Dehydropolymerization of H₃B·NMeH₂ Mediated by Cationic Iridium(III) Precatalysts Bearing κ³- ⁱ Pr-PN ^R P Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

The dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( ⁱ Pr-PN^HP)} fragment [ ⁱ Pr-PN^HP = κ³-(CH₂CH₂P ⁱ Pr₂)₂NH] is reported. Neutral Ir( ⁱ Pr-PN^HP)H₃ or Ir( ⁱ Pr-PN^HP)H₂Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F₂C₆H₄ solution. In contrast, addition of [NMeH₃][BAr^F ₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) to Ir( ⁱ Pr-PN^HP)H₃ 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(N-methylaminoborane) with M _n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir( ⁱ Pr-PN^HP)(H)₂(H₂)][BAr^F ₄]; σ-amine-borane [Ir( ⁱ Pr-PN^HP)(H)₂(H₃B·NMe₃)][BAr^F ₄]; and [Ir( ⁱ Pr-PN^HP)(H)₂(NMeH₂)][BAr^F ₄]. 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( ⁱ Pr-PN^MeP)(H)₂(H₃B·NMe₃)][BAr^F ₄] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H₃B·NMeH₂ 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( ⁱ Pr-N^HP)(H)₂(H₂)][BAr^F ₄], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH₂ (via B-N bond cleavage), results in the formation of essentially inactive Ir( ⁱ Pr-PN^HP)H₃, with a coproduct of [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺. While reprotonation of Ir( ⁱ Pr-PN^HP)H₃ results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺/catalyst in insoluble polyaminoborane. The role of [NMeH₃]⁺/[H₂B(NMeH₂)]⁺ as chain control agents is also noted.