Xanthenol Clathrates: Structures and SolidâSolid Reactions

CRYSTAL GROWTH & DESIGN

Xanthenol Clathrates: Structures and Solid-Solid Reactions Elizabeth Curtis,† Luigi R. Nassimbeni,*,† Hong Su,† and Jana H. Taljaard‡ Department of Chemistry, UniVersity of Cape Town, Rondebosch 7701, Cape Town, South Africa, and Department of Chemistry, Nelson Mandela Metropolitan UniVersity, Port Elizabeth 6000, South Africa

2006 VOL. 6, NO. 12 2716-2719

ReceiVed June 22, 2006; ReVised Manuscript ReceiVed September 14, 2006

W This paper contains enhanced objects available on the Internet at http://pubs.acs.org/crystal. ABSTRACT: The structures of the host compound 9-(4-methoxyphenyl)-9H-xanthen-9-ol (H) and of its inclusion compounds with naphthalene, anthracene, phenanthrene, pyrene and β-naphthol have been elucidated. The inclusion compounds are isostructural with respect to the host positions, and all have stoichiometries of H‚1/2guest with the guest molecules located on a center of inversion. The inclusion compounds can be formed by solid-solid reactions, and the kinetics of the reactions with naphthalene and β-naphthol have half-lives of 44 and 31 min at room temperature, respectively. Introduction Host-guest chemistry depends on the phenomenon of molecular recognition, whereby supramolecular structures are constructed by noncovalent interactions and self-assembly. This field is receiving current attention, with particular reference to crystal engineering1 and the synthesis of solids with specific properties. The different kinds of intermolecular interactions that influence molecular recognition have been reviewed2 and of these, hydrogen bonding is the most important.3,4 The field of crystalline molecular compounds, including host-guest inclusion complexes, is discussed intensively in the recent major twovolume tome by Herbstein,5 which reviews their structures and thermodynamic properties. The host 9-(4-methoxyphenyl)-9H-xanthen-9-ol (H) is an organic compound that conforms to Weber’s rules for host design6 in that it is bulky, rigid, and has an hydroxyl moiety that may act as a hydrogen-bond donor, as well as a pyranyl oxygen, which is a potential hydrogen-bond acceptor. In this work, we present the structures of the apohost H and of its inclusion compounds formed with a series of polycyclic unsaturated hydrocarbons, namely, naphthalene (NAP), anthracene (ANT), phenanthrene (PHE), pyrene (PYR), as well as β-naphthol (NOL). The compounds were also formed by direct grinding of the host and guest components, and the kinetics of the solid-solid reactions were analyzed. The atomic numbering for the compounds is shown in Scheme 1. W 3D rotatable images of W NAP, W ANT, and W PHE in XYZ format are available. Experimental Procedures Suitable crystals were obtained by dissolving stoichiometric quantities (2 mol:1 mol) of host and guest in minimal quantities of methanol, and the solution was allowed to evaporate slowly. Crystals of the apohost H crystallized from the solution with R-naphthol. Details of the crystal data, intensity data collection, and refinement parameters are given in Table 1. Cell dimensions were established from the intensity data measured on a Kappa CCD diffractometer using Mo KR radiation. The strategy for data collection was evaluated using COLLECT7 software, and for all the structures the intensity data were collected by the standard phi scan and omega scan technique and were scaled and †

University of Cape Town. Nelson Mandela Metropolitan University. * To whom correspondence should be addressed. E-mail: xrayluigi@ science.uct.ac.za. ‡

Scheme 1.

