Insights into the Factors Controlling the Origin of Activation Barriers in the [2 + 2] Cycloaddition Reactions of Heavy Imine-like Molecules Featuring a Ge=Group 15 Double Bond with Heterocumulenes

The [2 + 2] cycloaddition reactions of the heterocumulene (<b>N=C=N</b>) with the heavy imine-like molecule <b>Ge=G15-Rea</b> (G15 = Group 15 element) were examined using density functional theory (M06-2X-D3/def2-TZVP). The theoretical findings indicate that the doubly bonded Ge=G15 moiety in <b>Ge=G15-Rea</b> (L₁L₂Ge=G15L₃) is characterized by electron-sharing bonding between the triplet L₁L₂Ge and triplet G15-L₃ fragments. All five Ge=G15-based heavy imine analogues readily undergo [2 + 2] cycloaddition reactions with <b>N=C=N</b>. Energy decomposition analysis (EDA-NOCV) suggests that the [2 + 2] cycloaddition reaction between <b>Ge=G15-Rea</b> and <b>N=C=N</b> involves a donor-acceptor (singlet-singlet) interaction instead of an electron-sharing (triplet-triplet) interaction. Frontier molecular orbital (FMO) theory and the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) findings emphasize that the key bonding interaction involves the occupied p-π orbital of <b>Ge=G15-Rea</b> and the vacant p-π* orbital of <b>C=N=C</b>. Based on the activation strain model results, the activation barrier of the [2 + 2] cycloaddition reaction is predominantly controlled by the deformation energies of <b>Ge=G15-Rea</b> and <b>N=C=N</b>.