J. Phys. Chem. 1987, 91, 6691-6695
6691
Compensation Effect in Host-Guest Interactions. The Activation Parameters for the Inclusion Reactions of a-Cyclodextrin with Hydroxyphenylazo Derivatives of Naphthalenesulfonic Acids Noboru Yoshida and Masatoshi Fujimoto* Laboratory of Coordination Chemistry, Department of Chemistry 11, Faculty of Science, Hokkaido University, Sapporo 060, Japan (Received: April 6, 1987; In Final Form: July 8, 1987)
The activation parameters for the inclusion reactions with a-cyclodextrin (a-CDx) of the title guest azo molecules have been and the entropy of activation (M') for the determined by using a stopped-flowmethod. The enthalpy of activation (AH*) inclusion reactions are considerably influenced by the solvent structure around the inclusion site of the guest molecule. An exact compensation effect between AH*and M*is observed in the systems investigated here. The dependency of the change in A c t on substituent and/or solvent structure becomes much smaller. Distinct inflection in Arrhenius plot is observed only for the formation rate constant ( k ; ) at about 290 K, which is attributable to the structural change of the reactants and/or the change of rate-determining step in the reaction in the lower temperature range (C290 K).
Introduction
Cyclodextrin (CDx) forms inclusion complexes with a large number of organic and/or inorganic molecules.' The mechanisms of the inclusion reactions with CDx have been investigated from the standpoints of the structure,2 thermodynamic^,^ catalysis: dynamics5 of these inclusion complexes, and syntheses of modified CDx6 and from the theoretical standpoint^.^ The driving force for inclusion with a-CDx has been attributed to hydrogen bonding,4bvan der Waals force^,^^^"^^ hydrophobic interactions: (1) (a) Cramer, F. Einschlussuerbindungen;Springer-Verlag: Heidelberg, W. Germany, 1954. (b) Griffiths, W. D.; Bender, M. L. Adu. Catal. 1973, 23, 209. (c) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: Heidelberg, W. Germany, 1978. (d) Saenger, W. Angew. Chem., Int. Ed. Engl. 1980, 19, 344. (2) (a) Manor, P. C.; Saenger, W. J. Am. Chem. SOC.1974,96,3630. (b) Saenger, W.; Noltemeyer, N.; Manor, P. C.; Hinerty, B.; Klar, B. Bioorg. Chem. 1976,5, 187. (c) Lindner, K.; Saenger, W. Angew. Chem., Int. Ed. Engl. 1978, 17, 694. (3) (a) Otagiri, M.; Miyagi, T.; Uekama, K.; Ikeda, K. Chem. Phurm. Bull. 1976, 24, 1146. (b) Bergeron, R. J.; Channing, M. A.; Gilbeily, G. J.; Pillor, D. M. J . Am. Chem. SOC.1977, 99, 5146. (c) Gelb, R. I.; Schwartz, L. M.; Johnson, R. F.; Laufer, D. A. J . Am. Chem. Soc. 1979,101, 1869. (d) Gelb, R. I.; Schwartz, L. M.; Cardelion, B.; Fuheman, H. S.; Johnson, R. F.; Laufer, D. A. J . Am. Chem. SOC.1981, 103, 1750. (4) (a) Hennrich, N.; Cramer, F. J . Am. Chem. SOC.1965,87, 1121. (b) Cramer, F.; Kampe, J. J . Am. Chem. Soc. 1965,87, 1115. (c) VanEtten, R. L.; Sebastian, J. F.; Clowes, G. A.; Bender, M. L. J . Am. Chem. SOC.1967, 89, 3241. (d) VanEtten, R. L.; Clowes, G. A.; Sebastian, J. F.; Bender, M. L. J . Am. Chem. SOC.1967, 89, 3253. (e) Czarniecki, M. F.; Breslow, R. J. Am. Chem. SOC.1978,100, 7771. ( f ) Bergeron, R. J.; Burton, P. S.J . Am. Chem. SOC.1982, 104, 3664. (5) (a) Cramer, F.; Saenger, W.; Spatz, H.