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.

A compartmental ligand scaffold HL with two β-diketiminato binding sites spanned by a pyrazolate bridge gave a series of dinuclear nickel(II) dihydride complexes M[LNi(H)], M = Na (Na·2) and K (K·2), which were isolated after reacting the precursor complex [LNi(μ-Br)] (1) with MHBEt (M = Na and K). Crystallographic characterization showed the two hydride ligands to be directed into the bimetallic pocket, closely interacting with the alkali metal cation. Treatment of K·2 with dibenzo(18-crown-6) led to the separated ion pair [LNi(H)][K(DB18C6)] (2[K(DB18C6)]). Reaction of Na·2 or K·2 with D was investigated by a suite of H andH NMR experiments, revealing an unusual pairwise H/D exchange process that synchronously involves both Ni-H moieties without H/D scrambling. A mechanistic picture was provided by DFT calculations which suggested facile recombination of the two terminal hydrides within the bimetallic cleft, with a moderate enthalpic barrier of ∼62 kJ/mol, to give H and an antiferromagnetically coupled [LNi] species. This was confirmed by SQUID monitoring during H release from solid 2[K(DB18C6)]. Interaction with the Lewis acid cation (Na or K) significantly stabilizes the dihydride core. Kinetic data for the M[L(Ni-H)] → H transition derived from 2D H EXSY spectra confirmed first-order dependence of H release on M·2 concentration and a strong effect of the alkali metal cation M. Treating [LNi(D)] with phenylacetylene led to D and dinickel(II) complex 3 with a twice reduced styrene-1,2-diyl bridging unit in the bimetallic pocket. Complexes [LNi(H)] having two adjacent terminal hydrides thus represent a masked version of a highly reactive dinickel(I) core. Storing two reducing equivalents in adjacent metal hydrides that evolve H upon substrate binding is reminiscent of the proposed N binding step at the FeMo cofactor of nitrogenase, suggesting the use of the present bimetallic scaffold for reductive bioinspired activation of a range of inert small molecules.

During the last decades, β-diketiminates, also referred to as "nacnac" ligands, have raised substantial interest among chemists due to their simple preparation and elec tronic and steric tunability. Therefore, the combination of two of these versatile moieties with pyrazolate as a linker unit was envisioned to furnish a ligand capable of preorgani zation upon metal complexation. This chapter deals with the preparation and character ization of the trianionic, hexadentate ligand system and the establishment of a dinuclear nickel complex as the foundation for further promising chemistry.