Homoleptic mono-, di-, and tetra-iron complexes featuring phosphido ligands: a synthetic, structural, and spectroscopic study

Abstract

We report the first series of homoleptic phosphido iron complexes synthesized by treating either the β-diketiminato complex [(Dippnacnac)FeCl₂Li(dme)₂] (Dippnacnac = HC[(CMe)N(C₆H₃-2,6-iPr₂)]₂) or [FeBr₂(thf)₂] with an excess of phosphides R₂PLi (R = tBu, tBuPh, Cy, iPr). Reaction outcomes depend strongly on the bulkiness of the phosphido ligands. The use of tBu₂PLi precursor led to an anionic diiron complex 1 encompassing a planar Fe₂P₂ core with two bridging and two terminal phosphido ligands. An analogous reaction employing less sterically demanding phosphides, tBuPhPLi and Cy₂PLi yielded diiron anionic complexes 2 and 3, respectively, featuring a short Fe–Fe interaction supported by three bridging phosphido groups and one additional terminal R₂P⁻ ligand at each iron center. Further tuning of the P-substrates bulkiness gave a neutral phosphido complex 4 possessing a tetrahedral Fe₄ cluster core held together by six bridging iPr₂P moieties. Moreover, we also describe the first homoleptic phosphanylphosphido iron complex 5, which features an iron center with low coordination provided by three tBu₂P-P(SiMe₃)⁻ ligands. The structures of compounds 1–5 were determined by single-crystal X-ray diffraction and 1–3 by ¹H NMR spectroscopy. Moreover, the electronic structures of 1–3 were interrogated using zero-field Mössbauer spectroscopy and DFT methods.