Construction of four d10 coordination polymers containing binuclear rings as building blocks from 4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylic acid

Abstract Four coordination polymers based on 4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylic acid (H2TBPC), [Zn(μ3-TBPC)(H2O)]n (1), [Zn(μ3-TBPC)(Me2NH)]n (2), [Cd(μ3-TBPC)(bpy)]n (bpy = 4,4′-bipyridine, 3), and [Cd(μ4-TBPC)(H2O)]n (4), were constructed under hydrothermal conditions. The compounds are composed of M2(TBPC)2 binuclear ring as a building block. In 1–3, the binuclear rings are interconnected to three different 2-D networks with the same (4·82) topology. In 4, the binuclear rings are arranged into a 3-D framework with PtS-type topology. The results revealed that the structural diversity is mainly attributed to the coordination geometries of metal ion, the coordination modes of TBPC2−, and the auxiliary ligand. The thermal stabilities and luminescent properties of 1–4 have also been studied.


Introduction
Coordination polymers (CPs) have attracted attention because of their interesting topologies and properties, such as gas separation, luminescence, magnetism, and sensors [1][2][3][4][5][6][7][8][9][10][11][12]. Controllable synthesis of CPs is not clearly elucidated. Selecting a special CP for a particular application from a large number of CPs is still difficult. Therefore, investigating the syntheses and structures of CPs based on new ligands is necessary as ligands play an important role in the final structure. Both carboxylate and tetrazole groups are good donors, frequently used in CP syntheses. These groups display versatile coordination, suitable for the formation of secondary building blocks. A combination of these two functional groups may produce new ligands with interesting coordination behavior. However, CPs based on such ligands are limited. Several ligands, such as 4-(1H-tetrazol-5-yl)benzoic acid [13][14][15][16][17], tetrazole-5-carboxylate [18][19][20] and 5-(1H-tetrazol-5-yl)isophthalic acid [21][22][23][24], have been applied to construct CPs with interesting structures and properties. In this study, 4′-(2H-tetrazol-5-yl)biphenyl-4-carboxylic acid (H 2 TBPC), a ligand that possesses a long biphenyl spacer between the terminal tetrazole and carboxylate groups, was selected to prepare new functional CPs by following the same design strategy. Reactions of H 2 TBPC with Zn(II) and Cd(II) result in four new d 10 CPs. The structures and luminescence properties of these CPs were investigated. To the best of our knowledge, only a few H 2 TBPC-based CPs have been reported [25,26].

Materials and instrumentation
H 2 TBPC was bought from Jinan Camolai Trading Company. All other solvents and reagents were obtained from commercial venders and used as purchased. elemental analyses for C, H, and N were performed on a Perkin-elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. X-ray powder diffraction measurements were measured using a Bruker D8 Advance diffractometer at 40 kV, 40 mA with a Cu-target tube and a graphite monochromator. Thermogravimetric analyses were performed on a Perkin-elmer TGA7 analyzer with a heating rate of 10 °C min −1 in flowing air. The luminescent spectra for the solid state were recorded at room temperature on Hitachi F-2500 and edinburgh-FLS-920 instruments with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra, the pass width is 5 nm.

X-ray data collection and structure refinement
Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite-monochromated Mo K α radiation (λ = 0.71073 Å) for 1-4. Multi-scan absorption corrections were applied using SADABS [27]. Structural solutions and refinements based on F 2 for all complexes were performed with SHeLXL [28]. All non-H atoms were refined with anisotropic thermal parameters. All hydrogens were placed in idealized positions. Hydrogens on hydroxyl for 2 were not added but included in the formula. The crystal parameters, data collections, and refinements for 1-4 are listed in table (1). CCDC reference numbers are 1408241-1408244 for 1-4.

