applications of inorganic-organic hybrid architectures

Received: 16 October 2018 Revised: 30 November 2018 Accepted: 24 December 2018

REV I EW

DOI: 10.1002/aoc.4808

Applications of inorganic‐organic hybrid architecturesbased on polyoxometalates in catalyzed and photocatalyzedchemical transformations

Nahid Lotfian1 | Majid M. Heravi1 | Masoud Mirzaei2 | Bahareh Heidari1

1Department of Chemistry, School ofSciences, Alzahra University, Vanak,Tehran, Iran2Department of Chemistry, Faculty ofScience, Ferdowsi University of Mashhad,Mashhad, Iran

CorrespondenceMajid M. Heravi, Department ofChemistry, School of Sciences, AlzahraUniversity, Vanak, Tehran, Iran.Email: [email protected];[email protected]

Masoud Mirzaei, Department ofChemistry, Faculty of Science, FerdowsiUniversity of Mashhad, Mashhad, Iran.Email: [email protected];[email protected]

Funding informationAlzahra University, Tehran, Iran; IranNational Science Foundation

Appl Organometal Chem. 2019;33:e4808.https://doi.org/10.1002/aoc.4808

The scope of this article is to reveal the fruitful combination of POM species

with metal coordination complexes, leading to the construction of several effi-

cient multifunctional catalysts. In this review, we try to underscore various cat-

alytic and photocatalytic reactions catalyzed by POM‐based inorganic‐organic

hybrid. Notably, it has been well established that depending on the type of

the reaction, the activity and selectivity of these hybrid catalyst can be drasti-

cally improved by the rational and correct choice of the organic metal complex

and POM anion providing a marriage of convenience.

KEYWORDS

catalytic, chemical transformations, inorganic‐organic hybrid, photocatalytic, polyoxometalate

1 | INTRODUCTION

The chemical industry plays a key role in sustaining theworld economy and underpinning future technologies.The chemical industry involving the companies that pro-duce chemicals by converting various raw materials, suchas oil, natural gas, air, water, metals, and minerals intoseveral numbers of different products, mostly are essen-tial for our daily life. However, many chemical processescomprising the utilization of hazardous chemicals whichhave the high potential to cause many negative effectson the environment thus, putting our life in great dangerand destroying the plant kingdom.

As a matter of fact, nowadays it is well recognizedthat an ideal chemical reaction is a catalyzed one, fromdifferent points of view particularly, when it is performedat high scale. Indeed, utilization of green catalysts inchemical processes is circumventing part of the

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aforementioned problems, minimizes these adverseeffects in the chemical industry. Thus, in recent twodecades, the preparation of green, modified and improvedcatalysts has attracted much attention of the chemicalcommunity resulted in the introduction of a plethora ofenvironmentally benign and harmless catalysts. However,besides being green and harmless to the environments,for the design and preparation of such catalysts, someother aspects and issues must be taken to consideration.Their selectivities, activities and effectiveness, therequired energy during the reactions are performed, suit-ability and greenness of the selected solvents, simple sep-aration of catalysts from reaction mixture, several timesreusability of them are the main parameters which mustbe carefully considered for obtaining the products in highto excellent yields in acceptable purity without requiringexpensive and time‐consuming separation techniquessuch as chromatography. If these challenges are met

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2 of 25 LOTFIAN ET AL.

reasonably, the problems of environmental contamina-tion may simultaneously be abridged. Nowadays, the syn-thetic chemists, who are engaged in the design andpreparation of novel catalysts have focused on the elimi-nation or minimization of the risk bringing it to anacceptable degree from both greenness and economicalpoints of view. Conventionally, the risks imposed bychemical processes are minimized by limiting exposureby controlling so‐called, circumstantial factors, such asthe safe use, handling, treatment, and disposal ofchemicals. The existing legislative and regulatory frame-work that governs these processes focuses almost exclu-sively on the issue of safety and clean environments.

Against this background, what will be needed for thechemical industries to embrace efforts to make their pro-cesses even “greener”? Some of the issues raised by thedevelopment of “green chemistry” techniques to identifythe potential barriers against the implementation of envi-ronmentally benign chemical processes by chemicalindustries.

Nowadays, many synthetic chemists from various dis-ciplines have directed their research to the design andpreparation of green catalysts for being used in catalyticreactions due to their important role played in modernchemical industries. Catalysts only in tiny amounts areincreasing the rate of a chemical reaction, they are notconsumed during the progress of reactions, often, in theform of heterogeneous can be easily separated andreused, repeatedly. Thus, a catalyzed reaction is almostan ideal approach in academia as well as chemical indus-try from economic point of view. It would be more ideal ifthe principles of green chemistry are mandatory andindulged in chemical industry. Green’ in chemistry is uti-lization of chemicals/processes that are benign, eco‐friendly, the appropriate starting materials are eitherinexpensively available or readily accessible, preferablybeing reusable, recyclable, producing minimum wasteand avoid using toxic and hazardous chemicals.

Among an overgrowing useful number of novel andmodified catalysts, polyoxometalates (POMs), as anionicmetal–oxygen clusters, are significant inorganic materialswhich have attracted enormous attention, in the last fourdecades among synthetic chemists due to their extreme ver-satility of compositions, high structural stability and specialproperties.[1] Among the various precious utilization, thecatalysis is one of the first and the most valuable applica-tions of POMs due to numerous merits and advantages,achieved and reported. First of all, it is worthy to mentionthat, POMs show fast and reversible multi‐electron redoxbehaviors under mild reaction conditions. These qualitiesintroduce them as outstanding candidates for thecatalyzed‐ oxidation of various organic substrates.[2] Sec-ond, POMs showing high Lewis and Brønsted acidity thus

can be first choice for being used as promising and effectivesolid‐acid catalysts for many acid‐catalyzed organic trans-formations even at industrial level.[3] Thirdly, severalPOMs show basic properties, thus can be efficiently usedin common base‐catalyzed reactions.[4] Furthermore,POM anions in their ground states can be excited by UVor near‐visible light, which make them suitable for beingutilized as effective photo‐catalyst in photocatalytic reac-tions.[5] It should be mentioned, that the pure bulk POMsexperience some drawbacks. They have relatively smallspecific surface areas that hinder accessibility to their activesites thus, some limitations are imposed to their catalyticactivity. To circumvent such disadvantage, much efforthas been devoted. For example, POMs have beenimmobilized onto different porous solid matrixes such assilica with high‐surface areas,[6] activated carbon,[7] molec-ular sieves,[8] and graphene or graphene oxide[9] to improvetheir dispersion and heterogeneity. Nevertheless, the use ofthese systems is limited by several other drawbacks, includ-ing low POMs loading, POMs leaching, the conglomerationof POM particles and nonuniform active sites. Thus, it issignificant to develop new strategies to improve the stabil-ity, recycling, and catalytic activities of POMs.

It is well known that metal–organic coordinationcompounds usually possess good stability and relativelylow solubility in several common solvents.[10] A promis-ing strategy to develop POMs as highly active catalyticsystems is the combination of POMs with metal‐organiccoordination complexes in order to construct crystallineinorganic‐organic hybrid architectures based on POMswith metal‐organic framework (MOF) structures orhigh‐dimensional supramolecular networks. Owing tothe presence of numerous surface oxygen atoms, POMsin hybrid forms are either coordinated to metal centersof the metal‐organic skeleton via metal‐oxygen coordina-tion bonds or connected as templates/guests/counter ionsthrough non‐covalent interactions.[11] POMs coupledwith metal‐organic coordination complexes can take theadvantages of both components properties and their com-bination are most likely lead to desirable active catalyticsystem. Due to the importance of POM‐based coordina-tion complexes, many of these compounds were synthe-sized and their extensive structural diversity weresummarized in several useful reviews.[12] On the otherhand, because of the importance of POMs acting aspotential (photo) catalysts, several invaluable reviewscovering the topic have been published during years.[13]

We are interested in POMs‐catalyzed organic reactionsespecially those leading in the synthesis of heterocycliccompounds under heterogeneous and green conditions.[14]

We have also some experiences in photocatalyzed‐organictransformations.[15] Use of heterogeneous catalysis undergreen conditions has always been considered as the first

LOTFIAN ET AL. 3 of 25

criteria in our laboratory. During the recent decade, wecovered the applications of POMs in organic transforma-tions in three independent reviews.[16] Armed with theseexperiences and our continuous interest in utilization ofPOMs as heterogeneous and green catalysts, in this review,we try to underscore the applications POM‐basedinorganic‐organic hybrid materials which have been suc-cessfully employed in a wide variety organic transformationand expected to have a brilliant future in the field ofcatalysis.

It is worth mentioning that, based on our literaturesurvey there is no any comprehensive review in the fieldof the catalytic applications of inorganic‐organic hybridarchitectures‐based POMs. Thus, this review for the firsttime, highlights the development of POM‐basedinorganic‐organic hybrid materials in designing a seriesof highly efficient catalyst for the catalysis of variousorganic transformations including oxidation, acid‐catalyzed reactions, base‐catalyzed reactions, acid‐basebi‐functional reactions, alkene polymerization and cou-pling reactions. Besides, we disclose that inorganic‐organic hybrids are also have a great impact inphotocatalysis. Thus, this review also covers the use ofthese materials as photocatalysts in degradation oforganic dyes, H2 evolution, water oxidation, degradationof inorganic pollutants, photocatalytic organic synthesisand photocatalytic CO2 reduction. This review alsoreveals that depending on the type of the reaction, theintelligent choice of metal‐organic complex and POManion is crucial for a rational design of highly efficienthybrid catalyst for that certain type of transformation.

2 | APPLICATIONS OFINORGANIC ‐ORGANIC HYBRIDMATERIALS ‐BASED ON POMS ASHETEROGENEOUS CATALYSTS INCHEMICAL TRANSFORMATIONS

2.1 | Oxidation

Oxidation is a significant process for production of a widerange of useful and versatile functional groups, such ashydroxyl, carbonyl, and so forth. Therefore, is in greatimportance in academia as well chemical industry. Dueto the large number of metal ions with high oxidationstates, POMs can participate in several oxidation reac-tions as the green and efficient catalyst. On the otherhand, some metal‐organic compounds are renown asthe highly active catalysts for the aerobic oxidation of dif-ferent functional groups.[17] Therefore, preparation of theefficient inorganic‐organic catalyst by its combining withPOMs with suitable metal‐organic compounds has

opened a gateway for effective and eco‐friendly of oxida-tion of different functional groups.

