Brodie, Claire N. orcid.org/0000-0002-8896-0270, Sotorrios, Lia, Boyd, Timothy M. et al. (2 more authors) (2022) Dehydropolymerization of H3B·NMeH2Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me):An Unexpected Inner-Sphere Mechanism. ACS Catalysis. 13050–13064. ISSN 2155-5435
Abstract
The dehydropolymerization of H3B·NMeH2to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir(iPr-PNHP)} fragment [iPr-PNHP = κ3-(CH2CH2PiPr2)2NH] is reported. Neutral Ir(iPr-PNHP)H3or Ir(iPr-PNHP)H2Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F2C6H4solution. In contrast, addition of [NMeH3][BArF4] (ArF= 3,5-(CF3)2C6H3) to Ir(iPr-PNHP)H3immediately 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 Mn∼40,000 g/mol, D ∼1.5, i.e., dihydrogen/dihydride, [Ir(iPr-PNHP)(H)2(H2)][BArF4]; σ-amine-borane [Ir(iPr-PNHP)(H)2(H3B·NMe3)][BArF4]; and [Ir(iPr-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(iPr-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·NMeH2with 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(iPr-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(iPr-PNHP)H3, with a coproduct of [NMeH3]+/[H2B(NMeH2)2]+. While reprotonation of Ir(iPr-PNHP)H3results 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.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2022 The Authors. |
Keywords: | amine-borane,catalyst,dehydropolymerization,iridium,mechanism,metal-ligand cooperativity, polymer, kinetics |
Dates: |
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Institution: | The University of York |
Academic Units: | The University of York > Faculty of Sciences (York) > Chemistry (York) |
Funding Information: | Funder Grant number EPSRC EP/R51181X/1 EPSRC EP/V520561/1 EPSRC EP/M024210/2 |
Depositing User: | Pure (York) |
Date Deposited: | 29 Sep 2022 14:40 |
Last Modified: | 19 Dec 2024 00:15 |
Published Version: | https://doi.org/10.1021/acscatal.2c03778 |
Status: | Published |
Refereed: | Yes |
Identification Number: | 10.1021/acscatal.2c03778 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:191489 |
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Filename: acscatal.2c03778.pdf
Description: Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3 ‑ i Pr-PNR P Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism
Licence: CC-BY 2.5