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 () with the heavy imine-like molecule (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 (LLGe=G15L) is characterized by electron-sharing bonding between the triplet LLGe and triplet G15-L fragments. All five Ge=G15-based heavy imine analogues readily undergo [2 + 2] cycloaddition reactions with . Energy decomposition analysis (EDA-NOCV) suggests that the [2 + 2] cycloaddition reaction between and 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 and the vacant p-π* orbital of . Based on the activation strain model results, the activation barrier of the [2 + 2] cycloaddition reaction is predominantly controlled by the deformation energies of and .