Paddlewheel SBU based Zn MOFs: Syntheses, Structural Diversity, and CO2 Adsorption Properties

Four Zn metal–organic frameworks (MOFs), {[Zn2(2,6-ndc)2(2-Pn)]·DMF}n (1), {[Zn2(cca)2(2-Pn)]·DMF}n (2), {[Zn2(thdc)2(2-Pn)]·3DMF}n (3), and {[Zn2(1,4-ndc)2(2-Pn)]·1.5DMF}n (4), were synthesized from zinc nitrate and N,N′-bis(pyridin-2-yl)benzene-1,4-diamine (2-Pn) with naphthalene-2,6-dicarboxylic acid (2,6-H2ndc), 4-carboxycinnamic acid (H2cca), 2,5-thiophenedicarboxylic acid (H2thdc), and naphthalene-1,4-dicarboxylic acid (1,4-H2ndc), respectively. MOFs 1–4 were all constructed from similar dinuclear paddlewheel {Zn2(COO)4} clusters and resulted in the formation of three kinds of uninodal 6-connected non-interpenetrated frameworks. MOFs 1 and 2 suit a topologic 48·67-net with 17.6% and 16.8% extra-framework voids, respectively, 3 adopts a pillared-layer open framework of 48·66·8-topology with sufficient free voids of 39.9%, and 4 features a pcu-type pillared-layer framework of 412·63-topology with sufficient free voids of 30.9%. CO2 sorption studies exhibited typical reversible type I isotherms with CO2 uptakes of 55.1, 84.6, and 64.3 cm3 g−1 at 195 K and P/P0 =1 for the activated materials 1′, 2′, and 4′, respectively. The coverage-dependent isosteric heat of CO2 adsorption (Qst) gave commonly decreased Qst traces with increasing CO2 uptake for all the three materials and showed an adsorption enthalpy of 32.5 kJ mol−1 for 1′, 38.3 kJ mol−1 for 2′, and 23.5 kJ mol−1 for 4′ at zero coverage.


Introduction
Metal-organic frameworks (MOFs) are infinite arrays of metal ions connected by organic modules through coordination bonds [1]. Such materials possess fascinating structure variations and some can sustain considerable porosity and internal surface area, which is associated with intrinsic photo and magnetic properties. These properties have led to the evaluation of MOFs for various practical applications in gas adsorption/separation [2][3][4][5], ion exchange [6,7], sensing [8,9], catalysis [10], magnetism [11], drug delivery [12], etc. Thus, research on these kinds of superior materials have explosively advanced over the past decades and now have entered a new exciting stage.

Materials and Instruments
The ligand 2-Pn was synthesized as reported previously [32,33]. Other chemicals were purchased commercially and used as received without further purification. Infrared spectra were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrophotometer (PerkinElmer, Taipei, Taiwan) in the region 4000−400 cm −1 ; abbreviations used for the IR bands are w = weak, m = medium, s = strong, and vs = very strong. Powder X-ray diffraction measurements were performed at room temperature using a Siemens D-5000 diffractometer (Siemens, Berlin, Germany) at 40 kV and 30 mA for Cu K radiation (λ = 1.5406 Å) with a step size of 0.02° in θ and a scan speed of 1 s per step size. Thermogravimetric analyses were performed under nitrogen with a Perkin-Elmer TGA-7 TG analyzer (PerkinElmer, Inc., Billerica, MA, USA). Elemental analyses were performed using a Perkin-Elmer 2400 elemental analyzer (PerkinElmer, Inc., Billerica, MA, USA). Brunauer−Emmett−Teller analyses and low-pressure carbon dioxide (CO2) adsorption measurements were investigated with a Micrometrics ASAP 2020 system using research grade carbon dioxide (99.9995% purity) as the adsorbate at 195 K, 273 K, and 298 K. Prior to analysis, the materials were immersed in EtOH for three days, then the EtOH-exchanged materials were loaded into a sample tube of known weight and activated at 120 °C under a high vacuum for about 24 h to completely remove guest solvent molecules. After activation, the sample and tube were re-weighed to determine the precise mass of the evacuated sample.

