Structural Characterization of a Hydroperoxo Nickel Complex and Its Autoxidation: Mechanism of Interconversion between Peroxo, Superoxo, and Hydroperoxo Species

Dioxygen Activation and Pyrrole α-Cleavage with Calix[4]pyrrolato Aluminates: Enzyme Model by Structural Constraint

The present work describes the reaction of triplet dioxygen with the porphyrinogenic calix[4]pyrrolato aluminates to alkylperoxido aluminates in high selectivity. Multiconfigurational quantum chemical computations disclose the mechanism for this spin‐forbidden process. Despite a negligible spin–orbit coupling constant, the intersystem crossing (ISC) is facilitated by singlet and triplet state degeneracy and spin–vibronic coupling. The formed peroxides are stable toward external substrates but undergo an unprecedented oxidative pyrrole α‐cleavage by ligand aromatization/dearomatization‐initiated O−O σ‐bond scission. A detailed comparison of the calix[4]pyrrolato aluminates with dioxygen‐related enzymology provides insights into the ISC of metal‐ or cofactor‐free enzymes. It substantiates the importance of structural constraint and element–ligand cooperativity for the functions of aerobic life.

Activation of Dioxygen at a Lewis Acidic Nickel(II) Complex: Characterization of a Metastable Organoperoxide Complex

In metal-mediated O₂ activation, nickel(II) compounds hardly play a role, but recently it has been shown that enzymes can use nickel(II) for O₂ activation. Now a low-coordinate Lewis acidic nickel(II) complex has been synthesized that reacts with O₂ to give a nickel(II) organoperoxide, as proposed for the enzymatic system. Its formation was studied further by UV/Vis absorption spectroscopy, leading to the observation of a short-lived intermediate that proved to be reactive in both oxygen atom transfer and hydrogen abstraction reactions, while the peroxide efficiently transfers O atoms. Both for the enzyme and for the functional model, the key to O₂ activation is proposed to represent a concomitant electron shift from the substrate/co-ligand.

Spectroscopically Characterized Synthetic Mononuclear Nickel-Oxygen Species

Iron, copper, and manganese are the predominant metals found in oxygenases that perform efficient and selective hydrocarbon oxidations and for this reason, a large number of the corresponding metal-oxygen species has been described. However, in recent years nickel has been found in the active site of enzymes involved in oxidation processes, in which nickel-dioxygen species are proposed to play a key role. Owing to this biological relevance and to the existence of different catalytic protocols that involve the use of nickel catalysts in oxidation reactions, there is a growing interest in the detection and characterization of nickel-oxygen species relevant to these processes. In this Minireview the spectroscopically/structurally characterized synthetic superoxo, peroxo, and oxonickel species that have been reported to date are described. From these studies it becomes clear that nickel is a very promising metal in the field of oxidation chemistry with still unexplored possibilities.