High Efficiency Electrochemiluminescence for Copper(II) and Cadmium(II) Pyrazolate Polymers

Two new coordination polymers, namely, [Cu2(L)2(4,4′-bpy)]n (1) and [Cd(HL)Cl(1,10′-phen)]n (2) (H2L = 1H-pyrazole-3-carboxylic acid, 4,4′-bpy = 4,4′-bipyridine, 1,10′-phen = 1,10-phenanthroline) have been synthesized via solvothermal method and were structurally characterized by single-crystal X-ray diffraction, FT-IR, elemental analysis and PXRD. The two complexes are both infinite 1D chain structures, in complex 1 the H2L ligands are completely deprotonated and chelating the metal centers, and 4,4′-bipyridine link adjacent metal units to form a 1D chain structure. While, in complex 2, the auxiliary ligands chelating the metal center and H2L ligands play a role in chelating-bridging the metal centers to help build an infinite 1D chain structure. What’s more, the two complexes further extended to 3D supramolecular networks by hydrogen bonds and weak C–H⋯π interactions. The weak intermolecular interactions existed in the complexes structures were further studied by Hirshfeld surface analysis and 2D fingerprint plots. In addition, these two transition metal complexes exhibit high intense electrochemiluminescence (ECL) in N,N-dimethylformamide (DMF) and after ten circulations the ECL intensity still remains stable, which can be a useful guide for the construct of new polymers ECL materials.


Introduction
Coordination polymers (CPs) have attracted intense interest for their structural diversity and potential applications in many areas, such as gas adsorption/separation [1][2][3], catalysis [4], luminescence sensing [5,6], battery [7][8][9], supercapacitor [10,11], and magnetism [12,13]. The design and selection of organic ligands and metal ions are critical factors for the construction of CPs, due to the coordination mode of organic ligands and functional groups in them are directly affect the configuration and applications of the CPs. However, it is a great challenge to control the structures and construct the target molecules with desired properties [14], owing to temperature, pH value and solvent will also affect the structure of resulting crystals [15,16].
Among the numerous organic ligands, pyrazole carboxylate ligands obtained widely concern due to possessing N-donors in pyrazole ring and O atoms on the carboxylate which can provide potential coordination nodes and can exhibit flexible chelating and bridging coordination modes [17,18]. What's more, act as multiple proton donors, deprotonated ligands can increase the coordination site which can help to fabricate high-dimensional coordination polymers. Inspired by the afore-mentioned facts, our group has devoted ourselves to synthesis several intriguing coordination polymers based on pyrazole carboxylate ligands and have made some achievements [19][20][21][22].
Electrochemiluminescence (ECL) is a special luminescence phenomenon, which combining the respective merits of electrochemistry and chemiluminescence [23,24]. There are two fundamental mechanisms of ECL, one is annihilation ECL and the other is coreactant ECL, and the process of ECL including electrochemical excitation step and light radiation step [25]. Owing to its extremely low background noise, wide dynamic range and high sensitivity, ECL has been received considerable attention. As we known that, until now, most coordination polymers used as ECL luminophores contain Ru [26], Ir [27], and Re [28] precious metal, so it limited the application of these complexes due to high cost. As result, using transition metal complexes as ECL materials may be an alternative choice. What's more, researches in this area have also been reported before. Our group has been working on electroluminescence of complexes, especially pyrazole-based transition metal complexes [29,30]. As our ongoing work, in this paper, Cu(II) and Cd(II) were chosen as the inorganic nodes to coordinate with 1H-pyrazole-3-carboxylic acid (H 2 L) ligand and auxiliary ligand, and obtained two new complexes [Cu 2 (L) 2 (4,4′-bpy)] n (1) and [Cd(HL)Cl(1,10′-phen)] n (2). The photoluminescence properties and ECL properties of complexes 1 and 2 have been investigated.

