Structure and red-ox activity of Cu sites isolated in a nanoporous P4VP polymeric matrix

In the recent years metal-containing polymers are becoming very attractive owing to their possible applications as catalysts for organic synthesis. Several examples can be found in literature of functionalized microporous polystyrenes (PSs) characterized by a very low surface-area, where the reactive sites are accessible to the reactants only upon swelling by the solvent. Swelling is usually not occurring when dealing with gaseous species and this usually prevents the use of polymer-based catalysts for gas phase synthesis. This restriction can be however overcome by the use of macroporous polymeric matrices, where a permanent open texture is obtained by 20-40% cross-linking levels. The development of easy methods to immobilize organometallic species into these polymers, by maintaining in the same time their porous structure, will allow their application to catalytic reactions also in the absence of a swellable solvent. An example of this class of systems are the nitrogen-containing polymers, like poly(vinylpyridine) (PVP), which play important roles as basic catalysts, and are extensively used to generate metal complexes with transition metals. PVP has been used as a good support for immobilization of CuCl2[5] in the oxidative carbonylation of methanol to dimethylcarbonate (DMC), the oxidative coupling of 2,6-dimethylphenol, and the oxidation of tetralin.[8] In these processes, all conducted in the liquid phase, CuII is reduced to CuI and HCl is released. Even if it is usually accepted that the basic N atoms of the pyridine (Py) rings act as preferential sites for the CuII grafting and several models have been proposed in literature, there are not direct proofs of the structure of the active species during the red-ox process. In this work CuCl2 was molecularly immobilized inside a high cross-linked P4VP matrix characterized by a permanent porosity, that could allow catalysis also in the gas-phase, coupled with a detailed spectroscopic in situ investigation. The grafting procedure and the red-ox processes involving the Cu sites were investigated by means of several complementary in situ techniques (FTIR, UV-Vis, XANES and EXAFS), allowing the determination of the structure of the system in all the steps. Note that the capability to disclose the structure surrounding Cu sites has to be considered a non trivial result, due to the amorphous nature of the host matrix. The detailed knowledge of the structural changes upon red-ox reaction is a mandatory condition for the understanding of more complex catalytic processes.