Synthesis and Structure of a New Erbium Porous Complex Assembled from 2,3-Pyrazinedicarboxylic Acid and Ammonia

A new erbium porous complex, (NH 4 ) 2 [Er 2 (pzdc) 4 (H 2 O) 2 ], was synthesized through hydrothermal reaction of 2,3-pyrazinedicarboxylic acid and Er(NO 3 ) 3 ·6H 2 O with the pH value adjusted by ammonia. It was characterized by elemental analysis, FT-IR, TGA and further by single-crystal X-ray crystallography. It crystallizes in monoclinic, space group P2 1 /c with a = 14.125(3), b = 15.706(2), c = 12.726(3) Å, β = 95.714(4)°, V = 2809.4(10) Å 3 , Z = 4, D c = 2.385 g/cm 3 , µ(MoKa) = 6.05 mm −1 and F(000) = 2056. 2056 reections were measured and 6446 independent reections (R int = 0.0695) were used in further renement. The complex exhibits a 3D framework constructed from the [Er 2 (pzdc) 4 (H 2 O) 2 ] 2− building blocks. It is scarce that the NH 4+ cations are located in the channels and balance the charge of anion framework. Furthermore, the ion exchange property of this complex has also been studied. The NH 4+ cations can be replaced by Na + cations, which is an uncommon phenomenon for gadolinium-containing porous complex.


Introduction
The investigation of porous metal-organic frameworks (MOFs) has attracted much interest due to their structural diversity and promising applications for ion exchange, gas storage, separation, and catalysis [1][2][3][4][5] . The multicarboxylic ligands have been widely used as bridging ligands to construct MOFs with diverse structures and topologies [6] , owing to their various coordination modes, strong coordination ability, chemical stability and structure rigidity [7,8] . 2,3-pyrazinedicarboxylic acid (H 2 pzdc) is a commonly used multifunctional ligand [9][10][11][12][13] . However, much of the work has so far focused on the assembly of the dblock MOFs, while the lanthanon-based MOFs are relatively less developed [14,15] . [16][17][18][19] . In this paper, we report the syntheses, crystal structures, and ion exchange property of a new Erbium-based MOFs complex, (NH 4 ) 2 [Er 2 (pzdc) 4 (H 2 O) 2 ], which was assembled from 2,3-pyrazinedicarboxylic acid and ammonia under hydrothermal condition. Experimental 2. 1 Reagents and physical measurements All reagents and solvents were commercially purchased and used without further puri cation. Elemental contents of carbon, hydrogen and nitrogen were determined by a German Vario MICRO analyzer. IR spectrum of the complex was recorded on a SpectrumOne FT-IR spectrometer in the 4000-400 cm -1 region using KBr pellets at room temperature. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 449C thermal analyzer under air atmosphere with a heating rate of 10℃ min -1 . Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 Advance X-ray diffractometer using Cu-Kα radiation (l = 1.5418 Å), in which the X-ray tube was operated at 40 kV and 40 mA. The Inductively Coupled Plasma OES spectrometer (ICP) data were recorded on Ultima2. Working band: 190-800 nm, Wavelength scanning step: 0.001 nm, Detection limit (LD): can be as low as 0.5 ppb.

2 Structure determination
A colorless crystal of the title compound having approximate dimensions of 0.20mm × 0.20mm × 0.20mm was mounted on the top of a glass ber. X-ray crystallographic data collection for the complex was performed on a Rigaku-CCD diffractometer equipped with a graphite monochromated Mo-Ka radiation (l = 0.71073 Å) by using the ω-scan mode at 293 K. All absorption corrections were applied using the CrystalClear program [20] . The structures were solved by direct methods, the metal atoms were located from the E-maps, and other non-hydrogen atoms were derived from the successive difference Fourier peaks. The organic hydrogen atoms in the pzdca ligands were positioned geometrically, and allowed to ride on their parent C atoms. The hydrogen atoms on the water molecules and the ammonium cations were not located because the data is not good. The structure was re ned on F 2 by full-matrix least-squares using the SHELXTL-97 program package [21,22] . A summary of the crystallographic data of this complex was presented in tables 1 and 2.   (5) 4)The NH 4 + cations are located in the channels Not only do they balance the charges of anion framework but also they are hydrogen bonded to the carboxylate groups O atoms surrounding them. The anion framework is stabilized by these N-H…O hydrogen Bonds.

Thermal behavior
To evaluate the thermal stability of the complexes, their thermal gravimetric analysis (TGA) have been performed and found the complex can be seen that there is no signi cant mass loss up to about 325℃, and then has one step of continuous mass loss between325℃and 800℃, corresponding to the decomposition of the complex.The higher decomposition temperature indicates that the framework of the two complexes possesses good thermal stability.The TGA curve of the complex is shown in Fig. 5. It can be seen that the comple is stable up to about 325℃.

XRD patterns
The X-ray powder diffraction data of the complex was collected. As shown in Fig. 6, the experimental XRD pattern agree well with the simulated pattern generated on the basis of the single-crystal analyses for compound , suggesting the phase purity of the product.

Ion exchange property
Single-crystal X-ray diffraction shows that the complex exhibit 3D network structure based on the [Er 2 (pzdc) 4  conditions and exhibit 3D network structure. The complex has ion exchange property and present better thermal stability before 325°C.

Figure 2
The coordinate modes of ligand in complex Figure 3 The coordinate modes of Er 3+ in complex Page 12/13

Figure 4
The 3D structure of the complex along a and b axis Page 13/13 Figure 5 TG curve of the complex Figure 6 The XRD patterns of the complex : (a) simulated; (b) experimental.