Pyrazole-based PNN(H) Rh^I complexes are active catalysts for the dehydrocoupling of amine boranes to produce cyclic oligomers. Efficient dehydrohalogenation of the Rh chloride precursor complex and ligand deprotonation are essential for the realization of high catalytic activity.

Abstract

Coordination of a pyridine-pyrazole-based PNN(H) ligand to Rh^I produces a family of neutral (1) and cationic (2Cl) Rh^I complexes. Deprotonation of the parent Rh chloride complex with LiNiPr₂ results in formation of a dinuclear LiCl bridged species 3 bearing a pyrazolate fragment. Complexes 1, 2Cl and 3 were tested as precatalyst for the dehydrocoupling of amine boranes. All complexes studied show activity for the formation of cyclic oligomers with N-methylcyclotriborazane as the main product. Base activation of the neutral Rh chloride complex 1 produces catalyst systems that are significantly more active than the parent system, suggesting that dehydrohalogenation of the Rh chloride precatalyst 1 is one of the key steps for catalyst formation.