Design, Synthesis, and Antibacterial Assessment of Silver(I)-Based

Article pubs.acs.org/crystal

Design, Synthesis, and Antibacterial Assessment of Silver(I)-Based Coordination Polymers with Variable Counterions and Unprecedented Structures by the Tuning Spacer Length and Binding Mode of Flexible Bis(imidazole-2-thiones) Ligands Azizolla Beheshti,*,† Susan Soleymani-Babadi,† Peter Mayer,‡ Carmel T. Abrahams,§ Hossein Motamedi,∥,⊥ Damian Trzybiński,# and Krzysztof Wozniak∇ †

Department of Chemistry, Faculty of Sciences, ∥Department of Biology, Faculty of Sciences, and ⊥Biotechnology and Biological Science Research Center, Shahid Chamran University of Ahvaz, Ahvaz, Iran ‡ LMU München Department Chemie, Butenandtstrasse, 5-13 (D) 81377 München, Germany § Department of Chemistry, La Trobe University, Bundoora, Victoria 3086, Australia # Department of Chemistry, Biological and Chemical Research Centre, University of Warsaw, Ż wirki i Wigury 101, 02-089 Warsaw, Poland ∇ Chemistry Department, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland S Supporting Information *

ABSTRACT: An investigation of the impact of alkyl spacer length of the flexible ligands and influence of shape and coordination ability of the counterions has been done on a series of five silver(I)-bis(imidazole-2-thione) coordination polymers. The five compounds, namely, [Ag2L4(NO3)2]n, {[AgL4]2[CF3SO3]2}n, [Ag2L6(CF3SO3)2]n, [Ag2L6(NO3)(ClO4)]n, and [Ag2L6Br2]n (L4 = 1,1′-(butane-1,4-diyl)bis(3methylimidazoline-2-thione) and L6 = 1,1′-(hexane-1,4-diyl)bis(3-methylimidazoline-2-thione) have been characterized by elemental analysis, IR spectra, thermogravimetric analysis (TGA), powder X-ray diffraction, and single crystal X-ray diffraction. In the title polymers, the L4 and L6 ligands exhibit unprecedented coordination modes, and the Ag(I) centers adopt a range of coordination geometries. The single crystal structural analysis of the title compounds shows that polymers based on the L4 ligand predominately adopt AgS4 core structure which does not appear in the polymers containing L6 ligand. The TGA experimental data reveal that these compounds start to decompose in the temperature range of 240−341 °C. All of the synthesized compounds, in particular, polymer 2, possess antibacterial activity against the selected strain of Gram-positive (Escherichiacoli, Pseudomonas aeruginosa) and Gram-negative (Staphylococcus aureus, Bacillus subtilis) bacteria.

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topology when using flexible multidentate bridging ligands is more difficult than that of a rigid one because their flexibility and conformational freedom allow them to coordinate to metal centers in unpredictable coordination modes and form unexpected structures with specific properties. Silver(I) is a soft acid that exhibits a great affinity for sulfur donor ligands such as thioethers and thiones. In recent years, many researchers have undertaken considerable work on the design and synthesis of new silver(I) coordination polymers with soft and flexible dithioether ligands.42−47 However, the coordination chemistry of the flexible bis(imidazole-2-thione) ligands such as bis(3-methylimidazole-2-thione-1-yl) alkyl

INTRODUCTION In recent years, research on the design and synthesis of coordination polymers have been increasingly highlighted in the literature because of the possibility of forming novel structures with unique properties.1−15 The characteristics of these polymers depend upon the type of polymeric structures. In the investigation of coordination polymers, it is important to understand the major factors that affect the metal coordination and crystal packing of the final product. For a given metal and ligand set, the factors that play an important role in controlling the structure of coordination polymers include structure and conformation of the ligands,16−18 the coordinating potential of the anions,19−25 oxidation state26,27 and hard/soft properties of the metal ions, 28−30 the reaction conditions, 31−34 the solvent,35−39 the metal-to-ligand ratio,40,41 and so on. In the structural design of coordination polymers, predicting the © XXXX American Chemical Society

