Cd(II)/Mn(II)/Co(II)/Ni(II)/Zn(II) Coordination Polymers Built from Dicarboxylic Acid/Tetracarboxylic Acid Ligands: Their Structural Diversity and Fluorescence Properties

Six Cd(II)/Mn(II)/Co(II)/Ni(II)/Zn(II) coordination complexes are formulated as [Cd2(X2−)2(μ3-O)2/3]n (1), [Mn2(X2−)2(μ3-O)2/3]n (2), {[Co1.5(Y4−)0.5(4,4′-bpy)1.5(OH−)]·2H2O}n (3), {[Ni(X2−)(4,4′-bpy)(H2O)2]·4H2O}n (4), [Zn(m-bdc2−)(bebiyh)]n (5), and [Cd(5-tbia2−)(bebiyh)]n (6) (H2X = 3,3′-(2,3,5,6-tetramethyl-1,4-phenylene) dipropionic acid. H4Y = 2,2′-(2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene) dimalonic acid, bebiyh = 1,6-bis(2-ethyl-1H-benzo[d]imidazol-1-yl)hexane, m-H2bdc = 1,3-benzenedicarboxylic acid, and 5-H2tbia = 5-(tert-butyl)isophthalic acid) were obtained by hydrothermal reactions and structurally characterized. Complexes 1 and 2 have a 6-connected 3D architecture and with several point symbols of (36·46·53). Complex 3 features a 5-connected 3D net structure with a point symbol of (5·69). Complex 4 possesses a 4-connected 2D net with a vertex symbol of (44·62). Complex 5 is a 3-connected 2D network with a point symbol of (63). Complex 6 is a (3,3)-connected 2D network with a point symbol of (63)2. In addition, complexes 1 and 4 present good photoluminescence behaviors. The electronic structures of 1 and 4 were investigated with the density functional theory (DFT) method to understand the photoluminescence behaviors.


Introduction
The research and development of materials play vital roles in the development of modern society [1][2][3][4]. As a new type of functional material, complexes' structures are modifiable and thus have easy to modify functions [5][6][7]. In the field of materials chemistry, the complex has been a hot topic [8][9][10].
Structure determines performance. If you want to get the desired performance of the complex, you have to design it properly which is exactly what we have been trying to pursue [11][12][13]. Complexes are self-assembled by central metal ions or clusters (inorganic components) and organic ligands (organic components), so selecting appropriate metal ions and organic ligands can realize the design and construction of this material [14,15]. Among them, the variety of ligands is extremely large. The organic ligands with different configurations have an important influence on the synthesis and structure of complexes. In terms of the toughness of the ligand, ligands can be divided into rigid, flexible, or semirigid. Although the stability of the complex constructed by rigid ligands is good, rigid ligands cannot twist at will, which makes the structure of the complex monotonous. Although more complex, novel, and exotic sturctures with varied configurations can be obtained using flexible ligands, complex structures synthesized by the ligands are difficult to control. To our satisfaction, semirigid ligands have the characteristics of both rigid and flexible ligands.
To date, many polycarboxylic acid ligands have been employed to construct complexes due to the abundant coordination patterns of carboxylic acids [16][17][18][19][20][21][22]. Carboxylic acids have the following advantages: firstly, the O atom on the carboxylic acid group has a strong electron-donating ability and it is easy to coordinate with metal ions. Secondly, the coordination modes of carboxylic groups are flexible and varied. There are roughly three modes: single tooth, chelate, and bridge. When coordinating with more than one metal, the three types of double, three, and four teeth are displayed. In addition, the different orientations of coordination bonds between metal ions and O atoms can be expressed as cis-cis, cis-anti, and anti-anti patterns. The structural diversity of the coordination patterns of carboxylic acids is impressive. Thirdly, carboxyl groups are completely or partially deprotonated, rendering them hydrogen bond acceptors or hydrogen bond donors. In this way, hydrogen bonds can be formed with more electronegative atoms such as O, N, and F, thus contributing to the formation of a supramolecular structure. Fourthly, the conjugation property of the aromatic ring is conducive to electron transfer. Therefore, semirigid polycarboxylate ligands are our first choice followed by rigid ligands. In addition, the mixing strategy of the polycarboxylic acid and N-donor ligand is also an effective method in the synthesis of multi-dimensional structures [23][24][25][26][27][28].
ligands cannot twist at will, which makes the structure of the complex monotonous. hough more complex, novel, and exotic sturctures with varied configurations can be tained using flexible ligands, complex structures synthesized by the ligands are diff to control. To our satisfaction, semirigid ligands have the characteristics of both rigid flexible ligands.
To date, many polycarboxylic acid ligands have been employed to construct c plexes due to the abundant coordination patterns of carboxylic acids [16][17][18][19][20][21][22]. Carbo acids have the following advantages: firstly, the O atom on the carboxylic acid group a strong electron-donating ability and it is easy to coordinate with metal ions. Secon the coordination modes of carboxylic groups are flexible and varied. There are rou three modes: single tooth, chelate, and bridge. When coordinating with more than metal, the three types of double, three, and four teeth are displayed. In addition, the ferent orientations of coordination bonds between metal ions and O atoms can be pressed as cis-cis, cis-anti, and anti-anti patterns. The structural diversity of the co nation patterns of carboxylic acids is impressive. Thirdly, carboxyl groups are compl or partially deprotonated, rendering them hydrogen bond acceptors or hydrogen b donors. In this way, hydrogen bonds can be formed with more electronegative atoms as O, N, and F, thus contributing to the formation of a supramolecular structure. Four the conjugation property of the aromatic ring is conducive to electron transfer. There semirigid polycarboxylate ligands are our first choice followed by rigid ligands. In a tion, the mixing strategy of the polycarboxylic acid and N-donor ligand is also an effe method in the synthesis of multi-dimensional structures [23][24][25][26][27][28].

