Synthesis, Crystal Structure, and Properties of a New Coordination Polymer From 4-(4-Hydroxypyridinium-1-yl) Phthalic Acid

A copper supramolecular coordination polymer, {[CuL(H₂O)₂]₂}_n (1), has been hydrothermally synthesized and structurally characterized by IR, UV spectroscopy, and elemental analyses, and also by single-crystal X-ray diffraction (H₂L = 4-(4-hydroxypyridinium-1-yl) phthalic acid). The coordination around Cu center is distorted octahedron coordination geometry involving six O atoms, and each L^2– ligand links three center atoms in the μ_3-η¹: η¹: η¹ mode; Adjacent Cu(II) atoms are united by the monodentate carboxylate group and hydroxy group to form infinite 1D chains (···Cu1LCu2L···)_n, which are further linked by coordinated bonds between Cu1 and chelating carboxylate groups to produce 2D layers. These 2D layers are extended by interlayer hydrogen-bonded interactions to form 3D supramolecular framework.

Keywords: 4-(4-hydroxypyridinium-1-yl) phthalic acid; polymer; copper; supramolecular framework

In recent years, the design and construction of novel metal-organic frameworks (MOFs) have become an interesting area not only because of their intriguing esthetic structures, but also because of their promising applications as functional materials, such as gas adsorption, separation, catalytic activities, optoelectronic materials, luminescence, and magnetism.[[[1]]] The self-assembly of multidentate organic ligands and metal ions has resulted in many polymeric frameworks, whose structures are influenced by the subtle interplay of many factors such as geometric preference of metal ions, sizes and shapes of organic building blocks, templates, and solvent systems. The selection and design of suitable ligands containing certain features such as flexibility or versatile binding modes is crucial to the construction of metal-organic coordination polymers with novel frameworks.

As a carboxylate group can exhibit a rich variety of coordination modes, multicarboxylate organic ligands are good candidates in assembling coordination polymers with peculiar structures and new functionalities.[[8],[9]] We selected 4-(4-hydroxypyridinium-1-yl) phthalic acid (H2L) (Scheme 1) as a ligand to design and synthesize 3D supramolecular MOFs for the following reasons. First, the abundant functional groups on this ligand can adopt various coordinated modes such as terminal monodentate, chelating one metal ion, bridging two metal ions in a synsyn, syn-anti, and anti-anti, and in a tridentate form to build interesting polymer architecture. Second, the functional hydroxyl group on this ligand can act not only as a coordinated site but also as a hydrogen-bond acceptor to give rise to supramolecular architecture. Third, this ligand has a good water solubility and is beneficial to research the property of this ligand in hydrothermal conditions. Fourth, only a few coordination polymers constructed from H2L have been reported by the literatures,[[10]] and much work is still necessary to understand the coordination chemistry of H2L. From this point of view, we focus on the self-assembly of Cu(II) and H2L ligand to intriguing structure and obtained a new MOF, {[CuL(H2O)2]2}n (1), and the thermal analysis and UV-vis absorption spectra of 1 have also been investigated.

Graph: Scheme 1. Structural formula of 4-(4-hydroxypyridinium-1-yl) phthalic acid (H2L).

4-(4-Hydroxypyridinium-1-yl) phthalic acid was synthesized according to the literature.[[10]] The single crystal of complex 1 was obtained through hydrothermal method in a Teflon-lined stainless steel container. Other starting materials were of reagent grade and obtained commercially without further purification. The FT-IR spectra were recorded from KBr pellets in the range from 4000 to 400 cm−1 on a Nicolet NEXUS 470-FTIR spectrometer. Solid UV−visible spectra were obtained in the 200−800 nm range on a JASCO UVIDEC-660 spectrophotometer. Elemental analyses for C, H, and N were performed on a Perkin Elmer 240 elemental analyzer. Thermal analysis was performed on a SDT 2960 thermal analyzer from 40 to 750°C with a heating rate of 10°C/min under nitrogen flow.

Complex 1 was synthesized hydrothermally in a Teflon-lined stainless steel container by heating a mixture of H2L (0.0130 g, 0.05 mmol), Cu(OAc)2·H2O (0.0100 g, 0.05 mmol), and KOH (0.0056 g, 0.1 mmol) in 7 mL of distilled water at 140°C for three days. Subsequent cooling to room temperature yielded block crystals of 1 (70% yield based on copper). C13H11NCuO7 (356.77): Anal. Calcd. C 43.76, H 3.11, N 3.93; Found C 43.69, H 3.20, N 3.85. IR/cm−1 (KBr): 3227(m), 1592(s), 1546(s), 1428(s), 1401(s), 1340(s), 1291(w), 1199(w), 1147(m), 1039 (s), 850(w), 801(w), 746(w).

