LIN Wei-Chen GUO Wei-Gng WANG Li-Hu

MA Xiu-Linga XIANG Sheng-Changa, b ZHANG Zhang-Jinga, c②

a (College of Chemistry and Chemical Engineering, Fujian Provincial Key Laboratory of Polymer Materials, Fujian Normal University, Fuzhou 350007, China)

b (College of Life and Environmental Sciences, Minzu University of China, Beijing 100081, China)

c (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

1 INTRODUCTION

In recent years, citric acid, a natural α-hydroxyl tricarboxylic acid, has attracted great attention for its abundance in biological fluids and its coordination diversity to biologically relevant metal ions because of its conformational flexibility. Moreover, it has the ability to increase its metal ion solubility and enhance the potential metal ion bioavailability and subsequent absorption by biological tissues. The properties make it a good physiological binder with trace metal ions in human body. Such ions include titanium, iron, calcium, magnesium, vanadium,cobalt, zinc, nickel, and manganese[1-6]. Zinc, as a trace element, is essential to DNA and protein synthesis, cell division, immune and enzyme activity[7-9]. The deficiency of zinc will lead to growth retardation, cell-mediated immune dysfunction and cognitive impairment[10]. It is necessary to understand the physiological metabolic functions of zinc in human body. Various zinc-citrate complexes have been synthesized, for instance, K4[Zn(C6H5O7)2][11],[Zn3(C6H5O7)2(H2O)2][12]and[Zn3(C6H5O7)2(H2O)2][13]which were influenced by pH, molar ratio, temperature, and concentration of the reactants. Here we report the synthesis and structure of a new zinc-citrate complex with zigzag chains, exhibiting interesting photoluminescent properties.

2 EXPERIMENTAL

2. 1 Materials and physical measurements

All chemicals were commercially analytical reagents and used without further purification. All reactions were carried out in the air. Nanopurequality water was used throughout this work. Elemental analyses (C and H)were performed on a Perkin-Elmer 240C analyzer. Infrared spectra were recorded on a Nicolet 5700 FT-IR spectrometer in the region of 400～4000 cm-1, using KBr pellets.Thermogravimetric analyses were carried out on METTLER STDA 851° under a flow of nitrogen at a heating rate of 10 ℃·min-1. Fluorescence spectrum data were recorded with a FLS 920 fluorescence spectrophotometer equipped with a 900w Xe lamp as the excitation source.

2. 2 X-ray data collection and crystal structure determination

The unit cell determination and data collection for compound 1 were measured on a SuperNova, Dual,Cu at zero, Atlas diffractometer equipped with a graphite-monochromatized MoKα radiation (λ =0.7107 ?)at 293.9(2)K. Using Olex2[14], the structure was solved with the ShelXS structure solution program by direct methods and refined with the olex2.refine refinement package with the Gauss-Newton minimization[15]. All of the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were generated according to coordination geometries. The selected bond lengths and bond angles and the hydrogen bonding parameters for complex 1 are summarized in Tables 1 and 2,respectively. Crystal data for 1: C6H7KO8Zn, Mr=311.59, triclinic, space group P1, a = 7.3487(9), b =7.5397(6), c = 9.6772(8)?, α = 76.894(8), β =68.260(10), γ = 65.155(10)°, V = 450.34(7)?3, Z = 2,Dc= 2.298 g/cm3and μ = 3.217 mm-1. The final R =0.0338 for 148 parameters and 1899 unique reflections with I ＞ 2σ(I)and wR = 0.0779 for all 2156 reflections.