Received: June 6, 2017 Revised: July 24, 2017

A

DOI: 10.1021/acs.cgd.7b00784 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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1651m, 1567m, 1541m, 1478s, 1459s, 1414s, 1398s, 1373s, 1343m, 1333m, 1251s, 1207s, 1187s, 1131s, 1095m, 918m, 826m, 803s, 719s, 671s, 628w, 604w, 527s, 480m, 418m. CAUTION: Perchlorate compounds are potentially explosive and should be handled with care. [Ag2L4(NO3)2]n (1). The L4 ligand (24 mg, 0.1 mmol) and silver(I) nitrate (34 mg, 0.2 mmol) were placed in the main arm of a branched tube. A methanol−water mixture (3:1 v/v) was added to fill the arms. The tube was sealed and the arm containing the reagents was immersed in an oil bath at 60 °C, while the other arm was kept at ambient temperature. After 4 days, colorless filament crystals of [Ag2L4(NO3)2]n suitable for X-ray analysis were deposited in the cooler arm (yield: 38 mg, 85%). Anal. Calcd (%) for Ag2C12H18N6O6S2: C, 23.16; H, 2.91; N, 13.50. Found: C, 23.55; H, 3.08; N, 13.19. IR (KBr pellet, cm−1): 3163m, 3122m, 3105w, 3051w, 3030w, 2960w, 2935w, 1621s, 1571s, 1493s, 1397vs- 1307vs, 1238m, 1149m, 1100m, 819s, 754s, 680w, 644w, 534w, 509s, 486w, 418w. TGA data: 287 °C (dec.). {[AgL4]2[CF3SO3]2}n (2). This compound was prepared by the same method as described for polymer 1, using an equimolar ratio of silver(I) triflate (25 mg, 0.1 mmol) with L4 ligand (24 mg, 0.1 mmol) in a methanol−water mixture (3:1 v/v). After 3 days, needle-shaped crystals were deposited in the cooler arm. The crystals were filtered off, washed with chloroform, and finally air-dried (yield: 36 mg, 67%). Anal. Calcd (%) for Ag2C24H36F6N8O6S6: C, 29.24; H, 3.10; N, 10.55. Found: C, 28.95; H, 3.36; N, 10.39. IR (KBr pellet, cm−1): 3166m, 3138m, 31100m, 2946m, 2879w, 1570s, 1480s, 1420s, 1381m, 1356m, 1225−1285vs, 1118−1136vs, 1097m, 1033vs, 735s, 689s, 638vs, 574m, 518s, 481m, 418m. TGA data: 314 °C (dec.). [Ag2L6(CF3SO3)2]n (3). A solution of AgCF3SO3 (26 mg, 0.1 mmol) in 5 mL of MeOH was gently layered on the top of a solution of L6 (16 mg, 0.05 mmol) in 5 mL of CHCl3 in a test tube. After 3 days, colorless fine needle-shaped crystals appeared at the boundary between the MeOH and CHCl3 (yield: 33 mg, 80%). Anal. calcd (%) for C16H22Ag2F6N4O6S4: C, 23.30; H, 2.68; N, 6.79. Found: C, 23.77; H, 2.91; N, 6.54. IR (KBr pellet, cm−1): 3168m, 3140m, 3110m, 2939w, 2871w, 1570m, 1477m, 1416m, 1405m, 1225−1285vs, 1170vs, 1097m, 1031vs, 758m, 638s, 574w, 518m, 481m, 418m. TGA data: 313 °C (dec.). [Ag2L6(NO3)(ClO4)]n (4). A solution of NaClO4.H2O (28 mg, 0.2 mmol) and AgNO3 (17 mg, 0.1 mmol) in 5 mL of methanol was carefully layered on the top of a solution of L6 (16 mg, 0.05 mmol) in 5 mL of chloroform in a test tube. Colorless fine needle-shaped crystals were obtained after 3 days (yield: 25 mg, 75%). Anal. Calcd (%) for Ag2C14H22N5O7S2Cl: C, 24.44; H, 3.22; N, 10.18. Found: C, 24.39; H, 3.21; N, 10.22. IR (KBr pellet, cm−1): 3161m, 3136m, 3111m, 2939m, 2864w, 1571s, 1482m, 1464m, 1425s, 1389vs, 1300s, 1239m, 1050vs- 1125vs, 750s, 679m, 621s, 520vw, 487w, 420w. TGA data: 240 °C (dec.). [Ag2L6Br2]n (5). This compound was prepared by the same procedure as described for polymer 1, using equimolar ratio of silver(I) nitrate (35 g, 0.2 mmol) and potassium bromide (48 mg, 0.4 mmol) with the L6 ligand (31 mg, 0.1 mmol) in methanol as a solvent. After 3 days, suitable crystals for X-ray analysis were observed in the cooler arm. The crystals were filtered off, washed with methanol, and air-dried (yield: 46 mg, 68%). Anal. calcd. (%) for Ag2 Br2 C14 H22 N4 S2: C, 24.68; H, 3.25; N, 8.22. Found: C, 24.95; H, 3.56; N, 7.69. IR (KBr pellet, cm−1): 3149m, 3115m, 3095w, 2938m, 2854w, 1564s, 1481s, 1452s, 1404s, 1376m, 1238m, 1211m, 1118m, 1153m, 1094m, 758m, 737s, 691m, 667m, 510m, 408m. TGA data: 341 °C (dec.). X-ray Crystallography. Single crystal data collections and corrections were done at 100(2) K (for polymers 1, 2, and 5) and 130.01(10) K (for polymers 3 and 4) with Bruker D8Venture diffractometer using graphite monochromated MoKa (λ = 0.71073 Å). Changing the temperature from 100 to 295 K causes only an increase in the unit cell parameters. The structures were solved by direct method with SIR9775 and refined with full-matrix least-squares techniques on F2 with SHELXL-97.76 In the case of 5, the X-ray diffraction data were collected on the Agilent Technologies SuperNova Dual Source diffractometer with CuKα radiation (λ = 1.54184 Å)

ligands has not been studied in detail despite potentially useful applications that their coordination compounds might have in biomedical chemistry.48 Only a few complexes with flexible bis(imidazole-2-thione) ligands have appeared in the literature.49−60 In these polymers, the metal nodes are connected through bridging flexible bis(imidazole-2-thione) ligands with three different coordination modes. Among these conformations, the bidentate chelate mode is mostly commonly observed in the complexes that have been structurally characterized,49−53 although chelating, combining μ2-bridging54,55 and bis-monodentate bridging56−60 coordination modes are observed in a number of polymers. To the best of our knowledge, no complexes of 1,1′-(hexane-1,4-diyl) bis(3-methylimidazoline-2thione) (L6) and only one dinuclear compound of 1,1′(butane-1,4-diyl)bis(3-methylimidazoline-2-thione) (L4) ligand has been published in the literature (Chart 1). The presence of Chart 1

the L-methylimidazoline-2(3H)-thione moiety with well-known biological activity61−65 in the structure of the L4 and L6 ligands make them an important candidate for antibacterial studies. Therefore, in addition to the structural aspects, the silver-based coordination polymers with L4 and L6 ligands as a potential source of Ag+ ions and thione rings can be considered as valuable antibacterial agents against some Gram-positive and Gram-negative bacteria. The toxicity and antimicrobial activity of silver compounds can best be described by coordination of silver ions to N, O, or in particular S donor atoms of the hydroxyl, carbonyl, carboxyl, amino, and sulfhydryl functional groups present in DNA or the cell membrane of bacteria and leading to deactivation of their biological activities.66−71 In the present study, we have successfully designed and synthesized five silver(I)-based coordination polymers with antibacterial activity by varying the size and coordination ability of the counteranions as well as the length of the bis(imidazole-2thione) ligands.

