Unraveling Transfer Hydrogenation Mechanisms by Ammonia Borane to Alkenes over Self-Healing Copper Nanoparticles: The Complementary Role of N–H Bond, Surface, and Solvent

Abstract

Ammonia borane-based transfer hydrogenation mechanisms on copper nanoparticles (CuNPs) are identified and assessed by isotope labeling and Kohn–Sham density functional methods, using the hydrogenation of styrene to ethylbenzene under ambient conditions as the model reaction. The key role of protonic solvents in permitting ammonia borane decomposition is confirmed. Different dehydrogenation pathways are evidenced for the N–H and B–H bonds: while the metal surface always acts as an intermediary in the hydrogen transfer from the B–H bond to the organic substrate, the N–H bond can directly hydrogenate the most negatively charged carbon atom of the unsaturated bond. The styrene to ethylbenzene reaction is here proved to have a >99% conversion with 100% selectivity at ambient conditions, using methanol and pure water as the solvents. The CuNPs are obtained in situ by reduction of the copper source, SION-X (Cu₂[(BO)(OH)₂](OH)₃), by ammonia borane. The catalytic properties of these CuNPs are stable for at least 5 cycles without the need for reduction steps and upon their exposure to air in between subsequent cycles. This is due to ammonia borane’s ability to act simultaneously as the hydrogen source for the reaction and as the reducing agent of copper. Ammonia borane shows then a significant advantage over other hydrogen sources for transfer hydrogenation in combination with CuNPs, eliminating both the catalyst preparation and activation steps and reducing the complexity and operational cost of the process.