Closed Cage Constructed from

Apr 2, 2018 - Synopsis. Two new triangular nanosized polyoxometalates modified by benzyl arsonate ligands, [(C7H7AsO3)6W12O36]12−, were successfully...
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A Reversibly pH-Switchable Open/Closed Cage Constructed from Triangular Polyoxometalate Hybrid [(C7H7AsO3)6W12O36]12− Cluster Anions Exhibiting Supramolecular Chirality Hao Yuan,† Wei-Dong Yu,† Shuang Liang,† and Jun Yan*,†,‡ †

School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, P. R. China Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, Hunan, China

‡

S Supporting Information *

without chemical structural alteration.21,22 The regularly used organic phosphonate or arsonate anions always play the role of structural template.23 Complicated reversible motions with molecular reconfiguration, such as open/closed switching, have not been achieved in a POM hybrid system yet, while these motions are crucial to the further development of intelligent molecular devices.24 Therefore, the design and manufacture of POM-based molecules with a reversible configuration change is still a major challenge for chemists. Our group is interested in functional POM and POM hybrid molecules, and we have demonstrated that using benzylarsonate to construct the POM hybrid could introduce additional chirality into the hybrid compound in either the formation of a conventional bond or supramolecular packing.25 In an effort to understand the self-assembly process and configuration control, herein we extended the study to polyoxotungstate systems, and a new triangular POM hybrid cage cluster, [(C7H7AsO3)6W12O36]12− (1a), was isolated as Na2(C2H8N)4H6[(C7H7AsO3)6W12O36]·30H2O (1; CCDC 1814918). Unexpectedly, 1a reorganized into the isomer 2a when it was further protonated. The orientation of the organic group made 1a form a “closed” state containing two Na+ and three coordinated water molecules in the cavity, while the Na+ ion was released in the “half-open” form cage 2a and two water and two dimethylamine cations (DMAH+) located in the cage center. The 1H NMR monitoring results show that the cage transformation between 1a and 2a is reversible, which presents the first example of a pH-switchable open/closed POM hybrid cage system. The detailed transformation mechanism was clarified because of the affirmatory characterization of both states of the cage, and the aggregation process of 1 with solid supramolecular chirality was preliminarily explored. Compound 1 was synthesized by the direct mixing of a reactant in an orderly manner in aqueous solution, and then the pH of the solution was adjusted to about 5.0 (see the Supporting Information for details). Evaporation of the solution leads to the formation of crystals suitable for single-crystal X-ray diffraction of 1 in 3 weeks (Table S1). The pH scan in the synthesis shows that only the 1a cluster can be isolated under weak acidic conditions between pH 3.5 and 6.0, and no other hybrid clusters have been detected yet. Single-crystal X-ray diffraction analysis reveals that

ABSTRACT: A new nanosized polyoxometalate modified by benzylarsonate ligands, [(C7H7AsO3)6W12O36]12− (1a), was successfully isolated and structurally characterized as Na2(C2H8N)4H6[(C7H7AsO3)6W12O36]·30H2O (1). Controlled protonation of 1a led it to self-reorganize into isomer cage cluster [(C7H7AsO3)6W12O30(OH)4(H2O)2]4− (2a) with an organic ligand arranging in “half-open” mode. The reversibly switchable cage transform was monitored by 1 H NMR. Also, the origin of the solid supramolecular chirality in the crystal 1 and the aggregation process of 1a were preliminarily studied.

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eversibly switchable open/closed motions activated by external stimuli are some of the most basic functions in our lives. The incorporation of such mechanical movements into synthetic molecules is highly promising for the development of molecular devices and machines.1,2 Diverse artificial compounds, such as inorganic−organic hybrid molecules, have been prepared, and the control and tuning of switchable processes at the molecular or supramolecular level is a field of ever-growing interest.3−5 Organic hybrids of polyoxometalates (POMs) are a class of anionic nanosized metal−oxygen clusters of highly oxidized early-transition-metal cations (W6+, Mo6+, V5+) with grafting organic moieties.6 The introduction of an organic group onto the polyoxometallic backbones has opened up a wide structural diversity that has novel interesting properties, and POM hybrids have been proposed to have applications in metal coordination and catalysis, photochemistry, chiral recognition, imaging, etc.7−9 Also, the construction of a stable nanosize cavity containing guests is easily attained in rigid inorganic units of POMs, and open/closed switching of molecular cavities by external stimuli (e.g., heat, light, and pH) normally could be achieved by rotating, twisting, or moving flexible organic groups.10−13 Tremendous effort has been dedicated to the development of molecular POM hybrid materials.14,15 Remarkable studies on the preparation of POM-based compounds with switchable behavior have been reported by the groups of Müller,16 Cronin,17 Kortz,18 Yang,19 Wei,20 and Dolbecq.7 However, unlike other coordinated molecular switches, most of the switching behaviors of POM-based molecules are beyond the molecular level or derived by the redox processes of metal centers © XXXX American Chemical Society

