New Antibacterial Silver(I) Coordination Polymers Based on a Flexible Ditopic Pyrazolyl-Type Ligand

Article New Antibacterial Silver(I) Coordination Polymers Based on a Flexible Ditopic Pyrazolyl-Type Ligand

1, 2 3 1, Aurel Tăbăcaru * , Claudio Pettinari , Mariana Bus, ilă and Rodica Mihaela Dinică * 1 Faculty of Sciences and Environment, Department of Chemistry, Physics and Environment, “Dunarea de Jos”

University of Galati, 111 Domneasca Street, 800201 Galat, i, Romania 2 School of Pharmacy, University of Camerino, Via Madonna delle Carceri 9, 62032 Camerino MC, Italy; [email protected] 3 Department of Materials Science and Engineering, Faculty of Engineering, ”Dunarea de Jos” University of

Galati, 111 Domneasca Street, 800201 Galat, i, Romania; [email protected] * Correspondence: [email protected] (A.T.); [email protected] (R.M.D.)

Received: 1 October 2019; Accepted: 10 October 2019; Published: 15 October 2019

Abstract: In the last two decades, a tremendous amount of attention has been directed towards the design of antibacterial silver(I)-based materials, including coordination polymers (CPs) built up with a great variety of oxygen and nitrogen-containing ligands. Herein, a family of six new silver(I)-based CPs, having the general stoechiometric formula [Ag(H2DMPMB)(X)] (X = NO3, 1; CF3CO2, 2; CF3SO3, 3; BF4, 4; ClO4, 5; and PF6, 6) and incorporating the ﬂexible ditopic pyrazolyl-type ligand 4,40-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H2DMPMB), has been prepared by the chemical precipitation method involving the reaction of silver(I) salts with H2DMPMB in the 1:1 molar ratio, in alcohols, or acetonitrile at room temperature for two-hours. The new silver(I)-based polymeric materials were characterized by means of Fourier transform infrared spectroscopy (FTIR), elemental analysis (EA), and thermogravimetric analysis (TGA), allowing for the proposition that their structures comprise one-dimensional chains, with the silver(I) ions mostly assuming a T-shapped stereochemistry completed by the exo-bidentate ligands and counter-anions. The obtained silver(I) CPs showed a remarkable light insensitivity and stability in the air, are insoluble in water and in most common organic solvents, and possess appreciable thermal stabilities spanning the range 250–350 ◦C. The antibacterial activity of the obtained silver(I) CPs was tested against the Gram-negative bacteria Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus (S. aureus) using the Tetrazolium/Formazan test (TTC), by measuring the bacterial viability at diﬀerent time intervals. The complete reduction of both bacterial strains occurred after 24 h of exposure to all silver(I) CPs, the bacterial viability values for S. aureus reaching 8% for compounds 3, 5, and 6 after only two-hours.

Keywords: silver; coordination polymers; nitrogen ligands; thermal stability; antibacterial activity

1. Introduction Coordination polymers (CPs) [1–4], and the subset of metal-organic frameworks (MOFs) [5–7], represent the vast class of inorganic organic hybrid materials, which are made of metal ions or − metal-based clusters and polydentate ligands connected through coordination bonds, which develop into 1-D, 2-D, and 3-D inﬁnite structures. CPs and MOFs have gained a special place in ﬁelds of industrial, technological, economical, and environmental interest thanks to the great variety of functional properties so far achieved, ranging from gas storage and separation [8] to heterogeneous catalysis [9], luminescence [10], sensing [11], magnetism [12], conductivity [13], and biomedicine [14]. For their successful construction, the most typically used organic ligands have been rationally chosen

Polymers 2019, 11, 1686; doi:10.3390/polym11101686 www.mdpi.com/journal/polymers Polymers 2019, 11, 1686 2 of 17 Polymers 2019, 11, x FOR PEER REVIEW 2 of 17 frombeen therationally class of poly(carboxylates)chosen from the class [15 ,of16 ],poly( poly(azolates)carboxylates [17) –[1915],,16 pyrazines], poly(azolates) and bipyridines [17–19], [p15yrazines,16,20], andand phosphonatesbipyridines [15,16, [21].20], and phosphonates [21]. SpecialSpecial attention attention has has also also been been offered, offered, over the over past the two decades,past two to decades, the synthesis to the of silver(I)-based synthesis of CPs,silver(I) which-based were CPs found, which to show were fascinating found to structuralshow fascinating motifs [ 22structural–25] and motifs interesting [22– properties,25] and interesting ranging fromproperties, photoluminescence ranging from [ 26 photolu,27] tominescence electrical conductivity [26,27] to [ electrical28,29], magnetism conductivity [30, 31[28],, guest29], magnetism exchange and[30,31 sorption], guest [exchange32,33], and and catalysis sorption [34 [32,35,33].] The, and successful catalysis [ preparation34,35]. The successful of such polymeric preparation materials of such haspolymeric taken thematerials judicious has taken management the judicious of various management factors of into various consideration, factors into such consideration as the reaction, such conditionsas the reaction [36], the conditions silver-to-ligand [36], the ratio silver [37],- theto-ligand stereochemistry ratio [37], of thesilver stereochemistry ion coupled with of the silver ligands ion functionalitycoupled with [ 38the], ligands and the functionality nature of counter-anions [38], and the [ 39nature]. of counter-anions [39]. InIn the the last last decade, decade, silver(I)-based silver(I)-based CPs, CPs, built built upup eithereither withwith rigidrigid or or flexible flexible nitrogennitrogen andand oxygen-donoroxygen-donor ligands, ligands, have have gained gained popularity popularity due due to their to their potential potential as antibacterial as antibacterial agents, agents which, unlikewhich theunlike soluble the soluble silver(I) silver(I) complexes complexes [40], have [40 arisen], have to arisen minimize to minimize the problem the of problem reducing, of reducing as much, asas possible,much as thepossi amountble, the ofamount silver(I) of ionssilver(I) released ions released into the into environment, the environment thus relying, thus onrelying their on higher their stabilitieshigher stabilities and very and low very solubilities. low solubilities. The pioneering The pioneering works by work Nomiyas by Nomiya et al. have et reportedal. have reported positive resultspositive of results antimicrobial of antimicrobial and antifungal and antifungal activities activities of the azolyl-basedof the azolyl-based silver(I) silver(I) CPs [Ag(im)] CPs [Ag( [im)]41], [Ag(1,2,3-tz)(PPh[41], [Ag(1,2,3-tz)3)(PPh2] and3)2 [Ag(1,2,4-tz)(PPh] and [Ag(1,2,4-tz)(PPh3)2][423],)2 [Ag(tetz)(PPh] [42], [Ag(tetz3))2(PPh][43]3) (im2] [43]= 1,3-imidazolate, (im = 1,3-imidazolate, 1,2,3-tz =1,2,31,2,3-triazolate,-tz = 1,2,3- 1,2,4-tztriazolate,= 1,2,4-triazolate, 1,2,4-tz = 1,2,4 tetz-=triazolate1,2,3,4-tetrazolate,, tetz = PPh 1,2,3,43 =-tetrazolatetriphenylphosphine)., PPh3 = Thesetriphenylphosphine first results have). These then first stimulated results have the preparation then stimulated and characterizationthe preparation and of new, characterization other types ofof silver(I)new, other CPs types with of promising silver(I) CPs results with regarding promising the results antibacterial regarding activity the antibacterial against a wide activity spectrum against of a Gram-negativewide spectrum and of Gram Gram-positive-negative andpathogenic Gram-positive microorganisms. pathogenic In microorganisms this context, various. In this prototypes context, ofvarious silver(I) prototypes CPs have of been silver(I) tested CPs in thehave form been of tested powders in the [44 form–49], polymer-basedof powders [44– composites49], polymer [50-based–54], andcomposites surface coatings[50–54], [and55–59 surface], exhibiting coatings remarkable [55–59], biocidalexhibiting effects, remarkable either in biocidal suspension effects, or by either contact. in suspensionIn the or past by contact few years,. we have shown that the flexible ditopic pyrazolyl-type ligand 4,40-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenylIn the past few years, we have shown that the (H2 DMPMB,flexible ditopic Scheme 1 pyrazolyl)[ 60], and-type its shorterligand analogue4,4’-bis((3,5 1,4-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)benzene-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H2DMPMB, (H2BDMPX) Scheme [61 ],1) were [60], able and to its generate shorter cobalt(II)-,analogue zinc(II)-,1,4-bis((3,5 cadmium(II)-,-dimethyl-1H and-pyrazol copper(I)-containing-4-yl)methyl)benzene CPs displaying (H2BDMPX) high [61 thermal], were stabilities, able to coupledgenerate withcobalt(II) either-, zinc(II) pro-porous-, cadmium(II) properties-, orand photoluminescence. copper(I)-containing Herein, CPs displaying we sought high to thermal further explorestabilities the, coupled coordinative with potentiality either pro- ofporous the flexible properties ligand or H photo2DMPMBluminescence towards. the Herein, preparation we sought of new to antibacterialfurther explore silver(I)-based the coordinative CPs with potentiality different counter-anions, of the flexible which ligand are remarkably H2DMPMB light-insensitive towards the andpreparati highlyon insoluble. of new antibacterial silver(I)-based CPs with different counter-anions, which are remarkably light-insensitive and highly insoluble.

