Dynamic Conversion Between SeâN Covalent and Noncovalent

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On the Dynamic Conversion Between SeN Covalent and Non-Covalent Interactions Meng Xie, Lili Wang, Fang Liu, Dongju Zhang, and Jun Gao J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08003 • Publication Date (Web): 19 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

On the Dynamic Conversion Between Se-N Covalent and Non-Covalent Interactions Meng Xie1, Lili Wang3*, Fang Liu2, Dongju Zhang1, Jun Gao1,2* 1

School of Chemistry & Chemical Engineering, Shandong University, Jinan, 250100, P. R. China

2

Hubei Key Laboratory of Agricultural Bioinformatics, College of Informatics, Huazhong Agricultural University, Wuhan, 430070, P. R. China

3

Advanced Research Center for Optics, Shandong University, Jinan, 250100, P. R. China

Corresponding Author E-mail: [email protected] Telephone number: +86-027-87386901 (Jun Gao) [email protected] Telephone number: +86-531-88369968 (Lili Wang)

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ACS Paragon Plus Environment


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ABSTRACT Se-N dynamic covalent bond is a new dynamic covalent bond which has applications in the fabrication of stimuli responsive and self-healing functional materials. Although recent advances have been achieved in the experimental aspect, little is known about the formation mechanism of Se-N dynamic covalent bond. Here the structures and nature of Se-N dynamic covalent bond between three kinds of pyridine derivatives R-C5H4N, [pyridine (R=H), 4-methylpyridine (R=CH3), 4-dimethylamino-pyridine (R=N(CH3)2)] and phenylselenyl bromine (PhSeBr) have been analyzed using density functional theory. The interactions between Se atom in PhSeBr and N atom in pyridine or pyridine derivatives can be divided into three models: dissociation, nonbonding interaction and covalent bond interaction. Quantum chemical calculations on three series compounds show that these three models can convert mutually, which results in the generation of Se-N dynamic covalent bond. Solvent effects produced in polar solvents such as CH2Cl2 can make the conversion between Se-N covalent bond and Se···N nonbonding interactions easier. The kind of the substituents in pyridine ring can affect the conversion process: the stronger the electron-donating ability of the substituent is, the easier the structure transformation is.

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1. INTRODUCTION Selenium is an essential trace element and micronutrient in the human body. Recently, organoselenium compounds which containing an chemistry interaction between Se and heteroatoms Y (like N, O, I, and F) have attracted considerable interest, and they have potential applications in enzyme mimetic1, 2, 3, therapeutic agents for cancer4, 5, 6, the synthesis of organic compounds7,

8, 9, 10, 11

, controlled self-assembly or

disassembly12, 13 and so on. In these Se···Y non-bonded interactions, selenium usually exists in a divalent state with two covalently bonded substituents and two lone pairs of valence electrons.11 Selenium atom can interact with a nearby heteroatom Y and form a pseudo-high-valent selenium species. With the help of NMR spectroscopy and theoretical calculation analyses, Iwaoka et al founded that Se···O weak non-bonded interaction has a dominant covalent nature rather than an electrostatic nature, and Se···F may be slightly more electrostatic.14,

15

du Mont et al. reported that

selenium-iodine interactions can change from undisturbed single bonds via n- > σ* (Se-I) interactions to van der Walls contacts by appropriate choice of the particular substituents and ligands.16 Besides, intramolecularly coordinated organoselenium compounds incorporating a Se···N interaction have obtained much attention since 1984.11 Iwaoka et al. reported the existence of non-bonded interaction between the divalent selenium and an amino nitrogen in seven 2-selenobenzylamine derivatives,17 and this interaction is derived chiefly from the orbital overlap of donor–acceptor. Zhou

et

al.

reported

a

periodic

supramolecular 3


[Ag-(bsd)2(NO3)·0.5bsd,


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bsd=2,1,3-benzoselenadiazole], and it is constructed by Se···N non-bonded interactions.13 Crystal structure of this supramolecular shows that the Se···N separation is much shorter than the van der Walls radii, which is similar to non-bonded interactions in halogen bonding, pnicogen bonding and carbon bonding.16, 18, 19, 20