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EXPERIMENTAL SECTION

Materials and General Methods. All the experiments were carried out in air atmosphere. Starting materials were reagent grade and used as commercially obtained without further purification. Infrared spectra (4000−400 cm−1) were recorded from KBr disks with a BOMEN MB102 FT-IR spectrometer. Elemental analyses for C, H, and N were implemented on a Thermo Finigan Flash EA 1120 CHN analyzer. Thermal analyses were performed using a Bahr-STA 503 TG/DTA thermal analyzer. A ramp rate of 10 °C·min−1 in the range of 50−900 °C was used. X-ray powder diffraction (PXRD) patterns were recorded on a Philips X’Pert Pro diffractometer (Cu Kα radiation, λ = 1.54184 Å) in the 2θ range 5−50°. Syntheses of Ligands. The two bis(imidazole-2-thione) ligands L4 and L6 were prepared by the literature methods.72−74,54 The reaction of N-methylimidazole with the 1,4-dichlorobutane (for L4), 1,6-dichlorohexane (for L6) and S8 in the presence of K2CO3 gave the title ligands with reasonable yields (47% for L4 and 64% for L6). Anal. Calcd (%) for L4 (C12H18N4S2): C, 51.02; H, 6.42; N, 19.84. Found: C, 50.75; H, 6.26; N, 19.39 and for L6 (C14H22N4S2): C, 54.15; H, 7.14; N, 18.04. Found: C, 53.85; H, 7.02; N, 17.75. IR (KBr pellet, cm−1) for L4 and L6: 3155m, 3121m, 3097m, 2967m, 2949m, 2865w, B



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Figure 1. (a) Perspective view of 22-memberd metallo-macrocyclic ring and coordination environment of Ag(I) in polymer 1. Atomic displacement parameters are drawn at 50% probability level. Symmetry code: i = 1/2 − x, y, 1/2 + z. (b) The 1D coordination polymer of 1 extending parallel to the c-axis. (c) packing of the 1D-tube structure of 1 along the c-axis. Hydrogen atoms were omitted for clarity. using CrysAlis RED software.77 The analytical numerical absorption correction, using a multifaceted crystal model based on expressions derived by R. C. Clark and J. S. Reid78 implemented in SCALE3 ABSPACK scaling algorithm, was applied.77 The structural determination procedure was carried out using the SHELX package.79 The structure of 5 was solved with direct methods, and then successive least-squares refinement was carried out based on the full-matrix leastsquares method on F2 using the XLMP program.79 All H atoms were positioned geometrically with C−H equal to 0.93, 0.96, and 0.97 Å for the aromatic, methyl, and methylene H atoms, respectively, and constrained to ride on their parent atoms with Uiso(H) = xUeq(C), where x = 1.2 for the aromatic and methylene H atoms, and x = 1.5 for

the methyl H atoms. The molecular structure plots were prepared using DIAMOND and Mercury.79

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RESULTS AND DISCUSSION Description of Crystal Structures. [Ag2L4(NO3)2]n (1). Coordination polymer 1 crystallizes in the orthorhombic space group Pca21 with Z = 4 (Table S1). In the considered compound, two crystallographic independent Ag+ centers with two kinds of coordination geometry are linked together by two distinct L4 ligands with a doubly bridging fashion to form a nonplanar 22-membered [Ag(L4)2Ag]2+ ring (Figure 1a). The C



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Figure 2. (a) View of the coordination environment of Ag(I) and the coordination mode of the ligand in polymer 2. Ellipsoid probability 70% and symmetry codes: i = −x, −y, −z; ii = 1 − x, 1 − y, 1 − z; iii = x, 1/2 − y, 1/2 − z. (b) The perpendicular Ag2S2 rings are interconnected to form a chain parallel to the c-axis. The CF3SO3− anions and the thione rings of the L4 ligand are omitted for clarity. (c) Adjacent Ag2S2 chains of 2 are simultaneously linked together by organic linker ligand L4 in the direction of a and b-axis to form a cationic 3D network with 44-membered rings in which the L4 adopts anti-anti-anti conformation. Hydrogen atoms were omitted for clarity.

first kind of Ag+ centers (Ag2) adopts a pseudo-tetrahedral geometry coordinated by two S atoms from two independent

ligands and two O atoms from one nitrate anion, while the second one [Ag(1)] is three-coordinated by two S atoms of D



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Figure 3. (a) View of the coordination environment of Ag(I) in polymer 3. (b) The 1D chain structure of 3 extending parallel to the a-axis direction. (c) Packing of the 1D coordination polymer of 3 perpendicular to the bc-plane. Hydrogen atoms were omitted for clarity.

one-dimensional (1D) tube-like chain structure parallel to the c-axis of the orthorhombic cell (Figure 1b). In the 1D coordination polymer of 1, the mono- and bidentate nitrate ions are arranged in a gauche-conformation. In this structure, the flexible ligand adopts gauche−antigauche conformation with S···S separation of 4.88 Å. Each of the 1D tube structures are surrounded by six neighboring crystallographic equivalent tubes in a hexagonal arrangement where the intermolecular hydrogen bond interactions are observed between the oxygen atoms that are not engaged with the silver(I) center and the

individual ligands and an O atom from a nitrate anion showing a slightly distorted trigonal planar coordination geometry with the Ag+ center deviating from the coordination plane by ca. 0.27 Å (Table S2). The bond angles around the Ag(2) center range from 49.41 to 140.58° in the case of Ag(1) ranging from 103.03 to 137.58°. The CS (thione) bond length shows a partial reduction of π-character and thus a slight lengthening of the CS (thione) distances (1.72 and 1.73 Å) relative to the mean free L4 ligand distance (1.68 Å).80 The [Ag(L4)2Ag]2+ rings are joined together by sharing the ligand and formed a E



