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. 2025 Jun 19;64(26):13094–13102. doi: 10.1021/acs.inorgchem.5c01229

Modification of Hydrophobic and Hydrophilic Features in Isostructural 3D Pillar-Layered Coordination Polymers for Adsorption Studies

Jitti Suebphanpho †, Sujitra Tunsrichon †, Xin Zheng ,, Yun Chen , Shin-ichiro Noro ,,*, Jaursup Boonmak †,*
PMCID: PMC12247697  PMID: 40536829

Abstract

A series of new microporous three-dimensional (3D) pillar-layered coordination polymers, including [Zn­(pzt)­(R-ipa)0.5]n (R = H (1-H), NH2 (1-NH 2 ), and OH (1-OH)) and [Co­(pzt)­(ppa)0.5]n (2) (Hpzt = 5-(3-pyridyl)-1,3,4-oxadiazole-2-thiol, H2R-ipa = 5-position-substituted isophthalic acid, and H2ppa = 1,4-phenylenedipropionic acid), were synthesized using a solvothermal method. Single-crystal X-ray diffraction analysis revealed that 1-H, 1-NH 2 , 1-OH, and 2 exhibit an isoreticular framework. Through the bridging-chelating modes of different pillar dicarboxylates (R-ipa2– for 1 and ppa2– for 2), each coordination layer with heteroatom (N-, O-, and S-)-functionalized pzt– ligands is extended into a 3D framework with one-dimensional (1D) channels. The altered pore characteristics, influenced by different functional groups of pillar dicarboxylate linkers, present unique hydrophilic and hydrophobic features of microporous CPs. PXRD and TG analyses confirmed the high stability of the porous structures after activation at 100 °C. The water vapor uptake capacities can be tuned from 33.0 to 68.0 cm3 (STP)/g at 298 K (P e = 2.5 kPa), while the methanol vapor adsorption capacities are configurable from 14.0 to 46.3 cm3 (STP)/g at 298 K (P e = 12 kPa). Different adsorption isotherms were examined, each associated with a specific type of guest molecule and the hydrophilic or hydrophobic nature of the pore structure.

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1. Introduction

Porous coordination polymers (PCPs), specifically metal–organic frameworks (MOFs), are organic–inorganic hybrid materials composed of metal ions linked by organic bridging ligands, notable for their porous three-dimensional (3D) structures. − PCPs have attracted significant attention because of their remarkable structural variety and possible application due to their exhibiting high stability, large pore volume, and high surface area. , The regulation of the physical and chemical characteristics of porous materials remains a challenge for improving various functions, including catalysis, luminescence, gas adsorption, and separation. − Generally, the size of organic linkers and functional groups can control the pore sizes and characteristic pore surfaces of PCPs for specific industrial and environmental applications. − The hydrophobicity and hydrophilicity of PCPs are critical characteristics that have been the topic of study in recent years, aimed at improving the efficacy of PCPs in various fields, particularly for water-related applications. − The predominant method for attaining hydrophilic or hydrophobic PCPs involves the incorporation of hydrophilic and hydrophobic functional groups into linkers. , To date, several PCPs have been reported, but the design of systematic pore surfaces to adjust water vapor adsorption with high water stability still remains a challenge. , Consequently, this work aims to design hydrophilic and hydrophobic pore walls in PCPs that were determined to modify the guest adsorption properties.