Recently, Thomas et al. defined Se···N non-bonded interaction as chalcogen

bonding21 in their structural studies on the polymorphs of the organoselenium antioxidant ebselen and its derivatives.22 They presented the experimental charge density analysis using high-resolution X-ray data followed by topological analysis to unravel the nature of Se···N interactions in the chalcogen-bond synthon. Therefore, the widely accepted σ-hole model can be adopted to understand the nature of Se···N interactions.23, 24 Theoretical studies indicate that the nature of the Se···N interaction involves charge transfer from the nitrogen long pair to the σ* orbital of Se-N bond (nN→σ*Se-X orbital interaction) and is predominantly covalent.17,

25, 26, 27, 28

The dominant covalent

character is also discussed in Te···N interaction.29 Even though, these interactions are still named as non-covalent interactions. But the interactions between Se and N atom are not only non-bonded interaction. Yu Yi et al. reported a new dynamic covalent bond of Se-N formed between the Se atom in a phenylselenyl halogen species and the N atom in pyridine derivatives.30 The formation of the Se-N covalent bond was investigated by various experimental methods, including NMR, X-ray photoelectron spectroscopy (XPS) and dynamic light scattering (DLS). The bond energy estimated by Pauling equation is 46 kcal/mol, which is higher than ordinary Se···N 4


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non-covalent interaction energy.17 Furthermore, they discovered that Se-N bond exhibits multiple responses to external stimuli and can be reversibly formed or cleaved by treatment with other stronger electron-donating group. However, the detail information that how Se-N interaction converts from covalent state to non-covalent state, which are of great importance for us to understand and regulate the dynamic nature of self-assembly and disassembly.31 This promotes us to study the dynamic conservation process of Se-N interactions. Here, we focus on the following questions. Firstly, figure out what are the stable conformers of Se-N covalent and non-covalent interaction. Secondly, explore how these conformers mutually convert. Thirdly, try to discuss the regulation of this dynamic convert process. Finally, the effect factors such as substituent, solvent effect were also discussed. 2. MATERIAL AND METHODS In this work, three pyridine derivatives (pyridine [P], 4-methylpyridine [MP], 4-dimethylamino-pyridine [DMAP]) combining with PhSeBr were selected to form the Se-N interactions. Consequently, we got three Se-N dynamic covalent bond models (Scheme 1): P-PhSeBr, MP-PhSeBr, and DMAP-PhSeBr, which were named as p-system, m-system and d-system respectively.

Scheme 1. Se-N dynamic covalent bond formed between PhSeBr and pyridine derivatives.

All of the calculations were performed using GAUSSIAN 0932 program. The molecular geometries for possible conformers of Se-N interaction between PhSeBr 5



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and pyridine analogues were fully optimized at CAM-B3LYP33/6-311G++ (d,p) level. Single point energies were subsequently calculated at the level of MP2/6-311G++ (d,p). These levels can provide reliable results for structure optimization, energy calculation, frequency calculation and molecular orbital analysis.34, 35 Solvent effects were studied by Tomasi’s Polarized Continuum Model (PCM).36 The basis set superposition error (BSSE)37 was considered in order to calculate the binding energies between R-pyridine and PhSeBr accurately. It was calculated by MP2 method with the 6-311G++ (d,p) basis set, according to the formula: E(interaction) = E(AB)-E(A)-E(B)+E(BSSE). 3. RESULTS AND DISCUSSIONS 3.1 The Optimized Conformers of Se-N Covalent and Non-Covalent Interactions After a system searching and flexibility scanning, we got three stable conformers for each pair of interaction, and conformer 1 to 3 were used to name these structures. Figure 1 shows the detailed molecular morphology of these conformers, and we named these structures as p-1 to p-3 for p-system, m-1 to m-3 for m-system and d-1 to d-3 for d-system. Some important structural parameters like bond lengths and angles are listed in Table 1.

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Figure 1. The optimized structures obtained from quantum chemical calculations at the CAM-B3LYP/6-311G++ (d,p) level. Here R represents three substituents: H, -CH3 and -N (CH3)2. Conformer 1, 2, 3 are shown respectively in a, b, c.

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Table 1. Significant bond lengths (Å) and angles (゜) for three systems.

Conformer1

Conformer 2

Conformer 3

a

Br-Ha/Hb distance

Se-N distance

Se-Br distance

N-H1 distance

p-1

4.47

2.36

2.36

-

88.0

m-1

4.48

2.36

2.40

-

88.5

d-1

4.48

2.36

2.41

-

89.4

p-2

2.67

2.40

-a

-

179.8

m-2

2.64

2.41

-

-

179.9

d-2

2.57

2.42

-

-

179.9

p-3

1.92

4.22

-

2.71/2.64

54.1

m-3

1.92

4.24

-

2.74/2.63

54.6

d-3

1.91

4.83

-

2.38/2.69

65.3

Compound

∠BrSeN

These structure parameters have not be shown because they have no important significance in

corresponding conformers.

