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

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 metaldoped 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. ,e SO2 gas molecule could be stably absorbed onto the metal-doped covalent organic frameworks. ,e 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.


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][4][5].It should be pointed out that adsorption by porous nanomaterials is recognized as an efficient and economical approach for capture of low concentration SO 2 from mixture gases [6][7][8][9][10].Covalent organic frameworks (COFs) are ideal porous materials for gas capture due to their low density, good stability, and large surface area [11][12][13][14][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 SO 2 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][18][19][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 CO 2 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 SO 2 gases adsorption and separation on the metal-doped covalent organic frameworks and the interactions between the metal-doped covalent organic frameworks and SO 2 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 SO 2 gas.Considering the excellent doping effect of alkali metals and transition metals into the covalent organic framework for hydrogen and carbon dioxide [17][18][19][20][21][22][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 SO 2 gas molecule.e main focus is to understand the metal doping into covalent organic frameworks and their in uence on sulfur dioxide capture.

Computational Details
e cluster model of covalent organic frameworks presented in Figure 1 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.e cuto functional groups of the covalent organic framework cluster model are saturated by hydrogen.Considering the di erent 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.

Properties of the Metal-Doped COF-105 Complexes.
In this section, we rstly focus on the possible doping sites of the metal and the stability of the metal-doped covalent organic framework complexes.Table 1 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 o ers the most favorable doping sites for Li, Na, K, and Sc metals.e 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.
Figure 2 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.
ese 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.
e 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 signi cantly higher than those of alkali metal-doped COF-105 complexes.Figure 3 shows the electrostatic potential distributions of the metal-doped COF-105 complexes with the surface electronic density criterion being 0.001 e/bohr 3 .
e 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.Di erent from the alkali metaldoped 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.e electrostatic potential distribution results reveal that a single metal atom (including alkali metal Li, Na, K, and transition  2 Journal of Chemistry 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.

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 SO 2 /Li@COF-105, SO 2 /Na@COF-105, SO 2 /K@COF-105, and SO 2 /Sc@ COF-105, respectively, where sulfur dioxide gas molecule was absorbed onto the Li, Na, K, and Sc metal-doped COF-105 complexes.Figure 4 and Table 3 display all the optimized geometries of the adsorption complexes SO 2 /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 SO 2 /metal@COF-105, as shown in Table 4.
When a Li atom, doped at the top of the phenyls in the TBPS building block of COFs, is positively charged, it can absorb one SO 2 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 SO 2 /Li@COF-105.As shown in Figure 4, the SO 2 gas molecule nearly lies on the surface of Li@COF-105 with the O7-Li distance of 1.975 Å. e bond angle C1-Si-C7 is 99.81 °in SO 2 /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 BCP O7-Li within the adsorption complex SO 2 /Li@COF-105, with charge density being 0.0268 a.u.Further, NBO results presented in Table 5 indicate that the absorbed SO 2 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. ese results indicate that the modi cations of doping Li into COF-105 enhance the a nity of the host material to the sulfur dioxide gas molecule signi cantly, compared to the nondoped ones in which SO 2 /COF-105 interaction energy is −2.57kcal/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 SO 2 molecule and the TBPS building block of COF-105, with the charge density of BCP O7-C4 and BCP O8-C10 being 0.0028 a.u. and 0.0055 a.u., respectively.
Similarly, the Na-doped COF-105 and K-doped COF-105 complexes also can absorb one SO 2 molecule with the interaction energy of −21.87 and −19.14 kcal/mol, where SO 2 lies on the surface with the O7-Na and O7-K distance of 2.279 Å and 2.718 Å, respectively.Charge densities of BCP O7-Na and BCP O7-K in the adsorption complexes SO 2 /Na@COF-105 and SO 2 /K@COF-105 are 0.0205 a.u. and 0.0158 a.u., respectively, and are weaker than those of the adsorption complex SO 2 /Li@COF-105.
On the contrary, the interaction energy between the absorbed SO 2 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 SO 2 /Sc@COF-105.e SO 2 gas molecule nearly lies onto the surface of Sc@COF-105 with the O7-Sc distance of 1.963 Å. ere is a (3,−1) bond critical point BCP O7-Sc between O7 atom of the absorbed SO 2 molecule and Sc@ COF-105 within the adsorption complex SO 2 /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 SO 2 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 .

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 SO 2 gas.We have doped alkali (Li, Na, and K) and transition  4 Journal of Chemistry metals (Sc) doped into the COF-105 cluster model and investigated properties of the metal-doped covalent organic frameworks and their interactions with the SO 2 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.e 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 SO 2 /Li@COF-105, SO 2 /Na@COF-105, and SO 2 /K@COF-105, the calculated interaction energy between the absorbed SO 2 gas molecule and the metal-doped COF-105 complexes is −25.46,−21.87, and −19.14 kcal/mol, respectively.e SO 2 gas molecule nearly lies on the surface of the alkali metal-doped COF-105 cluster  In the adsorption complex SO 2 /Sc@COF-105, the total interaction energy between the absorbed SO 2 molecule and the Sc-doped COF-105 complex is −41.36 kcal/mol, much larger than those of the adsorption complexes SO 2 /Li@COF-105, SO 2 /Na@COF-105, and SO 2 /K@COF-105.e SO 2 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 SO 2 molecule could interact with Sc@COF-105 by

Table 1 :
e energy of optimized metal@COF-105 and interaction energy between the doped metal and the COF-105 cluster model.

Table 4 :
Properties of partial interactions within the adsorption complexes SO 2 /metal@COF-105.

Table 5 :
6BO analysis at B3LYP/6-31G(d) level of theory for the adsorption complexes SO 2 /metal@COF-105 (selected values)1.BD denotes the formally occupied 2-center bonding orbital.RY * denotes 1-center Rydberg.LP denotes 1-center lone pair.e unstarred and starred labels denote Lewis and non-Lewis NBOs, respectively.6Journal of Chemistry 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 BCP O7-Sc between O7 atom of the absorbed SO 2 molecule and Sc@COF-105 within the adsorption complex SO 2 /Sc@COF-105, with charge density and energy density being 0.1226 a.u. and −0.0750 a.u, respectively.