Inclusion of Aromatic Guests by a Xanthenol Host: Structures, Guest

Inclusion of Aromatic Guests by a Xanthenol Host: Structures, Guest Exchange, and Desorption Kinetics Ramon,†

Gaelle Anthony W. Benjamin Taljaard§

Coleman,‡

Luigi R.

Nassimbeni,*,†

and

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 6 2331-2335

Department of Chemistry, University of Cape Town, Rondebosch 7701, South Africa, IBCP, CNRS-UMR 5086, 7 passage du Vercors, 69367 Lyon, France, and Department of Chemistry, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth 6000, South Africa Received November 12, 2004;

Revised Manuscript Received March 14, 2005

ABSTRACT: The host 9-(3-chlorophenyl)-9H-xanthen-9-ol forms inclusion compounds with benzene, toluene, p-xylene, and pyridine. Their structures have been elucidated and correlated with their kinetics of desolvation and guest exchange. Introduction The field of inclusion phenomena is at the forefront of chemical research, with the bulk of the work being directed at the synthesis of novel host compounds with specific properties and the elucidation of their structures. Several books and monographs are available, including the authoritative 11 volume publication of Comprehensive Supramolecular Chemistry.1 The secondary interactions that occur in host-guest systems have been reviewed,2-4 while the chemistry and inclusion properties of various classes of host compounds have also been well-documented.5-7 The inclusion of a given guest by a host compound is a process of molecular recognition, this being the sum of the intermolecular forces between the various molecular entities, which occur under defined conditions of temperature, pressure, and concentration of the components. These in turn depend on the shape, size, and surface charge distribution of the molecules, which pack in a given manner to yield a crystal. Crystal structure analysis therefore allows detailed studies of these interactions and computation of the lattice energies of particular crystalline host-guest systems. These lattice energies in turn allow us to predict the thermal stabilities of related structures, while their topologies may help us understand their kinetics of desolvation and enclathration. We have studied a number of host-guest systems and have related their reactivity to their crystal structures, correlating selectivity, enthalpies of guest release, and activation energies of desorption to their crystal packing.8-11 We now present the results of the inclusion chemistry of the host H ) 9-(3-chlorophenyl)-9H-xanthen-9-ol with a number of small aromatic guests and discuss their structures, thermal stability, guest exchange, and kinetics of desorption. The atomic numbering scheme of the inclusion compounds is given in Scheme 1. The compound was prepared as described by Bordwell and co-workers, using 3-chlorophenyl-magnesium bromide, * To whom correspondence should be addressed. Prof. Luigi R. Nassimbeni, Department of Chemistry, University of Cape Town, Private Bag, Rondebosch, 7701, South Africa. E-mail: xrayluig@ science.uct.ac.za. Tel +27 21 6502569. Fax +27 21 6854580. † University of Cape Town. ‡ IBCP. § University of Port Elizabeth.

Scheme 1. Atomic Numbering Scheme of the Inclusion Compounds and Abbreviations for Guest Compounds

instead of the chloro Grignard reagent. Crystallization from benzene/petroleum ether gave H in 83% yield and had the expected spectral properties as reported.12 Experimental Section Crystal Structures. Suitable crystals of the apohost H were obtained by recrystallization from tetrahydrofuran at 4 °C. Crystals of the inclusion compounds were obtained by crystallizing saturated solutions of the host H in the respective guests: H‚0.5BZ and H‚0.5TOL at 4 °C, H‚0.5p-XYL and H‚ PYR at 25 °C. In all cases, the host-guest ratios were confirmed by thermal gravimetry (TG), and details of the crystal data, intensity data collections, and refinements are contained in Table 1. Cell dimensions were established from the intensity data measurements on a Nonius Kappa CCD diffractometer using graphite-monochromated Mo-KR radiation. The strategy for the data collections was evaluated using COLLECT software.13 For all structures, data were collected by the standard phi- and omega-scan techniques and were scaled and reduced using DENZO-SMN software.14 The structures were solved by direct methods using SHELX-8615 and refined by least squares with SHELX-9716 refining on F2. The program X-Seed17,18 was used as a graphical interface for the structure solution and refinement using SHELX as well as to produce the packing diagrams. In each of the structures, the positions of all non-hydrogen host atoms were obtained by direct methods, and the nonhydrogen guest atoms were located in difference electron density maps. All non-hydrogen atoms were refined with anisotropic temperature factors. Hydrogen atoms were placed

