Self-Assembly of Goldberg Polyhedra from a Concave [WV5O11

Nov 19, 2018 - Nanoscale regular polyhedra with icosahedral symmetry exist naturally as exemplified by virus capsids and fullerenes. Nevertheless, the...
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Self–assembly of Goldberg polyhedra from a concave [WV5O11(RCO2)5(SO4)]3– building block with 5–fold symmetry Yuteng Zhang, Hongmei Gan, Chao Qin, Xinlong Wang, Zhongmin Su, and Michael J. Zaworotko J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10866 • Publication Date (Web): 19 Nov 2018 Downloaded from http://pubs.acs.org on November 19, 2018

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Journal of the American Chemical Society

Self–assembly of Goldberg polyhedra from a concave [WV5O11(RCO2)5(SO4)]3– building block with 5–fold symmetry Yuteng Zhang,†,§ Hongmei Gan,†,§ Chao Qin,† Xinlong Wang,*,† Zhongmin Su,† and Michael J. Zaworotko*,‡ †National

& Local United Engineering Laboratory for Power Batteries, Key Laboratory of Polyoxometalate Science of Ministry of Education, Northeast Normal University, Changchun, Jilin, China. ‡Bernal Institute, Department of Chemical Sciences, University of Limerick, Limerick, V94 T9PX, Republic of Ireland Supporting Information Placeholder

ABSTRACT: Nanoscale regular polyhedra with icosahedral symmetry exist naturally as exemplified by virus capsids and fullerenes. Nevertheless, their generation by supramolecular chemistry through the linking of 5–fold symmetry vertices remains unmet because of the absence of 5–fold symmetry building blocks with the requisite geometric features. This situation contrasts with that of tetrahedral and octahedral symmetry metal–organic polyhedra (MOPs), for which appropriate triangular and square molecular building blocks (MBBs) that can serve as vertices or faces are readily available. Herein, we report isolation of a pentagonal [WV5O11(SO4)6]8– cluster and reveal its utility to afford the first four examples of nanoscale Goldberg MOPs, based upon 5–fold MBBs. Two 32–faced Gv(1,1) MOPs and two 42– faced Gv(2,0) MOPs were formed using linear or triangular organic ligands, respectively. The largest Goldberg MOP–4, exhibits a diameter of 4.3 nm, can trap fullerene C60 molecules in its interstitial cavities.

Convex polyhedra are of broad interest thanks to their aesthetic beauty, their relevance to geometry and their adoption by architects.1 Platonic, Archimedean,2 Kepler3,4 and Goldberg polyhedra5–7 are four well–known classes of convex regular polyhedra. Goldberg polyhedra,8 which exhibit icosahedral geometries, are comprised of pentagons and hexagons and exist naturally in the form of fullerenes9 and virus capsids.10 Metal– organic polyhedra (MOPs) with tetrahedral and octahedral symmetry can be designed by synthetic and supramolecular chemists from first principles through use of appropriate triangular or square molecular building blocks (MBBs).11–14 However, Goldberg MOPs, remain a largely unsolved design challenge to synthetic and supramolecular chemists. First, the large number of edges and vertices for a typical Goldberg polyhedron complicates their design and self–assembly; there are 90 edges and 60 vertices in the simplest Goldberg polyhedron. Second, there are not yet any suitable pentagonal MBBs, thereby hindering their accessibility by self–assembly of five–coordinated nodes. As noted by Michael O’Keeffe, a pioneer in reticular chemistry, “We have not found such examples for molecules based on icosahedral polyhedra and the preparation of these remains a nice challenge”.15 Indeed, to our knowledge there exists only one example of a self–assembled Goldberg polyhedron which, as reported by Fujita’s group in 2016, is elegantly prepared by a combination of triangles and squares.7