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Figure 4. (a) View of the coordination environment of Ag(I) in polymer 4. Atomic displacement parameters are drawn at the 30% probability level. (b) The 1D coordination polymer of 4 extending parallel to the c-axis direction. Hydrogen atoms were omitted for clarity.

bonded to the Ag(I) center are 1.14 and 7.24°. Parallel adjacent cationic chains are simultaneously interconnected by organic linker ligand L4 along the crystallographic a- and b-axis to form a three-dimensional network (3D) structure (Figure 2c). In the polymeric structure of 2, four silver(I) ions are joined by four L4 ligands with an anticonformation to generate a 44membered metallo-macrocyclic ring with Ag···Ag separations of 20.118 and 13.893 Å. These rings build channels parallel to the crystallographic c-axis. The channels are occupied by triflate anions as counterion for the cationic 3D network. The oxygen and fluorine atoms of the trapped CF3SO3− anions are hydrogen bonded by the hydrogen atoms of the ligand. As in the structure of 2 in the cationic 3D network structure of {[Ag3L6](CF3SO3)3}n (L= bis(methylthio)methane),84 each of the silver atoms is connected to four sulfur atoms from four distinct L building blocks. In contrast, coordination mode of the L ligand is different with that of the flexible ligand in polymer 2, so that each of the sulfur atoms is coordinated to only one silver atom. Consequently, compound 2 is the first example with Ag2S2 rings in its structure. The structural difference between these two polymers is best explained by the difference of the flexibility of the linker ligands. [Ag2L6(CF3SO3)2]n (3). In this structure, each pentadentate bridging chelate L6 ligand is connected to four different metal centers with an unprecedented coordination mode, so that one of the S donor atoms bridges two crystallographic similar Ag(2) centers, and the other one is bonded to three silver atoms [2Ag(1) and Ag(2)]. At the same time, these S donor atoms are chelated to one Ag(2) to form a 13-member metallomacrocycles ring (Figure 3a). In doing so, zigzag chains of rectangular Ag3S3 extending along the a-axis are obtained. In the helical chain structure, there are two types of highly distorted

hydrogen atoms of the methylene of the spacer group (Figure 1c). A structural comparison between the structure of 1 and the reported compounds [Ag4(L1)2(NO3)4](CHCl3)2, (L1 = 9,10bis[(npropylthio)methyl]anthracene), [Ag 4 (L 2) 2 (NO 3 ) 4 ](CHCl3)2, (L2 = 9,10-bis[(n-butylthio)methyl]anthracene), and [Ag4(L3)2(NO3)4], (L3 = 9,10-bis[(tert-butylthio)methyl]anthracene)81 reveals that the coordination geometry in 1 is similar to the mentioned tetranuclear molecular cages. The main difference between their topology is related to the influence of flexibility and nature of spacer of the neutral ligands used in these complexes. {[AgL4]2[CF3SO3]2}n (2). With a view to obtain further information on the influence of counteranions on the structure of complexes, in the preparation of 2, AgCF3SO3 was used instead of AgNO3. As depicted in Figure 2a, the Ag(I) ion involved in a distorted tetrahedral coordination geometry comprising of four S donor atoms from four distinct ligands with S−Ag−S bond angles ranging from 93.30 to 126.45°. The average Ag−S bond length of 2.59 Å is best described as distorted tetrahedral around the silver atom.82,56 In the cationic structure of 2, the two adjacent silver atoms are joined by two sulfur atoms of two distinct L4 ligand to form a dimer Ag(μ2S)2Ag as a basic building block with four different Ag−S distances (Table S2). The perpendicular Ag(μ2-S)2Ag units are connected together by sharing the metal ions, building a chain that propagates along the c-axis (Figure 2b). The Ag−S−Ag angles within the Ag2S2 rhomboids are smaller to that of the S− Ag−S angles. The Ag···Ag distance in the distorted square Ag2S2 rings is 3.55 Å, which is slightly longer than the sum of the van der Waals radii of silver atoms (3.44 Å). Therefore, there are no silver−silver contacts in these basic building blocks.83 The dihedral angles between the two thione rings F



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Figure 5. (a) Atomic numbering and coordination environment of Ag(I) in polymer 5 (ellipsoid probability 50%). (b) The natural sheet of 5 parallel to the ac-plane showing the four-membered Ag2S2, Ag2Br2 and 30-membered Ag4Br4L62 metallo-macrocycles. (c) View of the packing of 5 showing natural sheets parallel to the bc-plane with ABAB··· array. Hydrogen atoms were omitted for clarity.

bond angles ranging from 89.37 to 145.82°. By contrast, for the Ag(1)S2O2 center the Ag atom is four-coordinated with a seesaw coordination geometry formed by two μ3-S(1) sulfur atoms from two different flexible ligands and two oxygen atoms from two symmetry related triflate ligands with μ-O,O′

tetrahedral geometries for the silver environments, namely, Ag(1)S2O2 and Ag(2)S3O. In the Ag(2)S3O core, the Ag(2) atom is coordinated by three sulfur atoms with μ2-S(2) and μ3S(1) coordination modes from two distinctly different L6 ligands and one oxygen atom from a terminal triflate with G



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Table 1. Structural Comparison of Ag(I) Polymers with 1-n-Bis (1-methylthioimidazolyl) Alkane Ligandsa no. formula 1·Ag2L4(NO3)2