Scheme 1. Structure of 4,4 -bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H DMPMB). Figure 1. Structure of 4,4’0-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H2DMPMB).

The present paper is, therefore, aimed at synthesizing, through a simple chemical precipitation The present paper is, therefore, aimed at synthesizing, through a simple chemical precipitation and high-yielding method, a family of six new silver(I)-based CPs incorporating the ﬂexible ligand and high-yielding method, a family of six new silver(I)-based CPs incorporating the flexible ligand H2DMPMB with the general stoechiometric formula [Ag(H2DMPMB)(X)] (X = NO3, 1; CF3CO2, H2DMPMB with the general stoechiometric formula [Ag(H2DMPMB)(X)] (X = NO3, 1; CF3CO2, 2; 2; CF3SO3, 3; BF4, 4; ClO4, 5; and PF6, 6). Only two examples of silver nitrate and perchlorate CF3SO3, 3; BF4, 4; ClO4, 5; and PF6, 6). Only two examples of silver nitrate and perchlorate complexes complexes with the ethyl homologues of the ﬂexible H2DMPMB ligand were reported a few years with the ethyl homologues of the flexible H2DMPMB ligand were reported a few years ago [62]. ago [62]. Recently, two photoluminescent silver nitrate 1D coordination polymers with the bitopic Recently, two photoluminescent silver nitrate 1D coordination polymers with the bitopic ligand ligand 1,1,2,2-tetra(pyrazol-1-yl)ethane were structurally characterized [63]. While Fourier transform 1,1,2,2-tetra(pyrazol-1-yl)ethane were structurally characterized [63]. While Fourier transform infrared spectra (FTIR) were exhaustively analyzed to propose the structure of all the obtained infrared spectra (FTIR) were exhaustively analyzed to propose the structure of all the obtained silver(I) polymeric compounds, thermogravimetric analysis (TGA) revealed their appreciable thermal silver(I) polymeric compounds, thermogravimetric analysis (TGA) revealed their appreciable stabilities, lying in the range 250–350 C. The antibacterial activity of the obtained silver(I) CPs against thermal stabilities, lying in the range◦ 250‒350 °C. The antibacterial activity of the obtained silver(I) the Gram-negative bacteria Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus CPs against the Gram-negative bacteria Escherichia coli (E. coli) and Gram-positive bacteria Staphylococcus aureus (S. aureus) was tested using the Tetrazolium/Formazan test (TTC), by measuring the bacterial viability at different time intervals.

Polymers 2019, 11, 1686 3 of 17

(S. aureus) was tested using the Tetrazolium/Formazan test (TTC), by measuring the bacterial viability at diﬀerent time intervals.

2. Materials and Methods

2.1. General All reagents and solvents were purchased from Sigma-Aldrich (Darmstadt, Germany) and Alfa • Aesar (Kandel, Germany), and were used as received. The ligand 4,4 -bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H DMPMB) was synthesized • 0 2 according to an optimized method previously reported in the literature [60]. FTIR spectra were recorded from 4000 to 650 cm-1 with a Perkin–Elmer Spectrum 100 instrument • (Perkin-Elmer, Shelton, CT, USA) by total reflectance on a ZnSe crystal. In the following, the IR bands are classified as very weak (vw), weak (w), medium (m), strong (s), and very strong (vs). Elemental analyses (C, H, N, and S) were performed in-house with Fisons Instruments 1108 • CHNS-O Elemental Analyzer (Thermo Scientific, Waltham, MA, USA). Before performing the -4 analytical characterization, all samples were dried in vacuo (50 ◦C, ~10 bar) until a constant weight was reached. Termogravimetric analyses (TGA) were carried out from 30 to 700 C in a N stream with a • ◦ 2 Perkin–Elmer STA 6000 simultaneous thermal analyzer (Perkin-Elmer, Shelton, CT, USA) with the heating rate of 7 ◦C/min.

2.2. Syntheses

[Ag(H2DMPMB)(NO3)] (1). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of methanol under gentle warming (45 ◦C). Then, AgNO3 (0.085 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 2 h, obtaining a white precipitate that was filtered off, washed twice with methanol, and dried under a vacuum. Yield: 75%. 1 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and water. 1 IR (cm− ): 3199(mbr) ν(N–H); 3100-3000(vw) ν(C–Haromatic), 3000–2900(w) ν(CH3, CH2), 1748(w), 1582(w), 1528(w), 1499(w) ν(C=C+C=N), 1387(vs) νasym(NO3), 1347(vs) νasym(NO3), 1210(w), 1006(w), 905(w), and 754(vs). Elemental analysis calcd. for C24H26AgN5O3 (FW = 540.36 g/mol): C, 53.34; H, 4.85; N, 12.96%. Found: C, 53.03; H, 4.67; N, 12.38%. [Ag(H2DMPMB)(CF3CO2)] (2). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of ethanol under gentle warming (45 ◦C). Then, AgCF3CO2 (0.111 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 4 h, obtaining a white precipitate that was filtered off, washed twice with ethanol, and dried under a vacuum. Yield: 75%. 2 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and water. 1 IR (cm− ): 3186(mbr), ν(N–H), 3092(vw), 3028(vw) ν(C–Haromatic), 2921(vw) ν(CH3, CH2), 1668(vs) ν(C=O), 1579(vw), 1528(vw), 1494(m) ν(C=C+C=N), 1431(m), 1201-1140(vs) ν(CF3CO2), 1058(m), 1005(m), 905(m), 806(m), 719(s). Elemental analysis calc. for C26H26AgF3N4O2 (FW = 591.38 g/mol): C, 52.81; H, 4.43; and N, 9.47%. Found: C, 52.35; H, 4.25; N, 9.08%. [Ag(H2DMPMB)(CF3SO3)] (3). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of ethanol under gentle warming (45 ◦C). Then, AgCF3SO3 (0.129 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 4 h, obtaining a white precipitate that was filtered off, washed twice with ethanol, and dried under a vacuum. Yield: 70%. 3 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and water. 1 IR (cm− ): 3233(mbr), ν(N–H), 3024(vw) ν(C–Haromatic), 2923(vw) ν(CH3, CH2), 1586(w), 1526(vw), 1498(w) ν(C=C+C=N), 1240(vs), 1224(vs), 1162(vs), 1029(vs) ν(CF3SO3), 905(w), 839(w), and 759(m). Elemental analysis calcd. for C25H26AgF3N4O3S (FW = 627.43 g/mol): C, 47.86; H, 4.18; N, 8.93; S, 5.11%. Found: C, 47.85; H, 3.92; N, 8.34; S, 4.91%. Polymers 2019, 11, 1686 4 of 17

