Laser-Induced Europium(III) Luminescence and NMR Spectroscopic Characterization of Macrocyclic Diaza Crown Ether Complexes Containing Carboxylate Ligating Groups
The Eu3+ and Y3+ complexes of 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N, N' - diacetic acid (K22DA), 1,7-diaza-4,10,13-trioxacyclopentadecane-N, N' - diacetic acid (K21DA), and the open-chain analogue ethylene glycol bis (ß-aminoethyl ether) - N,-N,N',N'-tetraacetic acid (EGTA) have been characterized in solution by using Eu3+ laser-induced luminescence and 1H and 13CNMR spectroscopy. All of these ligands form 1:1 complexes with Eu3+ in solution with the luminescence lifetimes in H2O and D2O providing the number of coordinated water molecules. Stepwise changes in the coordination environment of the Eu3+ and Y3+ ions were monitored spectroscopically for each complex as a function of temperature. In addition, the Eu3+ spectral changes as a function of pH were examined. These results indicate that Eu3+ and Y3+ each coordinate all the ligating atoms of K21DA but that K22DA undergoes fluctional processes, suggesting that the macrocyclic cavity size is too large to accommodate the metal ion properly. The noncyclic EGTA ligand, by comparison, appears to undergo an equilibrium process of wrapping and unwrapping at room temperature. The luminescence and NMR results demonstrate the importance of a proper match between metal ion size and macrocyclic cavity size.