2.2 | Oxidation of alkenes

Over the last decade, numerous catalytic oxidation reac-tions by inorganic‐organic hybrids of POMs have beendiscovered and mostly used in the oxidation of alkenessince it has been valuable in chemical industry due tothe production of important intermediates and useful fin-ished products, needed in our daily life, under green con-ditions as well as with a high economic feasibility.[18]

Notably, oxidation of alkenes initially generates epoxides,the constrained cyclic ethers whose molecules contains athree‐membered ring involving an oxygen and two car-bon atoms, which easily undergo hydrolysis, leading tothe formation of diols in the presence of water. Whenepoxides are used as intermediates, its strain in thethree‐membered ring makes it much more reactive thana typical acyclic ether. Fruitful ring opening of an epoxideas intermediate requires high regioselectivity and in somecases stereoselectivity, for example via asymmetric shape-less epoxidation (ASE) must be in high concerns.[19]

In 2010, Ali's group synthesized two copper complex‐modified Anderson‐type POMs, [{Na4(H2O)14} {Cu (gly)}2][TeMo6O24] (gly = glycine) and [{Cu (en)2}3{TeW6O24}]•6H2O (en = ethylene‐diamine), and found that thesematerials exhibited moderate catalytic activity for theliquid‐phase partial epoxidation of styrene and cyclohex-ene using tert‐butylhydroperoxide (TBHP) as oxidant.[20]

The major products of this partial oxidation of both sty-rene and cyclohexene were their corresponding epoxides,respectively. At the initial stage of the reaction, the highselectivity for both epoxides were observed. With time,as the conversions reach their respective maxima, theepoxide selectivity goes down to some extent. This couldbe attributed to the hydrolysis of the epoxides resultingin the formation of the respective dialcohols as the finalproducts. This observation is common for the liquidphase partial oxidation of olefin over a heterogeneous cat-alyst, as water generated in the oxidation process, pro-moting the epoxide ring opening.[21]

As mentioned above, nowadays ASE as an example ofkinetic resolution has attracted much attention. Asymmet-ric dihydorxylation of an alkene is in great importance,especially in the total synthesis of natural products. In thisline, the combination of chiral metal‐organic frameworksand POM catalysts in the same hybrid materials is an effec-tive strategy to improve the asymmetric oxidation of thealkenes. In 2013, Duan and coworkers prepared two enan-tiomorphs Ni‐PYI1 and Ni‐PYI2 amphipathic catalysts forthe asymmetric dihydroxylation of aryl olefins, through

TABLE 1 Yields and enantiomeric excess (ee) values of the

asymmetric dihydroxylation about the aryl olefins with Ni‐PYI1

Entry substrate conversion (%) ee (%)

1 styrene (1) 75 >95

2 2‐chlorovinylbenzene (2) 76 67

3 3‐chlorovinylbenzene (3) 79 >95

4 4‐chlorovinylbenzene (4) 75 >95

5 3,5‐di‐tert‐butyl‐4′‐vinylbiphenyl (5)

<10 nd


incorporating the oxidation catalyst [BW12O40]5– (BW12)

and the required chiral element, L‐ or D‐pyrrolidin‐2‐ylimidazole (PYI), within a single MOF (Figure 1).[22]

The superior redox properties of the oxygen‐enriched sur-face of BW12 provided a driving force for the transforma-tion of catalyzed chiral precursors to the optically activeepoxide intermediate. Meanwhile, the chiral PYIs actedas cooperative catalytic sites that enhanced the activitiesof the oxidants and drove the catalysis with highstereoselectivity. The channels of Ni‐PYIs consolidatedby the organic ligands and the POM anions were wellmodulated with hydrophilic/hydrophobic properties toallow ingress and egress of molecules of both H2O2 andolefin. The transformation of styrene and its derivativesusing H2O2 as the oxidant and Ni‐PYI1 (0.7% mol ratio)as the heterogeneous catalyst gave the corresponding(R)‐products in satisfactory ee (67−95%), as shown inTable 1. The low conversion of bulky 3,5‐di‐tert‐butyl‐4′‐vinylbiphenyl is attributed to negligible adsorption byNiPYI1 due to its large molecule size compared to the sizeof the channels of MOF. It is suggested that the asymmet-ric dihydroxylations indeed occur in the channels of theMOF, and not on the external surface. From a mechanisticpoint of view, in the dihydroxylation process, the forma-tion of hydrogen bonds between the pyrrolidine N atomand the terminal oxygen atoms (Ot) of the BW12 play akey role by first activating the related W=Ot bonds togenerate an active peroxide tungstate intermediate usingas an oxidant H2O2. In fact, the hydrogen bonds enforcedthe proximity between the conventional electrophilicoxidant and the chiral inducer to provide additional stericorientation, driving the stereoselective catalysis to occur.

Apart from epoxides and diols, the oxidation ofalkenes with POMs hybrid catalysts can also generate

carbonyl compounds via the cleavage of the C=C bondunder suitable reaction conditions. Selective oxidation ofalkenes to aldehydes is one of the most valuable andcommercially important chemical reactions, since alde-hydes are generally reactive and have been widely usedin MCRs for the synthesis of a wide range of intermedi-ates being used in the total synthesis of natural productsas well as the mass production of pharmaceuticals, dyes,perfumes and fine chemicals as starting materials.[23]

Considering the unique catalytic activity of the Co ionin many oxidation reactions[24] Li's group synthesized twoPOM‐based Co‐MOFs with chemical formulas [Co(BBTZ)2][H3BW12O40]•10H2O and [Co3(H2O)6(BBTZ)4][BW12O40]•NO3•4H2O (BBTZ = 1,4‐bis(1,2,4‐triazol1‐ylmethyl)benzene) for the selective oxidation of alkenesto aldehydes.[25] The former exhibits a non‐interpenetratedthree‐dimensional (3D) cds‐type open framework with a3D channel system, while the latter possesses a 3Dpolyrotaxane framework with one‐dimensional channels.In order to go insight into the structural influence of

FIGURE 1 Synthetic procedure of Ni‐

PYI1, showing the guest exchange and the

potential amphipathic channel for the

asymmetric olefin dihydroxylation. Figure

reproduced from Ref.,[22] with permission

of the copyright holders


these POM‐based MOFs (POM‐MOFs) in catalysis,another known non‐porous POM‐based organic‐inorganic hybrid compound, [Co2.5(BBTZ)4(H2O)][BW12O40]•4H2O,

[26] was selected as the reference cata-lyst for the selective oxidation of styrene. This hybrid pos-sesses the similar chemical composition to that ofcompounds but has no solvent‐accessible void in its crys-tal structure. In spite of observing higher selectivity, bycomparison this oxidation was found slower than that ofcatalyzed by [Co (BBTZ)2][H3BW12O40]•10H2O (4 h)and [Co3(H2O)6(BBTZ)4][BW12O40]•NO3•4H2O (6 h).These results suggested that styrene oxidation catalyzedby reference catalyst just occurs on the surface of the cat-alyst. As for POM‐MOFs compounds, their inherentporous property exposes more catalytic sites both on theexternal surface and in interior channels, thus accelerat-ing the oxidation process. In order to study the generalapplicability of this catalytic system, selective catalyticoxidation of styrene derivatives was examined. Bothelectron‐withdrawing and electron‐donating substratescan be converted smoothly and selectively into the corre-sponding aldehydes in the presence of [Co (BBTZ)2][H3BW12O40]•10H2O as catalyst (Table 2). It is supposedthat the required different reaction times for the oxida-tion of various styrene derivatives may be related tothe molecular size of the substrates. With the increasingof substrate size, the complete conversion requires a lon-ger reaction time. This result suggests that the substrateswith a large molecular size may not match with thechannels in POM‐MOFs compound, thus decreasingthe rate of conversion. In the other words, the pore size

TABLE 2 Selective catalytic oxidation of styrene derivatives to the co

[H3BW12O40]•10H2O

Entry Substrate Product

1

2

3

4

5

6

7

8

of the catalysts shows an obvious influence on the cata-lytic activities in a selective oxidation of styrene deriva-tives to the corresponding aldehydes.

2.2.1 | Oxidation of alkanes

The selective conversion of alkanes into highly desirableoxygenated compounds is one of the most significant trans-formations in petrochemical industry.[27] In this regard, thecontrolled aerobic oxidation of methane is an importantand common process in petrochemical units, worldwide.Althoughmethane can be aerobically oxidized to methanolbiologically, using methane monoxygenase enzymes, thisenzymatic oxidation needs to be carried out in the presenceof the sacrificial reducing agents for oxygen activation.[28]

Therefore, for production of methanol, non‐biologicalprocess for selective activation of saturated C‐H bonds, isfrequently preferred..[29] As mentioned previously in thechemical literature, platinum (II) complexes can also beused for methane activation leading to its oxidation to pro-duce MeOH.[30] Mixed‐metal oxide catalysts, especiallythose containing vanadium as one of their elements, werealso found suitable candidates for being used in the selec-tive oxidation of alkanes.[31] From the Neumann group, amore attractive bifunctional catalyst, [Pt (Mebipym)Cl2](H4V2PMo10) (Mebipym = 2,2′‐methylbipyrimidine), hasbeen prepared and successfully being used in methane oxy-genation by incorporating a cationic Pt complex and disub-stituted [H4V2PMo10] POMs via electrostatic interaction.[32]

The combination of [Pt (Mebipym)Cl2] cation and

rresponding aldehydes with H2O2 in the presence of [Co (BBTZ)2]

Time (h) Con. [%] Select. [%]

4 >99 96

4 95 98

6 98 99

6 >99 99

6 97 99

7 96 97

7 95 94

7 94 96


[H4V2PMo10] anion increases the activity of each individualmoiety. The enhanced catalyst activity allows the utiliza-tion of more accessible O2 instead of H2O2. Therefore, utili-zation of this inorganic‐organic hybrid material makes thereaction safer, more effective, and economically more feasi-ble for being used as a catalyst for industrial applications. Inthe presence of the hybrid catalyst, methane easily is trans-formed to methanol along with partial formation of acetal-dehyde at 320−330 K, under 1−2 bar O2 pressure. Underthese mild reaction conditions, no significant formation ofCO, CO2, and acetic acid was observed due to the suppres-sion of further oxidation to acetaldehyde. The most proba-ble pathway for acetaldehyde formation is the oxidationof methane to formaldehyde via generation of methanol,followed by its coupling with methane. The presence ofthe POMs in the hybrid catalysts is a key parameter forthe readily occurrence of methane oxidation under mildaerobic conditions. The hybrid catalyst possiblymakes both(a) oxidation of Pt (II) to Pt (IV) intermediates and (b) theaddition of methane (also methanol) to a Pt (II) centerfacile, by providing a conduit for improved oxidation ofintermediate, hydride species.

Metalloporphyrins, as a unique class of biomimetic cat-alyst, exhibit high catalytic activity for the oxidation ofsome inactive organic substrates under mild reaction con-ditions.[33] The combination of metalloporphyrins andPOMs in the same hybrid compounds potentially createshighly efficient heterogeneous catalysts by emerging theirunique individual functions. The major difficulty insynthesizing POMs porphyrin‐based hybrids is their poorsolubility in either hydrophilic (POMs) or hydrophobic(Metalloporphyrins) solvents.[34] To overcome this contest,Wu and co‐workers developed an effective synthetic meth-odology in which POMs are reacted withmetalloporphyrinto form zwitterionic complexes, which then reacted withcadmium nitrate to afford a metalloporphyrin−POM‐

based hybrid framework {[Cd (DMF)2MnIII(DMF)2TPyP](PW12O40)}• 2DMF•5H2O (DMF = N,N‐dimethylfor-mamide; TPyP = tetrapyridylporphyrin).[35] This hybridsolid exhibits remarkable capability for heterogeneousselective oxidation of alkylbenzenes to the correspondingketones in high yields and 100% selectivity by using TBHPin water as the oxidant. Compared with constituent moie-ties (73.6% yield for MnIIICl‐TPyP and inactive for[PW12O40]

3−), this hybrid catalyst showed superior cata-lytic activity (92.7% acetophenone yield) in the oxidationof ethylbenzene to methyl phenyl ketone.

