Aliphatic Dicarboxylate Directed Assembly of Silver ... - ACS Publications

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Aliphatic Dicarboxylate Directed Assembly of Silver(I) 1,3,5-Triaza-7phosphaadamantane Coordination Networks: Topological Versatility and Antimicrobial Activity Sabina W. Jaros,† M. Fátima C. Guedes da Silva,*,† Magdalena Florek,‡ M. Conceiçaõ Oliveira,† Piotr Smoleński,*,§ Armando J. L. Pombeiro,† and Alexander M. Kirillov*,† †

Centro de Quı ́mica Estrutural, Complexo I, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal ‡ Department of Veterinary Microbiology, Wroclaw University of Environmental and Life Sciences, ul. Norwida 31, 50-375 Wroclaw, Poland § Faculty of Chemistry, University of Wrocław, ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland S Supporting Information *

ABSTRACT: The present work describes the facile synthesis, full characterization, and architectural diversity of three new bioactive silver-organic networks, namely 1D [Ag2(μ-PTA)2(μ-suc)]n·2nH2O (1), 2D [Ag2(μPTA)2(μ4-adip)]n·2nH2O (2), and 3D [Ag2(μ4-PTA)(μ4-mal)]n (3) coordination polymers, generated via a mixed-ligand strategy using PTA (1,3,5-triaza7-phosphaadamantane) as a main building block and flexible aliphatic dicarboxylic acids (succinic (H2suc), adipic (H2adip), or malonic (H2mal) acids) as an ancillary ligand source. The compounds 1−3 were isolated as moderately air and light stable crystalline solids and were fully characterized by IR and 1H and 31P{1H} NMR spectroscopy, elemental analysis, ESI(±)-MS spectrometry, and single-crystal X-ray crystallography. The type of aliphatic dicarboxylate plays a key role in defining the dimensionality and structural and topological features of the resulting networks, which are also driven by the PTA blocks that adopt unconventional N,P- or N3,P-coordination modes. The topological analysis of simplified underlying nets revealed that 1 possesses uninodal 3-connected chains with the SP 1-periodic net (4,4)(0,2) topology, 2 features a uninodal 4-connected layer with the skl topology, and 3 reveals a uninodal 4-connected metal−organic framework with the dia topology. The presence of the crystallization water molecules in polymers 1 and 2 gives rise to the extension of their metal−organic structures into 3D (1) or 2D (2) H-bonded networks that disclose rather rare topologies. All of the obtained silver(I) coordination polymers feature solubility in water (S25 °C ≈ 3−5 mg mL−1) and show significant antibacterial and antifungal activity against the selected strains of Gram-negative (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria and yeast (Candida albicans).

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INTRODUCTION The design and synthesis of new functional framework materials has been a subject of high attention during the last few years.1 Coupling inorganic and organic building blocks into coordination polymers (CPs) opens up the possibility of creating new materials with fascinating polymeric architectures and functional properties.1,2 In particular, a pronounced interest has been devoted to the design of new bioactive metal−organic framework materials (bioMOFs) as possible alternatives to conventional antimicrobial drugs that are becoming more microbe resistant.3 It is widely known that silver and its simple salts are effective antimicrobial agents.3−5 Among all the bioactive metals, silver exhibits the highest toxicity to bacteria, viruses, and other eukaryotic microorganisms, while simultaneously showing a relatively low toxicity to mammal cells.3 Up to now, a large number of bioactive silver complexes have been reported.4,5 © XXXX American Chemical Society

However, in spite of significant antimicrobial activity, in many cases the use of these compounds as potential drugs, creams, and wound dressings is limited by their light sensitivity and insolubility in aqueous media.4,5 It was observed that compounds having weak Ag−O and/or Ag−N bonds usually exhibit a significantly higher bioactivity, lower light stability, and poorer solubility in water than compounds possessing rather strong Ag−S or Ag−P bonds.4,5 Therefore, an interesting direction consists in applying a mixed-ligand strategy involving both O-carboxylate and N,P-aminophosphine building blocks, which may not only lead to intriguing and fascinating polymeric architectures but also can strongly influence the biological and physical properties of the resulting framework materials. Received: April 22, 2014 Revised: August 28, 2014

