Fe(II) Hydride Complexes for the Homogeneous Dehydrocoupling of Hydrazine Borane: Catalytic Mechanism via DFT Calculations and Detailed Spectroscopic Characterization

The catalytic dehydrocoupling of hydrazine borane (N2H4·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 11B solid-state NMR spectroscopy techniques combined with density functional theory (DFT) calculations of 11B 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 11B magic-angle-spinning (MAS) NMR experiments only reveal boron in 4-fold coordination.