КООРДИНАЦИОННАЯ ХИМИЯ, 2014, том 40, № 5, с. 305-314
УДК 541.49
CONSTRUCTION OF 0D TO 3D COPPER(II) MOFs BASED ON HETEROCYCLIC CARBOXYLIC ACID: SYNTHESIS, STRUCTURES, CYCLIC VOLTAMMOGRAMS, AND LUMINESCENT PROPERTIES © 2014 H. M. Zhang, Q. Q. You, L. Z. Wu, L. Liu, and L. R. Yang*
Henan Key Laboratory of Polyoxometalate, Institute of Molecule and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004 P.R. China
*E-mail: lirongyang@163.com Received May 16, 2013
The crystal structures of MOFs [Cu(PDA)(Phen)(H2O)]2 • 5H2O (I) and [Cu(PZCA)2(H2O)2] ■ 2H2O (II) (H2PDA = pyridine-2,6-dicarboxylic acid, Phen = 1,10-phenanthroline, HPZCA = pyrazine-2-carboxylic acid, H2PZDA = pyrazine-2,3-carboxylic acid) have been prepared under hydrothermal conditions. These MOFs have been characterized by element analysis, single-crystal X-ray diffraction, thermogravimetric analyses and IR spectroscopy. 3D frameworks of MOFs I and II are fabricated from zero-dimensional (0D) motifs through hydrogen bonds and п—п interactions. In MOF II, the PZCA ligand comes from in situ decarboxylation of the part of pyrazine-2,3-dicarboxylic acid (H2PZDA). Luminescent emissions bands of MOF I in methanol have been measured at room temperature and it displays selectivity to Zn2+, Cu2+, Pb2+, and Cd2+ ions. Cyclic voltammetry of MOFs I and II showed that the Cu(II/I) couple is irreversible.
DOI: 10.7868/S0132344X14040124
INTRODUCTION
In recent years, the design and synthesis of metal-organic-frameworks (MOFs) structures have received enormous attention, because of their novel and potential applications in host guest chemistry, catalysis, electrochemical, luminescent and magnetism [1—6]. MOFs assembly based on the molecular building block method which has been introduced as a suitable method to design and construct these kinds of materials [7]. In light of that, many new organic ligands contribute to the occurrence of the varieties of novel polymers. Some multidentate carboxylic acid ligands containing N and O donor atoms have been widely used to assemble MOFs [8—10]. One feasible strategy involving the use of multicarboxylate ligands as the organic building block, such as pyridine-2,6-dicarboxylic acid (H2PDA) and pyrazine-2,3-dicarboxylic acid (H2PZDA) and their deprotonated anions (HPDA-, PDA2-, HPZDA- and PZDA2- tuned under appropriate pH value) adopt flexible, multidentate coordination sites, and therefore may potentially provide various coordination modes which are in favor of the construction of higher-dimensional MOFs frameworks [8, 11-16]. Herein, we have focused on developing the rational design and synthesis of this kind of frameworks.
The present paper is to report the hydrothermal synthesis, characteristics, electrochemical results, thermal analysis and the crystal structures of the 3D coordination polymers of Cu(II) connected by hydrogen bonds and n-n stacking - [Cu(PDA)(Phen)(H2O)]2 • 5H2O (I)
and [Cu(PZCA)2(H2O)2] ■ 2H2O (II). Emission spectra of MOF I in the presence of Cd2+, Cu2+, Mg2+, Pb2+, and Zn2+ ions have been studied and compared with those of the assynthesized MOFs.
EXPERIMENTAL
Reagents and general techniques. All chemicals are analytical grade and used without further purification. Elemental analyzer was performed on a Perkin-Elmer 240C elemental analyzer. Infrared spectra were recorded in the 4000-400 cm-1 region using KBr pellets on an AVATAR 360FT-IR spectrometer, the crystal structure was determined on a Bruker Smart CCD X-ray single-crystal diffractometer. Fluorescent data were collected on F-7000 FL Spectrophotometer at room temperature. The TG and DTG experiment were performed using a PerkinElmer TGA7 thermo-gravimeter. The heating rate was programmed to be 10 K min-1 with the protecting stream of N2 owing at 40 mL min-1. Cyclic voltammograms were obtained on a CHI650 electrochemical analyzer at 25°C.
