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.

Among such organic molecules is dichloromethane (CH 2 Cl 2 ), which is one of the most commonly used organic solvents in chemical production, film production, and pharmaceutical fields; however, its large emissions lead to water and air pollution 28 . The detection of CH 2 Cl 2 still relies on gas chromatography, a quasi-quantitative detection technology that requires considerable time, labor, and cost. Therefore, for certain specific environments and the rational allocation of resources, developing a real-time, rapid detection method for CH 2 Cl 2 is urgent. The design, synthesis, and development of selective and sensitive organic molecule sensors are research hotspots [23][24][25][26][27] . although the rapid detection of many organic molecules such as tetrahydrofuran (THF) 27 . and acetonitrile 26 . has been achieved. However, methods with high selectivity and fast response to CH 2 Cl 2 remain necessary to establish.
Herein, we achieved an in situ Schiff base exchange reaction between (E)-2-((3-methoxy-2-oxidobenzylidene) amino)ethanesulfonate (L1) and 2-aminomethylpyridine induced by Ln(III) ions at room temperature to obtain [Ln 2 (L2) 2 (NO 3 ) 4 ] · 2CH 3 OH (Ln = Tb (1), Dy (2), Ho (3), and Er (4); L2 = (E)-2-methoxy-6-(((pyridin-2-ylmethyl)imino)methyl)phenol). We used a combination of crystallography and electrospray ionization mass spectrometry (ESI-MS) to track the multistep assembly process of the above-described dinuclear complexes and proposed its assembly mechanism. The Schiff base exchange mechanism accompanying the complex assembly process was tracked and proposed (Table S1). This method can serve as an invaluable reference for mechanistic research (Tables S1-S2). Complexes 1 and 2, dissolved in N,N-dimethylformamide (DMF), showed π-π* level transitions between ligands, as well as energy-level transitions from metal-ion-to-ligand charge transfer (MLCT). When 1 was dissolved in a different solvent to test its luminescence, it showed strong fluorescence in CH 2 Cl 2 and weak fluorescence in other solvents. When 1 was dissolved and dispersed in different organic solvents under a UV lamp, only CH 2 Cl 2 solution showed strong luminescence. Thus, the method was a rapid, easy, and effective way to detect CH 2 Cl 2 .

Results and Discussion
Single-crystal X-ray diffraction studies. X-ray diffraction results suggested that 1, 2, 3, and 4 crystallized in the triclinic crystal system with the P-1 space group (Table S3), in which the nine-coordinated (N 2 O 7 ) Ln(III) cores were surrounded by two N atoms, two µ 2 -O − ions, one O atom from two L2, and two bidentate chelating nitrate anions (Fig. 1a). The intracluster Ln(III) ions were bridged by two phenoxo oxygen atoms from L2 ligands. The bond length distances of Ln-O varied from 2.308 (2) (Table S4). The intracluster Ln•••Ln distances were 3.741, 3.730, 3.719, and 3.713 Å for 1, 2, 3, and 4, respectively. By studying the weak interactions among the Ln2 molecules, only two of the same type of weak interaction, i.e., C-H•••O hydrogen bonding, were found (Fig. 1b); however, for all compounds, a slight difference in distance was observed (Table S5).The connecting modes of the clusters through hydrogen bonds and the above-mentioned supramolecular weak-action distances were all within a logical range. Thus, they can be considered as a formation of 10-connected fcu net ( Fig. 1c) with distances of 9.976-12.590 Å for 1, 9.972-12.572 Å for 2, 9.947-12.531 Å for 3, and 9.944-12.527 Å for 4 between the centers of the Ln2 dimer. By using SHAPE, the calculated results suggested that the geometry of the nine-coordinated Ln(III) was a muffin for Ln2 (Tables S6-S9). The TGA curves of the four compounds are shown in Fig. S1. The phase purities of 1, 2, 3, and 4 were determined from their powder X-ray diffraction patterns (Fig. S2).