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Table 1. Crystal Data and Refinement Parametersa H molecular formula host/guest ratio molecular mass temp, K λ, Å crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z Dc, Mg m-3 µmm-1 F(000) approx crystal size, mm θmax for collected data, deg index ranges min/max h,k,l reflns collected independent reflns Rint data/parameters refined R(F) [I > 2σ(I)] no. of reflections with I > 2σ(I) wR (all F2) largest diff peak and hole, e/Å-3

H‚0.5BZ

H‚0.5TOL

H‚0.5p-XYL

H‚PYR‚0.5 H2O

308.74 173 0.71073 monoclinic C2/c 22.584(5) 11.033(2) 14.723(3) 90 129.19(3) 90 2843.3(16) 8 1.442 0.273 1280 0.30 × 0.40 × 0.40

C19H13ClO2‚ 0.5(C6H6) 1:0.5 347.80 173 0.71073 triclinic P1 h 8.6543(17) 9.2758(19) 11.457(2) 104.01(3) 97.00(3) 106.35(3) 838.3(4) 2 1.378 0.240 362 0.20 × 0.30 × 0.40



C19H13ClO2‚ C5H5N‚1/2H2O 1:1:0.5 396.87 173 0.71073 triclinic P1 h 8.6910(17) 9.0124(18) 14.231(3) 108.16(3) 93.10(3) 111.89(3) 964.3(5) 2 1.363 0.221 414 0.30 × 0.40 × 0.50

27.1

27.1

27.5

27.1

27.5

-28/28; -14/0; -18/18 5946 3106 0.014 3106/199

0/11; -11/11; -14/14 3610 3610

0/11; -11/11; -15/14 3835 3835

3610/226

-11/11; -11/11; -15/15 6887 3864 0.025 3864/235

3835/236

-11/11; -11:/11; -18/18 7842 4305 0.012 4305/247

0.0351 2889

0.0389 3299

0.0470 2837

0.0362 3230

0.0555 3879

0.0984 -0.32 and 0.56

0.1543 -0.41 and 0.39

0.1513 -0.41 and 0.59

0.1334 -0.35 and 0.32

0.2056 -1.06 and 0.97

C19H13ClO2

a Pairs of host molecules are stabilized by weak O-H‚‚‚Cl interactions (d O‚‚‚Cl ) 3.40 Å) about the center of inversion at Wyckoff position d.

Figure 1. Packing of the apohost viewed along [010]. with geometric constraints and refined with isotropic temperature factors. All structures were determined at low temperature (173 K). Thermal Analysis. Differential scanning calorimetry (DSC) was performed on a Perkin-Elmer PC7 series system and TG was performed on a Mettler Toledo TGA/SDTA 851e system. Finely powdered, air-dried specimens (2-5 mg) were placed in crimped, vented aluminum DSC pans or open aluminum TG sample pans. Dinitrogen was used as purging gas at a flow rate of 30 mL/min. All experiments were carried out over temperature ranges of 30-300 °C for DSC and 30-350 °C for the TG experiments at a constant heating rate of 20 °C/min. Guest Exchange. Guest exchange was performed at 4 °C on crystals of H‚0.5p-XYL exposed to benzene vapor allowing the formation of H‚0.5BZ. A similar procedure with H‚0.5p-

XYL crystals with toluene vapor gave H‚0.5TOL. The progress of these reactions was monitored by DSC. Kinetics of Desolvation. The kinetics of desolvation of H‚ 0.5p-XYL were studied by carrying out a series of isothermal TG runs, performed over the temperature range 60-100 °C taking readings at intervals of 10 °C. The data were converted to extent of reaction (R) versus time curves, which were fitted by trial and error to various kinetic laws.19

Results and Discussion Structures. Crystal data and refinement parameters are given in Table 1. The packing of the host molecules is shown in Figure 1. Pairs of host molecules are

