Synthesis, Crystal Structures and Luminescence Properties of Three New Cadmium 3D Coordination Polymers

The new rigid planar ligand 2,5-bis(3-(pyridine-4-yl)phenyl)thiazolo[5,4-d]thiazole (BPPT) has been synthesized, which is an excellent building block for assembling coordination polymer. Under solvothermal reaction conditions, cadmium ion with BPPT in the presence of various carboxylic acids including (1,1′-biphenyl)-4,4′-dicarboxylic acid (BPDC), isophthalic acid (IP), and benzene-1,3,5-tricarboxylic acid (BTC) gave rise to three coordination complexes, viz, [Cd(BPPT)(BPDA)](BPPT)n (1), [Cd(BPPT) (IP)] (CH3OH) (2), and [Cd3(BPPT)3(BTC)2(H2O)2] (3). The structures of 1, 2, and 3 were characterized by single crystal X-ray diffraction. The IR spectra as well as thermogravimetric and luminescence properties were also investigated. Complex 1 is a two-dimensional (2D) network and further stretched to a 3D supramolecular structure through π–π stacking interaction. The complexes 2 and 3 show 3D framework. The complexes 1, 2, and 3 exhibited luminescence property at room temperature.


Introduction
In the last twenty years, the design and synthesis of new coordination polymers (CPs) have become an important research area for their intriguing structures and interesting properties, such as catalysis [1][2][3][4][5][6], gas storage and separation [7][8][9], photoluminescence [10][11][12], biomedical uses [13], and other applications [14][15][16]. In order to make the CPs have special structures and properties, it is necessary to synthesize new ligands [17], in which the heteroaromatic thiazolothiazole unit can be employed as a building block incorporated into organic ligands; the strong π-π stacking and overlapping of the orbitals in thiazolothiazole unit can afford high electron and hole mobilities, which are crucial properties for efficient charge transfer in optoelectronic materials [18]; and the heteroaromatic thiazolothiazole unit has also shown high luminescence properties [19]. On the other hand, the luminescence properties of coordination polymers with d 10 metal centres Cd(II) have attracted intense interest because of the potential applications of these compolexes as luminescent sensing materials [20]. Therefore, the design and synthesis of diverse structural Cd-MOFs or Cd-CPs with optical properties are highly demanded [21,22].

The Structure of Complex 2
The X-ray structural analysis reveals that complex 2 crystallizes in the monoclinic space group P21/c, which is a three-dimensional (3D) coordination polymers constructed from BPPT and IP ligands (Figure 2a). The structural unit is made up of one Cd (II) atom, one BPPT ligand, and one IP ligand. Cd (II) ions are bridged by IP ligands to form a 2D wavelike network along the a axis ( Figure  2b). It is noteworthy that the Cd (II) is a seven coordinated ion (Figure 2c), in which five oxygen atoms from IP ligand are in equatorial plane to form a two-dimensional structure (Figure 2b). The Cd-O bond distances vary from 2.366 (16)

The Structure of Complex 2
The X-ray structural analysis reveals that complex 2 crystallizes in the monoclinic space group P21/c, which is a three-dimensional (3D) coordination polymers constructed from BPPT and IP ligands (Figure 2a). The structural unit is made up of one Cd (II) atom, one BPPT ligand, and one IP ligand. Cd (II) ions are bridged by IP ligands to form a 2D wavelike network along the a axis ( Figure 2b). It is noteworthy that the Cd (II) is a seven coordinated ion (Figure 2c), in which five oxygen atoms from IP ligand are in equatorial plane to form a two-dimensional structure (Figure 2b). The Cd-O bond distances vary from 2.366(16) to 2.393(17) Å. The O-Cd-O bond angles are 53.3(5) • -166.5(6) • , respectively. Two pyridyl nitrogen atoms coordinate with Cd (II) in the axial direction to form a three-dimensional structure with a Cd-N bond distance of 2.317(18) Å. The N-Cd-N bond angle is 176.5(7) • . That is to say, the Cd(II) ions are assembled via bridging carboxylate oxygen atoms in a 2D plane, and these planes are further interconnected by the BPPT ligand, giving rise to a 3D framework. The closest distance between two parallel π-stacked DPPZ ligands was around 3.55 Å, that is, within the range of π−π interaction. Topological analysis through the olex, the IP ligands, and BPPT ligands are considered as linkers, then the 3D structure can be classified as a 5-connected network with (4.8 4 )(4.6 4 .8 4 .10) topology (Figure 2d). Limiting indices

