Synthesis and characterisation of halide, separated ion pair, and hydride cyclopentadienyl iron bis(diphenylphosphino)ethane derivatives

Treatment of anhydrous FeX2 (X = Cl, Br, I) with one equivalent of bis(diphenylphosphino)ethane (dppe) in refluxing THF afforded analytically pure white (X = Cl), light green (X = Br), and yellow (X = I) [FeX2(dppe)](n) (X = Cl, I; Br, II; I, III). Complexes I-III are excellent synthons from which to prepare a range of cyclopentadienyl derivatives. Specifically, treatment of I-III with alkali metal salts of C5H5 (Cp, series 1), C5Me5 (Cp*, series 2), C5H4SiMe3 (Cp', series 3), C5H3(SiMe3)(2) (Cp '', series 4), and C5H3(Bu-t)(2) (Cp-tt, series 5) afforded [Fe(Cp-dagger)(Cl)(dppe)] 1Cl-5Cl, [Fe(Cp-dagger)(Br)(dppe)] 1Br-5Br, and [Fe(Cp-dagger)(I)(dppe)] 1I-5I (Cp-dagger = Cp, Cp*, Cp', Cp '', or Cptt). Dissolution of 1I-5I in acetonitrile, or treatment of 1Cl-5Cl with Me3SiI in acetonitrile (no halide exchange reactions were observed in other solvents) afforded the separated ion pair complexes [Fe(Cp-dagger)(NCMe)(dppe)][I] 1SIP-5SIP. Attempts to reduce 1Cl-5Cl, 1Br-5Br, and 1I-5I with a variety of reductants (Li-Cs, KC8, Na/Hg) were unsuccessful. Treatment of 1Cl-5Cl with LiAlH4 gave the hydride derivatives [Fe(Cp-dagger)(H)(dppe)] 1H-5H. This report provides a systematic account of reliable methods of preparing these complexes which may find utility in molecular wire and metal-metal bond chemistries. The complexes reported herein have been characterised by X-ray diffraction, NMR, IR, UV/Vis, and Mossbauer spectroscopies, cyclic voltammetry, density functional theory calculations, and elemental analyses, which have enabled us to elucidate the electronic structure of the complexes and probe the variation of iron redox properties as a function of varying the cyclopentadienyl or halide ligand.