Theoretical Study of Reaction Mechanisms of Carbon Dioxide with E–CH2–Z-Type Frustrated Lewis Pairs (E = C–Pb; Z = N–Bi)

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/P-Rea (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.

二氧化碳（CO₂）排放引发了诸多环境挑战，例如全球变暖以及对生物机体的损害。因此，在温和条件下开发高效的CO₂固定方法显得尤为迫切且必要。本研究以E（= C、Si、Ge、Sn与Pb）类路易斯酸（E/P型）以及Z（= N、P、As、Sb与Bi）类路易斯碱（Sn/Z型）受阻路易斯酸碱对（frustrated Lewis pairs, FLPs）为模型反应物，针对无金属CO₂结合反应开展了密度泛函理论计算研究。理论计算结果表明，在E/P型与Sn/Z型两类FLPs中，仅亚甲基桥连的Si/P-反应物（Rea=reactant）与Sn/P-反应物可结合CO₂，且该过程同时具备动力学与热力学可行性，并生成五元杂环加合物。能量分解分析-化学价自然轨道（energy decomposition analysis–natural orbitals for chemical valence, EDA-NOCV）分析显示，E/P型与Sn/Z型FLPs与CO₂之间的成键相互作用，可更准确地用「最高占据分子轨道（HOMO，来自Z位点）→最低未占据分子轨道（LUMO，来自CO₂）」的相互作用来描述，即FLP向CO₂的正向成键。而作为CO₂向FLP的反向成键的「LUMO(E) ← HOMO(CO₂)」相互作用，在该类CO₂活化反应中仅发挥次要作用。基于活化应变模型（activation strain model, ASM）分析可知，反应能垒的起源可归因于E或Z元素的原子半径：换言之，通过调控E与Z元素的原子半径，可分别优化E/P-反应物与Sn/Z-反应物类FLP化合物与CO₂之间的轨道重叠效果，进而影响反应能垒的高低。