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 range bonds. of Due experimental to the and chemicalinstability stimuli,of the non they-covalent can self-organize interactions, spontaneously the available molecular via the synthonsprocess 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 inorganic for the formation and metal–organic and ultimately systems for the observedand more functionality specifically of in a widepolyoxometalates, 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 on as thethe experimental wheel-shaped { conditionsMo154} oxide [ 4 cluster]. 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 mentioned attractive Van earlier, der the Waals self-assembly forces, hydrogen process bond depends forces, on and a wide repulsive 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 concentrationequilibria 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 identification approaches of short which-lived occasionally intermediate involve species real[6–8] time, theor monitoringetical calculations of chemical and simulation reactions and models also make it possible, in some cases, to extract crucial mechanistic information. Fujita et al. identification of short-lived intermediate species [6–8], theoretical calculations and simulation models reported the simulation of the formation process of the M6L8 coordination cage (M = Pd (II) and L= also make it possible, in some cases, to extract crucial mechanistic information. Fujita et al. reported the pyridine-capped tridentate ligands) in 3 stages (assembly, evolution, fixation), which is in agreement simulationwith the of the experimental formation process data [9] of. The the Mauthors6L8 coordination showed that cage the (M life = Pd time (II) of and theL= coordinatively pyridine-capped tridentateunsaturated ligands) intermediate in 3 stages species (assembly, play a key evolution, role in the self fixation),-assembl whichy process is of in the agreement final complete with the experimental data [9]. The authors showed that the life time of the coordinatively unsaturated Inorganics 2018, 6, 71 3 of 25

intermediate species play a key role in the self-assembly process of the final complete M6L8 cage. Inorganics 2018, 6, x FOR PEER REVIEW 3 of 26 Recently, the same group implemented the same simulation model to predict the formation of the larger Inorganics 2018, 6, x FOR PEER REVIEW 3 of 26 M12L24 cageM6L (M8 cage =. Pd Recently, (II) and the L same = pyridine-capped group implemented bidentate the same ligands) simulation following model to predicta similar the approach formation of the larger M12L24 cage (M = Pd (II) and L = pyridine-capped bidentate ligands) following (Figure2M)[6L810 cage]. In. Recently, this case, the the same self-assembly group implemented process the revealed same simulation the existence model of to kinetically predict the trapped a similar approach (Figure 2) [10]. In this case, the self-assembly process revealed the existence of structuresformation of lower of the nuclearity, larger M12L24 while cage (M the = Pd observed (II) and L behaviour= pyridine-capped depends bidentate on geometrical ligands) following parameters kinetically trapped structures of lower nuclearity, while the observed behaviour depends on a similar approach (Figure 2) [10]. In this case, the self-assembly process revealed the existence of such as thegeometrical bond angle parameters of the such ligands as the as bond well angle as the of the strength ligands ofas well the metal–ligandas the strength of bond. the metal This– is a very kinetically trapped structures of lower nuclearity, while the observed behaviour depends on importantligand observation, bond. This whichis a very needs important to beobservation taken into, which consideration needs to be taken when into consideration trying to understand when the geometrical parameters such as the bond angle of the ligands as well as the strength of the metal– underlyingtrying processes to understand that the govern underlying the formationprocesses that of govern large the clusters. formation of Very large often, clusters. kinetic Very ofte effectsn, make ligandkinetic bond. effects Th ismake is a veryprediction important of self observation-assembled ,structures which needs of higher to be takennuclearity into clustersconsideration extremely when prediction of self-assembled structures of higher nuclearity clusters extremely challenging. tryingchallenging to understand. the underlying processes that govern the formation of large clusters. Very often, kinetic effects make prediction of self-assembled structures of higher nuclearity clusters extremely challenging.

Figure 2. Structural changes during the self-assembly process. Adapted from ACS Nano 2014, 8(2), Figure 2. Structural1290–1296. Copyright changes (2014) during American the self-assemblyChemical Society.process. Adapted from ACS Nano 2014, 8(2),

1290–1296. Copyright (2014) American Chemical Society. FigureIdentification 2. Structural of intermediatechanges during species the self masked-assembly by theprocess. self- assemblyAdapted fromprocesses ACS Nanoduring 2014 the, 8“one(2), - pot”1290 reaction–1296. Copyright is crucial (2014)for the American understanding Chemical of Society.the formation mechanism. Recently, Cronin et al., Identificationreported an of alternative intermediate approach species by employing masked a synthetic by the self-assemblyprocess under continuous processes flow during conditions the “one-pot” reaction isforIdentification crucial the investigation for of the intermediate of understanding self-assemb speciesly driven masked of the formation by formation the ofself molybdenum-assembly mechanism. processes blue nano Recently,duringclusters the. T “oneCroninhe - et al., reportedpot” anconcurrent alternativereaction controlis crucial approach of fourfor the experimental by understanding employing variables a of synthetic the(pH, formation Mo process concentration, mechanism under reducing continuous. Recently, agent ,Cronin flowflow rate conditions et ) al., for reportedkept the an sys alternativetem far from approach equilibrium by employing for a given a synthetic period of process time which under proved continuous to be crucial flow conditions for the the investigation of self-assembly driven formation of molybdenum blue nanoclusters. The concurrent for identification the investigation and isolation of self -ofassemb the intermediately driven formationspecies [11] of. In molybdenum this case, it was blue concluded nanoclusters that the. T he control of four experimental variables (pH, Mo concentration, reducing agent, flow rate) kept the system concurrentformation control mechanism of four of experimentalthe {Mo(154−x)} familyvariables involves (pH, Mothe {concentration,Mo36} cluster which reducing acts asagent a structur, flow erate- ) far fromkept equilibriumdirecting the sys tem template forfar from a (F givenigure equilibrium 3 period). The more offor time a we given understand which period proved of the time mechanism towhich be crucial proved behind forto the be the self crucial-assembly identification for the and isolationidentification ofprocesses, the intermediate the and more isolation constructive species of the [11 will intermediate]. be In the this implementation case, species it was [11] concludedof. Ina design this case, element that it was the which formationconcluded will allow that mechanism the the of construction of chemical systems with intricate structures, unique properties and a large range of the {Moformation− } family mechanism involves of the the {Mo {Mo(154−x)} clusterfamily in whichvolves actsthe { asMo a36} structure-directing cluster which acts as template a structur (Figuree- 3). (154applications.x) 36 The moredirecting we understand template (F theigure mechanism 3). The more behind we understand the self-assembly the mechanism processes, behind the the more self constructive-assembly will processes, the more constructive will be the implementation of a design element which will allow the be the implementation of a design element which will allow the construction of chemical systems with construction of chemical systems with intricate structures, unique properties and a large range of intricateapplications. structures, unique properties and a large range of applications.

Figure 3. Formation of {Mo186}, {Mo150} and {Mo36} complexes. The flow-reaction conditions and the reducing environment are necessary factors for the isolation of {Mo36}. Colour scheme, yellow: {Mo1},

red; {Mo2}, blue; {Mo8}.

Figure 3. Formation of {Mo186}, {Mo150} and {Mo36} complexes. The flow-reaction conditions and the Figure 3. Formation of {Mo186}, {Mo150} and {Mo36} complexes. The flow-reaction conditions and the reducing environment are necessary factors for the isolation of {Mo36}. Colour scheme, yellow: {Mo1},

reducingred environment; {Mo2}, blue; {Mo are8}. necessary factors for the isolation of {Mo36}. Colour scheme, yellow: {Mo1}, red; {Mo2}, blue; {Mo8}. Inorganics 2018, 6, 71 4 of 25