Synthesis of MOF I. A mixture of Cu(Ac)2 ■ H2O (59.90 mg, 0.3 mmol), H2PDA (50.11 mg, 0.3 mmol), Phen (54.02 mg, 0.3 mmol) and water (10 mL) was homogenized by stirring for 30 min, then transferred into 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 150°C for 96 h. The reaction system was cooled to room temperature and then left at room temperature for 21 days. Blue prism crystals of
MOF I suitable for X-ray diffraction analysis were obtained with a yield of 78.6%.
For C38H36N6O15Cu2
anal. calcd., %: Found, %:
C, 48.31; C, 49.04;
H, 3.81; H, 3.45;
N, 8.90. N, 8.56.
IR spectrum (v, cm-1): 3424 b, 3091 w, 3013 w, 1622 s, 1590 s, 1519 m, 1426 s, 1364 s, 1275 m, 1147 w, 1110 w, 1068 w, 1033 w, 912 w, 857 m, 812 w, 778 w, 722 s, 690 w, 647 w, 448 w, 405 w.
Synthesis of MOF II. A mixture of Cu(Ac)2 ■ H2O (59.90 mg, 0.3 mmol), H2PZDA (50.4 mg, 0.3 mmol), and water (10 mL) was homogenized by stirring for 30 min, then transferred into 25 mL Teflon-lined stainless steel autoclave under autogenous pressure at 150°C for 96 h. After cooling the reaction system to room temperature, blue prism crystals of II suitable for X-ray diffraction analysis were isolated with a yield of 49.1%.
For C10H14N4O8Cu
anal. calcd., %: Found, %:
C, 31.43; C, 31.02;
H, 3.66; H, 3.24;
N, 14.67. N, 14.98.
IR spectrum (v, cm-1): 3450 b, 1636 s, 1600 s, 1572 m, 1475 w, 1450 w, 1414 s, 1363 s, 1292 s, 1269 w, 1176 m, 1159 m, 1101 w, 1032 w, 863 m, 796 m, 771 w, 711 s, 699 w, 661 w, 609 w, 458 w.
X-ray crystallographic determination. Single-crystal diffraction data I and II were collected suitable single crystals of the MOF on a Bruker Smart CCD X-ray single-crystal diffractometer with graphite monochro-mated Mo^-radiation (X = 0.71073 A) at 296(2) K. All independent reflections were collected in a range of 2.25°-25.00° for MOF I and 3.02°-24.98° for MOF II (determined in the subsequent refinement). Multi-scan empirical absorption corrections were applied to the data using the SADABS [17]. The crystal structure was solved by direct methods and Fourier synthesis. Positional and thermal parameters were refined by the full-matrix least-squares method on F2 using the SHELXTL software package [18]. The final least-square cycle of refinement gave R1 = 0.0275, wR2 = 0.0770 for I and R1 = 0.0300, wR2 = 0.0790 for
II, the weighting scheme w = 1/[a2(Fo2) + (0.0429P)2 +
+ 0.55P] for MOF I and w = 1/[a2(Fo2) + (0.0306P)2 +
+ 0.67P] for MOF II, where P = (Fo2 + 2Fc2)/3. A summary of the key crystallographic information is given in Table 1. Selected bond lengths and band angles for the MOFs I and II are listed in Table 2.
Supplementary material for the MOFs I and II has been deposited with the Cambridge Crystallographic Data Centre (nos. 826477 (I), 827162 (II); deposit@ccdc. cam.ac.uk or http://www.ccdc.cam.ac.uk).
