Research progress of Strandberg-type polyoxometalates

TIAN Yinbing,FANG Ning,LI Chunyan,MA Chenguang,LIU Hongling

(Henan Key Laboratory of Polyoxometalate Chemistry,College of Chemistry and Chemical Engineering,Henan University,Kaifeng 475004,Henan,China)

Research progress of Strandberg-type polyoxometalates

TIAN Yinbing,FANG Ning,LI Chunyan,MA Chenguang,LIU Hongling*

(HenanKeyLaboratoryofPolyoxometalateChemistry,CollegeofChemistryandChemicalEngineering,HenanUniversity,Kaifeng475004,Henan,China)

As an imporant branch of polyoxometalates(POMs)-based materials,Strandberg-type POMs{e.g.,the ([P2Mo5O23]6-)} have seized the attention of chemists in recent years due to their unique structures and properties relative to catalysis,medicine,electrochemistry,magnetism and fluorescence,etc.The [P2Mo5O23]6-clusters have a relatively small size potentially resulting in the formation of diverse architectures and stable clusters.The protrudent {PO4} fragments are also coordinated to metal-organic segments except for MoO6octahedra,which can be easily used for the construction of organic-inorganic hybrid materials.This review provides an overview of structural characteristics,synthesis methods,a range of important applications of Strandberg-type POMs and future prospects in this field.

Strangberg-type POMs; structure; synthesis; applications

Polyoxometalates (POMs),as a subset of polynuclear anionic clusters of transition metals in high oxidation states,are attracting an increasing attention due to their unique physical and chemical properties,which endow them with a wide range of application including medicine,catalysis,material,magnetism and photochemistry[1-7].In POM chemistry,the structures of POMs are diverse.A broad classification splits the POMs into isopolyanions ([HxMyOz]n-) and heteropolyanions ([XxMmOy]q-,X = heteroatom,e.g.,a first-row transition metal).The isopolyanions lack heteroatoms resulting in an increased instability compared with heteropolyanions.The metal-oxide framework in heteropolyanions contains a templating heteroatom/heteropolyanion and has been in focus due to the great structural diversity that they bring and the structures of POMs are versatile due to the possibilities of there being different combinations of transition metal atoms and hetero atoms such as Keggin,Dawson,Anderson,Waugh,Silverton and Lindqvist structures[8].

POMs exhibit a rich diversity of properties owing to their structural versatility.The main physicochemical properties of POMs are as follows:1) The thermal stability of POMs is very good and the molecular size is large and the relative molecular weight is high (103-104).2) The composition elements are rich,non-toxic,tasteless,non-volatile,easy to be dissolved in oxygen containing solvents such as water,ether,ethanol,acetone,etc.3) POMs have excellent redox properties and the majority of reduction shows blue,known as the “heteropoly blue”,oxidation will restore color.The redox potential can be easily tuned by adjusting the structure and composition of POMs.4) Surfactants can effectively prevent the aggregation of POMs and obtain a variety of organic/inorganic hybrid materials[9].As we know,progress in polyoxometalate chemistry has always been closely related to the functionalized POM materials[10].One of the most challenging tasks in current POM chemistry is to develop bridging units that can link POM building units into extended frameworks.This is of great interest not only from a structural point of view,but also because the resulting complexes are potentially interesting for applications in a range of different areas[11],The synthesis of new POM-based extended assemblies,such as chains,nets,and open frameworks also play an important role in the design of new materials with novel electronic,magnetic,and topological properties[12].

