Zirconium-Catalyzed Amine Borane Dehydrocoupling and Transfer Hydrogenation

kappa(5)-(Me3SiNCH2CH2)(2)N-(CH2CH2NSiMe2CH2)Zr (1) has been found to dehydrocouple amine borane substrates, RR'NHBH3 (R = R' = Me; R = Bu-t, R' = H; R = R' = H), at low to moderate catalyst loadings (0.5-5 mol %) and good to excellent conversions, forming mainly borazine and borazane products. Other zirconium catalysts, (N3N)ZrX [(N3N) = N-(CH2CH2NSiMe2CH2)(3), X = NMe2 (2), Cl (3), and (OBu)-Bu-t (4)], were found to exhibit comparable activities to that of 1. Compound 1 reacts with Me2NHBH3 to give (N3N)Zr-(NMe2BH3) (5), which was structurally characterized and features an eta(2) B-H sigma-bond amido borane ligand. Because 5 is unstable with respect to borane loss to form 2, rather than beta-hydrogen elimination, and 2-4 do not exhibit X ligand loss during catalysis, dehydrogenation is hypothesized to proceed via an outer-sphere-type mechanism. This proposal is supported by the catalytic hydrogenation of alkenes by 2 using amine boranes as the sacrificial source of hydrogen.