5-(3-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpzt) bridging ligand was used for synthesizing PCPs based on various reasons: (i) Hpzt is a multidentate bridging ligand with three functional groups, i.e., oxadiazole, pyridine, and thiolate/thione isomers. − (ii) The asymmetrical structure of Hpzt may generate connectivity between metal ions in many conformations, leading to unique and unusual structural features. , (iii) The heteroatoms-functionalized –O, –N, and –S together with π-systems in Hpzt may facilitate intermolecular interactions that stabilize the framework of PCPs. , Although CPs based on Hpzt ligands have been reported by our group , and others, , this study focuses on the design of the hydrophilic and hydrophobic PCPs of Hpzt with different pillar dicarboxylate ligands for guest adsorption studies. The organic ligands containing functional groups (such as –NH2, −OH, −COOH, −CH3, etc.) are recognized as a good strategy for introducing on the hydrophilic and hydrophobic surfaces of PCPs, which enhances the interaction between the active sites and guest molecules. − In this work, the synthesis of hydrophilic and hydrophobic PCPs has been accomplished using common pillar isophthalate ligands with different functional groups. ,

Herein, the synthesis and crystal structures of four new isoreticular PCPs of [Zn­(pzt)­(R-ipa)0.5]n (R = H (1-H), NH2 (1-NH 2 ), and OH (1-OH)) and [Co­(pzt)­(ppa)0.5]n (2) were reported (Scheme ). Each two-dimensional (2D) layer appears by the bridging of the pzt– ligand, which builds to a 3D porous framework through different pillar dicarboxylates. All PCPs can adsorb water and methanol vapors higher than N2 and CO2. Moreover, the adsorption profiles of water and methanol vapors show different adsorption isotherms depending on the hydrophilic and hydrophobic characteristics of the channels.

1. Synthetic Details of Porous Coordination Polymers.

1

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2. Experimental Section

2.1. Materials and Methods

All chemicals and solvents were reagent-grade and used in the synthesis process without further purification after commercial purchase. The attenuated total reflectance-Fourier transform infrared (ATR-FT-IR) spectra were obtained using a Bruker Tensor 27 FT-IR spectrophotometer over the range of 4000 to 600 cm–1. The elemental analyses were conducted on a Thermo FLASH 2000 CHNS/O analyzer. Thermogravimetric analysis (TGA) was measured using a TGA Q50 (TA Instruments) at a heating rate of 10 °C·min–1 in the temperature range of 30–700 °C under a N2 atmosphere with a flow rate of 60 mL·min–1. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker D2 PHASER second-generation diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature with the 2θ range from 3 to 50°. N2 (77 K) and CO2 (273 K) adsorption–desorption isotherms were performed using BELSORP-mini X and BELSORP-mini II (MicrotracBEL), respectively. Water and methanol vapor adsorption measurements were measured using BELSORP-aqua 3 (MicrotracBEL). Before gas adsorption measurements, the samples were pretreated at 100 °C in vacuum for 24 h using BELPREP-vac (MicrotracBEL).

2.2. Synthesis of Porous Coordination Polymers

2.2.1. Synthesis of {[Zn­(pzt)­(ipa)0.5]·H2O}n (1-H·H2O)

The single crystals of 1-H were prepared by mixing Zn­(NO3)2·6H2O (44.5 mg, 0.15 mmol), 5-(3-pyridyl)-1,3,4-oxadiazole-2-thiol (Hpzt) (17.9 mg, 0.1 mmol), and isophthalic acid (H2ipa) (33.2 mg, 0.2 mmol) in 4 mL of H2O/DMF/MeCN (1/1/2 mL). The solution combination was contained in a 20 mL glass vial, thereafter sealed, heated at 90 °C for 24 h, and then cooled to room temperature. The colorless agglomerated block-shaped crystals of 1-H were obtained. Yield: 46.2% (based on Hpzt). Anal. Calcd for C11H8ZnN3O4S (1-H·H 2 O): C, 38.45; H, 2.35; N, 12.23. Found: C, 38.24; H, 2.13; N, 12.43. FT-IR (ATR, cm–1): 1664 (m), 1603 (m), 1529 (s), 1483 (s), 1429 (s), 1400 (s), 1172 (m), 1091 (m), 1032 (m), 965 (w), 805 (w), 750 (m), 692 (m).

