Three New Supramolecular Coordination Polymers Based on 1H-pyrazolo[3,4-b]pyridin-3-amine and 1,3-benzenedicarboxylate Derivatives

Three new supramolecular coordination polymers, namely [Zn(1,3-BDC)(HL)]n (Polymer 1), [Zn3(1,3,5-BTC)2(HL)2(H2O)2]n (Polymer 2), and [Zn9(5-SO3-1,3-BDC)2(L)8(OH)4]n (Polymer 3), were synthesized under solvothermal conditions, based on 1H-pyrazolo[3,4-b]pyridin-3-amine (HL) along with 1,3-benzenedicarboxylate (1,3-BDC) and its derivatives, such as 1,3,5-benzenetricarboxylate (1,3,5-BTC) and 5-sulfo-1,3-benzenedicarboxylate (5-SO3-1,3-BDC). Polymers 1–3 were characterized by elemental analysis, IR spectroscopy, powder X-ray diffraction (PXRD), and single crystal X-ray diffraction analysis. Polymer 1 exhibited a two-dimensional (2D) 4-connected sql net. The neighboring 2D nets were further linked into a 3D supramolecular network by hydrogen-bonding interactions. Polymer 2 displayed a 3D (4, 4, 4)-connected network, which was further stabilized by R22(14) and S(9) hydrogen-bonding rings along with π–π interactions. The 2D sheet structure of Polymer 3 was constructed by novel quasi-linear nonanuclear Zn(II) units, which further extended into a 3D supramolecular structure by hydrogen-bonding interactions. The solid-state photoluminescence properties of Polymers 1–3 were also investigated.


Introduction
Crystal engineering, based on coordination polymers, is one of the most popular research fields, because supramolecular coordination polymers possessing covalent and noncovalent interactions (such as hydrogen-bonding interactions and π···π-stacking interactions) have the advantages of both coordination polymers and supramolecular materials. Many of them not only have diverse and biomimetic dynamic structures but also have widely promising applications in many fields, such as in structural transformation (SCSC), ionic diodes, and semiconductors [1][2][3][4][5][6][7][8][9][10]. Although there are many factors affecting the structure of supramolecular coordination polymers, such as pH, temperature, and solvents, the most important one is the structure of ligands. Aromatic acid ligands and N-heterocyclic ligands are the best choice among many ligands, because carboxylic acid ligands and N-heterocyclic ligands have a variety of coordination models; furthermore, they can perform hydrogen bonding and aromatic π-π stacking in the construction of polymers, which are superior conditions for the synthesis of supramolecular coordination polymers [11][12][13].

X-ray Crystallographic Determinations
Single crystal analyses for Polymers 1-3 were detected on a Bruker D8-QUEST diffractometer equipped with graphite monochromatic MoKa radiation (λ = 0.71073 Å). The crystal data, data collection, and structure refinement details of the three polymers are summarized in Table 1. The structures were solved using direct methods and refined using the SHELXL crystallographic software package [22,23]. Final refinements were carried out by the full matrix least-squares methods with anisotropic thermal parameters for nonhydrogen atoms on F 2 , while the hydrogen atom positions were located in calculated positions. The hydroxyl H atoms in Polymer 3 were placed in calculated positions and refined as riding, with the O−H distances fixed at 0.82 Å [24]. Selected bonds and angles for Polymers 1-3 are presented in Table 2. The hydrogen-bonding interactions of Polymers 1-3 are listed in Table S1 in the supporting information.

