SYNTHESIS, CRYSTAL STRUCTURE AND FLUORESCENCE PROPERTY OF RU-U HETEROMETALLIC COORDINATION POLYMER CONSTRUCTED BY MIXED LIGANDS

WU Hong-yan, LI Long-fei, LI Hao-hong, ZHANG Jian-han, LUO Ju-xiang

SYNTHESIS, CRYSTAL STRUCTURE AND FLUORESCENCE PROPERTY OF RU-U HETEROMETALLIC COORDINATION POLYMER CONSTRUCTED BY MIXED LIGANDS

*WU Hong-yan1, LI Long-fei1, LI Hao-hong2, ZHANG Jian-han1, LUO Ju-xiang1

（1. School of Resource and Chemical Engineering, Sanming University, Sanming, Fujian 365004, China; 2. College of Chemistry, Fuzhou University, Fuzhou, Fujian 350116, China）

A new heterometallic organic coordination polymer, i.e. [Ru(bipy)3][(UO2)2(BDC)3]·2H2O was synthesized by the reaction of ruthenium metal complex ion ( [Ru(bipy)3]2+) and aromatic organic dicarboxylic acid ligand(benzene-1,4-dicarboxylate acid, H2BDC) with uranyl ion under hydrothermal condition. The structure of the coordination polymer was characterized with single crystal X-ray diffraction, powder X-ray diffraction and infrared spectroscopy. Under the [Ru(bipy)3]2+templates, the complex formed a two-dimensional honeycomb structure with [(UO2)2(BDC)3]2-planar noninterpenetrated honeycomb layer. Intra- and inter-layer hydrogen bonding between the two dimensional honeycomb structure and guest cations contributed to the stabilization of the overall framework. This double layer was eventually connected into a quasi-three dimensional(3D) network. In addition, the solid state UV-Vis diffuse reflectance spectroscopy showed that the complex had semiconductor properties and was a potential photoelectric material. The solid state fluorescence measurement shows that the title complex exhibits strong luminescence.

mixed ligand; heterometallic coordination polymer; hydrothermal synthesis; fluorescence; uranyl

INTRODUCTION

In the past fifty years, uranyl-containing complexes have played a vital role in the research of actinide chemistry not only due to their interesting topological diversity[1], but also their potential application in ion exchange[2], photocatalytic degradation of organic pollutants[3-4], magnetic[5], luminescence[6], chemical sensors[7], ion exchange[8], and so on.

Uranium usually exist in the form of linear dioxide UO22+, the terminal nature of which promotes additional ligand coordination about the equatorial plane. Thus uranyl coordination polymers usually presents low dimensional structures rather than three-dimensional (3D) frameworks[9]. Three-dimensional uranyl coordination polymers usually exhibit superior thermal stability to low-dimensional structures and many outstanding properties, such as photoelectric effects[10], non linear optical properties[11],and porous adsorption[12].

To construct uranyl coordination polymers with three dimensional structures, there are at least three strategies of synthesis were employed. First, uranyl ions are bound in different directions by polycarboxylic acid ligands which contain multiple functional groups[13]. Second, organic or inorganic template agents are introduced to guide the formation of three dimensional uranyl coordination polymers[14]. Third, a large number of secondary metal cations were used to construct heterometallic uranyl complexes with three dimensional framework structures[15-17]. The third method is particularly effective because the second metal ion can provide additional connecting nodes which increase the possibilities of forming three dimensional frameworks and also bring additional physicochemical properties.

According to the hard-soft acid base (HSAB) theory, uranyl ions are generally considered to be hard acids which have a high binding affinity for O-donor groups such as carboxylate acid groups, while d-block transition metal ions are soft acids, which are more inclined to bind with some N-donor groups[15-17].

