Inclusion by a Xanthenol Host: Relating Structure to the Kinetics of

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 6 1003-1006

PerspectiVe Inclusion by a Xanthenol Host: Relating Structure to the Kinetics of Desolvation and Guest Exchange Ayesha Jacobs,*,† Nobathembu Faleni,† Luigi R. Nassimbeni,† and Jana H. Taljaard‡ Department of Chemistry, Faculty of Applied Sciences, Cape Peninsula UniVersity of Technology, P.O. Box 652, Cape Town 8000, South Africa, and Department of Chemistry, Nelson Mandela Metropolitan UniVersity, Port Elizabeth 6000, South Africa ReceiVed NoVember 9, 2006; ReVised Manuscript ReceiVed February 28, 2007

ABSTRACT: The inclusion compounds of the host 9-(4-methoxyphenyl)-9H-xanthen-9-ol, C23H16O2, with the guests cyclohexane (CHEX), 1,4-dioxane (DIOX), and N,N-dimethylformamide (DMF) have been investigated. Their crystal structures have been determined and correlated with their thermal behavior. Both dioxane and cyclohexane guests gave structures that were isostructural with respect to the host and displayed (Host)sOH‚‚‚Os(Host) hydrogen bonding. The guests were found on centers of inversion. For the DMF structure, we observed a different packing arrangement with both the host and the guest molecules in general positions, and for the first time, this host displayed (Host)-OH‚‚‚(Guest) hydrogen bonding. Kinetics of desolvation for the 1,4-dioxane and N,N-dimethylformamide compounds were studied. Guest-exchange reactions were also performed. Introduction The study of guest exchange in porous crystals is important because of its possible applications, which include the storage of small volatile molecules, component separation of mixtures, catalysis, and the optical properties of materials. The traditional compounds that exhibit porosity and are thermodynamically stable under a wide range of temperature and pressure are the zeolites, which enjoy wide applications such as petrochemical cracking and ion exchange. Recently, however, there has been increasing interest in new materials that mimic the properties of zeolites, the so-called “organic zeolites”,1 which fall into two classes: (a) metal-organic frameworks (MOFs) and (b) porous materials composed of purely organic moieties. The concept of porosity in crystals is not well-defined and Barbour, in a publication entitled “Crystal porosity and the burden of proof”,2 has pointed out that the term is subjective and sometimes misused. Porous MOFs are attractive host compounds in that they offer the flexibility of various coordination geometries inherent in metals and often retain the structure of the framework upon guest removal. Thus, the structure of magnesium formate forms an extended diamondoid lattice, which allows the exchange of various volatile guests such as dimethylformamide, acetone, benzene, ethanol, and methanol while retaining es* To whom correspondence should be [email protected]. † Cape Peninsula University of Technology. ‡ Nelson Mandela Metropolitan University.

addressed.

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sentially unchanged unit-cell parameters.3 Similar results have been obtained with the framework hosts Co(4,4′-bipy)1.5(NO3)24 and Ni2(4,4′-bipy)3(NO3)4,5 in which the frameworks show flexibility upon guest desorption or guest exchange. Many MOFs have been employed in gas absorption, particularly of hydrogen, the storage of which presents considerable technological challenges.6 The mechanism of guest exchange in inclusion compounds formed by purely organic hosts, however, is generally more complex. Two mechanisms are possible

(a) H‚nG1(s,β) + mG2(liquid or vapor) f H‚mG2(s,β) + nG1(liquid or vapor) in which the host-guest system retains its structure throughout the exchange, and so behaves like a zeolite, or

(b) H‚nG1(s,β) f H(s,R) + nG1(liquid or vapor) H(s,R) + mG2 f H‚mG2(s,β) Here, the host-guest system desorbs the original guest G1 to yield the apohost in its R, nonporous phase, which in turn forms a new inclusion compound with the incoming guest G2. Mechanism (a) is rare with organic hosts and is exemplified by the host gossypol, which forms a clathrate with dichloromethane as guest, retains its structure on desorption, and absorbs other small volatile guests.7 We have shown that mechanism (b), essentially a recrystallization, is obtained in several guest-