Structural Description of [Zn(1,3-BDC)(HL)]n (Polymer 1)
The single crystal X-ray diffraction analysis revealed that Polymer 1 crystallized in the monoclinic space group P2/c. The asymmetric unit contains two Zn(II) ions (Zn1 and Zn2), one 1,3-BDC 2− ligand, and one HL ligand. As shown in Figure 2, the Zn1(II) ion is four-coordinated with distorted tetrahedron geometry, which is completed by two carboxylic oxygen atoms from two different 1,3-BDC 2− ligands and two pyrazole-N atoms from two different HL ligands (Zn1-O = 1.9572(15) Å and Zn1-N = 2.0421(17) Å). The Zn2 atom is coordinated by two carboxylic oxygen atoms from two different 1,3-BDC 2− ligands and two pyridine-N atoms from two different HL ligands, also giving a distorted tetrahedron geometry (Zn2-O = 1.9445(15) Å and Zn2-N = 2.0496(17) Å). In Polymer 1, each 1,3-BDC 2− ligand adopts the same coordination fashion-bis-monodentate. The two carboxylate groups in the 1,3-BDC 2− ligands act as a monodentate-bridging ligand interlinking the Zn1(II) and Zn2(II) ions alternately into a long-wave chain along the a-axis. The neighboring chains are connected by the HL ligands into an ABAB sheet grid-type structure, as shown in Figure 3a (A with two HL ligands in the grid and B with two 1,3-BDC 2− ligands in the grid)-S(8) intramolecular hydrogen-bond rings and the π-π-stacking interactions of the aromatic rings further stabilized the grid. Taking each Zn(II) ion as a node, the topology of this structure can be simplified into a familiar 2D 4-connected sql net [25]. The edge lengths of the Zn···Zn separation within the {Zn4(HL)2(1,3-BDC)2} rectangular grid are 6.12 and 9.56 Å and the diagonal Zn···Zn separations are 11.33 and 11.37 Å for the A type grid and 10.57 and 12.08 Å for the B type grid. Viewed down the a-axis, the 2D sheet structure presents a zigzag chain along the c-axis, as shown in Figure 3b         Polymer 2 crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit contains two Zn(II) ions (Zn1 and Zn2), one BTC 3− ligand, one HL ligand, and one coordination water molecule. As shown in Figure 5, the Zn1(II) ion is four-coordinated with a slightly distorted tetrahedron geometry, which is completed by three carboxylic oxygen atoms from three different BTC 3− ligands and one pyrazole-N atom from a HL ligand (Zn1-O = 1.9239(13) − 2.0075(15) Å and Zn1-N = 2.0330(16) Å). The Zn2 atom is coordinated by two carboxylic oxygen atoms from two different BTC 3− ligands, two oxygen atoms from two coordination water molecules, and two pyridine-N atoms from two different HL ligands, giving an octahedral geometry with slight distortion (Zn2-O = 2.0596(13) − 2.1648(15) Å and Zn2-N = 2.1997(16) Å). In Polymer 2, each BTC 3− ligand adopted the same coordination fashion. There are two kinds of coordination mode of the three carboxylate groups in the BTC 3− ligand-two monodentate and one µ-1,2 [bis(monodentate)] (syn-anti). The Zn1 atoms are bridged by the tridentate BTC 3− ligand to give rise to an undulating 2D 6-connected sheet structure extending in the a-c plane. Viewed down the a-axis, the 2D network shows a square-wave chain structure, as depicted in Figure 6. Crystallographic symmetry of the HL ligands located in two neighboring square-wave-shaped chains links the Zn1(II) and Zn2(II) ions together by the pyrazole-N and pyridine-N atoms, respectively, and generates the 3D framework of Polymer 2. Two symmetrically related trans bridging bidentate carboxylic groups connect the Zn1(II) to Zn2(II) ions together, further strengthening the links between two neighboring layers, as shown in Figure 7. From the viewpoint of topology, if the Zn1(II), Zn2(II), and BTC 3− are considered as nodes, separately, this structure can be simplified into a 3D (4, 4, 4)-connected net with a Schläfli symbol of (4.5.6 3 .7)2 (5 2 .6 2 .8.9) (5 2 .6 4 ), as