Quantum Chemical Investigation of Covalent Adduct Formation in Five-Membered N-Heterocyclic Carbenes for CO2 Capture—Insights From Mechanistic and Topological Studies

Abstract

N-heterocyclic carbenes (NHCs) have been identified as promising CO2 capture agents due to their strong nucleophilicity, structural and reactive tunability, and the immense possibility of the resulting zwitterionic adducts to convert into a plethora of valuable compounds. Thus, in this work, we systematically explored NHC─CO2 interactions in 81 five-membered NHCs comprising pyrrole, pyrazole, imidazole, triazole, and tetrazole family, with a wide range of structural variations and nucleophilicity. The proton affinities (PA) of the NHCs and the interaction energies (ΔEint) of their complexes with CO2 correlate well with varying NHC structures, classifying these into three groups. The thermodynamic and IR data of the NHC─CO2 adduct formation pathway agree well with the characteristics of each group of NHCs. These observations were further verified by the topology of the C─C bond formation in NHC─CO2 complexes and adducts through QTAIM analysis. The results show that the presence or absence of N─H…CO2 secondary interactions and the orientation of CO2 (bent/perpendicular) play a vital role in characterizing the C─C bond during NHC─CO2 interaction. These insights should assist in the design, development and exploration of more such NHC-based CO2 capture systems addressing an urgent need for decarbonization, promising a sustainable future.