Theoretical exploration for recognition mechanism of two similar coumarin-based probes on Hg2+ and Cu2+

Journal of Molecular Structure 1198 (2019) 126870

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Journal of Molecular Structure journal homepage: http://www.elsevier.com/locate/molstruc

Theoretical exploration for recognition mechanism of two similar coumarin-based probes on Hg2þ and Cu2þ Haiyan Wang a, b, Shun Yao a, Qian Liu a, Kun Wang a, Haizhu Yu a, Xiaojiao Zhu a, c, Lin Kong a, c, **, Hongping Zhou a, c, * a

College of Chemistry and Chemical Engineering, Anhui University, 230601, Hefei, PR China Institutes of Physical Science and Information Technology, Anhui University, PR China Key Laboratory of Structure and Performance of Functional Hybrid Materials of Ministry of Education, Key Laboratory of Functional Inorganic Materials Chemistry of Anhui Province, Key Laboratory of Chemistry for Inorganic/ Organic Hybrid Functionalized Materials, Anhui Province, 230601, Hefei, PR China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 September 2018 Received in revised form 9 June 2019 Accepted 30 July 2019 Available online 31 July 2019

Two coumarin-based fluorescent probes with two or one pyridine groups, denoted as L1 and L2, were synthesized, which exhibited quick identification of Hg2þ and/or Cu2þ in water medium, respectively. In order to explore the difference in recognition performance caused by structure and to guide the structural tuning in the experiments, optimized structures, Natural Bond Orbital (NBO) atomic charges, Wiberg bond index (WBI), HOMO-LUMO gaps and complexation energies between probes and center cation were calculated by density functional theory (DFT) methods using BP86 with 6e31(d-p) and lanL2DZ basis sets. Combining the calculated parameters of different coordination positions and coordination numbers of Hg2þ and Cu2þ complexes with the experimental results, the most likely coordination modes are inferred, which provides the beneficial guidance for designing probes rationally. © 2019 Elsevier B.V. All rights reserved.

Keywords: Reaction mechanism Five-coordination mode Mercuric ion Copper ion Combination Theoretical calculations

1. Introduction It is well known that transition metals are of great concern, based on the fact that trace amounts of most transition metals play significant roles in living organisms, mainly because they have an extremely significant eco- and toxicological influence in both the environment and human beings as a result of bioaccumulation [1e4]. For instance, Hg2þ is a kind of nonbiodegradable pollutant [5,6] that can directly be absorbed by skin, the respiratory system and seriously threaten human health [7,8]. In particular, Cu2þ is the third most abundant essential trace element in the human body, which participates in fundamental physiological processes such as cellular respiration, bone formation, and connective tissue development. Excessive concentrations of free copper ions in living cells can cause various diseases [9]. Therefore, the effective and

* Corresponding author. College of Chemistry and Chemical Engineering, Anhui University, 230601, Hefei, PR China. ** Corresponding author. College of Chemistry and Chemical Engineering, Anhui University, 230601, Hefei, PR China. E-mail address: [email protected] (H. Zhou). https://doi.org/10.1016/j.molstruc.2019.126870 0022-2860/© 2019 Elsevier B.V. All rights reserved.

quantitative detection of Hg2þ [10e12] and Cu2þ [13e17] is of great significance in physiology, environmental science, and pharmacy. In many measurement methods, organic fluorescent probes are efficient means for the nondestructive, facile monitoring of target metal ions due to high sensitivity, specificity, low cost, and rapid tracking of analytes in biological [18e20], toxicological [21e23], and environmental samples [24e27]. In the last few years, various small molecular organic fluorescent probes have been developed for the recognition of Hg2þ [28e30] and Cu2þ [31e33]. Recently, our research group has synthesized two novel coumarin-based probes L1 and L2 (Fig. 1). It is worth mentioning that Hg2þ (turn on) was recognized by L1 in H2O and L2 in CH3OH/H2O, respectively, and Cu2þ (turn off) was recognized only by L1 in H2O. As is known, the bivalent cations (Hg2þ and Cu2þ) can serve to link ligands to form new coordination complexes with various coordination modes including different coordination numbers and binding sites. Taking Hg2þ for example, general patterns of coordination are four-coordinated in a tetrahedral geometry [34,35] and six-coordinated with a distorted octahedral geometry [36]. Similarly, Cu2þ exhibits several coordination geometries, among which the four-coordination is the most common due to sp3 or sp3d2

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Fig. 1. Molecular and crystal structure diagrams of prboes L1 and L2.

hybrids [37,38]. In general, revealing the configuration of the complex plays an important role in understanding the mechanism of the reaction, which are based on the single crystal structural data, the 1H NMR titration and Mass spectra. It is well-known that there is a certain luck for obtaining single crystals and 1H NMR or MS data often derive from the experiments, which would bring some degree of uncertainties in explaining mechanism. On this basis, the theoretical calculation comes into view of the researchers, which can be used to assist in determining the configuration of the complex by comparison of the bond angles, bond lengths, bond index and the binding energies of the predicted complexes. In this study, different coordination pathways of Hg2þ and Cu2þ complexes with different coordination numbers were built and calculated, which further inferred the reaction mechanism by combining experimental results.