[Ag(H2DMPMB)(BF4)] (4). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of acetonitrile under gentle warming (45 ◦C). Then, (MeCN)4AgBF4 (0.179 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 4 h, obtaining a white precipitate that was filtered off, washed twice with acetonitrile, and dried under a vacuum. Yield: 80%. 4 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and water. 1 IR (cm− ): 3316(mbr) ν(N–H), 3025(vw) ν(C–Haromatic), 2923(vw) ν(CH3, CH2), 1587(w), 1526(vw), 1494(w) ν(C=C+C=N), 1057(vs), 1005(vs) ν(BF4), 904(w), 845(m), 799(w), 768(m), and 692(mbr). Elemental analysis calcd. for C24H26AgBF4N4 (FW = 565.17 g/mol): C, 51.00; H, 4.64; N, 9.91%. Found: C, 50.86; H, 4.45; N, 9.36%. [Ag(H2DMPMB)(ClO4)] (5). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of ethanol under gently warming (45 ◦C). Then, AgClO4 (0.104 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 2 h, obtaining a white precipitate that was filtered off, washed twice with ethanol, and dried under a vacuum. Yield: 85%. 5 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and water. 1 IR (cm− ): 3267(mbr) ν(N–H), 3025(vw) ν(C–Haromatic), 2923(vw) ν(CH3, CH2), 1587(w), 1525(vw), 1495(w) ν(C=C+C=N), 1075(vs), 1040(vs) ν(ClO4), 845(m), 769(m), and 687(mbr). Elemental analysis calcd. for C24H26AgClN4O4 (FW = 577.82 g/mol): C, 49.89; H, 4.54; N, 9.70%. Found: C, 49.52; H, 4.46; N, 9.51%. [Ag(H2DMPMB)(PF6)] (6). H2DMPMB (0.093 g, 0.25 mmol) was dissolved in 30 mL of ethanol under gently warming (45 ◦C). Then, AgPF6 (0.125 g, 0.5 mmol) was added. The resulting white suspension was left under stirring at room temperature for 3 h, obtaining a white precipitate that was filtered off, washed twice with ethanol, and dried under a vacuum. Yield: 90%. 6 is insoluble in chlorinated solvents, alcohols, acetone, acetonitrile, dimethylformamide, dimethylsulfoxide, and 1 water. IR (cm− ): 3410(mbr) ν(N-H), 3027(vw) ν(C–Haromatic), 2926(vw) ν(CH3, CH2), 1585(w), 1525(vw), 1495(w) ν(C=C+C=N), 828(vs) ν(PF6), 768(m), and 667(mbr). Elemental analysis calcd. for C24H26AgF6N4P (FW = 623.33 g/mol): C, 46.25; H, 4.20; N, 8.99%. Found: C, 46.22; H, 3.98; N, 8.50%.

2.3. Bacteria Strain and Growth Conditions The pathogenic bacteria, Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 (clinical isolates), used as test microorganisms for the antibacterial activity study, were collected from the Microbial Culture Collection (Sf. Andrei Hospital of Galati, Romania). The cultivation medium for E. coli and S. aureus was Brain Heart Infusion Broth (BHI, Merck, Darmstadt, Germany). Bacterial culture for antibacterial activity was prepared by picking the colony in BHI agar plates and incubated for 24 h, followed by suspension in an appropriate medium (9 mL). The culture was grown aerobically for 20 h at 37 ◦C. For antibacterial assay, 1 mL of bacterial culture was diluted in 9 mL BHI medium to 106 CFU/mL.

2.4. Antibacterial Activity TTC (2,3,5-triphenyl tetrazolium chloride, Merck) was prepared by dissolving the powder in sterile water (5 mg/mL) at room temperature. In the presence of bacteria, TTC was reduced to red formazan, which was directly proportional to the activity and viability of the bacterial cells [64]. The TTC test was evaluated as a qualitative antibacterial method. For the susceptibility test using TTC, 1 mL of silver(I) CPs (suspended in sterile water, 5 mg/mL), 1 mL of AgNO3 used as positive control (dissolved in sterile water, 5 mg/mL), and 100 µL of bacteria suspension (106 CFU/mL) was added into 40 mL of BHI broth. The control sample contained only bacterial suspension and BHI broth. All flasks were incubated with shaking at 37 C at 200 g for 4 h. Then, 1 mL from each flask ◦ × containing the treated and control cultures was transferred to sterile Eppendorf tubes, and 100 µL of TTC was then introduced. All tubes were incubated at 37 ◦C for 20 min. The samples were centrifuged at 4000 g for 3 min, followed by decantation of the supernatants. The resulting formazan crystals × were resuspended in ethanol (70%) and centrifuged again. The red formazan solution obtained at the Polymers 2019, 11, 1686 5 of 17 end was measured by a microplate reader (Tecan Infinite 200PRO, Tecan Trading AG, Männendorf, Switzerland) at 480 nm. All experiments were done in triplicate and the relative cell viability (%) was represented by a percentage relative to the untreated control cells.