Ag(I) geometry

ligand coord. modes

Ag−S length and Ag−O length (av.)/ Å

dimension structure

S···S separation of ligand

membered metallomacrocycles

conformation

ref

trigonal and tetrahedral (AgS2O and AgS2O2) tetrahedral (AgS4)

bis-μ2 bridging

2.457 and 2.494

1D

4.881

22

g-a-g

this work

bis-μ2 bridging

2.598

3D

44 and 4

a-a-a

3·Ag2L6(CF3SO3)2

tetrahedral (AgS3O and AgS2O2)

2.557 and 2.503

1D

13 and 6

g-g-a-a-g

this work this work

4·Ag2L6(NO3)(ClO4)

2.477 and 2.644

1D

4.066

13 and 6

g-g-a-a-g

this work

5·Ag2L6Br2

T-shape and tetrahedral (AgS2O and AgS3O) tetrahedral (AgS2Br2)

chelating combining μ2 and μ3-bridging) chelating combining μ2 and μ3-bridging bis-μ2 bridging

8.309 and 9.275 4.137

2.675

2D

12.424

30 and 4

a-a-a-a-a

6·{[AgL4]2[ClO4]2}n

tetrahedral (AgS4)

bis-μ2 bridging

2.611

3D

44 and 4

7·{[AgL4]2[BF4]2}n

tetrahedral (AgS4)

bis-μ2 bridging

2.607

3D

a-a-a and g-a-g a-a-a and g-a-g

8·{[Ag(L1)2][BF4]}n

tetrahedral

2.590

2D

9·{[Ag(L1)2][PF6]}n

tetrahedral (AgS4)

bis-monodentate bridging bis-monodentate bridging

7.528 and 8.951 7.467 and 8.857 6.013 and 5.978

this work 90

2.605

2D

2·{[AgL4]2[CF3SO3]2}n

44 and 4

80

32

57

32

57

a 1

L = 1,1′-methylenebis(1,3-dihydro-3-methyl-2H-imidazole-2-thione).

Ag−S distance of 2.64 Å and one oxygen of a nitrate anion with an Ag−O length of 2.31 Å. The Ag (2) atom has a nearly linear coordination geometry of sulfur atoms with an S−Ag(2)−S angle of 161.96°. The third ligand is an oxygen atom from the perchlorate anion, which is roughly perpendicular to the AgS2 core (91.67° and 95.87°). The deviation of the S−Ag(2)−S angle from the ideal linear coordination geometry can be attributed to the large steric hindrance of the perchlorate anion. The sum of the angles around the Ag(2) atom (349.51°) demonstrate a trigonal pyramidal or T-shaped geometry for the central atom due to the weak coordinating ability of the ClO4− anion (Ag−O = 2.581 Å). As in the structure of 3, in the Ag3S3 trinuclear rings, the silver−silver distance is longer than the sum of van der Waals radii of the two silver (3.44 Å), which is a sign for the absence of any interaction between the Ag ions. Similar to the polymer 3 in this structure, the flexible ligand adopts gauche−gauche−anti−anti−gauche conformation with S···S separation of 4.066 Å (Figure 4b). In the same way as for polymers 1 and 3, each of the 1D chains in the structure of 4 connect to six neighboring crystallographic similar chains (Figure S3). [Ag2L6Br2]n (5). Single crystal X-ray diffraction analysis reveals that [Ag2L6Br2]n, crystallizes in the monoclinic space group P21/n with Z = 2 (Table S1). The 2D structure of 5 consists of an infinite double-stranded polymeric chains with alternating perpendicular inorganic rhomboid dimers [Ag(μS)2Ag] and [Ag(μ-Br)2Ag] aligning with the a-axis (Figure 5a). In each of the dimer units, the silver centers have a tetrahedral geometry that includes two μ2-S atoms from two different L6 ligands with an average Ag−S distance of 2.67 Å and is symmetrically bridged by two μ-bromide ligands (average Ag− Br bond length of 2.70 Å). The values for the bond angles around the central atom are in the range of 100.76−123.64°. The Ag···Ag separations in the [Ag(μ-S)2Ag] and [Ag(μBr)2Ag] motifs are 3.42 and 3.16 Å, respectively, which indicates the presence of Ag−Ag interactions in the rhomboid dimeric units. The adjacent [Ag(μ-Br)2Ag] rhomboid rings are linked by the parallel L6 ligands, generating a corrugated 2D coordination network extending along the ac-plane with two

coordination mode. The Ag−O distance in the cores are 2.52, 2.56, and 2.36 Å for the Ag(1) and Ag(2), respectively, lying in the normal range for an Ag−O bond length (Table S2).85 The whole single helix chain can be described as two single zigzag lines of [-Ag-μ2-(S)-Ag-]n bonded together by the chelating L6 ligand with the μ3-S donor atoms to form a zigzag chain of sixmembered rectangular Ag3S3 rings with Ag···Ag separations of 3.642, 4.158, and 4.985 Å (Figure 3b). In each of the zigzag chains, the dihedral angle between the two thione rings bonded to the Ag(1) and Ag(2) [-Ag-μ2-(S)-Ag-]n are 78.00° [for Ag(1)] and 7.65°[for Ag(2)]. Each of the 1D chains pack so that there are six neighboring crystallographic equivalent chains (Figure 3c). Intermolecular hydrogen bond interactions are observed between the nonbonded oxygen atoms and fluorine of the CF3SO3− anions with the hydrogen atoms of the methylene of the spacer group. As far as we know, there is no any coordination compound with the AgS2O2 and AgS3O cores in which the triflate coligands are bonded simultaneously to the Ag centers via the mono and bidentate coordination modes. A structural comparison between 3 and [Ag2L21‑Me(CF3SO3)2]n [L1‑Me = bis(methylthio)methane]44 shows that in contrast to polymer 3 in the structure of reported two-dimensional (2D) coordination network, all the Ag centers adopt the same coordination geometry. In this structure, the Ag atom is coordinated by three sulfur atoms with μ2-S coordination mode from three distinctly different L1‑Me ligands and the anion is bonded to the central atom through only one of its oxygen atoms. As in polymer 3 in the structure of [Ag2L21(CF3SO3)2]n [L1 = bis(phenylthio)methane], the Ag atoms adopt a distorted AgS2O2 tetrahedral coordination geometry and the triflate anion chelates to the Ag in a bidentate fashion.33 [(Ag2L6(NO3)(ClO4)]n (4). In the structure of 4, uncommonly the nitrate and perchlorate coligands are separately coordinated to the Ag(1) and Ag(2) atoms with a monodentate coordination mode. As in the structure of 3 in 4 (Figure 4a), the L6 ligand acts as a pentadentate bridging chelate ligand with two types of silver atoms [Ag(1) and Ag(2)]. The Ag(1) is surrounded by four atoms in a distorted tetrahedral arrangement: three sulfur atoms from two distinct ligands with average H



