Self-Assembly of Anionic Polyoxometalate–Organic Architectures

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Self-Assembly of Anionic Polyoxometalate−Organic Architectures Based on Lacunary Phosphomolybdates and Pyridyl Ligands Chifeng Li,† Noritaka Mizuno,† Kazuya Yamaguchi,*,† and Kosuke Suzuki*,†,‡

J. Am. Chem. Soc. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/23/19. For personal use only.

†

Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

metal species.20−23 Notably, various types of vacant sites with a wide range of numbers and directions of the coordination sites can be designed by utilizing lacunary POMs. Therefore, we envisaged that they would be promising building blocks for the construction of novel anionic POM−organic architectures through POM−pyridine coordination, and the achievement of unique properties could be possible. To date, several fully occupied POMs, such as vanadate,24−28 Ni-substituted POMs,29 {Mo6}30 and {XM6} (M = Mo or W),31 have been utilized for the construction of inorganic−organic architectures upon connection with organic ligands (e.g., carboxylate and phosphate ligands). However, multivacant lacunary POMs are metastable and frequently suffer from unexpected isomerization or decomposition reactions, and the construction of POM−pyridyl architectures has no precedent. Accordingly, we have recently developed a powerful method for the utilization of metastable multivacant lacunary POMs in organic media. Thus, the construction of giant assembled structures32 and multinuclear clusters33,34 by reacting with metal species and their functionalization with organic ligands35 has become possible. Herein, we report the successful self-assembly of inorganic− organic architectures comprising multivacant lacunary POMs and pyridyl ligands in organic media for the first time (Figure 1). As a lacunary POM building block, we chose a Keggin-type trivacant lacunary phosphomolybdate [A-α-PMo9O34]9− because of (i) the unique properties of phosphomolybdates36−45 and (ii) the stronger Lewis acidity of their vacant sites (i.e., higher affinity to pyridine) than those of other lacunary POMs (e.g., phosphotungstates).46 However, the low hydrolytic stability of lacunary phosphomolybdates makes it extremely difficult to isolate and utilize them.21,47−49 Consequently, the introduction of metal species and/or organic ligands into their vacant sites has not been successfully achieved to date. In the present study, we demonstrate that, by the introduction of pyridine moieties to the vacant sites, a trivacant lacunary phosphomolybdate [A-α-PMo9O34]9− can be remarkably stabilized in organic media. Moreover, by utilizing the pyridine (py)-coordinating phosphomolybdate (I) as a stable and reactive multivacant lacunary POM, selfassembled POM−organic architectures comprising lacunary POMs and pyridyl ligands can be constructed, namely a dimer

ABSTRACT: The development of novel systems for metal−organic architectures is an attractive research field because they are fascinating materials with unexplored functions. Lacunary polyoxometalates (POMs) offer structurally well-defined coordination sites with various coordination directions and numbers in addition to the designable properties; thus, lacunary POMs are ideal building blocks for inorganic−organic architectures. However, their utilization is currently limited by their low stability and difficulty in controlling the reactivity. Here, we report the successful self-assembly of anionic POM−organic architectures comprising multivacant lacunary POMs and pyridyl ligands. By introducing pyridine moieties to its vacant sites, the trivacant lacunary phosphomolybdate [A-α-PMo9O34]9− is significantly stabilized in organic solvents. Furthermore, the resultant structure can be utilized as a stable and reactive building block to synthesize a dimer pillared by 4,4′-bipyridyl and a tetramer bridged by two cofacial porphyrin ligands, which can intercalate aromatic molecules.

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etal−organic architectures have received increasing research interest owing to their outstanding properties for application in many fields of science, including molecular recognition, separation, catalysis, and energy conversion.1−6 Metal−pyridine coordination is one of the most suitable systems for the self-assembly of well-designed structures because of the labile and reversible bonds formed as well as the flexibility of ligand design.7−11 The properties of metal− organic architectures are highly dependent on their constituent metals, organic ligands, and structure (i.e., shape and size), and, therefore, the development of novel systems will lead to unexplored functions and applications for these fascinating materials. Polyoxometalates (POMs) are clusters of anionic metal oxides where the metals are high oxidation states (e.g., Mo6+, W6+, V5+), and their chemical and physical properties can be readily modulated by appropriate selection of the structures and metal atoms, resulting in a wide range of applications, such as catalysis, photocatalysis, sensors, batteries, energy conversion, and magnetism.12−19 Lacunary POMs, where one or more {MOx} units are removed, possess structurally welldefined and highly reactive vacant sites for organic ligands and © XXXX American Chemical Society

Received: March 7, 2019

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DOI: 10.1021/jacs.9b02541 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 1. Schematic representation of the self-assembly of POM− organic architectures (I, II, III) from trivacant lacunary phosphomolybdates and pyridyl ligands.

