Dehydropolymerization of HB·NMeH Mediated by Cationic Iridium(III) Precatalysts Bearing κ- Pr-PN P Pincer Ligands ( = H, Me): An Unexpected Inner-Sphere Mechanism.

The dehydropolymerization of HB·NMeH to form -methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( Pr-PNP)} fragment [ Pr-PNP = κ-(CHCHP Pr)NH] is reported. Neutral Ir( Pr-PNP)H or Ir( Pr-PNP)HCl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-FCH solution. In contrast, addition of [NMeH][BAr ] (Ar = 3,5-(CF)CH) to Ir( Pr-PNP)H immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(-methylaminoborane) with ∼ 40,000 g/mol, ∼1.5, i.e., dihydrogen/dihydride, [Ir( Pr-PNP)(H)(H)][BAr ]; σ-amine-borane [Ir( Pr-PNP)(H)(HB·NMe)][BAr ]; and [Ir( Pr-PNP)(H)(NMeH)][BAr ]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal-ligand cooperativity, and this is supported experimentally by N-Me substituted [Ir( Pr-PNP)(H)(HB·NMe)][BAr ] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional HB·NMeH with the N-H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir( Pr-NP)(H)(H)][BAr ], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH (via B-N bond cleavage), results in the formation of essentially inactive Ir( Pr-PNP)H, with a coproduct of [NMeH]/[HB(NMeH)]. While reprotonation of Ir( Pr-PNP)H results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH]/[HB(NMeH)]/catalyst in insoluble polyaminoborane. The role of [NMeH]/[HB(NMeH)] as chain control agents is also noted.