Strandberg-type POMs,as an important branch of the POM family,present unique properties.Firstly,relatively small size of these POMs increases the electron density of the whole clusters,which can result in the presence of more counterions,and thus potentially results in the formation of more diverse architectures and stable clusters.Secondly,the protrudent {PO4} fragments are also coordinated to metal-organic segments except for MoO6octahedra.Thirdly,this kind of POM cluster can be easily assembled through the reaction of simple starting materials under hydrothermal conditions[13].In addition,these molecules have been used for the construction of organic-inorganic hybrid materials based on mono-,bi-,or polymetal-complex fragments supported or bridged by polyoxometalate anions,while these polyanions have some particular characteristics,and the tetrahedraland octahedral oxygen atoms can also coordinate to metal atoms[14].Strandberg-type POMs have been extensively studied in recent years.The aim of this review is introducing strandberg-type POMs comprehensively,which describes the following points:Strandberg-type POMs’ structural characteristics,synthesis methods,a range of important applications in inorganic-organic hybrid materials,photoluminescence,catalysis,electrochemistry,magnetism,biochemistry and the future prospects of Strandberg-type POMs.

1 Structures of Strandberg-type POMs

1.1 Inorganic structure

[(HOPO3)2Mo5O15]4-,the pentamolybdodiphosphate ion,is well known as its structure was characterized by Strandberg already in 1973,and later verified by many people,which makes it a heteropoly anion owning a clear structure.This anion consists of a chiral ring of five alternating edge-and corner-sharing MoO6octahedra (point group C2) capped above and below by phosphate groups[15],and the number of coordination atoms in the central atom may be 3 or 4,having a triangular pyramid or tetrahedral configuration,respectively.The anionic polyhedral structure is shown in Fig.1.

Fig.1 Polyhedral structure of Strandberg-type polyoxoanions[(HOPO3)2Mo5O15]4-

Strandberg-type POMs structural features are:the coordination atom is a 6-coordinated octahedral structure,and the five octahedra are connected by common edge or common vertices.The central atom is connected to the atom via three common oxygen atoms,and the non-shared atom above the central atom is optional or other atoms or groups.Therefore,the general formula of the series of anions is [X2M5O(21-23)]n-[16].

1.2 Extended structures

POMs,such as Keggin,Wells-Dawson,Silverton and Lindquist type polyoxoanions serve as the building blocks have been successfully reported.However,the role of Strandberg-type polyoxoanions as the inorganic building units to construct such extended frameworks has not been so extensively studied[17].Strandberg-type polyoxoanions as the building units have developed fast over the past decade.In 2008,CARRARO et al synthesized the chiral Strandberg-type molybdates [(RPO3)2Mo5O15]2-,which is self-assembled fibrillar nanostructures (as shown in Fig.2)[18].In 2011,NIU and co-workers reported a new 2D network polyoxometalate constructed from Strandberg-type phosphomolybdates linked through binuclear Ca(II) clusters (as shown in Fig.3)[19].In 2013,ZHANG and co-workers synthesized a new three-dimensional (3D) compound [Ag12(OH)2(H2O)4KNa(P2Mo5O23)2],which is high-dimensional architecture from Strandberg-type polyoxometalate clusters and silver ions (Fig.4)[20].

Fig.2 Polyhedral/ball-and-stick representation of the two diastereomeric forms of (R,R)-[(R*PO3)2Mo5O15]2-，R*= CH3CH(NH3)crystallized as Rb salts.MoO6 octahedra orange,O red,P magenta,C gray,N blue,H white

Fig.3 2D network of 1a viewing a axis,showing the {P2Mo5} clusters linked through binuclear calcium clusters.Hydrogen atoms and lattice water are omitted for clarity

Fig.4 Polyhedral/ball-stick diagram showing the 3D frameworks of the compound

2 Synthesis of Strandberg-type POMs

2.1 Hydrothermal method

The hydrothermal method can be used to obtain the reaction in which the solid phase reaction is difficult to obtain and allows the reaction to proceed under relatively mild conditions.In the past several decades,polyoxometalates have been hydrothermally synthesized and more and more novel structures have been reported.