To study the stability of the dinuclear complexes in solution, we performed ESI-MS tests. For the mass spectra of 1, 2, 3, and 4, their peak location and frame peaks were similar, so only the details of 2 were provided in the ensuing discussion. According to the correlation between the solid and liquid structures, at m/z = 993.00 (the main frame peak of the structure), the molecular formula [Dy 2 (L2) 2 (NO 3 ) 3 ] + (calc. 993.01) was obtained by analysis and fitting. However, its peak strength was <0.1, indicating that 2 was unstable based on the mass spectrum in solution. A similar peak appeared at m/z = 1173.12. [Dy 2 (L2) 3 (NO 3 ) 2 ] + (calc. 1173.12) can be obtained by fitting, indicating that with a [L2] − ligand instead of a NO 3 − anion, its strength was lower. The highest intensity peak appeared at m/z = 312.09, and the bivalent peak of [Dy(L2)(DMF) 3 ] 2+ (calc.312.09) was obtained by fitting. Similar fragment peaks appeared at m/z = 275.56 and 348.62, and the molecular fragments showed divalence, which differed from the number of DMF. This finding indicated that 2 was broken into the half-structure of DyL2 Overall, under the ESI-MS condition, the above dinuclear complexes were more prone to homolysis to produce DyL2 semistructural fragments, and only a small amount of dinuclear structure lost one Dy(III) ion to produce Dy(L2) 2 fragments (Fig. 2). Although the binuclear framework of 1, 2, 3, and 4 was unstable under ESI-MS conditions, L2 in the above complexes was always stable (Figs S3-S13, Tables S10 and S11). The ESI-MS measurements for control experiments were performed in negative mode ( Fig. 2) but those for the Schiff-base exchange reaction were conducted in positive mode (Fig. 3a); the other positive or negative mode are shown in the Supporting  www.nature.com/scientificreports www.nature.com/scientificreports/ Information (Fig. S3,a,c). We also attempted to dissolve crystal 2 in methanol, and high-intensity peaks for the [Dy(L2) 2 (solv.)] molecular fragment were found (Fig. S4,b).
Interestingly, the L1 we used was not found in the structure,  (Figs 2 and S3). The above control experiments showed that the Schiff base replacement reaction did not occur without Ln(III) induction, i.e., 2-aminoethanesulfonic acid was first coordinated with Dy(III) ions in solution and then replaced with 2-aminomethylpyridine. Thus, the time-dependent tracking of the replacement process revealing its mechanism of replacement self-assembly was interesting. Notably, time-dependent assembly occurred at room temperature, which benefited slowing down the reaction rate and observing the process. It was followed by a 60 min time gradient. When L1, 2-aminomethylpyridine, and Dy(NO 3 ) 3 ·6H 2 O were added, no reaction occurred. About 1 min later, substitution occurred rapidly and the ligand after in situ replacement appeared at m/z = 243.11. The molecular formula was [(L2) + H] + (calc. 243.11) by fitting, but L1 ligand was reduced to 0.437 until it almost disappeared 3.5 min later. Moreover, with increased reaction time, the peak intensity of the L2 fragment gradually decreased to 60 min. Two weak peaks appeared at m/z = 715.13 and 646.12, and [Dy(L1)(NO 3 )(solv.)] + (calc. 715.13) and [Dy(L2) 2 ] + (calc.646.12) was obtained by fitting. However, with increased reaction time, the peak intensities of the two groups gradually increased. Figure 3 shows that the frame fragment of 646.12 obviously did not grow rapidly with 715.13. After 8 min, [Dy(L1)(NO 3 )(solv.)] + gradually increased to the highest. However, after 45 min, the peak intensity of the molecular fragment of [Dy(L2) 2 ] + gradually increased to the highest until 60 min when the [Dy(L1)(NO 3 )(solv.)] + framework fragment disappeared. Thus, driven by Dy(NO 3 ) 3 ·6H 2 O, 2-aminomethylpyridine gradually replaced taurine in L1 with increased time and then assembled into the 2 frame (Figs S14, S15, and Table S12). In summary, through ESI-MS analysis, we speculated that the possible formation mechanism of 2 was . In this work, the Schiff base replacement reaction, which accompanied the assembly of the lanthanide complex, was studied through a combination of crystallography and mass spectrometry. Moreover, the most suitable time and stage of Schiff base replacement were clearly explained.
