# Properties of Metal-Doped Covalent Organic Frameworks and Their Interactions with Sulfur Dioxide

Authors: Ju Wang; Jia Wang; Wenchang Zhuang; Xiaoqin Shi; Xihua Du
Journal: Journal of Chemistry
 (2018)

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## Abstract

Covalent organic frameworks are unique for their highly open architecture and attractive for use as promising gas adsorption and storage carriers. In this work, density functional theory calculations have been performed to investigate the properties of metal-doped covalent organic frameworks and their interactions with the SO2 gas molecule. It is found that a single metal atom (including Li, Na, K, and Sc) doped at the top of phenyls within the tetra(4-dihydroxyborylphenyl) silane (TBPS) building block of covalent organic frameworks can easily lose its valence electrons and can be positively charged. The SO2 gas molecule could be stably absorbed onto the metal-doped covalent organic frameworks. The absorbed SO2 molecule interacts with Li, Na, K, and Sc metal-doped covalent organic frameworks by the dominant donor-acceptor delocalization between 1-center lone pair of an oxygen atom within SO2 and 1-center non-Lewis lone pairs of the doped metal atom.


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## Body

## 1. Introduction

Sulfur dioxide gases, mainly resourced from industrial activities, are known as the major source of atmospheric pollution leading in particular to acid smog formation and acid rain [1, 2]. Reducing sulfur dioxide emission has become one of the most important social and environmental challenges [3–5]. It should be pointed out that adsorption by porous nanomaterials is recognized as an efficient and economical approach for capture of low concentration SO2 from mixture gases [6–10]. Covalent organic frameworks (COFs) are ideal porous materials for gas capture due to their low density, good stability, and large surface area [11–15]. Target covalent organic framework porous materials have been designed and synthesized for sulfur dioxide gases adsorption and separation. Lee et al. developed functionalized covalent organic frameworks reversible for SO2 and highly stable on repeated adsorption-desorption cycles [16].Doping of metals into covalent organic frameworks is one of the most effective modifications of covalent organic frameworks for gases capture [17–20]. Experimental and theoretical studies have been carried out to explore on the doping of metals into covalent organic frameworks in order to enhance their capture for hydrogen and carbon dioxide gases. Yang et al. doped metal Pd clusters onto COF-1 materials and enhanced their hydrogen storage properties under mild conditions [21]. Guo et al. reported the doping of Pt cluster onto covalent organic frameworks and investigated hydrogen spillover reaction mechanism [22]. Stegbauer et al. reported CO2 sorption properties in two isostructural azine-linked covalent organic frameworks based on 1,3,5-triformyl benzene (AB-COF) and 1,3,5-triformylphloroglucinol (ATFG-COF) and hydrazine building units, respectively [23]. However, there are few studies focused on SO2 gases adsorption and separation on the metal-doped covalent organic frameworks and the interactions between the metal-doped covalent organic frameworks and SO2 gases.In this work, density functional theory calculations have been performed to investigate the properties of metal-doped covalent organic frameworks and their interactions with SO2 gas. Considering the excellent doping effect of alkali metals and transition metals into the covalent organic framework for hydrogen and carbon dioxide [17–23], we thus choose alkali (Li, Na, and K) and transition metals (Sc) doped into covalent organic frameworks and investigated properties of the metal-doped covalent organic frameworks and their interactions with the SO2 gas molecule. The main focus is to understand the metal doping into covalent organic frameworks and their influence on sulfur dioxide capture.


## 2. Computational Details

The cluster model of covalent organic frameworks presented in Figure1 consisting of the tetra(4-dihydroxyborylphenyl) silane (TBPS) and 2,3,6,7,10,11-hexahydroxy triphenylene (HHTP) building blocks selected from COF-105 [24] is adopted to represent the real structure of COF-105 for the saving computational cost. The cutoff functional groups of the covalent organic framework cluster model are saturated by hydrogen. Considering the different doping sites, alkali (Li, Na, and K) and transition (Sc) metals were doped into the cluster model of COF-105, respectively. All geometry optimization and frequency analysis calculations were done by the hybrid density functional B3LYP [25] and 6-31G(d) basis set. Interaction energy calculations were performed at B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d) level of theory with counterpoise algorithm to eliminate the overlap error of basis functions [26]. AIM and NBO calculations based on the optimized geometries at B3LYP/6-31G(d) level of theory were further carried out to analyze, evaluate, and classify the nature of the interactions within the metal-doped COF-105 complexes and their interactions with the absorbed sulfur dioxide gas molecule. Gaussian 09 package [27], AIM 2000 software [28], and NBO 5.9 program implemented in the Gaussian 09 package have been used for all geometry optimization, frequency analysis, AIM, and NBO calculations.Figure 1
The cluster model of covalent organic frameworks (COF-105). Dark yellow: Si; dark grey: C; grey: H; pink: B; red: O.


