Mechanistic Insights into the Reduction of Carbon Dioxide with Silanes over N-Heterocyclic Carbene Catalysts

Exploring the reductive CO2 fixation with amines and hydrosilanes using readily available Cu(II) NHC-phenolate catalyst precursors

N-Methylation of amines is of great interest in the synthesis of pharmaceuticals and valuable compounds, and the possibility to perform this reaction with an inexpensive and non-toxic substrate like CO2 and its derivatives is quite appealing. Herein, the synthesis of four novel homoleptic Cu(II) complexes with hybrid NHC-phenolate (NHC = N-Heterocyclic Carbene) ligands is reported, and their use in the catalytic N-methylation of amines with CO2 in the presence of hydrosilanes is explored. Both bidentate or tetradentate ligands can be used in the preparation of the complexes provided that the structural requirement that the two NHC and the two phenolate donors in the metal coordination sphere are mutually in trans is fulfilled. A new reaction protocol to perform the N-methylation of secondary aromatic amines and dibenzylamine in high yield under mild reaction conditions is developed, using the ionic liquid [BMMIM][NTf2] (1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide) as solvent and the catalyst precursor [Cu(L2)2]. Reactivity studies indicate that the reaction follows two different pathways with different hydrosilanes, and that the starting Cu(II) complexes are reduced under the catalytic conditions.

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.

DFT Study of Functional Reduction of CO2 with BH3NMe3: The Real Role of Organic Catalyst TBD

The detailed mechanism of transition metal-free-catalyzed monomethylation of 2-naphthyl acetonitrile (1a) with CO2 in the presence of triazabicyclodecene (TBD) and BH3NMe3 was investigated using density functional theory. The C-methylation process proved to generate formaldehyde followed by the formation of the product via an alcohol rather than a methoxyborane intermediate. During the reaction, CO2 is activated to form the TBD-CO2 adduct and BH3NMe3 is changed into TBD-BH2 (IM2) in the presence of TBD. IM2 plays a real reducing role within the system due to the unique coordination capability of the B atom. In addition to enhancing the nucleophilicity of 1a through deprotonation by tBuOK, our research also indicates that the generated tBuOH not only assists in proton transfer to generate an alcohol intermediate but also promotes the regeneration of TBD.

A supported pyridylimine-cobalt catalyst for N-formylation of amines using CO2

N-Formylation of amines with CO2 as a cheap and non-toxic C1-feedstock and hydrosilane reducing agent is a practical and environment friendly method to synthesize formamides. This study describes an efficient and chemoselective mono-N-formylation of amines using CO2 and phenylsilane under mild conditions using a porous metal-organic framework (MOF)-supported single-site cobalt catalyst (pyrim-UiO-Co). The pyrim-UiO-Co MOF has a UiO-topology, and its organic linkers bear a pyridylimine ligated Co catalytic moiety. A wide range of aliphatic and aromatic amines are transformed into desired N-formamides in moderate to excellent yields under 1-5 bar CO2. Pyrim-UiO-Co is tolerant to various functional groups and could be recycled and reused at least 10 times. Mechanistic investigation using kinetic, spectroscopic and density functional theory studies suggests that the formylation of benzylamine proceeds sequentially via oxidative addition of PhSiH3 and CO2 insertion, followed by a turn-over limiting reaction with an amine. Our work highlights the importance of MOF-based Earth-abundant metal catalysts for the practical and eco-friendly synthesis of fine chemicals using cheap feedstocks.

Catalytic reduction of carbon dioxide by a zinc hydride compound, [Tptm]ZnH, and conversion to the methanol level

The zinc hydride compound, [Tptm]ZnH, may achieve the reduction of CO₂ by (RO)₃SiH (R = Me, Et) to the methanol oxidation level, (MeO)_xSi(OR)_4-x, via the formate species, HCO₂Si(OR)₃. However, because insertion of CO₂ into the Zn-H bond is more facile than insertion of HCO₂Si(OR)₃, conversion of HCO₂Si(OR)₃ to the methanol level only occurs to a significant extent in the absence of CO₂.

Hydrogenation of CO2 or CO2 Derivatives to Methanol under Molecular Catalysis: A Review

The atmospheric CO2 concentration has been continuously increasing due to fossil fuel combustion. The transformations of CO2 and CO2 derivatives into high value-added chemicals such as alcohols are ideal routes to mitigate greenhouse gas emissions. Among alcohol products, methanol is very promising as it fulfills the carbon neutral cycle and can be used for direct methanol fuel cells. Herein, we summarize the recent progress in the hydrogenation of CO2 or CO2 derivatives to methanol, and focus on those systems with homogeneous catalysts and molecular hydrogen as the reductant. Discussions on the catalytic systems, efficiencies, and future outlooks will be given.

