A multifunctional cadmium–organic framework comprising tricarboxytriphenyl amine: selective gas adsorption, liquid-phase separation and luminescence sensing

Abstract

The reaction of tricarboxytriphenyl amine (H₃TCA) with Cd^II and auxiliary ligand 4,4′-bis(1-imidazolyl)biphenyl (bimb) afforded [Cd₃(TCA)₂(bimb)·(DMA)₆]_n (1) containing a two-fold interpenetrating three-dimensional microporous framework. Compound 1 showed remarkable adsorption enthalpies for CO₂ and H₂ and good H₂/N₂ selectivity at low temperature. Moreover, activated 1 showed a highly selective adsorption towards CO₂ over N₂ and CH₄ under ambient conditions. Interestingly, compound 1 selectively removed methyl orange (MO) over methylene blue (MB) from contaminated water with good recyclability, and possessed the second highest adsorption capacity for MO among the MOFs reported to date. Remarkably, complex 1 showed strong guest-dependent luminescence behavior.

---

Introduction

Metal–organic frameworks (MOFs) with inherent structural diversities and functional properties, represent a class of crystalline material formed from organic electron-donor linkers and metal cations, demonstrating prominent applications in strategic storage and gas separation, chemical sensing, catalysis, and medical application.^1–7

Selective capture of CO₂ is very important for the purification of natural gases and the reduction of CO₂ emissions.^8,9 Employing basic nitrogen-containing organic groups into MOFs has been considered to be advantageous for the selective adsorption of CO₂.^10,11 Typically, the interaction involving the localized dipoles of the N-containing group and the quadrupole moment of CO₂ would induce the dispersion and electrostatic forces, thus improving the CO₂ adsorption and separation abilities of MOFs. Recently, using MOFs in liquid-phase adsorption of harmful dyes has drawn great attention,^12–14 due to their tunable pore structures,¹⁵ relatively heterogeneous surfaces,¹⁶ and potentially strong electrostatic interactions with the guest molecules.¹⁷ The selectivity and recyclability, adsorption capacity and stability are all crucial for high-performance liquid-phase separation using MOFs. Nowadays, the permanent porosity of some luminescent MOFs favored reversible storage and release of guest substrates, as well as provided differential recognition sites to guest species; and therefore, they were successful in sensing of ions and small molecules.^18–22 Even more remarkably, these applications were combined and integrated into individual frameworks to construct multifunctional MOFs.^23–25 Despite a tremendous progress in the field of MOFs, currently, it remains extremely challenging to insert more complex functionalities within the lattice in a rational and systematic manner.

In our effort aimed at multifunctional MOFs, tricarboxytriphenyl amine (H₃TCA, Scheme S1‡)^26–31 and Cd^II were selected in the presence of auxiliary ligand 4,4′-bis(1-imidazolyl)biphenyl (bimb) to synthesize MOFs based on the following reasons: (1) the collaborative interaction between d¹⁰ cadmium(II) and photoactive nitrilotribenzoate organic ligand facilitates the generation of the architecture with enhanced fluorescence; (2) Lewis base triphenylamine sites on the internal surface of framework may prompt the selective adsorption of CO₂. Herein, we successfully prepared a two-fold interpenetrating three-dimensional (3D) microporous MOF [Cd₃(TCA)₂(bimb)·(DMA)₆]_n (1) that demonstrates impressive adsorption enthalpies for CO₂ and H₂ and exhibits good H₂/N₂ selectivity at lower temperature. Moreover, activated 1 showed highly selective adsorption towards CO₂ over N₂ and CH₄ under ambient conditions. Interestingly, compound 1 selectively removed methyl orange (MO) over methylene blue (MB) from contaminated water with a good recyclability. To the best of our knowledge, 1 showed the second highest adsorption capacity for MO among the MOFs up to now. Remarkably, complex 1 showed a strong guest-dependent luminescence behavior, enabling it as a promising luminescent probe for detecting small molecules from their photoluminescence (PL) spectra.