Host and Guest Compounds with Numbering Scheme

reduced using the program DENZO-SMN.8 The structures were solved by direct methods using SHELX-979 and refined by full-matrix leastsquares on F2. The program X-Seed10 was used as a graphical interface. For all the structures, the positions of the non-hydrogen atoms of host were refined anisotropically. The oxygen O1G on guest β-naphthol (NOL) and the non-hydrogen atoms of guest phenanthrene (PHE) were refined isotropically, due to relatively high displacement parameters. Other guest non-hydrogen atoms were all refined anisotropically. All the aromatic hydrogen atoms were refined with geometric constraints of d(C-H) ) 0.95 Å and Uiso equal to 1.2 times the Ueq value of the parent carbon atom, while the methyl hydrogen atoms of host were refined with d(C-H) ) 0.98 Å and Uiso equal to 1.2 times the Ueq of the parent carbon atom. The hydroxyl hydrogen atoms of the host were located in the difference electron density maps and were refined with simple bond length restraints on d(O-H) ranging from 0.960 to 0.965 Å, as a function of O-H distances versus hydrogen-bonding O‚‚‚O distances.11 The hydroxyl hydrogen of NOL could not be located in difference electron density maps and was excluded from the final model.

Results and Discussion The host compound H crystallizes in the space group P21/c with Z ) 8, and there are two molecules in the asymmetric unit labeled with suffixes A and B and located in general

10.1021/cg060384q CCC: $33.50 © 2006 American Chemical Society Published on Web 10/28/2006

Xanthenol Clathrates

Crystal Growth & Design, Vol. 6, No. 12, 2006 2717 Table 1. Crystal Data and Refinement Parameters

structures

H

molecular formula

C20H16O3

Mr/g mol-1 data collect T/K crystal system space group a/Å b/Å c/Å R/° β/° γ/° V/Å3 Z Dc/g cm-3 µ(MoKR)/mm-1 F(000) range scanned θ/ ° range of indices no. reflns collected no. unique reflns no. restraints no. parameters extinction coef goodness of fit, S final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole Å-3

304.33 113(2) monoclinic P21/c 21.3666(6) 9.2617(2) 15.4760(3) 90 92.479(1) 90 3059.7(1) 8 1.321 0.088 1280 2.40-26.76 (27; -10,11; (19 11282 6388 2 425 0.0041(6) 0.962 R1 ) 0.0435 wR2 ) 0.0885 R1 ) 0.0945 wR2 ) 0.1039 -0.207/0.212

H‚1/2NAP

H‚1/2ANT

H‚1/2PHE

H‚1/2PYR

H‚1/2NOL

C20H16O3‚ 1/ (C H ) 2 10 8 368.41 113(2) triclinic P1h 8.3618(2) 9.0628(2) 12.8581(4) 95.611(1) 93.604(1) 109.750(1) 907.90(4) 2 1.348 0.088 388 2.60-27.13 (10; (11; (16 7682 3916 1 256 0.015(5) 1.051 R1 ) 0.0395 wR2 ) 0.0975 R1 ) 0.0561 wR2 ) 0.1062 -0.231/0.310

C20H16O3‚ 1/ (C H ) 2 14 10 393.44 113(2) triclinic P1h 8.3447(2) 9.2247(3) 13.3533(4) 91.857(2) 92.303(2) 108.133(1) 974.97(5) 2 1.340 0.086 414 2.33-27.54 0,10; (11; (17 9324 4448 1 277 0.018(3) 1.023 R1 ) 0.0451 wR2 ) 0.1018 R1 ) 0.0821 wR2 ) 0.1154 -0.204/0.202



C20H16O3‚ 1/ (C H O) 2 10 8 375.91 113(2) triclinic P1h 8.3914(2) 9.1177(2) 12.8162(3) 96.162(1) 93.767(1) 109.314(1) 914.67(4) 2 1.365 0.090 395 2.86-26.78 (10; (11; (16 10243 6951 1 263 0.011(6) 1.051 R1 ) 0.0570 wR2 ) 0.1541 R1 ) 0.0796 wR2 ) 0.1692 -0.417/1.041

positions. The packing is shown in Figure 1, which views the structure along [010]. Molecules A form centrosymmetric dimers, with the hydroxyl moiety of one molecule hydrogen bonded to the pyranyl oxygen (O2) of a neighboring molecule. Molecules B hydrogen bond through the hydroxyl moiety of one molecule to the methoxy oxygen O3 of a second molecule, forming chains running in the [001] direction. The metrics of the hydrogen bonding for this, and the other structures are reported in Table 2. The inclusion compounds all crystallize in the space group P1h with two host and one guest molecules in the unit cell. The host molecules again form centrosymmetric dimers via O1-H1‚‚‚O2 hydrogen bonds, and the guests are located on a center of inversion at Wyckoff position h. The packing of the structure H‚1/2PYR is shown in Figure 2 as a typical example.

