The dehydrogenation of ammonia-borane catalysed by dicarbonylruthenacyclic(II) complexes

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 NH(3)BH(3) within minutes at ca. 40 °C. The release of 3 eq. of H(2) 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 NH(3)BH(3) 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 NH(3)BH(3) 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.