ZHANG Mei-Li WANG Ji-Jiang CHEN Xiao-Li
(Department of Chemistry and Chemical Engineering, Yan′an University, Shaanxi 716000, China)
In recent years, the design and construction of metal-organic frameworks (MOFs) are interest in the field of supramolecular chemistry and crystal engineering due to their fascinating structures and great potential applications[1-4]. Among them, lanthanidebased metal-organic frameworks (LMOFs) are particularly desirable for their luminescence, electronic, optical, and chemical characteristics arising from the 4f electronic shells. The aromatic N- and O-containing polycarboxylic ligands are often regarded as an effective strategy for the construction of LMOFs[5,6]. In these kinds of ligands, rigid polycarboxylic acids, especially aromatic dicarboxylic acids, are frequently used to link lanthanide ions or lanthanide clusters[7-11]. Until now, most of the LMOFs are built from just one type of multicarboxylate ligands, if adding the auxiliary ligands in the synthesis of LMOFs may not only result in new structure types but also bring interesting properties[12-14]. With the above in mind, we would like to synthesize and explore new LMOFs with 1,3-phenylenediacetate and different auxiliary ligands, to study the influence of auxiliary ligands on the framework structures of their complexes. Herein, we report two new Pr3+coordination polymers with 1,3-pda, 1 and 2 which contain 1,3-pda spacers and nbca/2,2?-bpy.
All the solvents and reagents for syntheses were commercially available and used as received. The FT-IR spectra were recorded (KBr pellets) on a PerkinElmer Spectrum One FT-IR spectrometer.Elemental analyses were performed on a Perkin-Elmer 2400 Series II analyzer. Powder X-ray diffraction (PXRD) patterns were taken on a Rigaku D/max-2500 diffractometer for CuKα radiation (λ =1.5406 ?), with a scan speed of 2°·min–1and a step size of 0.02° in 2θ. TG analyses were carried out with a NETZSCH STA 449C microanalyzer in a nitrogen atmosphere at a heating rate of 10 ℃·min–1.
[Pr(1,3-pda)0.5(nbca)2(H2O)2] (1) was synthesized from the reaction mixture of Pr(NO3)3·6H2O (0.4 mmol), 1,3-H2pda (0.4 mmol), Hnbca (0.8 mmol),and water (10 mL) in a 25 mL Parr Teflon-lined stainless-steel vessel under autogenous pressure at 140 ℃ for 96 h, and then cooled to room temperature at a rate of 5 ℃/h. Green block crystals of 1 were obtained (yield 51% based on Pr). Anal. Calcd.for C31H24N2O12Pr: C, 49.15; H, 3.19; N, 3.70%.Found: C, 49.32; H, 3.25; N, 3.68%. IR data (KBr cm–1): 3458 (br), 2941(w), 1642(s), 1401(s),1226(w), 1118(s), 1014(m), 812(m).
[Pr2(1,3-pda)3(2,2?-bpy)2(H2O)2] (2). A synthetic procedure similar to that for 1 was used except that Hnbca was replaced by 2,2?-bpy for 2, with different yield (45% based on Pr). Anal. Calcd. for C47H44N4O14Pr2: C, 48.20; H, 3.79; N, 4.79%. Found:C, 48.52; H, 3.45; N, 4.48%. IR data (KBr cm–1):3404 (br), 1673(s), 1423(s), 1276(m), 779(w), 727 (w).
Single-crystal X-ray diffraction data for complexes 1 and 2 were collected on a RIGAKU RAXIS-RAPID diffractometer with MoKa radiation(λ = 0.71073 ?) at room temperature. Both structures were solved by direct methods using the SHELXS program and refined by full-matrix leastsquares methods with SHELXL[15,16]. Metal atoms in each complex were located from E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses, which were refined with anisotropic thermal parameters on F2. The H atoms of ligands were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. The H atoms of water molecules were located using difference Fourier method and refined freely. Complex 1 is of monoclinic system, space group C2/c with a = 27.2638(13), b =10.1066(5), c = 21.4387(10) ?, β = 94.258(1)o, V =5891.0(5) ?3, Z = 8, μ = 1.725 mm-1, reflections collected/unique data 15099/5487, crystal size 0.26mm × 0.23mm × 0.20mm, scan mode φ-ω, θ range for data collection 1.50~25.50o, Rint= 0.0140,GOOF = 1.134, Ra= 0.0271 and wRb= 0.0760.Complex 2 is of triclinic system, space group P1 with a = 10.7167(16), b = 10.9125(17), c =12.1124(18) ?, α = 115.901(2), β = 97.107(2), γ =103.802(2)o, V = 1194.6(3) ?3, Z = 1, μ = 2.085 mm-1, reflections collected/unique data 6238/4393,crystal size 0.23mm × 0.21mm × 0.18mm, scan mode φ-ω, θ range for data collection 1.94~25.50,Rint= 0.0151, GOOF = 1.078, Ra= 0.0264 and wRb=0.0674 (aR = Σ(||Fo| - |Fc||)/Σ|Fo|,bwR = [Σw(|Fo|2-|Fc|2)2/Σw(Fo2)2]1/2). Selected bond lengths and bond angles for complexes 1 and 2 are listed in Tables 1 and 2, respectively.
