1,4-Shifts in a Dinuclear Ni(I) Biarylyl Complex: A Mechanistic Study of C−H Bond Activation by Monovalent Nickel

The known aryne complex (PEt3)2Ni(η2-C6H2-4,5-F2) (1a) reacts with a catalytic amount of Br2Ni(PEt3)2 over 1% Na/Hg to afford the dinuclear Ni(I) biarylyl complex [(PEt3)2Ni]2(μ-η1:η1-3,4-F2C6H2-3‘,4‘-F2C6H2) (2a), which results from a combination of C−C bond formation and C−H bond rearrangement.
The dinuclear benzyne [(PEt3)2Ni]2(μ-η2:η2-C6H2-4,5-F2) (3) was obtained by the reaction of 1a with a
stoichiometric amount of Br2Ni(PEt3)2 over excess 1% Na/Hg, and 3 was found to catalyze the conversion
of 1a to 2a. The reaction of 1a with B(C6F5)3 produced the trinuclear complex (PEt3)3Ni3(μ3:η1:η1:η2-4,5-F2C6H2)(μ3:η1:η1:η2-4,5-F2C6H2-4‘,5‘-F2C6H2) (6). The addition of PEt3 to 6 produced 1 equiv of 1a and 1
equiv of [(PEt3)2Ni]2(μ-η1:η1-4,5-F2C6H2-4‘,5‘-F2C6H2) (7a). Both 6 and 7a were identified as intermediates
in the conversion of 1a to 2a. The analogue [(PEt3)(PMe3)Ni]2(μ-η1:η1-4,5-F2C6H2-4‘,5‘-F2C6H2) (7b) was
prepared by the addition of PMe3 to 6 and was structurally characterized. NMR spectroscopic evidence
identified the additional asymmetric biarylyl [(PEt3)2Ni]2(μ-η1:η1-4,5-F2C6H2-3‘,4‘-F2C6H2) (8a) during the
conversion of 1a to 2a. The initial observation of 2 equiv of 8a for every equivalent of 2a produced from
solutions of 7a suggests that 8a and 2a are formed from a common intermediate. A crossover labeling
experiment shows that the C−H bond rearrangement steps in the conversion of 1a to 2a occur with the
intermolecular scrambling of hydrogen and deuterium labels. The evidence collected suggests that Ni(I)
complexes are capable of activating aromatic C−H bonds.

已知的芳炔（aryne）配合物(三乙基膦(PEt₃))₂镍(Ni)(η²-C₆H₂-4,5-F₂)（标记为1a），在1%钠汞齐(Na/Hg)存在下，与催化量的Br₂Ni(PEt₃)₂反应，得到双核一价镍联芳基（biarylyl）配合物[(PEt₃)₂Ni]₂(μ-η¹:η¹-3,4-F₂C₆H₂-3’,4’-F₂C₆H₂)（标记为2a），该产物由C-C键形成与C-H键重排两步反应共同生成。
双核苯炔配合物[(PEt₃)₂Ni]₂(μ-η²:η²-C₆H₂-4,5-F₂)（标记为3）可通过1a与化学计量比的Br₂Ni(PEt₃)₂在过量1%钠汞齐下反应制备，实验证实3可催化1a向2a的转化。1a与三(五氟苯基)硼(B(C₆F₅)₃)反应则生成三核配合物(PEt₃)₃Ni₃(μ₃:η¹:η¹:η²-4,5-F₂C₆H₂)(μ₃:η¹:η¹:η²-4,5-F₂C₆H₂-4’,5’-F₂C₆H₂)（标记为6）。向6中加入三乙基膦(PEt₃)，可得到1当量的1a与1当量的[(PEt₃)₂Ni]₂(μ-η¹:η¹-4,5-F₂C₆H₂-4’,5’-F₂C₆H₂)（标记为7a）。6与7a均被鉴定为1a转化为2a过程中的关键中间体。
类似物[(PEt₃)(三甲基膦(PMe₃))Ni]₂(μ-η¹:η¹-4,5-F₂C₆H₂-4’,5’-F₂C₆H₂)（标记为7b）可通过向6中加入三甲基膦(PMe₃)制备，并完成了晶体结构表征。核磁共振光谱（NMR）分析证据显示，在1a转化为2a的过程中，还检测到了非对称联芳基配合物[(PEt₃)₂Ni]₂(μ-η¹:η¹-4,5-F₂C₆H₂-3’,4’-F₂C₆H₂)（标记为8a）。从7a的溶液中初始观测到，每生成1当量的2a，就会伴随2当量的8a生成，这表明8a与2a源自共同的反应中间体。交叉标记实验(crossover labeling experiment)结果表明，1a转化为2a过程中的C-H键重排步骤，存在氢与氘标记物的分子间同位素混排。综上所有实验证据，一价镍配合物能够实现芳香族C-H键的活化。