Materials and Methods
All reagents and solvents were purchased commercially except for H2X and H4Y In the region of 400-4000 cm −1 , FT-IR spectra were tested on an FTIR-7600 spectropho eter. The C, H, and N content was recorded on a FLASH EA 1112 elemental analyzer

Materials and Methods
All reagents and solvents were purchased commercially except for H 2 X and H 4 Y [29]. In the region of 400-4000 cm −1 , FT-IR spectra were tested on an FTIR-7600 spectrophotometer. The C, H, and N content was recorded on a FLASH EA 1112 elemental analyzer. The luminescence properties were studied using a Cary Eclipse fluorescence spectrophotometer.

X-ray Crystallography
Crystallographic data for 1-6 were collected using an Xcalibur Eos Gemini CCD diffractometer (Mo-Kα, λ = 0.71073 Å). Absorption corrections were applied by using a multi-scan program. The data were corrected for Lorentz and polarization effects. Structures were solved by immediate methods and refined with a full-matrix least-squares technique based on F 2 using the ShelXL software package [30]. Then, all of the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of ligands were assigned at perfect positions capitalizing on a riding model and then they were refined isotropically [30]. Crystallographic crystal data and structure refinement details for 1-6 are summarized in Table S1, while selected bond lengths and bond angles for 1-6 are listed in Table S2.
In 1, the ligand X 2− exhibits a coordination pattern (Figure 1a). In this pattern, two carboxyl groups appear as μ2-η 1 :η 1 and μ3-η 1 :η 2 , respectively, bridged with five Cd(II) ions. Based on this connection pattern, Cd1 and symmetrically related Cd1 atoms are bridged together by three carboxyl oxygen atoms and a μ3-O to produce a three-nucleated [Cd3O4] unit (SBU-A). The Cd2 atom and the symmetrically related Cd2 atom are also joined together by three carboxyl oxygen atoms and a μ3-O bridge to produce a three-nucleated [Cd3O4] unit (SBU-B). SBU-A and SBU-B are interchangeably connected by carboxyl oxygen atoms of X 2− , resulting in a 1D chain structure (Figure 2b). The 1D chain forms a 3D structure under the extension of X 2− (Figure 2c). Topologically, the [CdO] unit can be considered as a 6-connected node, which is connected to six equivalent nodes through six X 2− ligands. Each X 2− links two [CdO] units, so the X 2-can be simplified as links. Accordingly, the whole structure of 1 is related to a 6connected network with a Schläfli symbol of (3 6 ·4 6 ·5 3 ) topology (Figure 2d). Topologically, the [CdO] unit can be considered as a 6-connected node, which is connected to six equivalent nodes through six X 2− ligands. Each X 2− links two [CdO] units, so the X 2− can be simplified as links. Accordingly, the whole structure of 1 is related to a 6-connected network with a Schläfli symbol of (3 6 ·4 6 ·5 3 ) topology (Figure 2d). The asymmetric unit of 3 is composed of one and a half Co(II) atoms, half a Y 4− anion, one and a half 4,4′-bpy, a coordinated OH -, and two dissociative H2O molecules. Each Co1(II) atom has a hexagonal configuration formed by four carboxyl O atoms (O2, O3A, and O4A) from four Y 4-anions, one O1 atom from coordinated