Stable Amide-Functionalized Metal–Organic Framework with Highly

Jan 31, 2019 - Synopsis. A new amide-functionalized metal−organic framework (AFMOF) was synthesized via the combination of 6-connected ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Stable Amide-Functionalized Metal−Organic Framework with Highly Selective CO2 Adsorption Cong Chen,†,§ Mingxing Zhang,‡,§ Wenwei Zhang,† and Junfeng Bai*,†,⊥ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China ⊥ School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Department of Chemistry, Chongqing Normal University, Chongqing 401331, China

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 02/05/19. For personal use only.

S Supporting Information *

ABSTRACT: A new amide-functionalized metal−organic framework (AFMOF) with the combined feature of highly selective CO2 adsorption and high thermal and chemical stability, [Sc3(μ3-O)(L)1.5(H2O)3Cl]n [NJU-Bai49; NJU-Bai for Nanjing University Bai’s group, H4L = 5-(3,5-dicarboxybenzamido)isophthalic acid], was synthesized that exhibits 4.5 wt % CO2 uptake at 298 K and 0.15 bar and the highest selectivity for CO2/N2 (166.7) among all AFMOFs. Grand canonical Monte Carlo simulations further indicate that both the decorated amide group and the open metal site act as CO2 binding sites, which may contribute to its highly selective CO2 uptake.

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INTRODUCTION Climate change has become one of the greatest concerns for our society because of excessive carbon dioxide (CO2) emissions from human activities.1−3 In order to address this issue, porous materials including traditional zeolites and carbons, etc., have been intensively investigated for CO2 capture and sequestration.4−13 Recently, as a new class of crystalline porous materials, metal−organic frameworks (MOFs) built from organic linkers and metal ions/clusters have been considered to have the most potential in the application of CO2 capture because of their structural diversity and fine tunability.14−19 For future applications, the stability of MOFs has been recognized as the main obstacle. In order to solve such a problem, many MOFs based upon high-valence metal−oxo clusters (Cr3+, Al3+, Sc3+, Zr4+, etc.) have been investigated. For instance, the famous MIL-100-Cr is constructed from 6connected [Cr3(μ3-O)(COO)6] clusters, being stable in water for 12 months.20,21 UiO-66 is constructed from 12-connected [Zr6(μ3-O)4(μ3-OH)4(COO)12] clusters, which can survive after stirring in water for 24 h and even retains its crystallinity after immersion in pH = 1 or 14 aqueous solutions for 2 h.22,23 The outstanding stability of these MOFs is attributed to the © XXXX American Chemical Society

strong interactions between high-valence metal ions and carboxylate linkers according to the hard/soft acid−base principle.24,25 We are interested in constructing MOFs with novel structures and promising properties and finely tuning MOFs toward significantly improved performance.26,27 In particular, we have devoted significant efforts to constructing a subclass of amide-functionalized MOFs (AFMOFs).28−36 To further expand our work and also increase their thermal and chemical stability, based upon the 6-connected [Sc3(μ3-O)(COO)6] clusters and amide-functionalized carboxylate linker H4L (Figure 1), herein, a new AFMOF, [Sc 3 (μ 3 -O)(L)1.5(H2O)3Cl]n (NJU-Bai49; NJU-Bai for Nanjing University Bai’s group), was afforded. Interestingly, it shows improved CO2/N2 selectivity and thermal/chemical stability, its CO2 adsorption amounts are 4.5 and 17.3 wt % at 298 K under 0.15 and 1 bar, respectively, and it has the highest selectivity of CO2/N2 (166.7) in all AFMOFs reported so far. Received: November 28, 2018

A

DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Single-Crystal X-ray Crystallography. Single-crystal X-ray diffraction data of NJU-Bai49 were collected on a Bruker D8 venture diffractometer at 150 K using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The Bruker SAINT program was used for data reduction. Direct methods and refinement with the full-matrix least squares on F2 technique in the SHELXTL package38 were employed to determine the structure of NJU-Bai49. During the final cycles, non-H atoms were refined with anisotropic displacement parameters. Organic H atoms were fixed by geometrical considerations with isotropic displacement parameters set to 1.2Ueq of the attached atom. The free solvent molecules in the unit cell were disordered and could not be solved, so the solvent diffraction intensities were removed using PLATON/SQUEEZE39 and refined again using the generated data. Details of the crystal parameters, data collection, and refinements for NJU-Bai49 are summarized in Table S1.