A

dx.doi.org/10.1021/cg500557r | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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H3PO4 (85% in H2O), respectively. The powder X-ray diffraction patterns were recorded with a Bruker D8 ADVANCE diffractometer (Bragg−Brentano geometry, Cu radiation). Synthesis and Analytical Data. [Ag2(μ-PTA)2(μ-suc)]n·2nH2O (1). Silver oxide (0.1 mmol, 23 mg) and H2suc (0.25 mmol, 29.5 mg) were combined in a solution containing 7 mL of MeOH and 3 mL of H2O. The obtained cloudy mixture was stirred for 30 min in air at room temperature. Then PTA (0.2 mmol, 31.4 mg) was added and the obtained mixture was stirred further at room temperature for 40 min, resulting in the formation of a white precipitate that was then completely dissolved by dropwise addition of an aqueous 1 M NH3 solution (until pH 8−9; ca. 0.5 mL). The obtained solution was filtered off, and a transparent filtrate was left to slowly evaporate in air at room temperature, resulting in the formation of colorless X-rayquality single crystals. These were collected and dried in air to give 1 in 60% yield (based on Ag2O). S25 oC(in H2O) ≈ 3 mg mL−1. Anal. Calcd for C16H36Ag2N6P2O8 (1·2H2O, mol wt 682.15): C, 26.78; N, 11.70; H, 5.05. Found: C, 27.06; N, 11.52; H, 4.82. IR (KBr, cm−1): 3444 (s, br.) ν(H2O), 2965 (m) and 2938 (m) νas(CH), 2916 (w) νs(CH), 1685 (w, br), 1644 (s) νas(COO), 1559 (w, br), 1439 (m) and 1411 (m) νs(COO), 1384 (m), 1286 (s), 1236 (s), 1205 (w), 1169 (w), 1094 (m), 1040 (m), 1012 (s), 966 (vs), 952 (s), 898 (m), 808 (m), 794 (m), 749 (m), 728 (w), 642 (m) 595 (m), 586 (m), 564 (w), 545 (w), 477 (w), 450 (w). 1H NMR (300.13 MHz, D2O): δ 4.66 and 4.54 (2d, 12H, JAB = 13.3 Hz, NCHAHBN, PTA), 4.24 (d, 12H, 2JP−H = 2.5 Hz PCH2N, PTA), 2.36 (s, 4H, suc). 31P{1H} NMR (121.49 MHz, D2O): δ −78.4 (s, PTA). ESI(±)-MS (H2O/MeOH) (m/z (relative abundance >10%)): MS(+) m/z 421 (75%) [Ag(PTA)2]+, 647 (20%) [Ag2(PTA)2(Hsuc)]+; MS(−) m/z 117 (100%) [Hsuc]−, 342 (75%) [Ag(Hsuc)2]−, 555 (45%) [Ag3(suc)2]−. [Ag2(μ-PTA)2(μ4-adip)]n·2nH2O (2). Compound 2 was synthesized following the method described for 1, except that H2suc was replaced by H2adip. Colorless crystals were collected and dried in air to give 2 in 55% yield (based on Ag2O). S25 oC(in H2O) ≈ 5 mg mL−1. Anal. Calcd for C18H40Ag2N6P2O8 (2·2H2O, mol wt 710.20): C, 28.97; N, 11.26; H, 5.40. Found: C, 28.73; N, 10.86; H, 5.07. IR (KBr, cm−1): 3424 (s, br) ν(H2O), 2930 (m) νas(CH), 2865 (w) νs(CH), 1642 (w) δ(H2O), 1566 (vs) νas(COO), 1438 (m) and 1406 (s) νs(COO), 1330 (w), 1293 (m), 1241 (m), 1098 (m), 1015 (vs), 974 (vs), 961 (s), 807 (m), 749 (m), 509 (w). 1H NMR (300.13 MHz, D2O): δ 4.61 and 4.49 (2d, 12H, JAB = 13.2 Hz, NCHAHBN, PTA), 4.24 (d, 12H, 2JP−H = 2.3 Hz PCH2N, PTA), 2.15 (t, 4H, adip), 1.51 (m, 4H, adip). 31P{1H} NMR (121.49 MHz, D2O): δ −78.4 (s, PTA). ESI(±)-MS (H2O/ MeOH) (m/z (relative abundance >10%)): MS(+) m/z 421 (100%) [Ag(PTA)2]+, 264 (80%) [Ag(PTA)]+, 675 (30%) [Ag2(PTA)2(Hadip)]+, 781 (25%) [Ag3(PTA)2(adip)]+; MS(−) m/z 145 (55%) [Hadip]− , 253 (30%) [Ag(adip)]− , 610 (100%) [Ag3(adip)2]−, 862 (15%) [Ag4(Hadip)(adip)2]−. [Ag2(μ4-PTA)(μ4-mal)]n (3). Compound 3 was synthesized following the method described for 1, except that H2suc was replaced by H2mal. Colorless crystals were collected and dried in air to furnish 3 in 50% yield (based on Ag2O). S25 oC(in H2O) ≈ 5 mg mL−1. Anal. Calcd for C9H14Ag2N3PO4 (mol wt 474.9): C, 22.76; N, 8.85; H, 2.97. Found: C, 22.40; N, 8.72; H, 2.89. IR (KBr, cm−1): 3413 (s, br) ν(H2O), 2927(w) νas(CH), 2890 (w) νs(CH), 1567 (s, br) νas(COO), 1471 (w), 1430 (w) νs(COO), 1340 (w), 1287 (m), 1240 (m), 1203 (w), 1167 (w), 1109 (w), 1097 (w), 1037 (w), 1008 (vs), 960 (vs), 950 (s), 913 (w), 895 (w), 805 (s), 760 (w), 699 (m), 617 (w), 604 (w), 586 (w), 568 (w), 471 (w), 454 (w). 1H NMR (300.13 MHz, D2O): δ 4.69 and 4.56 (2d, 6H, JAB = 13.0 Hz, NCHAHBN, PTA), 4.26 (d, 6H, 2JP−H = 2.3 Hz, PCH2N, PTA), 3.15 (s, 2H, mal). 31P{1H} NMR (121.49 MHz, D2O): δ −78.9 (s, PTA). ESI(±)-MS (H2O/MeOH) (m/z (relative abundance >10%)): MS(+) m/z 421 (100%) [Ag(PTA)2]+, 633 (50%) [Ag2(PTA)2(Hmal)]+, 896 (10%) [Ag4(PTA)(Hmal)3]+, MS(−) m/z 313 (100%) [Ag(Hmal)2]−, 738 (20%) [Ag2(PTA)2(Hmal)(mal)]−, 844 (20%) [Ag3(PTA)2(mal)2]−. Refinement Details for the Single-Crystal X-ray Analyses. The X-ray diffraction data of 1−3 were collected using a Bruker AXSKAPPA APEX II diffractometer with graphite-monochromated Mo Kα radiation. Data were collected using ω scans of 0.5° per frame, and a

Recently, the cagelike aminophosphine 1,3,5-triaza-7-phosphaadamantane (PTA) and its derivatives have became an important class of ligands in organometallic chemistry,6−9 not only because of their interesting physicochemical properties including the water solubility, air stability, and ability to act as multidentate N,P-blocks but also due to very promising biomedical applications of the corresponding coordination compounds as antimicrobial and antitumor agents.6,7a Bearing these facts in mind, it is surprising that very little attention has been paid to PTA and derived building blocks when constructing functional metal−organic networks.6f,7 Up to now, only a few examples of silver coordination polymers bearing both PTA and polycarboxylate ligands have been reported.9 In these compounds, PTA has been successfully applied as a structure-determining ligand through controlled P,N coordination to the silver center. Moreover, it has been demonstrated that a slight modification of an arylcarboxylate ligand strongly affects the final structure and properties of silver-PTA coordination networks.9,10 As a continuation of our previous work,9,10 we have further modified the Ag/PTA system by incorporating a series of flexible aliphatic dicarboxylate linkers. The flexible nature of such linkers allows their aliphatic chain to be freely twisting and banding, thus resulting in coordination and conformation versatility and leading to unexpected and topologically unusual polymeric architectures.11−13 To the best of our knowledge, coordination networks composed of Ag, PTA, and aliphatic carboxylates have not yet been reported. Their synthesis thus was a main objective of the present work.14 Hence, we describe herein the facile synthesis, full characterization, and architectural diversity of three new silver−organic networks, namely 1D [Ag2(μ-PTA)2(μ-suc)]n·2nH2O (1), 2D [Ag2(μ-PTA)2(μ4-adip)]n·2nH2O (2), and 3D [Ag2(μ4-PTA)(μ4-mal)]n (3) coordination polymers, derived from PTA and succinic (H2suc), adipic (H2adip), and malonic (H2mal) acids, respectively. In addition, the antibacterial and antifungal activity of 1−3 has been tested, revealing that these compounds can become promising candidates toward the development of novel antimicrobial silver(I) materials.

