Synthesis and characterizations of three layered ZnII, CdII and MnII coordination polymers with a flexible ligand [3-(2-pyridyl)-1-pyrazolyl] acetic acid

Self-assembly of flexible [3-(2-pyridyl)-1-pyrazolyl] acetic acid (Hpypza) with ZnCl2·4H2O, CdCl2·4H2O, and Mn(OAc)2·4H2O affords three new coordination polymers [M(pypza)(HCOO)]n (M = Zn for 1; Cd for 2) and [Mn(pypza)(OAc)]n (3), which are significantly different from the previously reported complexes [Zn(pypza)Cl]2, [Cd(pypza)Cl]n, [Mn(pypza)Cl]2, and [Mn(pypza)Cl]n. These structural differences may be ascribed to the use of different solvents and anions. Single-crystal X-ray diffraction analyses of these polymers indicate that 1 and 2 are isostructural with a 2-D coordination network via bridging pypza− and HCOO− resulting from the hydrolysis of DMF, and 3 also displays a 2-D network connected via pypza− and OAc−. These complexes have been characterized by IR, microanalysis, and powder X-ray diffraction techniques. In addition, the solid fluorescence and thermal stability properties of 1–3 have also been investigated. Magnetic properties of the MnII complex have also been measured. Graphical Abstract Self-assembly of flexible [3-(2-pyridyl)-1-pyrazolyl] acetic acid (Hpypza) with ZnCl2·4H2O, CdCl2·4H2O, and Mn(OAc)2·4H2O affords three new coordination polymers [M(pypza)(HCOO)]n (M = Zn for 1; Cd for 2) and [Mn(pypza)(OAc)]n (3), which are significantly different from the previously reported complexes [Zn(pypza)Cl]2, [Cd(pypza)Cl]n, [Mn(pypza)Cl]2, and [Mn(pypza)Cl]n. These structural differences may be ascribed to the use of different solvents and anions.


Introduction
The design and synthesis of coordination polymers have received interest in recent years for their structural diversity and potential applications in catalysis, gas storage, optics, electronic conductivity, and magnetism [1][2][3][4][5][6]. Choice of organic building blocks plays an important role in such architectures [7][8][9]. Carboxylate and/or pyridyl-containing ligands constitute an important family of building blocks and have been selected to construct a variety of coordination architectures because of their robust and rich binding tendencies [10][11][12][13]. Pyrazole-based ligands have also been shown to form stable metal complexes due to elaboration of the pyrazole ring by various organic fragments [14][15][16]. Considering the above facts, we chose a pyrazole-derived ligand, [3-(2-pyridyl)-1-pyrazolyl] acetic acid (Hpypza), to construct coordination polymers with metal ions. As a pyrazole-derived ligand, Hpypza possesses pyridine, pyrazole, and carboxyl groups which should display diverse coordination, and the -CH 2spacer between the pyrazole ring and carboxylate group offers flexible orientations of the carboxylate arm, favoring formation of various framework structures. Hpypza can be considered as an excellent and versatile building block, which has already been chosen to construct new crystalline materials by many scientists [17][18][19]. Among them, Yang 2 , and [Mn(pypza)Cl] n from assembly of Hpypza and ZnCl 2 , CdCl 2 or MnCl 2 , respectively, in water [20].
The type of solvent and anion influence the architectures of the coordination polymers [21][22][23]. With these considerations in mind, we aim to study the solvent and/or anion effect based on Hpypza. In this work, we chose [3-(2-pyridyl)-1-pyrazolyl] acetic acid as the ligand to assemble with ZnCl 2 , CdCl 2 , and Mn(OAc) 2 in DMF to afford three similar 2-D coordination layers [M(pypza)(HCOO)] n (M = Zn for 1; Cd for 2) and [Mn(pypza)(OAc)] n (3) under hydrothermal reaction conditions. The formate in 1 and 2 is in situ generated by hydrolysis of DMF. However, the HCOO − is not observed in 3 due to stronger binding ability of OAc − relative to HCOO − . Complexes 1-3 have been characterized by IR, microanalysis, and powder X-ray diffraction (PXRD) techniques. Moreover, solid-state properties such as fluorescence, magnetic property, and thermal stability of all complexes have also been explored and discussed.

