Dehydropolymerization of H₃B·NMeH₂ Using a [Rh(DPEphos)]⁺ Catalyst: The Promoting Effect of NMeH₂

[Rh(κ²-PP-DPEphos){η²η²-H₂B(NMe₃)(CH₂)₂ ^tBu}][BAr^F ₄] acts as an effective precatalyst for the dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane (H₂BNMeH) . 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 Đ 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(κ²-PP-DPEphos)₂(μ-H)(μ-H₂BN=HMe)][BAr^F ₄], followed by the crystallographically characterized amidodiboryl complex [Rh₂(cis-κ²-PP-DPEphos)₂(σ,μ-(H₂B)₂NHMe)][BAr^F 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(κ²-DPEphos)(NMeH₂)₂][BAr^F ₄] and [Rh(κ²-DPEphos)(H)₂(NMeH₂)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(κ²-PP-DPEphos){η²-H₂B(NMe₃)(CH₂)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.