Computational study of titanocene-catalyzed dehydrocoupling of the adduct Me2NH•BH3:: An intramolecular, stepwise mechanism

The titanocene-catalyzed dehydrocoupling of the adduct Me2NH center dot BH3 has been investigated by density functional theory (B3LYP). Unlike the thermal dehydrogenation of ammonia-borane, which was previously found to occur via a concerted mechanism, such a catalytic dehydrogenation of Me2NH center dot BH3 has been found to be a stepwise process. Both intra- and intermolecular dehydrogenation pathways have been computed. The entire intramolecular dehydrogenation process is exergonic by 7.4 kcal/mol, and the intermolecular dehydrogenation is an endergonic process. The solvation free-energy barriers for the intramolecular dehydrogenations are 14.1 and 7.9 kcal/mol, respectively, which are lower than those of the intermolecular dehydrogenation pathway (24.1 and 9.2 kcal/mol, respectively). These results suggest that the intramolecular pathway is preferable both kinetically and energetically. The resulting monomeric aminoborane species, Me2NBH2, may undergo dimerization leading to the cyclic (Me2N-BH2)(2), which was experimentally observed.