From Carbon Dioxide to Methane: Homogeneous Reduction of Carbon Dioxide with Hydrosilanes Catalyzed by Zirconium−Borane Complexes

Carbon dioxide reduction to methane and coupling with acetylene to form propylene catalyzed by remodeled nitrogenase

A doubly substituted form of the nitrogenase MoFe protein (α-70(Val)(→Ala), α-195(His→Gln)) has the capacity to catalyze the reduction of carbon dioxide (CO(2)) to yield methane (CH(4)). Under optimized conditions, 1 nmol of the substituted MoFe protein catalyzes the formation of 21 nmol of CH(4) within 20 min. The catalytic rate depends on the partial pressure of CO(2) (or concentration of HCO(3)(-)) and the electron flux through nitrogenase. The doubly substituted MoFe protein also has the capacity to catalyze the unprecedented formation of propylene (H(2)C = CH-CH(3)) through the reductive coupling of CO(2) and acetylene (HC≡CH). In light of these observations, we suggest that an emerging understanding of the mechanistic features of nitrogenase could be relevant to the design of synthetic catalysts for CO(2) sequestration and formation of olefins.

Cascade catalysis for the homogeneous hydrogenation of CO2 to methanol

This communication demonstrates the homogeneous hydrogenation of CO(2) to CH(3)OH via cascade catalysis. Three different homogeneous catalysts, (PMe(3))(4)Ru(Cl)(OAc), Sc(OTf)(3), and (PNN)Ru(CO)(H), operate in sequence to promote this transformation.

Transformation of carbon dioxide with homogeneous transition-metal catalysts: a molecular solution to a global challenge?

A plethora of methods have been developed over the years so that carbon dioxide can be used as a reactant in organic synthesis. Given the abundance of this compound, its utilization in synthetic chemistry, particularly on an industrial scale, is still at a rather low level. In the last 35 years, considerable research has been performed to find catalytic routes to transform CO(2) into carboxylic acids, esters, lactones, and polymers in an economic way. This Review presents an overview of the available homogeneous catalytic routes that use carbon dioxide as a C(1) carbon source for the synthesis of industrial products as well as fine chemicals.

Trialkylborane-Assisted CO(2) Reduction by Late Transition Metal Hydrides

Trialkylborane additives promote reduction of CO(2) to formate by bis(diphosphine) Ni(II) and Rh(III) hydride complexes. The late transition metal hydrides, which can be formed from dihydrogen, transfer hydride to CO(2) to give a formate-borane adduct. The borane must be of appropriate Lewis acidity: weaker acids do not show significant hydride transfer enhancement, while stronger acids abstract hydride without CO(2) reduction. The mechanism likely involves a pre-equilibrium hydride transfer followed by formation of a stabilizing formate-borane adduct.

The catalytic role of N-heterocyclic carbene in a metal-free conversion of carbon dioxide into methanol: a computational mechanism study

A density functional theory study at the M05-2X(IEFPCM, THF)/6-311+G**//M05-2X/6-31G* level has been conducted to gain insight into the catalytic mechanism of the first metal-free N-heterocyclic carbene (NHC)-catalyzed conversion of carbon dioxide into methanol. Among the various examined reaction pathways, we found that the most favorable leads to the experimentally detected intermediates, including formoxysilane (FOS), bis(silyl)acetal (BSA), silylmethoxide (SMO), and disiloxane (DSO). However, our study also revealed that formaldehyde (CH(2)O), generated from the dissociation of BSA into DSO and CH(2)O via a mechanism somewhat similar to the Brook rearrangement, should be an inevitable intermediate, although it was not reported by the experimentalists. When NHC catalyzes the reactions of CO(2)/FOS/CH(2)O with silane, there are two activation modes. It was found that NHC prefers to activate Si-H bonds of silane and push electron density to the H atoms of the Si-H bonds in favor of transferring a hydridic atom of silane to the electrophilic C center of CO(2)/FOS/CH(2)O. This holds true in particular for the NHC-catalyzed reactions of silane with FOS/CH(2)O to produce BSA/SMO. The preferred activation mode can operate by first passing an energetically unfavorable NHC-silane local minimum via pi-pi interactions or by directly crossing a transition state involving three components simultaneously. The activation mode involving initial coordination of NHC with the electrophilic C atom of CO(2)/FOS/CH(2)O is less favorable or inoperable. The predicted catalytic mechanism provides a successful interpretation of the experimental observation that phenylsilane is more efficient than diphenylsilane in performing the conversion.

Tandem frustrated Lewis pair/tris(pentafluorophenyl)borane-catalyzed deoxygenative hydrosilylation of carbon dioxide

The frustrated Lewis pair system consisting of 2 equiv of 2,2,6,6-tetramethylpiperidine (TMP) and tris(pentafluorophenyl)borane [B(C(6)F(5))(3)] activates carbon dioxide to form a boratocarbamate-TMPH ion pair. In the presence of triethylsilane, this species is converted to a silyl carbamate and the known ion pair [TMPH](+)[HB(C(6)F(5))(3)](-), which recently was shown to react with CO(2) via transfer of the hydride from the hydridoborate to form the formatoborate [TMPH](+)[HC(O)OB(C(6)F(5))(3)](-). In the presence of extra B(C(6)F(5))(3) (0.1-1.0 equiv) and excess triethylsilane, the formatoborate is rapidly hydrosilated to form a formatosilane and regenerate [TMPH](+)[HB(C(6)F(5))(3)](-). The formatosilane in turn is rapidly hydrosilated by the B(C(6)F(5))(3)/Et(3)SiH system to CH(4), with (Et(3)Si)(2)O as the byproduct. At low [Et(3)SiH], intermediate CO(2) reduction products are observed; addition of more CO(2)/Et(3)SiH results in resumed hydrosilylation, indicating that this is a robust, living tandem catalytic system for the deoxygenative reduction of CO(2) to CH(4).

Conversion of carbon dioxide into methanol with silanes over N-heterocyclic carbene catalysts

Activate and reduce: Carbon dioxide was reduced with silane using a stable N-heterocyclic carbene organocatalyst to provide methanol under very mild conditions. Dry air can serve as the feedstock, and the organocatalyst is much more efficient than transition-metal catalysts for this reaction. This approach offers a very promising protocol for chemical CO(2) activation and fixation.

Trimetallic supported catalyst for renewable source of energy and environmental control through CO2 conversion

A supported catalyst and a catalytic process have been developed for the conversion of carbgas (CO2 + (100 ppm) H2O + 1% H2) as a renewable source of energy and as a measure for the control of carbon dioxide -- a greenhouse gas. The carbgas was passed over a trimetallic supported catalyst consisting of ruthenium (Ru), manganese (Mn) and cobalt (Co) dispersed on a high surface area titanium dioxide support at 673 K and at atmospheric pressure with a gas space velocity of 6000-7200/h. The catalytic reaction produces methanol and propyne in a fixed bed reactor system. The catalyst simultaneously splits water into hydrogen and oxygen, and carbon dioxide into carbon and oxygen under very mild reaction conditions and at atmospheric pressure. The oxygen generated during the reaction and the addition of hydrogen during the catalytic reaction not only generates a considerable amount of energy for the reaction to proceed but also sustains the oxidation states of Ru, Mn and Co. This process maintains the specific active oxidation states of the metals during the catalytic run -- a key step in the process.