Synthesis of [Cd(HL)Cl(1,10′-phen)] n (2)
H 2 L (0.3 mmol, 0.0336 g) and CdCl 2 ·6H 2 O (0.1 mmol, 0.0228 g) were dissolved in 8.0 mL DMF/H 2 O(1:1 volume ratio), and the solution was stirred at room temperature for 1 h, after that 4.0 mL CH 3 CN dissolved in 1,10′-phen (0.1 mmol, 0.0198 g) was added and the reaction mixture was stirred for another 15 min. Then, pouring the solution into a 25 mL Teflon-lined stainless steel vessel, and heated in an oven at 100 °C for 4 days. After slowly cooling to room temperature, colorless needle-like crystals were collected, washed with deionized H 2 O and dried in air (Yield: 60% based on H 2 L). Elemental analysis calculated for C 16

Single Crystal X-ray Crystallography
Single crystals of suitable size were placed on a Xcalibur, Atlas, Gemini ultra CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å), using the ω-θ scan mode in the ranges 3.4° ≤ θ ≤ 29.8° (1), 3.5° ≤ θ ≤ 25° (2). The structures were solved by direct methods using SHELXS-2014 and refined by full-matrix least-squares on F 2 using SHELXS-2014 [31,32]. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were positioned geometrically and refined as riding. The measured crystallographic data are shown in Table 1. The relevant hydrogen bonding parameters were provided in Table 2.

Description of [Cu 2 (L) 2 (4,4′-bpy)] n (1)
Single-Crystal X-ray diffraction analysis reveals that complex 1 crystallized in monoclinic system with a space group of P2 1 /c. The asymmetric unit of 1 contains one Cu(II), one L 2− ligand and a half of 4,4′-bipyridine ligand. As depicted in Fig. 1b, each Cu(II) center is four-coordinated by two N atoms from two independent L 2− ligands, another N atom from 4,4′-bipyridine ligand and a carboxylate O atom, forming a distorted tetrahedral Cu(N3O) which is consistent to those previously reported [33][34][35]. The Cu-N bond lengths vary from 1.948(8) to 1.996(8) Å, while Cu1-O1 bond length is 1.962(7) Å, which are all in the normal range. The L 2− ligand is completely deprotonated with three coordination atoms adopting monodentate and chelated modes, in the asymmetrical unit it adopts μ 2η 1 : η 2 coordination mode. In complex 1, two Cu(II) metal centers were chelated by two L 2ligands forming a dinuclear structure, while auxiliary 4,4′-bipyridine ligand plays a role in bridging the adjacent metal units, help expanding into a 1D chain. As shown in Fig. 1c, a 2D layer structure is formed by hydrogen bonds C(5)-H(5)⋯O(1) i (Symmetry code: (i) 2 − x, 1 − y, − z) between the carboxylate oxygen (O1) and carbon (C5) atom from 4,4′-bipyridine. The 2D layers are further outspread to 3D networks by the intermolecular interaction C(2) ii -H (2)

Description of [Cd(HL)Cl(1,10′-phen)] n (2)
Complex 2 crystallized in monoclinic system with P2 1 /c space group and exhibits a 1D chain structure. Each crystallographic independence unit comprises one Cd (II), one HL − ligand, one Cl − anion, and one 1,10′-phen. Cd1 center is six-coordinated and it is in a distorted octahedral environment with N1, N2 from the auxiliary 1,10′-phen ligand, O1A from a HL − ligand, N3 and O2 from another HL − ligand (Fig. 2a) [36]. The bond angles surrounding the Cd(II) ion vary from 68.89(6) to 148.50(7)°. Each HLligand connects two Cd atoms with one N atom and one carboxylate O atom chelating a Cd atom, and another carboxylate O atom links adjacent Cd atom to produce a 1D chain structure, the HLligand in the asymmetrical unit adopt μ 2 -η 1 : η 1 : η 1 coordination mode. From Fig. 2c, it is noteworthy that the polymeric chains are further assembled by interchain π⋯π packing interactions, resulting in a 2D crystal structure of 2.

Hirshfeld Surface Analysis
The weak intermolecular interaction such as hydrogen bond, π⋯π stacking interactions and so on play a vital role in the assemble of crystal structure. To study different types of intermolecular interactions existed in the crystal structure, a Hirshfeld surface analysis was carried out with Crystal Explorer program (Figs. 3 and 4) on their asymmetric units. Figure 3 demonstrated the Hirshfeld surface mapped over d norm (− 1.1281 to 0.9996); Shape Index(− 1.0 to 1.0) and Curvedness(− 4.0 to 4.0). The d norm is a normalized distance and displays a surface with a red-white-blue colour: the red spots represent close-contacts in the crystal structure of the studied complexes, such as C-H⋯O interactions, the blue and white regions represent the weak intermolecular interaction, like C⋯H and H⋯H interaction [37].The adjacent red/ blue triangles on the shape-index of the Hirshefld surface indicating the π⋯π stacking interactions.
The 2D fingerprint plots clearly show the quantitative of different types of intermolecular interaction. In complex 1, the maximum contribution in the 2D fingerprint plot is from H⋯H contacts (25.1%), which is reflected in the middle of the 2D fingerprint plot. The next is C⋯H contacts (18.7%) which result from the presence of C-H⋯π interactions. In complex 2, the highest interatomic contact contribution is between hydrogen atoms, H⋯H contacts (26.3%). The contribution of Cl⋯H/H⋯Cl contacts is 16.2%, its 2D fingerprint plots present two sharp spikes pointing towards the lower left, the above sharp spike corresponds to the donor of the hydrogen bond and the other is acceptor, which is accordance with the C-H⋯Cl hydrogen bond. The proportions of C⋯H/H⋯C, N⋯H/H⋯N, Cd⋯O/O⋯Cd is 24.7%, 6.5%, 4.7% of the total area of the Hirshfeld surfaces respectively (seen in Figure S3).