Experimental

Materials and measurements

All the commercially available chemicals were of reagent grade and used as received without further purification. Elemental analyses for C, H and N were determined on a Perkin-Elmer 240 elemental analyzer. IR spectra were recorded in the 4000–400 cm⁻¹ region on a Bruker Vector 22 spectrophotometer using KBr discs. Thermogravimetric analyses (TGA) were conducted on a TA-SDT 2960 at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. Powder X-ray diffraction (PXRD) data were collected on a Siemens D5005 diffractometer using CuK_α radiation over the 2θ range of 5–30°. UV-vis spectra were collected on a TU-1900 double-beam spectrophotometer. Fluorescent spectra were measured on a Varian Cary Eclipse fluorescence spectrophotometer. The 1-solvent emulsions were prepared by introducing 5 mg of 1 powder into 5.00 mL of methanol, ethanol, and water. Ultra high purity-grade (99.999%) N₂, H₂, CO₂, and CH₄ were used for gas sorption measurements, which were carried out on a Quantachrome IQ₂ automatic volumetric instrument. The methanol-exchanged samples were degassed at 393 K for 12 h under a dynamic vacuum to remove guest solvent molecules.

Synthesis of [Cd₃(TCA)₂(bimb)·(DMA)₆]_n (1)

A mixture of Cd(NO₃)·4H₂O (15.4 mg, 0.05 mmol), H₃TCA (4.9 mg, 0.01 mmol), and bimb (14.3 mg, 0.05 mmol) in a N,N-dimethylacetamide (DMA, 3 mL) was sealed in a 25 mL Teflon-lined autoclave and heated at 110 °C for 2 days and then slowly cooled to room temperature. Yellow crystals of 1 were obtained (81% yield based on Cd). Anal. calcd. for 1 (C₈₄H₉₂Cd₃N₁₂O₁₈): C, 53.24; H, 4.89; N, 8.87%; found: C, 53.56; H, 4.01; N, 8.70%. IR spectrum (cm⁻¹): 2930w, 1595s, 41m, 1311m, 1269m, 1173w, 1103w, 1064w, 1014w, 962w, 932w, 831w, 784m, 712w, 675w, 523w, 437w.

X-ray crystallography

The crystallographic data of 1 was collected using a Bruker Smart Apex CCD area-detector diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 173 K using the ω-scan technique. Data reductions and absorption corrections were respectively performed with SAINT³² and SADABS³³ software packages, respectively. The structure was solved by direct methods, and refined anisotropically with SHELXL-97 (ref. 34) using full-matrix least-squares procedures based on F². The disordered segments were subjected to geometric restraints during the refinements. The distribution of peaks in the channels of 1 was chemically featureless to refine using conventional discrete-atom models. To solve the problems, the free solvent molecules were removed by the SQUEEZE routine in PLATON.³⁵ The number of solvent molecules in 1 was obtained by element analysis and TG. The crystallographic data were deposited in the Cambridge Crystallographic Data Centre: CCDC 1057133. The relevant crystallographic data are presented in Table 1; the selected bond lengths and angles are given in Table 2.

Table 1 Crystallographic data and other pertinent information for 1

	1
Formula	C60H38Cd3N6O12
Formula weight	1372.16
Crystal system	Monoclinic
Space group	P2/n
a/Å	17.7101(9)
b/Å	13.2378(5)
c/Å	17.8690(9)
α/°	90
β/°	90.847(2)
γ/°	90
V/Å^3	4188.8(3)
Z	2
ρcalcd/g cm^−3	1.088
μ/mm^−1	0.800
Collected reflections	20 691
Unique reflections	7545
R1 [I > 2σ (I)]	0.0434
wR2 (all data)	0.1234

---

Table 2 Selected bond distances (Å) and angles (deg) for 1^a

a Symmetry codes for 1: #1 x, −1 + y, z; #2 3/2 − x, −1 + y, 5/2 − z; #3 3/2 − x, y, 5/2 − z; #4 2 − x, 1 − y, 2 − z; #5 −1/2 + x, 1 − y, 1/2 + z.
Cd1–O2	2.242(3)	Cd1–O5#1	2.242(4)
Cd1–O5#2	2.132(3)	Cd1–O2#3	2.242(3)
Cd1–O3#4	2.368(3)	Cd1–O3#5	2.368(3)
Cd2–O1	2.219(4)	Cd2–N2A	2.31(3)
Cd2–O6#1	2.218(5)	Cd2–O3#5	2.296(3)
Cd2–O4#5	2.517(4)	O2#3−Cd1–O5#1	164.47(12)
O3#4−Cd1#5	170.82(14)	O2–Cd1–O5#2	164.47(12)
O3#5−Cd2–N2A	144.8(6)	O1–Cd2–O6#1	94.17(15)
O3#5−Cd2–O4#5	54.44(12)

