Architectural chemistry of polyoxometalate-based coordination frameworks constructed from flexible N-donor ligands

Abstract

Polyoxometalate-based coordination frameworks (POMCFs) extend the families of material chemistry and structural chemistry due to their diverse properties and fascinating architectures. In the architectural field of POMCFs, the choice of organic ligand usually plays a key role in the self-assembling process. To date, plenty of organic ligands, such as carboxylates and N-donor ligands, have been designed and utilized in the construction of porous, multinuclear and polythreaded architectures. As far as N-donor ligands are concerned, their flexible and rigid nature can show important influences on the formation of various architectures. This perspective highlights the recent developments in polythreaded/penetrating, multinuclear, helical architectural chemistry of POMCFs constructed from flexible N-donor ligands, including bis-imidazole, bis-triazole, bis-pyridine, bi-mercaptotetrazole, bi-pyridyl-bis-amide and bis-pyridyltetrazole ligands. Here, we will describe the influence of different space lengths and coordination groups on flexible N-donor ligands, as well as the influence of pH and polyoxometalates on the architectural structures of POMCFs, which may offer some possible synthetic strategies for constructing functional POMCFs with novel architectures.

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Xiang Wang

Dr Xiang Wang received his B.S. degree in Chemistry from Bohai University (2003). He obtained his M.S. degree (2008) from Northeast Normal University, where he received his Ph.D. degree in 2014 under the direction of Professor Jun Peng with research on organic–inorganic hybrid materials based on polyoxometalates. Then, he joined the research group of Professor Xiuli Wang in Bohai University, and his research interests now focus on the synthesis and properties of metal–organic hybrids based on polyoxometalates.

Aixiang Tian

Dr Aixiang Tian obtained her B.S. degree from Hebei Normal University in 2004, and completed her M.S. and Ph.D. degrees under the supervision of Professor Jun Peng from Northeast Normal University in 2009. After her graduation, she has been working as an associate professor in Bohai University. Her current research interests include the modification of POMs and POM materials.

Xiuli Wang

Xiuli Wang graduated with her first degree in chemistry from Northeast Normal University (NENU) in 1993, and then received her M.S. (1996) and Ph. D. (2003) degrees under the direction of Professor Enbo Wang from NENU. In 2003, she took up an Associate Professorship in the Chemistry Department at Bohai University. After a postdoctoral appointment (2004–2006) in Professor Xianhe Bu's group at Nankai University, she was promoted to Professor in 2006. In 2013, she stayed in Dr Shengqian Ma's group at the University of South Florida as a visiting scholar. Her current research interests focus in multifunctional metal–organic complexes, particularly in the coordination assembly of polyoxometalate-based metal–organic complexes.

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1 Introduction

Polyoxometalates (POMs) is an unique class of inorganic building blocks, which are usually composed of early transition metal ions in their highest oxidization states, such as molybdenum (Mo^VI), tungsten (W^VI) and vanadium (V^V), and bridged by oxide anions, have attracted intense attention based on their fascinating architectures and excellent properties in the fields of catalysis, electrochemistry, ion-exchange, magnetism, proton conduction and medicine.^1–9 An early review was reported by Pope and Müller in 1991, which not only expatiated on such significant features but also expedited the expansion of this field.¹⁰ In recent years, numerous POM derivatives with novel architectures have been reported. This great development was well-documented by Cronin and Müller.¹¹ With the development of materials chemistry, a class of multifunctional materials united by POMs and metal organic complexes have emerged, which are usually defined as polyoxometalate-based coordination frameworks (POMCFs). In 2012, a review on organic–inorganic hybrids based on POMs was published in Chemical Reviews,¹² which presented a larger overview on the recent developments in two classes of hybrids, defined as Class I and Class II according to the nature of the interaction between organic and inorganic components. To date, the classical POMs as inorganic components, such as Keggin, Wells-Dawson, Anderson, Waugh, Silverton, and Lindqvist type, have been repeatedly introduced into POMCFs. First, owing to their abundant surface oxygen atoms, classical POMs as linkers have a remarkable ability to capture the metal centers of metal–organic skeletons with the aid of M–O covalent bonds (Class II), resulting in the formation of POMCFs with complicated architectures. Second, due to their high symmetry and regular configuration, classical POMs can display key roles in inducing porous or open POM-based frameworks, which act as either templates, pillars or simple counter ions (Class I).^13–19