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EXPERIMENTAL SECTION

Materials and Methods. All synthetic work was performed in air. 1,3,5-Triaza-7-phosphaadamantane (PTA) was prepared by a published method developed by Daigle and co-workers.15 All other solid reactants and solvents were purchased from commercial sources and used without further purification. The Microanalytical Service of the Instituto Superior Técnico or The Laboratory of Elemental Analysis at Faculty of Chemistry, University of Wrocław, carried out the C, N, and H elemental analyses. Infrared (IR) spectra were measured in the 4000−400 cm−1 range on a BIO-RAD FTS 3000MX or Bruker IFS 1113v instrument using KBr disks (abbreviations: vs, very strong; s, strong; m, medium; w, weak; br, broad; sh, shoulder). Mass spectra were obtained by using a 500-MS ion trap mass spectrometer equipped with an ESI ion source (Varian, Inc., Palo Alto, CA, USA). The ESI-MS spectra in positive and negative modes were measured with the following operating parameters: a needle voltage of ±5 kV, a flow rate of 20 μL/min, and a capillary voltage of 20 V. Nitrogen was applied as a nebulizing and drying gas (temperature 350 °C and pressure 35 psi). The MS2 and MS3 experiments (CID, collisioninduced dissociation) were measured at excitation amplitudes between 1.0 and 2.5 V, an excitation time of 10 s, and a selection window of 2 Da. The mass spectra were collected in the m/z 100−1000 range. 1H and 31P{1H} NMR spectra of compounds were run in D2O solution at room temperature (∼25 °C) on a Bruker 300 AMX spectrometer. The 1 H and 31P{1H} chemical shifts are relative to Me4Si and external B



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Antibacterial and Antifungal Activity Studies. The biological activity of 1−3 and AgNO3 was evaluated by the method of serial dilutions using Antibiotic Broth (component (g L−1 (H2O)): dextrose, 1.0; K2HPO4, 3.68; beef extract, 1.5; peptone, 5.0; KH2PO4, 1.32; NaCl, 3.5; yeast extract, 1.5), according to Grove and Randall.17a The strains Staphylococcus aureus PCM 2054 (ATCC 25923) and Escherichia coli PCM 2057 (ATCC 25922) were employed from the Polish Collection of Microorganisms of the Institute of Immunology and Experimental Therapy in Wroclaw, Poland, as well as Pseudomonas aeruginosa and Candida albicans isolated from clinical samples in the Department of Veterinary Microbiology, University of Environmental and Life Sciences, Wroclaw, Poland. The last two strains were identified using conventional methods and miniaturized identification systems (ID 32 C and API 20 NE (bioMérieux), respectively). An overnight culture of strain tested was diluted 1:1000 in Antibiotic Broth (AB). In a series of tubes containing appropriate amounts of AB, aqueous solutions of 1−3 were added to obtain 0.9 mL. In each well, 0.1 mL of a microbial suspension was pipetted. The following concentrations of tested compounds in water were obtained (μg mL−1): 60, 50, 40, 30, 20, 10, 9, 8, 7, 6, 5, 4, 3, 2, and 1. In addition, the antimicrobial activity of the “free” PTA was examined using the same method, showing that it is inactive at the maximum concentration. Broth sterility control and (for each strain) growth controls were included. The tubes were incubated at 37 °C for 24 h. The minimum inhibitory concentration (MIC, μg mL−1) was defined as the lowest concentration of the compound that fully inhibited the growth of bacteria or fungi. For the sake of comparison, the MIC values were normalized for the number of silver atoms in 1−3 and are also given on a nmol mL−1 scale (Table 2). The turbidity of well contents was controlled using a DEN-1B densitometer (Biosan, Latvia).17b

full sphere of data was obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT on all of the observed reflections. Absorption corrections were applied using SADABS.16a Structures were solved by direct methods using the SHELXS 97 program and refined with SHELXL 97.16b Calculations were performed with the WinGX System, Version 1.80.03.16c The hydrogen atoms were inserted at geometrically calculated positions and included in the refinement using the riding-model approximation; Uiso(H) values were defined as 1.2Ueq of the parent carbon atoms for methylene residues. The hydrogen atoms of the water molecules (in 1 and 2) were located from the final difference Fourier map, and the isotropic thermal parameters were set at 1.5 times the average thermal parameters of the associated oxygen atoms. There were disordered molecules present in the structure of 1 and 3. Since no obvious major site occupations were found for those molecules, it was not possible to model them. PLATON/SQUEEZE16d was used to correct the data, and potential volumes of 273 (1) and 40 Å3 (3) were found with 94 and 34 electrons, respectively, per unit cell worth of scattering. These were removed from the model and not included in the empirical formula. The modified data sets improved the R1 values by ca. 16 (1) and 53 (3) %. Crystal data and details of data collection for 1−3 are given in Table 1.

