Dehydropolymerization of H ₃B·NMeH ₂ Using a [Rh(DPEphos)] ⁺ Catalyst: The Promoting Effect of NMeH ₂

Abstract

[Rh(κ ²-PP-DPEphos){η ²η ²-H ₂B(NMe ₃)(CH ₂) ₂ ^tBu}][BAr ^F ₄] acts as an effective precatalyst for the dehydropolymerization of H ₃B·NMeH ₂ to form N-methylpolyaminoborane (H ₂BNMeH) _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 H ₂: with M _n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H ₂ evolution studies (1,2-F ₂C ₆H ₄ 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, [Rh ₂(κ ²-PP-DPEphos) ₂(μ-H)(μ-H ₂BN=HMe)][BAr ^F ₄], followed by the crystallographically characterized amidodiboryl complex [Rh ₂( cis-κ ²-PP-DPEphos) ₂(σ,μ-(H ₂B) ₂NHMe)][BAr ^F ₄]. Adding ∼2 equiv of NMeH ₂ 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 NMeH ₂ acts to form the  precatalysts [Rh(κ ²-DPEphos)(NMeH ₂) ₂][BAr ^F ₄] and [Rh(κ ²-DPEphos)(H) ₂(NMeH ₂) ₂][BAr ^F ₄], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ ²-PP-DPEphos){η ²-H ₂B(NMe ₃)(CH ₂) ₂ ^tBu}][BAr ^F 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.