The dehydrogenation of ammonia–borane catalysed by dicarbonylruthenacyclic(ii) complexes

Abstract

The reactivity of ruthenacyclic compounds towards ammonia–borane's dehydrogenation was investigated by considering both hydrolytic and anhydrous conditions. The study shows that the highly soluble μ-chlorido dicarbonylruthenium(II) dimeric complex derived from 4-tert-butyl,2-(p-tolyl)pyridine promotes, with an activation energy E_a of 22.8 kcal mol⁻¹, the complete hydrolytic dehydrogenation of NH₃BH₃ within minutes at ca. 40 °C. The release of 3 eq. of H₂ entails the formation of boric acid derivatives and the partly reversible protonolysis of the catalyst, which produces free 2-arylpyridine ligand and a series of isomers of “Ru(CO)₂(H)(Cl)”. Under anhydrous conditions, hydrogen gas release was found to be slower and the dehydrogenation of NH₃BH₃ results in the formation of conventional amino–borane derivatives with concomitant protonolysis of the catalyst and release of isomers of “Ru(CO)₂(H)(Cl)”. The mechanism of the protonolysis of the ruthenacycle was investigated with state-of-the-art DFT-D methods. It was found to proceed by the concerted direct attack of the catalyst by NH₃BH₃ leading either to the formation of a coordinatively unsaturated “Ru(CO)₂(H)(Cl)” species. The key role of “Ru(CO)₂(H)(Cl)” species in the dehydrogenation of ammonia–borane was established by trapping and quenching experiments and inferred from a comparison of the catalytic activity of a series of dicarbonylruthenium(II) complexes.