Enhanced Hydrogen-Transfer Catalytic Activity of Iridium N-Heterocyclic Carbenes by Covalent Attachment on Carbon Nanotubes

Oxidized multiwall carbon nanotubes (CNT) were covalently modified with appropriate hydroxyl-ending imidazolium salts using their carboxylic acid groups. Characterization of the imidazolium-modified samples through typical solid characterization techniques, such as TGA or XPS, allows for the determination of 16 wt % in CNT-1 and 31 wt % in CNT-2 as the amount of the imidazolic fragments in the carbon nanotubes. The imidazolium-functionalized materials were used to prepare nanohybrid materials containing iridium N-heterocyclic carbene (NHC)-type organometallic complexes with efficiencies as high as 95%. The nanotube-supported iridium-NHC materials were active in the heterogeneous iridium-catalyzed hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source. The iridium hybrid materials are more efficient than related homogeneous catalysts based on acetoxy-functionalized Ir-NHC complexes with initial TOFs up to 5550 h(-1). A good recyclability of the catalysts, without any loss of activity, and stability in air was observed.