CAO et al hydrothermally synthesized a Strandberg-type polyoxotungstate compound,(H2dien)2[W5O15(HPO4)2]H2O(dien = diethylenetriamine)[21].The compound of diphosphopentamolybdate {[Cu(en)(Hen)]2[P2Mo5O23]}·3H2O [en=ethylenediamine] has been obtained from the hydrothermal method by JIN’s team[22].

2.2 Conventional solution method

NAGAZI et al reported an inorganic compound formulated as Rb4[Se2Mo5O21·2H2O using Rb2CO3,(NH4)6Mo7O24·4H2O and SeO2through the conventional solution method[23].NIU et al synthesized a polyoxometalate cluster based on Strandberg-type phosphomolybdate [K3Ca(H2O)4(HP2Mo5O23)]·6H2O using Na2MoO4·2H2O,Na2HPO4·12H2O and CaCl2through the conventional solution method[19].

2.3 The liquid diffusion method

ZHANG et al synthesized a new three-dimensional (3D) compound [Ag12(OH)2(H2O)4KNa(P2Mo5O23)2] using Na2MoO4·2H2O,NaH2PO4and AgNO3through the liquid diffusion method[20].

3 Properties and applications of Strandberg-type POMs

3.1 Materials:assembly of inorganic-organic hybrid POMs

Inorganic-organic hybrid materials have been successively prepared due to their remarkable multifunctional properties.In the development of these materials with high application potentials,POMs have attracted considerable attention as the inorganic nano-cluster components because of POMs form a large class of composites with versatile properties by combination with organic components[24].The Strandberg-type POMs,as the typical subclass,have attracted significant attention due to their own structural characters and their higher charges.The Strandberg-type POMs have been used to construct inorganic-organic hybrid materials[10].Inorganic-organic hybrid materials not only demonstrate their fascinating structural diversities,but also provide new and exciting functional materials[21].

For example,NAGAZI et al prepared a new organic-inorganic hybrid compound (2-AMP)4[Se2Mo5O21]·2H2O[2-AMP = 2-aminopyridinium] and introduced its excellent electrochemical properties[25].LI et al synthesized three new inorganic-organic hybrid compounds based on Strandberg-type anions and Zn(II)-H2biim/H2O subunits in aqueous solutions (H2biim = 2,2′-biimidazole),which were used as efficient and reusable catalysts for the protection of carbonyl compounds,they also represent the first example where Strandberg-type POMs are used as acidcatalysts in an organic reaction[10].WANG et al reported two novel Strandberg-type organophosphomolybdate hybrid compounds [(Cu(H2O))2(ì-bipy)2(C6H5PO3)2Mo5O15]nand (Cu(H2O)2)2(ì-bipy)(C6H5PO3)2Mo5O15]n(bipy =4,4′-bipyridyl) and investigated their fluorescence properties and catalytic properties for the synthesis of cyclohexanone ethylene ketal[13].

3.2 Catalytic properties

The application of POMs as catalytists is increasing continuously in the catalytic field because of their high thermal stability,stronger acidity,high hydrolytic stability (pH 0-12),safety,quantity of waste,reusability,corrosiveness,separability,high oxidation potential and greenness[26].POMs can be used as photocatalysts and electrocatalysts,and they are most widely used as catalysts in organic synthesis,such as esterification reaction,oxidation reaction and protection of carbonyl compounds[10].MA et al[27]successfully assembled an organic-inorganic hybrid Strandberg-type organophosphotungstate,{[(Cu(H2O)(ì-bipy))2(C6H5PO3)2W5O15]}n(bipy = 4,4′-bipyridyl) using phenylphosphonic acid,simple tungstate and copper(II) compounds as starting materials,which exhibits good acid-catalytic activity for the synthesis of cyclohexanone ethylene ketal.The reaction was tested without any catalyst,the yield of cyclohexanone ethylene ketal was only 9%.At the mine time,the reaction was also tested with the compounds as catalyst,the yield of cyclohexanone ethylene ketal is 95%(Fig.5).LI et al synthesized three new inorganic-organic hybrid compounds based on Strandberg-type anions and Zn(II)-H2biim/H2O subunits in aqueous solutions (H2biim = 2,2′-biimidazole),which were used as efficient and reusable catalysts for the protection of carbonyl compounds,they also represent the first example where Strandberg-type POMs are used as acid catalysts in an organic reaction[10].

