Coordination Chemistry Reviews - جستجو در مقالات دانشگاهی و ...profdoc.um.ac.ir/articles/a/1052932.pdf · 2018-01-24 · Coordination Chemistry Reviews 309 (2016)

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Coordination Chemistry Reviews 309 (2016) 84–106

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

j ourna l h omepage: www.elsev ier .com/ locate /ccr

eview

uning the topology of hybrid inorganic–organic materials based onhe study of flexible ligands and negative charge of polyoxometalates:

crystal engineering perspective

omayeh Taleghania, Masoud Mirzaeia,∗, Hossein Eshtiagh-Hosseinia, Antonio Fronterab

Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 917751436, IranDepartament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca, Baleares, Spain

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 852. Rigid vs. flexible ligands in hybrid inorganic–organic compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853. The design of the flexible ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 874. An analysis of the topology and dimensionality in hybrid inorganic–organic compounds: the influence of the flexible ligands . . . . . . . . . . . . . . . . . . . 89

4.1. Entangled structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.1.1. Hybrids with polythreaded/penetrating architectures based on imidazole derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.1.2. Hybrids with polythreaded/penetrating architectures based on triazole, and tetrazole derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 904.1.3. Hybrids with polycatenated/polypseudo-rotaxane architectures based on triazole derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .914.1.4. Hybrids with polyrotaxane/penetrating architectures based on pyridyl derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

4.2. Other structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924.2.1. Hybrids based on imidazole derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 924.2.2. Hybrids with loop/helical subunits based on triazole, and pyridyl derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934.2.3. Hybrids with cluster subunits based on tetrazole derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.2.4. Other hybrids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

5. Different negative charges of polyoxometalate modulated self-assembly of hybrid architectures based on flexible ligands . . . . . . . . . . . . . . . . . . . . . . . .976. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

r t i c l e i n f o

rticle history:eceived 1 June 2015ccepted 18 October 2015vailable online 26 November 2015

eywords:olyoxometalateybrid inorganic–organicrystal engineeringlexible ligandsntangled structures

a b s t r a c t

Currently, the design and assembly of hybrid inorganic–organic compounds has become an area of rapidgrowth due to their structural diversities as molecular building blocks. Polyoxometalates (POMs) as asubset of metal oxides, that represent a tremendous range of inorganic clusters with unique physical andchemical performances, possess intriguing structures and abundant potential applications. In the self-assembling process of POM-based architectures, in addition to the important role of POMs, the choiceof adequate ligands is significant to modulate the properties of inorganic–organic hybrids. Moreover,flexible N-donor ligands, including imidazole, triazole, tetrazole, and pyridyl derivatives with strongcoordination capacity have been used for construction of POM-based compounds with attractive topolo-gies and different dimensionality. Thus, this review puts into perspective latest research on this topicfocusing to the construction of hybrid inorganic–organic materials based on flexible ligands. Remark-

harges ably, they provide to POM-based systems the flexibility and conformational freedom necessary to satisfythe coordination environment of the metal centers creating fascinating structural topologies, such as

cture
entangled and other stru work devoted to analyze howhybrid inorganic–organic com
∗ Corresponding author. Tel.: +98 05138805554; fax: +98 05138796416.E-mail address: [email protected] (M. Mirzaei).

ttp://dx.doi.org/10.1016/j.ccr.2015.10.004010-8545/© 2015 Elsevier B.V. All rights reserved.

s with various architectures. In addition, this review also describes recent
the negative charge of POMs influences the supramolecular assembly ofpounds.
© 2015 Elsevier B.V. All rights reserved.

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Chemi

1

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S. Taleghani et al. / Coordination

. Introduction

Polyoxometalates (POMs), as an outstanding class of inorganicetal-oxide clusters, have attracted extensive attention due to

heir controllable shape, size, composition, and structural diver-ity. Moreover, their promising properties, such as photochemicalctivity [1], magnetism [2], efficient adsorbents [3], macromolec-lar crystallography [4], medicine [5], biocatalysis [6] explain therowing interest in POMs by the scientific community. The rapidrowth of this field is also supported by the progress in technicaltructural analyses, such as X-ray diffraction, NMR, IR, and Ramanpectroscopies. The design and synthesis of inorganic–organicybrids based on POMs with various topologies are still challeng-

ng and interesting tasks in modern inorganic chemistry. Theseybrid compounds are conveniently classified in two main classes,epending on the number of components in the system. Class I:wo-component systems that comprise a combination of a POMnd an organic component. To this respect, interesting investiga-ions [7–10] have been devoted to study the combination of POMsith amino acids (as an important family of organic compounds)

o produce POM–amino acid hybrids with promising biologicalpplications [11–15]. In class II, a secondary metal (often a tran-ition metal) is present forming a complex, which is coordinatedo the organic component. This transition metal complex (TMC)an be either isolated (charge-balancing cation) or coordinatedirectly to the oxygen atoms of POM co-ligands [16,17]. In theseystems, POMs have two different roles: inorganic building blockso link transition-metal complexes, which lend itself to play theinkages role, and inorganic templates to induce the formation ofD inorganic–organic frameworks. Taking advantage of both func-ionalities, many interesting supramolecular architectures withigh dimensionality and connectivity have been obtained [19–22].hus, POMs play a prominent role facilitating the synthesis ofnorganic–organic hybrids. Recent investigations in this topic haveeen reviewed [16,18]. From the viewpoint of crystal engineering,he control of the dimensionality of the resulting hybrid com-ounds is still a great challenge nowadays, as the final structuresre frequently modulated by various factors. Much effort has beenade by POM researchers to rationalize and control the final

upramolecular architecture of the hybrid materials. For instancehe charge of the POMs is a significant parameter in the synthe-is of hybrid materials where combined with the flexibility of therganic ligand influences the final structure [23]. Besides choosinghe appropriate POM as the starting material, it is very importanto control other parameters such as the pH value [24], the length,igidity/flexibility of organic ligands [25], and the possible utiliza-ion of a second transition metal [26]. Among these factors, theelection of the ligand is crucial. Flexible organic ligands provide theecessary conformational freedom to origin more or less compli-ated structures [27], like entanglements [28–30], and supramolec-lar isomerism [31]. Entangled systems, as one of the major themesf supramolecular chemistry, are comprised of individual motifsorming, via interlocking or interweaving, a periodic architecturehat is infinite in at least one dimension. Different types of entan-lement including interpenetration, polycatenanes, polyrotaxanes,olypseudo-rotaxane, and self-penetration have attracted consid-rable attention to chemists because of their amazing assembliesnd their potential applications. To date, numerous examples basedn entangled coordination networks have been reported [30].he aim of this review is putting into perspective recent inves-igations devoted to study the topology and dimensionality ofybrid inorganic–organic compounds belonging to class II (three-
omponent systems). This review is developed under three maineadings. First, a comparison between flexible and rigid ligands inetermining the structure of the hybrid material, second, the dif-erent topologies of the hybrids focusing on entangled and related
stry Reviews 309 (2016) 84–106 85

systems, third, the role of the POMs charge influencing the solidstate architecture of hybrid systems containing flexible ligands.

2. Rigid vs. flexible ligands in hybrid inorganic–organiccompounds

To date, many POM-based hybrid compounds constructed frommetal ions and rigid/flexible ligands have been reported [16–31].Among the variety of organic ligands, N-heterocyclic carboxylateligands such as pyridine carboxylic acid, pyridine-dicarboxylic acid,and pyrazine-dicarboxylic acid have been widely used for theconstruction of POM-based hybrids inorganic–organic with highdimensionality. This is mainly due to their polydentate natureand diversity of coordination modes [32–34]. Flexible organic lig-ands usually consist of N-donor group (or coordination group) (seeScheme 1) are more commonly used than O-donor ones for thedesign and synthesis of new hybrids based polyoxometalate. TheN-donor ligands not only compensate charges, but also adapt them-selves to the geometrical requirements of both the metal center andPOMs yielding the final structure of the hybrid inorganic–organicmaterials. In these hybrids, POM does not coordinate to the liganddirectly, instead it only coordinates to the transition metal ions.Chen et al. have examined the Cambridge structural data base andconcluded that the reported flexible ligands used in these sys-tems usually incorporate aliphatic fragments (CH2)n as theirbackbones [34]. These flexible N-donor ligands in combinationwith several transition metal ions and POMs have been used tosynthesize hybrid compounds with unusual topologies, such asinterpenetration and polyrotaxanes [29,30].

It should be also noted that there is a clear differentiationbetween rigid and flexible ligands within the realm of crystal engi-neering. In principle, ligands are considered flexible if they havefree rotation around a single bond. In this review, the ligands withat least one sp3 hybrid atom (usually C, N, or O) in their back-bones are discussed, some of which were defined as “semi-rigidligands” in the original literature. The polydentate flexible N-donorligands have been employed extensively as chelating or bridginglinkers due to their diversity of connecting modes and the highstructural stability, and they are excellent synthons for the con-struction of functional porous coordination polymer materials. Aclear example is the bis-triazole family, where different spacers

(CH2)n (n = 2, 3, 4, 5, 6) have been used. This family is rich in Ncoordination sites with strong coordination capacity and the abil-ity to satisfy almost any conformational request. In general, rigidorganic ligands with N-donors (e.g. pyrazine, imidazole, bipyridine,1,10-phenanthroline, 2,3-dimethylpyrazine) are preferred to con-trol the structure of the final product [35–39]. However, by usingrigid ligands, it is difficult to obtain structures with high dimen-sional. Thus, the rigid N-donor ligands (Scheme 2) are usually called“terminators” since they impede the formation of interpenetratingstructures. Therefore, the majority of reported POM based com-pounds containing rigid ligands usually are multitrack and/or lowdimensional structures. In this regard, two interesting examplesof hybrid compounds {[Cu(bipy)]3[HGeMo12O40]}·0.5H2O (1), and[Cu2(phnz)3]2[SiW12O40]] (2) based on different rigid ligands (4,4′-bipyridine (bipy) and phenazine (phnz)) have been reported by Shaet al. In these cases, the rigid ligands are linked by copper atoms intomultitrack Cu N coordination polymeric chain-modified POMs, inwhich the polyoxoanions act as linkages (see Table 1) [40].

