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 mu-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 degrees 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 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.