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.

Consecutive Ligand-Based Electron Transfer in New Molecular Copper-Based Water Oxidation Catalysts.

Water oxidation to dioxygen is one of the key reactions that need to be mastered for the design of practical devices based on water splitting with sunlight. In this context, water oxidation catalysts based on first-row transition metal complexes are highly desirable due to their low cost and their synthetic versatility and tunability through rational ligand design. A new family of dianionic bpy-amidate ligands of general formula H2 LNn- (LN is [2,2'-bipyridine]-6,6'-dicarboxamide) substituted with phenyl or naphthyl redox non-innocent moieties is described. A detailed electrochemical analysis of [(L4)Cu]2- (L4=4,4'-(([2,2'-bipyridine]-6,6'-dicarbonyl)bis(azanediyl))dibenzenesulfonate) at pH 11.6 shows the presence of a large electrocatalytic wave for water oxidation catalysis at an η=830 mV. Combined experimental and computational evidence, support an all ligand-based process with redox events taking place at the aryl-amide groups and at the hydroxido ligands.

Proof of Phosphate Diester Binding Ability of Cytotoxic DNA-Binding Complexes.

We have rationally designed a family of dinuclear transition-metal complexes to bind two neighboring phosphate diester groups of DNA. The two metal ions are positioned at the distance of two neighboring phosphate diesters in DNA of 6-7 Å by a 1,8-naphthalenediol backbone. Two sterically demanding dipicolylamine pendant arms in the 2 and 7 positions stabilize coordination of the metal ions and prevent coordination to the less exposed nucleobases of DNA. Although the dinuclear NiII2 and CuII2 bind to DNA, inhibit DNA synthesis, and preferentially kill human cancer cells over fast proliferating human stem cells, the DNA binding mode was elusive. Here, we prove the principle phosphate diester binding ability of this family of dinuclear complexes by a new dinuclear NiII2 complex with dibenzimidazolamine pendant arms. The distance of the oxygen atoms of the coordinated phosphate diesters of 6.5 Å confirms the initial design and binding ability to two neighboring phosphate diesters of the DNA backbone. Moreover, the facile exchange of coordinated acetates by phosphate diesters indicates a preferential binding to phosphate diesters.

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.

Direct and remote control of electronic structures and redox potentials in µ-oxo diferric complexes.

Non-heme diiron enzymes activate O2 for the oxidation of substrates in the form of peroxo FeIII2 or high-valent FeIV2 intermediates. We have developed a dinucleating bis(tetradentate) ligand system that stabilizes peroxo and hydroperoxo FeIII2 complexes with terminal 6-methylpyridine donors, while the peroxo FeIII2 intermediate is reactive with terminal pyridine donors presumably via conversion to a fluent high-valent FeIV2 intermediate. We present here a derivative with electron-donating methoxy substituents at the pyridine donors and its diferric complexes with an {FeIIIX(µ-O)FeIIIX} (X- = Cl-, OAc-, and OH-) or an {FeIII(µ-O)(µ-OAc)FeIII} core. The complex-induced oxidation of EtOH with H2O2 provides µ-OAc-, and in acetone, the complex with mixed OH-/OAc- exogenous donors is obtained. Both reactivities indicate a reactive fluent peroxo FeIII2 intermediate. The coupling constant J and the LMCT transitions are insensitive to the nature of the directly bound ligands X- and reflect mainly the electronic structure of the central {FeIII(µ-O)FeIII} core, while Mössbauer spectroscopy and d-d transitions probe the local FeIII sites. The remote methoxy substituents decrease the potential for the oxidation to FeIV by ∼100 mV, while directly bound OH- in {FeIII(OH)(µ-O)FeIII(OH)} with a short 1.91 Å FeIII-OOH bond decreases the potential by 590 mV compared to {FeIII(OAc)(µ-O)FeIII(OAc)} with a 2.01 Å FeIII-OOAc bond. Interestingly, this FeIII-OH bond is even shorter (1.87 Å) in the mixed OH-/OAc- complex but the potential is the mean value of the potentials of the OH-/OH- and OAc-/OAc- complexes, thus reflecting the electron density of the central {FeIII(µ-O)FeIII} core and not of the local FeIII-OH unit.

Evaluation of the binding mode of a cytotoxic dinuclear nickel complex to two neighboring phosphates of the DNA backbone.

A family of dinuclear complexes based on 2,7-disubstituted 1,8-naphthalenediol-ligands has been designed to bind covalently to two neighboring phosphate diester groups in the backbone of DNA. The dinuclear CuII and NiII complexes bind to DNA resulting in the inhibition of DNA synthesis in PCR experiments and in a cytotoxicity that is stronger for human cancer cells than for human stem cells of the same proliferation rate. These experiments support but cannot prove that the dinuclear complexes bind as intended to two neighboring phosphate ester groups of the DNA backbone. Here, we evaluate the potential binding mode of the cytotoxic dinuclear NiII complex using simple phosphate diester models (dimethyl phosphate and diphenyl phosphate). Depending on the reaction conditions, the phosphate diesters bind to the NiII ions in a bridging or in a terminal coordination mode. The latter occurs by substitution of two coordinated acetates by the phosphate diesters. This reaction has been followed by NMR spectroscopy, which demonstrates that the substitution of acetate by phosphate is thermodynamically strongly favored, while the exchange with excess phosphate is fast on the NMR time scale. The molecular structure of the NiII complex with two coordinated diphenyl phosphates served as a model for the computational evaluation of the binding to the DNA backbone. This combined experimental and computational study suggests a monodentate coordination mode of the DNA phosphate diesters to the NiII ions that is assisted by hydrogen bonds with water ligands.

Enhancing the ferromagnetic coupling in extended phloroglucinol complexes by increasing the metal SOMO-ligand overlap: synthesis and characterization of a trinuclear Co(II)(3) triplesalophen complex.

The triplesalophen complex [(baron(Me))Co(II)(3)] has been synthesized and characterized. The low-spin Co(II) ions possess an (2)A2 ground state with the magnetic orbitals of dyz type. These are well oriented for a strong π overlap with the bridging phloroglucinol, which results in the strongest ferromagnetic interactions by the spin-polarization mechanism for a 3d phloroglucinol complex.