Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications

Coordination Chemistry Reviews xxx (2017) xxx–xxx

Contents lists available at ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications q Jiangwei Zhang a,b,1, Yichao Huang a,1, Gao Li b,⇑, Yongge Wei a,⇑ a b

Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, PR China State Key Laboratory of Catalysis & Gold Catalysis Research Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences (CAS), Dalian 116023, PR China

a r t i c l e

i n f o

Article history: Received 29 August 2017 Received in revised form 12 October 2017 Accepted 30 October 2017 Available online xxxx

Keywords: Polyoxometalates POMs-based materials Alkoxylation Organic functionalization Tris ligands

a b s t r a c t Polyoxometalates (POMs), an exceptional family of coordination clusters consisting of Mo, W, V, etc. early transition metal ions in their highest oxidation states, have received significant attention over recent years due to their structural versatility and unique and diverse chemical and physical properties. The functionalization of POMs with organic ligands provides a novel strategy to precisely incorporate POMs with advanced functional organic moieties on their surfaces and enhance their compatibility in organic media. Among the various organically functionalized synthetic strategies for POM-based organic–inorganic hybrid materials, alkoxylation of POMs stands for one of the hottest topics during the past decades, since the diverse and tunable alkoxyl ligands are able to anchor on the surface of many POMs clusters, forming novel and flexible organically functionalized POM clusters, which can be further exploited as building blocks to design various functionalized POM-based hybrids with charming catalytic properties, and biomedicine, energy, or functional materials applications. In this review, recent advances in alkoxylation chemistry of POMs from synthetic strategies, structural overviews to their functional applications have been discussed. Ó 2017 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic strategies and structural overviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. The alkoxylation chemistry of Anderson-type POMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. The alkoxylation chemistry of Lindqvist-type POMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. The alkoxylation chemistry of Dawson-type POMs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. The alkoxylation chemistry of other POMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00

Abbreviations: POMs, polyoxometalates; POV, polyoxovanadate; Ol, bridging O atom; Mo2, [Mo2O7]2
; Mo6, [Mo6O19]2
; Mo7, [Mo7O24]6
; PMo12, (NH4)3[PMo12O40]; V5, [Bu4N]3V5O14; V6, [V6O19]8
; V10, [H3V10O28]3
; P2W15V3, [H4V3P2W15O62]5
; D-V3, [H4P2V3W15O62]5
; EEDQ, 2-ethoxy-l-(ethoxycarbonyl)-l,2-dihydroquinoline; DCC, dicyclohexylcarbodiimide; NH2-{MnMo6}-NH2, {[NH2C(CH2O)3]2MnMo6O18}3
; Mo6„N-{XMo6}-N„Mo6, [XMo6O18{(OCH2)3CN(Mo6O18)}2]; C16-{MnMo6}-C16, [MnMo6O18 {(OCH2)3CNHCO (CH2)14CH3}2]; EG193-b-V57, poly (ethylene glycol-b-N-methyl-4-vinylpyridinium iodide); S480-b-V57, poly (styrene-b-N-methyl-4-vinylpyridinium iodide); COF, cluster organic frameworks; TM-POMs, transition metal substituted POMs; MHA, 16-mercaptohexadecanoic acid; Fmoc, 9-fluorenylmethyloxycarbonyl; {Mo6}-N-{MMo6}NH2, [Mo6O18NC-(OCH2)3MnMo6O18(OCH2)3CNH2]5
; CH2OH-{Cr(OH)3Mo6}, {[CH2OHC(CH2O)3]Cr(OH)3Mo6O18}3
; R-{Cr(OH)3Mo6}, {[RC(CH2O)3]Cr(OH)3Mo6O18}3
; COOH-{Al (OH)3Mo6}, {[HOOCCH2NHC(CH2O)3]Al(OH)3Mo6O18}3
; NH2-{Al(OH)3Mo6}, {[NH2C(CH2O)3]Al(OH)3Mo6O18}3
; MHC, metal halide cluster; POTs, polyoxotungstates; CH2OH{Ni(OH)3W6}, {[CH2OHC(CH2O)3]Ni(OH)3W6O18}4
; Fc, ferrocenyl; pyridyl-{V6}-pyridyl, TBA2[V6O13{(OCH2)3CNHC(O)-3-C5H4N}2]; Tris, tris(hydroxymethyl) aminomethane; NH2-{V6}-NH2, {V6O13[(OCH2)3CNH2]2}2
; {V6}-3NH2, [VIV3VV3O10{NH2C(CH2O)3}3]2
; pyridine-{V6}-pyridine, [V6O13{(OCH2)3C(4-CONHC5H4N)}2]2
; pyrene-{V6}-pyrene, [TBA]2[V6O13{(OCH2)3CNH-CH2-C16H9}2]; pyrene-CH2-NH-{V6}-NH2, [V6O13{(OCH2)3CNH(CO)CH2CH2CH2C16H9)}{(OCH2)3CNH2}]2
; pyrene-CH2-NH-{V6}-NH-CH2-pyrene, [V6O13{(OCH2)3-C(NH(CO)CH2CH2CH2C16H9)}2]2
; COOH-{V6}-COOH, [V6O13{(OCH2)3CCOOH}2]2
; OH-{V6}-OH, [V6O13{(OCH2)3CCH2OH}2]2
; C18-{V6}-C18, [V6O13{(OCH2)3CCH2OOC(CH2)16CH3}2]2
; C16-Ar-{V6}-Ar-C16, [V6O13(OCH2)3CNH2(OCH2)3CNHCH2C6H4COOC16H33]2
; R-COO-{V6}-COO-R, [V6O13{(OCH2)3CCH2OOC(CH2)2COOR}2]2
. q

We declared that the authors have no competing interests.

⇑ Corresponding authors. 1

E-mail addresses: [email protected] (G. Li), [email protected] (Y. Wei). These authors contributed equally to this publication.

https://doi.org/10.1016/j.ccr.2017.10.025 0010-8545/Ó 2017 Elsevier B.V. All rights reserved.

Please cite this article in press as: J. Zhang et al., Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications, Coord. Chem. Rev. (2017), https://doi.org/10.1016/j.ccr.2017.10.025

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

3.

4.

Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Biomedicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Functional materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Polyoxometalates (POMs) are an exceptional family of inorganic clusters consisting of Mo, W, V, etc. early transition metal ions in their highest oxidation states with structural versatility and a wide range of optical, electronic and magnetic properties and applications [1–3]. Since the first POMs, (NH4)3[PMo12O40] (PMo12), reported by Berzelius in 1826, the POMs chemistry, a special branch of coordination chemistry, has aroused wide attention with the development of structure characterization technology. Especially with the spread and mature of single X-ray diffraction, the atomically precise structure of POMs can be definitely confirmed [4]. The topology structures of Keggin, Anderson, Dawson, Waugh, Silverton, Lindqvist, Weakley, Standberg, Finke, and Preyssler have been discovered sequentially and formed into the ten basic structures of POMs [5] (Fig. 1). As inorganic multi-dentate coordination compounds with structural versatility, POMs are prime candidates for the design and construction of different architectures with judicious selection of appropriate organic molecules. The functionalization of POMs with organic ligands provides a novel strategy to precisely incorporate POMs with advanced functional organic moieties giving rise to POMs-based organic–inorganic hybrids and thus contributing to the diversity of POM structures and enhancing their compatibility in organic media [6–8]. Generally, POMsbased organic–inorganic hybrids can be categorized into two classifications according to the interactions between organic moieties and POMs: classification I (non-covalent interactions) and classification II (covalent interactions). POMs-based organic–inorganic hybrids (I) are assembled by ionic bond, hydrogen bonding, as well as/or intermolecular force, while POMs-based organic–inorganic hybrids (II) are formed through covalent grafting organic moieties onto POMs surface by replacing the oxygen atoms in POM clusters. In recent years, POMs-based organic–inorganic hybrids (II) gained intensively

00 00 00 00 00 00 00 00

attentions and dramatic development since that the covalent organic modification of POMs can incorporate POMs with advanced functional organic moieties on their surfaces in a reliable and predefined manner. This holds promise for the development of molecular hybrid materials that bridge the gap between organics moieties and inorganic POMs clusters. As could be expected, the intrinsic topology structure of POMs will remain while such organic–inorganic hybrid materials will not only combine the advantages of organic materials (good process abilities and diverse electronic properties), but also bring exciting synergistic effects due to the covalent bond interaction of delocalized organic ligands’ p-p electrons and the inorganic POM clusters’ d-p orbits [9,10], leading to so-called ‘‘value-adding’’ properties. Covalent organic functionalization has been proved to be an effective strategy to make the design of advanced functional materials more rational in order to conquer recent challenges in cuttingedge fields including catalysis, material, energy and biology. Organic modification and functionalization of POM clusters stand for one of the hottest topics among current frontier of POM investigations. The strategy of grafting organic moieties onto a POMs cluster requires an anchorage point ensuring the link between the two components. The current existed synthesis strategies are organoimidization, organoalkoxylation, organophosphonylation, organoarsonylation, organosilylation and organotin respectively. These synthesis strategies were also well summarized in the previous reviews [6–11] (Fig. 2). These six synthesis strategies can be branded into two modes according to coordination modes, including (A) covalently link organic moieties onto the POM cluster surface in direct mode (organoalkoxylation and organoimidization) and (B) extra elements substitution mode (organophosphonylation, organoarsonylation, organosilylation and organotin, et al.).

R M

N

N R M

R'

R

Organoimidization

Si O

O

M

M

R

O

O

O

O M

M

M

M

M

M

R

M

O O M

M

R

O

M

O

O

O O

O

M

M

M M

M

R

Organotin

X

Fig. 1. Ten basic structural topology of POMs clusters.

R

Organoalkoxylation

R Sn

R

M

Organosilylation

O

M

O

O

O

M

M

M

X = P, As Organophosphonylation Organoarsonylation

Fig. 2. Current existed synthesis strategies and main coordination modes to covalently link organic moieties onto surface of the POM clusters.

Please cite this article in press as: J. Zhang et al., Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications, Coord. Chem. Rev. (2017), https://doi.org/10.1016/j.ccr.2017.10.025

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Compared with organoimidization strategy, the synthetic protocols of organoalkoxylation is more effective and the arganoalkoxylated POMs products are more stable than organoimido-derivatized POMs products, since Mo„NR bonds of organoimido-derivatized POMs are sensitive and fragile to H2O while the Mo-O-R bonds derived from organoalkoxylation is compatible with H2O and organoalkoxyl groups may serve as the isoelectronic species of O2
to connect with POMs leading to covalent organically functionalized POMs. Moreover, the alkoxylation chemistry of POMs has obvious advantages from the following aspects: (a) The diverse and flexible coordination modes benefit from alkoxo ligands including the monoalcohol ligands (ROH, R = Me, Et, etc.) and a series of triol ligands with remote functionalized groups (RC(CH2OH)3, R = NH2, OH, etc.); (b) The robust of CAO covalent bond will great increase the chemical stability of the corresponding POM-based organic–inorganic hybrids; (c) Organoalkoxylation is a general strategy that can be widely employed not only for isopolyanions, such as [V6O19]8
(V6) and [H3V10O28]3
(V10) clusters etc, but also for heteropolyanions, like Anderson-type [Hy(XO6)M6O18]n
(y = 0–6, n = 2–8, M = Mo or W, X = Cr, Mn, Fe, Co, Ni and Pt etc.) and Dawson-type [H4V3P2W15O62]5
(P2W15V3) POMs clusters. In this review, we will focus on the recent advances in alkoxylation chemistry of polyoxometalates in terms of the current synthetic strategies, structural overviews and applications in the cutting-edge fields of catalysis, material science, energy and biomedicines. 2. Synthetic strategies and structural overviews Since the first formation of organoalkoxylation derivatives, [PMo12O39(OMe)]2
and [PW12O39(OMe)]2
reported by Knoth in 1981 [12]. The alkoxylation chemistry of POM has witnessed dramatic developments during the past 40 years especially in the last decades. In this section, synthetic strategies and structural overviews will be categorized into four parts, Anderson, Lindqvist, Dawson, and the other POM species, according to the types of POMs clusters. 2.1. The alkoxylation chemistry of Anderson-type POMs The organoalkoxylation derivatives of Anderson type POM clusters stand for one of the biggest branches in POM alkoxylation family. This was due to that the parent Anderson clusters were one of the most tuneable subclass structures in POMs family which benefited from the central heteroatom can be accessible to many elements in the periodic table [13]. The synthesis protocol of parent Anderson clusters was well summarized in the recent reviews by Rompel [14]. The Anderson cluster is consisted of ringlike building block of {Mo6O12(l2-O)6} or {W6O12(l2-O)6} and octahedral coordination central heteroatom building block of {X(l3-OH)6}. From the structural topology analysis, the organoalkoxylation derivatives of Anderson-type POMs can be regarding as plane parent Anderson clusters anchored by organoalkoxyl ligands, such as triol ligands and monoalcohol, etc. It has been reported that triol ligands can anchor on Anderson POMs, forming single-side {[RC (CH2O)3]X(OH)3MO18}3
[15–17], symmetry-side {[RC(CH2O)3]2XMo6O18}3
[18,19] and asymmetric-side {[R1C(CH2O)3] XM6O18[(OCH2)3CR2]}3
[20–22] triol-functionalized Andersontype POMs respectively. Besides, the Anderson-type POMs can also be anchored on monoalcohol ligands, forming monoolfunctionalized Anderson POMs [23–26] (Fig. 3). It is of significance to in-depth understand the reactive sites of parent Anderson-type POMs toward controllable synthesis of targeted organoalkoxyl-functionalized Anderson-type POMs. For monoalcohol ligands, methanol ligand occupies the major parts

