Macrocycles Bearing Ferrocenyl Pendants and their Electrochemical Properties upon Binding to Divalent Transition Metal Cations

Metal complexes of Cu²⁺, Co²⁺, Cd²⁺, Zn²⁺, and Ni²⁺ formed with the ligands [Fc(cyclen)](1) and [Fc(cyclen)₂](2) (Fc=ferrocene, cyclen=1,4,7,10-tetraazacyclododecane) are synthesised and characterised. The X-ray structure of the Cu²⁺ complex of 2, Fc([Cu(cyclen)(CH₃CN)]₂(ClO₄)₄, is reported, and shows that the two positively charged Cu²⁺-cyclen units have a coordination number of five, adopting a distorted trigonal-bipyramidal configuration. The Cu²⁺-cyclen units are arranged in a trans-like configuration with respect to the Fc group, presumably to minimise electrostatic repulsion. The voltammetric oxidation of the free ligands 1 and 2 in a CH₂Cl₂/CH₃CN (1:4) solvent mixture yields two closely spaced oxidation processes. Both electron-transfer steps are associated with the ferrocenyl moiety, implying strong communication between the cyclen nitrogen atoms and the ferrocenyl group. In contrast, cyclic voltammograms display only a simple reversible one-electron process if 1 and 2 are complexed with Cd²⁺, Cu²⁺, Zn²⁺, Ni²⁺, or Co²⁺. Binding of these metal ions produces a significant shift in the reversible midpoint potential (E_m). Except for Ni²⁺, E_m is linearly proportional to the charge density of the transition metal ion, demonstrating that 1 and 2 may undergo redox switching. The diffusion coefficients of Fc, DmFc, 1 and 2, and their metal ion complexes correlate well with their molecular weights.