Variation of the Molecular and Electronic Structures of μ‐Oxo Diferric Complexes with the Bridging Motive

The complexes [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}](ClO4)2, [(susan){FeIII(μ‐O)(μ‐OAc)FeIII}](ClO4)3, and [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 (susan = 4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) were synthesized and characterized. Prominent IR vibrations do not shift from the solid state to CH3CN solutions demonstrating dissolution without structural rearrangements or substitutions. The acetates in [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}]2+ are in a trans conformation resulting in a C2 symmetry of the central core and the overall complex. On the other hand, a second bridging ligand enforces a CS symmetry of the central core, which is incommensurable with the wrapping of the ligand around the central core resulting in only overall C1 symmetry. The Fe–OAc bonds are shorter for the terminal acetates (1.93 Å) than for the bridging acetate (1.97 and 2.07 Å). The bridging dianionic carbonate also results in shorter Fe–Ocarb bonds (1.91 and 1.97 Å). Mössbauer spectroscopy shows a lower quadrupole splitting for [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 in line with the shorter and thus more covalent Fe–Ocarb bonds. UV/Vis/NIR spectra differ in the d–d and in the LMCT regions for the mono‐ and doubly‐bridged complexes. The electrochemical characterization shows variations for the potentials for oxidations and reductions. This variation is discussed in light of the difference in π‐donor interactions and the overall charge of the complexes.