Lu et al. by utilizing 1,2,4-triazole and its derivatives as rigid lig-ands, designed hybrid inorganic–organic materials, [Ag4(dmtrz)4]
[Mo8O26] (dmtrz = 3,5-dimethyl-1,2,4-triazole) (3), [Ag6(3atrz)6][PMo12O40]2·H2O (4), and [Ag2(3atrz)2]2[HPMoVI
10MoV2O40] (5)

(3atrz = 3-amino-1,2,4-triazole) based on POMs and supramolec-ular coordination cages via covalent interactions. Furthermore,

86 S. Taleghani et al. / Coordination Chemistry Reviews 309 (2016) 84–106

or liga

Clt

esTosy(i(f

Scheme 1. The flexible N-don

ui et al. synthesized another kind of supramolecular metaligand coordination cage [PWVI

10.5WV1.5O40][Cu(2,2′-bpy)2]4.5 (6),

hrough non-covalent interactions using bipyridine ligand [41,42].The introduction of rigid N-donor ligands into interpen-

trating systems can be used to obtain un-interpenetratingtructures taking advantage of their terminal function ability.his strategy has been used by Tian et al. for the preparationf interpenetrating and un-interpenetrating POM-based hybridolid materials by combining the flexible 1,4-bis(1,2,4-triazol-1-lmethyl)benzene (bbtz) ligand and the rigid 1-H-1,2,4-triazole
trz) ligand [43]. They synthesized two new compounds using Cuon, and two different POMs, i.e. [Cu6(bbtz)6(HPMo12O40)]·2H2O7), and [Cu6(trz)2(bbtz)2(SiW12O40)] (8). The flexible bbtz ligand,acilitates the construction of a 2-fold interpenetrating structure in
Scheme 2. The rigid organic ligands employed to

nds mentioned in this review.

compound 7. But in 8, the coordination of CuII ions, and trz/bbtzligands results in a 2D Cu-trz-bbtz layers, which is extended by(SiW12O40)4− anions into a 3D framework (Fig. 1).

It should be mentioned that there is no direct link betweenthe rigidity/flexibility of the organic linker and the resulting prod-uct. Flexible hybrid inorganic–organic compounds can be builtby rigid or flexible ligands and usually, the framework flexibil-ity is connected to supramolecular host–guest interactions, andcan be derived from non covalent interactions (H-bonds, rota-tion or wobbling of free linkers, and van der Waals interactions)
[45–50]. The flexible ligands can be also used to construct bothflexible and robust hybrid compounds [44]. The surface oxygenatoms of POMs can form hydrogen bonding interactions. Theseweak interactions may play a positive role in dynamic framework
construct POMs-based hybrids in Section 2.

S. Taleghani et al. / Coordination Chemistry Reviews 309 (2016) 84–106 87

Table 1Fundamental building blocks and schematic POM-based frameworks in 1 and 2.

F m bb

F

tt(aer

3

tmfl

ig. 1. Representation of the two-fold interpenetrating architecture constructed fro

igure was reproduced from Ref. [43], with permission of the copyright holders.

ransformations. For example, an interesting compound withhe flexible ligand 4,4′-bis((1H-1,2,4-triazol-1-yl)methyl)biphenyl,btmbp) has been synthesized by Luo et al. with chemical formulas [Cu(btmbp)2(H2O)H(PW12O40)]·7H2O (9). The obtained hybrid,xhibits a dynamic structural transformability upon the reversibleemoving/adsorbing of guest water molecules (see Fig. 2) [51].

. The design of the flexible ligands

It is well known that POMs have the ability to coordinate toransition metals because of their high electronic density and

ultiple surface oxygen atoms. Therefore by means of designingexible ligands, it is feasible to control the POMs-based hybrid

tz ligand, and un-interpenetrating 3D framework from (bbtz/trz) ligands.

material formation under hydrothermal conditions. Moreover theright choice of the organic ligand based on its intrinsic proper-ties is a fundamental strategy for controlling and defining therole of POMs in the crystallization of new hybrids. At present,N-donor ligands are the preferred choice to assemble POM-basedcompounds. In particular, the (CH2)n connected bis(triazole)ligands have been widely used [52–55]. The important featuresof these ligands are: (i) the flexible bis(triazole) ligands havefour N-donor atoms that enhance their coordination ability to
transition metal (TM) ions, and facilitating the formation of thecorresponding transition metal complexes (TMCs). (ii) The flex-ibility and conformational freedom of (CH2)n spacers allowto adapt themselves to the requirements of the coordination


F amole

F

sFytapU((Luap

bd

F

F

ig. 2. Representation of hydrogen bonding interactions in 9, to generate a 3D supr


phere of the transition metal (TM) ions, and POMs easily.luconazole (1-(2,4-difluorophenyl)-1,1-bis[(1H-1,2,4-triazol-1-l)methyl]methanol)) (Hfcz) is an important derivative of theriazole ligand family. It is not only used as antifungal drug butlso as a good flexible ligand to construct inorganic–organic com-ounds with optical properties and medical applications [56–60].sing this ligand, three hybrids [CuII

2(Hfcz)4(SiW12O40)]·3H2O10), [CuII

4(fcz)4(H2O)4(SiMo12O40)]·6H2O (11), andEt3NH)2[CuI

2(Hfcz)2(SiW12O40)]·H2O (12) have been reported byi et al. The results illustrate that various transition-metal organicnits, and Keggin polyanions with different coordination modesre important for the formation of the different dimensional
roducts (Fig. 3) [61].
The 1,3-bis(imidazolyl)propane (bip) and 1,4-bis(imidazol)utane (bib) as flexible bidentate ligands are excellent candi-ates for the construction of novel structures and topologies

ig. 3. Schematic view of the different networks based on Hfcz ligand, and Keggin polyox


cular architecture.

namely, [Cu6(bip)12(PMoVI12O40)2(PMoVMoVI

11O40O2)]·8H2O (13)and [CoII

3CoIII2 (H2bib)2(Hbib)2(PW9O34)2(H2O)6]·6H2O (14) [62].

These compounds provide new examples of host-guest hybridsbased on flexible bis(imidazole) ligands. Compound 13, exhibits a3D framework, and PMo12O2 polyoxoanions as the guest moleculesare incorporated into the cages which are composed of the bip lig-ands and Cu2+ ions, while in 14, the {PW9} units are located in thechannels formed by the bib ligands (see Fig. 4) [62,63].

Another important family of flexible ligands is composedby tetrazole-functionalized thioether derivatives. Compared tothe triazole family containing (CH2)n groups, the thioetherligands exhibit better flexibility and can bend to a major
extend. For example, three structurally related bis-(tetrazole)-functionalized thioether ligands [1,1′-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane] (bmtm), [1,2-bis(1-methyl-5mercapto-1,2,3,4-tetrazole)ethane] (bmte), and [1,5-bis
oanions.


F ible b

F

(bfleahpl51bKc

F

F

ig. 4. Schematic representation of the 3D framework architectures formed by flex


1-methyl-mercapto1,2,3,4-tetrazole)pentane] (bmtp) haveeen reported. The introduction of sulfhydryl provides additionalexibility to the backbone. Moreover, these ligands possessxtra coordination sites provided by tetrazole groups and Stom making them attractive for the design of novel POM-basedybrids with interesting structures [64–67]. To this respect, theerformance of flexible tetrazole-functionalized (thioether)

igands bis(1-methyl-1H-tetrazol-5-yl)sulfane (bmps) and,5′-((2,2-bis(((1-methyl-1H-tetrazol-5-yl)thio)methyl)propane-
,3-diyl)bis(sulfanediyl))bis(1-methyl-1H-tetrazole) (bpbb), haveeen designed and tested [68]. Two new compounds based oneggin-type POMs have been synthesized under hydrothermalonditions, i.e. [Cu2(bmps)2(H2O)3(PMo12O40)]·(OH)·2.5H2O (15),
ig. 5. The architectures of the inorganic–organic hybrids constructed from tetrazole fun


identate ligands (bip, bib).

and [Cu4(bpbb)2Cl(PW12O40)] (16) [68]. In 15, the polyoxoanionsconnect the secondary building units based on Cu2+ ions and bmpsligands (Fig. 5a) as a result, to form a 2D pyramid-like network(Fig. 5b). But in 16, the bulky nano-scaled [Cu4(bpbb)4(�4-Cl)]calixarene cluster with limited flexibility (Fig. 5c) induce the POMsto act as templates located into a 2D square layer (Fig. 5d).

4. An analysis of the topology and dimensionality in hybridinorganic–organic compounds: the influence of the flexible
ligands
Hybrid inorganic–organic compounds can be classified into twomajor structural groups, including entangled and other structures

ctionalized flexible ligands (bmps and bpbb).


o-rotaS

bt

4

iIamasbnioOglaTspsistlfail

4o

fdabCwp[ts

Scheme 3. (a) Schematic representations of the reported POM-based polypseudcheme was reproduced from Ref. [79], with permission of the copyright holders.

ased on certain derivatives of flexible ligands such as imidazole,riazole, tetrazole, and pyridyl derivatives.

