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

Abstract

The dinuclear complex [(susan){Fe(OH)(μ-O)Fe(OH)}](ClO) (Fe(OH)(ClO); 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){FeF(μ-O)FeF}](ClO) (FeF(ClO)) have been synthesized and characterized in the solid state as well in acetonitrile (CHCN) and water (HO) 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 FeF and Fe(OH) retain their structural integrity in a CHCN 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 FeF to 0.79 V versus Fc/Fc in Fe(OH). Protonation/deprotonation studies in CHCN and aqueous solutions of Fe(OH) 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 HO. Titration experiments in aqueous solution at room temperature provide the p K values as p K = 4.9 and p K = 6.8. Kinetic studies by temperature- and pressure-dependent O NMR spectrometry revealed for the first time the water-exchange parameters [ k = (3.9 ± 0.2) × 10 s, Δ H = 39.6 ± 0.2 kJ mol, Δ S = -5.1 ± 1 J mol K, and Δ V = +3.0 ± 0.2 cm mol] and the underlying I mechanism for a {Fe(OH)(μ-O)Fe(OH)} core. The same studies suggest that in solution the monoprotonated {Fe(OH)(μ-O)Fe(OH)} complex has μ-O and μ-OH bridges between the two Fe centers.

Reactivity Differences for the Oxidation of FeIIFeII to FeIII(µ‐O)FeIII Complexes Caused by Pyridyl versus 6‐Methyl‐Pyridyl Ligands

“…The same reaction has been observed using the ligand tpa resulting in [(tpa)Fe III F(µ‐O)Fe III F(tpa)](BF 4 ) 2 , . The complex [(susan){Fe III F(µ‐O)Fe III F}] 2+ can also be synthesized under a controlled protocol by reaction of [Fe(OH 2 ) 6 ](ClO 4 ) 2 and (Bu 4 N)F · 3H 2 O as a source of fluoride affording [(susan){Fe III F(µ‐O)Fe III F}](ClO 4 ) 2 …”

“…The molecular structure of [(susan){Fe III F(µ‐O)Fe III F}] 2+ possesses crystallographically imposed C 2 symmetry in single‐crystals of [(susan){Fe III F(µ‐O)Fe III F}](ClO 4 ) 2 · H 2 O but is only of approximate C 2 symmetry in single‐crystals of [(susan){Fe III F(µ‐O)Fe III F}](BF 4 ) 2 · MeOH. However, there are no significant structural differences between these two molecular structures.…”

“…Solvents were used as received without further purification. The ligands susan and susan 6‐Me [13] and the complex [(susan){Fe III F(µ‐O)Fe III F}](ClO 4 ) 2 were synthesized according to the published procedures.…”

“…However, although these complexes exhibit low redox potentials for oxidation, the oxidations are ligand‐centered leading to coordinated phenoxyl radicals and the oxidized dinuclear complexes decay into mononuclear fragments , . Thus, we have developed a dinucleating bis(tetradentate) ligand system to coordinate a {Fe(µ‐O) 2 Fe} core (Schemeà) employing different terminal donors (carboxylates, phenolates, and pyridines) , , . The strong electron‐donating character of the bis‐µ‐oxo bridge should enable the oxidation to high‐valent species.…”

“…However, due to the limiting solubility in water, this high‐valent species cannot be stabilized by low temperatures. With the ligand susan, the thermodynamic sink appears to be the formation of the mono‐µ‐oxo‐bridged complexes [(susan){Fe III X(µ‐O)Fe III X}] 2+ (X = Cl – , F – , OAc – , or OH‐ depending on the composition of the reaction solution), which cannot be avoided in the reaction of the ligand susan with Fe II salts in the presence of a base , , …”