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

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.