Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic–Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates

Lacunary polyoxometalates (LPOMs) are key precursors for the synthesis of functional POMs. To date, reviews dedicated to behavioral studies of LPOMs often comprise the role of metal ions, including transition metal (TM) and rare earth (RE) ions, in extending and stability of high-nuclearity clusters. In contrast, the role of organic ligands in the structures and properties of lacunary-based hybrids has remained less explored. In this review, we focus on the role of organic fragments in the self-assembling process of POM-based architectures and discuss relationships between the nature and structure of organic ligand and properties such as the topology of hybrid inorganic–organic material in RE and TM-RE heterometallic derivatives of lacunary Keggin-type POMs. The effects of organic fragment in mixed ligand hybrids are also briefly reviewed.


Introduction
Polyoxometalates (POMs), a large group of fascinating polynuclear metal-oxo clusters of early transition metals such as Mo, W and V, constitute a marvelous class of inorganic systems, due to their intriguing structures and remarkable potential applications in electrochemistry, catalysis, magnetism, and medicine [1][2][3][4][5][6]. Lacunary POMs (LPOMs) with a set of remarkable properties such as high coordination reactivity, rigidity, oxidative and thermal stability are an important sub-class of POMs [7]. Typically, LPOMs imply that topologies gain by the loss of one single {MO} moiety or multiple {M x O 2x } moieties, resulting in the formation of the monolacunary or polylacunary POMs, respectively. Mainly, lacunary species are limited to polyoxotungstates, while lacunary POMs of polyoxomolybdates and polyoxovanadates are uncommon [8]. Tungsten skeleton of the POMs with saturated Keggin [XW 12 [9]. The lacunary species with high negative charge and nucleophilic oxygenenriched surfaces can interact with various cations. In fact, owing to the negative-charged surface and defect binding sites, LPOMs can act as outstanding multidentate nucleophilic ligands toward the electrophilic center. Transition metal (TM) or lanthanoide (Ln) cations can be incorporated into the defect sites of LPOMs to form metal-substituted POMs, which exhibit unique chemical properties that depend on the incorporated metal ions [10][11][12][13][14][15][16][17][18][19][20][21][22]. Metal-substituted POMs possess a higher negative charge density than the saturated parent POMs due to the substitution of a high oxidation state W 6+ ion by a low oxidation state M n+ ion (usually n = 1-3). One of the promising approaches toward the synthesis of this kind of material is combining metal-substituted POMs with organic or metal-organic fragments to make inorganic-organic hybrid-based LPOMs. Various POM-based inorganic-organic hybrids with interesting structural and functional properties were reported [23][24][25]. In this field, a certified fact is that the synthesis of such hybrids depends on the selection of POMs as inorganic building blocks and organic ligands as structure-directing and functional components. Therefore, the careful choice of POM species and organic ligands are vital for the synthesis of hybrids with intriguing topologies and improved properties. The utilization of organic fragments as a solvent or coordinated with metal centers in the metal-substituted POM compounds can improve physical and chemical properties of these compounds. Most metal-substituted POM hybrids contain N-donor organic ligands. In these structures, organic fragments not only act as charge-compensation cations, but also as multidentate chelating agents to coordinate TM or Ln cations. This highlights the significant role of organic ligands as stabilizing agents in forming hybrid structures. The general structures discussed in this review are summarized in Figure 1.
choice of POM species and organic ligands are vital for the synthesis of h triguing topologies and improved properties. The utilization of organic solvent or coordinated with metal centers in the metal-substituted POM c improve physical and chemical properties of these compounds. Most m POM hybrids contain N-donor organic ligands. In these structures, organi only act as charge-compensation cations, but also as multidentate chel coordinate TM or Ln cations. This highlights the significant role of org stabilizing agents in forming hybrid structures. The general structures d review are summarized in Figure 1.
In the present review, we highlight the most important investigati ic-organic hybrid compounds combining LPOMs with TM and Ln cente research reveals a great variety of N/O-donor ligands and their potential sembly of metal-substituted POMs that have not been previously review this review provides a comprehensive description of the different roles of in their interaction with these relevant inorganic clusters.

