Functionalized Aromatic Dicarboxylate Ligands in Uranyl-Organic Assemblies: The Cases of Carboxycinnamate and 1,2-/1,3-Phenylenedioxydiacetate.

Abstract

2-Carboxycinnamic acid (ccnH2) and the isomeric 1,2- and 1,3-phenylenedioxydiacetic acids (1,2- and 1,3-pddaH2) have been used to synthesize eight uranyl ion complexes under solvo-hydrothermal conditions. In the four complexes [PPh4]2[UO2(ccn)(NO3)]2 (1), [PPh4]2[UO2(ccn)(dibf)]2 (2), [UO2(ccn)(bipy)]2 (3), and [Ni(R,S-Me6cyclam)][UO2(ccn)(HCOO)]2 (4), the ccn2- dianion retains a nearly planar geometry, which favors the formation of the centrosymmetric [UO2(ccn)]2 dimeric unit. Additional terminal ligands, either neutral (bipy = 2,2'-bipyridine) or anionic (nitrate, dibf- = 1,3-dihydro-3-oxo-1-isobenzofuranacetate, and formate, the two latter formed in situ), complete the uranyl coordination sphere, leading in all cases to discrete, dinuclear species. Sodium(I) bonding to the carboxylate/ether O4 site of the 1,2-pdda2- dianion in the two complexes [UO2Na(1,2-pdda)(OH)] (5) and [(UO2)2Na2(1,2-pdda)2(C2O4)] (6) results in this ligand being planar. Further lateral coordination to uranyl and sodium bonding to a uranyl oxo group allow formation of heterometallic diperiodic networks containing monoperiodic uranyl-only subunits. In the absence of Na+ cations, 1,2-pdda2- adopts a conformation in which one carboxylate group is tilted out of the ligand plane in [UO2(1,2-pdda)2Ni(cyclam)] (7) and diaxial carboxylato bonding to nickel(II) unites uranyl-only monoperiodic subunits into a diperiodic network. The 1,3-pdda2- ligand in [UO2(1,3-pdda)(H2O)] (8) is also nonplanar with one tilted carboxylate group, and the bridging bidentate nature of both carboxylate groups allows formation of a triperiodic framework in which both metal and ligand are four-coordinated nodes. While the emission spectra of complexes 1 and 5 display the vibronic progression considered typical of uranyl ion, those of complexes 2, 4, and 8 show broad emission bands which in the case of complex 4 completely replace the uranyl emission and which appear to be ligand-centered. The low energy of these broad bands can be rationalized in terms of the close association of certain ligand pairs within the structures.