Increasing the electron donation in a dinucleating ligand family: molecular and electronic structures in a series of CoIICoII complexes.

We have developed a family of dinucleating ligands with varying terminal donors to generate dinuclear peroxo and high-valent complexes and to correlate their stabilities and reactivities with their molecular and electronic structures as a function of the terminal donors. It appears that the electron-donating ability of the terminal donors is an important handle for controlling these stabilities and reactivities. Here, we present the synthesis of a new dinucleating ligand with potentially strong donating terminal imidazole donors. As CoII ions are sensitive to variations in donor strength in terms of coordination number, magnetism, UV-Vis-NIR spectra, redox potentials, we probe the electron donation ability of this new ligand in CoIICoII complexes in comparison to the parent CoIICoII complexes with terminal pyridine donors and we synthesize the analogous CoIICoII complexes with terminal 6-methylpyridines and methoxy-substituted pyridines. The molecular structures show indeed strong variations in coordination numbers and bond lengths. These differences in the molecular structures are reflected in the magnetic properties and in the d-d transitions demonstrating that the molecular structures remain intact upon dissolution. The redox potentials are analyzed with respect to the electron donation ability and are the only handle to observe an effect of the methoxy-substituted pyridines. All data taken together show the following order of electron donating ability for the terminal donors: 6-methylpyridines ≪ pyridines < methoxy-substituted pyridines ≪ imidazoles.

Reactivities and Electronic Structures of µ-1,2-Peroxo and µ-1,2-Superoxo CoIIICoIII Complexes: Electrophilic Reactivity and O2 Release Induced by Oxidation.

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoOx or as {CoIII(µ-1,2-peroxo)CoIII} complexes, which are the oldest peroxo complexes known. The oxidation of {CoIII(µ-1,2-peroxo)CoIII} complexes had usually been described to form {CoIII(µ-1,2-superoxo)CoIII} complexes; however, recently the formation of {CoIV(µ-1,2-peroxo)CoIII} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {CoIII(µ-1,2-peroxo)(µ-OH)CoIII} and {CoIII(µ-OH)2CoIII} complexes. Oxidation of {CoIII(µ-1,2-peroxo)(µ-OH)CoIII} at -40 °C in CH3CN provides the stable {CoIII(µ-1,2-superoxo)(µ-OH)CoIII} species and activates electrophilic reactivity. Moreover, {CoIII(µ-1,2-peroxo)(µ-OH)CoIII} catalyzes water oxidation, not molecularly but rather via CoOx films. While {CoIII(µ-1,2-peroxo)(µ-OH)CoIII} can be reversibly deprotonated with DBU at -40 °C in CH3CN, {CoIII(µ-1,2-superoxo)(µ-OH)CoIII} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {CoIII(µ-1,2-superoxo)(µ-OH)CoIII} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {CoII(µ-OH)CoIII} core formed by the release of O2 from {CoIII(µ-1,2-superoxo)(µ-OH)CoIII} confirmed by a 100% yield of O2 upon photocatalytic oxidation of {CoIII(µ-1,2-peroxo)(µ-OH)CoIII}. This release of O2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.

Catalytic H2O2 Activation by a Diiron Complex for Methanol Oxidation.

In nature, C-H bond oxidation of CH4 involves a peroxo intermediate that decays to the high-valent active species of either a "closed" {FeIV(µ-O)2FeIV} core or an "open" {FeIV(O)(µ-O)FeIV(O)} core. To mimic and to obtain more mechanistic insight in this reaction mode, we have investigated the reactivity of the bioinspired diiron complex [(susan){Fe(OH)(µ-O)Fe(OH)}]2+ [susan = 4,7-dimethyl-1,1,10,10-tetrakis(2-pyridylmethyl)-1,4,7,10-tetraazadecane], which catalyzes CH3OH oxidation with H2O2 to HCHO and HCO2H. The kinetics is faster in the presence of a proton. 18O-labeling experiments show that the active species, generated by a decay of the initially formed peroxo intermediate [(susan){FeIII(µ-O)(µ-O2)FeIII}]2+, contains one reactive oxygen atom from the µ-oxo and another from the µ-peroxo bridge of its peroxo precursor. Considering an FeIVFeIV active species, a "closed" {FeIV(µ-O)2FeIV} core explains the observed labeling results, while a scrambling of the terminal and bridging oxo ligands is required to account for an "open" {FeIV(O)(µ-O)FeIV(O)} core.

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 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 Ka 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 [ kex298 = (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 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.

Reversible Carboxylate Shift in a µ-Oxo Diferric Complex in Solution by Acid-/Base-Addition.

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-η1:η1 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.

Design and synthesis of a dinucleating ligand system with varying terminal donor functions that provides no bridging donor and its application to the synthesis of a series of Fe(III)-µ-O-Fe(III) complexes.

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.