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Formation of the O-O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High-valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O-O bond in solution, from non-heme, N5 -coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta-chloroperbenzoic acid is administered in excess. Oxygen-isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)-initiated free-radical pathways of the peroxides, which are typical of catalase-like reactivity, and iron-borne O-O coupling, which is unprecedented for non-heme/peroxide systems. Interpretation in terms of [FeIV (O)] and [FeV (O)] being the resting and active principles of the O-O coupling, respectively, concurs with fundamental mechanistic ideas of (electro-) chemical O-O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.

Deprotonation of ligand-appended alkoxyl groups in mononuclear copper(II) complexes of N,O ligands L-1 and L-2, gave dinuclear complexes sharing symmetrical Cu2O2 cores. Molecular structures of these mono-and binuclear complexes have been characterized by XRD, and their electronic structures by UV/Vis, H-1 NMR, EPR and DFT; moreover, catalytic performance as models of catechol oxidase was studied. The binuclear complexes with anti-ferromagnetically coupled copper(II) centers are moderately active in quinone formation from 3,5-di-tert-butyl-catechol under the estab-lished conditions of oxygen saturation, but are strongly activated when additional dioxygen is administered during catalytic turnover. This unforeseen and unprecedented effect is attributed to increased maximum reaction rates v(max), whereas the substrate affinity KM remains unaffected. Oxygen administration is capable of (partially) removing limitations to turnover caused by product inhibition. Because product inhibition is generally accepted to be a major limitation of catechol oxidase models, we think that our observations will be applicable more widely.

Tris-o-semiquinonato cobalt complexes react with a tetrapodal pyridine-derived ligand to form dinuclear cobalt compounds of general formula (OMP)[CoQ2]2, where OMP = 2,2′-(pyridine-2,6-diyl)bis(N1,N1,N3,N3-tetramethylpropane-1,3-diamine), Q = mono- or dianion of 3,6-di-tert-butyl-o-benzoquinone (complex 1) and it derivatives: 3,6-di-tert-butyl-4,5-N,N′-piperazino-o-benzoquinone (complex 2), and 3,6-di-tert-butyl-4-Cl-o-benzoquinone (complex 3). Single crystal X-ray crystallography of 1 and 3 indicates two bis-quinonato cobalt units bound by an OMP ligand, which acts as a bridge. Each central cobalt atom is chelated by one N1,N1,N3,N3-tetramethylpropane-1,3-diamine and two o-quinonato fragments. The nitrogen atom of the pyridine ring is uncoordinated. All complexes were characterized by NIR-IR and EPR spectroscopy, precise adiabatic vacuum calorimetry, and by variable-temperature magnetic susceptibility measurements. All data indicate a reversible thermally driven redox-isomeric (valence tautomeric) transformation in the solid state for all complexes.