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

Abstract

The sigma amine-borane complexes [Rh(L1)(η2:η2-H3B·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 a 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 the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1–5 nm) nanoparticles is observed at the end of 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 of H3B·NMeH2 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-trimethylcyclotriborazane [H2BNMeH]3: the chemical repurposing of a main-group polymer.