Dehydropolymerization of HB·NMeH Using a [Rh(DPEphos)] Catalyst: The Promoting Effect of NMeH.

[Rh(κ-PP-DPEphos){ηη-HB(NMe)(CH) Bu}][BAr ] acts as an effective precatalyst for the dehydropolymerization of HB·NMeH to form -methylpolyaminoborane (HBNMeH) . 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: with ranging between 5 500 and 34 900 g/mol and between 1.5 and 1.8. H evolution studies (1,2-FCH solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]. Speciation studies at 10 mol % indicate the initial formation of the amino-borane bridged dimer, [Rh(κ-PP-DPEphos)(μ-H)(μ-HBN=HMe)][BAr ], followed by the crystallographically characterized amidodiboryl complex [Rh(-κ-PP-DPEphos)(σ,μ-(HB)NHMe)][BAr ]. Adding ∼2 equiv of NMeH in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh] is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., = 40 000 g/mol). Speciation studies suggest that NMeH acts to form the  precatalysts [Rh(κ-DPEphos)(NMeH)][BAr ] and [Rh(κ-DPEphos)(H)(NMeH)][BAr ], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ-PP-DPEphos){η-HB(NMe)(CH) Bu}][BAr ], 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.