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Electrochemical Comparison on New (Z)-5-(Azulen-1-Ylmethylene)-2-Thioxo-Thiazolidin-4-Ones

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Electrochemical Comparison on New (Z)-5-(Azulen-1-Ylmethylene)-2-Thioxo-Thiazolidin-4-Ones S S symmetry Article Electrochemical Comparison on New (Z)-5-(Azulen-1-Ylmethylene)-2-Thioxo-Thiazolidin-4-Ones Eleonora-Mihaela Ungureanu 1, Mariana Popescu (Apostoiu) 1, Georgiana-Luiza Tatu (Arnold) 1, Liviu Birzan 2, Raluca Isopescu 1, Gabriela Stanciu 3 and George-Octavian Buica 1,* 1 Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Gh. Polizu Street, 011061 Bucharest, Romania; [email protected] (E.-M.U.); [email protected] (M.P.); [email protected] (G.-L.T.); [email protected] (R.I.) 2 Institut of Organic Chemistry “C. D. Nenitzescu” of the Romanian Academy, 202B Spl. Independentei, 71141 Bucharest, Romania; [email protected] 3 Department of Chemistry and Chemical Engineering, Ovidius University, 124 Mamaia Blvd., 900527 Constantza, Romania; [email protected] * Correspondence: [email protected]; Tel.: +40-21-402-3930 Abstract: Three (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones are electrochemically char- acterized by cyclic voltammetry, differential pulse voltammetry, and rotating disk electrode voltam- metry. The electrochemical investigations revealed that the redox potential is influenced by the number and position of the alkyl groups, and the possible oxidation mechanism is proposed. These compounds, after their immobilization on glassy carbon electrodes during oxidative electropolymer- ization, were examined as complexing ligands for heavy metal ions from aqueous solutions through Citation: Ungureanu, E.-M.; adsorptive stripping voltammetry. Popescu (Apostoiu), M.; Tatu (Arnold), G.-L.; Birzan, L.; Keywords: (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones; electrochemical characteriza- Isopescu, R.; Stanciu, G.; Buica, G.-O. tion; modified electrodes; metal detection Electrochemical Comparison on New (Z)-5-(Azulen-1-Ylmethylene)-2- Thioxo-Thiazolidin-4-Ones. Symmetry 2021, 13, 588. https://doi.org/ 1. Introduction 10.3390/sym13040588 Azulene is an isomer of naphthalene, but they are essentially different. While naphtha- Academic Editor: Edwin lene has biaxial symmetry, azulene has single symmetry about the x-axis, which gives the Charles Constable azulene system an asymmetric charge distribution. This causes azulene to present impor- tant properties, such as a significant dipole moment and remarkable chemical, electronic Received: 10 March 2021 and optical behavior. As a result, such a system can have many technical applications Accepted: 29 March 2021 in optoelectronics [1,2], medicine [3], Li–S batteries [4], supercapacitors [5], photovoltaic Published: 2 April 2021 cells [6], sensing and imaging [7], etc. The ability of the electrons to move easily from the seven-member cycle to the five-member cycle [8] allows the azulene molecule to act as a Publisher’s Note: MDPI stays neutral donor and acceptor of electrons [9]. For the same reason, it is also possible to use azulenes with regard to jurisdictional claims in as building blocks for the development of new molecules [10–12]. Azulene can polymerize published maps and institutional affil- via electrochemistry resulting in polymers with high electrical conductivity [13]. If they are iations. properly substituted, they can complex specific targets. Moreover, if they are immobilized on an electrode surface, it could lead to materials that are able to enhance sensitivity and selectivity towards metal ions [14,15]. In line with our previous work [10–12,14,15], the results given in this paper are related Copyright: © 2021 by the authors. to the electrochemical characterization of new azulene compounds and highlight their Licensee MDPI, Basel, Switzerland. complexing properties towards metal ions [16]. The studied compounds are derivatives of This article is an open access article (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-one (L1). Its structure contains the two distributed under the terms and essential elements of molecular recognition: a complexing part (2-thioxo-thiazolidin-4-one conditions of the Creative Commons ring) [17] and a polymerizable part—azulene moiety. Attribution (CC BY) license (https:// The 2-thioxo-thiazolidin-4-one (rhodanine) is a very important heterocyclic compound creativecommons.org/licenses/by/ used to synthesize different compounds because of its biological activity. The applications 4.0/). Symmetry 2021, 13, 588. https://doi.org/10.3390/sym13040588 https://www.mdpi.com/journal/symmetry Symmetry 2021, 13, x FOR PEER REVIEW 2 of 12 The 2-thioxo-thiazolidin-4-one (rhodanine) is a very important heterocyclic com- Symmetry 2021, 13, 588 2 of 12 pound used to synthesize different compounds because of its biological activity. The ap- plications of its