H2O molecules, and two N atoms (N1 and N3) from two separate 4,4′-bpy (Figure 3a). The Co1-O bond length is between 2.054(5) and 2.159(6) Å. The Co1-N bond length is between 2.170(6) and 2.184(6) The asymmetric unit of 3 is composed of one and a half Co(II) atoms, half a Y 4− anion, one and a half 4,4 -bpy, a coordinated OH − , and two dissociative H 2 O molecules. Each Co1(II) atom has a hexagonal configuration formed by four carboxyl O atoms (O2, O3A, and O4A) from four Y 4− anions, one O1 atom from coordinated H 2 O molecules, and two N atoms (N1 and N3) from two separate 4,4 -bpy (Figure 3a). The Co1-O bond length is between 2.054(5) and 2.159(6) Å. The Co1-N bond length is between 2.170(6) and 2.184(6) Å. Each Co2(II) atom has a hexagonal configuration formed by four carboxyl O atoms (O6, O7, O6B, and O7B) and two N atoms (N2 and N2B). The Co2-O bond length is between 2.105(6) and 2.112 (7)
As depicted in Figure 3e, topological analysis is performed on 3. If the binuclear unit constituted by Co1 and symmetrically related Cd1 atoms is taken as a 5-connector, the Y 4− and Cd2 atoms can be defined as linkers, and the 3D framework of 3 can be classified as a 5-connected net with point symbol of (5·6 9 ). To further demonstrate the overall 2D structure of 4, we can consider each Ni(II) as a 4-connecting node which is linked to four equivalent nodes through two X 2− anions and two 4,4′-bpy. X 2− and 4,4′-bpy are simplified as linear linkers separately. The whole structure of 4 can be simplified to a 4-connected net with a vertex symbol of (4 4 ·6 2 ) ( Figure  4e).   X 2− adopts a trans-configuration and its two carboxyl groups adopt a single-tooth coordination mode (Figure 1c). The dihedral angle of the two pyridine rings of 4,4 -bpy is close to 90 • . In 4, 4,4 -bpy connects adjacent Ni(II) ions along an a-axis to generate a 1D straight chain structure (Figure 4b). Whereas, X 2− connects adjacent Ni(II) ions to generate a 1D wave-like chain structure along an a-axis (Figure 4c). Both 1D Ni(II)/4,4 -bpy chains and 1D Ni(II)/X 2− chains are alternately connected to form 2D layer structures (Figure 4d).
To further demonstrate the overall 2D structure of 4, we can consider each Ni(II) as a 4connecting node which is linked to four equivalent nodes through two X 2− anions and two 4,4 -bpy. X 2− and 4,4 -bpy are simplified as linear linkers separately. The whole structure of 4 can be simplified to a 4-connected net with a vertex symbol of (4 4 ·6 2 ) (Figure 4e).
To further demonstrate the overall 2D structure of 5, we can consider each Zn(II) as a 3-connecting node which is linked to three equivalent nodes through two m-bdc 2− anions and one 26-membered ring. The m-bdc 2− and 26-membered ring are simplified as linear linkers separately. The whole structure of 5 can be simplified to a 3-connected net with a vertex symbol of (6 3 ) ( Figure S2e).
To further demonstrate the overall 2D structure of 6, we can consider each Cd (II) (Cd1(II) and Cd2(II)) as a 3-connecting node. The 5-tbia 2− and bebiyh are simplified as linear linkers separately. The whole structure of 6 can be simplified to a (3,3)-connected net with a vertex symbol of (6 3 ) 2 ( Figure S3e).

