Zinc(II)-Directed Self-Assembly of Metal–Organic Nanocapsules

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Zinc(II)-directed Self-assembly of Metal-organic Nanocapsules Asanka S. Rathnayake, Charles L. Barnes, and Jerry L. Atwood Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00597 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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Zinc(II)-directed Self-assembly of Metal-organic Nanocapsules Asanka S. Rathnayake,† Charles L. Barnes,† and Jerry L. Atwood *† †

Department of Chemistry, University of Missouri-Columbia, 601, S. College Avenue, Columbia MO 65211 * Corresponding author

ABSTRACT: A metal-organic nanocapsule consisting of 24 Zn2+ ions and six pyrogallol[4]arene units, has been synthesized and structurally characterized. All the Zn2+ ions seaming the nanocapsule are disordered over two positions forming concave distortions on the framework.

Schematic representation of the synthesis of [Zn24(PgC5)6] nanocapsule. Axial ligands, solvent molecules, and hydrogen atoms are removed for clarity. Color code: Zn = teal, O = red, C = grey.

Jerry L. Atwood University of Missouri-Columbia Department of Chemistry 125 Chemistry Building Columbia, MO 65211 Phone: (573) 882-9606 Fax: (573) 882-2754 Email: [email protected] ACS Paragon Plus Environment

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Zinc(II)-directed Self-assembly of Metal-organic Nanocapsules Asanka S. Rathnayake,† Charles L. Barnes,† and Jerry L. Atwood *† †

Department of Chemistry, University of Missouri-Columbia, 601, S. College Avenue, Columbia MO 65211

Supporting Information Placeholder ABSTRACT: A metal-organic nanocapsule consisting of 24 Zn2+ ions and six pyrogallol[4]arene units, has been synthesized and structurally characterized. All the Zn2+ ions seaming the nanocapsule are disordered over two positions forming concave distortions on the framework.

Metal-directed molecular self-assembly plays a vital role in biological systems.1-6 Synthetic metal-organic nanostructures have been investigated to understand the proliferation and function of these natural molecular assemblies.1, 7-12 With regard to the synthesis of biologically relevant metal-organic assemblies, we have primarily focused on C-alkylpyrogallol[4]arenes (PgCn, where n is the number of carbon atoms in the pendant alkyl chains), as the

organic ligand. PgCn is a class of polyphenolic compounds, which may act as receptor molecules for metal ions, upon deprotonation of upper-rim phenolic groups. Several types of metal-PgCn complexes have been synthesized, however, we are specifically interested in metal-organic nanocapsules (MONCs).13-16 Two structural types of MONCs have been discovered: dimers, and hexamers, of which, respectively, two or six PgCn units are assembled into capsular structures seamed by eight or 24 divalent metal ions. To date, we have reported the synthesis and characterization of several different MONCs.13-15, 17-21 One of the initial discoveries related to PgCn-based MONCs was a Zn2+-seamed dimeric nanocapsule.16, 22 We now report the reaction conditions used to facilitate the self-assembly of the first Zn2+-seamed hexameric MONC, [Zn24(C-pentylpyrogallol[4]arene)6] (1), as well as the unexpected structural features found therein.

Figure 1. Synthesis of 1. Alkyl chains, hydrogen atoms, and solvent molecules are removed for clarity. Color code: Zn = teal, O = red, C = grey. Nanocapsule 1 was synthesized by reacting PgC5, Zn(NO3)2, and triethylamine in a CH2Cl2/ethanol mixture (Figure 1) (See Supporting Information). Dark blue X-ray quality crystals were

formed over a period of several weeks. Single crystal X-ray diffraction analysis revealed that the crystals are hexameric MONCs, formed by the self-assembly of six PgC5 units and 24 Zn2+ ions.23

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This coordination nanocapsule resembles an octahedron, of which each vertex consists of six membered [Zn3O3] units. The overall structure of 1 is similar to other reported hexameric MONCs, however, structure displays unique structural features. In hexameric MONCs characterized to date, the metal ions are normally co-planar with the coordinated oxygen atoms of the upper-rim phenolic groups.14, 15, 21 However, in 1, all the Zn2+ ions are disordered over two positions, above and below the capsule surface (Figure 2). This disorder may occur because the coordination requirements of the Zn2+ ions do not match the coordination geometry defined by the phenolic groups.

tions (O∙∙∙O = 3.789-3.798 Å). This might be due to wide separation of Zn2+ ions through phenolic oxygen atoms.

