Study on the Interaction Between Melamine-Cored Schiff Bases with Cucurbit[n]urils of Different Sizes and Its Application in Detecting Silver Ion

Three different complexes, TMeQ[6]-TBT, Q[7]-TBT and Q[8]-TBT, are constructed by three different cucurbiturils and synthesized guest melamine-cored Schiff base (TBT) through outer-surface interaction and host-guest interaction, where TBT and TMeQ[6] form complex TMeQ[6]-TBT through outer-surface interaction, while TBT and Q[7,8] form complexs Q[7]-TBT and Q[8]-TBT through host-guest interaction., and finally, Q[7]-TBT is selected as a UV detector for the detection of precious metal Ag. This work makes full use of the characteristics of each cucurbiturils and combines that of Schiff bases to construct a series of complexes and apply them to metal detection.

In this work, nitrogen-rich melamine is used as the center to synthesize the Schiff bases TBT through the Schiff base reaction and the nucleophilic reaction of haloalkane (Scheme 1 and S1) [24] . On the basis of retaining the abundant coordination sites of Schiff bases, TBT also modified with the carbon chain of appropriate length as the site of host-guest interaction and carboxyl group is used as the site of the outer-surface interaction. Then three kinds of cucurbit[n]urils with different size of cavities are chosen, tetramethylcucurbit [6]uril (TMeQ [6]), Q [7] and Q [8], to study their interaction with TBT [25][26][27][28][29][30] . Due to the small cavity size and the higher density of positive charge of TMeQ [6], TBT cannot enter the cavity of TMeQ [6], and instead has an outer-surface interaction with the exposed methyl group of it on the outer surface. The cavity of Q [7] 3 is just right for the carbon chain of TBT, so TBT can have host-guest interactions with Q [7]. With the addition of Q [8] with lager cavity into TBT, a supermolecule polymer Q[8]-TBT is constructed. Since Q [7] and TBT constitute a host-guest complex, the carboxyl group at the end of TBT and the carbonyl group of the Q [7] still have a strong ability to coordinate with metals. Therefore, Q[7]-TBT is selected for the detection of precious metal Ag + .

The outer-surface interaction of TMeQ[6]-TBT
In order to investigate the outer-surface interaction between TMeQ [6] and TBT, 1 H NMR titration is used. As shown in Figure 1, with the addition of TMeQ [6], the proton signal peak is shifted accordingly. For example, the signal of both Ha, Hb and Hc are shifted downfield, while the signal of Hd, He and Hf have almost unchanged in the presence of TMeQ [6]. Therefore, it can be preliminarily inferred that the interaction between TMeQ [6] and TBT is mainly driven by outer-surface interaction between the carboxylic carbon chain of TBT and the methyl or hydrogen of TMeQ [6] on its outer surface. In addition, when using UV-vis spectrum ( Figure S5 and S6) to investigate the interaction between them, it is found that the presence of TMeQ [6] did not affect the 4 absorbance of TBT, which infers that TMeQ [6] does not interact with the benzene ring or the melamine of TBT. On the contrary, their outer-surface interaction occurs on the outer surface of TMeQ [6] and the carboxyl group of TBT, which is consistent with the results of NMR.

The host-guest interaction of Q[7]-TBT
Using the same 1 H NMR titration method as above to investigate the interaction between Q [7] and TBT, it is found that their interaction changes significantly, from the outer-surface interaction to host-guest interaction because of the larger cavity of Q [7].
As shown in Figure 2, the proton signal peak of the entire TBT upfield with the increasing amount of Q [7]. For example, the signal of Ha shifted from δ= 1.88 ppm to 5 1.64 ppm, Hb from δ= 2.32 ppm to 2.25 ppm, Hc from δ= 3.97 ppm to 3.34 ppm, Hd from δ= 6.88 ppm to 5.87 ppm, He from δ= 7.68 ppm to 7.06 ppm and Hf from δ= 9.52 ppm to 9.29 ppm. Naturally, it can be inferred that Q [7] bound with the entire TBT with a strong host-guest interaction. In addition, we also used UV-vis spectrum to verify above inference and further investigate their molar radio in detail. Q [7] has a larger cavity compared with TMeQ [6], that can bind with TBT, so the absorbance of TBT gradually decreases and redshifts in the presence of Q [7] (Figure 3), which is mainly due to the π-π * and n-π * transition caused by the hydrophobic effect of the Q [7] cavity after binding with the phenyl and carboxyl groups of TBT. Meanwhile, the absorbance of TBT is gradually approaching the saturation state, when the amount of Q [7] reaches 3 times the amount of TBT.
6 Therefore, it can be inferred that Q [7] binds with the three "arms" of TBT at a molar ratio of 3:1 (NQ [7]:NTBT = 3:1) and forms a host-guest complex Q[7]-TBT. In addition, the ITC experiment also strongly supports the above results, which data can be fitted to a very suitable curve using the model of Sequential Three Site ( Figure S7), and corresponding binding ability (Ka) is 1.422×10 6 M -1 .

The host-guest interaction of Q[8]-TBT
Since Q [8] has a larger cavity than Q [7], it can definitely bind the entire TBT molecule like Q [7]. However, in the 1 H NMR titration experiment ( Figure S8), it's found that upon the addition of Q [8], the chemical shift value of TBT did not change significantly. But with the continuous increase concentration of Q [8], the proton signal of TBT began to weaken and the proton signal of Q [8] has not been detected during the whole experiment. In addition, UV-vis spectrum in Figure 4a shows that, as continuously increasing amount of Q [8], the absorbance of TBT keeps on decrease from A= 0.735 to 0.112 (ΔA=0.623), and is no red shift or blue shift. Both of above phenomena and experiment show that Q [8] interacted with the "arm" of TBT and produced corresponding precipitation due to aggregation, which is also the reason why the proton signal and absorbance of TBT in the 1

Detection of Ag + based on Q[7]-TBT
The guest molecule TBT contains three carboxyl groups and a wealth of lone-pair electrons, so it has a high binding ability to metals. In this study, Q[7]-TBT was selected as a UV detector to detect common metals. As shown in Figure 5 In order to further explore the detection limit (DL) and detection mechanism of Q[7]-TBT towards Ag + , UV-vis titration experiment was carried out. As shown in Figure 6, with the continuous addition of Ag + , the absorbance of Q[7]-TBT continues to increase at λmax=258 nm, which is caused by the n-π * transition of Q[7]-TBT. Therefore, it can be further inferred that Ag + mainly binds with the carboxyl group of TBT. In addition, we calculated the value of DL is 3.91×10 -6 M, and the corresponding fitting formula is y=-0.01+0.0182x with a high R 2 =0.997. The synthesis of TBT: compound 2 (0.780 g, 1.0 mmol)) and NaOH (0.27 g, 6.75 mmol) were combined in a 1:1 solution of acetonitrile: water (20 mL) and reflux for 4hs.
The mixture was concentrated under vacuum and then acidified it by HCl to pH=2 to 11 precipitate a white solid TBT. 1

Supporting Information
Supporting Information File 1: