Metal and Ligand Effect on the Structural Diversity of Divalent Coordination Polymers with Mixed Ligands: Evaluation for Photodegradation

Eight coordination polymers constructed from divalent metal salts, N,N′-bis(pyridin-3-ylmethyl)terephthalamide (L), and various dicarboxylic acids are reported, affording [Co(L)(5-ter-IPA)(H2O)2]n (5-tert-H2IPA = 5-tert-butylisophthalic acid), 1, {[Co(L)(5-NO2-IPA)]⋅2H2O}n (5-NO2-H2IPA = 5-nitroisophthalic acid), 2, {[Co(L)0.5(5-NH2-IPA)]⋅MeOH}n (5-NH2-H2IPA = 5-aminoisophthalic acid), 3, {[Co(L)(MBA)]⋅2H2O}n (H2MBA = diphenylmethane-4,4′–dicarboxylic acid), 4, {[Co(L)(SDA)]⋅H2O}n (H2SDA = 4,4-sulfonyldibenzoic acid), 5, {[Co2(L)2(1,4-NDC)2(H2O)2]⋅5H2O}n (1,4-H2NDC = naphthalene-1,4-dicarboxylic acid), 6, {[Cd(L)(1,4-NDC)(H2O)]⋅2H2O}n, 7, and {[Zn2(L)2(1,4-NDC)2]⋅2H2O}n, 8, which were structurally characterized by using single-crystal X-ray diffraction. The structural types of 1–8 are subject to the metal and ligand identities, showing a 2D layer with the hcb, a 3D framework with the pcu, a 2D layer with the sql, a polycatenation of 2-fold interpenetrated 2D layer with the sql, a 2-fold interpenetrated 2D layer with the 2,6L1, a 3D framework with the cds, a 2D layer with the 2,4L1, and a 2D layer with the (102⋅12)(10)2(4⋅10⋅124)(4) topologies, respectively. The investigation on the photodegradation of methylene blue (MB) by using complexes 1–3 reveals that the degradation efficiency may increase with increasing surface areas.


Introduction
Coordination polymers (CPs) have been intensively investigated by scientists in recent years because of their intriguing architectures and prospective applications in magnetism, luminescence, catalysis, gas storage, and sensing [1][2][3][4]. The coordination of spacer ligands to metal ions during the self-assembly process may result in the production of infinite one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) CPs, which are subject to the careful selection of metal ion and spacer ligands with diverse functionalities and flexibility. Despite the fact that many fascinating CPs have been reported, control of the structural variety remains a difficulty in the field of crystal engineering, and the factors that influence the structural diversity are less well understood [5,6].
The mixed ligand assembly technique has been employed to develop novel CPs [7]. In this context, mixed-ligand techniques including two distinct types of ligands with unique functions, such as polycarboxylate paired with a bis-pyridyl-bis-amide (bpba)-based N donor ligand, have been introduced as an effective way for adjusting structural diversity in CPs. Bpba ligands are remarkable ligands that may be modified to yield intriguing CPs [8], due to the fact that the majority of bpba ligands are flexible; however, others are semi-rigid.
Herein, we adopted the semi-rigid N,N -bis(pyridin-3-ylmethyl)terephthalamide (L), as shown in Figure 1, and differently substituted dicarboxylic acid as part of our ongoing research into understanding the relationship between the mixed show similar 2D cellular networks for the first two complexes, and the latter has a 4connected 3D structure with a semicircle 1D channel [9]. On the other hand, [Zn(L) ( 4 topologies and a 4-connected 3D framework and a 1D structure, respectively [10], whereas [Cd 3 (L) 2 (1,4-bdc) 3 ]·4H 2 O and [Cd(L)(1,4-bdc)]·2H 2 O are (3,5)-connected nets with the (3·7 2 )(3 2 ·4·7 5 ·8 2 ) topology [11], indicating that the types of the dicarboxylate play important role in determining the structural diversity.  2 (1,4-NDC) 2 ]·2H 2 O} n , 8, form the subject of this report. We observed that the roles of the dicarboxylate ligands and the metal atoms in the structural diversity of the CPs prepared thusly are significant. The governing factors of 1-3 in the degradation of methylene blue (MB) were also evaluated.