2.2.2 | Oxidation of arenes

Phenol, is an important commercially available startingmaterial for the synthesis of several medicines. It is also

used in the production of co‐ polymers such phenol‐ form-aldehyde. For such purpose, it is currently produced by athree‐step process starting from cumene which is presentin some crude oil extracted in the USA. This process sufferseconomically from high energy consumption, low yieldingof phenol and pollutions imposed to environment.[36]

Therefore, many attempts have been made to prepare thiscommercially important compound via a cost‐effective pro-cedure. To the purpose, several selective and efficient cata-lysts for direct hydroxylation of benzene have beendeveloped and examined at pilot plant and for the massproduction of phenol. After several trials and errors, it hasbeen found that vanadium‐substituted Keggin POMs arethe most effective catalyst for direct conversion of benzeneto phenol. It is presumed that the aforementioned KegginPOMs activate aromatic compounds by extracting an elec-tron from the hydrocarbon through the formation of aradical‐cation, followed by oxygen transfer from the POManions to the substrates.[37] In order to improve the cata-lytic activity of vanadium‐substituted Keggin POMs, Longet al. synthesized a heterogeneous ionic inorganic‐organichybrid catalyst, [(C3CNpy)2Pd(OAc)2]2 [HPMo10V2O40]for direct aerobic oxidation of benzene to phenol.[38] Thecomparison of the catalytic behavior of aforementionedhybrid with immobilized [Pd (OAc)2] and [PMoV2O40]onto a support,[39] demonstrated that connecting Pd(OAc)2 and [PMoV2O40] onto the ionic structure of hybridis more active due to remarkable synergistic effect betweenPd (OAc)2 and [PMoV2O40]. Furthermore, the facilitationof Pd (IV)–Pd (II) valence change in the catalytic cycle bythe conjugated [PMoV2O40] POM might be the other rea-son for the high catalytic efficacy of above‐mentionedhybrid.

Transition metal Schiff base complexes are anotherimportant class of redox catalysts for direct hydroxylationof benzene.[40] Chen's research group have successfullyachieved and reported the preparation of Schiff basemetal complexes (Fe (II), Co (II), Cu (II)) with [PMo12‐nVnO40] POM and used them as efficient catalysts forfruitful direct conversion of benzene to phenol..[41] Thesehybrid catalysts in combination with triphenylphosphine(PPh3) as additive showed significantly higher conver-sions of benzene to phenol in comparison with thoseobtained from the corresponding Schiff base metal com-plexes and [PMo12‐nVnO40] POMs alone.

Leng et al. reported the preparation of a catalytically‐active organometallic‐POM hybrid with V Schiff basecomplex as well as V‐containing Keggin‐type POM.[42]

This hybrid solid with two types of catalytically activevanadium moieties shows remarkable capability forheterogeneous catalysis of hydroxylation of benzene togive excellent yield for phenol (19.6% and 100%selectivity).


2.2.3 | Oxidation of alcohols

In recent years, there has been an overgrowing demandfor the preparation of efficient catalysts for the selectiveoxidation of alcohols to the corresponding carbonyl com-pounds as appropriate precursors in multistep reactions,the production of new materials and energy sources.[43]

The Pd (II)‐catalyzed oxidation of alcohols in the pres-ence of a base is a well‐known transformation.[44] Onthe other hand, V‐substituted POMs have proved to bemore active species in the selective catalyzed‐oxidationof alcohols than other TM‐substituted POMs.44c, 45 Inthis regard, Hu and co‐workers achieved and reportedthe synthesis of three Pd‐POM hybrids [Pd(dpa)2]3[PW12O40]2•12DMSO, [Pd (dpa)2]3[PMo12O40]2•12DMSO•2H2O and [Pd (dpa)(DMSO)2]2[HPMo10V2O40]•4DMSO (dpa = 2,2′‐dipyridylamine) with differentKeggin‐type POMs.[46] It is believed that, Pd complexesand POM anions are coupled through electrostatic inter-action. Comparison of the catalytic activity of thesehybrids indicates that the PMo10V2O40 hybrid is moreactive than the other hybrids in the selective aerobic oxi-dation of benzylic alcohols. Interestingly, during the cata-lytic reaction of these compounds, the unprecedentedintermediate, [Pd (dpa)2{VO (DMSO)5}2][PMo12O40]•4DMSO was isolated and characterized by single crystalX‐ray diffraction and shown to be the active species. It isworthwhile to mention that the anion of the active spe-cies differs from that of the catalyst precursor, revealingthat, during the oxidation of alcohols, VO2+ was removedfrom the parent HV2PMo10 anion and the cationic species[VO (DMSO)5]

2+ was formed, while at the same time theremaining fragment reassembles into PMo12O40. Similardetachments of mononuclear vanadium species from V‐containing POMs under acidic conditions have successfullybeen achieve and reported by Streb et.al[47] and Neumannresearch group,[48] independently. Thus, this intermediatecatalyst has three active centers, the Pd complex, themono-nuclear vanadium species, and the Keggin‐type POM,which all showed potential catalytic activity as well asbeing able to promote the catalysis, synergically. Havingmore active sites resulted in high catalytic activity for theselective aerobic oxidation of benzylic alcohols into the cor-responding aldehydes/ketones (98.1–99.8 % conversion,91.5–99.1 % selectivity). Encouraged, by the results show-ing the importance of V‐substituted POMs, Lin et al. com-bined the bicapped Keggin cluster [PMo8V6O46]

9‐ anionswith a nickel complex, to obtain the catalyst{[H3PMo8V6O46][Ni (en)2]}•2[Ni (en)2]•5H2O, whichshowed high catalytic activity for the selective oxidationof benzyl alcohol to benzaldehyde (56.5% conversion,70.1% selectivity).[49] However, in comparison with Pd‐POM hybrids, this new Ni‐POM hybrid showed less

catalytic activity in the selective oxidation of benzyl alcoholto benzaldehyde.

Molybdenum and copper have been previously used asefficient catalysts for the oxidation of alcohols to the corre-sponding carbonyl compounds.[50] Lysenko and co‐workersdesigned and synthesized two mixed molybdenum‐copperheterogeneous catalysts, namely [CuII2(tr2ad)4](Mo8O26)and [CuII4(μ4‐O)(tr2ad)2(MoO4)3]•7.5H2O(tr2ad = 1,3‐bis(1,2,4‐triazol‐4‐yl)adamantane).[51] These compoundsshowed high thermal and chemical stability and were usedas catalysts in the efficient oxidation of benzyl alcohol inthe presence of different types of oxidants. By the analysisof obtained results, these hybrids were proven to be supe-rior heterogeneous catalysts for the direct synthesis ofbenzoic acid from benzyl alcohol in the presence of TBHPas an effective oxidant.

2.2.4 | Oxidation of carbonyl compounds

Aldehydes, bearing carbonyl moiety can be oxidized tothe corresponding carboxylic acids in the presence of var-ious oxidizing agents. The catalytic properties of[Mo8O26]

4– as a POM catalyst for the oxidation of acetal-dehyde has extensively been studied.[52] Advantages ofoxidation of carbonyl compounds and pollutant removalof this process are eminently clear. Niu and co‐workerssuccessfully performed a probe oxidation of acetaldehydeusing H2O2 as common oxidant in the presence of thenovel hybrid compound {[Cu (DIE)2] [Mo8O26]0.5}∞([DIB = 1,2‐diimidazoloethane) as a catalyst.[53] Thishybrid has a rare supramolecular structure that containsoctamolybdate polymeric chains interconnected viaorganic ligands to form a 3D network (Figure 2). Theresults for this catalyzed‐oxidation demonstrated thatthe aforementioned hybrid has high catalytic activity aswell as high selectivity (nearly 100%) for the conversionof acetaldehyde to acetic acid.

2.2.5 | Oxidation of sulfur‐containingcompounds

Selective oxidation of organosulfur compounds is animportant organic transformation since the products ofsuch transformation are actually organic sulfur oxides,which are recognized as useful intermediates in the artof organic synthesis. The organic sulfur oxides exhibitdiverse biological activity and also have been used asintermediate in the production of several useful com-pounds at high scale industrial levels.[54] This transforma-tion also presents a promising strategy for desulfurizationof oil and aerobic decontamination of the toxic or pun-gent chemicals in agreement with green chemistry

FIGURE 2 (a) Packing structure of {[Cu (DIE)2] [Mo8O26]0.5}∞ hybrid generate a ladder‐like poly threaded structure; (b) 3D inorganic‐

organic hybrid framework of {[Cu (DIE)2] [Mo8O26]0.5}∞ hybrid. Figure reproduced from Ref.,[53] with permission of the copyright holders


principles. In this process, after oxidative desulfurization,sulfone was easily removed from fuel by simple extrac-tion. In recent years, the design and preparation of theefficient POMs‐based hybrid catalysts for oxidative desul-furization has stirred up the interest of chemical commu-nity. Various designed and synthesized POMs have beenutilized as hybrid forms, as efficient catalysts for the oxi-dation of sulfides.

Li's research group also explored and presented threenon‐porous Keggin‐type POM‐based metal‐organiccoordination networks (MOCNs) with chemical formulaeof [Co (BBTZ)1.5(HBBTZ)(H2O)2]{PW12O40]•H2O, [Co2.5(BBTZ)4(H2O)2][BW12O40]•4H2O and [Cu (BBTZ)2]5[BW12O40]2•4H2O(1).

[26] The first compound exhibits aPOM‐encapsulated 3‐D supramolecular network, whilethe second and third compounds display the POM‐

supported 3‐D coordination networks. The same researchgroup proved that all three hybrids exhibit effective cata-lytic activity for the oxidation of dibenzothiophene (DBT)to the corresponding sulfoxides and sulfones. Catalyticactivities of these novel compounds are much higher thanthat of parent POMs, indicating that the uniform disper-sion of POM units into the MOCNs may expose morePOMs active centers at the molecular level, thus remark-ably improve the catalytic activities. Furthermore, in thispaper, the surfactant‐assisted hydrothermal method wasestablished to prepare nanoscale MOF materials. Theresults clearly showed that by reducing the size of crystal-line catalysts, the specific surface area increased thus, thecatalytic activity improved, drastically.

