The Real Role of N-Heterocyclic Carbene in Reductive Functionalization of CO2: An Alternative Understanding from Density Functional Theory Study

The mechanisms of reductive functionalization of CO2 to formamide catalyzed by N-heterocyclic carbene (NHC) were comprehensively studied with DFT calculations. New activation mode with much lower energy barrier than those proposed before was discovered. In this reaction, NHC acts as neither a CO2 nor a silane activator, but as a precursor of the real catalyst, i.e., the in situ formed ionic liquid [NHCH](+)[Carbamate](-). In this loose contact ion pair, the negatively charged O atom of the carbamate anion becomes the new active site and is free to do nucleophilic attack. When amine is absent, CO2 will be converted into methanol. In this case, the NHC-CO2 adduct is the real catalytic species, the active site shifted from the carbene C atom to the negatively charged O atom. These new activation modes follow a pattern of "S(N)2@Si-Acceptor", in which the Si-H bond is activated via concerted backside S(N)2 nucleophilic attack by the negatively charged O atom, and the leaving hydride is directly accepted by a free CO2 molecule. The advantages of these new activation modes originate from the following points: (1) The ionic liquid [NHCH](+)[Carbamate](-) and NHC-CO2 adduct are thermodynamically more stable than NHC. (2) The active site of the NHC catalyst is extended outside a lot. Consequently, the large steric effect between the NHC arms and the substrates in transition state can be avoided to some extent. (3) The O atom has good silicon affinity. In addition, a free CO2 molecule, whose carbon atom is more electrophilic than those of the CO2 moieties in NHC-CO2 adduct and carbamate, acts as an efficient hydride acceptor.