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

Table 2. Hydrogen Bond Lengths (?)and Bond Angles (°)for Complex 1

2. 3 Syntheswas

{K[Zn(C6H5O7)(H2O)]}n(1)The mixture of Zn(NO3)2·6H2O (0.297 g, 1 mmol),citric acid monohydrate (C6H8O7·H2O, 0.21 g, 1 mmol), KOH (0.168 g, 3 mmol)and H2O (2.5 mL)was stirred for 10 min, then C2H5OH (1.5 mL)was added dropwise to the aqueous solution. After that,the mixture was moved into a 23 mL Teflon-lined stainless steel bomb which was sealed and held at 120 ℃ for 3 days. The reaction system was allowed to slowly cool to room temperature. Colorless block-like crystals were obtained after filtration and dried in air. Yield: 43.5 mg (14.0% based on Zn).Anal. Calcd. (%)for C6H7KO8Zn: C, 23.12; H, 2.26.Found (%): C, 23.30; H, 2.27.

3 RESULTS AND DISCUSSION

3. 1 Syntheses

Through the solvothermal reaction we have synthesized complexes 1 and 2, but the ratio of Zn(NO3)2·6H2O:cit:KOH, the pH and the reaction temperature were different. Compound 1 crystallizes from the solution with the Zn(NO3)2·6H2O:cit:KOH ratio of 1:1:3 at pH = 3.9 and 120 ℃, while 2 in the corresponding ratio of 1:1:2.5 at pH = 3.5 and 80 ℃.Che and Zhang obtained the same compound with 2 by different reaction conditions[12,16]. In literature[12],the complex was obtained at 140 ℃ with the ratio of Zn(NO3)2·6H2O and cit being 1:1 and the pH being adjusted to 4.0 with ammonia (30%). However, in literature[18], it was obtained at 150 ℃ with the ratio of 1:2 at pH = 6.0. From the above, we know that the ratio of K+ions plays an important role in crystallization, and increasing the amount of K+ion results in the formation of compound 1.

3. 2 Description of the crystal structure

X-ray single-crystal diffraction analysis reveals that 1 crystallizes in triclinic with P1 space group.Without considering weak interaction, 1 displays a zigzag chain. There are one crystallographically independent zinc ion and one citrate ligand (Fig. 1).The citrate is triply deprotonated and acts as a quadridentate ligand, which affords its α-hydroxyl group, α-carboxylate group and two β-carboxylate groups to chelate the Zn2+ions. The Zn(1)ion is five-coordinated to adopt a trigonal biyramidal coordination geometry. The equatorial plane is defined with the α-hydroxyl O(3)from one citrate ligand together with β-carboxylate group O(2)iand β-carboxylate group O(7)iifrom another two citrate ligands, respectively. The axial positions were occupied with the α-carboxylate atom O(5)and one coordinated water molecule O(1w), resulting in the O(1w)–Zn(1)–O(5)angle of 158.47(9)°. The Zn–O distances range from 1.969(2)to 2.183(2)?, similar to that in the previously reported zinc-citrate complex[17]. In 1, some α- and β-carboxylate groups bridge the discrete metal centers to from a zigzag chain with an edge-sharing mode. The K+ions occupy the spaces between zigzag chains and interact with the oxygen atoms of the citrate ligands.The K(1)ion has a disordered pentagonal bipyramidal environment of oxygen atoms arising from five citrate anions. The equatorial plane was defined with one α-carboxylate oxygen atom O(5)from one citrate ligand, one α-carboxylate oxygen atom O(4)iiiand one β-carboxylate oxygen atom O(6)iiifrom another citrate ligand, one β-carboxylate oxygen atom O(7)iifrom the third citrate ligand, and one β-carboxylate oxygen atom O(1)ivfrom the fourth citrate ligand. The axial positions are occupied with the β-carboxylate oxygen atom O(6)from one citrate ligand and β-carboxylate oxygen atom O(7)vfrom the other citrate ligand. The O(6)–K(1)–O(7)vangle is 172.46(9). The distances of K–O are from 2.624(2)to 3.146(3)?. As a result, the zigzag chains are then further connected into a 3D network by the K+ions.There are hydrogen bonding interactions between O(1W)··O(1W)(3.029(4)?), O(1W)··O(4)(2.698(3)?)and O(3)··O(1)(2.560(4)?), thus stabilizing the 3D structure (Fig. 2).