2. Polyoxometalate Systems

2.1. POM-Based Clusters and Supramolecular Aggregates Polyoxometalates are a class of discrete molecular inorganic clusters also known as polyoxoanions or polyanions. These are formed by metals (addenda atoms) such as tungsten, molybdenum and vanadium in their higher oxidation state; however, over the last decades, it has been shown that other metals can serve as the primary atoms within POM clusters, such as niobium [12,13], and tantalum [14,15]. POMs are composed of condensed metal-oxygen {MOx} polyhedra (x = 4 to 7). POMs can be formed mainly by vanadium, molybdenum and tungsten due to their appropriate ionic size and their ability to act as good acceptors of oxygen’s π electrons. The fundamental requirements for a transition metal to serve as addenda are the ability to adopt a variety of coordination modes (mainly 4 to 6) in response to acidification, have high positive charge, and are capable of pπ–dπ interactions. However, there are a few examples that deviate from this set of rules, as reported by Kortz et al., where the authors demonstrated the preparation and characterization of metal oxides made of noble metals such as Au [16,17] and Pd [18–22] in the presence of supporting organic or inorganic ligands. There are a number of archetypal POMs that are well documented in the literature and many other POMs contain structural features of these “classical” architectures. Such architectures are dominant in the field because of their high reproducibility, stability and the fact that they can be formed using several different types of addenda metal atoms. They can also incorporate a wide range of heteroatoms and still maintain their structure, a feature not present in most examples of polyoxometalates. The classical POMs are often used as starting materials for the construction of larger structures or for the manufacturing of POM-based materials due to their stability under specific experimental conditions. POM clusters can be classified in three general categories [23–25]; (1) heteropolyanions are the most explored category, and they consist of metal-oxide clusters of Mo, W, n− V, which contain XO4 type heteroanions, where X = B, Si, Ge, P, and S. Heteroanions induce stability to the clusters as well as to their lacunary derivatives generated by the removal of one or more addenda atoms. In the first category for example, belong the two well-known POM archetypes, Keggin and n− Dawson. The Keggin structure [(XO4)M12O36] (M = Mo, W; X = S, P, Si, et al.), which was confirmed in 1933, consists of {MO6} octahedra connected to each other via edge sharing of oxygen ligands forming {M3O13} triads, which are organised tetrahedrally around a central heteroatom. Similarly, n− the Dawson structure [(XO4)2M18O62] is the result of the assembly process of two lacunary Keggin n− monomers [XM9O34] ; (2) isopolyanions are POMs that are comprised entirely of addenda atoms. Typically, this means that only one type of metal is present; however, there are examples of mixed metal clusters where both metals are addenda and technically such clusters are also iso-POMs. Due to the lack of template heteroanion, isopolyanionic structures are less stable. An example for the second n− category is the Lindqvist anion [M6O19] (M = W, Mo, V, Nb, Ta), which is formed by 6 edge-sharing {MO6} octahedrals; (3) the Molybdenum Blues and Browns are the oldest class of POMs and were discovered by Scheele in 1793, but the structures could not be determined until the development of modern X-ray crystallographic analysis. They also represent the largest size of molecular POM clusters with some approaching the size of small proteins (the {Mo368} “blue lemon” is ~6 nm in diameter) [22]. Molybdenum Blues are defined by the fact that they contain mixed valence MoV/MoVI addenda and have delocalised electrons capable of intervalence charge transfer from MoV to MoVI facilitated by the π-orbitals of the bridging oxo ligands and this electronic interaction is responsible for their characteristic intense blue colour. Molybdenum Browns are further reduced comparing to Mo-Blues and have electrons localised between reduced MoV centres as Mo–Mo bonds which contribute to the brown colour of these clusters. The reduction of molybdenum addenda is only possible for {O = MoO5} containing structures POMs. This is due to the fact that the molecular orbitals of {MOL5} complexes contain a non-bonding t2g orbital capable of accepting electrons, while in cis {MO2L4} complexes all the t2g orbitals are associated with π-bonding to oxo ligands and as such there Inorganics 2018, 6, 71 5 of 25 are no available non-bonding orbitals for electrons to occupy and reduction would destabilise the clusters [26]. This rule induces certain constraints on the structural features of Molybdenum Blues and Browns.Inorganics The 2018 most, 6, x well-knownFOR PEER REVIEW Molybdenum Blue structures are the giant wheels, {Mo1545}[ of27 26] and {Mo176}[28], and the {Mo132} Keplerate cluster [29], which are all constructed using the same structural and Browns. The most well-known Molybdenum Blue structures are the giant wheels, {Mo154} [27] building block, {MoMo5}. The central building block is occupied by a pentagonal bipyramidal {MoO7} and {Mo176} [28], and the {Mo132} Keplerate cluster [29], which are all constructed using the same unit, which is surrounded by five edge-shared {MoO6} octahedra along the equator of the bipyramid. structural building block, {MoMo5}. The central building block is occupied by a pentagonal This pentagon is considered to be the fundamental unit responsible for the construction of elaborate bipyramidal {MoO7} unit, which is surrounded by five edge-shared {MoO6} octahedra along the architecturesequator of (see the Figurebipyramid.4). In This wheel pentagon structures, is considered the pentagonal to be the building fundamental blocks unit are responsible connected for into the ring architecturesconstruction and of are elaborate comprised architectures of two such (see rings Figure fused 4). In together. wheel structures Each ring, the is formed pentagonal from building two distinct buildingblocks blocks are connected known into as {Mo ring8 }architectures and {Mo2}. and The are {Mo comprised8} building of two block such incorporatesrings fused together. the pentagonal Each unit describedring is formed previously from two withdistinct an building additional blocks two known {MoO as6 {Mo} octahedra8} and {Mo connected2}. The {Mo8 to} building the pentagon block via incorporates the pentagonal unit described previously with an additional two {MoO6} octahedra corner sharing with four of the {MoO6} octahedra of the pentagonal unit. These additional molybdate connected to the pentagon via corner sharing with four of the {MoO6} octahedra of the pentagonal units on the pentagon connect to neighbouring {Mo8} units via corner and edge sharing to produce unit. These additional molybdate units on the pentagon connect to neighbouring {Mo8} units via the final ring-shaped structure. The rims of the wheel are supported by {Mo } units, which consist corner and edge sharing to produce the final ring-shaped structure. The rims2 of the wheel are of two corner-sharing {MoO6} octahedra. They connect to the {Mo8} building blocks via three sites supported by {Mo2} units, which consist of two corner-sharing {MoO6} octahedra. They connect to the 14− of corner{Mo8} sharing.building blocks Two representative via three sites of giant corner wheels—{Mo sharing. Two154 representative}, [Mo154O462 giantH14(H wheels2O)70—] {Mo[15427},] and 16− {Mo176[Mo},154 [MoO462176HO14(H5282O)H1670](H14− [27]2O) 80and] {Mo[28176]—have}, [Mo176O external528H16(H2O) diameters80]16− [28]—have of 3.4 external and 4.1 diameters nm, respectively. of 3.4 The {Moand 176 4.1} has nm, a respectively. smaller curvature The {Mo because176} has each a smaller ring possesses curvature an because additional each ring {Mo possesses8} building an block relativeadditional to the {Mo{Mo1548} building}. block relative to the {Mo154}.

Figure 4. Ball-and-stick representation of the {Mo6} pentagonal bipyramidal building block integral Figure 4. Ball-and-stick representation of the {Mo6} pentagonal bipyramidal building block integral to to Molybdenum Blue and Brown architectures beside polyhedral and ball-and-stick representations Molybdenum Blue and Brown architectures beside polyhedral and ball-and-stick representations of how of how the pentagonal units are arranged within the {Mo154} wheel and {Mo132} Keplerate. (Colour the pentagonalscheme: Mo units, blue; areO, red arranged; the central within Mo theof the {Mo pentagonal154} wheel units and has {Mo been132 highlighted} Keplerate. in (Colourpale blue) scheme:. Mo, blue; O, red; the central Mo of the pentagonal units has been highlighted in pale blue). Interestingly, POM-based structures such as Keggin, Dawson, Anderson, etc., can be used as Interestingly,secondary building POM-based blocks for structures the construction such of as larger Keggin, architectures Dawson,. There Anderson, are many etc., methods can be that used as secondarycan be building used for the blocks modification for the construction and functionalization of larger of architectures. POM clusters, including There are the many choice methods of that cancounterions be used, organic for the lig modificationands and transition and functionalization metals. The counterions of POM are clusters, necessary including for the existence the choice of of POMs as they stabilize the negative charge, but they can also influence the self-assembly process counterions, organic ligands and transition metals. The counterions are necessary for the existence during the formation of POM structures. Depending on their size, charge, solubility etc., can stabilize of POMs as they stabilize the negative charge, but they can also influence the self-assembly process the intermediate building blocks and “transitional” species in solution, which ultimately influence duringthe the structural formation features of POM of the structures. final product Depending [24]. The onorganic their ligands size, charge, and first solubility row transition etc., can metals stabilize the intermediatecan be used as building ligands and blocks metal and linkers. “transitional” Additionally, species the exchange in solution, of redox which “innocent” ultimately templates influence the structural(e.g., SO42− features and PO43− of) with the finalredox product active ones [24 ].(e. Theg., SO organic32−, SeO3 ligands2−) is an alternative and first row way transition to direct the metals can beassembly used as process ligands towards and metal the linkers. formation Additionally, of different thestructures exchange (e.g. of, lacunary redox “innocent” structures) templates and 2− 3− 2− 2− (e.g.,influence SO4 and the PO electronic4 ) with properties redoxactive of the clusters ones (e.g., [30– SO32]3. Lacunary, SeO3 POMs) is an can alternative be formed by way expulsion to direct the assemblyof some process atoms towards from the formation complete structure of different, which structures tends (e.g., to increas lacunarye the structures)nucleophilicity and influenceand the electronicconsequently properties the reactivity of the ofclusters the formed [30–32 clusters]. Lacunary towards POMs electrophiles can be formed. This provid by expulsiones a unique of some opportunity for the formation of larger architectures by reacting under appropriate experimental atoms from the complete structure, which tends to increase the nucleophilicity and consequently the conditions unsaturated POM structures with transitional metals, Figure 5, lanthanides and metal reactivity of the formed clusters towards electrophiles. This provides a unique opportunity for the complexes [25]. Finally, it is clear that the plethora of design approaches and wide range of formationexperimental of larger parameters architectures that can by reactinginfluence under the self appropriate-assembly process experimental allows the conditions preparation unsaturated of an POM structures with transitional metals, Figure5, lanthanides and metal complexes [ 25]. Finally, it is

Inorganics 2018, 6, 71 6 of 25

clear thatInorganics the plethora 2018, 6, x FOR of PEER design REVIEW approaches and wide range of experimental parameters6 of 26 that can influence the self-assembly process allows the preparation of an immense variety of POM architectures with differentimmense sizes, variety compositions of POM architectures and properties, with different in a controllable sizes, composition fashion.s and properties, in a controllable fashion.

Figure 5. Representation of the transition metal substituted POM structures derived from the {P2W15} Figure 5. Representation of the transition metal substituted POM structures derived from the {P2W15} lacunary species. From top left: {M3P2W15} [33–35]; {M4(P2W15)2} [36–39]; {M6Ln9(P2W15)6} [40]; lacunary species.{M12(P2W15) From4} [41]; {M top16(P left:2W15)4 {M} [423]P; {(Mo2W152O}[2S2)338(P–2W3515];)4} {M [43]4.(P (Colour2W15 scheme:)2}[36 –W,39 teal;]; {M O, 6red;Ln 9Sulfur,(P2W 15)6}[40]; {M12(P2Wpurple;15)4}[ 41Heteroatom,]; {M16(P lime2W 15green;)4}[ 42Transition]; {(Mo metals,2O2S2 or)8ange;(P2W Lanthanide,15)4}[43]. pale (Colour yellow). scheme: W, teal; O, red; Sulfur, purple; Heteroatom, lime green; Transition metals, orange; Lanthanide, pale yellow). Interestingly, polyoxometalate clusters have the tendency to interact constructively with other inorganic or inorganic moieties and form supramolecular aggregates. For example, since 2008 and Interestingly,the research polyoxometalate of Mizuno et al. who clustersreported the have first hybrid the tendency complexes to based interact on cucurbit[n]uril constructively (CB[n]) with other inorganicand or inorganic polyoxometalat moietieses [44] and (Figure form 6), supramolecularvarious hybrid molecular aggregates. solids based For example,on POMs and since CB[n] 2008 and the derivatives have been investigated. The hybrid system of POMs and CB[n] has gained great interest research of Mizuno et al. who reported the first hybrid complexes based on cucurbit[n]uril (CB[n]) due to distinctive structural features. Recently, Lü and co-workers reported the first example of and polyoxometalateshybrid solids based [44] (Figure on Lindqvist6), various-type POM hybrid anions molecular [W6O19]2− and solids decamethylcucurbit[5]uril based on POMs and CB[n] derivatives(Me have10CB[5]) been [45] investigated.. In the reported compound, The hybrid {[Na system2(W6O19)(Me of POMs10CB[5])(H and2O)]∙2H CB[n]2O]n has, the gainedsodium ions, great interest due to distinctive[W6O19]2− anions structural and Me features.10CB[5] form Recently, 1D chains Lü that and are co-workers held together reported through supramolecular the first example non- of hybrid bonding interactions such as C–H···π, dipole–dipole and hydrogen bonds resulting in a 3D solids based on Lindqvist-type POM anions [W O ]2− and decamethylcucurbit[5]uril (Me CB[5]) [45]. supramolecular host–guest network. The compound6 19 exhibits enhanced photocatalytic property for10 2− In the reporteddye degradation compound, under {[Navisible2 (Wlight,6O due19)(Me to cooperative10CB[5])(H effects2O)] induced·2H2 O]by nits, thecomponents. sodium ions, [W6O19] anions and Me10CB[5] form 1D chains that are held together through supramolecular non-bonding interactions such as C–H···π, dipole–dipole and hydrogen bonds resulting in a 3D supramolecular host–guest network. The compound exhibits enhanced photocatalytic property for dye degradation under visible light, due to cooperative effects induced by its components. Inorganics 2018, 6, x FOR PEER REVIEW 7 of 26