The architectural chemistry of POMCFs have attracted the attention of the scientists owing to their fascinating architectures, such as interpenetrating, porous, multinuclear, and helical structures, and potential applications. The synthetic strategy for such POMCFs is mainly dependent on organic ligands because the flexible and rigid character of organic ligands is an important factor for the formation of multiform architecture. In reported cases, N-donor ligands have been exploited and utilized in the assembling process of POMCFs due to their strong coordination ability with transition metal ions. N-donor ligands can be subdivided into three classes: rigid, semi-rigid and flexible ligands, depending on the nature of the linkage between the coordination groups. On the one hand, the rigid N-donor ligands, such as 2,2′-bipyridine (2,2′-bpy),^9,20 4,4′-bipyridine (4,4′-bpy),^21–25 and 3,5-bis(3-(pyrid-4-yl)-1,2,4-triazolyl)-pyridine (3,5-bptp),²⁶ have been chosen in the early stage of constructing POMCFs. For example, Wang et al. reported three POMCFs with five-fold interpenetrating, self-penetrating networks assembled from 4,4′-bpy/Cu metal–organic frameworks and polyoxoanions, , and .^27,28 In all these cases, the bpy ligands are linked by copper atoms into rigid metal–organic frameworks, in which the polyoxoanions act as linkages or templates. The porous POMCFs based on bpy ligands are partly summarized in a review published in Coordination Chemistry Reviews.²⁹ On the other hand, the reaction of the semi-rigid ligands, 1,4-bis(1,2,4-triazol-1-ylmethyl)benzene or 4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl, metal ions and polyoxoanions under hydrothermal conditions gave interpenetrating,^30–32 self-penetrating,^33–35 interdigitated³⁶ and porous architectures.³⁷ These cases indicate that the polyrotaxane networks in POMCFs are formed easily by the combination of semi-rigid ligands and metal ions, in comparison to the rigid N-donor ligands. Third, the flexible N-donor ligands usually consist of an N-donor group (or coordination group) and a flexible bridging backbone (or linkage) (Scheme 1). The flexibility can be adjusted by changing the space length of the –(CH₂)_n– linkage, it and plays a significant role in constructing fascinating POMCFs. To date, plenty of flexible N-donor ligands have been designed and introduced into POMCFs (Scheme 2), including bis-pyridine,^29,38–46 bis-imidazole,^47–61 bis-triazole,^62–70 bis-benzimidazole,⁷¹ bis-tetrazole,^72,73 bis-pyridyl-bis-amide,^16,74,75 bis[2-(4-pyridyl)benzimidazole] derivatives.⁷⁶ Moreover, to date, abundant POMCFs with diverse architectures modified by flexible N-donor ligands have been isolated. This article will mostly focus on the POMCFs modified by the flexible N-donor ligands with interpenetrating, multinuclear and helical architectures with the aim of demonstrating and discussing the influence of the flexible nature of N-donor ligands, pH of reaction system and POM anions on the architectures of target POMCFs.

Scheme 1 Schematic view of the flexible N-donor ligand.

Scheme 2 The flexible N-donor ligands employed to construct POMCFs in the literature.

2 POMCFs with polythreaded/penetrating architectures constructed from flexible N-donor ligands

Interpenetrating or self-penetrating architectures could not only strengthen the stability of POM-based frameworks, but also endow them with structural flexibility. In comparison with rigid N-donor ligands, the nature and conformational freedom of flexible N-donor ligands can not only adapt well to the coordination environment of transition metal ions, but can also surmount the effect of steric hindrance of polyoxoanions in self-assembling processes, inducing interpenetrating or self-penetrating architectures.

2.1 Polythreaded/penetrating architectures constructed from flexible bis-imidazole derivatives

In 2008, Su's group selected a flexible N-donor ligand, 1,1′-(1,4-butanediyl)bis(imidazole) (bbi), as an organic component and synthesized two POMCFs with polythreaded architectures.^54,55 In the absence of Et₃N (pH ≈ 2), the coordination of Cu^II ions and bbi ligands resulted in a 2D grid-like sheet, which was extended by β-octamolybdate anions into a two-fold interpenetrating structure (Fig. 1a). With a gradual increase in the amount of Et₃N and HNO₃, Cu^II ions transformed into Cu^I ions in different degrees. Then, two similar 3D polythreaded frameworks were obtained at the pH values of 2 and 4, both of which were constructed from a 3D network based on a Cu^I/Cu^II/bbi/α-octamolybdate anion penetrated by Cu^I/bbi chains (Fig. 1b and c). When the pH value was adjusted to 4–5 with dilute Et₃N solution and hydrochloric acid, the coordination of the octamolybdate anions, including α- and δ-isomers, bbi ligands and copper atoms, generated an interesting 2D sheet, which was further penetrated by another type of Cu/bbi metal organic chain (Fig. 1d). A ladder-like double chain was constructed by θ-octamolybdate anions, bbi ligands and copper atoms at pH 2, which was penetrated by the Cu/bbi chains (Fig. 1e). An investigation of these structures revealed that the transformation of Cu^II ions into Cu^I ions can be achieved by changing the pH of the POM/bbi reaction system, and that the octamolybdate anions in these structures exhibit different isomers. In a word, the use of the bbi ligand is feasible in the synthesis of polythreaded POMCFs. The different metal organic motifs formed by Cu^II/Cu^I cations with bbi ligands, and the various types and coordination modes of octamolybdate isomers are crucial for the construction of the final structures.

Fig. 1 (a) Representation of interpenetrating architecture constructed from β-octamolybdate anions, Cu^II ions and bbi ligands. (b and c) 3D networks based on Cu^I/Cu^II/bbi/α-octamolybdate penetrated by Cu^I/bbi chains. (d and e) Schematic views of polythreaded structures constructed from different octamolybdate isomers, bbi ligands and Cu^I/Cu^II cations. Reprinted with permission from ref. 54 and 55.

Ma et al. reported a self-threading CdSO₄-type framework in 2011.⁵⁶ The major feature of the architecture is that each window is stemmed from Co^II ions, and octamolybdate anions are threaded by two dangling bbi ligands from itself (Fig. 2a). Therefore, the flexible nature of bbi ligand plays a key role in the formation of a self-threading architecture. A two-fold interpenetrating framework based on the α-metatungstate and bbi ligand was obtained by Peng group.⁷⁷ Assembly of the bbi, Cu atoms and [H₂W₁₂O₄₀]⁶⁻ clusters create a 3D framework, which are penetrated into each other to form a two-fold 3D + 3D topology (Fig. 2b). Moreover, it represents first interpenetrating network based on the isopolytungstate. Subsequently, a Wells-Dawson-based POMCF with interpenetrating architecture was isolated in 2011.⁵⁷ In the structure, an infinite inorganic chain supported {Ag₂}²⁺ dimers and [P₂W₁₈O₆₂]⁶⁻ polyanions were extended by other Ag⁺ ions and bbi ligands into a 3D framework, which was penetrated by another 2D dimensional bbi/Ag layer with huge loops. The Ag^I⋯Ag^I interaction have a crucial effect on the formation of interpenetrating architecture (Fig. 2c).

