The design of conducting polymers with metal binding sites

Abstract

This study is concerned with the synthesis of conjugated polyheterocycles with potential metal binding sites for applications in sensors, catalysis and electronics. The first synthetic approach to polyheterocycles was based on the interfacial polycondensation of a dihydrazide derivative of pyridine with a diacid chloride to produce a precursor polymer. It was shown, however, the starting materials could not be easily prepared in high yield. Model studies confirmed the feasibility of the route but these studies also suggested that the precursor polymers were unlikely to be very soluble. The second precursor route explored began with the preparation of 2,6-diethynylpyridine. The intermolecular Glaser coupling of the ethyne groups afforded the precursor polymer, poly((2,6'-pyridyl)but-l,3-diyne), as a black powder which was insufficiently soluble to allow conversion to the poly heterocycles. A series of dimers and trimers containing various combinations of 2-furyl, 2-thienyl and 2-pyridyl moieties were prepared using two different coupling procedures that yielded compounds with the required 2,2'-heteroatom arrangement as required for metal binding. Some of these monomers were electropolymerised and the metal binding properties of these polymers was investigated by cychc voltammetry. In particular, the two trimers: 2,5-di-(2-thienyl) pyridine; and 2,6-di-(2-thienyl) pyridine showed potential metal coordination despite their hydrophobic nature and impermeability towards metal complexes. Evidence was presented to suggest that these polymers are protonated during the electropolymerisation reaction. X-ray analysis of the 2,5-di-(2-thienyl) pyridine showed that only the 2,2'-linked thiophene was coplanar with the pyridine due to a charge transfer interaction. This interaction insures that S and N atoms have a planar syn arrangement conducive to metal binding. Several oligothiophenes were prepared to investigate methods for enhancing the solubility of polyheterocycles. The knowledge gained from these investigations was used to prepare a series of regiochemically well-defined poly((3-alkyl)thiophenes). The regularity of these polymers was confirmed by NMR analysis. Related monomers were prepared containing the necessary solubilising alkyl groups as well as phenyl groups designed to act as acceptor ligands for the low-valent transition metals such as ruthenium(II). The electrochemistry of these novel thiophene monomers is reported.