Controlling oxidation states of noble-metal nanocatalysts and their structural characterization for catalytic organic reactions

Abstract

In the pharmaceutical and fine chemical industry, one of the most interesting point is to convert homogeneous catalytic reactions to heterogeneous catalytic systems. Due to Heterogeneous systems providing many advantages, some of which are not revealed by their homogeneous systems, such as convenience of separation from the reaction mixture, recyclability, sustainability, stability and useful in continuous flow catalytic systems. It is highly valuable to develop new systems that combine the many advantages of heterogeneous catalysis with the versatile application of homogeneous catalysts. At present, homogeneous catalytic processes have been employed in various fields and show high activity and selectivity in C-C coupling and C-H activation reactions. However, the lack of reuseability and difficulty of saparation of the catalysts from products are major limitation to carry out mass production of wide range of chemicals. Over the past few decades, various efforts have been focused on the heterogenization of homogeneous catalysts in fine chemicals and pharmaceutical industries. The development of novel synthetic methods and morphology control of nanoscale metal particles has attracted a great deal of attention owing to their unique catalytic properties with a variety of applications. However, catalytic activities of fine chemical production via heterogeneous pathways have often been limited as a result of zero oxidation states of metal surface comparing the possibility of the various oxidation states of homogeneous metal catalysts. Apparently, controlling the oxidation states would widen the applicability of metal nanoparticles as highly active catalysts in various reactions. In chapter I, we present a comprehensive introduction of catalytic organic reactions using heterogeneous noble metal nanocatalyst and recent investigations. In chapter II, we offer Pd NPs catalyzed C-H functionalization by controlling oxidation states via organic oxidants such as succinimide and hyperiodine derivertives. Pd(IV) nanoparticles by an oxidant, PhICl2, can promote the direct C-H halogenation reaction of bezoquinoline with outstanding activity comparing to the homogeneous Pd catalysts. Also, Pd(II) nanoparticles generated by the Ph2IBF4 treatment exhibit high activity for direct C-H arylation reaction of indole, benzofuran, benzothiophene and naphthalene. The oxidized Pd nano-catalysts were precisely characterized by TEM, XPS, XAFS and zeta potential mesearments. In chapter III, we introduce Rh(0)-Rh(III) core-shell nanodendrite hybrid catalysts, which were generated by the oxidation with NBS for cyclic addition reaction of carbon dioxide. In addition, the mechanisms of catalytic reaction pathways using oxidized Rh species were demonstrated using various characterization tools.