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1004 Crystal Growth & Design, Vol. 7, No. 6, 2007

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Table 1. Crystal Data, Experimental, and Refinement Parameters molecular formula guest host:guest ratio Mr (g mol-1) crystal symmetry space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) Z V (Å3) µ(Mo-KR) (mm-1) T (K) range scanned, θ (deg} index range no. of reflns collected no. of unique reflns no. of reflections with I > 2σ(I) data/restraints/ params GOF, S final R indices (I > 2σ(I)) R indices (all data) largest diff peak and hole (e Å-3)

H‚CHEX

H‚DIOX

H‚DMF

C20H16O3‚ 1/ C H 2 6 12 cyclohexane

C20H16O3‚ 1/ C H O 2 4 8 dioxane

1:1/2 346.41 triclinic P1h 8.4834(17) 9.1242(18) 12.712(3) 96.10(3) 104.73(3) 110.41(3) 2 871.0(4) 0.086 113(2) 2.44-25.64

1:1/2 348.38 triclinic P1h 8.4075(17) 9.0908(18) 12.442(3) 97.04(3) 101.33(3) 111.15(3) 2 849.9(4) 0.093 113(2) 2.68-26.37

C20H16O3‚ C3NOH9 N,N-dimethylformamide 1:1 377.42 triclinic P1h 9.0685(18) 9.6815(19) 12.311(3) 72.36(3) 73.95(3) 78.05(3) 2 980.9(4) 0.087 113(2) 3.02-25.69

h: -10 to 9, k: -10 to 11, l: -15 to 15 5676 3263 2572

h: -10 to 10, k: -11 to 11, l: 15 to 14 6199 3422 2419

h: 0 to 11, k: -11 to 11, l: -13 to 15 3695 3695 2813

3263/2/240

3422/2/240

3695/2/260

1.044 R1 ) 0.0383, wR2 ) 0.0955 R1 ) 0.0523, wR2 ) 0.1026 0.165; -0.235

1.030 R1 ) 0.0396, wR2 ) 0.0854 R1 ) 0.0682, wR2 ) 0.0943 0.223; -0.220

1.033 R1 ) 0.0416, wR2 ) 0.1010 R1 )0.0607, wR2 ) 0.1096 0.177; -0.247

Scheme 1

Experimental Section Structure Analysis. The 9-(4-methoxyphenyl)-9H-xanthen-9-ol host compound, H, was dissolved in excess 1,4-dioxane and N,N-dimethylformamide, respectively, to obtain crystals of H‚DIOX and H‚DMF. The host H is only slightly soluble in cyclohexane and a cosolvent, ethanol, was added until a clear solution was obtained to form H‚CHEX. Ethanol is not included by this host. Thermogravimetry (TG) was employed to determine the host:guest ratios. The crystal data, intensity data collection, and refinement details are given in Table 1. Cell dimensions were established from the intensity data measured on a Kappa CCD diffractometer using graphite-monochromated Mo-KR radiation. The strategy for the data collections was evaluated using COLLECT software,10 and for all structures, the intensity data were collected by the standard phi scan and omega scan techniques and scaled and reduced using the program DENZO-SMN.11 The structures were solved by direct methods using SHELX-8612 and refined by full-matrix least-squares with SHELX-97,13 refining on F2. The program X-Seed14 was used as a graphical interface. For all the structures, the nonhydrogen atom positions were obtained by direct methods. The hydroxyl hydrogens were located in the difference electron density maps and refined with simple bond-length constraints.15 The remaining hydrogen atoms were fixed in position and refined isotropically. Thermal Analysis and Kinetics of Desolvation. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were performed on a Perkin-Elmer Pyris 6 system. The TG and DSC experiments were performed over the temperature range 303-473 K at a heating rate of 10 K min-1 with a purge of dry nitrogen flowing at 30 mL min-1. The samples were crushed, blotted dry, and placed in open ceramic pans for TG and in crimped but vented pans for DSC. Isothermal TG experiments were also performed at selected temperatures for the kinetic studies. Powdered forms of the dioxane and the DMF compounds were grown for the analyses. The isothermal kinetics of desolvation for the cyclohexane compound could not be determined, as the TG trace gave two poorly defined steps that could not be analyzed separately. The percentage mass loss for each step varied with the scanning rate, and attempts at non-isothermal kinetics were unsuccessful. Guest Exchange. Crushed crystals of H‚DIOX were exposed to vapors of dimethylformamide in a sealed container at 298 K and the reaction monitored using DSC. Similar experiments were set up between H‚DIOX and cyclohexane, H‚DMF and dioxane, H‚DMF and cyclohexane, H‚CHEX and dioxane, and finally, H‚CHEX and dimethylformamide.