---

Dye adsorption

The adsorption isotherm of MO on compound 1 was measured in the dark by mixing 10 mg of compound 1 with 50.0 mL of MO aqueous solution at the initial concentrations (C₀) from 15 to 300 ppm. The suspension was first sonicated for 5 min and then shaken at a constant rate overnight. After the suspension was filtered through a membrane filter (pore size, 0.45 μm), the filtrate was analyzed using a TU-1900 double-beam spectrophotometer to get the equilibrium concentration C_e. The amount of adsorption Q_e, was calculated by dividing the decreased concentration of (C₀ − C_e) by the amount of adsorbent used and expressed in mol g⁻¹ of 1. The maximum adsorption capacity was calculated from the Langmuir adsorption isotherm.

Results and discussion

Description of crystal structure

As shown in Fig. 1a, there are two crystallographically independent Cd^II centers in the structure: the Cd1 atom at half occupancy, situated on an inversion center, adopts an octahedral coordination environment occupied by six carboxylate O atoms from six distinct TCA³⁻ anions; the Cd2 center sites in a trigonal bipyramid sphere was defined by four O atoms from three distinct TCA³⁻ anions and one N atom from a bimb ligand. In 1, the Cd–O bond lengths were in the range 2.218(5)–2.517(4) Å, and the Cd–N distances ranged from 2.219(4) to 2.31(3) Å. The TCA³⁻ moiety can be considered as a propeller because its phenyl rings are tilted relative to each other with dihedral angles varying from 74.2 to 87.1°. Interestingly, six carboxylate groups bridge three Cd^II atoms, generating the centrosymmetric trinuclear cluster Cd₃(CO₂)₆ as the secondary building unit (SBU) with nonbonding Cd1⋯Cd2 distance of 3.6362 Å (Fig. 1b) in complex 1. Apparently, each tricarboxytriphenyl amine (TCA³⁻) connects three Cd₃(CO₂)₆ clusters to serve as a 3-connected node (Fig. S1a‡), whereas each Cd₃(CO₂)₆ subunit in turn links another six Cd clusters and two TCA³⁻ to act as 8-connected node, therefore, the individual framework of compound 1 can be rationalized as a binodal (3,8)-connected tfz-d net with Schlafli symbol (4³)₂(4⁶·6¹⁸·8⁴) (Fig. 1c). The entire structure of 1 adopts three-dimensional (3D) porous architecture with two-fold interpenetrating belonging to class Ia.³⁶ Micropore windows were observed with a size of 6.1 × 4.7 Ų along the b axis in 1, considering van der Waals distances (Fig. 1d). The free accessible volume in the fully desolvated 1 is ca. 43.6% (1827 ų) as determined by PLATON.

Fig. 1 (a) Coordination environments of Cd(II) atoms with hydrogen atoms and disordered atoms for clarity of 1. (b) Trinuclear cluster [Cd₃(CO₂)₆] SBU in 1. (c) Schematic representation of (3,8)-connected network for single framework of 1: blue, TCA³⁻ ligand; grey-green, Cd1. (d) two-fold interpenetrated structure of 1 with microporous cavity. The symmetry codes refer to Table 2.

PXRD and thermal stability

A careful comparison of PXRD patterns of the as-synthesized and activated 1, as well as the simulated curve from the single-crystal data showed that the evacuated framework of 1 underwent no structural transformation after the activation (Fig. S2‡). The PXRD profiles of 1 also indicated that the framework maintained structural integrity after the recycling test for MO removal and immersion in different solvents. Compound 1 showed a weight loss of 23% below 290 °C, which can be attributed to the loss of guest DMA molecules (calcd 26%). The dehydrated phase remained stable up to 355 °C until the organic ligands were released (Fig. S3‡).