## 3. Results and Discussion

### 3.1. Properties of the Metal-Doped COF-105 Complexes

In this section, we firstly focus on the possible doping sites of the metal and the stability of the metal-doped covalent organic framework complexes. Table1 lists the energies of the stable metal-doped covalent organic framework complexes and the metal-COF interaction energies obtained by subtracting the energies of the metal atom and covalent organic frameworks from the energy of the Li, Na, K, and Sc metal-doped COF-105 complex, respectively. Much higher interaction energies when doping at TBPS sites than those at HHTP sites indicate that the phenyl of the TBPS building blocks in COF-105 offers the most favorable doping sites for Li, Na, K, and Sc metals. The main reason is that the tetrahedral structure of the TBPS building block provides more C and B atoms interacting with the doped metal atoms than those in the HHTP building block within COF-105.Table 1
The energy of optimized metal@COF-105 and interaction energy between the doped metal and the COF-105 cluster model.
Metal@COF-105
α
1
β
2
E (a.u.)
ΔE (kcal/mol)
E (a.u.)
ΔE (kcal/mol)
Li@COF-105
−2439.86
−60.13
−2407.26
−8.05
Na@COF-105
−2594.63
−42.51
−2516.35
−7.86
K@COF-105
−3032.26
−31.15
−3002.14
−6.59
Sc@COF-105
−3192.99
−141.38
−3176.89
−11.26
α
1: at the top of the phenyl within TBPS; β2: at the top of the phenyl within HHTP.Figure2 and Table 2 show the optimized geometries of the metal-doped complexes when Li, Na, K, and Sc metals are doped at the top of the phenyl of the TBPS building block of the covalent organic framework cluster model, regarded as Li@COF-105, Na@COF-105, K@COF-105, and Sc@COF-105, respectively. When alkali metals Li, Na, and K are doped at the top of the phenyl within the TBPS building blocks of COF-105 cluster, the interaction energy between the doped metal and covalent organic frameworks is −60.13 kcal/mol, −42.51 kcal/mol, and −31.15 kcal/mol, respectively. As shown in Figure 2, the bond angle C1–Si–C7 in the alkali metal Li, Na, K doped covalent organic framework complexes is 94.16°, 97.65°, and 100.64°, respectively. The distance (presented in Table 2) between C1 and the doped metal atom in the complexes Li@COF-105, Na@COF-105, and K@COF-105 is 2.454 Å, 2.728 Å, and 3.163 Å, respectively. The distance between C7 and the doped-metal atoms (Li, Na, K) in the complexes Li@COF-105, Na@COF-105, and K@COF-105 is 2.346 Å, 2.719 Å, and 3.214 Å, respectively. These results indicate that the smaller the angle C1–Si–C7 in the metal-doped complexes Li@COF-105, Na@COF-105, and K@COF-105, the greater is the interaction energy between the doped-metal and covalent organic frameworks.Figure 2
Optimized geometries of all the metal-doped COF-105 complexes. Dark yellow: Si; dark grey: C; grey: H; pink: B; red: O; purple: Li; blue: Na; green: K; breen: Sc. Partial atom numbers of C, B, and O atoms were presented. (a) Li@COF-105. (b) Na@COF-105. (c) K@COF-105. (d) Sc@COF-105.
(a)
(b)
(c)
(d)Table 2
Partial geometrical data of the metal-doped complexes (metal@COF-105).
Angle (°)/atom distances (Å)
Li@COF-105
Na@COF-105
K@COF-105
Sc@COF-105
C1–Si–C7
94.16
97.65
100.64
91.10
C1–metal
2.454
2.728
3.163
2.359
C2–metal
2.330
2.840
3.225
2.478
C3–metal
2.688
3.038
3.362
2.538
C4–metal
2.465
3.115
3.425
2.471
C5–metal
2.699
3.016
3.363
2.538
C6–metal
2.810
2.818
3.224
2.477
C7–metal
2.346
2.719
3.214
2.331
C8–metal
2.494
2.821
3.342
2.457
C9–metal
2.779
2.986
3.413
2.484
C10–metal
2.907
3.055
3.342
2.400
C11–metal
2.787
2.956
3.214
2.484
C12–metal
2.504
2.793
3.157
2.457
B1–metal
4.001
4.148
4.422
3.502The bond angle C1–Si–C7 is 91.10° in Sc@COF-105, with the distances of C1–Sc and C7–Sc being 2.359 Å and 2.457 Å, respectively, smaller than those in alkali metal-doped covalent organic framework complexes. In addition, the interaction energy of the transition Sc metal with the COF-105 cluster (−141.38 kcal/mol) is significantly higher than those of alkali metal-doped COF-105 complexes. Figure3 shows the electrostatic potential distributions of the metal-doped COF-105 complexes with the surface electronic density criterion being 0.001 e/bohr3. The maximum electrostatic potential of the metal-doped complexes Li@COF-105, Na@COF-105, and K@COF-105 is 225.63 a.u., 188.75 a.u., and 151.99 a.u., respectively. Different from the alkali metal-doped COF-105 complexes, there are two maximum electrostatic potential points in Sc@COF-105, with electrostatic potential being 162.98 a.u. and 164.85 a.u., respectively. The electrostatic potential distribution results reveal that a single metal atom (including alkali metal Li, Na, K, and transition metal Sc) doped at the top of phenyls within the TBPS building block can easily lose its valence elections and can be positively charged. And, the metal-doped COF-105 complexes Li@COF-105, Na@COF-105, K@COF-105, and Sc@COF-105 could exist stably.Figure 3
Electrostatic potential distribution of all the metal-doped complexes (metal@COF-105). (a) Li@COF-105. (b) Na@COF-105. (c) K@COF-105. (d) Sc@COF-105.
(a)
(b)
(c)
(d)