Inorganic-Organic Hybrids Based on TMSPs
TM-substituted polyoxometalates (TMSPs) are an important clas [26,27]. Compared to nonlacunary heteropolyoxometalates, the localized of LPOMs have made them reactive building blocks for the construction o They are commonly used as excellent precursors, since (i) the LPOMs are from intact materials in high yield, (ii) vacant sites are conducive to enca transition metals, including mixed-valent metal ions, and (iii) the robus makes it possible to predict the frameworks of the ultimate products. H ferent types of LPOMs saturated by TMs or even TM complexes have b recent years [28][29][30][31]. However, the introduction of an organic ligand i pivotal impact on the chemical/physical properties or the structural featur via synergetic effects between the properties of the POM and those of the fore, the role of the organic ligands as structure-directing agents is predo inorganic-organic hybrid constructions, a point that is less considered in containing polyoxometalate clusters. Because of the nucleophilic inter In the present review, we highlight the most important investigations of inorganicorganic hybrid compounds combining LPOMs with TM and Ln centers. This field of research reveals a great variety of N/O-donor ligands and their potential roles in the assembly of metal-substituted POMs that have not been previously reviewed. Therefore, this review provides a comprehensive description of the different roles of various ligands in their interaction with these relevant inorganic clusters.
The reactivity of other nitrogen heterocyclic ligands such as imi has also been studied. It has been shown that imi ligands connect to metal ions via M-N imi bonds, and no M-C imi bond in POMs has been observed to date [73][74][75][76][77]. In addition, numerous studies have shown that imi could interfere with DNA via weak interactions (hydrogen bonds, π-π stacking, etc.), and then halt cell growth and division [78,79]. Compound {[Ag 7 (H 2 biim) 5 ][PW 11 O 39 ]}·Cl·H 3 O displays a 2D network featuring dimerized monolacunary Keggin anions {PW 11 O 39 } 2 which are connected through hexanuclear silver clusters. Interestingly, besides {Ag 5 } 5+ clusters, there are other kinds of argentophilic {Ag 4 } 4+ clusters coexisting in this compound [80]. Liu et al. reported three hexa-nuclear-substituted sandwich-type arsenotungstates [81]. The transition metal ions (Ni II , Co II , and Mn II ) and Na + are alternately coordinated in the six-membered central belt by [α-AsW 9 O 33 ] 9− units, taz, and water molecules. The contribution of nitrogen atoms of the ligands in the formation of hydrogen bonding network leads to the fortification of the structures [81]. Htz as a rigid multifunctional ligand can provide four sequent electron-donating nitrogen atoms to coordinate to metals with the smaller steric hindrance. Many metal-organic frameworks based on Htz ligands exhibit intriguing topologies and interesting magnetic, absorptive, and photophysical properties, having diverse coordination/bridging modes [82,83]. For example, in {[Cu 8 (tz) 8 (Htz) 4 VI 10 Mo V O 39 ]}·~10H 2 O, the six-nuclear copper clusters are bridged by the tz ligands to form wave-like layers by the µ 2 -Htz ligands. The polyoxomolybdate anions act as the eight-connected node to link the layers into a 3D framework [84].

