Phase-pure crystalline thin films of a mixed-valence Ru(2)(II,III) metal-organic framework with 1,3,5-benzenetricarboxylate (btc) as a linker were solvothermally grown on amorphous alumina and silica surfaces. Based on the Rietveld refinement, the structure of Ru-MOF was assigned to be analogous to [Cu(3)(btc)(2)] (HKUST-1).

Based on a rational ligand design for stabilizing high-valent {Fe(μ-O)2Fe} cores, a new family of dinucleating bis(tetradentate) ligands with varying terminal donor functions has been developed: redox-inert biomimetic carboxylates in H4julia, pyridines in susan, and phenolates in H4hilde(Me2). Based on a retrosynthetic analysis, the ligands were synthesized and used for the preparation of their diferric complexes [(julia){Fe(OH2)(μ-O)Fe(OH2)}]·6H2O, [(julia){Fe(OH2)(μ-O)Fe(OH2)}]·7H2O, [(julia){Fe(DMSO)(μ-O)Fe(DMSO)}]·3DMSO, [(hilde(Me2)){Fe(μ-O)Fe}]·CH2Cl2, [(hilde(Me2)){FeCl}2]·2CH2Cl2, [(susan){FeCl(μ-O)FeCl}]Cl2·2H2O, [(susan){FeCl(μ-O)FeCl0.75(OCH3)0.25}](ClO4)2·0.5MeOH, and [(susan){FeCl(μ-O)FeCl}](ClO4)2·0.5EtOH, which were characterized by single-crystal X-ray diffraction, FTIR, UV-Vis-NIR, Mössbauer, magnetic, and electrochemical measurements. The strongly electron-donating phenolates afford five-coordination, while the carboxylates and pyridines lead to six-coordination. The analysis of the ligand conformations demonstrates a strong flexibility of the ligand backbone in the complexes. The different hydrogen-bonding in the secondary coordination sphere of [(julia){Fe(OH2)(μ-O)Fe(OH2)}] influences the C-O, C[double bond, length as m-dash]O, and Fe-O bond lengths and is reflected in the FTIR spectra. The physical properties of the central {Fe(μ-O)Fe} core (d-d, μ-oxo → Fe(III) CT, νas(Fe-O-Fe), J) are governed by the differences in terminal ligands - Fe(III) bonds: strongly covalent π-donation with phenolates, less covalent π-donation with carboxylates, and π-acceptation with pyridines. Thus, [(susan){FeCl(μ-O)FeCl}](2+) is oxidized at 1.48 V vs. Fc(+)/Fc, which is shifted to 1.14 V vs. Fc(+)/Fc by methanolate substitution, while [(julia){Fe(OH2)(μ-O)Fe(OH2)}] is oxidized ≤1 V vs. Fc(+)/Fc. [(hilde(Me2)){Fe(μ-O)Fe}] is oxidized at 0.36 V vs. Fc(+)/Fc to a phenoxyl radical. The catalytic oxidation of cyclohexane with TONs up to 39.5 and 27.0 for [(susan){FeCl(μ-O)FeCl}](2+) and [(hilde(Me2)){Fe(μ-O)Fe}], respectively, indicates the potential to form oxidizing intermediates.

The design and first application of a new dinucleating ligand system to mimic high-valent oxidation states of O(2)-dependent diiron enzymes is presented.

A reversible carboxylate shift has been observed in a μ-oxo diferric complex in solution by UV-vis-NIR and FTIR spectroscopy triggered by the addition of a base or an acid. A terminal acetate decoordinates upon the addition of a proton, resulting in a shift of the remaining terminal acetato to a μ-η:η bridge. The addition of a base restores the original structure containing only terminal acetates. The implications for metalloenzymes with carboxylate-bridged nonheme diiron active sites are discussed.

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.

The reaction of the new dinucleating ligand susan with Fe(BF)·6HO results in formation of the homovalent FeFe complex [(susan){Fe(μ-F)Fe}] and the mixed-valence FeFe complex [(susan){FeF(μ-F)FeF}] depending on the absence or presence of dioxygen, respectively. Complex [(susan){FeF(μ-F)FeF}] is the first molecular mixed-valence complex with a fluorido bridge. The short Fe-μ-F bond of 1.87 Å causes a large reorganization energy, resulting in a localized class II system with an intervalence charge-transfer band of high energy at 10000 cm.

No abstract

The complexes [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}](ClO4)2, [(susan){FeIII(μ‐O)(μ‐OAc)FeIII}](ClO4)3, and [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 (susan = 4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) were synthesized and characterized. Prominent IR vibrations do not shift from the solid state to CH3CN solutions demonstrating dissolution without structural rearrangements or substitutions. The acetates in [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}]2+ are in a trans conformation resulting in a C2 symmetry of the central core and the overall complex. On the other hand, a second bridging ligand enforces a CS symmetry of the central core, which is incommensurable with the wrapping of the ligand around the central core resulting in only overall C1 symmetry. The Fe–OAc bonds are shorter for the terminal acetates (1.93 Å) than for the bridging acetate (1.97 and 2.07 Å). The bridging dianionic carbonate also results in shorter Fe–Ocarb bonds (1.91 and 1.97 Å). Mössbauer spectroscopy shows a lower quadrupole splitting for [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 in line with the shorter and thus more covalent Fe–Ocarb bonds. UV/Vis/NIR spectra differ in the d–d and in the LMCT regions for the mono‐ and doubly‐bridged complexes. The electrochemical characterization shows variations for the potentials for oxidations and reductions. This variation is discussed in light of the difference in π‐donor interactions and the overall charge of the complexes.