Results and Discussion

exchange reactions. In the tetrahydrofuran clathrate of 1,1,6,6tetraphenylhexa-2,4-diyne-1,6-diol, the exchange with thiophene was monitored by IR spectroscopy and showed the continuous presence of the apohost.8 The exchange of p-xylene with benzene in the clathrates formed with 9-(3-chlorophenyl)-9Hxanthen-9-ol was followed by differential scanning calorimetry (DSC) and again proceeds by mechanism (b).9 We now present the results of the inclusion chemistry of the host 9-(4methoxyphenyl)-9H-xanthen-9-ol, its structures with cyclohexane (CHEX), 1,4-dioxane (DIOX), and N,N-dimethylformamide (DMF), their thermal stability, kinetics of desolvation, and guest exchange. The atomic numbering scheme of the inclusion compounds is given in Scheme 1.

All three structures were successfully solved in P1h. H‚DIOX and H‚CHEX are isostructural with respect to the host and display the expected packing motif that we have previously reported on for inclusion compounds between this host and benzene, toluene, the xylene isomers,16 and aniline.17 The packing of the structures is characterized by host dimers that are hydrogen-bonded via pairs of (Host)-OH‚‚‚O-(Host) bonds. These host dimers are located in columns along the [011] direction. The location of the p-methoxy phenyl moieties is such that they encapsulate the guest (dioxane or cyclohexane) that lies on a center of symmetry. In both cases, the guest exhibits a chair conformation. The packing is shown in Figure 1, which displays a host:guest ratio of 2:1, and the guests are located in highly constrictred channels. The metrics of the hydrogen bonds are given in Table 2. For the H‚DMF structure, both the host and the guest atoms were found in general positions with a host:guest ratio of 1:1. We also observed, for the first time for this host, hydrogen bonding between the host and guest molecules of the form (Host)-OH‚‚‚O-(Guest). This is illustrated in Figure 2. The dimethylformamide molecules are located in interconnected narrow channels running parallel to [100] and [010]. Both channels exhibit a zigzag arrangement. We have mapped the voids using the program SECTION18 and have found that the channels down [100] vary from 6.545 Å × 4.788 Å to 7.745 Å

Perspective

Crystal Growth & Design, Vol. 7, No. 6, 2007 1005 Table 2. Hydrogen Bonding Details

inclusion compd H‚CHEX H‚DIOX H‚DMF a

donor (D)

acceptor (A)

D‚‚‚A (Å)

D-H (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

O2 O2 O2

O1a

2.865(2) 2.868(2) 2.763(2)

0.961(1) 0.970(1) 0.960(1)

1.904(1) 1.900(1) 1.820(1)

178(2) 176(2) 167(1)

O1b O1Gc

Symmetry transformation: -x, -y + 1, -z + 2. b Symmetry transformation: -x, -y + 1, -z + 1. c Symmetry transformation: -x + 2, -y, -z.

Figure 3. Arrhenius plot for the first step in the desolvation of H‚DMF. Table 3. Thermal Analysis Data

Figure 1. Packing of H‚DIOX viewed down [100]. compd

H:G ratio TG calcd % mass loss exp % mass loss DSC Ton (K) Ton (K) Tba (K) Ton/Tb a

Figure 2. Packing of H‚DMF viewed down [100].

