Many Simple Molecular Cavitands Are Intrinsically Porous (Zero

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Many Simple Molecular Cavitands are Intrinsically Porous (Zero-dimensional Pore) Materials Christopher M. Kane, Onome Ugono, Leonard J. Barbour, and K. Travis Holman Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b02972 • Publication Date (Web): 01 Oct 2015 Downloaded from http://pubs.acs.org on October 6, 2015

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Chemistry of Materials

Many Simple Molecular Cavitands are Intrinsically Porous (Zero-dimensional Pore) Materials Christopher M. Kane,a Onome Ugono,a,† Leonard J. Barbour,b and K. Travis Holmana,* a

Department of Chemistry, Georgetown University, 37th and O Sts. NW, Washington D. C. 20057, USA Department of Chemistry, University of Stellenbosch, Private Bag XI, 7600, Stellenbosch, South Africa

b

Cavitands, Porous Materials, Zero-dimensional Pores, Clathrates, Void Space

ABSTRACT: The guest-free crystal forms of eight related small molecule cavitands (Scheme 1; simplified nomenclature: R,R’,Y) are investigated as candidate discrete molecule microcavity materials (DMMMs). Due to their rigid bowl-like molecular structures, many cavitands are incapable of efficient crystal packing in pure form, yielding zero-dimensionally porous apohost phases. By molecular modifications that eschew self-inclusion, emphasis is placed on engineering structures that exhibit uniform microcavities that are large enough to accommodate small molecules of interest (e.g., gases or volatile organic compounds). The most thermodynamically stable guest-free crystal forms of several cavitands—namely, H,H,CH2, H,Me,CH2, α-Me,H,CH2, Me,Me,CH2, Br,Me,CH2, Me,Et,CH2, Me,Et,SiMe2 , and Me,i-Bu,CH2—appear to be as-close-packed-as-possible, yet exhibit relatively large microcavities (or, zero-dimensional pores) in the range of 27-115 Å3. Where self-inclusion is ineffective, the microcavities predictably assimilate the intrinsic cavitand molecular cavity, yet the ultimate size and shape of cavities are also strongly influenced by crystal packing. It is demonstrated that some cavitand solvates, CH2Cl2@H,Me,CH2, xH2O@Me,Et,SiMe2 , and CH2Cl2@Me,iBu,CH2 (84:16 rccc:rcct) maintain host crystal packings that are equivalent to their empty, intrinsically porous phases and it is argued that the intrinsic pores of DMMMs are particularly suited to selective gas enclathration and/or storage. As a proof-ofconcept demonstrations, the porous phase of Me,Et,SiMe2 is shown to capture and temporarily hold Freon-41 (fluoromethane, bp = -78 °C) at room temperature. A single crystal of empty Me,Et,SiMe2 is shown to uptake CO2 gas at room temperature, allowing structure determination of xCO2@Me,Et,SiMe2, and single-crystal-to-single-crystal dehydration of xH2O@Me,Et,SiMe2 demonstrates its permeability to water.

1. INTRODUCTION Molecule-derived microporous materials—that is, coordination polymers (CPs) or metal-organic frameworks (MOFs),1-4 covalent organic frameworks (COFs),5 polymers of intrinsic microporosity (PIMs),6,7 intrinsically or extrinsically porous discrete-molecule materials,8,9,10,11 etc.—are of much contemporary interest due to the ease with which they may be structurally modified relative to inorganic microporous materials, and their myriad of potential applications, particularly related to small molecule (gas) sorption, separations, and/or storage.12,13,14 In this sense, microporous molecular materials exhibiting open, continuous one-dimensional (1D), 2D, or 3D pores, as defined by IUPAC,15 are principal structural targets, considering that such pores are generally readily accessible to external fluid sorbents. That is, guest loading into the evacuated pores typically occurs on the order of seconds, even at low temperatures. Such materials are typically characterized by inert gas sorption isotherms under cryogenic conditions, allowing assessment of surface area and other pore characteristics via quantitation of gas condensation within the pores. In contrast, microcavity materials (MMs)—those which possess zero-dimensional (0D), “closed”, or discrete pores and exhibit no apparent pore connectivity—are commonly regarded to be “ineffective”15 sorbents because the pores are thought to be inaccessible to fluids (Figure 1). Moreover, the structural demonstration of discrete, empty, molecule-sized void spaces

(> 25 Å3) in molecular crystalline solids remains rare,16,17,18 despite increasing academic interest in porous materials.

Figure 1. Guest sorption by 1D vs. 0D pore materials. Sorption into open, 1D (or higher dimensional) pores (orange) of a porous host material (transparent) tends to be rapid, with equilibration occurring on the order of seconds. For microcavity (0D pore) materials, the barrier for guest sorption is higher, but dynamic processes or other mechanisms may nonetheless allow permeation of sorbents, the process being highly dependent on structure and composition.

The 0D pores of rigid inorganic structures tend to be inaccessible. For example, inorganic clathrasils19 and certain zeolites (e.g., Cd-exchanged RHO20) entrap gases more-or-less permanently within their microcavities. The framework apertures are too narrow and too rigid to permit guest permeation at room temperature. Similarly, some discrete molecule microcavity materials (DMMMs) are capable of confining gases to unusually high temperatures. For example, the discrete

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microcavities of the empty form of calix[4]arene—which arise from inefficient crystal packing and are not intrinsic to the calix-shaped molecule—can be induced to trap Freons, methane, or other gases and hold them to high temperatures.17 And we have recently shown that the intrinsic molecular cavities of a particular crystal form of the cage compound cryptophane-111 can hold xenon up to unprecedented temperatures (~300°C).21,22 Still other DMMMs—e.g., hydroquinone,23 C60 (interstices)24—are known to form kinetically stable gas clathrates. Discrete molecule hosts, however, allow substrate release via low temperature treatment, such as simple dissolution of the host. Crystalline materials with zero-dimensional pores are not always impermeable, however, and this is particularly true for molecule-derived materials. The structures of moleculederived materials are often more flexible, or more dynamic, than inorganic materials and thereby tend to be more permeable. For example, Atwood, Barbour and coworkers showed that the low-density crystal form of p-tertbutylcalix[4]arene—exhibiting only intrinsic 0D pores—is permeable to liquids25 or gases8 on the timescale of several minutes. Fluid permeability is presumably facilitated by a gating mechanism involving dynamics of the tert-butyl groups. Similar behavior has been observed for solid forms of other calixarenes,8,26,27,28,29,30,31,32,33,34,35,36 including partially solvated phases.37 And over the past decade or so, several other classes of cage compounds exhibiting only 0D pores have been found to be dynamically porous toward gases, including hemicarcerands,38 cucubit[n]urils,39-42 organic and metalorganic macrocycles,43,44 and discrete molecule cages.45 All of these structures exhibit, from a static point of view, discrete, unconnected microcavities. Yet, akin to higher dimensional porous materials, the 0D pores are accessible to sorbents without inducing a phase change of the host. Similarly, certain crystalline polymers such as MOFs, and even polystyrene,46 can exhibit accessible 0D pores. ZIF-7 (SOD topology Zn(benzimidazolate)2)47 and the NBO-type triazolate [Cu(3,5diethyl-1,2,4-triazolate)]48 are exemplary MOFs that have been demonstrated to dynamically gate access to their 0D pores. And although zero dimensional pore materials have received relatively little specific attention, structural flexibility in porous materials has become an increasingly topical area of study.49,50 Indeed, it is becoming increasingly apparent that the 0D pores present in many molecule-derived materials are accessible under certain conditions and there has been a call to study more “intrinsically porous” discrete molecule materials in the context of porosity.10 Some advantages provided by discrete molecule porous materials, in contrast to porous networks, have been discussed by Cooper and others.9,10 These include chemical/synthetic diversity, solubility (processing advantages), the ability to mix-and-match components (e.g. cocrystals51 or solid solutions), and, importantly, the increased mobility of weakly connected components. We anticipate that discrete molecule microcavity materials (DMMMs), specifically, may offer certain additional advantages/properties over materials with higher dimensional pores. Separations, for example, are achieved on the basis of thermodynamic or kinetic permselectivity. From the perspective of thermodynamics, discrete microcavities

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complementary to individual sorbent molecules are likely to be more selective than 1D or non-uniform pores. In terms of sorption kinetics, it is clear that a wide range of timescales and temperature dependences will be available to microcavity materials, depending on the structure and composition of the material and the mechanism of sorption/desorption. Clearly, certain DMMMs may exhibit very high kinetic barriers to sorbent exchange and will be able to effectively confine/store volatiles. At the opposite end of the spectrum, certain DMMMs may provide kinetic sorption profiles useful for separations. The sorption kinetics may also be tunable, by, for example, introducing dynamic functional groups to lower kinetic barriers,8 slurrying, exploiting particle size effects,52 or, we suggest, employing mechanochemically53 assisted methods.54 Moreover, DMMMs may serve as sorbents under atypical conditions, such as relatively high temperatures.

Scheme 1. Schematic representation and simplified nomenclature of the cavitands whose guest-free crystal forms are reported and studied herein. Except where otherwise noted, all compounds are pure rccc stereoisomers.

Herein we report guest-free crystal forms of eight related small molecule cavitands55-57 (Scheme 1; simplified nomenclature: R,R’,Y) as candidate intrinsic discrete molecule microcavity materials (DMMMs). By molecular modifications that eschew self-inclusion, emphasis is placed on engineering structures that exhibit uniform microcavities that are large enough to accommodate small molecules of interest, without necessitating a change in crystal packing. The guest-free structures appear to be as-close-packed-as-possible, yet exhibit relatively large microcavities (or, zero-dimensional pores) in the range of 27-115 Å3. It is argued that the intrinsic pores of these DMMMs may be particularly suited to selective gas enclathration and/or gas storage. As a proof of concept, the porous phase of Me,Et,SiMe2 is shown to capture and store fluoromethane gas (bp = -78 °C) and is demonstrated to be permeable to water at high temperatures. 2. EXPERIMENTAL METHODS 2.1 Experimental Procedure General. All solvents were used as received from Fisher (Pittsburg, PA). Reagents were obtained from Acros (Pittsburgh, PA) or Aldrich (Milwaukee, WI) and were used without further purification. All reactions were performed under N2 atmosphere. Chromatography was carried out on silica gel (32-64µm) from Silicycle Chemical Division. The calix[4]resorcinarenes 58 59 H,H,OH , , Me,H,OH60, H,Me,OH73, Me,Me,OH73, 73 60 Br,Me,OH , and cavitands Me,H,CH2 , H,Me,CH273, Me,Me,CH273, Br,Me,CH273 were synthesized by published procedures and were isolated as various solvates that could be dried

