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

[Rh(κ 2 -PP-DPEphos){η 2 η 2 -H 2 B(NMe 3 )(CH 2 ) 2 t Bu}][BAr F 4 ] acts as an effective precatalyst for the dehydropolymerization of H 3 B·NMeH 2 to form N -methylpolyaminoborane (H 2 BNMeH) 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 2 C 6 H 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 2 BN=HMe)][BAr F 4 ], followed by the crystallographically characterized amidodiboryl complex [Rh 2 ( cis -κ 2 -PP-DPEphos) 2 (σ,μ-(H 2 B) 2 NHMe)][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 2 B(NMe 3 )(CH 2 ) 2 t Bu}][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.