Figure 1. Projection of the host structure H viewed along [010]. All hydrogen atoms are omitted except the hydroxyl hydrogen. Hydrogen bonds are drawn in red dotted lines. Symmetry related molecules are in the same color (black for molecules B and gray for molecules A). W A 3D rotatable image in XYZ format is available.

Table 2. Hydrogen-Bonding Detailsa

H H‚1/2NAP H‚1/2ANT H‚1/2PHE H‚1/2PYR H‚1/2NOL

O1A-H1AO2A#1 O1B-H1BO3B#2 O1-H1O2#3 O1-H1O2#3 O1-H1O2#3 O1-H1O2#3 O1-H1O2#3

O-H

HO

OO

< O-HO

0.97(1) 0.945(9) 0.960(1) 0.960(1) 0.960(1) 0.964(1) 0.960(1)

1.94(1) 1.87(1) 1.904(2) 1.896(2) 1.903(5) 1.936(5) 1.903(4)

2.906(2) 2.809(2) 2.862(1) 2.856(1) 2.859(2) 2.898(3) 2.859(2)

177(2) 174(2) 175(2) 179(2) 173(3) 175(4) 174(3)

a Symmetry code: #1 1 - x, 1 - y, -z; #2 x, -y + 1/ , z + 1/ ; #3 2 2 1 - x, -y, -z.

In the case of the H‚1/2PHE structure the phenanthrene molecule is disordered about two equivalent positions, while for the H‚1/2NOL structure, the center of the naphthyl moiety is located on a center of inversion and the hydroxyl group is disordered over two positions. There is possible additional hydrogen bonding between the hydroxyl groups of the β-naphthol guest molecules and also between the hydroxyl moiety of the β-naphthol and the methoxy oxygen of the host. This is illustrated in Figure 3. The conformation of the host molecules is somewhat different in the apohost structure versus that of the inclusion compounds. In the apohost, the tricyclic ring system is gently curved, and the dihedral angles Λ between the two side phenyl rings C1-C2-C3-C4-C5-C6 and C8-C9-C10-C11-C12-C13 are 2.47(8)° and 8.54(4)°. The torsion angles τ1 (O1-C7C14-C19) and τ2 (C20-O3-C17-C18) in the apohost are -1.7(2)° and 4.7(3)° for the molecule A and -4.9(2)° and 12.7(2)° for the molecule B. All these values are close to zero, so that the methoxy group takes on a cis-configuration with respect to the hydroxyl moiety. In the inclusion compounds, however, the dihedral angles Λ are larger and range from 12.48(7)° to 16.86(5)°, and the torsion angles τ1 and τ2 are such that the methoxy group has a trans-configuration with respect to the hydroxyl moiety. The dihedral and torsion angles are listed in Table 3. In all structures, the host central ring adopts a shallow

2718 Crystal Growth & Design, Vol. 6, No. 12, 2006

Curtis, E.

Figure 2. Projection of structure H‚1/2PYR viewed along [100]. All hydrogen atoms are omitted except the host hydroxyl hydrogen. Hydrogen bonds are shown as red dotted lines. The host is in black, and the guest pyrene is in blue. W A 3D rotatable image in XYZ format is available.