As the incorporation of lanthanoid (Ln) ions into aPOM framework enhances their catalytic properties, a Ln

hybrid supramolecular 3‐D framework, {(Yb (PDCH2)2(PDCH))•Na(H2O)2 (Na (PDCH)(H2O)2)}2[P2W18O62]•14H2O, based on Wells–Dawson cluster anions and cat-ionic Yb & Na complex units of pyridine‐2,6‐dicarboxylicacid (PDCH) was design and synthesized by Pradeep'sresearch group.[55] This hybrid is soluble in water, thus, itshomogeneous catalytic activity towards the oxidation ofvarious sulfides in presence of H2O2 could be studied. Thishybrid successfully catalyzed the sulfoxidation of sulfidesbearing various functional groups, involving bothelectron‐withdrawing ester group and the electron‐donating hydroxyl group to afford the desired sulfoxidesin moderate to excellent yields with good selectivity. Thesubstrates bearing electron‐donating functional groupswere found to give better yields in comparison with thoseof bearing electron‐withdrawing groups. It was also notedthat this Ln‐coordination polymer hybrid acts as an efficientand selective catalyst for the oxidation of sulfide functionalgroup in substrates bearing other functional groups such asthiophene, –CH2OH, –NH2 etc. A proposed mechanism forthe oxidation of sulfides, involves the possible interaction ofH2O2 with the POM cluster generating an electrophilicintermediate which leads to the electrophilic attack on theS atom of the sulfide resulting in the creation of the corre-sponding sulfoxide. The mechanism for the oxidation ofsulfoxide to sulfone comprises the formation of a POM‐

sulfoxide intermediate through the nucleophilic attack bythe oxygen of the sulfoxide on theW atom of the POM unit,which followed by the nucleophilic attack by H2O2 on thesulfur atom of the POM–sulfoxide intermediate.

Inspired by the significant catalytic activities of theEvans−Showell‐type POM [Co2Mo10H4O38]

6– and


copper‐based compounds in catalyzing the oxidation, Anand co‐workers combined copper−organic subunits withEvans−Showell‐type POMs to make the new high‐dimensional hybrid materials, featuring the high catalyticactivity of both aforementioned hybrids for the oxidationof sulfides and alcohols.[56]

Wang et al. reported the successful synthesis of alargely overlooked example of heteropolyvanadates‐basedinorganic‐organic hybrid, H6[(Cu(C5H4NCO2)(H2O)4]2[Mn2V22O64] •28H2O (C5H4NCO2 = isonicotinic acid),which shows high activity for the catalyzed‐ oxidation ofsulfur compounds such as DBT, 4,6‐dimethyldiben-zothiophene (4,6‐DMDBT) and benzothiophene (BT) inthe presence of TBHP as oxidant under mild reactionconditions.[57]

A polyrotaxane structure, {Ag8O(Htrz)4(4,4′‐bpy)2}{AgPMo12O40} (Htrz = 1,2,4‐triazole, 4,4′‐bpy = 4,4′‐bipyridine) (1), constructed from a purely inorganic 1Dchain and the 2D metal‐organic layer exhibited efficientcatalytic activities for oxidative‐desulfurization.[58]

Encapsulation of POM in chromium terephthalate(MIL‐101), copper trimesate (e.g., HKUST‐1), and cobalttriazole MOFs gives active catalysts for oxidative desulfur-ization. These catalysts have been obtained through theencapsulation of [PW12O40]

3– (PW12),[59] [A‐PW9O34]

9–,[60]

[PW11Zn(H2O)O39]5–,[61] [Ln (PW11O39)2]

11– (Ln =Tb andEu),[62] [HnPMo12O40]

(n–3)–63, 59d and [H3PV2Mo10O40]2–.[64]

In 2015, Li and co‐workers combined Keggin‐typePOMs with cationic cobalt triazole‐based MOF to pro-duce POM‐MOF hybrid, [Co (BBPTZ)3][HPMo12O40]•24H2O, that can act as effective and size‐selective het-erogeneous catalyst for the oxidative desulfurizationreaction.[63]

Hill and co‐workers used a combination of Keggin‐type [CuPW11O39]

5– and MOF‐199 (HKUST‐1) in air‐based oxidations.[65] The obtained catalyst effectively,chemo‐ and shape‐selectively and rapidly catalyzed oxida-tion of thiols to disulfides and, more significantly, therapid and sustained removal of toxic H2S via the reactiontook place as follows,

H2Sþ 1=2O2→1=8S8 þH2O

The immobilization of PW12 anions in the mesocages ofamine‐functionalized MIL‐101(Cr) was successfully

FIGURE 3 The structural

representation of PMoV ⊂ rho‐ZIF. Figure



accomplished and described by Cao and co‐workers.59c

This heterogeneous catalyst showed excellent activity foroxidative desulfurization under mild reaction conditions.As a consequence of the strong electrostatic interactionsof POM anions with the amine groups of MIL‐101(Cr)‐NH2, the above‐mentioned catalyst exhibited high stabilityand could be easily separated and recycled several timeswithout leaching or significant loss of activity. Encouragedby this work, Julião's research group prepared and used[PW11Zn(H2O)O39]

5– POM anion impregnated withinNH2‐MIL‐101(Al) for the removal of sulfur compoundsfrom diesel fuels.[66]

In a novel strategy, via a one‐potmechanochemical syn-thesis, zeolitic imidazolate frameworks with rho topology(rho‐ZIF) and large interior cavities and windows was usedas host matrix for encapsulating and immobilization ofvanadium‐substituted Keggin‐type POMs with high load-ing efficiency and chemical stability (Figure 3).[67] As thenovel catalysts, PMoV⊂rho‐ZIFs were found being effec-tive catalyst for selective oxidation of a series of sulfides tosulfoxides or sulfones, including bulky diphenyl sulfides.

2.3 | Acid catalysis

In POMs catalysts protons can act as Brønsted acids, whilethe metal ions on the skeleton of POMs possess unoccu-pied orbitals that can accept electrons thus, acting as Lewisacids. Therefore, the POMs are known as an unrivaled acidcatalyst in scientific communities. In inorganic‐organichybrid structures, the Lewis acidity of metal ions (espe-cially metal ions with high oxidation states, high coordina-tion numbers, and unsaturated coordination sites) in themetal‐organic complex could exacerbate the catalytic acid-ity of POMs. Furthermore, delocalization of negativecharge of POM anions in huge supramolecular hybridstructure could enhance the Lewis acidity of hybrid mate-rial. Thus, the nature of cations and structure of POM‐

based inorganic‐organic hybrids show great impact andhaving high effects in acid catalytic properties.

2.3.1 | Reactions involving C−C bondformation

The C‐C bond formation is the most important organicreaction and it is the nearest reaction to the heart of

TABLE 3 Results for the catalytic cyanosilyation of aldehydes in

the presence of [Cu (bpy)(H2O)5.5][H2W11O38]•3H2O•0.5CH3CN

entry Ar‐ Yield (%)

1 Phenyl 98.1

2 4‐methoxyphenyl 89.3

3 1‐naphthyl 87.8

4 2‐naphtyl 88.0

5 3‐formyl‐1‐phenylene‐(3,5‐di‐tert‐butylbenoate)

52.4


synthetic organic chemists. In multi‐step organic synthe-sis, often the protection/deprotection of certain functionalgroups is necessary. One of functional group which mustbe frequently protected during a multi‐step synthesis iscarbonyl group. In such protecton/deporotection the acidcatalysts play a vital role. Synthesis of inorganic‐organichybrids based on Strandberg‐type organophosphomoly-bdates and various transition metal‐complexes have beenwidely investigated by the Zhou and Zhang researchgroup. As part of this study, their acid‐catalytic perfor-mances for the synthesis of cyclohexanone ethylene ketalwere examined and disclosed.[68] The obtained resultsshowed that the hybrid based on Strandberg‐type POMsas the efficient heterogeneous acid catalysts can catalyzethe synthesis of cyclohexanone ethylene ketal. Later on,the same authors compared acid‐catalytic activity of aninorganic‐organic hybrid based on Strandberg‐type POMs,{[(Cu(H2O)(μ‐bipy))2(C6H5PO3)2W5O15]}n (bipy = 4,4′‐bipyridyl), with the inorganic‐organic hybrid based onthe hollow Keggin isopolytungstate, {[Cu (bipy)2((μ‐bipy)Cu (bipy))2(H2W12O40)]•12H2O}n, in the synthesis ofcyclohexanone ethylene ketal.[69] The comparative resultsshowed that the acid catalytic activity of the Strandberghybrid is slightly higher than that of the Kegginisopolytungstate hybrid. The differing catalyzed activitiescan be attributed to the differences in their structures.The former hybrid has an organophosphorus center in itsstructure, which may have a positive effect on the higheracid catalytic activity.

Cyanosilylation of carbonyl compounds is the otherfundamental carbon–carbon bond forming reaction inorganic chemistry, which is frequently used to synthesize,inter alia, cyanohydrins. The latter are important inter-mediates for the preparation of several fine chemicalsand synthesis of pharmaceuticals and some prescribeddrugs.[70] To obtain cyanohydrin molecules, the use of acatalyst with a Lewis acid or base character, capable ofactivating both substrates as well as the cyanide precur-sor, is necessary. The products of such reaction are cyano-hydrin trimethylsilyl ethers which can be used as highlyversatile intermediates in organic synthesis

A POM‐MOF constructed from an isolatedisopolyoxotungstate [H2W11O38]

8– cluster, [Cu(4,4′‐bpy)(H2O)5.5][H2W11O38]•3H2O•0.5CH3CN, was synthesizedunder solvothermal conditions by Niu's researchgroup.[71] In this hybrid structure, the [H2W11O38]

8–

anion acts as a hexadentate ligand is coordination withsix copper complexes to construct a 2D framework, and2D sheets further stacked in a parallel fashion resultingin the formation of 1D channels. The resulting POM‐

MOF was used as a heterogeneous Lewis acid catalystfor cyanosilylation of differently substituted aromaticaldehydes using cyanotrimethylsilane in CH3CN as

solvent at ambient temperature. As shown in Table 3,the yield of the products of this catalytic cyanosilylationdecreased when the molecular size of the substituentson aromatic aldehydes increased. This catalyzed reactionin the presence of bulky aldehyde 3‐formyl‐1‐phenylene‐(3,5‐di‐tert‐butylbenzoate) gave less than 52%conversion under the identical reaction conditions. Theadsorption experiments confirmed that bulky substrateswere too large to be adsorbed in the channels of thePOM‐MOF. It is well known that the cyanosilylationindeed occurs in the channels of the POM‐MOF.

In recent years, the high catalytic activities of Lewisacid–base catalysts of the Evans–Showell‐type POM[Co2Mo10H4O38]

6– attracted much attention of An'sresearch group. In 2016, they reported the synthesis offour inorganic‐organic hybrids based on the[Co2Mo10H4O38]

6– POM, namely [Zn2(H2O)5(4,4′‐bipy)3]H2[Co2Mo10H4O38]•5H2O, [Zn2(H2O)2(bpe)3]H2[Co2Mo10H4O38], [Zn(H2O)4(H2bpp)2][Co2Mo10H4O38]•2H2O, and[Zn3(Htrz)6(H2O)6][Co2Mo10O38H6]•12H2O (4bpe = 1,2‐bis(4‐pyridyl) ethane, bpp = 1,3‐di(4‐pyridyl)propane).[72]

These hybrid compounds acted effectively as heteroge-neous Lewis acid catalysts for the cyanosilylation of alde-hydes under solvent‐free conditions at ambienttemperature. The cyanosilylation reaction of differentlysubstituted benzaldehdes such as 2‐hydroxybenzal-dehyde, 4‐methylbenzaldehyde, or 4‐nitrobenzaldehydein the presence of this hybrid catalysts were successfullyachieved to afford the corresponding cyanohydrintrimethylsilyl ethers in high yields (up to about 94%).Using the large molecule such as 1‐napthaldehyde asthe substrate under identical reaction conditions gavethe lower yield of the corresponding product (75%) maybe due to the steric hindrance and electronic effects. Itis proposed that in the transition‐state, geometry of thealdehyde is not suitable for the reaction to proceedsmoothly to completion due to the Lewis acid effect ontransition metals. Later, in 2017, the same group in theircontinuous study on [Co2Mo10H4O38]

6– POM, successfully


obtained four novel compounds, based on Evans–Showell‐type POMs and alkaline earth metal cations.[73]

These hybrids also fruitfully catalyzed the cyanosilylationof carbonyl moieties obtaining the expected correspond-ing products in excellent yields.

