GUO Guo-Zhe ZHENG Xv-Dong ZHANG Yu-QuanZHU Ji-Hua LI Yan-Chun LI Zhi-Jun

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Two Novel Coordination Polymers of Schiff Base Ligands: Synthesis, Crystal Structures and Antibacterial Properties Studies①

GUO Guo-Zhe②ZHENG Xv-Dong ZHANG Yu-QuanZHU Ji-Hua LI Yan-Chun LI Zhi-Jun

(745000)

Two novel complexes [AgL1(NO3)H2O]n(1) and [PbL2(NO3)2]n(2) were synthesized by the evaporation reaction with metal salts and Schiff base ligands. They were characterized by elemental analyses, IR spectra and X-ray single-crystal diffraction. 1 crystallizes in monoclinic, space group21/with= 18.358(3),= 10.0395(15),= 13.4643(16) ?,= 91.749(12)o,= 2480.4(6) ?3,D= 1.597 g/cm3,M= 596.35,(000) = 1208,= 4,= 0.0772 and=0.0927. 2 crystallizes in monoclinic space group2/with= 15.8549(10),= 21.1988(17),= 17.7198(12) ?,= 105.829(8)o,= 5729.9(7) ?3,D= 1.645 g/cm3,M= 709.63,(000) = 2736,= 8,= 0.0541 and= 0.1175. X-ray single-crystal diffraction experiments of 1 and 2 display that extensive×××stacking interactions and hydrogen bonds construct into a 2D rectangular network and a 3D supramolecular framework. The antibacterial properties of L1, L2, 1 and 2 were also studied.

framework, antibacterial properties,crystal structure, synthesis;

1 INTRODUCTION

Double Schiff base ligands, due to their specific geometry, including the different relative orientation of N-donors and the zigzag conformation of the spacer moiety between the two terminal coordination groups, may result in coordination polymers with novel network patterns not achievable by other rigid linking ligands[1]. The finding that metal complexes based on Schiff base ligands can be widely applied in catalysis, magnetism and material chemistry[2], and that they are also ubiquitous in developing intriguing coordination models of main group and transition metals, is mainly due to their stability, easy pre- paration, structural variability and biological activity[3]. Especially, pyridine- and pyrazine-Schiff base ligands containing an additional nitrogen donor in the pyridine and pyrazine units can systematically be used to understand the features of the supramo- lecular architectures and to explore the fascinating properties of these supramolecular frameworks[4]. For example, Hannon et al.[5]have reported a series of metallo-supramolecular architectures based on pyridine- and pyrazine-Schiff base ligands containing rigid spacers. In these structures, additional donor groups were introduced into the pyridine- and pyrazine-based ligands system to link the distinct architectures into larger arrays.

On the other hand, assemblies of Ag(I) and Pb(Ⅱ) coordination polymers have attracted attention for a long time due to their interesting structure and potential physical and chemical functions[6]. The variable coordination numbers of Ag(I) and Pb(Ⅱ) and various supramolecular forces in the Ag(I) and Pb(Ⅱ) compounds such as metal-ligand, metal-metal and metal-anion interactions, increase the possibility of compounds forming complicated geometries, which also stimulate the study of polythreaded coordination networks. Taking inspiration from previous work on Ag(I) and Pb(Ⅱ) coordination polymers, herein we report two novel silver and lead complexes from the reaction of ligand (L1and L2) with nitrate (silver and lead) (see Scheme 1). The crystal structures are determined by single-crystal X-ray diffraction analyses. Complex 1 shows coor- dination by five nitrogen atoms from three ligands and has a distorted square pyramidal geometry, and complex 2 shows coordination by eight atoms and the Pb(Ⅱ) center is in a distorted hemidirected geo- metry. The antioxidant activities of L1, L2, 1 and 2 were also reported.

Scheme 1. Molecular structures of L1and L2

2 EXPERIMENTAL

2. 1 Materials and measurements

The regents and solvents were used as commercial sources without further purification. The ligand was prepared according to the literature[7]. Elemental analyses were performed on a Perkin-Elmer 2400C elemental analyzer. The IR spectra were recorded on a Bruker Vector 22 FTIR spectrophotometer with KBr pellets. The crystal data of the compounds were collected on a SuperNova, Dual, Cu at zero, Eos diffractometer.