Figure 3. Projection of structure H‚1/2NOL viewed along [100]. All hydrogen atoms are omitted except the host hydroxyl hydrogen. Hydrogen bonds are shown as red dotted lines. The host is in black, and the guest β-naphthol is in blue with the hydroxyl group disordered over two positions. W A 3D rotatable image in XYZ format is available.

boat conformation with the pyranyl oxygen O2 and tetrahedral carbon C7 as bow and stern. We have calculated the mirror plane asymmetry parameters12 ∆Cs01 bisecting the central ring through O1-C7, and these are also listed in Table 3. Their ∆Cs01 are all below 2.7°, attesting to the fact that the central ring is highly symmetrical. The inclusion compounds are isostructural with respect to the host positions, the only variation being the size of the guest

Figure 4. Plot showing the linear relationship between increasing unit cell volume and number of non-hydrogen atoms of the guest.

molecules, which are all located on a center of inversion. A comparison of the unit cell volumes as a function of the number of non-hydrogen atoms of the guest molecules is shown in Figure 4. We have added the parameters obtained from the structures of the inclusion compounds formed between the host and benzene (BEN), toluene (TOL), and the xylene isomers (XYL).13 We note that the two series of compounds fall along two distinct straight lines with the benzene guest, BEN, as a common point. The slope, measured in Å3 per guest nonhydrogen atom, is 24.6 for the substituted benzenes and 13.2 for the fused ring guests. This corresponds to the volume measurement given by Kitaigorodsky14 of 23.5 Å3 for a methyl group and 13.2 Å3 for an sp2 carbon. Solid-Solid Reactions. Solid-solid reactions, or reactions in the absence of a solvent, are interesting because they are environmentally attractive. Early work was carried out by Rastogi,15,16 who studied the kinetics of solid-solid reactions between selected hydrocarbons and picric acid. Toda17,18 has championed the use of such reactions for the efficient synthesis of novel organic compounds, and Tanaka19 has compiled an exhaustive list of such syntheses detailing their experimental conditions. We have monitored the reaction of 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol with benzophenone by measuring the changes in the infrared spectra of the hydrogen-bonding that occurs with complex formation,20 and this host has also been employed to carry out selective enclathration of solid aminobenzonitrile isomers.21 Bourne et al.22 used the technique of solid-solid grinding in the preparation of zinc coordination polymers with pyrazine, yielding compounds of differing stoichiometries. We carried out the solid-solid reaction between the host H and all the guests by grinding stoichiometric quantities of the solids in a brass mortar. All the reactions, except that with anthracene, yielded the desired product.

Table 3. Conformation Parameters of the Host Molecule

H-molecule A B H‚1/2NAP H‚1/2ANT H‚1/2PHE H‚1/2PYR H‚1/2NOL

dihedral angle Λ/°

∆Cs01/°

τ1 ) O1-C7-C14-C19/°

τ2 ) C20-O3-C17-C18/°

8.54(4) 2.47(8) 13.25(4) 16.86(5) 15.60(5) 14.9(1) 12.48(7)

2.3 1.3 2.5 2.7 1.9 1.3 2.5

-1.7(2) -4.9(2) 1.0(1) 2.9(2) 6.4(3) 11.5(3) 1.7(2)

4.7(3) 12.7(2) 179.9(1) 171.3(1) 175.4(2) 179.4(3) -179.6(2)

Xanthenol Clathrates

Crystal Growth & Design, Vol. 6, No. 12, 2006 2719

Conclusion The packing of the host molecule 9-(4-methoxyphenyl)-9Hxanthen-9-ol, which forms head-to-tail dimers linked by (host)O-H‚‚‚O(host) hydrogen bonds, remains remarkably consistent when it forms a series of inclusion compounds with various fused ring hydrocarbon guests. The inclusion compounds are readily formed by grinding the solid host and solid guest at room temperature.

Figure 5. PXRD traces: A, pure β-naphthol; B, pure host H; C, powder from grinding experiment of H and β-naphthol; D, calculated PXRD pattern from the coordinates of H‚1/2NOL structure. Arrows show the common peaks.

Figure 6. Plot of ln(peak intensity) versus time for the solid-solid reaction between host H and naphthalene (NAP), which was monitored by PXRD.

