Importance of Ligand Fine-Tuning: Alkyne Metathesis with Molybdenum Alkylidyne Complexes Supported by Phenyl-tert-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 kappa 2 O,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-n Ph n }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).