Figure 1. Structure of the organic ligand (H4L) used in the synthesis of NJU-Bai49.

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EXPERIMENTAL SECTION

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Materials and Methods. All of the reagents were from commercial sources without further purification. 1H NMR spectroscopy was performed on a Bruker DRX-400 spectrometer with tetramethylsilane as an internal reference. Elemental analyses (C, H, and N) were recorded on a PerkinElmer 240 analyzer. The IR spectra were collected on a VECTOR TM 22 spectrometer in the 4000−400 cm−1 region using KBr pellets. Thermogravimetric analyses (TGA) were measured in the range of 20−100 °C under a N2 atmosphere with a heating rate of 20 °C min −1 using a 2960 SDT thermogravimetric analyzer. X-ray photoelectron spectroscopy (XPS) analysis measurement was carried out on a PHI 5000 VersaProbe instrument. Powder X-ray diffraction (PXRD) data were collected over the 2θ range of 5−40° on a Bruker AXS D8 Advance diffractometer using Cu Kα radiation at room temperature. Variabletemperature PXRD (VT-PXRD) measurements were performed on a Bruker D8 Advance X-ray diffractometer over the 2θ range of 5−40°. Simulated PXRD spectral analysis of NJU-Bai49 was carried out by the single-crystal data and diffraction-crystal module of the Mercury program. Water stability measurements were carried out by directly immersing the as-synthesized samples into pure water at different temperatures or aqueous solutions with various pH values, and then samples were obtained for PXRD and N2 adsorption measurements after a period of time. Gas adsorption isotherms were measured on a Micromeritics ASAP 2020 under low pressure (0−1 bar). To obtain the fully activated NJU-Bai49, the as-synthesized sample was guestexchanged with anhydrous methanol every 8 h for 3 days and then degassed at 100 °C for 24 h under vacuum. All of the gases (N2, CO2, and CH4) used for the measurements were UHP-grade (99.999%). The temperatures were maintained at 77 (liquid-nitrogen bath), 273, and 298 K, respectively. Ultrahigh-purity helium gas (UHP grade 5.0; 99.999% purity) was used for warm and cold free-space correction measurements in all isotherms. Simulations about CO2 locations in NJU-Bai49 were performed by the sorption module of the Materials Studio 7.0 package.37 Synthesis of 5-(3,5-Dicarboxybenzamido)isophthalic acid (H4L). H4L was synthesized according to the literature35 and characterized by IR and 1H NMR. Selected IR (KBr, cm−1): 3319, 3081, 1701, 1655, 1607, 1449, 1402, 1331, 1274, 1209, 1126, 1000, 952, 910, 776, 758, 734, 662, 613, 576, 519, 478, 450. 1H NMR (DMSO-d6): δ 13.53 (br peak, 4H, COOH), 10.97 (s, 1H, CONH), 8.82 (s, 2H, ArH), 8.70 (s, 2H, ArH), 8.65 (s, 1H, ArH), 8.25 (s, 1H, ArH). Synthesis of [Sc3(μ3-O)(L)1.5(H2O)3Cl]n (NJU-Bai49). H4L (10 mg, 0.027 mmol) and ScCl3·6H2O (30 mg, 0.116 mmol) in 1.5 mL of N,N-dimethylformamide (DMF) with 0.5 mL of formic acid were mixed and stirred for 10 min to obtain a clear solution. Then, the obtained mixture was transferred into a sealed glass vessel (20 mL) and heated to 130 °C for 48 h. After cooling to room temperature at a rate 5 °C h−1, colorless block crystals of NJU-Bai49 were obtained and washed by fresh DMF (yield: 65% based on the ligand). Selected IR (KBr, cm−1): 3082, 2924, 2848, 1682, 1570, 1448, 1425, 1381, 1288, 1194, 1092, 1001, 933, 912, 864, 839, 781, 750, 735, 712, 660, 636, 592, 503, 478. Elem anal. Calcd for C43.5H62.5Sc3N7.5O25.5Cl [NJU-Bai49·6(DMF)6(H2O)2]: C, 41.18; H, 4.96; N, 8.28. Found: C, 41.05; H, 5.03; N, 8.42.

