Dehydropolymerization of H(3)B·NMeH(2) Mediated by Cationic Iridium(III) Precatalysts Bearing κ(3)-(i)Pr-PN(R)P Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

[Image: see text] The dehydropolymerization of H(3)B·NMeH(2) to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir((i)Pr-PN(H)P)} fragment [(i)Pr-PN(H)P = κ(3)-(CH(2)CH(2)P(i)Pr(2))(2)NH] is reported. Neutral Ir((i)Pr-PN(H)P)H(3) or Ir((i)Pr-PN(H)P)H(2)Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F(2)C(6)H(4) solution. In contrast, addition of [NMeH(3)][BAr(F)(4)] (Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) to Ir((i)Pr-PN(H)P)H(3) 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(H)P)(H)(2)(H(2))][BAr(F)(4)]; σ-amine-borane [Ir((i)Pr-PN(H)P)(H)(2)(H(3)B·NMe(3))][BAr(F)(4)]; and [Ir((i)Pr-PN(H)P)(H)(2)(NMeH(2))][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(Me)P)(H)(2)(H(3)B·NMe(3))][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 H(3)B·NMeH(2) 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(H)P)(H)(2)(H(2))][BAr(F)(4)], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH(2) (via B–N bond cleavage), results in the formation of essentially inactive Ir((i)Pr-PN(H)P)H(3), with a coproduct of [NMeH(3)](+)/[H(2)B(NMeH(2))(2)](+). While reprotonation of Ir((i)Pr-PN(H)P)H(3) results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH(3)](+)/[H(2)B(NMeH(2))(2)](+)/catalyst in insoluble polyaminoborane. The role of [NMeH(3)](+)/[H(2)B(NMeH(2))](+) as chain control agents is also noted.