Fig. 2 (a) Schematic representation of the self-threading CdSO₄-type framework. (b) Representation of the interpenetrating 3D + 3D framework. (c) Interpenetration of the huge single loop and the 3D framework. Reprinted with permission from ref. 56, 57 and 77.

2.2 Inter/self-penetrating architectures constructed from flexible bis-triazole derivatives

The imidazole group can be replaced with the triazole group, and the reaction between the flexible N-donor ligand, 1,4-bis(1,2,4-triazol-1-yl)butane (btb), transition metal ions and POMs can also present interpenetrating architectures. In 2011, our group synthesized a POMCF with a two-fold interpenetrating structure.⁶⁸ The 2D wave-like metal–organic layers constructed from Cd²⁺ ions and btb ligands are extended by [SiMo₁₂O₄₀]⁴⁻ anions into a 3D framework with two types of quadrate channels, further inducing a two-fold interpenetrating architecture (Fig. 3a). In the reaction system of [PMo₁₂O₄₀]³⁻ polyoxoanions, Ni²⁺ ions and btb ligands, the polyoxoanions connect the layers based on Ni²⁺ ions and btb ligands into a 3D framework, which exhibits a two-fold interpenetrating architecture with larger channels.⁷⁸ The discrete polyoxoanions are embedded in these channels (Fig. 3b). Another two-fold interpenetrating structure was prepared from Co²⁺ ions, btb ligands and [PMo₁₂O₄₀]³⁻ polyoxoanions in 2012,⁷⁹ which was formed in a sandwich-style mode (Fig. 3c). Two (4, 4, 6)-connected self-penetrating 3D frameworks based on the Wells-Dawson POM were obtained in the presence of Co²⁺ ions and btb ligands.⁶⁹ The results manifest that the flexible btb ligands can effectively surmount the steric hindrance of the polyoxoanions and form a self-penetrating architecture (Fig. 3d).

Fig. 3 Schematic representations of inter/self-penetrating architectures constructed from different POMs, metal ions and btb ligands. (a) Cd²⁺ ions and [SiMo₁₂O₄₀]⁴⁻ anions; (b) Ni²⁺ ions and [PMo₁₂O₄₀]³⁻ anions; (c) Co²⁺ ions and [PMo₁₂O₄₀]³⁻ anions; (d) Co²⁺ ions and Wells-Dawson POM anions. Reprinted with permission from ref. 68, 69, 78 and 79.

A novel 3D POMCF with a self-penetrating architecture, containing Mo–N covalent bonds, was isolated by our group in 2011.⁸⁰ In the structure, each γ-octamolybdate polyoxoanion connects two btb ligands through Mo–N bonds to form an organoimido polyoxoanion building block with dangling arms. Building blocks are connected to each other by Ni²⁺ ions and threaded by Ni/btb chains into a 2D self-threading skeleton, which is further extended into a 3D self-penetrating framework (Fig. 4a). In addition, the result revealed that the γ-[Mo₈O₂₆]⁴⁻ anion could be functionalized by bis(triazole) ligand through Mo–N bonds in hydrothermal conditions, which generated a new approach for synthesizing the organoimido derivatives of the POMs. Soon after, our group reported an interesting 2D → 3D polycatenated structure,⁸¹ in which [Mo₈Na₂O₂₈]⁶⁻ anions are sandwiched by 2D layers built from Ni²⁺ ions and 1,2-bis(1,2,4-triazol-1-yl)propane (btp) ligands to form a sandwich-like 2D columnar layer. The 2D layers are penetrated into each other to form a 2D → 3D polycatenated architecture (Fig. 4b).

Fig. 4 (a) Schematic view of the 3D self-penetrating architecture formed by γ-octamolybdate anions, Ni²⁺ cations and btb ligands. (b) View of the 3D polycatenated architecture formed by [Mo₈Na₂O₂₈]⁶⁻ anions, Ni²⁺ cations and btp ligands. Reprinted with permission from ref. 80 and 81.

2.3 Inter/self-penetrating architectures constructed from flexible bis-pyridyl derivatives

In 2005, a POMCF with a polythreaded architecture, [Cd(bpe)(α-Mo₈O₂₆)][Cd(bpe)(DMF)₄]·2DMF (bpe = 1,2-bis(4-pyridyl) ethane), was prepared.⁴⁵ The flexible bpe ligand reacts with the octamolybdate cluster and Cd²⁺ ion to yield a polypseudo-rotaxane architecture, in which the 1D [Cd(bpe)(DMF)₄]_n²ⁿ⁺ chains penetrate the 2D [Cd(bpe)(α-Mo₈O₂₆)]_n²ⁿ⁻ network (Fig. 5a). A 2D interpenetrating architecture was reported by our group in 2008.³⁹ Flexible 1,3-bis(4-pyridyl)propane (bpp) ligands are linked by copper ions to form a 2D interpenetrating framework with cube-like channels, and the [SiW₁₂O₄₀]⁴⁻ polyoxoanions reside in these channels (Fig. 5b).

