CO₂ Adducts of N-Heterocyclic Carbenes: Thermal Stability and Catalytic Activity toward the Coupling of CO₂ with Epoxides

Abstract

Thermal stability of CO2 adducts of N-heterocyclic carbenes (NHCs) was studied by means of in situ FTIR method with monitoring of the nu(CO2) region of the infrared spectra under various conditions. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-CO2) shows higher thermal stability compared with 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPr-CO2). The presence of free CO2 can significantly inhibit the decomposition of NHC-CO2 adducts, while the addition of an epoxide such as propylene oxide has a negative effect on stabilizing these adducts. As zwitterionic compounds, NHC-CO2 adducts were also proved to be effective organic catalysts for the coupling reaction of CO2 and epoxides to afford cyclic carbonates, for which a possible mechanism was proposed. Among these NHC-CO2 adducts, the relatively unstable IPr-CO2 exhibits the highest catalytic activity. The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components.

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

“…If the complex has a low-valent, nucleophilic metal locus, then CO 2 coordination occurs as an g 2 -CO 2 complex with the metal bound to the central carbon to form a carboxylate (36). This particular type of coordination is also observed in the presence of organic nucleophiles as described in Section 3.3 and is of particular interest in the field of electrocatalysis for CO 2 reduction with molecular species.…”

“…Thermal stability studies have yielded a series of data shown in Fig. 3 [35,36]. From the available data, we have observed a trend in the temperature (T) of CO 2 release of these zwitterionic species.…”

Sustainable conversion of carbon dioxide: the advent of organocatalysis

“…59 Being highly nucleophilic in nature, NHCs need to bind suitable electrophiles to form air-and moisture-stable adducts: a classic example is the formation of a zwitterionic NHC−CO 2 adducts (Scheme 3). 60 These adducts are labile in solution, making them suitable for catalytic turnover.…”

“…The nucleophilic attack of the CO 2 moiety bound to the NHC on the substrates is thought to be the rate-limiting step: this hypothesis seems to be supported by a significant positive effect of electron-donating alkyl substituents on the NHC heteroatoms. Previously, Lu and coworkers 60 reported on the use of 1,3-diarylimidazolium-2-carboxylates as catalysts in similar transformations. At a later stage, the same group has shown that a NHC-CO 2 adduct could also be immobilized on a MCM-41 support and the catalytic activity of this hybrid material in the coupling between CO 2 and epoxides was investigated.…”

Self-Assembly Fabrication of Hollow Mesoporous Silica@Co–Al Layered Double Hydroxide@Graphene and Application in Toxic Effluents Elimination

“…5 Imidazolium carboxylates have been shown to successfully catalyse reactions such as the coupling of epoxides and carbon dioxide to produce cyclic carbamates. 6 The more Lewis basic nature of imidazolium dithiocarboxylates has allowed them to be used successfully as catalysts for the cyanosilylation of aldehydes, 7 and in the Staudinger reaction to prepare β-lactams. 8 Delaude demonstrated that the NHC·CS 2 can also be used as ligand and coordinates as a κ 2 ,S,S′ chelate to the Ru II centre in the first examples of transition-metal complexes with such ligands (B in Chart 1).…”