### 3.2. Interactions between Metal@COF-105 and Sulfur Dioxide

In this section, we focus on the interactions between the metal-doped COF-105 complex and the absorbed sulfur dioxide gas molecule in order to investigate the effect of metal-doping into COF-105 for sulfur dioxide capture. Here, the adsorption complexes were regarded as SO2/Li@COF-105, SO2/Na@COF-105, SO2/K@COF-105, and SO2/Sc@COF-105, respectively, where sulfur dioxide gas molecule was absorbed onto the Li, Na, K, and Sc metal-doped COF-105 complexes.Figure4 and Table 3 display all the optimized geometries of the adsorption complexes SO2/metal@COF-105, in which the metals (Li, Na, K, and Sc) are doped at the top of the phenyl of the TBPS building blocks in the COF-105 cluster model. AIM calculations were further performed to investigate the weak interactions within the adsorption complexes SO2/metal@COF-105, as shown in Table 4.Figure 4
Optimized geometries of all the adsorption complexes SO2/metal@COF-105. Dark yellow: Si; black: C; grey: H; pink: B; red: O; purple: Li; blue: Na; green: K; khaki: Sc; yellow: S. Partial atom numbers of C, B, and O atoms were presented. (a) SO2/Li@COF-105. (b) SO2/Na@COF-105. (c) SO2/K@COF-105. (d) SO2/Sc@COF-105.
(a)
(b)
(c)
(d)Table 3
Partial geometrical data of the adsorption complexes SO2/metal@COF-105.
Angle (°)/atom distances (Å)
SO2/Li@COF-105
SO2/Na@COF-105
SO2/K@COF-105
SO2/Sc@COF-105
C1–Si–C7
99.81
99.08
100.25
92.02
O7–S–O8
116.45
116.23
115.85
110.08
O7–metal
1.975
2.279
2.718
1.963
O8–metal
3.742
4.062
4.537
3.916
S–metal
3.261
3.663
4.095
3.451
C1–metal
2.353
2.754
4.190
2.489
C2–metal
2.706
2.879
3.316
2.593
C3–metal
3.231
3.087
3.396
2.719
C4–metal
3.421
3.162
3.434
2.738
C5–metal
3.130
3.044
3.397
2.680
C6–metal
2.587
2.833
3.318
2.547
C7–metal
2.363
2.841
3.326
2.484
C8–metal
2.716
3.091
3.307
2.550
C9–metal
3.215
3.304
3.293
2.624
C10–metal
3.396
3.272
3.295
2.640
C11–metal
3.087
3.033
3.347
2.639
C12–metal
2.570
2.820
3.358
2.569Table 4
Properties of partial interactions within the adsorption complexes SO2/metal@COF-105.
BCP
ρ
∇
2
ρ
E
G
V
V
BCP
/
G
BCP
SO2/Li@COF-105
O7–C4
0.0028
0.0094
0.0004
0.0019
−0.0015
0.7747
O7–Li
0.0268
0.2060
0.0113
0.0402
−0.0290
0.7198
O8–C10
0.0055
0.0187
0.0008
0.0038
−0.0030
0.7796
Li–C1
0.0143
0.0644
0.0025
0.0136
−0.0110
0.8130
Li–C7
0.0139
0.0637
0.0026
0.0134
−0.0108
0.8083
SO2/Na@COF-105
S–O1
0.0140
0.0424
0.0009
0.0097
−0.0088
0.9093
O7–C11
0.0052
0.0148
0.0005
0.0032
−0.0027
0.8374
O7–Na
0.0205
0.1348
0.0063
0.0274
−0.0210
0.7686
Na–C1
0.0106
0.0472
0.0018
0.0100
−0.0082
0.8182
Na–C12
0.0095
0.0411
0.0016
0.0086
−0.0070
0.8122
SO2/K@COF-105
O7–O2
0.0062
0.0228
0.0009
0.0048
−0.0039
0.8087
O7–K
0.0157
0.0713
0.0023
0.0156
−0.0133
0.8552
K–C9
0.0069
0.0267
0.0013
0.0054
−0.0041
0.7635
K–C1
0.0070
0.0267
0.0012
0.0054
−0.0042
0.7711
SO2/Sc@COF-105
O8–C5
0.0081
0.0327
0.0014
0.0067
−0.0053
0.7877
O7–Sc
0.1226
0.2582
-0.0750
0.1395
−0.2145
1.5374
Sc–C11
0.0251
0.0935
0.0029
0.0204
−0.0175
0.8563
Sc–C7
0.0377
0.1146
0.0002
0.0285
−0.0283
0.9937
Sc–C1
0.0371
0.1146
0.0003
0.0284
−0.0281
0.9900When a Li atom, doped at the top of the phenyls in the TBPS building block of COFs, is positively charged, it can absorb one SO2 molecule with the interaction energy of −25.46 kcal/mol obtained by subtracting the energies of sulfur dioxide and the metal-doped covalent organic frameworks from the energy of the adsorption complexes SO2/Li@COF-105. As shown in Figure 4, the SO2 gas molecule nearly lies on the surface of Li@COF-105 with the O7–Li distance of 1.975 Å. The bond angle C1–Si–C7 is 99.81° in SO2/Li@COF-105, with the distances of C1–Li and C7–Li being 2.353 Å and 2.363 Å, respectively. AIM