Regular pentagons cannot tile a plane and so curvature or distortion from regular geometry is needed to generate curved or domed structures from pentagonal MBBs.16 Given that an appropriate pentagonal MBB is a prerequisite for such Goldberg MOPs, we have targeted inorganic clusters as potential MBBs. In earlier work, our groups independently succeeded in assembling triangular17,18 (Figure 1a) and tetragonal19–21 (Figure 1b) polyoxovanadate cluster MBBs into MOPs with tetrahedral and octahedral symmetries (Figure 1d–g). However, pentagonal MBBs remain elusive even though pentagonal moieties {M(M)5} (M = Mo, W, Ti)22,23 exist in inorganic clusters such as {Mo132}24 as reported by Müller. Herein, we address this issue by reaction of VOSO4 and Na2WO4, affording [C2NH8]7H2[WV5O11(SO4)6]·(SO4)0.5 as an

Figure 1. MOPs based on vanadium–clustered MBBs. (a) Triangular {V6S}. (b) tetragonal {V5Cl}. (c) pentagonal {WV5}. (d–g) Self–assembly of triangular and tetragonal MBBs with linear or triangular ligands into MOPs having tetrahedral and octahedral symmetry. (h, i) Self–assembly of the pentagonal MBB with linear organic ligands into icosahedral Goldberg polyhedron Gv(1, 1), and with triangular ligands into Gv(2, 0). (j–l) Ideal models of Goldberg polyhedron Gv(1, 1), Gv(1, 0) and Gv(2, 0).

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Figure 2. Above: ball-and-stick representation of the pentanuclear

cluster [WV5O11(SO4)6]8-. Below: side view of {WV5} revealing its concave nature and the orientation of the bridging sulphates.

isolated cluster. This cluster, {WV5}, has the symmetry and geometry to serve as vertex for Goldberg MOPs. Specifically, {WV5} assembles with linear or triangular ligands of different lengths to generate four examples of two distinct types of Goldberg MOPs. Goldberg MOP–1 and 2 are comprised of 12 {WV5} MBBs and 30 linear ligands and exhibit Gv(1, 1) geometries. Goldberg MOP–3 and 4 are comprised of 12 {WV5} MBBs and 20 triangular ligands to offer Gv(2, 0) geometries. Single–crystal X–ray diffraction reveals that {WV5} exhibits ‘pentastar’ symmetry (Figure 2 above) as five V cations are connected by sharing corners to form a {V5} ring. This {V5} ring is further linked with a central WO7 pentagonal bipyramid in edge–sharing mode, thereby affording the {WV5} core. The vanadium cations are almost coplanar with the average deviations from the {V5} plane of 0.03 Å. The W cation is located in center of the five V cations and is above the {V5} plane by 0.7035(2) Å. One SO42− ligand is coordinated to the central part of the {WV5} unit while the remaining five SO42− ligands are bound to its periphery in such a manner that the concave shape MBB (Figure 2 below) is suitable for constructing polyhedra. Overall bond valence sum (BVS)25 calculations indicate that V cations and the W cation exhibit +4 and +6 oxidation states, respectively (Table S1). Classic Goldberg polyhedra are generally denoted tri–Gv(h, k).6 According to this notation, tri–Gv(1, 1), one of the two simplest Goldberg polyhedra (Gv(1, 0) and Gv(1, 1)), requires that adjacent pentagons are related as in Figure 1j. This means that, from a self–assembly perspective, linear linker ligands should logically generate G(1,1) polyhedra. We introduced a linear organic ligand, 1,4–benzenedicarboxylic acid (H2BDC), during synthesis and thereby obtained crystals of [H2NMe2]21[(WV5O11SO4)12(BDC)30][W12V20O94]0.25 (Goldberg MOP–1, Figure 1h). The refined structure of Goldberg MOP–1 reveals that it is comprised of 12 {WV5} MBBs and 30 BDC ligands in which each {WV5} MBB is linked by BDC ligands to form a sphere of diameter 3.3 nm (Figure 3a). Goldberg MOP– 1 is the expected tri–Gv(1, 1) polyhedron, which is a truncated icosahedron with 90 edges and 60 vertices and can be classified as an Archimedean polyhedron with 32 faces, 12 pentagons and 20 hexagons. The angle between BDC ligands and the {WV5} pentagonal plane is 121.98°, very close to that of the ideal