R

R

μ3-O

X

{[RC(CH2O)3] X(OH)3MO18}3M

μ2-O [Pt(μ 3-OH)2Mo6O20(μ 2-OMe)2]4-

Pt

R1 Ringlike building block {M6O12(μ2-O)6}

R1 (R2)

R1

X

R1(R2) {[R1C(CH2O)3]XM6O18[(OCH2)3CR2]} 3Fig. 3. The alkoxylation chemistry of Anderson-type POM cluster from structural topology analysis.

R

(a)

X = MnIII, FeIII R = CH3, NO2 CH2OH, NH2

X(OAc)3 [Mo8O26]4- MeCN Refluxing

R

{[RC(CH2O)3]2XMo6O18}3R

(b) X 3+ H 2O [X(OH)6Mo6O18] n-

[Mo7O24]6X=

13 Al

24 Cr

25 Mn

26 Fe

27 Co

H2O Refluxing {[RC(CH2O)3]X(OH)3Mo6O18}n28 Ni

31 Ga

R = CH3, C2H5, NH2, CH2OH NHCH2COOH, pyridine

Fig. 4. The current synthetic routes to triol-functionalized Anderson-type POMs. (a) The organic solvent Mo8 reconstruction protocol; (b) The aqueous direct functionalization of Anderson-type POMs protocol.

due to its high reactivity as it can be employed to react with mononuclear MoO3 species forming the [Mo8O24(OMe)4]4
cluster [27]. It can be interfered that it is uncontrollable for isopolyanions to obtain such organoalkoxyl-functionalized POMs since the nucleophilicity of all O atoms is uniformly dispersed in isopolyanions, and thus the methoxyl groups can substitute both Ot atoms (terminal O atoms) and l2-O atoms. However, the situation in Andersontype heteropolyanions is improved, because Ol (bridging O) atoms possessed stronger nucleophilicity than Ot atoms, and thus the electrophilic alkoxy ligands are more prone to substituted the Ol atoms than Ot atoms, indicating Ol can be regarded as the specific reaction site. For methoxyl-functionalized Anderson-type POMs, only l2-O becomes the reactive site by the direct functionalized parent Anderson-type POMs clusters, forming methoxylfunctionalized POMs, [IMo6O23(l2-OMe)]3
[24] and methoxylfunctionalized [Pt(l3-OH)2Mo6O20(l2-OMe)2]4
[25,26]. It should be noted that the protonation of l2-O was the key factor to further enhance the reactivity leading to selective and controllable organoalkoxyl functionalization of Anderson POMs [15–20]. Compared with the monoalcohol ligands, the triol ligands are more flexible, tuneable and stable serving as specific POM-linker. More often, the three hydroxymethyl covalently tend to anchor on the triangle edge of octahedral central heteroatom

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Fig. 5. (a) The post-functionalization strategy employing tris-functionalized Anderson-type POMs, NH2-{MnMo6}-NH2, as ‘‘Anderson-Tris Linker”; (b) The prefunctionalization strategy employing triol ligands with functional group, [RNHC (CH2O)3], as specific ‘‘Functionalized-Tris Linker” to further anchored on the parent Anderson-type POMs.

O

O

NH

R

A

R

NH

{MnMo6O18[(OCH2)3CNHCOR)]2}

B H

NH2-{MnMo6}-NH2

R N

R

N

H

{MnMo6O18[(OCH2)3CN=CHR)]2} Fig. 6. The typical organic reaction strategies for the post-functionalization of NH2{MnMo6}-NH2. (A) The amidation reaction with acyl chloride and carboxylic acid ligands in the presence of EEDQ; (B) The aldimine condensation reaction with aldehyde ligands.

forming a very stable triol functionalized Anderson-type derivatives. While the remote group can be meticulously designed for further post-functionalization. The current synthetic routes to triol-functionalized Anderson-type POMs mainly contain (a) the organic solvent [Mo8O26]4
(Mo8) reconstruction protocol proposed by Hasenknopf [18,19] and (b) the aqueous direct functionalization of Anderson-type POMs protocol by our group [15–17,20] (Fig. 4). In 2011, Cronin’s group used the mass spectrometry to study the reaction mechanism and [Mo8O26]4
(Mo8) reconstruction self-assembly behaviors at real-time [28]. The extension of organic solvent Mo8 reconstruction synthetic strategy mainly focused in the following aspects: (1) Employing tris-functionalized Anderson-type POMs hybrids, {[NH2C(CH2O)3]2MnMo6O18}3
(denoted as NH2-{MnMo6}-NH2), as special POM-based amino synthon called ‘‘Anderson-Tris Linker”, defined as postfunctionalization strategy (Fig. 5a) [29]; (2) Grafting other organic functional moieties on the specific triol ligands called

‘‘Functionalized-Tris Linker”, which are further anchored on the parent Anderson-type POMs, defined as pre-functionalization strategy (Fig. 5b) [30]. As for the post-functionalization strategy, NH2-{MnMo6}-NH2 can serve as special POM-based synthon to further react with [Mo6O19]2
(Mo6) forming nanoscale chiral rod-like hybrids (Fig. 5a) [29], [XMo6O18{(OCH2)3CN(Mo6O18)}2], denoted as (Mo6„N-{XMo6}-N„Mo6, X = MnIII, FeIII), through the well-known dicyclohexylcarbodiimide (DCC)-dehydrated protocol. Interestingly, these chiral rod-like hybrid materials have been demonstrated to interesting self-recognition behavior by assembling into two types of homogeneous blackberries instead of mixed ones in their mixed solution. Such nanoscale chiral POM-based hybrid materials can serve as an ideal model to better understand the recognition behaviors and self-assembly of biomolecules [31]. The typical strategies for the post-functionalization of NH2{MnMo6}-NH2 include amidation and aldimine condensation reaction (Fig. 6). Through the amidation reaction, the hydrophobic alkyl chain can be anchored symmetrically on the NH2-{MnMo6}-NH2 synthon forming Anderson-type POMs-based amphiphilic hybrid surfactants, [MnMo6O18{(OCH2)3CNHCO(CH2)14CH3}2] (named C16-{MnMo6}-C16). It can slowly assemble into membrane-like vesicles in MeCN/water mixed solvents. The large and charged Anderson polar heads play a role in controlling the vesicle size [32]. While reverse vesicles can be found for C18-{MnMo6}-C18 due to the solvophobic/solvophilic interactions [33]. The number of C atoms in long alkyl-chains and the solvent content ratio controlled the morphologies and phase transition of Anderson-based amphiphilic hybrid surfactants, providing a better understanding of spontaneous self-organization process [34,35]. Through the aldimine condensation reaction (Fig. 6B), song synthesized series aromatic platforms-grafted Anderson-type hybrid materials to give 1-D, 2-D and 3-D architectures using directed hydrogen bond interactions [36]. They also covalently attached NH2-{MnMo6}NH2 to single-walled carbon nanotubes by amide bond and presented potential application in lithium ion batteries [37]. Meanwhile the cation modulation is also an effective strategy to modify the property of tris-functionalized Anderson-type POMs. Song discovered that C16-{MnMo6}-C16 Anderson-type POMs hybrids could assemble with cationic polyelectrolytes, poly (N-alkyl-4vinylpyridinium iodide) (alkyl = methyl, PMV and octadecyl, POV), poly (ethylene glycol-b-N-methyl-4-vinylpyridinium iodide) (EG193-b-V57) and poly (styrene-b-N-methyl-4-vinylpyridinium iodide) (S480-b-V57) via electrostatic interactions leading to the POM-based surfactant and block ionomer complexes [38]. Wang and his co-workers used poly (benzyl ether) (PBE; g1-COOH and g2-COOH) dendrons to covalently bond with NH2-{MnMo6}-NH2 directly, forming dendron-{MnMo6}-dendron hybrid [39]. They also grafted two dendritic poly(urethane amide) wings on NH2-{MnMo6}-NH2, forming Anderson-based organogels with nano-ribbons structure [40]. Furthermore, through amidation reaction, 2-ethoxy-l-(ethoxycarbonyl)-l,2-dihydroquinoline (EEDQ) as the activating reagent, biological active molecules including cholic acid (CA), Dehydrocholic acid (DHCA), Cholesterol (CHOL) and galactose (GAL) could be grafted on NH2-{MnMo6}NH2, forming POMs-biomolecule conjugates hybrids [41]. Moreover, the maleimide-grafted Anderson-type POMs, maleimide-{MnMo6}-maleimide, could also be obtained in metalfree Diels–Alder click reactions under mild conditions [42]. Subsequently, they selected three steroids (cholic acid, dehydrocholic acid and cholesterol) to anchor on NH2-{MnMo6}-NH2 to construct Anderson-type POMs hybrid, steroid-{MnMo6}-steroid, which displayed unexpectedly different supramolecular structures in the self-assembly process [43]. Additionally, cholesterol{MnMo6}-cholesterol conjugates could self-assemble into microrods or nanoribbons regulated by means of a