The distance between Se and N (dSe-N) is an important evidence to predicate bond types and the strength of the interaction between atoms. Here, dSe-N is compared with the sum of van der Waals radii (rw (Se) + rw (N) = 1.90 + 1.55 = 3.45 Å) and the sum of the single-bond covalent radii (rb (Se) + rb (N) = 1.14 + 0.7 = 1.84 Å) for Se and N atom.38 For conformer 1 (including p-1, m-1, d-1), dSe-N is about 4.48 Å, which is longer than 3.45 Å, indicating there is no interaction between Se and N in conformer 1. The distance between N atom and the H1 atom in benzene ring in conformer 1 is in the range of 2.41-2.36 Å, which is shorter than threshold of the hydrogen bond length (2.7 Å), so there exists hydrogen bond between N atom and H1 atom. We termed this 8


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state as dissociation state to specify there is no Se-N interaction between PhSeBr and pyridine analogues. For conformer 2 (including p-2, m-2 and d-2), dSe-N is in the range of 2.57-2.67 Å. It is shorter than the sum of the van der Waals radii but longer than the sum of the covalent radii, which indicates the existence of intermolecular Se···N non-bonding interaction between the Se and N in conformer 2. Furthermore, the angle of N···Se-Br is close to 180°and the geometry around the divalent selenium is T-shaped. These characteristics of conformer 2 fit very well with the Se···N non-bonding interactions described in the literatures.7, 25, 28 Here, Se···N non-bonding interaction is originated from the σ-hole interaction between Se and N atom (shown in Figure S1 in Supporting Information), which is similar to halogen bond.22, 39 For conformer 3 (p-3, m-3 and d-3), dSe-N is about 1.92 Å. It is much shorter than the sum of the covalent radii of Se and N atoms in conformer 2, which indicates that Se-N interaction is close to covalent bond interaction. At the same time, the distances between Br atom and nearby H atoms (including Ha and Hb) are shorter than the sum of the van der Waals radii (rw (Br) + rw (H) = 1.85 + 1.17 = 3.02 Å). So in the forming process of Se-N covalent bond, Br is far away from Se atom and form hydrogen bond with adjacent two H atoms which respectively in the pyridine ring and benzene ring. The existence of hydrogen bonding between Br and H atoms limits the degree of freedom for Br atom and increases the stability of conformer 3. Furthermore, bond order is one of the most important descriptors to describe the characteristic of electron distribution in molecules.40, 41 Wiberg bond order can be used to represent the covalence of chemical bond. At the same time, the value of bond 9



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energy represents the strength of the interaction between two atoms.42 They are used to estimate the strength of Se-N interactions. The binding energies between pyridine derivatives and PhSeBr in two conformers (2 and 3) were calculated with BSSE correction. Wiberg bond order was obtained from NBO analysis. The results show that the Se···N bond energy in conformer 2 (Table 2) is close to common hydrogen bonding (3-10 kcal/mol)43. So there exists strong intermolecular Se···N non-bonding interaction between Se and N atoms in conformer 2. The values of Wiberg bond orders are less than 0.1, which confirms the low covalence properties of Se···N non-bonding interaction in conformer 2. Interestingly, the bond energy of Se-N in conformer 3 is in the range of 59-72 kcal/mol, which is larger than 46 kcal/mol obtained from Pauling equation.30 The Wiberg bond orders of conformer 3 are around 0.7. It confirms that Se-N bond in conformer 3 is covalent bond. Table 2. The bond energy (kcal/mol) and wiberg bond order between Se and N atom in conformer 2 and conformer 3 of three systems. Compound

