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

ACS Catal. 2022 Oct 21;12(20):13050-13064. doi: 10.1021/acscatal.2c03778. Epub 2022 Oct 12.

ABSTRACT

The dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir( i Pr-PNHP)} fragment [ i Pr-PNHP = κ3-(CH2CH2P i Pr2)2NH] is reported. Neutral Ir( i Pr-PNHP)H3 or Ir( i Pr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4 solution. In contrast, addition of [NMeH3][BArF4] (ArF = 3,5-(CF3)2C6H3) to Ir( i Pr-PNHP)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-PNHP)(H)2(H2)][BArF4]; σ-amine-borane [Ir( i Pr-PNHP)(H)2(H3B·NMe3)][BArF4]; and [Ir( i Pr-PNHP)(H)2(NMeH2)][BArF4]. 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-PNMeP)(H)2(H3B·NMe3)][BArF4] 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-NHP)(H)2(H2)][BArF4], 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-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir( i Pr-PNHP)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.

PMID:36313521 | PMC:PMC9594342 | DOI:10.1021/acscatal.2c03778

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