# 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

## TL;DR

This paper investigates how the structure of certain heavy imine-like molecules affects their [2 + 2] cycloaddition reactions with N=C=N using computational methods.

## Contribution

The study reveals that activation barriers in these reactions are mainly due to deformation energies of the reacting molecules.

## Key findings

- The Ge=G15 moiety in the molecule forms electron-sharing bonds between triplet fragments.
- The [2 + 2] cycloaddition reaction involves a donor–acceptor interaction rather than an electron-sharing one.
- Deformation energies of the reactants control the activation barrier of the reaction.

## Abstract

The [2 + 2] cycloaddition reactions of the heterocumulene (N=C=N) with the heavy imine-like molecule Ge=G15-Rea (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 Ge=G15-Rea (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 N=C=N. Energy decomposition analysis (EDA–NOCV) suggests that the [2 + 2] cycloaddition reaction between Ge=G15-Rea and N=C=N 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 Ge=G15-Rea and the vacant p-π* orbital of C=N=C. Based on the activation strain model results, the activation barrier of the [2 + 2] cycloaddition reaction is predominantly controlled by the deformation energies of Ge=G15-Rea and N=C=N.