Substitutional Lability of Diphosphine Ligands in Tetrahedral Iron(II) Chloro Complexes

A series of iron(II) dihalogenide complexes with two different bisphosphinoethane ligands is reported. In the case of 1,2-bis(diphenylphosphanyl)ethane (dppe), depending on the stoichiometry, the tetrahedral [(μ-dppe)FeCl2]n and octahedral trans-[(dppe)2FeCl2] complexes are formed. The polymeric complex [(μ-dppe)FeCl2]n, with iron in a tetrahedral environment, preferentially reacts with chelating amines to give the octahedral diphosphine complex, trans-[(dppe)2FeCl2], and different octahedral amine complexes. With the sterically more demanding 1,2-bis(diisopropylphosphanyl)ethane (dippe), the monomeric and tetrahedral halogen complexes [(dippe)FeX2] are exclusively obtained (X = Cl, Br). These complexes react with chelating amines in a similar manner, to give free ligand and the corresponding octahedral amine complex. The present results suggest that the diphosphines in the investigated iron(II) complexes are bound too weakly to form productive catalyst precursors.