Dehydrogenation of amine-borane Me2NH·BH3 catalyzed by a lanthanum-hydride complex

The rare-earth-metal-hydride complexes [{(1,7-Me2TACD)LnH}4] (Ln=La 1 a, Y 1 b; (1,7-Me2TACD)H2 =1,7-dimethyl-1,4,7,10-tetraazacyclododecane, 1,7-Me2[12]aneN4) were synthesized by hydrogenolysis of [{(1,7-Me2TACD)Ln(η(3)-C3H5)}2] with 1 bar H2. The tetrameric structures were confirmed by (1)H NMR spectroscopy and single-crystal X-ray diffraction of compound 1 a. Both complexes catalyze the dehydrogenation of secondary amine-borane Me2NH·BH3 to afford the cyclic dimer (Me2NBH2)2 and (Me2N)2BH under mild conditions. Whilst the complete conversion of Me2NH·BH3 was observed within 2 h with lanthanum-hydride 1 a, the yttrium homologue 1 b required 48 h to reach 95% conversion. Further reactions of compound 1 a with Me2NH·BH3 in various stoichiometric ratios gave a series of intermediate products, [{(1,7-Me2TACD)LaH}4](Me2NBH2)2 (2 a), [(1,7-Me2TACDH)La(Me2NBH3)2] (3 a), [(1,7-Me2TACD)(Me2NBH2)La(Me2NBH3)] (4 a), and [(1,7-Me2TACD)(Me2NBH2)2La(Me2NBH3)] (5 a). Complexes 2 a, 3 a, and 5 a were isolated and characterized by multinuclear NMR spectroscopy and single-crystal X-ray diffraction studies. These intermediates revealed the activation and coordination modes of "Me2NH·BH3 " fragments that were trapped within the coordination sphere of a rare-earth-metal center.