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, E_m, is observed. The magnitude of the shift in the E_m values follows the order [Fc(cyclen)₂] ≈ [Fc(cyclam)₂] ≫ [R-Fc(cyclen)] ≈ [Fc₂(cyclen)] > [Fc₄(cyclen)]. The ability of the ligands to work as electrochemical sensors for the mentioned cations is discussed.