Article pubs.acs.org/crystal
4-Phenoxyphenol: A Porous Molecular Material Lynne H. Thomas,† Elaine Cheung,‡ Andrew O. F. Jones,‡,§ Andras A. Kallay,†,‡ Marie-Hélène Lemée-Cailleau,§ Garry J. McIntyre,# and Chick C. Wilson*,† †
Department of Chemistry, University of Bath, Bath, BA2 7AY, U.K. School of Chemistry and WestCHEM Research School, University of Glasgow, Glasgow, G12 8QQ, U.K. § Institut Laue-Langevin, BP 156, 38042, Grenoble Cedex 9, France # The Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee, DC New South Wales 2232, Australia ‡
S Supporting Information *
ABSTRACT: 4-Phenoxyphenol is a simple organic molecule that crystallizes as a porous material with channels running throughout the structure. The channels are constructed by a 6-fold hydrogen bonded ring and can host solvent molecules incorporated during crystal growth, with a minimum channel diameter of 5.8−5.9 Å; each channel usually contains a single solvent molecule per unit cell. The hydrogen bonded ring shows surprising flexibility, being able both to breathe and to sustain its crystalline integrity even when grown with empty pores. This is particularly surprising given that the remainder of the interactions within the crystal structure are C−H···π interactions and are weak in nature. It is also possible to grow “dry” porous 4-phenoxyphenol crystals by using a bulky solvent in the recrystallization.
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INTRODUCTION Porous storage materials such as zeolites have found widespread applications in a range of areas for many years1,2 and have become some of the most technologically relevant systems across materials chemistry. The value of micro- or nanoporous materials such as zeolites lies in several different options for functionality, including the creation of geometric features such as accessible cavities (with tunable dimensions depending on the detailed chemical composition) and contiguous linear pores. Physisorption into such cavities, benefiting in addition from the high exposed surface area available at the molecular level, renders such materials potentially valuable as gas storage systems. In addition, design of appropriate chemical functionality in such systems allows for a wide range of chemisorption potential, and microporous materials can also act as microscopic chemical reactors, in addition to their physisorption and gas transport capabilities. More recently, a much wider range of chemical systems have been under investigation as gas storage systems, as a potentially key development in the establishment of the “hydrogen economy”, the management of the greenhouse gas CO2 and its remediation, and in the storage and delivery of other fuel gases such as methane. For this reason, the properties of structures with large cavities somewhere in the solid are of huge current relevance, most recently in the area of metal−organic frameworks (MOFs), covalent organic frameworks (COFs), and other hydrogen bonded frameworks in which some of the most exciting current structural chemistry is being undertaken.3−5 The aim of this work in this application domain is to create systems in which cavities are present that © 2012 American Chemical Society
are suitable for gas storage, and in which gas ingress and egress can be achieved under appropriate conditions. Much of the recent work in developing such structures has thus involved inorganic or metal−organic systems, and a huge variety of extended systems have been identified as candidates. In many of these, genuine gas ingress and egress has been demonstrated, as well as gas storage capacities that are compatible with the requirements set out among target criteria for hydrogen storage systems by the U.S. Department of Energy (DoE) and commonly accepted as the international benchmark.6 There are many such materials which selfassemble around solvent-containing cavities to form solidstate structures, and in such cases the initial criterion for a potentially useful system will be the robustness of the molecular architecture to the removal of solvent, and the consequent preservation of a cavity of potential value for application in gas storage. The number of purely molecular porous or potentially porous systems remains rather lower than in the metal−organic area because such discrete molecules tend to pack in the most efficient manner possible, but such systems have potential advantages in terms of low density and lack of toxicity associated with metal content. A low density molecular microporous material would have advantages in meeting one of the DoE criteria in terms of mass ratio for gas take-up. Received: August 2, 2011 Revised: February 10, 2012 Published: February 28, 2012 1746
dx.doi.org/10.1021/cg200998u | Cryst. Growth Des. 2012, 12, 1746−1751
Crystal Growth & Design
Article
Figure 1. The channel structure adopted by 4-phenoxyphenol in the crystalline phase. The six molecule hydrogen bonded ring (left) and the relative orientations of the molecular tails viewed along the plane generated by the hydrogen bonded ring (right).