Sun and co‐workers successfully achieved and reportedthe use of a 3D network based on a Preyssler‐type[P5W30O110] POM, {[Cu12(pbtz)2(Hpbtz)2(OH)4(H2O)16][Na(H2O)P5W30O110]}•16H2O (H2pbtz = 5′‐(pyridin‐2‐yl)‐1H,2′H‐3,3′‐bi(1,2,4‐triazole)), as an acid catalyst topromote the cyanosilylation of compounds containingcarbonyl group.[74] In comparison, the ketones undergocyanosilylation slower than aldehydes. This observationcan be attributed to general lower reactivity of ketonesthan aldehydes. Another work by this group includes thesynthesis of a hybrid material based on a Keggin POMand a silver(I)–organic sheet, [Ag4(apym)4(SiW12O40)]n(apym = 2‐aminopyrimidine).[75] In fact when relevantAg ion is used, its low coordination number, resulting inthe open coordination sites. However, it acted as Lewisacid sites to catalyze the cyanosilylation of carbonyl group,fruitfully under mild reaction conditions, resulting in ahigh turnover number and turnover frequency.

2.3.2 | Reactions involving C−X bondformation

Transformation of carbonyl group as acetal or ketal is oneof the most important method for the protection of car-bonyl group present in aldehydes and ketones. A plethoraof catalyst reported in the literature for such conversion.[76]

Until 2014, there is no report in chemical literatureconcerning the utilization of Strandberg POM as catalystfor this protection. In 2014, Li et al. successfully accom-plished the preparation of inorganic‐organic hybrids basedon Strandberg‐type anions and Zn (II)‐H2biim/H2O sub-units, namely {H4(H2biim)3}[Zn(H2biim)(H3biim)(H2O)(HP2Mo5O23)]2•3H2O, {H9(H2biim)7}[(μ‐biim){(Zn(H2O)2)0.5(HP2Mo5O23)}2]•7H2O and {H7(H2biim)7}[Zn(H2biim)(H2O)2(HP2Mo5O23)][H2P2Mo5O23]•8H2O (H2biim =2,2′‐biimidazole) and used them as efficient heteroge-neous and reusable catalysts for the effective protectionof carbonyl groups.[77] Acid‐catalyzed synthesis of cyclo-hexanone ethylene ketal was used as a model reaction toevaluate the catalytic performances of these hybrids. Theaforementioned hybrid was also successfully used in theoxidation reaction of cyclohexanol to cyclohexanone withhigh conversion. The results indicated that theseStrandberg‐type POMs, showed better catalytic perfor-mances in an acid‐catalyzed reaction compared with theoxidation reaction of cyclohexanol to cyclohexanone.

2.3.3 | Esterification

Esterification is one of the most common acid‐catalyzedreactions. Jiang and co‐workers isolated a POM‐basedcoordination polymer, namely Na [Ag4(pyttz‐I)2][H2PMo12O40] (H2‐pyttz‐I = 3‐(pyrid‐2/3/4‐yl)‐5‐(1H‐

1,2,4‐triazol‐3‐yl)‐1,2,4‐triazolyl), which exhibits anunprecedented two‐fold interpenetrating structure witha helical feature.[78] This hybrid catalyst exhibited an effi-cient catalytic activity for the esterification reaction (as anexample synthesis of aspirin from salicylic acid). Thereactants can fully enter the interior hybrid crystals beingentirely in contact with the active sites during the initialphase of reaction which favors the catalytic activity ofthe catalyst. According to reaction kinetics analyses, thereaction between the carbonyl cation and salicylic acidis the key step in an acid‐catalyzed synthesis of aspirin.The large electronegativity on the POM surface can stabi-lize the carbocation intermediate, promoting the forma-tion of the aspirin. Furthermore, the charges of thePOMs in the hybrid are non‐localized, and the hydroxylproton exhibits strong mobility on the POM surface andhigh acidity, which plays an important role in thepseudo‐liquid phase catalytic behavior of the POMs.

Two new Keggin POM‐based hybrid compounds,[Ag6(btp)(pyttz)6(HSiMo12O40)2]•4H2O (btp = 5,5′‐di(pyridin‐4‐yl)‐1H,1′H‐3,3′‐bi(1,2,4‐triazole)) with the inorganicmeso‐helical channel and [Ag2(pyttz)4(H2SiMo12O40)2]•(TMA)2•4H2O TMA = tetramethylammonium) with 2Dhomological helical layer, have been explored as solid‐acid catalyst in esterification reaction.[79] The resultsdemonstrated that these hybrids possess high activityand stability as heterogeneous and environmentallybenign catalysts.

In the MIL‐100 MOF structure, the strong interactionbetween carboxylate groups of the small ligands andhigh‐valent metal ions (M3+) preserve their frameworkswith a superior physical and chemical stability, makingthem the ideal candidates being selected as an efficientcatalyst.[80] In this regard, [H3PW12O40] (HPW12) wasencapsulated into a mesoporous MIL‐100(Fe) by Zhang'sgroup.[81] The prepared HPW12@MIL‐100(Fe) demon-strated a significant prospect for being employed ashighly efficient in the liquid phase esterification of aceticacid with monohydric alcohols and acetalization reac-tions as solid acid catalysts. It is believed that the syner-gistic effect of POM and the mesoporous MIL‐100(Fe) isresponsible for high activity of this hybrid (Figure 4).The results clearly indicate that the POM molecules canbe incorporated into the mesoporous cages of the MIL‐100(Fe) framework while maintaining the integrity oftheir protonic acidity leading to imparting high catalyticactivity and excellent reusability to the supported catalyst.

FIGURE 4 The preparation and structural representation of HPW@MIL–100(Fe) hybrid. Figure reproduced from Ref.,[81] with permission



2.3.4 | Hydration

Hydrolysis of estersA series of remarkable crystalline POM‐MOF catalysts[Cu2(BTC)4/3(H2O)2]6[HnXM12O40]•(C4H12N)2 (X=Si,Ge, P, As; M = W, Mo; BTC = 1,3,5‐benzentricaboxylate)(NENU‐n) was synthesized by Su research group.[82]

Structural analysis revealed that catalytically‐activeKeggin POM units alternately arrayed as guests in thecuboctahedral cages of the Cu−BTC‐based MOF hostmatrix, while water and (CH3)4N

+ occupy the smallercavities of the MOF (Figure 5). The host‐guest materialcontaining HPW species was used as a representative acidcatalyst for hydrolysis of different carboxylic esters in

excess amount of water. This hybrid catalyst indicatedhigh catalytic activity and good reusability. Furthermore,these catalysts presented high selectivity depending onthe molecular size of the substrates and substrate accessi-bility to the pore surface.

Hydrolysis of phosphoestersThe rational design for the construction of catalysts forthe hydrolysis of phosphate diesters are still a challengein bioorganic chemistry.[83] One of the most interestingsubstrates for being hydrolyzed is famous biologicallyimportant molecule, DNA. However, due to itspolyanionic nature, DNA is resistant to hydrolysis. Thefirst report concerning the hydrolysis of DNA using

FIGURE 5 View of a sheet with two

kinds of pores in NENU‐n. The Cu‐BTC

framework and Keggin POMs are

represented by wireframe and polyhedral

models, respectively. Figure reproduced

from Ref.,[82] with permission of the

copyright holders


synthetic agents initially, appeared in literature in2006.[84] Since then, considerable attention has been paidto design the active homogeneous catalysts for the hydro-lysis of phosphate diesters in aqueous solution. Despitethe significant potential advantages of heterogeneouscatalysis, such as ease of separation, efficient recycling,and minimization of metal trace in the product in fact,heterogeneous catalysts for the hydrolysis of phosphatediesters were found being more favorable despite of theirsynthesis has been largely overlooked. In an attempt toobtain efficient catalysts for hydrolysis of phosphate dies-ters, Duan's research group prepared a POM‐based MOFhaving a lanthanoid‐based nanocage {[Ho4(dpdo)8(H2O)16BW12O40] (H2O)2}

7+ (dpdo = 4,4′‐bipyridine‐N,N′‐dioxide) as a secondary building block.[85] The Lewisacidity, high coordination numbers, fast ligand‐exchangerates, and absence of accessible redox chemistry of thelanthanide ions, provide an opportunity to mimic theactivity of hydrolases to bind and activate phosphateesters for being easily cleaved. The selection of 4,4′‐bipyridine‐N,N′‐dioxide as the longer spacer ligand isdue to the excellent hard acid/hard base complementarityof the lanthanoid cations/N‐oxide donor and also smallsteric size of this ligand to construct robust MOFs. It isexpected that the weak conjugate bases BW12 ascounteranions increase the Lewis acidity of the lantha-noid complex. The heterogeneous catalysis of resultedhybrid for phosphodiester bond cleavage of bis(4‐nitro-phenyl) phosphate (BNPP) was followed by monitoringof the increase in absorbance at 400 nm due to formationof the 4‐nitrophenoxide anion. Phosphodiester bondcleavage promoted by the heterogeneous inorganic‐organic hybrid suggests a pseudo‐first‐order hydrolyticcleavage reaction. In a comparison under different pHconditions, kobs is only slightly decreased with change ofthe pH of the solution, and the fastest cleavage is seenat pH 4.0. From the structural point of view, the presenceof water molecules as the labile ligands allows for sub-strate binding and the simultaneous provision of ametal‐bond nucleophile. The coordination of the BW12

to the holmium ions results in the nanosized hybrid withthe activity of the acid surface, which facilitate thetransesterification step by virtue of its additional labileligand sites, conceivable through the provision of ahydroxide ligand.[86]

Besides having the aforementioned characteristics of Lnions, Zn2+ also has some remarkable features such as hav-ing a ligand field stabilization energy and being nontoxic,which provide an opportunity to mimic the activity ofhydrolases to bind and activate the phosphate esters foreasy cleavage. Zn2+ is the only metal ion which frequentlycome across in both natural and artificial agents. Thus,development of Zn2+ based catalyst has currently been

highly valued. To seek efficient catalysts based on Zn‐cluster, Han et al. synthesized an inorganic‐organic hybridcomprising a Zn‐cluster and Keggin‐type PW12,{[Zn3Na2(μ‐OH)2(dpdo)6(H2O)16][PW12O40]2}•(dpdo)3•C2H5

OH•2H2O. This hybrid was employed for the fruitful hydro-lytic cleavage of BNPP.[87] The structural analysis revealsthat this hybrid composed of an S‐like complex[Zn3Na2(μ‐OH)2(bpdo)6(H2O)16]

6– with two PW12 andwater occupying several coordination sites to act as labileligands, allowing for easy binding of substrate and nucleo-phile. The results of this catalytic behavior demonstratethat cleavage of the phosphodiester bond proceeds viapseudo‐first‐order rate constant 6.7(±0.2) × 10‐7 s–1, givingan inorganic phosphate and p‐nitrophenol as the finalproducts of hydrolysis.