.1. Entangled structures

One interesting topic in crystal design and material engineer-ng is to build coordination networks based on polyoxometalates.n this research field, entangled coordination networks representn important and interesting subfamily of the coordination poly-er chemistry. Therefore, the synthesis of these compounds has

ttracted much attention in recent years [69–73]. In these synthe-es, one strategy is the utilization of transition-metal (TM) ions,ridging ligands (L), and POMs where TM and L usually act as theode and linker, respectively. POMs can play three different roles,

.e. template, pillar (or linker) and node. The POM units and typef metal influence the construction of novel entangled networks.ne of the most important strategies for constructing new entan-led networks is the design and choice of flexible organic bridgingigands [74]. These ligands can tolerate large structure twist andcquire the desirable coordination environment for metal ions.his provides advantages in designing intricate crystal structuresuch as interpenetration, polyrotaxanes, polypseudo-rotaxanes,olythreaded, and polycatenation (see Scheme 3) [75–79]. Theseystems are comprised of individual motifs forming, via interlock-ng or interweaving, interpenetrated and interlocked assembliesuch as catenanes and rotaxanes, in which components are heldogether by mechanical linkages rather than by chemical cova-ent bonds. These systems are archetypes that have been usedor constructing molecular machines [80], molecular motors [81],nd molecular knots [82], all of which have potential applicationsn information processing and storage, molecular electronics andight-driven molecular machines [83–85].

.1.1. Hybrids with polythreaded/penetrating architectures basedn imidazole derivatives

In this section, some examples of hybrid compounds obtainedrom imidazole derivatives are highlighted and their topology andimensions are examined. A typical example was reported by Pengnd co-workers in 2009, [Cu(bbi)]5H[H2W12O40] (bbi = 1,1′-(1,4-utanediyl)bis(imidazole)) (17) [86]. Assembly of the bbi ligands,uI cations, and [H2W12O40]6− clusters create a 3D framework,hich are penetrated each other to form a two-fold 3D + 3D inter-
enetrating structure (Fig. 6a). Also, a Wells-Dawson-based hybrid,Ag7(bbi)5(OH)(P2W18O62)] (18) [87] with interpenetrating archi-ecture was isolated in 2011. In 18, an infinite inorganic chainupported {Ag2}2+ dimers, and P2W18 polyanions is extended
xanes. (b) Schematic representations of the reported POM-based polythreaded.

by other Ag+ ions and bbi ligands into a 3D framework, whichis penetrated by another 2D dimensional bbi/Ag layer withhuge loops (Fig. 6b). Another two-fold interpenetrating archi-tecture [CuII(bbi)2(H2O)(�-Mo8O26)0.5] (19) was prepared fromCuII ions, bbi ligands, and �-octamolybdate in 2008 [94]. TheSu and Lan group obtained two supramolecular isomers withpolythreaded topology, [CuII(bbi)2(�-Mo8O26)][CuI(bbi)]2 (20) and[CuIICuI(bbi)3(�-Mo8O26)][CuI(bbi)] (21), both of which are con-structed from a 3D framework based on CuI/CuII/bbi/�-Mo8O26anion penetrated by CuI/bbi chains (Fig. 6c and d) [94].

The length and backbone of flexible ligands have a stronginfluence upon the structural properties of the hybrids[88–92]. By introducing the flexible isomeric 1,4-bis(imidazol-1-ylmethyl)benzene (bix) ligand (bixa = 1,2-bis(imidazol-1-ylmethyl)benzene; bixb = 1,3-bis(imidazol-1-ylmethyl)benzene)into a polyoxometalates (POMs) system, two POM-basedcompounds, [Ag(bixa)]2[Ag2(bixa)2(SiW12O40)] (22) and[Ag5bixb

5][K2(OH)P2W18O62]·H2O (23) were synthesized [93].Compound 22, shows a 2D supramolecular framework by theAg/bix chains, in which the polyoxoanions act as linkages. But23, features a 3D polythreading framework containing {POM-K}chains, and winding metal-organic layers, as shown in Fig. 7a. Inaddition, the complexes [CuI(bix)][(CuIbix)(�-Mo8O26)0.5] (24),and H(CuIbix)[(CuIbix)2(�-Mo8O26)]·2H2O (25), also display 3Dpolythreaded frameworks (Fig. 7b and c) [95].

4.1.2. Hybrids with polythreaded/penetrating architectures basedon triazole, and tetrazole derivatives

Replacing the imidazole group with triazole group, the reac-tion of the flexible bis-triazole ligands with different spacers

(CH2)n (n = 4 for btb, 6 for btx), transition metal ions, and POMsshow various architectures. In [Cd2(H2O)2(btb)4(SiMo12O40)](btb = 1,4-bis(1,2,4-triazol-1-yl)butane) (26), the 2D wave-likeCdII-btb layers are extended by [SiMo12O40]4− anions into a3D framework with two kinds of quadrate channels, furtherinducing a two-fold interpenetrating architecture (Fig. 8a) [154].A two-fold interpenetrating architecture with larger channels,[Ni3(btb)5][PMo12O40]2·14H2O (27) was reported by Wang et al.(Fig. 8b) [150]. Also, the first high-dimensional POM-organichybrid functionalized {[Ni(btb)2(H2O)][�-Mo8O26]0.5·H2O}n (28),was isolated in 2011. The major feature of the architec-ture is that organoimido polyoxoanions, joints Ni2+ ions, and
threaded by Ni/btb chains into a 2D self-threading skeleton, fur-ther giving rise to a 3D self-penetrating framework (Fig. 8c)[97]. In [Mn2(H2O)4(btx)3][SiMo12O40]·4H2O (btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene) (29), the coordination of Mn2+ ions,


F ures c I

[ and �

F ers.

aaoast[pbpetibmww

FC

F

ig. 6. Schematic representations of interpenetrating, and polythreaded architectH2W12O40]6− anions. (b) AgI ions, and Wells-Dawson POM anions. (c) CuII cations,

igure was reproduced from Refs. [86,87,94] with permission of the copyright hold

nd btx ligands results in a 2D net with rectangle channel,nd [SiMo12O40]4− anions act as a linker to help the formationf the interpenetrating structure which has both polyrotaxanend polycatenane characters (Fig. 8d). Similar complexes thathow the same entangled topology of 2D interpenetrated archi-ecture include: [Ni2(H2O)4(btx)3][SiMo12O40]·4H2O (30), andCo2(H2O)4(btx)3][SiMo12O40]·4H2O (31) [96]. In 2014, a 3Dolythreading network of [CuI(bbtz)]5[BW12O40]·H2O (bbtz = 1,4-is(1,2,4-triazol-1-lmethyl)benzene) (32) can be described as a 3Dolythreading network that consists of 2D pseudo-rotaxane lay-rs, and 1D Cu-bbtz chains, in which the POM units entrap inhe interspaces between two adjacent layers (Fig. 8e) [99]. Andn [Cu12(bmmtp)9(HSiW12O40)4]·0.5H2O (33), a metallacalix[4]uilding block is formed in the presence of 1,3-bis(1-methyl-5-ercapto-1,2,3,4-tetrazole)propane (bmmtp) ligand and CuI ions,
hich is linked by polyoxoanions into a 3D self-penetrating frame-ork (Fig. 8f) [104].
ig. 7. (a) Representation of polythreaded framework constructed from [P2W18O62]6− anuI/bix/different octamolybdate isomers.

igure was reproduced from Refs. [93,95] with permission of the copyright holders.

onstructed from different POMs, metal ions, and bbi ligands. (a) Cu cations, and-[Mo8O26]4− anions. (d) CuII/CuI cations, and �-[Mo8O26]4− anions.

4.1.3. Hybrids with polycatenated/polypseudo-rotaxanearchitectures based on triazole derivatives

In 2014, two unprecedented entangled coordina-tion polymers, [Co2(btmbp)5(H2O)2(SiW12O40)] (34), and[Cu2(btmbp)3(H2O)2(SiW12O40)]·H2O (35) (btmbp = 4,4′-bis((1H-1,2,4-triazol-1-yl)methyl)biphenyl) were reported [79]. In 34, the2D Cu-btmbp layers, are united by {SiW12}4− clusters to constructa 3D polypseudo-rotaxane architecture (Fig. 9a). But compound 35,is the first polycatenated POM-based coordination polymers withmulti-form helical chains (Fig. 9b). In 2010, Chen et al. successfullyisolated [CuII(btp)2(H2O)][�-Mo8O26]0.5·2H2O (btp = 1,3-bis(1,2,4-triazol-1-yl)propane) (36). In 36, the btp ligands are linkedby copper ions into 2D grid layer, and the �-Mo8 anions aresandwiched between a pair of layers, to form 3D polycatenatedframework (Fig. 9c) [98]. Compound [Cu(btx)]4[SiMo12O40] (37)
exhibits 1D + 2D polypseudo-rotaxane architecture based onCu/btx complexes, and SiMo12 clusters (Fig. 9d) [96].
ions, AgI ions, and bix ligands. (b and c) The 3D polythreaded frameworks based on


Fig. 8. Schematic views of polythreaded/penetrating architectures constructed from different POMs, metal ions, and triazole, and tetrazole derivatives. (a) [SiMo12O40]4−

a ligandM s. (f) [

F yrigh

4o

snbI[N[a3ii[bwt

4

loos

nions, Cd2+ cations, and btb ligands. (b) [PMo12O40]3− anions, Ni2+cations, and btbn2+ cations, and btx ligands. (e) [BW12O40]5− anions, Cu2+cations, and bbtz ligand

igure was reproduced from Ref. [96,97,99,104,150,154] with permission of the cop

.1.4. Hybrids with polyrotaxane/penetrating architectures basedn pyridyl derivatives

In 2012, the flexible bis-pyridyl-bis-amide ligand was used toynthesize the first POM-templated 2D metal-organic polyrotaxaneetwork, [Cu2(bpba2)3(SiMo12O40)(H2O)6]·9H2O [bpba2 = N,N′-is(3-pyridinecarboxamide)-1,4-butane] (38) (Fig. 10a) [112].n 2013, the first example of interpenetrating architecture,Ag5(bpba1)3(HSiWVI

10WV2O40)(H2O)2]·6H2O (39) based on

,N′-bis(3-pyridinecarboxamide)-1,2-ethane ligand, was obtained155]. The 2D layers constructed from flexible bpba1 ligands,nd Ag+ ions are connected by [SiW12O40]4− clusters into aD framework, which are penetrated each other into two-fold

nterpenetrating network (Fig. 10b). The other example withnterpenetrating architecture [Cu2(bpp)4(H2O)2](SiW12O40)·6H2Obpp = 1,3-bis(4-pyridyl)propane] (40) was reported in 2008. Thepp ligands are linked by copper ions into 2D cationic frameworksith cubic-like channels, where the [SiW12O40]4− clusters as

emplates reside in these channels (Fig. 10c) [156].