Materials and Instruments
The ligand 2-Pn was synthesized as reported previously [32,33]. Other chemicals were purchased commercially and used as received without further purification. Infrared spectra were recorded on a Perkin-Elmer Paragon 1000 FT-IR spectrophotometer (PerkinElmer, Taipei, Taiwan) in the region 4000-400 cm −1 ; abbreviations used for the IR bands are w = weak, m = medium, s = strong, and vs = very strong. Powder X-ray diffraction measurements were performed at room temperature using a Siemens D-5000 diffractometer (Siemens, Berlin, Germany) at 40 kV and 30 mA for Cu Kα radiation (λ = 1.5406 Å) with a step size of 0.02 • in θ and a scan speed of 1 s per step size. Thermogravimetric analyses were performed under nitrogen with a Perkin-Elmer TGA-7 TG analyzer (PerkinElmer, Inc., Billerica, MA, USA). Elemental analyses were performed using a Perkin-Elmer 2400 elemental analyzer (PerkinElmer, Inc., Billerica, MA, USA). Brunauer-Emmett-Teller analyses and low-pressure carbon dioxide (CO 2 ) adsorption measurements were investigated with a Micrometrics ASAP 2020 system using research grade carbon dioxide (99.9995% purity) as the adsorbate at 195 K, 273 K, and 298 K. Prior to analysis, the materials were immersed in EtOH for three days, then the EtOH-exchanged materials were loaded into a sample tube of known weight and activated at 120 • C under a high vacuum for about 24 h to completely remove guest solvent molecules. After activation, the sample and tube were re-weighed to determine the precise mass of the evacuated sample. closed glass tube and heated at 50 • C for 48 h. Deep purple block crystals of compound 1 were formed in 48% yield (21.5 mg, based on Zn 2+ ). Solid products were isolated on a filter, and crystals were then washed with DMF and EtOH and dried in air. Anal. Calc. for C 43

X-Ray Data Collection and Structure Refinement
Suitable single crystals of 1-4 were mounted on the tip of a glass fiber and placed onto the goniometer head for indexing and intensity data collection using a Bruker Smart APEX 2 CCD diffractometer equipped with graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). Collection of intensity data was conducted at 150(2) K. Empirical absorptions were applied using the multiscan method [34]. The structures were solved by direct methods with the SHELXS-97 [35] program and refined against F 2 by the full-matrix least-squares technique using the SHELXL-2014/7 [36] and WINGX [37] program packages. Nonhydrogen atoms were found from the difference Fourier maps, and most of them were treated anisotropically except where noted. Whenever possible, the amine hydrogen atoms were located on a difference Fourier map and fixed at the calculated positions, while the carbon-bound hydrogen atoms were geometrically placed and refined as a riding model. All of the hydrogen atoms were refined isotropically. In both 1 and 2, the 2-Pn ligand was treated disorderedly over two positions with equal occupancy sites. For 2, the cca 2− ligand was symmetrically disordered about an inversion center. For 4, one lattice DMF molecule was disordered over two positions with occupancy sites of 0.55 and 0.45, respectively, whereas another was partially occupied with an occupancy site of 0.50. Some of the disordered atoms in 1 and 2 and lattice DMF molecules in 4 were refined isotropically. For 1-3, there were lattice solvent molecules in the voids that were highly disordered and very difficult to model properly, in addition to these well-modeled DMF molecules. PLATON/SQUEEZE routine [38] indicated the presence of 217, 184, and 149 electrons per unit cell for 1-3, respectively, which, associated with the elemental and thermogravimetric (TG) analyses, suggested an approximate quantity of lattice solvent of one DMF molecule per formula in both 1 and 2, and three DMF molecules per formula in 3. CCDC 1879110 (1), 1879111 (2), 1879112 (3), and 1879113 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Experimental details for X-ray data collection and the refinements are summarized in Table 1.
Compound 2 crystallizes in the monoclinic space group C2/c, and the coordination environment of the Zn(II) center is shown in Figure S1. It is noted that the cca 2− ligand in 2 is symmetrically disordered about an inversion center, which results in similarity of spatial occupation between the dislocated aromatic rings of cca 2− and the well-located naphthalene ring of 2,6-ndc 2− (Scheme 2); this would lead to the formation of similar frameworks despite the use of different ligands. Thus, compound 2 has a molecular structure that is identical to that of 1 with 16.8% of the free extra-framework voids occupied by lattice DMF molecules.
Polymers 2018, 10, x FOR PEER REVIEW 5 of 13 coordination mode in a syn,syn-bridging bidentate manner. The 2-Pn ligand behaves as a μ2-bridge to link two Zn(II) centers through its two 2-pyridyl functions. The extended framework of 1 consists of dinuclear paddlewheel {Zn2(COO)4} clusters as SBUs, where the Zn•••Zn separation is 3.0008(11) Å, and anionic 2,6-ndc 2− and neutral 2-Pn ligands as linear linkers. The anionic 2,6-ndc 2− ligands bridge {Zn2(COO)4} SBUs to form a three-dimensional (3D) Zn−2,6-ndc network suiting a 4-connected diamondoid (dia) topology with Schläfli symbol of 6 6 , in which the {Zn2(COO)4} paddlewheel SBU adopts a topologically distorted tetrahedral node instead of a common square planar node (Figure 1b). The neutral 2-Pn ligands bridge {Zn2(COO)4} SBUs to form a one-dimensional (1D) chain structure (Figure 1c), which intersects the Zn−2,6-ndc diamondoid network to give a whole 3D uninodal 6-connected non-interpenetrated framework with a Schläfli symbol of 4 8 •6 7 topology (Figure 1d). When viewed down the crystallographic [101] direction, 1D channels were observed (Figure 1e), which suggests approximately 17.6% solvent-accessible volumes [39] to accommodate the lattice DMF molecules. Compound 2 crystallizes in the monoclinic space group C2/c, and the coordination environment of the Zn(II) center is shown in Figure S1. It is noted that the cca 2-ligand in 2 is symmetrically disordered about an inversion center, which results in similarity of spatial occupation between the dislocated aromatic rings of cca 2− and the well-located naphthalene ring of 2,6-ndc 2− (Scheme 2); this would lead to the formation of similar frameworks despite the use of different ligands. Thus, compound 2 has a molecular structure that is identical to that of 1 with 16.8% of the free extra-framework voids occupied by lattice DMF molecules.