In continuation of the above described work, Duan'sresearch group prepared a series of POM‐MOFs hybridsbased on Ln and Zn cluster, namely, {[Eu4(dpdo)9(H2O)16PW12O40]}(PW12O40)2•(dpdo)3•Cl3, {ZnNa2(μ‐OH)(dpdo)4(H2O)4[PW12O40]}•3H2O, {Zn3(dpdo)7}[PW12O40]2•3H2O, and [Ln2H(μ‐O)2(dpdo)4(H2O)2][PW12O40]•3H2O(Ln = Ho and Yb), and compared the catalytic activitiesof these novel hybrids with those of bearing differentmetal ions.[88] These compounds were also applied as cat-alysts to BNPP hydrolysis which being found acting, excel-lently. As shown in Table 4, slight changes in the BNPPaffects the cleavage rates. It was found that the Eu3+ andYb3+ hybrids catalyzed the hydrolysis of phosphate dies-ters in rate constants kobs with the same order of magni-tude and was found being higher than that of Zn2+

hybrids. The results show that metal ion radius (r (Ln3+)> r (Zn2+)) and coordination model or ligand field stabili-zation energy (LFSE) are determining factors for the BNPPcatalytic cleavage rate. Furthermore, for lanthanoidhybrids, the kobs increases as the hydrolysis tendency ofthe metal ion (as reflected in the pKa of the coordinatedwater) increases, which is determined by the ionic poten-tial (¢) of metal ions.

2.4 | Base catalysis

The oxygen atoms on the surface of POM anions have ahigh negative charge, which makes them basic enough toreact to abstract active protons from organic substrates.Thus, these surface oxygen atoms of POM catalysts canbe considered as the active sites in base‐catalyzed reac-tions. Notably, in the inorganic‐organic hybrid structures,nucleophilic oxygen atoms of POMs interact with metal‐organic complexes via coordination bonds and/or variousweak intermolecular interactions. The interactions of sur-face oxygen atoms with metal‐organic compounds anddelocalization of negative charge of POM in the whole

TABLE 4 Rate constants and half‐life time of hydrolysis with a series of POM‐MOFs hybrids based Ln and Zn cluster

(Eu) (Zn) (Zn) (Ho) (Yb)

kobs 1.00(±0.04)*10‐6 6.74(±0.22)*10‐7 8.80(±0.30)*10‐7 7.57(±0.36)*10‐7 1.19(±0.07)*10‐6

t1/2 6.93*105 1.03*106 7.88*105 9.15*105 5.82*105


hybrid structure are effective factors which decrease thebasicity of POMs in the hybrid materials. Therefore, thebasicity of inorganic‐organic hybrids has been lessexplored for potential catalytic activity. However, depend-ing on the nature of the metal‐organic complex, in partic-ular organic ligands, such hybrid compounds can act asbasic catalysts. For example, some of the Schiff baseligands in hybrid structures can act as active basic catalyst.Jun and co‐workers designed three inorganic‐organichybrid catalysts by anchoring of the Schiff base complexesof manganese (L‐Mn, L: N,N′‐disalicylidene‐1,6‐hexane-diamine (L1), N,N′‐disalicylidene‐ethanediamine (L2), orN,N′‐disalicylidene‐1,2‐phenylenediamine (L3)) onto the[H4SiW12O40] Keggin anion.[89] The resulting hybrids wereused as solid catalysts in the esterification of phosphoricacid with equimolar amounts of lauryl alcohol to generatethe monoalkyl phosphate ester (MAP). As it is well‐recognized, esterification is an acid‐ or base‐catalyzed reac-tion. Because of the basicity of the L1 ligand, the situationfor the L1‐containing catalysts can be different. It has beenproposed that in this reaction, nucleophilic bases probablyreact with phosphoric acid to form active intermediatespecies, resulting in the a more efficient phosphatemonoesters formation.[90] In the case of catalyst, L1‐Mn‐SiW12O40, the oxygen bonded to the benzene ring in L1

may have its lone‐pair of electrons attached to the nitrogenatom through the resonance of the benzene ring to modu-late the basicity of the nucleophilic nitrogen atom. There-fore, the resulting electron pair on the nitrogen atom canattack the phosphorus atom to form the active intermedi-ate, leading to nucleophilic substitution by the alcohol toproduce the MAP product. The contribution of the POMcomponent is stabilizing the L1‐Mn‐SiW12O40 catalyst bycoordination linkages; which enable the hybrid to act asa base catalyst in a heterogeneous catalysis and preventsthe decomposition of the L1‐Mn complex which occurs inthe absence of the POM component. The L2‐Mn‐SiW12O40

system produced only a low yield, probably because of theweaker basicity of L2 compared to that of L1 which arose

due to its shorter carbon chain and weaker electron induc-tive effect. In the L3‐Mn‐SiW12O40 catalyst, the large sterichindrance of the vertical plane of the benzene ring also canreduce the interaction of the L3‐Mn complex with the POManion, making it being rather unstable in the reaction.

2.5 | Acid‐base bi‐functional catalysts

As mentioned in the previous section, depending on themetal‐organic complex, hybrid compounds can be usedas the Lewis base catalysts in organic reactions. On theother hand, POM anions in hybrid structural form, usu-ally behave as Lewis acid catalysts. Therefore, someinorganic‐organic hybrids in some organic reactions areable to act as the acid‐base bi‐functional catalysts. Twoisostructural POM hybrids containing Schiff base com-plex, [M(H2O)2(DAPSC)]3{[M(H2O)(DAPSC)]2BW12O40}BW12O40•nH2O (M = Co, Zn; DAPSC = Schiff base 2,6‐diacetylpyridine bis‐(semicarbazone)) have been utilizedin the direct esterification of phosphoric acid with equi-molar amounts of lauryl alcohol.[91] These two above‐mentioned catalysts were found being highly efficient cat-alysts for esterification giving high yield of MAP. In bothstructures, two metal‐DAPSC complexes are covalentlybonded to one BW12 POM resulting in improvement ofthe electron transfer between the two moieties as wellas modulating the basicity of nucleophilic group (‐NH‐,‐NH2) of DAPSC ligand. Thus, the cooperation of both,BW12 and the metal‐DAPSC complex in the catalyzedesterification is highly probable.

An inorganic‐organic hybrid material based on aPOM and a copper coordination polymer, [CuI(4,4′‐bpy)]3[PMo10

VIMo2VO40{Cu

II(2,2′‐bpy)}] (2,2′‐bipy = 2,2‐bipyridine)[92] has been used as an efficient heterogeneousbi‐functional acid‐base catalyst in the known Knoevenagelcondensation of aromatic aldehydes or cyclohexanonewith malononitrile or ethyl cyanoacetate.[93] Mechanisticstudies suggested that a double interaction through acid


and base sites of the bi‐functional hybrid catalyst with thealdehyde andmalononitrile occurs. The former interactionstrongly increases the electrophilicity of the aldehyde,whereas the second interaction would cause direct depro-tonation of the methylene group of malononitrile, produc-ing the corresponding carbanion which can then attackthe susceptible activated aldehyde.

2.6 | Alkene polymerization

The use of POMs in the catalysis of the oligomerizationreactions of alkynes is well ‐established.[94] On the otherhand, the ruthenium half‐sandwich compounds areknown to be attractive catalysts for oligomerization reac-tions including carbon–carbon bond formation.[95] Inorder to obtain a highly active and selective catalyst forthe oligomerization of alkynes, Dingwal and co‐workersprepared an effective catalyst system via combination ofthe PW12 Keggin anion with an organometallic com-pound involving, ruthenium [Ru(η5‐C5H5)(PPh3)2Cl] inthe presence of NEt3 which afforded a Ru–tetheredPOM Keggin anion of formula [HNEt3][(Ru(η

5‐C5H5)

(PPh3)2)2(PW12O40)].[96] Tethering of the POM anion to

a Ru‐complex, through a Ru–O=W linker, provided arobust solid hybrid with active Ru metal centers This sys-tem was utilized as catalyst in oligomerisation of terminalphenyl acetylene which well‐tuned the selectivity, whilemaintaining the competitive activity.

2.7 | Coupling reactions

The presence of metal ions with the d10 electron config-uration was found being important in coupling reac-tions. Accordingly, POMs‐based Cu(I) inorganic‐organichybrid compound {[Cu (py)2]4[SiW12O40]} (py = pyri-dine) was prepared and examined as a catalyst in thecatalyzed‐ Ullmann diaryl ether synthesis.[97] The 1‐Dchain complex efficiently catalyzed the O‐arylation ofphenols with aryl halides in the presence of air‐stableCu(I) center. Significantly, the catalytic activity of thishybrid is higher than that of CuBr under the identicalreaction conditions.

Berardi et al. designed a hybrid palladium catalystthrough decoration of a POM surface with imidazolium‐

based NHC palladium complexes for palladium‐mediatedSuzuki coupling.[98] In the presence of this catalytic sys-tem and depending on the reaction conditions, bothcross‐coupling and dechlorination of aromatic com-pounds proceeded smoothly to completion. The activePd center of the catalyst is in the organic moiety of thehybrid structure and the inorganic POM moiety stabilizes

the organic Pd−NHC complex due to its bulk/charge,resulting in improved TONs.

3 | INORGANIC ‐ORGANIC HYBRIDMATERIALS ‐BASED ON POMS FORPHOTOCATALYSIS

POM clusters have also been extensively utilized asphotocatalysts in various organic transformations.[3,5,99]

POM anions in their ground states can be excited by theabsorption of light to produce an excited‐state POM*.Light absorption causes the ligand‐to‐metal charge trans-fer (LMCT) from an oxygen atom (O2−) to the d0 transi-tion metal ion (M6+), leading to the generation of thespecies with a hole center (O−) and trapped electron cen-ter (M5+) pair.[3] The excited POM clusters usually pro-vide better efficiency than of their ground states andcan act as both electron donors and electron acceptors.Therefore, some catalytically‐inactive POMs in theabsence of light may be converted into effective reagents,being capable of oxidizing or reducing a wide variety ofsubstrate upon exposure to the irradiation of UV ornear‐visible light. On the other hand, the strong photoox-idative ability of the holes and photoreductive ability ofthe electrons allows initiation of chemical reactionsunder mild reaction conditions. An important point tomention is that the light absorption by POMs generallyoccurs in the region of 200−500 nm. Therefore, photosen-sitization might be used as an effective strategy whichallows the use of visible light. Under the secured optimalphotocatalytic conditions, metal‐organic complexes canact as the photosensor. In this regard, the hybridizationof POMs with metal‐organic complexes to forminorganic‐organic hybrids was found to be one of themost promising strategies to optimize the performanceof the POM clusters. In addition, this combination,improved photoactivities due to the formation of thelarger surface areas between the catalysts and substratesfor the surface‐mediated electron‐transfer reactions alongwith the synergistic effect between the two parts. In thissection, we try to underline the latest advances in devel-opment of POM‐based hybrids for being utilized inphotocatalysis. In addition, worthy approaches for theefficient light gathering and active site engineering inPOM hybrid‐based photocatalysts are discussed.