Single-crystal X-ray diffraction data of 1 was collected on a Bruker Smart Apex CCD diffractometer[[11]] equipped with graphite-monochromatized MoKα radiation (λ = 0.71073 Å) at room temperature using the ω-scan technique. Empirical absorption corrections were applied to the intensities using the SADABS program.[[12]] The structure was solved by direct methods using SHELXS-97[[13]] computer program and refined by full-matrix least-squares methods on F2 with the SHELXL-97[[14]] program package. All non-hydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms of the organic ligands and the coordination water molecules were included in the structure factor calculation at idealized positions using a riding model and refined isotropically. The crystallographic data and selected bond distances and angles for complex 1 are listed in Tables 1 and 2, respectively.

Table 1. Crystallographic data for the complex 1

10001 R1 = Σ||Fo|−|Fc||/Σ|Fo|. bwR2 = [Σw(Fo2−Fc2)2/Σw(Fo2)2]1/2,w = [σ2(Fo2)+(AP)2+BP]−1, where P = (Max(Fo2,0)+2Fc2)/3 and A and B are constants adjusted by the program. cGoF = S = [Σw(Fo2−Fc2)2/(nobs − nparam)]1/2,where nobs is the number of data and nparam is the number of refined parameters.

Table 2. Selected bond lengths (Å) and bond angles (°)

3 Symmetry transformations used to generate equivalent atoms: #1 1 – x, y, 0.5 – z; #5 –x, y, 0.5 – z; #6 –1 – x,1 – y, –z; #7 1 + x, 1 – y, 0.5 + z.

In 1, {[CuL(H2O)2]2}n, the asymmetric unit consists of two half Cu centers [(Cu1)1/2 and (Cu2)1/2], two coordinated water molecules [O3w, (O1w)1/2, and (O2w)1/2], and one completely deprotonated L2− anion (Figure 1). As illustrated in Figure 1, the coordinated environment around the Cu(II) center can be described as a distorted octahedron coordination geometry [CuO6], where Cu1 ion is six-coordinated by four oxygen atoms (O1, O2, O1#5, and O2#5) from two different chelating carboxylate groups and two hydroxyl oxygen atoms (O5#6, O5#7) of other two different L2– ligands, and the Cu2 center coordinates to four water molecules (O1w, O2w, O3w, and O3w#1) and two carboxylate oxygen atoms (O3, O3#1) from two different asymmetric units. The Cu1−O and Cu2−O bond lengths fall in the range of 1.971(2)−2.6180(25) Å and 1.919(4)−2.4799(27) Å, respectively (Table 2), which are well in agreement with those reported in other Cu(II) complexes.[[15]]

Graph: Fig. 1. The coordination modes of L2– ligand and coordination environment of Cu(II) ions (Cu1, Cu2, O1w, and O2w occupied factors are all 0.5). All hydrogen atoms are omitted for clarity. Symmetric code: #1 1 – x, y, 0.5 – z, #5 –x, y, 0.5 – z, #6 –1 – x,1 – y, –z.

The L2– ligand adopts a twisted conformation, and the dihedral angle between pyridine ring and the phenyl ring is 39.448(91)°. Two carboxylate groups have a dihedral angle of 32.751(127)° and 55.970(234)° toward the plane of the corresponding linking phenyl rings, respectively. Each L2– ligand links three center atoms in the μ3-η1: η1: η1 mode with the 1-COO group in monodentate fashion, 2-COO group in bidentate chelating fashion and 4-hydroxyl in monodentate fashion, respectively. Adjacent Cu atoms are united by the L2– ligands to from an infinite 1D chains (···Cu1LCu2L···)n with Cu1···Cu2 = 13.7386(7) Å (Figure 2a). These chains are further linked by coordinated bonds, which are formulated by Cu1 and chelating carboxylate group oxygen atoms that are from two neighboring chains to form 2D layers, as shown in Figure 2b.

Graph: Fig. 2. (a) View of the bimetallic (Cu2L2)n chain in the structure of coordination polymer 1 (the grey dotted lines represent Interchain hydrogen bonds interactions). All irrelevant atoms are omitted for clarity. (b) View of the 2D layer structure of 1.