.2. Other structures

As mentioned in the previous section, the flexible ligands with
ong spacer usually lead to large voids that facilitate the formationf interpenetrating or entangled structures. However, the existencef the bulky groups of these flexible ligands may avoid entanglingtructures which may be attributed to the big steric hindrance of
s. (c) �-[Mo8O26]4− anions, Ni2+cations, and btb ligands. (d) [SiMo12O40]4− anions,H4SiW12O40]4− anions, Cu2+cations, and bmmtp ligands.

t holders.

these groups, and to form another structures. In this section, thesestructures are discussed.

4.2.1. Hybrids based on imidazole derivativesA remarkable complex [Cu2(bbimid)4(H2O)2](SiW12O40)·8H2O

[bbimid = 1,1′-(1,4-butanediyl)bis-1H-benzimidazole] (41) [90]was reported by Lu et al. Compound 41, shows a 2D grid-likemetal-organic layer templated by [SiW12O40]4− polyanions(Fig. 11a). Another examples belonging to this classification wasisolated in 2011, in compound [Ag8(pbpb)4(�-Mo8O26)(�-Mo8O26)(H2O)3]·H2O (pbpb = 1,1′-(1,3-propanediyl)-bis[2-(4-pyridyl)benzimidazole]) (42), the 2D polymeric silver(I)-organic layersare linked by (�-Mo8O26)4−, and (�-Mo8O26)4− anions into a 3Dframework structure (Fig. 11b), and compound [CuI

3.1CuII0.5(�-

Mo8O26)0.5(�-Mo7VIMoVO26)0.5(bbpb)2(H0.8bbpb)0.5] (bbpb = 1,

1′-(1,4-butanediyl)bis[2-(3-pyridyl)benzimidazole]) (43),is a rare 3D framework containing copper(I,II)-organiccages, and rectangular channels occupied by the (�-Mo8O26)4−, and (�-Mo7

VIMoVO26)5− anions [100]. Thecomplex [CuI

2bmmb2]2[Mo8O26], (bmmb = 1,3-bis(imidazol-l-ylmethyl)benzene) (44) [101], revealing a [4+4]metallomacrocycle, in which the [Mo8O26]4− anions connectthe metallomacrocycles to form a 2D neutral knitmesh-like
network (Fig. 11c).
In 2009, two new polyoxometalate-templated compounds,(bix)[Cu(bix)]3[PW12O40]·4H2O (45), and [Cu(bix)]3[PMo12O40](46) were hydrothermally synthesized [102]. In 45, the


F f the

a

F ers.

Kfnclaab((o(bopa

Ftt

F

ig. 9. (a) Representation of the 3D polypseudo-rotaxane structure of 34. (b) View orchitecture of 36. (d) The 2D polypseudo-rotaxane framework of 37.

igure was reproduced from Refs. [79,96,98] with permission of the copyright hold

eggin polyanions [PW12O40]3− direct the [Cu(bix)] chains toorm a 3D supramolecular framework with grid-like 2D chan-els (Fig. 12a). But in 46, the [Cu(bix)] coordination polymerichains array around the polyanions to form a 3D supramolecu-ar framework with 1D hexagon-like channels (Fig. 12b). Pengnd co-workers prepared several novel polyoxoanion-templatedrchitectures from [As8V14O42]4−, [V16O38Cl]6− as buildinglocks, and bbi ligands, namely [Co(bbi)2]2[As8V14O42(H2O)]47), [Cu(bbi)]4[As8V14O42(H2O)] (48), and [Cu(bbi)]6[V16O38Cl]49). Compound 47, exhibits 2D structure, in which the poly-xoanion induces a closed four-membered circuit of Co4(bbi)4Fig. 12c), and compound 48, is the first example of a 3D motif
ased on the [As8V14O42]4− building block, in which composedf two cationic ladders like double chains of [Cu(bbi)]n. But com-ound 49, is composed of 24-membered circuit of Cu24(bbi)24nd heptadentate [V16O38Cl]6− anions [103]. Das et al. in 2014,
ig. 10. (a) The [SiMo12O40]4− anions sandwiched by neighboring parallel-packed 2D poecture constructed from N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, Ag+ ions and [SiWemplated by [SiW12O40]4− anions.

igure was reproduced from Refs. [112,155,156] with permission of the copyright holder

3D (4,5)-connected polycatenated motif of 35. (c) Representation of polycatenated

reported a novel polyoxometalate-based ion-pair compound,[Cu(1,4-bpimb)(H2O)2][Mo8O26]·4H2O with a flexible 1,4-bpimblinker, (bpimb = bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene)[118] (50) that represents the first example of an isolatedmetallomacrocycle in the polyoxometalate matrix (Fig. 12d).

4.2.2. Hybrids with loop/helical subunits based on triazole, andpyridyl derivatives

The different spacer length of bis(triazole) ligands have agreat effect on the construction of the metal-organic subunitsand the final structures [108]. Recently, 3D POM-based AgI com-plexes [Ag8(btp)4(H2O)2(HPWVI

10WV2O40)2]·H2O (51), [Ag4(btb)2

(HPWVI10WV

2O40)] (52), and [Ag5(btx)4(PWVI10WV

2O40)] (53)(btp = 1,3-bis(1,2,4-triazol-1-yl)propane, btb = 1,4-bis(1,2,4-triazol-1-yl)butane, and btx = 1,6-bis(1,2,4-triazol-1-yl)hexane)[107], with various types of multinuclear loops are formed from

lyrotaxane layers in 38. (b) Representation of the two-fold interpenetrating archi-12O40]4− clusters. (c) View of interpenetrating [Cu2(bpp)4(H2O)2]n

4n+ framework

s.


Fig. 11. (a) View of the topology of the stagger-peaked double-sheet in compound 41. (b) Perspective view of the 3D framework in 42. (c) Schematic illustration of 3D(

F lders.

Aroalciti

A(w(3taadam([O([

Fr

F

3,4,6)-connected framework in 44.

igure was reproduced from Refs. [90,100,101] with permission of the copyright ho

g+ ions, and bis-triazole ligands, which Keggin anions play theole of multidentate linkages in these hybrids. In 2011, a seriesf hybrids, [Ni2(H2O)4(btx)4(Hbtx2)2][PMo12O40]2·2H2O (54),nd [Ni2(btb)4(SiW12O40)]·H2O (55) based on NiII/bis-triazoleigands/Keggin-type polyoxometalates were prepared. In twoompounds, although the bis-triazole ligands are linked by NiII

ons into 2D grid-like nets containing different multinuclear loops,he roles of POMs are different. The polyoxoanions act as templatesn 54, and tetradentate linkages in 55 (Fig. 13) [150].

Also, in the reaction system of [P2W18O62]6− polyoxoanions,gI ions, and bis-triazole ligands with different lengths (CH2)n

n = 2 for bte, 3 for btp, and 4 for btb), three metal-organic frame-orks are formed, [Ag7(bte)4(H2O)(HP2WVI

16WV2O62)]·2H2O

56), [Ag7(btp)5(HP2WVI16WV

2O62)]·H2O (57), and [Ag4(btb).5(P2W18O62)](H2btb)·2H2O (58). The results manifest that inhis compounds, the spacer length of flexible bis-triazole lig-nds play a key role in the construction of multinuclear looprchitectures, and the [P2W18O62]6− anions show different coor-ination modes in the 3D frameworks (Fig. 14a) [157]. Wangnd co-workers reported four novel Keggin-type POM-templatedetal-organic complexes based on flexible bis-pyridyl-bis-amide

bpba) ligands, i.e., [Cu2(bpba1)3(SiMo12O40)(H2O)6]·2H2O (59),
Cu2(bpba2)3(SiW12O40)(H2O)6]·6H2O (60), [Cu2(bpba3)3(SiMo12
40)(H2O)6]·4H2O (61), and [Cu2(bpba3)3(SiW12O40)(H2O)6]·4H2O62), [bpba1 = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane],bpba2 = N,N′-bis(3-pyridinecarboxamide)-1,4-butane], [bpba3 =

ig. 12. (a) View a 3D supramolecular framework of 45 with grid-like 2D channels. (bepresentation of the layer and binodal (4,6) connected in 47. (d) View of the metallomac

igure was reproduced from Refs. [102,103,118] with permission of the copyright holder

N,N′-bis(3-pyridinecarboxamide)-1,6-hexane]. In 59–62, threekinds of metal-organic loops are formed. The dimension of thequadrate (Cubpba)n loops increases by adjusting the space lengthsof ligands, and the polyoxoanions show different template roles infinal structures (Fig. 14b) [112,113].

The flexible bis(pyridyl-tetrazole) ligands have beenutilized to synthesis inorganic–organic hybrids with heli-cal architectures, due to their multiple coordinationsites. Therefore, two flexible bis-pyridyl-tetrazole ligands,1,4-bis(5-(4-pyridyl)tetrazolyl)butane(4-bptzb), and 1,4-bis(5-(3-pyridyl)tetrazolyl)butane(3-bptzb), with the aim of constructinghelical subunits in POMs system were designed. Compound{Ag2(4-bptzb)2(H2O)2[H2PMo12O40]2}4-bptzb·5H2O (63), exhibitsa dimeric structure, in which the [PMo12O40]3− polyoxoanion arelinked on both the sides of the dimers through Ag-O bonds.In [Ag4(3-bptzb)2(PMoVMoVI

11O40)]·2H2O (64), a meso-helixchain is constructed from 3-bptzb ligands and Ag+ ions, and thepolyoxoanions reside in the 3D metal organic framework. In{Ag3(3-bptzb)2.5(H2O)2[H3P2W18O62]} (65), a meso-helix loopconnecting loop 1D chains is formed, which are linked each otherby [P2W18O62]6− into a 2D sheet, further extended by the 3-bptzbligand into a 3D framework (Fig. 15) [158].

