Mg(II) Coordination Polymers Based on Flexible Isomeric Tetracarboxylate Ligands: Syntheses, Structures, Structural Transformation and Luminescent Properties

By using a new flexible tetracarboxylic acid, bis(3,5-dicarboxyphenyl) adipoamide, H4L1, and its isomer, bis(2,5-dicarboxyphenyl)adipoamide, H4L2, three Mg(II) coordination polymers, {[Mg2(L1)(H2O)2]·2EtOH·3H2O}n, 1, [Mg2(L1)(H2O)8]n, 2, and {[Mg2(L2)(H2O)6]·H2O}n, 3, have been hydrothermally synthesized and structurally characterized by single-crystal X-ray diffraction. Complexes 1 and 2 are the solvent ratio-dependent hydrothermally stable products. The tetracarboxylate ligand of complex 1 connects eight Mg(II) ions through nine oxygen atoms, resulting in a three-dimensional (3D) 5-connected uninodal net with a rare non-interpenetrating (44.66)-pcu-5-Pmna topology, whereas those of 2 and 3 link four Mg(II) ions through four oxygen atoms and six Mg(II) ions through six oxygen atoms, forming a 1D linear chain and a 3,6-connected 2-nodal 3D net having {4.62}2{42.610.83}-rtl topology, respectively. Complex 1 shows a series of structural transformations on heating to 200 °C and almost reversible structural transformation when the activated products were immersed in a mixture of ethanol and water or on hydrothermal. Likewise, complex 2 exhibits a reversible structural transformation on heating/hydrothermal, while 3 exhibits irreversible structural transformations. All three complexes exhibit blue light emissions, with that of complex 3 being much more intense.


Introduction
The investigation of coordination polymers (CPs) continues to be an intense area of research activity due to their intriguing structural topologies and promising potential applications as multifunctional materials [1][2][3][4][5][6][7][8]. The structural types of the resulting CPs are influenced by various factors, such as organic ligands, metal ions, counterions, metal-to-ligand ratio, solvent system, and the temperature and pH values of the solution, even though the control of the desired structural dimensionality of CPs remains a challenge [7,8]. Generally, the structural transformations of CPs can be induced by various methods, including external stimuli like heat, light and mechanical forces, removal and uptake of solvents, exposure to reactive vapors, and exchange of solvents and guest molecules, which are rare due to the breaking and forming of coordinate and/or covalent bonds in multiple directions [9,10]. The applications of such phenomena observed in CPs are significant for sensors and switches.
The construction of CPs from s-block metal centers has been explored to a lesser extent than in transition metals (TMs), and their coordination ability is inferior compared with that of TMs; furthermore, they are lighter (lower atomic weight) and bigger in size, and are filled shell orbitals in ionic form, which makes them difficult incorporate into CPs [11][12][13]. Among the alkali-earth metals, Herein, we report three Mg(II) CPs, {[Mg 2 (L 1 )(H 2 O) 2 ]·2EtOH·3H 2 O} n , 1; [Mg 2 (L 1 )(H 2 O) 8 ] n , 2; and {[Mg 2 (L 2 )(H 2 O) 6 ]·H 2 O} n , 3, having the different dimensional architectures, in which complexes 1 and 2 are the solvent ratio-dependent products. Additionally, the tetracarboxylate ligands of 1-3 connect eight, four and six Mg(II) ions, respectively. Moreover, all these complexes display structural transformations on heating, and Complex 3 shows a much more intense blue photoluminescence than the other two complexes. Scheme 2 presents the overall sketch for complexes 1-3.

