The bis(thiophosphinite) pincer complexes [(RPSCSPR)Rh(py)(H)(Cl)] (RPSCSPR= C6H4–2,6-(SPR2)2with R =iPr,2aand R = Ph,2b) are highly active precatalysts for the dehydropolymerisation of methylamine borane.

Dehydropolymerisation of methylamine borane (H3 B⋅NMeH2 ) using the well-known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH2 CH2 PiPr2 )2 ) (1) gives poly(aminoborane)s by a chain-growth mechanism. In toluene, rapid dehydrogenation of H3 B⋅NMeH2 following first-order behaviour as a limiting case of a more general underlying Michaelis-Menten kinetics is observed, forming aminoborane H2 B=NMeH, which selectively couples to give high-molecular-weight poly(aminoborane)s (H2 BNMeH)n and only traces of borazine (HBNMe)3 by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H3 B⋅NMe2 H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H3 B⋅N(CH2 SiMe3 )H2 was studied in homo- and co-dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

The field of amine borane chemistry has seen significant advances and fascinating developments in the last decade, mostly driven by the potential use of hydrogen-rich amine boranes as hydrogen storage media. Along with this, organometallic chemists have more and more looked into the mechanistic aspects of N-H and B-H bond activation to improve catalytic systems that are now known for most of the transition metals and several main group metals. Dehydrogenation of amine boranes and related phosphine boranes that looks more into the B-N and B-P products formed rather than the amount of hydrogen released is an interesting reaction that gives rise to aminoboranes and phosphinoboranes, formally isoelectronic main group analogues of olefins. Coupling of these allows for the production of "inorganic polyolefins", a class of compounds that has great potential for applications as high-performance polymeric materials, pre-ceramics or as precursors to functional B-N/P materials. In this review, a survey of catalysts for dehydropolymerisation of primary amine boranes and phosphine boranes is presented along with selected systems that are active for dehydrocoupling of secondary amine boranes and phosphine boranes to highlight mechanistic aspects of these conversions. (C) 2018 Elsevier B.V. All rights reserved.

The catalytic dehydrocoupling of hydrazine borane (N2H4 center dot BH3, HB) using two molecularly defined PNP Fe(II) hydride complexes [(PNHP)Fe(HBH3)(H)(CO)] (PNHP = HN[CH2CH2P(i-Pr)(2)], 1-BH3) and [(PNP)Fe-(H)(CO)] (PNP = N[CH2CH2P(i-Pr)(2)], 2) is reported. Both catalysts are highly active and recyclable, and they can potentially release up to 2.56 equiv of hydrogen from HB, giving a thermally unstable solid dehydrocoupling product. For the stable dehydrogenation of HB of up to 1.0 equiv of hydrogen, we have analyzed the resulting insoluble BxNyHz polymeric residue using B-11 solid-state NMR spectroscopy techniques combined with density functional theory (DFT) calculations of B-11 NMR parameters for a comprehensive library of molecular BxNyHz fragments compatible with the polymeric BxNyHz structure. These studies were further supported by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. On this basis, we propose a detailed mechanism for the stable dehydrogenation of HB of up to 1.0 equiv of hydrogen and the subsequent polymerization reaction, leading to the insoluble polymeric residue. This residue with the general chemical formula of BNH3 is shown to be consistent with a polymer repeat unit of five- or six-ring amino-borane structures, supported by the fact that the B-11 magic-angle-spinning (MAS) NMR experiments only reveal boron in 4-fold coordination.