Institut für Anorganische Chemie , Universität Göttingen , Tammanstrasse 4 , D-37077 Göttingen , Germany.
Inorg Chem. 2019 Apr 15;58(8):5154-5162. doi: 10.1021/acs.inorgchem.9b00256. Epub 2019 Mar 25.
The metal-mediated activation of PhNO represents an important starting point for understanding the reactivity patterns of nitrosoarenes in biological systems and catalysis. Here we report that the pyrazole-based dinickel(II) dihydride complex [KL(NiH)] (1) reacts with PhNO to eliminate dihydrogen concomitant with binding of the doubly reduced substrate in μ-κ(O):κ(N) mode in the bimetallic pocket of [KLNi(PhNO)] (2). The addition of [2,2,2]cryptand leads to the ionic complex [K(crypt)][LNi(PhNO)] (3). Structural and spectroscopic analyses evidence that interaction with the Lewis acidic K in 2 causes significant elongation and weakening of the substrate's N-O bond [ d = 1.487(12) Å in 2 vs 1.374(4) Å in 3]. Complex 2 (or 3) reacts with [FeCp*][PF] to give LNi(PhNO) (4), which is shown by electron paramagnetic resonance and IR spectroscopies and density functional theory calculations to feature two low-spin d nickel(II) ions and a bridging (PhNO) radical anion ligand, with the out-of-plane π*(NO) being the singly occupied molecular orbital. Cyclic voltammetry and UV-vis spectroelectrochemical experiments show that 4 and the anion of 3 can be reversibly interconverted at very low potential ( E = -1.53 V vs Fc/Fc). Protonation of 2 leads to the N-phenylhydroxylamine complex [LNi(ONHPh)] (5) with a long N-O bond of 1.464(2) Å, and titration studies suggest a p K of around 23-25 in tetrahydrofuran. This allows one to derive a bond dissociation energy of 62-65 kcal mol for the N-H bond of 5. Accordingly, 5 readily reacts with the phenoxy radical 2,4,6- BuCHO to yield 4. This work demonstrates the reductive binding of PhNO without prior formation of unstable nickel(I) species and the redox noninnocence of the PhNO ligand in the less common μ-κ(O):κ(N) bridging mode. Thermodynamic data for H-atom-abstraction chemistry at the activated PhNO may be valuable for understanding the reactivity patterns of the transient but biologically relevant nitroxyl (HNO) ligand.
金属介导的 PhNO 活化代表了理解生物体系和催化中亚硝基芳烃反应模式的重要起点。在这里，我们报告基于吡唑的二镍（II）二氢化物配合物 [KL(NiH)]（1）与 PhNO 反应，同时消除氢气，伴随亚硝基芳烃在双金属口袋中以 μ-κ(O):κ(N)模式的双重还原。[KLNi(PhNO)]（2）的配位。添加[2,2,2]穴醚导致离子配合物[K(crypt)][LNi(PhNO)]（3）的形成。结构和光谱分析表明，与路易斯酸性 K 的相互作用导致底物 N-O 键的显著伸长和弱化[在 2 中 d = 1.487(12) Å，而在 3 中 d = 1.374(4) Å]。配合物 2（或 3）与[FeCp*][PF]反应生成 LNi(PhNO)（4），电子顺磁共振和红外光谱以及密度泛函理论计算表明，4 具有两个低自旋 d 镍（II）离子和一个桥联（PhNO）自由基阴离子配体，其中面外 π*(NO)是单占据分子轨道。循环伏安法和紫外可见光谱电化学实验表明，4 和 3 的阴离子可以在非常低的电势（E = -1.53 V 相对于 Fc/Fc）下可逆地相互转化。2 的质子化导致 N-苯羟胺配合物[LNi(ONHPh)]（5）的形成，其中 N-O 键长为 1.464(2) Å，滴定研究表明在四氢呋喃中的 p K 约为 23-25。这使得人们可以为 5 中的 N-H 键推导 62-65 kcal mol 的键离解能。相应地，5 与苯氧基自由基 2,4,6- BuCHO 快速反应生成 4。这项工作证明了 PhNO 的还原结合，而无需先形成不稳定的镍（I）物种，并且在较少见的 μ-κ(O):κ(N)桥接模式中，PhNO 配体具有氧化还原非中性。PhNO 中 H-原子攫取化学的热力学数据可能有助于理解瞬态但具有生物学相关性的硝酰（HNO）配体的反应模式。