calculations further show that there is a bond critical point BCPO7–Li within the adsorption complex SO2/Li@COF-105, with charge density being 0.0268 a.u. Further, NBO results presented in Table 5 indicate that the absorbed SO2 molecule could interact with Li@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of atom Li. These results indicate that the modifications of doping Li into COF-105 enhance the affinity of the host material to the sulfur dioxide gas molecule significantly, compared to the nondoped ones in which SO2/COF-105 interaction energy is −2.57 kcal/mol derived from DFT calculations at B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d) level of theory. In addition, there are weak interactions between the absorbed SO2 molecule and the TBPS building block of COF-105, with the charge density of BCPO7–C4 and BCPO8–C10 being 0.0028 a.u. and 0.0055 a.u., respectively.Table 5
NBO analysis at B3LYP/6-31G(d) level of theory for the adsorption complexes SO2/metal@COF-105 (selected values)1.
Complex
Donor NBOs
Acceptor NBOs
E
(2) (kcal/mol)
E(j)− E(i) (a.u.)
F(i, j) (a.u.)
Principle delocalization types
SO2/Li@COF-105
BD∗(2) S–O8
LP∗(2) B1
2.87
0.01
0.009
Remote
BD (1) S–O8
LP∗(1)Li
1.73
0.78
0.034
Remote
BD (1) S–O7
LP∗(1)Li
6.85
1.00
0.076
Remote
CR (2) S
LP∗(1)Li
7.53
9.15
0.242
Remote
LP (1) S
LP∗(1)Li
18.79
0.76
0.109
Remote
LP (1) O7
LP∗(1)Li
26.25
0.91
0.141
Remote
SO2/Na@COF-105
BD (1) S–O8
LP∗(1)Na
1.05
1.02
0.030
Remote
BD (1) S–O7
LP∗(1)Na
3.75
1.00
0.056
Remote
CR (2) S
LP∗(1)Na
3.68
9.20
0.168
Remote
CR (1) O7
LP∗(1)Na
2.73
19.11
0.208
Remote
LP (1) S
LP∗(1)Na
9.35
0.75
0.076
Remote
LP (1) O7
LP∗(1)Na
19.47
0.90
0.120
Remote
SO2/K@COF-105
BD (1) S–O7
LP∗(1)K
1.60
0.99
0.036
Remote
CR (2) S
LP∗(1)K
1.66
9.33
0.113
Remote
CR (1) O7
LP∗(1)K
1.95
19.10
0.175
Remote
LP (1) S
LP∗(1)K
4.24
0.74
0.051
Remote
LP (1) O7
LP∗(1)K
14.56
0.90
0.103
Remote
SO2/Sc@COF-105
BD∗(1)S–Sc
RY∗(1) O8
0.76
1.36
0.032
Vicinal
BD∗(1)S–Sc
BD(1)S–Sc
6.74
0.14
0.064
Geminal
CR(3)Sc
BD∗(1)S–O7
2.70
2.77
0.080
Vicinal
LP(2) O7
LP∗(1)Sc
84.93
0.73
0.225
Remote
LP(2) O8
RY∗(1) Sc
0.97
1.35
0.034
Remote
LP∗(2)Sc
BD∗(1)S–O7
1.70
0.18
0.038
Vicinal
BD(1)S-Sc
LP∗(8)Sc
0.66
0.06
0.013
Geminal
1BD denotes the formally occupied 2-center bonding orbital. RY∗ denotes 1-center Rydberg. LP denotes 1-center lone pair. The unstarred and starred labels denote Lewis and non-Lewis NBOs, respectively.Similarly, the Na-doped COF-105 and K-doped COF-105 complexes also can absorb one SO2 molecule with the interaction energy of −21.87 and −19.14 kcal/mol, where SO2 lies on the surface with the O7–Na and O7–K distance of 2.279 Å and 2.718 Å, respectively. Charge densities of BCPO7–Na and BCPO7–K in the adsorption complexes SO2/Na@COF-105 and SO2/K@COF-105 are 0.0205 a.u. and 0.0158 a.u., respectively, and are weaker than those of the adsorption complex SO2/Li@COF-105.On the contrary, the interaction energy between the absorbed SO2 molecule and the Sc-doped covalent organic frameworks is −41.36 kcal/mol, obtained by subtracting the energies of sulfur dioxide and the metal-doped covalent organic frameworks from the energy of the adsorption complexes SO2/Sc@COF-105. The SO2 gas molecule nearly lies onto the surface of Sc@COF-105 with the O7-Sc distance of 1.963 Å. There is a (3,−1) bond critical point BCPO7–Sc between O7 atom of the absorbed SO2 molecule and Sc@COF-105 within the adsorption complex SO2/Sc@COF-105, with charge density and energy density being 0.1226 a.u. and −0.0750 a.u, respectively. Further, NBO results presented in Table 5 also indicate that the absorbed SO2 molecule could interact with Sc@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of atom Sc.