Dichloromethane is the most common organic solvent and is widely used in organic synthesis. Its rapid and easy detection is a huge challenge. Based on the above luminescence studies (Figs S16 and 17), we used 1 as a fluorescent probe to examine its potential for sensing organic molecules. The as-obtained products (5 mg) were ground and dissolved in 10 mL of various organic solvents (CHCl 3 , MeOH, t-BuOH, CH 2 Cl 2 , DMF, 1,4-dioxane, H 2 O, THF, EtOAc, acetone, EtOH, n-heptane, DMSO, and toluene). When we excited 1 in different organic solvents at an excitation wavelength of 333 nm, we found that the emission peak of 1 dissolved in CH 2 Cl 2 was remarkably higher than those of the other organic solvents. When 1 was dissolved and dispersed in CH 2 Cl 2 , it showed a strong, broad emission peak at 435 nm and strong emission peaks at 545 and 667 nm. Weak emission peaks were also observed at 490, 585, and 620 nm. When 1 was dissolved and dispersed in CHCl 3 , although it exhibited the same emission peak position as CH 2 Cl 2 , the luminescence intensity was weak. These emission peaks can be assigned to the π-π* level transition of the organic ligand L2, and the energy-level transition of the organic ligand (organic molecule) to the Tb(III) ion (Fig. 4a). To resolve CH 2 Cl 2 quickly and easily, we placed a solution containing different organic molecules of 1 in a portable UV lamp at 365 nm. The fluorescence in the CH 2 Cl 2 solution was clearly visible to the naked eye, whereas the emission in other organic solvents was weak. Therefore, the CH 2 Cl 2 solution was quickly and easily distinguished (Fig. 4b).
We subsequently performed a luminescence titration experiment to examine the probe sensitivity. A certain amount of CH 2 Cl 2 was separately added to the DMF solution containing 1 to test its fluorescence emission www.nature.com/scientificreports www.nature.com/scientificreports/ (Fig. 5a). Without CH 2 Cl 2 , the solution showed a weak luminescence with a strength of only 68, whereas the addition of 2% CH 2 Cl 2 solution led to a remarkable increase in luminescence intensity to 198. With gradually increased CH 2 Cl 2 content, the solution fluorescence gradually increased. With increased CH 2 Cl 2 content to 22%, the luminous intensity of the solution increased to 928 (Fig. 5b). To verify the detection effect of 1 fluorescent probe on CH 2 Cl 2 in sewage, we mixed a certain amount of CH 2 Cl 2 and H 2 O, and the fluorescence increased rapidly with increased CH 2 Cl 2 content in water (Fig. S18). 1, 2, 3, and 4 were conducted in a 1000 Oe field from 300 to 2 K (Fig. S19). At room temperature, the χ M T values of 23.75, 28.51, 28.22, and 23.13 cm 3 K mol −1 were consistent with the expected values for two isolated Tb(III), Dy(III), Ho(III), and Er(III) ions, respectively. These χ M T values gradually decreased with decreased temperature until 50 K before decreasing rapidly to reach 9.48, 8.92, 7.61 and 9.13 cm 3 K mol −1 at 2 K, respectively. The decreases may be attributed to the thermal depopulation of the Ln(III) Stark sublevels [29][30][31][32][33] . The maximum values of magnetization for 1, 2, 3, and 4 were not saturated at 5 T and 2 K (Fig. S20). The saturation was not reached because the orbital contribution to the magnetic moment was very important and led to strong magnetic anisotropy. Alternating current (AC) magnetic susceptibilities suggest that only 2 exhibited weak frequency-dependent behavior in zero DC field (Fig. S21). Usually, a small magnetic field is efficient to observe the slow relaxation of magnetization and suppress the possible quantum tunneling effect of magnetization. Only a more obvious frequency-dependent behavior at 2000 Oe DC field, i.e., the effect of dc field on magnetic relaxation, was observed 34,35 . The Cole-Cole plots fitted by Debye model are shown in Fig. S22. The fitting parameters τ and α are listed in Tables S13. The parameter α ranged within 0.24-0.34, which showed the relatively moderate distribution of each relaxation time. Afterwards, we analyzed the temperature-dependent relaxation time by Arrhenius law (τ = τ 0 exp(U eff /k B T)), and the energy barrier and relaxation time were found to be U eff1 = 17.9 K and τ 1 = 3.5 × 10 −7 s under a 2000 Oe dc field ( Fig. S22 and Table S13). An S-shaped magnetization hysteresis for 2 (Fig. S23) was observed with a steep rise at the crossing field H = 5 T.

