Dehydropolymerization of H3B·NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2.
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ABSTRACT: [Rh(?2-PP-DPEphos){?2?2-H2B(NMe3)(CH2)2 tBu}][BArF 4] acts as an effective precatalyst for the dehydropolymerization of H3B·NMeH2 to form N-methylpolyaminoborane (H2BNMeH) n . Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1-1 mol %, an inverse relationship) and use of the chain-modifying agent H2: with M n ranging between 5?500 and 34?900 g/mol and ? between 1.5 and 1.8. H2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]TOTAL. Speciation studies at 10 mol % indicate the initial formation of the amino-borane bridged dimer, [Rh2(?2-PP-DPEphos)2(?-H)(?-H2BN=HMe)][BArF 4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-?2-PP-DPEphos)2(?,?-(H2B)2NHMe)][BArF 4]. Adding ?2 equiv of NMeH2 in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh]TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M n = 40?000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh(?2-DPEphos)(NMeH2)2][BArF 4] and [Rh(?2-DPEphos)(H)2(NMeH2)2][BArF 4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(?2-PP-DPEphos){?2-H2B(NMe3)(CH2)2 tBu}][BArF 4], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.
SUBMITTER: Adams GM
PROVIDER: S-EPMC6454579 | biostudies-literature | 2019 Apr
REPOSITORIES: biostudies-literature
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