Electronic and mechanistic insights into the role of group 15 elements in the reactivity of dipnictogen dications featuring group 15group 15 double bonds in [2 + 2] cycloaddition reactions with phenylacetylene

A computational study at the M06-2X-D3/def2-TZVP level elucidates the [2 + 2] cycloaddition between phenylacetylene and G15-Rea ((L: → G15G15 ← :L)²⁺), featuring a doubly bonded G15G15 moiety (G15 = a group 15 element) and highlighting an electron-sharing interaction between two triplet G15 fragments. Among the systems examined, only dipnictogen dications containing SbSb and BiBi double bonds readily undergo [2 + 2] cycloaddition with phenylacetylene. Energy decomposition analysis–natural orbitals for chemical valence (EDA–NOCV) and frontier molecular orbital (FMO) analyses reveal that the key bonding interaction in the reaction involves electron donation from the filled p–π orbital of PhCCH into the vacant p–π* orbital of G15-Rea, while the reverse electron transfer is only marginal. The activation strain model (ASM) analysis suggests that the activation barrier for the capture of PhCCH by G15-Rea is primarily governed by the geometric deformation energies of both G15-Rea and PhCCH. A theoretical interpretation based on Shaik's model further indicates that the energy required to promote G15-Rea from its singlet to triplet state plays a significant role in determining the reaction barrier.