The Structure of Complex 3
According to the single crystal-XRD analysis, complex 3 also crystallized in the monoclinic crystal system with space group of P21/c and its asymmetric unit contains three Cd(II) ions, three BPPT ligands, two BTC ligands, and two water molecules. Complex 3 has a 3D structure (Figure 3a). The binding of Cd(II) ions to the organic BTC ligand generates the 2D layer coordination structure, one carboxylic acids group in the BTC coordinates to Cd (1) ion, and the other two carboxylic groups in the BTC coordinates to Cd (2) (Figure 3b). The 2D layer interconnected by the BPPT ligand gives rise to a 3D framework and the closest distance between the two parallel π-stacked DPTTZ ligands was around 3.57 Å. As shown in Figure 3c, the metal atom Cd (1) is six-coordinate and in an octahedral geometry coordination environment with two oxygen atoms (O3, O8) from BTC ligands, two water molecules, and two nitrogen atoms (N3, N3) from BPPT. The Cd (1)-O3 bond length is 2.  (17) • . The BTC ligands and BPPT ligands are considered as linkers, thus the 3D structure can be classified as a 4, 5-connected network with (4 2 .6.8 4 .9)(8.9 3 .10 2 )(4 2 .6 7 .8) topology ( Figure 3d).
Molecules 2020, 25, x FOR PEER REVIEW 3 of 11 dimensional structure with a Cd-N bond distance of 2.317(18) Å. The N-Cd-N bond angle is 176.5(7)°. That is to say, the Cd(II) ions are assembled via bridging carboxylate oxygen atoms in a 2D plane, and these planes are further interconnected by the BPPT ligand, giving rise to a 3D framework. The closest distance between two parallel π-stacked DPPZ ligands was around 3.55 Å, that is, within the range of π−π interaction. Topological analysis through the olex, the IP ligands, and BPPT ligands are considered as linkers, then the 3D structure can be classified as a 5-connected network with (4.8 4 )(4.6 4 .8 4 .10) topology ( Figure 2d).

The Structure of Complex 3
According to the single crystal-XRD analysis, complex 3 also crystallized in the monoclinic crystal system with space group of P21/c and its asymmetric unit contains three Cd(II) ions, three BPPT ligands, two BTC ligands, and two water molecules. Complex 3 has a 3D structure (Figure 3a). The binding of Cd(II) ions to the organic BTC ligand generates the 2D layer coordination structure, one carboxylic acids group in the BTC coordinates to Cd (1) ion, and the other two carboxylic groups in the BTC coordinates to Cd (2) (Figure 3b). The 2D layer interconnected by the BPPT ligand gives rise to a 3D framework and the closest distance between the two parallel π-stacked DPTTZ ligands was around 3.57 Å. As shown in Figure 3c, the metal atom Cd (1) is six-coordinate and in an octahedral geometry coordination environment with two oxygen atoms (O3, O8) from BTC ligands, two water molecules, and two nitrogen atoms (N3, N3) from BPPT. The Cd (1)

TGA
TGA was conducted to investigate the thermal stability of complexes 1, 2, and 3. As shown in Figure 4, the TG curve of 1 displayed no obvious weight loss before 380 °C, the weight loss of 49.1% can be distributed to the removal of BPPT molecule in the "Z" shape cavity (calcd. 35.9%), and the framework to decompose at 380-445 °C, the remaining weight corresponds to the constitution of CdO (calcd. 11.8%, found 9.2%) at 700 °C. The TGA for complex 2 shows a weight loss of 3.8% (calculated 4.1%) before 75 °C, which is associated with the loss of one free methanol molecule. When the temperature reaches 400 °C, the framework of 2 begins to break down gradually. The weight loss of 85.5% (calculated 81.1%) at 700 °C can be ascribed to the decomposition of organic matters in complex

