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Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments

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Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments inorganics Review Self-Assembly in Polyoxometalate and Metal Coordination-Based Systems: Synthetic Approaches and Developments Stamatis Passadis 1, Themistoklis A. Kabanos 1,*, Yu-Fei Song 2,* and Haralampos N. Miras 3,* ID 1 Section of Inorganic and Analytical Chemistry, Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece; [email protected] 2 Beijing Advanced Innovation Center for Soft Matter Science and Engineering, State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China 3 WestCHEM, School of Chemistry, University of Glasgow, University Avenue, Glasgow G12 8QQ, UK * Correspondence: [email protected] (T.A.K.); [email protected] (Y.-F.S.); [email protected] (H.N.M.); Tel.: +44-141-330-4375 (H.N.M) Received: 8 June 2018; Accepted: 9 July 2018; Published: 13 July 2018 Abstract: Utilizing new experimental approaches and gradual understanding of the underlying chemical processes has led to advances in the self-assembly of inorganic and metal–organic compounds at a very fast pace over the last decades. Exploitation of unveiled information originating from initial experimental observations has sparked the development of new families of compounds with unique structural characteristics and functionalities. The main source of inspiration for numerous research groups originated from the implementation of the design element along with the discovery of new chemical components which can self-assemble into complex structures with wide range of sizes, topologies and functionalities. Not only do self-assembled inorganic and metal–organic chemical systems belong to families of compounds with configurable structures, but also have a vast array of physical properties which reflect the chemical information stored in the various “modular” molecular subunits. The purpose of this short review article is not the exhaustive discussion of the broad field of inorganic and metal–organic chemical systems, but the discussion of some representative examples from each category which demonstrate the implementation of new synthetic approaches and design principles. Keywords: self-assembly; supramolecular chemistry; coordination chemistry; polyoxometalates; metal–organic frameworks; clusters 1. Introduction The term self-assembly is frequently used to describe an extended network of equilibria which can be exploited in synthetic chemistry in order to construct complex molecular structures from molecular synthons linked by covalent bonds. This area of research is governed by a specific set of rules which has attracted the interest of numerous research groups over the last decades. On the other hand, the supramolecular chemistry aspect is considered a complementary research area and extends beyond the molecular chemistry, has also attracted substantial interest and is responsible for the formation of chemical systems using building blocks of appropriate structural features and chemical properties interacting via non-covalent intermolecular forces. The first signs of this new field emerged in 1967 by the work of Jean-Marie Lehn in the design and study of alkali-metal cryptates, and the identification of the phenomenon of molecular recognition in chemical systems. This initial observation set the scene for the development of the field of supramolecular chemistry [1] and resulted in the award of the Nobel Prize in chemistry in 1987. Supramolecular chemistry investigates the interactions between molecular Inorganics 2018, 6, 71; doi:10.3390/inorganics6030071 www.mdpi.com/journal/inorganics Inorganics 2018, 6, 71 2 of 25 speciesInorganics and aims2018, 6, tox FOR shed PEER light REVIEW upon the underlying mechanisms which lead to the construction2 of 26 of highly complicated and functional chemical systems, constructed by constituents which are held by orthe temporarily interactions interactbetween molecul with intermolecularar species and aims bonds. to shed Due light to upon the instability the underlying of the mechanisms non-covalent interactions,which lead the to available the construction molecular of highly synthons complicated can connect and functional and disconnect chemical reversibly, systems, constructed by rearranging and re-organizingby constituents their which components are held by [ 2or]. temporarily In other words, interact depending with intermolecular on a wide rangebonds. ofDue experimental to the and chemicalinstability stimuli,of the non they-covalent can self-organize interactions, spontaneouslythe available molecular via the synthons process ofcan self-assembly connect and into disconnect reversibly, by rearranging and re-organizing their components [2]. In other words, well-defined