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

[Rh­(κ2-PP-DPEphos)­{η2η2-H2B­(NMe3)­(CH2)2tBu}]­[BArF4] 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 Mn 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)]­[BArF4], followed by the crystallographically characterized amidodiboryl complex [Rh2(cis-κ2-PP-DPEphos)2(σ,μ-(H2B)2NHMe)]­[BArF4]. 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., Mn = 40 000 g/mol). Speciation studies suggest that NMeH2 acts to form the precatalysts [Rh­(κ2-DPEphos)­(NMeH2)2]­[BArF4] and [Rh­(κ2-DPEphos)­(H)2(NMeH2)2]­[BArF4], 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)2tBu}]­[BArF4], 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.