Trapping Formaldehyde in the Homogeneous Catalytic Reduction of Carbon Dioxide

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.

Divergent CO2 Activation by Tuning the Lewis Acid in Iron-Based Bimetallic Systems

Bimetallic motifs mediate the selective activation and functionalization of CO2 in metalloenzymes and some recent synthetic systems. In this work, we build on the nascent concept of bimetallic frustrated Lewis pairs (FLPs) to investigate the activation and reduction of CO2 . Using the Fe0 fragment [(depe)2 Fe] (depe=1,2-bis(diethylphosphino)ethane) as base, we modify the nature of the partner Lewis acid to accomplish a divergent and highly chemoselective reactivity towards CO2 . [Au(PMe2 Ar)]+ irreversibly dissociates CO2 , Zn(C6 F5 )2 and B(C6 F5 )3 yield different CO2 adducts stabilized by push-pull interactions, while Al(C6 F5 )3 leads to a rare heterobimetallic C-O bond cleavage, and thus to contrasting reduced products after exposure to dihydrogen. Computational investigations provide a rationale for the divergent reactivity, while Energy Decomposition Analysis-Natural Orbital for Chemical Valence (EDA-NOCV) method substantiates the heterobimetallic bonding situation.

A masked form of an O-borylated Breslow intermediate for the diastereoselective FLP-type activation of aldehydes

Breslow intermediates are very often elusive species whose application in Frustrated Lewis Pair chemistry is unprecedented. We describe herein the use of a masked form of an O-Borylated Breslow (OBB) intermediate that performs FLP-type activation of the carbonyl function of five different benzaldehyde derivatives with complete diastereoselectivity. The resulting compounds are characterised in solution by NMR spectroscopy (compounds 4 - 8 ) and in solid state by X-Ray diffraction analysis (compounds 4 - 6 ). A combined kinetic and theoretical investigation reveals the associative nature of the rate determining step and suggests that the OBB intermediate part is never released during the whole process.

Selective Catalytic Synthesis Using the Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and Chemistry

The present Review highlights the challenges and opportunities when using the combination CO2 /H2 as a C1 synthon in catalytic reactions and processes. The transformations are classified according to the reduction level and the bond-forming processes, covering the value chain from high volume basic chemicals to complex molecules, including biologically active substances. Whereas some of these concepts can facilitate the transition of the energy system by harvesting renewable energy into chemical products, others provide options to reduce the environmental impact of chemical production already in today's petrochemical-based industry. Interdisciplinary fundamental research from chemists and chemical engineers can make important contributions to sustainable development at the interface of the energetic and chemical value chain. The present Review invites the reader to enjoy this exciting area of "catalytic chess" and maybe even to start playing some games in her or his laboratory.

Hydrophosphination of CO2 and subsequent formate transfer in the 1,3,2-diazaphospholene-catalyzed N-formylation of amines

Hydrophosphination of CO2 with 1,3,2-Diazaphospholene (NHP-H; 1) afforded phosphorus formate (NHP-OCOH; 2) through the formation of a bond between the electrophilic phosphorus atom in 1 and the oxygen atom from CO2 , along with hydride transfer to the carbon atom of CO2 . Transfer of the formate from 2 to Ph2 SiH2 produced Ph2 Si(OCHO)2 (3) in a reaction that could be carried out in a catalytic manner by using 5 mol % of 1. These elementary reactions were applied to the metal-free catalytic N-formylation of amine derivatives with CO2 in one pot under ambient conditions.

Carbon dioxide reduction to methylamines under metal-free conditions

The first metal-free catalysts are reported for the methylation of amines with carbon dioxide. Proazaphosphatrane superbases prove to be highly active catalysts in the reductive functionalization of CO2, in the presence of hydroboranes. The new methodology enables the methylation of N-H bonds in a wide variety of amines, including secondary amines, with increased chemoselectivity.

Metal-free reduction of CO2 with hydroboranes: two efficient pathways at play for the reduction of CO2 to methanol

Guanidines and amidines prove to be highly efficient metal-free catalysts for the reduction of CO2 to methanol with hydroboranes such as 9-borabicyclo[3.3.1]nonane (9-BBN) and catecholborane (catBH). Nitrogen bases, such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (Me-TBD), and 1,8-diazabicycloundec-7-ene (DBU), are active catalysts for this transformation and Me-TBD can catalyze the reduction of CO2 to methoxyborane at room temperature with TONs and TOFs of up to 648 and 33 h(-1) (25 °C), respectively. Formate HCOOBR2 and acetal H2C(OBR2)2 derivatives have been identified as reaction intermediates in the reduction of CO2 with R2BH, and the first C-H-bond formation is rate determining. Experimental and computational investigations show that TBD and Me-TBD follow distinct mechanisms. The N-H bond of TBD is reactive toward dehydrocoupling with 9-BBN and affords a novel frustrated Lewis pair (FLP) that can activate a CO2 molecule and form the stable adduct 2, which is the catalytically active species and can facilitate the hydride transfer from the boron to the carbon atoms. In contrast, Me-TBD promotes the reduction of CO2 through the activation of the hydroborane reagent. Detailed DFT calculations have shown that the computed energy barriers for the two mechanisms are consistent with the experimental findings and account for the reactivity of the different boron reductants.

Transition-metal-free catalytic reduction of carbon dioxide

Metal-free systems, including frustrated Lewis pairs (FLPs) have been shown to bind CO2. By reducing the Lewis acidity and basicity of the ambiphilic system, it is possible to generate active catalysts for the deoxygenative hydroboration of carbon dioxide to methanol derivatives with conversion rates comparable to those of transition-metal-based catalysts.