TMSPs with O-Donor Ligands
O-donor ligands have been comparatively less studied than flexible nitrogen donating ligands for the design and synthesis of new hybrids. As a peculiar branch of POMs, considerable attention has been directed toward POM-based metal carbonyl derivatives in recent years because of their unique structures and potential catalytic properties [85][86][87][88]. For example, Na 2 H 2 [(CH 3 ) 4 N] 6 6 } octahedra. Furthermore, two Re atoms and two central W atoms are located in the same plane and each carbonyl rhenium group fac-{Re(CO) 3 } + is in the "out-of-pocket" structural motif [88].
Acetate has been frequently used as a bridging ligand to connect different fragments. For example, in the monomeric structure of [γ-H 2 SiW 10 O 36 Pd 2 (OAc) 2 ] 4− , Pd atoms are bridged by two bidentate acetate ligands [89]. However, the synthesis is usually performed in acetate buffer solution [90]. A bidentate acetate ligand connects two diametrically opposed Ru atoms in a (µ 2 -η 1 :η 1 ) fashion. As far as we know, such a crown-shaped acetate-bridged Ru-substituted arsenotungstate is the first report that supports the structural novelty of this rare compound [91]. Kholdeeva, Kortz, and co-workers reported isolated polyanions with unprecedented hexazirconium and hexa-hafnium core and the metal ions occupying the vertices of an octahedron that is accommodated by two (B-α-AsW 9 O 33 ) fragments. The two {AsW 9 } units are not eclipsed, leaving a cavity perfectly suitable to host the M 6 unit. Furthermore, five bridging acetate ligands lead to stronger bonding between metal centers [92]. The first carboxylic group decorated arsenotungstate was reported in 2015 [93]. Each Mn II ion in the Na 15 [(Mn II (COOH)) 3 (AsW 9 O 33 ) 2 ]·15H 2 O is chelated with four oxygen atoms (µ 3 -O) from four {WO 6 } octahedra belonging to two different {AsW 9 } units and edge-sharing with two adjacent {NaO 6 } groups. The most interesting structural feature is that three carboxy groups are separately bonded to three manganese ions. O is functionalized by three types of organic ligands, which is rare in POMs. Furthermore, the nitrogen atoms of the Tris ligand incorporate in hydrogen bonding and help to expand the structure [94]. Subjoining of multi-dentate organic ligands such as tartaric acid and glycolic acid into tetranuclear sandwich-type [Zr 4 (H 2 O) 2 (µ 3 -O) 2 (GeW 10 O 37 ) 2 ] cluster not only helps in the fortification of the sandwich-type cluster, but also, in some cases, induces chirality in the polyoxometalates [95]. Utilization of the dicarboxylato ligands such as [OOC(CH 2 ) 4 COO] 2− (adipate), instead of monocarboxylate ligands, can lead to the oligomeric structure. As shown in the dimeric helical Na 2 K 12 [Ni(H 2 O) 6 ][(SiW 9 O 34 ) 2 (OH) 6 Ni 8 (C 6 H 8 O 4 ) 3 ]·40H 2 O structure, three adipato linkers connect two {SiW 9 Ni 4 } units ( Figure 2) [96]. Liu et al. reported unique nona-Mn II -encapsulated sandwich-type species in which the utilization of three oxalate ligands lead to the formation of planar hexagonal {Mn 6 } core. This structure was further connected to another three external Mn II cations and constructed a 1D oxalate-bridging high-nuclear Mn-sandwiched antimonotungstate chain [97]. The utilization of chiral ligands and transfer of chirality to achiral LPOM units has been reported by using L-or D-tartrate units (tart) in the large polyoxoanion compound [α-P 2 W 15 O 56 ] 12 [98]. In another interesting study, Ishimoto et al. successfully prepared a BINOL-functionalized lacunary Keggin-type POM for the asymmetric oxidation of thioanisole [99]. This is especially true in POMs, which contain multiple metal centers that are subject to rapid racemization via water exchange, partial hydrolysis, or fluxional rearrangements. As a result, it is often challenging to discriminate between enantiomers and/or diastereomers, and even more formidable to achieve partial or complete resolution.  [100]. While in it, the µ 2 -OH and terminal aqua ligands are replaced by the oxygens of the ligating malates (from carboxylate and hydroxo moieties) [101]. Changing organic units to D/L-mandelic acid has been seen in similar architecture [102]. Wang et al. also reported four similar chiral sandwich-type compounds consisting of tetra-Zr IV -substituted sandwich-type Keggin polyoxoanion and L-/D-mal fragments. The most striking structural feature of (NH 4 ) 3 Na 2 K 5 [Zr 4 (µ 3 -O) 2 (L-mal)(D-mal)(B-α-SiW 10 O 37 ) 2 ] relative to similar compounds is that each [Zr 4 (µ 3 -O) 2 (L-mal)(D-mal)(B-α-SiW 10 O 37 ) 2 ] 10− polyoxoanion joins adjacent six K + bridges and each K + bridge links to adjacent three [Zr 4 (µ 3 -O) 2 (L-mal)(D-mal) (B-α-SiW 10 O 37 ) 2 ] 10− polyoxoanions, leading to a 2D layer. Furthermore, adjacent 2D layers are interconnected by two Na + cations, forming a 3D framework [103].

