Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF₃)_nMe_3–n (n = 0–3)

The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesCM­{OC­(CF₃)_nMe_3–n}₃] (M = Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M = W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared by the reaction of the tribromides [MesCMBr₃(dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC­(CF₃)_nMe_3–n. The molecular structures of all complexes were established by X-ray diffraction analysis. The catalytic activity of the resulting alkylidyne complexes in the homometathesis and ring-closing alkyne metathesis of internal and terminal alkynes was studied, revealing a strong dependency on the fluorine content of the alkoxide ligand. The different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne. Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C₃Et₃)­Mo­{OC­(CF₃)₃}₃], which was characterized spectroscopically.