RESULTS AND DISCUSSION
MOFs I and II are stable in the solid state upon exposure to air and insoluble in common solvents, such as CH3COCH3, CH3CH2OH, CHCl3, CHCl2, CH3CN, THF or DMF, but they are soluble in DMSO and CH3OH. The solid state properties of the two MOFs were examined by IR spectra and recorded in the range of 4000-400 cm-1 suggest that the PDA2-, Phen (MOF I) and PZCA (MOF II) ligands coordinated to the central Cu2+ ions. The IR spectra of the MOFs show a broad band at 3424 cm-1 for MOF I and 3450 cm-1 for MOF II due to the OH groups of water molecules. Strong absorption bands in the ranges 1622-1519 cm-1 (I) and 1636-1572 cm-1 (II) correspond to COO- vibrations groups of the PDA2- and PZCA ligands [9]. Two strong bands are observed at 1426, 1364 cm-1 (I) and 1414, 1363 cm-1 (II) that corresponds to the C-O vibration of the PDA2- and PZCA ligands [19, 20], respectively. The absence of the characteristic bands around 1700 cm-1 indicates that the H2PDA and PZCA ligands of MOFs I and II are completely deprotonated in the form of PDA2- and PZCA-anions upon reaction with the metal ions. The weak bands 405 to 690 cm-1 (I) and 458 to 699 cm-1 (II) may be ascribed to Cu-N and Cu-O stretching vibrations. The conclusions above all are supported by the results obtained by X-ray diffraction measurements [21].
The unit of MOF I consists of two six-coordinated Cu(II) atoms, two independent PDA2- ligands, two Phen ligands and two coordinated water molecules (Fig. 1a). The independent Cu2+ ions present a distorted octahedral geometry. The equatorial sites of the polyhedron are occupied by N(1) donor sets from PDA2-, N(2), N(3) from Phen donor and O(1w), while the axial vertexes are occupied by O(2) and O(3) from PDA2-. The average distances of Cu(1)-O(2) and Cu(1)-O(3) (O(2) and O(3) belonging to PDA2-ligand) is 2.312 A, which is markedly longer than that of Cu(1)-O(1w) (2.029(15) A). The bond length of Cu(1)-N(1) (1.988(17) A) is slightly shorter than the average bond length (2.028 A) of Cu(1)-N(2) and Cu(1)-N(3) (N(2) and N(3) deriving from Phen ligand), suggesting that the tridentate chelating effect of PDA2- ligand is more stronger than that of Phen ligand. The lengths of Cu-N, Cu-O and Cu-O(W) are consistent with those in the similar MOFs reported [4, 5]. The adjacent 0D units are connected by the hydrogen bonds (O(5w)-H(5wP)-O(8)#6, O(7w)-H(7w^)-O(5w)#1, O(4w)-H(4wA)-O(7w) and O(4w)-H(4w w)---O(5)) to construct an infinite 1D chain, which are further arranged into 2D sheet through hydrogen bonds (Fig. 1b). The neighboring 2D sheets are connected through hydrogen bonds (O(6w)-H(6wA)-O(1)#2, O(5w)-H(5wA)-O(1), O(3w)-H(3wA)-O(4)#3, O(7w)-H(7wP)-O(5w)#4, O(6w)-H(6wP)-O(5)#5, O(3w)-H(3wP)-O(6w)#2,
Table 1. Crystallographic data and experimental details for MOFs I and II
Parameter Value
I II
Formula weight 943.81 381.79
Crystal system Triclinic Monoclinic
Space group P T P21/c
a, A 10.0603(13) 6.9088(5)
b, A 13.8635(18) 14.1990(11)
c, A 14.2575(19) 8.2807(6)
a, deg 97.601(2) 90
ß, deg 97.724(2) 112.435
Y, deg 98.103(2) 90
Z 2 2
Volume, A3 1927.5(4) 750.84(10)
Pcalcd mg m-3 1.626 1.689
/(000) 968 390
Crystal
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