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.