Synthesis
Complexes 1-8 were prepared by the hydro(solvo)thermal reactions of L with corresponding dicarboxylic acids and metal salts in different solvent systems at 100 • C for 48 h. Hydro(solvo)thermal synthesis enables a unique combination of pressure and temperature for crystallization of CPs. Characteristic FT-IR peaks for complexes 1-8 are N-H and C=O stretching which are from L. The range of N-H stretching is 3386-3483 cm −1 , probably coupled with the O-H stretching of the solvent molecule, while those around 1606-1653 cm -1 can be attributed to C=O stretching.

Crystal Structure of 1
The crystal structure of 1 conforms to the triclinic space group Pī and the asymmetric unit consists of one Co(II) cation, two halves of an L ligand, one 5-ter-IPA 2− ligand, and two coordinated water molecules. The Co(II) metal center is coordinated by two pyridyl nitrogen atoms of two L ligands [Co-N = 2.1346(12) − 2.1816(12) Å], two oxygen atoms from two 5-ter-IPA 2− ligands [Co-O = 2.0587(10) and 2.1362(10) Å], and two coordinated water molecules [Co-O = 2.0794(10) and 2.1411(10) Å], forming a distorted octahedral geometry, as in Figure 2a. Two Co(II) ions are bridged by two 5-ter-IPA 2− ligands to form dinuclear units, which are connected by L ligands to afford a 2D layer. Considering the Co(II) cations as 4-coordinated nodes, 5-ter-IPA 2 as 2-connected nodes, and L ligands as linkers, the structure of 1 can be regarded as a 2,2,4-connected net with the point symbol (12)(4·12 5 )(4) (standard representation), as in Figure 2b, determined using ToposPro [12]. Moreover, if the dinuclear units are considered as 3-coordinated nodes, the structure can be further simplified as a 3-connected net with the (6 3 )-hcb topology (cluster representation) [13], as in Figure 2c.

Crystal Structure of 2
The structure of 2 was solved in the triclinic space group Pī with one Co(II) cation, two halves of an L ligand, one 5-NO 2 -IPA 2− ligand and two co-crystallized water molecules in each asymmetric unit. The Co(II) metal center is coordinated by four oxygen atoms from three 5-NO 2 -IPA 2− ligands [Co-O = 2.007(2) -2.240(2) Å] and two pyridyl nitrogen atoms from two L ligands [Co-N = 2.149(3) -2.150(3) Å], resulting in a distorted octahedral geometry, as in Figure 3a. Two Co(II) ions are bridged by two 5-NO 2 -IPA 2− ligands to form dinuclear units, which are connected by L ligands to afford a 3D framework. Considering the Co(II) cations as 5-coordinated nodes, 5-NO 2 -IPA 2− as 3-coordinated nodes, and L ligands as linkers, the structure of 2 can be regarded as a 3,5-connected binodal 3D net with the point symbol of (4 2 ·6 5 ·8 3 )(4 2 ·6)-3,5T1 (standard representation), as in Figure 3b. Moreover, if the dinuclear units are considered as 6-coordinated nodes, the structure can be further simplified as a 6-connected net with the (4 12 ·6 3 )-pcu topology (cluster representation), as in Figure 3c.   Figure 4a. Two Co(II) ions are bridged by two 5-NH 2 -IPA 2− ligands to form dinuclear units, which are connected by L ligands to afford a 2D layer. Considering the Co(II) cations as 4connected nodes and 5-NH 2 -IPA 2− ligands as 3-connected nodes, with L ligands as linkers, the structure of 3 can be simplified as a 3,4-connected 2D net with the {4 2 ·6 3 ·8}{4 2 ·6}-bey topology (standard representation), as in Figure 4b. Moreover, if the dinuclear units are considered as 4-coordinated nodes, the structure can be further simplified as a 4-connected net with the (4 4 ·6 2 )-sql topology (cluster representation), as in Figure 4c.