Carbene catalyzed C(sp3)–Cl activation of chlorinated solvents for benzyne chlorination

A new mode of N-heterocyclic carbene (NHC) organocatalysis was discovered, in which the normally inert chlorinated solvents were activated by carbene to realize the chlorination of hexa-dehydro-Diels-Alder derived benzynes.

From CO2 activation to catalytic reduction: a metal-free approach

Over exploitation of natural resources and human activities are relentlessly fueling the emission of CO2 in the atmosphere. Accordingly, continuous efforts are required to find solutions to address the issue of excessive CO2 emission and its potential effects on climate change. It is imperative that the world looks towards a portfolio of carbon mitigation solutions, rather than a single strategy. In this regard, the use of CO2 as a C1 source is an attractive strategy as CO2 has the potential to be a great asset for the industrial sector and consumers across the globe. In particular, the reduction of CO2 offers an alternative to fossil fuels for various organic industrial feedstocks and fuels. Consequently, efficient and scalable approaches for the reduction of CO2 to products such as methane and methanol can generate value from its emissions. Accordingly, in recent years, metal-free catalysis has emerged as a sustainable approach because of the mild reaction conditions by which CO2 can be reduced to various value-added products. The metal-free catalytic reduction of CO2 offers the development of chemical processes with low cost, earth-abundant, non-toxic reagents, and low carbon-footprint. Thus, this perspective aims to present the developments in both the reduction and reductive functionalization chemistry of CO2 during the last decade using various metal-free catalysts.

A Resin-Supported Formate Catalyst for the Transformative Reduction of Carbon Dioxide with Hydrosilanes

A heterogeneous formate anion catalyst for the transformative reduction of carbon dioxide (CO₂ ) based on a polystyrene and divinylbenzene copolymer modified with alkylammonium formate was prepared from a widely available anion exchange resin. The catalyst preparation was easy and the characterization was carried out by using elemental analysis, Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state ¹³ C cross-polarization/magic-angle spinning nuclear magnetic resonance (¹³ C CP/MAS NMR) spectroscopy. The catalyst displayed good catalytic activity for the direct reduction of CO₂ with hydrosilanes, tunably yielding silylformate or methoxysilane products depending on the hydrosilanes used. The catalyst was also active for the reductive insertion of CO₂ into both primary and secondary amines. The catalytic activity of the resin-supported formate can be predicted from the FTIR spectra of the catalyst, probably because of the difference in the ionic interaction strength between the supported alkylammonium cations and formate anions. The ion pair density is thought to influence the catalytic activity, as shown by the elemental and solid-state ¹³ C NMR analyses.

Unprecedent formation of methylsilylcarbonates from iridium-catalyzed reduction of CO2 with hydrosilanes

The iridium complex [Ir(μ-CF₃SO₃)(κ²-NSi^Me2)₂]₂ (3) (NSi^Me2 = {4-methylpyridine-2-yloxy}dimethylsilyl) has been prepared by reaction of [Ir(μ-Cl)(κ²-NSi^Me2)₂]₂ (1) with two equivalents of AgCF₃SO₃. The solid structure of 3 evidenced its dinuclear nature, being a rare example of an iridium species with triflate groups acting as bridges. The 3-catalyzed reduction of CO₂ with HSiMe(OSiMe₃)₂ affords a mixture of the corresponding silylformate and methoxysilane together with the silylcarbonate CH₃OCO₂SiMe(OSiMe₃)₂ (4a). This is the first time that the formation of silylcarbonates has been observed from the catalytic reduction of CO₂ with silanes. Analogous behaviour has been observed when HSiMe₂Ph and HSiMePh₂ were used as reductants.

The formation of methylsilylcarbonates from the iridium-catalyzed hydrosilylation of CO₂ has been observed for the first time.

The next big thing for silicon nanostructures – CO2 photocatalysis

Silicon nanostructures for the catalytic conversion of CO₂ to value-added products.

Synthesis of fused-tetrahydropyrimidines: one-pot methylenation–cyclization utilizing two molecules of CO2

A methylenation–cyclization reaction employing cyclic enaminones with primary aromatic amines and two molecules of CO₂ to furnish fused-tetrahydropyrimidines has been developed.

