Kinetics, Thermodynamics, and Effect of BPh3on Competitive C−C and C−H Bond Activation Reactions in the Interconversion of Allyl Cyanide by [Ni(dippe)]

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Abstract
Reaction of [(dippe)Ni(μ-H)]2 with allyl cyanide at low temperature quantitatively generates the η2-olefin complex (dippe)Ni(CH2CHCH2CN) (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.