Syntheses, Structures, and Luminescent Properties of the ZnII and CdII 1-D Chain Polymers Assembled by Salicylhydroxamic Acid①

GAO Dn-Dn GAO Qin CHEN Yn-Mei LI Y-Hong② LI Wu②

a (Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources,Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China)

b (College of Chemistry, Chemical Engineering and Materials Science,Soochow University, Suzhou 215123, China)

1 INTRODUCTION

Hydroxamic acids (HAs) are a valuable class of bioligands and have been extensively employed in chemical biology[1]. They possess high metalchelating affinity and exhibit versatile coordination modes. These properties make hydroxamic acids to be powerful ligands to construct homometallic or heterometallic polynuclear clusters and chain complexes[2-5]. Recent advances in the coordination chemistry of hydroxamic acids reveal that the coordination behaviors of hydroxamic acid can be tuned by the incorporation of different functional groups, e.g., -NMe2[2], -OH[6], -NH2[7], etc., at ortho,meta and (or) para positions of the phenyl ring,generating coordination complexes with fascinating configurations and useful properties.

Salicylhydroxamic acid (H3shi) is a member of a big family of hydroxamic acids. It shows rich coordination modes[8-18](Scheme 1) due to the extra hydroxyl group at the meta position of phenyl ring.H3shi acted as a polydentate ligand to afford mo-nonuclear complexes with Ru[18], V[14]and Pt[13],generating polynuclear compounds with Cu[10],Mo[11], and V[12]as well as polynuclear heterometallic Mn-Ho and Mn-Dy complexes[10]. To our surprise, no polynuclear coordination complexes of ZnIIand CdIIare reported. As is well known, coordination complexes of ZnIIand CdIIhave attracted a great deal of interest due to their potential applications in fluorescence-emitting materials[19-21], such as light-emitting diodes (LEDs) and organic lightemitting diodes (OLEDs)[22], in biological systems,e.g. cleavage of DNA, RNA and amino acid esters,etc.[23-27], and in organic synthesis[28]. To further explore the coordination chemistry of salicylhydroxamic acid with ZnIIand CdII, we conducted the reaction of salicylhydroxamic acid with Zn(CH3COO)2·2H2O and Cd(CH3COO)2·2H2O,respectively. Two new 1-D polymeric chain complexes with formulas [Zn(H2shi)(CH3COO)]n(1) and[Cd(H2shi)2(H2O)]n(2) were synthesized. Herein, we report the syntheses, structures and luminesient properties of these two complexes.

Scheme 1. Coordination modes of H3shi

2 EXPERIMENTAL

2.1 Materials and physical measurement

All chemicals were purchased from commercial suppliers and used without further purification. The C, H and N microanalyses were carried out with a Carlo-Erba EA1110 CHNO-S elemental analyzer.FT-IR spectra were recorded from KBr pellets in the range of 400～4000 cm-1on a Nicolet MagNa-IR500 spectrometer. Crystal determination was performed with a Bruker SMART APEX ΙΙ CCDC diffractometer equipped with graphite-monochromatized MoKα radiation (λ = 0.71073 ?). The solid-state luminescence emission/excitation spectra were recorded on a FLS920 fluorescence spectrophotometer equipped with a continuous Xe-900 xenon lamp and a μF900 microsecond flash lamp.Powder X-ray diffraction (PXRD) was recorded on a Rigaku D/Max-2500 diffractometer.

2.2 Syntheses of the complexes

2. 2. 1 Synthesis of [Zn(H2shi)(CH3COO)]n(1)

A mixture of Zn(CH3COO)2·2H2O (0.0216 g,0.10 mmol), H3shi (0.0153 g, 0.10 mmol), and H2O(1.0 mL) was placed in a Pyrex-tube. The tube was heated at 80 ℃ for 6 days. After being cooled to room temperature, pale yellow crystals of the product were afforded. The crystals were collected by filtration, washed with H2O (2 mL) and dried in air. Yield: 40% (based on Zn). Anal. Calcd. (%) for C9H9NO5Zn: C, 39.09; H, 3.28; N, 5.06. Found (%):C, 38.96; H, 3.15; N, 5.01. IR (KBr, cm-1): 3274(s),1612(s), 1551(s), 1481(s), 1367(m), 1250(m),1153(s), 1059(s), 1031(m), 925(s), 812(m), 751(m).

2.2.2 Synthesis of [Cd(H2Shi)2(H2O)]n(2)

A mixture of Cd(CH3COO)2·2H2O (0.0207 g,0.10 mmol), H3shi (0.0153 g, 0.10 mmol), and H2O(1.0 mL) was placed in a Pyrex-tube. The tube was heated at 80 ℃ for 7 days. After being cooled to room temperature, pale yellow crystals of the product were afforded. The crystals were collected by filtration, washed with H2O (2 mL) and dried in air. Yield: 56% (based on Cd). Anal. Calcd. (%) for C14H14Cd2N2O7: C, 38.68; H, 3.25; N, 6.44. Found(%): C, 38.76; H, 3.21; N, 6.38. IR (KBr, cm-1):3468(m), 3015(w), 1613(s), 1549(s), 1481(s),1417(s), 1341(s), 1250(s), 1153(s), 1122(s), 1050(s),925(s), 812(m), 751(s), 659(s).

