Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF3)nMe3–n (n = 0–3)

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The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesC≡M{OC(CF 3 ) n Me 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≡MBr 3 (dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC(CF 3 ) n Me 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 [(C 3 Et 3 )Mo{OC(CF 3 ) 3 } 3 ], which was characterized spectroscopically.