× 8.549 Å. The dimensions of the channel down [010] range from 8.656 Å × 2.736 Å to 8.224 Å × 4.100 Å. There is also a notable difference in the host conformation in the position of the hydroxyl oxygen relative to the methoxy group. For H‚DIOX and H‚CHEX, the torsion angle defined by O2-C13O3-C20 is 176.8 and -176°, respectively. This differs from the -10.5° obtained for H‚DMF. The thermal analysis data is given in Table 3. DSC results for both H‚DIOX and H‚DMF show a single endotherm corresponding to dissolution of the host upon release of the

H‚CHEX 1:1/

2

12.1 13.7 380.2 390.4 354.2 1.073

H‚DIOX 1:1/

H‚DMF

12.6 12.1 366.2

1:1 19.4 19.7 331.14

375.2 0.976

426.2 0.777

2

Normal boiling point.

guest. The cyclohexane compound, H‚CHEX, gave two endotherms, the first due to the guest loss and the second due to the host melt. This difference is mirrored in the TG results, which show single steps for the dioxane and DMF compounds compared to the two steps of the cyclohexane compound. These results suggest that H‚CHEX undergoes a different, more complex mechanism for thermal decomposition than the other two structures. We have used the ratio of the onset of desolvation (Ton) and the normal boiling point (Tb) as a measure of thermal stability. The Ton/Tb values listed in Table 3 indicate similar thermal stabilities for H‚CHEX and H‚DIOX, which is expected because of their identical packing arrangements and is comparable to that of the benzene compund16 (Ton/Tb ) 1.039). H‚DMF yields a Ton/Tb value of 0.777, indicating a lessstable structure, which correlates with our experience of handling the crystals that proved particularly unstable. For the kinetics experiments, powders of both H‚DMF and H‚DIOX compounds were grown and the structures verified using powder X-ray diffraction. The measured powder patterns were compared to the calculated ones generated from the program LAZYPULVERIX19 and good agreement for all the major peaks, which were significantly above the experimental background, was obtained. The kinetics of desolvation of H‚DMF was followed by performing a series of isothermal experiments, measuring the mass loss by TG at temperatures varying from 323 to 338 K. The reactions all went to completion, and the resulting curves of the extent of the reaction, R, versus time curves were

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49.5, and 130, respectively. Thus, one would expect that the exchange between H‚DIOX, H‚DMF, and cyclohexane would be the fastest; however, this is not the case, which suggests that the reaction mechanism for the formation of H‚CHEX is complex. Also, the reactions proceeded whether the initial included guest was situated in a cavity, e.g., H‚DIOX, or when the guests occupied channels, albeit narrow ones, as was seen for H‚DMF. Acknowledgment. We thank the NRF (Pretoria) and the Cape Peninsula University of Technology for funding. Supporting Information Available: Crystallographic information in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. Figure 4. Plot of -log β vs 1/T for H‚DIOX.

