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–)

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 = But, 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 pKa 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 (e ≈ 60–100 dm3 mol–1 cm–1) band in the 7000–8000 cm–1 region of the electronic spectrum.