Figure 6. Representation of intermolecular interactions between the {V18} and cucurbit[n]uril Figure 6. Representationmolecules forming of intermoleculara 1D square shaped interactionschannel. between the {V18} and cucurbit[n]uril molecules forming a 1D square shaped channel. In a similar approach, cyclodextrins (CDs) are cyclic oligosacharides comprised of 6 to 8 glucose units exhibiting torus-shaped ring structure comparable to the CB moieties discussed above. An important distinction, though, is that the interior cavity of CDs is hydrophobic, while the exterior side is hydrophilic; due to this architectural conformation, they can incorporate organic and inorganic guest molecules of appropriate size. The main driving force in this case are dipole–dipole, electrostatic forces, hydrogen bonding and van der Waals interactions [46,47]. Very recently, Cadot et al. reported the isolation of the 1:1, 1:2, 1:3 adducts, which are formed due to the attractive interactions including electrostatic, ion-dipole and hydrogen bonding between the longitudinal side of Dawson anion [P2W18O62]6− and primary face of γ-CD, as well as a complex that is consisted of a cationic octahedral cluster [Ta6Br12(H2O)6]2+ and γ-CD, which is held together by intermolecular interactions [48]. Additionally, the authors demonstrated that the interaction of three constituents ([P2W18O62]6−, [Ta6Br12(H2O)6]2+, γ-CD) generate a three-component supramolecular hybrid system. In this case, the [Ta6@2CD]2+ unit acts as a ditopic ionic linker forming a tubular chain with periodic alternation of POMs and clusters. The same group reported a three-component hybrid assembly organized through non-covalent interactions [49]. The supramolecular aggregate is based on the nano-sized [Mo154O462H14(H2O)70]14− molybdenum blue wheel, which hosts a Dawson cluster, [P2W18O62]6− which is capped by two γ-CDs moieties. The three components are held together by intermolecular interactions (Figure 7).

Inorganics 2018, 6, 71 7 of 25

In a similar approach, cyclodextrins (CDs) are cyclic oligosacharides comprised of 6 to 8 glucose units exhibiting torus-shaped ring structure comparable to the CB moieties discussed above. An important distinction, though, is that the interior cavity of CDs is hydrophobic, while the exterior side is hydrophilic; due to this architectural conformation, they can incorporate organic and inorganic guest molecules of appropriate size. The main driving force in this case are dipole–dipole, electrostatic forces, hydrogen bonding and van der Waals interactions [46,47]. Very recently, Cadot et al. reported the isolation of the 1:1, 1:2, 1:3 adducts, which are formed due to the attractive interactions including electrostatic, ion-dipole and hydrogen bonding between the longitudinal side 6− of Dawson anion [P2W18O62] and primary face of γ-CD, as well as a complex that is consisted of 2+ a cationic octahedral cluster [Ta6Br12(H2O)6] and γ-CD, which is held together by intermolecular interactions [48]. Additionally, the authors demonstrated that the interaction of three constituents 6− 2+ ([P2W18O62] , [Ta6Br12(H2O)6] , γ-CD) generate a three-component supramolecular hybrid system. 2+ In this case, the [Ta6@2CD] unit acts as a ditopic ionic linker forming a tubular chain with periodic alternation of POMs and clusters. The same group reported a three-component hybrid assembly organized through non-covalent interactions [49]. The supramolecular aggregate is based on the 14− nano-sized [Mo154O462H14(H2O)70] molybdenum blue wheel, which hosts a Dawson cluster, 6− [P2W18O62] which is capped by two γ-CDs moieties. The three components are held together byInorganics intermolecular 2018, 6, x FOR interactions PEER REVIEW (Figure 7). 8 of 26

Figure 7. Representation of the [Mo154O462H14(H2O)70]14− molecul14− ar wheel which hosts a Dawson anion, Figure 7. Representation of the [Mo154O462H14(H2O)70] molecular wheel which hosts a Dawson [P2W18O62]6−, capped6− by two γ-CDs moieties. anion, [P2W18O62] , capped by two γ-CDs moieties.

2.2.2.2. POM-OFsPOM-OFs PolyoxometalatePolyoxometalate open open frameworks frameworks (POM-OFs) (POM-OFs) are extended are extended architectures architectures that can be that constructed can be usingconstructed POM-based using clustersPOM-based as buildingclusters blocks.as building The blocks interplay. The of interplay supramolecular of supramo interactionslecular (throughinteractions hydrogen (through bonds, hydrogen van der bonds, Waals van forces), der Waals as well forces as reactivity), as well betweenas reactivity POM between clusters POM and transitionclusters and metals transition or metal metals complexes, or metal can complex lead toes the, can formation lead to ofthe extended formation networks of exten ofded coordinatively networks of linkedcoordinatively 1D chains, linked 2D sheets 1D chains, and 3D 2D compounds. sheets and As3D showncompounds in the. figureAs shown below, in thethe POMfigure clusters below, canthe bePOM linked clusters through can transitional be linked metals through (TM) transitional via grafted metals organic (TM) units, via directly grafted through organic transition units, directly metals, throughthrough organictransition linkers metals, and through through organic organic linkers viaand transition through organic metals (Figure linkers8 )[via50 transition]. POM-OFs metals are porous(Figure materials8) [50]. POM with-OFs interesting are porous structural materials flexibility, with interesting stability and structural interesting flexibility physical, stability properties. and interesting physical properties. Moreover, the ability of POM species to accept and release electrons reversibly with marginal structural changes makes them exceptional candidates for catalytic applications. In 2017, Chang et al. following the above approach reported the synthesis of the networked compound (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38]·24H2O, which is the first 3D host–guest structure with an Evans-Showell type polyoxometalate as the guest (Figure 9), while the compounds (Hbim)2[{Cu(bim)2(H2O)2}2{Co2Mo10H4O38}]·5H2O and H2[Cu(dpdo)3(H2O)4][{Cu2(dpdo)3(H2O)4 (CH3CN)}2{Co2Mo10H4O38}2]·9H2O are the first 2D hybrid networks that include this type of POM archetype [51]. In contrast with other POM building blocks (Keggin, Dawson), the reported examples of Evans-Showell-based hybrids are quite rare. All three compounds showed promising catalytic efficiency in the oxidation of sulfides and alcohols.

Inorganics 2018, 6, 71 8 of 25

Moreover, the ability of POM species to accept and release electrons reversibly with marginal structural changes makes them exceptional candidates for catalytic applications. In 2017, Chang et al. following the above approach reported the synthesis of the networked compound (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38]·24H2O, which is the first 3D host–guest structure with an Evans-Showell type polyoxometalate as the guest (Figure9), while the compounds (Hbim)2[{Cu(bim)2(H2O)2}2{Co2Mo10H4O38}]·5H2O and H2[Cu(dpdo)3(H2O)4][{Cu2(dpdo)3(H2O)4 (CH3CN)}2{Co2Mo10H4O38}2]·9H2O are the first 2D hybrid networks that include this type of POM archetype [51]. In contrast with other POM building blocks (Keggin, Dawson), the reported examples of Evans-Showell-basedInorganics 2018, 6, x FOR PEER hybrids REVIEW are quite rare. All three compounds showed promising9 of catalytic 26 efficiencyInorganics in the oxidation2018, 6, x FOR PEER of sulfides REVIEW and alcohols. 9 of 26

Figure 8. The four types of POM–Ligand connectivities. FigureFigure 8. 8The. The four four typestypes of POM POM–Ligand–Ligand connectivities connectivities..

Figure 9. Representation of (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38] compound. 3D host–guest

framework (ball-and-stick) with two enantiomers of [Co2Mo10H4O38]6− (polyhedral representation) as

the guest. (Colour scheme: Mo, teal; O, red; Cu, cyan; Co, yellow; C, black; N, blue; Lanthanide, pale Figureyellow). 9. Representation of (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38] compound. 3D host–guest Figure 9. Representation of (en)[Cu3(ptz)4(H2O)4][Co2Mo10H4O38] compound. 3D host–guest framework framework (ball-and-stick) with two enantiomers of [Co2Mo6−10H4O38]6− (polyhedral representation) as (ball-and-stick) with two enantiomers of [Co2Mo10H4O38] (polyhedral representation) as theII guest. the guest.In a similar (Colour manner, scheme: Liu Mo, et al.teal; utilized O, red; [10] Cu, Anderson cyan; Co, a yellow;nd octamolybdate C, black; N,-based blue; clusters Lanthanide, and Cu pale- (Colourbased scheme: complexes Mo, teal;to prepare O, red; a Cu,new cyan;family Co, of yellow;hybrid networked C, black; N,compounds blue; Lanthanide, which exhibit pale a yellow). variety yellow). of dimensions: [Cu2(2-pdya)(CrMo6(OH)5O19)(H2O)2]·3H2O, [Cu(3-dpye)0.5(γ-Mo8O26)0.5(H2O)4]·H2O, [Cu(4-Hdpyp)2(β-Mo8O26)(H2O)2]·4H2O, [Cu4(μ3-OH)2(H2O)4(3-dpyh)(γ-Mo8O27)]·4H2O, [Cu2(4- In aIn similar a similar manner, manner, Liu Liu et et al. al. utilized utilized [10] Anderson [10] Anderson and octamolybdate and octamolybdate-based-based clusters and clusters CuII- and Hdpye)2(TeMo6O24)(H2O)6]·4H2O, [Cu3(3-dpyb)2(TeMo6O24)(H2O)8]·4H2O, [Cu2(4-Hdpyb)2 II based complexes to prepare a new family of hybrid networked compounds which exhibit a variety Cu -based(TeMo complexes6O24)(H2 toO) prepare6]·4H2O, (3 a- newdpye family = N,N′- ofbis- hybrid(3-pyridinecarboxamide) networked compounds-1,2-ethane, which 4-dpye exhibit = N,N′- a variety of of dimensions: [Cu2(2-pdya)(CrMo6(OH)5O19)(H2O)2]·3H2O, [Cu(3-dpye)0.5(γ-Mo8O26)0.5(H2O)4]·H2O, dimensions:bis(4-pyridinecar [Cu2(2-pdya)(CrMoboxamide)-1,26-(OH)ethane,5O 419-dpyp)(H2 O) = N2],·N3H′-bis(42O,-pyridinecarboxamide) [Cu(3-dpye)0.5(γ-Mo-1,3-8propane,O26)0.5(H 3-2O)4]·H2O, [Cu(4dpyb-Hdpyp) = N,N2(β′-bis(3-Mo-8pyridinecarboxamide)O26)(H2O)2]·4H2O, -1,4 [Cu-butane,4(μ3-OH) 3-dpyh2(H2 O)4(3= N,N′-bisdpyh)(γ (3-pyridinecarboxamide)-Mo8O27)]·4H2O, -1,6 [Cu- 2(4- [Cu(4-Hdpyp)2(β-Mo8O26)(H2O)2]·4H2O, [Cu4(µ3-OH)2(H2O)4(3-dpyh)(γ-Mo8O27)]·4H2O, [Cu2(4- Hdpye)hexane)2(TeMo [52]6O. 24All)(H compounds2O)6]·4H2O, exhibit good [Cu3 (3electrocatalytic-dpyb)2(TeMo activities6O24)(H for2O) 8the]·4H reduction2O, of [Cu BrO2(43– -andHdpyb) 2 Hdpye)2(TeMo6O24)(H2O)6]·4H2O, [Cu3(3-dpyb)2(TeMo6O24)(H2O)8]·4H2O, [Cu2(4-Hdpyb)2 H2O2, and photocatalytic activity towards the degradation of harmful organic dyes such as methylene (TeMo6O24)(H2O)6]·4H2O, (3-dpye = N,0N′-bis-(3-pyridinecarboxamide)-1,2-ethane, 4-dpye = N,N′0- (TeMo6O24blue)(H2 andO)6 rhodamine]·4H2O, (3-dpye B. = N,N -bis-(3-pyridinecarboxamide)-1,2-ethane, 4-dpye = N,N -bis(4- bis(4-pyridinecarboxamide)-1,2-ethane, 4-dpyp0 = N,N′-bis(4-pyridinecarboxamide)-1,3-propane, 30- pyridinecarboxamide)-1,2-ethane,dpyb = N,NAnother′-bis(3 - interestingpyridinecarboxamide) example4-dpyp =ofN cooperativity,-N1,4-bis(4-pyridinecarboxamide)-1,3-propane,-butane, between3-dpyh supramolecular= N,N′-bis (3-pyridinecarboxamide) interactions 3-dpyb and self =- N-1,6,N--bis(3- assembly processes is the case of the β-octamolybdate isomer, [β-Mo8O26]4−. The octamolybdate hexane) [52]. All compounds exhibit good electrocatalytic activities for the reduction of BrO3– and cluster was used initially to coordinate through its terminal oxygen binding sites to many first row H2O2, transitionand photocatalytic metal complexes. activity Examples towards the include degradation coordination of harmful to various organic complexes dyes such of firstas methyle row ne blue and rhodamine B.