3.1 | Photocatalytic degradation of organicdyes

Inefficient waste disposal, water and soil pollution are fre-quently due to inefficiency in disposal or destruction ofwaste. Long term exposure to polluted air and water causes


chronic health problems, creating the matter of industrialpollution into a severe issue. With rapid industrialization,after the Industrial Revolution, a period from between1820 and 1840, new chemical manufacturing and iron pro-duction processes finding economical and effective tech-niques for mass production of useful compounds. In thisline, the removal of organic pollutants from wastewaterhas become a hot research topic due to its environmentalimportance. In recent years, the use of inorganic‐organichybrid‐based POMs as photocatalysts to decompose wasteorganic molecules such as Methylene Blue (MB),[100]

Methyl Orange (MO),[101] Rhodamine B (RhB),[102] thio-phene,[103] etc., have been widely reported. Several MOFsconstructed with POMs showed good photocatalytic activ-ities in the degradation of organic dyes in order to removedisposal in dye industry. In 2014, these photocatalyzeddegradations were reviewed by Wang et al.[104]

Li et al. synthesized and reported two POM‐based coor-dination polymers (POMCPs), [Ag4(H2pyttz‐I)(H2pyttz‐II)(Hpyttz‐II)][HSiW12O40]•4H2O and [Ag4(H2pyttz‐II)(Hpyttz‐II)2][H2SiW12O40]•3H2O (H2pyttz‐I = 3‐(pyrid‐2‐yl)‐5‐(1H‐1,2,4‐triazol‐3‐yl)‐1,2,4‐triazolyl; H2pyttz‐II =3‐(pyrid‐4‐yl)‐5‐(1H‐1,2,4‐triazol‐3‐yl)‐1,2,4‐triazolyl), withsimilar structures and different channels between compo-nents (Figure 6).[105] The photocatalytic degradation exper-iments of MB revealed that the photocatalytic propertiesare highly influenced by the structure of the fabricatedhybrids. The larger cavities in the second compoundincrease the contact area for catalysts and crude materialsand also promoting more active sites to participate in thereactions. Thus, the catalytic properties of the catalystsare improved. The proposed mechanism for increasingphotocatalytic activity in these hybrids is shown inFigure 7. In the first step, UV excitation of POMs inducesa ligand to metal charge transfer (LMCT) by elevating anelectron from the highest occupied molecular orbital(HOMO) to the lowest unoccupied molecular orbital

(LUMO). This can be regarded as a parallel process to theband‐gap excitation in the photocatalyst. In the secondstep, Ag–O clusters as bridging units facilitate the electrontransfer in POM anions due to the distortion of the MO6

units, namely, Ag‐pyttz which can act as photosensitizerunder UV light and promote the transition of electronsonto POMs. Therefore, the POM has higher charge densityand exerts considerable impact on the pseudo‐liquid phaseproperty of POMs.

Wang's group reported the successful preparation offour POM‐based on metal‐organic complexes [Cu2L2(PMoVI11MoVO40)(H2O)2]•H2O, [Cu2L2(PW

VI11W

VO40)(H2O)6]•H2O, [Cu2L2(SiW12O40)(H2O)6]•H2O and [Cu2L2(H2K2Mo8O28)•H2O)2] (L = N,N′‐bis(3‐pyridinecarbo-xamide)‐piperazine), which showed high photocatalyticactivivity in the decomposition of MB under UV/visiblelight.[106] The results obtained for the photocatalyticactivity of these hybrids suggest that the POM‐basedmetal‐organic complexes containing tungsten ions, whichshow higher photocatalytic activity than those of contain-ing molybdenum ions, which may be due to theirdifferent compositions and structures.[107]

[{Cl4Cu10(pz)11}{As2W18O62}]•1.5H2O (pz = pyrazine)was constructed from the Wells–Dawson POM[As2W18O62]

6‐ has two different channels in form a 3Dtopology framework. This is another inorganic‐organichybrid, showing high photocatalytic activity in the degra-dation of RhB dye, as reported by Su and co‐workers.[108]

Upon UV irradiation of this compound for 80 min,the photocatalytic decomposition rate was found ashigh as 96.8%. Compared with the [Cu7(3‐btz)16(OH)2(H2O)4(P2W18O62)2]•16H2O (btz = 1‐benzyl‐1H‐(1,3,4)tri-azole)[109] and [{CuII6Cu

I10(H2O)5(pzc)10(pz)6}{P2W18O62}2]

•4H2O[110] hybrids, the degradation rate was found to be

about 58% for RhB dye after 105 min and 92.18% forRhB dye after 120 min. The higher photocatalytic activityof this hybrid may be ascribed to the synergistic effect of

FIGURE 6 Representation of the

[Ag4(H2pyttz‐I)(H2pyttz‐II)(Hpyttz‐II)]

[HSiW12O40] and [Ag4(H2pyttzII)

(Hpyttz‐II)2][H2SiW12O40] compounds

with similar underlying frameworks but

different tunnels. Figure reproduced from

Ref.,[105] with permission of the copyright

holders

FIGURE 7 Representation of the

photocatalytic mechanisms for

[Ag4(H2pyttz‐I)(H2pyttz‐II)(HpyttzII)]

[HSiW12O40] and [Ag4(H2pyttz‐II)

(Hpyttz‐II)2][H2SiW12O40]. Figure




the [As2W18O62]6‐ POM and {Cl–Cu–pz} MOFs, the {Cl–

Cu–pz}. MOFs act as a photo‐sensitizer under UV irradi-ation to promote transition of electrons onto POMs. To bemore specific, the larger channels in this hybrid increasethe contact area between the catalyst and substrate,which is vital for efficient catalysis.

3.2 | Photocatalytic H2 evolution

Photo‐driven hydrogen evolution to facilitate the dis-placement of fossil fuel is a hot topic in the chemistryresearch community. Various POMs and hybrid mate-rials have been developed for promoting the hydrogenevolution reactions.[111] The layered double hydroxide(LDH) structure was also used to design 3D frameworkswith large channels, hosting the plenary Kegginanion.[112] The hybrid material exhibited a 3D CeIII

hydrotalcite‐like structure constructed from 2D cationic[{Ce(H2O)5}2{Ce (pdc)2(H2O)4} {Ce (pdc)3]

2+ layers, inwhich pdc = pyridine‐2,6‐dicarboxylate, pillared by

FIGURE 8 Projection of the 3D framework down the a axis (a) and th

(PW12O40)]. Figure reproduced from Ref.,[112] with permission of the c

[PW12O40]3– anions (Figure 8). This hybrid was active

for photocatalytic H2 evolution in aqueous methanolsolution when exposed to UV irradiation.

In 2015, Wang and co‐workers designed three com-plexes based on Keggin‐type POMs (HTEA)2{[Na(TEA)2] H [SiW12O40]}•5H2O, (HTEA)2{[Na (TEA)2][PW12O40]}•5H2O and (HTEA)2{[Na (TEA)2] H[GeW12O40]}•4H2O (TEA = triethanolamine) with differ-ent central atoms in order to further investigate the influ-ence of POMs on photocatalytic hydrogen evolution.[113]

These compounds exhibit 1D chain structures consistingof Keggin building blocks and {Na (TEA)2} linkers. Theresults demonstrated that these compounds exhibit pho-tocatalytic activity under Xe lamp irradiation. The ratesof H2 evolution catalyzed by these hybrids showed no sig-nificant differences. Furthermore, their photocatalytichydrogen evolution activities were compared with thoseof its homologous Keggin‐type POM, for recognizing theinfluence of the sacrificial electron donors TEA. Theresults indicate that the effective existence of the TEA

e b axis (b) in polymer H[{Ce(H2O)5}2{Ce (pdc)2(H2O)4} {Ce (pdc)3}

opyright holders


counter cation improves the catalytic activity, drastically(Figure 9). TEA can also be used as the sacrificial electrondonor in photocatalytic hydrogen evolution, whichexhibits higher activity than methanol.

3.3 | Photocatalytic water oxidation

The oxidation of water to oxygen is one of the most impor-tant chemical process in nature. This oxidation in fact is acritical step in converting sunlight to chemical energies.Thus, great efforts have been made for design and prepara-tion of the efficient water oxidation catalysts (WOCs)preferably, based on earth abundant metals in order toenable economical and scalable production of solarfuels.[114] Among the organic ligands, 2,2′‐bpy is the mostestablished ligand for synthesizing molecular WOCsowing to its oxidative stability.[115] On the other hand,POMs with oxygen‐enriched surfaces have also beenexamined as oxidative stable ligands for designing activeWOCs in the past few years.[116] A number of Co‐ andNi‐based POM WOCs have been used for visible‐light‐driven water oxidation reactions.[117] In an intelligentstrategy, Lin and co‐workers combined the well‐established bpy ligand and Co metal ion with stableKeggin‐type POMs to design robust organic‐inorganichybrid molecular WOCs.[118] The CoII‐capped ε‐Kegginanion {[CoII(2,2‐bpy)]2[PMoV4MoVI8O40]}

3– oxidizes H2Oto generate O2 under visible‐light irradiation using[Ru(2,2‐bpy)3]

2+ as the photosensitizer and S2O82– as the

sacrificial electron acceptor. The stability of WOC hybridunder photocatalytic conditions was demonstrated by

FIGURE 9 Comparison the photocatalytic hydrogen evolution activiti

hybrid structures (black) with its homologous Keggin‐type POM (red). F

holders

dynamic light scattering and extraction experiments, aswell as UV‐visible and Fourier transform infrared spectros-copy. However, it should be noted that mixtures of 2,2‐bipyridine and CoIII/II oxide, which can conceivably beformed under these conditions, are also known as compe-tent H2O oxidation catalysts. This work demonstrated thatPOM anions have the ability to enhance the WOC activityof bipyridine‐substituted cobalt complexes and opens apromising new gateway in designing hybrid molecularWOCs using POM and metal‐bpy building blocks.

3.4 | Photocatalytic degradation ofinorganic pollutants

Increasing pollution of water systems by toxic heavymetal ions has become a serious problem faced by man-kind across the world. As a result, and the importanceof matter, the efficient removal of inorganic pollutantsfrom wastewater and purification of water resourcesshould be an overgrowing concern of all society and itsachievement in fact has become a serious obligation andduty for all scientific research groups of variousdisciplines. Among the various methods for removal ofreducible toxic metal ions from aqueous solutions,photocatalysis was found to be the most suitable, feasibleand practical strategy.[119] In this regard, POMs and theirmodified derivatives as appropriate electron reservoirsdemonstrated efficiency in photocatalysis, especially if itis practically being done under the visible light.[120] How-ever, POM‐based inorganic‐organic hybrid photocatalysisunder visible light for this goal are exceedingly rare being

es of three (HTEA)2{[Na (TEA)2] H0or1[MW12O40]} (M = Si, P, Ge)

igure reproduced from Ref.,[113] with permission of the copyright


operation so far. In particular, the examples of theremoval of toxic CrVI by the POMs‐based photocatalystwith solar light energy utilization are still quite foundlimited in number, even in chemical literature.