2.2.2. Synthesis of {[Zn2(pzt)2(ipa-NH2)]·1.2H2O}n (1-NH2·1.2H2O)

The single crystals of 1-NH 2 were synthesized in the same way as 1-H, except 5-amino isophthalic acid (H2NH2-ipa) was used instead of H2ipa. The yellow agglomerated block-shaped crystals of 1-NH 2 were obtained. Yield: 28.3% (based on Hpzt). Anal. Calcd for C22H16.4Zn2N7O7.2S2 (1-NH 2 ·1.2H 2 O): C, 38.36; H, 2.40; N, 14.23. Found: C, 38.36; H, 2.11; N, 13.97. FT-IR (ATR, cm–1): 3416 (w), 3352 (m), 1659 (m), 1609 (m), 1528 (s), 1485 (m), 1425 (s), 1383 (s), 1173 (m), 1093 (m), 1033 (m), 981 (w), 893 (w), 736 (m), 691 (m).

2.2.3. Synthesis of {[Zn2(pzt)2(ipa–OH)]·1.8H2O}n (1-OH·1.8H2O)

The single crystals of 1-OH were synthesized in the same way as 1-H, except 5-hydroxy isophthalic acid (H2OH-ipa) was used instead of H2ipa. The colorless, agglomerated block-shaped crystals of 1-OH were obtained. Yield: 41.3% (based on Hpzt). Anal. Calcd for C22H15.6Zn2N6O8.8S2 (1-OH·1.8H 2 O): C, 37.77; H, 2.25; N, 12.01. Found: C, 37.58; H, 2.18; N, 12.09. FT-IR (ATR, cm–1): 3416 (w), 1661 (m), 1610 (m), 1522 (s), 1486 (m), 1427 (s), 1381 (s), 1171 (m), 1094 (m), 1032 (m), 967 (w), 896 (w), 736 (m), 692 (m).

2.2.4. Synthesis of {[Co­(pzt)­(ppa)0.5]·2H2O}n (2·2H2O)

The bulk crystals of 2 were prepared by mixing Co­(NO3)2·6H2O (43.6 mg, 0.15 mmol), Hpzt (17.9 mg, 0.1 mmol), and 1,4-phenylenedipropionic acid (H2ppa) (22.2 mg, 0.1 mmol) in 4 mL of H2O/DMF/MeOH (1/1/2 mL). The solution mixture was transferred and placed in a 20 mL glass vial, which was then sealed and heated at 90 °C for 24 h to obtain dark purple agglomerated block-shaped crystals of 2. Yield: 26.5% (based on Hpzt). Anal. Calcd for C13H14CoN3O5S (2·2H 2 O): C, 40.74; H, 3.68; N, 10.96. Found: C, 40.24; H, 3.96; N, 10.67. FT-IR (ATR, cm–1): 1665 (m), 1578 (m), 1531 (s), 1450 (m), 1419 (s), 1171 (m), 1096 (m), 1032 (m), 967 (w), 848 (w), 811 (m), 692 (m).

2.3. X-ray Data Collection and Structure Determination

Single X-ray diffraction data of all compounds were collected on a Bruker D8 Quest PHOTON100 CMOS detector with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 298(2) K. All data were recorded by the APEX III program. The raw data frame was integrated using the SAINT program. The empirical adsorption, Lorentz, and polarization effects were corrected by the SADABS program. The structure was solved with the SHELXT structure solution program using combined Patterson and dual-space recycling methods. All compounds were refined by least-squares using SHELXL and OLEX2. Hydrogen atoms were assigned to geometrically calculated positions and isotropically refined with a riding model. The solvent molecules in the channel of PCPs were removed from the electron density map using the solvent marking option in OLEX2 software. The lattice solvent molecules of all compounds are further determined by thermogravimetric (TGA) and elemental analyses. The details of the crystallographic data and refinement parameters of all compounds are summarized in Table . The selected bond lengths and angles are shown in Tables S1 and S2, respectively. The hydrogen bonds and centroid-centroid distances are calculated by PLATON software. , The details of the hydrogen bonds for 1-NH 2 and 1-OH are summarized in Table S3.

