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

J. Cross, C. N. Brodie, D. G. Crivoi, J. C. Goodall, D. E. Ryan, A. J. Martínez-Martínez, A. Johnson, A. S. Weller, Chem. Eur. J. 2023

Thesigma    amine-borane    complexes[Rh(L1)(h2:h2-H3B·NRH2)][OTf]  (L1=  2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]-pyridine, R = Me, Et, nPr) are described, alongside [Rh(L1)(NMeH2)]-[OTf]. UsingR   =   Meas apre-catalyst(1   mol%)the dehydro-polymerization  of  H3B·NMeH2gives[H2BNMeH]nselectively.  AddedNMeH2,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  (1-5nm) nanoparticles is observed at the end ofcatalysis, but studies are ambiguous as to whether the catalysis is solelynanoparticle promoted or if there is a molecular homogeneous component. [Rh(L1)(NMeH2)]-[OTf] is  shown  to operate at  0.025mol% loadings on  a  2  g  scale of H3B·NMeH2to give polyaminoborane [H2BNMeH]n[Mn= 30,900 g/mol, Ð = 1.8]that can be purified to alow residual [Rh] (6 μg/g). Addition of Na[N(SiMe3)2] to [H2BNMeH]nresults in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane [H2BNMeH]3: the chemicalrepurposing of a main-group polymer.