Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

International audienceThe reaction of the molybdenum alkylidyne complex [MesC=Mo{OC(CF3)(3)}(3)] (MoF9, Mes = 2,4,6-trimethylphenyl) with the potassium siloxides KOSi(OtBu)(3) and KOSi(OtBu)(2) (OMes) furnished the mixed fluoroalkoxy-siloxy alkylidyne complexes [MesC Mo{OC(CF3)(3)}(2) {OSi(OtBu)(3)}] (MoSiF9) and [MesC Mo{OC(CF3)(3)}(2) {OSi(OtBu)(2) (OMes)}] (MoSi*F9). A treatment of MoF9, MoSiF9, and MoSi*F9 with an excess of 3-hexyne (EtC CEt) afforded labile metallacyclobutadiene (MCBD) complexes with a (C3Et3)Mo core, which are in equilibrium with the corresponding propylidyne (EtC Mo) complexes in solution. Thermodynamic parameters for these [2 + 2]-cycloaddition/cycloreversion reactions were determined by van't Hoff plots, revealing that the nature of the ancillary siloxide ligand exerts a significant effect on the MCBD stability. An X-ray diffraction analysis of MoSi*F9-MCBD provided one of the first accurate crystal structures of a molybdenacyclobutadiene. MoF9, MoSiF9, and MoSi*F9 proved active catalysts for the metathesis of internal alkynes and diynes, with MoSi*F9 showing unprecendented selectivity in the conversion of sterically encumbered 1,3-pentadiynes into symmetrical 1,3,S-triynes and 2-butyne