Reaction of a Zirconocene–Carboryne Complex with Pyridines: Ligand C–H Activation

Reactions of Cp₂Zr(μ-Cl)(μ-C₂B₁₀H₁₀)Li(OEt₂)₂ (1) with various N-heterocycles derived from pyridine were studied. Treatment of 1 with pyridine, 2-bromopyridine, 2,4-lutidine, quinoline, and 2-(1-hexynyl)pyridine generated α-C–H activation (σ-bond metathesis) products Cp₂Zr(η²-C,N-C₅H₄N)(σ-C₂B₁₀H₁₁) (2), Cp₂Zr[η²-C,N-(6-Br-C₅H₃N)](σ-C₂B₁₀H₁₁) (3), Cp₂Zr[η²-C,N-(4,6-Me₂-C₅H₂N)](σ-C₂B₁₀H₁₁) (4), Cp₂Zr(η²-C,N-C₉H₆N)(σ-C₂B₁₀H₁₁) (5), and Cp₂Zr{η²-C,N-[6-(ⁿBuCC)-C₅H₃N]}(σ-C₂B₁₀H₁₁) (7), respectively. On the other hand, reaction of 1 with acridine gave the addition product 1,2-[Cp₂Zr(10,9-C₁₃H₉N)]-1,2-C₂B₁₀H₁₀ (6) in 85% isolated yield. Complex 1 reacted with 3-(1-hexynyl)pyridine to afford α-C–H activation species Cp₂Zr{η²-C,N-[5-(ⁿBuCC)C₅H₃N]}(σ-C₂B₁₀H₁₁) (8a) and Cp₂Zr{η²-C,N-[3-(ⁿBuCC)C₅H₃N]}(σ-C₂B₁₀H₁₁) (8b) in a molar ratio of 42:58, as determined by the ¹H NMR spectrum. In the presence of CuI, however, the CC insertion products zirconacyclopentenes 1,2-[Cp₂ZrC(2-C₅H₄N)CR]-1,2-C₂B₁₀H₁₀ [R = Buⁿ (9), Ph (10)] were obtained in 74–77% yields. It is suggested that the coordination of pyridine to the Zr atom is crucial for α-C–H activation (σ-bond metathesis). The presence of CuI can alter the reaction path by preventing the coordination of pyridine to the Zr atom, which blocks the α-C–H activation path, leading to the alkyne insertion reaction. All complexes were characterized by ¹H, ¹³C, and ¹¹B NMR spectra as well as elemental analyses. Their structures were further confirmed by single-crystal X-ray analyses.