Bond energy

Wiberg bond order

p-2

8.47

0.0746

m-2

8.82

0.0810

d-2

10.25

0.1041

p-3

59.27

0.7092

m-3

61.01

0.7106

d-3

71.70

0.7141

10


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In general, the geometry, bond energy and bond order analysis indicated that there are three interaction models of conformers. Conformer 1 is in dissociation state, Conformer 2 is in nonbonding (non-covalent) interaction state and Conformer 3 is in covalent bond interaction state. So, the subsequent question is that how these three conformers convert mutually. 3.2 Conversion between Covalent and Non-covalent Interaction and Solvent Effect In order to figure out how these conformers convert mutually, Intrinsic Reaction Coordinate (IRC) method44 was used to get the reaction paths and transition states between any two conformers. For each structure pair in one system, we scanned one transition state connecting two conformers, and their structures are shown in Figure 2. Hence every system has three transition states, and different conformers in one system can contact to each other through relevant transition states. The potential energy surfaces for every system are shown in Figure 3. ΔE1-2, ΔE1-3 and ΔE2-3 are used to represent the transition energy barriers of corresponding two conformers in Table 3. In gas phase, ΔE1-2 is about 1 kcal/mol that means dissociation state can convert to non-bonding interaction state easily. While, the initial energy barrier ΔE2-3 and ΔE1-3 for all system is in the range of 27-42 kcal/mol, which indicates it is difficult to convert from conformer 2 to conformer 3 and from conformer 1 to conformer 3 in gas phase. This result is not consistent with experimental results.30 It should be noticed that Se-N interaction could be enhanced in polarity solvent, so how solvation effects the energy barriers was considered.25, 26,

28

Here, energy barriers in four kinds of

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solvent conditions including CCl4, CHCl3, CH2Cl2, H2O (their permittivity show an increasing trend) were calculated also at the level of MP2/6-311G++ (d,p). And these values were compared with energy barriers in gas phase, as shown in Table 3.

Figure 2. The structures of transition states for three conformers at the CAM-B3LYP/6-311G++ (d,p) level of theory.

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Figure 3. The potential energy surfaces in gas phase and in CH2Cl2 for p-system, m-system, d-system, respectively at the level of MP2/6-311G++ (d,p). The energy of conformer 2 in gas phase is considered as zero point of energy for every system.

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Table 3. The energy barriers ΔE (kcal/mol) of three systems in different solvents.

p-system

m-system

d-system

a

Gas

CCl4

CHCl3

CH2Cl2

H2O

(Ɛa =1)

(Ɛ=2.23)

(Ɛ=4.71)

(Ɛ=8.93)

(Ɛ=78.36)

ΔE1-2

1.08

1.12

1.19

1.25

1.34

ΔE2-3

41.19

28.45

21.32

17.90

14.22

ΔE1-3

29.04

27.29

26.21

25.65

20.03

ΔE1-2

0.99

1.06

1.16

1.23

1.33

ΔE2-3

39.99

27.14

20.01

16.60

12.96

ΔE1-3

28.58

26.88

25.85

25.32

24.74

ΔE1-2

0.94

1.07

1.22

1.32

1.47

ΔE2-3

35.09

21.60

14.31

10.88

7.25

ΔE1-3

27.62

25.97

25.03

24.57

24.07

Ɛ is the permittivity of solvent.

Our result shows that solvation effects have different impacts on energy barriers of mutual transformation between conformers. Some of them decrease and some of them increase. The energy barrier between conformer 2 and conformer 3 (ΔE2-3) is reduced obviously with the increasing polarity of solvent. For example in polarity solvents like H2O, ΔE2-3 decreases about 27 kcal/mol (especially for d-system, this value decreases from 35.09 kcal/mol to 7.25 kcal/mol). So converting mutually for conformer 2 and conformer 3 can be realized easily as long as selecting suitable polarity solvents. The changes of ΔE1-2 and ΔE1-3 in solvation conditions are weaker than ΔE2-3. ΔE1-2 has increase slightly but still less than 1.5 kcal/mol. Therefore, conformer 1 and conformer 14


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2 still can convert to each other freely. ΔE1-3 has slightly reduced with the increase of polarity of solvent. It is predicted that conformer 1 and 3 can convert mutually only in strong polar solvent. The most favorable convert path from conformer 1 to 3 is a two steps convert, from 1 to 2 and then 2 to 3. In a word, the solvation effects make the mutual conversion easier than in gas phase. The stronger the polarity of the solvent is, and the easier the mutual conversion of conformer occurs. Combined with the moisture sensitive of reactant PhSeBr, it can be concluded that these conformers can convert mutually in strong polar organic solvent. Furthermore, why solvation has greater effect on ΔE2-3 than on ΔE1-2 and ΔE1-3 was analyzed. The difference of dipole moments that makes these conformers present different stability and energies in polarity solvents (in Figure 4). In each system, the value of dipole moment of molecules increases with the polarity of solvent, and when the dielectric constant of solvent is more than a certain value, the value of dipole moment of molecules starts to change slowly (shown in Figure S2). At the same time, with the increase of dipole moment, the energies of conformers will decrease in different degree. Take d-system for instance, two structures including d-TS2-3 and conformer d-3 have bigger value of dipole moment, and their energies decrease more quickly than others with the increase of solvent polarity. The degree of energy reduction is closed to 40 kcal/mol and 30 kcal/mol in H2O for d-TS2-3 and d-3 respectively, which compared with the energy in gas, as shown in Figure S3. This makes the energies of d-2 and d-3 close to each other, which increases the possibility

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of mutual conversion between d-2 and d-3. The other two systems have the same change trends with d-system.

