CSIRO PUBLISHING Full Paper

www.publish.csiro.au/journals/ajc Aust. J. Chem. 2010, 63, 709–711

CPMAS Study of Ring Inversion of cis-Decalin in theSolid State when Complexed with β-Cyclodextrin

Helena DodziukA,B and Mariusz PietrzakA

AInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland.BCorresponding author. Email: dodziuk@ichf.edu.pl

Cyclohexane rings in cis-decalin are known to invert at room temperature in the liquid state and in solution. However,according to conventional wisdom such a large amplitude movement should be frozen in the solid state. In this paper,cis-decalin buried inside the β-cyclodextrin cavity has been shown by variable temperature 13C CPMAS NMR to invertin the solid state even at room temperature.

Manuscript received: 29 September 2009.Manuscript accepted: 24 January 2010.

Formation of a supramolecular complexes is known to mod-ify the properties of the host and guest.[1,2] Stabilization of theotherwise short-lived cyclobutadiene in the hemicarcerand,[3] anobservation of the highly unusual sodium anion Na− after captur-ing its cation in a cryptand cage[4] and that of an isolated nitrogenatom inside C60,[5] testify to a remarkable influence that themutual host and guest interactions can have on their properties.In the current study we report that, due to the inclusion complexformation with β-cyclodextrin, β-CD 1, a large-amplitude move-ment of cis-decalin 2 inversion in solution is to a great extentpreserved in the solid state. This observation is incompatiblewith conventional wisdom according to which cyclohexane-typering inversion should be frozen in the solid state.[6,7] However, afew solid complexes with mobile cyclohexane-type guests havebeen already studied. They encompass a cyclohexane clathratestudied by X-ray and solid state NMR[8] and some of its deriva-tives including cis-decalin inside thiourea.[9] In this paper, a newexample of a mobile cis-decalin guest inside a β-CD moleculein the solid state is presented and characterized.

O

O

O

O

OO

2a@1 2b@1

O

O

O

O

O

O

O

O

O

OO

O

O

O

O

OO

O

H

H

O

O

O

O

OH

OH

OH

OHOH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OH

OHOHOH

OH

OH

OHHO

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

HO

H

H

HO

HO

HO

Scheme 1. cis-Decalin inversion between its enantiomers inside the β-CD cavity.

CD complexes are the subject of numerous studies[2] on theone hand, because they have found many marketed applica-tions. On the other hand, they are formed by rather weak forcesand therefore their structure and properties can be modified byexperimental conditions. The ultimate example of this kind aredifferent modes of entrance of nitrophenol into the permethy-lated β-CD cavity in the solid state and in solution.[10] NativeCDs, such as β-CD, 1, have long been erroneously thought tohave a rigid, truncated-cone structure in spite of several experi-mental evidences[11] and theoretical arguments[12] in favour oftheir flexibility. For instance, NMR studies of the CD complexeswith aromatic guests have shown that the aromatic ring canexecute a 180◦ flip in the solid state.[11c] However, in these casesdespite the movement, the structures before and after the fliphave been identical. This is not the case for the 2@1 complex.

cis-Decalin 2 is known to form relatively strong complexeswith β-CD 1[13] and (similarly to the cyclohexane behaviourat room temperature) in solution inside the β-CD cavity it isknown to undergo a rapid ring inversion as depicted in Scheme 1,

© CSIRO 2010 10.1071/CH09525 0004-9425/10/040709

710 H. Dodziuk and M. Pietrzak

120

150 ∼250 K

∼273 K

283 K

290 K

299 K

∼308 K

∼320 K

∼350 K

∼335 K

900

1500

2100

2700

5000

15000

50000

200000

C9 C3

35 30 25 [ppm] 35 30 25 [ppm] 40 20 0 �20 [ppm]

*

* *

*C7C4C8

C2C6

C1 C9 C3C7

C2C6

C1C5C10

C4C8C5C10

320

600

900

1800

4400

C9C10

C9

H 5

10

6

8

7

9

1

2

3

4

H

k [s�1]

C10

C2, C3

Solid 2@1

(a) (b) (c)

Liquid 2 207Pb NMRPb(NO3)2

C6, C7C1, C4C5, C8

C2, C3C6, C7

C1, C4C5, C8 360

Averagedtemp. 340 320 300 280 260 [K]

k [s�1]