Table 1. Crystal Data and Structure Refinement Details for Compounds 1−3 empirical formula fw T (K) λ (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g/cm3) μ(Mo Kα) (mm−1) no. of collected rflns no. of indep rflns Rint final R1,a wR2b (I ≥ 2σ) GOF on F2

1

2

3

C8H16AgN3O3P

C9H20AgN3O4P

C9H14Ag2N3O4P

341.08 150(2) 0.71069 monoclinic P21/c 6.8789(3) 17.7009(8) 11.7679(6) 90 105.110(2) 90 1383.35(11) 4 1.638 1.570

373.12 150(2) 0.71073 triclinic C2/c 26.089(3) 6.8425(7) 18.7014(19) 90 127.053(2) 90 2664.4(5) 8 1.860 1.644

474.94 150(2) 0.71073 orthorhombic Abm2 7.1411(7) 12.8457(12) 14.2823(14) 90 90 90 1310.2(2) 4 2.408 3.124

9951

10165

3430

2528

3396

1278

0.0430 0.0817, 0.1796

0.0371 0.0390, 0.0916

0.0255 0.0237, 0.0563

1.054

1.089

1.153

R1 = ∑||F o | − |F c ||/∑|F o |. ∑[w(Fo2)2]]1/2. a

b

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RESULTS AND DISCUSSION Synthesis and Characterization. The combination in MeOH/H2O medium at 25 °C of the equimolar amounts of silver(I) oxide, PTA, and an aliphatic dicarboxylic acid (succinic acid (H2suc) for 1, adipic acid (H2adip) for 2, and malonic acid (H2mal) for 3), followed by the alkalization of the reaction mixture by an aqueous NH3 solution, leads to the self-assembly generation of three distinct silver−organic networks. Therefore, depending on the type of aliphatic dicarboxylate block, 1D (1), 2D (2), and 3D (3) coordination polymers driven by the N,Por N3,P-PTA moieties have been generated (Scheme 1). All of the products have been isolated as water-soluble and moderately light stable solids in ∼50% yields and fully characterized by IR and 1H and 31P{1H} NMR spectroscopy, elemental analysis, ESI(±)-MS spectrometry, and single-crystal X-ray crystallography. The IR spectra of 1−3 show the expected bands, revealing the characteristic asymmetric (1644−1566 cm−1) and symmetric (1439−1406 cm−1) vibrations of carboxylic groups and the PTA ligand (1180−900 cm−1).18 The calculated frequency difference (Δ = 205−233 cm−1) between the asymmetric (1644 cm−1) and symmetric (1439 and 1411 cm−1) COO stretching in 1 is in agreement with a η1:η0 mode of carboxylate groups in

wR2 = [∑[w(F o 2 − F c 2 ) 2 ]/

Table 2. Antimicrobial Activity of Compounds 1−3 Expressed as Minimum Inhibitory Concentration (MIC) MIC (μg mL−1)

a

normalized MIC (nmol mL−1)a

entry

strain

1

2

3

AgNO3

1

2

3

AgNO3

1 2 3 4

E. coli P. aeruginosa S. aureus C. albicans

6 20 6 40

6 20 40 >50

7 6 8 30

9 20 9 40

17 59 18 117

17 56 113 >141

30 25 34 126

53 118 53 236

These MIC values were normalized for the number of silver atoms in each compound. C



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the δ 3.19−1.15 range are attributed to the methylene groups of dicarboxylate moieties. The 31P{1H} NMR spectra exhibit a characteristic broad singlet at δ −78.4 (1), −78.4 (2), or −78.9 (3) expected for the monophosphine derivatives,18c which is shifted in comparison with the signal for “free” PTA (δ −95.0).18e The ESI(+)-MS spectra of MeOH/H2O solutions of 1−3 are rather similar, showing an intense peak at m/z 421 attributed to [Ag(PTA)2]+. In addition, a less intense peak is observed at m/z 647, 675, or 633 with the characteristic isotopic distribution pattern, which corresponds to the [Ag2(PTA)2(Hsuc)]+ (1), [Ag2(PTA)2(Hadip)]+ (2), or [Ag2(PTA)2(Hmal)]+ (3) species, respectively. The negative mode ESI(−)-MS spectra feature the characteristic [Ag4(Hadip)(adip)2]− species in 2 and the [Ag2(PTA)2(Hmal)(mal)]− and [Ag3(PTA)2(mal)2]− fragments in 3. The ESI(+)-MS data suggest that the [Ag(PTA)2]+ ion with m/z 421 is the most stable species in solution and can be considered as a precursor ion and secondary building unit (SBU) in the process of crystal growth. The formation of the same species was reported in prior work regarding the first PTA-driven 3D metal−organic frameworks.9a In these compounds PTA was applied as a structure-determining ligand, while the aromatic carboxylic acids were used as an ancillary ligand source.9a A thorough analysis of ESI-MS data for 1−3 and related derivatives9a allowed us to propose a probable mechanism of the self-assembly formation of silver(I)-PTA coordination polymers bearing different polycarboxylate ligands. The first step probably involves the generation of the most stable [Ag(PTA)2]+ ions, which adopt an alternating P,Ncoordination mode, leading to the formation of PTA(P)−Ag− (N)PTA building blocks. The next step most likely includes an addition of the partially deprotonated carboxylate ligand and an extra Ag+ ion to the precursor cation via strong coordination bonds, resulting in the generation of the [Ag2(PTA)2(Hcarboxylate)]+ fragments. Further addition of Ag+ ions to these [Ag2 (PTA)2 (Hcarboxylate)]+ fragments gives the [Ag3(PTA)2(carboxylate)]+ moieties. An alternating repetition of all these steps leads to a slow polymeric chain growth and the formation of the final structural architecture. The [Ag2(PTA)2(Hcarboxylate)]+ fragments, namely [Ag2(PTA)2(Hbpca)]+ (H2bpca = biphenyl-4,4′-dicarboxylic acid),9a [Ag2(PTA)2(H3pma)]+ (H4pma = pyromellitic acid),9a [Ag2(PTA)2(Hsuc)]+ (in 1), [Ag2(PTA)2(Hadip)]+ (in 2), and [Ag2(PTA)2(Hmal)]+ (in 3) have been observed in the ESI(+)MS spectra of different Ag-PTA coordination polymers. The CID (collision-induced dissociation) experiments for the ion at m/z 421 provide further information on the organization of [Ag(PTA)2]+ in the solution mixture (Figure S5, Supporting Information). The MS2 spectrum of this precursor ion shows a group of peaks at m/z 264/266 with the characteristic isotopic pattern of a silver atom, assigned to [Ag(PTA)]+. The MS3 spectrum of the fragment ion at m/z 264 exhibits four main groups of peaks at m/z 237/235, 223/ 221, 196/194, and 179/177, attributed to consecutive fragmentations of the PTA ligand. The fragment at m/z 237/ 235 is due to the loss of one CH2NH moiety from the PTA cage. The fragment ions at m/z 223/221, 196/194, and 179/ 177 resulted from the elimination of CH2NCH3, C3H6N2, and C3H9N3 moieties from the PTA ligand, respectively. These results not only confirm the stability of the [Ag(PTA)2]+ precursor ion but also supply additional evidence for the P,N-coordination mode of PTA ligand. Furthermore, the data

Scheme 1. Simplified Representation of Self-Assembly Synthesis and Structural Formulas of Compounds 1−3

the μ-suc moiety (Scheme 2).18d The Δ values below 200 cm−1 between the νas(COO) (1566 cm−1 (2); 1567 cm−1 (3)) and Scheme 2. Schematic Comparison of the Coordination Modes of Ag(I) Atoms, PTA, and Aliphatic Dicarboxylate Ligands in 1−3a

a

Color code Ag (magenta), N (blue), O (red), P (orange), C (cyan).