4.2.3. Hybrids with cluster subunits based on tetrazole derivativesThe flexible N-donor ligands have exhibited the satisfied flex-

ibility in synthesis of inorganic–organic hybrids. However, in

) The 2D POM-templated supramolecular layer in compound 46. (c) Polyhedralrocycle [Cu(1,4-bpimb)(H2O)]2

4+ in 50.

s.


Fig. 13. Representations of different architectures by the flexible bis-triazole ligands (btp, btb, btx) with different space lengths in multinuclear loop mode in Ag/KegginP

F rs.

teccmibabpmutlS

s[

F

F

OM, and Ni/Keggin POM systems.

igure was reproduced from Refs. [107,150] with permission of the copyright holde

his section, we consider that the ligands containing sulfhydrylxhibit better flexibility and can bend to a larger twist inomparison with the (CH2)n linkages, as a result, to formompounds with cluster subunits. Two POM-based compoundsodified by such ligands with cluster subunits were reported

n 2010 [111]. In compound [Cu4(bmtm)4][SiW12O40]·2H2O, 1,1′-is(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane (bmtm) (66),

one-dimensional (1D) chain, is generated from two differentinuclear CuI units. Each of that are bridged by [SiW12O40]4−

olyoxoanions into a 2D layer (Fig. 16a). When the 1,2-bis(1-ethyl-5-mercapto-1,2,3,4-tetrazole)ethane (bmte) as ligand is

sed, compound [Cu4(bmte)3.5][SiW12O40] (67) is formed. In 67,wo equivalent tetranuclear cluster subunits are bridged by bmteigands and to form a 1D chain. Finally, a 3D framework is built from
iW12 clusters (Fig. 16b).
The influence of the space length of ligands on thetructures of SiMo12–Ag system is further investigated. InAg4(bmte)2(H2O)2(SiMo12O40)] (68), a tetranuclear cluster

ig. 14. The architectures of the hybrids constructed from (a) bis-triazole (bte, btp, btb), a


and SiMo12 anions arrange alternately forming a one-dimensional (1D) chain (Fig. 16c). The structure of compound[Ag4(bmtp)2(H2O)2(SiMo12O40)] (69) is similar to that of 68,(Fig. 16d), but the Ag ions show different coordination modesin tetranuclear clusters because of the longer skeleton of bmtpligand. In [Ag4(bmtb)3(SiMo12O40)] (70), a 2D layer templated by[SiMo12O40]4− polyoxoanions is formed by binuclear silver clus-ters, and bmtb ligands with the longest (CH2)4 alkyl skeleton(Fig. 16e) [159]. Thus, the thioether bond, and length of flexibleligand may play an important role in forming cluster subunits.

4.2.4. Other hybridsA remarkable approach for constructing multifunctional mate-

rials has been reported that exploits the coordination ability
of polyanions to different transition metal organic units [105].Among multidentate N-donor ligands, hexakis(1,2,4-triazol-ylmethyl)benzene (Htrb) is a good candidate for the constructionof POM-supported compounds because the six triazole groups
nd (b) bis-pyridyl-bis-amide (bpba1, bpba2, bpba3) flexible ligands.


F l-tetra

F

iss(Mdassc1p

F

F

ig. 15. Schematic view of the inorganic–organic hybrids by the flexible bis-pyridy

igure was reproduced from Ref. [158] with permission of the copyright holders.

ncorporated in the ligand can provide multiple coordinationites. Various hybrids using this ligand have been synthe-ized by Zhang et al., namely [Cu6Na2(Htrb)4(Mo6O19)(MoO4)6]71) [Zn3(Htrb)(Mo10O34)]·8H2O (72), and [Zn2(Htrb)(�-

o8O26)(H2O)2]·6H2O (73) [106]. In 71–73, due to twoifferent coordination modes of ligand (1,3,5-up/2,4,6-down,nd 1,2,4-up/3,5,6-down fashions), the Htrb ligand bridgesix metals, forming structures with relatively high dimen-
ionality (see Fig. 17a). The Xu’s group prepared a novelompound [Ag4(ttmb)2(�-Mo8O26)] (ttmb = 1,3,5-tri(1,2,4-triazol--ylmethyl)-2,4,6-trimethylbenzene) (74) in the appropriateH range [109]. Crystal structural analysis reveals in this
ig. 16. The POM-based hybrids by the flexible bis-mercaptotetrazole ligands with cluste


zole ligands in helical architectures.

species, the wave-like Ag-ttmb layers are linked by 1D Ag-[�-Mo8O26]4− chains to form a 3D framework. In 2011, a newhybrid [CuI

2(tmp)3(Mo8O26)0.5] (tmp = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine) (75) was reported with the bottom-up designprinciple, in which the Cu-tmp subunits through Mo-N bonding toyield 1D chain, which is further extended to a 2D supramolecularvia �· · ·� interactions (Fig. 17b) [110].

Currently, a great deal of POM-based complexes having pyri-
dine, imidazole, triazole, and tetrazole based ligands and classicalKeggin or Dawson-type POMs have been reported [114–116].Recently, based on Anderson-type POMs two novel metal-organicframeworks, H{Cu2(�2-OH)2bpbp1[CrMo6(OH)6O18]}·4H2O,
r subunits, in CuI, and AgI/Keggin-type POM systems.


Fig. 17. (a) View of the 3D frameworks based on Htrb ligand in 71–73. (b) Schematic view of two compounds (74 and 75) based on derivatized octamolybdate polyoxoanion.

Figure was reproduced from Refs. [106,109,110] with permission of the copyright holders.

F ′ ecarb ′

p

F

[{bshtts

ig. 18. Representation of the 3D framework in 76 based on N,N -bis(3-pyridinyridinecarboxamide)-piperazine.


bpbp1 = N,N′-bis(3-pyridinecarboxamide)-piperazine] (76),Cu2bpbp2[CrMoVI

5MoV(OH)6O18](H2O)4}·4H2O, [bpbp2 = N,N′-is(4-pyridinecarboxamide)-piperazine] (77) have beenynthesized. The different coordination modes of POM polyanions,exadentate linker (in 76), and quadridentate linker (in 77), and
he isomeric bis(pyridylformyl)piperazine ligands play key roles inhe construction of the title frameworks (Fig. 18) [117]. In Table 2,ome relevant hybrid compounds are summarized.
oxamide)-piperazine, and view of the 2D network in 77 based on N,N -bis(4-

5. Different negative charges of polyoxometalatemodulated self-assembly of hybrid architectures based onflexible ligands

In recent years, the introduction of transition-metal complexes
(TMCs) to POMs has become an appealing field, aiming for construc-tion and assembly of new hybrid materials with novel topology.The properties of the resulting assemblies depend on the nature of

98

S. Taleghani

et al.

/ Coordination

Chemistry

Review

s 309

(2016) 84–106

Table 2Summary of inorganic–organic compounds based on flexible ligands in Section 4.

Formula Flexible ligandsa The role of ligand The property of compound Topology Dimensionality Ref.

[Cu(bbi)]5H[H2W12O40] bbi All the bbi molecules, show bidentatebridging coordination mode, to linktwo CuI cations through the imidazolenitrogen atoms

Electrocatalytic activity toward thereduction of bromate, and nitrite.Further, the non-isothermal kinetics ofthe thermal decomposition ofcompound provides the dynamicparameters E (activation energy), andA (pre-exponential factor) for thepyrolytic reaction of the organicligands

Interpenetrating 3D [86]

[Ag7(bbi)5(OH)(P2W18O62)] bbi The bbi ligands exhibit three kinds ofcoordination fashions: (i) two terminalnitrogen atoms coordinate to two Ag+

ions; (ii) an imidazole groupcoordinates to a third Ag+ ion; (iii) oneterminal nitrogen atom coordinates toa Ag+ ion, and the other one to a{Ag2}2+ dimer

Electrocatalytic activity in theelectrical reductions of bromate, andshows a photocatalytic action fordegradation of Rhodamine B (RhB)


[CuII(bbi)2(H2O)(�-Mo8O26)0.5] bbi Two kinds of bbi ligands exhibit TTTand GTG conformations, coordinate toCuII cations to generate a 2D (4,4) sheet

Anhydrous composition begins todecompose at 253 ◦C, and ends above559 ◦C


[CuII(bbi)2(�-Mo8O26)][CuI(bbi)]2 bbi Three kinds of bbi ligands adopt bismonodentate coordination modes, thatshow TGT, TTT, and TTT conformations

Photoluminescent property (theemission spectra of compound isblue-shifted compared with free bbimolecule), and the anhydrouscomposition begin to decompose at319 ◦C, and ends above 700 ◦C

Polythreaded 3D [94]

[CuIICuI(bbi)3(�-Mo8O26)][CuI(bbi)] bbi Three kinds of bbi ligands exhibit GTT,TTT, and GTG conformations

Photoluminescent property (theemission spectra of compound isblue-shifted), and the anhydrouscomposition begin to decompose at317 ◦C, and ends above 692 ◦C


[Ag(bixa)]2[Ag2(bixa)2(SiW12O40)] bix Two bix ligands adopt the bridgingcoordination mode with “U”-type, and“�” conformations in [Ag2(bixa)2]n

2n+,and [(Ag bixa)2]n

2n+, respectively

Luminescent property (blue-shiftemission compared with free bixmolecule), and decomposition of bixligands occur in the temperature rangeof 359–692 ◦C

2D [93]

