Design of ligands containing the o-hydroxybenzyl group. Metal-complexing properties of N,N″-bis(2-hydroxybenzyl)diethylenetriamine-N,N′,N″-triacetic acid

Abstract

The ligand N,N″-bis(2-hydroxybenzyl)diethylenetriamine-N,N′,N″-triacetic acid (H₅L) has been synthesised and the protonation constants for L determined by potentiometric methods in 0.5 mol dm^–3 NaNO₃, and spectrophotometric methods in 0.5 and 0.1 mol dm^–3 NaCl, all at 25 °C. The sites of protonation have been inferred from ¹H NMR studies in D₂O. The complex formation constants of Ca^II, Zn^II Cd^II, Cu^II, Pb^II and Bi^III have been determined at 25 °C by potentiometric methods in 0.5 mol dm^–3 NaNO₃, and spectrophotometric methods in 0.5 mol dm^–3 NaCl. The results show that at biological pH the hydroxybenzyl groups tend to remain protonated, and over most of the pH range protonated complexes dominate, with fully deprotonated complexes occurring only at pH values above 9 or 10. The ligand L is, compared to some of its analogues, effectively a weak complexing agent. This is rationalised in terms of the six-membered chelate rings formed in the complex, which include the hydroxybenzyl group. The six-membered chelate rings destabilize complexes of the larger metal ions with which the octadentate ligand should prefer to co-ordinate.