Hydrogen bonding interactions are generally very important for generating supramolecular architectures.[[16]] There are three sorts of hydrogen-bonded interactions in 1 crystal: (a) the hydrogen bonding interactions in the same asymmetric unit. They are formed by coordinated water molecules (O2W, O3w, and O3w#1) and coordinated or uncoordinated carboxylate group oxygen atoms (O2, O2#1, O4, and O4#1); (b) interchain hydrogen bonding interactions. They are constructed through coordinated water molecules (O3w and O3w#1) and coordinated carboxylate group oxygen atoms (O1#5 and O1#4) from the adjacent chains; and (c) interlayer hydrogen bonding interactions. They are between coordinated water molecules (O3w and O3w#1) and two hydroxyl oxygen atoms (O5#2 and O5#3) from the adjacent 2D layer (Table 3 and Figure 3), these interlayer weak interactions further extend the 2D sheet into 3D supramolecular framework (Figure 4) Yellow, grey and Green dotted lines represent the same asymmetric unit, interchain and interlayer hydrogen-bonded interactions in Figure 3, respectively. Symmetry codes were #1 1 – x, y, 0.5 – z; #2 1.5+x, 0.5 – y, 0.5+z; #3 -0.5 – x, 0.5 – y, –z; #4 1 + x, y, z; #5 –x, y, 0.5 – z..

Table 3. Hydrogen bond geometry (Å, °)

4 Symmetry codes: #1 1 – x, y, 0.5 – z; #2 1.5 + x, 0.5 – y, 0.5 + z; #3 –0.5 – x, 0.5 – y, –z; #4 1 + x, y, z; #5 –x, y, 0.5 – z.

Graph: Fig. 3. Interlayer hydrogen bond interactions and intralayer hydrogen bond interactions (yellow dotted lines, grey dotted lines, and green dotted lines represent the same asymmetric unit hydrogen bond, interchain hydrogen bond, and interlayer hydrogen bond interactions, respectively). Symmetry codes: #1 1 – x, y, 0.5 – z; #2 1.5 + x, 0.5 – y, 0.5 + z; #3 –0.5 – x, 0.5 – y, –z; #4 1 + x, y, z; #5 –x, y, 0.5 – z.).

Graph: Fig. 4. The 3D net connected by interlayer hydrogen bond (dotted lines) interactions.

Thermogravimetric analysis (TGA) was conducted to study the thermal stability of coordination polymer 1, which is an important aspect of metal–organic framework. TGA was performed on crystalline samples of complex 1 in the range of 40–750°C, as depicted in Figure 5. The framework of complex 1 can be stable up to 50°C. The compound lost its coordinated water molecules from 50 to 235°C (obsvd. 9.61%, calcd. 10.09%). Then the compound begins to rapidly decompose at 250°C, as shown in Figure 5.

Graph: Fig. 5. The TGA curve of the coordination polymer 1.

The diffuse-reflectance UV−vis spectra reveal the absorption features of 1 and H2L (Figure 6), and the two spectra consist of absorption components in the UV and Vis regions. The H2L ligand itself displays strong absorption bands in the UV spectral region from 241 to 309 nm, arising from the π−π* transitions of the ligand,[[17]] whereas the main UV absorption bands at 347 nm for 1 can be attributed to ligand-to-metal charge transfer. These bands are not strongly perturbed upon its coordination to copper(II), suggesting that coordination of metal ions hardly alters the intrinsic electronic properties of H2L. In the case of complex 1, the additional clear absorption band in the visible region observed after 630 nm, which probably respectively originate from the d-d spin-allowed transition of the Cu2+ (d9) ion in distorted octahedral coordination configuration, whereas the absorption peaks of the band has not been observed in Figure 6, it may be moved into infrared region for the weak coordination from coordinated water molecules and carboxylate of L2– ligand.

Graph: Fig. 6. UV-vis absorption spectra for the complex 1 and H2L ligands.

We have successfully synthesized and structurally characterized a copper 3D supramolecular coordination polymer based on 4-(4-hydroxypyridinium-1-yl)phthalic acid (H2L) ligand. Comparing with the reported complexes in the literature,[[10]] coordination polymer 1 displays 3D supramolecular architecture through interlayer hydrogen-bonded interactions. Each L2– ligand links three center atoms in chelating bidentate fashion, monodentate fashion, and monodentate fashion, respectively. Structural difference of these complexes constructed by the present ligand indicates that center ions play an important role in complex assembly.

The authors express thanks to Education Chamber of Henan Province (No.15A150068) and Key Laboratory of Applied Chemistry of Pingdingshan University (No.201201).