1. Crystal Data and Structure Refinement for 1-H, 1-NH 2 , 1-OH, and 2 .

crystal data 1-H 1-NH 2 1-OH 2
formula C11H6N3O3SZn C22H13N7O6S2Zn2 C22H12N6O7S2Zn2 C13H10N3O3SCo
molecular weight 325.65 666.29 667.28 347.23
temperature (K) 298(2) 298(2) 298(2) 298(2)
crystal system orthorhombic orthorhombic orthorhombic monoclinic
space group Pnma Pnma Pnma C2̅ /c
a (Å) 7.1479(14) 7.2083(7) 7.1604(7) 39.983(4)
b (Å) 33.120(7) 33.049(4) 33.197(4) 11.0806(10)
c (Å) 10.856(2) 10.8315(11) 10.8506(13) 6.7624(6)
α (deg) 90 90 90 90
β (deg) 90 90 90 95.817(3)
γ (deg) 90 90 90 90
V (Å3) 2570.1(9) 2580.3(5) 2579.3(5) 2980.5(5)
Z 8 4 4 8
ρcalc (g cm–3) 1.683 1.715 1.690 1.548
μ (Mo Kα) (mm–1) 2.079 2.074 2.066 1.303
F(000) 1308.2 1340.2 1320.0 1094.5
data collected 33342 40339 22360 18268
unique data, R int 3249, 0.0694 3272, 0.0393 3246, 0.0379 2930, 0.2105
R1/wR 2[I > 2(Iσ)] 0.0360/0.1331 0.0344/0.1515 0.0400/0.1604 0.0648/0.1109
R1/wR 2 [all data] 0.0408/0.1464 0.0485/0.1787 0.050/0.1815 0.1481/0.1374
goodness of fit, GOF 1.053 1.033 1.071 0.987
maximum/minimum electron density (e Å–3) 0.87/–0.68 1.41/–1.53 0.93/–1.10 1.33/–1.30
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a

R = ∑||F0|−|Fc||/∑|F0|.

b

Rw = {∑[w­(|F0|−|Fc|)]­2/∑[w|F0|2]}­1/2.

3. Results and Discussion

3.1. Crystal Structures of 1-H, 1-NH 2 , and 1-OH

Single-crystal X-ray diffraction analysis revealed that 1-H, 1-NH 2 , and 1-OH are isostructural metal–organic frameworks. All compounds are crystallized in an orthorhombic system with a Pnma space group. The asymmetric unit consists of one Zn­(II) ion, one deprotonated pzt–, and half of ipa2–, NH2-ipa2–, or OH-ipa2– for 1-H, 1-NH 2 , and 1-OH, respectively (Figures a, S1a, and S2a). The Zn­(II) centers display distorted octahedral geometry with a ZnO3N2S coordination sphere surrounded by two nitrogen atoms of pyridyl and oxadiazole rings, one sulfur atom of the thiolate group from three individual deprotonated pzt–, and three oxygen atoms from two different deprotonated R-ipa2– (Figures b, S1b, and S2b). The bond distances of Zn–Noxa, Zn–Npy, and Zn–Sthiolate from pzt– are 2.037(2)-2.040(3), 2.281(3)-2.299(3), and 2.408(9)-2.4105(9) Å, while those of Zn–O1 and Zn–O2 from R-ipa2– are 1.989(2)-1.994(3) and 2.200(3)-2.248(4) Å, respectively. The pzt– and R-ipa2– display tridentate-bridging (μ3-η1(Npy):η1(Noxa):η1(Sthiolate)) and chelating-bridging (μ4-η1(O1): η2(O2): η1(O1): η2(O2)) modes, respectively (Figure S3). The adjacent Zn­(II) centers of isostructural 1 are initially bridged by three different μ3-pzt ligands, resulting in dinuclear 16-membered rings (A) with a Zn···Zn distance of 9.017(7) Å and tetranuclear 26-membered rings (B) with the Zn···Zn distances of 4.103(4) and 11.195(5) Å, respectively (Figure S4). Each dinuclear and tetranuclear unit is further connected by μ3-pzt ligands, generating a 2D layer along with the ac plane, as shown in Figures c, S1c, and S2c for 1-H, 1-NH 2 , and 1-OH, respectively. Moreover, the Zn­[pzt]n 2D layer is stabilized by intralayer π-π interactions between oxadiazole and pyridine rings with a distance of 3.781(2), 3.776(2), and 3.771(2) Å for 1-H, 1-NH 2 , and 1-OH, respectively (Figures S5a, S6a, and S7a). Furthermore, each 2D layer of Zn­[pzt]n is linked together with the pillar dicarboxylate R-ipa2– ligand, resulting in the formation of a 3D framework along the crystallographic b-axis. The shortest Zn···Zn distances through μ4-ipa2–, μ4-NH2-ipa2–, and μ4–OH-ipa2– are 8.686(7), 8.675(1), and 8.741(1) Å for 1-H, 1-NH 2 , and 1-OH, respectively (Figures d, S1d, and S2d).

