Importance of Ligand Fine-Tuning: Alkyne Metathesis with Molybdenum Alkylidyne Complexes Supported by Phenyl-<i>tert-</i>butoxysilanolate Ligands
Angelika Neitzel, Tobias Ludwig, Dirk Bockfeld, Thomas Bannenberg, Matthias Tamm
Abstract
High Resolution Image Download MS PowerPoint Slide The phenyl- tert -butoxysilanols Ph( t BuO) 2 SiOH and Ph 2 ( t BuO)SiOH were prepared and used for the synthesis of the molybdenum 2,4,6-trimethylbenzylidyne complexes [MesC≡Mo{OSi(O t Bu) 2 Ph} 3 ] ( 1 ) and [MesC≡Mo{OSi(O t Bu)Ph 2 } 3 ] ( 2 ) from mer- [MesC≡MoBr 3 (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 κ 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 [(Et 3 C 3 )Mo{OSi(O t Bu)Ph 2 } 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(O t Bu) 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).