Inspired by the above considerations, we are interested in exploring uranyl complex that can be formed in the mixed ligands system containing BDC molecules and neutral N-donor ligands. Herein, we report the uranyl complex [Ru(bipy)3] [(UO2)2(BDC)3]·2H2O with BDC as the main linker and bipy as the auxiliary ligand. The complex has been structurally determined by single crystal X-ray diffraction and characterized by FT-IR, UV-vis and photoluminescent spectroscopy.

1 EXPERIMENTAL SECTION

Ruthenium(III) chloride anhydrous(RuCl3), uranyl nitrate hexahydrate ((UO2(NO3)2·6H2O, 99%), anhydrous ethanol(99.5%)and benzene-1,4-dicarboxylate acid (99%),2,2'-bipyridine, lithium chloride(LiCl),N, N-dimethylformamide (DMF) were purchased from Aladdin Chemical Reagents Co. Ltd. Allthe reactants used in the syntheses were of analytical grade. Power XRD data was collected on a PANalytical X'Pert Pro MPD diffractometer equipped with a graphite-monochromated Cu-Kradiation. The data was collected with the 2in the range from 5 to 50˚ at room temperature. The FT-IRspectrumwas recorded on a Nicolet Co. Magna-IR750 Infrared spectrometeras KBr pellets. The data was collected over the range from 400 cm−1to 4000 cm−1. Solid state UV-visible spectrum was takenon a Perkin-Elmer lambda 900 UV/Vis spectrophotometer equipped with an integratingsphere. Measurement data was collected in the wavelength ranging from 800 to 200 nm. The absorption spectrum was obtained from reflectance spectrum using the Kubelka-Munk function[18]. Solid state luminescent spectrumwas collected at ambient condition with a PW2424 spectrometer with an excited wavelength of 374 nm.

1.1 Synthesis of ruthenium complex [Ru(bipy)3]Cl2

[Ru(bipy)3]Cl2complex were prepared according to literature methods[19]. RuCl3(0.3900 g, 1.5 mmol), 2,2'-bipyridine (0.7050 g, 4.5 mmol) and lithium chloride (0.4200 g, 1.5 mmol) were successively added to 100 mL three-neck round bottom flask and dissolved in 20 mL N, N-dimethylformamide (DMF). Under the protection of nitrogen, heat and stir the reflux for 12 h. When there is a large amount of red solid in the solution, stop heating, cool to room temperature, add 50 mL acetone, freeze overnight. After filtration, the precipitate was washed with ice water and cold acetone, then dried in vacuum to obtain 0.4705 g red powder. The yield was 65 %.

1.2 Preparation of the title complex[Ru(bipy)3] [(UO2)2(BDC)3]·2H2O

Ru(bipy)3Cl2(0.0640 g, 0.1 mmol) was dissolved in 10 mL deionized water, and stirred for 10 min with a red solution obtained. Afterwards, uranyl nitrate hexahydrate (0.0500 g, 0.3 mmol) and benzene-1,4-dicarboxylate acid (0.1055g, 0.2 mmol) were added and kept under stirring for half an hour. Then the mixture was sealed in a 25 mL Teflon lined stainless steel container, heated at 160 ℃ for 4 days, and then slowly cooled to room temperature at a rate of 3 ℃/h. After suction filtering, dark red block crystals of the title uranyl complex [Ru(bipy)3] [(UO2)2(BDC)3]·2H2O.The yield was 26.7% (calculated by UO2(NO3)2·6H2O).

1.3 X-ray crystallography

Suitable single crystals of the title complex were selected manually under an optical microscope, and mounted on glass fibers for X-ray single crystal diffraction. The single-crystal X-ray data sets were collected at 296(2)K on a Rigaku 4-circle diffractometer (MoKα, λ=0.71073 Å). The structures were solved by direct methods using SHELXS-97[20]and refined by full-matrix least-squares techniques on2using SHELXL-97[21]. The carbon-bound H-atoms were generated geometrically. All non-hydrogen atoms were refined by full-matrix least-squares techniques.

