Importance of Ligand Fine-Tuning: Alkyne Metathesis with Molybdenum Alkylidyne Complexes Supported by Phenyl-<i>tert-</i>butoxysilanolate Ligands

The phenyl-tert-butoxysilanols Ph(tBuO)2SiOH and Ph2(tBuO)SiOH were prepared and used for the synthesis of the molybdenum 2,4,6-trimethylbenzylidyne complexes [MesC≡Mo{OSi(OtBu)2Ph}3] (1) and [MesC≡Mo{OSi(OtBu)Ph2}3] (2) from mer-[MesC≡MoBr3(dme)] (dme = dimethoxyethane, Mes = 2,4,6-trimethylphenyl). The molecular structures of 1 and 2 were determined by X-ray diffraction analysis, revealing a secondary molybdenum–oxygen interaction and a chelating κ2O,O′ coordination mode of one of the siloxide ligands in the case of 1. Treatment of 1 and 2 with an excess of 3-hexyne (EtC≡CEt) afforded the corresponding propylidyne (EtC≡Mo) complexes, which are in equilibrium with labile metallacyclobutadiene complexes as evidenced by variable-temperature NMR spectroscopy. Single crystals of [(Et3C3)Mo{OSi(OtBu)Ph2}3] (2-MCBD) have been isolated at low temperature, providing a rare example of a crystallographically characterized molybdenacyclobutadiene. The alkylidyne complexes 1 (n = 1) and 2 (n = 2) complete the series of available alkyne metathesis (pre-)catalysts [RC≡Mo{OSi(OtBu)3–nPhn}3] (n = 0–3), and their catalytic performance in the metathesis of internal and terminal alkynes was investigated and compared, if available, with the previous members of this series (n = 0, 3).