[Ag5bixb5][K2(OH)P2W18O62]·H2O bix All the bix ligands acting as bidentate

linkers, with “S”-type, and “U”-typeconformations

Luminescent property (red-shiftemission), and to the release of bixligands occur in the temperature rangeof 348–776 ◦C


[CuI(bix)][(CuIbix)(�-Mo8O26)0.5] bix Two kinds of bix ligands as linkagesand show GG and TT conformations

Chemical decomposition starts at270 ◦C, and ends at 480 ◦C, equivalentto the loss of two bix molecules


H(CuIbix)[(CuIbix)2(�-Mo8O26)]·2H2O bix Two kinds of bix ligands as linkagesexhibit GG conformations

Compound shows the electrochemicalproperty and weight loss from 380 to540 ◦C corresponds to the removal ofbix ligands


[Cd2(H2O)2(btb)4(SiMo12O40)] btb The btb ligands exhibit transconformation like a “S”, and synconformation like a “U”

Differential thermal analysis (DTA)gives the starting decompositiontemperature (DT) 363 ◦C


[Ni3(btb)5][PMo12O40]2·14H2O btb Three types of btb ligands showdifferent conformation modes (btbA,btbB, and btbC)

Decomposition of btb molecules startat 300 ◦C, and end at 600 ◦C. Compoundshows good electrocatalytic activitytoward the reduction of nitrite


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99

{[Ni(btb)2(H2O)][�-Mo8O26]0.5·H2O}n btb All the btba ligands show monodentatecoordination mode to link Ni(II) ions,and the btbb ligands as spacer, tofunctionalize [Mo8O26]4− anions

Fluorescence property andelectrochemical activity (redox of thepolyanion)

Self-penetrating 3D [97]

[Mn2(H2O)4(btx)3][SiMo12O40]·4H2O btx Two btx molecules exhibit V-shapedconformations, and another oneexhibits Z-shaped conformations

Compound is stable up to 310 ◦C, anddecomposition does not end untilheating to 600 ◦C

Polyrotaxane andpolycatenanecharacters

2D [96]

[Ni2(H2O)4(btx)3][SiMo12O40]·4H2O btx Two btx molecules exhibit V-shapedconformations, and another oneexhibits Z-shaped conformations



2D [96]

[Co2(H2O)4(btx)3][SiMo12O40]·4H2O btx Two btx molecules exhibit V-shapedconformations, and another oneexhibits Z-shaped conformations



2D [96]

[CuI(bbtz)]5[BW12O40]·H2O bbtz The bbtz ligands as linkers to connectto the POM units, and Cu atoms inloop-containing 1D chains

Compound exhibit excellentphotocatalytic property for thedegradation of (methylene blue) MB


[Cu12(bmmtp)9(HSiW12O40)4]·0.5H2O bmmtp Three bmmtp ligands as a bridgingligands adopt two cis conformations,and one trans conformations

Electrocatalytic activity toward thereduction of nitrite

Self-penetrating 3D [104]

[Co2(btmbp)5(H2O)2(SiW12O40)] btmbp Four btmbp ligands showmonodentate bridging coordinationmode to link two Co(II) ions throughthe triazole nitrogen atoms

Photocatalytic property for thedegradation of (methylene blue) MB

Polypseudo-rotaxane 3D [79]

[Cu2(btmbp)3(H2O)2(SiW12O40)]·H2O btmbp Three btmbp ligands adoptmonodentate coordination mode, andlies on an inversion center (at themidpoint of the biphenyl moiety)


Polycatenaned 3D [79]

[CuII(btp)2(H2O)][�-Mo8O26]0.5·2H2O btp Four btp ligands show monodentatecoordination mode

Photoluminescence property (theemission spectra of compound isblue-shifted)

Polycatenaned 3D [98]

[Cu(btx)]4[SiMo12O40] btx Four btx ligands in [Cu(btx)]nn+ show

the Z-shaped conformationsLuminescent property (the emissionbands are bathochromic) andcompound remains intact until it isheated to 314 ◦C

Polypseudo-rotaxane 2D [96]

[Cu2(bpba2)3(SiMo12O40)(H2O)6]·9H2O bpba2 Two bpba2 ligands with a “V”-typeconformation to form a squareCu2(bpbaa

2)2 loop, and bpbab2 ligands

serve as bidentate ligands to bridgeadjacent Cu2(bpbab

2)2 loops into a 1Dchain

Weight loss in the range of 300–650 ◦Cis equivalent to the loss of bpba2

ligands, and compound exhibitsphotocatalytic activity toward thedegradation of methylene blue (MB)

Polyrotaxane 2D [112]

[Ag5(bpba1)3(HSiWVI10WV

2O40)(H2O)2]·6H2O bpba1 The bpba1a ligands show GAG cis

conformation, and bpba1b is

�3-bridging

Photoluminescence property (red-shiftemission)


[Cu2(bpp)4(H2O)2](SiW12O40)·6H2O bpp Four bpp ligands show monodentatecoordination mode

Electrocatalytic activity in theelectrical reductions of nitrite, and theweight-loss step after 255 ◦C isattributed to the oxidization of ligandsbpp


[Cu2(bbimid)4(H2O)2](SiW12O40)·8H2O bbimid Each bbimid ligands acting as abidentate ligand bridges two Cu(II)

cations

Electrocatalytic activity toward thereduction of nitrite, and decompositionof bbimid molecules occur in thetemperature range of 300–700 ◦C

3D [90]

[Ag8(pbpb)4(�-Mo8O26)(�-Mo8O26)(H2O)3]·H2O

pbpb Each pbpb ligands acts as atetradentate ligand shows a distortedtrans-conformation

Photoluminescent property (theemission spectra of compound isred-shifted) and exhibitsphotodegradation of (methylene blue)MB

3D [100]

[CuI3.1CuII

0.5(�-Mo8O26)0.5(�-Mo7

VIMoVO26)0.5(bbpb)2(H0.8bbpb)0.5]bbpb The bbpb ligands act as a tridentate or

tetradentate show three kinds ofconformations (trans, cis and distortedcis)

Photoluminescent property (theemission spectra of compound isblue-shifted)

3D [100]

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Table 2 (Continued )


[CuI2bmmb2]2[Mo8O26] bmmb Two bmmb ligands as a bidentate

ligands bridge two Cu(II) cations toform a [4+4] oval metallomacrocycle

Compound, in contrast to the bmmbligand, exhibits a blue-shift, which istuned by the metal-ligand interactions

3D [101]

(bix)[Cu(bix)]3[PW12O40]·4H2O bix All the bix ligands with “Z”-typetrans-conformation to form two kindsof Cu(bix) coordination polymericchains

Compound keeps the redox propertiesof their parent polyanions and theweight loss of at 320–550 ◦C may beascribed to the decomposition of thebix ligands, and POMs

3D [102]

[Cu(bix)]3[PMo12O40] bix 1/3 and 2/3 parts of the bix ligandsadopt “Z”-type trans, and “U”-typesyn-conformations between Cu1 andCu2 ions, respectively

Compound keeps the redox propertiesof their parent polyanions, and exhibitsa one step weight loss, starting at350 ◦C, and ending at 500 ◦C

3D [102]

[Co(bbi)2]2[As8V14O42(H2O)] bbi Each bbi acting as a bidentate ligandbridges two Co(II) cations and showTTG conformations

Magnetic property (a common featureof this compound is the presence ofstrong antiferromagnetic couplinginteractions)

2D [103]

[Cu(bbi)]4[As8V14O42(H2O)] bbi Each bbi acting as a bidentate ligandbridges two Cu(II) cations and showthree kinds of conformations(GTG, GTT,TGT) in the ladder like double-chain

3D [103]

[Cu(bbi)]6[V16O38Cl] bbi Each bbi acting as a bidentate ligandbridges two Cu(II) cations and show thefive sorts of conformations in the[Cu(bbi)]n chain (GTG, GGT, TGT, TTG,GGG)

3D [103]

[Cu(1,4-bpimb)(H2O)2][Mo8O26]·4H2O bpimb Two bpimb ligands acting as abidentate linker in cis configuration

Supramolecular interactions generatedby octamolybdate thermally stabilizethe metallo-macrocycle compositematrix to the high temperature (up to330 ◦C) in comparison to conventionalmetallomacrocycles

[118]

[Ag8(btp)4(H2O)2(HPWVI10WV

2O40)2]·H2O btp All the potential coordination N donorsin each btp ligand with two types ofconformation modes: the “Z”-type, andthe “U”-type are utilized to coordinatewith four AgI ions

This compound may be suitablecandidates for potential fluorescentmaterials (the emission spectra isred-shifted compared), The secondweight loss step of compound in therange of 300–600 ◦C can be attributedto the loss of btp ligands, and exhibitsphotodegradation of (methylene blue)MB

3D [107]

[Ag4(btb)2(HPWVI10WV

2O40)] btb The btb ligand exhibits only a singleconformation mode by offering four Ndonors to link four Ag ions

Photoluminescent property (theemission spectra of compound isred-shifted). This compound exhibitsonly one weight loss step below 600 ◦C,which is ascribed to the loss of btpligands, and exhibits photodegradationof (methylene blue) MB

3D [107]

[Ag5(btx)4(PWVI10WV

2O40)] btx Four btx ligands show �2-, and�3-coordination modes

Photoluminescent property (theemission spectra of compound isred-shifted). Compound shows onlyone weight loss step below 600 ◦C,which is ascribed to the loss of btxligands, and exhibits photodegradationof (methylene blue) MB

3D [107]

[Ni2(H2O)4(btx)4(Hbtx2)2][PMo12O40]2·2H2O btx Three types of btx ligands showdifferent conformation modes (btxA,btxB, and btxC)

Decomposition of btx ligands occur inthe temperature range of 300–660 ◦C

3D [150]

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[Ni2(btb)4(SiW12O40)]·H2O btb Three types of btb ligands showdifferent conformation modes (btbA,btbB, and btbC)

The second weight loss step in therange of 300–660 ◦C can be attributedto the decomposition of btb ligands