Photoluminescence Properties
We investigated the fluorescence spectrum of complexes 1 and 4 and the free ligand H 2 X ( Figure S4). H 2 X shows an emission band at 300 nm (λ ex = 282 nm). The emission band of 4,4 -bpy is 428 nm (λ ex = 350 nm) [33]. The fluorescence emission peaks were observed at 301 nm for complex 1 (λ ex = 281 nm) and 306 nm for complex 4 (λ ex = 265 nm), respectively. The emission peak of 1 is similar to that of H 2 X, which may be mainly attributed to the endoligand emission of H 2 X. The emission band of complex 4 is red-shifted by 6 nm, thus corresponding to the emission band of H 2 X. As for 4,4 -bpy, the emission band of complex 4 is blue-shifted by 122 nm. This may be due to coordination with metal ions. To better understand the photoluminescent properties of complexes 1 and 4, we further performed theoretical investigations on their model systems as shown in Figure 5. We optimized the four geometries at the theoretical level of M06L/6-31G(d,p) under a vacuum, where the SDD effective core potential was applied for the metallic elements. We further calculated the excited properties with the time dependent density function theory (TDDFT) method [34], where the option of nstates for TDDFT was set as 10 [35]. The calculated Cd2-O bond length is 2.558 Å, which is similar to the experimental results (between 2.210(6) and 2.451(6) Å). The calculated emission wavelength of complex 1 is 278 nm and the oscillator strength is as large as 0.228, which is consistent with the experimental results. Moreover, the relevant orbitals for the excited process are HOMO, LUMO+7, and LUMO+8, with corresponding energies of −4.45 eV, 0.37 eV, and 0.53 eV, respectively (Figure 5a). The luminescent processes are related to the frontier orbitals including HOMO, LUMO+3, and LUMO+4. For complex 4, as shown in Figure 5b, the calculated luminescent properties are both relevant to the metal center, which is indicative of their crucial roles. As shown in Figure 5c,d where we gave the calculated emission spectrum of complexes 1 and 4, the oscillator strength of 1 is arguably larger than 4. This indicates that the emission of 4 is weaker than 1, which is in line with the experimental observations.

Conclusions
Six new Cd(II)/Mn(II)/Co(II)/Ni(II)/Zn(II)-containing coordination complexes based on the dicarboxylic acid/tetracarboxylic acid ligands were synthesized. Complexes 1, 2, and 3 feature several 3D net structures. Complex 4, 5, 6 possesses a 2D layer structure, severally. The structure of the ligand has an important effect on the configuration of the complex, leading to the formation of different beautiful topologies. The theoretical calculation results indicate that the luminescence could be mainly related to the metal

Conclusions
Six new Cd(II)/Mn(II)/Co(II)/Ni(II)/Zn(II)-containing coordination complexes based on the dicarboxylic acid/tetracarboxylic acid ligands were synthesized. Complexes 1, 2, and 3 feature several 3D net structures. Complex 4, 5, 6 possesses a 2D layer structure, severally. The structure of the ligand has an important effect on the configuration of the complex, leading to the formation of different beautiful topologies. The theoretical calculation results indicate that the luminescence could be mainly related to the metal center for complexes 1 and 4, while the oscillator strength of 1 is larger than 4.