Another interesting example of cooperativity between supramolecular interactions and self- assembly processes is the case of the β-octamolybdate isomer, [β-Mo8O26]4−. The octamolybdate cluster was used initially to coordinate through its terminal oxygen binding sites to many first row transition metal complexes. Examples include coordination to various complexes of first row

Inorganics 2018, 6, 71 9 of 25 pyridinecarboxamide)-1,4-butane, 3-dpyh = N,N0-bis (3-pyridinecarboxamide)-1,6-hexane) [52]. All compounds – exhibit good electrocatalytic activities for the reduction of BrO3 and H2O2, and photocatalytic activity towards the degradation of harmful organic dyes such as methylene blue and rhodamine B. Another interesting example of cooperativity between supramolecular interactions and 4− self-assembly processes is the case of the β-octamolybdate isomer, [β-Mo8O26] . The octamolybdate cluster was used initially to coordinate through its terminal oxygen binding sites to many first row transition metal complexes. Examples include coordination to various complexes of first row transition metals (Co, Ni, Cu and Zn), allowing the formation of 2D and 3D networks [53–55]. Another prominent 4− example of the coordination behaviour adopted by the [β-Mo8O26] anion is its ability to coordinate silver (I) cations using its terminal oxygen ligands enabling it to behave as a bi-, tetra-, or hexadentate ligand [56–58]. For example, work in this area by Cronin et al. revealed that reaction of the 2− molybdenum Lindqvist anion, [Mo6O19] with silver (I) cations in a variety of coordinating solvents led to the isolation of various architectures involving, specifically the aggregation of (Ag{Mo8}Ag) synthons [57]. The use of rigid, sterically bulky cations such as tetraphenylphosphonium ions in DMSO solvent allowed isolation of the structure (Ph4P)2[Ag2Mo8O26((CH3)2SO)4], which is composed of ‘monomers’ of this (Ag{Mo8}Ag) building-block [58]. In comparison, the use of varying chain-length alkylammonium cations, i.e., tetrapropyl-, tetrabutyl-, tetrahexyl-, and tetraheptylammonium ions in a range of solvents such as acetonitrile, DMSO and DMF, led to the isolation of a variety of architectures, ranging from chains, to grids and 2D networks. The generation of these different POM architectures was shown to be governed mainly by the steric requirements of the organic cations or coordinated solvent molecules. Another important feature of these results was the identification of the unusual {Ag2} dimers positioned between the {Mo8} cluster units, which are a result of the repeating (Ag{Mo8}Ag) building-block units within these structures. This linking motif is uncommon in POM chemistry and is a rare example of d10 (i.e., filled d-shell) bridging units which are held together by significant argentophilic interactions, i.e., where the Ag-Ag distance is less than the sum of the van der Waals radii (3.44 Å). Moreover, POM-based clusters also attracted the attention of research groups whose work was focused on exploring their interactions in biological systems. The implemented approach involved the functionalization of POMs with organic ligands which exhibit biological activity. A family of organic compounds with these characteristics are biphosphonates, with general formula H2O3PC(OH)(R)PO3H2, which have been studied as anti-bacterial and anti-cancer agents [59], and various amino acids, like glycine and proline [60,61]. Initial exploration of the parameter space (pH, concentration, counterions effect, ionic strength, etc.) yielded a large variety of compounds with different properties which can interact with biological systems [62–67]. Utilization of a similar approach focused a lot of research efforts on exploiting the constructive interactions of the POM-OFs constituents in an effort to introduce specific functionality to the final material. For example, a lot of investigations have been dedicated into the reduction of gas emission, as well as, development of materials for water purification. Very recently, · Ma et al. reported the synthesis of a 2D compound [Co2L0.5V4O12]·3DMF 5H2O (L = wheel-like resorcin[4]arene ligand), which was used as heterogeneous catalyst and exhibited high efficiency for the cycloaddition of CO2 with epoxides and for oxidative desulfurization of sulfides [68]. It was shown that the compound’s catalytic efficiency originates from the exposed vanadium sites residing in the channels. Also, in 2016 Yang et al. synthesized a family of three compounds [PMo12O40]@ [Cu6O(TZI)3(H2O)9]4·OH·31H2O (H3TZI = 5-tetrazolylisophthalic acid), [SiMo12O40]@ [Cu6O(TZI)3(H2O)9]4·32H2O, and [PW12O40]@[Cu6O(TZI)3(H2O)6]4·OH·31H2O under “one-pot” solvothermal conditions, which involved the immobilization of Keggin clusters within the cavities of the rht-MOF-1 [69]. The cooperative effects due to the co-existence of MOF and POM structures proved to be beneficial for the adsorption of organic pollutants and their subsequent oxidation to useful products. Keggin-based species have been proven to be very useful building blocks for the construction of POM-OF compounds. For example Dolbecq et al. [70] reported the preparation of four 1D Inorganics 2018, 6, 71 10 of 25

V VI and 2D networks. More specifically, (TBA)3{PMo 8Mo 4O36(OH)4Zn4}[C6H4(COO)2]2 is a 2D compound which incorporates Zn-capped Keggin units connected via 1,3 benzenedicarboxylate (isop) linkers and tetrabutylammonium (TBA) counter-cations occupying the space between the V VI planes. In the case of (TPA)3{PMo 8Mo 4O37(OH)3Zn4}[C6H3(COO)3](TPA[e(trim)]∞), the Zn-capped Keggin units form 1D inorganic chains which are linked further via 1,3,5-benzenetricarboxylate V VI (trim) ligands into an overall 2D architecture. Alternatively, in (TBA){PMo 8Mo 4O40Zn4}(C7H4N2)2 (C7H5N2)2·12H2O(e(bim)4) compound the Zn capped Keggin units are connected to benzimidazole V VI (bim) ligands. Finally, the (TBA)(C10H10N4)2(HPO3)PMo 8Mo 4O40Zn4}2(C10H9N4)3(C10H8N4) 2− (e2(pazo)4) incorporates dimeric Zn capped Keggin units bridged by [HPO3] anions and para-azobipyridine (pazo) ligands completing the coordination sphere. The group also showed the use of these compounds as environmentally friendly reducing agents for the reduction of graphite oxide (GO)Inorganics to graphene 2018, 6, x FOR (G) PEER under REVIEW mild conditions. The obtained materials’ (POM@G)11 of large 26 surface area and noteworthy stability under various experimental conditions made them promising candidates for numerousazobipyridine applications (pazo) suchligands as completing photo/electro the coordination catalysis, sphere electrode. The group materials also showed and the sensors. use of these compounds as environmentally friendly reducing agents for the reduction of graphite oxide Another(GO) to set graphene of representative (G) under mild conditions. examples The obtained are the material all-inorganics’ (POM@G) large POM-OF surface area compounds reportedand by noteworthy Cronin etstability al. under [71] various where experimental the connectivity conditions made between them promising the POM-based candidates building blocks wasfor numerous served applications exclusively such byas photo/electro transitions catalysis, metals. electrode The materials Mn-linked and sensors. cubic framework, Another set of representative examples are the all-inorganic POM-OF compounds reported by II K18Li6[Mn8(H2O)48P8W48O184]·108H2O, incorporates 8-connected {P8W48} units, bridged by Mn Cronin et al. [71] where the connectivity between the POM-based building blocks was served centres, locatedexclusively to the externalby transitions surfaces of metals the rings. The (Figure Mn 10-linked). The cubic cubic sub-units framework, formed by the perpendicularK18Li6[Mn orientation8(H2O)48P8W of48O the184]∙108H {P8W2O,48 } incorporates rings define 8-connected an infinite {P8W array48} units, nano-sized bridged bymolecular MnII cubes, enclosingcentres, roughly located spherical to the external cavities surfaces of ca. of 7.2the nmrings3 .(Figure The authors10). The cubic demonstrated sub-units formed the by accessibility the of these cavitiesperpendicular by divalent orientation transition of the metals. {P8W48} rings In a similardefine an manner, infinite array the samenano-sized group molec reportedular cubes shortly, after enclosing roughly spherical cavities of ca. 7.2 nm3. The authors demonstrated the accessibility of these a whole familycavities of by POM-OFs divalent transition based on metals the. connectivity In a similar manner, of {P8 Wthe48 same} building group reported blocks withshortly other after transitionala 2+ metals (e.g.,whole Co family). The of POM authors-OFs showedbased on the that connectivity the conformational of {P8W48} building flexibility blocks with of the other crystalline transitional framework, which canmetals undergo (e.g. eight, Co2+) different. The authors crystal-to-crystal showed that the transformations conformational flexibility without of compromising the crystalline either its structuralframework stability,or which its crystallinity. can undergo Finally, eight different it was shown crystal- thatto-crystal observed transformations adsorption with propertiesout are compromising either its structural stability or its crystallinity. Finally, it was shown that observed directly associatedadsorption withproperties the are conformational directly associated flexibility with the conformation of the material.al flexibility of the material.

