Pd- and Au-Induced Circular and Fibrous Polymer Gelation via Thiocarbonyl Groups and High Pd Catalyst Activity

Self-assembled gelation behavior of a hydrophilic polymer bearing a metal-coordination unit with metal ions (Pd-II or Au-III) was investigated upon addition of a dispersed aqueous solution of the polymer to an aqueous solution of metal ions. Self-assembled gels with different morphologies were formed depending on the metal ions used. Spherical and fibrous gels were formed by gelation with Pd-II and Au-III ions, respectively. The mechanism for formation of the different gel morphology was examined by measuring cross-linking rates, examining coordination sites, and determining mechanical properties. The Pd-II self-assembled gels efficiently catalyzed the Mizoroki-Heck cross-coupling reaction of a variety of aryl halides and olefins with a turnover number (TON) and turnover frequency (TOF) of greater than 2,762,000 and 138,100 h(-1), respectively. These are the greatest TON and TOF values for heterogeneous polymeric catalysts of the Mizoroki-Heck reaction. The Au-III self-assembled fibrous gels, which had nano-order diameter, contained uniformly dispersed gold nanoparticles and could be synthesized by a facile method, i.e., the dropwise addition of a dispersed aqueous solution of the polymer to an aqueous solution of Au-III ion.