RESULTS AND DISCUSSION Single-Crystal Structure Analysis. Single X-ray crystal structure analysis revealed that NJU-Bai49 crystallizes in the trigonal space group R3̅ with soc topology. In the framework, each ScIII atom is coordinated by six O atoms, of which four are carboxylate O atoms (dSc−O = 2.091−2.151 Å) from four independent L4− ligands, one is the μ3-O atom (dSc−O = 2.011−2.049 Å) shared by three Sc atoms, and one is an O atom from a terminal H2O molecule (dSc−O = 2.110−2.161 Å). The whole coordination geometry can be viewed as a distorted octahedron. Each pair of Sc atoms are bridged by two carboxylate groups from separate ligands to form the [Sc3(μ3O)(COO)6] cluster (Figure 2a), for which the Sc···Sc separations range from 3.505 to 3.518 Å. Then the clusters are bridged by six independent ligands to afford the 3D cationic framework with soc topology (Figure 2d). The balance charge is provided by Cl− ions, which was confirmed by the XPS experiment (Figure S5).

Figure 2. (a) 6-connected [Sc 3(μ3 -O)(COO)6] cluster and deprotonated ligand L4− in the X-ray crystal structure of NJUBai49. (b) Cuboidal cage constructed from eight [Sc3(μ3-O)(COO)6] clusters and six L4−. (c) Two types of intersecting channels in NJUBai49: hydrophilic channel, rose; hydrophobic channel, pale blue. (d) Soc topology of NJU-Bai49. Color code: C, black; O, red; N, turquoise; Sc, blue; H, gray. Cl− ions, partial H atoms, and coordinated H2O molecules are omitted for clarity. B

DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Summary of Selective CO2 Adsorption Properties of NJU-Bai49, SNNU-61, and Some Typical AFMOFs CO2 uptakea MOFs 47

HHU-2 NJU-Bai49 SNNU-6140 NJU-Bai2134 LIFM-11(Cu)48 NJFU-149 NJU-Bai030 HNUST-133 LIFM-10(Cu)48 NJU-Bai2234 eea-MOF-450 NJU-Bai2334 NJU-Bai332 NJU-Bai4′31 NJU-Bai431

−1

N2 uptakea 1 bar (wt %)

Sb

0.6 0.4

0.5

50.2c 166.7

0.9

1.0

Qst (kJ mol )

pore/channel size (Å)

0.15 bar (wt %)

1 bar (wt %)

0.75 bar (wt %)

30.0 33.4 27.0 25.9 53 27.2 26.3 31.2 29 25.6 27.5 25.1 36.5 24.4 24.0

11/8 10/4 10.2/6.1 12/11/7.6 12/8 9.6/8.0 18.7/12/11.6 16/11 12/9 19/12/12 × 7 × 7 10.5/7.8 19/12/17 × 8 × 8 16/14/11 23/15/12 23/15/12

6.3 4.5 4.0 4.4 4.3 4.5 4.2 3.7 3.3 3.1 2.5 2.1 2.0

21.1 17.3 18.3 22.6 15.3 17.3 23.8 18.8 12.9 14.3 13.2 9.6 11.8 10.9 10.8

0.6 1.0

0.7

0.8

0.4

0.5

93 68.9 38.3d 22 39.8 14.5 81 19.2e 72 60f 34.3g 34.3g

a

Data at 298 K. bIAST-predicted selectivity for a CO2/N2 (15/85) mixture. cSelectivity calculated from the ratio of the adsorption amounts of CO2 at 0.15 bar and N2 at 0.75 bar. S = (qCO2/qN2)/(PCO2/PN2). dIAST-predicted selectivity at 273 K. eIAST-predicted selectivity for CO2/N2 (10/90). f IAST-predicted selectivity at P = 20 bar. gSelectivity calculated from the ratio of the initial slopes based upon the isotherms.