Figure 4. The 3D relationship among relative energy, dipole moment and permittivity of solvent for d-system.

In the same solvent, different systems have various abilities of structure transformation, which shows the effect of substituent. In this work, the order of electron donating ability of substituent on pyridine ring of these three systems is d-system (R = N (CH3)2) > m-system (R = CH3) > p-system (R = H). In order to study the effect of the substituent, the energies of each structure for three systems in the same solvent were compared in Figure 5. The trend of ΔE2-3 is d-system (10.88 kcal/mol) < m-system (16.60 kcal/mol) < p-system (17.90 kcal/mol), which is contrary with the ability of electron donating of substituent on pyridine ring. Hence, the stronger electron donating ability of the substituent on pyridine ring is more favorable for the converting of structures.

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Figure 5. The potential energy surfaces for p-system, m-system and d-system in CH2Cl2.

3.3 Thermodynamics Stability and IR Spectroscopy Property Analysis In order to further confirm the stability of covalent bond state and non-bonding interaction state accurately, the energies of conformer 2 and conformer 3 in dichloromethane solvent condition were calculated in high level methods. For p-system and m-system, the energies of conformer 3 are higher than energies of conformer 2 in these different calculation levels (in Table 4). But for d-system, basis set has a big effect on the energy differences between conformer 2 and conformer 3. When the basis set is aug-cc-pVDZ, the energies of conformer 3 are lower than energies of conformer 2 and when the basis set is 6-311G++ (d,p), the energies of conformer 3 are higher than energies of conformer 2. The energy difference for d-system keeps within 5 kcal/mol and has a trend of decrease with the improvement of calculation method. So, considering the calculation errors from precision of calculation methods, simplification of calculation model and the differences with the true reaction environment, there don’t have a consistent and certain conclusion for which structure is more stable between covalent bond state and nonbonding 17



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interaction state. It may be need a further research at higher accurate methods and more realistic simulation environment. Table 4. The energy differences ΔE (kcal/mol) between conformer 2 and conformer 3 at the different levels. Calculation level

ΔEpa

ΔEm

ΔEd

MP2/6-311G++ (d,p)

13.40

11.90

4.33

MP3/6-311G++ (d,p)

14.71

12.70

3.35 (3.20)b

MP2/aug-cc-pVDZ

6.64

5.35

-1.36

MP3/aug-cc-pVDZ

8.16

6.33

-2.62

a

ΔE= E (conformer 3) – E (conformer 2)

b

Calculated at CCSD/6-311G++ (d,p) level

In addition, the IR spectroscopy for the Se-N dynamic covalent bond is explored as a probe to judge the dynamic properties in the experimental research. In order to further investigate the properties of the Se-N dynamic covalent bond, it is also essential to examine the IR spectroscopy of the DMAP-PhSeBr dimers. In this work, the IR spectroscopy of d-system was calculated in the level of CAM-B3LYP/6-311G++ (d,p). According to Figure 6, the IR spectroscopy for d-1 is closed to d-2, because DMAP and PhSeBr are linked by weak intermolecular interaction, which have little influence on each other in these two complexes. So infrared absorption peaks for d-1 and d-2 can be seen as sum of absorption peaks for alone DMAP and PhSeBr molecular. But for the existence of interactions, the position of some absorption peaks occur an offset. Most clearly, the position peak for the skeleton vibration of pyridine ring shows 18


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bathochromic effect from 1675 cm-1 (in DMAP) to 1678 cm-1 (in d-1) and 1684 cm-1 (in d-2) respectively. Because DMAP and PhSeBr are linked by Se-N covalent bond interaction, the position and intensity of many peaks have changed in complex d-3, which leads to the largest changes than the former two complexes. In addition, the stretching vibration of Se-N bond at 594 cm-1 further verifies the existence of Se-N covalent bond in d-3.