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Inherently porous molecular structures7 can take two forms: those with intrinsic and extrinsic porosity.8 Intrinsically porous materials result from the use of molecules which in themselves contain a porous structure, for example, a cage or a bowl. Such systems rely on the creation of covalent bonds in the formation of a porous solid material. Materials that are extrinsically porous only acquire this property on self-assembly into the solid state. Consequently, designing extrinsically porous materials is more challenging and unpredictable. One of the most widely studied extrinsically porous organic materials is the clathrate, Dianin’s compound (4-(p-hydroxyphenyl)-2,2,4-trimethylchroman);9 this adopts a porous structure with hourglass channels through the formation of hexagonal hydrogen bonded rings. Dianin’s compound has been shown to act as a host to a number of different solvent molecules,9−11 and it is also possible to adsorb various gas molecules into the channels.9,12 Guest-free Dianin’s compound can also be obtained through recrystallization from dodecane.13 The isopropanol clathrate has been observed to undergo phase transitions involving ordering of the solvent molecules on cooling.11 Other derivatives of Dianin’s compound are also known to have similar properties.9,14 β-Hydroxyquinone also forms a porous structure templated by a hexagonal hydrogen bonded ring; it adopts a trigonal structure, with hourglass channels.15 Various solvent16 and gas17 molecules have been incorporated into the channels. There are two other known polymorphs that pack more efficiently, and thus on removal of solvent from the channels, the structure reverts to its most stable nonporous form.18 We note that other phenolic compounds reported show a different route for containing solvent molecules and are not directly relevant to the title compound. For example, biphenol clathrate compounds in the literature do not have the same hexagonal hydrogen bonded channel, instead interacting with the solvent molecules directly through relatively strong hydrogen bonds.19 We report here a new example of an extrinsically porous organic material, 4phenoxyphenol, adding to the small number previously discovered and characterized.
EXPERIMENTAL SECTION
Crystals of 4-phenoxyphenol were grown by slow evaporation of the solvent of a supersaturated solution of 4-phenoxyphenol in each of the solvents at room temperature, the recrystallizations typically taking periods of a few days to a week or two. Solvents used included methanol, ethanol, acetonitrile, diethyl ether, and acetone. The crystals were block shaped and clear in color. 4-Phenoxyphenol crystallizes in the space group R3̅ with unit cell parameters at 100 K of a = 29.0220(12) Å, c = 5.8586(12) Å when crystallized from methanol at room temperature. X-ray data were collected at 100 K on a Bruker Nonius Kappa CCD diffractometer with the exception of the acetone solvate which was collected on a Bruker Apex II CCD diffractometer, again at 100 K. The structures were solved using SHELXS-97 and refined using SHELXS-9720 within the WinGX program suite.21 The program SQUEEZE22 was used to estimate the quantity of disordered solvent molecules contained within the pores. Neutron data were collected for the acetonitrile clathrate of 4phenoxyphenol at 100 K on the Very Intense Vertical Axis Laue diffractometer (VIVALDI)23 at the Institut Laue-Langevin reactor source in Grenoble, France, using a trigonal prismatic single crystal of dimensions 1.5 × 1.5 × 2.5 mm3. Nine Laue patterns at typically 20° intervals of rotation of the crystal about an axis perpendicular to the incident neutron beam were collected, each pattern for 30 min. Orientation matrices were determined using the program LAUEGEN.24 Unit cell parameters were assumed to be the same as those determined from X-ray data at 100 K as the Laue method at a continuous neutron source only allows relative linear cell dimensions to be determined. Reflections were integrated using ARGONNE_BOXES25 and normalized to a common wavelength using LAUE4.26 The final refinement was carried out using SHELXS-9720 within the WinGX program suite21 taking initial atomic coordinates from the refined X-ray structure at 100 K. All hydrogen positional and anisotropic thermal parameters have been fully refined. Correction for absorption was deemed unnecessary in view of the small crystal size.
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RESULTS AND DISCUSSION 4-Phenoxyphenol is a simple organic material that adopts a porous crystal structure, with channels running along the crystallographic c-axis, when crystallized from a wide variety of solvents. It shows no evidence of polymorphism in the extensive experiments carried out to date. Six molecules form a hydrogen bonded ring, R66(12), with a channel dimension of 1747
dx.doi.org/10.1021/cg200998u | Cryst. Growth Des. 2012, 12, 1746−1751
Crystal Growth & Design
Article
Table 1. Summary of the Effect of Solvent and Temperature on the Crystallographic Data and the Hydrogen Bonded Channela 1A methanol dried at (°C) a (Å) c (Å) V (Å3) potential solvent volume (Å3) electron density in pores solvent molecules per channel max channel diameter (Å) O−H···O (Å) a
1B
1D
2
3
4
neutron
methanol
recrystallized from melt
diethyl ether
acetone
acetonitrile
acetonitrile
29.0220(12) 5.8586(12) 4273.5(9) 184.8 (4.3% of unit cell) 54
75 29.0154(18) 5.8446(18) 4261.3(14) 181.6 (4.3% of unit cell) 51
90 29.0243(19) 5.8381(19) 4259.2(14) 186.1 (4.4% of unit cell) 15
29.0335(13) 5.8410(2) 4264.0(3) 180.3 (4.2% of unit cell) 18
28.9950(13) 5.9450(3) 4328.4(4) 213.2 (4.9% of unit cell) 69
28.7848(6) 5.9868(6) 4295.9(4) 200.8 (4.7% of unit cell) 66
28.785(5) 5.987(5) 4296(4)
1
1
0
0