Description of crystal structures of 1 and 2
Single-crystal X-ray diffraction analysis indicates that 1 and 2 are isostructural, and therefore, only the crystal structure of 1 will be described in detail. Notably, the attempt to assemble Hpypza with M(OAc) 2 /M(NO 3 ) 2 (where M = Zn or Cd) in place of MCl 2 afford no X-ray diffraction suitable crystals. The asymmetric unit of 1 is composed of one Zn II center, one pypza − , and one formate hydrolyzed from DMF. As depicted in figure 1(a), each distorted octahedral Zn II center is surrounded by three oxygens (O1A and O2A from the chelating carboxyl of pypza − and O4B from formate) and one nitrogen (N1 from the pyridyl ring of pypza − ) in the basal plane, and one nitrogen (N2 from the pyridyl ring of  reported the framework compound (Me 2 NH 2 )[Zn(HCO 2 ) 3 ] in which both formate ligands and dimethylammonium cations arise from DMF hydrolysis [37]. For 3, HCOO − is not observed in the structure, due to the stronger binding ability of OAc − relative to HCOO − . In addition, we added HCOO − to reaction system of 3 affording the same block single crystals as 3, which further confirm the stronger binding ability of OAc − relative to HCOO − . Hpypza has been investigated frequently to assemble with several metal salts previously [17][18][19]. Hpypza has different bridging coordination such as bidentate, tridentate, and quadridentate modes. The pypza − ligands are quadridentate chelating in 1-3. In this complex, the Cd II is pentagonal bipyramidal, different to the octahedral Cd II in 2. In addition, Leciejewicz et al. [40] reported a mononuclear Mn II complex based on 2-pyrazinecarboxylic acid. In this complex, Mn II also adopts octahedral geometry, defined by pyrazine and carboxyl groups of ligand, and water ligands.

IR, PXRD, fluorescence, TGA, and magnetic properties
In IR spectra of 1-3, the absence of the characteristic absorption at 1650-1700 cm −1 indicates complete deprotonation of carboxyl. The phase purities of the complexes were also identified by PXRD patterns, which show essential similarity to the corresponding calculated patterns (see Supporting Information). It should be pointed out that the provided PXRD patterns are from several batches of crystals because of the low yield. Solid-state fluorescent properties of 1-3 were also measured at room temperature (see figure 3). Excitation of the microcrystalline samples leads to different fluorescent emissions showing emission maxima at 496 nm (λ ex = 290 nm) for 1, 581 nm (λ ex = 337 nm) for 2, and 496 nm (λ ex = 337 nm) for 3. To explore the origin of these emission bands, the fluorescence spectra of Hpypza has also been measured (see Supporting Information), which show the emission peak at 375 nm (λ ex = 322 nm). In comparison to that of Hpypza, the significant redshift for the maximum emissions of 1-3 may be ascribed to ligand-centered transitions caused by metal-ligand coordinative interactions [41][42][43][44][45]. An obvious decrease of intensity is observed for these emission bands as compared to the free ligand, especially for Mn II complexes. TGA experiments were carried out to explore their thermal stability, which is an important parameter for coordination polymers (see figure 4). The TGA curves of 1-3 are similar due to their isostructural nature. The first weight loss of 0.7% for 1, 1.8% for 2, and 0.7% for 3, from room temperature to 269, 213, and 306°C, respectively, may be ascribed to water absorption for 1-3. The remaining substances follow a series of sharp weight losses and hold a weight of 42.1%, 8.5%, and 39.3% of the total samples, respectively, until further heating to 600°C. The material loses almost 92% for 2, which can be ascribed to sublimation of CdO.
Magnetic susceptibility measurements for 3 were performed from 2 to 300 K with an applied field of 1000 Oe. The plot of χ m T versus T is presented in figure 5. As can be seen from plots of experimental data, the experimental χ m T value at 300 K is 4.55 cm 3 ·M −1 ·K, slightly larger than the spin-only value of 4.38 cm 3 ·M −1 ·K for one highspin Mn(II) (g = 2, S = 5/2). The χ m T value remains almost constant from 300 to 60 K,   and then decreases on further cooling, reaching a value of 1.05 cm 3 ·M −1 ·K at 2 K. This behavior indicates a dominant antiferromagnetic interaction in 3.

Conclusion
Three new 2-D Zn II , Cd II , and Mn II coordination complexes based on Hpypza have been synthesized and characterized using different anions compared to the reported literature. The HCOO − ligands in 1 and 2 are in situ generated by hydrolysis of DMF, which is not observed in 3. Our study shows that the change of solvents and anions can influence the subtle variables that lead to coordination polymers with different structures. Systematic exploration of effects on the construction of coordination polymers based on Hpypza is underway in our group.