Redox properties of titanocene-pyrrole derivative and its electropolymerization

A new titanocene dichloride derivative in which one cyclopentadienyl ligand (Cp) is functionalized with a pyrrolyl ring, Tc3Py (Cl2TiCpC5H4(CH2)(3)NC4 H4), has been synthesized and characterized with NMR. Its redox properties have been studied by CV in acetonitrile (AN), tetrahydrofurane (THF) and di chloromethane (DCM), in comparison with unsubstituted titanocene dichloride (Tc) and pyrrole. Ti(IV/III) transition observed within the negative poten tial range is a quasi-reversible reaction in THF and DCM (but without a com plete recuperation of the initial reagent in the back scan) while the reoxi dation peak in AN is strongly shifted in the positive direction. These resu lts are interpreted within the framework of a 'square scheme' where the ele ctrochemical reduction step is accompanied by the rapid substitution of one chloride ligand by the solvent molecule. The back electron transfer follow s the same reaction path for weakly coordinating media (THF, DCM) whereas t his process is shifted to a more positive potential in the case of strongly coordinating solvents (AN). In the positive potential range, the derivativ e, Tc3Py, is irreversibly oxidized (similar to pyrrole) in AN or DCM so tha t one can deposit a conducting polymer film with the use of different elect rochemical regimes, Redox properties of its polymer matrix resemble alkyl-m odified polypyrroles. The redox activity of pendant Tc groups is restricted in AN since only a thin layer of the film near the electrode surface can b e reduced. A more pronounced response of immobilized Tc is observed in DCM for films polymerized in AN. Reduction peak current and reduction charge of the film depend on the potential scan rate. For sufficiently thin films an d slow potential sweeps, one can reach an almost complete redox transformat ion of titanocene groups inside the film as evidenced by the theoretically expected ratio of polymer matrix and titanocene redox charges. Possible rea sons of 'prepeak' appearance are discussed. (C) 2001 Elsevier Science Ltd. All rights reserved.