Variable coordination modes and catalytic dehydrogenation of B-phenyl amine-boranes.

The chemistry of N-substituted amine-boranes and their reactivity towards transition metal centres is well established but the chemistry of B-substituted amine-boranes is not. Here we present the coordination chemistry of H2PhB·NMe3 towards a range of Rh(i) fragments with different P-Rh-P ligand bite angles, {Rh(P(i)Pr3)2}(+), {Rh(P(i)Bu3)2}(+), {Rh((i)Pr2P(CH2)3P(i)Pr2)}(+), {Rh(Ph2P(CH2)nPPh2)}(+) (n = 3, 5), as characterised by NMR spectroscopy and single-crystal X-ray diffraction. This reveals a difference in the coordination mode of the amine-borane, with large bite angle fragments favouring η(2)-coordination through a sigma-interaction with BH2, whereas fragments with small bite angles favour η(6)-coordination through the aryl group of the amine-borane. The catalytic dehydrocoupling of H2PhB·NMe2H is also explored, with the aminoborane HPhB[double bond, length as m-dash]NMe2 found to be the sole dehydrogenation product. Stoichiometric reactivity with H2PhB·NMe2H again showed small bite angle fragments to prefer η(6)-aryl coordination, while the larger bite angle {Rh(P(i)Pr3)2}(+) gave rapid dehydrogenation to form a mixture of the Rh(iii) dihydride [Rh(P(i)Pr3)2(H)2(η(2)-H2PhB·NMe2H)][BAr(F)4] and the low coordinate aminoboryl complex [Rh(P(i)Pr3)2(H)(BPhNMe2)][BAr(F)4]. These results suggest that precatalysts which η(6)-bind arenes strongly should be avoided for the dehydrocoupling of amine-boranes bearing aryl substituents.