Crystal Structure of {[Zn2(thdc)2(2-Pn)]•3DMF}n (3)
Single-crystal X-ray diffraction analysis reveals that compound 3 crystallized in the monoclinic space group C2/c. The asymmetric unit consists of one Zn(II) center, one thdc 2− ligand, one half 2-Pn ligand, and one lattice DMF molecule. The Zn(II) center has a square pyramidal geometry, composed of four oxygen atoms of four distinct thdc 2− ligands in the basal plane and one nitrogen atom of a 2-Pn ligand at the axial position (Figure 2a). In turn, the thdc 2− ligand adopts a μ4-bridge mode to link four Zn(II) centers where each of the two carboxylate groups adopts a syn,syn-bridging bidentate to show a μ2-η 1

Crystal Structure of {[Zn 2 (thdc) 2 (2-Pn)]·3DMF} n (3)
Single-crystal X-ray diffraction analysis reveals that compound 3 crystallized in the monoclinic space group C2/c. The asymmetric unit consists of one Zn(II) center, one thdc 2− ligand, one half 2-Pn ligand, and one lattice DMF molecule. The Zn(II) center has a square pyramidal geometry, composed of four oxygen atoms of four distinct thdc 2− ligands in the basal plane and one nitrogen atom of a 2-Pn ligand at the axial position (Figure 2a). In turn, the thdc 2− ligand adopts a µ 4 -bridge mode to link four Zn(II) centers where each of the two carboxylate groups adopts a syn,syn-bridging bidentate to show a µ 2 -η 1 :η 1 coordination mode. The 2-Pn ligand that serves as a µ 2 -bridge links two Zn(II) centers through its two 2-pyridyl functions. The extended framework of 3 is constructed by the dinuclear paddlewheel {Zn 2 (COO) 4 } SBUs, where the Zn···Zn separation is 3.0610(9) Å, and anionic thdc 2− and neutral 2-Pn linkers. Each {Zn 2 (COO) 4 } SBU is linked by four anionic thdc 2− ligands in a square planar manner to form a 2D gridlike Zn-thdc layer, which suits a topologic 4 4 -sql net (Figure 2b). The gridlike layers are connected by 2-Pn ligands as pillars in two different orientations, resulting in the formation of a new type of 3D pillared-layer framework. The whole 6-connected non-interpenetrated framework has a Schläfli symbol of 4 8 ·6 6 ·8 topology (Figure 2c). There are 39.9% solvent accessible volumes [39]. When viewed down the crystallographic [101] direction, the packing diagram shows 1D channels where lattice DMF molecules reside (Figure 2d).