Received: January 12, 2018

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DOI: 10.1021/acs.inorgchem.8b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

crystallizes in a chiral space group of I432. Because of the highly symmetric space group, the organic cations and solvents cannot be well located in the solid crystal structure. Chemical and thermogravimetric analyses show that the charge of 1a was balanced by four DMAH+ ion and six protons, and there are 30 solvent water molecules per cluster in 1. Fascinatingly, recrystallization of 1 in a stronger acidic solution leads to further protonation of 1a, and an isomer anion [(C7H7AsO3)6W12O30(OH)4(H2O)2]4− (2a) was isolated as (DMAH)4[(C7H7AsO3)6W12O30(OH)4(H2O)2]·32H2O (2; CCDC 1814837). As shown in Figure 2, 2a also consists of

1a is a triangular hybrid cluster. As shown in Figure 1, every W center is six-coordinated and forms a {WO6} octahedron with

Figure 1. Ball-and-stick representation of the nanosized cluster of [(C7H7AsO3)6W12O30(OH)6]6− in compound 1. Inset: Rotation mode of the phenyl group.

two terminal O ligands. All {WO6} octahedra at vertex positions link other {WO6} octahedra by the edge-sharing mode, and the three {WO6} octahedra on the side positions link each other in the corner-sharing mode. The rotation of these octahedra leads to C3 symmetry of the whole inorganic triangle POM unit. Each benzylarsenic group binds to one vertex {WO6} and three side {WO6} of the triangle ring by the AsO3 unit. All benzyl groups are located on the side of the POM framework and are from a rare organic−inorganic−organic “sandwich”-type structure. Alternatively, the cluster also can be described as three hybrid {W3O10(C7H7AsO3)2} units alternately connected by three {WO6} linkers in cis mode (Scheme 1). All benzyl rings and

Figure 2. Ball-and-stick representation of 2a in compound 2. Inset: Three rotation modes of the phenyl group.

three {W3O10(C7H7AsO3)2} units and three {WO6} linkers. The major structural difference between 1a and 2a is the connecting mode of the {WO6} linker. Two of the three linker bond to the hybrid fragment in the trans mode in 2a due to protonation (Scheme 1), and the protons locate according to the bondvalence sum on the terminal O ligands (Tables S4 and S5). Consequently, the orientation of the benzyl rings changes, and the “closed” cavity becomes “half-open”, and two DMAH+ ions and two water molecules occupy the central positions of the cavity (Figure S3). Also, the rotation mode of the phenyl rings disperse into three patterns in 2a, and the supramolecular chirality is lost in the solid structure of 2. To further understand the transformation process in solution, 1 H NMR was utilized to follow the self-assembly procedure and protonation process. As condensation between tungstate and benzylarsonate occurs, some O ligands on the POM fragment get close to the −CH2− group and an intramolecular hydrogen bond is generated. Different linking modes between the AsO3 group and {WO6} units affect the hydrogen-bond strength (Figures S4 and S5 and Tables S2 and S3). The terminal ligand O1 has a stronger interaction with H on the −CH2− group than the bridge ligand O7 in 1a, which moves and splits the H signal into 4.24 and 3.88. With the formation of a supramolecular aggregate, the signal further splits by a weak interaction difference between the methylene group and ordered solvent and organic cations (Figure S6). Consequently, the H signal of −CH2− in 1a contains three single peaks with similar integral strength at 3.50, 3.88, and 4.22 in 1a. Instead, the H signals of −CH2− in 2a merge at 3.85 because of the configuration difference and weaker intramolecular hydrogen bonding compared with 1a. The 1H NMR scan at different pH values shows that the transformation of 1a happens when the pH is lower than 3.5 and no other intermediate clusters are detected (Figures S7 and S8). Further, the switching process between 1a and 2a is also observed in aqueous solution by using the −CH2− signal as an indicator. As shown in Figure 3, the cluster is fully in 1a form under the weak acidic conditions of pH 5.2, and it transforms into 2a at pH 1.5

