Quantitatively Analyzing Metathesis Catalyst Activity and Structural Features in Silica-Supported Tungsten Imido-Alkylidene Complexes

A broad series of fully characterized, well-defined silica-supported W metathesis catalysts with the general formula [( SiO)W(=NAr)(=CHCMe2R)(X)] (Ar = 2,6-iPr(2)C(6)H(3) (Ar-1Pr) 2,6-Cl2C6H3 (Ar-Cl), 2-CF3C6H4 (Ar-CF3), and C6F5 (AT(F5)); X = OC(CF3)(3) (OtBu(F9)); OCMe(CF3)(2) (OtBU(F6)); OtBu, OSi(OtBu)(3), 2,5-dimethylpyrrolyl (Me(2)Pyr) and R = Me or Ph) was prepared by grafting bis-X substituted complexes [W(NAr)(=CHCMe2R)(X)(2)] on silica partially dehydroxylated at 700 degrees C (SiO2(700)), and their activity was evaluated with the goal to obtain detailed structure activity relationship. Quantitative influence of the ligand set on the activity (turnover frequency, TOP) in self-metathesis of cis-4-nonene was investigated using multivariate linear regression analysis tools. The TOF of these catalysts (activity) can be well predicted from simple steric and electronic parameters of the parent protonated ligands; it is described by the mutual contribution of the NBO charge of the nitrogen or the IR intensity of the symmetric N-H stretch of the ArNH2, corresponding to the imido ligand, together with the Sterimol B-5 and pK(a) of HX, representing the X ligancl. This quantitative and predictive structure-activity relationship analysis of well-defined heterogeneous catalysts shows that high activity is associated with the combination of X and NAr ligands of opposite electronic character and paves the way toward rational development of metathesis catalysts.