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

Abstract

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(PⁱPr₃)₂}⁺, {Rh(PⁱBu₃)₂}⁺, {Rh(ⁱPr₂P(CH₂)₃PⁱPr₂)}⁺, {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 η²-coordination through a sigma-interaction with BH₂, whereas fragments with small bite angles favour η⁶-coordination through the aryl group of the amine–borane. The catalytic dehydrocoupling of H₂PhB·NMe₂H is also explored, with the aminoborane HPhBNMe₂ found to be the sole dehydrogenation product. Stoichiometric reactivity with H₂PhB·NMe₂H again showed small bite angle fragments to prefer η⁶-aryl coordination, while the larger bite angle {Rh(PⁱPr₃)₂}⁺ gave rapid dehydrogenation to form a mixture of the Rh(III) dihydride [Rh(PⁱPr₃)₂(H)₂(η²-H₂PhB·NMe₂H)][BAr^F₄] and the low coordinate aminoboryl complex [Rh(PⁱPr₃)₂(H)(BPhNMe₂)][BAr^F₄]. These results suggest that precatalysts which η⁶-bind arenes strongly should be avoided for the dehydrocoupling of amine–boranes bearing aryl substituents.