Metal Complexes of Oxadiazole Ligands

International Journal of Molecular Sciences Review Review Metal Complexes of Oxadiazole Ligands: An Overview Giovanni Salassa 1,* and Alessio Terenzi 2,* Giovanni Salassa 1,* and Alessio Terenzi 2,* 1 Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1 Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, 1211 Geneva 4, 1211 Geneva 4, Switzerland Switzerland 2 Donostia International Physics Center, Paseo Manuel de Lardizabal 4, 20018 Donostia, Spain 2 Donostia International Physics Center, Paseo Manuel de Lardizabal 4, Donostia 20018, Spain * Correspondence: [email protected] (G.S.); [email protected] (A.T.) * Correspondence: [email protected] (G.S.); [email protected] (A.T.) Received: 30 June 2019; Accepted: 13 13 July July 2019; 2019; Published: Published: date 16 July 2019 Abstract: OxadizolesOxadizoles are are heterocyclic heterocyclic ring ring systems systems that that find find application in different different scientificscientific disciplines, from medicinal chemistry to optoelectronics.optoelectronics. Coordination with metals (especially the transition ones)ones) proved proved to to enhance enhance the the intrinsic intrinsic characteristics characteristics of these of these organic organic ligands ligands and many and many metal metalcomplexes complexes of oxadiazoles of oxadiazoles showed showed attractive attractive characteristics characteristics for different for research different fields. researchIn this fields.review, In wethisprovide review, awe general provide overview a general on overview different on metal different complexes metaland complexes polymers and containing polymers oxadiazolecontaining oxadiazolemoieties, reporting moieties, the reporting principal the synthetic principal approaches synthetic adopted approaches for their adopted preparation for their and showingpreparation the varietyand showing of applications the variety they of applications found in the they last 40found years. in the last 40 years. Keywords: oxadizole; 1,2,4-oxadizole; 1,3,4-oxadizole; metal complexes 1. Introduction Introduction Oxadiazoles are are an an interesting interesting class class of offive-membered five-membered heterocyclic heterocyclic compounds compounds containing containing two atomstwo atoms of nitrogen of nitrogen and one and atom one of atom oxygen. of oxygen. They exist They in existfour different in four di regioisomericfferent regioisomeric forms, namely forms, 1,2,3-,namely 1,2,4-, 1,2,3-, 1,2,5-, 1,2,4-, 1,2,5-,and 1,3,4-oxadiazoles and 1,3,4-oxadiazoles (Figure (Figure 1). 1,2,4-1). 1,2,4- and and 1,3,4-isomers 1,3,4-isomers are are by by far moremore represented inin literature, literature, while while the th 1,2,5-isomer,e 1,2,5-isomer, which which displays displays a di ffaerent different orientation orientation of the of side the chains side (chainsR1 and (R R12 and), is significantlyR2), is significantly less reported less reported [1]. 1,2,3-Oxadizole [1]. 1,2,3-Oxadizole cycles are cycles instead are quite instead unstable quite andunstable these andcompounds these compounds are usually are observed usually in observed their diazoketone in their diazoketone tautomeric formtautomeric and are form relatively and are complicated relatively complicatedto synthesize to [2 synthesize]. It is interesting [2]. It is to interesting note that bothto note 1,2,4- that and both 1,3,4-oxadiazoles 1,2,4- and 1,3,4-oxadiazoles satisfy the Hückel satisfy rule, the Hückelmaking rule, them making aromatic them molecules. aromatic Nevertheless, molecules. 1,2,4-isomersNevertheless, are1,2,4-isomers the least aromatic are the five-memberedleast aromatic five-memberedheterocyclic systems heterocyclic and they systems are better and describedthey are better as a conjugated described dienes,as a conjugated while 1,3,4-oxadiazole dienes, while 1,3,4-oxadiazolederivatives show derivatives far greater show aromaticity far greater [2,3]. aromaticity [2,3]. Figure 1. StructuresStructures of the differ differentent oxadiazole isomers. In the past 40 years, oxadiazole-based ligands stimulated the curiosity of many researchers, In the past 40 years, oxadiazole-based ligands stimulated the curiosity of many researchers, creating a vast literature that spans from synthesis to different applications. In fact, oxadiazoles exhibit creating a vast literature that spans from synthesis to different applications. In fact, oxadiazoles a broad range of uses: in medicinal chemistry they were employed as drug candidates for several exhibit a broad range of uses: in medicinal chemistry they were employed as drug candidates for diseases, in organic synthesis as useful intermediates, and in material science as building blocks for new several diseases, in organic synthesis as useful intermediates, and in material science