Metathesis of alkali-metal amidoborane and FeCl3 in THF

Abstract

Metathesis of LiNH₂BH₃ and FeCl₃ in THF solution was investigated in detail. Instead of formation of expected Fe amidoborane i.e., 3LiNH₂BH₃ + FeCl₃ → 3LiCl + Fe(NH₂BH₃)₃, 1.5 equiv. H₂/LiNH₂BH₃ together with LiCl and a black precipitate was produced as a result of salt metathesis and reduction of Fe³⁺ by BH₃. The hydrogen was desorbed in two steps involving a homogeneous interaction of the two starting chemicals to form [Fe(H₂NBH₂)₃] precipitate and subsequent solid-state dissociation of [Fe(H₂NBH₂)₃] to yield a polymeric product, [Fe(HNBH)₃]_n, respectively. FTIR evidenced the persistence of B–H and N–H stretches in the above two solid products and following the dissociation of [Fe(H₂NBH₂)₃] to release 1 equiv. H₂/LiNH₂BH₃ the B–N bond strengthened. Mössbauer and XAFS both indicated that Fe atoms in these solids are in very similar chemical environments, linking to the neighbouring N and B atoms and bearing slightly positive charge. Most likely, H₂NBH₂ in [Fe(H₂NBH₂)₃] binds to Fe as a π-bound ligand. The mechanism of salt metathesis and reduction of Fe³⁺ was confirmed based on simulation work on the homogeneous reaction process.