Carbon dioxide (CO2) emission poses several environmental challenges, such as global warming and harm to living creatures. Therefore, developing efficient CO2-fixing methods under mild conditions is particularly urgent and essential. In this study, a metal-free CO2 binding reaction using E (= C, Si, Ge, Sn, and Pb) Lewis acid (E/P-based) and a Z (= N, P, As, Sb, and Bi) Lewis base (Sn/Z-based) frustrated Lewis pairs (FLPs) as model reactants was theoretically investigated using density functional theory calculations. The theoretical results suggested that in both E/P-based and Sn/Z-based FLPs, a five-membered heterocyclic adduct was produced only from CH2-bridged Si/P-Rea and Sn/PRea (Rea = reactant) that can bind CO2, both kinetically and thermodynamically. An energy decomposition analysis-natural orbitals for chemical valence analysis revealed that the bonding interactions between E/P-based and Sn/Z-based with CO2 are better described in terms of the highest occupied molecular orbital (HOMO) (Z) -> lowest unoccupied molecular orbital (LUMO) (CO2) interaction, which is the FLP-to-CO2 forward bonding. However, the LUMO(E) <- HOMO (CO2) interaction, which is the CO2-to-FLP back-bonding, plays a minor role in such CO2 activation reactions. According to the activation strain model, it was found that the origin of the reaction barrier could be due to the atomic radius of either the E or Z elements. That is, obtaining a better orbital overlap between the E/P-Rea and Sn/Z-Rea FLP-type compounds and CO2 influences the barrier heights through the atomic radius of E and Z, respectively.