Fig. 5 (a) Representation of [Cd(bpe)(α-Mo₈O₂₆)]_n²ⁿ⁻ networks penetrated by [Cd(bpe)(DMF)₄]_n²ⁿ⁺ chains. (b) View of interpenetrating [Cu₂(bpp)₄(H₂O)₂]_n⁴ⁿ⁺ framework templated by [SiW₁₂O₄₀]⁴⁻ anions. (c) Representation of the two-fold interpenetrating architecture constructed from N,N′-bis(3-pyridinecarboxamide)-1,2-ethane, Ag⁺ ions and [SiW₁₂O₄₀]⁴⁻ clusters. Reprinted with permission from ref. 39, 45 and 82.

In 2013, our group utilized flexible bis-pyridyl-bis-amide ligands to synthesize a POMCF with a two-fold interpenetrating architecture.⁸² The 2D layers constructed from flexible N-donor ligand N,N′-bis(3-pyridinecarboxamide)-1,2-ethane and Ag⁺ ions, are connected by [SiW₁₂O₄₀]⁴⁻ clusters into a 3D framework, which are penetrated into each other to form a two-fold 3D + 3D topology (Fig. 5c). Moreover, this represents the first example of interpenetrating POMCF based on flexible bis-pyridyl-bis-amide ligands.

3 POMCFs with multinuclear architectures constructed from flexible N-donor ligands

3.1 POMCFs with multinuclear loop architectures constructed from flexible bis-triazole derivatives

The coordination of flexible bis-triazole ligands and metal ions can not only result in polythreaded architectures, but also multinuclear loop architectures due to its rich coordination sites in comparison with bis-imidazole and bis-pyridine groups, leading to 3D frameworks with novel architectures. Peng's group synthesized four Keggin POM-templated POMCFs by tuning the molar ratio of the flexible bis-triazole ligand to a Cu^II ion, or by changing the lengths of the flexible bridging backbone.⁸³ For a 1 : 3 molar ratio of 1,2-bis(1,2,4-triazol-1-yl)ethane (bte) to Cu^II ions, a circle-connecting-circle chain is constructed by the bte ligands and Cu^I ions (Fig. 6a), which is extended by [SiW₁₂O₄₀]⁴⁻ anions into a 3D framework. When the molar ratio is decreased to 1 : 15, a 2D circle-connecting-circle sheet is formed by bte ligands and copper ions (Fig. 6b), and the [SiW₁₂O₄₀]⁴⁻ anions are sandwiched by sheets. Tuning the space length of the ligands, 1,4-bis(1,2,4-triazol-1-yl)butane (btb) reacts with copper ions at the molar ratio of 1 : 5 in the same POM system, and a circle-connecting-circle chain is formed by btb ligands and copper ions (Fig.6c), which are linked to each other by [SiW₁₂O₄₀]⁴⁻ anions into a 2D layer. At a 1 : 14 molar ratio of btb to Cu^II ions, a hexanuclear loop with dimensions of ca. 14.82 × 13.75 Å is generated by btb ligands and copper ions (Fig. 6d), which is extended into a 3D framework with hexagonal channels. [SiW₁₂O₄₀]⁴⁻ anions are incorporated into these channels. Structural analysis indicated that the molar ratio of ligand to metal ion results in different metal–organic motifs with multinuclear loops in the same POM system. The polyoxoanions exhibit different roles in the final structures with changes of the loop sizes.

Fig. 6 Representations of different architectures constructed from Cu^I ions, bte or btb ligands and [SiW₁₂O₄₀]⁴⁻ anions at different ratios. Reprinted with permission from ref. 83.

In 2011, a series of POMCFs based on Ni^II/flexible bis-triazole ligand complexes, supported by Keggin-type polyoxometalates, were prepared in our group.⁷⁸ It is worth noting that although a common feature is that the bis-triazole ligands are linked by metal ions into 2D grid-like nets containing different multinuclear loops (Fig. 7), the roles of POMs in these structures are completely different. The polyoxoanions act as templates in supramolecular networks based on (1,4-bis(1,2,4-triazol-1-yl)butane)(btb)/Ni²⁺/[PMo₁₂O₄₀]³⁻ and as tetradentate linkages in the btb/Ni²⁺/[SiW₁₂O₄₀]⁴⁻ system; they not only act as linkages but also as templates in the presence of 1,6-(bis(1,2,4-triazol-1-yl)hexane)(btx), [PMo₁₂O₄₀]³⁻ and Ni²⁺ ions. In a word, changing the lengths of flexible bis-triazole ligands results in different 2D or 3D architectures with multinuclear loops, and the polyoxoanions have different roles in the final structures.

Fig. 7 Architectures modified by the flexible bis-triazole ligands (btb, btx) with different space lengths in multinuclear loop mode in Ni/Keggin-type POM system. Reprinted with permission from ref. 78.

The influence of the space length of ligands on the structures of the Ag/POM reaction system is further investigated. First, in the [PW₁₂O₄₀]³⁻/Ag system, a series of multinuclear loops are generated from Ag⁺ ions and bis-triazole ligands with different spacers –(CH₂)_n– (n = 3 for btp, 4 for btb, 6 for btx) (Fig. 8).⁸⁴ As expected, three distinct 3D POM-based frameworks with multinuclear loop architectures are isolated successfully. Different types of multinuclear loops with distinct sizes are formed by changing the spacer length of bis-triazole ligands. The Keggin anions act as multi-dentate linkages in these compounds. The spacer length of flexible bis-triazole ligands plays a key role in the construction of multinuclear Ag^I loops, as well as the final high-dimensional POM-based complexes.