General Procedures
Solvents were dried and deoxygenated by refluxing over the appropriate reagents before use. Elemental analyses were obtained from a PE 2400 series II CHNS/O analyzer (PerkinElmer, Boston, MA, USA) or an Elementar Vario EL cube analyzer (Elementar Americas Inc., Ronkonkoma, NJ, USA). 1 H-NMR and TOF-MS (ESI-MS) were recorded on a Bruker 400 MHz NMR and a Bruker MicrOTOF II (Bruker, Billerica, MA, USA), respectively. Powder X-ray diffraction patterns were obtained from a Bruker D2 PHASER diffractometer (Bruker, Billerica, MA, USA) with CuK α (λ α = 1.54 Å) radiation. IR spectra (KBr disk) were recorded on a Jasco FT/IR-460 plus spectrometer (JASCO, 28600 Mary's Court City, MD, USA). The UV-Vis absorption spectra and emission spectra were obtained in the solid state at room temperature by using an SP-1901 UV-Vis spectrophotometer (Shimadzu, Kyoto, Japan) and a Hitachi F-4500 spectrometer (Hitachi, Tokyo, Japan), respectively. Thermal gravimetric analysis (TGA) was carried out on a SII EXSTAR6000 TG/DTA 6200 (Seiko Instruments Inc., Tokyo, Japan) under N 2 atmosphere at a heating rate of 10 • C min −1 in the temperature range of 30 to 1000 • C.

Preparations
Adipoyl chloride (1.00 g, 5.47 mmol) in 10 mL DMF was added slowly to a 10 mL DMF solution of 5-amino-isophthalic acid (1.98 g, 10.94 mmol) kept in the reflux flask facilitated with the magnetic stirrer and the mixture was stirred for 30 min. Then, triethylamine (1.11 g, 10.94 mmol) in 10 mL DMF was carefully added dropwise to the stirring solution mixture which was refluxed for one day. After cooling, the mixture was poured in 300 mL water to induce the precipitation. Finally, the solution was filtered, and the residue was washed with 300 mL distilled water to obtain a white powder. Yield: 1.40 g (54%). Anal Calcd for C 22 Figure S1. 13  Typically, only one kind of solvent is used in the reflux process during the synthesis of the ligand, but here, two different solvents (Pyridine and DMF) were used to synthesize a modified version of H 4 L 2 . 10 mL pyridine solution of triethylamine (1.11 g, 10.94 mmol) was added slowly to the 15 mL pyridine solution of 2-amino-isophthalic acid (1.98 g, 10.94 mmol) kept in the reflux flask facilitated with the magnetic stirrer, and the mixture was stirred for 30 min. Adipoyl chloride (1.00 g, 5.47 mmol) in 15 mL DMF was carefully added dropwise to the stirring solution mixture, which was refluxed for one day. After cooling, the mixture was poured into 300 mL ether to induce the precipitation, and the solution was stirred for some hours. Finally, the solution was filtered, and the residue was washed with 3 × 300 mL ether to obtain the white powder. Yield: 2.51 g (97%). Anal Calcd for C 22 11.06 (s, 2H; NH), Figure S4. 13

[Mg 2 (L 1 )(H 2 O) 8 ] n , 2
The synthetic procedure for 2 is similar to that for 1, except that the solvent system adopted is 4:1, 10 mL, instead of 5:1, 6 mL. Two types of crystals were obtained: block crystals 1 and light yellowish plate crystals 2, which were manually separated under a microscope. The crystals were collected individually, washed with ethanol, and then dried under vacuum. Yield of 1: 0.031 g, (51%). Yield of 2: 0.024 g (36%). Anal Calcd for C 22  The synthetic procedure for 3 is similar to that for 2, except that H 4 L 2 (0.047 g, 0.10 mmol) is used instead of H 4 L 1 , without passing N 2 gas to the solution. Heating temperature (120 • C) was changed to 140 • C for 48 h and the oven was cooled naturally. Colorless block crystals of 3 were collected, washed with ethanol, and then dried under vacuum. Yield: 0.055 g (86%). Anal Calcd for C 22

X-ray Crystallography
The diffraction data for complexes of 1-3 were collected on a Bruker AXS SMART APEX II diffractometer at 296 K, which was equipped with a graphite-monochromated MoKα (Kα = 0.71073 Å) radiation. Data collection and reduction were performed by standard methods with use of well-established computational procedures [32,33]. Positions of some of the heavier atoms were located by the direct or Patterson method and the remaining atoms were found in a series of alternating difference Fourier maps and least-square refinements [34]. Basic information pertaining to crystal parameters and structure refinement is summarized in Table 1. Selected bond distances and angles are listed in Table S1.