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temperature-mediated approach [44]. Zhang and his co-workers selected methacrylic anhydride (MA) to anchor on NH2{MnMo6}-NH2 to construct MA-{MnMo6}-MA conjugates as special monomer. Then POM-based PMMA-MAPOM hybrid polymer could be prepared through the copolymerization of MA-{MnMo6}-MA with methyl methacrylate (MMA) [45]. Pradeep also selected methacrylic anhydride (MA) to anchor on NH2-{MnMo6}-NH2 to construct MA-{MnMo6}-MA conjugates as special monomer. And then MA-{MnMo6}-MA was copolymerized with photoresponsive organic monomer (methacryloyloxy) phenyl dimethylsulfoniumtriflate (MAPDST) to form POM-based hybrid polymer, MAPDST-MAPOM [46]. They also synthesized a series of aromatic organic moieties naphthalen (Ar1), quinoline (Ar2), carbazole (Ar3) grafted Anderson-type POMs hybrid materials Ar-{MnMo6}Ar [47]. Cronin’s group synthesized a series of carboxylic acid grafted Anderson-type hybrid materials, carboxylic acid-{MnMo6}carboxylic acid, by the reaction of anhydride precursors and NH2-{MnMo6}-NH2 [48]. Likewise, alkoxyl-functionalized Anderson-type POMs hybrid, azide-{MnMo6}-azide [49], boronic acid-{MnMo6}-boronic acid [50] bipyridine-{MnMo6}-bipyridine [51], could be obtained via the reaction of NH2-{MnMo6}-NH2 and the corresponding triol ligands. It should be noted that some of the as-prepared alkoxyl-functionalized Anderson-type POMs hybrid like bipyridine-{MMo6}-bipyridine (M = Mn3+, Fe3+, Co3+) could further serve as synthon to in situ coordinate with bulky Cu(I) complexes, forming Cu(I)-functionalized Anderson POM anions, which exhibited rich redox-chemistry properties [52]. Moreover, the bipyridine-{MMo6}-bipyridine (M = Mn3+, Fe3+, Co3+) could combine with iridium dimer [Ir(ppy)2(m-Cl)]2 (ppy = 2-phenylpyridine) to give rise to the pure photosensitizerPOM dyads (n-Bu4N)[MMo6O18{(OCH2)3CNCH(IrC33 H26N4)}2] (Ir-POMMn, Ir-POMFe, and Ir-POMCo) [53]. In general, the post-functionalization using ‘‘Anderson-Tris Linker”, NH2-{MnMo6}-NH2, as a synthon is proved to be an efficient strategy to achieve the synergistic covalent interactions between the organic functional groups and Anderson-type POMs clusters, leading to rational and controllable synthesis of POMsbased organic–inorganic hybrids. For the pre-functionalization strategy, the specific functional triol ligands, ‘‘Functionalized-Tris Linker”, should be synthesized firstly by typical organic reaction strategies and then anchored on the Anderson-type POMs, forming POMs-based organic–inorganic hybrids (Fig. 5b). With pre-functionalization strategy, Wu grated azobenzene (Azo) group on tris ligands and Azo-{MnMo6}Azo Anderson-type cluster hybrid materials could be obtained via reconstruction synthetic strategies [30]. Furthermore, chiral selfassembly of Azo-{MnMo6}-Azo with b-Cyclodextrin through selfcrosslinking by the electrostatic and host–guest interactions was achieved [54]. They also grafted ferrocene on tris ligand forming [(HOCH2)3CNHCH2C16H9] denoted as Tris-Fe(Cp), and finally obtained Fe(Cp)-{MnMo6}-Fe(Cp) hybrid materials [55]. In the similar protocol, they prepared pyridine-grafted Anderson-type hybrids, pyridine-{MnMo6}-pyridine, which further combined with dicarboxylic acids through hydrogen bonding forming chain-like supramolecular gels [56]. Moreover, the adenine-grafted Anderson-type hybrids, Adenine-{MnMo6}-Adenine, possessed thermal-induced dynamic self-assembly behaviors [57,58]. The Anderson-type hybrids, Thymine-{MnMo6}-Thymine, and Adenine-{MnMo6}-Adenine are complementary base pair, and therefore, the hydrogen bonds between the complementary base pairs drive the formation of the chain-like supramolecules while the Anderson-type POMs cluster provide electrostatic interaction sites for anchoring side chains and cross-linking [59]. Hasenknopf firstly synthesized a triol ligands containing pyridine group through aldimine condensation reaction, and the as-

(a)

(b) CuI

pyridine-{MnMo6}-pyridine Fig. 7. (a) Ab initio effective synthesis strategy for triol functionalization via Cannizzaro reaction; (b) The synthesis strategy of POM-based COF hybrid materials. Figure was reproduced from Ref. [73] with permission of the copyright holders.

prepared triol was then grafted on Anderson-type POMs by using Mo8 reconstruction protocol (Fig. 4a), forming Anderson-type hybrid material, {MnMo6O18[(OCH2)3CN@C(4-C5H4N)]2}, abbreviated as pyridine-{MnMo6}-pyridine, which was further coordinated with PdII yields a transparent and birefringent gel [60]. They further used electropolymerization to combine pyridine{MnMo6}-pyridine with porphyrin, forming POMs-based organic– inorganic copolymers [61,62]. Furthermore, terpyridine-based tris derivative ligand, Tris-TPy, was used to obtain TPy-{MnMo6}-TPy, which was also further used to coordinate with porphyrin, forming supramolecular hybrid materials [63,64]. Cronin synthesized highly delocalized aromatic pyrene-grafted Anderson-type POMs, pyrene-{MnMo6}-pyrene, with nanoporous framework [65]. They also synthesized C9 alkyl ligands grafted Anderson-type hybrid materials C9-{MnMo6}-C9, through Langmuir-Blodgett deposition [66]. Moreover, a library of alkyl ligands grafted Anderson-type hybrid materials, Cx-{MnMo6}-Cx, with different chain distance (X = 4, 10, 12, 14, and 18 respectively) were also synthesized in their group using the prefunctionalization strategy [67]. Song synthesized triol-functionalized Anderson-type POMs hybrid, coumarin-{MnMo6}-coumarin [68] by using coumarincontaining triol ligand as precursor through Mo8 reconstruction protocol. Coronado synthesized 2,6-di(pyrazol-1-yl)-pyridine (bpp)-grafted Anderson-type hybrid materials bpp-{MnMo6}-bpp [69] and further coordinated with FeII gives rise to a 2D cationic network forming bimetallic MnIII–FeII hybrid complexes. Draper and co-workers synthesized Iodobenzene-grafted Anderson-type hybrid materials Iodobenzene-{MnMo6}-Iodobenzene and used as a new platform to generate a series of tunable rigid-molecular rods including thiophene-{MnMo6}-thiophene, pyrene-{MnMo6}pyrene and phenylacetylene-{MnMo6}-phenylacetylene via Sonogashira cross coupling of the corresponding alkyne derivatives [70]. Parac-Vogt synthesized chloride-grafted Anderson-type hybrid materials Cl-{MnMo6}-Cl and employ as nucleophiles for effective nucleophilic substitution reaction as a versatile post functionalization method. Using this method, several types of different nucleophiles including primary and secondary amines, carboxylates, and thiolates were efficiently coupled to Cl-{MnMo6}-Cl in high yields and purity [71].

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

R1

R

R1

R-{MnMo6}-NH2

R2 R1-{MnMo6}-R2

Asymmetric Product Yield 14%

R2 [Mo8O26]4-

R1

+ Mn(OAc)3

R2

Symmetric

NH2-{MnMo6}-NH2

Organic Funconal Ligands

Byproduct R1

R2

Fig. 8. The synthesis of asymmetrically and symmetrically triol-functionalized Anderson-type POMs hybrids via organic solvent Mo8 reconstruction protocol.

SPCOOH

Except for employing remote active amino group to afford ‘‘Functionalized-Tris Linker”, recently, another Ab initio effective synthesis strategy for triol functionalization via Cnnizzaro reaction has also been reported (Fig. 7a). Through Cnnizzaro reaction, the functionalized organic moieties including benzene and pyridine can be directly anchored on triol ligands by CAC bonding, forming stable functionalized triol ligands (Triol-benzene and Triolpyridine) [72]. Employing this strategy, Yang’s group synthesized pyridine-grafted Anderson-type hybrid materials pyridine{MnMo6}-pyridine [73], which was further served as linker to coordinate with copper forming heterometallic cluster organic frameworks (COF) as shown in Fig. 7b, indicating the linkage of different types of clusters with distinct properties is an effective method to obtain multifunctional materials, and it is possible to develop new classes of POMs-based multifunctional materials by using various transition metal substituted POMs clusters (TM-POMs) as structural building blocks. Admittedly, through organic solvent Mo8 reconstruction protocol (Fig. 4a), a series of organoalkoxylation derivatives of Anderson-type POMs hybrid materials could be synthesized. The controllability has been improved since only the l3-O atoms served as precise and specific reactive sites. However, the obvious drawback of such [Mo8O26]4
reconstruction protocol was that the reactivity of six l3-O atoms was identical. Thus both sides of an Anderson POMs cluster can be simultaneously functionalized by triol ligands. Only symmetrically triol functionalize Andersontype hybrid materials can be obtained. The extension of Mo8 reconstruction protocol to asymmetrically triol functionalized Anderson-type POMs hybrid materials is still a very important but knotty task that needed to be conquered [20]. At the early stage, Cronin developed a strategy to synthesize triol asymmetrically functionalized Mn-Anderson hybrids by adding two types of triol ligands simultaneously and isolated the asymmetric products (highest optimization 14% Yield) from the mixtures containing byproducts by monitoring crystallization and sorting the clusters with mass spectrometry [21] (Fig. 8). They further developed a chromatographic methodology to simplify this isolation [22]. Employing this synthesis strategy, pyrene and 16mercaptohexadecanoic acid (MHA) moieties could be grafted asymmetrically on the ‘‘Anderson-Tris Linker”, NH2-{MnMo6}NH2, forming triol asymmetrically functionalized Anderson-type POMs hybrids, pyridine-{MnMo6}-MHA, stamped on gold surface via Au–S bonds [74]. Furthermore, highly delocalized aromatic pyrene and aliphatic C9 alkyl chain could also be grafted asymmetrically on NH2-{MnMo6}-NH2, leading to pyrene-{MnMo6}-C9, which was found to show protein-like fiber anisotropic nanostructure architectures due to the interactions between aromatic and aliphatic moieties [75]. They further proposed a general synthetic route to symmetrical and asymmetrical triol-functionalized Anderson-type POMs with peptide chains by using NHS-

SNCOOH

BODIPY

Fig. 9. Strategy for the post-functionalization of Anderson-type POMs hybrids by using NH2-{MnMo6}-NH2 as the precursor.

Fig. 10. The extension of Mo8 reconstruction protocol toward other heteroatom such as Zn, resulting v isomer of triol-functionalized Anderson-type POMs and ZnN coordination compounds.

{MnMo6}-NHS (NHS = nhydroxysuccinimide ester) as a precursor to conjugate with pre-synthesized peptide chains. Furthermore, an unnatural amino acid with an activated C terminus and an Fmoc-protected N terminus could also be synthesized from asymmetrical triol-functionalized Anderson-type POMs precursor, NHS{MnMo6}-NR (R = 9-fluorenylmethyloxycarbonyl (Fmoc)). These methods may open the way for the synthesis of peptides and perhaps even proteins that contain POMs-based ‘‘inorganic” amino acids [76]. Additionally, alkyne and azide have been also grafted asymmetrically and symmetrically on the ‘‘Anderson-Tris Linker”, giving rise to triol-functionalized Anderson-type POMs hybrids, alkyne-{MnMo6}-NH2, azide-{MnMo6}-NH2, alkyne-{MnMo6}alkyne and alkyne-{MnMo6}-alkyne, which have been employed to afford monodisperse linear Anderson-type POMs-based oligomers with controllable sizes and structures ranged from two to five clusters through Cu-catalyzed alkyne-azide cycloaddition [77]. Another strategy for the post-functionalization of Andersontype POMs hybrids is using NH2-{MnMo6}-NH2 as precursor as shown in Fig. 9. Single side post-functionalization of Andersontype POMs, SP-{MnMo6}-NH2 (SP = spiropyran), could be obtained by controlling ratio of SPCOOH/NH2-{MnMo6}-NH2, [78]. SP-{MnMo6}-NH2 is modifiable, which can be grafted with an additional functional ligand. Therefore, asymmetrically triolfunctionalized Anderson-type POMs hybrids, SP-{MnMo6}-SN, SP{MnMo6}-BODIPY, and SP-{MnMo6}-C16 (SN = spironaphthoxazine; BODIPY = Difluoro {2-[(2H-pyrrol-2-ylidene-jN) methyl]-1H-pyr rolato-jN}boron), could be well synthesized using SP-{MnMo6}NH2 as the precursor [79–81]. Besides, Wang synthesized

