Structural Elucidation and Electrochemiluminescence on a 3D Cadmium(II) MOF with 5-c Topology

A novel cadmium(II) based MOF, [Cd(μ–ppza)]n (1) (H2ppza = 3–(pyridin–4–yl)–1H–pyrazole–5–carboxylic acid) was synthesized under solvothermal condition. The single-crystal X-ray diffraction reveals that the title MOF 1 crystallizes in the triclinic system with the P–1 space group, and each Cd(II) is coordinated with five (Hppza)2− ions forming a distorted octahedron. The carboxylic group in the ligand acts as a bridge linking the Cd(II) ions into a 3D structure. MOF 1 has a 5-c {46.64} topological structure. Furthermore, MOF 1 exhibits good electrochemiluminescence (ECL) behavior.


Introduction
In the last two decades, the design and synthesis of metal-organic frameworks (MOFs) has attracted more and more attentions due to their a unique structure and potential applications in the fields of magnetism [1,2], electrochemiluminescence [3,4], supercapacitors [5][6][7], etc. Generally, to obtain metal organic framework materials with interesting topological structure and performance, many influencing factors should be considered, such as metal ions, organic ligand, temperature, pH value, etc., among which the selected ligand is crucial in the construction system. Because it can modulate the coordination mode and the flexibility of molecular skeleton to construct metal-organic framework materials [8]. Among the candidate organic ligands, pyrazole carboxylic acid ligands have excellent coordination abilities and flexible coordination modes in the assembly of MOFs structure. Pyrazole carboxylic acid ligands can not only be acted as hydrogen bond donors, but also be as a acceptors to bring its topological structures have many diversities, so this type of ligand is widely used to construct MOFs with intriguing topological structures and outstanding performances [9].
Electrochemiluminescence (ECL) is a phenomenon that the luminescent body forms excited species on the electrode surface through high-energy electron transfer reaction, and then rapidly returns to the ground state through energy relaxation. It is an analytical technology combining electrochemical methods and chemiluminescence methods [10], and has been received considerable attention. As we know that, the Ir-, Ru-and Re-based complexes in the field of ECL has been used widely, but they are precious metals, so it limited their applications [11][12][13][14]. Therefore, it is a good choice to use transition metal complexes as ECL materials for the wide range and inexpensive. Niu reported a novel Cd-MOF a fluorescent probe for antibiotics in water [15], but they adopted explosive Cd(NO 3 ) 2 ·4H 2 O as metal ions. Hong synthesized a threefold interpenetrating Cd(II)-MOF via hydrothermal reaction and studied its topological structure [16]. As our continuing work, our group have reported a series of transition metal framework based on pyrazole carboxylic ligands with excellent ECL performace [17,18]. Thus, the use of transition metal ions and organic ligands to synthesize MOFs with highly luminescent efficiency has become an urgent issue.

Materials and Methods
All the reagents and solvents were purchased from commercial grade and used without purification. 3-(pyridin-4-yl)-1H-pyrazole-5-carboxylic acid (H 2 ppza) was prepared according to the literature [7]. Elemental analyses (C, H, and N) were made with a Thermo Quest Flash EA1112 microanalyzer. IR spectra were recorded with a Spectrum One Perkin-Elmer FT-IR spectrophotometer (KBr disc) from 4000 to 400 cm −1 . The thermogravimetry (TGA) was taken on a Netzsch STA 409PC differential thermal analyzer by heating the crystalline sample from 25 to 900 °C at a rate of 20 °C min −1 in air.

X-ray Crystallography
Single-crystal X-ray diffraction measurement for MOF 1 was performed using a SuperNova diffractometer using graphitemonochromated Mo-Kα radiation (λ = 0.71073 Å) with the ω-θ scan technique at 293 K. The structure was solved with SHELXS-97 employing by direct methods, and refined by full matrix least squares on F 2 with SHELXS-2014 [19]. All nonhydrogen atoms were refined with anisotropic thermal parameters. H atoms linked to C were placed at the geometric positions and refined with an isotropic displacement parameter. Detailed data collection and refinements were summarized in Table S1. Selected bond lengths and angles were listed in Table S2. Crystallographic data has been deposited with the Cambridge Crystallographic Data Center (CCDC numbers: 2123339).

Electrochemistry
Electrochemistry was carried on an electrochemical working station (IVIUM Vertex1, Netherlands). Using a standard threeelectrode cell at ambient temperature. Taking Ag/AgCl as a reference electrode, a platinum wire as the auxiliary electrode, and a glassy carbon electrode (GCE) the working electrode with a diameter of 1 mm (CV in DMF). The supporting electrolyte was employing potassium peroxydisulfate (K 2 S 2 O 8 , 0.1 M) in DMF.