Crystal Structure of {[Zn2(thdc)2(2-Pn)]•3DMF}n (3)
Single-crystal X-ray diffraction analysis reveals that compound 3 crystallized in the monoclinic space group C2/c. The asymmetric unit consists of one Zn(II) center, one thdc 2− ligand, one half 2-Pn ligand, and one lattice DMF molecule. The Zn(II) center has a square pyramidal geometry, composed of four oxygen atoms of four distinct thdc 2− ligands in the basal plane and one nitrogen atom of a 2-Pn ligand at the axial position (Figure 2a). In turn, the thdc 2− ligand adopts a μ4-bridge mode to link four Zn(II) centers where each of the two carboxylate groups adopts a syn,syn-bridging bidentate to show a μ2-η 1

Crystal Structure of {[Zn2(1,4-ndc)2(2-Pn)]•1.5DMF}n (4)
Single-crystal X-ray diffraction analysis revealed that compound 4 crystallizes in the triclinic space group P1 . The asymmetric unit consists of two Zn(II) centers, two 1,4-ndc 2-ligands, one 2-Pn ligand, and three partially-occupied lattice DMF molecules with occupancy sites of 0.55, 0.50, and 0.45, respectively. Both of the two distinct Zn(II) centers adopt a square pyramidal geometry, with four oxygen atoms belonging to four distinct 1,4-ndc 2-ligands occupying the basal plane and one nitrogen atom from a 2-Pn ligand occupying the apex (Figure 3a). The 1,4-ndc 2-ligand displays a bis(syn,syn-bridging bidentate) coordination mode (i.e., a μ4-bridge mode) linking four Zn(II) centers, where each of the two carboxylate groups show a μ2-η 1 (Figure 3b), which extends to a 3D non-interpenetrated pillared-layer framework through the connection of 2-Pn pillars in the same orientation. The whole framework adopts a 6-connected distorted pcu-net with Schläfli symbol of 4 12 •6 3 topology ( Figure  3c). When viewed down the crystallographic a-axis, the packing diagram shows 1D channels having 30.9% solvent accessible volumes [39] that are occupied by lattice DMF molecules ( Figure  3d).

Powder X-Ray Diffraction and Thermogravimetric Analysis
Powder X-ray diffraction (PXRD) analysis was carried out to determine the purity of the compounds. The experimentally obtained patterns of compounds 1-3 matched well with the simulated patterns calculated from the single-crystal data (Figure 4), confirming phase purity. In addition, compounds 1 and 2 showed their stability in the atmosphere and in the presence of water over 4 days ( Figure S2). Nevertheless, 4 showed an experimental pattern that did not match with the simulated pattern very well. A likely explanation for such differences is simply that the crystal packaging of the crystalline solid of 4, after leaving the mother solution, might be distorted and/or partially collapse as a result of the release of the lattice DMF solvents before loading the sample for PXRD measurement and during the period of PXRD measurement.
Thermogravimetric (TG) analysis of 1 showed a weight loss between room temperature and ca. 175 • C, whereas that of 2 revealed a weight loss between room temperature and ca. 185 • C, corresponding to the escape of lattice DMF molecules ( Figure 5). Both the solvent-free frameworks of 1 and 2 remained thermally stable up to a temperature of approximate 283 and 335 • C, respectively, followed by a decomposition process ending at approximately 530 and 650 • C, respectively. The TG curves of 3 and 4 both show that the lattice DMF molecules were gradually released upon heating from room temperature to about 318 and 360 • C, respectively, followed by a decomposition process ending at about 720 and 510 • C, respectively.

Powder X-Ray Diffraction and Thermogravimetric Analysis
Powder X-ray diffraction (PXRD) analysis was carried out to determine the purity of the compounds. The experimentally obtained patterns of compounds 1−3 matched well with the simulated patterns calculated from the single-crystal data (Figure 4), confirming phase purity. In addition, compounds 1 and 2 showed their stability in the atmosphere and in the presence of water over 4 days ( Figure S2). Nevertheless, 4 showed an experimental pattern that did not match with the simulated pattern very well. A likely explanation for such differences is simply that the crystal packaging of the crystalline solid of 4, after leaving the mother solution, might be distorted and/or partially collapse as a result of the release of the lattice DMF solvents before loading the sample for PXRD measurement and during the period of PXRD measurement.
Thermogravimetric (TG) analysis of 1 showed a weight loss between room temperature and ca. 175 C, whereas that of 2 revealed a weight loss between room temperature and ca. 185 C, corresponding to the escape of lattice DMF molecules ( Figure 5). Both the solvent-free frameworks of 1 and 2 remained thermally stable up to a temperature of approximate 283 and 335 C, respectively, followed by a decomposition process ending at approximately 530 and 650 C, respectively. The TG curves of 3 and 4 both show that the lattice DMF molecules were gradually released upon heating from room temperature to about 318 and 360 °C, respectively, followed by a decomposition process ending at about 720 and 510 C, respectively.

