Mixed-ligand uranyl ion complexes with two flexible, pyridinium-based dicarboxylate zwitterions and simple anionic dicarboxylates

Abstract

Two new dizwitterionic dicarboxylates, E-bis(N-(2′-carboxylatoethyl)pyridinium-4-yl)ethene (L1) and E-bis(3-carboxylatopyridiniomethyl)ethene (L2) have been reacted with uranyl nitrate hexahydrate under solvo-hydrothermal conditions, in the presence of dianionic dicarboxylates, yielding a series of 7 complexes which have been characterized by their crystal structure and luminescence properties. Both [(UO₂)₂(L1)(1,2-pda)₂]·2H₂O (1) and [(UO₂)₂(L1)(1,4-pda)₂]·H₂O (2), where 1,2- and 1,4-pda²⁻ are 1,2- and 1,4-phenylenediacetates, crystallize as monoperiodic coordination polymers, either two-stranded and ladder-like or sinuous and daisychain-like, respectively. [(UO₂)₂(L1)(t-1,4-chdc)₂] (3), where t-1,4-chdc²⁻ is trans-1,4-cyclohexanedicarboxylate, is a diperiodic assembly with the hcb topology. In situ formation of oxalate anions (ox²⁻) produces [(UO₂)₂(L2)(ox)(OH)₂] (4), a diperiodic coordination polymer containing dihydroxo-bridged, dinuclear subunits. Simple chains are found in [(UO₂)₂(L2)(pht)₂(H₂O)₂]·2H₂O (5), where pht²⁻ is phthalate, while [(UO₂)₂(L2)(ipht)₂]·2H₂O·2CH₃CN (6), where ipht²⁻ is isophthalate, is another hcb network. In all these cases, each dicarboxylate ligand connects two metal centres. Finally, [(UO₂)₂(L2)(t-1,4-chdc)₂] (7) is a triperiodic framework with the unusual mog topology, in which t-1,4-chdc²⁻ is either bis(κ²O,O′-chelating) or bis(μ₂-κ¹O:κ¹O′-bridging). Bond valence calculations reveal no very significant difference in donor strength between the two types of ligands. The importance of weak interactions (hydrogen bonding, π–π stacking) is discussed. Only complex 5 is strongly emissive in the solid state, with a photoluminescence quantum yield of 19%, and 6 is weakly emissive (4%), while 1–3 and 7 are non-emissive. The spectra of 5 and 6 display the usual vibronic fine structure, the peak positions being dependent on the uranyl ion equatorial environment.