Synthesis and Structural Study of Tetravalent (Zr4+, Hf4+, Ce4+, Th4+, U4+) Metal Complexes with Cyclic Hydroxamic Acids

Six- and seven-membered cyclic hydroxamic acids, such as 1-hydroxypiperidine-2-one (1H, 1,2-PIPOH) and 1-hydroxyazepan-2-one (2H), have recently been identified in some mixed siderophores as one of their three chelating subunits. Compared to their ubiquitous noncyclic counterparts, cyclic hydroxamates are preorganized for metal binding. Surprisingly, the coordination chemistry of these bidentate, monoanionic ligands remains virtually unknown, even in the case of iron(III). We report herein the first structural study of the complexes of 1- and of 6-, an unsaturated seven-membered ring analog of 2-, with tetravalent cations of transition metals (zirconium and hafnium), lanthanide (cerium), and actinides (thorium and uranium). Structural characterization by means of X-ray crystallography of the corresponding ML4 complexes evidenced distorted square antiprismatic coordination geometries with the exception of U4+, which favors a dodecahedral arrangement.