Preparation and Characterization of a Formally Ni^IV–Oxo Complex with a Triplet Ground State and Application in Oxidation Reactions

Abstract

High-valent first-row transition-metal–oxo complexes are important intermediates in biologically and chemically relevant oxidative transformations of organic molecules and in the water splitting reaction in (artificial) photosynthesis. While high-valent Fe– and Mn–oxo complexes have been characterized in detail, much less is known about their analogues with late transition metals. In this study, we present the synthesis and detailed characterization of a unique mononuclear terminal Ni–O complex. This compound, [Ni(TAML)(O)(OH)]3–, is characterized by an intense charge-transfer (CT) band around 730 nm and has an S t = 1 ground state, as determined by magnetic circular dichroism spectroscopy. From extended X-ray absorption fine structure (EXAFS), the Ni–O bond distance is 1.84 Å. Ni K edge XAS data indicate that the complex contains a Ni(III) center, which results from an unusually large degree of Ni–O π-bond inversion, with one hole located on the oxo ligand. The complex is therefore best described as a low-spin Ni(III) complex (S = 1/2) with a bound oxyl (O•–) ligand (S = 1/2), where the spins of Ni and oxyl are ferromagnetically coupled, giving rise to the observed S t = 1 ground state. This bonding description is roughly equivalent to the presence of a Ni–O single (σ) bond. Reactivity studies show that [Ni(TAML)(O)(OH)]3– is a strong oxidant capable of oxidizing thioanisole and styrene derivatives with large negative ρ values in the Hammett plot, indicating its electrophilic nature. The intermediate also shows high reactivity in C–H bond activation of hydrocarbons with a kinetic isotope effect of 7.0(3) in xanthene oxidation.

Nanoscale Ni–NiO–ZnO Heterojunctions for Switchable Dehydrogenation and Hydrogenation through Modulation of Active Sites

“…Notably, ZN–O–A-7 catalyzed dehydrogenation of toluene in the presence of oxygen (10 bar) at 100 °C failed to produce any desired products. High-valent nickel–oxo (Ni III –oxo/peroxo and Ni IV –oxo) species are believed to be possible intermediates in a number of nickel-based oxidation reactions involving discrete homogeneous nickel-based catalysts. , Meanwhile, the generation of these species in the presence of oxygen is more feasible in the case of lower oxidation states (0, +1) of nickel in comparison to Ni­(II). , In the present case, the generation and stability of these active species are expected to be inhibited by strong charge transfer between Ni–NiO–ZnO heterojunctions in ZN–O–A-7 . For catalytic activity, toluene was used as the reagent and solvent, and its reaction was performed in the presence of TBHP (200 μL 5–6 M decane solution) at 110 °C for 12 h. The conversion of the reaction was calculated based on the amount of TBHP used for the reaction with decane as the internal standard.…”

“…Highvalent nickel−oxo (Ni III −oxo/peroxo and Ni IV −oxo) species are believed to be possible intermediates in a number of nickelbased oxidation reactions involving discrete homogeneous nickel-based catalysts. 45,46 Meanwhile, the generation of these species in the presence of oxygen is more feasible in the case of lower oxidation states (0, +1) of nickel in comparison to Ni(II). 47,48 In the present case, the generation and stability of these active species are expected to be inhibited by strong charge transfer between Ni−NiO−ZnO heterojunctions in ZN−O−A-7.…”

C–H Bond Activation by an Antimony(V) Oxo Intermediate Accessed through Electrochemical Oxidation of Antimony(III) Tetrakis(thiocyano)corrole

“…4−6 The molecular complex [Ni-(TAML)(O)(OH)] 3− (TMAL = tetraamido macrocyclic ligand) has been used for the oxidation of organic compounds like thioanisole and styrene derivatives in which Ni III -oxyl was involved in the oxidation process. 7 Also, the rich chemistry of metal−oxo complexes inspired the synthesis of C−H oxidation catalysts. 8,9 The previous report showed that chelating ligands could enhance the stability of metal−oxo intermediates, thereby enabling precise modulation of chemo-and regioselectivity during the electron-transfer and oxo-transfer stages.…”

The elusive active species in nickel(ii)-mediated oxidations of hydrocarbons by peracids: a NiII–oxyl species, an aroyloxy radical, or a NiII–peracid complex?

“…, cobalt and nickel) after group 8 in the periodic table of elements are extensively employed. 29–49 Due to the oxo-wall, an additional electron locates in an antibonding orbital to convert a metal–oxo species to a metal–oxyl species, which makes the active species containing metals after group 8 more robust. 50–52 In this text, we focus on the elusive active species and reaction mechanism of nickel( ii )-mediated hydrocarbon oxidations with meta -chloroperbenzoic acid ( m CPBA).…”

“…50–52 In this text, we focus on the elusive active species and reaction mechanism of nickel( ii )-mediated hydrocarbon oxidations with meta -chloroperbenzoic acid ( m CPBA). 29–49…”