Solvomorphism of Double-Layered Networks through Single-Crystal

Communication Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Solvomorphism of Double-Layered Networks through Single-Crystalto-Single-Crystal Transformation Sangseok Lee, Seo Young Hwang, Haeri Lee, and Ok-Sang Jung* Department of Chemistry, Pusan National University, Pusan 46241, Republic of Korea S Supporting Information *

ABSTRACT: Self-assembly of Hg(CF3SO3)2 with benzene-1,3,5-triyltris(methylene)tripicolinate (L) as a C3-symmetric tridentate ligand gives rise to a unique double-layered honeycomb two-dimensional (2D) structure of [Hg3L4](CF3SO3)6 with an hcb, net/(6,3) topology. In this communication, a unique 2D double-layered network’s four unprecedented polymorph patterns formed under various temperature, time, and solvent conditions are described. The double-layered 2D structure with π···π interaction affords two solvent-dependent morphs via its flexible conformation and sliding nature. Of the two morphs, a morph from acetone/dioxane changes to two different morphs according to time and temperature in single-crystal-to-single-crystal (SCSC) fashion. fficient construction of open-coordination networks from molecular building blocks has resulted in exciting advances in the field of task-specific functionalities such as those of photoelectronic devices, ion exchangers, desiccants, molecular sieves, sensors, catalysts, biomimetics, negative Poisson’s ratio materials, guest-preorganization, and unusual state-transition phenomena.1−14 Among the numerous functional motifs, formation of desirable two-dimensional (2D) networks as a site-flexible template for reversible adsorption/desorption has been an intriguing topic.15−17 Accordingly, new tridentate tectonics as spacers have been employed as triangular modules for the construction of interesting 2D networks.17−21 During the construction of useful 2D or 3D structures, solvation/ desolvation in crystallization via self-assembly of central metal ions with appropriate spacer ligands is a significant process enabling understanding of adsorption/desorption, heterogeneous catalysis, photoreactors, and porous templates for structure determination of liquid compounds.22−30 Thus, the polarity, protic properties, coordinating ability, and bulkiness of solvents molecules can be critical factors to both crystallization and construction of functional 2D networks. Furthermore, a noteworthy conceptual feature of 2D molecular materials is the possibility of an interlayer sliding and stretching nature.16,17,31−35 For such a flexible 2D porous system, therefore, polymorphism can significantly occur, as dependent on various conditions, which indeed can be momentous factors in the formation of desirable crystalline materials. Polymorphism usually results from the existence of different conformers of the same molecule, hydration, or solvation. This is more correctly referred to as solvomorphism, as different solvates have different chemical formulas.36,37 For instance, solvent-dependent polymorphism is relevant to the fields of pharmaceuticals,

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© XXXX American Chemical Society

agrochemicals, pigments, dyestuffs, foods, and explosives because it can determine morphology, crystal habit, amorphous fraction, and crystallographic defects.38−41 In this communication, a unique 2D double-layered network’s four unprecedented polymorph patterns formed under various temperature, time, and solvent conditions are described. This is a unique pseudopolymorphic system constructed from self-assembly of Hg(CF3SO3)2 with a flexible C3-symmetric nitrogen donor. Herein, we report the solvomorphism and four morphs of a conceptually advanced doublelayered 2D network of [Hg3L4](CF3SO3)6 (L = benzene-1,3,5triyltris(methylene)tripicolinate) constructed via assembly of tetrahedral geometrical Hg(II) with C3-symmetric L. The present study undertook proof-of-concept experiments on the delicate modulation of the porous nature of the flexible 2D system via single-crystal-to-single crystal (SCSC) solvomorphism. A schematic of the construction of the unusual doublelayered molecular network and the related solvomorphism is provided in Scheme 1. The tridentate N-donor, benzene-1,3,5triyltris(methylene)tripicolinate (L), is a key C3-symmteric tectonic for the double-layered 2D network, coordinating with the mercury(II) ion. On this schematic basis, the self-assembly of Hg(CF3SO3)2 with L in two solvent systems in 3:4 stoichiometry afforded two morph crystals of [Hg3L4](CF3SO3)6·xSolvent (I and II) composition in high yields. The reactions were originally carried out in the 1:1 mol ratio of Hg(II) to L. However, the product formation was not Received: December 8, 2017 Revised: January 16, 2018 Published: January 29, 2018 A

