Woven, Polycatenated, or Cage Structures: Effect of Modulation of Ligand Curvature in Heteroleptic Uranyl Ion Complexes

Abstract

Combining the flexible zwitterionic dicarboxylate 4,4′-bis(2-carboxylatoethyl)-4,4′-bipyridinium (L) and the anionic dicarboxylate ligands isophthalate (ipht2–) and 1,2-, 1,3-, or 1,4-phenylenediacetate (1,2-, 1,3-, and 1,4-pda2–), of varying shape and curvature, has allowed isolation of five uranyl ion complexes by synthesis under solvo-hydrothermal conditions. [(UO2)2(L)(ipht)2] (1) and [(UO2)2(L)(1,2-pda)2]·2H2O (2) have the same stoichiometry, and both crystallize as monoperiodic coordination polymers containing two uranyl–(anionic carboxylate) strands united by L linkers into a wide ribbon, all ligands being in the divergent conformation. Complex 3, [(UO2)2(L)(1,3-pda)2]·0.5CH3CN, with the same stoichiometry but ligands in a convergent conformation, is a discrete, binuclear species which is the first example of a heteroleptic uranyl carboxylate coordination cage. With all ligands in a divergent conformation, [(UO2)2(L)(1,4-pda)(1,4-pdaH)2] (4) crystallizes as a sinuous and thread-like monoperiodic polymer; two families of chains run along different directions and are woven into diperiodic layers. Modification of the synthetic conditions leads to [(UO2)4(LH)2(1,4-pda)5]·H2O·2CH3CN (5), a monoperiodic polymer based on tetranuclear (UO2)4(1,4-pda)4 rings; intrachain hydrogen bonding of the terminal LH+ ligands results in diperiodic network formation through parallel polycatenation involving the tetranuclear rings and the LH+ rods. Complexes 1–3 and 5 are emissive, with complex 2 having the highest photoluminescence quantum yield (19%), and their spectra show the maxima positions usual for tris-κ2 O,O′-chelated uranyl cations.

4-(Ammoniomethyl)benzoate, a protic zwitterionic bifunctional linker in uranyl ion carboxylate complexes

“…In contrast to previous work involving aprotic dizwitterionic dicarboxylates of large size, 3–11 which are conducive to the formation of mixed-ligand complexes with periodicities varying from 0 to 3 displaying entanglement in several cases, the present work with the Hamb monozwitterionic ligand has only yielded monoperiodic coordination polymers and one tetranuclear discrete species. The reduced metal ion-bridging ability of Hamb is of course a drawback here, but, due to the presence of the primary ammonium group, its bifunctionality is more obvious than with pyridinium-based zwitterionic carboxylates.…”

“…3,4 The intention there was to determine how the structure of uranyl ion coordination polymers based on polycarboxylate ligands might be modified by the co-coordination of neutral oligozwitterionic carboxylates. To avoid possible complications in synthesis due to additional acid–base equilibria and coordination modes, the oligozwitterions employed in this work 5–11 were those with quaternary nitrogen centres, thus lacking dissociable protons. This has the disadvantage of eliminating usage of the positively charged centre for strong hydrogen bonding interactions, a factor which can strongly influence the role of a zwitterionic ligand as a structure determinant.…”

Oligozwitterions in coordination polymers and frameworks – a structural view

“…Although as yet unexplored in regard to any possible applications, it has been demonstrated 17,82,101–104 that carboxylate di- or tri-zwitterions and anionic polycarboxylates can form mixed ligand coordination polymers in which both ligands act as bridges, seemingly consistent with the different donor species having almost identical bonding interactions. Bond lengths to zwitterion carboxylate-O and anionic carboxylate-O are similar but the common adoption of a bis or tris(κ 1 O ) mode of bonding by the zwitterions may indicate that they have a weaker structural influence and in the particular case of thiophene-2,5-dicarboxylate as the anion, 103 the isolated crystals have the composition of a mixed-ligand species but the structure in fact involves independent polymer networks of the two ligands (Fig.…”

“…Hence, the appropriate choice of zwitterion might be a means of controlling guest inclusion in such complexes. Added to these considerations, it has also been shown recently 104 that the moderately flexible dizwitterion 4,4′-bis(carboxylatoethyl)-4,4′-bipyridinium (L23) in combination with 1,3-phenylenediacetate (1,3-pda 2− ) can form a chiral, binuclear cage species [(UO 2 ) 2 (L23)(1,3-pda) 2 ]. Cage complexes are relatively rare in U( vi ) coordination chemistry 105 but they bring with them the posssibility of selective inclusion of substrates susceptible to reaction with photoexcited U( vi ) as well as the possibility of sufficient solubility for use as homogeneous catalysts.…”