Figure 3. The overall views of the structures of Goldberg MOPs 1–

4. Nearly spherical molecular structures with icosahedral symmetry were unveiled. Pentagonal MBBs are presented by polyhedra and organic ligands are shown by sticks.

truncated icosahedron (121.68°), and the dihedral angle between {BDC}6 hexagon and {WV5} pentagon is 142.576°, also close to the ideal value (142.599°) (Figure S2). This specific polyhedron is common in everyday life as exemplified by soccer balls and Buckminsterfullerene.26 If the {WV5} MBB is simplified into a 5– fold vertex and the linear ligand is treated as edge then Goldberg MOP–1 can be regarded as an icosahedron, the largest Platonic solid. The relationship between an icosahedron and truncated icosahedron is presented in Figure S3a. Given that tri–Gv(1, 1) represents a family of related polyhedra, we replaced H2BDC with an extended version, 2,6– naphthalenedicarboxylic acid (H2NDC), to obtain the second member of this family, [H2NMe2]21[(WV5O11SO4)12(NDC)30][W12V20O94]0.25 (Goldberg MOP–2). Goldberg MOP–2 exhibits a diameter of 3.7 nm (Figure 3b). The interior of Goldberg MOP–1 and 2 is occupied by the inorganic cluster {W12V20O94} (Figure S2), a regular dodecahedron (Figure 1k). All vertices are trivalent in Goldberg polyhedra and triangular organic ligands should thus also be suitable candidates to link {WV5} MBBs, in which case Gv(2, 0) polyhedra are expected (Figure 1l). 1,3,5–benzenetricarboxylic acid (H3BTC) and its expanded analogue 4,4’,4’’–s–triazine–2,4,6–triyl–tribenzoic acid (H3TATB) indeed afforded two crystalline compounds [H2NMe2]12[(WV5O11SO4)12(BTC)20] (Goldberg MOP–3) and [H2NMe2]12[(WV5O11SO4)12(TATB)20] (Goldberg MOP–4) (Figure 1i). Goldberg MOP–3 and 4 are isostructural and comprised of 12 {WV5} MBBs and 20 organic ligands. They exhibit diameters of 2.9 and 4.3 nm, respectively (Figure 3c, d). Their shapes resemble the carbon–based fullerene C80. Gv(2, 0) polyhedron is the truncated rhombic triacontahedron with 120 edges and 80 vertices, whose 42 faces include 12 pentagons and 30 hexagons (Figure S4). If the {WV5} MBB is simplified to a 5–c

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Journal of the American Chemical Society Overall, four TATB ligands surround and stabilize C60 molecule. Included C60 molecules are released when Goldberg MOP–4·C60 is immersed in toluene for 12 hours, at which point toluene had turned from colorless to violet (Figure S10 insert). The characteristic absorption bands of C60 were observed in the UV– vis spectrum of the toluene solution (Figure S10).

Figure 4. Fullerene C60 inclusion in Goldberg MOP–4. (a, b) C60 trapped in crystal lattice of MOP–4. (c, d) Raman and UV–vis spectra of C60, MOP–4 and MOP–4·C60.

