Mechanistic Insights into the Coupling of Hydrosilanes and Alcohol-Amines for Hydrogen Production

The production of hydrogen on-demand via the dehydrogenative coupling of hydrosilanes and alcohol-amines, conducted at room temperature without solvents nor catalysts, marks a significant advancement in the pursuit of greener energy sources. This work deals with the reaction mechanism of the dehydrogenative coupling of phenyl silane (PhS) and 2-(methylamino)-ethanol (MEA) as well as the interference of hydrolysis/condensation and disproportionation reactions when alkoxy-containing hydrosilanes, such as triethoxysilane (TES), are used. DFT methods were used to calculate the thermodynamic properties of the reactions involved (Δ_r G°, Δ_r H°, and Δ_r S°), the hydride donating ability of the reactants and intermediate products (HDA) as well as the Mulliken charges. These theoretical data complemented NMR and GC experiments to identify the most likely reaction pathway. Our results suggest the occurrence of multiple reaction pathways involving the nucleophilic attack of OH groups, and to a lesser extent NH groups, on the Si-H bond, leading to the formation of Si-O and Si-N containing products and the release of hydrogen. However, due to the low stability and high entropic contribution in the formation of Si-N containing products compared to their Si-O containing counterparts, their formation seems unfavorable, and no products containing only Si-N bonds were detected. However, the product with mixed Si-N and Si-O bonding, PhSiH-(O-)-(N-), was detected as a minor product. The high hydride donating ability of Si-O containing products led to such species undergoing consecutive nucleophilic attacks on their Si-H bonds by the hydroxyl group in MEA until complete dehydrogenation, ultimately leading to the formation of PhSi-(O-)₃ as the main reaction product.