Figure 3. (a) Synthesis of II from two I and three bpy ligands. (b) Crystal structure of TBA-II. (c) ESI mass spectrum of TBA-II in acetonitrile. Insets: a spectrum in the range of m/z 1795−1825, and a simulated pattern for [TBA9(PMo9O31)2(bpy)3]3+; a spectrum in the range of 2570−2620, and a simulated pattern for [TBA8(PMo9O31)2(bpy)3]2+. (d) 31P NMR spectrum and (e) 1H NMR spectrum of TBA-II in nitromethane-d3 (●, 4,4′-bipyridyl; ▲, TBA; ◊, nitromethane; ○, 1,4-dioxane; ◆, water).

in situ formed Na3H6[A-α-PMo9O34] with TBABr, (TBA = tetra-n-butyl ammonium), we attempted to synthesize an organic-solvent-soluble salt TBA3H6[A-α-PMo9O34].48 However, 31P NMR spectroscopy showed that the purity of the crude sample was ca. 86% (Figure S1a). Furthermore, TBA3H6[A-α-PMo9O34] is very unstable and easily undergoes isomerization into TBA3[PMo12O40] (a fully occupied structure) and dimerization into TBA6[P2Mo18O62] at room temperature within 5 min, even in organic solvents such as acetonitrile (Figures S1b, S2a,b). We have recently reported that alkoxo groups can be utilized as protecting groups for the highly reactive lacunary POM [Aα-SiW9O34]10−.35 However, decomposition and isomerization of TBA3H6[A-α-PMo9O34] proceeds immediately, even in the presence of alcohols (Figure S2c). In contrast, the stability of the lacunary phosphomolybdate is enhanced dramatically in the presence of pyridine, and we found that, upon treating TBA3H6[A-α-PMo9O34] with pyridine, pyridyl ligands are introduced to the vacant sites (TBA-I, see Supporting Information for detailed synthetic methods).

Figure 2. (a) Synthesis of I from TBA3H6[A-α-PMo9O34] and pyridine. Orange arrows indicate the coordination sites for pyridine. (b) Crystal structure of TBA-I. Polyhedra: P = purple, Mo = green. Spheres: C = black, H = pink, N = light-blue. 31P NMR spectra of (c) as-prepared TBA-I in acetonitrile-d3, (d) TBA-I in acetonitrile-d3 after being stored at room temperature for 7 days, and (e) TBA-I in a mixture of acetonitrile-d3 and pyridine-d5 (9/1, v/v) after being stored at room temperature for 7 days.

(II) pillared by three 4,4′-bipyridyl (bpy) ligands, and a tetramer (III) bridged by two 5,10,15,20-tetra(4-pyridyl)porphyrin (tpyp) ligands (Figure 1). Notably, III possesses a double-layered cofacial porphyrin structure, and aromatic molecules such as acenaphthenequinone (ANQ) can be intercalated between the two cofacial porphyrins. As discussed in the introduction, the Keggin-type trivacant lacunary phosphomolybdate [A-α-PMo9O34]9− is metastable in aqueous media. Therefore, by the cation exchange reaction of B

DOI: 10.1021/jacs.9b02541 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 4. (a) Synthesis of III from four I and two tpyp ligands and intercalation of ANQ into III. (b, c) Crystal structure of TPP-III ((b) top view; (c) side view). (d) Crystal structure of TPP-III·ANQ. (e) 1H NMR spectrum of TBA-III in acetonitrile-d3 (◆, pyridine). (f) ESI mass spectrum of TPP-III in acetonitrile. Insets: a spectrum in the m/z range 3040−3080 and a simulated pattern for [TPP16(PMo9O31)4(tpyp)2]4+. (g) ESI mass spectrum of a mixture of TPP-III and ANQ (100 equiv with respect to TPP-III) in acetonitrile. Insets: a spectrum in the m/z range 3085−3120 and a simulated pattern for [TPP16(PMo9O31)4(tpyp)2·ANQ]4+.