1.

1

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(a) Asymmetric unit with the selected atom labeled 1-H, showing a 50% probability level of ellipsoid. (b) Coordination environment around the Zn­(II) center of 1-H. All hydrogen atoms have been omitted for clarity. (c) 2D layer of 1-H in the ac plane. (d) 3D pillar-layered network along the crystallographic b-axis.

Moreover, the 3D frameworks of 1-H, 1-NH 2 , and 1-OH are staked by a C–H···π interaction among the centroid and hydrogen atom of the adjacent R-ipa2– ligand with C5–H5···Cg distances of 3.040(6), 2.895(6), and 2.979(6) Å, respectively (Figures S5b, S6b, and S7b). Finally, 1-NH 2 shows the hydrogen bonds of N4–H4a···O1 = 2.558(5) Å and N4–H4b···O1 = 2.569(5) Å between amino groups and coordinated carboxylate oxygen atoms of ipa-NH2 2–, while 1-OH shows the hydrogen bond of O3–H3···O1 = 2.270(1) Å between hydroxyl groups and coordinated carboxylate oxygen atoms of ipa–OH2–, further stabilizing the 3D framework of 1-NH 2 and 1-OH (Figures S6b and S7b). Interestingly, 1-H exhibits a square pore shape, while 1-NH 2 and 1-OH show rectangle pore shapes in the 1D channel along the a-axis (Figure S9). The pore sizes are calculated to be 3.8 × 5.4, 3.3 × 5.4, and 3.4 × 5.4 Å2 for 1-H, 1-NH 2 , and 1-OH, respectively, as shown in Figure . A PLATON , calculation shows that the effective volumes in 1-H, 1-NH 2 , and 1-OH are about 487.9, 442.6, and 465.0 Å3 per unit cell, comprising 19.0, 17.2, and 18.0% of the crystal volume being occupied by guest molecules. Either isostructural 1-NH 2 or 1-OH has smaller pore sizes than 1-H because the amino and hydroxy functional groups minimize the volume of space in the pore structure. ,

2.

2

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Capped and sticks and space-filling models of the 3D framework with the 1D channels along the a-axis for (a) 1-H, (b) 1-NH 2 , and (c) 1-OH, respectively.

