polymers 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 [ 26photolu,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 the silver 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 thethe lastlast decade,decade, silver(I)-basedsilver(I)-based CPs,CPs, builtbuilt upup eithereither withwith rigidrigid oror flexibleflexible 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 ofproblem 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 = PPh1,2,3,43 =-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 andof Gram Gram-positive-negative and pathogenic 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 1pyrazolyl)[ 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