2. 2 Synthesis of 1

AgNO3(0.15 mmol, 25.5 mg) andL1(0.15 mmol, 61.3 mg)were dissolved inMeOH (15 mL), and the yellow slurry was stirred for 5 min at room temperature. A solution of strong ammonia (3 drops) was then added and the resulting orange solution was stirred for 1 h at room temperature. Brown and block crystal was obtained by evaporation after one week, and washed with menthol. Yield: 57 wt%. Anal. Calcd. for 1 (%): C, 48.44; H, 3.86; N, 16.22. Found (%): C, 48.34; H, 3.72; N, 16.44. IR(KBr):= 3382(m), 3361(s), 1630(m), 1385(m), 1217(w), 455(s) cm-1.

2. 3 Synthesis of 2

A yellow solution of L2(50.9 mg, 0.15 mmol) in CH3CN (5 mL) was slowly added to the solution of Pb(NO3)2(49.7 mg, 0.15 mmol) in MeOH (5 mL), and the light yellow slurry was stirred for 5 min at room temperature. A solution of strong ammonia (3 drops) was then added and the resulting yellow solution was stirred for 1 h at room temperature. Slow evaporation of the solvent at room temperature gave colourless and block crystal of compound 2 suitable for X-ray analysis. The crystals were collected by filtration, washed with cold acetonitrile, and dried under vacuum. Yield: 51 wt%. Anal. Calcd. for 2 (%): C, 40.53; H, 2.82; N, 11.62. Found (%): C, 40.62; H, 2.56; N, 11.84. IR(KBr):= 3452(w) 1589(s), 1491(s), 1386(s), 1238(m), 832(m), 774(m) cm-1.

2. 4 Crystal structure determination and refinement

Diffraction intensity data for single crystals of these two complexes were collected and mounted on a SuperNova, Dual, Cu at zero, Eos diffractometer. Data were collected at 295.42(10) K by using a graphite-monochromator with Moradiation (= 0.71073 ?) in the-scanning mode. Data collection, reduction and absorption correction were performed by Olex2[8]. The structure was solved by direct methods using the ShelXS[9]and refined by Least-Squares minimization techniques ShelXL[10]. The non-hydrogen atoms were refined anisotro- pically. The hydrogen atoms were determined with theoretical calculations and refined isotropically. The selected bond lengths and bond angles are given in Tables 1 and 2, respectively. The hydrogen bond lengths and bond angles are listed in Table 3.

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

Symmetry codes: i: 1–, 1–, 1–; ii: 1–, –0.5+, 0.5–; iii: 1–, 0.5+, 0.5–

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for 2

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Compounds 1 and 2

Symmetry codes: i: 1–,, 0.5–; ii: 1–, –0.5+, 0.5–; iii: 1–, 0.5+, 0.5–

3 RESULTS AND DISCUSSION

3. 1 Structure description

The silver(I) coordination polymer 1 was con- firmed by single-crystal X-ray diffraction analysis. The Ag–N(pyrazine) distances are in the range of 2.347～2.514 ? and the Ag–N(C=N) distances fall in the 2.267～2.441 ? range[11]. The bond angles N–Ag–N are 67.8°, 71.1°, 83.6°, 100.1° and 103.9°, respectively[11]. A perspective view of the title complex is depicted in Fig. 1. The title complex 1 reveals that the central silver ion is five-coordinated by five nitrogen atoms from three ligands, forming a distorted square pyramidal geometry. Two of the ligands bridge both the Ag(1) centres; one ligand molecule passes above the Ag(1)–Ag(1i) axis and the other beneath.

It is interesting that there is one ether group in the spacer favoring a bent conformation of the ligand yielding an internal mirror plane and a mesocate structure, and forms a two-dimensional sheet (Fig. 2). As the molecular box occupies an inversion center, the opposite two phenyl rings are parallel to each other. The dihedral angles of A and B are ca. 6.3°, the center-to-plane separation ca. 3.79 ? and the shortest interplanar atom-atom separation ca. 3.45 ?. These distances are similar to the standard distance for a-stacking interaction between two aryl rings. The two Ag(I) are separated intramolecularly by 11.73 ?. 1 displays that extensive×××stacking interactions and hydrogen bonds construct into a 2D rectangular network (Fig. 3).