Fig. 8 Architectures modified by flexible bis-triazole ligands (btp, btb, btx) with different space lengths in multinuclear loop mode in Ag/Keggin POM systems. Reprinted with permission from ref. 84.

Second, in [P₂W₁₈O₆₂]⁶⁻/Ag systems,⁸⁵ bis-triazole ligands with different lengths (–(CH₂)_n–) coordinate with Ag^I ions to give divers multinuclear loop architectures (Fig. 9). A hexanuclear [Ag₆(bte)₂]⁶⁺ loop is formed when bte (n = 2) is chosen, and [P₂W₁₈O₆₂]⁶⁻ anions, acting as 10-dentate inorganic building blocks, are incorporated into a 3D framework. A [Ag₅(btp)₂]⁵⁺ loop is formed in the presence of btp, as well as a [Ag₆(btb)₂]⁶⁺ loop is formed in the presence of btb, and the [P₂W₁₈O₆₂]⁶⁻ anions act as 10-dentate and 5-dentate inorganic ligands in the final 3D frameworks, respectively. The results illustrate that the different connecting modes of the flexible bis-triazole ligands with Ag^I ions result in diverse multinuclear loop architectures with different ligand lengths, which also induce different coordination behaviours of the [P₂W₁₈O₆₂]⁶⁻ anions in the 3D frameworks.

Fig. 9 Architectures modified by flexible bis-triazole ligands (bte, btp and btb) in multinuclear loop mode in Ag/Wells-Dawson systems. Reprinted with permission from ref. 85.

3.2 POMCFs with multinuclear loop architectures constructed from flexible bis-pyridyl-bis-amide ligands

The exploitation of new, flexible N-donor ligands is also very important in constructing POMCFs with multinuclear motifs. In 2012, three flexible bis-pyridyl-bis-amide ligands with –(CH₂)_n– linkages, namely, N,N′-bis(3-pyridinecarboxamide)-1,2-ethane (n = 2), N,N′-bis(3-pyridinecarbox-amide)-1,4-butane (n = 4), and N,N′-bis(3-pyridinecarboxamide)-1,6-hexane (n = 6), were synthesized by our group. Moreover, five POMCFs based on Keggin-type POMs and bis-pyridyl-bis-amide ligands with different spacer lengths were isolated for the first time.⁷⁴ The final architectures of these POMCFs have two common structural features (Fig. 10 right). On the one hand, three types of metal–organic loop motifs with different sizes can be formed by Cu^II ions and bis-pyridyl-bis-amide ligands by adjusting the space lengths of ligands. A quadrate loop with a dimension of ca. 7.716 × 13.41 Å is formed for n = 2, a larger loop with a dimension of ca. 13.026 × 13.38 Å is formed for n = 4, and the largest quadrate loop with a dimension of ca. 39.536 × 43.26 Å is formed for n = 6. In other words, the flexibility and the spacer length of the ligand have a significant effect on the formation of the multinuclear loop architectures. On the other hand, the polyoxoanions exhibit different template roles along with the changes of the sizes of the loops in the 3D supramolecular structures.

Fig. 10 POMCFs modified by flexible bis-pyridyl-bis-amide ligands with different space lengths in multinuclear loop mode. Reprinted with permission from ref. 74, 86 and 87.

To investigate the influences of the size and the charge density of polyoxoanions on multinuclear loop architectures, the octamolybdate anion was chosen due to its small size and low charge density compared to Keggin-type POMs, and two octamolybdate based hybrids modified by flexible bis-pyridyl-bis-amide ligands were further isolated in 2013.⁸⁶ Interestingly, two 1D meso-helical chains, instead of the metal–organic loops in the Keggin POM system, are obtained in the final structures (Fig. 10 top), which are linked by octamolybdate polyoxoanions into 2D layers and a 3D framework. Replacing the flexible –(CH₂)_n– bridging backbones with a semi-rigid piperazine linkage or a semi-rigid bis-pyridyl-bis-amide ligand, N,N′-bis(3-pyridine-carboxamide)piperazine, was synthesized. Moreover, four POM-based complexes modified by this ligand were reported.⁸⁷ In these structures, the semi-rigid ligands can also be connected by Cu^II ions into metal–organic loops similar to those formed from flexible bis-pyridyl-bis-amide ligands, which are extended by polyoxoanions into a 1D chain, a 2D layer and a 3D framework structure by different polyoxoanions (Fig. 10 left). The results manifest that the flexible and semi-rigid bis-pyridyl-bis-amide ligands can be aggregated easily by transition metal ions to form multinuclear loop architectures in POM-based complexes, which give a new synthetic strategy for constructing metal–organic loop motifs. In addition, the choice of polyoxoanions is also an important factor in generating multinuclear motifs.