depicted in Figure 8. Furthermore, intramolecular R 2 2 (14) hydrogen-bonding interaction rings are found between the coordination water molecules and the carboxylic oxygen atoms. The N-amino atom from the HL ligands and the carboxylic oxygen atoms also form intramolecular S(6) hydrogen-bonding rings. The π-π interactions are found between the pyrazole ring of the HL ligands and the aromatic ring of the BTC 3− ligands with a centroid-to-centroid distance of 3.50 Å, further enhancing the stability of Polymer 2 ( Figure 9). Figure 5, the Zn1(II) ion is four-coordinated with a slightly distorted tetrahedron geometry, which is completed by three carboxylic oxygen atoms from three different BTC 3− ligands and one pyrazole-N atom from a HL ligand (Zn1-O = 1.9239(13) − 2.0075(15) Å and Zn1-N = 2.0330(16) Å). The Zn2 atom is coordinated by two carboxylic oxygen atoms from two different BTC 3− ligands, two oxygen atoms from two coordination water molecules, and two pyridine-N atoms from two different HL ligands, giving an octahedral geometry with slight distortion (Zn2-O = 2.0596(13) − 2.1648(15) Å and Zn2-N = 2.1997(16) Å). In Polymer 2, each BTC 3− ligand adopted the same coordination fashion. There are two kinds of coordination mode of the three carboxylate groups in the BTC 3− ligand-two monodentate and one μ-1,2 [bis(monodentate)] (syn-anti). The Zn1 atoms are bridged by the tridentate BTC 3− ligand to give rise to an undulating 2D 6-connected sheet structure extending in the a-c plane. Viewed down the a-axis, the 2D network shows a square-wave chain structure, as depicted in Figure 6. Crystallographic symmetry of the HL ligands located in two neighboring square-wave-shaped chains links the Zn1(II) and Zn2(II) ions together by the pyrazole-N and pyridine-N atoms, respectively, and generates the 3D framework of Polymer 2. Two symmetrically related trans bridging bidentate carboxylic groups connect the Zn1(II) to Zn2(II) ions together, further strengthening the links between two neighboring layers, as shown in Figure 7. From the viewpoint of topology, if the Zn1(II), Zn2(II), and BTC 3− are considered as nodes, separately, this structure can be simplified into a 3D (4, 4, 4)-connected net with a Schläfli symbol of (4.5.6 3 .7)2 (5 2 .6 2 .8.9) (5 2 .6 4 ), as depicted in Figure 8. Furthermore, intramolecular 14

As shown in
hydrogen-bonding interaction rings are found between the coordination water molecules and the carboxylic oxygen atoms. The N-amino atom from the HL ligands and the carboxylic oxygen atoms also form intramolecular S(6) hydrogen-bonding rings. The π-π interactions are found between the pyrazole ring of the HL ligands and the aromatic ring of the BTC 3− ligands with a centroid-to-centroid distance of 3.50 Å, further enhancing the stability of Polymer 2 ( Figure 9).  As shown in Figure 5, the Zn1(II) ion is four-coordinated with a slightly distorted tetrahedron geometry, which is completed by three carboxylic oxygen atoms from three different BTC 3− ligands and one pyrazole-N atom from a HL ligand (Zn1-O = 1.9239(13) − 2.0075(15) Å and Zn1-N = 2.0330(16) Å). The Zn2 atom is coordinated by two carboxylic oxygen atoms from two different BTC 3− ligands, two oxygen atoms from two coordination water molecules, and two pyridine-N atoms from two different HL ligands, giving an octahedral geometry with slight distortion (Zn2-O = 2.0596(13) − 2.1648(15) Å and Zn2-N = 2.1997(16) Å). In Polymer 2, each BTC 3− ligand adopted the same coordination fashion. There are two kinds of coordination mode of the three carboxylate groups in the BTC 3− ligand-two monodentate and one μ-1,2 [bis(monodentate)] (syn-anti). The Zn1 atoms are bridged by the tridentate BTC 3− ligand to give rise to an undulating 2D 6-connected sheet structure extending in the a-c plane. Viewed down the a-axis, the 