Crystal structure of 1-4
Single crystal X-ray diffraction shows that the compounds possess different structures, but contain an M 2 L 2 ring as the building block. The coordination angle between carboxylate and N on the 2-position of tetrazole is approximately 70°, which may be suitable for constructing a small ring. In 1-4, two H 2 TBPC ligands and two metal ions are bridged via carboxylate and N on the 2-position of tetrazole to form a binuclear grid (scheme 1).
The formation of a binuclear ring cannot use all the coordination sites of the metal ions, and some coordination atoms remain uncoordinated on TBPC ligands. Hence, such rings may act as building units to assemble further into a higher dimensional network. each Zn(II) in 1 is five-coordinate with two nitrogens and two carboxylate oxygens from three individual TBPC 2− ligands and one oxygen from Table 1. Crystallographic data and structure refinement summary for 1-4. coordinated water, forming a distorted square-pyramidal geometry ( figure 1(a)). each binuclear ring is connected to four adjacent binuclear rings by Zn-N bonds between Zn(II) ions and three nitrogens on TBPC 2− dianions, resulting in a 2-D network structure in the bc plane ( figure 1(b)). each μ 3 -TBPC 2− connects to three Zn(II) ions and can be considered as a 3-connecting node, while each Zn(II) ion is coordinated by three μ 3 -TBPC 2− ligands and also can be regarded as a 3-connecting node. Therefore, the resulting structure of 1 can be described as a binodal ( 2(b)). The TBPC 2− is a tridentate ligand that connects three Zn(II) ions. However, apart from the coordination donors that are used in the binuclear ring, TBPC 2− using the N4 rather than the N3 (observed in 1) coordinate to the third Zn(II) ion. Therefore, the interconnection of binuclear rings via Zn-N bonds between Zn(II) and N4 forms a 2-D layer, in which the binuclear rings are almost perpendicular to the plane with an angle of 87.4° ( figure 2(c)). These 2-D layers are stacked along the a direction in an -AAAA-sequence, and no noticeable interaction exists between the neighboring layers. The topological method was also performed to further analyze the 2-D framework of 2. As mentioned above, each μ 3 -TBPC 2− bonds to three Zn(II) ions and each Zn(II) ion also links to three μ 3 -TBPC 2− ligands, thus both of them can be viewed as 3-connected nodes. Therefore, the whole structure of 2 can be represented as (3,3)-connected 2-D FeS-type topology ( figure 2(d)), which is the same as compared with 1.
Compound 3 crystallizes in the monoclinic space group P2(1)/c. Its asymmetric unit comprises one Cd(II), one TBPC 2− , and one bpy neutral ligand. As illustrated in figure 3(a), each Cd(II) is six-coordinate in a distorted octahedral [CdN 4 O 2 ] geometry. This configuration is surrounded by two tetrazole nitrogens and two carboxylate oxygens from three different TBPC 2− ligands, and two pyridine nitrogens from one bpy chelating ligand. The coordination mode of the tetrazole group on TBPC 2− is the same as that in 2, hence, a similar 2-D layer based on the binuclear ring and Cd1-N4 bonds is formed ( figure  3(b)). A (4·8 2 ) network topology is also formed when both the metal ion and TBPC 2− anion serve as 3-connected nodes ( figure 3(c)). The coordinated bpy molecules are located on the sides of the 2-D layer. As a result, a 3-D supramolecular framework is formed by linking these 2-D layers through weak π⋯π stacking interactions between bpy rings from neighboring layers, with a centroid-centroid distance of 3.867 Å ( figure 3(d)).  Single crystal X-ray diffraction shows that 4 crystallizes in the monoclinic system with P2(1)/c space group and the asymmetric unit contains only one crystallographically independent Cd(II), one TBPC 2− , and one coordinated water. The local coordination geometry around the Cd(II) ion can be described as a [CdN 3 O 3 ] slightly distorted octahedron [29,30], with the axial positions occupied by one nitrogen from one TBPC 2− and one oxygen from one coordinated water ( figure 4(a)). The equatorial plane consists of two nitrogens and two carboxylate oxygens from the other three different TBPC 2− ligands.
In 4, each TBPC 2− has a unique μ 4 -kO,O′: kN, kN′, kN″ quadridentate bridging mode. Thus, the binuclear units are connected by Zn-N bonds between Zn and N2 and N4, giving a 3-D pillar-layered framework ( figure 4(b)). The 2-D layer constructed by Zn(II) and tetrazole groups is shown in figure 4(c). Two tetrazole groups simultaneously bridge two neighboring Cd(II) ions to form a binuclear [CdN 2 ] 2 unit. These units are further connected to a 2-D network extending along the bc plane ( figure 4(c)). The topological analysis approach was employed for 4. each μ 4 -TBPC 2− bonds to four Cd(II) ions and each Cd(II) also links to four μ 4 -TBPC 2− ligands, thus both can be viewed as 4-conneted nodes ( figure 4(d)). On the basis of this simplification, the resulting structure of 4 can be described as a binodal (4,4)-connected PtS-type 3-D framework [31][32][33] with the Schlfli symbol of (4 2 ·8 4 ) analyzed by TOPOS ( figure 4(e)).
Thus, a binuclear ring is often formed in 1-4, regardless the of metal ion, anion, and coligand used. This indicates that H 2 TBPC possesses the ability to form a binuclear unit easily during the assembly process, which may help predict the structures of new CPs from H 2 TBPC ligand.

PXRD and thermogravimertric analyses of 1-4
PXRD experiments show that the phase purity of 1-4 is high (figures S1-S4). Thermogravimertric analyses (TG) of 1-4 were carried out under air with a heating rate of 10 °C min −1 . As described in figure (5), the TG curve for 1 indicates a gradual weight loss of 5.6% from room temperature to 270 °C, which can be ascribed to the removal of water (Calcd 5.2%) in the asymmetric unit. Then a major weight loss above 330 °C is attributable to the decomposition of the whole coordination framework. Seen from the TG diagram of 2, a gradual weight loss of 10.4% from room temperature to 300 °C may be due to the release of N(CH 3 ) 2 (Calcd 12.6%), and the framework begins to decompose above that temperature. For 3, there is no noticeable weight loss before 370 °C, and further heating leads to decomposition of the framework. For 4, the coordination water is lost before 260 °C (observed 5.0%, Calcd 4.7%), and the framework is stable to about 360 °C.

Conclusion
Four new d 10 CPs were constructed based on a new tetrazole-based carboxylate ligand with biphenyl spacer. All compounds contain a binuclear ring as building blocks and display different structures and topologies. Compounds 1-3 are 2-D networks with 4·8 2 topology, whereas 4 is a 3-D framework with PtS-type topology. This study confirms that H 2 TBPC is suitable for constructing CPs with interesting structures.

Disclosure statement
No potential conflict of interest was reported by the authors.