3. Results and Discussion

3.1. Synthesis and FTIR Spectroscopy

Compounds 1–6, having the general stoechiometric formula [Ag(H2DMPMB)(X)] (X = NO3, 1; CF3CO2, 2; CF3SO3, 3; BF4, 4; ClO4, 5; and PF6, 6), hypothesized with the help of elemental analyses, have been obtained from the room temperature reaction carried out in a 2:1 molar ratio of two equivalents of silver(I) salts and one equivalent of H2DMPMB, in alcohol for compounds 1–3, 5, and 6, and acetonitrile for compound 4. Their stoechiometry was found to be independent of the amount of silver(I) salts or ligand used in the syntheses, since the 1:1 molar ratio was always obtained, which was also observed in other bis(pyrazolyl)-based silver(I) compounds, such as [Ag(H2BPZ)(X)] (X = NO3, ClO4, BF4, PF6, CH3SO3, CF3SO3;H2BPZ = 4,40-bipyrazole) [54], [Ag(H Me BPZ)(NO )] CH OH [65], and [Ag(H Me BPZ)(X)] (X = ClO , CF SO ,C F CO ; 2 4 3 · 3 2 4 4 3 3 2 5 2 H2Me4BPZ = 3,30,5,50-tetramethyl-4,40-bipyrazole) [66]. All compounds were in the form of white powders, and were remarkably light insensitive and air stable, even after twelve months, thus relying on the strength of silver-ligand bonds that may be imparted to the inertness of the overall structure. In fact, no change of their aspect was noticed, and both elemental analyses and FTIR spectra were preserved. Their polymeric nature was indicated by the observed insolubility, both in water and in most common organic solvents, along with the occurrence of high thermal decomposition temperatures revealed by the thermogravimetric analysis (Section 3.2). A preliminary powder X-ray diffraction analysis showed the amorphous nature of the obtained silver(I) CPs, and therefore, their molecular structures are yet to be revealed. Nevertheless, a deeper analysis of their FTIR spectra allowed the proposition of a structural characterization for the obtained H2DMPMB-based silver(I) CPs, as will be shown in the following lines. FTIR spectroscopy was employed in order to reveal both the changes in the absorbtion bands of the ligand upon coordination to silver(I) ions and the binding modes of the counterions. The FTIR spectrum of the free H2DMPMB ligand is given in Figure1, while the FTIR spectra of silver(I) CPs 1–6 are given in Figures2 and3. The infrared spectrum of the free ligand showed a strong and broad band 1 in the region 2500–3200 cm− , which was specific to the intermolecular hydrogen bond interactions mediated by the N-H functions of the pyrazolyl rings. Upon coordination to silver(I) ions, the infrared pattern of the ligand was significantly changed, indicating the success of the synthetic methodology adopted for obtaining the new silver(I) CPs. Polymers 2019, 11, x FOR PEER REVIEW 6 of 17

95,0 10094

90 18 96,4

86 16 12,7 94 3,8 84 90 15 20,3 82 12 65,4 80

78 90 0,3

76 15 85,5 14 00,0 Polymers 2019, 11, 1686 13 79,5 11 40,2 6 of 17 74 Polymers80 2019, 11, x FOR PEER REVIEW 12 95,1 70 3,96 of 17 72 68 1,2 14 13,8 12 07,2 29 79,2 %T 70 31 80,0 95,0 28 73,2 14 68,0 10094 30 22,1 14 35,8 68 31 35,2 30 85,2 10 52,8 92 66 29 20,6 90 18 96,4 7064 Transmittance(%) 88 10 03,8 62

86 16 12,7 60 14 98,4 94 3,8 84 9058 15 20,3 82 79 8,1 56 76 5,0 12 65,4 82 6,6 80 6054 78 90 0,3 52 76 50 15 85,5 14 00,0 13 79,5 11 40,2 74 8048 12 95,1 70 3,9 46,672 68 1,2 14 13,8 12 07,2 4000,0 3600 3200 29 79,2 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0 %T 31 80,0 400070 3600 3200 280028 73,2 2400 2000 cm-11800 1600 14 68,01400 1200 1000 800 650 30 22,1 14 35,8 68 31 35,2 -1 30 85,2 Wavenumber (cm ) 10 52,8 66 29 20,6 7064 Transmittance(%) Figure 1. Fourier transform infrared spectroscopy (FTIR) spectrum of the free H2DMPMB10 03,8 ligand. 62

60 14 98,4 The58 infrared spectra of all silver(I) CPs were generally consistent with the proposed 79 8,1 56 76 5,0 formulations, and present all of the required absorbtion bands for both the ligand82 6,6 and the 6054 counterions.52 The presence of absorption bands, with a generally medium intensity in the range of 3180–341050 cm−1, was specific for the stretching vibrations of the N–H function, confirming that the 48 ligand 46,6maintained its neutrality in all compounds. The weak absorption bands located in the range 4000,0 −1 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0 1579–15874000 cm 3600 were due 3200 to the 2800 so - called 2400 “breathing” 2000 cm-11800 of the 1600 neutral 1400 pyrazolyl 1200 rings 10 00[67] , which800 650 was Wavenumber (cm-1) significantly reduced compared to the free ligand, as a consequence of coordination, and in consideration of the influence manifested by the different anions. Figure 1. Fourier transform infrared spectroscopy (FTIR) spectrum of the free H DMPMB ligand. Figure 1. Fourier transform infrared spectroscopy (FTIR) spectrum of the free H22DMPMB ligand.

The infrared spectra of all silver(I) CPs were generally consistent with the proposed 1527,8 2923,5 1581,6 1052,6 formulations, and present3198,7 all of the required absorbtion 1499,0 bands for 1179,1 both the ligand904,6 and the 1210,5 1005,8 666,9 800,3 counterions. The presence of absorption bands, with a generally medium intensity in 823,4the range of 3180–3410 cm−1, was specific for the stretching vibrations of the N–H function, confirming that the ligand maintained its neutrality in all compounds. The weak absorption bands located in the range 1387,1 −1 1346,7 1579–1587 cm were due to the so-called “breathing” of the neutral pyrazolyl rings [67], which754,2 was significantly reduced compared to the free 1917,0 ligand, as a consequence of coordination, and in 3186,2 3091,7 2920,5 1578,7 1401,8 1293,5 consideration of the influen3028,1 ce manifested by the different anions1528,1 . 1377,5

1493,9 905,4 %T 1004,9 1057,5 776,7 1431,4 835,7 759,7 1667,7

797,8 1140,0 718,7

1527,8 1184,2 Transmittance(%) 2923,5 1581,6 1170,5 1052,6 805,8 3198,7 1499,0 1201,11179,1 904,6 1210,5 1005,8 666,9 800,3 823,4 3024,0 1586,0 1398,3 905,1 2923,0 1526,5 1437,8 797,2 3232,9 1498,4 1378,4 839,3 1005,6 706,2

1387,1 1346,7

759,0754,2

1917,0 1161,7 1028,6 3186,2 1224,0 3091,7 2920,5 1578,7 1401,8 1240,21293,5 4000,0 3600 3200 3028,1 2800 2400 2000 1800 1600 1528,1 14001377,5 1200 1000 800 650,0 4000 3600 3200 2800 2400 2000 cm-11800 1600 1400 1200 1000 800 650 1493,9 905,4 %T 1004,9 Wavenumber (cm-1) 1057,5 776,7 1431,4 835,7 759,7 1667,7

797,8 Figure 2. FTIR spectra of silver(I) CPs 1 (black), 2 (red), and 3 (blue).1140,0 718,7

Figure 2. FTIR spectra of silver(I) CPs 1 (black), 2 (red), and 3 1184,2(blue). Transmittance(%) 1170,5 805,8 1201,1

3024,0 1586,0 1398,3 905,1 2923,0 1526,5 1437,8 797,2 3232,9 1498,4 1378,4 839,3 1005,6 706,2

759,0

1161,7 1028,6 1224,0 1240,2 4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0 4000 3600 3200 2800 2400 2000 cm-11800 1600 1400 1200 1000 800 650 Wavenumber (cm-1) Figure 2. FTIR spectra of silver(I) CPs 1 (black), 2 (red), and 3 (blue).