Figure 10. Representation of Mn-linked cubic framework, K18Li6[Mn8(H2O)48P8W48O184]∙108H2O. The Figure 10. Representation of Mn-linked cubic framework, K18Li6[Mn8(H2O)48P8W48O184]·108H2O. {P8W48} units are connected through MnII centres to form a face-directed cube, with internal cavity II The {P8W48volume} units of areca. 7.2 connected nm3. The repeated through cubic Mn unitscentres form an to infinite form a 3D face-directed lattice. (Colour cube, scheme: with {P8W internal48}, cavity 3 volume oforange ca. 7.2 wired nm representation;. The repeated MnII cubic, blue spheres). units form an infinite 3D lattice. (Colour scheme: {P8W48}, orange wired representation; MnII, blue spheres). Another sub-category of the hybrid POM-MOF-based materials is the polyoxometalate- lanthanide organic frameworks (POM-LOF), which have attracted the interest of various research groups due to their modular optical (e.g., luminescence) and magnetic properties originating from the interactions with 4f electrons of LnIII cations. Very recently, the compound [Ce4(BINDI)2(DMA)16]·[SiW12O40]·3DMA (BINDI = N,N′-bis(5-isophthalate)-1,4,5,8- naphthalenediimide, DMA = N,N-dimethylacetamide) was reported, which is the first example of a visible-light-responsive photochromic POM-LOF hybrid material. This compound exhibits unusual

Inorganics 2018, 6, 71 11 of 25

Another sub-category of the hybrid POM-MOF-based materials is the polyoxometalate-lanthanide organic frameworks (POM-LOF), which have attracted the interest of various research groups due to their modular optical (e.g., luminescence) and magnetic properties originating from the interactions with 4f electrons of LnIII cations. Very recently, the compound [Ce4(BINDI)2(DMA)16]·[SiW12O40]·3DMA (BINDI = N,N0-bis(5-isophthalate)-1,4,5,8-naphthalenediimide, DMA = N,N-dimethylacetamide) was reported, whichInorganics is the 2018 first, 6, x example FOR PEER REVIEW of a visible-light-responsive photochromic POM-LOF hybrid12 material.of 26 This compound exhibits unusual four-fold interpenetration of 3D cationic frameworks that further four-fold interpenetration of 3D cationic frameworks that further encapsulate non-coordinating encapsulate non-coordinating Keggin [SiW O ]4− species (Figure 11)[72]. The interactions between Keggin [SiW12O40]4− species (Figure 1211)40 [72]. The interactions between the π-acidic the πnaphthalenediimide-acidic naphthalenediimide moieties and moieties the anionic and POM the clusters anionic play POMed a key clusters role in played the self- aorganization key role in the self-organizationof the components of the, which components, led to the whichformation led of to the the final formation product of. the final product.

Figure 11. Keggin anions [SiW12O40]4− immobilized via anion–π interactions developed between Figure 11. Keggin anions [SiW O ]4− immobilized via anion–π interactions developed between POMs and NDIs (left). Structural12 analysis40 revealed two kinds of double-stranded helical chains with POMs and NDIs (left). Structural analysis revealed two kinds of double-stranded helical chains opposite handedness surrounded by NDI–NDI–POM–NDI–NDI sandwich arrays (right). Adapted withfrom opposite Dalton handednessTrans. 2017, 46, surrounded 4898–4901. Copyright by NDI–NDI–POM–NDI–NDI (2017) Royal Society of Chemistry. sandwich arrays (right). Adapted from Dalton Trans. 2017, 46, 4898–4901. Copyright (2017) Royal Society of Chemistry. 3. Metal–Organic Coordination Frameworks 3. Metal–OrganicThe family Coordination of porous solids Frameworks is another category of self-assembled materials which exhibit Theinteresting family properties of porous and solids wide is another range of category applications. of self-assembled Their functionality materials is based which mainly exhibit on interestingtheir propertieschemical and and wide physical range properties, of applications. such as porosity, Their surface functionality area and isthermal based stability. mainly A on sub their-category chemical of solid materials, which developed dramatically during the last decades, is the metal–organic and physical properties, such as porosity, surface area and thermal stability. A sub-category of solid frameworks (MOFs). MOFs are porous crystalline materials, which are constructed by metal ion materials,centres which or clusters developed and organic dramatically linkers throughduring the coordination last decades, bonding. is the The metal–organic term “metal– frameworksorganic (MOFs).framework” MOFs are was porous introduced crystalline for the materials, first time which in 1995 are fromconstructed Yaghi [73] by. metal The structure ion centres and or the clusters and organicproperties linkers of MOFs through depend coordinations on the choice bonding. of metal The term centres “metal–organic, as well as the framework” metal’s coordination was introduced for thenumber first time, which in 1995 consequently from Yaghi influences [73]. The the structure shape and and the the pore properties size. Also, of MOFsthe organic depends ligands on theplay choice of metalan equally centres, important as well as role the due metal’s to their coordination chemical reactivity number,, physical which consequently properties, geometry, influences and the the shape and thenon pore-covalent size. interactions Also, the organic that can ligands develop play [74,75] an equally. important role due to their chemical reactivity, physical properties,The implementation geometry, of and some the designnon-covalent approaches interactions can lead that to can construction develop [of74 functionalized,75]. Thestructures implementation with a wide of range some of design applications approaches. The can two leadcommon to construction methods utilized of functionalized for the functionalization of MOFs are the pre-synthetic modification, where the components are pre- structures with a wide range of applications. The two common methods utilized for the functionalized and form the desired MOF-based architecture, and the post-synthetic modification, functionalizationwhere the modification of MOFs takes are place the after pre-synthetic the preparation modification, of the MOF-based where progenitor the components. An example are pre-functionalizedof pre-synthetic and modification form the is desired the synthesis MOF-based of the rht architecture,-type MOF structure, and the post-synthetic [Co24(TPBTM)8 modification,(H2O)24], wherewhere the modificationthe TPBTM is takes an amide place functionalizedafter the preparation trigonal of carboxylate the MOF-based linker [76] progenitor.. This compound An example of pre-syntheticexhibited greater modification affinity for is CO the2 synthesismolecules ofin compariso the rht-typen with MOF the structure, compound [Co [Co2424(TPBTM)(btei)8(H28O)(H242],O) 24], due to dipole-quadrupole interactions and hydrogen bonds developed between the acylamide groups and the CO2 molecules. On the other hand, Kitagawa et al. reported an elegant example of post-synthetic modification via coordinative surface ligand exchange [57] of two Zn-based MOFs: [Zn2(1,4-bdc)2(dabco)]n and [Zn2(1,4-ndc)2(dabco)]n (Figure 12) [77].

Inorganics 2018, 6, 71 12 of 25 where the TPBTM is an amide functionalized trigonal carboxylate linker [76]. This compound exhibited greater affinity for CO2 molecules in comparison with the compound [Co24(btei)8(H2O)24], due to dipole-quadrupole interactions and hydrogen bonds developed between the acylamide groups and the CO2 molecules. On the other hand, Kitagawa et al. reported an elegant example of post-synthetic modification via coordinative surface ligand exchange [57] of two Zn-based MOFs: [ZnInorganics2(1,4-bdc) 2018, 62, (dabco)]x FOR PEERn and REVIEW [Zn2 (1,4-ndc) 2(dabco)]n (Figure 12)[77]. 13 of 26

Figure 12.12. Exchange of surface ligands,ligands, 1,4-benzenedicarboxylate1,4-benzenedicarboxylate units (1,4 (1,4-bdc)-bdc) by the fluorescentfluorescent boron dipyrromethene linkers. (Colour scheme: Zn, purple; O, O, red; B, pink; Co, yellow; C, dark dark grey; grey; N, blue; F, green).green).

MOF materials can be synthesized with various methods, and each one offers its own MOF materials can be synthesized with various methods, and each one offers its own advantages. The synthetic procedures typically involve the following: (a) solvothermal synthesis: the advantages. The synthetic procedures typically involve the following: (a) solvothermal synthesis: most commonly used method, in which the reagents are heated in solution in closed vessels under the most commonly used method, in which the reagents are heated in solution in closed autogenous pressure; (b) slow evaporation method: this does not need a supply of additional energy, vessels under autogenous pressure; (b) slow evaporation method: this does not need a supply but requires more time than other techniques, and is rarely used due to solubility issues; (c) of additional energy, but requires more time than other techniques, and is rarely used due electrochemical synthesis: the synthesis is carried out under mild reaction conditions, where real- to solubility issues; (c) electrochemical synthesis: the synthesis is carried out under mild time control/modification of the reaction parameters is possible; (d) mechanochemical synthesis: a reaction conditions, where real-time control/modification of the reaction parameters is possible; solvent-free, economical chemical reaction driven by mechanical force; (e) microwave-assisted (d) mechanochemical synthesis: a solvent-free, economical chemical reaction driven by mechanical synthesis: rapid method for MOF synthesis, and it has been used to produce nanosized metal oxides; force; (e) microwave-assisted synthesis: rapid method for MOF synthesis, and it has been used and (f) sonochemical synthesis: implementation of intensive ultrasonic radiation that triggers to produce nanosized metal oxides; and (f) sonochemical synthesis: implementation of intensive chemical and physical changes, which leads to the reduction of crystallization times. As a new class ultrasonic radiation that triggers chemical and physical changes, which leads to the reduction of of porous solid materials, MOFs are attractive candidates for a variety of industrial applications due crystallization times. As a new class of porous solid materials, MOFs are attractive candidates for to their physical and chemical properties such as high porosity, high surface area, and modular a variety of industrial applications due to their physical and chemical properties such as high porosity, surface. Some representative applications are summarized below: high surface area, and modular surface. Some representative applications are summarized below:

3.1.3.1. Catalysis andand Gas Gas Storage Storage Catalysis is is one one of of the the most most studied studied area area of applications of applications for forMOF MOF-based-based materials materials [78–80] [78. –The80]. Theporous porous nature nature of MOF of MOF structures structures allows allows the the efficient efficient diffusion diffusion of of the the substrates substrates towards towards the the catalytic sites (Figure 1313).). Taking into consideration the fact that the catalytic sites,sites, as well as the interactions within the pores,pores, can be tuned by employingemploying rational structure design approaches,approaches, thesethese are ideal candidates for catalytic applications. The compound MIL-101(Cr) is used in catalytic reactions due to the presence of free Lewis acid sites when water molecules are removed (Figure 14). Another important application field for MOF-based materials is the storage of small gas molecules [81–83]. Also, in this case, utilizing design principles is crucial for tailoring the structure for specific application such as engineering of the pores’ size and the chemical environment within the pores in order to optimize the affinity of the material towards specific substrates [84].

