Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence

Abstract

Metal ligand cooperativity is a powerful strategy in transition metal chemistry. This type of mechanism for the activation of O<sub>2</sub> is best exemplified by heme centers in biological systems. While aerobic oxidations with Fe and Cu are well precedented, Ni-based oxidations are frequently less common due to less-accessible metal-based redox couples. Some Ni enzymes utilize special ligand environments for tuning the Ni(II)/(III) redox couple such as strongly donating thiolates in Ni superoxide dismutase. A recently characterized example of a Ni-containing protein, however, suggests an alternative strategy for mediating redox chemistry with Ni by utilizing ligand-based reducing equivalents to enable oxygen binding. While this mechanism has little synthetic precedent, we show here that Ni complexes of the redox-active ligand<i><sup> t</sup></i><sup>Bu,Tol</sup>DHP (<i><sup>t</sup></i><sup>Bu,Tol</sup>DHP = 2,5-bis((2-<i>t</i>-butylhydrazono)(<i>p</i>-tolyl)methyl)-pyrrole) activate O<sub>2</sub> to generate a Ni(II) superoxo complex via ligand-based electron transfer. This superoxo complex is competent for stoichiometric oxidation chemistry with alcohols and hydrocarbons. This work demonstrates that coupling ligand-based redox chemistry with<b> </b>functionally redox-inactive Ni centers enables oxidative transformations more commonly mediated by metals such as Fe and Cu.

Electrocatalytic Semihydrogenation of Terminal Alkynes Using Ligand-Based Transfer of Protons and Electrons

“…Complex 1 was synthesized following a previously reported procedure. 59 All manipulations were carried out under an atmosphere of N 2 by using standard Schlenk and glovebox techniques. Glassware was dried at 180 °C for a minimum of 2 h and cooled under vacuum prior to use.…”

“…The EPR spectrum is distinct from that of complex 1 which is formally Ni(II) with a ligand radical. 59 These combined data, and particularly the need to add an additional reductant to 3 to react with alkynes, supports that a reduction to a Ni(I) species ( 4) is critical to initiating catalysis. With evidence supporting the necessity of the two-electron reduction of 1 in the presence of acid before the system is catalytically active, we then investigated the chronoamperometry of bulk electrolyses to gain additional mechanistic insights.…”

“…We initially targeted the previously reported complex ( tBu,Tol DHP)Ni(II) (1) as a precatalyst. 59 The cyclic voltammogram (CV) of 1 in acetonitrile shows two reversible redox events at peak potentials (E 1/2 ) of −0.1 and −1.1 V vs Fc 0/+ . We assign these features to ligand-based oxidation and reduction of the starting complex to form 1 + and 1 − , respectively (Figure 2A).…”

Appended Lewis Acids Enable Dioxygen Reactivity and Catalytic Oxidations with Ni(II)

“…The CW X-band EPR spectra of O 2 reactions with both 1 and 2 display signals with slight rhombic distortion (1: g av = 2.24, 2: g av = 2.23), where the lowfield signal value is shifted to lower magnetic field compared to reported Ni−O 2 adducts. 29,31,32,47,91 The CW X-band EPR spectrum of the reaction between 10 and O 2 at 130 K displays a signal (g av = 2.20), with resolved hyperfine coupling. 92 Spin quantification data using a TEMPO standard established that the EPR signals above accounted for at most 37% of the nickel.…”

Steric Effects on the Oxygen Reduction Reaction with Cobalt Porphyrin Atropisomers

“…1−6 Extensive efforts have been made to design and develop new efficient catalysts for ORR and to understand catalyst structural effects on their activities. 7−11 On one hand, these efforts lead to the identification of a large number of metal complexes, including those of Mn, 12−15 Fe, 16−19 Co, 20−24 Ni, 25,26 and Cu, 27−29 as active catalysts for ORR. More importantly, fundamental knowledge to correlate the structure and the catalytic performance of ORR catalysts is learned.…”