## 3.1. Properties of the Metal-Doped COF-105 Complexes

In this section, we firstly focus on the possible doping sites of the metal and the stability of the metal-doped covalent organic framework complexes. Table1 lists the energies of the stable metal-doped covalent organic framework complexes and the metal-COF interaction energies obtained by subtracting the energies of the metal atom and covalent organic frameworks from the energy of the Li, Na, K, and Sc metal-doped COF-105 complex, respectively. Much higher interaction energies when doping at TBPS sites than those at HHTP sites indicate that the phenyl of the TBPS building blocks in COF-105 offers the most favorable doping sites for Li, Na, K, and Sc metals. The main reason is that the tetrahedral structure of the TBPS building block provides more C and B atoms interacting with the doped metal atoms than those in the HHTP building block within COF-105.Table 1
The energy of optimized metal@COF-105 and interaction energy between the doped metal and the COF-105 cluster model.
Metal@COF-105
α
1
β
2
E (a.u.)
ΔE (kcal/mol)
E (a.u.)
ΔE (kcal/mol)
Li@COF-105
−2439.86
−60.13
−2407.26
−8.05
Na@COF-105
−2594.63
−42.51
−2516.35
−7.86
K@COF-105
−3032.26
−31.15
−3002.14
−6.59
Sc@COF-105
−3192.99
−141.38
−3176.89
−11.26
α
1: at the top of the phenyl within TBPS; β2: at the top of the phenyl within HHTP.Figure2 and Table 2 show the optimized geometries of the metal-doped complexes when Li, Na, K, and Sc metals are doped at the top of the phenyl of the TBPS building block of the covalent organic framework cluster model, regarded as Li@COF-105, Na@COF-105, K@COF-105, and Sc@COF-105, respectively. When alkali metals Li, Na, and K are doped at the top of the phenyl within the TBPS building blocks of COF-105 cluster, the interaction energy between the doped metal and covalent organic frameworks is −60.13 kcal/mol, −42.51 kcal/mol, and −31.15 kcal/mol, respectively. As shown in Figure 2, the bond angle C1–Si–C7 in the alkali metal Li, Na, K doped covalent organic framework complexes is 94.16°, 97.65°, and 100.64°, respectively. The distance (presented in Table 2) between C1 and the doped metal atom in the complexes Li@COF-105, Na@COF-105, and K@COF-105 is 2.454 Å, 2.728 Å, and 3.163 Å, respectively. The distance between C7 and the doped-metal atoms (Li, Na, K) in the complexes Li@COF-105, Na@COF-105, and K@COF-105 is 2.346 Å, 2.719 Å, and 3.214 Å, respectively. These results indicate that the smaller the angle C1–Si–C7 in the metal-doped complexes Li@COF-105, Na@COF-105, and K@COF-105, the greater is the interaction energy between the doped-metal and covalent organic frameworks.Figure 2
Optimized geometries of all the metal-doped COF-105 complexes. Dark yellow: Si; dark grey: C; grey: H; pink: B; red: O; purple: Li; blue: Na; green: K; breen: Sc. Partial atom numbers of C, B, and O atoms were presented. (a) Li@COF-105. (b) Na@COF-105. (c) K@COF-105. (d) Sc@COF-105.
(a)
(b)
(c)
(d)Table 2
Partial geometrical data of the metal-doped complexes (metal@COF-105).
Angle (°)/atom distances (Å)
Li@COF-105
Na@COF-105
K@COF-105
Sc@COF-105
C1–Si–C7
94.16
97.65
100.64
91.10
C1–metal
2.454
2.728
3.163
2.359
C2–metal
2.330
2.840
3.225
2.478
C3–metal
2.688
3.038
3.362
2.538
C4–metal
2.465
3.115
3.425
2.471
C5–metal
2.699
3.016
3.363
2.538
C6–metal
2.810
2.818
3.224
2.477
C7–metal
2.346
2.719
3.214
2.331
C8–metal
2.494
2.821
3.342
2.457
C9–metal
2.779
2.986
3.413
2.484
C10–metal
2.907
3.055
3.342
2.400
C11–metal
2.787
2.956
3.214
2.484
C12–metal
2.504
2.793
3.157
2.457
B1–metal
4.001
4.148
4.422
3.502The bond angle C1–Si–C7 is 91.10° in Sc@COF-105, with the distances of C1–Sc and C7–Sc being 2.359 Å and 2.457 Å, respectively, smaller than those in alkali metal-doped covalent organic framework complexes. In addition, the interaction energy of the transition Sc metal with the COF-105 cluster (−141.38 kcal/mol) is significantly higher than those of alkali metal-doped COF-105 complexes. Figure3 shows the electrostatic potential distributions of the metal-doped COF-105 complexes with the surface electronic density criterion being 0.001 e/bohr3. The maximum electrostatic potential of the metal-doped complexes Li@COF-105, Na@COF-105, and K@COF-105 is 225.63 a.u., 188.75 a.u., and 151.99 a.u., respectively. Different from the alkali metal-doped COF-105 complexes, there are two maximum electrostatic potential points in Sc@COF-105, with electrostatic potential being 162.98 a.u. and 164.85 a.u., respectively. The electrostatic potential distribution results reveal that a single metal atom (including alkali metal Li, Na, K, and transition metal Sc) doped at the top of phenyls within the TBPS building block can easily lose its valence elections and can be positively charged. And, the metal-doped COF-105 complexes Li@COF-105, Na@COF-105, K@COF-105, and Sc@COF-105 could exist stably.Figure 3
Electrostatic potential distribution of all the metal-doped complexes (metal@COF-105). (a) Li@COF-105. (b) Na@COF-105. (c) K@COF-105. (d) Sc@COF-105.
(a)
(b)
(c)
(d)


