Experimental and Theoretical Approaches Toward Anion-Responsive Tripod-Lanthanide Complexes: Mixed-Donor Ligand Effects on Lanthanide Complexation and Luminescence Sensing Profiles

A new series of tripods were designed to form anion‐responsive, luminescent lanthanide complexes. These tripods contain pyridine, thiazole, pyrazine, or quinoline chromophores combined with amide carbonyl oxygen and tertiary nitrogen atoms. Crystallographic and EXAFS studies of the 10‐coordinated tripod–La(NO3)3 complexes revealed that each La3+ cation was cooperatively coordinated by one tetradentate tripod and three bidentate NO3− anions in the crystal and in CH3CN. Quantum chemical calculations indicated that the aromatic nitrogen plays a significant role in lanthanide complexation. The experimentally determined stability constants of complexes of the tripod with La(NO3)3, Eu(NO3)3, and Tb(NO3)3 were in good agreement with the theoretically calculated interaction energies. Complexation of each tripod with lanthanide triflate gave a mixture of several lanthanide complex species. Interestingly, the addition of a coordinative NO3− or Cl− anion to the mixture significantly influenced the lanthanide complexation profiles. The particular combination of tripod and a luminescent Eu3+ center gave anion‐selective luminescence enhancements. Pyridine‐containing tripods exhibited the highest NO3− anion‐selective luminescence and thus permit naked‐eye detection of the NO3− anion. Bright sparks! A series of tripod–lanthanide complexes (see graphic) were experimentally and theoretically characterized. Some of them exhibited anion‐responsive luminescence signals and can thus be used for the naked‐eye detection of the NO3− anion.