Enantiomerically Pure Chiral Coordination Polymers:  Synthesis, Spectroscopy, and Electrochemistry in Solution and on Surfaces

The two enantiomerically pure bridging ligands (±)-[ctpy-x-ctpy] have been prepared employing a two-fold stereospecific alkylation reaction of the enantiomerically pure, chiral terpyridyl-type ligands (±)-ctpy. The reaction of each of the enantiomerically pure bridging ligands with Fe2+ gives rise to chiral coordination polymers whose formation and stoichiometry were followed spectrophotometrically. An assignment of the absolute configuration of the formed helical polymeric structures was carried out on the basis of circular dichroism studies. Highly ordered domains (as determined from STM imaging) of the enantiomerically pure chiral redox polymers could be prepared via the interfacial reaction, over an HOPG substrate, of the bridging ligand in CH2Cl2 and FeSO4 in water. The degree of polymerization was estimated to be up to 60 from analysis of the STM images of the highly ordered domains on HOPG. The helicality of the domains was compared to the configuration obtained from the circular dichroism studies. The electrochemical properties of the polymers were investigated using cyclic voltammetry and the results compared to those of the respective monomeric complexes. The redox behavior of the iron centers in the polymer was comparable to that of the monomeric complex [Fe((−)-ctpy)2](PF6)2 as well as to that of [Fe(tpy)2](PF6)2. The polymeric materials undergo electrodeposition following the two-electron reduction of each bridging ligand unit (one electron per terpyridine group). No ligand-mediated metal−metal interactions were evident from the cyclic voltammetric measurements, suggesting that the metal centers act independently. Moreover, oxidation of the metal centers within the polymeric materials did not give rise to electrodeposition.