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 B_xN_yH_z 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 B_xN_yH_z fragments compatible with the polymeric B_xN_yH_z 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.