Crystal Structure of 4
Single crystal X-ray diffraction of 4 conforms to the orthorhombic space group Ibca, and the asymmetric unit consists of half of a Co(II) ion, half of an L ligand, half of an MBA 2− ligand, and one co-crystallized water molecule. Figure 5a shows the coordination environment around the Co(II) metal center, which is six coordinated by two nitrogen atoms from two L ligands [Co-N = 2.083(3)] and four oxygen atom from two MBA 2− ligands [Co-O = 2.064(2)-2.263(3) Å], resulting in a distorted octahedral geometry. The Co(II) ions are interlinked by the L and MBA 2− ligands to give highly undulated 2D nets, as in Figure 5b. Topological analysis reveals that complex 4 forms 2-fold parallelly interpenetrated layers with the {4 4 ·6 2 }-sql topology, as in Figure 5c. In addition, layers of the 2-fold interpenetrated 2D layers polycatenated with other sql layers to form a final 2D → 3D entanglement, as in Figure 5d.

Crystal Structures of 5
In the space group Pī, the structure of complex 5 was solved. The asymmetric unit consists of one Co(II) ion, half of an L ligand, one SDA 2− ligand, and one co-crystallized water molecule. Figure 6a shows the coordination environment around the dinuclear Co(II) centers with a Co-Co distance of 2.8143 (5). Both Co(1) and Co(2) are 5-coordinated by one pyridyl nitrogen atom of the L ligand [Co-N = 2.0564(17) Å] and four oxygen atoms of four SDA 2− ligands [Co-O = 2.0211(17) Å-2.0515(17) Å], resulting in a distorted square pyramidal geometry. Two Co(II) ions are bridged by four carboxylate groups of the SDA 2− ligands to form dinuclear paddlewheel units, which are further linked by the L ligands to form a 2D layer. If the dinuclear units are considered as 6-connected nodes, the SDA 2− ligands as 2-connected nodes, and the L ligands as linkers, the structure of 5 can be simplified as a 2D net with the (4 2 ·6 8 ·8·10 4 )(4) 2 -2,6L1 topology, as in Figure 6b, which shows a 2-fold interpenetration, as in Figure 6c.

Crystal Structures of 6
Crystals of 6 conform to the monoclinic space group P2 1 /c with each asymmetric unit consisting of one and two halves of a Co(II) cation, two L ligands, two 1,4-NDC 2− ligands, two coordinated water molecules, and five co-crystallized water molecules. Figure 7a shows the coordination environment of the Co(II) metal centers, which are all 6-coordinated. The Co (1)

Crystal Structures of 7
Single-crystal X-ray diffraction analysis shows that 7 crystallizes in the monoclinic space group P2 1 /c. The asymmetric unit is comprised of one Cd(II) cation, one L ligand, one 1,4-NDC 2− ligand, one coordinated water, and two lattice water molecules.

Crystal Structures of 8
The crystals of complex 8 conform to the triclinic space group Pī with two Zn(II) ions, two L ligands, two 1,4-NDC 2− ligands, and two lattice water molecules in the asymmetric unit. Figure 9a shows the coordination environment of the Zn(II) centers. Both of the Zn(1) and Zn (2)