Based on the aforementioned consideration, a family ofPOM‐based inorganic‐organic hybrids, namely [Ag4(H2O)(L)3(SiW12O40)], [Zn(L)(H2O)]2[SiW12O40]•3H2O, [Cu(L)(H2O)]2 [SiW12O40], and [Cu2(L)2(HPWVI

10WV2O40)]

•4H2O (L = 1,4‐bis(3‐(2‐pyridyl)pyrazol)butane), havebeen designed and synthesized.[121] Strikingly, Ag‐L‐SiW12 hybrid can serve as an active and recyclablephotocatalyst for the reduction of CrVI with isopropanolas scavenger at ambient temperature. In comparison withAg‐L‐SiW12, three other compounds exhibit relativelyweak photocatalytic CrVI reduction under visible light.The authors concluded that the POM and the much larger[Ag4(H2O)(L)3]

4+ unit support the transport of excitedholes/electrons to the surface,[122] and then they togetherinitiate an effective photocatalytic degradation of CrVI

under the irradiation of visible light. A plausible mecha-nism for the reduction of CrVI, is proposed in which ini-tially, the silver organic fragment is excited underinfluence of visible light, and then the excited state elec-trons on the organic ligand are inclined to transfer toSiW12. At the same time, the isopropanol on the surfaceof photocatalyst forms reducing radicals and captures thephotoinduced holes produced by the photocatalyst. Ulti-mately, the photoinduced holes are scavenged by theisopropanol, yielding CO2, H2O and etc. Thus, this chargetransfer sustains the recombination of holes and electrons.In fact, they are the electrons accumulated on the SiW12

are responsible for the reduction of CrVI to CrIII.

3.5 | Photocatalytic organic synthesis

MOFs are quite suited for catalysis of organic reactionsbecause they can impose and dictate the size and shape

FIGURE 10 A new approach to merge

Cu‐catalysis/Ru‐photocatalysis within one

single MOF for oxidative C–C bond

formation. Figure reproduced from

Ref.,[123] with permission of the copyright

holders.

selectivity through readily fine‐tuned pores or channels.The combination of MOFs with the pohotocatalyticallyactive POMs is actually expected to be a promisingapproach due to the creation of synergism for photo-catalysis. Compared to the intensively studied degrada-tion of organic dyes, photocatalysis have been largelyoverlooked in organic synthesis, using inorganic‐organichybrids, probably, the fruitful photocatalytic demandmore control over the properties and strictures, inexcited‐state during a photocatalyzed–organic synthesis

The combination of photoactive catalysts with theMOFmetal and using the outcome, is expected to be more prom-ising due to synergism effect. Duan and co‐workers usedthis approach to merge Cu‐catalysis/Ru‐photocatalysiswithin one single MOF by incorporation of the photoredoxcatalyst ruthenium (III)‐substituted [SiW11O39Ru(H2O)]

5–

into the pores of copper (II)‐bipyridine MOFs.[123] Theresulting hybrid material (referred to as CR‐BPY1) exhib-ited an absorption band centered at 398 nm in the solid‐state UV‐Vis spectrum. The hybrid catalyzed oxidativeC–C bond formation between N‐phenyl‐tetrahydroiso-quinoline and acetophenones took place in excellent con-version and size selectivity (Figure 10). The resultsindicated the significant contribution of cooperative effectsof the individual parts within one single MOF. The N‐phe-nyl‐tetrahydroisoquinoline substrate could be activated bythe POMmoiety to generate the iminium species under vis-ible light irradiation. At the same time, the acetophenonesubstrate coordinated to the copper node to form an enolintermediate, which acted as a nucleophile in the oxidativeC–C bond coupling.

In another study, a solvothermal synthetic method isused to obtain a decatungstate‐based MOF with 1D chan-nels. The MOF of [Cu4(4,4′‐bpy)6Cl2(W10O32)]•3H2Oexhibited remarkable photocatalytic activities for the β‐or γ‐site C–H alkylation of aliphatic nitriles under mildreaction conditions.[124] The photoactive [W10O32]

4–

POMs are embedded in the pores of Cu–BPY via


electrostatic attraction. The 1D hydrophilic/hydrophobicchannels are beneficial for the adsorption of the reactionsubstrates (Figure 11). The high catalytic efficiency, excel-lent size selectivity, high structural stability and goodrecyclability of this photocatalyst offer an idealenvironmental‐friendly route for widening the scope ofaccessible nitriles in both the laboratory and industry.In addition, this work provides a superior route to stabi-lize and constrain the highly active and metastablePOM moieties in the confined space to improve the cata-lytic efficiency for a synthetic organic transformation in aheterogeneous manner.

3.6 | Photocatalytic CO2 reduction

The photocatalytic recycling of CO2 by using solar lightirradiation is a scientific and technological challenge.Nowadays this strategy has attracted much attention ofchemists because it is no addition of extra energy andno destructive effect on the environment. Therefore, itis has stirred up the interest of research group to designthe desired visible light‐sensitive photocatalysts, beingable in transformation of CO2. To the purpose, fre-quently a common strategy is utilization of a transition‐metal coordination compound as a photocatalyst.[125] Inparticular, ReI organometallic complexes were found asthe most efficient catalyst systems for photoreduction ofCO2. However, in this protocol, the adsorption of thelight is limited only to the near‐UV region and inclusionof the sacrificial electron donors are also needed.[126] Onthe other hand, POMs were found as potentially effectivephotocatalysts for the transformation of CO2, since theycan absorb from a very broad absorption spectrum, cov-ering almost the entire visible region and also are known

FIGURE 11 Schematic representation of the photocatalytic reaction

catalytic substrate encapsulation. Figure reproduced from Ref.,[124] with

to act as electron reservoirs. Thus, the outcome fromcombination of ReI organometallic complexes withPOMs provides a powerful tool for the design and syn-thesis of multifunctional inorganic‐organic hybridphotocatalysts that take advantage of the requiredfeatures of both substructures. In 2011, Neumann andco‐workers developed an approach for the coupling of aphotoactive rhenium complex, [ReI(L)(CO)3(CH3CN)]

+,with a Keggin anion, [MHPW12]

–, via a crown ether‐functionalized phenanthroline ligand L (L = 5,6‐(15‐crown‐5)‐1,10‐phenanthroline; M = Na, H3O

+) to forma supramolecular hybrid catalyst.[127] The synergisticeffect of those two aforementioned components resultedin an ideal conversion of the CO2. The rhenium complexrepresents the photoactive site, while the Keggin anionacts as the redox shuttle, which provides two electronsfor the reduction of CO2 to CO. In the first step, thePOM moiety undergoes a two‐electron reduction in thepresence of Pt/C by H2 in which a sacrificial reducingagent such as an amine is non‐required. Photoexcitationof the reduced supramolecular catalyst results in anintramolecular one‐electron transfer from the POManion to the ReI center, resulting in the formation of aone‐electron‐reduced POM anion and a Re0 species.Then, CO2 undergoes an oxidative addition to the Re0

center with simultaneous electron transfer of theremaining POM‐based electron, resulting in the forma-tion of a [ReI(L)(CO)3(CO2H)] species. Subsequently,cleavage of the C−O bond leading to the release of CO.After that, DFT theoretical calculation study has alsoconfirmed this proposition.[128] In this plausible reactionmechanism for the photoreduction of CO2 by arhenium(I)‐POM hybrid compound, it is believed thatPOM anion acts as photo‐sensitizer, electron reservoirand electron donor.

and the crystal‐type I to crystal‐type II transformation induced by

permission of the copyright holders


4 | CONCLUSIONS ANDPROSPECTS

Design and preparation of the active, selective, recyclableand environmentally benign catalysts has become anattractive issue in recent scientific research in the areaof catalysis as well as photocatalysis. In this regard, POMsare frequently considered as unrivaled and outstandingcandidates due to their unique and fascinating posses-sions, such as tunable redox and acid‐base properties,inherent resistance to oxidative decomposition, high ther-mal stability, and impressive sensitivity to light becauseof their multiple active sites, including protons, metalions, and oxygen atoms. One of the most appealing strat-egies to optimize and create more functionalities based onthese intriguing clusters comprising, the introduction ofPOMs into the network of metal‐organic complex to gen-erate inorganic‐organic hybrid materials. In this reviewwe tried to underscore various important reactions, mostof them practically being done in chemical industry.These reactions were POM‐based inorganic‐organichybrids catalyzed‐ oxidation, acid‐catalyzed reactions,base‐catalyzed reactions, photocatalytic reactions, etc.Increasing the specific surface area and number of activesites, decreasing the solubility, and synergistic interactionof two moieties in inorganic‐organic hybrid compoundsresulted in better catalytic efficiency than parent compo-nent. The selected examples clearly indicated thatdepending on the reaction type, the intelligent choicefor combination of diverse metal ion, organic ligand,and POM is crucial for a rational design and facile prepa-ration of highly efficient hybrid catalysts.

The variety of POMs and metal‐organic complexes inthe hybrid structures means that inorganic‐organichybrids can promote other organic transformationsbesides the examples, emphasized in this review. Manyefforts have been made and being examined for theinorganic‐organic hybrids‐ catalyzed organic transforma-tions and encouraging outcomes will impel scientists todevelop more efficient and examine them as superior cat-alysts in various organic reactions organic hybrids. It isnoteworthy, to mention that literature survey revealedthat nowadays there has been a longstanding interest inthe design and preparation of novel inorganic‐organichybrids based on POMs. Inorganic synthetic chemists, par-ticularly have focused on, how POMs bind to metal‐organic compounds in hybrid structures in the designedtargets while, organic chemists have wondered how touse these precious catalysts as efficient and superior cata-lysts in virous organic reactions especially those have beenalready by POMs or different organic metal complexes bythemselves, in sake of comparison their efficiencies andactivities. On the other hand, environmental scientists

are keener of the results of the inorganic‐organic hybridscatalyzed photochemical transformations which are well‐recognized, partly circumventing the environmental pol-lution as the serious problems, caused by overgrowingchemical industries which provides materials, productionsome of them are highly demanding for our daily life.Therefore, we believe that the POM‐based inorganic‐organic materials which have a great impact also will havebright future across the field of catalysis.

ACKNOWLEDGEMENTS

NL and MMH appreciate partial financial support fromAlzahra University, Tehran, Iran. MMH is also gratefullyacknowledged the individual research chair granted byIran National Science Foundation (INSF). MM is thank-ful to research council of Ferdowsi University of Mash-had, Mashhad, Iran.

ORCID

Majid M. Heravi https://orcid.org/0000-0003-2978-1157Masoud Mirzaei https://orcid.org/0000-0002-7256-4601

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How to cite this article: Lotfian N, Heravi MM,Mirzaei M, Heidari B. Applications of inorganic‐organic hybrid architectures based onpolyoxometalates in catalyzed and photocatalyzedchemical transformations. Appl OrganometalChem. 2019;33:e4808. https://doi.org/10.1002/aoc.4808



applications of inorganic-organic hybrid architectures

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