Fig. 1. Ag coordination environment of complex 1 at 30% probability displacement ellipsoids. H atoms, water molecules and nitrate anions have been omitted for clarity (Symmetry codes: (i) 1–, 1–, 1–; (ii) 1–, –0.5+, 0.5–; (iii) 1–, 0.5+, 0.5–; (iv), 0.5–, 0.5+; (v), 1.5–, 0.5+)

Fig. 2. 2D Crystal structure of complex 1. H atoms, water molecules and nitrateanions have been omitted for clarity

Fig. 3. Crystal packing of complex 1

As shown in Fig. 4, the crystal structure of 2 reveals that each Pb(II) is bonded to four nitrogen atoms from two L2and four oxygen atoms from three nitrates. In one asymmetric unit, the nitrates exhibit two different coordination modes; two nitrates are bidentate to Pb(II) and the other as tetra-dentate chelating and bridging links four L2to form a 1D infinite chain (Fig. 5). The Pb(1)-Pb(1i) separation is 5.5781(7) ?. Coordination number of Pb(II) of 2～5 shows hemidirected stereochemistry, high coordination number (9,10), holodirected geometry, and coordination number (6～8) either hemidirected or holodirected geometry. The coor- dination sphere of each Pb(II) is hemidirected. All the Pb–O and Pb–N bond distances are below 2.9 ?, such bond distances are reasonable and can find a nearly ideal value assumed for oxidation state II on the Pb(II) ions. The dihedral angle of C and D is ca. 20.4°, the center-to-plane separation ca. 4.74 ? and the shortest interplanar atom-atom separation ca. 3.60 ?. These distances are similar to the standard distance for a weak-stacking interaction between two aryl rings. Hydrogen bonds interactions (C–H×××O and C–H×××N) and offset×××stacking of the neighboring phenyl and pyridine rings with a sandwich conformation (Fig. 6) stabilize the crystal lattice in a 3D supramolecular framework in the solid state.

Fig. 4. Pb coordination environment of complex 2. H atoms have been omitted for clarity. Symmetry code: (i) 1–,, 0.5–

Fig. 5. 1D infinite chain structure of crystal 2. H atoms have been omitted for clarity

Fig. 6. Crystal packing of complex 2. Hydrogen bonds are shown as dashed lines

3. 2 Antibacterial property studies

The activities of the complexes and ligands against several bacteria have been studied using the well diffusion method on beef extract-peptone medium. Paper disc diffusion method was employed on these compounds dissolved in CHCl3(~1 mM) against test organisms, where the paper discs were prepared by immersion to these different solutions, and the antimicrobial performance of the compounds towards two bacterial pathogens. Proteusbacillus vulgaris and bacillus subtilis were determined by measuring the size of inhibition zone diameters (IZDs). The zone of inhibition was measured after 24 h of incubation. The results are presented as inhibition zone diameters in Table 4.It is evident that 1 exhibits considerable higher activity against bacillus subtilis which is significantly better than L1. Complex 1 was found to be highly active against bacillus subtilis. The antibacterial activity of complex 2 is close to L2.

Table 4. Antibacterial Activities (IZD Values) of the Complexes and Ligands

4 CONCLUSION

In the present work, two novel coordination com- pounds [AgL1(NO3)H2O]n(1) and [PbL2(NO3)2]n(2) were synthesized based on the N-containing group ligands and characterized. In this structure, complex 1 has a distorted square pyramidal geometry, and complex 2 has a distorted hemidirected geometry. Moreover, 1 and 2 display that extensive×××stacking interactions and hydrogen bonds construct into a 2D rectangular network and a 3D supramo- lecular framework. The antibacterial activities of L1, L2, 1 and 2 afford a guiding role for potential antibiotic resistance.

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16 November 2017;

19 March 2018 (CCDC 1443583 for 1 and 1561175 for 2)

① This work was supported by the University Project of Gansu Province (2017A-095) and the 13th Five-Year Period Education Plan of Gansu Province (GS[2017]GHB0360)

. Born in 1988, majoring in functional coordination chemistry. E-mail: 2660859870@qq.com

10.14102/j.cnki.0254-5861.2011-1892