J Am Chem Soc. 2018 Apr 11;140(14):4929-4939. doi: 10.1021/jacs.8b01468. Epub 2018 Mar 29.
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.
与生物上更相关的铁或铜体系相比，镍配合物中二氧（合）物的活化研究要少得多，但由于镍独特的配位化学，有望产生新的反应模式。在这里，我们报告了一种基于吡唑的二镍（II）二氢化物配合物[KL（Ni-H）]（1a）可通过还原氢消除与 O 平稳反应，生成μ-1,2-过氧二镍（II）配合物[KLNi（O）]（2a），并且在用二苯并[18]-冠-6 处理后，生成分离的离子对[K（DB18C6）][LNi（O）]（2b）；这些是第一个通过 X 射线衍射表征的μ-1,2-过氧二镍中间体。在 2a 中，发现 K 以侧位与过氧单元结合，表明 O-O 键明显减弱：d（O-O）=1.482（2）Å 在 2a 中，而在 2b 中为 1.465（2）Å；ν̃（O-O）=720 cm 在 2a 中，而在 2b 中为 755 cm。1a（或 2a/2b）与过量 O 的反应可干净地得到[LNi（O）]（3），通过 X 射线晶体学（d（O-O）=1.326（2）Å）、电子顺磁共振和拉曼光谱（ν̃（O-O）=1007 cm）、磁性测量和密度泛函理论计算表明，它具有两个低自旋 d 镍（II）离子和一个真正的μ-1,2-超氧配体，其中未配对电子在面外π*轨道中。这些μ-1,2-超氧和μ-1,2-过氧物种都包含在二镍（II）核的裂口中的 O 衍生单元，可以通过化学和电化学在非常低的电位（E=-1.22 V 相对于 Fc/Fc）下可逆地相互转换。初步反应性研究表明，2a 的质子化，或 3 与 TEMPO-H 的反应，最终得到μ-羟基金属镍（II）配合物[LNi（μ-OH）]（4）。这项工作提供了一整套全新的密切相关且异常坚固的 Ni/O 中间体，避免使用不稳定的镍（I）前体，但将氧化还原当量储存在镍（II）氢化物键中，以备还原 O 结合。