-Ch. J. Am. Chem. SOC.1967, 89, 14. (b) Behr, J. P.; Lehn, J. M. J . Am. Chem. SOC.1976,98, 1743. (c) Rohrbach, R. P.; Rodrigues, L. J.; Eyring, E. M.; Wojcik, J. F. J . Phys. Chem. 1977,81,944. (d) Bergeron, R. J.; Channing, M. A. J. Am. Chem. SOC.1979, 101, 2511. (e) Yoshida, N.; Fujimoto, M. Chem. Lett. 1980, 231. ( f ) Yoshida, N.; Fujimoto, M. Chem. Lett. 1980, 1377. (g) Yoshida, N.; Fujimoto, M. Bull. Chem. SOC.Jpn. 1982,55,1039. (h) Turro, N. J.; Okubo, T.; Chung, C.-J. J. Am. Chem. SOC.1982, 104, 1789. (i) Yoshida, N.; Seiyama, A.; Fujimoto, M. Chem. Lett. 1984, 703. 6) Hersey, A.; Robinson, B. H. J. Chem. SOC.,Faraday Trans. 1 1984,80,2039. (k) Clarke, R. J.; Coates, J. H.; Lincoln, S. F. Curbyhydr. Res. 1984,127, 181. (I) Yoshida, N.; Fujimoto, M. J . Chem. Res. 1985, 224. (m) Seiyama, A.; Yoshida, N.; Fujimoto, M. J . Inclusion Phenom. 1984, 2, 765, (n) Seiyama, A.; Yoshida, N.; Fujimoto, M. Chem. Lett. 1985, 1013. ( 0 ) Orstan, A.; Wojcik, J. F. Curbohydr. Res. 1985, 143, 43. (6) Tabushi, I.; Shimokawa, K.; Shimizu, N.; Shirakata, H.; Fujita, K. J . Am. Chem. SOC.1976, 98, 7855. Emert, J.; Breslow, R. J. Am. Chem. SOC. 1975, 97,670. Breslow, R. Acc. Chem. Res. 1980,13, 170. Tabushi, I. Acc. Chem. Res. 1982, 15, 66. (7) (a) Harata, K. Bull. Chem. Soc. Jpn. 1976, 49, 2066. (b) Berjeron, R.; Pillor, D. M.; Gilbeily, G. J.; Roberts, W. P. Bioorg. Chem. 1978, 7, 263. (c) Tabushi, I.; Kiyosuke, Y.; Sugimoto, T.; Yamamura, K. J . Am. Chem. SOC.1978, 100, 916. (d) Matsui, Y. Bull. Chem. SOC.Jpn. 1982, 55, 1246. (8) Cramer, F. Angew. Chem. 1967, 73, 49.
0022-3654/87/2091-6691$01.50/0
relaxation of the conformational strain in the macrocyclic ring,1° and release of partially hydrogen-bonded water molecules from the cavity." Theoretical approach to the driving force for the inclusion with a-CDx on the basis of the calculation of A P and hSe has suggested that (1) van der Waals interaction between a-CDx and guest molecule is critical in the stabilization of the inclusion complex, (2) the stabilization is largely compensated for by the term of water cluster, and (3) the conformational change of a-CDx is unfavorable to the formation of the inclusion comp l e ~ On . ~ the ~ other hand, from the correlation of A I P vs ASe and the displacement of anomeric C1 resonance of a-CDx vs AfF' data, Laufer et al. concluded that hydrophobic contribution to the binding cannot play a major role and dipolar interactions appear to be the main binding force.3d These concepts about the driving force provide a static feature of the inclusion process, but not a dynamic feature of the inclusion process. The exact values of the rate constants and the activation parameters are indispensable for the clarification of the reaction mechanism and the driving force for inclusion. We report here the determination of the rate constants and the activation parameters in host (aCDx)-guest (azo compound) interactions, which enable us to advance our knowledge and understanding of the dynamic aspects of the host-guest interactions. Experimental Section
