Time-controlled synthesis of the 3D coordination polymer U(1,2,3-Hbtc)2 followed by the formation of molecular poly-oxo cluster {U14} containing hemimellitate uranium(iv).

Abstract

Two coordination compounds bearing tetravalent uranium were synthesized in the presence of tritopic hemimellitic acid in acetonitrile with a controlled amount of water (H2O/U ≈ 8) and structurally characterized. Compound 1, [U(1,2,3-Hbtc)2]·0.5CH3CN is constructed around an eight-fold coordinated uranium cationic unit [UO8] linked by the poly-carboxylate ligands to form dimeric subunits, which are further connected to form infinite corrugated ribbons and a three-dimensional framework. Compound 2, [U14O8(OH)4Cl8(H2O)16(1,2,3-Hbtc)8(ox)4(ac)4] ({U14}) exhibits an unprecedented polynuclear {U14} poly-oxo uranium cluster surrounded by O-donor and chloride ligands. It is based on a central core of [U6O8] type surrounded by four dinuclear uranium-subunits {U2}. Compound 1 was synthesized by a direct reaction of hemimellitic acid with uranium tetrachloride in acetonitrile (+H2O), while the molecular species ({U14} (2)) crystallized from the supernatant solution after one month. The slow hydrolysis reaction together with the partial decomposition of the starting organic reactants into oxalate and acetate molecules induces the generation of such a large poly-oxo cluster with fourteen uranium centers. Structural comparisons with other closely related uranium-containing clusters, such as the {U12} cluster based on the association of inner core [U6O8] with three dinuclear sub-units {U2}, were performed.