Synthesis, Crystal Structure, Gas Absorption, and Separation Properties of a Novel Complex Based on Pr and a Three-Connected Ligand

Abstract

A novel Pr complex, constructed from a rigid three-connected H₃TMTA and praseodymium(III) ion, has been synthesized in a mixed solvent system and characterized by X-ray single crystal diffraction, infrared spectroscopy, a thermogravimetric analysis, an element analysis, and powder X-ray diffraction, which reveals that complex 1 crystallizes in a three-dimensional porous framework. Moreover, the thermal stabilities and the fluorescent and gas adsorption and separation properties of complex 1 were investigated systematically.

1. Introduction

During the past few decades, a lot of effort has been devoted to the rational design and synthesis of coordination polymers (CPs) in the field of chemical and material science due to their fascinating architectures and topologies together with their potential applications [1,2,3,4,5,6,7,8]. Besides the N-containing ligands, rigid multi-carboxylate ligands are intriguing components owing to their easily predictable and stable resulting framework [9,10,11,12,13,14,15,16,17]. Among all of the multi-carboxylate ligands, many C₃-symmetric tricarboxylate ligands have been extensively investigated to construct CPs with interesting architectures and properties, including H₃TATB and H₃BTB (TATB denotes 4,4′,4″-s-triazine-2,4,6-triyltribenzoate and BTB denotes benzene-1,3,5-tribenzoate) [18,19,20]. At the same time, with its three carboxylate groups almost perpendicular to the central benzene ring, a nonplanar ligand H₃TMTA (TMTA denotes 4,4′,4″-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoate) has also been applied to build CPs with appealing topologies [21,22,23].

On the other hand, thousands of CPs based on the transition metal ions have been intensively investigated. Compared with transition metal ions, there exists a kind of rare earth metal ion, which possesses abundant luminescent properties. It should be pointed out that although quite a lot of coordination complexes have been developed using different ligands in the past years, to the best of our knowledge, porous frameworks built from rigid three-tricarboxylate ligands and rare earth ions are still rare.

In the present paper, a novel rare earth complex was constructed from a rigid three-connected H₃TMTA ligand and a praseodymium(III) ion, (Pr(TMTA)(H₂O)₂]·[DMF·2EtOH·4H₂O] [1, H₃TMTA = 4,4′,4″-(2,4,6-trimethylbenzene-1,3,5-triyl)tribenzoic acid). Interestingly, complex 1 shows permanent porosity and a moderate adsorption heat of CO₂ (21.6 kJ·mol⁻¹), which can be used as a platform for the selective adsorption of CO₂/CH₄ (3.56).

2. Experimental

2.1. Materials and Methods

All chemicals were used as commercially received without further purification. The FT-IR spectra were collected from 400 to 4000 cm⁻¹ using the KBr pellet method. The elemental analyses (for C, H, or N) were performed on a Perkin-Elmer 240 elemental analyzer ((PerkinElmer, Billerica, MA, USA). The powder X-ray diffraction measurements were performed with a Bruker AXS D8 Advance instrument (Karlsruhe, Germany). The thermogravimetric analysis was recorded on a Mettler Toledo instrument (Mettler Toledo, Zurich, Swiss). The gas uptake was performed on the surface area analyzer ASAP-2020 (Micromeritics, Norcross, GA, USA).

2.2. Synthesis of [Pr(TMTA)(H₂O)₂]·[DMF·2EtOH·4H₂O] (1)

H₃TMTA (2 mg, 0.0045 mmol) and Pr(NO₃)₃·6H₂O (9.2 mg, 0.02 mmol) were dissolved in mixed solvents, DMF:EtOH:H₂O (v:v:v = 1:1:1; 1 mL). The resulting green solution was sealed in a glass tube, heated to 75 °C in 5 h, kept for 40 h, then slowly cooled to 30 °C in 8 h. The green rod crystals were collected, washed with EtOH, and dried in the air (yield: 40%). Elemental analysis calcd (%) for 1: C 49.84, H 5.88, N 1.57; found: C 48.98, H 5.77, N 1.74%. IR (KBr): ν (cm⁻¹) = 3349 (m), 1618 (m), 1554 (s), 1419 (s), 1367 (s), 1273 (w), 1101 (w), 894 (w), 839 (m), 771 (m), 724 (s), 640 (m).