conclusions
A rare in situ Schiff base exchange reaction was found and its process was examined by crystallography and mass spectrometry. The reaction was accompanied by a multistep assembly of lanthanide metal ions to afford a series of lanthanide-metal-complex fragment. The exchange of the Schiff base occurred after the original ligand L1 was coordinated and then multistep assembly occurred to give the product, i.e.,  2 . Furthermore, the quick, easy, and selective identification of CH 2 Cl 2 was realized using 1 as fluorescence probe. To the best of our knowledge, this study is the first to explore the quick and easy fluorescence sensing of CH 2 Cl 2 . This work shows the great potential of lanthanide metal ions for the in situ Schiff base exchange and as a fluorescence probe. Our research method described an in situ reaction during complex self-assembly, which may provide new insights into the quick and easy detection of CH 2 Cl 2 and thus the foundation for the discovery of highly effective and convenient detection methods.

experimental Section
Materials and measurements. All reagents were obtained from commercial sources and used without further purification. Elemental (C, H, and N) analysis was conducted on an Elementar Micro cube elemental analyzer. Thermal analysis was performed in N 2 at a heating rate of 5 °C/min using Labsys Evo TG-DTG/DSC. IR spectra with KBr pellet were recorded on PE Spectrum Two FT/IR spectrometer (400-4000 cm −1 ). PXRD measurements were recorded on a Rigaku D/max-IIIA diffractometer. Magnetic susceptibility was measured with a MPMS SQUID-XL magnetometer equipped with 5 T magnet within the temperature range of 2-300 K. Diamagnetic corrections were estimated using Pascal's constants. AC susceptibility was measured and data were www.nature.com/scientificreports www.nature.com/scientificreports/ collected at increasing temperatures from 2 K to 10 K within frequencies ranging from 1 Hz to 1000 Hz and a drive frequency of 2.5 Oe.
Single-crystal X-ray crystallography. The diffraction data of all complexes were obtained on a Bruker SMART CCD diffractometer (Mo Kα radiation and λ = 0.71073 Å) in Φ and ω scan modes. All structures were solved by direct methods followed by difference Fourier syntheses and then refined by full-matrix least-square techniques on F 2 using SHELXL 36 . All other non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were placed in the calculated position and refined in the isotropic direction using a riding model. Table S1 summarizes X-ray crystallographic data and refinement details of the complexes. Complete details can be found in the CIF files provided in the Supporting Information. The CCDC reference numbers are  1879618 for 1, 1879615 for 2, 1879617 for 3, and 1879616 for 4. High-resolution eSi-MS test. High-resolution ESI-MS were performed at the capillary temperature of 275 °C, and the solution was injected at a rate of 0.3 mL/h. The ESI-MS used for the measurements was a ThermoExactive, and data were collected in positive and negative ion modes. The spectrometer was previously calibrated with the standard tune mix to give a precision of ca. 2 ppm within the range of 200-2500 m/z. The capillary voltage was 50 V, the tube lens voltage was 150 V, and the skimmer voltage was 25 V. The in-source energy was set within the range of 0-100 eV with a gas flow rate at 10% of the maximum.