title: Heterogeneous dehydrocoupling of amine-borane adducts by skeletal nickel catalysts.
abstract: 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 Me(2)NH·BH(3) (1) and Et(2)NH·BH(3) (5) was demonstrated using 5 mol % skeletal Ni catalyst at 20 °C and produced [Me(2)N-BH(2)](2) (2) and [Et(2)N-BH(2)](2)/Et(2)N═BH(2) (6), respectively. The related adduct iPr(2)NH·BH(3) (7) was also dehydrogenated to afford iPr(2)N═BH(2) (8) but with significant catalyst deactivation. Catalytic dehydrocoupling of MeNH(2)·BH(3) (9) was found to yield the cyclic triborazane [MeNH-BH(2)](3) (10) as the major product, whereas high molecular weight poly(methylaminoborane) [MeNH-BH(2)](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 NH(3)·BH(3) (12), which produced cyclic oligomers and insoluble polymers, [NH(2)-BH(2)](x) (14), under catalytic and stoichiometric Ni loadings, respectively. Catalyst recycling was hindered by gradual poisoning. A study of possible catalyst poisons suggested that BH(3) 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 MeNH(2)·BH(3) (9). Skeletal Fe was found to be completely inactive toward amine-borane dehydrogenation.
authors: School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS United Kingdom.
authors: Alasdair P M
authors: AP
authors: Robertson
authors: Riccardo
authors: R
authors: Suter
authors: Laurent
authors: L
authors: Chabanne
authors: George R
authors: GR
authors: Whittell
authors: Ian
authors: I
authors: Manners
mesh_terms: D000497:Alloys / Q000737:chemistry; D000535:Aluminum / Q000737:chemistry; D000588:Amines / Q000737:chemistry*; D001880:Boranes / Q000737:chemistry*; D002384:Catalysis; D006859:Hydrogen / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*
chemical_list: D000497:Alloys; D000588:Amines; D001880:Boranes; D009532:Nickel; D006859:Hydrogen; D000535:Aluminum