzed by various ruthenium bidentate phosphine complexes was explored by DFT. In addition to the ligand dmpe (Me2 PCH2 CH2 PMe2 ), which was studied experimentally previously, a more bulky diphosphine ligand, dmpp (Me2 PCH2 CH2 CH2 PMe2 ), together with a more electron-withdrawing diphosphine ligand, PN(Me) P (Me2 PCH2 N(Me) CH2 PMe2 ), have been studied theoretically to analyze the steric and electronic effects on these catalyzed reactions. Results show that all of the most favorable pathways for the hydrogenation of CO2 catalyzed by bidentate phosphine ruthenium dihydride complexes undergo three major steps: cis-trans isomerization of ruthenium dihydride complex, CO2 insertion into the Ru-H bond, and H2 insertion into the ruthenium formate ion. Of these steps, CO2 insertion into the Ru-H bond has the lowest barrier compared with the other two steps in each preferred pathway. For the hydrogenation of CO2 catalyzed by ruthenium complexes of dmpe and dmpp, cis-trans isomerization of ruthenium dihydride complex has a similar barrier to that of H2 insertion into the ruthenium formate ion. However, in the reaction catalyzed by the PN(Me) PRu complex, cis-trans isomerization of the ruthenium dihydride complex has a lower barrier than H2 insertion into the ruthenium formate ion. These results suggest that the steric effect caused by the change of the outer sphere of the diphosphine ligand on the reaction is not clear, although the electronic effect is significant to cis-trans isomerization and H2 insertion. This finding refreshes understanding of the mechanism and provides necessary insights for ligand design in transition-metal-catalyzed CO2 transformation.

Phosphazenes: efficient organocatalysts for the catalytic hydrosilylation of carbon dioxide

Phosphazene superbases are efficient organocatalysts for the metal-free catalytic hydrosilylation of carbon dioxide. They react with CO2 to form the respective phosphine oxides, but in the presence of hydrosilanes, CO2 can be selectively reduced to silyl formates, which can in turn be reduced to methoxysilanes by addition of an extra loading of silanes. Activities reach a TOF of 32 h(-1) with a TON of 759. It is also shown that unexpectedly, N,N-dimethylformamide can reduce CO2 to a mixture of silyl formates, acetals and methoxides in the absence of any catalyst.

Reducing CO₂ to methanol using frustrated Lewis pairs: on the mechanism of phosphine-borane-mediated hydroboration of CO₂

The full mechanism of the hydroboration of CO2 by the highly active ambiphilic organocatalyst 1-Bcat-2-PPh2-C6H4 (Bcat = catecholboryl) was determined using computational and experimental methods. The intramolecular Lewis pair was shown to be involved in every step of the stepwise reduction. In contrast to traditional frustrated Lewis pair systems, the lack of steric hindrance around the Lewis basic fragment allows activation of the reducing agent while moderate Lewis acidity/basicity at the active centers promotes catalysis by releasing the reduction products. Simultaneous activation of both the reducing agent and carbon dioxide is the key to efficient catalysis in every reduction step.