Electrochemical properties of cyclen and cyclam macrocycles bearing ferrocenyl pendants and their transition metal complexes

The ligands [R-Fc(cyclen)], [Fc(cyclen)2], [Fc(cyclam)2], [Fc2(cyclen)] and [Fc4(cyclen)] (R = -H or -CH2OH; Fc = ferrocene; cyclen = 1,4,7,10-tetraazacyclododecane; cyclam = 1,4,8,11-tetraazacyclotetradecane) and their respective Cu2+, Co2+, Cd2+, Zn2+, and Ni2+ metal complexes have been synthesised and electrochemically characterised. The voltammetry of the free ligands in a CH2Cl2/CH3CN (1:4) solvent mixture containing [Bu4N] [PF6] or [Bu4N](B(C6F5)4] as the supporting electrolyte yields two closely spaced oxidation processes. The first one is Fc based, and the other is related to the interaction of the Fc with the nitrogen component. Details of the mechanism were established by studying the oxidation of N,N-dimethylaminomethylferrocene by cyclic voltammetry and bulk electrolysis with product isolation. However, cyclic voltammetries exhibit a single Fc-based reversible oxidation process when the ligands form metal complexes with Cu2+, Co2+, Cd2+, Zn2+, or Ni2+. Upon metal ion binding, an important positive shift in the reversible midpoint potential, Em, is observed. The magnitude of the shift in the Em values follows the order [Fc(cyclen)2] similar to [Fc(cyclam)2] >> [R-Fc(cyclen)] similar to [Fc2(cyclen)] > [Fc4(cyclen)]. The ability of the ligands to work as electrochemical sensors for the mentioned cations is discussed.