DOI: 10.1021/cg9008009
Molecular Capsules in Modular Frameworks
2009, Vol. 9 3859–3861
Yuzhou Liu and Michael D. Ward* Department of Chemistry and the Molecular Design Institute, New York University, 100 Washington Square East, New York, New York 10003-6688 Received July 13, 2009
ABSTRACT: In a rare example of crystal engineering through empirical design, the metric and symmetry complementarity between the four sulfonates appended to the upper rim of tetrasulfonated calix[4]arenes and the sulfonate nodes of the guanidinium-sulfonate hydrogen-bonded sheet permits the formation of single crystals comprising antiparallel interdigitated arrays of calixarene capsules embedded in discrete bilayers wherein each calixarene is suspended from a single guanidinium-sulfonate sheet through four sulfonate attachments. A hydrate of the parent compound tetraguanidinium calix[4]arenetetrasulfonate, in which water occupies the calixarene as well as positions in the hydrogen-bonded sheet, undergoes a single crystal-single crystal phase transition with loss of all water molecules, but with remarkable retention of crystallinity. These compounds suggest a path to a new class of crystalline materials with molecular capsules that can be tailored for selective guest inclusion for separations and regulation of solid-state properties through the encapsulation of functional guests. The design and synthesis of crystalline organic materials (aka crystal engineering) largely relies on empirical strategies that combine molecular symmetry, metric complementarity, and structure-directing interactions to guide assembly of molecular components into preordained solid-state structures.1 Modular approaches based on the interchange of topologically similar components are promising,2 but these often can be thwarted by the delicate and noncovalent nature of the intermolecular forces responsible for crystal packing. Our laboratory has demonstrated that this obstacle can be surmounted in molecular frameworks built from a structurally robust two-dimensional hydrogenbonded network of guanindinium ions (G) and the sulfonate (S) moieties of a wide range of organomonosulfonates or disulfonates.3 These frameworks display a remarkable capacity to behave as crystalline hosts, forming inclusion compounds for numerous combinations of organosulfonates and guest molecules. A similar propensity for guest inclusion has been observed for numerous calixarenes, which can form discrete molecular “capsules” that include guest molecules, in both soluble and solid forms.4 Herein we describe the design and synthesis of unusual crystalline host frameworks in which tetrasulfonated calixarenes are tethered to the GS sheet as a consequence of topological and metric compatibility, forming molecular capsules embedded in a 2D layered framework. The GS sheet usually exhibits an ideal “quasihexagonal” motif, in which each guanidinium ion and sulfonate ion participate in six hydrogen bonds. The sulfonate-sulfonate distance in the flat GS sheet, as measured by the distance between the sulfur atoms in numerous compounds, is dS 3 3 3 S ≈ 7.5 A˚ (Figure 1). The Cambridge Structural Database (Version 5.29, August 2008) contains 83 entries for calix[4]arenetetrasulfonate (C4TS), with dS 3 3 3 S values ranging from 5.95-8.41 A˚, with an average value of dS 3 3 3 S= 7.25 A˚. The similar dS 3 3 3 S values for the GS sheet and C4TS suggest a metric compatibility between a quad of sulfonate nodes on the GS sheet and the sulfonate nodes of C4TS. Moreover, the wide range of dS 3 3 3 S values observed for C4TS suggested an inherent compliance that would permit the calixarene to adapt to the sulfonate positions on the GS sheet, thus forming a layer of calixarene “capsules” suspended from the GS sheet. The ability to vary the depth of the capsule through well-developed synthetic pathways suggests a new family of crystalline hosts with tunable
Figure 1. Calix[4]arenetetrasulfonate (left) and a simplified representation of the quasihexagonal GS sheet (right), illustrating the similar S 3 3 3 S distances. The sulfonate nodes are denoted as “S” and the guanidinium nodes are located at the other vertices. The lower panel depicts the anticipated antiparallel packing of calixarene capsules suspended from GS sheets in a bilayer architectures.
*To whom correspondence should be addressed. E-mail: mdw3@nyu. edu.
“endo-inclusion” cavities while retaining the structural integrity and reliability of the GS network. Slow evaporation of aqueous solutions of calix[4]arene tetrasulfonic acid and guanidine hydrochloride produced large, colorless crystals of G4C4TS 3 1.2 H2O (1) with a block-like habit. Single crystal X-ray diffraction revealed a bilayer architecture in which all four sulfonate groups for each calixarene are attached to the hexagonal nodes of a single GS sheet, resulting in a 2D array of suspended calixarene capsules that are interdigitated with an identical array on an opposing sheet (Figure 2).5 The calixarene is suspended below a 3.3 A˚-diameter aperture in the GS sheet. This bilayer architecture is a topological blend of the discrete bilayers observed for many guaninidium organomonosulfonates6 and the antiparallel packing arrangement reported for a few crystalline
r 2009 American Chemical Society
Published on Web 07/21/2009
pubs.acs.org/crystal
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Crystal Growth and Design, Vol. 9, No. 9, 2009
Figure 3. Single crystal structure of G4C4TS 3 H2O 3 3(CH2Cl2). Only the CH2Cl2 guest molecules embedded in the calix[4]arene cavities are displayed (space filling); the remaining CH2Cl2 guests and water molecules are omitted for clarity.
Figure 2. (A) The single crystal structure of 1, as viewed parallel to the GS sheets. For clarity, only the endo-included water guests are shown. (B) Calix[4]arene capsules attached to the GS sheet beneath in 1.
calixarene compounds.7 The quasihexagonal character of the GS sheet is apparent, although it is perturbed slightly by three crystallographically distinct water molecules that form hydrogen bonds with the network, two that occupy positions in the GS sheet with occupancies of 100% and 12%. The remaining position corresponds to an “endo-included” water molecule that is partially embedded within the calixarene cavity with 8% occupancy. The calixarene cavity, which has a free volume of 30 A˚3, can easily accommodate one water molecule (V = 20 A˚3). Notably, single crystals of 1 transformed to single crystals of anhydrous G4C4TS (2) within 24 h under ambient conditions, with no apparent change in crystal integrity and crystallinity. Compound 2 was refined in the same space group as 1 (P1), with similar lattice parameters, a slight reduction in unit cell volume (
