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

Abstract

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)(CH3 CN)]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 CH2 Cl2 /CH3 CN (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.