Tetrahydrofurantetracarboxylic Acid: An Isomerizable Framework-Forming Ligand in Homo- and Heterometallic Complexes with UO22+, Ag+, and Pb2+

(2R*,3R*,4S*,5S*)-Tetrahydrofurantetracarboxylic acid (H(4)thftc) has been used as a ligand to synthesize five uranyl ion complexes, three of them including additional silver(I) or lead(II) metal cations. The complex [C(NH2)(3)](2)[UO2(H(2)thftc)(2)] (1), obtained in water at room temperature, is a discrete mononuclear species in which the uranyl cation is bound to the tridentate coordination site (involving the ether oxygen atom and the two adjoining carboxylate groups) of two ligands, and extensive hydrogen bonding is present. All the other complexes were obtained under (solvo)-hydrothermal conditions giving rise to higher degrees of ligand deprotonation. [(UO2)(3)(Hthftc)(2)(H2O)(2)]center dot 2CH(3)CN (2) crystallizes as a two-dimensional (2D) network with the V2O5 topological type, whereas in the heterometallic complex [(UO2)(3)Ag-2(thftc)(2)(H2O)(2)]center dot 2H(2)O (3), similar 2D layers are assembled into a three-dimensional (3D) framework by bridging Ag-2 moieties. Lead(II) replaces uranyl in the tridentate coordination site in the two complexes [UO2Pb(thftc)(H2O)] (4) and [UO2Pb(thftc)(H2O)(2)]center dot H2O (5), and the high connectivity of the ligand, bound to seven metal cations through diverse chelating and bridging interactions, ensures that both are 3D frameworks. Bonding of a uranyl oxo group to either silver(I) or lead(II) is apparent in complexes 3 and 5. The homometallic complexes [Ag-3(Hthftc)] (6) and [Pb-2(thftc)(H2O)] (7), devoid of uranyl cations, are both 3D frameworks in which the ligand is bound to 11 or 9 metal cations, respectively. Complex 6 is the single instance in this series in which the ligand, originally in the trans,cis,trans form, has undergone isomerization into the chiral cis,trans,trans (2R*,3S*,4S*,SS*) form through a process probably involving an ene-diol intermediate. Only complexes 1, 2, and 4 display intense and well-resolved emission bands under excitation at 420 nm in the solid state, the uranyl emission of complexes 3 and 5 being largely quenched.