3D [150]

[Ag7(bte)4(H2O)(HP2WVI16WV

2O62)]·2H2O bte Four bte ligands act as a tridentatebridging/chelate ligand, tridentatelinkage, and tetradentate linkagebridging, with two types ofconformation modes (the “Z”-, and“U”-type)


3D [157]

[Ag7(btp)5(HP2WVI16WV

2O62)]·H2O btp The btp ligands act as a bridging ligand,and also shows two types ofconformation modes: the “U”-, and“Z”-type

Antibacterial activity againstStaphylococcus aureus, and E. coli.Photocatalytic property for thedegradation of (methylene blue) MB

3D [157]

[Ag4(btb)3.5(P2W18O62)](H2btb)·2H2O btb The btb ligands (“Z”-type) act as abidentate linkage, and tridentatelinkage (“U”- and “S”-type)

Antibacterial activity againstStaphylococcus aureus, and E. coli.Photocatalytic property for thedegradation of (methylene blue) MB

3D [157]

[Cu2(bpba1)3(SiMo12O40)(H2O)6]·2H2O bpba1 Two bpba1 ligands show two types ofconfigurations (“V” type bpbaa

1andlinear bpbab

1), acting as a bidentatebridging ligand to coordinate with twoCuII ions

Weight loss in the range of 300–600 ◦Ccorresponds to the loss of three bpba1

ligands, and compound showselectrocatalytic activity toward thereduction of nitrite, andphotodegradation of methylene blue(MB)

2D [112]

[Cu2(bpba2)3(SiW12O40)(H2O)6]·6H2O bpba2 The bpba2 ligands with a “V”-typeconformation exist in squareCu2(bpbaa

2)2 loop, and bpbab2 ligands

serve as bidentate ligands to bridgeadjacent Cu2(bpbab

2)2 loops into a 1Dchain

Electrocatalytic activity toward thereduction of nitrite and the removal oforganic molecules occur in the range of300–750 ◦C, and compound showsphotocatalytic activity toward thedegradation of methylene blue (MB)

Polyrotaxane 2D [112]

[Cu2(bpba3)3(SiMo12O40)(H2O)6]·4H2O bpba3 The bpba3 ligands acting as a bidentatecoordination mode and show twotypes of configurations (bpbaa

3 andbpbab

3). The bpbaa3 to link Cu1 and

Cu2 ions to generate wave-like chain.The bpbab

3 to link Cu2 ions toconstruct an undulating 2D network

Weight loss in the temperature rangeof 200–550 ◦C can be assigned to therelease of organic ligands andcompound shows photocatalyticactivity toward the degradation ofmethylene blue (MB)

2D [112]

[Cu2(bpba3)3(SiW12O40)(H2O)6]·4H2O bpba3 All the bpba3 ligands acting as abidentate coordination mode, andshow two types of configurations(bpbaa

3 and bpbab3)

Weight loss occurring from 300 to650 ◦C is attributed to the organicmolecules, and compound showsphotocatalytic activity toward thedegradation of methylene blue (MB)

2D [112]

{Ag2(4-bptzb)2(H2O)2[H2PMo12O40]2}4-bptzb·5H2O

4-bptzb Trans-4-bptzb ligand, acting as abidentate bridging ligand, to form[Ag2(trans-4-bptzb)2]2+ subunit

Photocatalytic activity toward thedegradation of methylene blue (MB),and Rhodamine B (RhB)

3D [158]

[Ag4(3-bptzb)2(PMoVMoVI11O40)]·2H2O 3-bptzb The “S”-type, and “U”-type 3-bptzb

ligands (tetradentate linkage) connectwith Ag1 ions alternately, giving acharming meso-helix chain

This compound may be regarded as apotential fluorescence sensor to detectsome aromatic molecules

3D [158]

{Ag3(3-bptzb)2.5(H2O)2[H3P2W18O62]} 3-bptzb The 3-bptzb ligands show three typesof coordination modes: �1, �2

′ , and�4

′ modes

Photocatalytic activity toward thedegradation of methylene blue (MB),and Rhodamine B (RhB)

3D [158]

[Cu4(bmtm)4][SiW12O40]·2H2O bmtm Two kinds of bmtm ligands (bmtm1,bmtm2) act as a tridentate �2(1, 2),and a tetradentate �3(1, 2, 1

s)linkages, respectively

The removal of organic componentsand collapse of the SiW12 clustersoccur at 280 and 530 ◦C, respectively

1D [111]

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Table 2 (Continued)


[Cu4(bmte)3.5][SiW12O40] bmte All the bmte ligands to chelate CuI ions,forming stable nine-memberedchelating rings

To release of the organic ligands andcollapse of the SiW12 clusters occur at300 and 570 ◦C, respectively

2D [111]

[Ag4(bmte)2(H2O)2(SiMo12O40)] bmte Two bmte ligands act as a linkages, andto generate [Ag4(bmte)(H2O)2]4+


1D [159]

[Ag4(bmtp)2(H2O)2(SiMo12O40)] bmtp Two bmtp ligands act as a linkages, andto form [Ag4(bmtp)(H2O)2]4+


1D [159]

[Ag4(bmtb)3(SiMo12O40)] bmtb The bmtb ligands act as a bridgingligands exist in [Ag2(bmtb)]2+


2D [159]

[Cu6Na2(Htrb)4(Mo6O19)(MoO4)6] Htrb Eight Htrb ligands bridge six Cu(II)

atoms in hexadentate modes to form ametal-organic Cu6(Htrb)8 cage

Photodegradation of methylene blue(MB), and luminescent property (theemission spectra of compound isblue-shifted, may result from thecoordination effects of the Htrb ligandsto the metal atoms)

3D [106]

[Zn3(Htrb)(Mo10O34)]·8H2O Htrb Htrb as a hexadentate ligand bridgesneighboring layers to form a 3Dframework

Photodegradation of methylene blue(MB), and luminescent property (theemission spectra of compound isblue-shifted)

3D [106]

[Zn2(Htrb)(�-Mo8O26)(H2O)2]·6H2O Htrb Htrb ligands in 1,3,5-up/2,4,6-downfashions bridge adjacent chains to givea 3D framework

Photodegradation of methylene blue(MB), and luminescent property (theemission spectra of compound isblue-shifted)

3D [106]

Ag4(ttmb)2(�-Mo8O26) ttmb The ttmb ligands can adopt differentcoordination conformations toconstruct coordination polymers

Compound displays strong greenfluorescence

2D [109]

[CuI2(tmp)3(Mo8O26)0.5] tmp N atoms from the imidazole ring of the

tmp ligands graft onto octamolybdateso as to obtain tecton

Compound is stable up to 280 ◦C, andthen begins to decompose until about580 ◦C

2D [110]

H{Cu2(�2-OH)2bpbp1[CrMo6(OH)6O18]}·4H2O bpbp1 Each bpbp1 ligands display a�2-bridging mode connecting two CuII

which act as the “supporter”

Weight loss at 350–585 ◦C is ascribedto the loss of organic molecules, andcompound shows photocatalyticactivity toward the degradation ofmethylene blue (MB)

3D [117]

{Cu2bpbp2

[CrMoVI5MoV(OH)6O18](H2O)4}·4H2O

bpbp2 Each bpbp2 ligands display a�4-bridging mode directly extendingthe 1D Cu-POM chain

All of the bpbp2 ligands decompose inthe temperature range of 380–642 ◦C,and compound exhibits photocatalyticactivity toward the degradation ofmethylene blue (MB)

3D [117]

a Abbreviations: bbi = 1,1′′-(1,4-butanediyl)bis(imidazole); bix = 1,4-bis(imidazol-1-ylmethyl)benzene; btb = 1,4-bis(1,2,4-triazol-1-yl)butane; btx = 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene; bbtz = 1,4-bis(1,2,4-triazol-1-lmethyl)benzene; bmmtp = 1,3-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)propane; btmbp = 4,4′-bis((1-1,2,4-triazol-1-yl)methyl)biphenyl; btp = 1,3-bis(1,2,4-triazol-1-yl)propane; bpba1 = N,N′-bis(3-pyridinecarboxamide)-1,2-ethane; bpba2 = N,N′-bis(3-pyridinecarboxamide)-1,4-butane; bpba3 = N,N′-bis(3-pyridinecarboxamide)-1,6-hexane; bpp = 1,3-bis(4-pyridyl)propane; 4-bptzb = 1,4-bis(5-(4-pyridyl)tetrazolyl)butane; 3-bptzb = 1,4-bis(5-(3-pyridyl)tetrazolyl)butane; bbimid = 1,1′-(1,4-butanediyl)bis-1H-benzimidazole; pbpb = 1,1′-(1,3-propanediyl)-bis[2-(4-pyridyl)benzimidazole]; bbpb = 1,1′-(1,4-butanediyl)bis[2-(3-pyridyl)benzimidazole]; bmmb = 1,3-bis(imidazol-l-ylmethyl)benzene; bpimb = bis[2-(2-pyridyl)imidazol-1-ylmethyl]benzene; bmtm = 1,1′-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane; bmte = the1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane;bmtp = the1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)propane; bmtb = the1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)butane; Htrb = hexakis(1,2,4-triazolylmethyl)benzene; ttmb = 1,3,5-tri(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene; tmp = 3-((1H-1,2,4-triazol-1-yl)methyl)pyridine; bpbp1 = N,N′-bis(3-pyridinecarboxamide)-piperazine; bpbp2 = N,N′-bis(4-pyridinecarboxamide)-piperazine.