CO2 Adsorption Properties
Prior to the gas adsorption experiments, ethanol was used to exchange guest DMF molecules. The TG curves of EtOH-exchanged materials show the success of complete substitution of DMF with EtOH according to the observations on both percentage and temperature of weight loss

CO 2 Adsorption Properties
Prior to the gas adsorption experiments, ethanol was used to exchange guest DMF molecules. The TG curves of EtOH-exchanged materials show the success of complete substitution of DMF with EtOH according to the observations on both percentage and temperature of weight loss ( Figure S3). PXRD patterns of EtOH-exchanged materials confirm the maintenance of structural integrity and crystallinity after solvent exchange for 1 and 2, but show slight alternation of crystal packing after solvent exchange for 3 and 4 ( Figure 4). The EtOH-exchanged materials of 1, 2, and 4 were degassed under a high vacuum at 120 • C for about 24 h to give the activated materials, denoted as 1 , 2 , and 4 , for the use of CO 2 capture studies. The structural integrity and crystallinity of these activated materials was maintained ( Figure S2). Sorption studies showed that all the three activated materials exhibited as typical reversible type I isotherms with moderate CO 2 uptakes of 55.1, 84.6, and 64.3 cm 3 g −1 for 1 , 2 , and 4 , respectively, at 195 K (Figure 6a). These values are comparable with those shown by other Zn MOFs (Table 2) [40][41][42][43][44][45]. For 1 and 2 , featuring identical framework topologies, the slight decrease of CO 2 uptake for the former in comparison to that of the latter can be ascribed to its lower porosity. The Brunauer-Emmett-Teller (BET) analysis verified the lower surface area of 1 , which was estimated to be 135 m 2 g −1 (Langmuir surface area = 203 m 2 /g −1 ) compared to that of 2 with a BET surface area of 233 m 2 g −1 and Langmuir surface area of 347 m 2 g −1 . Compound 4 showed a BET surface area of 173 m 2 g −1 and Langmuir surface area of 230 m 2 g −1 , which is consistent with the CO 2 uptakes among the three materials. To study the affinity between compounds and CO 2 molecules, the coverage-dependent isosteric heat of CO 2 adsorption (Q st ) was calculated by the virial method based on the CO 2 uptakes at 273 and 298 K ( Figure S4), which gave commonly decreased Q st traces with increasing CO 2 uptake for all the three materials (Figure 6b). At zero coverage, the adsorption enthalpy for 2 exhibited a stronger binding affinity of 38.3 kJ mol −1 for CO 2 , compared to 1 at 32.5 kJ mol −1 and 4 at 23.5 kJ mol −1 . However, the values are higher than that of liquefaction of CO 2 (17 kJ mol −1 ) [46], suggesting significant interactions (possible dipole-quadruple interactions) between the framework surface and the CO 2 molecules with the gas-gas affinity at higher temperature [27][28][29].   Abbreviations

Conclusions
In this work, we successfully synthesized four Zn MOFs featuring a topologic 4 8 ·6 7 net for 1 and 2, a pillared-layer open structure of topologic 4 8 ·6 6 ·8 net for 3, and a pcu-type pillared-layer open structure of 4 12 ·6 3 -topology for 4. These fascinating framework topologies were all constructed from similar 6-connected paddlewheel {Zn 2 (COO) 4 } SBUs connected by dicarboxylate links and 2-Pn bridges. As a result, these paddlewheel SBU-based Zn MOFs demonstrate cases showing interesting structure diversity that might be ascribed to the influences of dicarboxylate links with different spacers and 2-Pn bridges with varying ligating orientations. In addition, these MOFs also exhibited CO 2 uptake abilities with noticeable binding affinity.