Synthesis, Structures, and Water Adsorption of Two Coordination Polymers Constructed by M(II) (M = Ni (1) and Zn (2)) with 1,3-Bis(4-Pyridyl)Propane (bpp) and 1,2,4,5-Benzenetetracarboxylate (BT4−) Ligands

Two coordination polymers (CPs), with chemical formulas {[Ni2(bpp)2(BT)(H2O)6] 1.5(EtOH) 1.5H2O}n (1) and [Zn(bpp)(BT)0.5]·5H2O (2) (bpp = 1,3-bis(4-pyridyl)propane, and BT4− = tetraanion of 1,2,4,5-Benzenetetracarboxylic acid), have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. In compound 1, the coordination environments of two crystallographically independent Ni(II) ions are both distorted octahedral bonded to two nitrogen donors from two bpp ligands and four oxygen donors from one BT4- ligand and three water molecules. Both bpp and BT4− act as bridging ligands with bis-monodentate and 1,4-bis-monodentate coordination modes, respectively, connecting the Ni(II) ions to form a 2D layered metal-organic framework (MOF). Adjacent 2D layers are then arranged orderly in an ABAB manner to complete their 3D supramolecular architecture. In 2, the coordination environment of Zn(II) ion is distorted tetrahedral bonded to two nitrogen donors from two bpp ligands and two oxygen donors from two BT4− ligands. Both bpp and BT4- act as bridging ligands with bis-monodentate and 1,2,4,5-tetrakis-monodentate coordination modes, respectively, connecting the Zn(II) ions to form a 3D MOF. The reversible water de-/adsorption behavior of 1 between dehydrated and rehydrated forms has been verified by cyclic Thermogravimetric (TG) analyses through de-/rehydration processes. Compound 1 also exhibits significant water vapor hysteresis isotherms.


General Procedures
All chemicals were used as commercially obtained. Elemental analyses were conducted with a Perkin-Elmer 2400 elemental analyzer. IR spectra (KBr disk) were recorded on a Nicolet FT IR, MAGNA-IR 500 spectrometer. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer 7 Series/UNIX TGA7 analyzer. Single-phased powder samples were loaded into alumina pans and heated with a ramp rate of 5 °C/min from room temperature to 700 °C under a nitrogen atmosphere. Scheme 2. Coordination modes of (a) 1,4-bis-monodentate, (b) 1,2,4,5-tetrakis-monodentate of BT 4− ligand used in 1 and 2, respectively.

General Procedures
All chemicals were used as commercially obtained. Elemental analyses were conducted with a Perkin-Elmer 2400 elemental analyzer. IR spectra (KBr disk) were recorded on a Nicolet FT IR, MAGNA-IR 500 spectrometer. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer 7 Series/UNIX TGA7 analyzer. Single-phased powder samples were loaded into alumina pans and heated with a ramp rate of 5 • C/min from room temperature to 700 • C under a nitrogen atmosphere. The water vapor adsorption isotherm at 298 K was measured in the gaseous state by using BELSORP-max volumetric adsorption equipment from BEL, Osaka, Japan. The adsorbent sample (~100-150 mg), which was prepared at 150 • C and 10 −2 Pa for around 24 h, was placed into the sample cell, and then the change in pressure was monitored and the degree of adsorption was determined by the decrease in pressure at equilibrium state.

X-ray Crystallography and Refinements
The diffraction data for compounds 1 and 2 were collected on a Siemens SMART diffractomer with Mo radiation (λ = 0.71073 Å) at 150 K. Cell parameters were retrieved using SMART [37] software and refined with SAINT [38] on all observed reflections. Data reduction was performed with the SAINT [39] software and corrected for Lorentz and polarization effects. Absorption corrections were applied with the program SADABS [39]. Direct phase determination and subsequent difference Fourier map synthesis yielded the positions of all atoms. The final full-matrix, least-squares refinement on F 2 was applied for all observed reflections [I > 2σ(I)]. All calculations were performed by using the SHELXTL-PC V 5.03 software package [40]. Crystal data and details of the data collection and structure refinements for 1 and 2 are summarized in Table 1. CCDC-2022795 and 2,022,797 for 1 and 2, respectively, contain the supplementary crystallographic data for this paper (Figures S1 and S2). These data can be obtained free of charge from www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; email: deposit@ccdc.cam.ac.uk.

In Situ X-ray Powder Diffraction
The powder X-ray diffraction measurement of compounds 1 and 2 was performed at the BL01C2 beamline of the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The X-ray energy was selected at 1.0332 Å. The diffraction pattern was recorded with a Mar345 imaging-plate detector and typical exposure duration of 2 min. The one-dimensional powder diffraction profile was converted with GSAS-II program [41]. The diffraction angles were calibrated according to Bragg positions of lanthanum boride (SRM660b) standards.