Polymers 2019, 11, 1686 7 of 17 Polymers 2019, 11, x FOR PEER REVIEW 8 of 17

2923,3 1586,6 1525,7 1399,6 1286,6 3316,1 1494,2 1380,4 1185,3 903,9 799,0 1438,7

845,0

691,6 757,7

767,5 1057,1 1004,5

2050,7 1657,3 3024,8 1525,1 2923,4 1586,6 1400,1 1287,8 927,7 3267,6 1494,6 1381,3 904,8 1438,9 1185,3 799,6 1420,5 845,0 %T 758,0 686,9 1005,4

768,9

1074,5 1039,7 Transmittance(%)

3027,4 1655,3 1132,8 2926,1 1585,4 1400,0 1210,2 1524,7 1438,5 1291,9 1005,8 706,8 3410,0 1495,3 1381,1 1184,4 1417,1

739,2

666,5

767,7 828,4

4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0 4000 3600 3200 2800 2400 2000 cm-11800 1600 1400 1200 1000 800 650 Wavenumber (cm-1) Figure 3 3.. FTIRFTIR spectra spectra of of silver(I) silver(I) CPs 4 (black),(black), 5 (red)(red),, and 6 (blue).

InThe the infrared IR spectrum spectra of of 4 all, the silver(I) fine splitting CPs were of generallythe very strong consistent and withbroad the band proposed in the formulations,region 1000– 1100and presentcm−1, together all of the with required the medium absorbtion sharp bands band for bothat 768 the cm ligand−1 and and the the medium counterions. broad Theband presence at 692 1 cmof absorption−1, were assigned bands, to with the aweakly generally coordinat mediuming intensity tetrafluoroborate in the range group of 3180–3410 (Figure 7)cm [76]− ,, which was specific could befor involved the stretching in long vibrations interactions of the with N–H the function, silver(I) confirming ions to complete that the a ligand T-shaped maintained stereochemistry its neutrality, as 1 wasin all also compounds. observed in The the weak silve absorptionr(I) CP [Ag(H bands2BPZ)(BF located4)] [54] in the. The range IR spectrum 1579–1587 of cm5 show− wereed two due strong to the broadso-called bands “breathing” at 1040 and of the 1075 neutral cm−1 pyrazolyl, along with rings the [ 67medium], which sharp was significantly band at 769 reducedcm−1 and compared a medium to broadthe free band ligand, at 687 as acm consequence−1, which were of coordination, attributed to andthe incoordinated consideration perchlorate of the influence group (Figure manifested 8) [76 by], thusthe di completingfferent anions. a T-shaped stereochemistry of the silver(I) ions, as was also observed in the silver(I) CPs [Ag(HA deeper2BPZ)(ClO analysis4)] of[54 the] and infrared [Ag(Me spectra4BPZ)(ClO of all4)] synthesized[66]. Finally, silver(I) in the IR CPs spectrum was also of performed6, the very strongin order, broad to obtain absorption clues about band thelocate possibled at 828 interactions cm−1, together in which with the the medium anions are sharp involved band at within 768 cm the−1 andcorresponding the medium structures. broad bandAs such, at in667 the cm IR−1 spectrumcould be of assigned1, the asymmetric to the weakly stretching coordinating modes of 1 hexafluorophosphatethe nitrate anion were anions denounced, which by are the in slightly fact situated split bandsbetween at 1387the cationic and 1347 sheets cm− instead. Moreover, of being the 1 insingle, direct very interaction weak band, with almostthe silver(I) indistinguishable ions (Figure 9 from) [77]. noise, in the overtone region at 1748 cm− , suggested that the nitrate anion was in a non-coordinated form [68]. Such a suggestion was further strengthened by the absence of the band associated with symmetric stretching nitrate vibrations 1 1 near 1049 cm− [69]. The broad band present in the region 2700–3400 cm− was indicative of the hydrogen bond interactions between the nitrate anion and the N–H function of the ligand, spreading + along adjacent linear chains formed by the [Ag(H2DMPMB)] units. However, although the nitrate anion was not directly bound to the silver(I) ion, weak Ag-O interactions could be considered so that the silver(I) ion assumes a T-shaped stereochemistry (Figure4), as was already observed in the silver(I) CPs [Ag(H BPZ)(NO )] [54] and [Ag(Me BPZ)(NO )] CH OH [65], and in the silver(I) 2 3 4 3 · 3 complexes AgNO3:tz2(CH2):ER3:MeCN (tz2(CH2) = bis(1,2,4-triazol-1-yl)methane; E = P, As, Sb; MeCN = acetonitrile) [70]. Figure 4. Proposed polymeric structure of [Ag(H2DMPMB)(NO3)] (1), evidencing the T-shaped stereochemistry of silver(I) ions assured by the nitrate anions and the neutral ligands coordinated in a bridging exo-bidentate mode.

Polymers 2019, 11, x FOR PEER REVIEW 8 of 17

2923,3 1586,6 1525,7 1399,6 1286,6 3316,1 1494,2 1380,4 1185,3 903,9 799,0 1438,7

845,0

691,6 757,7

767,5 1057,1 1004,5

2050,7 1657,3 3024,8 1525,1 2923,4 1586,6 1400,1 1287,8 927,7 3267,6 1494,6 1381,3 904,8 1438,9 1185,3 799,6 1420,5 845,0 %T 758,0 686,9 1005,4

768,9

1074,5 1039,7 Transmittance(%)

3027,4 1655,3 1132,8 2926,1 1585,4 1400,0 1210,2 1524,7 1438,5 1291,9 1005,8 706,8 3410,0 1495,3 1381,1 1184,4 1417,1

739,2

666,5

767,7 828,4

4000,0 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 650,0 4000 3600 3200 2800 2400 2000 cm-11800 1600 1400 1200 1000 800 650 Wavenumber (cm-1) Figure 3. FTIR spectra of silver(I) CPs 4 (black), 5 (red), and 6 (blue).

In the IR spectrum of 4, the fine splitting of the very strong and broad band in the region 1000– 1100 cm−1, together with the medium sharp band at 768 cm−1 and the medium broad band at 692 cm−1, were assigned to the weakly coordinating tetrafluoroborate group (Figure 7) [76], which could be involved in long interactions with the silver(I) ions to complete a T-shaped stereochemistry, as was also observed in the silver(I) CP [Ag(H2BPZ)(BF4)] [54]. The IR spectrum of 5 showed two strong broad bands at 1040 and 1075 cm−1, along with the medium sharp band at 769 cm−1 and a medium broad band at 687 cm−1, which were attributed to the coordinated perchlorate group (Figure 8) [76], thus completing a T-shaped stereochemistry of the silver(I) ions, as was also observed in the silver(I) CPs [Ag(H2BPZ)(ClO4)] [54] and [Ag(Me4BPZ)(ClO4)] [66]. Finally, in the IR spectrum of 6, the very strong, broad absorption band located at 828 cm−1, together with the medium sharp band at 768 cm−1 and the medium broad band at 667 cm−1 could be assigned to the weakly coordinating

Polymershexafluorophosphate2019, 11, 1686 anions, which are in fact situated between the cationic sheets instead of 8being of 17 in direct interaction with the silver(I) ions (Figure 9) [77].