Figure 3. Side view of 1 showing the metal-ligand arrangement at major occupancy level. Two facets of 1 are bent inwards forming slight concave distortions on the framework. The expanded view shows the coordination of axial ligand of two adjacent facets at one concave sites. Alkyl chains, hydrogen atoms, and solvent molecules are removed for clarity. Color code: Zn = teal, O = red, C = grey.

Figure 2. (A) Side view of a [Zn3O3] array with disordered Zn2+ ions, and (B) distorted square pyramidal geometry around each of the two disordered Zn2+ ions. Alkyl chains and hydrogen atoms are removed for clarity. Color code: Zn = teal, O = red, C = grey. All the metal ions in 1 have been modelled using two different occupancy values. At major occupancy level, a close look at 1 shows that two sides of the octahedron bent inwards forming, slight, concave distortions on the framework. These two distortions have been occurred on two vertices opposite each other, through an inversion center. With regard to the coordination geometry, all the Zn 2+ ions show distorted square pyramidal geometry by coordinating to four equatorial phenolic groups [Zn-O(planar) = 1.948-2.084 Å] and one axial water ligand, [Zn-O(axial) = 2.098-2.147 Å]. It is important to note that these axial ligands are located either inside or outside of the capsule surface, depending on the position of the coordinated Zn2+ ions. For example, at major occupancy level, axial ligands at two concave sites are located inside the capsule, whereas the other axial ligands reside on the exterior of the capsule (Figure 3). The square pyramidal coordination geometry around each Zn 2+ ion can be explained by considering their coordination to upperrim phenolic groups. For instance, this structurally rigid, planar coordination site, between upper-rim phenolic groups, limits or restricts the flexibility of Zn2+ ions to rotate around.24 Therefore, upper-rim phenolic groups tend to arrange equatorially, forming distorted square planar geometry around Zn2+ ions. This positioning allows an additional ligand to come along the axial direction and coordinates to Zn2+ site forming a distorted square pyramidal geometry. This is the similar type of coordination geometry observed in [Zn2+-(salphen)]- and [Zn2+-(porphyrin)]-type complexes.25-29 The O∙∙∙O contact between axial water molecules, in cyclic arrays, suggested that they do not form hydrogen bonding interac-

To quantitatively evaluate the coordination geometry around Zn2+ ions, using bond angle calculations, tau-5 (τ5) values have been determined (See Supporting Information).30 The τ5 value usually intermediates between zero and one, where zero indicates perfect square pyramidal geometry and one implies perfect trigonal bipyramidal geometry. The calculated τ5 values, for all disordered Zn2+ ions in the asymmetric unit, fit into almost perfect square pyramidal geometry (average τ5 = 0.0083). This large coordination nanocapsule is further stabilized by intramolecular hydrogen bonding interactions formed through upper-rim phenolic groups (O∙∙∙O = 2.442-2.523 Å). Therefore, 1 consists of 96 coordination bonds and 24 hydrogen bonding interactions. In the solid state, the internal volume of this nanocapsule is calculated to be ~1300 Å3 (Internal volume is calculated using MSRoll with sphere radius of 1.25 Å). This suggests that, despite having slight distortions on the framework, 1 still has the ability to trap fairly large guest species. In conclusion, we have reported the synthesis and structural characterization of the first hexameric Zn2+-coordinated MONC. Metal-ligand arrangement of this nanocapsule closely resembles that of a typical hexamer, however, all the Zn 2+ ions are disordered over two positions, below and above the capsule surface. This structural feature is due to the imperfect match of the coordination requirements of the Zn2+ ions with the structural demands imposed by the upper-rim phenolic groups of the pyrogallol[4]arenes.

ASSOCIATED CONTENT Supporting Information Detailed synthetic method, crystal data, single-crystal X-ray information file (CIF) for 1 (CCDC# 1543763), bond angle calculations, UV-visible spectrum, MALDI-TOF mass spectrum, DLS data, and a figure showing the crystal packing of 1. The Supporting material is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION

Corresponding Author *Email: [email protected]. Telephone: (573) 882-9606

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interests.

