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

Abstract

2-Carboxycinnamic acid (ccnH 2 ) and the isomeric 1,2-and 1,3-phenylenedioxydiacetic acids (1,2-and 1,3-pddaH 2 ) have been used to synthesize eight uranyl ion complexes under solvo-hydrothermal conditions. In the four complexes 4), the ccn 2− dianion retains a nearly planar geometry, which favors the formation of the centrosymmetric [UO 2 (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 O 4 site of the 1,2-pdda 2− dianion in the two complexes [UO 2 Na(1,2-pdda)(OH)] ( 5) and [(UO 2 ) 2 Na 2 (1,2pdda) 2 (C 2 O 4 )] (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-pdda 2− adopts a conformation in which one carboxylate group is tilted out of the ligand plane in [UO 2 (1,2-pdda) 2 Ni(cyclam)] ( 7) and diaxial carboxylato bonding to nickel(II) unites uranyl-only monoperiodic subunits into a diperiodic network. The 1,3-pdda 2− ligand in [UO 2 (1,3-pdda)(H 2 O)] ( 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.

Contrasting Structure-Directing Effects in the Uranyl–Phthalate/Isophthalate Isomer Systems

“…The organic cosolvent was either DMF (complexes 1, 3, 4, 8 and 9), acetonitrile (2, 5 and 6), or THF (7), but it only affected directly the nature of the complex formed in the cases of 8 and 9 which incorporate formate anions generated from DMF hydrolysis, as often observed. 65,66 Complex 7 was obtained together with the previously reported [UO2Cu(C2O4)2(bipy)] 54 (see Experimental Section);…”

Optimizing Photoluminescence Quantum Yields in Uranyl Dicarboxylate Complexes: Further Investigations of 2,5‐, 2,6‐ and 3,5‐Pyridinedicarboxylates and 2,3‐Pyrazinedicarboxylate

“…[ 15,23d,23e ] In regard to interactions within the stacks of aromatic units, examination of the Hirshfeld surfaces does not provide evidence of interactions beyond dispersion, indicating that they are not unusually strong and where, in different complexes, there do appear to be additional, specific interactions, these lead not to enhancement of uranyl emission but to its replacement by emission with the characteristics of an organic fluorophore. [ 44 ]…”

“…[15,23d,23e] In regard to interactions within the stacks of aromatic units, examination of the Hirshfeld surfaces does not provide evidence of interactions beyond dispersion, indicating that they are not unusually strong and where, in different complexes, there do appear to be additional, specific interactions, these lead not to enhancement of uranyl emission but to its replacement by emission with the characteristics of an organic fluorophore. [44] Emission from hydroxo/oxo-bridged oligomers of uranyl ion can have an unusual form taken to be indicative of direct electronic interactions between close uranyl centers, [45] and an alternative explanation of the strong emission from (Cation) 2 -[UO 2 (2,6-pydc) 2 ] species initially considered was that the relatively long U•••U separations (~7 Å) found in the crystal structures might signify an absence of perturbations shortening excited state lifetimes due to U•••U interactions. There is a rough correlation between the minimum U•••U distances and the PLQY values, with the PLQY generally increasing with increasing separation but this is clearly not an argument which could be extended to explain the high PLQY values in complexes 3 (71 %), [UO 2 (1,3,5-btcH)] (58 %, btc = benzenetricarboxylate), [5b] [NMe 4 ] 2 [(UO 2 ) 4 (C 2 O 4 ) 4 (succinate)] (49 %), [5a] 6 (36 %) or [PPh 4 ][UO 2 (1,2,4-btc)] (35 %), [13] where the minimum U•••U separations are 3.85, 5.23, 5.55, 3.85 and 5.12 Å, respectively.…”

Stepwise Introduction of Flexibility into Aromatic Dicarboxylates Forming Uranyl Ion Coordination Polymers: a Comparison of 2‐Carboxyphenylacetate and 1,2‐Phenylenediacetate

“…These polymer units are linked into diperiodic sheets parallel to (011) by the coordination to the two inequivalent Ni II sites, both located on inversion centres. Uranium atoms are 4-c nodes, cpa 2are 3-c nodes and nickel atoms are simple links, and the network has the vertex symbol {4.6 2 } 2 {4 2 .6 2 .8 2 } and the topological type 3,4L13, previously found in some transition metal ion complexes, [26,27] and also in some uranyl compounds, [9,28,29] with one particular example involving 1,4-pda 2-. [13] No significant π-stacking interaction is apparent in this case, the only weak interactions worthy of note being the hydrogen bonds between the amine groups of R,S [13] The centrosymmetric, diprotonated dmaepH 2 2 + cation, where the piperazine ring has a chair CN (6) where the coordination polymer is triperiodic, the first found to be formed with the 1,2-pda 2À ligand (Figure 6).…”