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where necessary. New calix[4]resorcinarenes and cavitands were made by methods analogous to the methods in the literature55,60-62 and their protocols are described below. Except where otherwise noted, all compounds are stereopure (>95%) rccc isomers. Synthesis of H,H,CH2. Calix[4]resorcinarene H,H,OH (4.88 g, 10.0 mmol), bromochloromethane (76 g, 39 mL, 0.59 mol), and Cs2CO3 (17.9 g, 54.9 mmol) were added to DMF (100 mL) and stirred for 3 days at 92 °C. After cooling, the reaction was poured into 2 M HCl(aq.) (300 mL) and stirred until the remaining bromochloromethane evaporated (≤ 24 hours). The suspension was filtered and washed with water. The solid was redissolved in boiling CHCl3 and any insoluble material was removed by filtration. The solvent was then removed in vacuo and the residue was purified by passing through a plug of silica gel using chloroform as an eluent to give 0.91 g of a colorless solid (17%) after drying in an oven at 150°C for one day. 1H NMR (400 MHz, CDCl3): δ 7.14 (s, 4H, ArHbottom), 6.52 (s, 4H, ArHtop), 5.75 (d, 4H, 3JHH= 7.2 Hz, O-CH2(out)), 4.49 (d, 4H, 3JHH = 7.2 Hz, O-CH2(in)), 4.45 (d, 4H, 2JHH = 12.4 Hz, CH2(out)), 3.29 (d, 4H, 2JHH = 12.4 Hz, CH2(in)) ppm (Figure S1). 13C NMR (100 MHz, CDCl3): δ 155.2, 135.6, 128.4, 117.0, 100.0, 33.2 ppm (Figure S2). Synthesis of Me,Et,OH. The synthesis of Me,Et,OH was reported by Aoki et al., but issues of stereochemistry were not discussed.63 In our hands, the Me,Et,OH material isolated from standard octol synthesis, as follows, was not stereopure. 2Methylresorcinol (20.0 g, 161 mmol) was added to ethanol (80 mL), water (80 mL) and HCl (40 mL). Propionaldehyde (9.35 g, 11.5 mL, 161 mmol) was added dropwise at room temperature and the reaction was heated to 50°C for two hours. The reaction was allowed to cool to room temperature and was stirred for 2 days. The resulting solid was removed by filtration, triturated with water and filtered to give 7.43 g of an off-white solid (28%). 1 H NMR spectroscopic analysis revealed the isolated material to be a stereoisomeric mixture of rccc-Me,Et,OH (89%) and rcctMe,Et,OH (as deduced by analysis of the cavitand product, 11%, Figure S3-S4). A sample of pure rccc-Me,Et,OH (>95% rccc) was obtained by altering the work-up. Filtration of the reaction mixture while hot and washing the solid with cold methanol (25 mL) gave 0.61 g (5%) of rccc-Me,Et,OH (Figure S5). 1H NMR rccc-Me,Et,OH (400 MHz, CD3OD): δ 7.07 (s, 4H, ArHbottom), 4.23 (t, 4H, 3JHH = 7.9 Hz, CH), 2.18 (dt, 8H, 3JHH = 7.2 Hz, CH2), 2.01 (s, 12H, ArCH3), 0.89 (t, 12H, 3JHH = 7.1 Hz, CH3) ppm. 13C NMR (100 MHz, δ6-DMSO): δ 149.3, 124.8, 120.0, 112.0, 36.4, 26.7, 11.7, 8.4 ppm. Synthesis of Me,Et,CH2. Me,Et,OH (3.0 g, 4.5 mmol; 89:11 rccc:rcct), bromochloromethane (34.25 g, 17.7 mL, 0.266 mol), and Cs2CO3 (8.2 g, 25.1 mmol) were added to DMF (45 mL) and stirred for 3 days at 92 °C. After cooling, the reaction was poured into 6% HCl solution (250 mL) and stirred for 2 hours. The suspension was filtered and washed with water. The solid was dissolved in boiling CHCl3 and insoluble material was removed by filtration. The solvent of the filtrate was removed in vacuo and the residue was purified by passing through a plug of silica gel using chloroform as an eluent to afford 1.48 g of a white solid (44%) with a rccc:rcct stereoisomeric ratio of ca. 76:24% (Figure S6-S7), consistent with the greater reactivity of non-rccc stereoisomers in cavitand-forming reactions.64 A sample of nearstereopure rccc-Me,Et,CH2 was synthesized by the above procedure using near-stereopure rccc-Me,Et,OH (0.30 g, 0.5 mmol), bromochloromethane (3.50 g, 1.81 mL, 27 mmol), Cs2CO3 (1.30 g, 4.0 mmol) and DMF (15 mL), yielding, after workup (including drying in an oven at 150 °C for one day), 0.17 g (48%) of the compound in a 92:8 rccc:rcct stereoisomeric ratio (Figure S8). 1H NMR rccc-Me,Et,CH2 (400 MHz, CDCl3): δ 6.98 (s, 4H, ArHbot-

2 3 tom), 5.89 (d, 4H, JHH = 6.9 Hz, O-CH2(out)), 4.68 (t, 4H, JHH = 8.2 Hz, CH), 4.27 (d, 4H, 2JHH = 6.9 Hz, O-CH2(in)), 2.25 (dt, 8H, CH2), 1.98 (s, 12H, ArCH3), 1.00 (t, 12H, 3JHH = 7.2 Hz, CH3) ppm. 13C NMR (100 MHz, CDCl3): δ 153.3, 137.8, 123.8, 117.4, 98.5, 38.9, 23.1, 12.4, 10.3 ppm. Synthesis of Me,Et,SiMe2. Me,Et,OH (2.0 g, 3.1 mmol, 89:11 rccc:rcct), was dissolved in pyridine (90 mL) at room temperature while stirring. Dichlorodimethylsilane (3.93 g, 3.67 mL, 30.5 mmol) was then added to the solution in one portion and the mixture was stirred overnight. Pyridine was then removed from the reaction by distillation and methanol was added to quench any remaining dichlorodimethylsilane. The suspension was filtered to give 0.96 g of the silyl cavitand (35%) with a rccc:rcct stereoisomeric ratio of approximately 68:32% (Figure S9-S10). A sample of near-stereopure rccc-Me,Et,SiMe2 (95% rccc) was obtained by the above procedure using near-stereopure rccc-Me,Et,OH (0.30 g, 0.5 mmol), pyridine (40 mL), and dichlorodimethylsilane (0.65 g, 0.61 mL, 5 mmol), yielding, after workup, 0.40 g (91%) of the compound (Figure S11). 1H NMR rccc- Me,Et,SiMe2 (400 MHz, CDCl3): δ 7.17 (s, 4H, ArHbottom), 4.47 (q, 4H, Ar2CH, 3JHH = 8.0 Hz,), 2.23 (dq, 8H, 3JHH = 7.4 Hz, CH2), 1.92 (s, 12H, ArCH3), 0.92 (t, 12H, 3JHH = 7.2 Hz, CH3), 0.52 (s, 12H, SiCH3(out)), -0.67 (s, 12H, SiCH3(in)) ppm. 13C NMR (100 MHz, CDCl3): δ 148.4, 131.1, 119.8, 119.2, 37.5, 26.1, 12.6, 10.6, -3.0, -6.2 ppm. Synthesis of Me,i-Bu,OH. The synthesis of purportedly stereopure rccc-Me,i-Bu,OH was reported by Miao et al.65 In our hands, however, the material isolated from this procedure was not stereopure. The product appeared to be a mixture of stereoisomers, consisting of ~87% of rccc-Me,i-Bu,OH and ~13% of another stereoisomer, seemingly the rcct form based on the stereoidentity of the Me,i-Bu,CH2 cavitand mixture derived from this material (Figure S12). A sample of nearly stereopure rccc-Me,iBu,OH (>95%) was obtained by combining 2-methylresorcinol (6.2 g, 50 mmol), isovaleraldehyde (5.4 mL, 50 mmol) in a solution of ethanol and conc. HCl (50 mL 95% ethanol, 10 mL HCl(aq.)) and heating at 65°C for 12 hours. After cooling, the precipitate was filtered and triturated in warm methanol (< 100 mL) to selectively remove the more soluble stereoisomeric impurity, giving 2.5 g of near stereopure rccc-Me,i-Bu,OH as a colorless solid (27% yield). 1H NMR rccc-Me,i-Bu,OH (400 MHz, (CD3)2CO, Figure S13): δ 7.41 (s, 4H, ArHbottom), 4.52 (t, 4H, 3 JHH = 8.1 Hz, CH), 2.80 (s, 12H, ArCH3), 2.15 (dd (~t), 8H, 3JHH = 7.5 Hz, CH2), 1.44 (m, 4H, 3JHH = 6.9 Hz, CH(CH3)2), 0.93 (d, 24H, 3JHH = 6.6 Hz, CH3) ppm. Synthesis of Me,i-Bu,CH2. Crude Me,i-Bu,OH (3.0 g, 3.95 mmol, ~87:13 rccc:rcct), bromochloromethane (30.1 g, 15.6 mL, 0.233 mol), and Cs2CO3 (7.07 g, 21.7 mmol) were added to DMF (45 mL) and stirred for 3 days at 92 °C. After cooling, the reaction was poured into 6% HCl solution (250 mL) and stirred until the remaining bromochloromethane had evaporated. The suspension was filtered and washed with water. The crude solid product was dissolved in hot CHCl3 and any insoluble material was removed by filtration. The solvent of the filtrate was removed in vacuo and the residue was purified by passing through a plug of silica gel using methylene chloride as an eluent. Removal of the solvent afforded CH2Cl2@Me,i-Bu,CH2 (Figure S14) as an offwhite solid. Trituration in methanol was found to improve the color, affording 3.17 g (89%) Me,i-Bu,CH2 after oven drying. The isolated product appeared to be an 80:20 mixture of rccc and rcct stereoisomers, respectively (Figure S14). An isolated single crystal of CH2Cl2@Me,i-Bu,CH2 (Table S1) proved to be a solid solution of the rccc and rcct stereoisomers, the ratio refining to 84:16. Recrystallization from ethyl acetate (EtOAc) slightly improved the bulk stereopurity to 86:14 (Figure S16), though an

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isolated single crystal of EtOAc@Me,i-Bu,CH2 grown by slow evaporation appeared to be ~97% stereopure according to SCXRD (Table S1). A sample of near-stereopure rccc-Me,iBu,CH2 was synthesized by the above procedure using nearstereopure rccc-Me,i-Bu,OH (2.0 g, 2.6 mmol), bromochloromethane (20.0 g, 10.4 mL, 155 mmol), Cs2CO3 (6.9 g, 21 mmol) and DMF (60 mL), yielding, after workup, MeOH@Me,i-Bu,CH2 (93:7 rccc:rcct, Figure S17) with a dry weight of 1.7 g (81 %). 1H NMR rccc-Me,i-Bu,CH2. (400 MHz, CDCl3): δ 6.97 (s, 4H, ArHbottom), 5.88 (d, 4H, 2JHH = 6.9 Hz, O-CH2(out)), 4.91 (t, 4H, 3 JHH = 8.1 Hz, CH), 4.27 (d, 4H, 2JHH = 6.9 Hz, O-CH2(in)), 2.09 (t, 8H, 3JHH = 7.5 Hz, CH2), 1.97 (s, 12H, ArCH3), 1.56 (m, 4H, CH(CH3)2), 1.01 (d, 24H, 3JHH = 6.6 Hz, CH3) ppm. 13C NMR (100 MHz, CDCl3): δ 153.2, 137.9, 123.6, 117.9, 98.5, 39.0, 34.7, 26.1, 22.9, 10.3 ppm. Sublimation of Cavitands. Single crystals of guest-free cavitands were grown by thermally induced sublimation of the desolvated cavitand materials under dynamic vacuum (~0.5 mTorr). Specifically, a few milligrams of desolvated cavitand were loaded into a Pasteur pipette that had been melted and sealed at one end. The open end was connected to a vacuum pump via vacuum tubing and the pipette was positioned at an incline over the nozzle of a heat gun so as to heat the end containing the solid. The temperature of the heat source was raised until sublimation started to occur. Over the course of several seconds to hours, this procedure yielded X-ray quality single crystals of the cavitands.

