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 (Em). Except for Ni2+, Em 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.