Structure of the {U₁₃} polyoxo cluster U₁₃O₈Cl (MeO)_38- (x = 2.3, MeO = methoxide)

The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U₁₃}, is reported. The complex crystallized from methanol containing tetra-valent uranium (U^IV) with a basic organic ligand, and was characterized as di-chloridoocta-cosa-μ₂-methano-lato-octa-kis-(methano-lato)octa-μ₄-oxido-trideca-uranium, [U₁₃(CH₃O)_35.7Cl_2.3O₈] or [U₁₃(μ₄-O_oxo)₈Cl (MeO)_38- ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U₁₃} polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO₂. The U-O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO₂ but also in another analogue in the {U₃₈} cluster. This shortening of U-O bonds, together with BVS calculations and the overall negative charge (2-) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the penta-valent state (U^V) from the original tetra-valent state (U^IV). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of U^V.