Table 1. Selected Bond Lengths (?) for Complexes 1 and 2
Table 2. Selected Bond Angles (o) for Complexes 1 and 2
The structures were determined by single-crystal X-ray diffraction analyses. Complex 1 is monoclinic with space group C2/c. Complex 2 is triclinic with space group P1.
Fig. 1. Coordination environments of Pr atom in complex 1.All hydrogen atoms are omitted for clarity. Symmetry codes: #1: –x, –y + 1, –z + 1; #2: –x, –y, –z + 1
In 1, each Pr atom is surrounded by eight oxygen atoms from one 1,3-pda, four individual nbca ligands and two water molecules, forming a distorted square-antiprism geometry (Fig. 1). The Pr–O distances are from 2.427(2) to 2.925(3) ?, which drop into the normal scope of Pr–O bond lengths[17].Two such Pr centers are bridged by a pair of carboxylate groups (from nbca ligands) with the η-O,O?-μ-O,O binding mode, furnishing a centrosymmetric bimetallic unit with Pr···Pr separation of 4.648 ?. The bimetallic nodes are linked by two μ2-nbca ligands to generate a 1D wavelike double chain, extending along the b axis. These chains are interconnected through the trans-bis(μ2-η1:η1)-1,3-pda ligands to result in the infinite corrugated 2D layer (Fig. 2).
Fig. 2. 2D network structure of complex 1 (Hnbca ligands were omitted)
In 2, there is one crystallographically independent Pr atom with the 9-coordinated distorted triangle tetrakaidecahedron geometry (Fig. 3). Pr is coordinated by one O atom (O(1)) of one monodentate carboxylate, five O atoms (O(3), O(4), O(5), O(6)and O(6)#1) of four bridging carboxylates, one O atom (O(7)) of one water ligand, and two N atoms(N(1) and N(2)) of one chelating 2,2?-bpy. The Pr–N distances range from 2.679(3) to 2.699(3) ?, and Pr–O distances vary from 2.413(3) to 2.637(3) ?,which are similar to those found in other related examples[18]. Two such Pr centers are bridged by four carboxylate groups, furnishing a centrosymmetric bimetallic unit with Pr···Pr separation of 4.044 ?. The bimetallic nodes are linked by two cis-μ4-(η1:η0)-(η1:η1)-1,3-pda ligands to generate a 1D wavelike double-chain, extending along the b axis. The auxiliary terminal bipy ligands locate at both sides of the double chain. These chains are interconnected through the trans-bis(μ2-η2:η1)-1,3-pda ligands to result in the infinite corrugated 2D layer (Fig. 4).
Fig. 3. Coordination environments of Pr atom in complex 2. All hydrogen atoms are omitted for clarity. Symmetry codes: #1: –x + 1, –y,–z; #2: x, y – 1, z; #3: –x + 1, –y + 1, –z
Fig. 4. 2D network structure of complex 2
Thermal stability of 1 and 2 was studied by thermogravimetric analysis (TGA) under air atmosphere in the 20~800 ℃ region at a heating rate of 10 ℃·min-1(Fig. 5), and they have similar patterns.Therefore, only the structure of 1 is discussed in detail as a representative (Fig. 5). For 1, the first weight loss of 5.20% (calcd.: 4.75%) from 20 to 80 ℃ indicates the removal of two coordination water molecules. The second weight loss is 73.56%at 350~600 ℃, assigned to the decomposition of Hnbca and 1,3-pda ligands (calcd. 73.98%). The final stage can be considered as the decomposition of anhydrous composition to yield praseodymium oxide.
Fig. 5. TG curves of complexes 1 (left) and 2 (right)
The emission spectra of complexes 1 and 2 in the solid state at room temperature are shown in Fig. 6.To understand the nature of emission band, the photoluminescence properties of H2pda, Hnbca and 2,2?-bpy ligand were analyzed. It was found that three emissions at 370, 470 and 350 nm could be observed for free H2pda, Hnbca and 2,2?-bpy ligands,respectively. The emission of Hnbca and 2,2?-bpy ligand is omitted because of their weak emission spectra. Complexes 1 and 2 exhibit broad bands near ca. 464 and 461 nm. In comparison to the free H2pda,the emission peak of two complexes has a visible red shift, which can be attributed to intraligand π-π*transition. The fluorescence spectra for the two complexes are different, which may be related to a different auxiliary ligand.
Fig. 6. Solid-state emission spectra of complexes 1 (blue) and 2 (red) and H2pda (blank) at room temperature (The emission of Hnbca and 2,2?-bpy was omitted because of their weak emission spectra)
In conclusion, we report the syntheses and crystal structures of two new coordination complexes, 1 and 2. In 1, the Hnbca ligands bridge the Pr3+ions to form a 1D chain. Moreover, these 1D chains are united together through the 1,3-pda ligands to afford a 2D layer. Contrast to complex 1, the auxiliary ligand (2,2?-bpy) of 2 acts as a terminal ligand,locating at both sides of the chain. So, a 2D layer of complex 2 is formed through only bridging deprotonated 1,3-pda. Compared with free 1,3-pda ligand,the room temperature solid-state fluorescence spectra of 1 and 2 showed that the emission peaks of two complexes had a visible red shift, which could be attributed to intraligand π-π* transition. The fluorescence spectra of two complexes are different because of a different auxiliary ligand.
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