Gold Catalysis Using Calixarene-Bound Nanoparticles

Gold catalysis represents a promising and emerging area that has been used to enhance the selectivity and activity of chemical reactions, which are often difficult or impossible to accomplish using other means. Much of gold catalysis relies on active metal nanoparticles and hinges on the use of either inorganic and/or organic modifiers.  This study aims to use calixarene-based modifiers as ligands for gold and uses these ligands as tools to elucidate mechanisms of catalysis. Because of its size and steric bulk, the calixarene does not completely block reactants' access to the surface atoms.  Instead we have shown it previously to synthesize pockets of accessibility on the metal surface, which allows reactants access.  We have also developed methods to measure the amount of accessible surface area on calixarene-bound gold nanoparticles by fluorescence previously using 2-napthalenethiol (2-NT) binding.[1]

The catalytic performance of homogeneous calixarene-modified Au nanoparticles was demonstrated using 4-nitrophenol (4-NP) hydrogenation with sodium borohydride as reductant.  Synthesis is used as a tool to enable comparative studies employing different surface ligands on the gold nanoparticles.  Au nanoparticles that have accessible surface area with bound calixarene molecules are bound with 2-NT to remove surface area that would otherwise be accessible to reactants such as 4-NP.  Even without this accessible surface area, in the combined calixarene-2-NT bound particles, the hydrogenation rates are similar to the particles with only the calixarene bound.   However, the organic ligand plays a distinct and unique role in controlling the induction time before onset of catalytic activity.  These results provide a unique perspective into the identity of the active sites of these catalysts.

References

1.    (a) Ha, J.-M., Solovyov, A., and Katz, A.  Langmuir  25, 10548 (2009). (b) de Sliva, N., Ha, J.-M., Solovyov, A., Nigra, M. M., Ogino, I., Yeh, S. W., Durkin K. A., and Katz, A. Nat. Chem. 2, 1062 (2010).