Molecular heterogeneous catalysts derived from bipyridine-based organosilica nanotubes for C-H bond activation
Heterogeneous metal complex catalysts for direct C−H activation with high activity and durability have been ever desired for transforming raw materials into feedstock chemicals. This paper describes the design and synthesis of one-dimensional organosilica nanotubes containing 2,2'-bipyridine (bpy) ligands in the frameworks (BPy-NT) and their postsynthetic metalation to provide highly active and robust molecular heterogeneous catalysts. Through adjusting the ratios of organosilane precursors, the very short BPy-NT with ~50 nm in length could be controllably obtained. The postsynthetic metalation of bipyridine-functionalized nanotubes respectively with [IrCp*Cl(µ-Cl)]2 (Cp* = η5-pentamethylcyclopentadienyl) and [Ir(cod)(OMe)]2 (cod = 1,5-cyclooctadiene) afforded solid catalysts IrCp*-BPy-NT and Ir(cod)-BPy-NT, which were applied for C−H oxidation of heterocycles and cycloalkanes as well as C-H borylation of arenes. The cut-shorted nanotube catalysts displayed enhanced activities and durability compared to the analogous homogeneous ones and other conventional heterogeneous catalysts, benefiting from the isolated active sites as well as the fast transport of substrates and products. A detailed characterization for Ir-immobilized BPy-NT after the reactions by TEM, SEM, nitrogen adsorption, UV/vis, XPS and 13C CP MAS NMR indicates the molecular nature of the active species as well as the quite stable structure of nanotube scaffolds. This work demonstrates the great potential of BPy-NT with short length as an integration platform for the construction of efficient heterogeneous catalytic systems of organic transformations.