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

The dinuclear complex [(susan){Fe^III(OH)(μ-O)Fe^III(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){Fe^IIIF(μ-O)Fe^IIIF}](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 Å). UV-vis-near-IR (NIR) and Mössbauer 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 reversible acid-base equilibria. UV-vis-NIR, Mössbauer, and cryo electrospray ionization mass spectrometry experiments show conservation of the mono(μ-oxo) bridging motif, while the terminal OH^- ligands are protonated to H₂O. Titration experiments in aqueous solution at room temperature provide the p K_a values as p K₁ = 4.9 and p K₂ = 6.8. Kinetic studies by temperature- and pressure-dependent ¹⁷O NMR spectrometry revealed for the first time the water-exchange parameters [ k_ex²⁹⁸ = (3.9 ± 0.2) × 10⁵ s^-1, Δ H^⧧ = 39.6 ± 0.2 kJ mol^-1, Δ S^⧧ = -5.1 ± 1 J mol^-1 K^-1, and Δ V^⧧ = +3.0 ± 0.2 cm³ mol^-1] and the underlying I_d mechanism for a {Fe^III(OH₂)(μ-O)Fe^III(OH₂)} core. The same studies suggest that in solution the monoprotonated {Fe^III(OH)(μ-O)Fe^III(OH₂)} complex has μ-O and μ-O₂H₃ bridges between the two Fe centers.