Synthesis Discussion
The synthesis method is described as in the experiment section. The ratio of ligand H 2 ppza and CdCl 2 is 1:1, which is corresponding with the asymmetric unit. We used the solvothermal method to synthesize the target product, and tried to change the temperature to control the product, found that crystalline materials were obtained at 130 °C, but single crystals were produced at 150 °C. In addition, in the solvothermal reaction, the formation of the phase, the size and shape of the particle size can also be controlled, and the dispersibility of the product is better. The SEM image of MOF 1 was presented in Figure S2, giving details on nanocrystals assembled microblocks on the 3D network structure.

Crystal Structure of [Cd(μ-ppza)] n (1)
MOF 1 crystallizes in the triclinic system with the P-1 space group, and each asymmetric unit consists of one Cd(II) ion and one completely deprotonated (Hppza) 2− as shown in Fig. 1. Figure 2  centers and the N atom on pyradine also acts as a bridge to connect the other two Cd(II) atoms. It is precisely because of this unique chelating-bridging coordination mode that makes 1 has a three-dimensional structure, seen in Fig. 3. Additionally, there exists a twisted ribbon structure in 1, which formed by a four-membered ring Cd 2 O 2 and a sixmembered ring Cd 2 O 4 C 2 , and the adjacent Cd···Cd distances are 3.973(3) Å and 5.721(9) Å. Compared with cgh6 topological Cd-MOF by Cui [20], the Cd···Cd distances in our work are much shorter than theirs (10.471(10) Å). Considering from the principle of topology, With each Cd 2 O 2 taken for a node, and the ligand for a line, MOF 1 can be simplified as a 5-c net with {4 6 .6 4 } Schläfli symbol (Fig. 4), which is different with a (4,4)-connected topological compound [Cd(2,6-ndc)(DMF)] [21]. After all, there also exists intramolecular interactions in MOF 1, such as π···π and C-H···π stacking, with centroids distances 3.430(2) Å and 3.596(3) Å, respectively. This interactions are also found in the similar structure {[Cd(L)(5-hip)]·2.66H 2 O} n recently [20].

Electrochemiluminescence for [Cd(μ-ppza)] n (1)
Electrochemiluminescence (ECL) is the process where species generated at electrodes undergo electron-transfer reactions to form excited states that emit light. ECL has become a powerful tool with near zero background, excellent sensitivity, fast response and wide dynamic range. Up to now, most of the coordination polymers used as electrochemiluminescent groups contain noble metals such as Ru, Ir, and Re. Due to their high cost, the application of these coordination polymers has been limited. Therefore, the use of transition metal complexes as ECL materials is a potential choice [22]. Based on this point of view, we studied the electrochemiluminescence properties of MOF 1.
In this paper, we tested the ECL behavior of the title MOF in DMF solution and the results were showed in Fig. 5, and the solubility of MOF 1 in DMF is 0.161 g/L. In order to understand the redox properties of the complex, we also carried out a cyclic voltammetry test on the MOF 1. From Figure S3, it is obviously to find that the oxidation peak in the cathodic current at − 0.8 V and the reduction peak at − 1.24 V. The ECL intensity of 1 is approximately to 1150 Fig. 1 The asymmetry of 1 with thermal ellipse at the 30% probability level Fig. 2 The coordination environment of Cd(II), all hydrogen atoms were omitted for clarity a.u. which is higher than reported [23], and much more stable. The possible function of ECL behavior is as following [24].
Cathode: H 2 ppza + 2e → (ppza) 2− + H 2 . Anode: Cd 0 -2e → Cd 2+ Solution: (ppza) 2− + Cd 2+ → Cd(ppza)•DMF. Taken the Ru(bpy) 3 3+ as the standard [25], the ECL yield of title MOF is 0.25. This yield is similar to Yang's work [26], but slightly lower than the literature reported by Tang [27]. This good ECL performance may due to the intersecting 3D structure of MOF 1, and its luminescence properties are mainly affected by the central metals, as well as the Cd···Cd interactions providing channels for carrier transferring [28]. Hence, as the good ECL performance, the title MOF with interesting topological structure can be used to design OLED devices for the future. Figure S4 gives the TGA curve of MOF 1, which can be seen a long stage, and implies that the 1 is very stable before 500 °C. After 500 °C, the MOF skeleton began to collapse, the weight loss is about 80% at 580 °C, and the last remaining maybe CdO. To confirm whether successful formation of the final products, the X-ray powder diffraction (PXRD) was tested in Figure S5. The PXRD pattern reveals that the peak positions of the obtained crystalline sample of MOF 1 matched well with the simulated.

Conclusions
In summary, by the use of 3-(pyridin-4-yl)-1H-pyrazole-5-carboxylic acid as a ligand, one novel 3D Cd(II) MOF (1) has been successfully synthesized by solvothermal method. Single-crystal X-ray diffraction reveals MOF 1 displays a 3D structure with a 5-c topology. The electrochemiluminescent properties of 1 has been studied, show that 1 could be anticipated as good candidates for obtaining solid state photoluminescent materials or OLED materials. And we wish that our research would attract more crystallographers' attentions on the filed of electrochemiluminescence.