2D network shows a square-wave chain structure, as depicted in Figure 6. Crystallographic symmetry of the HL ligands located in two neighboring square-wave-shaped chains links the Zn1(II) and Zn2(II) ions together by the pyrazole-N and pyridine-N atoms, respectively, and generates the 3D framework of Polymer 2. Two symmetrically related trans bridging bidentate carboxylic groups connect the Zn1(II) to Zn2(II) ions together, further strengthening the links between two neighboring layers, as shown in Figure 7. From the viewpoint of topology, if the Zn1(II), Zn2(II), and BTC 3− are considered as nodes, separately, this structure can be simplified into a 3D (4, 4, 4)-connected net with a Schläfli symbol of (4.5.6 3 .7)2 (5 2 .6 2 .8.9) (5 2 .6 4 ), as depicted in Figure 8. Furthermore, intramolecular 14 hydrogen-bonding interaction rings are found between the coordination water molecules and the carboxylic oxygen atoms. The N-amino atom from the HL ligands and the carboxylic oxygen atoms also form intramolecular S(6) hydrogen-bonding rings. The π-π interactions are found between the pyrazole ring of the HL ligands and the aromatic ring of the BTC 3− ligands with a centroid-to-centroid distance of 3.50 Å, further enhancing the stability of Polymer 2 ( Figure 9).               Polymer 3 crystallizes in the triclinic space group P1. The asymmetric unit contains five Zn(II) ions (Zn1, Zn2, Zn3, Zn4, and Zn5), one 5-SO 3 -1,3-BDC 3− ligand, three L − ligands, and two hydroxyl groups. As shown in Figure 10, Zn1(II), Zn2(II), Zn3(II), and Zn4(II) ions are all four-coordinated with slightly distorted tetrahedron geometries: the Zn1(II) ion is completed by one carboxylic oxygen atom from the 5-SO 3 -BDC 3− ligand, one oxygen atom from the sulfo group of the 5-SO 3 -BDC 3− ligand, and two pyrazole-N atoms from the L − ligand (Zn1-O = 1.934(3) -1.997(4) Å and Zn1-N = 1.991(4) -2.006(4) Å); the Zn2(II) ion is coordinated by one µ 2 -hydroxyl oxygen atom, two pyrazole-N atoms, and one pyridine-N atom from three different HL ligands; the Zn3(II) ion is coordinated by one carboxylic oxygen atom from the 5-SO 3 -BDC 3− ligand, one µ 2 -hydroxyl oxygen atom, and two pyrazole-N atoms from two different HL ligands; and the Zn4(II) ion is coordinated by one µ 2 -hydroxyl oxygen atom, two pyrazole-N atoms, and one pyridine-N atom from three different L − ligands. The Zn5(II) ions present a six-coordinated mode surrounded by two carboxylic oxygen atoms from two different 5-SO 3 -BDC 3− ligands, two µ 2 -hydroxyl oxygen atoms, and two pyridine-Ns, giving an octahedral geometry with considerable distortion. The distances of the Zn-O/N and angles of Zn-O/N-Zn are presented in Table 2. Zn1(II), Zn2(II), Zn4(II), and Zn3(II) ions and their related symmetry, Zn1a(II), Zn2a(II), Zn4a(II), Zn3a(II), are distributed on the two sides of the Zn5(II) ion. These nine metal ions are arranged in a quasi-linear form of Zn1-Zn2-Zn4-Zn3-Zn5-Zn3a-Zn4a-Zn2a-Zn1a (a = 2 − x, 1 − y, −z), forming a {Zn 9 (L) 8 (µ 2 -OH) 4 (OCO) 2} secondary building unit ({Zn 9 } unit), as shown in Figure 11. Although some nonanuclear polymers have been reported [26][27][28][29], most of them are based on rare earth ions [27,[30][31][32], while those based on the Zn(II) ion are relatively rare [33,34]. Furthermore, most of the reported structures of nonanuclear units are a square antiprismatic arrangement or hourglass structure [33,35]. As far as we know, those based on the quasi-linear array of a nonanuclear zinc polymer have not been reported. The adjacent {Zn 9 (L) 8 (µ 2 -OH) 4 (OCO) 2 } units are connected to a two-dimensional planar structure by the 5-SO 3 -BDC 3− ligand. Intramolecular hydrogen-bonding S(6) and S(8) rings as well as π-π interactions between the neighboring pyrazole-pyrazole rings of two L − ligands with a centroid-to-centroid