Figure 4. Proposed polymeric structure of [Ag(H DMPMB)(NO )] (1), evidencing the T-shaped Figure 4. Proposed polymeric structure of [Ag(H2 2DMPMB)(NO33)] (1), evidencing the T-shaped stereochemistrystereochemistry of of silver(I) silver(I) ions ions assured assured by by the the nitrate nitrate anions anions and and the the neutral neutral ligands ligands coordinated coordinated in ain bridging exo-bidentate mode. a bridging exo-bidentate mode. 1 1 The IR spectra of 2 showed a strong band at 1668 cm− and a medium one at 1431 cm− that were characteristic of the carboxylate asymmetric (νa) and symmetric stretching (νs), respectively. 1 The difference ∆ν = νa – νs = 237 cm− likely indicates a unidentate coordination of trifluoroacetate anion [71], thus conferring to the silver(I) ions a T-shaped stereochemistry, as was also observed in the silver(I) CP [Ag(Me4BPZ)(C2F5CO2)] [66] and the binaphthylbis(amidopyridyl)-based silver(I) CP 1 [Ag(C20H12{NHC(O)-3-C5H4N}2)(CF3CO2)] [72]. The broad band located in the region 2700–3400 cm− was also found in the IR spectrum of 2, which was indicative of the hydrogen bond interactions between the trifluoroacetate anion and the N–H function of the ligand (Figure5). Strong absorbtion 1 bands present in the range 1140–1201 cm− were assignable to different vibration modes of the C-F 1 bonds, while the sharp band at 719 cm− could be attributed to the C-O vibration [73]. The IR 1 spectra of 3 shows two very strong and broad absorption bands in the 1200–1300 cm− region, due to asymmetric and symmetric vibrations of the SO3 group from the unidentate trifluoromethanesulfonate 1 group, along with the strong and broad absorption at 1162 cm− due to the C-F vibrations, and a 1 sharp absorption at 1029 cm− , which was assignable to the S-O vibration [74,75]. The structure + of 3 would therefore be made up of linear chains formed by [Ag(H2DMPMB)] units, in which the silver(I) ions assume a T-shaped stereochemistry assured by the exo-bidentate ligands and the trifluoromethanesulfonate anions, as was also observed in the silver(I) CPs [Ag(H2BPZ)(CF3SO3)] [54] andPolymers [Ag(Me 2019, 411BPZ)(CF, x FOR PEER3SO REVIEW3)] [66] (Figure6). 9 of 17

FigureFigure 5.5.Proposed Proposed polymericpolymeric structurestructure of of [Ag(H [Ag(H2DMPMB)(CF2DMPMB)(CF33COCO22)] ((22),), evidencingevidencing thethe T-shapedT-shaped stereochemistrystereochemistry of of silver(I) silver(I) ions ions assured assured by by the the triﬂuoroacetate trifluoroacetate anions anions and and the the neutral neutral ligands ligands coordinatedcoordinated inin aa bridgingbridging exo-bidentateexo-bidentate mode.mode.

Figure 6. Proposed polymeric structure of [Ag(H2DMPMB)(CF3SO3)] (3), evidencing the T-shaped stereochemistry of silver(I) ions assured by the trifluoromethanesulfonate anions and the neutral ligands coordinated in a bridging exo-bidentate mode.

Figure 7. Proposed polymeric structure of [Ag(H2DMPMB)(BF4)] (4), evidencing the T-shaped stereochemistry of silver(I) ions assured by the tetrafluoroborate anions and the neutral ligands coordinated in a bridging exo-bidentate mode.

Polymers 2019, 11, x FOR PEER REVIEW 9 of 17

Figure 5. Proposed polymeric structure of [Ag(H2DMPMB)(CF3CO2)] (2), evidencing the T-shaped Polymersstereochemistry2019, 11, 1686 of silver(I) ions assured by the trifluoroacetate anions and the neutral ligands9 of 17 coordinated in a bridging exo-bidentate mode.

Polymers 2019, 11, x FOR PEER REVIEW 9 of 17 Polymers 2019, 11, x FOR PEER REVIEW 9 of 17

Figure 6. Proposed polymeric structure of [Ag(H DMPMB)(CF SO )] (3), evidencing the T-shaped Figure 6. Proposed polymeric structure of [Ag(H2 2DMPMB)(CF33SO33)] (3), evidencing the T-shaped stereochemistrystereochemistryFigure 5. Proposed of of silver(I) silver(I)polymeric ions ions structure assuredassured of by by [Ag(H the the trifluoromethanesulfonatetrifluoromethanesulfonate2DMPMB)(CF3CO2)] (2), evidencing anions anions and and the the the T neutral-shapedneutral Figure 5. Proposed polymeric structure of [Ag(H2DMPMB)(CF3CO2)] (2), evidencing the T-shaped ligandsstereochemistry coordinated of in silver(I) a bridging ions exo-bidentate assured by the mode. trifluoroacetate anions and the neutral ligands ligandsstereochemistry coordinated of silver(I)in a bridging ions exo assured-bidentate by the mode. trifluoroacetate anions and the neutral ligands coordinated in a bridging exo-bidentate mode. Incoordinated the IR spectrum in a bridging of 4 exo, the-bidentate fine splitting mode. of the very strong and broad band in the region 1 1 1000–1100 cm− , together with the medium sharp band at 768 cm− and the medium broad band at 1 692 cm− , were assigned to the weakly coordinating tetrafluoroborate group (Figure7)[ 76], which could be involved in long interactions with the silver(I) ions to complete a T-shaped stereochemistry, as was also observed in the silver(I) CP [Ag(H2BPZ)(BF4)] [54]. The IR spectrum of 5 showed two strong broad 1 1 bands at 1040 and 1075 cm− , along with the medium sharp band at 769 cm− and a medium broad 1 band at 687 cm− , which were attributed to the coordinated perchlorate group (Figure8)[ 76], thus completing a T-shaped stereochemistry of the silver(I) ions, as was also observed in the silver(I) CPs

[Ag(H2BPZ)(ClO4)] [54] and [Ag(Me4BPZ)(ClO4)] [66]. Finally, in the IR spectrum of 6, the very strong, Figure 7. Proposed polymeric structure1 of [Ag(H2DMPMB)(BF4)] (4), evidencing the T-shaped1 broad absorption band located at 828 cm− , together with the medium sharp band at 768 cm− and the stereochemistry of silver(I) ions assured by the tetrafluoroborate anions and the neutral ligands mediumFigure broad 6. Proposed band at 667polymeric cm 1 couldstructure be assigned of [Ag(H2 toDMPMB)( the weaklyCF3SO coordinating3)] (3), evidencing hexaﬂuorophosphate the T-shaped Figure 6. Proposed polymeric− structure of [Ag(H2DMPMB)(CF3SO3)] (3), evidencing the T-shaped stereochemistrycoordinated in a ofbridging silver(I) exo ions-bidentate assured mode. by the trifluoromethanesulfonate anions and the neutral anions,stereochemistry which are in fact of silver(I) situated ions between assured the by cationic the trifluoromethanesulfonate sheets instead of being anions in direct and interaction the neutral with ligands coordinated in a bridging exo-bidentate mode. the silver(I)ligands ionscoordinated (Figure 9in)[ a77 bridging]. exo-bidentate mode.

