Synthesis, characterisation and electrochemical reductions of oxo-centred, carboxylate-bridged triiron complexes, [Fe3(µ3-O)(µ-O2CR)6L3]X (R = Me, But, Ph, CH2Cl, CCl3, CH2CN or 4-NO2C6H4; L = py, 3-H2Npy, 4-H2Npy, 3-NCpy, 4-NCpy or 4-CH2CHpy; X = ClO4– or NO3–)

Abstract

A range of oxo-centred, carboxylate-bridged triiron complexes of general formula [Fe₃O(O₂CMe)₆L₃]X (L = py, 3-H₂Npy, 4-H₂Npy, 3-NCpy, 4-NCpy or 4-CH₂CHpy, X = ClO₄^–) and [Fe₃O(O₂CR)₆(py)₃]X (R = Bu^t, Ph, CH₂Cl, CCl₃, CH₂CN or 4-NO₂C₆H₄, X = ClO₄^– or NO₃^–) has been synthesized, some for the first time. The [Fe₃O(O₂CPh)₆(py)₃]NO₃ complex (as its dichloromethane solvate) has been the subject of a room-temperature single-crystal X-ray study. The redox behaviour of these complexes has been investigated by cyclic voltammetry in 0.2 mol dm^–3 [NBu₄][PF₆]–dichloromethane, in the presence and absence of free L (where L is pyridine or substituted pyridine). These complexes in general display a chemically reversible one-electron reduction to the neutral mixed-valence species at +0.5 to –0.2 V, vs. Ag–AgCl, provided an excess of free L is present. The reversible potential of the [Fe₃O(O₂CMe)₆L₃]^+/0 and [Fe₃O(O₂CR)₆(py)₃]^+/0 reductions varies linearly with the pK_a of L and ^–O₂CR. A second reduction was also detected at more negative potentials. Only for R = CH₂CN the second reduction becomes chemically reversible in the presence of free pyridine, such that the monoanion, [Fe₃O(O₂CR)₆(py)₃]^–, is stable on the timescale of the voltammetric experiment. Where solubility and stability permitted, the [Fe₃O(O₂CMe)₆L₃] and [Fe₃O(O₂CR)₆(py)₃] complexes have been electrogenerated in situ. The mixed-valence species display an extremely broad, low intensity (ε ≈ 60–100 dm³ mol^–1 cm^–1) band in the 7000–8000 cm^–1 region of the electronic spectrum.