Inclusion of Aromatic Guests by a Xanthenol Host

Figure 2. Packing of H‚0.5BZ viewed along [100].

stabilized by weak O-H‚‚‚Cl interactions (d O‚‚‚Cl ) 3.40 Å) about the center of inversion at Wyckoff position d. The structures of H‚0.5BZ, H‚0.5TOL, and H‚0.5pXYL are similar. The packing is shown in Figure 2, in which a pair of host molecules are hydrogen-bonded via (host) O-H‚‚‚O (host) bonds with d O‚‚‚O ) 2.899(4) Å, and the guest lies on a center of inversion at Wyckoff position c. In the case of H‚0.5TOL, the toluene is disordered about the center of inversion. In both H‚0.5TOL and H‚ 0.5p-XYL structures, the guests lie in the channels running parallel to [010], as shown in Figure 3. In contrast, the channels of the H‚0.5BZ structures are highly constricted, and the benzene guests effectively lie in cavities, making this a true clathrate structure. The H‚PYR‚0.5H2O exhibits a different packing mode, in that the host is stabilized by a (host) O-H‚‚‚N (Pyr) hydrogen bond (d O‚‚‚N ) 2.765(5) Å) and also to the guest water molecule via a (host) O‚‚‚O (water) hydrogen bond (d O‚‚‚O ) 2.829(5) Å). The water molecule is disordered over two sites located about the center of inversion at Wyckoff position b. This is shown in Figure 4a. The packing of the H‚PYR‚0.5H2O structure is characterized by a double ribbon of host molecules inter-

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leaved by layers of pyridine and water guests lying in the ab plane, as shown in Figure 4b. Kinetics of Desolvation. The results of the kinetics of desolvation of H‚0.5p-XYL are shown in Figure 5a, which displays the extent of reaction, R, versus time for a series of isothermal runs carried out at temperatures ranging from 333 to 373 K. These fitted the Avrami Erofe’ev deceleratory law A2: -ln(1 - R)1/2 ) kobst over virtually the complete range of reaction. The resulting Arrhenius plot shown in Figure 5b, yielded an activation energy Ea ) 87 kJ mol-1. Similar results have been obtained for the thermal desolvation of the inclusion compound formed between cyclotriveratrylene and 1,1,2,-trichloropropane, which exhibits deceleratory desorption kinetics with an activation energy of 84 kJ mol-1.20 The thermal decompositions of a series of binaphthol inclusion compounds with the volatile guests dimethylsulfoxide, 1,4-dioxane, and acetone in various host/guest ratios have also been investigated, and their activation energies were found to vary between 61 and 94 kJ mol-1.21 Guest Exchange. The progress of the exchange reactions (i) and (ii) were monitored by DSC experiments by exposure of small crystals of the starting compounds to the vapor of the incoming guest at selected times. These DSC results are shown in Figure 6a,b.

(i) H‚0.5p-XYL (s) + benzene (vapor) f H‚0.5BZ (s) (ii) H‚0.5p-XYL (s) + toluene (vapor) f H‚0.5TOL (s) Both sets of curves start with exhibiting an endotherm due to the loss of p-xylene, followed by the endotherm due to the melt of the host. The p-xylene endotherm disappears and is followed by the appearance of the endotherm associated with the incoming guest. Thus, for the first reaction (i), an endotherm is obtained attributed to the incorporated benzene (Tpeak ) 66 °C) after 6 h, while for the reaction (ii) the corresponding toluene endotherm is broader, showing the appearance of two peaks (Tpeak ) 66 °C, 84 °C) after 192 h. We surmise that the rate of the reaction is dependent on the vapor pressure of the incoming guest: at the

Figure 3. Packing of H‚0.5p-XYL structure viewed along [010] showing the host molecules with their van der Waals radii and the guest molecules as sticks.

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Figure 4. (a) Hydrogen bonding motif of the H‚PYR‚0.5H2O structure. (b) Packing of H‚PYR‚0.5H2O viewed along [010].