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

asymmetrically triol-functionalize Anderson-type POMs hybrid materials, DBA-{MnMo6}-NH2, by grafting coumarin 3,5-bis (tetradecyloxy)benzoic acid (DBA) on the NH2-{MnMo6}-NH2 precursor [82]. A series of covalently bonded hybrid compounds composed of Anderson type polyoxometalate (POM) moiety and porphyrin moiety could also be synthesized and by the postfunctionalization of Anderson-type POMs hybrids using NH2{MnMo6}-NH2 as precursor [83]. It is obvious that central heteroatom in most of Anderson-type POMs clusters using organic solvent Mo8 reconstruction protocol is Mn atom (Fig. 8). The tunable and flexible nature of Andersontype POMs clusters cannot be completely employed and exploited. The extension to other central heteroatoms and the controllable synthesis cannot be promised due to the coordination reactivity of heteroatoms with alkoxyl ligands. Though at the early stage, Hasenknopf tried to extend the Mo8 reconstruction protocol from early transition metal, Mn and Fe, to late transition metal such as Ni, Zn as central heteroatoms [18,19], leading to isomer of symmetric triol functionalized Anderson-type POMs derivative, whose triol ligands replaced two l3-OH and one l2-O atoms in both sides of NiII/ZnII-Anderson POMs clusters as shown in Fig. 10. To distinguish from the topologic structures, the triol-functionalized Anderson POMs cluster that triol ligands replaced all the l3-OH to anchor entirely on the planar of central heteroatom octahedron is defined as d isomer. Such a new topology is defined as v isomer, v-{[RC (CH2O)3]2ZnMo6O18}4
. The special structure of v isomer indicates that under specific situation, l2-O can become reactive sites like l3-O atoms. However, such Mo8 reconstruction protocol is more accidental rather than controllable. The attempts to apply Mo8 reconstruction protocol to obtain v isomer of tris-functionalized Anderson-type POMs with remote amino group, v-NH2-{XMo6}NH2, were confirmed to be infeasible and only simple coordination compounds could be found (Fig. 10), due to the complex Mo8 reconstruction and the coordination activity nature of N atoms of tris ligands toward late transition metal (Ni and Zn, etc.) [19]. Thus, the regioselective activation of l2-O atoms may not be achieved using N-containing triol ligands during Mo8 reconstruction protocol process. Recently, Wu extend the Mo8 reconstruction protocol toward Cu as heteroatom and similar v-{[RC(CH2O)3]2CuMo6(l3OH)2O16}2
triol functionalize Anderson-type hybrid species was obtained with two remained protonated l3-O atoms as reactive sites [84]. Such l3-OH reactive sites can be further functionalized by methanol ligand forming triol and monool co-functionalized Anderson-type POMs hybrid, {[RC(CH2O)3]2CuMo6(l3OMe)2O16}2
. Using methanol instead of MeCN as solvent can directly lead to such triol and monool co-functionalized Anderson-type POMs hybrid. They also modified Mo8 reconstruction protocol by transfer Mo8 precursor in two small species [Mo2O7]2
(Mo2) that can forming an new isomer of asymmetric triol functionalized Anderson POMs derivative where one side of triol ligand is anchored on Anderson cluster in d isomeric mode while the triol ligand at other side is in v isomeric mode. This novel topology can be defined as x isomer, x-{[RC(CH2O)3]2CuMo6 (l3OH)O17}3
. Additionally, extra proton introduction can further transferred it into v isomer of triol-functionalized Anderson-type hybrids, v-{[RC(CH2O)3]2CuMo6(l3-OH)2O16}2
, indicating that for Mo8 reconstruction protocol, solvent effect, starting precursor and the proton introduction all has significant impact on the final formation of alkoxylated Anderson-type POMs hybrids (Fig. 11). However, it cannot be applied for the triol ligand with remote amino group due to the coordination activity nature of N atoms with Cu. Wu also extended this modified Mo8 reconstruction protocol to the heteroatom of Co, leading to different isomers in variety of solvents, including d isomer in DMF, d-{[CH3C (CH2O)3]2CoIIMo6O18}4
, v isomer in MeOH, v-{[CH3C(CH2O)3]2-

7

Fig. 11. The revised Mo8 reconstruction protocol for v isomer of alkoxylfunctionalized Anderson-type POMs hybrids, including v isomer and x isomer.

Fig. 12. The structural topology of parent Anderson cluster. A type Anderson POMs without protons; B type Anderson POMs without six protons in l3-O atoms.

CoIIMo6(l3-OH)2O16}2
, and x isomer in MeCN, x-{[CH3C(CH2O)3]2CoIIMo6(l3-OH)O17}3
[85]. Ritchie further developed a microwave-assisted Mo8 reconstruction protocol to synthesize novel mono-organoimido functionalized Anderson-type POMs hybrids with a pendant remote amino group, [Mo6O18NC(OCH2)3MnMo6O18(OCH2)3CNH2]5
, denoted as {Mo6}-N-{MMo6}NH2), which may serve as a useful synthon considering the well development of the post-functionalization of ‘‘Anderson-Tris Linker” as a special POM-based synthon mentioned above [86]. From the Mo8 reconstruction protocol, it is not disputed that both sides of an Anderson POMs cluster can be simultaneously functionalized by triol ligands. The selectively anchored triol ligands on Anderson POMs cluster in specific mode seemed impossible. Thus, it is of great breakthrough to develop a direct and controllable strategy for the alkoxylation of Anderson-type POMs. It should be noted that Anderson-type POMs have been historically classified in two categories: those ones without any protons in frameworks are A type and those ones with six protons are B type. Both these two types of Anderson POMs cluster have six l3-O and l2-O atoms. The l3-O atoms can be regarded as more basic atoms among these l-O atoms since, in all reported literature, protons in B type Anderson clusters are almost attached to the l3-O atoms (Fig. 12). These protonated l3-O atoms are more reactive than other Ol atoms to react with alkoxyl ligands [13]. To precisely control the reaction sites in the parent Anderson-type POMs cluster, and obtain the single-side triol-functionalized Anderson-type POMs selectively, our group first develop the direct alkoxylation route by using B type parent Anderson cluster [(XOH)6M6O18]n
as precursor and aqueous reaction media which protons can be easily and precisely introduced and controlled to created abundant

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

proton reaction environment. The direct alkoxylation of parent Anderson cluster has advantages as the following: (1) the central heteroatom is first encapsulated into the parent Anderson POMs cluster before triol functionalization. Thus, the possibility of coordinate reactivity of central heteroatom with triol ligand bearing the remote active moiety leading to the failure of the corresponding triol functionalization will be completely ruled out, and the restriction on the central heteroatoms and the remote active moiety on triol ligands would be entirely removed. (2) Regarding the mature of parent Anderson cluster synthesis which has been well reviewed by Rompel [14], the most tunable and flexible nature of Anderson POMs cluster can be completely employed and exploited to design the alkoxyl-functionalized Anderson-type POMs hybrids with desire heteroatoms and alkoxyl ligands in controllable and rational manner. It has been demonstrated that suitable protonation of l3-O can make it become l3-OH reactive site while excess protons in the aqueous reaction media will lead to the temporary passivation of l3-OH for further alkoxylation [16]. This makes the possibility to selectively anchor triol ligand on one side of Anderson POMs cluster while avoid the reaction of l3-OH on other side. According to the above discussions, single-side triolfunctionalized Anderson-type POMs, {[CH2OHC(CH2O)3]Cr(OH)3Mo6O18}3
(CH2OH-{Cr(OH)3Mo6}) has been obtained in our group for the first time [16]. Furthermore we also extend the direct parent Anderson POMs alkoxylation protocol to other triol ligands, RC (CH2OH)3 (R = CH3,C2H5,NH2,CH2OH). A series of single-side triol functionalized Anderson-type POMs, {[RC(CH2O)3]Cr(OH)3Mo6O18}3
, denoted as R-{Cr(OH)3Mo6} can also be synthesized (Fig. 4b). Interestingly, the as-prepared single-side triolfunctionalized Anderson-type POMs hybrids possess intrinsic chirality derived from the symmetry reduction by triol modification [17]. Enantiopure crystals can be obtained by adjusting the polarity of solvents to assist the spontaneous resolution occurred upon crystallization. We also incorporated other metals including the transition metal Mn, Fe and main group metal Al as central heteroatoms to afford a series of single-side triol-functionalized Anderson-type POMs, {[RC(CH2O)3]Mn(OH)3Mo6O18}3
, {[RC (CH2O)3]Fe(OH)3Mo6O18}3
and {[RC(CH2O)3]Al(OH)3Mo6O18}3
respectively [20]. Later on, Wu, Yang and Rompel group followed this direct parent Anderson POMs alkoxylation protocol to design novel single-side triol functionalized Anderson-type POMs for various applications. Wu prepared a triol-functionalized Anderson POMs with triol ligand containing carboxyl group, {[HOOCCH2NHC (CH2O)3]Al(OH)3Mo6O18}3
, denoted as COOH-{Al(OH)3Mo6} [87]. Wu’s group also employed {[NH2C(CH2O)3]Al(OH)3Mo6O18}3
, abbreviated as NH2-{Al(OH)3Mo6}, as synthon to synthesize single-side triol-functionalized Anderson POMs hybrid, Azo-{Al (OH)3Mo6} with azobenzene (Azo) moieties via the postfunctionalization strategy. The novel umbrella-like hybrid, Azo-{Al(OH)3Mo6}, displays interesting host–guest recognition via electrostatic interactions in a three component supramolecular system with a-CD and MB dye cations [88]. Yang also extended parent Anderson POMs alkoxylation protocol to prepare novel single side triol-functionalized POMs, pyridine-{Cr(OH)3Mo6} with pyridine containing triol ligands synthesized by Cnnizzaro reaction (Fig. 7b), and employed it to coordinated with metal halide cluster (MHC) systems forming an unprecedented super cluster built up from one high-nuclear MHC cation, [Cu8I6]2+, and eight pyridine{Cr(OH)3Mo6} Anderson-type anionic POMs by Cu–N bonding through a feasible step-by-step synthetic route, which is general and shows the great potential for creating a large family of novel POM–MHC-based composite functional materials [89]. Rompel further extended direct parent Anderson POMs alkoxylation protocol to other central heteroatoms including FeIII, GaIII and NiII to synthesize single-side triol-functionalized Anderson-type POMs, {[RC

R L= R = CH3, NH2, CH2OH R {[RC(CH2O)3]2XMo6O18}n-

2L

R [X(OH)6Mo6O18] n-

X=

13 Al

24 Cr

H2O Refluxing

L=

25 Mn

27 Co

26 Fe

28 Ni

31 Ga

R {[RC(CH2O)2(COO)]2XMo6O18}nFig. 13. The direct parent Anderson POMs alkoxylation protocol for symmetry-side alkoxyl-functionalized Anderson-type POMs.

(CH2O)3]X(OH)3 Mo6O18}n
(denoted as R-{X(OH)3Mo6}, X = FeIII, GaIII and NiII) respectively [90,91]. Moreover, single side triolfunctionalized tungsten Anderson-type polyoxotungstates (POTs), {[CH2OHC(CH2O) 3]Ni(OH)3W6O18}4
, denoted as CH2OH-{Ni (OH)3W6}, with NiII as central heteroatom could also be synthesized using the similar direct alkoxylation protocol [92]. The introduction of tris-ligand to Anderson POTs suggests the existence of rich tris-functionalization chemistry of POTs that will be elucidated in the future. Furthermore, our group discovered the modification mode could also be simply controlled by adjusting the ratio of the triol ligands to the parent Anderson POMs cluster and the proton remained in the aqueous media simultaneously. When the pH of the aqueous reaction media was adjusted to 7 and the ratio of the ligands to the parent Anderson POMs cluster was set to 2, symmetry-side triol-functionalized Anderson POMs hybrids, {[RC (CH2O)3]2XMo6O18}, could be obtained (Fig. 13) [91]. Besides, a novel but nonclassical tripodal ligand which has carboxyl group, RC(CH2OH)2(COOH), can also be anchored on Anderson cluster forming symmetry-side tripodal-functionalized Anderson-type POMs, {[CH3C(CH2O)2(COO)]2CrMo6O18}. It was inaccessible from Mo8 reconstruction protocol since coordinate reactive carboxyl group can coordinate with the central heteroatom and the introduced protons from aqueous solvent would transfer Mo8 to a more stable structural topology of Mo6 under acidic condition [93]. Symmetry-side triol functionalized Anderson hybrids {[CH3C(CH2O)3]2CoIIIMo6O18} could also be obtained by Wu in such direct parent Anderson POMs alkoxylation protocol [85]. By removing proton and increasing the triol ligands ratio, the yields and selectivity of symmetry-side triol functionalized Anderson hybrids could be improved. Then we also introduced extra protons to further regioselectively protonated and activated l2-O and transferred it to become l2-OH reactive sites that can be further functionalized by triol ligands leading to an unprecedented triol modification mode. A series of v isomers of single-side triol-functionalized Andersontype POMs, {[RC(CH2O)3]M(l3-OH)4Mo6O17}2
(M = Cr, Mn R = CH3, C2H5, NH2, CH2OH), with different central heteroatom and remote reactive group in triol ligands have been obtained in our group [13]. In v isomers of single-side triol-functionalized Anderson-type POMs, the triol ligand substituted one l2-OH reactive site instead of l3-OH reactive site. Proton-controlled isomer transformation between the d and v isomers of single-side triol-functionalized Anderson-type POMs was observed. This general protocol will open up a new era regarding controllable

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

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Fig. 16. The flat hexagonal and butterfly-shaped structural topology of parent Anderson POMs cluster isomer.