Gas adsorption

To evaluate the porosity of activated 1, gas sorption studies were conducted. The type-I H₂ isotherms on the desolvated 1 were collected at 77 and 87 K, as shown in Fig. 2a. Obviously, both desorption isotherms exhibited significant hysteresis loops, indicating a small amount of mesoporosity that can be attributed to the intercrystalline voids.^37,38 Complex 1 showed a maximum H₂ uptake of 62.2 cm³ g⁻¹ (0.55 wt%) at 77 K/1 bar and 32.8 cm³ g⁻¹ (0.29 wt%) at 87 K/1 bar, respectively. Moreover, the isosteric heat (Q_st) of H₂ adsorption calculated by the virial method³⁹ from the adsorption isotherms measured at 77 and 87 K was ca. 8.58 kJ mol⁻¹ at zero loading, which is one of the highest values found in MOFs without open metal sites^40,41 and significantly higher than those of well-known MOFs, such as NOTT-400 (5.96 kJ mol⁻¹), NOTT-401 (6.65 kJ mol⁻¹),⁴² SNU-6 (7.74 kJ mol⁻¹),⁴³ MOF-646 (7.8 kJ mol⁻¹)⁴⁴ and MOF-5 (7.6 kJ mol⁻¹).⁴⁵ Moreover, the value was obviously superior to that of the typical van der Waals type interactions.⁴⁶ The Q_st decreased slowly with increasing H₂ loading and reached ∼5.12 kJ mol⁻¹ at the maximum coverage. Apparently, this high Q_st value can be attributed to its narrow pores after the double interpenetration, indicating a strong interaction between H₂ molecules and the framework. In contrast, the evacuated 1 only showed an external surface N₂ adsorption at 77 K. In this case, the preferential adsorption of H₂ over N₂ might be due to the size-exclusion effect, where the small windows limit the diffusion of larger N₂ (3.64 Å) molecules into the pores, resulting in a lower adsorption over the diffusion time of the measurement, whereas smaller H₂ molecules (2.9 Å) entered into the pores under the given conditions.

Fig. 2 (a) H₂ (77 K), H₂ (87 K) and N₂ (77 K) sorption isotherms for desolvated 1, where the filled and open shape represent the adsorption and desorption, respectively. (b) The isosteric heat of H₂ adsorption (Q_st) for desolvated 1. (c) Sorption isotherms for CO₂, CH₄, and N₂ at 273 K of desolvated 1 (adsorption and desorption branches are shown with filled and empty shape, respectively).

Interestingly, the maximum CO₂ uptake for desolvated 1 at 273 K/1 bar was 16.7 cm³ g⁻¹ (Fig. 2c), whereas the corresponding CH₄ and N₂ uptakes were only 5.51 and 0.989 cm³ g⁻¹, respectively, highlighting 1 as a promising material for highly selective separation of CO₂/CH₄ and CO₂/N₂ at room temperature. The separation ratios of CO₂ vs. CH₄ and N₂ were calculated from the ratios of the initial slopes based on the single-component sorption isotherms (Fig. S5a‡).⁴⁷ The calculated CO₂/N₂ and CO₂/CH₄ selectivities were 53 : 1 and 5.9 : 1 at 273 K, respectively. The values were superior to those of SYSU (CO₂/N₂: 25.5, CO₂/CH₄: 4.7),⁴⁸ Cu₆(BTTC)₄(H₂O)₆·xS (CO₂/N₂: 31, CO₂/CH₄: 4.9)⁴⁹ and [Cu₂(obb)₂(bpy)_0.5(DMF)]·2DMF (CO₂/N₂: 30).⁵⁰ The isosteric heat (Q_st) of CO₂ adsorption was calculated using the experimental isotherm data at 273 and 298 K (Fig. S5b and c‡). Compound 1 exhibited strong binding affinity for CO₂ (35.6 kJ mol⁻¹) at zero coverage (Fig. S5d‡), which is comparable to the superior values of MOFs containing amine functionality (35–41 kJ mol⁻¹).^51–54 The high initial Q_st of CO₂ can be mainly attributed to the strong interactions between Lewis basic amino sites along the small channels and acidic CO₂ molecule. Moreover, the high quadrupole moment of (13.4 × 10⁻⁴⁰ C m²) compared to those of N₂ (4.7 × 10⁻⁴⁰ C m²) and CH₄ (nonpolar) could enhance specific interactions with the host framework, leading to the selectivity of CO₂ vs. CH₄ and N₂.

