Visual detection of Hg2+ without interference by alkali and alkaline earth metal ions using an iron(iii) complex in a hetero-bimetallic environment

Abstract

In a new approach to detect Hg²⁺ in solution, we employed the effect of Hg²⁺ binding on an Fe(III) complex (1) and its effect on the intense phenolate to iron(III) charge transfer (CT) band. The Fe(III) complex acts as a tetradentate ligand and binds Na⁺(2), K⁺(3), Ca²⁺(4), and Ba²⁺(5) in either a 1 : 1 or 1 : 2 ratio depending on the charge on the cations. This is determined from the crystal structures of the corresponding hetero-bimetallic complexes. The CT band of these complexes, except for Ca²⁺ (blue shift, 8 nm), red-shifted from 3 to 9 nm from that of 1. The addition of mercury(II) salt shifts the charge transfer band by ∼100 nm, leading to a distinct visual change in color. The Na⁺, K⁺, Ca²⁺, and Ba²⁺ ions, commonly present in water, do not interfere with the shift. The mercury(II) complex was isolated but could not be crystallized. Based on the characterization data, a structure has been proposed. The synthesis of the complexes 2–5, from 1, also serves as a simpler way to form multi-center heterometallic chiral complexes.