supramolecular architectures. Self-assembly processes and supramolecular interactions depending on a wide range of experimental and chemical stimuli, they can self-organize havespontaneously been identified via as the the process main of driving self-assembly forces into responsible well-defined for thesupramolecular formation andarchitectures. ultimately Self for- the observedassembly functionality processes and of asupramolecular wide range ofinteractions chemical have systems. been identified Thus, we as the will main focus driving our discussionforces on aresponsible subset of inorganicfor the formation and metal–organic and ultimately systems for the observed and more functionality specifically of ina wide polyoxometalates, range of metal–organicchemical systems. frameworks Thus, (MOFs)we will andfocus metal our discussion coordination on a cages.subset of inorganic and metal–organic Thesystems process and ofmore self-organization specifically in polyoxometalates, is generally considered metal– toorganic proceed frameworks over three (MOFs) stages: and (1) metal molecular recognition;coordination (2) growth cages. through the connection of multiple constituents; (3) termination, where the process isThe completed process [of3]. self A representative-organization is example generally of considered this process to canproceed be in polyoxometalateover three stages: systems,(1) whichmolecular can lead recognition to the formation; (2) growth of a widethrough range the ofconnection intricate of and multiple functional constituents architectures.; (3) termination, For example, where the process is completed [3]. A representative example of this process can be in the self-assembly process is responsible for the formation of the family of molybdenum blue nano-sized polyoxometalate systems, which can lead to the formation of a wide range of intricate and functional clusters,architectures. such as For the example, wheel-shaped the self-assembly {Mo154} process oxide clusteris responsible where for intermolecular the formation of interactionsthe family of can promotemolybdenum the formation blue nano of vesicles-sized clusters, depending such onas thethe experimentalwheel-shaped { conditionsMo154} oxide [ 4cluster]. The where formation of polyoxometalate-basedintermolecular interactions vesicles can promote is the result the formation of a delicate of vesicles balance depending between on short-range the experimental attractive Van derconditions Waals [4] forces,. The hydrogenformation of bond polyoxometalate forces, and-based repulsive vesicles electrostatic is the result interactions of a delicate betweenbalance the anionicbetween clusters. short As-range we mentionedattractive Van earlier, der theWaals self-assembly forces, hydrogen process bond depends forces, onand a widerepulsive range of externalelectrostatic parameters interactions (pH, concentration, between the anionic ligands, clusters. templates, As we temperature, mentioned earlier pressure,, the self etc.);-assembly even small variationsprocess of depends these parameters on a wide can range affect of external the complex parameters network (pH, of concentration equilibria established, ligands, templates, initially in the reactiontemperature, mixture andpressure consequently, etc.); even small trigger variations the formation of these parameters of different can species affect the in solution.complex network Muller et al. of equilibria established initially in the reaction mixture and consequently trigger the formation of reported that the {Mo72Fe30} clusters in dilute aqueous solution behave as nano-sized weak inorganic different species in solution. Muller et al. reported that the {Mo72Fe30} clusters in dilute aqueous acidssolution and can behave be deprotonated, as nano-sized accordingweak inorganic to the acids pH, and which can be leads deprotonated, to the formation according of to different the pH, size molecularwhich nanoobjectsleads to the formation (Figure1 )[of 5different]. size molecular nanoobjects (Figure 1) [5]. Figure 1. Structure of the spherical {Mo72Fe30} Keplerate cluster. Mo, Blue and light blue polyhedral; Figure 1. Structure of the spherical {Mo Fe } Keplerate cluster. Mo, Blue and light blue polyhedral; Fe, red polyhedral; Oxygen, red spheres.72 30 Fe, red polyhedral; Oxygen, red spheres. The self-assembly process is usually system-specific, which makes our efforts to unveil the Theunderlying self-assembly reactions processand understand is usually better system-specific, the overall process which extremely makes challenging our efforts. Apart to unveil from the underlyingthe expe reactionsrimental approaches and understand which betteroccasionally the overall involve process real time extremely monitoring challenging. of chemical Apart reactions from the experimentaland
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