Like other MOFs with soc topology, there exits cubic cages constructed from eight [Sc3(μ3-O)(COO)6] clusters and six ligands with 10 × 10 Å2 diameter (Figure 2b) in the framework. Interestingly, the cubic cages are encapsulated by two types of intercrossed channels (Figure 2c), one hydrophobic (pale-blue channel) and the other hydrophilic (rose channel), with coordinated H2O molecules pointing inside the channel. Notably, the two channels have the same channel size (4 Å diameter), which is the smallest in all AFMOFs reported so far (Table 1). From crystallographic analysis, Cl− ions distribute inside the cages close to the Sc3 clusters and the center of the hydrophilic channels. Compared with isoreticular SNNU-61,40 which consists of 3,3′,5,5′-azobenzenetetracarboxylate (ABTC4−; Figure S6) and [Fe3(μ3-OH)(COO)6] clusters, NJU-Bai49 shows a similar cubic cage size (10 × 10 Å2 vs 10.2 × 10.2 Å2) but more narrow intercrossed channels (4 Å vs 6.1 Å). We attribute this to the flexibility of the amideinserted H4L, although L4− and ABTC4− are similar in length. Thermal and Chemical Stability. TGA, PXRD, VTPXRD, and N2 adsorption measurements were conducted to investigate the stability of NJU-Bai49. According to the TGA (Figure S2) plots, NJU-Bai49 has lost about 44.4% weight in the range of 25−420 °C, which can be attributed to the removal of free guest molecules. When the temperature is above 550 °C, a sharp weight loss appears, which suggests that the framework is decomposed. VT-PXRD (Figure S3) measurements display that NJU-Bai49 reatins its crystallinity up to 420 °C, indicating the excellent thermal stability of the framework. Very interestingly, NJU-Bai49 also exhibits good chemical and water stability. As shown in Figure 3, NJU-Bai49 could preserve its structural integrity in pH = 1 to 10 aqueous solutions for 24 h. Moreover, it can survive after being soaked in water at 100 °C for 24 h and even retains its crystallinity in water for 30 days. Besides, N2 adsorption measurements of NJU-Bai49 after water or acid−base treatments showed that the N2 adsorption isotherms were nearly the same as that of the fresh sample and further confirmed the stability of NJUBai49 (Figure S13). It should be pointed out that most AFMOFs decompose before 350 °C and could not survive in water for more than 2 days. The excellent thermal and

Figure 3. PXRD patterns of NJU-Bai49 after different treatments.

chemical stability of NJU-Bai49 could be attributed to the strong interactions between Sc ions and carboxylate from ligand H4L. Permanent Porosity. To obtain the fully activated framework and further investigate the permanent porosity of NJU-Bai49, the methanol-exchanged sample was degassed under high vacuum at 100 °C for 24 h. As can be seen in Figure 4a, the N2 adsorption isotherms of NJU-Bai49 at 77 K show reversible type I adsorption behavior of microporous materials, which is consistent with the crystal structure analysis. The maximum N2 uptake amount of NJU-Bai49 is 300 cm3 g−1, which is slightly smaller than that of SNNU-61 (328 cm3 g−1). The corresponding Brunauer−Emmett−Teller and Langmuir surface areas were calculated to be 1189 and 1316 m2 g−1, respectively. On the basis of the maximum N2 uptake, the total pore volume was estimated to be 0.46 cm3 g−1, which is also smaller than that of SNNU-61 (0.49 cm3 g−1). Pore-size distribution analysis using the adsorption isotherm with the density functional theory (DFT) method indicates a narrow micropore of around 1 nm distributed in NJU-Bai49. Gas Storage and Separation. The permanent porosity and decorated amide functional groups motivated us to check into its gas adsorption performance. The low-pressure (0−1 bar) adsorption isotherms for CO2, CH4, and N2 were C

DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX

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to 2.3 and 1.3 wt %, respectively. Very interestingly, NJU-Bai49 adsorbs limited amounts of N2 (8.1 and 4.3 cm3 g−1, amounting to 1.0 and 0.5 wt % at 273 and 298 K under 1 bar, respectively), which is the smallest in all AFMOFs and could be attributed to its smallest channel size. On the basis of the experimental CO2, CH4, and N2 adsorption isotherms, the selectivities for CO2/N2 (15/85) and CO2/CH4 (50/50) at different temperatures were calculated using the ideal adsorbed solution theory (IAST).41 As shown in Figure 4c, the selectivity for CO2/N2 (15/85) is 166.7 at 298 K and 1 bar, which is the highest for all reported AFMOFs (Table 1) and higher than those of NJU-Bai21 (91), LIFM-11(Cu) (68.9), NJU-Bai0 (22), HNUST-1 (39.8), NJUBai22 (81), and NJU-Bai23 (72) under the same conditions. Compared with the MOFs without amide groups, the CO2/N2 selectivity of NJU-Bai49 (166.7) is higher than those of many MOFs, such as SIFSIX-2-Cu-i (140), HKUST-1 (101), ZnMOF-74 (87.7), UTSA-85a (62.5), and ZIF-78 (41.4) but still lower than those of SIFSIX-3-Zn (1818), UTSA-16 (314.7), and Mg-MOF-74 (182.1).10,42−46 Moreover, the calculation for CO2/CH4 (50/50) is 7.5 at 298 K and 1 bar, which is moderate because of the moderate CO2 adsorption capacity of NJU-Bai49 at 0.5 bar. To estimate the interaction between the absorbed molecules and framework and further understand the highly selective CO2 adsorption of NJU-Bai49, isosteric heats (Qst) for CO2 and CH4 of NJU-Bai49 were calculated using the experimental isotherm data at 273 and 298 K. As shown in Figure 5, the

Figure 4. (a) Adsorption and desorption isotherms of N2 at 77 K for NJU-Bai49. Inset: Pore-size distribution plot of NJU-Bai49 using the DFT method. (b) CO2, CH4, and N2 adsorption isotherms of NJUBai49 at 273 and 298 K. (c) IAST-predicted selectivity for CO2/N2 (15/85) and CO2/CH4 (50/50) mixtures of NJU-Bai49 at 298 K.

Figure 5. CO2 and CH4 adsorption enthalpies of NJU-Bai49.

collected at 273 and 298 K. As shown in Figure 4b, the CO2 uptake of NJU-Bai49 at 273 and 298 K under 0.15 bar are 43.3 and 23.1 cm3 g−1, corresponding to 8.4 and 4.5 wt % [wt % = 100 × (mass of adsorbed gas)/(mass of MOF)], respectively, which is larger than those of many AFMOFs under the same conditions such as NJU-Bai21 (4.4 wt %), LIFM-11(Cu) (4.3 wt %), NJU-Bai0 (4.2 wt %), HNUST-1 (3.7 wt %), NJUBai22 (3.1 wt %), and isoreticular SNNU-61 (4.0 wt %) and only smaller than that of HHU-2 (6.3 wt %; Table 1). In addition, NJU-Bai49 can adsorb 137.5 and 88.2 cm3 g−1 CO2 at 273 and 298 K under 1 bar, corresponding to 27.0 and 17.3 wt %, which is higher than those of LIFM-11(Cu) (15.3 wt %), NJU-Bai22 (14.3 wt %), and NJU-Bai3 (11.8 wt %) but smaller than those of HNUST-1 (18.8 wt %), HHU-2 (21.1 wt %), NJU-Bai0 (23.8 wt %), and the isoreticular SNNU-61 (18.3 wt %). In contrast, its CH4 uptake amounts at 273 and 298 K under 1 bar are only 32.6 and 18.8 cm3 g−1, amounting

CO2 adsorption enthalpy for NJU-Bai49 is calculated to be 33.4 kJ mol−1 at zero loading, which shows the strong interaction between the CO2 molecules and NJU-Bai49 framework. This Qst value is among the top range of AFMOFs and higher than HHU-2 (30.0 kJ mol−1), NJU-Bai21 (25.9 kJ mol−1), NJFU-1 (27.2 kJ mol−1), LIFM-10(Cu) (29 kJ mol−1), and the isoreticular SNNU-61 (27.0 kJ mol−1). However, with more CO2 molecule loading, the enthalpy values gradually reduce over the entire loading range, which indicates the heterogeneity of CO2 binding sites within the framework. Therefore, the highly selective CO2 adsorption of NJU-Bai49 could be attributed to the strong interaction toward CO2 molecules and multiple CO2 binding sites provided by the framework. In addition, the adsorption enthalpy for CH4 adsorption of NJU-Bai49 is estimated to be 18.3 kJ mol−1, which indicates the moderate interaction between the framework and adsorbed CH4 molecules. D

DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX

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shorter than those of the exposed metal sites and CO2 molecule (3.301−3.332 Å). With more CO2 loading, CO2 molecules start to occupy the secondary adsorption sites. The distances between the exposed metal sites and CO2 molecule at the secondary adsorption site (3.200−3.248 Å) are shorter than those at the primary adsorption site (3.301−3.332 Å). Besides the primary and secondary adsorption sites, some CO2 molecules also distribute inside the cages around the Sc3 clusters. Therefore, both the decorated amide group and open metal site participate in the formation of CO2 binding sites, which may provide strong interaction toward CO2 and contribute to the highly selective CO2 adsorption.

To better understand the interaction between the CO2 molecules and framework and predict the possible binding sites of CO2 molecules, grand canonical Monte Carlo (GCMC) simulations were carried out through the sorption module of the Materials Studio 7.0 package according to the literature.51 The unit-cell framework of NJU-Bai49 was constructed from experimental crystal X-ray diffraction data. The Locate and Metropolis methods52 were used. The maximum loading and production steps were set as 1 × 105 and 1 × 107, respectively. The simulations were done by utilizing one unit cell, and on the basis of the experimental data, CO2 molecules were chosen to be 18 (0.15 bar) and 70 (1 bar). During the simulation, the CO2 molecules and framework were considered to be rigid. All atom charges were assigned by the COMPASS force field.53 The Ewald summation method was used for electrostatic terms. Atom based on van der Waals was included with a 18.5 Å cutoff radius. Different from other Cu-based AFMOFs, in NJU-Bai49, the cage window composed of the CO bond and exposed metal sites serves as the primary adsorption site (Figure 6a), while

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CONCLUSIONS In summary, on the basis of the 6-connected [Sc3(μ3O)(COO)6] clusters and amide-functionalized tetratopic carboxylate linker H4L, NJU-Bai49, a highly stable AFMOF with soc topology was afforded. Very interestingly, it shows the combining feature of highly selective CO2 capture and excellent thermal/chemical stability. Furthermore, its selectivity for CO2/N2 is the highest among all AFMOFs reported so far. Its highly selective CO2 uptake is due to both the decorated amide and open metal binding sites, which have been confirmed by GCMC simulations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03308. Crystallographic data for NJU-Bai49, PXRD, TGA, VTPXRD, IR, and XPS analysis, calculations of the isosteric heats of gas adsorption, IAST calculations, and N2 adsorption measurements after water or acid−base treatments (PDF) Accession Codes

CCDC 1864012 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID Figure 6. (a) Preferential CO2 molecule binding sites simulated with GCMC at 0.15 bar. (b) Preferential CO2 molecule binding sites simulated with GCMC at 1 bar. Color code: C, black; O, red; N, turquoise; Sc, blue; Cl, green; H, gray. Partial H atoms are omitted for clarity.

Cong Chen: 0000-0001-8191-3773 Wenwei Zhang: 0000-0002-7482-3013 Junfeng Bai: 0000-0002-6487-7085

the opposite cage window consists of the N−H bond and exposed metal sites serves as the secondary adsorption site (Figure 6b). At low CO2 loading, collaboration of the two exposed metal sites and one amide group at the primary adsorption site offers stronger binding interaction toward CO2, which is consistent with the Qst calculation. Before the pressure of 0.15 bar, CO2 molecules mainly adsorbed at the primary adsorption site. Interestingly, the distance between the amide group and CO2 molecule (CO···C) is 3.217 Å, which is

Notes

Author Contributions §

These authors contributed equally.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Cheung Kong Scholars Program, the Hundred Talents Program of Shaanxi Province, the National Natural Science Foundation of China (Grant 21771121), and the State Key Laboratory of Coordination Chemistry (Grant SKLCC1812) for their support. E

DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b03308 Inorg. Chem. XXXX, XXX, XXX−XXX