2.2 Analytical Methods NMR Spectroscopy: 1H (400 MHz) and 13C (100 MHz) NMR spectra were carried out on a Varian 400-MR spectrometer at 9.4 T. Spectra were recorded at 298 K unless otherwise specified. MestReNova version 5.2.5-4119 software was used for data analysis. Deuterated solvents were used as received from Cambridge Isotope Laboratories, Inc. Chemical shifts given are based upon on the residual solvent peaks. X-ray Crystallography: Single crystal X-ray diffraction data were collected at 100(2) K (or room temperature, as indicated) on either a Siemens SMART three-circle X-ray diffractometer equipped with an APEX II CCD detector (Bruker-AXS), a Bruker APEX II Duo diffractometer, or an Bruker D8 Quest, equipped with an Oxford Cryosystems 700 Cryostream, using Mo-K radiation (0.71073 Å). The crystal structures were solved by direct methods using SHELXS, and all structural refinements were conducted using SHELXL-2014/7.66 All hydrogen atoms were placed in calculated positions and were refined using a riding model with coordinates and isotropic displacement parameters being dependent upon the atom to which they are attached. 1.8(NO2CH3)@Me,Et,CH2 was treated with SQUEEZE to model disordered solvent.67 The program X-Seed68 was used as a graphical interface for the SHELX software suite and for the generation of figures. Crystal data for all solvated and sublimed cavitands are reported in Tables S1-2 and S3-4, respectively. Cavity/Pore Volumes and Porosity, ε. Microcavity (0D pore) volumes (Vcav) are determined from the atomic coordinate data provided by SCXRD analysis. By computationally probing each unique cavity with a sphere of a defined probe radius, the volume of space that can be encompassed by the sphere can be summed over all achievable positions of the sphere. Cavity volumes were calculated using the X-seed interface68 to MSRoll,69 employing accepted van der Waals atomic radii and a 1.4 Å probe radius (van der Waals radius of a helium atom). All C-H bonds were normalized to a distance of 1.08 Å before pore analysis. The resulting Connelly surfaces are imaged in orange or blue and display the available void space. Packing fractions (PF) were

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determined from PLATON.70 In all cases, PLATON gave microcavity/void volumes comparable (±3 Å3) to the MSRoll calculations. Porosity (ε) is defined by IUPAC as the fraction of the apparent volume of the sample which is attributed to the pores detected by the method used.15 For the reported compounds, the method of pore detection is structural analysis by X-ray diffraction, as described. Porosity, ε, is therefore simply the sum of the volumes of the microcavities in the unit cell (ΣVcav) divided by the total unit cell volume (Vcell), expressed as a percent. Notably, common methods to assess porosity (e.g., those using a fluid, such as low temperature gas sorption techniques) have the capacity to detect only the “open” or accessible pores whereas other methods (e.g., X-ray structure determination) are capable of also assessing the so-called "closed pores." Thus, low temperature gas sorption isotherms of the 0D porous cavitands reported here would not be expected to exhibit any measurable uptake of gases over the timescale of the measurement. The issue of pore accessibility is one of temperature/timescale and means. Ingress and egress of fluids can be observed for certain reported compounds (e.g., with Me,Et,SiMe2), but at relatively high temperatures (room temperature and above) and over relatively long timescales. The 0D pores of the cavitands may be accessed, however, by other means. For example, their solubility allows for capture of substrates within the pores by slurrying in appropriate liquids.

Powder X-ray diffraction (PXRD). All PXRD data were collected with graphite monochromated Cu-K radiation (λ = 1.5418 Å). Most data were acquired in transmission using a Rigaku R-Axis Rapid diffractometer equipped with a curved image plate detector and a 0.5 mm collimator. The samples were mounted in a 0.5 mm capillary tube and were irradiated for 30 mins. while rotating about the φ axis. The diffraction data were integrated using AreaMax v. 1.15 (5-60° 2θ, with a 0.02° step size) and were further manipulated using MDI Jade 5.0. The PXRD pattern of empty Me,i-Bu,CH2 was acquired in reflection mode (zero background sample holder) using a Rigaku Ultima IV X-ray powder diffractometer employing a D/teX silicon strip detector. The PXRD patterns of EtOAc@Me,i-Bu,CH2 and empty Me,Et,SiMe2 were acquired in reflection mode (zero background sample holder) using a Bruker D2 Phaser desktop diffractometer equipped with a scintillation counter detector. The PXRD pattern of empty Me,H,CH2 was acquired in transmission mode using the Bruker-AXS APEX II Duo diffractometer. The samples were irradiated for 20 mins from 3-60° in 2θ with no spin. Simulated PXRD patterns were calculated from single crystal structure data using the Lazy Pulverix71 program suite via the X-Seed interface.68 Thermogravimetric analysis (TGA). TGA data were acquired using a TA Instruments Q5000IR TGA or a Q500 thermogravimetric analyzer. Samples were placed in platinum pans and heated at a rate of 3˚C/min for guest-loss studies, and a rate of 5˚C/min to 500˚C for sublimation studies under He atmosphere (10 mL/min).

3. RESULTS AND DISCUSSION Hypotheses. Pioneered by Cram and coworkers in 1982, cavitands are broadly defined as synthetic compounds with enforced cavities large enough to accommodate simple molecules or ions,55-57 but, more narrowly, are the ubiquitous and readily available

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family of bowl-shaped molecular vessels shown in Scheme 1.72-78 Hundreds of synthetic derivatives are known, including dimeric caged forms known as (hemi)carcerands.79-81 These molecules have been developed, primarily, for exploitation of the rich molecular recognition chemistry exhibited by their shape-persistent concave cavities.

Figure 2. Hypothesis: Even the lowest energy, as-close-packedas-possible crystal forms of certain cavitands are pre-disposed to exhibit molecule sized zero-dimensional pores (microcavities). Orange spaces represent cavities intrinsic to the concave molecular structure, regardless of its crystal packing. Blue spaces represent extrinsic cavities that could arise due to the specific, but difficult-to-predict, crystal packing. In this hypothetical, the extrinsic void is contiguous with the intrinsic one, resulting in a structure with uniform, uniquely shaped microcavities.

Our investigation of the cavitands in the context of porosity is based upon a number of hypotheses and observations. Firstly, cavitands are shape persistent, bowl-shaped structures, like the (porous) calix[n]arenes,17,25-34,37,82 yet they are, in many ways, synthetically more diverse and generally more amenable to simple functional group modifications. The upper ‘rims’ (R), ‘feet’ (R’), and ‘bridges’ (Y) (Scheme 1) of the bowl-like moieties are readily amenable to chemical modification. Moreover, the R’ feet can be arranged in different stereochemical modifications, the all-cis (rccc) and the relative-cistrans-trans (rctt) stereoisomers being the most common. We hypothesized that many simple cavitand derivatives will be simply incapable of close-packing and, even within the Kitaigorodskii paradigm of being as-close-packed-aspossible,16 their pure crystal forms ought to exhibit intrinsic microcavities (or higher dimensional pores) wherein the specific dimensions of the pores will reflect, in part, on the bowl shape of the cavitand (Figure 2). It was also anticipated that crystal packing would play a role in governing the cavity/pore size and shape at the open end of the intrinsic cavitand molecular cavities. Thus, extrinsic voids were also deemed likely; these voids could be contiguous with the intrinsic molecular cavity or could appear simply as interstitial sites that arise from the inability of these compounds to close-pack, like in C60. So, while one end of the intrinsic cavities may be well defined a priori, the overall size/shape of the pores (0D, 1D, pore uniformity, etc.) of an empty cavitand host structure are difficult to forecast, and must be determined experimentally, or via crystal structure prediction (CSP) approaches. Notably, lacking conformational degrees of freedom, the simplest rigid cavitands may in fact be candidates for CSP.83,84 Importantly, due to the anticipated inability of certain cavitands to close-pack in pure form, we further hypothesized that porous cavitand structures will be the most thermodynam-

ically stable crystal forms of these molecules. That is, porous cavitand structures ought to be both thermally stable and insusceptible to structural collapse, even under aggressive conditions. This feature would be in notable contrast to many classes of porous materials (most MOFs, zeolites, many discrete molecule compounds), which are commonly metastable structures in their empty, porous forms, and are therefore susceptible to framework “collapse” under conditions of stress. Accordingly, we anticipate that the lowest energy, incollapsible crystal forms of certain cavitands will be capable of readily (under some conditions) and selectively, encapsulating, absorbing, and/or storing even the most weakly interacting substrates, such as light gases, within the otherwise vacant microcavities. We further anticipate that solid gas clathrates of certain cavitands may exhibit high kinetic stability, similar to the remarkable stability of the extrinsic freon clathrates of calix[4]arene8 or the intrinsic xenon clathrate of cryptophane111 recently reported by our group,21,22 and related materials. Thus, in our view, determination of the crystal and pore structures of empty cavitand phases is important. Background and CSD Search. Given the bowl-like shape of the cavitands (Scheme 1), it is perhaps obvious that, in pure form, some of these molecules will be incapable of yielding close-packed structures in the solid state (i.e. packing so as to fill all of the accessible space). Shape-persistent compounds that cannot pack efficiently are particularly prone to forming solvates and/or other inclusion compounds when crystallized from solution. Indeed, the principal of inefficient packing has long been employed to design inclusion hosts (e.g. wheel and axle compounds, etc.).85-87 Cavitands are exemplary hosts in this regard. In fact, Cram’s seminal early work with cavitands and their derivatives (e.g. (hemi)carcerands79-81) highlights this design feature and provides the crystal structures of several bowl-occupied solvates.55-74 It was noted early on that many simple cavitands essentially always crystallize from solution with included solvent, some forming highly stable solvates. For example, Cram and coworkers found that the aryl-footed cavitand H,4CH3C6H4,CH2 strongly holds ethyl acetate and apparently could only be emptied by sublimation at 450 °C under vacuum.74,88 Other cavitands form less stable solvates, whereas still other are purported to crystallize without solvent. For example, H,Me,CH2 was said to “crystallize only as [a] solvate” whereas Br,Me,CH2 was said to crystallize “only free of solvent”,55 though, later, the crystal structure of CHCl3@Br,Me,CH2 was reported.73 Moreover, many of the cavitands (and/or their (hemi)carcerand derivatives) display the remarkable ability to bind, albeit weakly, small neutral molecules and gases in solution.72 Such solutions can probably be classified as porous liquids.89,90 Indeed, the propensity of most cavitands to form solvates is so high that, remarkably, over 25 years later, the guest-free crystalline structures of even the simplest of cavitands appear to remain unknown. Some time ago we performed a search of the Cambridge Structural Database (CSD, Nov. 2012) for cavitands (Scheme 1, with covalent “Y” linkers). Of the 235 reported crystal structures, 96% displayed occupied molecular cavities. Approximately 81% of these exhibit either solvent-occupied cavities (75%), ion-occupied cavities (3%), or have their cavities occupied by other molecular guests (3%). Approximately

