A Dinuclear Lanthanide Complex for the Recognition of Bis(carboxylates): Formation of Terbium(III) Luminescent Self-Assembly Ternary Complexes in Aqueous Solution

Abstract

The synthesis and photophysical properties of a coordinatively unsaturated cationic dinuclear terbium complex, 2·Tb₂, that can detect the presence of mono- or bis(carboxylates) in buffered aqueous solution at physiological pH is described. Full ligand synthesis and structural characterization of 2·Na₂ are also described. Spectroscopic measurements determined that each Tb(III) metal center has two metal-bound water molecules (q = 2). The recognition or sensing of N,N-dimethylaminocarboxylic acid, 4, and the bis(carboxylate) terephthalic acid, 5, which can also function as sensitizing antennae, was found to occur through the binding of these carboxylates to the metal center via the displacement of the metal bound water molecules. This gave rise to the formation of luminescent ternary complexes in solution in 2:1 or 1:1 (ion:2·Tb₂) stoichiometry, respectively. Aliphatic bis(carboxylates) also bind to 2·Tb₂ where the selectivity for the ion recognition and stoichiometry was dictated by the structure of the anion, being most selective for pimelic acid, 6. Binding of either l- or d-tartaric acid gave rise to the formation ternary complex formation, with 2:1 stoichiometry, where the ion recognition resulted in quenching of the lanthanide emission.