Liquid-phase separation

The removal of dyes from wastewater is of paramount importance as many dyes are toxic and even carcinogenic.^55,56 Because of its high efficiency and economic feasibility combined with simple operation, the removal of dyes by adsorption technology has becoming one of the competitive methods⁵⁷ among many available physical, chemical and biological strategies.

Methyl orange (MO) and methylene blue (MB) (Scheme S2‡) represent well-known anionic and cationic dyes, respectively. To investigate the adsorption ability of compound 1 for hazardous dyes, 10 mg crystals of 1 were added to an aqueous solution of MO (20 ppm, 3 mL) at room temperature. After 20 minutes, the MO was removed from the aqueous solution with the yellow MO solution fading to colorless and the corresponding concentration decreased to 0.758 pm. In the case of MB, no apparent color change was observed after 20 minutes, and the MB concentration mostly remained unchanged (Fig. S6‡). The electrostatic interactions between the metal cation centers with a strong positive electric field effect and anionic MO in addition to the π–π interaction between the benzene rings of MOF and dyes may be responsible for the selective adsorption of MO over MB in water for compound 1.⁵⁸

Interestingly, after the MO-loaded 1 was soaked in a fresh DMA for 20 minutes at room temperature, the recovery of adsorbent 1 was completed. Remarkably, the colorless DMA solution changed to yellow; meanwhile, the color of absorbent 1 almost shifted back to the original color (Fig. S7‡). Furthermore, a recyclability test with three consecutive runs was carried out for 1 to determine whether the adsorbent could be reused for subsequent removal of MO (Fig. S8‡). Specifically, the removal ability of 1 for MO did not decrease for all the three runs, indicating the applicability of MOFs in the adsorptive removal of anionic dyes from contaminated water.

As shown in Fig. 3, the MO adsorption isotherm of 1 follows the Langmuir equation C_e/Q_e = C_e/Q_max + 1/(KQ_max), where Q_e is the amount of MO adsorption at C_e, the equilibrium concentration of MO in the bulk solution, Q_max is the maximum amount of adsorption, and K is the adsorption constant. Therefore, Q_max can be obtained from the reciprocal of the slope of a plot of C_e/Q_e vs. C_e. The Q_max value was 748 mg g⁻¹, higher than the adsorption capacity of MOF-235,⁵⁹ NH₃⁺-MCM-41,⁶⁰ LDH⁶¹ and activated carbon,⁶² indicating the high efficient adsorption of 1 for MO. To the best of our knowledge, 1 has the second highest adsorption capacity among the MOFs reported to date (Table 3). Moreover, the Gibbs free energy change ΔG was calculated to be −21.02 KJ mol⁻¹ using the following equation: ΔG = −RT ln K (where R is gas constant), which suggests spontaneous adsorption of MO over 1 under the experimental conditions.

Fig. 3 (a) Adsorption isotherm for the adsorption of MO over 1. (b) Langmuir plot of the isotherm.

Table 3 Comparison of MO adsorption capacity in various MOFs

Adsorbents	Adsorption capacity (mg g^−1)	References
MOF-235	477.0	59
MIL-53(Cr)	57.9	15
PED-MIL-101	194.0	15
MIL-100(Cr)	211.8	17
MIL-100(Fe)	1045.2	17
1	748	This work

