In situ ligand synthesis with the image cation under hydrothermal conditions

A novel uranium (VI) coordination polymer, (UO2)2(C2O4)(C5H6NO3)2 (1), has been prepared under the hydrothermal reaction of uranium nitrate hexahydrate and l-pyroglutamic acid. Compound 1 (monoclinic, C2/c, a=22.541(6) Å, b=5.7428(15) Å, c=15.815(4) Å, β=119.112(4)°, Z=4, R1=0.0237, wR2=0.0367) consists of uranium pentagonal bipyramids linked via l-pyroglutamate and oxalate anions to form an overall two-dimensional (2D) structure. With the absence of oxalic acid within the starting materials, the oxalate anions are hypothesized to form in situ whereby decarboxylation of l-pyroglutamic acid occurs followed by coupling of CO2 to form the oxalate linkages as observed in the crystal structure. Addition of copper (II) to this system appears to promote oxalate formation in that synthetic moolooite (Cu(C2O4)·nH2O; 0⩽n⩽1) and a known uranyl oxalate [(UO2)2(C2O4)(OH)2(H2O)2·H2O], co-crystallize in significant quantity. Compound 1 exhibits the characteristic uranyl emission spectrum upon either direct uranyl excitation or ligand excitation, the latter of which shows an increase in relative intensity. This subsequent increase in the intensity indicates an energy transfer from the ligand to the uranyl cations thus illustrating an example of the antenna effect in the solid state.