Fig. 14. Proton-controlled transformation cycling between d, v and w isomers of single-side alkoxyl-functionalized Anderson-type POMs.

Fig. 15. The direct Anderson POMs alkoxylation protocol for single-side and symmetry-side bis(triol)-functionalized Anderson POMs.

regioselective activation and further develop chemical reactivity of l2-O atoms in Anderson POMs cluster. Proton can serve as an optional and sensitive switch to control whether d or v isomer will be generated and the isomer transformation [13]. When the ratio of the proton to the parent Anderson POMs cluster reached 3, triol (HOCH2)3CNH2 and diol ligand (HOCH2)2(C2H5)2CNH2 can both form single-side diol-functionalized Anderson-type POMs hybrids, w-{[R1R2C(CH2O)2]XMo6O18(l3-OH)4}3
, defined as w isomer. The protonation of amino as remote reactive site plays a key role to form H bonding while one l2-O atom in the Anderson POMs cluster acts as an acceptor for NAH. . .O hydrogen bonding interaction. Such a H bonding interaction act as an anchor forcing the third foot of triol ligand pending, and thus unable to anchor on the Anderson POMs cluster. This proton-controlled protocol will lead to the controllable alkoxylation of Anderson-type POMs and obtain targeted single-side diol-functionalized Anderson-type POMs hybrids [13]. Such a diol functionalization mode not only works for some specific triol ligands but also can readily be extended to the diol ligands, which will greatly enrich the species of alkoxyl-derivatized Anderson POMs clusters [15]. Furthermore, the introduced proton may also serve as a key optional to control the formation of w, d or v isomer (Fig. 14).

Additionally, double-triol ligand was also used to obtain singleside and symmetry-side bis(triol)-functionalized Anderson POMs hybrids, {[XMo6O18(OH)3]2[(CH2O)3CCH2OCH2C(CH2O)3]}6
, {[XMo6O18(OH)3][(CH2O)3CCH2OCH2C(CH2OH)3]}3
and {[XMo6O18] [(CH2OH)3CCH2OCH2C(CH2O)3]2}3
with three modification mode respectively [94]. It can be imagined that bis(triol)-functionalized Anderson POMs clusters tend to form POM-L, POM-L-POM, L-POML, L-POM-L-POM (L = double-triol) hybrids, etc., which are much more complex than the mono(triol)-functionalized Anderson POMs cluster. Bis(triol)-functionalized Anderson POMs hybrids in these three modifications modes all hang reactive sites (l3-OH and LOH), which can serve as powerful synthon to construct POMsbased organic–inorganic hybrids (Fig. 15). From the standpoint of reactivity, single-side triol functionalized Anderson-type POMs hybrids, {[RC(CH2O)3]Cr(OH)3Mo6O18}3
, having three l3-OH reactive sites on the other side, can be further functionalized with other alkoxyl ligands. Recently, Song and his co-workers developed a step-by-step hydrothermal protocol in aqueous solution to obtain asymmetric-side triol-functionalized Anderson-type POMs hybrids, {[R1C(CH2O)3]XMo6O18[(OCH2)3CR2]}3
[95]. Interestingly, we found that asymmetric-side triolfunctionalized Anderson-type POMs hybrids, {[R1C(CH2O)3]MMo6 O18[(OCH2)3CR2]}3
, can be efficiently synthesized in high purity and good yields by using EtOH as solvent [20]. Besides, the asprepared asymmetric-side triol functionalized Anderson POMs derivatives possess intrinsic chirality due to the symmetry reduction by such stepwise triol modification. The synthesis strategy is general and will remove a significant obstacle to exploiting POM-based asymmetric functionalized hybrids as functional synthons, including the design of novel chiral POMs hybrids. Except for the well-known flat hexagonal structural topology of Anderson POMs clusters, there also exists another butterfly-shaped structural topology as [Mo7O24]6
(Mo7). The nomenclature of Anderson-type anions with A and B type is to distinguish those ones without any protons in frameworks (A type) from those with six protons (B type), while nomenclature of a and b isomer is to distinguish the dominated flat hexagonal topology (a isomer) from rare butterfly-shaped topology (b isomer) (Fig. 16). However, compared with the big ‘‘Anderson-Evans” POMs family with flat topology, the b isomers are rather rare. Up to date, only two isolated b isomers with the high-valence central heteroatoms, [Pt(OH)4Mo6O20]4
and [Sb(OH)2Mo6O22]5
, were discovered [96,97]. Attempts to seek for the butterfly-shaped b isomers with the low-valence metal as central heteroatoms have still never been achieved yet. Recently, our group demonstrated that triol ligands can stabilize such butterfly-shaped structural topology units. We employ the B type a parent Anderson POMs, a-[X(OH)6Mo6O24], to react with triol ligand in hot DMF solvent under N2 atmosphere, forming symmetry-side triol-functionalized butterfly-shaped b

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Fig. 17. The direct parent Anderson alkoxylation protocol for symmetry-side triolfunctionalized butterfly-shaped b isomers of Anderson POMs hybrids.

Fig. 18. The alkoxylation chemistry of Lindqvist-type [V6O19]8
(V6) cluster from structural topology analysis.

isomers of Anderson POMs (Fig. 17), b-{[RC(CH2O)3]2XMo6O18}3
, which represented the first triol-functionalized butterfly-shaped b isomers of Anderson-type POMs clusters in the POM family [98]. It should be noted that besides triol ligand, extra introduced molybdenum source of [MoO4]2
can also stabilized B type b-[X (OH)6Mo6O18]3
parent Anderson cluster and forming Waugh type [XMo9O32]6
cluster with atomically dispersed Co3+, which has been proved to serve as photosensitizers for photocatalytic oxygen molecule activation [99]. This is the first time to further elucidate the structural topology relationship between Anderson and Waugh type cluster. It is a general and controllable protocol to afford novel symmetry-side triol-functionalized b isomers of the Anderson POMs with butterfly-shaped structural topology.

2.2. The alkoxylation chemistry of Lindqvist-type POMs The structural topology of Lindqvist-type POM clusters that can be employed for alkoxylation are relatively immobilized compared with Anderson POMs clusters. For alkoxylation of lindqvist-type POMs, alkoxyl ligands are mainly grafted on the Lindqvist polyoxovanadate (POV), [V6O19]8
(V6). From the structural topology analysis, the Lindqvist-type V6 cluster derivatives can be divided into two parts: (1) the monoalcohol ligands directly anchor on the l2-O reactive sties of V6; (2) the triol ligands tend to anchor on the l2-O reactive sties of trivanadates (V3) to form {[RC(CH2O)3]V3O9} synthon and further to assemble the triol-functionalized V6 [100] (Fig. 18). For alkoxyl derivatives of V6, monoalcohol functionalized V6 are one of the major species due to the high reactivity of monoalcohol ligands. The formation of [V6O12(OMe)7]
from [H3V10O28]3
(V10) in refluxing methanol indicates that V6 core could be stabilized by the incorporation of monodentate alkoxyl ligands, which was first reported by Hill in 1993 [101]. Kessler employed EtOH instead of MeOH to obtain monool-

functionalized V6 hybrids, [V6O7(OEt)12], and all the l2-O atoms of V6 were replaced by EtOH ligands [102]. Hartl reported monool-functionalized V6 hybrids with mixed V valence, [VIV 4 V2 O7(OCH3)12], which possessed redox-active property under solvothermal conditions by reaction of [VO(OtBu)3] and methanol [103]. Temperature dependence of the EPR spectrum further indicates an electron transfer behavior between VIV and VV in V [VIV 4 V2 O7(OCH3)12] molecule [104]. Their highly symmetrical molecular structures make them particularly interesting as model compounds for the investigation of intervalence charge transfer and electron delocalization in the hexanuclear core [105]. The VIV to VV unpaired electron transfers together with isotropic Heisenberg exchange were discovered to determine the total spin states compoV sition and the intracluster dynamics in such mixed-valence [VIV 4 V2 O7(OEt)12] [106]. Monool-functionalized mixed-valenced V6 V hybrid, [VIV 4 V2 O8(OCH3)11] was also synthesized, and all but one Ol ligands in this molecule are substituted by methoxyl, which could serve as exceptional model compounds for the investigation of magnetic exchange interactions in spin frustrated systems. These cluster series were proved of great interest for research and applications in materials science as well as in homogeneous and heterogeneous catalysis research [107]. Hayashi synthesized monool-functionalized V6 hybrid, [V6O13 (OCH3)6]2–, which was further utilized as synthon for the [V6 + V6] coupling reaction to form the bowl-type dodecavanadates, [V12O42]4
[108]. Adach synthesized monool-functionalized V 2
V6 hybrid, [VIV , which was served as anion [X] of 2 V4 O11(OCH3)8] cobalt (II) scorpionate-like complexes, {[CoII(NCS)LS]n+[X]n
} [109]. Hu synthesized monool-functionalized V6 hybrids, [V6O13(OCH3)6]2
, and [V6O11(OCH3)8] and employ Pd as cation to assemble alkoxohexavanadate-based Pd-Polyoxovanadates hybrid material through the electrostatic interactions [110]. Then V [V6O13(OCH3)6]2–, [VIV 2 V4 O12(OCH3)7] and [V6O11(OCH3)8] were assemble with Cu and Co cations through electrostatic interactions forming alkoxohexavanadate-based Cu- and Co-Polyoxovanadates hybrid materials [111]. Triol-functionalization of V6 (V6-Tris) has also attracted increasingly attention and a series of V6-Tris bearing different functional organic groups can now be prepared owing to the efforts by Zubieta [112,113], Muller [100], Hill [101,114] and Hasenknopf [62], etc. Compared to the Anderson-type POMs mentioned above, the triol-functionalization of V6 is still less explored due to some difficulties, such as the frequent redox side reactions and particularly low yield. The triol-functionalization of V6 can also be described by two main strategies: direct functionalization and post-functionalization [7]. For direct functionalization, the V6-Tris products are mainly obtained by using [Bu4N]3V5O14 (V5) [115], NaVO3 or NH4VO3 [116,117], V10 [118,119] as precursors to react with triol ligands. Besides, Stepnicka and co-workers first prepared an triol ligand, amide FcCONHC (CH2OH)3 (Fc = ferrocenyl), from fluorocarbonyl ferrocene and tris(hydroxymethyl)methylamine, and then reacted with V10 in N,N-dimethylacetamide forming a bis(Tris) capped hexavanadate anion cluster bearing two ferrocenyl groups, {[FcC (O)NHC(CH2O)3]2V6O13}2
[120]. Hasenknopf reported a bis(Tris)functionalized V6, TBA2[V6O13{(OCH2)3CNHC(O)-3-C5H4N}2], denoted as pyridyl-{V6}-pyridyl, with two pyridyl functional groups, which was prepared by the reaction of pyridyl-triol ligands and V10 (Fig. 19) [121]. The as-prepared pyridyl-{V6}-pyridyl was further combined with PdCl2 to form a supramolecular triangle. Furthermore, POMs-porphyrin copolymeric films could be obtained by the electrooxidation of 5,15-ditolyl porphyrin and zinc-b-octaethyl porphyrin in the presence of pyridyl-{V6}pyridyl [122]. Primary alkyl amines are functional groups that could participate in many organic reactions, like alkylation, acylation,

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

Fig. 19. Synthesis for pyridyl-triol ligands and bis(tris)-functionalized Lindqvist V6, pyridyl-{V6}-pyridyl. Figure was reproduced from Ref. [121] with permission of the copyright holders.

