Formation of Pd-nanoparticles within the pores of ring opening metathesis polymerization-derived polymeric monoliths for use in organometallic catalysis

Polymeric monolithic supports displaying micro-, meso- and macroporosity have been prepared via ring opening metathesis polymerization (ROMP) from (Z)-9-oxabicyclo[6.1.0]non-4-ene (OBN) and tris(cyclooct-4-en-1-yloxy)methylsilane (CL) by the use of the 2(nd)-generation Grubbs-initiator RuCl2(Py)(2)(IMesH(2))(CHPh) 1 in the presence of a phase separation-enforcing macroporogen, i.e. 2-propanol, and a microporogen, i.e. toluene. The porous supports were then subject to pore size-selective functionalization by hydrolyzing the epoxy-groups within pores >6 nm to the corresponding vic-diol with the aid of poly(styrenesulfonic acid) (PS-SO3H). The remaining epoxide moieties located within the small pores (diameter <6 nm) were then reacted with N,N-dipyrid-2-ylamine to yield the corresponding dipyrid-2-ylamin-functionalized monolithic supports. Typical loadings with functional monomer were in the range of 6-7 mu mol/g. These chelating ligands located within the small pores were then used for the immobilization of Pd-II. Quantitative metal loadings (approx. 1 mg/g) could be accomplished. Reduction of the Pd-II with NaBH4 resulted in the formation of Pd nanoparticles <4 nm in diameter that were still exclusively located within the small pores. These Pd-loaded monoliths were used in a series of Sonogashira-Hagihara and Suzuki-type couplings. Scanning electron microscopy measurements on the metal loaded monoliths prior to the catalytic reactions revealed that the metal nanoparticles that formed were immobilized within the small pores. These findings are supported by the comparably low metal content in the coupling products. Thus, metal leaching was <0.13 %.