A Cationic Ruthenium Olefin Metathesis Catalyst Supported on Weakly Coordinating Silica

Silica functionalized with Lewis acidic silylium aluminate ion-pairs ([iPr3Si][(RFO)3Al–OSi≡] (RF = C(CF3)3)) abstract chloride from the second-generation Grubbs-Hoveyda catalyst (SIMes)Cl2Ru=CH(o-iPrO-C6H4) (GH-II) to form supported [(SIMes)ClRu=CH(o-iPrO-C6H4)][(RFO)3Al–OSi≡] (1) and iPr3SiCl. Solid-state NMR measurements at 100 K, desorption of Ru from the support, and X-ray adsorption spectroscopy (XAS) are consistent with formation of a monomeric Ru-benzylidene cation that is electrostatically immobilized on the modified silica surface. 1 is active and selective in ring closing metathesis (RCM), self-metathesis, ring opening metathesis polymerization (ROMP), acyclic diene metathesis (ADMET), cross metathesis, and ethenolysis reactions. 1 is generally more active than cationic [(SIMes)ClRu=CH(o-iPrO-C6H4)]2[BArF4]2 (GH-II+; BArF4 = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) in these reactions and gives higher yields with higher selectivity than neutral GH-II under equivalent conditions. DFT calculations comparing models of GH-II+ and 1 suggest that cationic Ru=CH2+ is stabilized by interactions with the anion and accounts for the increase in reaction rates observed using 1 compared to GH-II+.