## 3.2. Interactions between Metal@COF-105 and Sulfur Dioxide

In this section, we focus on the interactions between the metal-doped COF-105 complex and the absorbed sulfur dioxide gas molecule in order to investigate the effect of metal-doping into COF-105 for sulfur dioxide capture. Here, the adsorption complexes were regarded as SO2/Li@COF-105, SO2/Na@COF-105, SO2/K@COF-105, and SO2/Sc@COF-105, respectively, where sulfur dioxide gas molecule was absorbed onto the Li, Na, K, and Sc metal-doped COF-105 complexes.Figure4 and Table 3 display all the optimized geometries of the adsorption complexes SO2/metal@COF-105, in which the metals (Li, Na, K, and Sc) are doped at the top of the phenyl of the TBPS building blocks in the COF-105 cluster model. AIM calculations were further performed to investigate the weak interactions within the adsorption complexes SO2/metal@COF-105, as shown in Table 4.Figure 4
Optimized geometries of all the adsorption complexes SO2/metal@COF-105. Dark yellow: Si; black: C; grey: H; pink: B; red: O; purple: Li; blue: Na; green: K; khaki: Sc; yellow: S. Partial atom numbers of C, B, and O atoms were presented. (a) SO2/Li@COF-105. (b) SO2/Na@COF-105. (c) SO2/K@COF-105. (d) SO2/Sc@COF-105.
(a)
(b)
(c)
(d)Table 3
Partial geometrical data of the adsorption complexes SO2/metal@COF-105.
Angle (°)/atom distances (Å)
SO2/Li@COF-105
SO2/Na@COF-105
SO2/K@COF-105
SO2/Sc@COF-105
C1–Si–C7
99.81
99.08
100.25
92.02
O7–S–O8
116.45
116.23
115.85
110.08
O7–metal
1.975
2.279
2.718
1.963
O8–metal
3.742
4.062
4.537
3.916
S–metal
3.261
3.663
4.095
3.451
C1–metal
2.353
2.754
4.190
2.489
C2–metal
2.706
2.879
3.316
2.593
C3–metal
3.231
3.087
3.396
2.719
C4–metal
3.421
3.162
3.434
2.738
C5–metal
3.130
3.044
3.397
2.680
C6–metal
2.587
2.833
3.318
2.547
C7–metal
2.363
2.841
3.326
2.484
C8–metal
2.716
3.091
3.307
2.550
C9–metal
3.215
3.304
3.293
2.624
C10–metal
3.396
3.272
3.295
2.640
C11–metal
3.087
3.033
3.347
2.639
C12–metal
2.570
2.820
3.358
2.569Table 4
Properties of partial interactions within the adsorption complexes SO2/metal@COF-105.
BCP
ρ
∇
2
ρ
E
G
V
V
BCP
/
G
BCP
SO2/Li@COF-105
O7–C4
0.0028
0.0094
0.0004
0.0019
−0.0015
0.7747
O7–Li
0.0268
0.2060
0.0113
0.0402
−0.0290
0.7198
O8–C10
0.0055
0.0187
0.0008
0.0038
−0.0030
0.7796
Li–C1
0.0143
0.0644
0.0025
0.0136
−0.0110
0.8130
Li–C7
0.0139
0.0637
0.0026
0.0134
−0.0108
0.8083
SO2/Na@COF-105
S–O1
0.0140
0.0424
0.0009
0.0097
−0.0088
0.9093
O7–C11
0.0052
0.0148
0.0005
0.0032
−0.0027
0.8374
O7–Na
0.0205
0.1348
0.0063
0.0274
−0.0210
0.7686
Na–C1
0.0106
0.0472
0.0018
0.0100
−0.0082
0.8182
Na–C12
0.0095
0.0411
0.0016
0.0086
−0.0070
0.8122
SO2/K@COF-105
O7–O2
0.0062
0.0228
0.0009
0.0048
−0.0039
0.8087
O7–K
0.0157
0.0713
0.0023
0.0156
−0.0133
0.8552
K–C9
0.0069
0.0267
0.0013
0.0054
−0.0041
0.7635
K–C1
0.0070
0.0267
0.0012
0.0054
−0.0042
0.7711
SO2/Sc@COF-105
O8–C5
0.0081
0.0327
0.0014
0.0067
−0.0053
0.7877
O7–Sc
0.1226
0.2582
-0.0750
0.1395
−0.2145
1.5374
Sc–C11
0.0251
0.0935
0.0029
0.0204
−0.0175
0.8563
Sc–C7
0.0377
0.1146
0.0002
0.0285
−0.0283
0.9937
Sc–C1
0.0371
0.1146
0.0003
0.0284
−0.0281
0.9900When a Li atom, doped at the top of the phenyls in the TBPS building block of COFs, is positively charged, it can absorb one SO2 molecule with the interaction energy of −25.46 kcal/mol obtained by subtracting the energies of sulfur dioxide and the metal-doped covalent organic frameworks from the energy of the adsorption complexes SO2/Li@COF-105. As shown in Figure 4, the SO2 gas molecule nearly lies on the surface of Li@COF-105 with the O7–Li distance of 1.975 Å. The bond angle C1–Si–C7 is 99.81° in SO2/Li@COF-105, with the distances of C1–Li and C7–Li being 2.353 Å and 2.363 Å, respectively. AIM