PXRD and TG Analyses
In order to confirm the phase purities of complexes 1 and 2, powder X-ray diffraction experiments have been performed ( Figure S1). It is obvious that the experimental PXRD patterns are in accordance with the simulated ones, indicating samples crystallize in the pure phase. The thermogravimetric analysis (TGA) under a nitrogen atmosphere increasing from 25 to 800 °C with a rate of 10 °C/min were carried out in a platinum crucible to investigate the thermal behaviors of complexes 1 and 2. The TG curves are shown in Fig. 5. For complex 1, no obvious weight loss can be observed until 250 °C, at which point it begins to break down. In the temperature range of 250-320 °C a rapid weight loss of 47.9% occurred, which can be described to the decomposition of a L 2− ligand (calculated: 44.52%) in complex 1. For complex 2, there is a platform before 320 °C, after that followed by the framework's collapse. The first mass loss occurred in 350 °C and showed about 13.28% weight loss that can be assigned to the loss of Cl atom, calc. 15.35%. With heating, the structure suffered an obvious weight loss of 38.18% (calc. 45.14%) at 480 °C, which can be relevant to the removal of the 1, 10′-phen ring. Based on the staring decomposition temperatures, it showed that complex 2 is relatively stable.

ECL Properties of 1 − 2
The ECL properties of pure complex 1 and complex 2(the phase purity of 1-2 has been checked by PXRD) were assessed in DMF solution and the results were showed in Fig. 6. Cyclic voltammetry (CV) was studied to investigation the redox properties of 1-2. All experiments used a standard  Fig. 6a, when 0.1 mol/L K 2 S 2 O 4 has been added into the solution, an oxidation peak in the cathodic current at −1.2 V for complex 1 and −1.4 V for complex 2, a reduction peak in the cathodic current are all at −2.0 V.
For complexes 1 and 2 the maximum luminous intensity are about 1870 and 1286 a.u.. After ten circulations, the ECL intensity still remain stable, it can be arise from the complexes did not collapse during tests. Compared with complex 2, complex 1 has higher the maximum luminous intensity. Taken Ru(bpy) 3 3+ as the standard for ECL efficiency [38], resulting in ECL yields of 0.42 and 0.29. It is tentatively suggested that the introduction of Cu(II) and Cd(II) ions could strengthen the spin-orbit coupling effect and enhance the ECL emission [21]. Compared to Cu(II) complexes [39] and Cd(II) complexes [40] reported before, the two complexes exhibit a stronger ECL emission and higher stability, and this result maybe help us to construct new ECL materials.

Conclusion
In summary, two new coordination polymers have been successfully assembled based on 1H-pyrazole-3-carboxylic acid by adding auxiliary ligands. The 1D chain structures of these two complexes are further extended to 3D supramolecular networks by weak intermolecular interaction. The weak intermolecular interaction such as hydrogen bond, C-H⋯π stacking interactions existed in the crystal structures can be quantification by Hirshfeld surface analysis and its fingerprint plots. The Hirshfeld surface analysis results indicate that the most important contributions for the two complexes are from H⋯H and C⋯H contacts. Additionally, these two transition metal complexes manifest stronger ECL emission and higher stability than that reported before, which reveal that these complexes may offer new ECL materials.

Supplementary Material
Crystallographic data for the structures in this work have been deposited to the Cambridge Crystallographic Data Centre, CCDC 1941495 and 2036652. These data can be obtained free of charge via http:// www. ccdc. cam. ac. uk/ data_ reque st/ cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12,Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.