2.3 X-ray crystal structure determination

The single crystals of complexes 1 and 2 were determined with MoKα radiation using a Bruker SMART APEX-II CCD diffractometer at 296(2) K for 1 and 293(2) K for 2 using the ω-2θ scan mode.For complex 1, in the range of 2.15≤θ≤28.42o,a total of 13821 reflections were obtained with 2490 unique ones and used in the refinements. For complex 2, in the range of 3.39≤θ≤27.49o, a total of 5355 reflections were obtained with 1573 unique ones and used in the refinements. The structures were solved by direct methods and refined with full-matrix least-squares on F2using SHELXS-97[29]and SHELXL-97[30]. The selected bond lengths and bond angles of complexes 1 and 2 are listed in Table 1.

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

3 RESULTS AND DISCUSSION

3.1 Structure descriptions of the complexes

3. 1. 1 Structure of complex 1

The crystal structure analysis reveals that 1 crystallizes in the tetragonal crystal system, space group I41/a. The asymmetric unit (Fig. 1) consists of one crystallographically independent ZnIIatom,one H2shi-ligand and one acetate ion. The ZnIIion is penta-coordinated by one carboxyl oxygen atom,one μ2-hydroxamate oxygen atom and one μ1-hydroxamate oxygen atom from two H2shi-ligands,and two oxygen atoms from acetate ions, displaying distorted tetragonal geometry. The adjacent ZnIIions are doubly connected by two oxygen atoms of the acetate ion and one hydroxamate oxygen atom to generate an infinitive 1-D chain structure (Fig. 2).The Zn–O/N bond lengths are in the range of 1.983(2)～2.095(3) ?. The ZnII···ZnIIdistance is 3.6506(7) ?. The H2shi-ligands possess the coordination mode F (Scheme 1).

Fig. 1. Coordination environment of the Zn ion in 1. Hydrogen atoms have been omitted for clarity.Color scheme: blue, ZnII; red, oxygen; dark blue, nitrogen; yellow, carbon

Fig. 2. 1-D chain structure of 1. Hydrogen atoms are omitted for clarity.Color scheme: blue, ZnII; red, oxygen; dark blue, nitrogen; yellow, carbon

3. 1. 2 Structure of complex 2

The crystal structure analysis reveals that 2 crystallizes in monoclinic, space group P2/c. The crystal structure of 2 (Fig. 3) consists of one CdIIion and two singly deprotonated H2shi-ligands and one coordinated water molecule. The adjacent CdIIions are bridged by the hydroximate oxygen atoms to form a one-dimensional linear chain (Fig. 4).Each CdIIion is bound to seven oxygen atoms (O1,O1A, O3A, O3B, μ2-O1B, μ2-O1C, and O1W)originated from four different H2shi-ligands and one water molecule. Each H2shi-ligand chelates one CdIIion through a μ1-carbonyl oxygen atom and a μ2-hydroxamate oxygen atom (coordination mode F in Scheme 1), with the Cd–O distances ranging from 2.341(2) to 2.432(16) ?. The Cd···Cd distance is 3.7392(6) ?.

Remarkably, complexes 1 and 2 were prepared by using similar starting materials, but their compositions and structures are totally different, indicating that metal ions play key roles in the construction of coordination polymers.

Complexes 1 and 2 join a big family of coordination polymers of ZnII/CdII[31-36]. However, the ZnIIand CdII1-D chain complexes supported by the H3shi ligand are never reported.

Fig. 3. Coordination environment of the Cd ion in 2. Hydrogen atoms have been omitted for clarity.Color scheme: green, CdII; red, oxygen; blue, nitrogen; yellow, carbon

Fig. 4. 1-D chain structure of 2. Hydrogen atoms are omitted for clarity.Color scheme: green, CdII; red, oxygen; blue, nitrogen; yellow, carbon

3.2 Powder X-ray diffraction (PXRD)

In order to check the phase purity of complexes 1 and 2, the powder X-ray diffractions (PXRD) have been measured and compared with those simulated from the single-crystal structure data. As can be seen from Figs. 5 and 6, the experimental PXRD patterns and simulated peaks match well, indicating the purities of the complexes.

3.3 Luminescent property

The luminescent properties of the H3shi ligand and complexes 1 and 2 were investigated in the solid state at room temperature (Fig. 7). The H3shi ligand shows photoluminescence emission at 344 nm. Complex 1 shows emission with the maximum peak at 356 nm. Compared with the H3shi ligand,the red-shifted emission of 1 may be ascribed as the intraligand charge transfer (π-π*)[33,37]. The pronounced fluorescence emission of complex 1 reveals its potential application in photoactive materials.

Fig. 5. Powder XRD patterns of complex 1

Fig. 6. Powder XRD patterns of complex 2

Fig. 7. Emission spectra of the H3shi ligand and complex 1 in the solid state at room temperature (Emission slit = 1 nm)

No emission was observed for complex 2. The water in lattice may prevent an efficient intraligand charge transfer.

4 CONCLUSION

In summary, two 1D chain ZnIIand CdIIcomplexes of compositions [Zn(H2shi)(CH3COO)]n(1)and [Cd(H2shi)2(H2O)]n(2) have been prepared under solvothermal conditions. The luminescent properties have been investigated. Complex 1 shows red-shifted luminescence emission. The pronounced fluorescence emission of 1 reveals its potential applications for photoactive materials.

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