Synthesis and IR Spectroscopy
Compounds 1 and 2 were synthesized by direct mixing of M(II) salts, bpp and H 4 BT with a molar ratio of 2:2:1 to obtain blue rod-like crystals of 1 and colorless needle-like crystals of 2, respectively. The synthetic representation is shown in Scheme 3. The most relevant IR features are those associated with the BT 4− and bpp ligands [19,20]. Absorption bands in the range of 1400-1700 cm −1 can be related to the carboxylate groups of BT 4− ligand and bpp ligand, since the main vibrational frequencies of the two ligands appear in the same spectral region. However, a strong band centered at around 1571 cm −1 in the spectrum can be attributed to vibrational modes representing the C-C and C-N mixing stretching motions and is in agreement with the characteristics of the bpp ligand. Additional broad bands appear in the range of 3100-3500 cm −1 in 1 and 2, indicating the presence of the O-H stretching vibration from water molecules.  [20] and crystallizes in the monoclinic Cc space group, in which the asymmetric unit is composed of two Ni(II) centers, two bpp, one BT 4− ligand, six coordinated H2O molecules and one and a half solvated H2O and C2H5OH   [20] and crystallizes in the monoclinic Cc space group, in which the asymmetric unit is composed of two Ni(II) centers, two bpp, one BT 4− ligand, six coordinated H 2 O molecules and one and a half solvated H 2 O and C 2 H 5 OH molecules. The molecular structures of 1 are shown in Figure 1a, in which two crystallographically independent Ni(II) ions are both six coordinate bonded to four oxygen donors of one BT 4− ligand and three H 2 O molecules and two nitrogen donors of two bpp ligands located at the cis position, forming a distorted octahedral geometry. Bond lengths and angles around the Ni(II) ions are listed in Table S1 (included in the Supporting Information). In 1, two crystallographically independent bpp ligands, adopting TT and TG conformations, respectively, both act as bridging ligands with bis-monodentate coordination mode connecting the Ni(II) ions to form one-dimensional (1D) zigzag chains (Figure 1b). Adjacent chains are inter-linked via the connectivity between Ni(II) ions and BT 4− ligands with 1,4-bis-monodentate coordination mode [20] to generate a 2D layered MOF (Figure 1b Table S2 (included in the Supporting Information).

Structural Characterization
The thermal stability and temperature-dependent structural variation for 1 were studied by thermogravimetric (TG) analysis and in situ powder X-ray diffraction (PXRD) measurements, respectively. During the heating process, the TG analysis of 1 displayed a two-step weight loss (Figure 2a), while the first weight loss of 20.3% corresponded to the weight losses of six coordinated water molecules, 1.5 guest ethanol and 1.5 water molecules (calc. 21.1%), occurring in the range of approximately 33.9-156.9 • C and then remaining thermally stable up to 198.4 without any weight loss. On further heating, these samples decomposed. It is important to note that, during the heating processes, crystals in 1 were accompanied by morphological changes. The simultaneous and gradual color changes of the crystals were followed by optical microscopy, as shown in Figure 2a, during the de-solvation processes at some specific temperatures. A well-formed blue crystal of 1 was obtained. As the temperature was raised, the colors of dried crystals gradually turned from blue to pale-blue, and many chaps with random cracks on the crystal surface were observed. In order to gain more structural information, in situ temperature-dependent synchrotron PXRD patterns of 1 were collected from 25 • C to 410 • C, and the results at some specific temperatures are shown in Figure 2b. The PXRD patterns of as-synthesized samples 1 at RT matched well with the simulated one based on the single crystal structures. As the temperature increased, solvent de-solvation processes were initialized at 100 • C for 1 and converted to a meta-stable phase before entire structure collapse. However, due to the poor long-range ordering of PXRD data, the unit cells of dehydrated form 1 could not be determined directly by using synchrotron PXRD data. The temperature-dependent PXRD results are consistent with the TGA results and morphological changes. The thermal stability and temperature-dependent structural variation for 1 were studied by thermogravimetric (TG) analysis and in situ powder X-ray diffraction (PXRD) measurements, respectively. During the heating process, the TG analysis of 1 displayed a two-step weight loss (Figure 2a), while the first weight loss of 20.3% corresponded to the weight losses of six coordinated water molecules, 1.5 guest ethanol and 1.5 water molecules (calc. 21.1%), occurring in the range of approximately 33.9-156.9 °C and then remaining thermally stable up to 198.4 without any weight loss. On further heating, these samples decomposed. It is important to note that, during the heating processes, crystals in 1 were accompanied by morphological changes. The simultaneous and gradual color changes of the crystals were followed by optical microscopy, as shown in Figure 2a, during the de-solvation processes at some specific temperatures. A well-formed blue crystal of 1 was obtained. As the temperature was raised, the colors of dried crystals gradually turned from blue to pale-blue, and many chaps with random cracks on the crystal surface were observed. In order to gain more structural information, in situ temperature-dependent synchrotron PXRD patterns of 1 were collected from 25 °C to 410 °C, and the results at some specific temperatures are shown in Figure 2b. The PXRD patterns of as-synthesized samples 1 at RT matched well with the simulated one based on the single crystal structures. As the temperature increased, solvent de-solvation processes were initialized at 100 °C for 1 and converted to a meta-stable phase before entire structure collapse. However, due to the poor long-range ordering of PXRD data, the unit cells of dehydrated form 1