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13% of the structures are self-included, where peripheral functional groups of one cavitand fill the intrinsic molecular cavity of an adjacent cavitand, and, finally, ~2% of the structures exhibit collapsed cavities attributable to flexible “Y” linkers. Remarkably, only a handful of these cavitand structures (< 4%) appear to exhibit vacant molecular cavities in the solid state, and only a few of these appeared to have empty cavities of a size and shape large enough to accommodate small molecules. Some notable good-quality crystal structures that apparently exhibit appreciably sized empty intrinsic microcavities include YUFBUX91 (~70 Å3 voids), ITIMAA92 (~39 Å3 voids), MIGVAZ93 (~100 Å3 voids in an empty carcerand-like structure), and NUMDEE94 (~49 Å3 voids), though all of these examples also contain included solvents of crystallization. The structures are phosphonate/thiophosphonate bridged cavitands and are already being exploited for their sorptive, porous-like properties (vide infra). For example, crystals of ITIMAA will absorb methanol into the vacant cavity while simultaneously displacing included water and acetone in a single crystal-to-single-crystal fashion.92 Relatedly, the voids of YUFBUX can be occupied by methanol without affecting the crystal packing.91 To summarize, the CSD search clearly confirmed that: i) the crystalline structures of solvent/guestfree cavitands (e.g. any combination of R, R’ = H, Me, aryl, halogen and Y = -(CH2)-, or -SiR2-) are indeed unknown, and ii) empty cavitand cavities of significant volume (> 25 Å3) are exceedingly rare. Importantly, lack of knowledge of the structures of empty cavitand bulk phases has not limited their materials applications. In particular, Dalcanale and coworkers have extensively employed various cavitand thin films in the development of gas/vapor sensors.75-78 Notably, Me,H,CH2, one of the cavitands described herein, was found to slowly and more-or-less irreversibly absorb CH3CN. The lack of reversibility is presumably due to the crystalline nature of the material and a phase change of the host induced by absorption. They found, however, that long alkyl feet (R’, Scheme 1) tended to facilitate formation of amorphous films that permit more rapid sorption/desorption of gases as compared to crystalline films, though the extended feet also increase the undesired effects of non-specific absorption. Non-specific absorption has been addressed in part by the introduction of additional recognition functionality, such as in the (thio)phosphonate cavitands.75-78 Relatedly, Rudkevich and coworkers studied reversible gas sorption (CO2, N2O, H2 and N2) by long-alkyl footed hemicarcerands.38 Beyond cavitands, the crystallographic demonstration of empty, molecule-sized void spaces greater than ~25 Å3—and particularly discrete (microcavity) volumes of this size—in molecular crystalline solids remains generally rare.18 We attribute the paucity of such structures partly to the challenge of crystal structural determination of molecular crystalline powders by PXRD, which often requires sophisticated instrumentation (e.g. a high flux, monochromatic X-ray source) and a more specialized skills set.95 For reasons of convenience, the vast majority of single crystals are grown from solution and shape-persistent compounds that cannot pack efficiently, including cavitands, typically crystallize as solvates—so-called β-phases.96 Importantly, solvated crystals have a high tendency to fracture upon desolvation. The relative instability of the (often hypothetical) empty β0-phase as compared to a more

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close-packed, and more thermodynamically stable apohost αphase typically results in structural rearrangement of the host upon desolvation, thereby fracturing the crystal. The experimentalist is left with a guest-free powder and the challenge of structure determination by PXRD, even though, especially for shape persistent molecules, the apohost structure may be porous.97 The CSD is thereby, in our opinion, highly biased toward crystal structures that are essentially close-packed and the guest-free structures of most shape-persistent compounds that are incapable of close-packing remain unknown. Moreover, the situation is exacerbated by experimental porosity measurements, which are capable of revealing the porous nature of solids exhibiting 1D or higher dimensional pores, but may erroneously suggest a lack of porosity for materials that exhibit 0D pores or microcavities that are less kinetically accessible, particularly at the cryogenic temperatures of standard analyses. Cavitands and their Synthesis. Systematic study of guest-free cavitands as microcavity materials began with the simplest of cavitands, namely H,H,CH2, which is a novel compound. Seeking to explore how simple changes in the cavitand molecular structure affect the crystal packing of the guest-free crystal phases and the corresponding size/shape of the anticipated pores, a handful of other simple cavitands were prepared according to literature procedures: H,Me,CH273, Me,H,CH260, Me,Me,CH273, and Br,Me,CH273. Relative to H,H,CH2, these compounds replace hydrogen atoms (2 Å3) with simple methyl (23 Å3) or bromo (26 Å3) substituents so as to extend the depth of the cavitand bowl, modify the electronic structure, or add rigid “pegs” to the feet of the bowl, or both. Intuitively, and supported by the CSD search results, we envisioned that lengthening the alkyl feet of the bowl may result in crystal packing where the feet of one bowl fill the intrinsic cavity of adjacent cavitands; we sought to test this hypothesis by studying Me,Et,CH2. Similarly, we sought to study compounds with a smaller cavity aperture and the dimethyl silyl derivative Me,Et,SiMe2 seemed a logical choice. Though ethyl-footed resorcinarenes have been widely studied, Me,Et,CH2 and Me,Et,SiMe2 are novel compounds. Lastly, we wished to determine whether self-inclusion could be stymied by the use of bulkier, branched feet and so we prepared and studied the novel cavitand Me,iBu,CH2. Each of the aforementioned cavitands was derived from its corresponding resorcinarene octol—H,H,OH58,59, 60 73 Me,H,OH , H,Me,OH , Me,Me,OH73, Br,Me,OH73, Me,Et,OH,63 or Me,i-Bu,OH65—by methods similar to the preparation of other known –CH2-bridged (Me,H,CH260, H,Me,CH273, Me,Me,CH273, Br,Me,CH273) or -SiMe2bridged62 cavitands. In our hands, however, octols rcccMe,Et,OH and rccc-Me,i-Bu,OH were difficult to obtain in stereopure form. The compounds isolated after following published procedures contained as much as 13% of other stereoisomers, seemingly the rcct form. Unfortunately, the stereoisomeric impurities are known to react more quickly in their conversion to cavitands,64 exacerbating the stereopurity issues for the resulting cavitands. Ultimately, near stereopure rcccMe,Et,OH and rccc-Me,i-Bu,OH, and thereby stereopure cavitands Me,Et,CH2, Me,Et,SiMe2, and Me,i-Bu,CH2 could

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be obtained, albeit in lower overall yield, by modification of the work-up. Cavitand Solvates. It seemed plausible that cavitands, and perhaps compounds that pack inefficiently in general, may be particularly susceptible to polymorphism. Thus, in order to ensure reproducibility we sought to prepare bulk samples of guest-free cavitand crystal forms from well-characterized solvates. The following solvates were isolated by slow evaporation of solutions of the corresponding cavitands: NO2CH3@H,H,CH2, CHCl3@H,Me,CH2, NO2CH3@Me,H,CH2·NO2CH3, EtOAc@Me,Me,CH2, 1.8(NO2CH3)@Me,Et,CH2, EtOAc@Me,i-Bu,CH2, CH2Cl2@Me,i-Bu,CH2 (rccc:rcct = 84:16), EtOAc@Br,Me,CH2⋅EtOAc and 0.28(H2O)@Me,Et,SiMe2 (from EtOAc). The “@” symbol preceding the cavitand denotes that the guest preceding the symbol occupies the intrinsic cavity of the cavitand whereas the “⋅” symbol implies that the solvent can be found in extrinsic sites. Each of these solvates was characterized by single crystal X-ray diffraction (SCXRD, Table S1 and Figures S18S27), thermal gravimetric analysis (TGA, Figures S28-S29, S31, S33-S37), and powder X-ray diffraction (PXRD, Figures S28-S29, S33-S37). In all cases, the PXRD pattern of the bulk solvates matched well with the calculated patterns derived from the single crystal structures. Additionally, single crystals of several other solvates were isolated, but their structures are not being formally reported here: EtOAc@H,Me,CH2, NO2CH3@H,Me,CH2, pxylene@H,Me,CH2·1.5(p-xylene), CCl4@Me,H,CH2, p-xylene@Me,Me,CH2, Et2O@Me,Me,CH2, CCl4@Br,Me,CH2·CCl4, p-xylene@Br,Me,CH2, mxylene@Me,Et,CH2, and MeOH@Me,i-Bu,CH2. Moreover, of the eight cavitands in this study (Scheme 1), four have previously been crystallographically characterized, all as solvates. These are CH2Cl2@H,Me,CH2 (GEDKEF)73, CHCl3@Me,H,CH2·0.5(CHCl3) (QUCWIU)60 (GEDKOP)73, (CH2)6@Me,Me,CH2·C6H6 73 CH3CN@Me,Me,CH2 (GEDKIJ) , and CHCl3@Br,Me,CH2 (GEDKUV)73. Unfortunately, with the exception of QUCWIU, the atomic coordinates of these structure determinations, reported by Cram and coworkers, are not publicly available, though unit cell, space group, and figures of the host-guest complexes are published, revealing solvents within the intrinsic cavitand cavities. Lastly, we also observed that, during synthetic work-up, most of the cavitands readily form solvates (e.g., with CH2Cl2), but we did not generally seek to characterize these solvates.

Figure 3. Thermal ellipsoid plots of the solvent@cavitand complexes from the crystal structures of the reported new solvates. Solvent molecules extrinsic to the cavitand cavities have been omitted. Some of the solvent molecule are disordered, but could be adequately modelled. The electron density associated with the solvent molecules in 1.8(NO2CH3)@Me,Et,CH2 was modelled with SQUEEZE and the modelled volume is depicted in orange.

From the above observations, it is clear that, with the exception of Me,Et,SiMe2, all of the cavitands of this study essentially always crystallize as solvates from simple organic solvents. SCXRD analysis of the organic solvates reported here revealed structures that are generally unremarkable in the current context of porosity and so they are discussed only in the Supporting Information. Briefly, as expected, the solvents occupy the intrinsic cavities of the cavitands. Some of the compounds include additional solvents molecules in extrinsic sites—channels or cavities—that are defined by the host packing. 0.28(H2O)@Me,Et,SiMe2. In general, the organic solvent molecules complexed by the cavitand solvates protrude considerably from the intrinsic molecular cavities of the cavitands and likely, therefore, influence the crystal packing of the solvates (see discussion in Supporting Information). This is seemingly not the case, however, for Me,Et,SiMe2, which crystallizes as a 0.28(H2O)@Me,Et,SiMe2 partial hydrate from ethyl acetate, notably without enclathrating the organic solvent. Though the water content appears to be only fractional, its presence is clearly indicated by a 3.0 e-/Å3 electron density peak in the difference Fourier map, and a nearly 1% improvement in the R1-factor that accompanies modelling the peak as a partial occupancy oxygen atom. TGA analysis of the bulk 0.28(H2O)@Me,Et,SiMe2 sample shows no obvious mass loss before sublimation, though the expected mass loss for 0.28 equivalents of water is only 0.15%. Apparently, the small cavity aperture of Me,Et,SiMe2, guarded by the dimethylsilyl bridging groups, prohibits penetration of the intrinsic cavity by the ethyl acetate solvent and the conformational freedom of the ethyl groups allows the molecule to otherwise pack efficiently in the solid state. The appearance of water within the ostensibly hydrophobic, ~28 Å3 cavities of Me,Et,SiMe2 illustrates the cavitand’s ability to scavenge appropriate guests in the absence of more appropriate small molecules. The deep position of the water molecule within the cavity—O⋅⋅⋅C(arene) intermolecular contact distances as low as 3.64 Å, and O⋅⋅⋅arene(centroid) distances as low as 3.59 Å—is suggestive of O-H⋅⋅⋅arene(π) hydrogen bonds such as those observed in related, water-occupied ca-

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lixarenes.98 Similar behaviour has been observed for Me,H,SiMe2, which purportedly crystallizes from CH2Cl2 as xH2O@Me,H,SiMe2, though the quality of the structure determination is poor (R1 > 9%) and the water molecule is modelled as being disordered over two sites.99 The above observations are mostly consistent with Cram’s pioneering work concerning the solution phase binding properties of H,Me,SiR2 cavitands.55 Cram demonstrated that these cavitands weakly bind small, linear molecules such as CS2 (Ka = 0.8 M-1 H,Me,SiMe2 at 250 K), CH3C≡CH, and even O2 in chloroform solution, and reported the crystal structure of the CS2@H,Me,SiMe2⋅CS2 solvate.55 Despite attempts to observe it, however, they uncovered no evidence for binding of other, less complementary guests, including H2O, CO2, and CH2C12. The crystal structure of 0.28(H2O)@Me,Et,SiMe2 reported here suggests that the dialkylsilyl-bridged cavitands do weakly bind water—indeed, scavenge water—in organic solvents. The structure suggests, however, that under rigorously dry conditions, it must be possible to crystallize an isostructural empty form, exhibiting unoccupied, and potentially exploitable, microcavities (vide infra). Moreover, we find, not surprisingly, that the empty crystal form of Me,Et,SiMe2 is isostructural to this partial hydrate; its structure and additional features are described in the following section.

except Me,Et,SiMe2 (vide infra)—are microcrystalline and no crystals of sufficient size for SCXRD analysis could be identified. Thus, the cavitand organic solvates behave similarly to the vast majority of other crystalline organic solvates: desolvation of the so-called β-phase solvate invokes a structural “collapse” of the host packing that results in destructive fragmentation of the single crystal and a significant reduction in crystallite size.100 Seeking to determine the crystal structures of the guest-free cavitand apohost phases (so-called α-phases), the compounds were subjected to sublimation by heating under vacuum and the isolated single crystals were analysed by SCXRD.

