Two Unsupported Terminal Hydroxido Ligands in a μ-Oxo-Bridged Ferric Dimer: Protonation and Kinetic Lability Studies

The dinuclear complex [(susan){FeIII(OH)(μ-O)FeIII(OH)}](ClO4)2 (Fe2(OH)2(ClO4)2; susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10-tetraazadecane) with two unsupported terminal hydroxido ligands and for comparison the fluorido-substituted complex [(susan){FeIIIF(μ-O)FeIIIF}](ClO4)2 (Fe2F2(ClO4)2) have been synthesized and characterized in the solid state as well in acetonitrile (CH3CN) and water (H2O) solutions. The Fe–OH bonds are strongly modulated by intermolecular hydrogen bonds (1.85 and 1.90 A). UV–vis–near-IR (NIR) and Mossbauer spectroscopies prove that Fe2F22+ and Fe2(OH)22+ retain their structural integrity in a CH3CN solution. The OH– ligand induces a weaker ligand field than the F– ligand because of stronger π donation. This increased electron donation shifts the potential for the irreversible oxidation by 610 mV cathodically from 1.40 V in Fe2F22+ to 0.79 V versus Fc+/Fc in Fe2(OH)22+. Protonation/deprotonation studies in CH3CN and aqueous solutions of Fe2(OH)22+ provide two rev...