Heterogeneous Dehydrocoupling of Amine-Borane Adducts by Skeletal Nickel Catalysts

Skeletal Ni, produced by the selective leaching of Al from a Ni/Al alloy, has been successfully employed in the catalytic dehydrogenation of various amine borane adducts. The combination of low cost and facile single-step synthesis make this system a potentially attractive alternative to the previously described precious metal and other first-row metal catalysts. The heterogeneous nature of the catalyst facilitates convenient product purification, and this is the first such system to be based on a first-row transition metal. Catalytic dehydrocoupling of Me2NH center dot BH3 (1) and Et2NH center dot BH3 (5) was demonstrated using 5 mol % skeletal Ni catalyst at 20 degrees C and produced [Me2N-BH2](2) (2) and [Et2N-BH2](2)/Et2N=BH2 (6), respectively. The related adduct iPr(2)NH center dot BH3 (7) was also dehydrogenated to afford iPr(2)N=BH2 (8) but with significant catalyst deactivation. Catalytic dehydrocoupling of MeNH2 center dot BH3 (9) was found to yield the cyclic triborazane [MeNH-BH2](3) (10) as the major product, whereas high molecular weight poly(methylaminoborane) [MeNH-BH2](n) (11) (M-w = 78 000 Da, PDI = 1.52) was formed when stoichiometric quantities of Ni were used. Similar reactivity was also observed with NH3 center dot BH3 (12), which produced cyclic oligomers and insoluble polymers, [NH2-BH2](x) (14), under catalytic and stoichiometric Ni loadings, respectively. Catalyst recycling was hindered by gradual poisoning. A study of possible catalyst poisons suggested that BH3 was the most likely surface poison, in line with previous work on colloidal Rh catalysts. Catalytic dehydrogenation of amine-borane adducts using skeletal Cu and Fe was also explored. Skeletal Cu was found to be a less active dehydrogenation catalyst for amine-borane adducts but also yielded poly(methylaminoborane) under stoichiometric conditions on reaction with MeNH2 center dot BH3 (9). Skeletal Fe was found to be completely inactive toward amine-borane dehydrogenation.