Ligand Conformations and Coordination Modes
The L ligands in complexes 1-8 display various conformations which can be defined as follows: (A) the cis and trans conformations can be given if the two C=O groups are in the same and the opposite direction, respectively; (B) due to the different orientations adopted by the pyridyl nitrogen atoms and the amide oxygen atoms, three more conformations, namely syn-syn, syn-anti, and anti-anti, can also be found for bpba [8]. Table 1 lists the ligand conformations and coordination modes of the organic ligands in complexes 1-8. The L ligands in 1-8 bridge two metal ions through two pyridyl nitrogen atoms, adopting five different conformations including trans anti-anti, cis syn-syn, trans syn-syn, trans syn-anti and cis anti-anti. On the other hand, the dicarboxylate ligands in 1-8 bridge two to four metal ions with various coordination modes.   The metal effect on the structural diversity is shown in 6-8 by changing the metal atom from Co, Cd, to Zn, giving cds, 2,4L1, and (10 2 ·12)(10) 2 (4·10·12 4 )(4) topologies, respectively.

Photodegradation
The governing role of CPs in the photodegradation of organic pollutants has been a subject of current interest [14][15][16][17][18][19]. Complexes 1-3, which differ in the fifth position of the phenyl ring of the dicarboxylate ligands, i.e., the tert-butyl, NO 2 and NH 2 groups, respectively, thus, provide a unique opportunity to compare the substituent effect on the photodegradation. Methylene blue (MB, C 16  The intensity of the peculiar absorption band at 663 nm was utilized to precisely monitor the degradation process of MB. Figure 10 illustrates the variations in the A t /A 0 of MB solutions vs irradiation time for complexes 1-3, showing that the absorption intensities of MB reduced gradually with increasing reaction time, where A 0 is the initial absorbance of the MB solution and A t is the absorbance of the solution after illumination at time t. Degradation efficiency (DE) of MB was calculated by using DE % = [(A 0 − A t )/A 0 ] × 100. Additionally, the DE % with the mean values and standard deviations were evaluated, Tables S1-S8. After 120 min, the DE of MB for the various strategies are as follows: 3% (blank), 53.46% (MB + H 2 O 2 ), 6% (MB + complex 1), 10.42% (MB + complex 2), 16 complex 3), demonstrating that the DE % of MB by the complexes participated with H 2 O 2 follows the pattern of 1 < 2 < 3. Moreover, the Brunauer-Emmett-Teller (BET) surface areas obtained from the N 2 adsorption experiments were 4.96, 6.12, and 9.95 m 2 /g for 1-3, respectively, as in Figures S9-S11. The PXRD patterns of complexes 1-3 succeeding photodegradation processes were examined. No noticeable alterations were found for 1 and 2, whereas significant change has been observed for 3, as illustrated in Figures S12-S14. The structural change in 3 may enhance the photodegradation efficiency. Structural modification was also observed for 3 after the N 2 adsorption and desorption, as in Figures S15-S17, indicating that complex 3 was not stable during the experiments. Although the role of the tert-butyl, NO 2 , and NH 2 groups in determining the DE is complicated, the different BET surface areas of the original 1-3 resulting from the different substituent groups can be influential. The hydroxyl radical (OH·) has been considered as the major oxidant which decomposes the organic dye with a good efficiency [15]. High surface area reflects a higher adsorption quantity of H 2 O 2 that led to the formation of (OH·) and, thus, implies more MB can be degraded. For comparisons it is noted that the CPs {[Zn(L2)(AIPA)]·2H 2 O} n (L2 = N,N -bis(3-pyridinyl)terephthalamide; H 2 AIPA = 5-acetamidoisophthalic acid) and {[Zn(L3)(AIPA)]·2H 2 O} n (L3 = N,N -di(3pyridyl)adipoamide), which adopted self-catenated 3D frameworks with the (4 24 ·6 4 )-8T2 and the (4 4 ·6 10 ·8)-mab topologies, respectively, promoted the MB degradation, and the DE were 81.56 and 85.46%, respectively [20]. On the other hand, the four topologically identical CPs having the 2-fold interpenetrating 3D net with the mog topology,  4 ]·3H 2 O} n (M = Co and Ni; L6 = bis(N-pyrid-3-ylmethyl) suberoamide) also display good photodegradation performance toward MB, and the Co(II) CPs display better catalytic ability than the Ni(II) ones [21].

