New intermediate-spin chloroiron(III) complex with a mixed nitrogen– sulfur co-ordination sphere‡

Abstract

A new tetradentate proligand, 6,6′-bis(2,2-diphenyl-2-sulfanylethyl)-2,2′-bipyridine (H₂L), involving two nitrogen bases and two thiols, has been synthesized. The crystal structure of its chloroiron(III) complex [FeL(Cl)] exhibits a five-co-ordinate iron in a distorted square-pyramidal structure including a long Fe–Cl axial bond. Its effective magnetic moment (µ_eff = 3.8 µ_B from 10 to 300 K) and its Mössbauer parameters [δ = 0.328(5) mm s^–1 and ΔE_Q = 2.543(9) mm s^–1 at 293 K] are consistent with a pure intermediate S =  spin state for the iron(III). This complex exhibits a sulfur-to-iron charge transfer absorption at 490 nm and a one-electron reduction wave at –415 mV (vs. SSCE). Upon reaction with tetrabutylammonium hydroxide it is converted into the µ-oxo complex [Fe₂L₂O], which has been characterized by mass spectral and elemental analyses. Its formation is reversible in dmf upon addition of trifluoroacetic acid in the presence of chloride anion, as shown by ¹H NMR and UV/VIS spectroscopy and electrochemistry. The [FeL(Cl)] complex appears to be the first example of an iron(III) complex with a mixed nitrogen–sulfur co-ordination sphere in a pure S =  ground spin state.