Ligand-Centered Oxidation in a Diiron s-Indacene Complex

The reaction of 1,3,5,7 tetra-tert-butyl-s-indacene (Ic‘) with [Fe₂(CO)₉] gives the bimetallic complex [{Fe(CO)₃}₂Ic‘], 1, where the two Fe(CO)₃ fragments are coordinated to the same face of the Ic‘ ligand. The cyclic voltammogram of 1 displays two quasi-reversible molecular oxidation waves and an irreversible molecular reduction wave. 1 can be chemically oxidized to its monocation with silver tetrafluoroborate. Comparison of the crystal structures of 1 and [{Fe(CO)₃}₂Ic‘]⁺[BF₄]^-·CH₂Cl₂, 2, shows little change in bond lengths on oxidation. Although, at first glance, 2 may appear to be an Fe⁰/Fe^I mixed-valence compound, Mössbauer and EPR spectroscopy suggest that that the ionized electron originates from a largely indacene-based orbital. The structural and spectroscopic features of 1 and 2 have been rationalized by density functional calculations and compared to those of related systems.