Tracking the Multistep Formation of Ln(III) Complexes with in situ Schiff Base Exchange Reaction and its Highly Selective Sensing of Dichloromethane

Four complexes, namely, [Ln2(L2)2(NO3)4]. 2CH3OH (Ln = Tb (1), Dy (2), Ho (3), Er (4), and L2 = (E)-2-methoxy-6-(((pyridin-2-ylmethyl)imino)methyl)phenol), were obtained by reacting (E)-2-((3-methoxy-2-oxidobenzylidene)amino)ethanesulfonate (L1), Ln(NO3)3·6H2O, and 2-aminomethylpyridine at room temperature under solvothermal conditions in methanol for 12 h. The new Schiff base L2 was generated in situ based on the organic ligand L1 and 2-aminomethylpyridine through Schiff base exchange reaction by using lanthanide salts as inductor. A combination of crystallography and mass spectrometry was performed to track the exchange reaction, and the underlying mechanism accompanied by the complex assembly process was clearly presented. The multistep formation mechanism of the above dinuclear complex was also proposed, i.e., [L1] → Dy[L1]/[L2] → Dy[L2] → Dy[L2]2 → Dy2[L2]2. Luminescence test of 1 showed that it had extremely high selectivity to dichloromethane (CH2Cl2). Therefore, we established a quick, simple, and efficient method of detecting CH2Cl2 that enabled strong-luminescence observation with the naked eye. Tests for small amounts of CH2Cl2 in water further indicated the potential of 1 as a test strip for CH2Cl2 fluorescence detection in water samples. Alternating-current magnetic susceptibility studies indicated the field-induced single-molecule magnet behavior of 2.