Synthesis
The crystals of Complex 1 were obtained from the solvent system of 5 mL ethanol and 1 mL water (ratio of EtOH:H 2 O = 5:1, total volume 6 mL), whereas those of both 2 and 3 were obtained from 8 mL ethanol and 2 mL water (ratio of EtOH:H 2 O = 4:1, total volume 10 mL), keeping all other experimental condition the same in both cases. It is interesting to note that the subtle differences in ratio and volume of the solvent system are significant for the formation of the single crystals of 1 and 2. Further, we have attempted to investigate the possibility of transformation between 1 to 2 with their mutual exchange of solvent system. However, no transformation between 1 and 2 can be observed, indicating that both 1 and 2 are hydrothermally stable products.

Crystal Structure of 1
Complex 1 crystallizes in the orthorhombic space group Pna2 1 , and its asymmetric unit contains two Mg(II) ions, one (L 1 ) 4− ligand, two bonded water, two lattice ethanol and three lattice water. Figure 1a depicts the coordination environment of the two Mg(II) metal centers. It can be shown that the Mg (1)  Notably, each tetracarboxylate ligand connects eight Mg(II) ions through nine oxygen atoms to form a 3D net. Topologically, both the Mg 2 and (L 1 ) 4− can be regarded as 5-connected nodes and afford a uninodal 5-connected net with the non-interpenetrating (4 4 .6 6 )-pcu-5-Pmna topology, Figure 1c, determined using ToposPro [35]. While several 3-fold interpenetrated CPs that show pcu-5-Pmna topology can be found, only two with non-interpenetrating modes have been reported, as shown in the TTD database of ToposPro. A PLATON calculation [36] indicates 30.5% accessible free voids for 1 after the removal of guest molecules.

Crystal Structure of 2
Single-crystal X-ray diffraction analysis reveals that Complex 2 crystallizes in the monoclinic space group P2 1 /n and its asymmetric unit contains one Mg(II) ion, half (L 1 ) 4− ligand and four bonded water molecules. Figure 2a shows the arrangement around the Mg(II) metal center, which is six-coordinated by two oxygen atoms of the carboxylate groups of the different ligands and four oxygen atoms of the water molecules, resulting in a distorted octahedral geometry. The equatorial plane is formed by the coordination of O(1), O(4A), O(8) and O(9) atoms, where the axial positions are occupied by O(6) and O(7) atoms. In Complex 2, each of the four carboxylate groups of the (L 1 ) 4− ligand coordinates to one Mg(II) ion through one of the two oxygen atoms, which is repeated in the linear fashion to form a 1D chain, Figure 2b. Another notable feature is that the 1D linear planar structures of 2 interact with each other through the π-π interaction (3.68 Å), forming a 2D supramolecular structure, Figure 2c

Crystal Structure of 3
Single-crystal X-ray diffraction studies reveal that 3 crystallizes in the monoclinic space group P2 1 /c, featuring a 3D framework. Its asymmetric unit contains one Mg(II) ion, half (L 2 ) 4− ligand, three coordinated H 2 O molecules and one crystallized water molecule. As shown in Figure 3a

Coordination Modes and Ligand Conformation
From Figure 4, it is clear that the tetracarboxylate ligands (L 1 ) 4− and (L 2 ) 4− exhibit various coordination modes with Mg(II) metal centers. The (L 1 ) 4− ligands show drastically different bonding modes in 1 and 2. More interestingly, the (L 1 ) 4− ligand in 1 bridges eight Mg(II) ions through nine oxygen atoms, which is participated in by four different coordination modes from carboxylate groups and carbonyl oxygen atoms, including two chelating/bridging (carboxylate group), one bridging (carboxylate group), one chelating (carboxylate group) and one monodentate coordination (carbonyl oxygen atom). Such a bonding mode is unique for the tetracarboxylate ligands as linkers in the CPs, presumably due to the existence of the amide carbonyl oxygen atoms in the (L 1 ) 4  The A and G conformations are symbolized when the C-C-C-C torsion angle (θ) is 180 ≥ θ > 90 • and 0 ≤ θ ≤ 90 • , respectively. Based on this descriptor [7], the ligands (L 1 ) 4− and (L 2 ) 4− can be arranged in anti-anti-anti (AAA), anti-anti-gauche (AAG), anti-gauche-anti (AGA), anti-gauche-gauche (AGG), gauche-anti-gauche (GAG) and gauche-gauche-gauche (GGG) conformations. Accordingly, the (L 1 ) 4− ligand of 1 can be assigned as AAG while both (L 1 ) 4− of 2 and (L 2 ) 4− of 3 as AAA. Likewise, each conformation can adopt cis or trans arrangements based on the relative orientation of the C=O (or N-H) groups. Accordingly, both ligands in 1-3 can be assigned as trans. Hence, the conformations of both ligands, as a whole, can be assigned as AAG trans conformation in 1, while AAA trans conformation in 2 and 3. These ligand conformations also differ in the dihedral angle between the two phenyl rings, i.e., the two rings are coplanar or twisted about the C-N bonds. The dihedral angles between the two phenyl rings of (L 1 ) 4− ligands in 1 is 34.7 • while for the case of 2 and 3, the pair of phenyl rings of the ligands is coplanar, i.e., 0 • .