Guest-free Cavitand Phases. Bulk, guest-free cavitand phases were prepared by simply heating the phase-pure cavitand solvates— NO2CH3@H,H,CH2, CHCl3@H,Me,CH2, EtOAc@Me,Me,CH2, NO2CH3@Me,H,CH2·NO2CH3, NO2CH3@Me,Et,CH2·0.8(NO2CH3), EtOAc@Me,i-Bu,CH2, EtOAc@Br,Me,CH2⋅EtOAc, and 0.28(H2O)@Me,Et,SiMe2—in an oven at the minimum temperature necessary to effect complete desolvation, according to TGA (generally > 150 °C). The empty cavitands were then resubjected to TGA under nitrogen atmosphere (Figures S28S29, S31, S33-S37). Though Br,Me,CH2 exhibited some level of decomposition at temperatures above 400 °C, the cavitands are generally thermally stable and sublime cleanly; visual inspection of the samples during attempted melting point determinations (capillary) confirmed that the cavitands studied here do not melt, but, rather, sublime under atmospheric pressures. Table 1 summarizes the sublimation temperatures of the cavitands, as defined by the temperature at their maximum rate of sublimation (Tmax). Perhaps not surprisingly, the heaviest of the cavitands studied, Br,Me,CH2, exhibits the highest sublimation temperature (Tmax = 470 °C). At the low end of the series, however, it is curious that the alkyl-footed cavitands exhibit the lowest sublimation temperatures (Tmax = 358-381 °C) despite their being heavier than many of the rigid cavitands. We hypothesize that the conformational dynamics accessible to the ethyl and isobutyl groups may facilitate the sublimation process by disrupting some of the intermolecular interactions that stabilize the crystal packing. As expected, PXRD analysis of the bulk, desolvated cavitand powders (Figures S28-S29, S31, S33-S37) demonstrated that the resulting guest-free cavitand phases are crystalline, but, with the exception of Me,Et,SiMe2, adopt crystal structures that differ significantly from the prepared solvates. Optical microscopy revealed that the desolvated samples—again,

Figure 4. A view of the polar cavitand layers in the crystal packing of H,H,CH2. Adjacent layers are related by an inversion center. The unoccupied intrinsic microcavities (Vcav = 44 Å3) are depicted in orange.

H,H,CH2, H,Me,CH2, Me,H,CH2, and MeMeCH2. The H,H,CH2, H,Me,CH2, Me,H,CH2, and MeMeCH2 cavitands constitute a simple series wherein the upper and lower rims of the cavitand bowls are systematically deepened and/or extended by replacement of hydrogen atoms (~2 Å3) with relatively rigid methyl groups (~23 Å3) . Upper rim substitution (R) formally extends the depth of the intrinsic cavitand cavity and lower rim substitution (R’) was expected to increase separation between the molecules in the solid state, thereby lowering overall packing fraction. Table 1. Thermal Data and Microcavity Summary for 0D Porous Cavitand Phases. Empty Cavitand

PFa

Porosity, ε (%) (% b

Vcav (Å3)a

Tmax (°C)c 427

0.69

6.9

44

0.68

8.2

61

0.67d

8.4d

63d

α-Me,H,CH2

0.72

3.8

27

390

Me,Me,CH2

0.69

5.0

38

401

H,H,CH2 H,Me,CH2

410

α-Br,Me,CH2

0.68

2.0

17

β-Br,Me,CH2

0.69

4.7

39

Me,Et,CH2

0.68

3.4

41e

358

Me,Et,SiMe2

0.66

2.3

28

374

470

8


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Me,i-Bu,CH2

Chemistry of Materials 0.64

9.5

115

d

d

d

0.63

9.8

122

381

a

Packing fraction and empty microcavity volumes (Vcav) at 100(2) K. bPorosity (ε)15 = (ΣVcav)/Vcell × 100%. cTemperature at maximum rate of sublimation (°C); heating rate: 5 °C/min. d At 298(2) K. eAn extrinsic microcavity. H,H,CH2. Sublimation of H,H,CH2 yielded single crystals suitable for SCXRD analysis. The crystals adopt the orthorhombic space group Pnma with one half of a molecule, residing on a mirror plane, in the asymmetric unit. Importantly, the single crystal structure corresponds that indicated by the PXRD pattern of H,H,CH2 obtained after thermal desolvation of NO2CH3@H,H,CH2 (Figure S28). The cavitands adopt an alternating (a-glide), canted (46°) feet-in-bowl type arrangement that is simply translated in the c-direction to sustain polar cavitand layers in the ac-plane. The packing seemingly optimizes penetration of the intrinsic cavitand cavity by the adjacent bowl (Figure 5) and there are no obvious directional C-H⋅⋅⋅O interactions within the layers. Adjacent layers are related along the b-axis by inversion symmetry and interlayer C-H⋅⋅⋅O interactions (dC(H)···O = 3.28 Å and ∡CHO = 145°) originating from the methylenic OCH2O moieties are apparent. As anticipated, the crystal structure is highlighted by the ineffective space-filling of the seemingly as-close-packed-as-possible (or nearly so) H,H,CH2 molecules. Ineffective penetration of the cavitand cavity by its bulky neighbor leaves spheroidal intrinsic microcavities that measure approximately Vcav = 44 Å3 at 100 K (Table 1), corresponding to a porosity (ε) of 6.9%. The packing fraction of the H,H,CH2 crystal is calculated to be 0.69 at 100 K, which is only somewhat low for organic solids, but not so low that the structure might be expected to be amorphous.16 Importantly, electron density analysis (SQUEEZE, no peaks > 0.22 e-/Å3 in the difference Fourier map) reveals that the intrinsic microcavities of H,H,CH2 are truly empty, despite there clearly being enough available space for at least one molecule of water or one of the constituents of air. Indeed, the observation of the empty microcavity suggests that air or water vapor sorption into the pores is either thermodynamically unfavorable or, more likely, sufficiently slow at room and lower temperatures that the pores do not take up these species during the crystal selection, mounting, or (low temperature) data collection processes. Indeed, relatively slow guest exchange kinetics in expected to be a feature of these DMMMs. H,Me,CH2. Seeking to explore the effect of subtle changes in cavitand molecular structure on crystal packing and porosity and microcavity characteristics, H,Me,CH2 was sublimed and single crystals suitable for SCXRD analysis were obtained. Perhaps not surprisingly, the crystal packing of H,Me,CH2 differs significantly from H,H,CH2. The former adopts a monoclinic, C2/c packing arrangement with one molecule in the asymmetric unit (Figure 5). Importantly, however, the determined single crystal structure of H,Me,CH2 corresponds to the same structure as is indicated by the PXRD pattern of the H,Me,CH2 material obtained after thermal desolvation of CHCl3@H,H,CH2 (Figure S31). The structure seems to arise from an optimization of packing efficiency, and no obviously directional C-H⋅⋅⋅O interactions appear to be present. The molecules pack in a significantly offset bowl-atop-

bowl type arrangement, giving rise to polar cavitand columns that repeat along the b-axis. The polar column close-packs with an adjacent (relative to the c-axis), parallel column by relation of a 21 screw axis. The next pair of polar columns (relative to the c-direction) runs anti-parallel to the first, being related by the c-glide. The result is a layer of alternating cavitand double-columns. Adjacent layers along the a-axis are related by inversion symmetry. Introduction of methyl feet at the lower rim of the cavitand leads to a slightly less efficiently packed apohost in H,Me,CH2 (PF = 0.68 at 100 K) as compared to H,H,CH2. The H,Me,CH2 structure is highlighted by relative large, roughly triangular prismatic microcavities that measure approximately Vcav = 61 Å3 at 100 K, corresponding to a porosity (ε) of 8.2% (Table 1). The relatively large, but empty microcavity of H,Me,CH2 (SQUEEZE analysis, no peaks > 0.20 e/Å3 in the difference Fourier map)—which may be considered a conjoined hybrid of the intrinsic concave cavity provided by the cavitand and extrinsic interstices arising from ineffective filling of space in the crystal—is clearly of sufficient volume and space to be capable of enclathrating small molecules and suggests possible applications of this material in gas capture/storage. Like H,H,CH2, however, the cavities of Me,H,CH2, are found to be devoid of electron density, suggesting that air or water vapor sorption into the microcavity is either slow or unfavorable under ambient conditions. A room temperature structure determination (Table 1, S1) also reveals an empty, but, of course, slightly expanded structure, with Vcav = 63 Å3 and ε = 8.4%.

Figure 5. One of the cavitand layers in the crystal packing of H,Me,CH2. Adjacent layers are related by an inversion center. The unoccupied microcavities (Vcav = 61 Å3) are depicted in orange/blue.

