Structure–stability relationships of 3-hydroxypyridin-4-one complexes

Abstract

The structural features of the ligand 1-ethyl-3-hydroxy-2-methylpyridin-4-one HL¹ and its tris complexes [ML¹₃]·3H₂O (M = Fe 1, Ga 2 and Al 3) have been characterized by single-crystal X-ray diffraction. The crystal structures of complexes 1–3 are isomorphous. The comparisons among HL¹ and 1–3 indicate that several resonance forms play an important role in determining their structures. It is concluded that the absolute stabilities of the complexes of Al³⁺ and Ga³⁺ are similar and both greater than that of the iron(III) complex. A structural comparison between ligand HL¹ and the closely related 1-butyl-3-hydroxypyridin-2-one provides an explanation for the higher affinity of 3-hydroxypyridin-4-ones for Fe³⁺ over 3-hydroxypyridin-2-ones. The complexes of Al³⁺ and Ga³⁺ with HL¹ can be crystallized both as tri- and dodeca-hydrates. It is shown by comparing the same complex in different environments that neither the interplanar angle nor the twist angle is a reliable structural parameter to characterize a metal complex.