Fig.5 Catalytic activity of {[(Cu(H2O)(ì-bipy))2(C6H5PO3)2W5O15]}n(bipy=4,4′-bipyridyl) used after four runs

3.3 Photoluminescent properties

Polyoxometalates are extremely versatile inorganic building blocks for the construction of functional materials.The relevant properties related to developing functional materials including photochromism,electrochromism,and luminescence.The photoluminescence of the inorganic-organic hybrid POM materials can be enhanced after the introduction of a transition metal and an organic component into the structure[28].MA et al successfully synthesized the {[(Cu(H2O)(ì-bipy))2(C6H5PO3)2W5O15]}nand investigated its solid-state fluorescence properties:two main broad emission bands at 400 nm and 470 nm with Eex= 290 nm and the emissions of the compounds are mainly caused by the coordination of {(C6H5P)2W5} to Cu2+ions[28],WANG et al synthesized two novel Strandberg-type organophosphomolybdate hybrid compounds,whose solid-state fluorescence properties are studied at room temperature.The results indicate that the two compounds may be excellent candidates for photoactive materials[13].

3.4 Electrochemistry

POMs own rich electrical and optical properties,one of which is indeed their electron-accepting capability.The reduction process is usually reversible and occurs with marginal structural variations[29].There are rising concerns about their electrochemical properties on them,which will have great potential for applications in electrochemistry.Strandberg-type POMs,as the typical POMs,are attractive molecular clusters due to their potential application in electrochemistry.DEY et al reported a Strandberg-type POM and Cu(II) based metal organic framework (MOF),namely [Mo5P2O23] [Cu(phen)(H2O)]3·5H2O (Cu3Mo5P2).The most important feature of Cu3Mo5P2is its proton conducting nature.The proton conducting materials are important due to their application in electrochemical device fabrication and fuel cells[30].

3.5 Magnetic behaviors of Strandberg-type POMs

Design and synthesis of new SMM (single-molecule magnets) materials have become a hot topic recently due to their potential applications in high-density information storage and quantum computation[8].POMs,compared with other coordination compounds,present some advantageous chemical,structural and electronic features that make them suitable for this goal,namely:ⅰ) They are robust molecules that keep their integrity in the solid state,in solution and probably also on surfaces.ⅱ) They can accommodate magnetic ions or groups of magnetic ions at specific sites of the rigid POM structure,leading to magnetic molecules and large magnetic clusters with specific topologies and highly symmetric environments.ⅲ) They can accept various numbers of electrons,while keeping almost intact their structure,leading to mixed-valence systems in which the extra electrons are extensively delocalizedover the whole POM framework[31].Strandberg-type POMs,as a crucial branch of POMs,can also own unique magnetic behavior.

JIN’s team investigated the magnetic behaviours of four inorganic-organic complexes based on polymolybdate anions and copper-organic subunits in the temperature range of 2-300 K at 1 000 Oe[32].AMMARI’s team measured the magnetism of the novel Strandberg type molybdophosphonate complex,(C5H7N2)6[Cu(H2O)3HP2Mo5O23]2·4H2O from 100 K to 2 K,which indicate that the presence of an antiferromagnetic coupling between the Cu (II) ions in complex,resulting in a maximum of an antiferromagnetic-paramagnetic transition atTn= 7 K.Furthermore,magnetic susceptibility data indicate an antiferromagnetic Curie-Weiss behavior in the studied temperature range,and molecular field theory gives the (J/kB) values of the nearest neighbor interactions between copper ions[33].