Fig. 20. The synthesis procedure for [V6O13{(OCH2)3CNH2}2]2
. V, orange; C, black; O, red; N, blue; H, light gray. Figure was reproduced from Ref. [124] with permission of the copyright holders.

sulfonation, diazotization, imine formation, etc. As one of the most important triol ligands with primary alkyl amine group, Tris (hydroxymethyl) aminomethane (Tris) has been successfully incorporated into Anderson-type POMs by Hasenknopf and our group, et al. [76–80]. However, it is difficult to obtain the bis (tris)-functionalized V6 hybrid, {V6O13[(OCH2)3CNH2]2}2
(denoted as NH2-{V6}-NH2) with desired yields due to the strong oxidation

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Fig. 21. The synthesis procedure for [V6O13{(OCH2)3COOH}2]2
and the POMsbased 2D nanosheet material. V, orange; Zn, light blue; C, black; O, red; N, blue; H, light gray. Figure was reproduced from Ref. [129] with permission of the copyright holders.

ability of vanadium [123]. Recently, our group developed an effective synthetic strategy for the amino-containing organic derivative of V6, NH2-{V6}-NH2, by introducing 4-anisaldehyde as an aminoprotecting ligand (Fig. 20) [124]. Hill and co-workers reported the preparation of a pyridineterminated bis(trialkoxo)-functionalized V6, [V6O13{(OCH2)3C(4CONHC5H4N)}2]2
(denoted as pyridine-{V6}-pyridine), by the direct functionalization of V10 with pyridine-containing ligands and the assembly of pyridine-{V6}-pyridine with divalent transition-metal cations, M = MnII, CoII, NiII, or ZnII, to form POMbased coordination polymers was also investigated [114]. Multitriol-functionalization of V6 can also be synthesized by the direct functionalization protocol using vanadium oxide precursors, like V2O3, KVO3 and NaVO3, to react with triol ligands, RC(CH2OH)3 (R = CH3, CH2CH3), forming a series of polyalkoxyoxovanadium clusters, [V6O7(OH)3{(OCH2)3C CH3}3]2
(denoted as {V6}-3CH3), [V6O7{(OCH2)3CCH2CH3}4]2
(denoted as {V6}-4CCH2CH3), and [V6O6F(OH)3{(OCH2)3CCH3}3]
(denoted as {V6F}-3CCH3) [113]. Recently, a hexavanadate V6 derivative with three amino groups V attached onto the POV cluster with mixed valence, [VIV 3 V3 O10{NH2C(CH2O)3}3]2
(denoted as {V6}-3NH2), was also synthesized by Wei, Wu and Xiao et al. They further demonstrated that {V6}-3NH2 could be served as building block for further functionalization [125]. Hill and Han reported an interesting V6-based open framework coordination network with large pores, Tb[V6O13{(OCH2)3C(NH2CH2C6H4-4-CO2)}{(OCH2)3C-(NHC H2C6H4-4-CO2)}2] (Tb1) [126]. The acid-tris ligand, 4-HOOCC6H4)CH2NHC(CH2OH)3 (acid tris), was first synthesized by the reaction of chloromethyl benzoic acid and tris (hydroxymethyl) amino methane. And then acid tris further reacted with V10 to yield [V6O13{(OCH2)3C (NHCH2C6H4-4-CO2)}2]4
(1), which was mixed with Tb(NO3) and Co(NO3) to generate Tb1 and Co1 [126,127]. Considering the NH2 group of Tris can combine with an organic functional group via aldimine condensation reaction (Fig. 6B), while the Tris(hydroxymethyl) groups can be directly anchored on the surface of POM units. Li and co-workers used Tris ligands to connect with luminescent pyrene groups to construct

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pyrene-Tris ligand, which was further anchored on the Lindqvist V6 unit, forming a new POMs-based organic–inorganic hybrid, [TBA]2[V6O13{(OCH2)3CNH-CH2-C16H9}2], denoted as pyrene-{V6}pyrene [128]. Recently, our group reported a facile bottom-up protocol to obtain carboxyl-triol-functionalized V6, [V6O13{(OCH2)3CCOOH}2]2
(denoted as COOH-{V6}-COOH), by using [V6O13{(OCH2)3CCH2OH}2]2
(denoted as OH-{V6}-OH) as precursor [129], which could be further formed POMs-based 2D material by combination of Zn2+ (Fig. 21). Furthermore, COOH-{V6}-COOH could also be used as precursor to further functionalized with various alcohol using N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquino line (EEDQ) as activating agent [130]. The post-functionalization is also a promising strategy to construct complicated POM-based hybrids with multiple functional groups, which firstly introduces a bridging active ligand into the POM cluster to obtain a POMs-based hybrid precursor that can be further modified (also called step-by-step functionalization). This strategy has already been successfully applied to the post functionalization of Anderson-type POMs [32–38]. Recently, Hill and Liu’ groups have successfully prepared two triolfunctionalized V6 derivatives with one or two covalently linked pyrene fluorescent groups, [V6O13{(OCH2)3CNH(CO)CH2CH2CH2C16H9)}{(OCH2)3CNH2}]2
(denoted as pyrene-CH2-NH-{V6}-NH2) and [V6O13{(OCH2)3-C (NH(CO)CH2CH2CH2C16H9)}2]2
(denoted as pyrene-CH2-NH-{V6}-NH-CH2-pyrene) using NH2-{V6}-NH2 as precursor [123]. Furthermore, our group discovered that OH{V6}-OH could be easily esterified with acid anhydrides using 4(N,N-dimethylamino) pyridine (DMAP) as a catalyst [131]. Thus, a series of hydrophobic long chain alkyl groups could be functionalized on Lindqvist V6 POMs. Subsequently, Wei and Liu employed such OH-{V6}-OH as precursor to prepare a novel V6-based organic–inorganic amphiphilic molecule, [V6O13{(OCH2)3CCH2OOC(CH2)16CH3}2]2
(denoted as C18-{V6}-C18) and studied its counterion dependent fluorescent properties [132]. Furthermore, single side V6-based organic–inorganic amphiphilic molecule, C18-{V6}, could also be obtained by controlling molar ratio of stearic acid/hexavanadate [133]. In 2014, Liu and Wei further synthesized long-alkyl-chain-functionalized POM hybrid molecules, [V6O13{(OCH2)3CNH2(OCH2)3CNHCH2C6H4C OOC16H33}2]2
(C16Ar-{V6}-Ar-C16) via post functionalization of NH2-{V6}-NH2 with cetyl 4-(chloromethyl) benzoate [134]. Wei and Wu et al. recently reported a step-by-step strategy to construct Lindqvist V6 POMs-based organic–inorganic hybrids [135]. Firstly, carboxyl-containing V6 derivatives, COOH-{V6}COOH, was prepared mixing the TRIS-V6-OH and succinic anhydride in an acetonitrile solution. And then the COOH-{V6}-COOH was further varied aliphatic amines, forming [V6O13{(OCH2)3CCH2O OC(CH2)2COOR}2]2
(denoted as R-COO-{V6}-COO-R). Following such carboxyl-generated protocol, they further synthesized a series of carboxyl-containing V6-based building blocks, which could be utilized to build more complex structures [136]. Employing direct-functionalization and the postfunctionalization, a series of alkoxyl-functionalized Lindqvist POMs could be successfully synthesized. Furthermore, both NH2{V6}-NH2, OH-{V6}-OH, and COOH-{V6}-COOH are powerful synthons for further functionalized and thus giving rise to more POMs-based organic–inorganic hybrids with functional organic moieties. However, it will be of interests to extend the alkoxylation of mixed-metal POMs clusters.

2.3. The alkoxylation chemistry of Dawson-type POMs The alkoxylation of Dawson-type POMs is relatively rare compared to that of Anderson-type POMs clusters and the Lindqvist POMs clusters, and most of the alkoxyl ligands are anchored on

Fig. 22. Calculated structures of the three possible conformations that can be generated by rotation around the C3–C4 bonds for both the trans dumbbell (Znfree) and cis dumbbell (Zn complex). Figure was reproduced from Ref. [150] with permission of the copyright holders.

the {V3}-capped Dawson POMs cluster, [H4P2V3W15O62]5
(denoted as D-V3), which is redox and catalytically active [1–8]. Hill and coworkers first reported the methanol functionalization of D-V3, forming trimethyl D-V3, [H(OCH3)3P2V3W15O59]5
. Furthermore, two dendrimers each with four tris(hydroxymethyl) termini were anchored on the {V3}-cap, giving rise to two novel dendritic tetra(POMs) molecules. It can be inferred that given more vanadium atoms in Dawson POMs, and various tris ligands, it is possible to obtain POM-bound polymers with catalytic oxidation activity [137]. Subsequently, Cronin employed a series of triol ligands, RC(CH2OH)3 (R = NH2, NO2, CH3) to functionalize the D-V3, forming [RC(CH2O)3P2V3W15O59]6
, whose solution supramolecular assembly behaviors were also investigated [138]. Furthermore, they synthesized a series of nanometer-sized organic-organic hybrid POMs clusters by the direct functionalization on D-V3 with a series of tris(hydroxymethyl) aminomethane (TRIS)-based linear (bis(TRIS)) and triangular (tris-(TRIS)) ligands prepared beforehand [139]. Such bis(TRIS)-functionalized D-V3 with different length were synthesized by the similar protocol, whose amphiphilic properties were further studied by Cronin and Liu et al. [140]. They also synthesized a tris-functionalized D-V3 with long alkyl chain [P2V3W15O59(OCH2)3C NHCOC15H31]6
and its concentration-, polarity-, counterion-, and pH-dependent selfassembly behaviors in solution were further investigated [141]. Hasenknopf reported a new functionalization of D-V3 POMs with amide derivatives of 2-amino-2-ethyl-1,3-propanediol, forming [P2V3W15O59{(OCH2)2-C(Et)NHCOCH3}]5
, representing the first example of insertion of amides into a POM [142]. Wang and co-workers reported a triol-functionalized D-V3 with a polystyrene (PS) chain and poly(ethylene glycol) (PEG) chain which were synthesized via atom transfer radical polymerization (ATRP) and amidation reaction respectively [143,144]. They also reported a new approach to create linear poly(POMs) that combine the advantages of polymers and POM clusters [145]. Hasenknopf and Ruhlmann further reported an electrocopolymerization strategy for Dawson POMs-porphyrin copolymers [146–148]. Dawson-type polyoxophosphovanado-tungstate bearing two pyridyl groups was synthesized first and then applied to combine with zinc octaethylporphyrin (ZnOEP) through electro-oxidation. Covalently functionalized porphyrin-POMs,

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J. Zhang et al. / Coordination Chemistry Reviews xxx (2017) xxx–xxx

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Fig. 23. Functionalization of the Dawson phosphovanadotungstate with a bipyridine ligand, and post-functionalization with a rhenium complex [151].

Fig. 24. Grafting scheme of triol ligands to the Ni6PW9 cluster. Ethylenediamine molecules and acetic groups are omitted for clarity. WO6: blue, NiO6/NiO4N2: green, PO4: pink. Figure was reproduced from Ref. [154] with permission of the copyright holders.

Fig. 25. Ball-and-stick views of representative POV-based carboxylate SBUs: (a) {V3};11 (b) {V4Cl};12 (c) {V5Cl};13 (d) {V6S}, in this work. Color code: V, light blue; O, red; C, gray; Cl, yellow; S, orange; H, white. Figure was reproduced from Ref. [156] with permission of the copyright holders.

