Hexanuclear actinide(IV) carboxylates formed by competition between ligation and hydrolysis

Tetravalent actinides show a strong tendency toward hydrolysis already at low pH. Below the onset of hydrolysis they commonly form monomeric complexes with carboxylate ligands. As the pH reaches the hydrolysis limit, olation and oxolation occur as competing reactions. We observed under these circumstances the formation of several well-defined hexanuclear carboxylate complexes. Recent thermodynamic studies suffer from the lack of structural information on such solution species. EXAFS spectroscopy is well suited to gain this information and has been applied to study U(IV), Th(IV), Np(IV) and Pu(IV) carboxylates and amino-carboxylates in aqueous solution and solid state. A comparison with the corresponding single crystal data supports the structure analysis of the solution species. We found that the carboxylic group acts as terminating ligand and stabilizes nanosized hexanuclear clusters in solution as well as in solid state. It is important to note that this reaction prevents the formation of hydrolysis products as well as the formation of An(IV) hydrous oxide colloids.