Kinetics, Thermodynamics, and Effect of BPh₃ on Competitive C−C and C−H Bond Activation Reactions in the Interconversion of Allyl Cyanide by [Ni(dippe)]

Reaction of [(dippe)Ni(μ-H)]2 with allyl cyanide at low temperature quantitatively generates the η2-olefin complex (dippe)Ni(CH2CHCH2CN) (1). At ambient temperature or above, the olefin complex is converted to a mixture of C−CN cleavage product (dippe)Ni(η3-allyl)(CN) (3) and the olefin-isomerization products (dippe)Ni(η2-crotonitrile) (cis- and trans-2), which form via C−H activation. The latter are the exclusive products at longer reaction times, indicating that C−CN cleavage is reversible and the crotononitrile complexes 2 are more thermodynamically stable than η3-allyl species 3. The kinetics of this reaction have been followed as a function of temperature, and rate constants have been extracted by modeling of the reaction. The rate constants for C−CN bond formation (the reverse of C−CN cleavage) show a stronger temperature dependence than those for C−CN and C−H activation, making the observed distribution of C−H versus C−CN cleavage products strongly temperature-dependent. The activation parameters for the C−CN formation step are also quite distinct from those of the C−CN and C−H cleavage steps (larger ΔH⧧ and positive ΔS⧧). Addition of the Lewis acid BPh3 to 1 at low temperature yields exclusively the C−CN activation product (dippe)Ni(η3-allyl)(CNBPh3) (4). Independently prepared (dippe)Ni(crotononitrile-BPh3) (cis- and trans-7) does not interconvert with 4, indicating that 4 is the kinetic product of the BPh3-mediated reaction. On standing in solution at ambient temperature, 4 decomposes slowly to complex 5, with structure [(dippe)Ni(η3-allyl)(N⋮C−BPh3), while addition of a second equivalent of BPh3 immediately produces [(dippe)Ni(η3-allyl)]+[Ph3BC⋮NBPh3]- (6). Comparison of the barriers to π−σ allyl interconversion (determined via dynamic 1H NMR spectroscopy) for all of the η3-allyl complexes reveals that axial cyanide ligands facilitate π−σ interconversion by moving into the P2Ni square plane when the allyl group is σ-bound.二(μ-氢)双[1,2-双(二异丙基膦基)乙烷(dippe)合镍]二聚体在低温下与烯丙基氰发生反应，以定量收率生成η²-烯烃配合物(dippe)Ni(CH₂=CHCH₂CN)（1）。在室温及更高温度下，该烯烃配合物会转化为C-CN键断裂产物(dippe)Ni(η³-烯丙基)(CN)（3）与烯烃异构化产物(dippe)Ni(η²-巴豆腈)（顺式和反式2）的混合物，后者通过C-H活化过程生成。延长反应时间后，体系仅会生成上述异构化产物，这表明C-CN键断裂是可逆过程，且巴豆腈配合物2的热力学稳定性高于η³-烯丙基物种3。本研究对该反应的动力学进行了温度依赖性研究，并通过反应建模提取了速率常数。C-CN键形成（即C-CN键断裂的逆反应）的速率常数相较于C-CN键断裂与C-H活化的速率常数，表现出更强的温度依赖性，因此观测到的C-H活化与C-CN键断裂产物的分布具有显著的温度依赖性。C-CN键形成步骤的活化参数（更大的活化焓ΔH⧧与正的活化熵ΔS⧧）也与C-CN键断裂和C-H活化步骤的活化参数存在显著差异。在低温下向配合物1中加入路易斯酸三苯基硼(BPh₃)，仅会生成C-CN键活化产物(dippe)Ni(η³-烯丙基)(CNBPh₃)（4）。独立制备的(dippe)Ni(巴豆腈-BPh₃)（顺式和反式7）无法与配合物4发生相互转化，这表明配合物4是BPh₃介导反应的动力学产物。配合物4在室温溶液中静置时，会缓慢分解为结构为[(dippe)Ni(η³-烯丙基)(N≡C−BPh₃)]的配合物5；若向体系中加入第二当量的BPh₃，则会立即生成[(dippe)Ni(η³-烯丙基)]⁺[Ph₃BC≡NBPh₃]⁻（6）。通过动态¹H核磁共振波谱法测定所有η³-烯丙基配合物的π-σ烯丙基互变能垒，结果表明：当烯丙基以σ键配位时，轴向氰基配体可通过迁移至P₂Ni平面正方形配位平面来促进π-σ互变过程。