νs(COO) (1438, 1406 cm−1 (2); 1471, 1430 cm−1 (3)) bands evidence the syn-syn-μ2-η1:η1 and syn-anti-μ2-η1:η1 coordination modes of carboxylate groups in μ4-adip and μ4-mal moieties in 2 and 3, respectively.18d The presence of water of crystallization in 1 and 2 is clearly indicated by strong and broad bands with maxima in the 3430−3413 cm−1 range.18a−c The 1H NMR spectra of 1−3 in D2O exhibit a set of typical resonances due to the methylene protons of PTA and linear carboxylate ligands that are slightly shifted in comparison with the corresponding uncoordinated molecules, thus supporting the formation of silver(I) complexes in solution. In the δ 4.69−4.24 region, there are characteristic signals corresponding to the methylene (NCH2N and PCH2N) protons of PTA.18b The signals in D



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Figure 1. Structural fragments of 1 showing (a) an ellipsoid plot (50% probability), (b) the 1D metal−organic chain, (c) a topological representation of an underlying uninodal 3-connected 1D coordination network with the SP 1-periodic net (4,4)(0,2) topology, and (d) a topological representation of an extended 3D H-bonded network classified as a binodal 3,4-connected net with the sqc185 topology (view along the a axis). Further details: (a, b) H atoms and crystallization H2O molecules omitted for clarity, color code Ag (magenta), N (blue), O (red), P (orange), and C (cyan); (b, c) view along the c axis; (c, d) Ag1 nodes (magenta), centroids of PTA linkers (c), or nodes (d) (cyan), centroids of suc linkers (c) or nodes (d) (yellow), (d) centroids of H2O linkers (red). Selected distances (Å) and angles (deg): Ag1−P1 2.344(3), Ag1−N12ii 2.395(8), Ag1−O1 2.275(10), Ag1···Ag1i 6.8789(11), Ag1···Ag1ii 9.4817(14); P1−Ag1−N12v 120.2(2), P1−Ag1−O1 136.9(3), N12v−Ag1−O1 102.2(3). Symmetry codes: (i) −1 + x, y, z; (ii) −1 + x, y, z; (iii) 3 − x, −y, 1 − z; (iv) 1 − x, −y, 1 − z; (v) 1 + x, y, z.

indicate that the relative strength of an Ag−P bond and, at the same time, the high lability of an Ag−N bond in solution can be a crucial parameter responsible for antimicrobial action of AgPTA coordination compounds. The data reported herein are a summary of various ESI-MS experiments that give a preliminary indication of the most probable mechanism of the self-assembly process. However, further studies should be carried out to confirm the proposed mechanism and its scope. Structural Description and Topological Analysis. [Ag2(μ-PTA)2(μ-suc)]n·2nH2O (1). Single-crystal X-ray diffraction analysis reveals that 1 is a 1D coordination polymer that crystallizes in the monoclinic P21/c space group. The structure contains one silver atom, half of a μ-suc moiety, one μ-PTA ligand, and one water of crystallization water molecule per asymmetric unit (Figure 1a). The tricoordinate Ag1 atoms have a slightly distorted T-shaped geometry, filled by the O1 atom of the μ-suc linker (Ag1−O1 2.275(10) Å), and the N12 and P1 atoms from two different μ-PTA ligands with Ag1−N12 and Ag1−P1 distances of 2.395(8) and 2.344(3) Å, respectively. The carboxylate groups are in the nonbridging monodentate binding mode, their least-squares planes being perfectly parallel. In 1, the μ-PTA adopts an alternating P- or N-coordination mode (Scheme 2) bridging the neighboring Ag1 atoms and forming −Ag1−PTA−Ag1−PTA− linear chain motifs (Figure

1b) with an Ag1···Ag1 separation of 6.879(1) Å. Two adjacent motifs are additionally pillared by the μ-suc moieties, giving rise to a double 1D chain composed of the repeating −Ag1−PTA− Ag1−suc−Ag1−PTA−Ag1−suc− rings. Such double chains are further extended into an intricate 3D supramolecular network via intermolecular H-bonding interactions (O1W−H1W···N11, O1W−H2W···O1) between the crystallization H2O molecule (O1W) and μ-suc and μ-PTA moieties. For the sake of topological classification of the 1D coordination and 3D H-bonded networks in 1, we have carried out their topological analysis using TOPOS 4.0 software19 and following the concept of the simplified underlying net.20,21 Hence, a simplified 1D net has been generated by contracting the μ-PTA and μ-suc blocks to their centroids. This net (Figure 1c) is built from the 3-connected Ag1 nodes and 2-connected μ-PTA and μ-suc linkers, thus revealing a uninodal 3-connected network with the SP 1-periodic net (4,4)(0,2) topology19,22 and the point symbol of (42.6). Following a similar procedure, we have also simplified and analyzed an underlying 3D H-bonded network in 1 (Figure 1d). It is composed of the 4-connected suc nodes, 3-connected Ag1 and PTA nodes (topologically equivalent), and 2-connected H2O linkers. This binodal 3,4connected framework features an uncommon sqc185 topology (EPINET database)23−25 described by the point symbol of E