3. 3 Thermal behavior

The TGA curve was carried out to study the thermal behavior of complex 1. As shown in Fig. 3,complex 1 was thermostable up to 222 ℃. The first weight loss happens upon heating to 272 ℃ with the weight loss of 5.5%, corresponding to the departure of coordinated water (the calculated value was 5.7 %). Above 272℃, the anhydrous complex begins to decompose.

Fig. 1. An ORTEP view of polymeric complex 1 showing the coordination environments of Zn and K ions. Hydrogen atoms have been omitted for clarity

Fig. 2. View of 1 showing the 3D structure, where hydrogen atoms have been omitted for clarity (Dashed lines represent the O–H··O hydrogen bonds)

3. 4 IR spectrum

The FT-IR spectrum of 1 in KBr reveals the presence of vibrationally active carboxylate group(Fig. 4). Both antisymmetric and symmetric vibrations for the carboxylate groups of the coordinated citrate ligands are present. The asymmetric stretching appears between 1627 and 1571 cm-1, and the symmetric stretching between 1444 and 1381 cm-1.All of the bands were shifted to lower frequencies compared with the corresponding vibrations in free citric acid, indicating changes in the vibrational status of the citrate anion upon coordination to the metal ion[18]. The absence of the typical absorption band of unassociated carboxylic acid group at 1700～1760 cm-1shows that all the carboxylic acid groups were fully deprotonated in complex 1, in consonance with other metal citrate complexes[18,19],which was further approved by X-ray crystal structure of 1. The difference between the symmetric and antisymmetric stretching, Δ(vas(COO-)– vs(COO-)),was greater than 200 cm-1, which shows that the carboxylate groups of the citrate ligand are either free or coordinate to the Zn2+ions in a monodentate fashion. The latter was confirmed by the X-ray crystal structure of 1. Similar trends have been found in the FT-IR spectra of citrate complexes with other metal ions[20].

3. 5 Fluorescent property

The emission spectra of complex 1 as well as citric acid monohydrate in the solid state at room temperature are shown in Fig. 5. Upon excitation at 335 nm, the free citric acid monohydrate shows a strong emission peak at 424 nm. Upon excitation at 350 nm, complex 1 displays an emission peak at 409 nm, which produces a slight blue shift by 15 nm compared to the band shown by the free ligand.

Fig. 3. TGA curve of complex 1 under N2 atmosphere

Fig. 4. FT-IR spectrum of complex 1 (blue)and citric acid monohydrate (red)

Fig. 5. Solid-state emission spectra at room temperature: blue–complex 1, red–citric acid monohydrate

4 CONCLUSION

In summary, through hydrothermal reaction, we have synthesized a novel zinc-citrate complex in which the zigzag chains are further connected into a 3D network by K+ions. The fluorescence reveals that complex 1 is potential optical material.

(1)Deng, Y. F.; Jiang, Y. Q.; Hong, Q. M.; Zhou, Z. H. Speciation of water-soluble titanium citrate: synthesis, structural, spectroscopic properties and biological relevance. Polyhedron 2007, 26, 1561–1569.

(2)Kaliva, M.; Giannadaki, T.; Salifoglou, A. A new dinuclear vanadium(v)-citrate complex from aqueous solutions. Synthetic, structural, spectroscopic,and pH-dependent studies in relevance to aqueous vanadium(v)-citrate speciation. Inorg. Chem. 2002, 41, 3850–3858.

(3)Swanson, R.; Ilsley, W. H. Crystal structure of zinc citrate. J. Inorg. Biochem. 1983, 18, 187–194.

(4)Deng, Y. F.; Zhou, Z. H. Manganese citrate complexes: syntheses, crystal structures and thermal properties. J. Coord. Chem. 2009, 62, 778–788.

(5)Xiang, S. C.; Wu, X. T.; Zhang, J. J.; Fu, R. B.; Hu, S. M.; Zhang, X. D. A 3D canted antiferromagnetic porous metal-organic framework with anatase topology through assembly of an analogue of polyoxometalate. J. Am. Chem. Soc. 2005, 127, 16352–16353.

