Dehydropolymerization of H3B·NMeH2 Using a [Rh(DPEphos)] + Catalyst: The Promoting Effect of NMeH 2

[Rh(k2–PP–DPEphos){h2-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 catalyst loading (an inverse relationship) and use of the chain termination/transfer agent H2: with Mn ranging between 5,500 to 34,900 g/mol and Đ between 1.5-1.8. H2 evolution studies (1,2-F2C6H4 solvent) reveal an induction period that gets longer with higher catalyst loading, and complex kinetics with a non– integer order in [catalyst]. Speciation studies indicate the initial formation of the amino–borane bridged dimer, [Rh2(k2–PP–DPEphos)2(µ–H)(µ–H2BNHMe)][BArF4], followed by the crystallographically characterized amidodiboryl [Rh2(cis–k2–PP–DPEphos)2(µ-H)(s,µ (H2B)2NHMe)][BArF4]. Adding ~2 equivs NMeH2 in THF solution to the precatalyst removes this induction period, pseudo first order kinetics are observed, and a half order relationship to catalyst is revealed, with regard to dehydrogenation and polymer molecular weights are increased (e.g. Mn = 40,000 g/mol). Speciation studies suggest NMeH2 acts to form the active precatalysts [Rh(DPEphos)(NMeH2)2][BArF4] and [Rh(DPEphos)(H)2(NMeH2)2][BArF4], which were independently synthesized and shown to follow very similar dehydrogenation kinetics and produce polymer of molecular weight comparable with the amine–doped systems. 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.