2. Experimental section As shown in Fig. 1, two novel coumarin-based probes, named L1 and L2 [39], were reported by our group, where one or two 2pyridine units were introduced into the ether chain by nucleophilic substitution. Firstly, the introduction of ether chain enhances

water solubility. Secondly, the target compounds can recognize the corresponding metal ions from CH3OH/H2O media to H2O by making fine adjustments of the structure (Detailed experimental process are showed in supporting information Scheme S1). But recognition mechanism via coordination modes of ligands together with metal ions and the role played by 2-pyridine unit needs to be studied further.

3. Methods All the calculations are performed with Gaussian09 package [40]. The geometry optimizations of all compounds in the work are performed by BP86 method [41] based on the density functional theory (DFT) calculations. In the calculation, 6-31G (d-p) [42] basis set was assigned to all elements with the exception of metal ions (Hg2þ and Cu2þ), for which the LanL2DZ basis set [43] was employed. All the frequencies were obtained at the same theoretical level in order to calculate the thermodynamic properties. And the solutions of metal ions were prepared from Cu(NO3)2$3H2O and HgCl2.

Fig. 2. Structure diagram of complex of L1-HgCl2-1, L1-HgCl2-2 and L1-HgCl2-3 upon four-coordinated models.

H. Wang et al. / Journal of Molecular Structure 1198 (2019) 126870 Table 1 NBO atomic charges and WBI of L1-HgCl2-1, L1-HgCl2-2 and L1-HgCl2-3 upon fourcoordinated models. Compounds

NBO charges(a.u.)

Wiberg bond index

L1-HgCl2-1

Hg1 1.13 Cl2 -0.65 Cl3 -0.62 O4 -0.54 O5 -0.65 Hg1 1.18 Cl2 -0.65 Cl3 -0.64 O4 -0.51 O5 -0.65 Hg1 1.11 Cl2 -0.68 Cl3 -0.67 N4 -0.53 N5 -0.53

Hg1e Hg1e Hg1e Hg1e

L1-HgCl2-2

L1-HgCl2-3

Cl2 0.49 Cl3 0.53 O4 0.05 O5 0.10

Hg1e Cl2 0.50 Hg1e Cl3 0.52 Hg1e O4 0.03 Hg1eO5 0.13 Hg1e Hg1e Hg1e Hg1e

Cl2 0.46 Cl3 0.48 N4 0.16 N5 0.16

4. Results and discussions 4.1. Four-coordination complexes analysis of L1/HgCl2 Through the Job's plot analysis (Figs. S20eS21 of Reference [39]) and the mutual effect between L1 and Cu2þ by MAlDI-TOF mass spectrometry (Fig. S22 of Reference [39]), the binding stoichiometric ratio of L2-HgCl2, L1-HgCl2 and L1-Cu(NO3)2 complexes were determined to be 1:1, respectively. Because HgCl2 is a typical sp hybrid covalent compound, according to the loci of lone pair electrons, L1-HgCl2 is most likely to adopt four coordination and five coordination modes, including HgeCl covalent bonds, three