Figure 5. (a) Curves of the extent of the reaction, R, versus time. (b) Semilogarithmic plot of ln k versus reciprocal temperature.

temperature of the exchange reaction, 4 °C, the vapor pressure of benzene is 24 Torr, while that of toluene is 4 Torr. It was also observed that the DSC results for both reactions feature a DSC curve halfway through the reaction, which shows only the melt endotherm of the host, with no evidence of an endotherm for either the p-xylene or the incoming guests, suggesting that the exchange reaction goes via a recrystallization step. DSC and IR spectroscopic studies gave analogous results for the exchange reaction of the guests tetrahydrofuran and thiophene in their inclusion compounds with the host 1,1,6,6-tetraphenylhexa-2,4-diyne-1,6-diol.21 Conclusion The structures of H‚0.5BZ, H‚0.5TOL, and H‚0.5pXYL are similar and are isostructural with respect to

Figure 6. (a) DSC results of the xylene benzene exchange reaction. (b) DSC results of the xylene toluene exchange reaction.

the host atom positions. In the H‚0.5TOL and H‚0.5pXYL structures the guest molecules reside in channels, and this topology explains the observed facile guest exchange and the relatively low activation energy of desorption of the p-xylene from the channels. Supporting Information Available: Crystallographic information files (CIF) are available free of charge via the Internet at http://pubs.acs.org.

References (1) Atwood, J. L.; Davies, J. E. D.; Mac Nicol, D. D.; Vo¨gtle, F., Eds.; Comprehensive Supramolecular Chemistry; Pergamon: Oxford, 1996; Vols. 1-11. (2) Steed, J. W.; Atwood, J. L. In Supramolecular Chemistry; Wiley: Chichester, 2000; Ch 1. (3) Vo¨gtle, F. In Supramolecular Chemistry; Wiley: Chichester, 1991: Ch 1.

Inclusion of Aromatic Guests by a Xanthenol Host (4) Desiraju, G. R.; Sharma, C. V. K. In The Crystal as a Supramolecular Entity; Desiraju, G. R., Ed.; Wiley: Chichester, 1996; Chapter 2. (5) Gutsche, C. D. Calixarenes; The Royal Society of Chemistry: Cambridge, 1989. (6) Gokel, G. W. Crown Ethers and Cryptands; The Royal Society Chemistry; Cambridge, 1991. (7) Diederich, F. Cyclophanes; The Royal Society Chemistry: Cambridge, 1991. (8) Caira, M. R.; Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Am. Chem. Soc. 2000, 122, 9367-9372. (9) Nassimbeni, L. R.; Kilkenny, M. L. J. Chem. Soc., Dalton Trans. 2001, 1172-1175. (10) Barbour, L. J.; Caira, M. R.; Le Roex, T.; Nassimbeni, L. R. J. Chem. Soc., Perkin Trans. 2 2002, 1973-1979. (11) Caira, M. R.; Chang, Y. P.; Nassimbeni, L. R.; Su, H. Org. Biomol. Chem. 2004, 2, 655-659. (12) Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.; Cornforth, F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCollum, G. J.; Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006-7014. (13) COLLECT, Data Collection Software; Nonius: Delft, The Netherlands, 1998.

Crystal Growth & Design, Vol. 5, No. 6, 2005 2335 (14) Otwinowski, Z.; Minor, W. In Methods in Enzymology: Macromolecular Crystallography; Carter, C. W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Part A, Vol. 276, pp 307-326. (15) Sheldrick, G. M. In SHELX-86: Crystallographic Computing; Sheldrick, G. M., Kruger, C., Goddard, R., Eds.; Oxford University Press: Oxford, UK, 1985; Vol. 3, p 175. (16) Sheldrick, G. M. In SHELX-97: Program for Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997. (17) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189. (18) Atwood, J. L.; Barbour, L. J. Cryst. Growth Des. 2003, 3, 3. (19) Brown, M. E. Introduction to Thermal Analysis; Chapman and Hall: London, 1988. (20) Caira, M. R.; Jacobs, A.; Nassimbeni, L. R. Supramol. Chem. 2004, 16, 337-342. (21) Nassimbeni, L. R.; Su, H. New J. Chem. 2002, 26, 989995. (22) Caira, M. R.; Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Chem. Soc., Perkin Trans. 2 2001, 2119-2124.

CG049615O

Inclusion of Aromatic Guests by a Xanthenol Host: Structures, Guest

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