DOI: 10.1021/acs.cgd.7b01718 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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compounds are shown in Figure S1 in the Supporting Information. All of the crystal structures are unique double-layered 2D networks of [Hg3L4](CF3SO3)6·xSolvent composition, as shown in Figures 1, 2, S2 and S3 (Supporting Information); their relevant bond distances and angles are listed in Table S1 in the Supporting Information. All crystal structures are basically similar double-layered networks except for solvate molecules (Figure 1). Accordingly, their unit cell volumes slightly differ: 15555(1) Å3 for crystal I, 16689.6(5) Å3 for II, 16071(2) Å3 for III, and 17466.3(5) Å3 for IV. Each mercury(II) ion is a distorted tetrahedral arrangement with four ortho-pyridyl moieties from four ligands (N−Hg−N = 86.561(1)−141.256(2)°) (see in Figure S4 in the Supporting Information). The distortion is partly due to the weak interaction of Hg(II) with the oxygen of the ester moiety (Hg···OC, 2.645(8)−2.824(6) Å); thus, the local geometry is a pseudo antisquare prism. The 2D network consists of unique double-layered hexagonal motifs of hcb, net/(6,3) topology43 (Hg6L12; intradouble-layer distance = 3.67(4), 3.75(2) Å, diameter Hg···Hg = 25.4683(9), 26.9994(7) Å for I; intradouble-layer distance = 3.77(4), 3.81(4) Å, Hg···Hg = 25.6607(4), 26.8650(4) for II; intradouble-layer distance = 3.70(2), 3.74(2) Å, Hg···Hg = 25.781(2), 26.921(1) for III; intradouble-layer distance = 3.68(1), 3.85(3) Å, Hg···Hg = 25.8422(4), 26.9518(4) Å for IV), resulting in the formation of a 102-membered ring in which all of the Hg(II) ions are coplanar in the solid state owing to the presence of the weak Hg···OC interactions. All of the hexagonal open cavities are filled for I, II, III, and IV, respectively, by the following: four acetonitrile molecules, four dioxane molecules, three dioxane molecules, and five dioxane molecules along with triflate anions. The amount of solvate molecules was estimated by 1H NMR spectra and TG analyses as shown in Figures S5 and S6 in the Supporting Information. The interdouble-layer distances for the I, II, III, and IV crystals are 6.41(3), 6.88(9), 6.59(4), and 7.14(4), respectively, indicating the high sensitivity of distance and flatness to the crystallization condition. The flatness of the

Scheme 1. Schematic Diagram of Construction of Unusual Double-Layered Molecular Network and Related SCSC Solvomorphism

significantly affected by the change of the mole ratio or concentration, which indicated that the skeleton is a thermodynamically stable structure. The triflate (CF3SO3−) anion plays a partial role in the construction of the 2D networks, acting as a counteranion rather than a coordinating moiety.42 Furthermore, the noncoordinating triflate anion induces the present unique double-layered 2D skeleton. In particular, II crystal was changed to III crystal and IV crystal by long-time standing in the mother liquor and 50 °C temperature, respectively, in a single-crystal-to-single-crystal (SCSC) manner, whereas I crystal is relatively stable under those conditions. The four polymorphic crystals have similar skeletons, but their delicate structures including packing diagrams are different under each condition, as will be discussed in detail. The formed crystalline solids suitable for X-ray single-crystallography slowly lost their crystallinity owing to the evaporation of the solvated molecules on the surface in air and were insoluble in water and common organic solvents (Table 1). They were dissociated in Me2SO and N,Ndimethylformamide. Infrared (IR) spectra for the present Table 1. Crystallographic Data formula Mw (g mol−1) cryst. System space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z μ (mm−1) F(000) data collected Rint parameters completeness (%) GoF on F2 R1 [I > 2σ(I)]a wR2 (all data)b a

I

II

III

IV

C122H96F18Hg3N16O42S6 3594.28 monoclinic I2/c 17.7304(5) 47.6156(16) 18.4253(8) 90.458(3) 15554.9(10) 4 3.129 6744 15311 0.0876 879 100 1.051 0.0702 0.2297



C134H124F18Hg3N12O52S6 3870.60 monoclinic I2/a 18.5284(3) 47.3052(9) 20.0353(4) 95.945(2) 17466.3(6) 4 1.304 2.787 17193 0.0772 879 100 1.041 0.0695 0.2230

R1 = Σ∥F0| − |Fc∥/Σ|F0|. bwR2 = (Σ[w(F02 − Fc2)2]/Σ[w(F02)2])1/2. B



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Figure 2. Helicity of tridentate ligand in [Hg3L4](CF3SO3)6·xSolvate showing PMMP··· mode.