H,Me,CH2 is Isostructural to CH2Cl2@H,Me,CH2: Implications and Gas Clathrates. It has been noted in the literature that H,Me,CH2 crystallizes out of chloroform in the presence of CH3CN, CH2Cl2, and SO2 to form clathrates of these guests.73 This observation is intriguing considering our observation that H,Me,CH2 forms a chloroform solvate. The crystal structure of the CHCl3@H,Me,CH2 complex, however, sheds some light on this curious observation (see Supporting Information discussion, and Figure S21). In short, the CHCl3 molecule is too bulky to deeply penetrate the H,Me,CH2 cavity. The smaller CH3CN, CH2Cl2, and SO2 guests may not only be more complementary to the molecular cavity, but they may also be more complementary to the intrinsic pores of the low energy apohost crystal phase. Indeed, comparison of the pre-

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viously reported unit cell parameters of the CH2Cl2@H,Me,CH2 clathrate73 with those of empty H,Me,CH2 reveals that the methylene chloride clathrate adopts essentially the same host phase—the so-called αphase—as the empty apohost, which may be appropriately termed the α0-phase. As the atomic coordinates for CH2Cl2@H,Me,CH2 are not published, the crystal structure was re-determined, albeit with some difficulty,101 and is reported here. At 100 K, the unit cells of H,Me,CH2 and CH2Cl2@H,Me,CH2 (see Table S1) differ by less than 4% in volume (232 Å3, or 29 Å3 per formula unit), despite the introduction of eight molecules of CH2Cl2 (8 × 56 = 448 Å3) that require 656 Å3 of space in the close-packed crystal structure of pure CH2Cl2 at 153 K.102 Figure S32 depicts an overlay of the host structures in H,Me,CH2 and CH2Cl2@H,Me,CH2, which are nearly superimposable. These observations support the hypothesis that the empty H,Me,CH2 apohost phase is functionally porous. That is, the empty pores of H,Me,CH2 can be occupied under some conditions, by a molecule as large as methylene chloride (56 Å3), without affecting the host packing. Importantly, the data also show that the empty apohost phases are capable of accommodating a modest degree of host expansion without inducing a phase change of the host, implying that the zero-dimensional pores of these DMMMs may be functionally considerably larger than what is depicted by Table 1. For example, CH2Cl2 (56 Å3) is accommodated by an ostensibly 61 Å3 cavity (ε = 8.2%, before expansion). The crystal, however, expands by 4 % (measured at 100 K) to accommodate methylene chloride; the majority of the crystal expansion serves to affect the 0D pores, expanding them considerably (~38 %) to Vcav = 84 Å3 (ε = 10.9%, after inclusion). In contrast, the accommodation of larger guests, such as chloroform, requires reorganization of the host packing to an alternative β-phase. The energetic penalty of this host rearrangement must be offset by H,Me,CH2⋅⋅⋅CHCl3 and CHCl3⋅⋅⋅CHCl3 interactions and removal of CHCl3 results in collapse of the host lattice to the porous apohost α0-phase. Observations concerning H,Me,CH2 have important implications for the potential use of cavitands as gas sorbents or capture/storage materials (Figure 6). Like the vast majority of solvates, the host structures of most cavitand solvates (e.g. CHCl3@H,Me,CH2) will “collapse” upon desolvation. As is shown herein, however, many empty cavitand phases—and, we presume many other, as yet to be discovered, “collapsed” phases of shape-persistent hosts—retain an unusual degree of zero-dimensional porosity, exhibiting uniform, molecule-sized microcavities (e.g. the α0 form of H,Me,CH2). Being porous, the empty hosts are inherently capable of, under some conditions, forming isostructural clathrates. In principal, even the most volatile, weakly interacting gases can be accommodated, provided the gas can fit within the confines of the native host microcavity. Thus, the existence of essentially the same host phase packing in the CH2Cl2@H,Me,CH2 solvate and its empty apohost form suggests that guests/gases similar in size to CH2Cl2 may be bound by H,Me,CH2 under some conditions. Indeed, preliminary studies in our laboratory have established that the α-phase of H,Me,CH2 captures propene, dimethyl ether and other weakly interacting gases from solution, and holds them at room temperature;103 these results will be de-

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scribed in more detail in a future communication.

Figure 6. Illustration highlighting the potential for cavitand DMMMs, such as H,Me,CH2, as gas sorbent/storage media. Inclusion of guests that are not complementary to the empty cavity of the intrinsically porous apohost (e.g. CHCl3) require reorganization of the host structure (β-phase); the energetic penalty for host reorganization must be paid by sufficiently strong host-guest and guest-guest interactions. Being porous, the empty apohost phases (α0-phase) can accommodate weakly interacting guests (e.g. gases) that are complementary to the pre-existing microcavity, yielding α-phase gas clathrates.

Me,H,CH2. Crystals of α-Me,H,CH2—which is extended at the upper rim of the parent H,H,CH2 bowl, deepening the intrinsic cavity as compared to the lower-rim extension of H,Me,CH2—adopt yet another kind of packing. The orthorhombic Pnma packing of α-Me,H,CH2 (Figure 7) is loosely similar of that of H,H,CH2 in that the structures are composed

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of polar, orthogonal ac-layers of cavitands that are connected about an inversion center via C-H⋅⋅⋅O interactions (dC(H)···O = 3.61 Å and ∡CHO = 149°) originating from the OCH2O moieties. α-Me,H,CH2 and H,H,CH2 thereby have similar b-axis dimensions (Table S1), the former being slightly larger due to the greater width of the methyl-functionalized Me,H,CH2 cavitand. The local cavitand arrangement in α-Me,H,CH2, however, is a side-to-bowl type assembly that propagates along the a-glide, with the upper rim methyl groups partially protruding into the bowls of adjacent cavitands. The resulting oblong-shaped, intrinsic microcavities are significantly smaller in α-Me,H,CH2 (Vcav = 27 Å3) than in H,H,CH2 or H,Me,CH2, and the crystal is more efficiently packed (PF = 0.72) and less porous (ε = 3.8 %). Though the cavity is tiny, it may nonetheless be large enough to accommodate a small molecule, such as water (18 Å3). Importantly, the PXRD pattern of the pure Me,H,CH2 material that arises from thermal desolvation of the NO2CH3@Me,H,CH2·NO2CH3 (heated to 220°C in the TGA pan) is a good, yet noticeably imperfect match for the αMe,H,CH2 structure revealed by single crystal diffraction. Indeed, the bulk Me,H,CH2 material obtained in this way invariably exhibits 2-3 additional features in its diffractogram (at 8.8°, 10.1°, and 16.2°) that cannot be attributed to the αMe,H,CH2 structure (Figures S29,S30). As there is no evidence of chemical decomposition, and the material is apparently fully desolvated (1H NMR, TGA), these peaks are likely indicative of the presence of a small amount of a polymorphic form of Me,H,CH2. Notably, after heating the empty Me,H,CH2 material for eleven days at 150 °C, the PXRD peaks attributable to the likely polymorph are no longer present, indicating conversion of this form to α-Me,H,CH2 (Figure S29).

Figure 7. A view of the polar cavitand layers observed in the isostructural crystal packings of α-Me,H,CH2 (top) and Me,Me,CH2 (bottom). Adjacent layers are related by an inversion center. The oblong, unoccupied intrinsic microcavities (Vcav = 27 and 38 Å3, respectively) are depicted in orange.

Me,Me,CH2. Me,Me,CH2 is slightly extended at both the upper and lower rims as compared to H,H,CH2. Interestingly, single crystals of Me,Me,CH2 obtained by sublimation were analyzed by SCXRD and found to be isostructural to αMe,H,CH2, albeit ~0.4-1.0 Å larger in unit cell dimensions due to the additional methyl groups (Figure 7). The C-H⋅⋅⋅O interactions between the layers are preserved (dC(H)···O = 3.61 Å and ∡CHO = 149°). Importantly, the PXRD pattern of Me,H,CH2 obtained after thermal desolvation of phase-pure EtOAc@Me,Me,CH2 corresponds to the determined single crystal structure of Me,Me,CH2 (Figure S33). Thus, while the introduction of methyl substituents at the lower and upper rims of H,H,CH2 leads to different modifications in the host packing, introduction of methyl groups at both positions gives rise to crystal packing that is similar to the upper rim modified, αMe,H,CH2 (though this compound appears to be polymorphic). Not surprisingly, introduction of the methyl feet interferes slightly with the local side-to-bowl motif observed in αMe,H,CH2, separating the molecules somewhat and resulting in a slightly larger intrinsic microcavity (Vcav = 38 Å3), a lower overall packing fraction, and a slightly greater porosity (PF = 0.69, ε = 5.0%). The cavity appears to be complementary in size/shape to a molecule of dioxygen.

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Figure 8. Crystal packing in two polytypic forms of Br,Me,CH2. (bottom) The α-Br,Me,CH2 form exhibits an …ABCD… interlayer repeat sequence and a close interlayer spacing (l = 1.34 Å) that minimizes the microcavity volume (Vcav = 17 Å3). The pistaking distance, d = 3.51 Å, is shown. (top) The β-Br,Me,CH2 form exhibits a simple, orthogonal ...ABAB… repeat sequence, but a slightly larger interlayer spacing (l = 1.60 Å), giving rise to larger microcavities (Vcav = 39 Å3). d = 3.46 Å.

Br,Me,CH2. The Br,Me,CH2 cavitand is sterically similar to Me,Me,CH2 (Br = 26 Å3 vs. Me = 23 Å3), but electronically quite different. Sublimation of Br,Me,CH2 yielded single crystals of two polytypic crystal forms, α and β (Figure 8). The PXRD pattern of the Br,Me,CH2 material obtained from thermal desolvation of EtOAc@Br,Me,CH2⋅EtOAc is unusually broad and, although it exhibits features that could be attributable to either form, it appears to greater resemble αBr,Me,CH2. Thus, it is unclear whether the bulk powder is phase pure or a mixture of polymorphic forms, possibly including other polytypes. From the 100 K unit cell volumes, α− and β -Br,Me,CH2 appear to have nearly the same density (ρ = 1.804 and 1.800 g/cm-3, respectively), though the αBr,Me,CH2 crystal was twinned and the quality of the refinement model is modest. The higher quality crystal structure determination, that of β−Br,Me,CH2, reveals orthorhombic Cmca packing. The structure is seemingly dominated by di-

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pole-dipole interactions, consisting of egg-carton-like layers of cavitands arranged in an alternating, antiparallel fashion in the (0 0 1) plane. The cavitands are π-π stacked within the layer, exhibiting arene⋅⋅⋅arene interplanar spacings, d, of 3.43 Å and 3.46 Å along the a and b axes, respectively. Adjacent layers are stacked such that one bromo substituent of each cavitand occupies a nearly central position at the opening of the other, defining the intrinsic cavity volume (Vcav = 39 Å3, corresponding to ε = 4.7 %). The closest intermolecular Br⋅⋅⋅Br distances involve these bowl-positioned bromine atoms, but measure only 4.12 Å, which is significantly greater than the sum of the van der Waals radii (3.70 Å). The distance between the layers, as defined by the bromine atoms, measures 1.60 Å. Crystals of α-Br,Me,CH2 exhibit almost identical cavitand layers, though the pi-staking distances between the cavitands along the two unique directions are slightly longer (3.51 and 3.55 Å), resulting in a slightly lower layer density as compared to β−Br,Me,CH2. The interlayer stacking in αBr,Me,CH2 is also similar to β-Br,Me,CH2, but follows an …ABCD… repeating pattern, resulting in a monoclinic C2/m packing. Additionally, the interlayer distance is shorter in αBr,Me,CH2 (l = 1.34 Å), resulting in a minimally sized intrinsic cavity (Vcav = 17 Å3). Apparently, optimization of layer density competes with optimization of stacking density in these unusual organic polytypes. We hypothesize that the inclusion of small molecules within the cavitand bowl may a means by which to bias the crystal packing toward the largercavity β-phase. Me,Et,CH2. It was hypothesized that extending the length of the cavitand “feet” (R’) might, at some point, be counterproductive to the achievement of a porous structure. Intuitively, extended alkyl feet ought to be able to penetrate/occupy the intrinsic cavities of adjacent cavitands and likely facilitate close packing. Indeed, examples of such self-included structures were found in the CSD search. Notably, however, selfinclusion would not preclude the formation of extrinsic voids arising from inefficient packing, such as is observed in crystals and gas clathrates of calix[4]arene.17 To explore this hypothesis, Me,Et,CH2 was synthesized and its empty structure determined by SCXRD. Guest-free Me,Et,CH2 crystallizes in the monoclinic P21/c space group. Importantly, the PXRD pattern of the empty Me,Et,CH2 material obtained from thermal desolvation of NO2CH3@Me,Et,CH2·0.8(NO2CH3) matches the empty single crystal structure. There are two symmetry-unique cavitands in the asymmetric unit, differing mainly with respect to the conformational arrangement the ethyl moieties. Not surprisingly, it is found that the intrinsic cavities of each cavitand are occupied by one of the ethyl feet of an adjacent cavitand in the structure (Figure 9). Nonetheless, appreciable volumes of contiguous void space are still observed in the form of extrinsic cavities (Vcav = 41 Å3, ε = 3.4 %) that arise from inefficient packing. The microcavities are surrounded by hydrocarbon moieties, including ethyl groups, methyl groups, and the methylene moieties.