TGA
TGA was conducted to investigate the thermal stability of complexes 1, 2, and 3. As shown in Figure 4, the TG curve of 1 displayed no obvious weight loss before 380 • C, the weight loss of 49.1% can be distributed to the removal of BPPT molecule in the "Z" shape cavity (calcd. 35.9%), and the Molecules 2020, 25, 2465 5 of 11 framework to decompose at 380-445 • C, the remaining weight corresponds to the constitution of CdO (calcd. 11.8%, found 9.2%) at 700 • C. The TGA for complex 2 shows a weight loss of 3.8% (calculated 4.1%) before 75 • C, which is associated with the loss of one free methanol molecule. When the temperature reaches 400 • C, the framework of 2 begins to break down gradually. The weight loss of 85.5% (calculated 81.1%) at 700 • C can be ascribed to the decomposition of organic matters in complex 2. According to the TGA results, complex 3 exhibited no obvious weight loss before 310 • C. After that temperature, the framework begins to decompose. The weight loss of 84.1% (calculated 79.4%) at 700 • C can be ascribed to the decomposition of organic matters in complex 3.

TGA
TGA was conducted to investigate the thermal stability of complexes 1, 2, and 3. As shown in Figure 4, the TG curve of 1 displayed no obvious weight loss before 380 °C, the weight loss of 49.1% can be distributed to the removal of BPPT molecule in the "Z" shape cavity (calcd. 35.9%), and the framework to decompose at 380-445 °C, the remaining weight corresponds to the constitution of CdO (calcd. 11.8%, found 9.2%) at 700 °C. The TGA for complex 2 shows a weight loss of 3.8% (calculated 4.1%) before 75 °C, which is associated with the loss of one free methanol molecule. When the temperature reaches 400 °C, the framework of 2 begins to break down gradually. The weight loss of 85.5% (calculated 81.1%) at 700 °C can be ascribed to the decomposition of organic matters in complex 2. According to the TGA results, complex 3 exhibited no obvious weight loss before 310 °C. After that temperature, the framework begins to decompose. The weight loss of 84.1% (calculated 79.4%) at 700 °C can be ascribed to the decomposition of organic matters in complex 3.

IR Analysis
In the IR spectra of complexes 1, 2, and 3 ( Figure 5), the broad bands at about 3482 cm -1 for 2 and 3399cm −1 for 3 are assigned to the presence of methanol (2) water molecules (3). All three complexes displayed similar FT-IR spectra with only a little variation in the peak position with regards to the -C=N (1019 cm −1 ) and -C-S bonds (693 cm −1 and 650 cm −1 ) of the BPPT [23]. The characteristic peaks of

IR Analysis
In the IR spectra of complexes 1, 2, and 3 ( Figure 5), the broad bands at about 3482 cm −1 for 2 and 3399cm −1 for 3 are assigned to the presence of methanol (2) water molecules (3). All three complexes displayed similar FT-IR spectra with only a little variation in the peak position with regards to the -C=N (1019 cm −1 ) and -C-S bonds (693 cm −1 and 650 cm −1 ) of the BPPT [23]. The characteristic peaks of the deprotonated -COO-symmetry band and the asymmetric band appeared at 1600 cm −1 and 1423 cm −1 for 2 and 1609 cm −1 and 1438 cm −1 for 3, respectively.

