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(ItBu′)2Cl] (2) and [Ir(ItBu′)2][PF6] (3) at 50 ppm catalyst loading led to reactions evolving approximately 3 equiv of H2 within 10 min at 40 °C. The activation parameters of the reaction were measured to be ΔH⧧ = 11.1 kcal/mol and ΔS⧧ = 30.9 eu. Mechanistic investigations revealed an equilibrium between [Ir(H)(NH3)(ItBu)(ItBu′)][PF6] (4) + H2 and [Ir(H)2(ItBu)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)(ItBu′)2][PF6] (5) are given.