Patterned Grafted Lewis-Acid Sites on Surfaces: Olefin Epoxidation Catalysis Using Tetrameric Ti(IV)-Calix[4]arene Complexes

Tetrameric Ti(IV)-calix[4]arene complexes were synthesized and characterized as well as grafted on a hydroxylated SiO2 inorganic support as an example of patterned Lewis-acid sites on an inorganic oxide surface. These complexes consist of a novel calixarene organic ligand with varying lengths of tethers to a central aromatic core, and with the calixarene coordinating four titanium(IV)-cations via tetrahedral recognition on the lower rim. Catalysis of these materials was investigated with a probe reaction consisting of the epoxidation 1-octene with tert-butylhydroperoxide as the oxidant, and a comparison of their hydrolytic stability was performed. The oligomeric calix[4]arenes showed similar behavior in catalysis to the monomeric control, with the exception of the material with the shortest tether length, which shows a 1.3-fold higher activity that may be due to a modest cooperativity effect. In hydrolytic leaching tests, the oligomeric complexes showed higher stability compared to the monomeric complex, and this stability appeared to be more thermodynamic rather than kinetic in nature. We hypothesize that encapsulatation of the tetrameric active site within a silica mesopore of the support contributes to this stability.