Inorganic-Organic Hybrids Based on RESPs
It is apparent that most mentioned RE-POMs are purely inorganic, and the investigations of organic-inorganic hybrids based on RE-POMs are relatively scarce. Although the simultaneous presence of organic and LPOM ligands that are bound to RE centers is also rare, one can expect that their properties are conveyed to novel hybrid molecules [110]. Occasionally, organic solvents can be connected to metal centers in the POM-based structures. For example, solvents such as acetone, DMSO, and DMF can play the role of a ligand in the dimeric, 1D, and 2D inorganic-organic hybrids based on RESPs by coordinating to metal centers [111,112]. By considering structural features, organic ligands with their functional groups reduce distances between RE centers and facilitate the formation of polynuclear fragments. Thus, in this section, we discuss the role of these interactions in the stability and structural features of rare-earth-substituted inorganic-organic hybrids. The tendency of RE centers to carboxylate groups results in effective interactions between these fragments. Naruke and co-workers reported [Ce 3 (CO 3 )(SbW 9 [113]. The presence of carbonate in the structure led to the isolation of a compound that was isomorphous to another structure, previously reported by the authors [114].
Acetic acid has been frequently used as a bridging agent in the synthesis of inorganicorganic hybrids because of its small size and tendency to RE metals [115]. In the dimeric structures of {RE(α-XW 11 O 39 )(H 2 O)} 2 [X = Si, Ge, P], acetic acid units act as linkers via (η 2 , µ-1,1) fashion, connecting two RE centers. The atomic radius of metals has an important role in the construction of such compounds (  [120]. Amino acids, as a type of carboxyl-and-amino-containing flexible multidentate ligand, are outstanding candidates for performing as organic modifiers in the building of novel structures [121,122]. In this regard, four gly amino acids linked two {RE(α-BW 11  Lu III ), concluding in a four-leaf-clover-shaped tetrameric structure. However, the major inconsistency in the ala-decorated W-O-RE heterometallic clusters lies in the number of ala molecules, which may result from the different coordination geometries of RE ions and the various construction modes of W-O-RE heterometallic clusters. It should be mentioned that the carboxyl groups of ala ligands only coordinate with the W centers in the compounds containing Dy III , Ho III , Er III , Yb III , and Lu III ions, and they not only link the W centers together but also combine RE ions in the other compounds [124]. In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.

Inorganic-Organic Hybrids Based on PBTREHDs
In recent years, the design and synthesis of 3d-4f inorganic-organic hybrids based on RESPs has increasingly become an emerging field of research due to an undeniable 4 In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.

Inorganic-Organic Hybrids Based on PBTREHDs
In recent years, the design and synthesis of 3d-4f inorganic-organic hybrids based on RESPs has increasingly become an emerging field of research due to an undeniable 4 In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.