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different metallo-cyclic, namely, a 30-membered Ag4Br4L62 and square [Ag(μ-S)2Ag] rings (Figure 5b). The 2D structures are further stabilized through the C−H···Br and C−H···N hydrogen bonds existing between neighboring sheets generating a network structure with ABAB··· sequencing (Figure 5c). The presence of voids and channels in the structure of polymers 1−5 offers an opportunity for these compounds to be used in various applications such as gas storage and guest transport. The voids and channels are suitable sites for occupation by the magnetic nanoparticles which can then be readily isolated from aqueous solution via magnetic decantation. In the continuation of this research we have planned to consider the mentioned applications on the considered compounds. Synthesis and General Characterizations. The silver(I)based polymers 1−5 with good yields were synthesized in air atmosphere via one-pot reactions in appropriate solvents by a metal-to-ligand ratio of 2:1. In the case of compound 2, its final product with a metal-to-ligand ratio of 2:1 and 1:1 is the same. In the syntheses of polymers 4 and 5, Na+ and K+ ions act as counterions for the anions, which have been used in the design of these compounds. Therefore, they have not any other role in the preparation and final morphology of the mentioned complexes. Infrared spectroscopy and elemental analysis confirmed that these compounds are stable in air and atmospheric moisture but slightly sensitive to light and are sparingly soluble in polar solvents such as DMF and DMSO, so that it was not possible to record their NMR spectra. In view to confirm the phase purity of the synthesized polymers, PXRD experiments were carried out for compounds 1−5 in which the results of the simulations were consistent with the experimentally observed spectra (Figure S4). It is worth to note that in the preparation of some of the title polymers two different salts were used. By this reason and by considering the light sensitivity of these compounds, it is probable that the powder of some of the samples are contaminated with a trace of impurity. This may cause a slight difference between the experimental and simulated PXRD pattern of the compounds. Infrared Spectroscopy. Each of the flexible building blocks L4 and L6 has two sulfur donor coordination sites suitable for coordination to the silver(I) ions. In complexes of these ligands the S-donor atoms adopt monodentate49−60 μ2-S and μ3-S bridging modes. So that, the Ag−S bond distances increase with increasing the number of coordinated S atoms around Ag(I) ion (Table 1). Consequently, the CS bond distance increases and the absorption bands shifted to lower frequencies. Hydrogen bonding between the flexible ligands L4 and L6 and counterions is another factor that is closely linked to the starching vibration of the CS band. The IR spectra of the free L4 and L6 ligands exhibit a medium CS stretching vibration at 527 cm−1. This band is shifted to 509, 518, 518, 520, and 510 cm−1 for compounds 1−5, respectively. The observed absorption bands at 1287 and 1433 cm−1 are assigned to the presence of the nitrate ion in polymer 1. In polymers 2 and 3, the unambiguous assignment of the vibration modes of the CF3SO3− anion is difficult because the mixing of the CF3, SO3, and dithione stretching vibrations occur in the region of 1000 to 1300 cm−1.86 However, the bands which appear at the region of 1033 and 1258 cm−1 for polymers 2 and 3 can tentatively be assigned to the stretching vibrations of the triflate anion.87,88 It is worth pointing out that the IR spectra of compounds 2 and 3 are nearly similar to each other, so it is difficult to distinguish the coordination modes of the triflate anions in these

complexes. The observed absorption band extending from 1039 to 1127 cm−1 with a maximum at 1088 cm−1 is assigned for the perchlorate anion.89 Thermal Analysis. To characterize the thermal stabilities of compounds 1−5, their thermal decomposition behaviors were investigated by TGA (Figure 6). The experiments were

Figure 6. TGA curves of the coordination polymers 1−5 recorded at heating rate of 10 °C/min.