Figure 1. Crystal structures of double-layered network ((a) top view; (b) side view), (c) [Hg3L4](CF3SO3)6·4CH3CN (I), (d) [Hg3L4](CF3SO3)6·4dioxane (II), (e) [Hg3L4](CF3SO3)6·3dioxane (III), and (f) [Hg3L4](CF3SO3)6·5dioxane (IV) hexagonal units. The triflate anions are presented in an ellipsoid mode. The solvate molecules are omitted for clarity and SQUEEZE handling.

Figure 3. PXRD patterns of I (black), II (red), III (green), and IV (blue).

double-layered 2D network is in the order IV > II > III > I (Figure S2 in the Supporting Information). For all of the crystals, each 2D network is packed in a PMMP··· helicity (Figure 2). A suitable combination of mercury(II) cation and new C3symmetric tridentate ligand is, needless to say, a significant factor in the formation of the unique double-layered 2D network. The planar C3-symmetric tridentate ligand can be ascribed to formation of a network consisting of hexagonal units, as has been seen, usually, in ornamental designs. The network framework has sustainable nanosized hexagonal open cavities that are occupied by solvate molecules or anions. In particular, the unique double-layered network in the crystalline state seems to be induced by the π···π interaction between the central benzene moieties of L. Furthermore, the sliding nature and stretching flexibility of the 2D network enables the doublelayered structure to exhibit solvomorphism via shifting and stretching of each layer. Crystals I and II were grown in different systems, resulting in different solvated molecules, acetonitrile and dioxane, respectively. Thus, their structures

slightly differ according to the solvate molecules within the open cavities. Although the roles of solvated molecules in the formation of the 2D framework have yet to be clarified, the solvents used in the present synthetic procedure deserve additional attention. In attempting to account for them, it is tempting to invoke the general solvent nature. Crystal I with acetonitrile solvate molecules is rigid under particular temperature and time condition, whereas crystal II with dioxane solvate is easily changed to other morphs in a SCSC fashion. Crystal II is transformed into III via long-time standing in mother liquor and is also interconverted into IV via thermal energy. The rigidity of crystal I with acetonitrile solvate molecules can be explained in terms of TGA and DSC curves (see Figure S6 in the Supporting Information). Such morphs have different stabilities, and indeed, they can spontaneously convert from metastable to stable form under a particular condition, the lattice energy difference being very small.41 Various conditions in the crystallization process can be significant causes of the formation of different morphic forms. C



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Figure 4. PXRD patterns of (a) [Hg3L4](CF3SO3)6·4dioxane (II) and (b) that left standing in mother liquor at 50 °C for 12 h, (c) 50 °C for 24 h (IV), (d) 50 °C for 48 h, and (e) 50 °C for 72 h.

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These conditions can be solvent effects, impurities, concentrations, temperatures, geometries, and stirring conditions. According to Ostwald’s rule,44 it is not the most stable but rather the least stable morph that crystallizes first. This can be explained on the basis of irreversible thermodynamics, structural relationships, or a combination of statistical thermodynamics and structural variation with temperature. Thus, the flatness of the skeletal structures (order: IV > II > III > I) might be involved in thermodynamical stability and is proportional to the size of unit cell volumes. The unusual solvomorphism can be explained, variously, in terms of solvation, the d10-electron configuration of the Hg(II) ion, flexible π···π interaction, and kinetic versus thermodynamic control, among other factors. Thus, crystalline [Hg3L4](CF3SO3)6 exists in four crystalline morphs; Figure 3 plots their PXRD patterns, Figure 4 monitors the SCSC transformation process at high temperature in terms of the PXRD patterns, and Figure S7 shows the PXRD pattern monitoring at room temperature in the mother liquor. As can be seen, they are slightly different from each other even though they are basically the same double-layered 2D network. In conclusion, this communication introduces solvomorphism that demonstrates the subtle role of solvents in the crystallization of a unique double-layered 2D network, as well as successive SCSC solvomorphism via time and thermal energy. This is, to our best knowledge, an unusually delicate 2D solvomorphism system showing four morphs. Further experiments and pervasive applications of solvomorphism are in progress, the results of which will help to open up intriguing and promising avenues in the fields of molecular recognition, sensors, separation, and solvate release.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ok-Sang Jung: 0000-0002-7218-457X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean Government [MEST] (2016R1A2B3009532 and 2016R1A5A1009405).

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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/acs.cgd.7b01718. Experimental details, ORTEP drawings, IR spectra, 1H NMR spectra in (CD3)2SO, and thermal analyses (PDF) Accession Codes

CCDC 1589956−1589959 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 D



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Solvomorphism of Double-Layered Networks through Single-Crystal

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