distance of 3.72 Å stabilized the 2D sheet of Polymer 3, as depicted in Figure 12 The π-π interactions between the neighboring pyridine−pyridine rings of two L − ligands with a centroid-to-centroid distance of 3.51 Å further stabilized the 3D supramolecular structure of Polymer 3 ( Figure 13). Polymer 3 crystallizes in the triclinic space group P1 . The asymmetric unit contains five Zn(II) ions (Zn1, Zn2, Zn3, Zn4, and Zn5), one 5-SO3-1,3-BDC 3− ligand, three L − ligands, and two hydroxyl groups. As shown in Figure 10, Zn1(II), Zn2(II), Zn3(II), and Zn4(II) ions are all four-coordinated with slightly distorted tetrahedron geometries: the Zn1(II) ion is completed by one carboxylic oxygen atom from the 5-SO3-BDC 3− ligand, one oxygen atom from the sulfo group of the 5-SO3-BDC 3− ligand, and two pyrazole-N atoms from the L − ligand (Zn1-O = 1.934(3) -1.997(4) Å and Zn1-N = 1.991(4) -2.006(4) Å); the Zn2(II) ion is coordinated by one μ2-hydroxyl oxygen atom, two pyrazole-N atoms, and one pyridine-N atom from three different HL ligands; the Zn3(II) ion is coordinated by one carboxylic oxygen atom from the 5-SO3-BDC 3− ligand, one μ2-hydroxyl oxygen atom, and two pyrazole-N atoms from two different HL ligands; and the Zn4(II) ion is coordinated by one μ2-hydroxyl oxygen atom, two pyrazole-N atoms, and one pyridine-N atom from three different L − ligands. The Zn5(II) ions present a six-coordinated mode surrounded by two carboxylic oxygen atoms from two different 5-SO3-BDC 3− ligands, two μ2-hydroxyl oxygen atoms, and two pyridine-Ns, giving an octahedral geometry with considerable distortion. The distances of the Zn-O/N and angles of Zn-O/N-Zn are presented in Table 2 Figure 11. Although some nonanuclear polymers have been reported [26][27][28][29], most of them are based on rare earth ions [27,[30][31][32], while those based on the Zn(II) ion are relatively rare [33,34]. Furthermore, most of the reported structures of nonanuclear units are a square antiprismatic arrangement or hourglass structure [33,35]. As far as we know, those based on the quasi-linear array of a nonanuclear zinc polymer have not been reported. The adjacent {Zn9(L)8(μ2-OH)4(OCO)2} units are connected to a two-dimensional planar structure by the 5-SO3-BDC 3− ligand. Intramolecular hydrogen-bonding S(6) and S(8) rings as well as π-π interactions between the neighboring pyrazole-pyrazole rings of two L − ligands with a centroid-to-centroid distance of 3.72 Å stabilized the 2D sheet of Polymer 3, as depicted in Figure 12

Comparison of the Structures of Polymers 1-3
As mentioned above, it can be seen that covalent interactions, hydrogen-bonding interactions, as well as π···π-stacking interactions play important roles in constructing the structures of supramolecular coordination polymers. The 5-positioned substituted functional group of 1,3-benzenedicarboxylic acid plays a significant role in determining the resulting structures. The different coordination modes of the HL ligand allow it to play different roles in constructing the three polymers. In Polymers 1 and 2, the pyridine-N atom and one of the pyrazole-N atoms of the HL ligand are involved in coordination and play a pivotal role in increasing the dimensionality of Polymers 1 and 2. In Polymer 3, in order to balance the positive charge of the metal ions, each HL ligand in Polymer 3 is deprotonated. The pyridine-N atom and two pyrazole-N atoms in each L − ligand are all coordinated with Zn(II) centers and make the novel {Zn 9 } units of Polymer 3.

PXRD and Photoluminescence Properties of the Three Polymers
The phase purity of the three polymers was detected by using powder X-ray diffraction (PXRD) experiments. The peaks of the experimental and simulated PXRD patterns coincide with each other, which indicate the purity of the three polymers ( Figures S1-S3).