antiviral, anticancer, and antimicrobial activity were recently reviewed of[18]. its It antiviral, is also known anticancer, for its and complexing antimicrobial prop activityerties towards were recently heavy metals reviewed [17]. [18 The]. It azulene is also knownderivative for itsof rhodanine complexing can properties be used towardsfor accurate heavy metal metals determination [17]. The azulene in the derivativesame way ofthat rhodanine the p-dimethylaminophenylidenrhodanine can be used for accurate metal determination was used to accurately in the same determine way that Cu, the Ni, p- dimethylaminophenylidenrhodanineFe, and Zn [19], or the way in which wastriarylamine used to accurately rhodanine determine derivatives Cu, were Ni, Fe,used and as Zncolorimetric [19], or the sensors way in to which detect triarylamine Ag and Hg rhodanineions [20]. derivatives were used as colorimetric sensorsElectrochemical to detect Ag and polymerization Hg ions [20]. of the ligand on the electrode results in a high den- sity ofElectrochemical complexing groups, polymerization which allows of the accu ligandmulation on the electrodeof metal resultsions on in its a highsurface density [21– of24]. complexing groups, which allows accumulation of metal ions on its surface [21–24]. TheThe threethree (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones(Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones (Figure 11)) were were comparedcompared byby thethe curvescurves obtainedobtained throughthrough electrochemicalelectrochemical methodsmethods (cyclic(cyclic voltammetryvoltammetry (CV),(CV), differential pulse pulse volta voltammetrymmetry (DPV), (DPV), and and rotating rotating disk disk electrode electrode (RDE) (RDE) voltamme- voltam- metry).try). This This comparison comparison allowed allowed the the establishm establishmentent of of the the main main characteristics for theirtheir polymerizationpolymerization processes. Then, they were used as ligands for the preparation of com- plexingplexing modifiedmodified electrodes.electrodes. The The obtained obtained modified modified electrodes electrodes were were compared compared in termsin terms of heavyof heavy metal metal sensing sensin propertiesg properties [25 [25].]. FigureFigure 1.1. Structure ofof (Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones;(Z)-5-(azulen-1-ylmethylene)-2-thioxo-thiazolidin-4-ones; R: R: H H ( L1(L1);); 4,6,8-Me 4,6,8- 3 (MeL2);3 ( 3,8-MeL2); 3,8-Me2-5-iPr2-5-iPr (L3). (L3). 2.2. Materials and Methods TheThe azulene derivatives derivatives were were synthesized synthesized according according to the to thepreviously previously published published pro- procedurecedure [26]. [26 The]. The electrochemical electrochemical experiments experiments were were performed performed either either in non-aqueous in non-aqueous sup- supportingporting electrolyte electrolyte (0.1 (0.1 M) M) (o (obtainedbtained from from acetonitrile acetonitrile (Sigma (Sigma Aldrich, Aldrich, electronic gradegrade 99.999%)99.999%) andand tetra-n-butylammoniumtetra-n-butylammonium perchlorateperchlorate (TBAP)(TBAP) (Fluka,(Fluka, puriss,puriss, electrochemicalelectrochemical gradegrade >>99%)) 99%)) or or in in 0.1 0.1 M acetate buffer solution (obtained from sodium acetateacetate (Roth,(Roth, 99.99%)99.99%) andand acetic acetic acid acid (Fluka, (Fluka, >99.0%, >99.0%, trace trace select)). select)). The The ionic ionic metal metal solution solution was obtainedwas ob- fromtained the from corresponding the corresponding metal salts. metal Purified salts. Purified water (18.2 water M (18.2Wcm), M wasΩcm), obtained was obtained from a from Mil- liporea Millipore Direct-Q Direct-Q 3UV water3UV water purification purification system system and used and to used prepare to prepare the aqueous the aqueous solutions. so- lutions.A PGSTAT 12 AUTOLAB potentiostat (Metrohm/Eco Chemie) was used to perform the electrochemicalA PGSTAT 12 experimentsAUTOLAB potentiostat by connecting (Metrohm/Eco it to a classical Chemie) 3-electrode was cell. used The to workingperform electrodethe electrochemical was a glassy experiments carbon disk by (3 connecting mm, from it Metrohm), to a classical and 3-electrode the counter cell. electrode The work- was aing Pt electrode wire. Ag/10 was mMa glassy AgNO carbon3 in 0.1 disk M (3 TBAP, mm, CHfrom3CN Metrohm), and Ag/AgCl, and the 3M counter KCl referenceelectrode electrodeswas a Pt wire. were Ag/10 used mM for experimentsAgNO3 in 0.1 performed M TBAP, CH in non-aqueous3CN and Ag/AgCl, and aqueous 3M KCl solutions, reference respectively.electrodes were For used the for electrochemical experiments perfor studiesmed performed in non-aqueous in the organic and aqueous solvent, solutions, all the potentialsrespectively. were For reported the electrochemical from the potential studies ofperformed the ferrocene/ferrocenium in the organic
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