Synthesis and Crystallographic Characterization of the Tetravalent Actinide-DOTA Complexes [AnIV(κ8-DOTA)(DMSO)] (An = Th, U).

The reaction of UCl4 or ThCl4(DME)2 (DME = dimethoxyethane) with 1,4,7,10-tetrazacyclododecane-1,4,7,10-tetraacetic acid (H4DOTA), and 6 equiv of triethylamine, in dimethyl sulfoxide (DMSO) results in the formation of [AnIV(κ8-DOTA)(DMSO)] (1, An = U; 2, An = Th), which can be isolated in moderate yields after workup. Complexes 1 and 2 are the first structurally characterized actinide-DOTA complexes to feature the κ8 binding mode for the DOTA ligand. In addition, we isolated a few crystals of [U(κ4-H2DOTA)(DMSO)4][Cl]2 (3). Crystallographic characterization of this material reveals that the [H2DOTA]2- ligand in 3 is only coordinated to U4+ by its four carboxylate arms, generating an overall κ4 binding mode. Similar complexes have been previously proposed as intermediates along the H4DOTA complexation pathway, but this intermediate had not been structurally characterized until now.