Intermolecular [2 + 2 + 2] Cycloaddition Reactions of Alkynes and Alkenes Mediated by Cobalt: X-ray Crystal Structures of Two Isomeric (η⁵-Cyclopentadienyl)(η⁴-1,3-cyclohexadiene)cobalt Complexes

Treatment of bicyclic alkenes (norbornylene, endo-dicyclopentadiene) with alkynes (diphenylacetylene, dimethyl acetylenedicarboxylate) in the presence of CpCo(CO)₂produced CpCo(η⁴-1,3-cyclohexadiene) complexes (1, 2, 6, 7, 10, 11) in good yields. Oxidation of complexes 1, 2, 6, 7, 10, and 11 using CuCl₂·²H₂O afforded the free diene ligands 4, 8, and 13. Further oxidation of 4 and 8 using ceric ammonium nitrate produced the aromatic derivatives 5 and 9. The isomeric complexes 1 and 2, which were obtained from endo-dicyclopentadiene and diphenylacetylene, were characterized by two-dimensional NMR spectroscopic techniques as well as X-ray crystallography. Complex 1 isomerized to 2 at 154 °C in a first-order process with an observed rate constant of (1.5 ± 0.1) × 10^-5 s^-1. Treatment of norbornylene with diphenylacetylene in the presence of (η⁵-C₅H₄CH₃)Co(CO)₂produced two isomeric (η⁵-C₅H₄CH₃)Co(η⁴-1,3-cyclohexadiene) complexes (14, 15). A 1:1 mixture of 1 and 14 isomerized intramolecularly to 2 and 15 in xylene-d₁₀ at 155 °C. Both 1 and 2 are triclinic, P1̅; 1 has unit cell parameters a = 10.943 (2) Å, b = 11.439 (1) Å, c = 14.954 (6) Å, α = 71.94 (3)°, β = 87.00 (3)°, γ = 62.27 (1)°, and D(calcd) = 1.30 g cm^-3for Z = 2. These values for 2 are a = 10.752 (1) Å, b = 12.640 (3) Å, c = 13.754 (3) Å, α = 69.89 (3)°, β= 66.83 (1)°, γ = 68.38 (1)°, and D(calcd) = 1.31 g cm^-3for Z = 2. Both were refined via least-squares, 1 to a conventional R value of 0.041 based on 4605 independent observed reflections ([F_o≥ 5σ(F_o)] and 2 to 0.041 utilizing 3530 reflections. Each cobalt atom is η⁵-coordinated to the cyclopentadienyl ligand (Co-Cp(av) = 2.09 (2) Å for 1 and 2.08 (3) Å for 2) and η⁴-bonded to the 1,3-cyclohexadiene ligand. The two exterior Co-C distances average 2.08 (3) Å in 1 and 2.07 (2) Å in 2, whereas the two interior Co-C values average 1.97 (1) and 1.984 (8) Å, respectively.