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(14) McKinlay, R. M.; Cave, G. W. V.; Atwood, J. L., Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 5944-5948. (15) McKinlay, R. M.; Thallapally, P. K.; Atwood, J. L., Chem. Commun. 2006, 2956-2958. (16) Power, N. P.; Dalgarno, S. J.; Atwood, J. L., New J. Chem. 2007, 31, 17-20. (17) Atwood, J. L.; Brechin, E. K.; Dalgarno, S. J.; Inglis, R.; Jones, L. F.; Mossine, A.; Paterson, M. J.; Power, N. P.; Teat, S. J., Chem. Commun. 2010, 46, 3484-3486. (18) Fowler, D. A.; Mossine, A. V.; Beavers, C. M.; Teat, S. J.; Dalgarno, S. J.; Atwood, J. L., J. Am. Chem. Soc. 2011, 133, 1106911071. (19) Fowler, D. A.; Rathnayake, A. S.; Kennedy, S.; Kumari, H.; Beavers, C. M.; Teat, S. J.; Atwood, J. L., J. Am. Chem. Soc. 2013, 135, 12184-12187. (20) Jin, P.; Dalgarno, S. J.; Atwood, J. L., Coord. Chem. Rev. 2010, 254, 1760-1768. (21) Rathnayake, A. S.; Feaster, K. A.; White, J.; Barnes, C. L.; Teat, S. J.; Atwood, J. L., Cryst. Growth Des. 2016, 16, 3562-3564. (22) Power, N. P.; Dalgarno, S. J.; Atwood, J. L., Angew. Chem. 2007, 119, 8755-8758. (23) C288H312O96Zn24, M = 6878.24, dark blue prism, 0.30 x 0.25 x 0.20 mm3, trigonal space group = R-3, a = 41.9823(9) Å, b = 41.9823(9) Å, c = 42.3607(11) Å, α = 90°, β = 90°, γ = 120°, V = 64659 (3) Å3, Z = 6, Dc = 1.060 g/cm3, F000 = 21168, Cu Kα λ = 1.5418 Å, T = 100(2) K, 2θmax = 59.1º, 215986 reflections collected, 10362 unique (Rint = 0.053). Final GooF = 1.120, R1 = 0.1190, wR2 = 0.436, R indices based on 8056 reflections with I > 2σ(I) (refinement on F2), 649 parameters, 53 restraints. Lp and absorption corrections applied (μ = 1.91 mm-1). . (24) Brock, S. L.; Kauzlarich, S. M., lnorg. Chem. 1994, 33, 24912492. (25) Belmonte, M. M.; Wezenberg, S. J.; Haak, R. M.; Anselmo, D.; Escudero-Adán, E. C.; Benet-Buchholz, J.; Kleij, A. W., Dalton Trans. 2010, 39, 4541-4550. (26) Huang, C.-Y.; Su, Y. O., Dalton Trans. 2010, 39, 8306-8312. (27) Hui, J. K. H.; Yu, Z.; MacLachlan, M. J., Angew. Chem. 2007, 119, 8126-8129. (28) Kleij, A. W., Chem. Eur. J. 2008, 14, 10520-10529. (29) Kleij, A. W.; Kuil, M.; Lutz, M.; Tooke, D. M.; Spek, A. L.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H., Inorg. Chim. Acta. 2006, 359, 1807-1814. (30) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C., J. Chem. Soc., Dalton Trans. 1984, 1349-1356.

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Synopsis: The synthesis and structural characterization of a novel zinc-seamed hexameric nanocapsule, [Zn24(PgC5)6] is presented. The overall structure of this nanocapsule is similar to other hexameric nanocapsule reported so far. However, two sides of this capsule bent inward forming, slight, concave distortions on the framework.

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Figure 1. Synthesis of 1. Alkyl chains, hydrogen atoms, and solvent molecules are removed for clarity. Color code: Zn = teal, O = red, C = grey. 100x56mm (300 x 300 DPI)

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Figure 2. (A) Side view of a [Zn3O3] array with disordered Zn2+ ions, and (B) distorted square pyramidal geometry around each of the two disordered Zn2+ ions. Alkyl chains and hydrogen atoms are removed for clarity. Color code: Zn = teal, O = red, C = grey. 62x47mm (300 x 300 DPI)

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Figure 3. Side view of 1 showing the metal-ligand arrangement at major occupancy level. Two facets of 1 are bent inwards forming slight concave distortions on the framework. The expanded view shows the coordination of axial ligand of two adjacent facets at one concave sites. Alkyl chains, hydrogen atoms, and solvent molecules are removed for clarity. Color code: Zn = teal, O = red, C = grey. 42x21mm (300 x 300 DPI)

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