Inorganics 2018, 6, 71 13 of 25 candidates for catalytic applications. The compound MIL-101(Cr) is used in catalytic reactions due to the presence of free Lewis acid sites when water molecules are removed (Figure 14). Another important application field for MOF-based materials is the storage of small gas molecules [81–83]. Also, in this case, utilizing design principles is crucial for tailoring the structure for specific application such as engineering of the pores’ size and the chemical environment within the pores in order to optimize the affinityInorganicsof 2018 the, 6, material x FOR PEER towards REVIEW specific substrates [84]. 14 of 26 Inorganics 2018, 6, x FOR PEER REVIEW 14 of 26

Figure 13.13. RepresentativeRepresentativetypes types of of MOF MOF structures structure whichs which exhibit exhibit high high gas gas storage storage properties. propertie MOF-200:s. MOF- Figure 13. Representative types of MOF structures which exhibit high gas storage properties. MOF- 200: H2; MOF-74: CO2, NO, CH4; MIL-101(Cr): H2, CO2; HKUST-1: CO2, NO, CH4. H2; MOF-74: CO2, NO, CH4; MIL-101(Cr): H2, CO2; HKUST-1: CO2, NO, CH4. 200: H2; MOF-74: CO2, NO, CH4; MIL-101(Cr): H2, CO2; HKUST-1: CO2, NO, CH4.

Figure 14. The coordinated water molecules are removed from MIL-101(Cr) generating free Figure 14. The coordinated water molecules are removed from MIL-101(Cr) generating free Figurecoordination 14. The sites coordinated (highlighted water with molecules black arearrows) removed during from the MIL-101(Cr) activation generatingprocess. Subsequently, free coordination the coordination sites (highlighted with black arrows) during the activation process. Subsequently, the sitesactivated (highlighted MIL-101(Cr) with black catalyses arrows) the during cyanosilylation the activation of benzaldehyde. process. Subsequently,the (Colour scheme: activated Fe, MIL-101(Cr)lime green; activated MIL-101(Cr) catalyses the cyanosilylation of benzaldehyde. (Colour scheme: Fe, lime green; catalysesO, red). the cyanosilylation of benzaldehyde. (Colour scheme: Fe, lime green; O, red). O, red). 3.2.3.2. Drug DeliveryDelivery Systems Systems (DDS) (DDS) 3.2. Drug Delivery Systems (DDS) The last decade, in an effort to exploit the inherent properties of MOFsMOFs,, such such as as porosity porosity,, The last decade, in an effort to exploit the inherent properties of MOFs, such as porosity, modularmodular surface chemistry, large surface areas, and tunable pore sizes, variousvarious research groups modular surface chemistry, large surface areas, and tunable pore sizes, various research groups explored the the potential potential uses uses of MOF-based of MOF-based materials materials as drug as delivery drug delivery systems systems [85,86]. The[85,86] low. chemical The low explored the potential uses of MOF-based materials as drug delivery systems [85,86]. The low andchemical aqueous and aqueous stability stability of some of MOF some structures MOF structures render themrender highly them highly promising promising candidates candidates for drug for chemical and aqueous stability of some MOF structures render them highly promising candidates for deliverydrug delivery applications, applications, taking into taking consideration into consideration that MOF structural that MOF units structural need to be units biodegraded need to after be drug delivery applications, taking into consideration that MOF structural units need to be thebiodegraded drug release. after the drug release. biodegraded after the drug release. Very recently, Forgan et al. reported the synthesis and characterization of a Zr-based family of Very recently, Forgan et al. reported the synthesis and characterization of a Zr-based family of MOFs. This family of MOFs are made of biocompatible components which can penetrate the cells’ MOFs. This family of MOFs are made of biocompatible components which can penetrate the cells’ boundaries, and therefore can be used as potential materials for DDS. In this case, The MOF structures boundaries, and therefore can be used as potential materials for DDS. In this case, The MOF structures have been loaded with fluorescent molecules (e.g., calcein), and demonstrated the use of mechanical have been loaded with fluorescent molecules (e.g., calcein), and demonstrated the use of mechanical amorphisation processes for the controlled delivery of the guest molecules (Figure 15). The research amorphisation processes for the controlled delivery of the guest molecules (Figure 15). The research results showed that the fine balance between the material’s pores and the guest molecules is crucial results showed that the fine balance between the material’s pores and the guest molecules is crucial for achieving effective release of the guest molecules [87]. for achieving effective release of the guest molecules [87].

Inorganics 2018, 6, 71 14 of 25

Very recently, Forgan et al. reported the synthesis and characterization of a Zr-based family of MOFs. This family of MOFs are made of biocompatible components which can penetrate the cells’ boundaries, and therefore can be used as potential materials for DDS. In this case, The MOF structures have been loaded with fluorescent molecules (e.g., calcein), and demonstrated the use of mechanical amorphisation processes for the controlled delivery of the guest molecules (Figure 15). The research results showed that the fine balance between the material’s pores and the guest molecules is crucial for

Inorganicsachieving 2018 effective, 6, x FOR release PEER REVIEW of the guest molecules [87]. 15 of 26

Inorganics 2018, 6, x FOR PEER REVIEW 15 of 26

Figure 15. Structure of the organic linkers used in the MOF synthesis (a), and the structure of Zr-L6 Figure 15. Structure of the organic linkers used in the MOF synthesis (a), and the structure of Zr-L6

MOF ( b)).. (Colour (Colour scheme: scheme: Zn, Zn, cyan; cyan; O, O, red; red; C, C, black; black; N, N, blue). blue). Figure 15. Structure of the organic linkers used in the MOF synthesis (a), and the structure of Zr-L6 MOF (b). (Colour scheme: Zn, cyan; O, red; C, black; N, blue). 44.. Supramolecular Supramolecular Interactions in LDH Materials Layered4. Supramolecular double hydroxides Interactions (LDHs) in LDH are Materials a cla classss of of two two-dimensional-dimensional clays, clays, consisting consisting of of positively positively charged brucite-like layers and interlayer compensating anions. Their composition is described by charged brucite-likeLayered double layers hydroxides and interlayer (LDHs) are compensating a class of two- anions.dimensional Their clays, composition consisting of is positively described by the general formula [MII1II−xMIIIx(OH)III 2]x+[Axm+−x/m]m·n−H2O (Figure 16), where MII, MIII areII divalentIII (e.g., Mn, the generalcharged formula brucite-like [M layers1−xM andx(OH) interlayer2] [A compensatingx/m]·nH2 anions.O (Figure The 16ir composition), where M is,M describedare divalent by m− 2− 2− Zn) and trivalent (e.g., Al,II Fe) IIImetal ions,x+ mwhile− A representsm− interlayerII anionsIII (e.g., SO4 , CO32−). (e.g., Mn,the general Zn) and formula trivalent [M (e.g.,1−xM x Al,(OH) Fe)2] metal[A x/m ions,]·nH2 whileO (Figure A 16)represents, where M , interlayer M are divalent anions (e.g. (e.g.,, Mn, SO 4 , The 2 formationZn)− and trivalent of LDH (e.g.-,based Al, Fe) materialsmetal ions, while is driven Am− represents by weak interlayer interlayer anions in teractions(e.g., SO42−, COand32− ). as a CO3 ). The formation of LDH-based materials is driven by weak interlayer interactions, and as consequencea consequence,The formation, they they offer of offer LDH an an excellent-based excellent materials opportunity opportunity is driven for for the theby development weakdevelopment interlayer of of composite in compositeteractions ,compounds compoundsand as a by by consequence, they offer an excellent opportunity for the development of composite compounds by exchaexchangingnging the the anionic anionic and and solvent solvent content content of of the the interlayer interlayer cavities cavities with with the the desirable desirable components. components. exchanging the anionic and solvent content of the interlayer cavities with the desirable components. For example, exploitation exploitation of of supramolec supramolecularular interactions interactions between between the the LDH LDH layers layers and and POM POM clusters, clusters, For example, exploitation of supramolecular interactions between the LDH layers and POM clusters, have given a wide range of of materials with with inter interestingesting catalytic properties tailored for applications in epoxidations,have given N a-oxidations wide range andof materials desulfurisations with intere [88sting–92] catalytic. properties tailored for applications in epoxidations,epoxidations, N-oxidations N-oxidations and and desulfurisations desulfurisations [[8888––9292]]..

FigureFigure 16. Schematic 16. Schematic representation representation of the of the classical classical LDHs LDHs structure structure [88 [88]]. Adapted. Adapted from from Catalysts Catalysts2017 2017, 7,, 260. Figure7, 1260.6. Schematic representation of the classical LDHs structure [88]. Adapted from Catalysts 2017, 7, 260. As describedAs described earlier, earlier, MOFs MOFs are are idealideal candidates candidates for for the the development development of membrane of membrane materials materials for for gasAsgas separation,described separation, earlier, due to to MOFstheir their pore poreare size ideal size flexibility, candidates flexibility, and andfortheir the their sorption development sorption properties properties of [93,94]membrane. [However,93,94 materials]. However, the for gasthe separation, limitedlimited progress due toobserved observed their pore in the in size field the flexibility, fieldof MOF of-based MOF-based and membranes their sorption membranes is mainly properties due is mainlyto the [93,94] weak due. interfacialHowever, to the weak the bonding between MOF-based membranes and chemical inert substrates, such as α-Al2O3. An efficient limited progress observed in the field of MOF-based membranes is mainly due to the weak interfacial means employed for the immobilization of MOF clusters on substrates, is the use of an LDH network. bonding between MOF-based membranes and chemical inert substrates, such as α-Al2O3. An efficient Recently, a novel seeding method for the preparation of ZIF-8 membrane on the porous substrate α- means employed for the immobilization of MOF clusters on substrates, is the use of an LDH network. Al2O3, was reported [95]. The research group demonstrated the self-organization of a network made Recently,of crystallographically a novel seeding verticallymethod for aligned the preparation LDH walls, whic of ZIFh prevent-8 membred theane detachment on the porous of ZIF -substrate8 seeds. α- Al2O3The, was prepared reported ZIF [95]-8 membrane. The research exhibit grouped H2 permeancedemonstrated and highthe self H2 selectivity.-organization of a network made of crystallographicallyMoreover, Han vertically et al. reported aligned the LDHconstruction walls, whicof LDH@ZIFh prevent-8 composite,ed the detachment by in situ of growth ZIF-8 ofseeds . The preparedZIF-8 on Zn ZIF–Al-8– membraneLDH, without exhibit addinged any H2 zincpermeance precursor and (Figure high 1 7H).2 Theselectivity. resulting material exhibits Moreover,CO2 adsorption Han capacity et al. reported of 1 mmol the·g− 1construction at room temperature of LDH@ZIF and 1 bar,-8 composite, which is higher by in than situ Zn growth–Al– of ZIF-8 on Zn–Al–LDH, without adding any zinc precursor (Figure 17). The resulting material exhibits CO2 adsorption capacity of 1 mmol·g−1 at room temperature and 1 bar, which is higher than Zn–Al–