(6)Xiang, S. C.; He, Y. B.; Zhang, Z. J.; Wu, H.; Zhou, W.; Krwashna, R.; Chen, B. L. Microporous metal-organic framework with potential for carbon dioxide capture at ambient conditions. Nat. Commun. 2012, 3, 954.

(7)Corbo, M. D.; Lam, J. Zinc deficiency and its management in the pediatric population: a literature review and proposed etiologic classification. J. Am.Acad. Dermatol. 2013, 69, 616–624.

(8)Kelleher, S. L.; McCormick, N. H.; Velasquez, V.; Lopez, V. Zinc in specialized secretory tissues: roles in the pancreas, prostate, and mammary gland. Adv. Nutr. 2011, 2, 101–111.

(9)Jomovaa, K.; Valko, M. Advances in metal-induced oxidative stress and human disease. Toxicology 2011, 283, 65–87.

(10)Prasad, A. S. Discovery of human zinc deficiency: its impact on human health and disease. Adv. Nutr. 2013, 4, 176–190

(11)Kim, Y.; Koo, H. G.; Shin, D. H.; Park, L.O.; Lee, J. H.; Jang, H. G.; Kim, C. Zinc citrate with alkali metal and ammonium cations: crystal structure of K4[Zn(Citrate)2]. Russ. J. Struct. Chem. 2010, 51, 382–385.

(12)Che, P.; Fang, D. Q.; Zhang, D. P.; Feng, J.; Wang, J. P.; Hu, N. H.; Meng, J. Hydrothermal synthesis and crystal structure of a new two-dimensional zinc citrate complex. J. Coord. Chem. 2005, 58, 1581–1588.

(13)Li, X. H.; Chen, W. L.; Wang, E. B. A second modification of poly[diaquadi-μ-citrato(3-)-trizinc(II)]. Acta Cryst. 2009, E65, m183.

(14)Dolomanov, O. V.; Bourhwas, L. J.; Gildea, R. J.; Howard. J. A. K.; Puschmann. H. Olex2: a complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339–341.

(15)Sheldrick, G. M. A short history of Shelx. Acta Cryst. 2008, A64, 112–122.

(16)Zhang, G. Q.; Yang, G. Q.; Ma, J. S. Versatile framework solids constructed from divalent transition metals and citric acid: syntheses, crystal structures, and thermal behaviors. Cryst. Growth. Des. 2006, 6, 375–381.

(17)Deng, Y. F.; Zhou, Z. H. Synthesis and crystal structure of a zinc citrate complex [Zn(H2cit)(H2O)]n.J. Coord. Chem. 2009, 62, 1484–1491.

(18)Kefalas, E. T.; Panagiotidwas, P.; Raptopoulou, C. P.; Terzwas, A.; Mavromoustakos, T.; Salifoglou, A. Mononuclear titanium(iv)-citrate complexes from aqueous solutions: pH-specific synthesis and structural and spectroscopic studies in relevance to aqueous titanium(iv)-citrate speciation. Inorg.Chem. 2005, 44, 2596–2605.

(19)Wang, W. G.; Zhang, X. F.; Chen, F.; Ma, C. B.; Chen, C. N.; Liu, Q. T.; Liao, D. Z.; Li, L. C. Homo- and hetero-metallic manganese citrate complexes: syntheses, crystal structures and magnetic properties. Polyhedron 2005, 24, 1656–1668.

(20)Kotsakwas, N.; Raptopoulou, C. P.; Tangoulwas, V.; Terzwas, A.; Giapintzakwas, J.; Jakusch, T.; Kwass, T.; Salifoglou, A. Correlations of synthetic,spectroscopic, structural, and speciation studies in the biologically relevant cobalt(II)-citrate system: the tale of the first aqueous dinuclear cobalt(II)-citrate Complex. Inorg. Chem. 2003, 44, 22–31.