A Zn(II) Coordination Polymer Based on a Flexible Polycarboxylate Ligand 5-(4-Hydroxypyridinium-1-ylmethyl)-isophthalic Acid: Synthesis, Structure, and Property①

LI Fu-An YANG Wei-Chun LI Qing-Bin

(College of Chemistry and Environmental Engineering,Pingdingshan University, Pingdingshan 467000, China)

1 INTRODUCTION

Because of the structural richness and promising applications in the field of material science, coordination polymers[1-3]have attracted great interest for chemists[4-7].The assembly of these compounds can be influenced by many factors, such as temperature,solvent, reactant concentration, etc., as well as some internal factors like coordination geometries of central metals, configurations and flexibility of the organic ligands[8-11].Among them, the most important influencing factors are the structures and properties of organic ligands.In recent years, the polycarboxylic acid ligands are most widely used linkers for the assembly of various coordination polymers[12-15].They can adopt a variety of coordination modes to conform to the requirements of coordination geometries of metal centers, and thus result in diverse multidimensional architectures.Therefore, in order to obtain the target frameworks with specific structures and functions, a number of organic aromatic polycarboxylate ligands with flexible or angular structures have been synthesized and reported by coordination chemistry.For example,a flexible organic aromatic ligand 4-carboxy-1-(3,5-dicarboxy-benzyl)pyridinium chloride has been reported by Li et al.[16].The organic ligand 5-(imidazol-1-ylmethyl)isophthalic acid with flexible and angular structure has also been synthesized and reported[17,18].Recently, we have also obtained a flexible and angular ligand 5-(4-hydroxypyridinium-1-ylmethyl) isophthalic acid (H2L), and some coordination polymers from H2L ligand have been reported[19].Continuing our effort in this area, a 3D Zn(II) supramolecular coordination polymer,{[Zn2(L)2(m-bix)(H2O)]·6H2O}n(1), with an interesting 1D → 2D polythreading array from H2L and the ancillary nitrogen-containing ligand 1,3-bis(imidazol-1-ylmethyl)benzene(m-bix), has been hydrothermally synthesized and structurally characterized by X-ray diffraction analyses.Here, the title compound should be investigated in detail.

Scheme 1. Coordination mode of the L2? anion in coordination polymer 1

2 EXPERIMENTAL

2.1 Materials and measurements

H2L was synthesized according to the literature[19],and other starting materials were of reagent quality and obtained from commercial sources without further purification.Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240 elemental analyzer.The FT-IR spectra were recorded from KBr pellets in the range from 4000 to 400 cm?1on a Nicolet NEXUS 470-FTIR spectrometer.Thermal analysis was performed on a SDT 2960 thermal analyzer from room temperature to 800?C at a heating rate of 10?C/min under nitrogen flow.Powder X-ray diffraction (PXRD) for compound 1 was measured at 293 K on a Rigaku D/max-3B diffractometer equipped with Cu-Kα (λ = 0.15406 nm)radiation (45 kV, 200 mA).The crushed single crystalline powder samples were prepared by crushing the crystals and the 2θ scanning angle range was from 5 to 50° with a step of 0.1 °/s.Luminescence spectra for the solid samples were recorded on a Hitachi 850 fluorescence spectrophotometer.

2.2 Synthesis of 1

Compound 1 was synthesized hydrothermally in a 23 mL Teflon-lined autoclave by heating a mixture of 1,3-bis(imidazol-1-ylmethyl)benzene (m-bix)(0.0119 g, 0.05 mmol), Zn(OAc)2·2H2O (0.022 g, 0.1 mmol), H2L (0.0168 g, 0.05 mmol) and LiOH(0.0042 g, 0.1 mmol) at 160 °C in 6 mL H2O for three days.Colorless block-wise crystals of 1 were obtained in 82% yield based on H2L.Anal.Calcd.for C42H46N6O17Zn2(1037.59): C, 48.62; H, 4.47; N,8.10.Found: C, 48.56; H, 4.59; N, 8.17.IR/cm-1(KBr): 3406(m, br), 3126(m), 1634(s), 1621(s),1583(w), 1521(s), 1361(s), 1313(w), 1238(w), 1022(w), 850(w), 772(m), 724(m).

