Donor–acceptor complexes of inorganic analogs of benzene

GRAPHICAL ABSTRACT

ABSTRACT

Results of experimental and theoretical studies of inorganic benzene analogs: borazine, substituted borazines, polyborazines, alumazene, and their donor–acceptor complexes are summarized. Structural and energetic aspects of complex formation and thermal stability of heterocycles and their complexes are discussed. A mechanism for the gas-phase acetonitrile polymerization in the presence of alumazene is proposed on the basis of computational studies. It is experimentally shown that solution of B,B′,B″-tribromborazine in deuterobenzene undergoes fast (within minutes) H/D exchange in the presence of Lewis acid AlBr₃. The proposed electrophilic substitution mechanism for the exchange is supported by quantum-chemical computations. In contrast, in the presence of AlBr₃, unsubstituted borazine in deuterobenzene polymerizes with hydrogen evolution without H/D exchange. The absence of the H/D exchange may be explained by larger stability of the borazonium ion B₃N₃H₇⁺ which prevents operation of the catalytic cycle. Quantum-chemical computations at B3LYP/TZVP level of theory indicate that upon complexation with AlCl₃ both endothermicity and activation energies of hydrogenation processes of borazine and polyborazines are significantly reduced. The use of Lewis acids as catalysts in the processes of regeneration of spent hydrogen fuel is recommended.

Keywords:

• Borazine
• polyborazines
• alumazene
• aromaticity
• complex formation
• reactivity
• quantum-chemical studies
• Lewis acids

Acknowledgments

Research was carried out using computational resources provided by Resource Center “Computer Center of SPbSU.” NMR studies were performed at Resource Center “Center for Magnetic Resonance of SPbSU.”

Funding

Financial support from St. Petersburg State University (grants 12.38.255.2014 and 12.50.1563.2013) is acknowledged. A.Y.T. is grateful to the Alexander von Humboldt foundation for continuous support.