Crystallographic data and structure refinement details of the complex are summarized in Table 1. The important bond distances and bond angles are listed in Table 2. More details of crystallographic data can be found in the Cambridge Crystallographic Data Center (CCDC) with the CCDC numbers of 879538 for the complex.

Table 1 Crystallographic data for the complex

a1=‖0|-|c‖/|0|, w2=[w(02c2)2/[w(02)2]1/2

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

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

Symmetry codes #1 -x+1,y,-z+1/2

2 RESULTS AND DISCUSSION

2.1 Crystal structure of the complex

Single-crystal X-ray diffraction analysis reveals the title complex is a neutral molecule that crystallizes in the2/space group, monoclinic system. The asymmetric unit contains three crystallographically independent UO22+, three deprotonated carboxylate ligands BDC2−, one Ru transition metal/bipy complex cation [Ru(bipy)3]2+, and two free water molecules(Fig.1). The coordination polyhedra around all UO22+units can be visualized as hexagonal bipyramid geometries of UO8. Axially, the O=U=O angles are 177.3(3)°,179.1(4)° and 179.2(4)° for U1, U2 and U3, respectively. The U=O bond lengths are in the range of 1.767 ～ 1.774 Å. Equatorially, tris-chelated U atoms are six-coordinated with six carboxylate oxygen atoms from three BDC2-ligands with U-O bonds ranging from 2.436 to 2.491 Å. The O-U-O bond angles fall in 52.15(16)-69.41(17)°. These are comparable with those reported in the literature[3-6]. All the carboxylic acid groups in BDC2−ligands are deprotonated and bound with UO22+by chelating coordination mode. As a result, BDC2−ligands act as a bridging linker to connect neighboring UO22+into a 2D layered anionic framework in ab plane (Fig.2). Hydrogen bonds interactions among guest molecules and anionic framework can be detected for structural stabilization (Fig.3, Table 3). Furthermore, two adjacent honeycomb layers are combined to form a double layer with C-H···O non-covalent interactions. This double layer is eventually connected into a quasi-3D network. The parallel interpenetration of the layers is observed because the two dimensional layers are coplanar (Fig.4).

Fig. 1 The asymmetric unit of the title complex(the hydrogen atoms are omitted for clarity)

Fig.2 2D layered anionic framework

Fig. 3 Hydrogen bonds interactions between guest molecules and anionic framework

Fig. 4 The quasi-3D structure of the title complex

Table 3 Hydrogen bonds interactions(C-H···O) of the title complex(distances in Å and angles in º)

2.2 Powder X-ray diffraction analyses

To check the phase purity of the title complex, powder X-ray diffraction (PXRD) pattern was obtained at ambient condition. As shown in Fig.5, the peak positions of the simulated and experimental PXRD patterns are basically the same, indicating that the complex is formed in pure phases.

Fig. 5 Powder X-ray diffraction patterns for the complex

2.3 Infrared spectrum

The infrared spectrum study for the the complex is shown in figure 6.The peaks around 3059 cm−1is assigned to the pyridine ring C-H stretching vibration.The bands at 1678 cm−1is attributed to stretching vibrations of C=N. The bands at 1508 cm−1and 1400 cm−1are attributed to symmetrical and asymmetrical stretches of C=O bonds. Absorptions at 947 cm−1and 824 cm−1are the symmetrical and asymmetrical stretches of U=O, respectively. The bands at 1285 cm−1、1016 cm−1and 734 cm−1are attributed tothe pyridine ring C-H in and out of plane bending vibrations. The bands at 511 cm−1is attributed to stretching vibrations ofRu-N.These assignment are consistent with previously reported complexes[3].

