Dioxygenase‐Like Reactivity of an Isolable Superoxo–Nickel(II) Complex

Abstract

Although O(2) activation by metals such as iron and copper has been a matter of intensive research in the last decades, this type of chemistry for nickel systems is still in its infancy. Moreover, studies regarding the oxidizing ability of the resulting "Ni(n)-O(2)" species towards exogenous substrates are scarce. In this work, we report on the reactivity of an isolable and thermally stable mononuclear superoxo-nickel compound [Ni(II)(beta-diketiminato)(O(2))] (1) towards different types of organic substrates. In addition, we have been able to prove that the beta-diketiminato ligand can undergo partial intramolecular oxidation due to close proximity between the isopropyl groups of the beta-diketiminato-aryl and the superoxo subunits. Compound 1 performs hydrogen-atom abstraction from O-H and N-H groups and most importantly it shows an unprecedented dioxygenase-like reactivity in the oxidation of 2,4,6-tri-tert-butylphenol. The latter reaction most likely occurs through the mediation of a putative [Ni(III)-oxo] intermediate, affording an unprecedented oxidation product of the phenol that incorporates two oxygen atoms from a single O(2) subunit. Results presented herein provide evidence of the striking oxidizing ability of dioxygen-nickel species and further support the viability to use such systems as oxidation catalysts analogous to its heavy metal congener, palladium.

Mechanistic Insights into the Oxidation of Substituted Phenols via Hydrogen Atom Abstraction by a Cupric–Superoxo Complex

“…Further, Nishinaga's work provides precedent for the proposed (Scheme 4) reaction of [(DMM-tmpa)Cu II (O 2 ˙ − )] + ( 2 ) with the initially generated phenoxyl radical intermediate; 51 Nishinaga 51b also obtained a cobalt peroxy species Co III -OO-(ArO'), and its X-ray structure for a para - t Bu phenol substrate (Scheme 7). Further, in order to explain the products observed in very recent investigations by Driess and co-workers, 52 involving phenol reactions with a new [Ni II (O 2 ˙ − )] + complex, an intermediate analogous to A' was proposed to form from p -R-DTBP's (R = H, Me, t Bu) substrates.…”

From Enzymes to Functional Materials—Towards Activation of Small Molecules

“…For example, the [Ni II (b-diketiminato)(O 2 )] superoxide complex 35 is able to effectively catalyze the oxidation of alcohols, the activation of NÀHb onds and the oxidation of PPh 3 . [182] Furthermore, heterobimetallic complexes that contain metal-oxygenb ridges were recently shown to be powerful for hydrogen atom transfer reactions (HAT) from hydroxyl groups forming oxygen radicals ( Figure 14). [183,184] Notably,t he second metal center controlled the HATr eactivity of the complexes, showingadecrease in activity according to NiCu > NiNi > NiCo > NiFe.…”

Spectroscopically Characterized Synthetic Mononuclear Nickel–Oxygen Species

“…In contrast to [Ni II (O 2 )(PhTt Ad )], the unpaired electron in [Ni II (O 2 )(Nacnac)] resided in the superoxide ligand. Apart from oxygen‐atom transfer to triphenylphosphine, this compound could perform hydrogen‐atom abstraction from O−H and N−H groups …”

“…Singlecrystalssuitable for X-ray analysisrevealed that the nickel atom was planart etracoordinated and thus, the O 2 moiety was bound in as ide-on fashion.T he measured OÀO distance of 1.347 was fully consistent with as uperoxo character of the O 2 ligand.I nc ontrastt o[ Ni II (O 2 )(PhTt Ad )],t he unpaired electron in [Ni II (O 2 )(Nacnac)] resided in the superoxide ligand.A part from oxygen-atom transfer to triphenylphosphine,t his compound could perform hydrogen-atom abstraction from OÀHa nd NÀHg roups. [34]…”