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Competing H2 versus Intramolecular C-H Activation at a Dinuclear Nickel Complex via Metal-Metal Cooperative Oxidative Addition
Journal of the American Chemical Society ( IF 15.0 ) Pub Date : 2020-03-12 , DOI: 10.1021/jacs.0c00758
Hendrik Stevens 1 , Peng-Cheng Duan 1 , Sebastian Dechert 1 , Franc Meyer 1
Affiliation  

Nickel(I) metalloradicals bear great potential for the reductive activation of challenging substrates but are often too unstable to be isolated. Similar chemistry may be enabled by nickel(II) hydrides that store the reducing equivalents in hydride bonds and reductively eliminate H2 upon substrate binding. Here we present a pyrazolate-based bis(-diketiminato) ligand [LPh]3- with bulky m-terphenyl substituents that can host two Ni-H units in close proximity. Complexes [LPh(NiII-H)2]- (3) are prone to intramolecular reductive H2 elimination, and an equilibrium between 3 and orthometallated dinickel(II) monohydride complexes 2 is evi-denced. 2 is shown to form via intramolecular metal-metal cooperative phenyl group C(sp2)-H oxidative addition to the dinickel(I) intermedi-ate [LPhNiI2]- (4); while NiI species have been implicated in catalytic C-H functionalization, discrete activation of C-H bonds at NiI complexes has rarely been described. The reversible H2 and C-H reductive elimination/oxidative addition equilibrium smoothly unmasks the powerful 2-electron reductant 4 from either 2 or 3, which is demonstrated by reaction with benzaldehyde. A dramatic cation effect is observed for the rate of interconversion of 2 and 3, and also for subsequent thermally driven formation of a twice orthometallated dinickel(II) complex 6. X-ray crystallographic and NMR titration studies indicate distinct interaction of the Lewis acidic cation with 2 and 3. The present system allows for unmasking of a highly reactive [LPhNiI2]- intermediate 4 either via elimination of H2 from dihydride 3 or via reductive C-H elimination from monohydride 2. The latter does not release any H2 byproduct and adds a distinct platform for metal-metal cooperative two-electron substrate reductions while circumventing the isolation of any unstable superreduced form of the bimetallic scaffold.

中文翻译:

通过金属-金属协同氧化加成在双核镍配合物中竞争 H2 与分子内 CH 活化

镍 (I) 金属基在还原活化具有挑战性的底物方面具有巨大潜力,但通常太不稳定而无法分离。类似的化学反应可以通过镍 (II) 氢化物实现,镍 (II) 氢化物将还原当量储存在氢化物键中,并在底物结合时还原性地消除 H2。在这里,我们提出了一种基于吡唑酯的双(-二酮亚氨基)配体 [LPh]3- 具有庞大的间三联苯取代基,可以在附近容纳两个 Ni-H 单元。配合物 [LPh(NiIII-H)2]-(3) 易于发生分子内还原性 H2 消除,并且证明了 3 与正金属化二镍 (II) 一氢化物配合物 2 之间的平衡。图2显示通过分子内金属-金属协同苯基C(sp2)-H氧化加成到二镍(I)中间体[LPhNiI2]-(4)形成;虽然 NiI 物质与催化 CH 官能化有关,但很少描述 NiI 复合物中 CH 键的离散激活。可逆的 H2 和 CH 还原消除/氧化加成平衡顺利地从 2 或 3 中揭示了强大的 2 电子还原剂 4,这通过与苯甲醛的反应得到证明。观察到 2 和 3 的相互转化率以及随后热驱动形成两次正金属化二镍 (II) 配合物 6 的显着阳离子效应。 X 射线晶体学和 NMR 滴定研究表明路易斯酸性阳离子的明显相互作用使用 2 和 3。本系统允许通过从二氢化物 3 中消除 H2 或通过从一氢化物 2 中还原 CH 消除来揭开高反应性 [LPhNiI2]-中间体 4。
更新日期:2020-03-12
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