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.

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.