3.6 Nonlinear optical (NLO) materials

Nonlinear optical (NLO) materials have been attracting continuous interest because of their potential applications in telecommunications,optical computing,optical data storage,and optical information processing[34].The totally inorganic compounds have high thermal stability relative to the organic molecules,and hence they are natural candidates for NLO materials.POMs possess unique chemical,electronic,medical,and optical properties and there are researches show that totally inorganic POM clusters with larger static first hyperpolarizabilities relative to our previous compounds can be applied to NLO materials.Strandberg-type POMs,as recent research focus,are excepted to design efficient NLO materials[35].

WANG’s team[36]synthesized a noncentrosymmetric chiral Strandberg-type polyoxometalate based material,Cs4Mo5P2O22,which shows a moderate second harmonic generation (SHG) response in nonlinear optical (NLO) materials.KONG’s team systematic explored the four new compounds featuring Strandberg-type [Mo5O15(SeO3)2]4-polyanions,which is the new second-order NLO materials in purely Mo(VI)-rich alkali metal selenites[37].

3.7 Biochemical applications.

Polyoxometalates are anionic complexes that have high thermodynamic and/or kinetic stability in aqueous solutions at biological pH.In addition to well-developed applications in catalysis,separations,analysis and as electron-dense imaging agents,these complexes have been shown to exhibit biological activity ranging from selective enzyme inhibition to antitumour/antiviral properties[38].Antitumoral,antiviral,and antibiotic properties have been evidenced for several POMs,and specific POM-protein interactions have been established,which opens opportunities in biomedical studies[31],Strandberg-type POMs have the prospect to be applied in biochemistry due to their unique properties.SUN et al applied capillary zone electrophoresis(CZE) to investigate the solution stability and equilibria of pentamolybdobis,[(C6H5PO3)2Mo5O15]4-,which is a Strandberg-type of polyoxometalate (POM) with considerable promise as an antiviral and anti-HIV chemotherapeutic agent[39].

4 Outlook and perspectives

The reasearch on Strandberg-type POMs has increased rapidly over the past years.However,POMs usually have low surface area and dissolve,lose in some polar solvents,which limit their own large-scale industrial applications.Many novel functionalized materials remain to be explored within Strandberg-type POMs based compounds,the assembly of nanomaterials and Strandberg-type POMs,Strandberg-type POMs doping metallics,Strandberg-type POMs incorporated metal salts which are all in theory accessible.The synthesis of new extended structures of Strandberg-type POMs is also a good method to assemble functionalized Strandberg-type POMs.Furthermore,exploring the possible applications of Strandberg-type POMs in different aspects and in real life need to be done.

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Strandberg-type型多金属氧酸盐的研究进展

田银兵，方 宁，李春艳，马晨光，刘红玲*

(河南省多酸化学重点实验室,河南大学 化学化工学院,河南 开封 475004)

作为多金属氧酸盐材料的重要分支，Strandberg型多金属氧酸盐(例如，[P2Mo5O23]6-)近些年来由于其独特的与催化、药物、电化学、磁学、荧光相关的结构和性质引起了化学家的广泛关注.Strandberg型多金属氧酸盐有相对小的尺寸，这可能形成不同的架构和稳定的团簇，除了六氧化钼八面体，突出的{PO4}部分仍然和金属-有机部分配位,这使得它易于构建有机-无机复合材料.本文对Strandberg型多酸的结构特征、合成方法、一系列重要应用和在该领域的应用前景进行了综合的概述.

Strangberg-type型多金属氧酸盐；结构；合成；应用

Received date：2016-10-03.

Foundation item：Key Scientific Research Projects of Henan Province Colleges and Universities,Foundation of Education Department of Henan Province (16A150002).

Biography：TIAN Yinbing (1996-)，female，majoring in polyoxometalate-based functional materials.*

,E-mail:hlliu@henu.edu.cn.

O612.6 Document code：A Article ID：1008-1011(2017)01-0127-08