---

Luminescence sensing

The PL spectra of complex 1 and free ligand H₃TCA in the solid state were recorded at room temperature. Complex 1 exhibited an intense emission peak at 470 nm upon excitation at 360 nm, which can be largely assigned to the ligand-centered luminescent process because a similar emission was observed at 453 nm for the free ligand H₃TCA (Fig. S10‡). Interestingly, complex 1 showed strong guest-dependent luminescence properties: this MOF showed a purple emission at 423 nm in methanol, a significantly red-shifted indigo emission at 461 nm in H₂O, and a further red-shifted cyan emission at 490 nm in ethanol, indicating that it is a promising luminescent probe for detecting small molecules through the shift of the PL spectra (Fig. 4). Moreover, the PL spectra did not shift when the same experiment was performed with only H₃TCA (Fig. S11‡). This indicates that this discrimination can be attributed to complex 1. The shifts in the PL spectra can be ascribed to the formation of exciplexes (excited complexes) by the interactions between analytes and MOFs in the excited states,^63,64 which is less investigated in MOFs.⁶⁵

Fig. 4 Emission spectra of compound 1 in MeOH, EtOH and H₂O. Inset: color changes upon the addition of different solvents.

Conclusions

We successfully prepared a two-fold interpenetrating three-dimensional (3D) microporous MOF [Cd₃(TCA)₂(bimb)·(DMA)₆]_n (1) that showed remarkable adsorption enthalpies for CO₂ and H₂ gas molecules and exhibited good separation ratios of H₂ over N₂ at lower temperature. In particular, activated 1 was efficient for the selective sorption of CO₂ over N₂ and CH₄ under ambient conditions. Interestingly, compound 1 exhibited highly selective and recyclable property in the removal of dyes MO from contaminated water. To the best of our knowledge, 1 has the second highest adsorption capacity for MO among the MOFs reported to date. Notably, complex 1 displayed strong guest-dependent luminescence behavior, indicating that it could be a promising luminescent probe for detecting small molecules.

Acknowledgements

This work was financially supported by the National Nature Science Foundation of China (No. 21171062 and 21371065), Self-Determined Research Funds of CCNU from the Colleges' Basic Research and Operation of MOE (CCNU15A05053).