ligands are labile (Figure 2a). The stability of TBA-I is enhanced compared to that of TBA3H6[A-α-PMo9O34], and by adding excess pyridine, isomerization is completely prevented, even over 7 days (Figure 2d,e). A tetraphenyl phosphonium (TPP) salt of I (TPP-I) could also be synthesized by using TPPBr instead of TBABr (Table S1, Figures S7−S10). As mentioned above, the coordinated pyridine molecules in TBA-I are labile. Therefore, I could be a useful precursor for the synthesis of more complex POM−organic architectures. Accordingly, we examined the reaction of I and multidentate pyridyl ligands. By reacting TBA-I with 4,4′-bipyridyl (bpy) in acetonitrile, single crystals of TBA-II were obtained. The anion of TBA-II comprises two A-α-Keggin-type trivacant lacunary phosphomolybdates pillared by three bpy ligands (Figures 3a,b and S11, Table S1). The electrospray ionization (ESI) mass spectrum of TBA-II in acetonitrile presents sets of signals at

Single-crystal X-ray structural analysis successfully revealed that three pyridyl ligands are introduced on the vacant sites of TBA-I through Mo−N bonds (Figures 2a,b and S3, Table S1). The average Mo−N bond length in TBA-I (2.31 Å, Table S8) is slightly longer than those of previously reported Mo-pyridine complexes (2.20 to 2.30 Å),50 indicating the stronger σ property of the bonds. 31P NMR spectrum of TBA-I in acetonitrile-d3 revealed a single signal at −0.26 ppm (Figure 2c). 1H NMR spectrum of TBA-I in acetonitrile-d3 revealed 1H signals for pyridine at 8.82, 7.83, and 7.35 ppm (Figure S4), which are shifted slightly downfield compared with those of free pyridine (8.57, 7.73, and 7.33 ppm). The cold-spray ionization (CSI) mass spectra of TBA-I in acetonitrile and a mixture of acetonitrile and pyridine (9/1, v/v) both present a set of signals at m/z 2360 assignable to [TBA4PMo9O31]+ (Figures S5 and S6), indicating that the coordinated pyridyl C

