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

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journal contribution
posted on 31.07.2018, 16:39 by Thomas Philipp Zimmermann, Thomas Limpke, Nicole Orth, Alicja Franke, Anja Stammler, Hartmut Bögge, Stephan Walleck, Ivana Ivanovic-Burmazovic, Thorsten Glaser
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 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 pKa values as pK1 = 4.9 and pK2 = 6.8. Kinetic studies by temperature- and pressure-dependent 17O NMR spectrometry revealed for the first time the water-exchange parameters [kex298 = (3.9 ± 0.2) × 105 s–1, ΔH = 39.6 ± 0.2 kJ mol–1, ΔS = −5.1 ± 1 J mol–1 K–1, and ΔV = +3.0 ± 0.2 cm3 mol–1] and the underlying Id mechanism for a {FeIII(OH2)­(μ-O)­FeIII(OH2)} core. The same studies suggest that in solution the monoprotonated {FeIII(OH)­(μ-O)­FeIII(OH2)} complex has μ-O and μ-O2H3 bridges between the two Fe centers.