Structural Description of {[Zn(bpp)(BT)0.5]·5H2O}n (2)
Compound 2 crystallizes in the monoclinic P 21/c space group, in which the asymmetric unit is composed of one Zn(II) ion, one bpp, half of a BT 4− ligand and five guest water molecules. The

Structural Description of {[Zn(bpp)(BT) 0.5 ]·5H 2 O} n (2)
Compound 2 crystallizes in the monoclinic P 2 1 /c space group, in which the asymmetric unit is composed of one Zn(II) ion, one bpp, half of a BT 4− ligand and five guest water molecules. The coordination geometry of Zn(II) ion in 2 is distorted tetrahedral (Figure 3a) bonded to two oxygen donors from two BT 4− ligands and two nitrogen donors from two bpp ligands. Relevant bond lengths and angles around the Zn(II) ion are listed in Table S3 (included in the Supporting Information). In 2, BT 4− acts as a bridging ligand with 1,2,4,5-tetrakis-monodentate coordination mode [28,29] connecting the Zn(II) ions to form a 2D sinusoidal-like [Zn 2 (BT)] n layered framework (Figure 3b). Adjacent layers are inter-linked via the connectivity between Zn(II) ions and bpp ligands with bis-monodentate coordination mode to complete its 3D MOF (Figure 3c), in which five guest water molecules are located at the vacant sites (Figure 3d, right). The Zn(II)···Zn(II) separation via the TG-bpp bridge is 10.681(1) Å and via BT 4− bridges is 6.170(1), 9.208(1), 11.111(3), 11.057(2) Å (Figure 3c, right), respectively. It is noteworthy that five guest water molecules are assembled together to generate a 2D sinusoidal-like hydrogen-bonded water layer (Figure 3d, left and middle), which is built up by two water cluster units-one is a (H 2 O) 6 chair-like water ring and the other is a (H 2 O) 18 Table S4 (included in Supporting Information).
During the heating process, the TG analysis (see Figure 4a) revealed that 2 underwent a two-step weight loss, while the first weight loss of 18.1% corresponded to the loss of five guest H 2 O molecules (calc. 18.8%), occurring in the range of approximately 30.6-64.3 • C, and then remaining thermally stable up to 291.7 • C without any weight loss. On further heating, the sample decomposed. It is important to note that, during the heating processes, crystal 2 was accompanied by morphological changes. The simultaneous and gradual changes of the crystals were followed by optical microscopy, as shown in Figure 4a, during the dehydration process at some specific temperatures. A well-formed colorless crystal of 2 was obtained. As the temperature was raised, the color of the crystals gradually turned from colorless to white and many chaps with random cracks on the crystal surface were observed. To gain more information on the temperature-dependent structural variation, in situ synchrotron PXRD patterns of 2 were collected from 25 • C to 410 • C, and the results at some specific temperatures are shown in Figure 4b. The PXRD patterns of as-synthesized sample 2 matched well with the simulated pattern based on the single crystal structure. As the temperature increased, a new phase was transformed at 110 • C and could be sustained up to 320 • C. According to the cell refinement results, the cell volume reduced from 2078.88 Å 3 to 1847.39 Å 3 , roughly a 12% reduction. The symmetry was kept in monoclinic lattice, and corresponding cell parameters of hydrated and dehydrated forms were    sustained up to 320 °C. According to the cell refinement results, the cell volume reduced from 2078.88 Å 3 to 1847.39 Å 3 , roughly a 12% reduction. The symmetry was kept in monoclinic lattice, and corresponding cell parameters of hydrated and dehydrated forms were a = 12.5099 Å, b = 14.7892 Å, c = 10.8858 Å, β = 113.469° and a = 10.6860 Å, b = 16.5976 Å, c = 12.4611 Å, β = 109.845°, respectively. The temperature-dependent PXRD results are consistent with the TGA results and morphological changes.
(a) (b) Figure 4. (a) Thermogravimetric (TG) analysis of 2 associated with morphological changes at selected temperatures. (b) Powder X-ray diffraction patterns at RT and selected temperatures and its simulation from single-crystal diffraction data associated with morphological changes at selected temperatures of 2. The baselines for each temperature were shifted for clarity.