2.3 Crystallographic data collection and structure determination

Single-crystal X-ray diffraction data of compound 1 were collected on a Bruker SMART APEX CCD diffractometer[20]equipped with graphite-monochromatized MoKα radiation (λ = 0.71073 ?) at room temperature using the φ-ω scan mode in range of 3.08≤θ≤25.00o(–13≤h≤13, –14≤k≤15,–19≤l≤19).Empirical absorption corrections were applied to the intensities using the SADABS program[21].The structures were solved using the program SHELXS-97[22]and refined with the program SHELXL-97[23].All non-hydrogen atoms were refined anisotropically.The hydrogen atoms of coordination water molecules and the ligands were included in the structure factor calculation at idealized positions by using a riding model and refined isotropically.The hydrogen atoms of solvent water molecules were located from difference Fourier maps, then restrained at the fixed positions and refined isotropically.Analytical expressions of neutral atom scattering factors were employed, and anomalous dispersion corrections were incorporated.A total of 7990 reflections were collected, of which 6079 were independent (Rint= 0.0365) and 4240 were observed with I ＞ 2σ(I).The final refinement gave R= 0.0600, wR = 0.1736 (w = 1/[σ2(Fo2) + (0.1149P)2+ 2.5993P], where P = (Fo2+ 2Fc2)/3), S = 1.006.It crystallizes in triclinic, space groupwith a =11.0180(7), b = 13.3260(7), c = 16.4598(7) ?, α =102.738(4), β = 90.424(4), γ = 104.827(5)°, V =2273.5(2) ?3, Z = 2, Mr= 1037.63 and Dc= 1.516 g/cm3.The selected bond lengths and bond angles for 1 are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Compound 1

3 RESULTS AND DISCUSSION

3.1 Crystal structure of{[Zn2(L)2(m-bix)(H2O)]·6H2O}n (1)

A single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic system with space group.The asymmetric unit of 1 contains two crystallographically independent Zn(II) ions, two L2?anions, one m-bix ligand, one coordinated water molecule, and eight free lattice water molecules with the total site occupancy of 6(Fig.1a).Zn(1) is in a trigonal bipyramidal coordination sphere, which is defined by one nitrogen atom (N(3)) from one m-bix ligand, three carboxylate oxygen atoms (O(1), O(2), and O(6)) from two L2?anions, and one oxygen atom (O(1W)) from one coordinated water molecule.Three atoms O(1), O(2),and O(1W)comprise the equatorial plane, while the other two atoms (O(6), and N(3)) occupy the axial positions.Zn2 is tetrahedrally coordinated by three oxygen atoms (O(3)#1, O(9)#2, and O(10)) from three different L2?ligands and one nitrogen atom (N6)from one m-bix ligand.The Zn?O lengths fall in the range of 1.964(4)～2.711(2) ?, and the Zn?N lengths are 1.984(4) and 2.003(4) ?.

In 1, it is interesting that the L2?anions show two different coordination modes (Scheme 1a and 1b).(L2?)aconnects three Zn(II) ions through two O atoms of the carboxylate groups in μ1-η1:η0modes and one O atom of monodentate hydroxyl group(Scheme 1a); while (L2?)bbridges two Zn(II) ions through two carboxylate groups with different coordination modes: one in a μ2-η1:η1coordination mode and the other in a μ1-η1:η0coordination fashion(Scheme 1b).Neglecting the (L2-)banions, two Zn(II)ions and two (L2?)aanions comprise of a dinuclear metallamacrocycle unit with the diagonal Zn ions distance of 10.70 ?.Then, two (L2?)aanions and two(L2?)banions connected four Zn atoms (Zn(1),Zn(1)#2, Zn(2)#2, and Zn(2)#3) to form a tetranuclear metallamacrocycle unit.Two kinds of metallamacrocycle units are linked through sharing a pair of symmetry-related isophthalic groups of the L2?ligands to yield an infinite 1D tubular structure (Fig.1b).These 1D tubes are interconnected by the m-bix spacers to form a 2D framework in the ab plane (Fig.1c).Notably, the hydropyridine groups in (L2?)banions are just like open arms protruding from both sides of the 1D tubes, and threaded by two arms above and below, thus resulting in a rare 1D → 2D polythreading array along the b axis.We propose that strong π-π interactions (centre···centre A = 3.88 ?, B = 3.52 ?, C = 3.74 ?) between pyridine rings or phenyl rings are potential driving force for the 1D → 2D polythreadding architecture (Fig.1d).