The preparation and the purification of guest azo compounds (Figure 1) are described in a previous paper.51Careful purification of the sample is needed for solution studies. a-Cyclodextrin (Tokyo Kasei) was used without further purification. The purity was checked by means of high-performance liquid column chromatography with an ERMA ERC 7520 R I detector (CH,CN/H,O; 70/30 (v/v); ERC-NH1171 column). Water was deionized and distilled. A Hitachi-Horiba pH meter F-7ss was used for the determination of pH values. Acid-dissociation constants were determined spectrophotometrically with a Hitachi recording spectrophotometer Model EPS-3T. The absorption spectra of the guest azo molecules a t varying a-CDx concentrations showed isosbestic points both in acidic (pH 4.2, phosphate buffer) and in alkaline (pH 11.5, phosphate buffer) region. The ionic strength was adjusted at I = 0.1 mol dm-3 (KNO3).I1 The temperature was maintained at 25 0.1 OC with a Lauda Type K2R thermostat. The absorption spectrum of the azo compound is shifted
*
(9) NCmethy, G.; Scheraga, H. A. J. Chem. Phys. 1962, 36, 3401. (10) Manor, P. C.; Saenger, W. Nature 1972, 237, 392. Saenger, W.; Macmullan, P. K.; Fayos, J.; Mootz, D. Acta Crystallogr. Sect. B 1974, 30, 2019. (1 1) We choose KNOg as a medium salt because of weak interaction with
CDx. See also ref 5c.
0 1987 American Chemical Society
6692 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Yoshida and Fujimoto
TABLE I: Rate and Association Constants for the Inclusion Reaction with a-Cyclodextrin" guest molecule kf/mol-' dm' s-I kb/s-' f?/mol-' dm' k'f/mol-L dm' s-I 1 e e 391d e 2 e e 280d 780 3 e e 271d 230 4 8.5 X 10' 39 220 43 5 1.4 x 104 60 230 58
k b/s-' e
K'C/mol-' dm' 21 I d 460 180 726 320
1.7 1.3 0.06 0.18
"At 25 OC and I = 0.1 mol dm-' (KNO'). b p H 4.6 (phosphate buffer). C p H 11.1 (phosphate buffer). dDetermined by UV-vis spectroscopy. Error 5-876. C T ~fast o to measure by the stopped-flow method. 'The intercept in the plot of kobsdvs the concentration of a-CDx is very small. k 6 = 0.06 f 0.03
*
TABLE 11: Thermodynamic Parameters for the Inclusion Reactions with a-Cyclodextrin" TASf' T u b* guest molecule AG: AHf* (AS,') AGb* AHb' (ub') A2-(2) 56.6 24.8 -31.8 71.7 51.7 -20.0 (-1 06.6) (-67.0) A2-(3) 59.4 31.6 -27.8 72.1 76.1 4.1 (-93.4) (13.8) HA'(4)b 51.3 32.5 -18.8 62.6 57.1 -5.1 (-63.1) A2-(4)b 65.4 57.8 -7.6 78.5 76.6 -1.9 (-25.7) (-6.3) HA75) 49.2 13.3 -35.9 62.7 33.7 -29.0 (-120.5) (-97.4) A2-(5)b 62.9 73.0 10.1 77.3 90.9 13.6 (33.9) (45.7)
TAP
(ae)
AHe
AGe -15.1
-27.0
-12.7
-44.5
-11.3 (-17.0) -13.1
-24.6
-13.5
-20.4
-14.4
-17.9
-1 1.8 -(39.6) -31.9 (- 107.2) -13.7 (-46.1) -5.8 (-19.4) -6.9 (-23.1) -3.5 (-1 1.8)
-18.8
" H , G, TAS/kJ mol-' and S / J mol-' K-' at 25 OC. AG', @, and ASe are obtained to be AG: - AGb*, AH: - A&*, and ASf* - h s b ' , respectively. Error limits are estimated to be not larger than *20% in any of the determination of AH* and AS*. In the case of HA-(5), the error is very large (*50%) because of the poor S / N ratio of a small signal amplitude. bReference 51.
R,
I
,
Figure 1. Structural formula of hydroxyphenylazo derivatives of naphthalenesulfonic acids as a guest molecule. (1) R2 = H, R, = -SO3-, (2) Rz = C1, Rs = -SO