Water De-/Adsorption Behaviors of CPs 1 and 2 by Cyclic TG Analysis
In order to certify the reversibility of water de-/adsorption behaviors in 1 and 2 under water vapor, cyclic TG measurements were performed under de-/rehydration procedures by heating/cooling processes. Compound 1 showed a 20.0% weight decrease, corresponding to the losses of six coordinated H2O molecules, 1.5 guest EtOH and 1.5 H2O molecules, to obtain a dehydrated form of 1 after heating up to 120 °C (Figure 5a). When the dehydrated samples 1 were exposed to the water vapor and gradually cooled down to RT, the dehydrated samples showed a weight-increasing of 9.3%, approximately equal to 5.0 H2O molecules, to form the rehydration ones. Such heating and cooling processes were repeated for five times (Figure 5a), showing almost equally weighted increasing/decreasing percentages in the range of 9.3~10.1%, to prove the stable reversibility of the water de-/rehydration behavior. On the contrary, compound 2 shows irreversible water de-/adsorption behavior during the cyclic TG measurements shown in Figure 5b, in which a 20.0% weight decrease was found in the first cycle, corresponding to the losses of five guest H2O

Water De-/Adsorption Behaviors of CPs 1 and 2 by Cyclic TG Analysis
In order to certify the reversibility of water de-/adsorption behaviors in 1 and 2 under water vapor, cyclic TG measurements were performed under de-/rehydration procedures by heating/cooling processes. Compound 1 showed a 20.0% weight decrease, corresponding to the losses of six coordinated H 2 O molecules, 1.5 guest EtOH and 1.5 H 2 O molecules, to obtain a dehydrated form of 1 after heating up to 120 • C (Figure 5a). When the dehydrated samples 1 were exposed to the water vapor and gradually cooled down to RT, the dehydrated samples showed a weight-increasing of 9.3%, approximately equal to 5.0 H 2 O molecules, to form the rehydration ones. Such heating and cooling processes were repeated for five times (Figure 5a), showing almost equally weighted increasing/decreasing percentages in the range of 9.3~10.1%, to prove the stable reversibility of the water de-/rehydration behavior. On the contrary, compound 2 shows irreversible water de-/adsorption behavior during the cyclic TG measurements shown in Figure 5b, in which a 20.0% weight decrease was found in the first cycle, corresponding to the losses of five guest H 2 O molecules, to obtain the dehydrated form 2 after heating up to 150 • C (Figure 5b). When the dehydrated samples 2 were exposed to the water vapor and cooled down to RT, the water molecules could not be re-adsorbed. The results described above show reversible water de-/adsorption behaviors between dehydrated and rehydrated forms, but 2 shows irreversible water de-/adsorption behavior. The poor water adsorption ability of dehydrated 2 may be attributed to the structural change of the dehydrated form, which may prevent the re-adsorption of water molecules via the synergistic hydrogen bonding interactions between water molecules and [Zn 2 (BT)] n layers.
Polymers 2020, 12, x FOR PEER REVIEW 10 of 14 molecules, to obtain the dehydrated form 2 after heating up to 150 °C (Figure 5b). When the dehydrated samples 2 were exposed to the water vapor and cooled down to RT, the water molecules could not be re-adsorbed. The results described above show reversible water de-/adsorption behaviors between dehydrated and rehydrated forms, but 2 shows irreversible water de-/adsorption behavior. The poor water adsorption ability of dehydrated 2 may be attributed to the structural change of the dehydrated form, which may prevent the re-adsorption of water molecules via the synergistic hydrogen bonding interactions between water molecules and [Zn2(BT)]n layers.

Water Sorption Studies of CP 1
According to cyclic TG analyses, compound 1 shows the reversible de-/adsorption behavior of approximately five water molecules. In order to explore the water adsorption ability of 1, water  Figure 5. TG measurements of (a) 1 and (b) 2 during the cyclic de-/rehydration processes, which were repeated five times. Solid red line: the variation in weight loss with time; blue dash-dot line: the variation in temperature with time.

