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: → G15G15 ← :L)²⁺), featuring a doubly bonded G15G15 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 SbSb and BiBi 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 PhCCH 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 PhCCH by G15-Rea is primarily governed by the geometric deformation energies of both G15-Rea and PhCCH. 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.