3.3 POMCFs with multinuclear cluster architectures constructed from flexible bis-tetrazole derivatives

Among the reported cases, although the flexible N-donor ligands with –(CH₂)_n– linkages have exhibited satisfactory flexibility in self-assembling processes, the twist degree of the –(CH₂)_n– linkages in these ligands is still restricted. However, we consider that the thioether bond may exhibit better flexibility and a larger twist degree in comparison with the –(CH₂)_n– linkages. In addition, compared with imidazole, triazole and pyridine coordination groups of the flexible ligands, tetrazole group with rich potential coordination sites is an ideal candidate for the formation of multinuclear cluster architectures.^{16,75,88–101} On the basis of these points, four flexible bis-tetrazole derivatives with a –(CH₂)_n– bridging backbone decorated by thioether bonds were designed, and a series of POMCFs modified by such ligands with multinuclear cluster architectures were reported in 2010.^72,73 In the presence of 1,1′-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)methane (bmtm), a 1D chain is constructed from two types of binuclear copper clusters, which may be attributed to the flexible nature of the bmtm ligand. Each 1D chain is bridged by [SiW₁₂O₄₀]⁴⁻ polyoxoanions to form a 2D layer (Fig. 11a). When the 1,2-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole)ethane (bmte) ligand is chosen, a tetranuclear cluster is formed by bmte ligands and Cu^I ions due to the torsion of the flexible backbone. Finally, a 3D network is built from [SiW₁₂O₄₀]⁴⁻ polyoxoanions and 2D layers stemming from tetranuclear clusters, bmte ligands and Cu^I ions (Fig. 11b). Moreover, a metallacalix[4] building block is formed in the presence of 1,3-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole) propane (bmtp) ligands and Cu^I ions, which is linked by polyoxoanions into a 3D self-penetrating architecture (Fig. 11c). When 1,5-bis(1-methyl-5-mercapto-1,2,3,4-tetrazole) pentane (bmtp-1) is used as a ligand, a 3D framework with a type of large cavity and two types of small windows is built from bmtb/Cu^I binuclear clusters owing to the long backbone of the bmtp-1 ligand, and the polyoxoanions locate in the large cavities of the framework (Fig. 11d). The results exhibit that the thioether bond and the length of the backbone may play a regulatory role in the formation of multinuclear motifs.

Fig. 11 POMCFs modified by the flexible bis-mercaptotetrazole ligands in multinuclear cluster modes in Cu⁺ and Ag⁺/Keggin-type POM systems. Reprinted with permission from ref. 72, 73 and 103.

Furthermore, we investigated the influence of the different flexible bis-tetrazole ligands on the frameworks in a Ag/POM system.¹⁰² As is shown in Fig. 11e, a tetranuclear cluster formed by bmte ligands and Ag⁺ ions is linked by [SiW₁₂O₄₀]⁴⁻ polyoxoanions to form a 1D chain. A similar structure is obtained by bmtp ligands and Ag ions (Fig. 11f), but the Ag ions show different coordination modes in tetranuclear clusters because of the longer skeleton of the bmtp ligand. Changing the number of methylenes to four, a 2D layer templated by [SiW₁₂O₄₀]⁴⁻ polyoxoanions is formed by binuclear silver clusters and bmtb ligands (Fig. 11g). As far as such a flexible bis-tetrazole ligand is concerned, the best length of the –(CH₂)_n– bridging backbones for the formation of multinuclear silver clusters in Ag/POM systems was two or three methylene groups, but only a binuclear silver cluster was formed using the bmtb ligand with the longest –(CH₂)₄– linkage. Thus, the flexible nature and thioether bond of the ligand are very critical to the formation of multinuclear silver clusters in Ag/POM systems.

To verify further whether different coordination natures of transition metal ions have an influence on the construction of multinuclear structures, Ag⁺, Cu⁺, Cu²⁺ and Ni²⁺ ions were introduced into the same bmtb/POM system (Fig. 12).¹⁰³ First, Ag⁺ ion was chosen, and a binuclear silver motif was given. Secondly, when the Cu⁺ ion was selected, a trinuclear copper motif was formed. Third, when Cu²⁺ or Ni²⁺ ions were present, two similar 2D layers with mononuclear structures were yielded, in which both Cu²⁺ and Ni²⁺ ions display six-coordinated modes, and these are different from those based on Cu⁺ and Ag⁺ ions. Thus, the different coordination natures of transition metal ions significantly influence the formation of multinuclear subunits.

Fig. 12 POMCFs in the same flexible bis-mercaptotetrazole ligands and POM systems formed by the use of different transition metal ions. Reprinted with permission from ref. 104.

Kang's group reported four Keggin type POM-based POMCFs constructed from two tetrazole-functionalized flexible ligands, bis(1-methyl-1H-tetrazol-5-yl)sulfane (bmps) and 5,5′-((2,2-bis(((1-methyl-1H-tetrazol-5-yl)thio)methyl)propane-1,3-diyl) bis(sulf-anediyl))bis(1-methyl-1H-tetrazole) (bpbb).¹⁰⁴ In the [PMo₁₂O₄₀]³⁻/bmps system, a mononuclear unit and a binuclear unit are formed by the bmps ligand and Cu ions, respectively (Fig. 13a). The polyoxoanions are linked together by two types of units to result in a 2D pyramid-like network (Fig. 13b). When the Ag ion is chosen, T-shaped trimeric [Ag₃(bmps)₃] units are obtained (Fig. 13c), which are linked with each other to form a fence-shaped chain. The polyoxoanions are anchored into U-type grooves (Fig. 13d). In the [PW₁₂O₄₀]³⁻/bpbb system, a nano-scaled [Cu₄(bpbb)₄(μ₄-Cl)] calixarene cluster is formed (Fig. 13e), which is further extended into a 2D square layer. The polyoxoanions are arranged within the pores (Fig. 13f). However, a pentanuclear [Ag₅(bpbb)₅] cyclodextrin cluster is obtained in the presence of Ag ion (Fig. 13g), and the polyoxoanions are located in the cavities of the cyclodextrin clusters from a 2D fold-shaped layer (Fig. 13h). These structures demonstrate that it is feasible to obtain multinuclear cluster architectures by utilizing tetrazole-functionalized flexible ligands. Moreover, the POMs play a key role in inducing the formation of structures under hydrothermal conditions.

