Highly Active Iridium(III)-NHC System for the Catalytic B-N Bond Activation and Subsequent Solvolysis of Ammonia-Borane

A series of coordinatively unsaturated NHC-Ir(III) complexes are examined in terms of their catalytic activity toward the solvolysis of ammonia-borane (AB). Utilization of [Ir((IBu)-Bu-t')(2)Cl] (2) and [Ir((IBu)-Bu-t')(2)][PF6] (3) at 50 ppm catalyst loading led to reactions evolving approximately 3 equiv of H-2 within 10 min at 40 degrees C. The activation parameters of the reaction were measured to be Delta H double dagger= 11.1 kcal/mol and Delta H double dagger = 30.9 eu. Mechanistic investigations revealed an equilibrium between [Ir(H)(NH3)((IBu)-Bu-t)((IBu)-Bu-t)(2)][PF6] (4) + H-2 and [Ir(H)(2)((IBu)-Bu-t)(2)][PF6] (6) + NH3. Spectroscopic data suggest that complexes 4 and 6 are the resting state of the active catalyst. X-ray structures of complex 4 and [Ir(NH3)((IBu)-Bu-t')(2)][PF6] (5) are given.