General Procedures
Elemental analyses involving C, H, and N atoms were performed on a PE 2400 series II CHNS/O (PerkinElmer instruments, Shelton, CT, USA) or an Elementar Vario EL-III analyzer (Elementar Analysensysteme GmbH, Hanau, Germany). Infrared spectra were obtained from a JASCO FT/IR-460 plus spectrometer with pressed KBr pellets (JASCO, Easton, MD, USA). Powder X-ray diffraction patterns were carried out with a Bruker D8-Focus Bragg-Brentano X-ray powder diffractometer equipped with a CuKα (λ α = 1.54178 Å) sealed tube (Bruker Corporation, Karlsruhe, Germany).

Powder X-ray Analysis and IR Spectra
In order to check the phase purity of the product, powder X-ray diffraction (PXRD) experiments were carried out for complexes 1-8. As shown in Figures S18-S25, the peak positions of the experimental and simulated PXRD patterns were in a good agreement with each other, indicating their bulk purities. The IR spectra of complexes 1-8 are provided in the Supplementary Materials as Figure S26.

Procedures for Photodegradation
The experiments were carried out in a homemade photodegradation box ( Figure S27). For the experiments, test tube 1 (blank), tube 2 (0.1 mL H 2 O 2 ), tube 3 (10 mg complex), and tube 4 (10 mg complex + 0.1 mL H 2 O 2 ) were prepared. A total of 10 mL of a 10 ppm MB solution was added to each tube, which was prepared by diluting 10 mg MB with deionized water in a 1000 mL quantitative bottle. Each tube was then irradiated with the 365 nm UV light for 20, 40, 60, 80, 100, and 120 min, respectively, and then their absorption spectra were measured. Tube 3 and tube 4 were first stirred in the dark for 15 min to confirm the physical adsorption of the complex.

X-ray Crystallography
Single-crystal X-ray diffraction data for complexes 1-8 were collected on a Bruker AXS SMART APEX II CCD diffractometer with graphite-monochromated MoKα (λ α = 0.71073 Å) radiation at 296 K [23]. Data reduction and absorption correction were performed by using standard methods with well-established computational procedures. Some of the heavier atoms were located by the direct or Patterson method, and the remaining atoms were found in a series of Fourier maps and least-squares refinements, while the hydrogen atoms were added by using the HADD command in SHELXTL [24]. Table 2 lists the basic information pertaining to crystal parameters and structure refinement. CCDC no. 2238099-2238106 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk; or at: http://www.ccdc.cam.ac.uk.

Conclusions
Eight divalent CPs constructed from L and various dicarboxylic acids have been successfully accomplished. The changes in the substituted group at the fifth position of the phenyl rings of the dicarboxylic acids from tert-butyl and NO 2 to the NH 2 group drastically alters the structural types, affording the simplified structures with the hcb, pcu, and sql topologies for complexes 1-3, respectively. The use of the angular dicarboxylic acids, such as H 2 MBA and H 2 SDA, gave entangled CPs 4 and 5, showing a polycatenation of a 2-fold interpenetrated 2D layer with the sql and a 2-fold interpenetrated 2D layer with the 2,6L1 topologies, whereas the metal effect on the structural diversity can be shown in complexes 6-8 by changing the metal atom from Co, Cd to Zn, affording a 3D framework with the cds, a 2D layer with the 2,4L1, and a 2D layer with the (10 2 ·12)(10) 2 (4·10·12 4 )(4) topologies, respectively. The structural diversity of the semi-rigid L-based divalent CPs is, thus, subject to the identities of the metal atom and the dicarboxylic acid. The degradation efficiency toward MB that follows 1 < 2 < 3 can be ascribed to their increasing surface areas, resulting from the different substituent groups of tert-butyl, NO 2 , and NH 2 at the fifth position of the phenyl ring of the respective dicarboxylate ligands.