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Figure 2. Structural fragments of 2 showing: (a) an ellipsoid plot (50% probability), (b) the 2D metal−organic layer, (c) a topological representation of an underlying uninodal 4-connected 2D coordination net with the sql (Shubnikov tetragonal plane net) topology, and (d) its further reinforced Hbonded network classified as a binodal 3,6-connected net with the 3,6L18 topology. Further details: (a, b) H atoms and crystallization H2O molecules omitted for clarity, color code Ag (magenta), N (blue), O (red), P (orange), and C (cyan); (b−d) views along the a axis; (c, d) topological networks correspond to the second simplification with the adjacent Ag1 atoms being treated as Ag2 nodes (magenta), centroids of PTA linkers (c), or nodes (d) (cyan), centroids of adip linkers (c) or nodes (d) (yellow), and (d) centroids of (H2O)2 nodes (red). Selected distances (Å) and angels (deg): Ag1−P1 2.3838(10), Ag1−N11 2.373(3), Ag1−O1 2.405(3), Ag1−O2 2.402(2), Ag1−Ag1iv 3.1516(6); P1−Ag1−O2 122.78(7), O2−Ag1−N11 96.28(9), N11−Ag1−O1 91.77(9), O2−Ag1−O1 111.74(9), O1−Ag1−Ag1iv 66.07(6), Ag1iv−Ag1−P1 100.99(3), N11−Ag1−Ag1iv 141.20(7), P1−Ag1−N11 117.15(7). Symmetry codes: (i) 1 − x, −1 + y, 1.5 − z; (ii) 1 − x, y, 1.5 − z; (iii) x, 1 − y, 0.5 + z; (iv) 1 − x, 1 − y, 1 − z; (v) x, −1 + y, z; (vi) 1 − x, 1 + y, 1.5 − z, (vii) x, 1 + y, z; (viii) x, −y, 0.5 + z.

(83)4(86), wherein the (83) and (86) indices are those of the 3connected Ag1/PTA and 4-connected suc nodes, respectively. [Ag2(μ-PTA)2(μ4-adip)]n·2nH2O (2). The structure of 2 crystallizes in the triclinic P1 space group and exhibits a 2D coordination network. The asymmetric unit includes one metal atom, half of an adip moiety, one PTA ligand, and two crystallization water molecules (Figure 2a). The Ag1 center is surrounded by the O1 and O2 atoms from two μ4-adip moieties (Ag1−O1 2.405(3), Ag1−O2 2.402(2) Å), the P1 and N11 atoms from two μ-PTA ligands (Ag1−P1 2.3838(10), Ag1− N11 2.373(3) Å), and the adjacent Ag1 atom, forming a distorted-square-pyramidal geometry (τ5 = 0.31).26 Although the Ag1−Ag1 distance of 3.1516(6) Å is longer than that in metallic silver (2.884 Å),27 it is well below the sum of the van der Waals radii of two silver atoms (∼3.44 Å).28 The carboxylate groups are in the syn-syn-type bridging bidentate binding mode, their least-squares planes making an angle of 25.66°. As in 1, the μ-PTA blocks connect the adjacent Ag1 atoms forming the −Ag1−PTA−Ag1−PTA− linear chain motifs (Figure 2b), which are transformed, via the Ag1−Ag1 interactions, into the double-chain motifs. These are further extended into an intricate 2D metal−organic layer by means of the μ4-adip pillars. Moreover, the metal−organic network of 2 is reinforced via a series of intermolecular H bonds involving the crystallization H2O molecules (O20−H20B···O10, O10−

H10A···O2, O10−H10B···O1, and O20−H20A···N13), thus resulting in a more complex 2D H-bonded layer. An interesting feature consists of the formation of discrete water dimers via the O20−H20B···O10 interactions.29 To better understand the intricate 2D metal−organic network of 2, we have performed its topological analysis by adopting the aforementioned concept of the underlying net.19,20 Thus, the first simplification procedure (μ4-adip and μ-PTA blocks were contracted to their centroids) resulted in a binodal 4,5-connected net (Figure S1a, Supporting Information) with the 4,5L55 topology19 and the point symbol of (32.43.54.6)2(32.64), wherein the (32.43.54.6) and (32.64) notations concern the Ag1 and μ4-adip nodes, respectively. Interestingly, although the present topological type has been theoretically predicted,19 no compounds with the 4,5L55 topology have been identified. Thus, the coordination network of 2 represents the first such example. Further simplification of this net by treating two adjacent Ag1 atoms as disilver(I) nodes furnished an alternative topological network (Figure 2c) composed of the 4-connected Ag2 nodes and the 2-connected adip and PTA linkers. This uninodal 4-connected net features the sql (Shubnikov tetragonal plane net) topology with the point symbol of (44.62). Furthermore, the topological analysis of the 2D supramolecular network in 2 driven by the presence of the (H2O)2 F



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Figure 3. Structural fragments of 3 showing: (a) an ellipsoid plot (50% probability), (b) a 2D Ag-PTA motif (view along the c axis), (c) the 3D metal−organic framework, and (d) a topological representation of an underlying 3D net with the dia topology. Further details: (a−c) H atoms omitted for clarity, color code Ag (magenta), N (blue), O (red), P (orange), and C (cyan); (c, d) view along the a axis; (d) 4-connected Ag nodes (magenta), centroids of 4-connected μ4-PTA (cyan), and μ4-mal (yellow) nodes. Selected distances (Å) and angles (deg): Ag1−P1 2.462(2), Ag1− N12 2.462(8), Ag1−O2 2.461(5), Ag2−N11 2.437(5), Ag2−O1 2.374(5), Ag1ii···Ag1iii 7.297(1), Ag1···Ag1ii 7.141(1), Ag1···Ag2 6.9209(9), Ag1ii··· Ag2i 6.0978(9), Ag1iv···Ag2 5.1051(7), Ag1iv···Ag2ii 6.0644(7); P1−Ag1−O2 126.14(11), O2−Ag1−N12 79.00(16), P1−Ag1−N12 124.17(18), O2−Ag1−O2 104.3(2). O1−Ag2−O1vi 129.0(3), O1−Ag2−N11 111.69(17), O1−Ag2−N11vi 96.53(16). Symmetry codes: (i) −x, −0.5 + y, z; (ii) −1 + x, y, z; (iii) −x, 0.5 − y, −0.5 + z; (iv) −1 + x, 0.5 + y, −0.5 + z; (v) −1 − x, 1 − y, z; (vi) −x, 1 − y, z; (vii) 1 + x, y, z; (viii) 1 + x, 1 − y, 0.5 + z; (ix) 1 + x, −0.5 + y, 0.5 + z.

[Ag4(PTA)4]4+ square blocks (Figure 3b). These motifs are then assembled by the μ4-mal pillars into a 3D metal−organic framework (Figure 3c). The 3D metal−organic framework of 3 has been simplified (μ4-PTA and μ4-mal blocks were reduced to their centroids) to furnish an underlying net constructed from the 4-connected Ag1, Ag2, μ4-PTA, and μ4-mal nodes that are all topologically equivalent (Figure 3d). Its topological analysis reveals a uninodal 4-connected framework with a common dia (diamond) topology19,24 and the point symbol of (66). A number of silver MOFs with the dia topology have been reported.30 Effect of Aliphatic Dicarboxylate Ligands on the SelfAssembly of Ag-PTA Coordination Polymers. In the present study, we selected three different aliphatic dicarboxylic acids (H2suc, H2adip, and H2mal) and applied them in a mixedligand synthetic system in order to monitor the effect of their size, shape, and flexibility on the self-assembly generation of AgPTA coordination polymers. Although glutaric acid has also been tested, all attempts to crystallize the resulting product have not been successful. Although compounds 1−3 are constructed from the same main building block (PTA) and different ancillary ligands, they display distinct coordination, structural, and topological features. Interestingly, the length of the −(CH2)n− (n = 1, 2, 4) backbone chain has a strong effect on the polymeric

clusters has been carried out. From the topological viewpoint, this network (Figure S1b, Supporting Information) can be considered as a tetranodal topologically unique 3,3,5,8connected net19 which, upon further simplification, gave rise to a binodal 3,6-connected net (Figure 2d) with the 3,6L18 topology and the point symbol of (43)2(46.66.83). [Ag2(μ4-PTA)(μ4-mal)]n (3). The compound 3 crystallizes in an orthorhombic Abm2 space group and features a 3D metal− organic framework structure. It consists of two symmetrynonequivalent Ag1 and Ag2 atoms, one μ4-mal moiety, and two μ4-PTA blocks (Figure 3a). The four-coordinate Ag1 atoms adopt a geometry intermediate between a {AgNPO2} trigonal pyramid and seesaw (τ4 = 0.76),26a which is filled by the N11 and P1 atoms from two adjacent μ4-PTA ligands (Ag1−N12 2.462(8), Ag1−P1 2.462(2) Å) and a pair of the O2 atoms from two different μ4-mal moieties (Ag1−O2 2.461(5) Å). The four-coordinate Ag2 atoms possess a {AgN2O2} trigonalpyramidal environment (τ4 = 0.85), defined by the two N11 atoms from two distinct μ4-PTA ligands (Ag2−N11 2.437(5) Å) and the two O1 atoms from two different μ4-mal moieties (Ag2−O1 2.374(5) Å). The carboxylate groups are in the synanti-type bridging bidentate binding mode, their least-squares planes making an angle of 81.10°. A notable feature of 3 concerns a very rare N3,P-coordination mode of PTA ligands, which are bound to two Ag1 and two Ag2 atoms, thus giving rise to 2D layered motifs composed of the repeating G



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aureus, with the MIC values for 1 and 3 (6−8 μg mL−1) being significantly lower than that of 2 (40 μg mL−1). It should also be emphasized that when the MIC of Staphylococcus aureus is expressed in nmol mL−1, the activities of 1 and 3 are essentially higher than that of AgNO3. A similar behavior is detected for Candida albicans, since 1−3 inhibited its growth in a concentration lower than that for AgNO3 salt (MIC = 236 nmol mL−1). In general, the antimicrobial activities of the tested compounds are also significantly superior to those exhibited by some other silver coordination compounds.5,31 However, some caution should be taken while comparing the obtained results with the literature data, because the MIC values can be influenced by several factors such as different bacterial strains, temperature, contact time, and microbial cell density; therefore, the results can be directly incomparable. As shown in Scheme 2, compounds 1−3 have geometrically different structures; however, each Ag1 center is surrounded by the same types of O, N, and P atoms with mixed metal−ligand binding properties. It is known that the antimicrobial effect of Ag(I) compounds can depend on the type of coordination environment of the silver(I) atom (i.e., Ag−N, Ag−O, Ag−P, Ag−S).3,4 Nomiya and co-workers have reported that the Ag− N and Ag−O compounds exhibit a wide and effective spectrum of antibacterial and antifungal activity.5b For example, the Ag− O derivative {[Ag(Hacgly)]2}n (Hacgly = N-acetylglycine) showed low activity against P. aeruginosa (MIC 125 μg mL−1) and S. aureus bacteria (MIC 62.5 μg mL−1) and higher activity against E. coli (15.7 μg mL−1) and C. albicans (15.7 μg mL−1).5d The Ag−N compounds [Ag(5-nqu)2]NO3 (5-nqu = 5nitroquinoline) and [Ag(Hqca)2]NO3 (Hqca = 6-quinolinecarboxylic acid) were poorly effective against the Gram-negative (S. aureus, MIC 256 and 32 μg mL−1) and Gram-positive bacteria (S. aureus MIC 64 and 256 μg mL−1).31c Abu-Youssef et al. showed that the mixed Ag−N and Ag−O complexes are very active against all tested pathogens in comparison to compounds that possess only {AgO} or {AgN} coordination environments.31e For example, [Ag(ethyl isonicotinate)(NO3)] exhibited good activity against S. aureus (MIC 8−16 μg mL−1) and slightly weaker activity against P. aeruginosa (MIC 32−64 μg mL−1).31e The same behavior was observed by Ö hrström and co-workers when studying the [Ag(8-nqu)2]NO3·H2O (8nqu = 8-nitroquinoline) complex possessing Ag−O and Ag−N bonds, which was very active against E. coli and S. aureus strains (MIC 32 μg mL−1).31c Moreover, it was detected that compounds with the {AgP} or {AgNP} coordination environment usually show weak or no activity against bacteria, mold, and yeast.5h,31f The complex [Ag(tetrz)(PPh3)2]n (tetrz = tetrazole, PPh3 = triphenylphosphine), is inactive against all tested pathogens,5h whereas compounds with the {AgNPO} coordination environment exhibit variable activity with MIC values in the 500−31.4 μg mL−1 range against all tested pathogens, depending on the number of PPh3 ligands coordinated to the silver atom.31f An analysis of our experimental and literature data suggests that the difference in the antimicrobial efficiencies of compounds 1−3 is mostly related to the coordination sphere of each silver atom. In addition, the recurrent pattern of the ESI-MS spectra confirms such an assumption, suggesting that the antimicrobial action of 1−3 most likely arises from a slow release of free Ag+ ions into solution. Given the moderate stability of 1−3, these compounds can be considered as a reservoir of bioactive Ag+ ions. These can be slowly released into the solution due to the presence of O and N donor atoms