as building polymers. A detailed discussion on the synthetic strategies and applications concerning oxadiazoles is blocks for new polymers. A detailed discussion on the synthetic strategies and applications out of the scope of this article and different reviews covering these topics are available elsewhere [1–8]. concerning oxadiazoles is out of the scope of this article and different reviews covering these topics are available elsewhere [1–8]. At the same time, considering the importance of coordination Int. J. Mol. Sci. 2019, 20, 3483; doi:10.3390/ijms20143483 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, x; doi: FOR PEER REVIEW www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, 3483 2 of 17 AtInt. theJ. Mol. same Sci. 2019 time,, 20,considering x FOR PEER REVIEW the importance of coordination chemistry in medicinal and material2 of 17 sciences, is somehow surprising to find out that an overview on the interaction between oxadiazole ligandschemistry and in metal medicinal ions is and missing, material at least sciences, to the is best some ofhow our knowledge.surprising to The find scope out that of this an review overview article on isthe to interaction summarize between some of oxadiazole the most important ligands and achievements metal ions in is the missing, development at least of to metal the best complexes of our containingknowledge. oxadiazole The scope ligands. of this For review the ease article of the is reader, to summarize this contribution some of is dividedthe most in fourimportant main sections.achievementsIn the in firstthe development one, we discuss of metal the use complexes of oxadiazole-based containing oxadiazole scaffolds ligands. for the construction For the ease of inorganicthe reader, compounds, this contribution either is coordination divided in four polymers main orsections. mononuclear In the first complexes. one, we Indiscuss the second the use part of, weoxadiazole-based highlight those scaffolds metal complexes for the containingconstruction oxadiazole of inorganic ligands compounds, with biological either activity. coordination In the thirdpolymers part, or we mononuclear show several complexes. examples ofIn oxadiazole-basedthe second part, materialswe highlight that havethose beenmetal employed complexes in optoelectronics.containing oxadiazole Finally, ligands we describe with biological how oxadiazole activity. In ligands the third were part, employed we show in several the preparation examples of severaloxadiazole-based metal ion sensors.materials that have been employed in optoelectronics. Finally, we describe how oxadiazole ligands were employed in the preparation of several metal ion sensors. 2. Supramolecular Assemblies and Synthetic Aspects 2. Supramolecular Assemblies and Synthetic Aspects Notwithstanding that oxadiazole rings have been used for many years in classical organic chemistry leadingNotwithstanding to molecules withthat severaloxadiazole applications, rings have their been use used as sca forff oldsmany for years inorganic in classical compounds organic is relativelychemistry recent.leading The to groupmolecules of Miao with Du, several in the applications, early 20000s, their was basicallyuse as scaffolds the first for to establishinorganic a successfulcompounds research is relatively line based recent. on The the group synthesis of Miao and characterizationDu, in the early of2000 coordination′s, was basically polymers the first where to pyridyl-substitutedestablish a successful 1,3,4-oxadiazole research line were based used on as the the synthesis main building and blockscharacterization [9–16]. The of idea coordination of Du and colleaguespolymers where was to pyridyl-subs coordinate 1,3,4-oxadiazoletituted 1,3,4-oxadiazole ligands to were different used metals as the in main order building to control blocks the design [9–16]. of multidimensionalThe idea of Du and infinite colleagues architectures was to coordinate with potential 1,3,4-oxadiazole interest for new ligands materials. to different Of course, metals prediction in order ofto acontrol metal–organic the design framework of multidimensional topology is not infini trivial,te butarchitectures Du and collaborators with potential worked interest extensively for new on thesematerials. systems, Of course, providing prediction a good repertoireof a metal–organic of structures framework varying the topology oxadiazole is not substituents, trivial, but the Du metal and ionscollaborators involved, worked and the correspondingextensively on counterions. these systems, providing a good repertoire of structures varyingThe the first oxadiazole system was substituents, developed the using metal Cu(II) ions and involved, the ligand and 2,5-bis(4-pyridyl)-1,3,4-oxadiazolethe

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