Fig. 13 The architectures of the POMCFs constructed from tetrazole-functionalized flexible ligands (bmps and bpbb). Reprinted with permission from ref. 104.

4 POMCFs with helical architectures constructed from flexible N-donor ligands

POMCFs with helical architectures have been of particular interest because of their attractive structural features and potential applications. For example, some POMCFs with helical features modified by rigid and semi-rigid N-donor ligands have been reported.^33,105,106 It has been mentioned that the proper configuration and coordination mode of organic ligands is advantageous for the preparation of POMCFs with helical architectures, especially achiral organic ligands. In 2007, Su reported two chiral POMCFs modified by flexible bis-imidazole ligands (bbi).⁴⁹ In these structures, the bbi ligands link Cu^I ions into a 2D sheet, which is pillared by [V₁₀O₂₆]⁴⁻ polyoxoanions into a 3D framework containing channels. Moreover, Cu^I ions are connected by the bbi ligands to generate two types of helical chains, including left-handed and right-handed helices. Such helical chains are inserted into the channels (Fig. 14). Later, several 3D chiral POMCFs based on helical vanadate chains modified by flexible N-donor ligands (bbi) were prepared again.^107,108 The successful isolation of these POMCFs not only provides several intriguing examples based on achiral POMs and ligands but also provides a possible method for the preparation of chiral POMCFs using flexible N-donor ligands.

Fig. 14 A 3D framework containing two types of channels, into which left-handed helices and right-handed helices are inserted. Reprinted with permission from ref. 49.

Pyridyl-tetrazolate ligands have been extensively utilized to synthesize POMCFs in recent years due to their multiple coordination sites.^{91,93,95–97,109,110} The coordination of the pyridyl-tetrazolate ligand with metal ions usually shows diverse multinuclear building motifs, but not helical structures because of their small sizes and rigid nature. Considering the coordination natures of the flexible ligands, we try to introduce the flexible –(CH₂)₄– linkage into pyridyl-tetrazolate ligands for the sake of controlling their rigidity for constructing helical motifs in a POMs system. Two flexible bis-pyridyltetrazole ligands, 1,4-bis(5-(4-pyridyl)tetrazolyl)butane(4-bptzb) and 1,4-bis(5-(3-pyridyl)tetrazolyl)butane(3-bptzb), were designed and synthesized in our study.¹¹¹ In a Keggin POM/Ag reaction system, the coordination of two types of ligands with Ag⁺ ions shows two distinct architectures. Two 4-bptzb ligands can be bi-aggregated by two Ag⁺ ions through the four N atoms from two tetrazolyl groups in the [PMo₁₂O₄₀]³⁻/Ag reaction system (Fig. 15a), but helix motifs are not formed. The [PMo₁₂O₄₀]³⁻ polyoxoanions are linked on both the sides of the dimers through Ag–O bonds.

Fig. 15 POMCFs modified by flexible bis-pyridyltetrazole ligands in helical architectures. Reprinted with permission from ref. 111.

However, a meso-helix chain is successfully constructed from 3-bptzb ligands and Ag⁺ ions in the same reaction system (Fig. 15b), in which the 3-bptzb ligand exhibits two types of configurations, and the polyoxoanions reside in the 3D metal organic framework. Moreover, a meso-helix loop connecting loop chain is also obtained in the Ag/3-bptzb/[P₂W₁₈O₆₂]⁶⁻ system. These are linked to each other by [P₂W₁₈O₆₂]⁶⁻ anions into a 2D layer, which is further extended by the 3-bptzb ligands into a 3D framework (Fig. 15c). The results confirm that the synthetic strategy is feasible, and both the flexibility of –(CH₂)₄– bridging backbones and the N donor site of the pyridyl group have crucial influences for inducing the helical architectures.

5 The properties of POMCFs constructed from flexible N-donor ligands

The redox activities of POMs have attracted considerable attention in recent years because they make POMs attractive as redox catalysts for indirect electrochemical processes. An early review organized by Steckhan and co-workers has been published in Chemical Reviews in 1998,¹¹² which not only summarizes the electrochemical behaviors of the POMs, but also reviews the application of POMs as reductive and oxidative electrocatalysts. Thus, it is very critical for the POMCFs as functional materials that the redox abilities of the polyoxoanions can be maintained in the final structures. To date, extensive research suggests that the polyoxoanions in POMCFs that is constructed from different organic ligands still exhibit good redox abilities.^19,36,113 Thus, the corresponding electrochemical properties of carbon paste electrodes modified by POMCFs in an aqueous solution of 0.1 M H₂SO₄ or in an aqueous solution 0.1 M H₂SO₄ + 0.5 M Na₂SO₄ are summarized here, including interpenetrating,^57,78 multinuclear loops,^84,85 and multinuclear cluster architectures of POMCFs.^72,103

Previous investigations suggest that POMs exhibit good photocatalytic activity towards the degradation of organic dyes.¹¹² However, compared to the secondary-pollution created by typical POM salts due to their water solubility, the POMCFs can display the advantages of stability, less contamination and easy recovery. Thus, the photocatalytic activity of the POMCFs for the degradation of organic dyes, such as methylene blue (MB) and Rhodamine B (RhB), is also usually investigated under UV or visible-light irradiation.^114–118