Scheme 1. (Left) Illustration of the Speculated Connecting Mode Transformation of the {WO6} Linker in a Reversible pH-Switching Process between 1a and 2a and (Right) Balland-Stick Representation of {{W3O10(C7H7AsO3)2}} Hybrid Units

vertical axes of the triangle plane ring have an interfacial angle of 77.5(3)°. Thus, these benzyl rings approach each other and form a closed cavity with the inorganic part of the cluster. During the synthesis, two Na+ cations and three coordinated water molecules were “locked” inside the cavity (Figure S1). Further, as shown in the inset of Figure 1, all of the benzyl rings uniformly slant to the right during the solid packing, which results in eight 1a units forming a supramolecular octamer subunit containing a chiral cavity (Figure S2). These cavities were filled by DMAH+ and solvent water molecules, and therefore compound 1 B

DOI: 10.1021/acs.inorgchem.8b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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aggregates can be observed in both the solid state and solution, they do not quantitatively match the UV−vis data and supramolecular chirality control has not yet been reached (Figures S25 and S26). In summary, we report the discovery of a remarkable triangular POM hybrid cluster, built from a commercialized benzyl arsenate ligand and a polyoxotungstate core specifically formed through a one-pot procedure. This paper presents the first reversibly pHswitchable open/closed POM hybrid cage. The structural characterization of two states of the cage implies that more states, such as a “fully-open” state, could exist but have not been isolated yet in this hybrid system. The transformation mechanism accompanying the protonation/deprotonation process of the POM unit also becomes clear. The origin of solid supramolecular chirality in the crystal is also clear, while the aggregation process of such a POM hybrid system is still not well understood, especially in targeting chirality tuning and control. Given the scope of this area, it becomes palpable that such a preliminary study should open new windows toward extensive and challenging molecular devices.

Figure 3. Representation of the monitoring results of reversible switching between 1a and 2a by selected 1H NMR using the −CH2− group signal as an indicator.

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with a conversion ratio of over 99%. When the pH decreases to 5.2 again, 2a converts to 1a again with a conversion ratio of over 99% within a few minutes. The process is repeatable at least in three cycles, which indicates that the open/closed switching of the cage cluster is reversible. To gain further insight into configuration control, aggregation experiments were preliminarily explored. 1a exhibits solubility in polar solvents, and good stability is confirmed by the recrystallization process and UV−vis spectrum (Figures S9− S11). Both 1a and 2a exhibits three board peaks in the UV−vis spectrum. The peaks at 200 and 264 nm correspond to classic W−O and WO bond absorption in the inorganic part, and the peak at 220 nm belongs to the phenyl rings. A weak peak at 325 nm also is detected in the spectrum of 2, which could contribute by the W−H2O charge-transfer process of the POM units in 2a. Accompanied by the formation of a nanosized aggregate, the absorption intensity around 264 nm could significantly increase because of the stronger supramolecular interaction between clusters. At about pH 5 in aqueous solution, the cluster is in monomeric form, as evidenced by the dynamic light scattering peaks (Table S6). The clusters form aggregates of about 171.4 nm at pure ethanol (EtOH) in minutes. The average sizes of the aggregates are up to 338 nm at H2O−EtOH (80:20, v/v) in hours. However, because of the multiple protonation balance between the anions, the detail structures of the aggregates are poorly defined in an acidic aqueous solution. The electrospray ionization mass spectrometry scan also does not show molecular peaks of 1a or the relevant aggregation peaks. Only some fragment peaks were detected, which indicated that the cluster was decomposed during the measurement (Figure S12 and Table S7). Preliminary kinetic studies in a H2O−EtOH solution show that the aggregation process is affected by the solvents and concentration (Table S8 and Figures S13−18). The absorption versus time profiles show the aggregation behavior that can be fit by an equation mode that is widely used in the aggregation of porphyrin and other related macrocycles.26 The “aggregate growth rate” parameter n is close to 0 in the mixture, which indicates that the equation takes on a conventional first-order form and the concentration is crucial for aggregation promotion. Also, changing the pH of the solution or adding EtOH could tune the aggregation size in principle. Additionally, the weak supramolecular chirality generated by the rotation of benzylarsonate is sensitive to disturbance in crystallization. Although circular dichroism chiral signals of the prepared nanosized

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00084. Synthesis and characterization details, including Figures S1−S26 and Tables S1−S8 (PDF) Accession Codes

CCDC 1814837 and 1814918 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jun Yan: 0000-0002-6158-0614 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Recruitment Program of Global Youth Experts, Innovation-Driven Project of Central South University (No. 2016CX037).

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DOI: 10.1021/acs.inorgchem.8b00084 Inorg. Chem. XXXX, XXX, XXX−XXX