Chemi

tTifiamitoaIacctaldndloaccdtctfiesmsoctwomfPcwptlbopt(

hcmoHsdPhbv


he individual components (POMs and organic ligands) [119–123].hey are divided into two different types according to the kind ofnteraction between the organic and inorganic components. Therst type (type I) refers to the systems where non covalent bondsre established between the organic and inorganic parts, com-only electrostatic interactions, hydrogen bonds, or van der Waals

nteractions are involved [124–127]. In the second type (type II),he organic and inorganic moieties are linked via strong covalentr iono-covalent bonds. The anionic character of POMs naturallyllows their association with organic counter cations forming type

hybrids. In type II hybrids, the organic ligand usually substitutesn oxo group of the POM and is directly linked to the metallicenter. Grafting an organic ingredient onto an inorganic system islosely dependent on the chemical nature and electronic proper-ies of the inorganic entity. The nucleophilic character of the oxygentoms localized on the surface of the POMs can also lead to cova-ent interactions with electrophilic groups (e.g. nitrosyl, hydrazido,iazoalkyl and imido) of the ligands [128–133]. Therefore, a largeumber of POM-based self-assembled structures with variousimensions have been synthesized via non-covalent, and cova-

ent interactions such as zero dimensional micelles and capsules,ne-dimensional fibers, tubes and belts, two-dimensional filmssemblies (Langmuir–Blodgett and layer-by-layer films) or singlerystals as three-dimensional. In addition, POMs can interact withationic hydrophobic components via electrostatic interactionsue to its hydrophilic head groups. For example, the interac-ion of hydrophilic POMs with hydrophobic surfactants producesore–shell like assemblies, such as surfactant-encapsulated clus-ers (SECs) or surfactant-encapsulated-POMs (SEPs), which canurther self-assembled to give a variety of nanostructures on var-ous surfaces/interfaces [134–136]. POM clusters can react withlectrically active materials such as CNTs, graphenes, metals,emiconductors and polymers, to develop POM-based electronicaterials [137,138]. POMs are very versatile according to their

tructure and chemical properties which can be fine-tuned in favorf the crystallization of bio macromolecules. Therefore POMs areonsidered as a promising candidate in the field of protein crys-allography which are most probably based on their interactionsith diverse proteins. Since POM–protein interactions are mainly

f electrostatic nature, the interaction of negatively charged POMolecules to positively charged protein induce different crystal

orms for the same protein. For example, interaction Keggin typeOM, Na2H[PW12O40], with prion protein with a low negativeharge favored the formation of rods, whereas (NH4)6[H2W12O40]ith a higher negative charge, the stronger the binding to therotein and favored the assembly of 2D crystals of the prion pro-ein. Also, Rompel et al. solved the crystal structures of both theatent and active forms of mushroom tyrosinase PPO4 from Agaricusisporus by co-crystallization with Na6[TeW6O24]·22H2O poly-xoanion. Then in order to confirm the charge interplay betweenroteins and POMs as the main driving force in crystal forma-ion, they crystallized the model protein hen egg white lysozymeHEWL) with the same polyoxoanion [151–153].

A common drawback in the synthesis of inorganic–organicybrids is the different solubility of the organic and inorganicomponents in the same solvent. Since the X-ray diffraction is theain method to identify these compounds, it is fundamental to

btain single crystal of good quality for the study of the hybrids.ydrothermal reactions facilitate the solubility of the primary

pecies increasing the chances to obtain suitable crystals for X-rayiffraction. Therefore, the assembly of hybrid materials based onOMs is often done in hydrothermal conditions. The formation of
ybrids in the black-box hydrothermal environment is affectedy a series of factors such as temperature, reaction time and pHalue. However, the modulation of the negative charge of the
stry Reviews 309 (2016) 84–106 103

POM units strongly influences the final structural assembly. Ifthe negative charges of the polyoxoanion increases, the num-ber of metal nodes and the organic bridging ligands is greater,thus increasing the possibility to build complicated 3D openframeworks. Moreover the electrostatic interaction between thepolyoxoanions and the metal cations is stronger. Consequently themetal ions are much easier to coordinate with the surface O atomsof POMs. The role of the POM changes from guest template to theinorganic linker. This change provides more chances and routesto design and synthesize new hybrid materials with complicated(entangled) and/or 3D open frameworks. To this respect, differentcoordination networks based on flexible ligands and differentnegative charges have been hydrothermally prepared [139].For example, the CuI/bbtz/POM reaction system, (bbtz = 1,4-bis-(1,2,4-triazol-1-ylmethyl)benzene) has been used toconstruct POM-based hybrid compounds: [Cu(I)bbtz]3[PMo12O40](78), [Cu(I)bbtz][Cu(I)(Hbbtz)2][SiMo12O40]·2.5H2O (79), and[Cu(I)bbtz]6[SiWV

2WVI10O40]·2H2O (80). The structural trans-

formation from host-guest frameworks in 78 and 79 to openframework in 80, is modulated by the charge of the Keggin-typepolyoxoanions. The charge is modulated by changing the cen-tral heteroatom of the POMs, e.g. [PMo12O40]3−, [SiMo12O40]4−,and [SiWV

2WVI10O40]6−. In this system, with the increase of

negative charge of the polyoxoanion, increases the numberof Cu(I) nodes, and the neutral organic bridging ligands (bbtz)promoting the possibility of building complicated 3D open frame-works. Similarly, new entangled coordination networks using ahydrothermal reaction system with transition-metal (TM) ions,bbtz ligands and various charge-tunable Keggin-type polyoxome-talates has been reported, [Co(II)(Hbbtz)(bbtz)2.5][PMo12O40](81), [Cu(II)(bbtz)]3[AsWV

3WVI9O40]·10H2O (82), and

[Cu(II)5(bbtz)7(H2O)6][P2W22Cu2O77(OH)2]·6H2O (83) [99]. In

81–83, with the enhancement of POM negative charges and theuse of different TM types, the number of nodes in the coordinationnetworks increased and the basic metal-organic building motifschanged from a 1D zipper-type chain (in 81) to a 3D diamond-likeframework (in 82) and finally to a 3D self-penetrating framework(in 83), as shown in Fig. 19.

The synthetic strategies to obtain Keggin-based transitionmetal monosubstituted POMs (Keggin-TPOMs), and heteropolyblue POMs (reduced POMs) based hybrids have been reported[144–148]. Zhang et al. synthesized four high-dimensional POM-based complexes by utilizing substituted or reduced Keggin anionswith high charge density, [Ag5(btp)4(H2O)2][PCuW11O39]·2H2O(84), [Ag4.33Na0.67(btp)4(H2O)2][PMnMo11O39]·H2O (85),[Cu4(btp)4Na(H2O)2][PMnMo11O39]·2H2O (86), and[Ag5(btp)4][PWVI

10WV2O40] (87). In structures, the different

coordination modes of POMs affected by the POM charge,in which the Keggin cluster act as decadentate ligand in84–86, and hexadentate linkages in 87 [149] (Fig. 20a). Thedesign and synthesis of supramolecular architectures basedon transition metal complexes (TMCs) is a topic of continu-ous interest in crystal engineering [140–142]. Recently, Wangand co-workers turned their attention to investigate the effectof the charge of POMs on the structures of TMCs by using(bpmb = 1,4-bis(pyrazol-1-ylmethyl)benzene). When the POManions possess two or five negative charges the TMCs in com-pounds [CuI

2bpmb2][Mo6O19] (88) or [CuI5bpmb5][BW12O40] H2O

(89) present 1D zigzag chain structures, whereas, when the POManions possess three or four negative charges the TMCs in com-pounds [CuI

3bpmb3]2[PMo12O40]2 (90), [CuI4bpmb4][SiMo12O40]

(91), [CuI4bpmb4][SiW12O40] (92), [CuI

4bpmb4][GeMo12O40] (93)
present macrocyclic structures. Thus, the charge of the POMas structure-directing influences the components of the TMCs(Fig. 20b) [143].


Fig. 19. Schematic view of the relationship between different architectures, and negative charge of POM units in compounds 78–83.

Figure was reproduced from Refs. [99,139] with permission of the copyright holders.

Fig. 20. (a) View of the 3D frameworks with coordination details of POM clusters: (left) PW11Cu in 84, and (right) PW12 in 87. (b) Representations of various architecturesconstructed from CuI ion, bpmb ligand with different charge polyoxometalates.

Figure was reproduced from Refs. [143,149] with permission of the copyright holders.

Chemi

6

vpPp(waotWtachttqfesrr

A

fACfi

R


. Concluding remarks

During the last decade the POMs research field has developedery rapidly as demonstrated by the increasing number of articlesublished in this regard. In the process of design and synthesis ofOM based hybrids several factors influence the final structure: (i)H, (ii) reaction temperature, (iii) molar ratios of starting materials,iv) selection of POM anions and ligand molecules. In this review,e highlight the role of flexible N-donor ligands, including imid-

zole, triazole, tetrazole, and pyridyl derivatives in the synthesisf the hybrid inorganic–organic compounds. Since the selection ofhe organic ligands is the key factor for constructing these hybrids.

e described the influence of different space lengths and coordina-ion groups of flexible N-donor ligands on the formation of variousrchitectures which may offer some possible synthetic strategies toonstruct inorganic–organic hybrids with novel architectures. Weave selected relevant works from the recent literature to illus-rate this matter. Remarkably, the topology and dimensionality ofhe compounds can be controlled and adjusted by using the ade-uate flexible N-donor ligands. This is likely due to the fact that theyacilitate the construction of entangled structures, such as interpen-tration, polycatenanes, polyrotaxanes, polypseudo-rotaxane, andelf-penetration. In the final section of this review the importantole of the POMs charge dictating the synthesis of the hybrid mate-ial and influencing the final structure of the product is emphasized.

cknowledgments

MM would like to thank the Ferdowsi University of Mashhador financial supporting this study (grant no. 28396/3-2013/10/02).F thanks MINECO of Spain (projects CONSOLIDER INGENIO 2010SD2010-00065 and CTQ2014-57393-C2-1-P, FEDER funds) fornancial support.

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Coordination Chemistry Reviews - جستجو در مقالات دانشگاهی و ...profdoc.um.ac.ir/articles/a/1052932.pdf · 2018-01-24 · Coordination Chemistry Reviews 309 (2016)