Inorganic-Organic Hybrids Based on PBTREHDs
In recent years, the design and synthesis of 3d-4f inorganic-organic hybrids based 4 In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.    In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.    In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.  In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   In some cases, organic ligands not only do not contribute to the increasing nuclearity of a structure but also acts vice versa.   Although ox ligands, such as the aforementioned ligands containing carboxylate groups, act as a linkage in the dimer of {RE(α-XW 11 O 39 )} subunits, their distances between RE centers are different (Table 2) [125]. In some cases, this ligand imposes polynuclear structures and deduces tetrameric moieties [126,127]. Tartaric acid compared to oxalic acid is more flexible, but their carboxylate groups can be completely or partially deprotonated. Hence, two {RE 2 (AsW 9 O 33 )} subunits can be linked by two series of non-equivalent tartaric acid segments in an unusual fashion [128]. For the dimeric polyoxotungstate [Ho(tart)(α-PW 11 O 39 )] 2 16− , the tartrate anion connects two Ho III ions by one carboxyl O atom and one hydroxyl O atom from each end of the tart ligand. Furthermore, two tartrate anions displayed a type of mesomeric configuration with the co-existence of the D-tartrate anion and L-tartrate anion, which played a significant role in understanding the organic carboxylic acid functionalization.  [130]. Two Dy ions occupy non-adjacent sites of lacunary {AsW 10 O 38 } polyanion, and each of them is coordinated to a tridentate citric acid ligand. Strong hydrogen bonding resulting from lattice water molecules has fortified the architecture [131]. In the dimeric structure of K 20 (Figure 4a). Although both sides of this polyanion were supported by two [RE(H 2 O) 5 ] III groups, the structure was extended to a 1D heterometallic double chain through the connection of the mentioned atoms in the same direction (Figure 4b). Similar structures were also observed for a series of compounds containing La III , Pr III , Nd III , Sm III , and Eu III cations, in which the connection of one O atom of a carboxylate moiety and one RE III ion of the adjacent unit and the interconnection of these chains constructed a 3D extended framework (Figure 4d) [179].  Because of high affinity of RE cation linkers to the anionic surface of polyanions, their utilization in synthesis is challenging and almost leads to precipitation [180,181]. By introducing O-donor ligands to the reaction medium Zhao et al. [182]

Inorganic-Organic Hybrids Based on PBTREHD with Mixed Organic Ligands
As mentioned before, O-donor ligands prefer to interact with RE cations in the presence of TM ions; therefore, the introduction of organic fragments containing carbox- Because of high affinity of RE cation linkers to the anionic surface of polyanions, their utilization in synthesis is challenging and almost leads to precipitation [180,181]. By introducing O-donor ligands to the reaction medium Zhao et al. [182] successfully established the 3D K 4 Na 4 [Ce 2 (ox) 3 [184].

Conclusions
The localized surface charge and diverse structures of LPOMs have made them reactive building blocks for the construction of inorganic-organic architectures. The structures of the lacuna itself as well as the main coordination modes of organic fragments are key parameters that need to be considered. The topology of the final hybrid depends on the number of vacancies generated in the parent structure. Organic fragments, on the  [198].
[H 2 N(CH 3 10 (β-TeW 9 O 33 ) 2 ] 4− fragments in which two external Fe III ions are substituted by two RE complexes. Among the two O-donor ligands in the structure, only 2,5-pdc as a linkage, incorporating N and O atoms, conduce tetrameric and then 1D chain arrangement [199].

Conclusions
The localized surface charge and diverse structures of LPOMs have made them reactive building blocks for the construction of inorganic-organic architectures. The structures of the lacuna itself as well as the main coordination modes of organic fragments are key parameters that need to be considered. The topology of the final hybrid depends on the number of vacancies generated in the parent structure. Organic fragments, on the other hand, can work as ornaments, bridges, and stabilizers in the whole structure. Since oxo groups in POM surfaces can only bond with limited species of transition metals, the utilization of organic ligands is crucial. In the present work, we highlighted the most important reports of inorganic-organic hybrid structures combining LPOMs with TM and Ln ions and compared the different roles of various ligands in their interaction with relevant Keggin-type POMs. Our results demonstrate a great variety of organic ligands and their important roles in the architecture of metal-substituted POMs.