performed under nitrogen atmosphere at a heating rate of 10 °C min−1 in the temperature range of 50−900 °C. The combustion of the flexible ligands and network break down of these polymers occur at a temperature range of 240−341 °C. The TGA curves described that polymers 1, 2, 4, and 5 decompose in two steps: the first step is due to the combustion of the flexible ligands, and the second one is assigned to decomposition of the silver salts to the metallic silver. In the case of polymer 3, in the first step it loses simultaneously the flexible ligand together with some of the triflate. The second step of 3 is the same as the rest of the title polymers. Structural Comparison between the Silver Coordination Polymers Based on the Bis(imidazole-2-thione) Ligands. As described in Table 1 the ionic polymers 8 and 957 with a distorted AgS4 tetrahedral coordination geometry have the same core structure as 2, 6,90 and 7.80 In contrast to polymers 2, 6, and 7, in complexes 8 and 9 the L1 ligand bridges a pair of silver(I) centers. These observations explain the influences of the spacer length of L1 and L4 upon the coordination mode of the ligands and dimension of the polymers. In the structures 6 and 7, the L4 ligand displays two different conformation modes, anti−anti−anti and gauche− antigauche with different S···S distances, while in the structure of 2 this ligand exhibits only anti (anti−anti−anti) conformation. The difference between the conformation behaviors of the L4 ligand in these polymers can be attributed to the size of the counterions. Structural analysis between the 3D ionic compounds 2, 6, 7, and 1D neutral structure of polymer 1 with the same flexible ligand and variable counterions demonstrates the impact of the coordination strength of the counterions on the structure of these polymers. In contrast to the polymers 2, 6, and 7 in which the anions CF3SO3−, ClO4−, and BF4− were used to balance or neutralize the charge of the cationic polymers. In polymer 1 with AgS2O and AgS2O2 donor sets, the nitrate group was coordinated to the metal center as a coligand. In order to evaluate the influence of spacer length on the topology of the networks, in polymers 3−5, L6 was used instead of L4. As pointed out in the crystal structure description of polymers 3 and 4, the coordination mode of the L6 ligand in these polymers is completely different with that observed for L4 in its complexes. It is important to I



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Figure 7. Antibacterial activity results of free ligands and their silver polymers.

note that in the L6 complexes the counterions have a noticeable impact in directing the coordination geometry around the silver atoms so that when the spacer length increases from L4 to L6 attempts to design and synthesis 3D networks with the AgS4 core failed. In the L4 and L6 complexes there is a competition between the coordination ability of nitrate and perchlorate anions. In polymer 4, these anions are coordinated to the Ag centers, while an attempt to synthesize analogous complexes with the L4 ligand was failed. Instead, compound 6 was isolated, with the perchlorate anion acting as counterion. In the 2D structure of polymer 5, the bromide ion, has a critical role in directing the structure of this compound compared to the 1D structures of 3 and 4. In the structure of 5, coordination mode of the L6 ligand is bis-μ2 bridging, while in 3 and 4 this ligand is coordinated to the silver atom via chelating combining μ2 and μ3-bridging modes. On the other hand, the conformation of L6 ligand in 5 is completely different from those in 3 and 4. In conclusion, polymers based on the L4 ligand frequently show an AgS4 core structure, while the Ag centers in compounds with the L6 ligand cannot adopts this core structure and complete their coordination environment by anions such as perchlorate ion. This observation can be rationalized by the fact that in the case of polymers based on the L4 ligand the anions predominantly use as counterion, whereas for the L6 ligand with a longer spacer length these anions coordinated to the metal center as a coligand. Antibacterial Activity Assay. Figure 7 presents the antibacterial results of the free ligands and their silver polymers (see Experimental Method in S5). On the basis of these results, all the tested compounds possess a broad spectrum of antibacterial activity against both Gram-positive and Gramnegative target bacteria. These results also confirmed that the inhibition ability of polymers 1−5 is higher than that of the free ligands L4 and L6. These polymers have negative and positive charged centers which are suitable for the interaction with the charged centers present in the cell membrane and outer membrane of the tested bacteria and hence able to disrupt membrane of the target bacteria. In comparison with the other

silver polymers, polymer 2 showed more antibacterial activity against target bacteria specially Staphylococcus aureus and Pseudomonas aeruginosa that are commonly considered as the most clinical resistant species to antibacterial agents. This observation can be rationalized by the fact that as the number of charged centers in the mentioned polymers increase the number of the interactions with multitarget molecules in bacteria should also be increased and cause inactivation of these bacteria.91−95 Therefore, the charged sites play a key role in the antibacterial activities of these compounds. Furthermore, the antimicrobial activity of the active agents is enhanced by increasing their size; hence the L6 ligand is more active than L4. By the same reason, the larger size of 3D polymer 2 causes a more disturbance in the cell membrane integrity relative to the other polymers because it increases the distance between the polar head groups in the phospholipids of the bacterial membranes and disturb the packing and integrity of the phospholipids. MIC and MBC Indices. Table S6 describes the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) results for the title compounds. The MIC and MBC of all compounds were considerable against Escherichia coli and S. aureus. For a compound, when it is MIC and MBC values are the same it can be concluded that this compound is primarily a bactericidal agent that facilitates its application for bacterial control and eradication. As much as the compound diffuse through bacterial membrane or cell wall its MIC and MBC values will be diminished. DNA Cleavage Assessment. Figure S7 reveals the DNA cleavage assay for polymers 1−5. The untreated DNA sample from each bacterial species remained unaffected in its well, while DNA in the H2O2 treated samples was degraded. All of the compounds were able to affect the DNA and break it, and as it can be seen from Figure S7, no DNA bond is remained in its well. These results suggest that the title polymers when they have access to DNA can degrade it and cause bacterial death. This can be due to the disruption of the hydrogen bonds between the AT or GC nucleotides or breaking the DNA J