2.3. X-ray Crystallography

The single-crystal structure of the complex 1 was collected by an Agilent Xcalibur Eos Gemini diffractometer (Agilent Technologies, CA, USA) with a (Cu) X-ray Source (Cu-Kα λ = 1.54184 Å). The multi-scan program SADABS was applied to do the absorption corrections [24]. SHELXS-97 and SHELXL-97 were used to solve and refine the final structure of complex 1 by direct methods [25,26]. PLATON was used to add the symmetry of complex 1. [27].

Table 1. Crystal data for complex 1.

Empirical Formula	C30H25O8Pr
Formula weight	654.41
Temperature/K	298.15
Crystal system	monoclinic
Space group	P21/n
a/Å	9.531(3)
b/Å	16.417(5)
c/Å	27.409(8)
α/°	90.00
β/°	93.098(6)
γ/°	90.00
Volume/Å3	4282(2)
Z	4
ρcalcmg/mm3	1.015
m/mm−1	1.169
F(000)	1312.0
Index ranges	−10 ≤ h ≤ 10, 0 ≤ k ≤ 18, 0 ≤ l ≤ 30
Reflections collected	6198
Independent reflections	6198[R(int) = 0.1019]
Data/restraints/parameters	6198/906/354
Goodness-of-fit on F2	1.002
Final R indexes [I >= 2σ (I)]	R1 = 0.1012, wR2 = 0.2613
Final R indexes [all data]	R1 = 0.1277, wR2 = 0.2752
Largest diff. peak/hole/e Å−3	5.28/−1.63

contains the crystallographic details of complex 1 and

Table 2. Selected bond lengths (Å) and angles (°) for complex 1.

Pr1-O1	2.390(8)	Pr1-O1w	2.496(8)	Pr1-O2 1	2.384(8)
Pr1-O2w	2.488(8)	Pr1-O3 2	2.535(8)	Pr1-O4 2	2.570(8)
Pr1-O5 3	2.445(8)	Pr1-O6 4	2.480(8)	Pr1-O6 3	2.967(8)
O1-Pr1-O1w	77.9(3)	O1-Pr1-O2w	78.6(3)	O1-Pr1-O3 1	76.5(3)
O1-Pr1-O4 1	124.3(3)	O1-Pr1-O5 2	155.4(3)	O1-Pr1-O6 2	138.1(3)

¹ 1 − X, −Y, −Z; ² −1/2 + X, −1/2 − Y, −1/2 + Z; ³ –1 + X, 1 + Y, +Z; ⁴ 1 − X, −1 − Y, −Z.

collects the selected bond lengths and angles for complex 1.

CCDC 1582391 contains the supplementary crystallographic data of complex 1 for this paper. These data could be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; E-mail: [email protected]).

3. Results and Discussion

3.1. Crystal Structure of Complex 1

Complex 1 was obtained in mixed solvents of DMF:EtOH:H₂O by a hydrothermal reaction of H₃TMTA and Pr(NO₃)₃·6H₂O at 75 °C. The single-crystal X-ray analysis shows that complex 1 crystalizes in a monoclinic crystal system with a p21/n space group. The asymmetry unit of complex 1 contains a praseodymium ion, a TMTA³⁻ ligand, and two coordinated water molecules. The Pr-O distances are 2.384(8) Å and 2.967(8) Å, and the distances of Pr-Ow are 2.488(8) Å and 2.496(8) Å, respectively. As shown in Figure 1a, the Pr(III) ion in complex 1 adopts a nine-coordinated mode forming a distorted {PrO₉} coordination sphere. It is interesting that the carboxylic groups in 1 adopt three different coordination modes: μ₁-η¹-η¹, μ₂-η¹-η¹, and μ₂-η¹-η². The carboxylic groups connect with the Pr(III) ion to form a one-dimensional infinite chain, and then the chains are linked by the TMTA³⁻ ligand to construct a three-dimensional framework (Figure 1b).

3.2. The Fluorescent Property

Because of the presentation of rare earth ions and a rigid carboxylate group, the luminescent property of complex 1 was tested in the solid state at 298 K. The emission band centered at 362 nm (λ_ex = 320 nm) for H₃TMTA, which could be assigned to the electronic transition based on ligand-centered, which means the π*→n or π*→π electronic transition [28]. The emission of complex 1 was observed at 358 nm upon excitation at 320 nm for 1, which can be attributed to the emission of H₃TMTA ligands (Figure 2). There was no characteristic emission of rare earth ions.