Hydrogen bonding interactions are generally very important for generating supramolecular architectures.A number of hydrogen bonds have been observed in 1 crystal formed by free water molecules,coordinated water molecules and coordinated or uncoordinated carboxylate group oxygen atoms.Three sorts of hydrogen-bonded interactions are found in 1 crystal: (1) Intra-asymmetric-unit hydrogen bonds of O(1w)–H(1wA)···O(4w), O(2w)–H(2wA)···O(7), O(3w)–H(3wA)···O(5w), O(3w)–H(3wB)···O(4w), O(5w)–H(5wB)···O(6w) and O(8w)–H(8wB)···O(7w); (2) Seven intralayer hydrogen bonds: O(1w)–H(1wB)...O(5)#8, O(2w)–H(2wB)···O(8w)#1, O(4w)–H(4wA)···O(9)#6, O(5w)–H(5wA)#8···O(7w)#1, O(6w)–H(6wA)···O(4)#7,O(6w)–H(6wB)···O(10)#4, and O(7w)–H(7wA)···O(8)#6; (3) One interlayer hydrogen bond interaction O(8W)–H(8WA)···O(3W)#5. These hydrogenbonded interactions not only stabilize the 2D layer net, but also further extend the 2D sheets into a 3D supramolecular framework (Fig.2, Table 2).

Table 2. Hydrogen-bond Geometry (?, °)

Fig.1. (a) Metal coordination and atom labeling in compound 1 (thermal ellipsoids at 50% probability level).Hydrogen atoms and the solvent water molecule are omitted for clarity.Symmetry codes: #1: 1 + x, y, z; #2: 2 ? x, 2 ? y, 1 ? z; #3: 3 ?x, 1 ? y, 1 ? z; #4: 2 ? x, 1 ? y, 1 ? z; #6: –1+x, y, z.(b) Dinuclear and tetranuclear metallamacrocycles are linked together to give rise to a 1D tubular structure.(c) 2D structure of 1.(d) Schematic illustration of the 1D → 2D polythreading array in 1

Fig.2. Hydrogen bonds in coordination polymer 1

3.2 Powder XRD studies

To confirm whether the crystal structures are truly representative of the bulk materials, X-ray powder diffraction (PXRD) experiments are carried out for 1.As shown in Fig.3, the experimental PXRD patterns correspond well with the results simulated from the single-crystal data, indicating the bulk synthesized materials and the measured single crystals are the same.The difference in reflection intensities between the simulated and experimental patterns was due to the variation in preferred orientation of the powder samples during the collection of experimental PXRD data.

Fig.3. XRD patterns of the single crystal of complex 1

Fig.4. TGA curve of complex 1

3.3 Thermal stability of coordination polymer 1

To ascertain the thermal stability of coordination polymer 1, its thermogravimetric analysis (TGA)was subjected in a N2atmosphere, and the TGA curve of 1 was recorded from 30 to 850 ℃ (Fig.4).The TG curve for compound 1 shows the initial weight loss in the temperature range of 30～112 °C,which can be ascribed to the removal of lattice and coordinated water molecules (observed: 12.55%,calculated: 12.15%).Further weight loss indicates the decomposition of coordination framework after 374 ℃.

4.4 Photochemical property of the coordination polymer 1

Luminescent complexes of d10metal centers from organic ligands are of great interest because of their potential applications[24,25].The solid-state emission spectra of coordination polymer 1, H2L and m-bix used in this work have been investigated at room temperature (Fig.5).Here, intense bands were observed at 412 nm (λex= 317 nm) for H2L, 395 nm(λex= 330 nm) for m-bix, and 418 nm (λex= 316 nm)for complex 1.The emission bands of 1 are similar to that of the free H2L, which can be probably assigned to intraligand fluorescent emission[26,27].Compared to the free ligand, the different locations and profiles of their emission/excitation peaks of 1 probably due to the chelating coordination effectively increased the rigidity of the ligand and reduced the loss of energy by radiationless decay[28,29].

Fig.5. Solid-state emission spectra of free H2L, m-bix and compound 1 at room temperature

4 CONCLUSION

Herein we report a novel 3D supramolecular coordination polymer, {[Zn2(L)2(m-bix)(H2O)]·6H2O}n(1), based on a flexible and angular organic aromatic polycarboxylate ligand 5-(4-hydroxypyridinium-1-ylmethyl) isophthalic acid (H2L) ligand with a N-donor auxiliary ligand 1,3-bis(imidazol-1-ylmethyl)benzene (m-bix).Structure analysis for 1 reveals that the Zn(II) center atoms have two different coordination geometries, and the L2-ligands also show two different coordination modes.In 1, Zn(II)atoms are linked together by L2-ligands forming an infinite 1D tubular structure with two different metallamacrocycle units, and further connected by the m-bix spacers to yield a 2D framework.The 3D supramolecular network of 1 is further assembled by week interaction forces, such as hydrogen bonds and π-π interactions.Further experiments are underway to investigate other fascinating structures by reacting the H2L with rare-earth(III) salts, and other transition-metal salts are ongoing in our laboratory.

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