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.

Kinetic hydricity of silane hydrides in the gas phase

Herein, gas phase studies of the kinetic hydricity of a series of silane hydrides are described. An understanding of hydricity is important as hydride reactions figure largely in many processes, including organic synthesis, photoelectrocatalysis, and hydrogen activation. We find that hydricity trends in the gas phase differ from those in solution, revealing the effect of solvent. Calculations and further experiments, including H/D studies, were used to delve into the reactivity and structure of the reactants. These studies also represent a first step toward systematically understanding nucleophilicity and electrophilicity in the absence of solvent.

CO2 to methanol conversion using hydride terminated porous silicon nanoparticles

Porous silicon nanoparticles (Si-NPs) prepared via magnesiothermic reduction were used to convert carbon dioxide (CO₂) into methanol. The hydride surface of the silicon nanoparticles acted as a CO₂ reducing reagent without any catalyst at temperatures above 100 °C. The Si nanoparticles were reused up to four times without significant loss in methanol yields. The reduction process was monitored using in situ FT-IR and the materials were characterized using SEM, TEM, NMR, XPS, and powder XRD techniques. The influence of reaction temperature, pressure, and Si-NP concentration on CO₂ reduction were also investigated. Finally, Si particles produced directly from sand were used to convert CO₂ to methanol.

Catalyst-free N-methylation of amines using CO2

Recently, utilizing CO₂ as a methylation reagent to construct functional chemicals has attracted significant attention. However, the conversion of CO₂ is still a challenge due to its inherent inertness. In this study, we have developed a catalyst-free N-methylation of amines to prepare numerous methylamines using CO₂ as a methyl source. By utilizing 2 eq. PhSiH₃ as the reductant, amines could undergo N-methylation under 1 atm of CO₂ in DMF at 90 °C. Aliphatic and aromatic amines were compatible, generating the desired products in up to 95% yield.

Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

A simple quaternary ammonium perrhenate salt catalyses the hydrosilylation of aldehydes, ketones, and carbon dioxide, and the methylation of amines using carbon dioxide. DFT calculations show that a perrhenate hypervalent silicate interacts directly with CO₂.

Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)₂ ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

Heterogeneous reduction of carbon dioxide by hydride-terminated silicon nanocrystals

Silicon constitutes 28% of the earth's mass. Its high abundance, lack of toxicity and low cost coupled with its electrical and optical properties, make silicon unique among the semiconductors for converting sunlight into electricity. In the quest for semiconductors that can make chemicals and fuels from sunlight and carbon dioxide, unfortunately the best performers are invariably made from rare and expensive elements. Here we report the observation that hydride-terminated silicon nanocrystals with average diameter 3.5 nm, denoted ncSi:H, can function as a single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide, at a rate of hundreds of μmol h⁻¹ g⁻¹. The large surface area, broadband visible to near infrared light harvesting and reducing power of SiH surface sites of ncSi:H, together play key roles in this conversion. Making use of the reducing power of nanostructured hydrides towards gaseous carbon dioxide is a conceptually distinct and commercially interesting strategy for making fuels directly from sunlight.

Elemental silicon is widely studied for photovoltaic applications. Here, the authors report that hydride-terminated silicon nanocrystals can also function as single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide.

N-Heterocyclic Olefins as Robust Organocatalyst for the Chemical Conversion of Carbon Dioxide to Value-Added Chemicals

In this report, the activity of N-heterocyclic olefins (NHOs) as a newly emerging class of organocatalyst is investigated for the chemical fixation of carbon dioxide through reactions with aziridines to form oxazolidinones and the N-formylation of amines with polymethylhydrosiloxane (PMHS) or 9-borabicyclo[3.3.1]nonane (9-BBN) as the reducing agent under mild conditions. The exocyclic carbon atoms of NHOs are highly nucleophilic owing to the electron-donating ability of the two nitrogen atoms. This high nucleophilicity of the NHOs activates CO2 molecules to form zwitterionic NHO-carboxylate (NHO-CO2 ) adducts, which are active in formylation reactions as well as the carboxylation of aziridines to oxazolidinones.

Tuning the activity and selectivity of iridium-NSiN catalyzed CO2 hydrosilylation processes

Catalyst design for iridium-catalyzed CO₂ hydrosilylation processes: improvement of the selectivity and reduction of the reaction time.