Mechanism of cyclization-degradation of silazanes: A DFT study

The present calculations at the B3LYP/6-31G (d) level confirm that the cyclization-degradation of silazanes proceeds via two mechanisms: Si-N bond interchange and hydrogen abstraction. The energy barrier for hydrogen abstraction is approximately 20 kcal/mol lower than that of the Si-N bond interchange pathway. Substituent effects critically influence the thermal stability of silazanes. For the Si-N bond interchange mechanism, factors that enhance nucleophilic attack or reduce Si-N bond strength destabilize silazanes. This implies that substituents with the electron-donating inductive effects, electron-donating conjugation effects, or bulky steric profiles should be avoided to improve thermal stability. In contrast, hydrogen abstraction predominantly occurs at chain termini. To suppress this pathway, substituents with low basicity and weak reducing capacity are recommended as terminal blocking groups. Based on these findings, using phenyl groups as terminal substituents represents an economically viable and thermodynamically favorable strategy for enhancing silazane stability.