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(5) Biradha, K.; Sarkar, M.; Rajput, L. Crystal engineering of coordination polymers using 4,49-bipyridine as a bond between transition metal atoms. Chem. Commun. 2006, 4169.416910.1039/ B606184B (6) Biradha, K.; Dennis, D.; MacKinnon, V. A.; Sharma, C. V. K.; Zaworotko, M. J. Supramolecular Synthesis of Organic Laminates with Affinity for Aromatic Guests: A New Class of Clay Mimics. J. Am. Chem. Soc. 1998, 120, 11894−11903. (7) Hu, D. C.; Fan, Y.; Si, C. D.; Wu, Y. J.; Dong, X. Y.; Yang, Y. X.; Yao, X. Q.; Liu, J. C. A series of Zn/Cd coordination polymers constructed from 1,4-naphthalenedicarboxylate and N-donor ligands: Syntheses, structures and luminescence sensing of Cr3+ in aqueous solutions. J. Solid State Chem. 2016, 241, 198−204. (8) Li, Q. Y.; Quan, Y.; Wei, W.; Li, J.; Lu, H.; Ni, R.; Wang, X. J. Synthesis, crystal structure and gas uptake properties of a ureafunctionalized rht-type metal−organic framework. Polyhedron 2015, 99, 1−6. (9) Bu, X. H.; Du, M.; Zhang, L.; Liao, D. Z.; Tang, J. K.; Zhang, R. H.; Shionoya, M. Novel nickel(II) complexes with diazamesocyclic ligands functionalized by additional phenol donor pendant(s): synthesis, characterization, crystal structures and magnetic properties. Dalton Trans. 2001, 593−598. (10) Li, Y. W.; Li, J. R.; Wang, L. F.; Zhou, B. Y.; Chen, Q.; Bu, X. H. Microporous metal−organic frameworks with open metal sites as sorbents for selective gas adsorption and fluorescence sensors for metal ions. J. Mater. Chem. A 2013, 1, 495−499. (11) You, L.; Zhu, W.; Wang, S.; Xiong, G.; Ding, F.; Ren, B.; Dragutan, I.; Dragutan, L.; Sun, Y. High catalytic activity in aqueous heck and Suzuki−Miyaura reactions catalyzed by novel Pd/Ln coordination polymers based on 2,2′-bipyridine-4,4′-dicarboxylic acid as a heteroleptic ligand. Polyhedron 2016, 115, 47−53. (12) Yu, S. Y.; Kusukawa, T.; Biradha, K.; Fujita, M. Hydrophobic Assembling of a Coordination Nanobowl into a Dimeric Capsule Which Can Accommodate up to Six Large Organic Molecules. J. Am. Chem. Soc. 2000, 122, 2665−2666. (13) Yang, Y.; Wang, K. Z.; Yan, D. Ultralong Persistent Room Temperature Phosphorescence of Metal Coordination Polymers Exhibiting Reversible pH-Responsive Emission. ACS Appl. Mater. Interfaces 2016, 8, 15489−15496. (14) Huo, J. Z.; Su, X. M.; Wu, X. M.; Liu, Y. Y.; Ding, B. Hydrothermal synthesis and characterization of a series of luminescent Ag(I) coordination polymers with two new multidentate bis-(1,2,3triazole) ligands: structural diversity, polymorphism and photoluminescent sensing. CrystEngComm 2016, 18, 6640−6652. (15) Pham, H. Q.; Mai, T.; Pham-Tran, N. N.; Kawazoe, Y.; Mizuseki, H.; Nguyen-Manh, D. Engineering of Band Gap in Metal− Organic Frameworks by Functionalizing Organic Linker: A Systematic Density Functional Theory Investigation. J. Phys. Chem. C 2014, 118, 4567−4577. (16) Plonka, A. M.; Banerjee, D.; Parise, J. B. Effect of Ligand Structural Isomerism in Formation of Calcium Coordination Networks. Cryst. Growth Des. 2012, 12, 2460−2467. (17) Li, J. R.; Zhang, R. H.; Bu, X. H. Structural Diversities of Silver(I) Coordination Compounds with Flexible Dithioether Ligands Based upon Changing the Ligand Spacers. Cryst. Growth Des. 2003, 3, 829−835. (18) Bu, X. H.; Chen, W.; Lu, S. L.; Zhang, R. H.; Liao, D. Z.; Bu, W. M.; Shionoya, M.; Brisse, F.; Ribas, J. Flexible meso-Bis(sulfinyl) Ligands as Building Blocks for Copper(II) Coordination Polymers: Cavity Control by Varying the Chain Length of Ligands. Angew. Chem., Int. Ed. 2001, 40, 3201−3203. (19) Awaleh, M. O.; Badia, A.; Brisse, F.; Bu, X. H. Synthesis and Characterization of Silver(I) Coordination Networks Bearing Flexible Thioethers: Anion versus Ligand Dominated Structures. Inorg. Chem. 2006, 45, 1560−1574. (20) Withersby, M. A.; Blake, A. J.; Champness, N. R.; Hubberstey, P.; Li, W. S.; Schroder, M. Anion Control in Bipyridyl silver(I) Networks: A Helical Polymeric Array. Angew. Chem., Int. Ed. Engl. 1997, 36, 2327−2329.

backbone. The obtained results are in agreement with the MBC data that show all compounds are bactericidal agents.

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CONCLUSIONS Five new silver(I)-based coordination polymers of the bis(imidazole-2-thione) ligands with varying dimensionality have been synthesized. Although the L4 and L6 ligands have the same number of sulfur donor atoms, by changing the spacer length of the ligands and varying the counterions, each of them displays a different type of coordination mode to the Ag centers. Additionally, polymers based on the L4 ligand predominately adopt a AgS4 core structure which is not possible for the L6 ligand to have this moiety in its polymers. Apart from interesting structural and topological features, these compounds (in particular polymer 2) exhibit significant antibacterial activity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00784. Table of the bond distances and angles involving the silver(I) ions, crystallographic data, the crystal packing of polymer 3, comparison of experimental and simulated Xray diffraction peaks, experimental method for antibacterial activity, the MIC, MBC, and DNA cleavage assay for 1−5 are all expressed as S1−S7 (PDF) (PDF) Accession Codes

CCDC 1514542−1514543 and 1526091−1526093 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Azizolla Beheshti: 0000-0001-8190-9563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Shahid Chamran University of Ahvaz for financial support (Grant Number 95/3/02/31400). REFERENCES

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Design, Synthesis, and Antibacterial Assessment of Silver(I)-Based

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