3.2. Crystal Structure of 2

Compound 2 exhibits an isoreticular framework similar to that of 1, but the crystal system, space group, and pore size are all different. Compound 2 crystallizes in the monoclinic system with the C2̅ /c space group. The asymmetric unit contains one Co­(II), one pzt–, and half of a ppa2– ligand (Figure a). The Co­(II) center has a distorted octahedral geometry (CoO3N2S) with the same coordination environment as that of 1, but the coordinated oxygen atoms are from ppa2– instead of R-ipa2–, as shown in Figure b. The bond distances of Co–Noxa, Co–Npy, and Co–Sthiolate are 2.082(4), 2.199(4), and 2.4808(2), while the Co–O1 and Co–O2 distances are 2.132(4) and 2.247(4), respectively. The coordination mode of pzt– in 2 is μ3-pzt, which is the same as 1, generating the 2D layer along the bc plane with the Co···Co distances of 9.063(1) and 9.386(1) Å as shown in Figures c and S8. Moreover, the 2D layer of 2 is stabilized by intralayer π-π interactions between oxadiazole and pyridine rings with a distance of 3.867(3) Å (Figure S8). Each of the 2D layers of 2 is further expanded by the pillar ppa2– ligand with the same coordination mode as R-ipa2– in 1 (Figure S3), giving rise to the 3D framework (Figure d). Significantly, the longer ppa2– ligands in 2 create a larger space between metal nodes, resulting in a longer Co···Co distance of 15.175(2) Å. The 3D framework of 2 contains a rectangular pore shape in the 1D channels along the crystallographic c-axis with a dimension of 3.4 × 8.6 Å2 (Figures e and S9). The accessible void volume was calculated to be 18.9% (564.1 from 2980.6 Å3). However, compound 2 has an isoreticular framework similar to that of 1 but with a longer ppa2– as a pillar linker. As a result, the M···M distance and pore size are obviously longer than those of 1.

3.

3

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(a) Asymmetric unit with the selected atom labeled 2, showing a 50% probability level of ellipsoid. (b) Coordination environment around the Zn­(II) center of 2. All hydrogen atoms are omitted for clarity. (c) View of the 2D layer of 2 formed by pzt– ligands in the bc plane. (d) 3D framework of 2 formed by pillar ppa2– linkers along the a-axis. (e) Capped and sticks. (f) Space-filling models of the 3D framework with the 1D channels along the c-axis for 2.

3.3. Characterizations

The FT-IR spectra of 1 and 2 are illustrated in Figure S10. FT-IR spectra of isostructural 1-H, 1-NH 2 , and 1-OH are closely comparable, but 1-NH 2 and 1-OH display characteristic peaks at 3352 and 3416 cm–1, which are assigned to νas(N–H) of the amino group and νas(O–H) of the hydroxyl group, respectively. The FT-IR spectra of all compounds display vibrational peaks at 1659–1665, 1531–1610, 1419–1429, and 1096–1173 cm–1 that are attributed to ν­(C = N), νas(COO–), νs(COO–), and νs(C–O), respectively. TGA profiles of isostructural 1 exhibit a step of weight loss around 70–200 °C, which corresponds to the release of solvent lattice molecules, including H2O (found 5.35%, calc. 5.53%), 1.2H2O (found 3.30%, calc. 3.24%), and 1.8H2O (found 4.94%, calc. 4.86%) for 1-H, 1-NH 2 , and 1-OH, respectively (Figure S11a). Compound 2 exhibits a step of weight loss around 65–150 °C, which corresponds to the loss of two water molecules in the lattice (found 10.14%, calc. 10.37%) (Figure S11b). The anhydrous products of isoreticulars 1 and 2 are stable up to 300 and 320 °C, respectively. After that, their structures start to decompose. Powder X-ray diffraction (PXRD) analyses were conducted to verify the phase purity of the products. The PXRD patterns of as-synthesized bulk products are identical to the simulated patterns derived from single-crystal X-ray diffraction. The PXRD results indicate a high crystalline phase purity in isoreticular 1 and 2 (Figure S12).