UV/Vis Analysis
From the spectrum of UV/vis (Figure 6), the maximum absorption of BPPT, 1, 2, and 3 occurred at 330 nm, 340 nm, 330 nm, and 344 nm, respectively. The bands could be assigned to characteristic π-π* transitions centered on BPPT. Compared with the free ligand BPPT, the absorption peaks of 1, 2, and 3 have slightly red-shift, probably attributed to the coordination of the ligands to the Cd ion, which effectively increases the conjugated extent of the complexes [24].

UV/Vis Analysis
From the spectrum of UV/vis (Figure 6), the maximum absorption of BPPT, 1, 2, and 3 occurred at 330 nm, 340 nm, 330 nm, and 344 nm, respectively. The bands could be assigned to characteristic π-π* transitions centered on BPPT. Compared with the free ligand BPPT, the absorption peaks of 1, 2, and 3 have slightly red-shift, probably attributed to the coordination of the ligands to the Cd ion, which effectively increases the conjugated extent of the complexes [24].

UV/Vis Analysis
From the spectrum of UV/vis (Figure 6), the maximum absorption of BPPT, 1, 2, and 3 occurred at 330 nm, 340 nm, 330 nm, and 344 nm, respectively. The bands could be assigned to characteristic π-π* transitions centered on BPPT. Compared with the free ligand BPPT, the absorption peaks of 1, 2, and 3 have slightly red-shift, probably attributed to the coordination of the ligands to the Cd ion, which effectively increases the conjugated extent of the complexes [24].

Luminescence Properties
The luminescence properties of complexes 1, 2, and 3 were investigated in the solid state at room temperature. The emission spectra are shown in Figure 7, and the complexes 1, 2, and 3 exhibit luminescence properties, with an emission at 483 nm (excited at 340 nm), 463 nm (excited at 330 nm), and 489 nm (excited at 344 nm), respectively. To understand the nature of the emission spectra, the luminescence property of free BPPT ligand was also investigated in the solid state under the same experimental conditions. The BPPT ligand exhibits one emission band at 455 nm upon excitation at 330 nm. This suggests that the emission of 1, 2, and 3 originated from the π-π* electronic transition of the ligand [25]. By comparing the emission spectra of complexes 1, 2, and 3 and the free BPPT ligand, we can conclude that the enhancement of luminescence in 1, 2, and 3 may be attributed to the ligation of ligand to the metal center, which effectively increases the rigidity and reduces the loss of energy by radiation less decay [26,27]. A detailed spectroscopic study of a possible structure-related luminescent is underway.

Luminescence Properties
The luminescence properties of complexes 1, 2, and 3 were investigated in the solid state at room temperature. The emission spectra are shown in Figure 7, and the complexes 1, 2, and 3 exhibit luminescence properties, with an emission at 483 nm (excited at 340 nm), 463 nm (excited at 330 nm), and 489 nm (excited at 344 nm), respectively. To understand the nature of the emission spectra, the luminescence property of free BPPT ligand was also investigated in the solid state under the same experimental conditions. The BPPT ligand exhibits one emission band at 455 nm upon excitation at 330 nm. This suggests that the emission of 1, 2, and 3 originated from the π-π* electronic transition of the ligand [25]. By comparing the emission spectra of complexes 1, 2, and 3 and the free BPPT ligand, we can conclude that the enhancement of luminescence in 1, 2, and 3 may be attributed to the ligation of ligand to the metal center, which effectively increases the rigidity and reduces the loss of energy by radiation less decay [26,27]. A detailed spectroscopic study of a possible structure-related luminescent is underway.

Material and Physical Measurements
The organic ligand BPPT was synthesized by modifying the reported procedure using 3-(pyridine-4-yl)benzaldehyde [28]. 3-Bromobenzaldehyde, 4-Pyridineboronic acid, dithiooxamide (Macklin Biochemical Co., Ltd., Shanghai, China) and other reagents (Fuyu Chemical Co., Ltd., Tianjin, China), were used without further purification. The structures of ligands used in the syntheses of the coordination polymers are shown in Scheme 1. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA) at the analysis center of Nanjing University. FT-IR (Fourier transform infrared) spectra were recorded in the range of 400-4000 cm −1 on a Bruker Vector 22 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany) using KBr pellets. Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu-Kα radiation (λ = 1.5418 Å). Thermal gravimetric analyses (TGAs) were taken on a Mettler-Toledo thermal analyzer (Mettler-Toledo, Greifensee, Switzerland) under an N2 atmosphere with a heating rate of 10 °C·min−1. The luminescence spectra were measured on a Perkin Elmer LS-55 fluorescence spectrophotometer (PerkinElmer, Waltham, MA, USA).

