Reductive O₂ Binding at a Dihydride Complex Leading to Redox Interconvertible μ-1,2-Peroxo and μ-1,2-Superoxo Dinickel(II) Intermediates

Abstract

Dioxygen activation at nickel complexes is much less studied than for the biologically more relevant iron or copper systems but promises new reactivity patterns because of the distinct coordination chemistry of nickel. Here we report that a pyrazolate-based dinickel(II) dihydride complex [KL(Ni-H)] (1a) smoothly reacts with O via reductive H elimination to give the μ-1,2-peroxo dinickel(II) complex [KLNi(O)] (2a) and, after treatment with dibenzo[18]-crown-6, the separated ion pair [K(DB18C6)][LNi(O)] (2b); these are the first μ-1,2-peroxo dinickel intermediates to be characterized by X-ray diffraction. In 2a, the K is found side-on associated with the peroxo unit, revealing a pronounced weaking of the O-O bond: d(O-O) = 1.482(2) Å in 2a versus 1.465(2) in 2b; ν̃(O-O) = 720 cm in 2a versus 755 cm in 2b. Reaction of 1a (or 2a/2b) with an excess of O cleanly leads to [LNi(O)] (3), which was shown by X-ray crystallography ( d(O-O) = 1.326(2) Å), electron paramagnetic resonance and Raman spectroscopy (ν̃(O-O) = 1007 cm), magnetic measurements, and density functional theory calculations to feature two low-spin d nickel(II) ions and a genuine μ-1,2-superoxo ligand with the unpaired electron in the out-of-plane π* orbital. These μ-1,2-superoxo and μ-1,2-peroxo species, all containing the O-derived unit within the cleft of the dinickel(II) core, can be reversibly interconverted chemically and also electrochemically at very low potential ( E = -1.22 V vs Fc/Fc). Initial reactivity studies indicate that protonation of 2a, or reaction of 3 with TEMPO-H, ultimately gives the μ-hydroxo dinickel(II) complex [LNi(μ-OH)] (4). This work provides an entire new series of closely related and unusually rugged Ni/O intermediates, avoiding the use of unstable nickel(I) precursors but storing the redox equivalents for reductive O-binding in nickel(II) hydride bonds.

Spectroscopic Characterization of μ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters

“…Our assignment of a bridging peroxo species in C2 at 756 cm -1 is supported by previously reported lower frequencies for such bands compared with mononuclear complexes, 30,32,47,48 and our observed frequency is in the range of frequencies observed for O-O vibrations assigned to bridging peroxo ligands, as reported for a number of complexes; [32][33][34][35]42 examples are complexes of iron, copper, cobalt, nickel, and platinum dimers. This range is more than 80 cm -1 lower in frequency compared with an η 2 peroxo O-O stretch (expected to occur at about 840 cm -1 ; 885 cm -1 for atomic Ir).…”

“…This frequency is consistent with reported 16 O- 16 O stretches in peroxo bands. [32][33][34][35]42 C3 consists of a shoulder at 729 cm -1 , and a strong intensity gain at 701 cm -1 . This shoulder and strong intensity enhancement are not present in C2, and we infer that these are a consequence of 18 O-labeling, resulting in a mean frequency red shift of 41 cm -1 from the 756 cm -1 band, which coincides with DFT-calculated shifts based on a harmonic model with different functionals (Table S2, ESI).…”

Generation and Reactivity of a NiIII2(μ-1,2-peroxo) Complex

“…The Ni-O bond lengths are shorter than those of the recently isolated Ni II 2(μ-1,2-peroxo) complex, which has Ni-O bond lengths of 1.834(2) Å and a dihedral angle of 89.9(2)°. 41 The shorter Ni-O distances are consistent with a higher oxidation state of Ni III in 2.…”

“…47 Meanwhile, the O-O bond lengths in two previously isolated Ni2(μ-1,2-superoxo) complexes are 1.33 and 1.35 Å. 33,41 The solid-state structure of 2 shows that the Ni III centers adopt a seesaw geometry, with an Ni-O bond length of 1.79(1) Å and a Ni-O-O-Ni dihedral angle of 161.8(5)°. The Ni-O bond lengths are shorter than those of the recently isolated Ni II 2(μ-1,2-peroxo) complex, which has Ni-O bond lengths of 1.834(2) Å and a dihedral angle of 89.9(2)°.…”

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

“…35,36 Nickel-superoxo species are generally rare even in synthetic systems, and Ni mediated oxygen activation without accessing Ni(I) or Ni(III) oxidation states has little synthetic precedent. [37][38][39][40][41][42][43][44][45] Therefore, the proposed enzymatic mechanism for quercetin dioxygenase motivates studies to examine whether ligand cooperativity is a viable strategy for Ni systems to activate O2 and mediate oxidative transformations.…”