3.4. Guest Adsorption Properties

Before the guest adsorption experiments, all PCPs were pretreated at 100 °C for 24 h under a vacuum to produce the guest-free frameworks. The PXRD patterns of activated samples 1-H, 1-NH 2 , 1-OH, and 2 are mostly the same as those of the as-synthesized ones (Figure S13a), which indicates that the structures of all PCPs are stable after activation. Furthermore, TG analysis can confirm the removal of guest molecules from the frameworks. The TGA curves of activated samples show a disappearance of weight loss around 30–300 °C (2 started the weight loss from 200 °C) (Figure S13b), implying that the guest molecules are removed after activation. However, the porosity of activated PCPs could not be measured by using N2 adsorption–desorption isotherm analysis at 77 K. All PCPs exhibit poor N2 uptake (horizontal axis unit: P/P 0, vertical axis unit: Vads (cm3(STP)·g–1)) (Figure S14), suggesting only surface adsorption. The result may be attributed to the strong interaction between N2 molecules and the pore windows of the PCPs at cryogenic temperature, which inhibits the access of other gas molecules into the pore. In addition, the CO2 adsorption measurements at 273 K of 1-H and 2 exhibited small amounts of adsorption, as shown in Figure , while 1-NH 2 and 1-OH adsorbed very little CO2 gas, while the CO2 adsorption/desorption isotherms of isostructural 1 at 195 K show the same results as at 273 K (Figure S15). This is because smaller pore sizes for 1-NH 2 and 1-OH show a pore-blocking effect for CO2, which has a smaller kinetic diameter (3.3 Å) than N2 (3.64 Å) at 273 K. The activated 1-H and 2 start to take up CO2 gradually from the low-pressure region, with maximum adsorption capacities of 10.0 and 21.9 cm3 (STP)/g at P e = 100 kPa. The type I isotherm shape observed for 1-H and 2 is characteristic of rigid microporous materials. , The adsorption capacity of 2 is higher than that of 1-H, which is attributed to the larger size of the channels. The CO2 hysteresis was observed in the adsorption/desorption isotherms for 1-H and 2 because of the long time required to reach the adsorption equilibrium.

4.

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CO2 adsorption isotherm at 273 K of 1-H, 1-NH 2 , 1-OH, and 2 (full and unfilled symbols represent adsorption and desorption, respectively).

To study another guest adsorption property, water vapor adsorption was investigated at 298 K. Adsorption data have been shown as a function of absolute pressure (Figure ). The water adsorption profiles of 1-NH 2 and 1-OH showed type I-like isotherms, according to the IUPAC classification. , The type I profiles show high water uptake at low pressure and a slight increase at high pressure, and the adsorption capacities are 33.0 and 49.0 cm3 (STP)/g (P e = 2.5 kPa), corresponding to 1.29 and 1.92 H2O molecules per formula unit for 1-NH 2 and 1-OH, respectively. In the case of 1-H and 2, the water adsorption shows type V-like isotherms , with low water uptakes at low pressure and a sudden water uptake at high pressure. The adsorption amounts are 58.3 and 68.0 cm3 (STP)/g (P e = 2.5 kPa), approximately equal to 2.29 and 2.67 H2O molecules per formula unit for 1-H and 2, respectively. The results observed for isotherms of different shapes are consistent with the characteristics of the substituents introduced into the ligands. In the case of water adsorption/desorption, the type I isotherm can be described as a hydrophilic surface, while the type V isotherm can be described as a hydrophobic or weakly hydrophilic surface. For example, very hydrophilic zeolites show that clear type I isotherms for water, and very hydrophobic porous carbon materials exhibit type V isotherms for water. , 1-NH 2 and 1-OH have hydrophilic substituents of NH2 and OH on the ipa2– ligands, and the channels are decorated by these hydrophilic substituents, causing the formation of a hydrophilic surface (Figure S16). On the other hand, there are no hydrophilic substituents in 1-H and 2, resulting in the formation of a hydrophobic or weakly hydrophilic surface (Figure S16). The order of adsorption amounts, 2 > 1-H > 1-OH > 1-NH 2 , depended on the pore volume. Nevertheless, the desorption isotherm exhibits poor consistency with the adsorption isotherm, displaying a slight hysteresis loop characterized by incomplete desorption. This behavior may be ascribed to the hydrogen bond interactions between water molecules and functional groups in the hydrophilic channels , (not only hydrophilic 1-NH 2 and 1-OH but also hydrophobic 1-H and 2 showed slight hysteresis). The strength of the hydrogen bonding interaction between 1-OH and 1-NH 2 depends on the Lewis basicity. , The oxygen lone pair of 1-OH exhibits more electron density, facilitating stronger hydrogen bonding relative to the amino group of 1-NH 2 , which enhances the strength of the hydrogen bond between adsorbed water molecules. The PXRD patterns of PCPs after water adsorption match well with the simulated patterns (Figure S17). However, only 1-NH 2 remains stable in saturated water vapor for 48 h, while others are nonstable with structures changed, as confirmed by different PXRD patterns from the original samples (Figure S18).