References

1. H. Furukawa, N. Ko, Y. B. Go, N. Aratani, S. B. Choi, E. Choi, A. Ö. Yazaydin, R. Q. Snurr, M. O'Keeffe, J. Kim and O. M. Yaghi, Science, 2010, 329, 424.
2. M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev., 2012, 112, 782.
3. S. Bourrelly, P. L. Llewellyn, C. Serre, F. Millange, T. Loiseau and G. J. Feérey, J. Am. Chem. Soc., 2005, 127, 13519.
4. J. T. Jia, L. Wang, F. X. Sun, X. F. Jing, Z. Bian, L. X. Gao, R. Krishna and G. S. Zhu, Chem.–Eur. J., 2014, 20, 9073.
5. Z. G. Xie, L. Q. Ma, K. E. deKrafft, A. Jin and W. B. Lin, J. Am. Chem. Soc., 2010, 132, 922.
6. L. E. Kreno, K. Leong, O. K. Farha, M. Allendorf, R. P. van Duyne and J. T. Hupp, Chem. Rev., 2012, 112, 1105.
7. (a) J. Y. Lee, O. K. Farha, J. K. Roberts, A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450; (b) X. L. Zhao and W. Y. Sun, CrystEngComm, 2014, 16, 3247.
8. (a) J. B. Lin, W. Xue, J. P. Zhang and X. M. Chen, Chem. Commun., 2011, 47, 926; (b) B. Bhattachary and D. Ghoshal, CrystEngComm, 2015, 17, 8388.
9. B. S. Zheng, Z. Yang, J. F. Bai, Y. Z. Li and S. H. Li, Chem. Commun., 2012, 48, 7025.
10. L. T. Du, Z. Y. Lu, K. Y. Zheng, J. Y. Wang, X. Zheng, Y. Pan, X. Z. You and J. F. Bai, J. Am. Chem. Soc., 2013, 135, 562.
11. Z. J. Zhang, Y. G. Zhao, Q. H. Gong, Z. Li and J. Li, Chem. Commun., 2013, 49, 653.
12. H. L. Jiang, Y. Tatsu, Z. H. Lu and Q. Xu, J. Am. Chem. Soc., 2010, 132, 5586.
13. Y. X. Tan, Y. P. He, M. Wang and J. Zhang, RSC Adv., 2014, 4, 1480.
14. C. Zou, Z. J. Zhang, X. Xu, Q. H. Gong, J. Li and C. D. Wu, J. Am. Chem. Soc., 2012, 134, 87.
15. E. Haque, J. E. Lee, I. T. Jang, Y. K. Hwang, J.-S. Chang, J. Jegal and S. H. Jhung, J. Hazard. Mater., 2010, 181, 535.
16. S. H. Huo and X. P. Yan, J. Mater. Chem., 2012, 22, 7449.
17. M. M. Tong, D. H. Liu, Q. Y. Yang, S. Devautour-Vinot, G. Maurin and C. L. Zhong, J. Mater. Chem. A, 2013, 1, 8534.
18. Y. J. Cui, Y. F. Yue, G. D. Qian and B. L. Chen, Chem. Rev., 2012, 112, 1126.
19. S. Pramanik, C. Zheng, X. Zhang, T. J. Emge and J. Li, J. Am. Chem. Soc., 2011, 133, 4153.
20. Y. N. Gong, L. Jiang and T. B. Lu, Chem. Commun., 2013, 49, 11113.
21. T. K. Kim, J. H. Lee, D. Moon and H. R. Moon, Inorg. Chem., 2013, 52, 589.
22. X. Y. Xu and B. Yan, ACS Appl. Mater. Interfaces, 2015, 7, 721.
23. Y. B. He, B. Li, M. O'Keeffe and B. L. Chen, Chem. Soc. Rev., 2014, 43, 5618.
24. L. Qin, M. X. Zheng, Z. J. Guo, H. G. Zheng and Y. Xu, Chem. Commun., 2015, 51, 2447.
25. S. L. Qiu and G. S. Zhu, Coord. Chem. Rev., 2009, 253, 2891.
26. H. K. Chae, J. Kim, O. D. Friedrichs, M. O'Keeffe and O. M. Yaghi, Angew. Chem., Int. Ed., 2003, 42, 3907.
27. E. Y. Lee, S. Y. Jang and M. P. Suh, J. Am. Chem. Soc., 2005, 127, 6374.
28. M. P. Suh, Y. E. Cheon and E. Y. Lee, Chem.–Eur. J., 2007, 13, 4208.
29. P. Y. Wu, J. Wang, Y. M. Li, C. He, Z. Xie and C. Y. Duan, Adv. Funct. Mater., 2011, 21, 2788.
30. P. Y. Wu, C. He, J. Wang, X. J. Peng, X. Z. Li, Y. L. An and C. Y. Duan, J. Am. Chem. Soc., 2012, 134, 14991.
31. P. Y. Wu, J. Wang, C. He, X. L. Zhang, Y. T. Wang, T. Liu and C. Y. Duan, Adv. Funct. Mater., 2012, 22, 1698.
32. SAINT, version 6.2, Bruker AXS, Inc, Madison, WI, 2001.
33. G. M. Sheldrick, SADABS, University of Göttingen, Göttingen, Germany, 1997.
34. G. M. Sheldrick, SHELXTL, version 6.10; Bruker Analytical X-ray Systems, Madison, WI, 2001.
35. A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7.