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[P2V3W15O59{(OCH2)3CNHCO(ZnTPP)}]6
, could also be prepared by anchoring porphyrin-triol ligand on the D-V3 [64,149]. Liu and Cronin also prepared a series of dumbbell-shaped polyoxometalate–organic hybrid molecules comprising two Dawsontype polyoxometalates linked by a 2,20 -bipyridine unit, which can be reversibly transformed into the cis-dumbbell through coordination upon the addition of ZnCl2 into a DMSO solution (Fig. 22) [150]. In 2014, Hasenknopf reported a triol-functionalized D-V3 with one remote bipyridine coordination site, which was further with the neutral {Re(CO)3Br} moiety (Fig. 23) [151]. Recently, Pradeep and co-workers prepared a new polyoxometalate (POM)based hybrid monomer, [P2V3W15O59{(OCH2)3CNHCO (CH3) C@CH2}]6
, which was further grafted with polymerizable organic units to generate POMs-containing hybrid polymers [152]. To understanding of the photochemical reactivity of coumarinfunctionalized POMs, Song prepared a triol-functionalized D-V3 with coumarin-containing triol ligands, [C12H9O4NHC(CH2 O)3P2W15V3O59]6
[153]. 2.4. The alkoxylation chemistry of other POMs Additionally to alkoxylation of Anerson POMs, Lindqvist POMs and Dawson POMs, some recent alkoxyl-functionalization of other POMs cluster has also been reported. For example, Yang reported a series of new POMs, [Ni(l3-OH)3(H2O)6(en)3(B-a-PW9O34)] (en = ethylenediamine), capped by different triol ligands and amino acid (Fig. 24) [154,155]. Recently, Wang and Su reported a series of unprecedented alkoxyl-functionalized POV-based metal–organic polyhedral (Fig. 25) [156]. Interestingly, unprecedented Anderson-like alkoxyl-functionalized POV, [V6O6(OCH3)9(l6-SO4)(COO)3]2
polyanion cluster could also be obtained, as shown in Fig. 25d. Kuznetsov reported a series of novel cluster compounds with molybdenum in a low valence state, [Mo4Cl4O2(OCH3)6(CH3OH)4], [Mo3Cl3(OCH3)7(CH3OH)3], [Mo4Cl4(OCH3)10(CH3OH)2], [Mo4Cl3O (OCH3)9(CH3OH)3], [Mo4Cl2(OCH3)12(CH3OH)2] and [Mo6Cl4O6(O CH3)10(CH3OH)2] by using [Mo+4 2 Cl4(OCH3)4 (CH3OH)2] as precursor to react with CH3OH and stoichiometric amounts of magnesium methoxide, leading to disproportionation [157]. Besides, Khan also reported a new POV cluster, [VIV 6 O6{(OCH2CH2)2N(CH2CH2OH)}6], n+ and a Anderson-like POV, [XVIV 6 O6{(OCH2CH2)2N(CH2CH2OH)}6] , (X = Li, Na, Mg, Mn, Fe, Co, Ni), by the reaction of Tris and [NH4]6{V10} in MeCN and ethanol solution [158]. Additionally, novel organically functionalized Weakley-type POMs, [Ln{Mo5O13(OMe)4NNC6H4-p-NO2}2]3
{LnIII = Tb, Dy, Ho, Er, Yb, Nd} with eight functionalizd MeOH ligands [159,160]. Moreover, alkoxylation of Ti-containing POMs, [Ti2O(OEt)8(EtOH)EuIIICl]2, could also be prepared by solvothermal reaction of Ti(OEt)4 with EuCl3 in dry ethanol [161]. 3. Applications The rational design and synthesis of the alkoxyl-functionalized POMs-based organic–inorganic hybrids in molecular and material sciences lay a foundation for their applications in catalysis, biomedicine, energy, and functional materials. Suitable alkoxylation can stabilize the POMs structures. Adequate alkoxylation of POMs with suitable functional alkoxyl moieties leads to organic–inorganic hybrid systems that feature the combined intrinsic properties of POMs and organic functional moieties, as well as new properties derived from the combination of these two components. 3.1. Catalysis In recent years, the dominative reports on alkoxylfunctionalized POMs catalysts have been related to chemical

oxidation but the reports are not limited to this. As for oxidation reactions of organic compounds, oxidation desulfurization receives the most attention. Although reviews concerning POMs catalysis are available [1–3], herein, we mainly focus on the advances of alkoxyl-functionalized POMs catalysts in the past 5 years. Hu’s group made four alkoxyl-functionalized V6-based Pd-POVs catalysts, including [Pd(dpa)(acac)]2[V6O13(OMe)6] (1), [Pd(dpa) (acac)]2[V6O11(OMe)8] (2), [Pd(dpa)(acac)]2[V6O11(OMe)8]H2O (3), and [Pd(DMAP)2(acac)]2[V6O11(OMe)8]H2O (4) (dpa = 2,20 -dip yridine amine; DMAP = 4-dimethylaminopyridine; acac = acetyla cetone), which exhibit excellent heterogeneous catalytic performance in the oxidation of benzyl-alkanes to the corresponding ketones with high conversion and selectivity (conv. up to 99%, selec. up to 99%) in the presence of t-butylhydroperoxide as oxidant (Fig. 26a) [110]. They further synthesized a series alkoxyl-functionalized Cu(Co)-POV catalysts, [Cu(dpa)(acac)(H2O)]2[V6O13(OMe)6] (1), [Cu(phen)(acac) (MeOH)]2[V6O13 (OMe)6] (2), [Co(dpa)-(acac)2]2[V6O13(OMe) 6]2MeOH (3), V [Co(phen)(acac)2]2[V6O13(OMe)6] (4), [Cu-(dpa) (acac)]2[VIV 2 V4 O12 IV V (OMe)7] (5), [Cu(dpa)(acac)(MeO H)]2[V2 V4 O11(OMe)8] (6) (phen = 1,10-phenanthroline), which were found to exhibit excellent heterogeneous catalytic performance in oxidative desulfurization and CEES ((2-chloroethyl) ethyl sulfide, a sulfur mustard simulant) in the presence of H2O2 as oxidant. Furthermore, compound 6 showed the highest catalytic property with conv. of DBT (dibenzothiophene) up to 100% in 6 h; conv. of CEES reached 100% and selectivity of CEESO ((2-chloroethyl) ethyl sulfoxide) up to 85% after 4 h, which can also be reused without losing its activity (Fig. 26b) [111]. Besides, heterogeneous aerobic oxidation catalyst based on redox-active POV cluster with three-dimensional openframework were also reported, which is so sufficiently robust that the pore solvent molecules can be removed and catalyze the sulfoxidation by peroxides [128] Furthermore, amphiphilic triolfunctionalized hexavanadates were also proven to be highly efficient emulsion catalysts with high stability for deep oxidation desulfurization [133]. By adjusting the hydrophobic, electrostatic, and hydrogen bonding interactions, the assembly of 1D nanobelt of triol-functionalizd hexavanadates could be achieved, which were highly efficient heterogeneous catalysts for oxidation of organic sulfides, because of their nanoscale thickness and the exposure of most of the redox active hexavanadates onto the surface [134]. Recently, our group also provided a new strategy for the preparation of a 2D POMs nanosheet nanomaterial using COOH{V6}-COOH as precursor to combine with Zn2+, which exhibited highly catalytic activity for the aerobic oxidation of PrSH to its corresponding disulfide under very mild conditions [129]. Apart from oxidation desulfurization of triol-functionalized POV clusters, triol-functionalized Anderson-type POMs copolymer materials that composed of pyridine-{MnMo6}-pyridine with porphyrin by electropolymerization, were discovered to possess photocatalytic property of reduction of AgI in visible light [61] and reversible redox processes [62]. Additionally, Chen et al. reported two novel Anderson-type POMs built-in conjugated microporous polymers (CMPs), Bn-Anderson-CMP and Th-Anderson-CMP prepared via the Sonogashira-Hagihara cross-coupling of 1,3,5-triethynylbenzene and tetrabromo-bifunctionalized Anderson-type POMs, including (TBA)3{MnMo6O18[(OCH2)3CNH (C7H3Br2O)]2} (1) and (TBA)3{MnMo6O18[(OCH2)3CNH(C5HBr2OS)]2} (2), which exhibited outstanding heterogeneous photocatalytic activities toward degrading organic dyes in water [162]. More importantly, employing tris-functionalized Andersontype POMs, {[NH2C(CH2O)3]Fe (OH)3Mo6O18}3
, abbreviated as NH2-{Fe(OH)3Mo6} as the oxidation catalyst, selective and effective aerobic oxidation of diverse functionalized aldehydes to the corresponding carboxylic acids in aqueous media were achieved. Its operational simplicity, gram-scale oxidation, and good cycle

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Fig. 26. Monoalcohol-functionalized V6-based Pd-POVs hybrids as heterogeneous catalysts for selective oxidation of (a) benzyl-alkanes and (b) DBT and CEES. Figure was reproduced from Ref. [110,111] with permission of the copyright holders.

NH2-{Fe(OH)3Mo6} 0.1 mol% 25 mmol

Na2CO3, 2mL H2O 1 atm O2, 50 oC

Yield: 91%

Fig. 27. The aerobic oxidation of aldehydes to the corresponding carboxylic acids in aqueous media employing NH2-{Fe(OH)3Mo6} as catalyst. Figure was reproduced from Ref. [163] with permission of the copyright holders.

{MnMo6}-pyrene, which has been used for noncovalent functionalization of carbon nanotubes (CNTs) to form Pyrene-Anderson-CNT nanocomposites through p-p interactions. They further employed the as-prepared nanocomposites to serve as anode material for lithium-ion batteries, exhibiting high discharge capacity of 665.3 mAhg
1 for up to 100 cycles at the current density of 0.5 mAcm
2 (Fig. 28) [165]. They also covalently attached NH2-{MnMo6}-NH2 to single-walled carbon nanotubes (SWCNT) by amide bonds, giving rise to SWNT/Anderson nanocomposite, which were further employed as electrode materials in lithium ion batteries that displays improved performance as an anode material compared with the individual components [37]. Streb and his co-workers synthesized bipyridine grafted Anderson-type hybrid materials bipyridine-{MnMo6}-bipyridine and further coordinated with iridium photosensitizer forming pho tosensitizer-bipyridine-{MnMo6}-bipyridine dyads catalyst for visible-light-driven hydrogen evolution reaction (HER) [51]. Methacrylic anhydride (MA) was also selected to anchor on NH2{MnMo6}-NH2 and the as-prepared MA-{MnMo6}-MA was served as special monomer to form POM-based MAPDST-MAPOM hybrid polymer, possessing photoresponse properties by fabricating an ITO/MAPDST-MAPOM/Al device which showed improved I–V characteristics [46].

3.3. Biomedicine

Fig. 28. The representation of the noncovalent functionalization of CNTs by pyrene{MnMo6}-pyrene clusters. Figure was reproduced from Ref. [165] with permission of the copyright holders.

performance make this new methodology environmentally benign and cost effective (Fig. 27) [163]. 3.2. Energy Recent studies have demonstrated that POMs-based materials, especially their composites with carbon nanotubes and graphene, have shown great potential to meet the challenges in energy storage and conversion as well as other cutting-edge technologies [164]. Herein, we mainly focus on the recent advance of alkoxylfunctionalized POMs-based materials. Recently, Song and Streb’s groups prepared a triolfunctionalized Anderson-type POMs with pyrene group, pyrene-

The previous studies have demonstrated that POMs have potential and inexpensive applications in biomedicine serving as inorganic drugs with attractive anti-tumor, -viral, and -bacterial activities [166–168,41,169]. Recently, Wang’s group reported a new approach to create POMs-based hybrid drugs potentially for cancer therapeutics [41]. They used NH2-{MnMo6}-NH2 as linker to attach the bioactive ligands via simple amidation reaction, forming POM-biomolecule conjugates. The cytotoxicity study showed that rationally selected ligands with cancer-cell targeting ability on POM-biomolecule conjugates can enhanced their anti-tumor activities and selectivity, which open a new route to develop novel POM-biomolecule hybrid drugs with the potential synergistic effect: increased bioactivity and lower side effect (Fig. 29). They even synthesized maleimide-grafted Anderson-type hybrid materials maleimide-{MnMo6}-maleimide and employ it in metal-free Diels–Alder click reactions under mild conditions with high coupling efficiencies to further convenient diverse biological active moieties attachment [42].

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Fig. 29. Conjugation of a Tris-modified POM cluster (Tris-POM-Tris) with several biological and organic molecules resulting in CA-POM-CA, DHCA-POM-DHCA, CHOL-POMCHOL and GAL-POM-GAL conjugates and AA-POM-AA hybrid. CA: cholic acid; DHCA: dehydrocholic acid; CHOL: O-succinyl-cholesterol; GAL: 6-O-(3-carboxypropanoyl)1,2:3,4-di-O-isopropylidene-b-D-galactopyranose; AA: adipic acid. Insert is the structure of Tris-POM-Tris in which three countercations (TBA) are not shown. Figure was reproduced from Ref. [41] with permission of the copyright holders.