5.1 Electrochemical properties of POMCFs constructed from flexible N-donor ligands

Four POMCFs, Ag₇(bbi)₅(OH)(P₂W₁₈O₆₂) and [Ni₃(btb)₅][PMo₁₂O₄₀]₂·14H₂O with interpenetrating architectures,^57,78 with multinuclear loop architecture,⁸⁴ and [Cu₄(bmte)_3.5][SiW₁₂O₄₀] with multinuclear cluster architecture,⁷² were isolated successfully by Peng's and our group. The corresponding electrochemical behaviors of these POMCFs recorded in an aqueous solution 0.1 M H₂SO₄ show consecutive two electron processes of W/Mo centers (Fig. 16).^119,120 With the increase in scan rates, the cathodic peak potentials shift towards the negative direction and the corresponding anodic peak potentials shift to the positive direction. Moreover, the peak currents are proportional to the scan rates, which indicate that the redox processes are surface controlled. In a word, the results confirm that the redox ability of the polyoxoanions in POMCFs constructed from flexible N-donor ligands can still be maintained.

Fig. 16 Cyclic voltammograms of POMCFs based on flexible N-donor ligands in an aqueous solution of 0.1 M H₂SO₄ at different scan rates. (a) Ag₇(bbi)₅(OH)(P₂W₁₈O₆₂); (b) [Ni₃(btb)₅][PMo₁₂O₄₀]₂·14H₂O; (c) ; and (d) [Cu₄(bmte)_3.5][SiW₁₂O₄₀]. Reprinted with permission from ref. 57, 72, 78 and 84.

POMs have been extensively studied as electrocatalysts for ethanol oxidation and reductions of nitrite, bromate and hydrogen peroxide.^121–123 However, the abovementioned POMCF bulk-modified carbon paste electrodes also exhibit electrocatalytic activities towards the reduction of nitrite,^{57,78,87,102,111} bromate^57,87 and hydrogen peroxide.⁸⁷ A POMCF with two-fold interpenetrating architecture constructed from flexible bbi ligands and two POMCFs with multinuclear loop architectures based on bis-pyridyl-bis-amide ligands were reported.^77,82,87 The electrocatalytic behaviors of the POMCF bulk-modified carbon paste electrodes towards the reduction of nitrite, bromate and hydrogen peroxide were reported in an aqueous solution of H₂SO₄ (Fig. 17). With the addition of nitrite, bromate and hydrogen peroxide, the corresponding reduction peak currents gradually increase, while the corresponding oxidation peak currents gradually decrease; however, such a response is not observed for bare carbon paste electrodes under the same conditions. Therefore, the POMCFs show good electrocatalytic activities towards the reduction of hydrogen peroxide, nitrite and bromate.

Fig. 17 Cyclic voltammograms of POMCF bulk-modified carbon paste electrodes in aqueous solutions of 1 and 0.01 M H₂SO₄ containing KNO₂ (a)/KBrO₃ (b)/H₂O₂ (c). Reprinted with permission from ref. 77, 82 and 87.

5.2 Photocatalytic properties of POMCFs constructed from flexible N-donor ligands

The photocatalytic activities of the POMCFs for the degradation of MB or RhB, including interpenetrating,^57,69,79 multinuclear loop,^84,85,87 multinuclear cluster¹⁰² and helical architectures,¹¹¹ have also been reported. In 2011 and 2012, two interpenetrating POMCFs constructed from flexible bbi and btb ligands were reported, both displayed photocatalytic activities towards the degradation of RhB and MB.^57,79 Ma's group synthesized a POMCF with a multinuclear loop architecture constructed from flexible N-donor ligands, as well as multinuclear cluster POMCF,^124,125 which possessed excellent photocatalytic activities for the degradation of MB (Fig. 18). The investigations of the stability and reversibility of photocatalysts suggest that the POMCFs are stable and reusable during catalytic reactions. The results imply that such POMCFs may be anticipated as potential photocatalysts for the degradation of organic dyes.

Fig. 18 Absorption spectra of RhB (a) and MB (b, c, d) solutions during the decomposition reaction under UV irradiation in the presence of the reported POMCFs. Reprinted with permission from ref. 57, 79, 124 and 125.

6 Conclusions and outlook

We mainly described the recent developments in POMCFs modified by flexible N-donor ligands in different architectures, especially the flexible bi-mercaptotetrazole, bi-pyridyl-bis-amide, and bis-pyridyltetrazole ligands, by our group. Discussion on the influence of the space length and natures of ligands on the frameworks may offer some prospective opportunities to construct functional POMCFs with novel topologies and properties. Although plenty of flexible N-donor ligands have been utilized to construct POMCFs to date, the exploitation of new flexible N-donor ligands is still of significance for the development of the structural chemistry and material chemistry, including tuning the space lengths and changing the configurations and coordination numbers of the apical groups. Depending on early achievements and future research, the ideal ligands could be explored in the near future for constructing functional POMCFs, which will be an inspiring achievement in the fields of polyoxometalate chemistry and functional materials.

Acknowledgements

The supports of the National Natural Science Foundation of China (no. 21171025, 21471021), New Century Excellent Talents in University (NCET-09-0853), Program of Innovative Research Team in University of Liaoning Province (LT2012020) and General Program Fund for Education Department of Liaoning Province (L2014449) are gratefully acknowledged.

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