DOI: 10.1021/jacs.9b02541 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society m/z 1810.569 and 2594.706, assignable to [TBA9(PMo9O31)2(bpy)3]3+ (theoretical m/z: 1810.571) and [TBA8(PMo9O31)2(bpy)3]2+ (theoretical m/z: 2594.713), respectively (Figure 3c). 31P NMR spectrum of TBA-II in nitromethane-d3 reveals a single signal at −0.70 ppm (Figure 3d). Furthermore, the 1H NMR spectrum of TBA-I in acetonitrile-d3 presents 1H signals for bpy at 9.14 and 7.67 ppm (Figure 3e), which are shifted slightly compared with those of free bpy (8.71 and 7.70 ppm). These results show that bidentate bpy ligands enhanced the stability of the hybrid, and the structure was maintained in the solvent. These results, elemental analyses, and thermogravimetric and differential thermal analysis (TG-DTA) data all indicate a formula of TBA6[(A-α-PMo9O31)2(bpy)3]·3(1,4-dioxane)·3H2O. Porphyrins are key motifs in a diverse range of biological processes and are thoroughly studied in coordination chemistry.51−54 Furthermore, cofacial porphyrins have been extensively studied as host molecules for guest recognition and other applications.55−60 Therefore, we were inspired to introduce porphyrins as organic linkers in the synthesis of cofacial porphyrin-based hybrid structures. By reacting TPP-I with 5,10,15,20-tetra(4-pyridyl)porphyrin (tpyp) (0.5 equiv with respect to TPP-I) in a mixture of N,Ndimethylacetamide (DMA) and 1,2-dichloroethane (DCE) (1/ 1, v/v) at 50 °C for 1 h followed by addition of toluene, TPPIII was obtained as purple crystals. The anion III is comprised of two tpyp molecules in a cofacial arrangement and four [A-αPMo9O31]3− units on the corners (Figure 4a,b,c, Table S2). The size of III is ca. 23.5 × 22.9 Å (Figure S12a). The pyridyl groups of tpyp coordinate two of the three reactive Mo atoms in vacant sites, while py coordinates the other Mo atom. Two tpyp units in III are located at a distance of ca. 6.6 Å (Figure S12b). Notably, the ESI mass spectrum of TPP-III in acetonitrile shows a set of signals at m/z 3057.365, assignable to [TPP16(PMo9O31)4(tpyp)2]4+ (theoretical m/z: 3057.356, Figure 4f), indicating that the cofacial porphyrin structure is stable in the solution. A similar structure (TBA-III) can also be synthesized by using TBA-I as a precursor (see Supporting Information for details). Due to the low solubility of TPP-III, TBA-III was used for NMR and UV−vis spectroscopy (Figures 4e, S13−S17). The 31P NMR spectrum of TBA-III in acetonitrile-d3 show a single signal at −0.24 ppm (Figure S13), also indicating the high stability of the structure in the solvent. Based on the above-mentioned results, elemental analyses, and TG-DTA data, the formula of TPP-III is TPP12[(A-α-PMo9O31)4(tpyp)2(py)4]·5DMA·12H2O and the formula of TBA-III is TBA10H2[(A-αPMo9O31)4(tpyp)2(py)4]·2py·7DMA·5DCE. Considering the interplanar distances (3.3 to 3.4 Å) of π−π stacked aromatic compounds, TPP-III possesses cofacial porphyrins (separated by ca. 6.6 Å) ideal for the intercalation of aromatic compounds. Indeed, the ESI mass spectrum of the acetonitrile solution of TPP-III in the presence of acenaphthenequinone (ANQ) shows a set of signals at m/z 3102.103, assignable to [TBA16(PMo9O31)4(tpyp)2·ANQ]4+ (theoretical m/z: 3102.109, Figure 4g), indicating the intercalation of ANQ. Similarly, the intercalation of other guests such as 1,4dihydroxynaphthalene, 1,4-naphthoquinone, and 1,8-naphthalic anhydride was also indicated by ESI mass spectroscopy (Figure S18). Single-crystal X-ray structural analysis revealed that ANQ is sandwiched by two porphyrins in TPP-III·ANQ (Figures 4a,d, S19, S20 Table S2).

In conclusion, we successfully demonstrated the selfassembly of anionic POM−organic architectures using lacunary phosphomolybdates and pyridyl ligands. In this paper, we report two important findings: First, the pyridyl groups protect the highly reactive vacant sites of the metastable lacunary phosphomolybdates and suppress their undesired isomerization and dimerization. We synthesized and isolated pyridine-coordinating A-α-Keggin-type trivacant lacunary phosphomolybdate (I) by introducing pyridines as protecting groups to the reactive vacant sites. Although phosphomolybdates are important materials in a wide range of fields, including catalysis, batteries, electronic devices, and energy conversion,36−45 functionalization and/or metal substitution have not been investigated, due mainly to the low stability of the lacunary species. Therefore, our stabilization method presents new research and application possibilities for phosphomolybdates. Second, the introduced pyridyl groups are moderately labile. Therefore, I can be utilized as a precursor in the synthesis of anionic POM−organic architectures. Notably, tetramer III possessing two cofacial porphyrin ligands can accommodate various aromatic molecules. Thus, the method reported here demonstrates a synthetic protocol for stabilizing and utilizing multivacant lacunary POMs as building blocks for the synthesis of novel POM−organic architectures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b02541. Detailed experimental procedures, crystallographic data, NMR, mass, and absorption spectra (PDF)

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Data for TPP-I, TBA-I, TBA-II, TPP-III, and TPP-III· ANQ (CIF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Kazuya Yamaguchi: 0000-0002-7661-4936 Kosuke Suzuki: 0000-0002-8123-1462 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from JST PRESTO Grant Number JPMJPR18T7, JSPS KAKENHI Grant Numbers 17H03037 and 18H04500, and the Ogasawara Foundation for the Promotion of Science & Engineering.

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Communication

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DOI: 10.1021/jacs.9b02541 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX