Electrochemical properties of metallocene hydroxo and oxo complexes of Ta(V): [Cp(CpR)TaOHCl]Cl, R=H, SiMe3 or (CH2)3NC4H4, Cp(Cp(CH2)3NC4H4)TaOCl Electrochemical deposition of conducting polymer film with incorporated tantalocene complexes

Redox properties of three cationic tantalocene hydroxo complexes, [Cp*(CpR)TaVOHCl]+, with one pentamethylated cyclopentadienyl ligand, Cp*=eta5-C5Me5, Me=CH3, and one monosubstituted cyclopentadienyl ligand, CpR, Cp=C5H4, R=H, SiMe3 or (CH2)3NC4H4, as well as of an oxo complex with the pyrrole-containing ligand, Cp*(Cp(CH2)3NC4H4)TaOCl, have been studied in acetonitrile solutions with cyclic voltammetry in the ranges of the Ta(V) reduction to Ta(IV) and to Ta(III). Both transitions are irreversible due to the dissociation of reduced complexes with the loss of their chloride ligand. The mechanism of hydroxo complex reduction is discussed in terms of its reversible deprotonation/protonation equilibrium with the corresponding oxo complex, Cp*(CpR)TaOCl. The variation of the reduction potential for the Ta(V)/Ta(IV) transition in the negative direction in the series of tantalocene complexes: dichloride > hydroxo > oxo, as well as the difference in the behavior of the reduced (Ta(IV)) complexes (stable for dichloride but losing their chloride ligand for hydroxo and oxo complexes) was explained by the increase of the electronic density at the central metal in this series, correlated with the weakening of the Ta-Cl bond and its easier dissociation. The tantalocene centers were immobilized inside a polypyrrole matrix on the electrode surface by electropolymerization of the cationic complex [Cp*(Cp(CH2)3NC4H4)TaOHCl]+. This was possible after removal of the 'free' chloride ions from the polymerization bath by their precipitation upon addition of TlPF6. The corresponding polymer, poly[Cp*(Cp(CH2)3NC4H4)TaOHCl], was found to be electroactive in contact with a background electrolyte, giving a redox response both in the polymer matrix and tantalocene potential ranges. The mechanism of the immobilized tantalocene complex transformation was the same as that for the same species dissolved in solution.