36. I. A. Baburin, V. A. Blatov, L. Carlucci, G. Ciani and D. M. Proserpio, J. Solid State Chem., 2000, 178, 2452.
37. A. Vishnyakov, P. I. Ravikovitch, A. V. Neimark, M. Bulow and O. M. Wang, Nano Lett., 2003, 3, 713.
38. J. Y. Lee, J. Li and J. Jagiello, J. Solid State Chem., 2005, 178, 2527.
39. J. L. C. Rowsell and O. M. Yaghi, J. Am. Chem. Soc., 2006, 128, 1304.
40. M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev., 2012, 112, 782.
41. D. F. Sava, V. C. Kravtsov, F. Nouar, L. Wojtas, J. F. Eubank and M. Eddaoudi, J. Am. Chem. Soc., 2008, 130, 3768.
42. I. A. Ibarra, S. Yang, X. Lin, A. J. Blake, P. J. Rizkallah, H. Nowell, D. R. Allan, N. R. Champness and P. Hubberstey, Chem. Commun., 2011, 47, 8304.
43. H. J. Park and M. P. Suh, Chem.–Eur. J., 2008, 14, 8812.
44. S. Barman, H. Furukawa, O. Blacque, K. Venkatesan, O. M. Yaghi and H. Berke, Chem. Commun., 2010, 46, 7981.
45. H. Kim, S. Das, M. G. Kim, D. N. Dybtsev, Y. Kim and K. Kim, Inorg. Chem., 2011, 50, 3691.
46. A. G. Wong-Foy, O. Lebel and A. J. Matzger, J. Am. Chem. Soc., 2007, 129, 15740.
47. Y.-S. Bae, O. K. Farha, A. M. Spokoyny, C. A. Mirkin, J. T. Hupp and R. Q. Snurr, Chem. Commun., 2008, 4135.
48. S. L. Xiang, J. Huang, L. Li, J. Y. Zhang, L. Jiang, X. J. Kuang and C. Y. Su, Inorg. Chem., 2011, 50, 1743.
49. Y. M. Huang, B. G. Zhang, J. G. Duan, W. L. Liu, X. F. Zheng, L. L. Wen, X. H. Ke and D. F. Li, Cryst. Growth Des., 2014, 14, 2866.
50. Y. X. Tan, Y. P. He and J. Zhang, Inorg. Chem., 2012, 51, 9649.
51. J. An, S. J. Geib and N. L. Rosi, J. Am. Chem. Soc., 2010, 132, 38.
52. R. Vaidhyanathan, S. S. Iremonger, K. W. Dawson and G. K. H. Shimizu, Chem. Commun., 2009, 5230.
53. K. C. Stylianou, J. E. Warren, S. Y. Chong, J. Rabone, J. Basca, D. Bradshaw and M. J. Rosseinsky, Chem. Commun., 2011, 3389.
54. S. Couck, J. F. M. Denayer, G. V. Baron, T. Rémy, J. Gascon and F. Kapteijn, J. Am. Chem. Soc., 2009, 131, 6326.
55. G. Crini, Bioresour. Technol., 2006, 97, 1061.
56. A. Mittal, A. Malviya, D. Kaur, J. Mittal and L. Kurup, J. Hazard. Mater., 2007, 148, 229.
57. M. Rafatullah, O. Sulaiman, R. Hashim and A. Ahmad, J. Hazard. Mater., 2010, 177, 70.
58. L. L. Wen, X. Y. Xu, K. L. Lv, Y. M. Huang, X. F. Zheng, L. Zhou, R. Q. Sun and D. F. Li, ACS Appl. Mater. Interfaces, 2015, 7, 4449.
59. E. Haque, J. W. Jun and S. H. Jhung, J. Hazard. Mater., 2011, 185, 507.
60. Q. Qin, J. Ma and K. Liu, J. Hazard. Mater., 2009, 162, 133.
61. Z. M. Ni, S. J. Xia, L. G. Wang, F. F. Xing and G. X. Pan, J. Colloid Interface Sci., 2007, 316, 284.
62. S. Chen, J. Zhang, C. Zhang, Q. Yue, Y. Li and C. Li, Desalination, 2010, 252, 149.
63. S. Pramanik, Z. C. Hu, X. Zhang, C. Zheng, S. Kelly and J. Li, Chem.–Eur. J., 2013, 19, 15964.
64. S. R. Zhang, D. Y. Du, J. S. Qin, S. J. Bao, S. L. Li, W. W. He, Y. Q. Lan, P. Shen and Z. M. Su, Chem.–Eur. J., 2014, 20, 3589.
65. K. Jayaramulu, P. Kanoo, S. J. George and T. K. Maji, Chem. Commun., 2010, 46, 7906.

---

Footnotes

† This paper is dedicated to Prof. Thomas C. W. Mak on the occasion of his 80th birthday.

‡ Electronic supplementary information (ESI) available: X-ray crystallographic file (CIF), additional crystal figures, Q_st calculation details, dye adsorption and separation, emission spectra, TGA data, PXRD patterns for complex 1. CCDC 1057133. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra21980a

---