Redox properties of titanocene-pyrrole derivative and its electropolymerization

Abstract

A new titanocene dichloride derivative in which one cyclopentadienyl ligand (Cp) is functionalized with a pyrrolyl ring, Tc3Py (Cl 2TiCpC 5H 4(CH 2) 3NC 4H 4), has been synthesized and characterized with NMR. Its redox properties have been studied by CV in acetonitrile (AN), tetrahydrofurane (THF) and dichloromethane (DCM), in comparison with unsubstituted titanocene dichloride (Tc) and pyrrole. Ti(IV/III) transition observed within the negative potential range is a quasi-reversible reaction in THF and DCM (but without a complete recuperation of the initial reagent in the back scan) while the reoxidation peak in AN is strongly shifted in the positive direction. These results are interpreted within the framework of a ‘square scheme’ where the electrochemical reduction step is accompanied by the rapid substitution of one chloride ligand by the solvent molecule. The back electron transfer follows the same reaction path for weakly coordinating media (THF, DCM) whereas this process is shifted to a more positive potential in the case of strongly coordinating solvents (AN). In the positive potential range, the derivative, Tc3Py, is irreversibly oxidized (similar to pyrrole) in AN or DCM so that one can deposit a conducting polymer film with the use of different electrochemical regimes. Redox properties of its polymer matrix resemble alkyl-modified 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 be 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 and slow potential sweeps, one can reach an almost complete redox transformation of titanocene groups inside the film as evidenced by the theoretically expected ratio of polymer matrix and titanocene redox charges. Possible reasons of ‘prepeak’ appearance are discussed.