Fig. 6 The infrared spectrum of the the complex

2.4 UV-Visible spectrum

The solid state diffuse reflectance UV-vis spectroscopy was performed for the as-synthesized complex to reveal its photo-response regions (Fig. 7). The peak at about 247 nm can be assigned to the π–* charge transfer of 2,2'-bipyridine, The absorption at 316 nm is very strong which can be assigned to the absorption of electron transitions in the typical U=O double bonds. This has been shown to be an active center for photocatalytic properties[4]. The absorptions in the range of 420～480 nm are due to the ligand to metal charge transfer (LMCT) between the O atoms of benzene-1,4-dicarboxylate acidand empty orbitals of the U(VI) centers[3]. The optical absorption spectrum ofthe title complexhas been measured by the diffuse-reflectance experiment (Fig. 7b). The absorption edge is 3.16 eV which indicates that it has wide gap semiconductor property(The value of Egwas obtained by direct extrapolation method).

2.5 Photoluminescence properties

The solid state fluorescence of the title complex was measured at ambient condition under an excitation wavelength of 374 nm(Fig. 8). As depicted in Fig.8, The peak at 392 nm is due to the π-π* transition of 2,2'-bipyridine. There are five well-resolved emission peaks at 492、517、540、568、596 nm, which are ascribed to the typical uranyl electron transitions. Compare with the emission bands of the benchmark reference of uranyl nitrate (488、510、534、560、588 nm), the complex exhibit a slight red shift. The intense luminescence emission bands originate from the electronic and vibrational transitions (S11−S00and S10−S0v(= 0−4)) of UO22+[3]. The peak at 732 nm is attributed to the L→M transition of bipyridine to ruthenium.

Fig.8 Room-temperature solid-state emission spectrum (Ex=374 nm)

3 CONCLUSION

In summary, a heterometallic Ru-U complex has been synthesized through a hydrothermal reaction and characterized by single-crystal X-ray diffraction. Crystal data analysis shows that the title complex crystallizes in monoclinic space group2and exists as a 2-D layered anionic framework in ab plane. In the title complex, there are hydrogen bonds attraction, yielding a quasi-3D supramolecular structure. Our results provide a new method for constructing uranyl-bearing heterometallic coordination polymers. The central issue we want to address in our ongoing studies is the incorporation of other transition metals cations to explore structural variations and potential properties.

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混合配体构筑的钌-铀酰异金属配位聚合物的合成和晶体结构及荧光性能

*吴红燕1，李龙飞1，李浩宏2，张建汉1，罗菊香1

（1.三明学院资源与化工学院，福建，三明 365004；2.福州大学化学学院，福建，福州 350116）

利用钌金属络合物离子和芳香有机二羧酸配体与铀酰离子在水热条件下反应，合成了一例新的异金属有机配位聚合物，也即[Ru(bipy)3][(UO2)2(BDC)3]·2H2O，利用单晶X-射线衍射方法、粉末X-射线衍射方法、红外光谱对该配位聚合物结构进行了基本表征。该配合物是由金属配合物阳离子客体[Ru(bipy)3]2+、二维蜂窝状平面结构的聚阴离子主体框架[(UO2)2(BDC)3]n2-之间通过分子内和分子间C-H···O氢键的相互作用形成准三维超分子结构。此外，固体紫外-可见漫反射光谱测试表明此配合物具有半导体性质，是潜在的光电材料。固体荧光测量表明配合物表现出强烈的发光性能。

混合配体；异金属配位聚合物；水热合成；荧光；铀酰

O641

A

10.3969/j.issn.1674-8085.2023.05.006

O641 Document Code：A DOI：10.3969/j.issn.1674-8085.2023.05.006

2022-11-29；

Modified date：2023-02-06.

the National Natural Science Foundation of China (51703120); Natural Science Foundation of Fujian Province (2021J011117);Education and Research Project of Fujian Province (JAT200640, B202031);Innovation and Entrepreneurship Training Program for College Students(202111311017)

1674-8085（2023）05-0034-07

Biographies:*WU Hong-yan(1985-), female, born in fuzhou, Jiangxi Province, experimentalist, master, major in synthesis of structural functional materials (E-mail:wuhy1985@126.com).