Grafted Metallocalixarenes as Single-Site Surface\nOrganometallic Catalysts

Abstract

Metallocalixarenes were grafted onto silica using a surface organometallic approach and shown\nto be active and selective catalysts for epoxidation of alkenes using organic hydroperoxides. Calixarene−Ti^IV precursors were anchored at surface densities from 0.1 to near-monolayer coverages (0.025−0.25\ncalixarene nm^-2). Several spectroscopic methods independently detected calixarene−Ti^IV connectivity before\nand after epoxidation catalysis. Kinetic analyses of cyclohexene epoxidation confirmed that the active sites\nwere anchored on the silica surface and were significantly more active than their homogeneous analogues.\nThe steric bulk and multidentate binding of the calixarenes led to structural stability and to single-site behavior\nduring epoxidation catalysis. Rate constants were independent of surface density for cyclohexene\nepoxidation with <i>tert</i>-butyl hydroperoxide (11.1 ± 0.3 M^-2 s^-1) or cumene hydroperoxide (25 ± 2 M^-2 s^-1).\nThe materials and methods reported here allow the assembly of robust surface organometallic structures\nin which the active sites behave as isolated species, even near saturation monolayer coverages. In turn,\nthis makes possible the rational design and synthesis of a class of heterogeneous oxide catalysts with\natomic-scale precision at the active site.