Figure 8. Proposed polymeric structure of [Ag(H2DMPMB)(ClO4)] (5), evidencing the T-shaped stereochemistry of silver(I) ions assured by the perchlorate anions and the neutral ligands FigureFigure 7. 7. ProposedProposed polymeric polymeric structure structure of of [Ag(H[Ag(H2DMPMB)(BFDMPMB)(BF4)] ( (44),), evidencing evidencing the the T-shaped T-shaped Figure 7. Proposed polymeric structure of [Ag(H22DMPMB)(BF44)] (4), evidencing the T-shaped stereochemistrycoordinated in a bridging of silver(I) exo ions-bidentate assured mode. by the tetrafluoroborate anions and the neutral ligands stereochemistrystereochemistry of of silver(I) silver(I) ions ions assured assured by by the the tetraﬂuoroborate tetrafluoroborate anions anions and and the the neutral neutral ligands ligands coordinated in a bridging exo-bidentate mode. coordinatedcoordinated in in a a bridging bridging exo-bidentate exo-bidentate mode. mode.

Figure 8. Proposed polymeric structure of [Ag(H DMPMB)(ClO )] (5), evidencing the T-shaped Figure 8. Proposed polymeric structure of [Ag(H22DMPMB)(ClO44)] (5), evidencing the T-shaped Figure 8. Proposed polymeric structure of [Ag(H2DMPMB)(ClO4)] (5), evidencing the T-shaped stereochemistrystereochemistry of of silver(I) silver(I) ions ions assured assured by the by perchlorate the perchlorate anions and anions the neutral and the ligands neutral coordinated ligands stereochemistry of silver(I) ions assured by the perchlorate anions and the neutral ligands incoordinated a bridging in exo-bidentate a bridging exo mode.-bidentate mode. coordinated in a bridging exo-bidentate mode.

Polymers 2019, 11, 1686 10 of 17 Polymers 2019, 11, x FOR PEER REVIEW 10 of 17 Polymers 2019, 11, x FOR PEER REVIEW 10 of 17

Figure 9. 9. ProposedProposed polymeric polymeric structure of [Ag(H2DMPMB)(DMPMB)(PFPF6)])] ( (66)),, evidencing evidencing the digonal digonal Figure 9. Proposed polymeric structure of [Ag(H22DMPMB)(PF66)] (6), evidencing the digonal stereochemistry of silver(I) ions assured by the neutral ligands coordinated in a bridging stereochemistrystereochemistry of of silver(I) silver(I) ions ions assured assured by by thethe neutralneutral ligands ligands coordinated coordinated in in a a bridging bridging exo-bidentateexo-bidentate mode. exo-bidentate mode. 3.2. Thermogravimetric A Analysisnalysis 3.2. Thermogravimetric Analysis The thermogravimetricthermogravimetric analyses analyses (TGA) (TGA) of ofsilver(I) silver(I) polymeric polymeric species species1–6 were1–6 were performed performed in order in The thermogravimetric analyses (TGA) of silver(I) polymeric species 1–6 were performed in toorder investigate to investigate their thermal their thermal behavior behavior upon heating upon heating from 30 from to 700 30◦C, to under700 °C, nitrogen, under nitrogen, and the resulting and the order to investigate their thermal behavior upon heating from 30 to 700 °C, under nitrogen, and the TGAresulting curves TGA were curves collectively were collectively gathered gathered in Figure in10 Figure. Compound 10. Compound1 showed 1 anshow appreciableed an appreciable thermal resulting TGA curves were collectively gathered in Figure 10. Compound 1 showed an appreciable stability,thermal stability, resistant resist up toant 275 up◦C, to after275 °C, which after a which weight a lossweight of about loss of 12% about in the12% range in the 275–325 range 275◦C– was325 thermal stability, resistant up to 275 °C, after which a weight loss of about 12% in the range 275–325 recorded,°C was recorded corresponding, correspond to theing removal to the ofremoval one nitric of one acid nitric molecule, acid molecule which was, which the result was ofthe the result proton of °C was recorded, corresponding to the removal of one nitric acid molecule, which was the result of transferthe proton from transfer one pyrazolyl from one moiety pyrazolyl (theoretical moiety weight(theoretical loss: 11.66%).weight loss: This 11.66%). loss was This immediately loss was the proton transfer from one pyrazolyl moiety (theoretical weight loss: 11.66%). This loss was followedimmediately by a followed progressive by a decomposition progressive decomposition of the remaining of the species. remaining species. immediately followed by a progressive decomposition of the remaining species.

100 100

90 90

80 80

70 70

Weight (%) Weight 60 Weight (%) Weight 60

50 50

40 40

30 3300 100 200 300 400 500 600 700 30 100 200 300 400 500 600 700 Temperature ( C) Temperature ( C) Figure 10. TThermogravimetrichermogravimetric analysis analysis ( (TGA)TGA) curves curves of of silver(I) silver(I) CPs CPs 1 1(green),(green), 2 2(black),(black), 3 (fuchsia),3 (fuchsia), 4 Figure 10. Thermogravimetric analysis (TGA) curves of silver(I) CPs 1 (green), 2 (black), 3 (fuchsia), 4 4(red),(red), 5 5(blue)(blue),, and and 6 6(purple).(purple). (red), 5 (blue), and 6 (purple). CompoundCompound 2 was thermally stable up to 250 °C, which then underwent a weight loss of about Compound 2 was thermally stable up to 250 ◦°C, which then underwent a weight loss of about 20%20% until 300 °C,C, which is i inn a good agreement with the e expectedxpected theoretical weight loss of 19.28 19.28%,%, 20% until 300 ◦°C, which is in a good agreement with the expected theoretical weight loss of 19.28%, corresponding to to the the evolution evolution of of one one trifluoroacetic trifluoroacetic acid acid molecule, molecule which, which was the was result the ofresult the protonof the corresponding to the evolution of one trifluoroacetic acid molecule, which was the result of the transferproton transferfrom the fromorganic the ligand. organic This ligand. event wasThis further event continued was further by a continuedprogressive by decomposition a progressive of proton transfer from the organic ligand. This event was further continued by a progressive thedecomposition remaining species. of the remaining species. decomposition of the remaining species. Compounds 3 and 5 displayeddisplayed the highest thermal stabilities, which were resist resistantant up to 350 Compounds 3 and 5 displayed the highest thermal stabilities, which were resistant up to 350 and 325 °C,C, respectively, overcomingovercoming thethe values reported for, e.g.,e.g., thethe 3-D3-D supramolecularsupramolecular isomer and 325 ◦°C, respectively, overcoming the values reported for, e.g., the 3-D supramolecular isomer α--[Ag[Ag2(Me(Me4BPZ)]BPZ)] [ 78]78] and thethe neutralneutral CP CP [Ag [Ag2(DMPMB)]2(DMPMB)] [ 60[60],] which, which were were both both resistant resistant up up to 300to 300◦C, α-[Ag2(Me4BPZ)] [78] and the neutral CP [Ag2(DMPMB)] [60], which were both resistant up to 300 or°C, the or 1-Dthe CP1-D [Ag(pz)] CP [Ag(pz)] (pz = (pzpyrazole), = pyrazole), which which was stable was stable up to 270up toC[ 27079 ].°C Such [79] remarkable. Such remarkable results °C, or the 1-D CP [Ag(pz)] (pz = pyrazole), which was stable up ◦to 270 °C [79]. Such remarkable mayresults be ascribedmay tobe the ascribed stronger electron-withdrawing to the stronger eff electronect of both-withdrawing trifluoromethanesulfonate effect of both and results may be ascribed to the stronger electron-withdrawing effect of both trifluoromethanesulfonate and perchlorate anions, imparting more inertness to the Ag‒N trifluoromethanesulfonate and perchlorate anions, imparting more inertness to the Ag‒N

Polymers 2019, 11, 1686 11 of 17 perchlorate anions, imparting more inertness to the Ag-N coordinative bonds, and thus to the overall polymeric structures. Compounds 4 and 6 showed the lowest thermal stability, which, although still signiﬁcant, peaked up to 250 ◦C, followed then by a progressive decomposition. In all cases, at the end of the heating process, black residues, possibly containing a mixture of carbonaceous species and metallic silver/silver(I) oxide, were recovered. Other than the eﬀect of the anions upon the thermal robustness of the obtained silver(I) polymeric species, the dependency of such property on the ligand itself should not be neglected.