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Figure 9. Crystal packing in Me,Et,CH2, illustrating a) the filling of the intrinsic Me,Et,CH2 cavity by the ethyl “feet” of an adjacent cavitand, and b) the extrinsic microcavities (blue, Vcav = 41 Å3) arising from inefficient packing. For clarity, the carbon atoms of the two symmetry-independent Me,Et,CH2 molecules are given different colors.

Me,Et,SiMe2: Porosity, Permeability to Water and CO2, and Capture of Freon-41. As intended by Cram’s original design, dimethylsilyl-bridged cavitands possess more narrow cavity openings as compared to the -CH2-bridged cavitands. The narrow cavity opening of Me,Et,SiMe2 thus precludes access of most functional groups to the cavity. Consequently, while –CH2-bridged cavitands form solvates when crystallized from EtOAc, Me,Et,SiMe2 apparently precludes the relatively bulky solvent, scavenging water and crystallizing as a partial hydrate. It can therefore be reasonably anticipated that most dimethlylsilyl-bridged cavitands, regardless of the “feet” (R’), may be incapable of self-inclusion and ought to crystallize with intrinsic (albeit small) microcavities that may be exploitable in the context of porosity. Thus, while the ethyl feet of Me,Et,CH2 are capable of filling and thereby negating the intrinsic cavity of the methylene–bridged cavitand, it was hypothesized that the ethyl feet of Me,Et,SiMe2 may be precluded from the more narrow dimethylsilyl bridged cavity, yielding an intrinsically porous structure.

Figure 10. (a) Empty Me,Et,SiMe2 as obtained from singlecrystal-to-single-crystal dehydration of xH2O@Me,Et,SiMe2. The crystal is isostructural to the hydrate. Intrinsic microcavities measure Vcav = 28 Å3 (ε = 2.3 %) (b) TGA analysis of the isostructural Freon-41 clathrate xCH3F@Me,Et,SiMe2 illustrating kinetic confinement of this gas within the porous Me,Et,SiMe2 apohost phase. A loss of 3.0 wt% corresponds to x = 0.80. c) Structure of an isolated single crystal of 0.69CH3F@Me,Et,SiMe2 (an approximately 69:31 solid solution of rccc:rcct host isomers). For clarity, the minor occupancy ethyl positions of the rcct isomer have been removed. d) Structure of 0.20CO2@Me,Et,SiMe2 ob-

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tained after pressurizing an empty, stereopure single crystal of Me,Et,SiMe2 with 35 bar of CO2 at room temperature for 6.5 days.

The PXRD pattern of empty Me,Et,SiMe2, obtained after oven treatment of 0.28(H2O)@Me,Et,SiMe2 (150 °C) indicates that there is no observable change in the host crystal packing. Clearly, water can be removed without a change in the crystal packing of the host. The PXRD data therefore suggests that empty Me,Et,SiMe2 must contain small, unoccupied microcavities. In attempts to validate this hypothesis, samples of x(H2O)@Me,Et,SiMe2 were subjected to sublimation under vacuum. Interestingly, the single crystals obtained were structurally almost indistinguishable from 0.28(H2O)@Me,Et,SiMe2 grown from solution and remained partially hydrated, though the water context was generally lower (0.05 < x < 0.25, for multiple crystals). The fully activated, empty structure (Figure 10) was obtained by heating the original 0.83 × 0.54 × 0.52 mm 0.28(H2O)@Me,Et,SiMe2 single crystal (150 °C, 2 days), and then re-determining the structure at 100 K. The empty structure is experimentally indistinguishable from the partial hydrate, except that it lacks any significant electron density within the intrinsic cavity (Vcav = 28 Å3) and is therefore porous (ε = 2.3 %). Notably, the crystal suffered essentially no deterioration in diffraction quality (88% and 86% of reflections are observed below 56° 2θ, before and after heating, respectively). The results demonstrate that: i) even relatively small (e.g., 28 Å3) microcavities and crystals of low total porosity are capable of trapping small molecule guests, ii) Me,Et,SiMe2 is functionally permeable to small molecules such as water, at least at 150 °C, and iii) the presence of water in the sublimed crystals is due to co-sublimation. Water vapour uptake by Me,Et,SiMe2 must be relatively slow at room temperature, otherwise the empty Me,Et,SiMe2 would have taken up moisture from the atmosphere during processing for the diffraction experiment. Capture of Freon-41. The equivalent host structures of empty and water-occupied Me,Et,SiMe2 suggest that the small Vcav = 28 Å3 microcavities may be useful for the capture and/or confinement of small gases. To test this hypothesis, and to illustrate the emerging properties of 0D porous cavitand phases, we sought to use Me,Et,SiMe2 to capture Freon-41 (fluoromethane, bp = -78 °C, 32 Å3), which appeared to us as though it may be small enough to be accommodated by the otherwise empty apohost phase of Me,Et,SiMe2. Moreover, Cram and coworkers have shown that dimethyl-silyl bridged cavitands very weakly bind small linear guests such as propyne, dioxygen, and dinitrogen in organic solvents, though no solid gas complexes were reported.72 Freon-41 is employed as an etchant in reactive-ion etching microfabrication technologies and, along with other hydrofluorocarbons, is a potent greenhouse gas (490 times the 20-year Global Warming Potential of CO2) that has been targeted for control by the Kyoto protocol. A saturated, room-temperature ethyl acetate solution of Me,Et,SiMe2 was treated with Freon-41, resulting in formation of xCH3F@Me,Et,SiMe2 as a crystalline powder. PXRD analysis of the product illustrates that the crystal packing of the Freon-41 clathrate is isostructural to empty Me,Et,SiMe2 (Figure S39). The solid was found by 1H NMR spectroscopy to contain approximately 0.57 equivalents of

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fluoromethane (Figure S38). TGA analysis reveals that the clathrate is remarkably stable, exhibiting a mass loss of about 3.0% between room temperature and ~150 °C (Tmax = 98 °C), corresponding to about 0.8 equivalents of CH3F. Loss of fluoromethane does occur slowly, however, at room temperature. NMR analysis of the clathrate powder after ten days reveals that only about 0.04 equivalents of Freon-41 remain (Figure S38). Since there is no structural change in the host during off-gassing, it is clear that the material is permeable to fluoromethane, even at room temperature, albeit on the hours-days timescale. Single crystals of xCH3F@Me,Et,SiMe2 were grown by a similar procedure as the stereopure xCH3F@Me,Et,SiMe2 powder, but using the more available stereoimpure rccc:rctt mixture. Despite the extreme volatility of the guest, no special precautions needed to be taken in mounting the crystal for Xray analysis. The 100 K X-ray structure determination indicated that the isolated crystal was formally a solid solution consisting of an estimated (refined) 69:31 ratio of rccc and rcct host forms and the fluoromethane occupancy independently refined to 0.69. Remarkably, despite the extent of the stereoimpurity, the 0.69CH3F@Me,Et,SiMe2 crystal is nonetheless isostructural to empty Me,Et,SiMe2, except that the intrinsic cavity is partially occupied by a molecule of fluoromethane. The fluoromethane molecule appears fully ordered at 100 K and appears to reside mainly or exclusively “methyl-down” with its C-F dipole coinciding almost perfectly (within ~0.2°) with the pseudo-C4 axis of the cavitand. The 100 K unit cell volume of 0.69CH3F@Me,Et,SiMe2 is slightly larger than that of empty Me,Et,SiMe2 (~35 Å3, 0.7 %), though it is unclear at this time whether this difference is due to the presence of the Freon, or the presence of the rcct stereoisomeric impurities. And, although the Freon occupancy is approximately equal to that of the rccc stereoisomer in this single crystal solid solution, the fact that a similar occupancy is measured for the stereopure clathrate (~0.57-0.8) suggests it is unlikely that the Freon and rccc-Me,Et,SiMe2 occupancies are correlated. Due to rather extensive variation in bond lengths involving fluorine, the covalent radius of fluorine has been the subject of considerable discussion. The C-F bond length, for example, varies from ~1.32 to ~1.39 in the series of fluromethanes. To our knowledge, 0.69CH3F@Me,Et,SiMe2 constitutes the first reported crystal structure determination of an ordered fluoromethane molecule. The X-ray-determined C-F bond length is 1.389(3) Å, in excellent agreement with the spectroscopic value of 1.391(5) Å and the value of 1.389 Å calculated at the MP2/6-311+G** level.104 Sorption of CO2. Single-crystal-to-single-crystal dehydration of x(H2O)@Me,Et,SiMe2 demonstrates that Me,Et,SiMe2 is permeable to small molecules at high temperatures, yet Freon-41 experiences some degree of kinetic confinement at room temperature. CO2 has a relatively small kinetic diameter (3.3 Å) and it was hypothesized that it may be capable of diffusing into the Me,Et,SiMe2 crystal on a measurable timescale. Also, being composed of three heavy atoms, it was thought to be easily identifiable by crystallographic methods. To test this hypothesis, and unequivocally demonstrate gas permeability in Me,Et,SiMe2, the following experiment was conducted. An empty single crystal of stereopure Me,Et,SiMe2 was placed under a constant 35 bar pressure of

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CO2 at room temperature in a stainless steel bomb. After 6.5 days, the crystal was removed from the vessel and immediately mounted on the single crystal diffractometer. Low temperature structure determination (Table S1, Figure 10) clearly revealed, as anticipated, the presence of a fractional amount of an ordered CO2 molecule in the 0D pores (~0.20 equivalents). Oddly, unlike CH3F, the CO2 molecule does not align its long axis with the pseudo C4-axis of the host, but, rather, is found to be tilted 57° from the C4-axis (Figure S40) and slightly “pushes open” one of the dimethyl silyl moieties, though it may also be weakly interacting with the silicon atom (O⋅⋅⋅Si = 3.62 Å). With the exception of the CO2 inducing concomitant disorder of one of the dimethylsilyl moieties, the host structure is practically unchanged compared to empty Me,Et,SiMe2. Clearly, Me,Et,SiMe2 is capable of absorbing CO2, and likely other smaller gases, though the mechanism of diffusion is as yet not understood. Experiments conducted on other crystals, but over shorter times, reveal a lower CO2 occupancy, implying that CO2 sorption occurs very slowly, on the daysweeks timescale. This, in turn, suggests that the composition of the xCO2@Me,Et,SiMe2 crystal has not yet reached equilibrium, even after 6.5 days, and the crystal can likely accommodate up to one equivalent of CO2 at high pressures. Given the demonstrated permeability of Me,Et,SiMe2 toward small gases, it seems clear that Me,Et,SiMe2, and related compounds, have some potential to serve as gas sorbents and/or gas storage materials at modestly high temperatures and/or pressures. Though the gas capacities of these materials are not high relative to many MOFs—Me,Et,SiMe2 is in principal capable of holding a modest 3.7% wt/wt CH3F and 6.7% wt/wt of CO2 when fully occupied—capacity is not the most important feature from the perspective of separations or other potential applications. Thus, there may be opportunities for 0D porous cavitands in the realm of kinetically selectivity gas sorption/permeation, in the capture/storage of volatiles, or the protection of reactive small molecules. Moreover, detailed studies of gas sorption kinetics should shed light on mechanisms of diffusion in 0D porous and related materials (e.g. barrier polymers). Me,i-Bu,CH2. As narrowing the intrinsic cavitand cavity was demonstrated to prevent inclusion of alkyl feet into the dimethylsilyl-bridged cavitand Me,Et,SiMe2, it was conceivable that increasing the bulk of the cavitand feet may similarly prevent foot inclusion into the more open methylene-bridged cavities, thereby yielding structures with larger pores. Thus, we sought to determine the structure of guest-free Me,iBu,CH2, which exhibits relatively bulky isobutyl feet. Single crystals of empty, stereopure Me,i-Bu,CH2 were obtained by sublimation, crystallizing in the orthorhombic space group Pnma (Figure 11). Importantly, the single crystal structure corresponds to that indicated by the PXRD pattern of the guest-free Me,i-Bu,CH2 material obtained from thermal desolvation of EtOAc@Me,i-Bu,CH2 (Figure S37). The structure consists of polar, canted layers of cavitands in the (0 1 0) plane. The layers are bisected by a mirror plane and adjacent layers are related by an inversion center. Remarkably, the structure of guest-free Me,i-Bu,CH2 appears to be dominated by as-close-as-possible-packing, yet the overall packing fraction is the lowest yet of this cavitand series (PF = 0.64, Porosity, ε = 9.5 % at 100(2) K) as there exists a large, empty