5.

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Water vapor adsorption isotherms of 1-H, 1-NH 2 , 1-OH, and 2 at 298 K with absolute pressure (full and unfilled symbols represent adsorption and desorption, respectively).

The adsorption properties of another vapor, methanol, were also investigated. At 298 K, all PCPs show a type I isotherm, as shown in Figure . The adsorption capacities are 32.5, 14.0, 28.0, and 46.3 cm3 (STP)/g for 1-H, 1-NH 2 , 1-OH, and 2, respectively (P e = 12 kPa). 2 shows the highest amount of adsorption due to the larger size of the channels. Moreover, the observed hysteretic and incomplete desorption curves also implied obvious noncovalent interactions between the PCPs and methanol molecules (not only hydrophilic 1-NH 2 and 1-OH but also hydrophobic 1-H and 2 showed slight hysteresis). Interestingly, 1-H and 2 exhibit different adsorption isotherms between water (type V) and methanol (type I). This can be explained by the fact that methanol has a higher hydrophobicity than water, which allows for relatively easy adsorption, even in weakly hydrophilic or hydrophobic channels of 1-H and 2. The PXRD patterns of all PCPs after the methanol adsorption experiment are consistent with the original samples before adsorption, implying that the structures are highly stable to methanol vapor (Figure S19).

6.

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Methanol vapor adsorption at 298 K of 1-H, 1-NH 2 , 1-OH, and 2 at 298 K (full and unfilled symbols represent adsorption and desorption, respectively).

4. Conclusions

Four isoreticular PCPs have been successfully synthesized by modifying the pore surface with various functional groups of dicarboxylate pillar linkers. All PCPs constructed a 2D layer using the bridging mode of pzt– ligand, which afterward expanded into a 3D framework employing pillar dicarboxylate linkers. The PCPs have 1D channels with varying pore dimensions based on the pillar linkers. The activated forms of isoreticular 1 and 2 exhibit more effective adsorption capacities for water and methanol vapor compared to those of N2 and CO2. The different kinetic sizes and hydrophilic and hydrophobic natures of guest molecules are responsible for the explanation of this finding. The characteristics of the substituent in the R-ipa2– linkers are found to be sufficient to make their adsorption isotherms entirely reasonable. Water and methanol adsorption mechanisms involve hydrogen bonds and noncovalent interactions between guest molecules and frameworks.

Supplementary Material

ic5c01229_si_001.pdf (4.1MB, pdf)

Acknowledgments

The funding for this study was granted by the National Research Council of Thailand (NRCT) and Khon Kaen University (Grant No. N42A680255) and the Material Chemistry Research Center, Khon Kaen University. J.S. expresses gratitude to the Development and Promotion of Science and Technology Talents Project (DPST) for a Ph.D. scholarship.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c01229.

  • X-ray crystallographic data of 1-H, 1-NH 2 , 1-OH, and 2 in CIF format; crystallographic data were deposited with the CCDC nos. 2428634–2428637. Supporting Information available: Crystal structure and packing structures of all PCPs; FT-IR spectra; thermograms; PXRD patterns; N2 adsorption/desorption isotherms, and other data (Tables S1–S3, Figures S1–S16) (PDF)

The authors declare no competing financial interest.

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