Molecular and Silica-Supported Mo and W d0 Imido-Methoxybenzylidene Complexes: Structure and Metathesis Activity

5-Coordinated methoxybenzylidene complexes M(=NAr)(=CH-C6H4-o-OMe)((OBuF3)-Bu-t)(2) (Ar=2,6-(Pr2C6H3)-Pr-i; Bu-t(F3)=CMe2(CF3)) of Mo (1m(Mo)) and W (1m(W)) were synthesized by cross-metathesis from the corresponding neophylidene/neopentylidene precursors and o-methoxystyrene. 1m(Mo) and 1m(W) were grafted onto the surface of silica partially dehydroxylated at 700 degrees C to give well-defined silica-supported alkylidenes (equivalent to SiO)M(=NAr)(=CH-C6H4-o-OMe)((OBuF3)-Bu-t) (M=Mo (1(Mo)), W (1(W))). Supported methoxybenzylidene complexes were tested in metathesis of cis-4-nonene, 1-nonene, and ethyl oleate, and compared to their molecular precursors and supported classical analogs (equivalent to SiO)M(=NAr)(=CHCMe2R)((OBuF3)-Bu-t) (M=Mo, R=Ph (2(Mo)), M=W, R=Me (2(W))). Both grafted complexes 1(Mo) and 1(W) show significantly better performance as compared to their molecular precursors 1m(Mo) and 1m(W) but are less efficient than the classical 4-coordinated alkylidenes 2(Mo) and 2(W). Noteworthy, both 1(Mo) and 1(W) can reach equilibrium conversion in metathesis of cis-4-nonene at catalyst loadings as low as 50 ppm.