Inorganics 2018, 6, 71 15 of 25

interfacial bonding between MOF-based membranes and chemical inert substrates, such as α-Al2O3. An efficient means employed for the immobilization of MOF clusters on substrates, is the use of an LDH network. Recently, a novel seeding method for the preparation of ZIF-8 membrane on the porous substrate α-Al2O3, was reported [95]. The research group demonstrated the self-organization of a network made of crystallographically vertically aligned LDH walls, which prevented the detachment of ZIF-8 seeds. The prepared ZIF-8 membrane exhibited H2 permeance and high H2 selectivity. Moreover, Han et al. reported the construction of LDH@ZIF-8 composite, by in situ growth of ZIF-8 on Zn–Al–LDH, without adding any zinc precursor (Figure 17). The resulting material · −1 exhibits COInorganics2 adsorption 2018, 6, x FOR capacity PEER REVIEW of 1 mmol g at room temperature and 1 bar, which16 is of higher26 than Zn–Al–LDH or ZIF-8 [96] and is the result of cooperative effects developed between the LDH and ZIF-8 componentsLDH or ZIF of-8 the [96] assembled and is the result material. of cooperative effects developed between the LDH and ZIF-8 components of the assembled material.

Inorganics 2018, 6, x FOR PEER REVIEW 16 of 26

LDH or ZIF-8 [96] and is the result of cooperative effects developed between the LDH and ZIF-8 components of the assembled material.

Figure 17. In situ growth of ZIF-8 nanocrystals on Zn–Al–LDH. (Colour scheme: Zn, purple; C, black; Figure 17. In situ growth of ZIF-8 nanocrystals on Zn–Al–LDH. (Colour scheme: Zn, purple; C, H2O, blue spheres; An−, cyan spheres). Adapted from Dalton Trans. 2016, 45, 12632–12635. Copyright n− black; H2O,(2016) blue Royal spheres; Society of A Chemistry., cyan spheres). Adapted from Dalton Trans. 2016, 45, 12632–12635. Copyright (2016) Royal Society of Chemistry. 5. Transition Metal Coordination Cages Figure 17. In situ growth of ZIF-8 nanocrystals on Zn–Al–LDH. (Colour scheme: Zn, purple; C, black; 5. Transition MetalHSupramolecular2O, blue Coordination spheres; coordination An−, cyanCages spheres complexes). Adapted (CCS from) are Dalton finite Trans. supramolecular 2016, 45, 12632 compound–12635. Copyrights formed by the self(2016)-assembly Royal Soci ofety metal of Chemistry. centres in the presence of various ligands with appropriate binding sites Supramolecularand geometrical coordination features. The complexesformation of this(CCS) family are of finite compounds supramolecular is driven by self compounds-assembly formed by the self-assembly5processes. Transition, during Metal of metalwhich Coordination the centres thermodynamically Cages in the presence favoured of variousproduct is ligandssynthesized. with The appropriate constituents binding sites and geometricalof variousSupramolecular coordination features. coordination Theabilities formation complexesand geometries of (CCS this) thatare family finite co-exist supramolecular of compounds in the reaction compound is mixture drivens formed have by self-assembly theby tendency to undergo a self-sorting process via a network of dynamic equilibria which appears to processes,the during self-assembly which of the metal thermodynamically centres in the presence favoured of various product ligands with is synthesized. appropriate binding The constituents sites of and“correct” geom etrical mis-alignments features. Theof building formation blocks of this or familynon-efficient of compounds types of is coordinationdriven by self, -ultimatelyassembly various coordinationprocessespromoting, during the abilities formation which andthe of thermodynamically the geometries most stable that species favoured co-exist in solution. product in the In is reaction 1990, synthesized. Fujita mixture et The al. reportedconstituents have the the tendency to undergoofrational avarious self-sorting synthesis coordination of processa tetragonal abilities via metal and a network complex geometries, which of that dynamic consists co-exist of equilibria infour the linear reaction ligands which mixture (4,4′ appears-bipyridine) have the to “correct” mis-alignmentstendencyheld together of to building undergo by four [enPd(II)]a blocks self-sorting orunits non-efficient process (Figure via 18 )a [97] network types. The p ofalladium dynamic coordination,-based equilibria building ultimately which block appears exhibits promoting to a the “correct”90° coordination mis-alignments angle, which of buildingultimately blocks drives or th none formation-efficient of type thes specific of coordination compound., ultimately Thus, the formationstructural of the most features stable and species coordination in solution. angles of Inthe 1990, components Fujita present et al. reported in the reaction the rationalmixture are synthesis of promoting the formation of the most stable species in solution. In 1990, 0 Fujita et al. reported the a tetragonalrationalcrucial metal for synthesis directing complex, of athe tetragonal whichassembly consistsmetal process complex towards of four, which the linear formation consists ligands of of four a wide linear (4,4 range-bipyridine) ligands of compounds (4,4′-bipyridine) held, such together by ◦ four [enPd(II)]heldas cages together units and (Figurecapsulesby four [enPd(II)] [98]18)[. 97]. units The (F palladium-basedigure 18) [97]. The palladium building-based block building exhibits block a exhibits 90 coordination a angle, which90° coordination ultimately angle drives, which the formationultimately drives of the thespecific formation compound. of the specific Thus, compound. thestructural Thus, the features and coordinationstructural angles features of and the coordination components angles present of the incomponents the reaction present mixture in the reaction are crucial mixture for are directing the crucial for directing the assembly process towards the formation of a wide range of compounds, such assembly processas cages and towards capsules the [98] formation. of a wide range of compounds, such as cages and capsules [98].

Figure 18. Representation of the tetragonal metal complex construction.

Both the metal’s coordination angle and the ligand’s binding site angle play a key role in the formation of new structures with different geometries. Shortly after, the same group reported the synthesis of a M24L48 rhombicuboctahedron, by simply controlling the angle between the binding sites Figure 18. Representation of the tetragonal metal complex construction. of the dipyridylFigure donor 18. Representation ligand [99]. The of use the of tetragonal ditopic building metal complexblocks led construction. to the formation of 2D convexBoth polygons, the metal while’s coordination the combination angle ofand ditopic the ligand and tritopic’s binding building site angle blocks play led a to key the role forma in titheon formationof 3D polygons of new (F igurestructuress 19 and with 20 )different [100]. Also, geometries. the presence Shortly of capping after, the ligands same is group very importantreported thefor synthesis of a M24L48 rhombicuboctahedron, by simply controlling the angle between the binding sites of the dipyridyl donor ligand [99]. The use of ditopic building blocks led to the formation of 2D convex polygons, while the combination of ditopic and tritopic building blocks led to the formation of 3D polygons (Figures 19 and 20) [100]. Also, the presence of capping ligands is very important for

Inorganics 2018, 6, 71 16 of 25

Both the metal’s coordination angle and the ligand’s binding site angle play a key role in the formation of new structures with different geometries. Shortly after, the same group reported the synthesis of a M24L48 rhombicuboctahedron, by simply controlling the angle between the binding sites of the dipyridyl donor ligand [99]. The use of ditopic building blocks led to the formation ofInorganics 2D convex 2018, 6, polygons,x FOR PEER REVIEW whilethe combination of ditopic and tritopic building blocks led17 to of the 26 formationInorganics 2018 of, 6, 3Dx FOR polygons PEER REVIEW (Figures 19 and 20)[100]. Also, the presence of capping ligands17 of 26 is verythe implementation important for theof design implementation elements and of designsynthesis elements of diverse and compounds, synthesis of since diverse they compounds, prevent the the implementation of design elements and synthesis of diverse compounds, since they prevent the sinceformation they prevent of infinite the formation arrays and of infinite 1D coor arraysdination and chains 1D coordination, while they chains, introduce while they the introduce desirable formation of infinite arrays and 1D coordination chains, while they introduce the desirable thedirectionality. desirable directionality. directionality.

Figure 19. Formation of 2D architectures constructed by self-assembly of ditopic building blocks. Figure 19.19. Formation of of 2D 2D architectures constructed by by selfself-assembly-assembly of ditopic buibuildinglding blocks. Adapted from Chem. Rev. 2011, 111, 6810–6918. Copyright (2011) Royal Society of Chemistry. Adapted from Chem. Rev. 2011,, 111,, 6810–6918.6810–6918. CopyrightCopyright (2011)(2011) RoyalRoyal SocietySociety ofof Chemistry.Chemistry.

Figure 20. Formation of 3D architectures constructed by self-assembly of ditopic and tritopic building Figure 20. Formation of 3D architectures constructed by self-assembly of ditopic and tritopic building Figureblocks. 20.AdaptedFormation from ofChem. 3D architectures Rev. 2011, 111 constructed, 6810–6918 by. Copyright self-assembly (2011) of ditopicRoyal Society and tritopic of Chemistry. building blocks. Adapted from Chem. Rev. 2011, 111, 681 6810–6918.0–6918. Copyright (2011) Royal Society of Chemistry. Supramolecular coordination complexes have attracted the attention of various research Supramolecular coordination complexes have attracted the attention of various research [101,102]Supramolecular groups due coordination to their applications complexes havein catalysis attracted [103] the attention, recognition of various and research separation [101 [104],102], [101,102] groups due to their applications in catalysis [103], recognition and separation [104], groupsstabilization due toof theirsensitive applications species [105] in catalysis, etc. The [103 ability], recognition of incorporating and separation different [ 104funct],ional stabilization groups ofto stabilization of sensitive species [105], etc. The ability of incorporating different functional groups to sensitivecoordination species cages [105 during], etc. The the ability self-assembly of incorporating process different renders functional the final groups compounds to coordination excellent coordination cages during the self-assembly process renders the final compounds excellent candidates for rational design of supramolecular nanoreactors [106]. Very often, the guest molecules candidates for rational design of supramolecular nanoreactors [106]. Very often, the guest molecules are encapsulated centrally due to their solvophobicity and size complementarity between guests and are encapsulated centrally due to their solvophobicity and size complementarity between guests and hosts. Occasionally, the nanocages are able to accommodate more than one guest depending on their hosts. Occasionally, the nanocages are able to accommodate more than one guest depending on their size of the host cage and the guest molecule. Nitschke et al. reported the synthesis of a new class of size of the host cage and the guest molecule. Nitschke et al. reported the synthesis of a new class of supramolecular MII6L4 pseudo-octahedra which can interact with guest molecules both internally and supramolecular MII6L4 pseudo-octahedra which can interact with guest molecules both internally and externally [107]. Moreover, the group demonstrated the importance of peripheral guests which externally [107]. Moreover, the group demonstrated the importance of peripheral guests which template the formation of the CuII6L4 structure; they showed that the cage would be able to form template the formation of the CuII6L4 structure; they showed that the cage would be able to form based only on the self-assembly process or central template considerations. based only on the self-assembly process or central template considerations.