Water Sorption Studies of CP 1
According to cyclic TG analyses, compound 1 shows the reversible de-/adsorption behavior of approximately five water molecules. In order to explore the water adsorption ability of 1, water vapor sorption isotherms of pre-treated dehydrated form 1 were measured at 298K. The isotherm curve ( Figure 6) of dehydrated 1 shows a steady increase in adsorbed water vapor at 0 < relative P/P 0 < 0.89, with maximum value of 210.7 cm 3 g −1 at relative P/P 0 equal to 0.89, approximately equal to 8.20 H 2 O molecules, indicating six coordinated and two guest H 2 O molecules in 1 being re-adsorbed. It is worth noting that the desorption curve did not trace the adsorption curve, exhibiting a significant hysteresis loop with a value of 148.5 cm 3 g −1 , approximately equal to 5.8 H 2 O molecules at lower relative P/P 0 equal to 0.22. The water ad-/desorption isotherms of dehydrated form 1 reveal that the vacant sites of Ni(II) ions provide the opportunity for bond re-formation with water molecules as the dehydrated samples being exposed to water vapor. Recently, MOFs with high porosity have been examined for their water capture properties and found to be highly promising materials [42][43][44]. However, water capture properties applied to 3D supramolecular architectures assembled via CPs or MOFs are seldom detected and only a few cases have been investigated [45][46][47]. The water sorption isotherm with large hysteresis loops found in 1 is interesting and may be exploited as a water harvesting material.

Conclusions
Two coordination polymeric frameworks, {[Ni2(bpp)2(BT)(H2O)6] 1.5(EtOH)·1.5H2O}n (1) and [Zn(bpp)(BT)0.5]·5H2O (2), have been structurally investigated. In compound 1, the 3D supramolecular architecture is built up via the assembly of 2D MOFs, which are constructed through the bridges of Ni(II) with 1,4-bis-monodentate bpp and BT 4− ligands. Compound 2 is a 3D MOF constructed through the bridges of Zn(II) with bis-monodentate bpp and 1,2,4,5-tetrakis-monodentate BT 4− ligands. Interestingly, the guest water molecules in 2 are assembled together to generate a 2D sinusoidal-like hydrogen-bonded water layer, which is built up by two water cluster units-one is a (H2O)6 chair-like water ring and the other is a (H2O)18 water ring-via the intermolecular O-H⋅⋅⋅O hydrogen bonding interactions. Notably, compound 1 exhibits interesting water hysteresis isotherms and undergoes reversible water de-/adsorption behavior between the dehydrated and rehydrated species during thermal re-/dehydration processes. Water capture uptake observed in 1 displayed significant hysteresis loops in water ad-/desorption isotherms, which may be developed for potential application as a water harvesting material.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1, Figure S1: (a) TG measurement of 1, (b) PXRD patterns at RT and selected temperatures and their simulation from single-crystal diffraction data of 1, Figure S2: (a) TG measurement of 2, (b) PXRD patterns at RT and selected temperatures and their simulation from single-crystal diffraction data of 2, Table S1: Bond lengths (Å) and angles (°) around Ni(II) ions in 1, Table S2: Related parameters of hydrogen bonds in 1, Table S3: Bond lengths (Å) and angles (°) around Zn(II) ion in 2, Table S4: Related parameters of hydrogen bonds in 2.

Conclusions
Two coordination polymeric frameworks, {[Ni 2 (bpp) 2 (BT)(H 2 O) 6 ] 1.5(EtOH)·1.5H 2 O} n (1) and [Zn(bpp)(BT) 0.5 ]·5H 2 O (2), have been structurally investigated. In compound 1, the 3D supramolecular architecture is built up via the assembly of 2D MOFs, which are constructed through the bridges of Ni(II) with 1,4-bis-monodentate bpp and BT 4− ligands. Compound 2 is a 3D MOF constructed through the bridges of Zn(II) with bis-monodentate bpp and 1,2,4,5-tetrakis-monodentate BT 4− ligands. Interestingly, the guest water molecules in 2 are assembled together to generate a 2D sinusoidal-like hydrogen-bonded water layer, which is built up by two water cluster units-one is a (H 2 O) 6 chair-like water ring and the other is a (H 2 O) 18 water ring-via the intermolecular O-H···O hydrogen bonding interactions. Notably, compound 1 exhibits interesting water hysteresis isotherms and undergoes reversible water de-/adsorption behavior between the dehydrated and rehydrated species during thermal re-/dehydration processes. Water capture uptake observed in 1 displayed significant hysteresis loops in water ad-/desorption isotherms, which may be developed for potential application as a water harvesting material.