calculations further show that there is a bond critical point BCPO7–Li within the adsorption complex SO2/Li@COF-105, with charge density being 0.0268 a.u. Further, NBO results presented in Table 5 indicate that the absorbed SO2 molecule could interact with Li@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of atom Li. These results indicate that the modifications of doping Li into COF-105 enhance the affinity of the host material to the sulfur dioxide gas molecule significantly, compared to the nondoped ones in which SO2/COF-105 interaction energy is −2.57 kcal/mol derived from DFT calculations at B3LYP/6-311 + G(d,p)//B3LYP/6-31G(d) level of theory. In addition, there are weak interactions between the absorbed SO2 molecule and the TBPS building block of COF-105, with the charge density of BCPO7–C4 and BCPO8–C10 being 0.0028 a.u. and 0.0055 a.u., respectively.Table 5
NBO analysis at B3LYP/6-31G(d) level of theory for the adsorption complexes SO2/metal@COF-105 (selected values)1.
Complex
Donor NBOs
Acceptor NBOs
E
(2) (kcal/mol)
E(j)− E(i) (a.u.)
F(i, j) (a.u.)
Principle delocalization types
SO2/Li@COF-105
BD∗(2) S–O8
LP∗(2) B1
2.87
0.01
0.009
Remote
BD (1) S–O8
LP∗(1)Li
1.73
0.78
0.034
Remote
BD (1) S–O7
LP∗(1)Li
6.85
1.00
0.076
Remote
CR (2) S
LP∗(1)Li
7.53
9.15
0.242
Remote
LP (1) S
LP∗(1)Li
18.79
0.76
0.109
Remote
LP (1) O7
LP∗(1)Li
26.25
0.91
0.141
Remote
SO2/Na@COF-105
BD (1) S–O8
LP∗(1)Na
1.05
1.02
0.030
Remote
BD (1) S–O7
LP∗(1)Na
3.75
1.00
0.056
Remote
CR (2) S
LP∗(1)Na
3.68
9.20
0.168
Remote
CR (1) O7
LP∗(1)Na
2.73
19.11
0.208
Remote
LP (1) S
LP∗(1)Na
9.35
0.75
0.076
Remote
LP (1) O7
LP∗(1)Na
19.47
0.90
0.120
Remote
SO2/K@COF-105
BD (1) S–O7
LP∗(1)K
1.60
0.99
0.036
Remote
CR (2) S
LP∗(1)K
1.66
9.33
0.113
Remote
CR (1) O7
LP∗(1)K
1.95
19.10
0.175
Remote
LP (1) S
LP∗(1)K
4.24
0.74
0.051
Remote
LP (1) O7
LP∗(1)K
14.56
0.90
0.103
Remote
SO2/Sc@COF-105
BD∗(1)S–Sc
RY∗(1) O8
0.76
1.36
0.032
Vicinal
BD∗(1)S–Sc
BD(1)S–Sc
6.74
0.14
0.064
Geminal
CR(3)Sc
BD∗(1)S–O7
2.70
2.77
0.080
Vicinal
LP(2) O7
LP∗(1)Sc
84.93
0.73
0.225
Remote
LP(2) O8
RY∗(1) Sc
0.97
1.35
0.034
Remote
LP∗(2)Sc
BD∗(1)S–O7
1.70
0.18
0.038
Vicinal
BD(1)S-Sc
LP∗(8)Sc
0.66
0.06
0.013
Geminal
1BD denotes the formally occupied 2-center bonding orbital. RY∗ denotes 1-center Rydberg. LP denotes 1-center lone pair. The unstarred and starred labels denote Lewis and non-Lewis NBOs, respectively.Similarly, the Na-doped COF-105 and K-doped COF-105 complexes also can absorb one SO2 molecule with the interaction energy of −21.87 and −19.14 kcal/mol, where SO2 lies on the surface with the O7–Na and O7–K distance of 2.279 Å and 2.718 Å, respectively. Charge densities of BCPO7–Na and BCPO7–K in the adsorption complexes SO2/Na@COF-105 and SO2/K@COF-105 are 0.0205 a.u. and 0.0158 a.u., respectively, and are weaker than those of the adsorption complex SO2/Li@COF-105.On the contrary, the interaction energy between the absorbed SO2 molecule and the Sc-doped covalent organic frameworks is −41.36 kcal/mol, obtained by subtracting the energies of sulfur dioxide and the metal-doped covalent organic frameworks from the energy of the adsorption complexes SO2/Sc@COF-105. The SO2 gas molecule nearly lies onto the surface of Sc@COF-105 with the O7-Sc distance of 1.963 Å. There is a (3,−1) bond critical point BCPO7–Sc between O7 atom of the absorbed SO2 molecule and Sc@COF-105 within the adsorption complex SO2/Sc@COF-105, with charge density and energy density being 0.1226 a.u. and −0.0750 a.u, respectively. Further, NBO results presented in Table 5 also indicate that the absorbed SO2 molecule could interact with Sc@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of atom Sc.


