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)(η² :η² -H₃ B⋅NRH₂ )][OTf] (L1=2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine, R=Me, Et, ⁿ Pr) are described, alongside [Rh(L1)(NMeH₂ )][OTf]. Using R=Me as a pre-catalyst (1 mol %) the dehydropolymerization of H₃ B ⋅ NMeH₂ gives [H₂ BNMeH]_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 H₃ B ⋅ NMeH₂ to give polyaminoborane [H₂ BNMeH]_n [M_n =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 [H₂ BNMeH]_n results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane [H₂ BNMeH]₃ : the chemical repurposing of a main-group polymer.