Isomerism in Benzenetricarboxylates: Variations in the Formation of Coordination Polymers with Uranyl Ion

Abstract

A series of uranyl ion complexes with fully or partially deprotonated 1,2,4-benzenetricarboxylic acid (H3btc) and involving either organic countercations or additional metal cations has been synthesized under solvo-hydrothermal conditions. The complexes [PPh4][UO2(btc)] (1) and [PPh4]2[(UO2)2(Hbtc)3]H2O (2) crystallize as monoperiodic coordination polymers, while [PPh3Me][UO2(btc)]H2O (3) is a diperiodic network with the fes topology. Monoperiodic organization is also found in [H2DABCO][(UO2)2(btc)2]2H2O (4) (DABCO = 1,4-diazabicyclo[2.2.2]octane), but [Hquin]2[(UO2)5(btc)4]2H2O (5) (quin = quinuclidine) is a triperiodic framework. Incorporation of azamacrocyclic complexes of d-block metal cations gives [(UO2)2(btc)2Ni(cyclam)] (6) and [(UO2)2(btc)2Cu(R,S-Me6cyclam)] (7) (cyclam = 1,4,8,11-tetraazacyclotetradecane, R,S-Me6cyclam = 7(R),14(S)-5,5,7,12,12,14-hexamethylcyclam), two diperiodic networks with the same V2O5 topology, but differing in the diaxial bonding of the 3d metal cation, either to uranyl oxo groups or to carboxylato groups, respectively. Triperiodic polymerization occurs in [UO2Ag2(Hbtc)2(H2O)2] (8) and [(UO2)2Ag2(btc)2(CH3CN)1.5(H2O)0.43] 1.5H2O (9), with both oxo-and carboxylato-bonding of the bridging silver(I) cations. The isomorphous complexes [UO2Rb(btc)(H2O)] (10) and [UO2Cs(btc)(H2O)] (11) also crystallize as triperiodic frameworks with bonding of the alkali metal cations to oxo and carboxylato groups. In 10 and 11, uranyl cations and btc 3ligands alone give a 2-fold interpenetrated triperiodic framework with utp topology. Emission spectra in the solid state display the usual vibronic fine structure for 1-5, 10 and 11, while uranyl emission is quenched in 7. Photoluminescence quantum yields range from 1.3 to 17.4%, less than that for solid UO2(NO3)26H2O, except for 1 which has the unusually large value of 35%. Comparisons are drawn with previous studies of uranyl ion complexes of all known benzenetricarboxylate isomers.

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

“…This topological type is frequently encountered when monoperiodic uranyl‐containing chains are linked by additional cations. [ 13,33 ] One of the NH groups of the macrocycle is hydrogen bonded to the formate oxygen atom bound to uranium ( R 1 1 (6) ring), and the other makes a bifurcated hydrogen bond with the formate oxygen atom bound to Ni II and a uranyl oxo group [N ··· O 2.853(3)–3.098(3) Å, N–H ··· O 110(2)–164(3)°]. The packing is of the bump‐to‐hollow type, the layers being offset with respect to one another (KPI 0.70), with a possible inter‐sheet π‐stacking interaction [centroid ··· centroid 4.4647(19) Å, dihedral angle 0°, slippage 2.51 Å].…”

“…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. These five complexes have features which clearly distinguish them from the (Cation) 2 [UO 2 (2,6‐pydc) 2 ] family.…”

“…They are all true coordination polymers of low solubility and all contain pentagonalbipyramidal, not hexagonal-bipyramidal, U VI centers. Three of the five, 3, [UO 2 (1, [13] containing monoperiodic chains seemingly only slightly different, although in this instance containing hexagonal-bipyramidal uranium centers, are only weakly luminescent, so that exactly what features of the chain structures may be most important is uncertain.…”

“…This topological type is frequently encountered when monoperiodic uranyl-containing chains are linked by additional cations. [13,33] One of the NH groups of the macrocycle is hydrogen bonded to the formate oxygen atom bound to uranium (R 1 1 (6) ring), and the other makes a bifurcated . The structures of complexes 4 and 5 provide further examples of the different ways in which a given tetra-azamacrocycle complex may contribute to a crystal array.…”

“…In this regard, uranyl complexes with high solid state photoluminescence quantum yields (PLQYs) are highly desirable, but these values are seldom reported and, when they are measured, they are often low. If the PLQY of crystalline uranyl nitrate hexahydrate (24 %) [ 12 ] is taken as a reference, only five carboxylate complexes, all polymeric, have comparable or higher PLQYs, these complexes involving the ligands (1 R ,3 S )‐(+)‐camphorate (23 %), [ 12 ] 1,2,4‐benzenetricarboxylate (35 %), [ 13 ] 2,6‐pyridinedicarboxylate (42 %), [ 12 ] succinate (49 %), [ 5a ] and 1,3,5‐benzenetricarboxylate (58 %). [ 5b ] It is notable however that a PLQY close to 100 % has been measured for a photoluminescent borate framework in which uranyl ions were introduced as activators.…”