Dehydropolymerization of Amine-Boranes using Bis(imino)pyridine Rhodium Pre-Catalysis: σ-Amine-Borane Complexes, Nanoparticles, and Low Residual-Metal BN-Polymers that can be Chemically Repurposed.

The sigma amine-borane complexes [Rh(L1)(h2:h2H3B·NRH2)][OTf] (L1 = 2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine, R = Me, Et, nPr) are described, alongside [Rh(L1)(NMeH2)][OTf]. Using R = Me as the pre-catalyst (1 mol%) the dehydropolymerization of H3B·NMeH2 gives [H2BNMeH]n selectively. Added NMeH2, or the direct use of [Rh(L1)(NMeH2)][OTf], is required for initiation of catalysis, which is suggested to operate through formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed during catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. [Rh(L1)(NMeH2)][OTf] is shown to operate at 0.025 mol% loadings on a 2 g scale to give polyaminoborane [H2BNMeH]n [Mn = 30,900 g/mol, Ð = 1.8] that can be purified to a low residual [Rh] (6 µg/g). Addition of Na[N(SiMe3)2] to [H2BNMeH]n results in selective depolymerization to form the eee-isomer of N,N,N-trimethylborazane [H2BNMeH]3: the chemical repurposing of a main-group polymer.