Bis[1,1′-N,N′-(2-picolyl)aminomethyl]ferrocene as a redox sensor for transition metal ions

Abstract

The compound bis[1,1′-N,N′-(2-picolyl)aminomethyl]ferrocene, L¹, was synthesized. The protonation constants of this ligand and the stability constants of its complexes with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ were determined in aqueous solution by potentiometric methods at 25 °C and at ionic strength 0.10 mol dm⁻³ in KNO₃. The compound L¹ forms only 1 ∶ 1 (M ∶ L) complexes with Pb²⁺ and Cd²⁺ while with Ni²⁺ and Cu²⁺ species of 2 ∶ 1 ratio were also found. The complexing behaviour of L¹ is regulated by the constraint imposed by the ferrocene in its backbone, leading to lower values of stability constants for complexes of the divalent first row transition metals when compared with related ligands. However, the differences in stability are smaller for the larger metal ions. The structure of the copper complex with L¹ was determined by single-crystal X-ray diffraction and shows that a species of 2 ∶ 2 ratio is formed. The two copper centres display distorted octahedral geometries and are linked through the two L¹ bridges at a long distance of 8.781(10) Å. The electrochemical behaviour of L¹ was studied in the presence of Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺, showing that upon complexation the ferrocene–ferrocenium half-wave potential shifts anodically in relation to that of the free ligand. The maximum electrochemical shift (ΔE_1/2) of 268 mV was found in the presence of Pb²⁺, followed by Cu²⁺ (218 mV), Ni²⁺ (152 mV), Zn²⁺ (111 mV) and Cd²⁺ (110 mV). Moreover, L¹ is able to electrochemically and selectively sense Cu²⁺ in the presence of a large excess of the other transition metal cations studied.