The d 10 coordination polymers were proved to have good photoluminescence properties. Therefore, the solid-state photoluminescence properties of Polymers 1-3, as well as the free ligand, were studied at ambient temperature. As shown in Figure 14, when the excitation spectrum was excited at 350 nm, the free HL gave an optical emission bond at 467 nm. The photoluminescent spectra of the free H 2 BDC, H 3 BTC, and 5-SO 3 Na-H 2 BDC ligands were also investigated-strong emission bands were observed at 393 nm and 560 nm for the free H 2 BDC and H 3 BTC ligands, respectively (λex = 349 nm for the H 2 BDC ligand; λex = 301 nm for the free H 3 BTC ligand). No luminescence spectrum was observed for the free 5-SO 3 Na-H 2 BDC ligands-this phenomenon may be due to the presence of the SO 3 Na group. The optimal emission bands were detected at 558 nm for Polymer 1, 485 nm for Polymer 2, and 543 nm for Polymer 3 (λ ex = 373 nm for Polymer 1, λ ex = 368 nm for Polymer 2, and λ ex = 370 nm for Polymer 3). The emission peaks of Polymers 1-3 were similar to that of the HL ligand, therefore, their emissions may be ascribed to the ligand-centered π-π* transitions of the HL ligand [36]. As mentioned above, it can be seen that covalent interactions, hydrogen-bonding interactions, as well as π···π-stacking interactions play important roles in constructing the structures of supramolecular coordination polymers. The 5-positioned substituted functional group of 1,3-benzenedicarboxylic acid plays a significant role in determining the resulting structures. The different coordination modes of the HL ligand allow it to play different roles in constructing the three polymers. In Polymers 1 and 2, the pyridine-N atom and one of the pyrazole-N atoms of the HL ligand are involved in coordination and play a pivotal role in increasing the dimensionality of Polymers 1 and 2. In Polymer 3, in order to balance the positive charge of the metal ions, each HL ligand in Polymer 3 is deprotonated. The pyridine-N atom and two pyrazole-N atoms in each L − ligand are all coordinated with Zn(II) centers and make the novel {Zn9} units of Polymer 3.

PXRD and Photoluminescence Properties of the Three Polymers
The phase purity of the three polymers was detected by using powder X-ray diffraction (PXRD) experiments. The peaks of the experimental and simulated PXRD patterns coincide with each other, which indicate the purity of the three polymers ( Figures S1-S3).
The d 10 coordination polymers were proved to have good photoluminescence properties. Therefore, the solid-state photoluminescence properties of Polymers 1-3, as well as the free ligand, were studied at ambient temperature. As shown in Figure 14, when the excitation spectrum was excited at 350 nm, the free HL gave an optical emission bond at 467 nm. The photoluminescent spectra of the free H2BDC, H3BTC, and 5-SO3Na-H2BDC ligands were also investigated-strong emission bands were observed at 393 nm and 560 nm for the free H2BDC and H3BTC ligands, respectively (λex = 349 nm for the H2BDC ligand; λex = 301 nm for the free H3BTC ligand). No luminescence spectrum was observed for the free 5-SO3Na-H2BDC ligands-this phenomenon may be due to the presence of the SO3Na group. The optimal emission bands were detected at 558 nm for Polymer 1, 485 nm for Polymer 2, and 543 nm for Polymer 3 (λex = 373 nm for Polymer 1, λex = 368 nm for Polymer 2, and λex = 370 nm for Polymer 3). The emission peaks of Polymers 1-3 were similar to that of the HL ligand, therefore, their emissions may be ascribed to the ligand-centered ππ* transitions of the HL ligand [36].

Thermogravimetric Analyses (TGA) of the Three Polymers
To investigate the thermal stabilities of Polymers 1-3, thermogravimetric analyses were performed in N 2 atmosphere. As shown in Figure S4 in the supporting information, Polymer 1 was stable up to 420 • C. The loss of 4.02% (calcd 3.94%) in 185-341 • C of Polymer 2 corresponds to the removal of coordination water molecules. The loss of 3.04% (calcd 3.08%) in 185-341 • C of Polymer 3 corresponds to the removal of hydroxyl in the molecules. Their further obvious weight losses were ascribed to the collapse of their frameworks.

Conclusions
In conclusion, three new supramolecular coordination polymers with different architectures were successfully synthesized by assembling multidentate HL ligand and Zn 2+ ions in the presence of different aromatic polycarboxylate ligands. Although the three polymers were synthesized under similar conditions, their structures were quite different, thus it can be seen that the 5-substituted functional group of 1,3-benzenedicarboxylate had a great influence on the final structures. Even so, the influence of noncovalent bonds (hydrogen-bonding interactions and π···π -stacking interactions) on the structure of coordination polymers should not be underestimated. This work provides a promising approach on the construction of novel supramolecular coordination polymers with multidentate N-containing ligands and aromatic polycarboxylates.
Author Contributions: Y.X. is responsible for the whole project, including the ideas, resources, funding acquisition, and writing; Q.-H.D. and F.D. synthesized materials and most of the characterization; R.A. and F.D. analyzed the materials and characterization; and D.L. and T.-F.M. proposed constructive amendments and partial financial support.