PXRD Patterns and Thermogravimetric Analysis
To identify the bulk purities of 1-3, the experimental pattern of powder X-ray diffraction (PXRD) is compared with the simulated pattern obtained from the single-crystal X-ray data. As shown in Figures S8-S10, the experimental patterns match quite well with their corresponding simulated ones, which demonstrates that complexes 1-3 show bulk purities. Figure S11 represents the TGA curves of Complexes 1-3. From the TGA curve of 1, it is clear that the first wt. loss in 25-92 • C is due to the removal of lattice ethanol (obsd. 12.5%, calcd. 13.17%), followed by the 2nd wt. loss of lattice water (obsd. 7.68%, calcd. 7.73%) at 93-155 • C. In the temperature range from 155 to 413 • C, the bonded water molecules were completely lost; and finally, the (L 1 ) 4− ligand started to decompose at temperatures above 413 • C. The TGA curve of 2 shows a weight reduction of 21.33% (calcd. 21.8%) in the temperature range from 25 to 190 • C, corresponding to the release of bonded water and, after that, the residual framework maintains a long stable period up to 420 • C, above which the entire skeleton of 2 deteriorates. Compound 3 exhibits a slight weight reduction of 3.23% (calcd. 2.79%) between 25 and 74 • C due to the loss of lattice water, followed by the 2nd weight loss of bonded water of 17.05% (calcd. 16.79%) between 75 and 187 • C; and above 378 • C, the complex started to decay. Comparatively, the dehydrated phase of 2 is more stable than the others up to approximately 420 • C.

Structural Transformation
After verification of the phase purity of Complex 1, we heated the powder of 1 for 4 h under vacuum at various temperatures from 50 to 250 • C to observe the thermal stability, Figure 5a. Firstly, the powder of 1 was activated by heating at 50 • C and then 80 • C. The powder patterns at both temperatures were found to be similar, but were different from that of the original synthesized 1, indicating the first-step structural transformation from 1 to 1a. Similarly, powder 1a was further heated successively at 95, 115, 155 and then 170 • C. The PXRD patterns at these temperatures were about the same, but were typically different from those of 1a or 1, representing the second-step structural transformation from 1a to 1b. Furthermore, third-step structural transformation from 1b to 1c was also noticed when the powder of 1b was heated successively at 180 • C and then 200 • C. When powder 1c was further activated at 215, 230 and 250 • C, we observed only one peak shifted slightly toward the left, which may represent the collapse of the framework. To monitor whether the activated samples can be reversed to the original one or not, the activated sample 1b (at 155 • C) was immersed in ethanol or the mixture of ethanol and water for 5 days, Figure 5b. The powder patterns obtained from the both solvent systems look nearly identical to the original synthesized 1, which indicate the reversible structural transformation from 1b to 1. Comparatively, the PXRD pattern obtained from the mixed solvent system was slightly better than that obtained from ethanol only. Likewise, the powder pattern obtained after immersing 1c (at 200 • C) in the same solvent mixture for 5 days seemed nearly identical to the synthesized 1, indicating the possibility of a reversible structural transformation from 1c to 1. Likewise, activated 1c can also be transformed into original 1 under hydrothermal conditions by employing the same solvent ratio as for 1. Overall, from the variable temperature powder patterns of 1, it can be shown that the structure of Complex 1 shows a series of stepwise structural transformations from 1 to 1a to 1b and to 1c on heating, and nearly reversible structural transformation from 1b and 1c to 1 on immersing the activated samples in solvent mixtures or by the hydrothermal process.