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four-coordinated complex models were constructed just as shown in Fig. 2. The structure of L1 and HgCl2 were optimized including the corresponding NBO atomic charges in the case of the solvation effect. Based on the above chemical measurement ratio, the complex models were set up, followed by the structure and energy calculations. Fig. 2 showed that HgeCl bond lengths of L1-HgCl2-3 are 2.70 Å and 2.76 Å, and the corresponding HgeN bond lengths are 2.53 Å and 2.54 Å. These values are all in the reasonable range of coordination bond lengths [44], which are relatively more reasonable compared with the HgeCl and HgeN bond lengths in L1-HgCl2-1 and L1-HgCl2-2. The same conclusion can be obtained from NBO atomic charges and WBI [46] (Table 1). NBO analysis is a method for studying hybridization and covalent effects in polyatomic molecular systems, which was proposed by Alan E. Reed, Weinstock and Weinhold in 1984 [45]. WBI has been proposed by Wiberg as a calculated bond property using orthonormalized atomic orbitals which are a basis set for semi-empirical molecular orbitals. WBI analysis is a measure of electron population overlap between two atoms [46]. Both of them are indicators of hybridization and covalence effects in polyatomic molecular systems. The larger NBO atmoc charges and WBI are, the higher degree of covalence of the molecule has. On the contrary, the interaction between atoms may be mainly Kulun effect (interaction between two static point charges) [47], such as electrostatic interaction. As shown in Table 1, the major WBI data of L1-HgCl2-3 come from Hg attached by coordination bonds to nonmetal ions (0.46, 0.48, 0.16, 0.16, respectively), which are attributed to four relatively balanced coordination bonds. In addition, bond angles of L1-HgCl23 were 101.23 (Cl2eHg1eCl3), 117.23 (Cl2eHg1eN4), 101.00 (N4eHg1eN5), 111.15 (N5eHg1eCl3) respectively, which

Fig. 3. HOMO-LUMO orbital energies diagram of L1-HgCl2-1, L1-HgCl2-2 and L1-HgCl2-3 upon four-coordinated models.

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Fig. 4. Complexation energies of L1-HgCl2-1, L1-HgCl2-2 and L1-HgCl2-3 upon four-coordinated models.

obviously shows that L1-HgCl2-3 has tetrahedral structure formed by sp3 hybrids. However, the tiny Wiberg bond index of Hg1eO4, 0.05(L1-HgCl2-1) and 0.03(L1-HgCl2-2), show that covalent bonds are weak. Among the three complexes, Hg1 NBO charges of L1HgCl2-3 (1.11) are the lowest, which implies that L1-HgCl2-3 has higher covalency. From the above, it can be concluded that L1HgCl2-3 has the strongest covalence and the highest bond level among them. To further verify the above conclusions, the HOMO-LUMO gaps (Fig. 3) and complexation energies (Fig. 4) continued to be studied. HOMO-LUMO gap or the energy difference between the HOMO and LUMO is generally the lowest energy electronic excitation that is possible in the molecule [48]. A large HOMO-LUMO gap implies high stability for the molecule in chemical reactions [49]. As displayed in Fig. 3, L1-HgCl2-3 has the maximum thermal stability. Complexation energies [50] refer to the energies emitted by the binding of free molecules or atoms, which can be expressed as E(complexation) ¼ E(1)þE(2)-E(M). E(1)and E(2) refer to the energies of fragmental molecules (HgCl2 and L1), and E(M) refers to

the energy of the whole molecule. The lower the E(complexation) is, the more stable the whole molecule is. The calculated complexation energies (kcal/mol) of L1-HgCl2-1, L1-HgCl2-2, L1-HgCl2-3 were 17.46, 22.35, 42.02, respectively. Obviously, thermodynamic data indicate that L1-HgCl2-3 has relatively higher stability and coordination tendency. 4.2. Five-coordinated complexes analysis of L1/HgCl2 According to the spatial configuration of L1, there is another N atom besides the two N atoms in the pyridine rings as the possible binding side, which provides possibility for five-coordination. And then five-coordinated complex models of L1/HgCl2 was hypothesized, and the corresponding chemical bond data and complexation energies were calculated. As shown in Fig. 5, the typical bond lengths, NBO atomic charges and Wiberg bond index data indicate that five-coordinated mode is very stable. Furthermore, the HOMOLUMO orbital energy gap is 1.92 ev (Fig. S1) and the complexation energy of L1-HgCl2-4 is 52.29 kcal/mol, which is lower than the

Fig. 5. The optimized structures, typical bond lengths (black), NBO atomic charges (red) and WBI (blue) of L1-HgCl2-4 upon five-coordinated models.

Fig. 6. Single crystal and molecular structure diagrams of L1-HgCl2-4.