Material and Physical Measurements
The organic ligand BPPT was synthesized by modifying the reported procedure using 3-(pyridine-4-yl)benzaldehyde [28]. 3-Bromobenzaldehyde, 4-Pyridineboronic acid, dithiooxamide (Macklin Biochemical Co., Ltd., Shanghai, China) and other reagents (Fuyu Chemical Co., Ltd., Tianjin, China), were used without further purification. The structures of ligands used in the syntheses of the coordination polymers are shown in Scheme 1. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA) at the analysis center of Nanjing University. FT-IR (Fourier transform infrared) spectra were recorded in the range of 400-4000 cm −1 on a Bruker Vector 22 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany) using KBr pellets. Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker D 8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu-Kα radiation (λ = 1.5418 Å). Thermal gravimetric analyses (TGAs) were taken on a Mettler-Toledo thermal analyzer (Mettler-Toledo, Greifensee, Switzerland) under an N2 atmosphere with a heating rate of 10 • C·min −1 . The luminescence spectra were measured on a Perkin Elmer LS-55 fluorescence spectrophotometer (PerkinElmer, Waltham, MA, USA). Chemical Co., Ltd., Tianjin, China), were used without further purification. The structures of ligands used in the syntheses of the coordination polymers are shown in Scheme 1. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C Elemental Analyzer (PerkinElmer, Waltham, MA, USA) at the analysis center of Nanjing University. FT-IR (Fourier transform infrared) spectra were recorded in the range of 400-4000 cm −1 on a Bruker Vector 22 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany) using KBr pellets. Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu-Kα radiation (λ = 1.5418 Å). Thermal gravimetric analyses (TGAs) were taken on a Mettler-Toledo thermal analyzer (Mettler-Toledo, Greifensee, Switzerland) under an N2 atmosphere with a heating rate of 10 °C·min−1. The luminescence spectra were measured on a Perkin Elmer LS-55 fluorescence spectrophotometer (PerkinElmer, Waltham, MA, USA). .

X-Ray Crystallography
Diffraction data for the complex were collected at 293 (2) K, with a Bruker Smart 1000 CCD diffractometer using Mo-Kα radiation (λ = 0.71073 Å) with the ω-2θ scan technique. An empirical absorption correction was applied to raw intensities [29]. The structure was solved by direct methods (SHELX-97) and refined with full-matrix least-squares technique on F 2 using the SHELX-97 [30]. The hydrogen atoms were added theoretically, and riding on the concerned atoms and refined with fixed thermal factors. Crystal data, data collection, and structure refinement details are summarized in Table 1. Suitable single crystals of 1, 2, and 3 were selected and mounted in air onto thin glass fibers. In these structures, H atoms bonded to C atoms were treated as riding in geometrically idealized positions, with C-H = 0.93 Å and U iso (H) = 1.2U eq (C), and the selected bond lengths and angles with their estimated standard deviations are listed in Table 2.

Conclusions
The new rigid planar ligand BPPT has been synthesized, by introducing different rigid carboxylic acids BPDC, IP, and BTC as auxiliary ligands; three Cd(II) CPs have been synthesized successfully under solvothermal conditions. The complexes 1, 2, and 3 show a 3D framework and have good thermal stability. In addition, complexes 1, 2, and 3 exhibit luminescence properties. We hope that this new BPPT ligand can obtain more novel structures. Studies toward the preparation of new CPs or MOFs, especially further studies of luminescence properties including the three complexes, are underway.