Allylidene Complexes from Tungsten Alkylidynes and Allyl Bromides: Structural Comparison and Dynamic behavior

The complexes [W(CPhCHCHR)Br₂ (CO)₂ (NC₅H₄Me-4)], 2a (R = H) and 2b (R = Me), and [W(CPhCHCHMe)Br₂ (CO)₂ (NC₅H₄NMe₂-4)], 2c, were prepared by reaction of [W (CPh) Br (CO)₂ (NC₅H₄Me-4)₂] and [W (CPh) Br (CO)₂ (NC₅H₄NMe₂-4)₂] with allyl or 2-methylallyl bromide in 18–50% yield. The solid–state structures of all three compounds were determined by X-ray crystallography. Crystal data for 2a: Space group P2₁, a = 7.549(3) Å,b = 12.648(5) Å, c = 9.931(4) Å, β = 90.65(3)°, Z = 2, 1746 unique observed data, F > 3π(F), R = 0.036,R_w = 0.037. Crystal data for 2b: Space group P2₁/c, a = 8.352(2) Å, b = 14.529(4) Å,c = 18.209(6) Å,β = 111.45(2)°, Z = 4, 4048 unique observed data, F > 3σ(P), R = 0.051, R_w = 0.039. Crystal data for 2c; Space group [formula omitted] (No. 2), a = 7.109(2) Å, b = 10.816(7) Å, c = 14.942(5) Å, α = 76.62(4)°, β = 88.20(3)°, γ = 71.01(3)°, Z = 2, 3123 unique observed data, I > 3σ, R = 0.044, R_w = 0.039. All three complexes displayed fluxional behavior in their variable-temperature NMR spectra. The detailed molecular motions associated with the solution dynamic processes are deduced from a comparison of the solid-state structures.