H. Wang et al. / Journal of Molecular Structure 1198 (2019) 126870

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Table 2 Experimental and calculated coordination bond lengths (A) and angles  () for L1-HgCl2-4 upon five-coordinated models. Comparison items Bond lengths(Å) Hg1eCl2 Hg1eCl3 Hg1eN4 Hg1eN5 Hg1eN6 Bond angles() Cl2eHg1eN4 Cl1eHg1eCl2 N4eHg1eN5 N5eHg1eN6 Cl2eHg1eN6

Experimental values

Calculated values BP86/6-31 g(d,p)/lanL2DZ solvent:H2O

2.43 2.42 2.36 2.67 2.39

2.68 2.71 2.56 2.78 2.56

100.99 130.84 67.54 66.96 99.32

89.35 101.84 66.33 66.95 102.41

Fig. 7. The optimized structures, typical bond lengths(black), NBO atomic charges (red) and WBI(blue) of L1-Cu(NO3)2 upon five-coordinated models.

comlexation energies of L1-HgCl2-1 (17.46 kcal/mol), L1-HgCl2-2 (22.35 kcal/mol) and L1-HgCl2-3 (42.02 kcal/mol). 4.3. Comparison of experimental and calculated values for L1HgCl2-4 Fortunately, the single crystal (Table S1 of Reference [39]) of complex by L1 combining with HgCl2 was obtained, whose structure is the same as that of L1-HgCl2-4 (Fig. 6). As is known, the experimental parameters (solvent, temperature, and etc) would influence the single crystal structure, whose structure parameters (bond lengths, bond angles and coordination) could be slightly different from those of theoretical structure. As shown in Table 2, judging from the difference between the same group data, the theoretical calculation is quite consistent with the changing trend of the experimental values, which confirms the credibility of the previous calculations.

4.4. Five coordinated complex analysis of L1/Cu(NO3)2 As demonstrated above, the experimental results of L1-HgCl2-4 are highly consistent with the theoretical value, which implies that such a coordination pattern is more reasonable in these similar ligands including pyridine rings. On the basis, L1-Cu(NO3)2 was hypothesized, and the corresponding chemical bond data and complexation energies were calculated, which was formed by the similar pattern. Just as showed in Fig. 7, nature of Cu1eN2 and Cu1eN3 is ion bonds, obviously manifested by Wiberg bond index (Cu1eN2 0.0037, Cu1eN3 0.0027). The bond lengths of Cu1eN4, Cu1eN5 and Cu1eN6 are 2.24 Å, 2.52 Å and 2.03 Å, respectively, which are relatively reasonable. In the same way, the HOMO-LUMO orbital energy gap (1.93 ev) and complexation energy (62.82 kcal/ mol) are both calculated as shown in Fig. S2.

Fig. 8. The optimized structures, typical bond lengths (black), NBO atomic charges (red) and WBI (blue) of L2-HgCl2-4 upon five-coordinated models.

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Fig. 9. The optimized structures, typical bond lengths (black), NBO atomic charges (red) and WBI (blue) of L2-Cu(NO3)2 upon five-coordinated models.

4.5. Five coordinated complex analysis of L2/HgCl2 and L2/Cu(NO3)2

References

As displayed in Fig. 8, the five coordinated complexes of Hg2þ and L2 were constructed. Because L2 lacks one pyridine ring compared with L1, oxygen in H2O was involved in fivecoordination, which is accordance with the results of the NMR titration experiment (Fig. S17 of Reference [39]). The calculated complexation energy of L2-HgCl2-4 is 50.84 kcal/mol, however, the complexation energy of four coordinated complex L2-HgCl2-3 (Fig. S3) is calculated to be 37.67 kcal/mol, which proved that the five-coordinated mode is also suitable for L2 and Hg2þ. With the same method, the complexes of Cu2þ and L2 were constructed repeatedly, just as exhibited in Fig. 9. The complexation energy is 39.09 kcal/mol, which is obviously higher than that of L2-HgCl2-4 (50.84 kcal/mol) and L1-Cu(NO3)2 (62.82 kcal/mol). Therefore, one could clearly see that the five coordination modes for the similar pyridine ring systems combining with Cu2þ had lower stabilities compared with that of Hg2þ, which was consistent with the previous described experimental phenomena that L2 could not recognize Cu2þ.

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5. Conclusion In summary, a series of theoretical structure models of coumarin-based complexes were presented, which were formed by probes L1/L2 and Hg2þ/Cu2þ. By combining the experimental results, coordination models located at different binding sites were built and the corresponding chemical bond data were calculated. The most reasonable five-coordination model of L1 combined with Hg2þ was obtained, which was in good agreement with the experimental results. Meanwhile, five-coordination models of L1 with Cu2þ, L1 with Hg2þ and Cu2þ in the same method were constructed, which explained the coordination modes. Acknowledgment This work was supported by the National Natural Science Foundation of China (51772002, 51472002 and 21805001), Changjiang Scholars and Innovative Research Team in University, Science and Technology Plan of Anhui Province (1604b0602016). Authors would thank Anhui University Research Start-up Fund and Natural Science Fund of Education Department of Anhui province (KJ2018A0024). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.126870.

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