CO ₂ activation without metals enabled by Lewis acid/base-free G13P double bonds

CO₂ is unequivocally recognized as a greenhouse gas. In this study, we carried out a theoretical investigation of CO₂ capture reactions mediated by Lewis acid/base-free G13P double bonds embedded within the Ter-G13P-Ter framework (Ter = 2,6-Dipp₂-C₆H₃; Dipp = diisopropylphenyl). Several theoretical approaches were employed to evaluate their reaction barriers and chemical reactivity. Our DFT results indicate that only Ter-AlP-Ter, Ter-GaP-Ter, and Ter-InP-Ter exhibit the ability to capture CO₂. The EDA analysis suggests that the bonding interaction between Ter-G13P-Ter and CO₂ in the transition state is best described by a donor-acceptor (singlet-singlet) interaction rather than an electron-sharing (triplet-triplet) interaction. FMO and NOCV analyses reveal two distinct bonding characteristics: (i) strong forward donation from the lone pair on phosphorus to the p-π* orbital of CO₂ (P → CO₂), and (ii) relatively weak back-donation from the filled p-π orbital of CO₂ into the vacant orbital of the G13 center (CO₂ → G13). ASM analysis further reveals that the geometrical strain energy of CO₂ plays a decisive role in determining the activation barrier of the [2 + 2] cycloaddition reaction between Ter-G13P-Ter and CO₂.