Inorganics 2018, 6, 71 17 of 25 cages during the self-assembly process renders the final compounds excellent candidates for rational design of supramolecular nanoreactors [106]. Very often, the guest molecules are encapsulated centrally due to their solvophobicity and size complementarity between guests and hosts. Occasionally, the nanocages are able to accommodate more than one guest depending on their size of the host cage and the guest molecule. Nitschke et al. reported the synthesis of a new class of supramolecular II M 6L4 pseudo-octahedra which can interact with guest molecules both internally and externally [107]. Moreover, the group demonstrated the importance of peripheral guests which template the formation II ofInorganics the Cu 20186, L6,4 x structure;FOR PEER REVIEW they showed that the cage would be able to form based only on18 of the 26 self-assembly process or central template considerations. The environment inside the cavity is very different from the bulk solution,solution, and this is the main reason that that the the encapsulated encapsulated molecules molecules demonstrate demonstrate different different chemical behaviour.chemical behaviour Under appropriate. Under conditions,appropriate uniqueconditions, reactions unique can reactions be carried can out be withincarried the out cage within which the arecage not which generally are not favoured generally in thefavoured reaction in medium;the reaction this medium; is extremely this important is extremely in several important applications, in several including applications, catalysis including [108]. Fujitacatalysis et al.[108] investigated. Fujita et the al. encapsulationinvestigated the of dinuclearencapsulation compounds of dinuclear which compounds exhibit weak which metal–metal exhibit bondsweak metal utilizing–metal a bonds cage-type utilizing nanoreactor a cage-type (Figure nanoreactor 21). In(Figure this case,21). In the this ruthenium case, the ruthenium complex, [(complexη-5-indenyl)Ru(CO), [(η-5-indenyl)Ru(CO)2]2, adopts2] a2, CO-bridgedadopts a COcis-bridgedconfiguration cis configuration and an unexpected and an enhancement unexpected ofenhancement its photostability of its photostability is observed [is109 observed]. Furthermore, [109]. Furthermore, the confined the space confined of the space nanocage of the nano preventscage theprevents dissociation the dissociation of the metal-metal of the metal bond,-metal which bond, is which generally is generally favoured favoured out of theout cage.of the Itcage. has It been has reportedbeen reported that the compoundthat the compound [(Me4Cp)Ru(CO) [(Me24]Cp)Ru(CO)2 (Cp = cyclopentadienyl)2]2 (Cp = cyclopentadienyl) undergoes photosubstitution undergoes ofphotosubstitution a CO ligand by of an a alkyne CO ligand without by an dissociation alkyne without of the dissociation Ru–Ru bond, of andthe aRu Ru–alkyne–Ru bond,π and–complex a Ru– isalkyne formed π–complex [110]. is formed [110].

IIII Figure 21.21. Representation of the M 6LL44 typetype cage cage reported reported by by Fujita Fujita et et al. al. Colour Colour code: code: Pd, Pd, red red spheres; spheres; C, black; N, blue.blue.

The solubility and host/guest capabilities of supramolecular coordination systems renders them The solubility and host/guest capabilities of supramolecular coordination systems renders them highly promising candidates for biomedical applications. Stang et al. synthesised eight tetranuclear highly promising candidates for biomedical applications. Stang et al. synthesised eight tetranuclear rectangles employing a coordination self-assembly approach between arene–Ru-based acceptors and rectangles employing a coordination self-assembly approach between arene–Ru-based acceptors and 3-bipyridyl donors. Interestingly, the research group investigated the in vitro cytotoxicities relative 3-bipyridyl donors. Interestingly, the research group investigated the in vitro cytotoxicities relative to to cis-platin and doxorubicin. Four of these compounds exhibited notable activity (Figure 22) [111]. cis-platin and doxorubicin. Four of these compounds exhibited notable activity (Figure 22)[111]. Finally, the CCSs nano-structures can also be used as selective molecular sensors. Chi and co- Finally, the CCSs nano-structures can also be used as selective molecular sensors. Chi and workers reported the synthesis of two heterometallic self-assembled molecular squares and studied co-workers reported the synthesis of two heterometallic self-assembled molecular squares and studied their potential functionality as sensors for the selective detection of picric acid [112] (Figure 23). their potential functionality as sensors for the selective detection of picric acid [112] (Figure 23). It is worth noting at this point that the examples we discussed above do not constitute an exhaustive list of available families of clusters. There are numerous other interesting metal coordination cages with remarkable structural features and properties [113–117]. Any effort to discuss every available family of clusters, their general features and functionality would have been fruitless and goes beyond the scope of the present article.

Inorganics 2018, 6, 71 18 of 25

It is worth noting at this point that the examples we discussed above do not constitute an exhaustive list of available families of clusters. There are numerous other interesting metal coordination cages with remarkable structural features and properties [113–117]. Any effort to discuss every available family of clusters, their general features and functionality would have been fruitless andInorganicsInorganics goes 20182018 beyond,, 66,, xx FORFOR the PEERPEER scope REVIEWREVIEW of the present article. 1919 ofof 2626

FigureFigure 22.2222.. SelfSelf-assemblySelf--assemblyassembly ofof tetranucleartetranuclear molecularmolecular rectangles.rectangles.

Figure 23. Heterometallic self-assembled square-shaped molecular sensor. (Colour scheme: Pd/Pt, Figure 23.23. HeterometallicHeterometallic self-assembled self-assembled square-shapedsquare-shaped molecular molecular sensor. sensor. (Colour(Colour scheme: scheme: Pd/Pt, Pd/Pt, teal; Fe, dark yellow; C, grey; N, blue; O, red; S, yellow; P, orange; F, bright green). Adapted from teal; Fe, dark yellow;yellow; C, grey; N, blue; O, red; S, yellow; P, orange;orange; F,F, brightbright green).green). Adapted from Chem. Rev. 2012, 41, 3046–3052. Copyright (2011) Royal Society of Chemistry. Chem. Rev. 20122012,, 41,, 3046–3052.3046–3052. Copyright (2011)(2011) RoyalRoyal SocietySociety ofof Chemistry.Chemistry. 6. Conclusions 6.6. Conclusion Conclusionss In summary, supramolecular non-covalent interactions and molecular self-assembly has proven In summary, supramolecularsupramolecular non-covalent non-covalent interactions interactions and and molecular molecular self-assembly self-assembly has has proven proven to to be powerful tools in synthetic chemistry, which have been employed for the construction of beto powerfulbe powerful tools tool in synthetics in synthetic chemistry, chemistry, which have which been have employed been employed for the construction for the construction of compounds of compounds made by primary and secondary (cluster-based) building blocks. Both processes are madecompounds by primary made and by secondary primary and (cluster-based) secondary building(cluster-based blocks.) bu Bothilding processes blocks. are Both driven processes by specific are driven by specific physical or chemical parameters, such as recognition processes, templating effects physicaldriven by or specific chemical physical parameters, or chemical such parameters as recognition, such processes, as recognition templating processes effects, templating and chemical effects and chemical reactivity. Efforts to understand and ultimately control such dynamic behaviour are reactivity.and chemical Efforts reactivity. to understand Efforts to and understand ultimately and control ultimately such control dynamic such behaviour dynamic are behaviour challenging, are challenging, but also exciting, since the embedded error correction mechanism offered by the butchal alsolenging exciting,, but since also exciting the embedded, since error the embed correctionded mechanismerror correction offered mechanism by the reversibility offered by of the reversibility of the metal–ligand bonding, in combination with the geometrically defined metal metal–ligandreversibility ofbonding, the metal in combination–ligand bonding with the, ingeometrically combination defined with the metal geometrically coordination defined environment, metal coordinationcoordination environmentenvironment,, rendersrenders thethe selfself--assemblyassembly processesprocesses aa particularlyparticularly fruitfulfruitful approachapproach forfor thethe constructionconstruction ofof complexcomplex structures.structures. IncrementalIncremental understandingunderstanding andand controlcontrol overover thethe covalentcovalent andand nonnon--covalentcovalent interactionsinteractions,, andand theirtheir subsequentsubsequent utiutillizationization asas syntheticsynthetic tooltoolss inin chemicalchemical systemssystems,, ledled toto the the development development of of new new families families of of compounds compounds with with impressive impressive functionalities. functionalities. As As discussed discussed above,above, polyoxometalate, polyoxometalate, metal metal organicorganic frframeworksameworks andand supramolecularsupramolecular coordinationcoordination compoundscompounds areare all all famil familiieses of of compounds compounds with with unique unique physical physical and and chemical chemical properties properties whosewhose formation formation is is basedbased on on thethe cooperative cooperative effect effect of of self self--assemblyassembly and and supramolecular supramolecular interactions interactions.. The The implementaimplementatitionon of of design design principlesprinciples using using appropriate appropriate building building blocks blocks led led to to the the emergence emergence of of

Inorganics 2018, 6, 71 19 of 25 renders the self-assembly processes a particularly fruitful approach for the construction of complex structures. Incremental understanding and control over the covalent and non-covalent interactions, and their subsequent utilization as synthetic tools in chemical systems, led to the development of new families of compounds with impressive functionalities. As discussed above, polyoxometalate, metal organic frameworks and supramolecular coordination compounds are all families of compounds with unique physical and chemical properties whose formation is based on the cooperative effect of self-assembly and supramolecular interactions. The implementation of design principles using appropriate building blocks led to the emergence of functionalities based on cooperative effects developed between the self-assembled constituents. The properties that these materials exhibit range from gas storage, drug delivery, selective sensors and membranes to catalysis, stabilization of short-lived species and chemical conversions to useful products. Moreover, the coupling of these processes between the solution and solid-state offers a unique opportunity for the future development of materials with desirable functionality. We are confident that the observed exponential growth of well-defined architectures in the area of self-assembled inorganic and metal–organic systems in combination of the deeper understanding of the underlying processes, will be followed by an explosion in functionalities; taking advantage of design principles for the construction of building blocks with appropriate geometry, physical and chemical behaviour, will allow us to engineer highly sophisticated chemical systems with unprecedented properties.

Author Contributions: The paper was written jointly with equal contributions from S.P., T.A.K., Y.-F.S. and H.N.M. Funding: This research received no external funding. Acknowledgments: This work has been supported by The University of Glasgow. Conflicts of Interest: The authors declare no conflict of interest.

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