Absorption and Luminescence Properties
Not only the d 10 metal centers, but also the Mg(II) CPs have the potential ability to quench, shift and enhance the emission wavelength of organic ligands through the coordination of metal centers, and therefore, they may also be regarded as promising candidates for prospective luminescent applications as light-emitting diodes (LEDs), probes and sensors [37][38][39][40][41]. Figure S13 depicts the solid-state UV/vis spectra of 1-3 and ligands (H 4 L 1 and H 4 L 2 ), which display sharp absorption bands ranging from 200 to 400 nm. Likewise, the solid-state emission spectra for all complexes and ligands (using equimolar quantities) were measured at room temperature under identical experimental conditions, as shown in Table 2 and Figure 7a. The free ligands H 4 L 1 and H 4 L 2 exhibit emission peaks at 365 nm (with a shoulder peak at 408 nm) and 391 nm upon excitation at 278 nm and 284 nm, respectively, in the solid state, while emissions at 353 nm (in EtOH) and 351 nm (in DMF) for H 4 L 1 and 389 nm (in EtOH) and 388 nm (in DMF) are observed for H 4 L 2 , Figures S14 and S15. Since the emission wavelengths of both ligands in solution and solid state are nearly similar, the effect of intermolecular interactions is negligible, and they may be tentatively ascribed to the intraligand n→π* or π→π* transitions [41]. In addition, 1 and 2 show broad emission bands at 384 nm and 399 nm upon excitations at 287 nm and 283 nm, respectively, while 3 displays an emission maximum at 423 nm upon excitation at 289 nm. By comparing the emission spectra of 1-3 and ligands, all of the complexes exhibit red shifts (19 nm for 1, 34 nm for 2 and 32 nm for 3) with respect to their corresponding free ligands, which may be attributed to the increase of the ligand conjugation degree after coordination with Mg 2+ ions [42,43]. Since Mg(II) ions, having 1s 2 2s 2 2p 6 electronic configuration, are difficult to oxidize or reduce in nature, the emission is most probably due to the ligand-centered n→π* or π→π* transition. Furthermore, the images of powders of 1-3 under UV lamp at 365 nm, Figure 7b, indicate that Complex 3 emits the more intense blue light than 1 and 2. Based on this observation, the (L 2 ) 4− in 3 can be assumed to be more rigid [22] than (L 1 ) 4− in 1 and 2, which can also be ascribed to the different ligand isomerism, although both of them have the same -(CH 2 ) 4 -spacer.

Conclusions
We have successfully synthesized a new tetracarboxylic acid H 4 L 1 , and its isomer H 4 L 2 has also been prepared in a modified version. The reactions of H 4 L 1 with Mg(II) salt under hydrothermal conditions gave 1 and 2, which possess a 3D net with a rare non-interpenetrating (4 4 .6 6 )-pcu-5-Pmna topology and a 1D chain, respectively. Here, the ratio and volume of the solvent system play vital roles in the formation of the crystals of 1 and 2. On the other hand, the solvothermal treatment of H 4 L 2 with Mg(II) salt produces Complex 3, which has a rutile-type 3D framework. Complexes 1-3 can be regarded as unique examples of Mg(II) CPs from flexible isomeric tetracarboxylate ligands. The different structures of 1-3 are presumably due to the structure-directing effect of isomeric ligands and the different experimental conditions. Notably, the tetracarboxylate ligand in 1 connects eight Mg(II) ions through nine oxygen atoms, whereas those of 2 and 3 link four Mg(II) ions through four oxygen atoms and six Mg(II) ions through six oxygen atoms, respectively. In addition, Complex 1 displays a stepwise structural transformation on heating, and the activated product can be reversibly transformed to the original 1 on hydrothermal or immersion in the solvent system. While such reversible structural transformations were also observed in 2, Complex 3 exhibited an irreversible structural transformation. The reversible and irreversible transformations in 1-3 can most probably be ascribed to the different donor positions between the isomeric (L 1 ) 4− and (L 2 ) 4− ligands. Furthermore, all three complexes exhibited visible blue light emissions under UV light, with that of Complex 3 being found much more intense.