## 4. Conclusions

In this work, density functional theory calculations have been performed to investigate the properties of metal-doped covalent organic frameworks and their interactions with SO2 gas. We have doped alkali (Li, Na, and K) and transition metals (Sc) doped into the COF-105 cluster model and investigated properties of the metal-doped covalent organic frameworks and their interactions with the SO2 gas molecule. It is found that that a single metal atom (including Li, Na, K, and Sc) doped at the top of phenyls in the TBPS building block can easily lose its valence electrons and can be positively charged and stably attached to the frameworks of the COF-105 cluster model. The metal-doped COF-105 complexes Li@COF-105, Na@COF-105, K@COF-105, and Sc@COF-105 could exist stably.In the adsorption complexes SO2/Li@COF-105, SO2/Na@COF-105, and SO2/K@COF-105, the calculated interaction energy between the absorbed SO2 gas molecule and the metal-doped COF-105 complexes is −25.46, −21.87, and −19.14 kcal/mol, respectively. The SO2 gas molecule nearly lies on the surface of the alkali metal-doped COF-105 cluster model with the O7–Li, O7–Na, and O7–K atom distance of 1.975 Å, 2.273 Å, and 2.719 Å, respectively. The absorbed SO2 molecule could interact with alkali metal@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of alkali metal atom. Further, AIM calculations indicate charge densities of BCPO7–Li, BCPO7–Na, and BCPO7–K being 0.0268 a.u., 0.0205 a.u., and 0.0158 a.u., respectively.In the adsorption complex SO2/Sc@COF-105, the total interaction energy between the absorbed SO2 molecule and the Sc-doped COF-105 complex is −41.36 kcal/mol, much larger than those of the adsorption complexes SO2/Li@COF-105, SO2/Na@COF-105, and SO2/K@COF-105. The SO2 gas molecule nearly lies onto the surface of Sc@COF-105 with the O7-Sc distance of 1.963 Å. NBO calculations indicate that the absorbed SO2 molecule could interact with Sc@COF-105 by the dominant donor-acceptor delocalization between 1-center lone pair of atom O7 and 1-center non-Lewis lone pair of atom Sc. AIM calculations also indicate there is a (3,−1) bond critical point BCPO7–Sc between O7 atom of the absorbed SO2 molecule and Sc@COF-105 within the adsorption complex SO2/Sc@COF-105, with charge density and energy density being 0.1226 a.u. and −0.0750 a.u, respectively.



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