Pradeep also synthesized a series of alkoxyl-functionalized Anderson-type POMs hybrid materials, Ar-{MnMo6}-Ar, with various aromatic organic moieties, such as naphthalen, quinoline and carbazole. The genotoxic effects of these Ar-{MnMo6}-Ar hybrid materials were evaluated by studying their effects on Allium cepa cells [47]. Rompel prepared single- and double-sided alkoxylfunctionalized Anderson-type POMs hybrid with GaIII and FeIII positioned as central heteroatoms, CH2OH-{Ga(OH)3Mo6}, NH2{Ga(OH)3Mo6}, CH2OH-{Fe(OH)3Mo6}, and NH2-{Fe(OH)3Mo6}, whose hydrolytic stability and charge inversions on the charged surface of BSA were investigated. They showed that CH2OH-{Ga (OH)3Mo6} has the highest negative surface charge induced charge inversions on the positively charged surface of bovine serum albumin (BSA) protein [90]. In 2016, they also discovered that the CH2OH-{Ni(OH)3W6} could also induce charge inversions on the surface of human serum albumin (HSA) protein [91].

(c) 3.4. Functional materials

1 In recent years, a lot of POMs-based functional materials, especially alkoxyl-functionalized POMs with novel properties of selfassembly, photoresponsive, photochromic, electrochromism and fluorescent, etc. have been well developed. Regarding the alkoxyl-functionalized POMs hybrid materials have remote active groups for hydrogen bonding host–guest interactions and electrostatic interactions, like OH, NH2, COOH or alkyl chains, recent reports demonstrated that these alkoxylfunctionalized POMs possessed dynamic features under light, thermal, chemical or other kinds of stimulus. Wu’s group discovered that alkoxyl-functionalized Anderson-type POMs hybrid, Azo{MnMo6}-Azo possessed photoresponsive smart self-assembly behavior under photoirradiation with the control of reversible destruction and rebuilding of hydrogen bonds [30]. Furthermore, they also found that self-crosslinking-driven chiral self-assembly of such Azo-{MnMo6}-Azo with b-Cyclodextrin could be achieved by the electrostatic and host–guest interactions [51].

2 3 4 Fig. 30. The anion cluster structure of (a), SP-{MnMo6}-SN and (b) SN-{MnMo6}-SN; (c) Photographs of powders of (1) SP-{MnMo6}-SN, (2) SN-{MnMo6}-SN, (3) SN-Tris and (4) (SP)3-SN-{MnMo6}-SN at different times (in min) during the coloration process under 365 nm UV irradiation (left), and the fading process under ambient light at room temperature (right). Let us notice that before UV irradiation, the pinkish color of SP-{MnMo6}-SN is attributed to a small amount of the opened ‘‘merocyanine” form of the SP group. Figure was reproduced from Ref. [81] with permission of the copyright holders.

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Fig. 31. Z-scan data curves for porphyrin grafted triol-functionalized Anderson-type POMs hybrid materials in DMF under open-aperture at 532 nm. Stars indicate the measured data, and the solid lines represent the theoretical fit. Figure was reproduced from Ref. [83] with permission of the copyright holders.

Hasenknopf and Ruhlmann prepared two porphyrin-containing POMs, porphyrin-{MnMo6}-porphyrin and porphyrin-D-V3, which showed independent redox processes for the organic and inorganic parts without electron transfer in the ground state. Photophysical studies reveal an electron transfer from the excited porphyrin to the Dawson POMs in porphyrin-D-V3, but not to the Anderson POMs in porphyrin-{MnMo6}-porphyrin [64]. Moreover, POMporphyrin copolymeric films were also found to exhibit significant photocurrent response [148,149]. In 2014, Song synthesized coumarin-grafted Anderson-type hybrid materials coumarin-{MnMo6}-coumarin with photosensitive property of reversible light-driven polymerization [68]. Mialane and co-workers reported BODIPY grafted asymmetrically and symmetrically triol-functionalized Anderson-type POMs hybrid materials that exhibited high fatigue resistant, photoswitchable fluorescent served as molecular switches [80]. As shown in Fig. 30, they also synthesized three BODIPY and SN grafted triol-functionalized Anderson-type POMs hybrid, (TBA)3 [{(OCH2)3CNHC20H23N2O}Mn-Mo6O18{(OCH2)3CNHC24H21N2O2}] denoted as SP-{MnMo6}-SN (Fig. 30a), (TBA)3[MnMo6O18{(OCH2)3CNHC24H21N2O2}2] denoted as SN-{MnMo6}-SN (Fig. 30b) and (C20H23N2O)3[MnMo6O18{(OCH2)3CNHC24H21N2O2}2] denoted as (SP)3-SN-{MnMo6}-SN, which possess multielectrochromic and photochromic properties and these electrochromic systems can adopt reversibly up to six different colored states, which correspond to different combinations of the redox states of the manganese, molybdenum and organic molecule(s) constituting the dyad (Fig. 30c) [81]. They also synthesized alkylchain grafted asymmetrically and symmetrically triol functionalize Anderson-type hybrid materials SP-{MnMo6}-C16, which exhibited reversible switching of the hydrophobic spiropyran fragment to the hydrophilic merocyanine that can be easily achieved under light irradiation at different wavelengths. This switch changes the amphiphilic feature of the hybrid, leading to a lightcontrolled self-assembly behavior in solution [81]. Zhou synthesized a series of porphyrin grafted triolfunctionalized Anderson-type POMs hybrid materials, which were discovered to exhibited third-order optical nonlinearities rendering as promising candidate materials for device applications in

photonics and optoelectronics (Fig. 31). They found that the hybrid wherein POM is coupled to porphyrin with shorter bridge has an NLO response superior to those bonded with longer bridge to porphyrin, and the hybrid having two porphyrins connected to POM shows more enhancement than the hybrid having single porphyrin [83]. In 2016, Li and co-workers design and synthesize a new POMpyrene compound, [V6O13{(OCH2)3CNH-C17}2]2
(1), which is employed as a negatively-charged fluorescent material to prepare a new class of fluorescent microspheres by a layer-by-layer electrostatic self-assembly method [128]. Wang reported dendrons of poly (benzyl ether) (PBE; g1-COOH and g2-COOH) could be covalently bonded to NH2-{MnMo6}-NH2 forming dendron-{MnMo6}-dendron hybrids, which could selfassemble into highly ordered layers served as novel solid proton conductors [39]. Recently, they further reported that cholesterol{MnMo6}-cholesterol conjugates could self-assemble into microrods or nanoribbons regulated by means of a temperaturemediated approach [44]. Pooi prepared a POM-based PMMA-MAPOM hybrid polymer through the copolymerization with methyl methacrylate(MMA), which could serve as multiple redox for high-density data storage memory devices material [45]. Wu and his co-workers prepared a ferrocene-grafted POMs hybrid, Fe(Cp)-{MnMo6}-Fe(Cp), which was further found to maintain redox-responsive and amphiphilicity of ferrocene. Such an interesting property can be used as a stimulus-responsive site for controlling the self-assembled structure and morphology [55]. Yang selected three steroids (cholic acid, dehydrocholic acid, cholesterol) to anchor on NH2-{MnMo6}-NH2 to construct Anderson-type hybrid materials steroid-{MnMo6}-steroid conjugates, which displayed unexpectedly different supramolecular structures in the self-assembly process [43]. Coronado synthesized 2,6-di(pyrazol-1-yl)-pyridine(bpp)-graf ted Anderson-type hybrid materials bpp-{MnMo6}-bpp and further coordinated with FeII gives rise to a 2D cationic network forming bimetallic MnIII-FeII hybrid complexes and they observed a fieldinduced slow relaxation of magnetization in the MnIII centres and a photoinduced spin-crossover in the FeII centres [69].

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Cronin and co-workers found that C9 alkyl ligands grafted Anderson-type hybrid materials C9-{MnMo6}-C9 could selfassemble into 2D hexagonal nanostructures with fascinating dielectric behavior and reversible capacitive properties as effective smart nanodielectrics [66]. They also discovered that the presence of acids change the hydrophilicity of the carboxylic acid-{MnMo6}carboxylic acid [48]. Furthermore, a highly delocalized aromatic pyrene-grafted Anderson-type POMs hybrid materials, pyrene{MnMo6}-pyrene, was found to possess nanoporous framework with nanoscale solvent-accessible 1D channels and guest-uptake property [65]. The assembly studies on alkyl ligands grafted Anderson-type hybrid materials Cx-{MnMo6}-Cx with different chain distance (X = 4, 10, 12, 14, and 18 respectively) revealed that such Cx-{MnMo6}-Cx hybrid materials could assemble into a range of interesting architectures including hexagonal structures, nanofibers and other supramolecular forms at the air/water interface [67].

4. Outlook To summarize, the synthetic strategies and structures of novel alkoxyl-functionalized POMs have been discussed in this review. Their applications as efficient oxidation catalysts, electrodes for Li-ion batteries and HER, drugs for anti-tumor, and anti-bacterial, and functional materials with self-assembly, photoresponsive, photochromic, electrochromism and fluorescent, etc. properties are highlighted. Furthermore, we have discussed the potential research interests with respect to alkoxylation of POMs. Although many triol ligands with functional groups, especially photochemical active moieties, hydrophobic alkyl chains, have been employed to functionalized Anderson, Lindqvist V6 and Dawson-type POMs, more novel alkoxyl ligands with functional groups and the extension of other POMs units are still need to be developed. Moreover, controllable and effective as well as novel alkoxyl-functionalized strategies to obtain alkoxyl-functionalized POMs are highly desired. Actually, examples using diol and multi-alcohol ligands with functional groups are still rare. As we know, one of the important application fields for POMs is catalysis, however, the exploration of catalysis of alkoxyl-functionalized POMs is rare, mainly concentrating on the oxidative desulfurization. It is worth exploring to develop more alkoxyl-functionalized POMs as catalysts for useful reactions, because alkoxyl-functionalized POMs have multiple POMs active sites and special properties derived from functional groups of alkoxyl ligands as well as cooperativities. The charge transfer properties between POMs and organic moieties in alkoxyl-functionalized POMs molecules have also attracted widely attentions. Using alkoxyl ligands with conjugated groups to functionalize POMs may enhance the ability of electron transfer to improve the intrinsic redox properties of POMs. It can be concluded with confidence that the persistent and dedicated efforts of chemists over the past 5 years have revealed a vast advance in alkoxylation of POMs, which may in fact just be the reason why the alkoxylation chemistry of POMs research field has flourished in recent years. Of course, more POMs-based materials with novel structures, interesting and functional properties remain to be explored and the alkoxylation chemistry of POM still has great growing opportunities in the future. Acknowledgements We thank the financial support by the National Natural Science Foundation of China (NSFC Nos. 21471087, 21631007, 21225103, and 21701168), the fund of the ‘‘Thousand Youth Talents Plan” and Liaoning Natural Science Foundation (No. 20170540897).

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Mr. Yichao Huang received his B.S. from Beijing University of Chemical Technology in 2014. Now, he is a Ph.D. candidate under the supervision of Professor Yongge Wei from Department of Chemistry, Tsinghua University. His research interests is the design and synthesis of polyoxometalates (POMs)-based organicinorganic hybrids materials, especially the organoimido derivatives of POMs and their applications for energy such as electrochemical and photoelectrochemical splitting of water.

Professor Gao Li, received his B.S. from Hunan Normal University in 2004, and PhD (2011) from Shanghai Jiao Tong University. After his postdoctoral research in Carnegie Mellon University (2011–2014), he joined State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences as a professor in 2014. His current research interests focus on the preparation and catalytic application of metal nanoclusters.

Professor Yongge Wei received his B.S. from Central China Normal University in 1988, M.S. from Wuhan University (1995). He became a faculty member of the Chemistry Department of Peking University in 1995, and was promoted to associate professor in 1999. During 2000–2001, he was a research associate at University of Missouri, Kansas City (USA), and joined Tsinghua University in 2005. His research interests focused on the chemical modification, reaction chemistry and controllable assembly of POMs.

Assistant Professor Jiangwei Zhang received his B.S. from Beijing University of Chemical Technology in 2011 and his Ph.D. under the supervision of Professor Yongge Wei from Department of Chemistry, Tsinghua University in 2016. He joined state key laboratory of catalysis & gold catalysis research center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences as Assistant Research Fellow in 2016. His research interests is controllable functionalization of clusterbased organicinorganic hybrids synthesis and their corresponding catalytic applications.

Please cite this article in press as: J. Zhang et al., Recent advances in alkoxylation chemistry of polyoxometalates: From synthetic strategies, structural overviews to functional applications, Coord. Chem. Rev. (2017), https://doi.org/10.1016/j.ccr.2017.10.025