References analyzed. At the lower temperatures, the R-time curves clearly exhibit a fast first decay step followed by a slow second step. At higher temperatures, the first step dominated the entire reaction. We restricted ourselves to the analysis of the first desolvation mechanism, which followed the deceleratory contracting volume model, R3: 1 - (1 - R)1/3 ) kt.20 A plot of ln k versus 1/T yielded an activation energy of 143(15) kJ mol-1 (Figure 3). Although isothermal analyses are preferred, the decomposition of H‚DIOX at selected temperatures did not lead to consistent overall percentage mass losses, presumably due to host decomposition that occurs immediately after guest release. The modelfree non-isothermal method of Flynn and Wall21 was used to study H‚DIOX. Ramped TG experiments at selected scan rates gave a series of log β vs 1/T curves, where β is the heating rate (Figure 4). It is usual for this technique to give an activation energy range, which for this series was 133-162 kJ mol-1. Guest-exchange experiments were conducted to determine whether H‚CHEX, H‚DIOX and H‚DMF could interconvert via exposure to the vapor of a different guest. The progress of the exchange reactions was monitored using DSC. Common to all of these is the loss of the included guest to yield desorbed host followed by uptake of the incoming guest. For the guestexchange reaction between H‚DMF and dioxane, at the start of the experiment (time ) 0 min), we observed the endotherm due to the melt of H‚DMF. After t ) 15 min, this peak is gradually diminished and the host endotherm appears. The reaction is complete after 70 min, when only the H‚DIOX is detected. The timeframes for the individual reactions varied from 70 min to form H‚DIOX from the DMF and cyclohexane compounds to approximately 2 weeks for the exchange reactions between H‚DIOX, H‚DMF, and cyclohexane. The rate of any exchange reaction is generally dependent on particle size and vapor pressure of the incoming guest. The vapor pressures (in mbars)22 of DMF, dioxane, and cyclohexane at 298 K are 4.39,

(1) Hertzsch, T.; Hulluiger, J.; Weber, E.; Sozzani, P. Encyclopedia of Supramolecular Chemistry; Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker, Inc.: New York, 2004; Vol. 2, pp 996-1005. (2) Barbour, L. J. Chem. Commun. 2006, 1163-1168. (3) Rood, J. A.; Noll, B. C.; Henderson, K. W. Inorg. Chem. 2006, 45, 5521-5528. (4) Halder, C. J.; Kepert, C. J. J. Am. Chem. Soc. 2005, 127, 78917900. (5) Kepert, C. J.; Rosseinsky, M. J. Chem. Commun. 1999, 375-376. (6) Jacoby, M. Chem. Eng. News 2005, August 22, 42-47. (7) Ibragimov, B. T.; Talipov, S. A.; Aripov, T. F. J. Inclusion Phenom. Mol. Recognit. Chem. 1994, 17, 317-324. (8) Caira, M. R.; Nassimbeni, L. R.; Toda, F.; Vujovic, D. J. Chem. Soc., Perkin Trans. 2001, 2, 2119-2124. (9) Ramon, G.; Coleman, A. W.; Nassimbeni, L. R.; Taljaard, B. Cryst. Growth Des. 2005, 5, 2331-2335. (10) COLLECT, Data Collection Software; Nonius: Delft, The Netherlands, 1998. (11) Otwinowski, Z.; Minor, W. Methods in Enzymology, Macromolecular Crystallography, Part A; Carter, C. W., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol. 276, pp 307-326. (12) Sheldrick, G. M. SHELXS; Acta Crystallogr., Sect. A 1990, 46, 467. (13) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Deteremination; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (14) Barbour, L. J. X-Seed, Graphical Interface for SHELX Program; J. Supramol. Chem. 2003, 1, 189. (15) Olovsson, I.; Jo¨nsson, P. The Hydrogen BondsStructure and Spectroscopy; Schuster, P., Zundel, G., Sardify, C., Eds.; North Holland Publishing Company: Amsterdam, 1975. (16) Jacobs, A.; Nassimbeni, L. R.; Su, H.; Taljaard, B. Org. Biomol. Chem. 2005, 3, 1319-1322. (17) Jacobs, A.; Nassimbeni, L. R.; Taljaard, J. H. CrystEngComm. 2006, 7, 731-734. (18) Barbour, L. J. SECTION, A Computer Program for the Graphical Display of Cross Sections through a Unit Cell; J. Appl. Cryst. 1999, 32, 353. (19) Yvon, K.; Jeitschko, W.; Parthe, E. J. Appl. Cryst. 1977, 10, 73. (20) Brown, M. E.; Introduction to Thermal Analysis; Chapman and Hall: New York; Chapter 13. (21) Flynn, J. H.; Wall, L. A. J. Polym. Sci, Part B: Polym. Lett. 1966, 4, 323. (22) CRC Handbook of Chemistry and Physics, 87th ed.; CRC Press: Boca Raton, FL, 2006

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