3.3. Powder X-ray Diffraction Analysis

The powder X-ray diffraction pattern was used to certify the phase purity of complex 1 (Figure 3). Almost all of the peak positions of the simulated and experimental patterns match very well with each other. The preferred orientation of the powder samples accounts for the differences in intensity.

3.4. IR Spectra

The FT-IR spectrum of compound 1 was also tested. As depicted in Figure 4, the sharp bands at 1554 cm⁻¹ and 1419 cm⁻¹ stand for the asymmetric and symmetric stretching vibrations of the carboxylic group, respectively [29].

3.5. Thermogravimetric Analyses

As shown in Figure 5, the thermogravimetric analysis (TGA) property of complex 1 was detected under an N₂ atmosphere. Complex 1 has two identifiable weight loss stages: the first stage is similar to the removal of seven uncoordinated and two coordinated solvent molecules (obsd 26.37%, calcd 27.91%), which arises between room temperature and 273 °C. The second stage belongs to the collapse of the framework, which appears at temperatures higher than 500 °C, which means that the present complex 1 shows moderate thermal stability.

3.6. Gas Sorption and Separation Measurements

Gas adsorption–desorption measurements of N₂, CO₂, CH₄, and H₂ on complex 1 were collected on a Micromeritics ASAP 2020 surface area and pore size analyzer at different temperatures: 77 K (liquid nitrogen bath), 273 K (ice-water bath), and 298 K (room temperature). The Brunauer-Emmett-Teller (BET) surface area and pore size distribution data were calculated from the N₂ adsorption isotherms at 77 K.

The as-synthesized crystals of complex 1 were exchanged three times with dry methanol. The activated phases samples were degassed at 353 K for 10 h for the gas sorption measurements. As can be seen from Figure 6, the active phase is highly crystalline and remains almost identical to its as-synthesized phase. The permanent porosity of complex 1 was confirmed by the reversible N₂ sorption measurements at 77 K and 1 atm, which showed a type I adsorption isotherm performance with a saturated adsorption amount of 106 cm³ g⁻¹. The values of the Brunauer-Emmett-Teller (BET) and Langmuir surface areas are 327.4 and 422.7 m² g⁻¹, respectively, calculated from the N₂ sorption isotherm. The pore size distribution is determined with NLDFT and calculated from N₂ adsorption isotherms at 77 K, corresponding to the pore size of 4.3 Å for complex 1, which matches well with the crystal data.

We also tested the low-pressure H₂, CO₂, and CH₄ uptakes of a desolvated sample of complex 1 by using volumetric gas adsorption measurements. Complex 1 can adsorb 89.5 cm³ g⁻¹ of H₂ molecules. Thus, the CO₂ uptake of complex 1 is 26.2 cm³·g⁻¹ (5.158 wt %) at 273 K and 17.6 cm³·g⁻¹ (3.46 wt %) at 298 K under 1 bar, respectively (Figure 7). The adsorption heat (Q_st) of CO₂ of complex 1 is 21.6 kJ·mol⁻¹ calculated from the Clausius-Clapeyron equation, indicating a moderate adsorbate-adsorbant interaction. Furthermore, the CH₄ uptake of complex 1 is 11.6 cm³·g⁻¹ at 273 K and 7.5 cm³·g⁻¹ at 298 K under 1 bar, respectively.

Since CO₂ is a dominant component of greenhouse gas and a main contaminant of natural gas, it is meaningful to investigate the capacity of CO₂ and the selectivity of CO₂/CH₄. The higher CO₂ uptake capacity of complex 1 prompted us to further investigate the selectivity of CO₂ adsorption over CH₄. According to the calculation results over a 10:90 and 50:50 CO₂/CH₄ mixed gas, the CO₂/CH₄ selectivitie at 273 K and 298 K are 3.2 and 3.56, respectively. These values are comparable to ZIF-79 (CO₂/CH₄: 5.4) [30], SIFSIX-2-Cu (CO₂/CH₄: 5.3) [31], and PCN-88 (CO₂/CH₄: 5.3) [32] (Figure 8). The results show that compound 1 may be a candidate for CO₂ capture and separation from natural gas.

4. Conclusions

In conclusion, A novel Pr complex, constructed from a rigid three-connected H₃TMTA and a praseodymium(III) ion, has been constructed under solvothermal conditions. Thus, the thermal stabilities and the fluorescent and gas adsorption and separation properties of complex 1 were investigated systematically. Complex 1 can be used as a candidate for CO₂ capture and separation from natural gas.