Figure 5. Temperature dependence of χM (●) and χMT (○) versus T plots of Goldberg MOP–3. The red solid line corresponds to the calculated behavior of compound. Inset: schematic topology of the magnetic interaction J between the VIV centres. vertex and BTC or TATB is treated as a 3–c vertex, then Goldberg MOP–3 and 4 are rhombic triacontahedra, a Catalan polyhedron (V3.5.3.5).27 The relationship between Catalan, Goldberg and Archimedean polyhedra is illustrated in Figure S3b. This series of Goldberg MOPs has good solubility and stability in most solvents as confirmed by ESI-MS, DLS and NMR spectra (Figure S5-S7). Goldberg MOP–4, the largest Goldberg MOP herein, was studied as a potential inclusion compound. Formation of Goldberg MOP–4 in the presence of C60 indeed resulted in an isostructural inclusion compound, Goldberg MOP–4·C60, in which the color had changed from green to purple. The inclusion of fullerene was probed by Raman spectroscopy with 488 nm excitation (Figure 4c), which revealed peaks attributable to both Goldberg MOP–4 and C60, including the Hg(1) 274 nm, Ag(1) 497 nm, Ag(2) 1465 nm and Hg(8) 1576 nm vibrations of C60, the Ag(2) vibration being characteristic of C60. Moreover, the solid– state UV–vis spectra also support inclusion of fullerene with absorption peaks of C60 at 244 nm and 337 nm (Figure 4d). The single crystal X–ray structure of Goldberg MOP–4·C60 reveals (Figure 4a, b and S8) that C60 molecules lie in interstitial space between adjacent cages. This rhombic window is created by adjacent TATB ligands of one {WV5} MBB form (Figure S9). The geometry of these two TATB moieties is somewhat similar to porphyrin rings, which are well–known fullerene receptors.

The temperature–dependent magnetic susceptibility of MOP–3 was conducted over the range 2−300 K. The plots of molar magnetic susceptibility χM and χMT versus T under a constant magnetic field of 1000 Oe are shown in Figure 5. The value of χMT at 300 K is 22.7 cm3 K mol−1, in good agreement with the expected value for sixty uncoupled V(IV) ions (S = 1/2, g = 2.00). Upon cooling, χMT increases slowly down to 50 K and then increases sharply to reach 36.92 cm3 Kmol−1 at about 4.0 K before decreasing again to 35.97 cm3 K mol−1 at 2.0 K. The sharp increase indicates intramolecular ferromagnetic interactions between the V(IV) ions in the {WV5} subunit and the decrease of χMT value below 4.0 K can be ascribed to the presence of antiferromagnetic interactions between {WV5} subunits. The magnetic susceptibility data was fitted on the basis of spin–only Hamiltonian, Ĥ = −2J(ŜV1·ŜV2 + ŜV2·ŜV3 + ŜV3·ŜV4 + ŜV4·ŜV5+ ŜV5·ŜV1), using the PHI program,20 and the best–fit parameters obtained are J = 5.66 cm−1, g = 1.98 and the intermolecular antiferromagnetic interactions zj = −0.085 cm−1 (Figure 5, red line). The fitting result indicates the presence of ferromagnetic interactions between the V(IV) ions In summary, the first example of a pentagonal MBB suitable for Goldberg polyhedra was isolated under solvothermal conditions. When this pentagonal MBB was reacted with carboxylate ligands, two families of Goldberg MOPs were afforded. We consider these results to be noteworthy for several reasons. First the approach we have taken is likely to be effective for other carboxylate ligands and related clusters and other Gv(1, 1) polyhedra, Gv(2, 0) are likely to be readily accessible. Indeed, the isomorphic clusters, {MoV5}, {NbV5} and {TaV5} have also been prepared. Second, the the compositions of the four Goldberg MOPs reported herein imparts stability and water solubility, enabling them to be readily crystallised and to serve as host frameworks for guests as large as C60. Finally, the organisation of pentagons and hexagons herein is akin to cowpea chlorotic mottle virus and Semliki Forest virus, respectively (Figure S11). The approach we have taken is an extension of work conducted independently by our groups on the use of 4–fold symmetry polyoxometallates to serve as vertices. It is likely to be general enough to be applied systematically to generate other families of Goldberg MOPs.

ASSOCIATED CONTENT Supporting Information. Experimental details, PXRD, TG spectra and other materials. These material are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] ORCID Xinlong Wang: 0000–0002–5758–6351 Zhongmin Su: 0000–0003–0156–7191 Michael J. Zaworotko: 0000–0002–1360–540X

Author Contributions

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§ These authors contributed equally

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the NSFC of China (No. 21471027, 21671034). We thank Prof. Jinkui Tang from Changchun Institute of Applied Chemistry for his help during magnetic measurement and insightful discussions.

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