3.3. Antibacterial Activity The assessment of antibacterial activity of all prepared silver(I) CPs against Gram-negative bacteria E. coli and Gram-positive bacteria S. aureus was performed by using the Tetrazolium/Formazan test (TTC) as a qualitative assay, by measuring the bacterial viability at different time intervals (2, 4, 6, and 24 h). The tetrazolium salts are known for their ability to form, within the bacterial culture medium, highly colored products called formazans [80]. From the point of view of the mechanism of antibacterial action, it was shown that, from within the silver-based topical antibacterial agents, the silver(I) ions are those that manifest the bactericidal effects. More exactly, the silver(I) ions prevented the performance of the function of bacterial cells by direct interaction with the bacterial enzymes and DNA, thus interrupting the cell division and replication [81]. On the other hand, the silver(I) ions are involved in the interaction with the negatively charged peptidoglycan from the bacterial cell wall, thus affecting the cell breathing, which finally leads to cell death through the release, into the surrounding environment, their fluid and electrolyte content [82,83]. Figure 11 shows the bacterial viability of the two bacterial strains, S. aureus and E. coli, in the presence of silver(I) CPs 1–6, and silver(I) nitrate (AgNO3) used as a standard antibacterial agent, after 2, 4, and 6 h. The TTC test indicated, as expected in the presence of an antibacterial agent, the bacterial reduction in time with respect to the control (untreated cells). In the case of S. aureus, after 2 h, the values of bacterial viability significantly decreased to 8–19%, and after 6 h the values went down to 1–4%. A markedly different behavior was appreciated against E. coli, that is, all six silver(I) CPs were less effective in the first period of exposure with a slower action rate compared to S. aureus. As such, after 2 h, the values of bacterial viability decreased to 37% (1), 60–64% (2, 4, and 6), and 79–81% (3 and 5). The decreasing trend was maintained after 4 h, and after 6 h, the values decreased to 2% (1), 21–22% (2, 4, and 6), 37% (3), and 42% (5). After 24 h of exposure, a complete bacterial reduction was observed. However, no neat influence of the counter-anions upon the bactericidal effect of the related silver(I) CPs could also be noticed. For the standard AgNO3, the bactericidal effect against S. aureus was strong during the first 2 h of exposure (8%), while the bacterial reduction was complete after only 6 h, compared to E. coli, where the bacterial reduction was only 62% after 2 h and 5% after 6 h of exposure. Thus, a difference in the rate of action as a function of the bacteria types could be appreciated. It can be observed that the bactericidal effect of the standard AgNO3 was comparable with that of our silver(I) CPs. However, the high insolubility of these CPs would bring the advantage of applying them for much longer time intervals, compared to the highly soluble AgNO3 (2160 g/L at 20 ◦C), which underwent faster silver(I) ions release into the environment, thus leading to an increased level of toxicity [52–54]. Polymers 2019, 11, 1686 12 of 17

Polymers 2019, 11, x FOR PEER REVIEW 12 of 17

Bacterial viability after 2 h [%] 100 80 60 40 20 0

E. coli

Bacterial viability after 4 h [%] 100 80 60 40 20 0

E. coli S. aureus

Bacterial viability after 6 h [%] 100 80 60 40 20 0

E. coli S. aureus

Figure 11. BacterialBacterial viability viability of S. of aureusS. aureus and andE. coliE. in coli the inpresence the presence of silver(I) of silver(I)CPs 1–6 CPsand standard1–6 and AgNO3. standard AgNO3.

4.4. Conclusions

Coupling the the flexible flexible ditopic ditopic ligandligand 4,4 4,4’0-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl-bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H(H2DMPMB) to to different different silver(I) silver(I) salts salts afforded afforded the the preparation preparation of of six sixnew new,, light light insensitive, insensitive, air airstable stable,, and and highly highly insoluble insoluble silver(I) silver(I) coordination coordination polymers polymers (CPs) (CPs) with with t thehe gegeneralneral formula formula [Ag(H2DMPMB)(X)]DMPMB)(X)] (X (X = NO33,, 1;; CF3COCO22, ,22;; CF3SOSO33, ,33;; BF BF44, ,44;; ClO44,, 5;; and PFPF6,, 6).). The obtainedobtained silver(I) CPs CPs demonstrated demonstrated important important antibacterial antibacterial performances performances against againstE. coli E.and coliS. and aureus S. ,aureus for which, for awhich complete a complete reduction reduction of both of bacterial both bacterial strains occurredstrains occurred after 24 after h of exposure24 h of exposure to all silver(I) to all CPs,silver(I) the bacterialCPs, the bacterial viability valueviability for valueS. aureus for reachedS. aureus 8%reach fored compounds 8% for compounds3, 5, and 63,after 5, and only 6 after 2 h. Theonly high 2 h. The high insolubility displayed by the new silver(I) CPs presents the advantage, if clinical applications are sought, of reapplying them for much longer time intervals, compared to the highly soluble silver(I) nitrate used as a standard antibacterial agent. The results thus highlight the quality

Polymers 2019, 11, 1686 13 of 17 insolubility displayed by the new silver(I) CPs presents the advantage, if clinical applications are sought, of reapplying them for much longer time intervals, compared to the highly soluble silver(I) nitrate used as a standard antibacterial agent. The results thus highlight the quality of the new silver(I) CPs as interesting platforms to explore new antibacterial agents. The antibacterial properties proven by our silver(I) CPs, on the inhibition of antibiotic-resistant strains, such as E. coli and S. aureus, are promising tools that we hope oﬀer a more powerful solution to the worldwide problem that has become a worrying issue: The multiplication of highly multidrug-resistant bacteria in clinical medicine. Future studies will be devoted to the application of these silver(I) CPs into antimicrobial cellulose papers for food packaging protection.

Author Contributions: Conceptualization, A.T., C.P. and R.M.D.; methodology, A.T., C.P., M.B. and R.M.D.; investigation, A.T., M.B. and R.M.D.; resources, C.P. and R.M.D.; writing—original draft preparation, A.T. and M.B.; writing—review and editing, A.T., C.P. and R.M.D.; visualization, C.P. and R.M.D.; supervision, C.P. and R.M.D.; project administration, A.T. and R.M.D.; funding acquisition, A.T and R.M.D. Funding: This work was supported by a grant of Ministery of Research and Innovation, CNCS—UEFISCDI, project number PN-III-P1-1.1-PD-2016-0409, within PNCDI III. Acknowledgments: University of Camerino (UNICAM) is gratefully acknowledged for the assistance with the necessary scientific instruments. Conflicts of Interest: The authors declare no conflict of interest.

References

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