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

Activation of Electrophilic Reactivity by Protonation and Oxidation of a Peroxo CoIIICoIII Complex: Reactivity and Molecular Structures of Hydroperoxo, Superoxo, and Peroxo CoIIICoIII Complexes

In nature, oxidizing metalloenzymes usually bind O2 in the form of peroxo intermediates that frequently requires for activation a further protonation, for which the formation of hydroperoxo species is proposed. In dinuclear metalloenzymes, which typically feature Fe, Mn, and Cu, structures and electrophilic reactivity of peroxo, superoxo, and hydroperoxo intermediates are generally difficult to study due to their reactive and transient nature, which is why more stable peroxo CoIIICoIII complexes might be helpful models. Here, we present the molecular structures of a series of μ-1,2-peroxo, μ-1,2-superoxo, and μ-1,1-hydroperoxo CoIIICoIII complexes and the variation of their electrophilic hydrogen-atom-transfer (HAT) and oxygen-atom-transfer (OAT) reactivity. The μ-1,2-peroxo complex exhibits no electrophilic reactivity, whereas oxidation to the μ-1,2-superoxo complex activates electrophilic HAT reactivity, while protonation to the μ-1,1-hydroperoxo complex activates electrophilic OAT reactivity. The latter occurs by an associated mechanism with the μ-1,1-hydroperoxo ligand being the electrophilic OAT agent. Protonation to the μ-1,1-hydroperoxo elongates the O-O bond, while oxidation to the μ-1,2-superoxo shortens it relative to the μ-1,2-peroxo. The protonation of the μ-1,2-peroxo complex could be made accessible by the introduction of remote electron-donating substituents into a recently reported μ-1,2-peroxo CoIIICoIII complex that increases the μ-1,2-peroxo basicity by more than five pKa units. Hence, the initially counterintuitive increase of electron donation by the ligand environment increases the μ-1,2-peroxo basicity allowing its protonation to a more electrophilic μ-1,1-hydroperoxo ligand. More generally, HAT and OAT reactivity is correlated with accessibility of the following intermediate.

Isolation and Crystallographic Characterization of an Octavalent Co2O2 Diamond Core

High-valent cobalt oxides play a pivotal role in alternative energy technology as catalysts for water splitting and as cathodes in lithium-ion batteries. Despite this importance, the properties governing the stability of high-valent cobalt oxides and specifically possible oxygen evolution pathways are not clear. One root of this limited understanding is the scarcity of high-valent Co(IV)-containing model complexes; there are no reports of stable, well-defined complexes with multiple Co(IV) centers. Here, an oxidatively robust fluorinated ligand scaffold enables the isolation and crystallographic characterization of a Co(IV)2-bis-μ-oxo complex. This complex is remarkably stable, in stark contrast with previously reported Co(IV)2 species that are highly reactive, which demonstrates that oxy-Co(IV)2 species are not necessarily unstable with respect to oxygen evolution. This example underscores a new design strategy for highly oxidizing transition-metal fragments and provides detailed data on a previously inaccessible chemical unit of relevance to O-O bond formation and oxygen evolution.

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.

Generation and Reactivity of μ-1,2-Peroxo CuIICuII and Bis-μ-oxo CuIIICuIII Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide

Tetradentate-N4 ligands stabilize dinuclear {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} species, and CuII complexes of these ligands were reported to catalyze the oxidation of benzene with H2O2. Here, we report {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} intermediates of dinucleating bis(tetradentate-N4) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {CuICuI} precursors with O2 or from {CuIICuII} precursors with H2O2 and NEt3. Both intermediates are not stable even at low temperatures, but they show no electrophilic HAT reactivity with DHA. Catalytic investigations on the hydroxylation of benzene with excess H2O2 between 30 and 50 °C indicate that both radical-based and {Cu2On}-based mechanisms depend strongly on the catalytic conditions. In the presence of a radical scavenger, TONs of ∼920/∼720 have been achieved without/with the 6-methyl group of the ligand. Although {CuII(μ-OH)CuII} reacts with excess H2O2 at -40 °C to {CuII(OOH)}2 species, these are only stable for seconds at 20 °C and cannot account for catalytic oxidations over a period of 24 h at 30-50 °C.