Figure 6. IR spectrums for d-system. Here epsilon is molar absorption coefficient, means the absorbance of solution when the concentration of solution is 1mol/L and thickness of solution is 1cm.

3.4 Toward Regulating Se-N Dynamic Property Combining with our calculation results and the experimental data based on the work of Yu Yi et al30, the factors which can regulate dynamic properties of Se-N dynamic covalent bond and their impact mechanisms are shown in Figure 7. Firstly, the polarity of solvent can change the polarity and dipole moment of molecules, and then change the energies of molecules and energy barriers of structure transformation. Strong polarity of solvent is beneficial to structure conversion. Secondly, the strong 19



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electron-donating ability of the substituent can increase the electron-donating ability of N atom on pyridine ring, and increase the bond energy and wiberg bond order of Se-N covalent bond, which reduce the energy barriers of structure converting and promote the stability of Se-N covalent bond. This is in accord with the experiment result of exchange reaction of Se-N bond reported by Yu Yi et al30. Thirdly, the addition of acid and alkali reagents can induce the reversible formation and cleavage of Se-N dynamic covalent bond. Take CF3COOH for instance, the proton H+ dissociate from CF3COOH at solution state is added to the conformer 3 which has formed Se-N bond, then the proton will attack to N atom in the Se-N bond and induce the break of Se-N and form N-H bond. It is agreed with the Se-N cleavage mechanism in ebselen when it generates complexation with reactive oxygen species such as the hydroxyl radical, superoxide radical and hydroxide anion, which was mentioned by Thomas et al.22 In this process, the energy of whole system is decreased by 65.27 kcal/mol finally with the reduction of distance between H+ and N atom (in Figure S4), which indicates the attacking of proton H is very easy. Furthermore, when the pyridine is added subsequently, pyridine can reform Se-N bond with PhSeBr. This induces the reversible formation and cleavage. Lastly, change temperature. The vibration of Se-N dynamic covalent bond can intensify with the increase of temperature, so it is easy to breakdown to R-pyridine and PhSeBr under the condition of heating.

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Figure 7. The sketch map of relationship among three structure states: Se-N dissociation state, Se···N nonbonding interactions state and Se-N covalent bond state.

4. CONCLUSIONS We have successfully explored the formation mechanism of Se-N dynamic covalent bond. According to calculation results, the complexes achieved from the reactions between pyridine derivatives and phenylselenyl bromine have three structure states: Se-N dissociation state, Se···N nonbonding interactions and Se-N covalent bond state. In Se-N dissociation state, there is no interaction between Se and N and only have hydrogen bond between N and nearest H. For Se···N nonbonding interaction state, it is stabilized by orbital interaction involving the delocalization of nitrogen lone pair electrons into the suitably aligned Se-Br antibonding orbital (nN→σ*Se-Br, shown in Figure S5). In Se-N covalent bond state, Se and N atom bond to each other by covalent bond. These three conformers can transform to each other through transition state structures. In the process of transformation, solvent effects and substituent effects on pyridine ring can impact the energy barriers between conformers. Organic 21



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solvent (CH2Cl2) with strong polarity can make the conversion between Se-N covalent bond and Se···N nonbonding interactions easier. The stronger the electron-donating ability of the substituent (N(CH3)2 > CH3 > H) on pyridine ring is, the easier the transformation is. This research provides new important theoretical support for Se-N and other selenium-containing dynamic covalent bond. It is hoped that this work will contribute to a greater understanding of organoselenium chemistry. ASSOCIATED CONTENT Supporting Information Figure S1, Electrostatic potential maps of pyridine and phenylselenyl bromine; Figure S2, The change trend of dipole moment with the permittivity of solvent for d-system; Figure S3, The linear relationship between relative energy and dipole moment with the permittivity of solvent for d-system; Figure S4, The curve of energy (kcal/mol) of m-3 changing with the distance (Å) between N atom of Se-N covalent bond and proton H atom; Figure S5, Orbital interaction between the occupied lone pair of N and unoccupied Se-Br σ* antibonding orbital in p-2. ACKNOWLEDGMENS This work is supported by the National Natural Science Foundation of China (No. 21373124), and also supported by Huazhong Agricultural University Scientific & Technological Self-innovation Foundation (Program No.2015RC008) and Project 2662015PY113 Supported by the Fundamental Funds for the Central Universities. The authors also thank the support of Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase). 22


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