microcavity of about Vcav = 115 Å3 (Figure 11). The isobutyl groups are found well above, and to the side of the intrinsic cavitand cavity, ostensibly being too large to effectively penetrate it. Fortuitous alignment of the intrinsic cavitand cavity and the extrinsic free volume that arises from inefficient packing yields microcavities that are far larger than the cavities observed in the other cavitand materials, constituting 9.5% of the Me,i-Bu,CH2 crystal volume (Table 1). A room temperature structure determination (Table 1, S1) also reveals an empty, though, of course, slightly expanded structure (PF = 0.63; Vcav = 122 Å3). Interestingly, empty Me,i-Bu,CH2 is essentially isostructural to the nitromethane solvate of Me,Et,CH2, namely NO2CH3@Me,Et,CH2·0.8(NO2CH3) (Figure S24). The unit cell dimensions are, of course, somewhat larger for Me,iBu,CH2, mainly along the a and c axes due to the larger size of the isobutyl groups, but the observation suggests that it may be possible to isolate a lower density polymorphic form of Me,Et,CH2 that is isostructural to its nitromethane solvate.

Figure 11. The crystal structures of (top) empty, stereopure Me,i-Bu,CH2 at 100(2) K, illustrating the large, 115 Å3 microcavities, and (bottom) the isostructural solvate CH2Cl2@Me,iBu,CH2 (84:16 rccc:rcct) wherein the microcavities are almost

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completely filled by CH2Cl2.

The relatively large microcavity in empty Me,i-Bu,CH2 suggests that small molecules with volumes of up to 70-80 Å3 might be easily accommodated by this intrinsically porous host phase. Indeed, while exploring the propensity of Me,iBu,CH2 to form solvates, single crystals of CH2Cl2@Me,iBu,CH2 were isolated and later found to be isostructural to empty Me,i-Bu,CH2, except that the microcavities are found to be occupied by methylene chloride molecules (Figure 11). Formally, the analysed CH2Cl2@Me,i-Bu,CH2 single crystal is a solid solution of the rccc and rcct cavitands, the relative occupancies refining to 84% and 16%, respectively. The rccc:rcct ratio from the single crystal refinement is remarkably consistent with the stereopurity of the bulk material as measured by 1H NMR spectroscopy (80:20). Apparently, crystallization of Me,i-Bu,CH2 from CH2Cl2 does little to improve the stereopurity of the host, whereas crystallization from ethyl acetate improves it (single crystals of EtOAc@Me,i-Bu,CH2 obtained from a stereoimpure host solution appeared to be stereopure). Interestingly, the methylene chloride molecules are not large enough to completely fill the pore of empty Me,iBu,CH2 as unoccupied voids of about 10-15 Å3 persist in the region of the isobutyl groups. Another interesting feature of CH2Cl2@Me,i-Bu,CH2 is its unit cell volume, which is 48 Å3 smaller than empty Me,i-Bu,CH2, the greatest difference being between the a axes of the two structures. Notably, from the crystal packing it appears that the a-axis would be most affected by the presence of the rcct stereoimpurities. Therefore, the smaller unit cell of CH2Cl2@Me,i-Bu,CH2 as compared to empty Me,i-Bu,CH2 is probably due to the presence of rcct-Me,i-Bu,CH2 in the crystal. 4. CONCLUSIONS Predictably, the guest-free crystal structures of many simple shape-peristent cavitands are formally porous, and exhibit zero-dimensional pores (microcavities) with contiguous pore volumes that are exceptionally large for discrete molecule materials. Eight new examples are reported here, with microcavities that range in volume from 27-115 Å3 and constitute between ~2.5-10% of the total crystal volume. The sizes and shapes of the cavities are dictated in part by the intrinsic molecular cavity, but are also largely affected by the specific crystal packings. Importantly, though polymorphism was exhibited by some cavitands, the reported porous structures are almost certainly the most thermodynamically stable crystal forms of these compounds, being approximately as-closepacked-as-possible. Consequently, the structures are incollapsible up to their sublimation temperatures (> 250 °C), a feature in notable contrast to many metastable porous molecule-derived materials. That the solvates CH2Cl2@H,Me,CH2, CH2Cl2@Me,iBu,CH2, and xH2O@Me,Et,SiMe2 are isostructural to their empty, porous apohost phases, demonstrates that small molecules can be accommodated by the porous host phases. Moreover, the apohost phases can tolerate a modest degree of expansion (a few percent) in accommodating substrates; the majority of the expansion is manifested as an increase in the microcavity volume. It is asserted that the porous apohost phases may therefore be suitable for the sorption, encathration, and/or storage of highly volatile compounds. In a proof-of-concept

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experiment, one of the reported cavitand apohosts (Me,Et,SiMe2) has been shown to form a stable clathrate with Freon-41 (bp = -78 °C). Moreover, the Me,Et,SiMe2 apohost has been demonstrated to be permeable to water and fluoromethane at elevated temperatures. Additionally, H,Me,CH2 forms stable clathrates with propene, dimethyl ether, and other gases, the details of which will be described in more detail elsewhere. At room temperature, the materials described herein appear to be more or less impermeable to most atmospheric gases, at least on the minutes timescale. The traditional assumption that solvated molecular solids "collapse" into close-packed structures upon desolvation is probably wrong, at least with respect to shape persistent compounds that are incapable of efficient crystal packing. The distinction between close-packed structures and those that exhibit zero-dimensional pores—that are less obviously accessible to substrates—is an important one; porous apohosts are capable of confining guests and/or exchanging them under some conditions. Moreover, discrete molecule microcavity materials (DMMMs) offer the unusual benefit of being soluble and therefore solution processible, offering a mechanism of pore access/exchange generally not available to frameworktype materials. Like other molecule-derived porous materials, there exist many opportunities to tune the pore and materials properties of these and related DMMMs to suit emerging applications.

ASSOCIATED CONTENT Supporting Information Available: NMR spectra (1H, 13C) of novel compounds, TGA, PXRD patterns, and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC depository numbers 1407164-1407184 and 1414905 contain the single crystal X-ray structure data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033.AUTHOR INFORMATION

Corresponding Author a

Address: Georgetown University, Department of Chemistry, 37th and O Sts. NW, Washington, DC, USA 20057. * E-mail: [email protected] b Address: Stellenbosch University, Department of Chemistry and Polymer Science, Matieland, 7602, Stellenbosch, South Africa. † Current address: Halliburton Energy Services Inc., 3000 N Sam Houston Pkwy E, Houston, TX, USA 77032-3219.

Funding Sources This work was partially supported by the U.S. National Science Foundation (NSF), under DMR-1106266 and CHE-1337975, and Georgetown University.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT CMK acknowledges support from the IMI Program of the National Science Foundation under Award No. DMR 08-43934.

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Crystal Transformation through Selective Guest Exchange. Chem. Eur. J. 2011, 17, 3064-3068 93 Bibal, B.; Tinant, B.; Declercq, J.-P.; Dutasta, J.-P. A new supramolecular assembly obtained form the combination of silver(I) cations with a thiophosphorylated cavitand. Chem. Commun. 2002, 432-433. 94 Shkarina, E. V.; Maslennikova, V. I.; Vasyanina, L. K.; Lyssenko, K. A.; Antipin, M. Yu.; Nifantiev, E. E. Phosphocavitands – II – Synthesis of P-V-Phosphocavitands in Reactions of P-IIIPhosphocavitand Oxidation – Stereotrend of the Process. Russ. J. Gen. Chem. 1997, 67, 1980-1986. 95 Karki, S.; Fábián, L; Friščić, T; Jones, W. Powder X-ray Diffraction as an Emerging Method to Structurally Characterize Organic Solids. Org. Lett. 2007, 9, 3133-3136. 96 Nassimbeni, L. R. Physiochemical Aspects of Host-Guest Compounds. Acc. Chem. Res. 2003, 36, 631-637. 97 Cruz-Cabeza, A. J.; Day, G. M.; Jones, W. Predicting Inclusion Behavior and Framework Structures in Organic Crystals. Chem. Eur. J. 2009, 15, 13033-13040. 98 Fucke, K.; Anderson, K. M.; Filby, M. H.; Henry, M.; Wright, J.; Mason, S. A.; Gutmann, M. J.; Barbour, L. J.; Oliver, C.; Coleman, A. W.; Atwood, J. L.; Howard, J. A. K.; Steed, J. W. The Structure of Water in p-sulfonatocalix[4]arene. Chem. Eur. J. 2011, 17, 10259-10271. 99 Lara-Ochoa, F.; Garcia, M. M.; Teran, R.; Almaza, R. C.; Espinoza-Perez, G.; Chen, G.; Silaghi-Dumitrescu, I. A New Tubular Arrangement of a Dimethylsilyl Bridged Calix[4]resorcinarene. Supramol. Chem. 2000, 11, 263-273. 100 Halasz, I. Single-Crystal-to-Single-Crystal Reactivity: Gray, Rather Than Black or White. Cryst. Grow. Des. 2010, 10, 2817–2823. 101 Crystals of CH2Cl2@H,Me,CH2 are polymorphic and habitually twinned. One crystal form, described here, matches that reported by Cram and coworkers, though the twinning affects the quality of the structure determination. 102 Kawaguchi, T.; Tanaka, K.; Takeuchi, T.; Watanabe, T. The Crystal Structure of Methylene Bichloride, CH2Cl2. Acta Bull. Chem. Soc. Jpn. 1973, 46, 62-66. 103 Kane, C. M.; Holman, K. T. Cavitand compositions and methods of use thereof. WO 2013191778 A3, Mar 22, 2013. 104 Lien, P.-Y.; You, R.-M.; Hu, W.-P. Theoretical modeling of hydrogen abstraction reaction of fluoromethane by the hydroxyl radical. J. Phys. Chem. A 2001, 105, 2391-2400.

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Many Simple Molecular Cavitands Are Intrinsically Porous (Zero

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