The success of the reaction was monitored by powder X-ray diffraction (PXRD), and a typical result is shown in Figure 5. This displays the diffraction pattern of β-naphthol (A), the host (B), the product powder (C) and the computed pattern derived form the coordinates of the H‚1/2NOL structure (D). Grinding was carried out over a period of 2 h at 298 K. There is a good match between the peaks of the diffraction patterns C and D. We monitored the kinetics of the solid-solid reaction between the host and naphthalene by measuring the decrease of the unique naphthalene peak at 2θ ) 19.75° with time. We interrupted the grinding experiment at selected times and added fixed quantities of diamond powder to the samples. This allowed us to measure the intensities of the decreasing naphthalene peak at 2θ ) 19.75° using the (111) peak of diamond as a calibrant. A plot of ln(peak intensities) versus time yielded a straight line, with a rate constant of 1.57 × 10-2 min-1 and a half-life of 44 min. This is shown in Figure 6. Similar results were obtained for the reaction of H with β-naphthol, yielding a rate constant of 2.27 × 10-2 min-1 and a half-life of 31 min. The mechanism of such solid-solid reactions is not well established, and we note that some of these reactions undergo a liquid melt phase during grinding.22 However, no liquid melt was observed during the course of our experiments.

Supporting Information Available: Crystallographic information files (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

References (1) Tiekink, E. R. T.; Vittal, J. J. Frontiers in Crystal Engineering; Wiley: Chichester, 2006. (2) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry, Wiley: Chichester, 2000; Chapter 1. (3) Jeffrey, G. A. An Introduction to Hydrogen Bonding, Oxford University Press: Oxford, 1997. (4) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond, Oxford University Press: Oxford, 1999. (5) Herbstein, F. H. Crystalline Molecular Complexes and Compounds, Oxford University Press: Oxford, 2005; Vols. 1 and 2. (6) Weber, E. Inclusion Compounds; Atwood, J. L.; Daves, J. E. D.; MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1991; Chapter 5. (7) COLLECT, data collection software; Nonius: Delft, The Netherlands, 1998. (8) Otwinowski, Z.; Minor W. in Methods in Enzymology, Macromolecular Crystallography; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: 1997; Part A, Vol. 276, pp 307-326. (9) Sheldrick, G. M. SHELX-97: Program for Crystal Structure Refinement; University of Go¨ttingen, Germany, 1997. (10) Barbour, L. J. X-Seed: A Software Tool for Supramolecular Crystallography, J. Supramol. Chem. 2001, 1, 189. (11) Olovsson, I.; Jo¨nsson, P. The Hydrogen Bond - Structures and Spectroscopy; Schuster, P.; Zundel, G.; Sarderfy, C., Eds.; NorthHolland Publishing Company: The Netherlands. (12) Atlas of Steroid Structure; Duax, W. L., Norton, D. A., Eds.; IFI/ Plenum: New York, 1975; Vol 1, p 18. (13) Jacobs, A.; Nassimbeni, L. R.; Su, H.; Taljaard, B. Org. Biomol. Chem. 2005, 3, 1319-1322. (14) Kitaigorodsky, A. I. Molecular Crystal and Molecules; Academic Press: New York, 1973, p 20. (15) Rastogi, R. P.; Bassi, P. S.; Chadha, S. L. J. Phys. Chem. 1962, 62, 2707-2708. (16) Rastogi, R. P.; Bassi, P. S.; Chadha, S. L. J. Phys. Chem. 1963, 67, 2569-2573. (17) Tanaka, K.; Toda, F. Chem. ReV. 2000, 1025-1074. (18) Toda, F. Cryst. Eng. Commun. 2002, 4, 215-222. (19) Tanaka, K. SolVent-free Organic Synthesis; Wiley: Weinheim, 2003. (20) Bond, D. R.; Johnson, L.; Nassimbeni, L. R.; Toda, F. J. Solid State Chem., 1991, 92, 68-79. (21) Caira, M. R.; Nassimbeni, L. R.; Toda, F. Vujovic, D. J. Am. Chem. Soc. 2000, 122, 9367-9372. (22) Bourne, S. A.; Kilkenny, M.; Nassimbeni, L. R. J. Chem. Soc. Dalton Trans. 2001, 1176-1179. (23) Rothenberg, G.; Downie, A. P.; Raston, C. L.; Scott, J. L. J. Am. Chem. Soc., 2001, 123, 8701-8708.

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Xanthenol Clathrates: Structures and SolidâSolid Reactions

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