architecture of 1−3. In 1, the 2-connected succinate ligand with the −(CH2)2− backbone chain acts as a pillar between two AgPTA motifs, resulting in the double-chain 1D coordination polymer. In 2, the adipate ligand with the −(CH2)4− backbone chain acts as the 4-connected pillar, with its carboxylic groups adopting the syn-syn-type bridging bidentate coordination modes, thus assembling the double-chain [Agn(PTA)n]+ subunits into a complex 2D layer. Hence, the combination of dicarboxylate ligands with elongated aliphatic chains (n = 2, 4) along with PTA that acts as a μ2-linker causes a tendency to increase the size of the voids between the metal−organic layers, thus decreasing the overall dimensionality of the coordination polymers 1 (1D) and 2 (2D). In contrast, the use of malonic acid with a short −(CH2)− backbone leads to a 3D metal− organic structure. In fact, compound 3 consists of two different four-coordinate silver atoms with the {AgNPO2 } and {AgN2O2} environments, while a highly flexible μ4-mal spacer possesses both carboxylic groups in the syn-anti-type bridging bidentate binding mode. Most likely, the μ4-mal moieties force the PTA ligand to adopt an unusual N3,P coordination mode, thus resulting in a three-dimensional structure. The formation of structurally distinct dicarboxylate-driven coordination polymers was also observed by Wen and coworkers. They reported a series of [Cun(bpt)n(L)n] and [Cdn(bpt)n(L)]n complexes (bpt = 2,5-bis(4-pyridyl)-1,3,4thiadiazole) with an aliphatic ancillary ligand (H2L = oxalic, malonic, succinic, glutaric, and adipic acids),13e demonstrating that the slight modification of the length and flexibility of employed aliphatic polycarboxylate is a key factor toward the structural discrepancy of the obtained coordination polymers.13e Taking into account all the aforementioned results, it can be concluded that the flexibility related to the presence of different −(CH2)n− backbone chains in dicarboxylate ligands has a great influence on the self-assembly process. In addition, the identification of the precursor ion in the solution mixture by the ESI-MS technique supplies additional evidence that the architectural versatility of 1−3 is related to distinct structural and geometrical properties of aliphatic acids. Antibacterial and Antifungal Activity. Bacterial strains that are multiresistant to conventional antibiotics have become a worldwide public health issue.3,4 The need for new effective antiseptic agents has encouraged chemists to design and investigate new antimicrobial materials. However, in spite of their recognized bioactivity, silver-based compounds are still undervalued as potential antimicrobial drugs by the pharmaceutical industry due to their insolubility in water and not fully understood mechanism of action.3−5 Bearing these points in mind, we have investigated the antibacterial and antifungal properties of the aqua-soluble coordination polymers 1−3. Their antimicrobial activity has been tested in vitro using the serial dilutions method with Antibiotic Broth17 against selected Gram-negative (Escherichia coli, Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus) bacteria and yeast (Candida albicans). The obtained results are presented as minimal inhibitory concentrations (MIC, μg mL−1) in Table 2. The free PTA ligand is inactive against the tested bacteria and yeast strains. Although compounds 1−3 showed a notable antimicrobial activity against the probed pathogens, the most promising results were obtained with 1 and 3. All of the tested compounds exhibit comparable antimicrobial activities against Escherichia coli bacteria and Candida albicans yeast with MIC values in the 6−7 and 30−50 μg mL−1 ranges, respectively. However, a remarkable difference is found for Staphylococcus H



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possessing a weak binding affinity to the metal center that increases the probability of organic ligand replacement by biological moieties. The ESI-MS spectra showed the tremendous strength of the Ag−P bond. On the basis of these observations, we can speculate that the [Ag2(PTA)2(Hsuc)]+ (in 1), [Ag2(PTA)2(Hadip)]+ (in 2), and [Ag2(PTA)2(Hmal)]+ ions (in 3) are responsible for the antimicrobial action of 1−3. Although the [Ag(PTA)2]+ species is the most stable ion in solution, it cannot be responsible for biological activity, due to the significant differences in the MIC values observed for 1−3.

CONCLUSIONS In the present work we synthesized and fully characterized three novel silver−organic networks obtained by a mixed-ligand strategy involving PTA and various flexible aliphatic dicarboxylic acids as building blocks. To our knowledge, the derivatives 1−3 not only represent the first examples of Ag-PTA coordination networks that bear aliphatic dicarboxylate moieties but also widen a still limited group of coordination compounds with unconventional N,P or N3,P coordination modes of PTA.9,10 The nature of flexible aliphatic dicarboxylate ligands appears to play a key role toward the formation of coordination polymers that show distinct dimensionality and diverse structural and topological features. In fact, the 1D structure of 1 revealed a uninodal 3-connected network with a rare SP 1-periodic net (4,4)(0,2) topology and the 2D structure of 2 disclosed a binodal 4,5-connected net with the unique 4,5L55 topology, whereas a uninodal 4-connected 3D framework with the dia topology was identified in 3. Furthermore, the metal−organic chains of 1 or layers of 2 are further extended into 3D (1) or 2D (2) H-bonded networks, which adopt very rare (1) or unprecedented (2) topologies. Apart from interesting structural and topological features, the obtained compounds also represent rare examples of aquasoluble silver(I) coordination polymers that exhibit significant antibacterial and antifungal activity, thus opening up their potential exploration as novel bioactive materials. ASSOCIATED CONTENT

S Supporting Information *

Figures and CIF files giving additional topological representations, PXRD patterns, ESI-MS plots, and crystallographic data for 1−3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.F.C.G.d.S.: e-mail, [email protected]. *P.S.: e-mail, [email protected]. *A.M.K: e-mail, [email protected]; tel, +351 218417178. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Foundation for Science and Technology (FCT) (projects PTDC/QUI-QUI/121526/2010 and PEst-OE/QUI/UI0100/2013 and REM 2013) of Portugal, as well as by the NCN program (Grant No. 2012/07/B/ST5/ 00885), Poland. We also thank Dr. J. Król for assistance in antimicrobial tests. I



Article

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