Fig. 1. cis-Decalin portion of the 13C CPMAS NMR spectra of the solid 2@1 complex (a), 13C MAS NMR spectra of liquid 2 and the correspondinginterconversion rates (b) at different temperatures (DMF signals are marked by asterisks, the rate constants of ring inversion of cis-decalin in the complexand in the liquid state calculated from the line shapes are included) and 207Pb NMR spectra of Pb(NO3)2 recorded prior or directly after the measurement ofthe solid 2@1 complex and liquid 2 (c). On the right, the horizontal scale shows relative 207Pb chemical shifts (zero represents isotropic δ207Pb of Pb(NO3)2

at 299 K, i.e. −3487.4 ppm versus tetramethyl lead), whereas the vertical scale shows real temperatures inside the rotor.[18]

with the invertomers being enantiomers as well. Freezing theinversion down to approximately 213 K allowed us to observemanifestations of chiral recognition of the enantiomers of 2 by1 in 13C NMR[14] spectra in DMF-d7 solution. However, the1H NMR solution spectrum upon freezing the inversion yieldedatypically[15] a very complicated pattern,[16] which we wereunable to assign. The inclusion character of the complex hasbeen proven by X-ray analysis at 100 K.[17] However, the dis-order of the solvent molecules even at such a low temperaturerequires further refinement but two different positions of theDMF molecules in the complex revealed by X-ray are consistentwith our NMR observation (see below). Nevertheless, the diffi-culties encountered by the interpretation of the room temperatureX-ray results prompted us to suspect that the ring inversion in 1could be, at least partly, preserved in the solid state. Therefore,solid state CPMAS 13C NMR spectra have been measured fromapproximately 250 K to 350 K to analyze the dynamic behaviourof the guest and host.

Internal movements typical of liquids and solutions areusually suppressed in the solid state. The rapid inversion ofcyclohexane rings in the solution and liquid state of cyclohexanederivatives, cis-decalin, and other similar molecules is known tobe frozen upon crystallization. This is not necessarily so if theguest molecule is inserted into a host cavity, as shown by thesolid state NMR studies of the complexes mentioned above[8,9]

or in our case of the cis-decalin@β-CD complex. Here, the guestinverts even in the solid state and the effect can be monitoredusing the CPMAS technique.

The temperature dependence of the decalin portion of theCPMAS spectra of the complex 2@1 is presented in Fig. 1together with the corresponding spectra of cis-decalin itself.Similarly to the solution spectra of 2@1 in DMF-d7, three sig-nals are observed in the spectra at 350 K and five at 250 K. Acoalescence of the signals appearing at around room temperature

shown in Figs 1a and b indicates that the cis-decalin rings’ inver-sion takes place not only in solution but in the solid state as well.The β-CD portion of the spectrum of the complex 2@1 and thatof 1 itself (see Accessory Publication) exhibits in the measuredtemperature range some broadening but, due to the low barrierto the macrocycle’s internal movement, no splitting of the CDsignals could be observed, even at 250 K.

It is worth stressing that such a pattern of 13C NMR signalsof cis-decalin can be explained only in terms of the ring inver-sion. Other types of movement, such as rotation or vibration ofthe guest molecule in the 2@1 complex, do not influence thecoalescence of NMR signals observed and analyzed here.

The quantitative 13C NMR spectrum of the 2@1 complex(see Accessory Publication) confirms its 1:1 composition. Thereis an excess of DMF molecules in the sample, i.e. 2@1:DMFratio is ∼1:5. All three solvent peaks are doubled pointing to twodifferent positions in the crystal lattice, e.g. inside and outsidethe β-CD cavity.

The inversion rates of cyclohexane rings in 2 in the free stateand in the complex have been calculated by fitting the line shapesof the C1-C8 signals using the gNMR program[19] on the basis ofthe Alexander-Binsch model.[20] Using the Eyring equation[21]

ln(k/T ) = −(�‡H ◦/R) ∗ 1/T + (�‡S◦/R) + ln(kB/h)

the characteristics of the inversion barrier shown in Table 1 havebeen obtained. In addition, the activation parameters for 2 mea-sured by Dalling et al.[22] and those for 2@thiourea reported byMüller[9] are given.

Surprisingly, the �‡G◦ values determined for inversion ofthe cis-decalin ring are almost identical in all cases: in neat2 and in the solid inclusion complexes 2@1 and 2@thiourea.On the other hand, the corresponding �‡H ◦ values differ to acertain extent. These differences are compensated, however, by

CPMAS Study of Ring Inversion of cis-Decalin 711

Table 1. Characteristics of the inversion barrier of 2

�‡H ◦ �‡S◦ �‡G◦ A

[kJ mol−1] [J mol−1 K] [kJ mol−1]

2@1B Solid 70 ± 8 53 ± 23 54 ± 152B,C Liquid 49 ± 6 −11 ± 15 52 ± 102[22] Liquid 57 ± 3 15 ± 13 53 ± 72@thiourea[9] Solid 46 ± 2 −25 ± 8 53 ± 4

ACalculated from �‡H ◦ and �‡S◦ values.BThis work.CMeasured in an NMR rotor under MAS conditions (3.7 kHz).

analogical changes in �‡S◦ leading to the very close valuesof �‡G◦ for all systems under scrutiny. As discussed in detailby Rekharsky and Inoue,[23] enthalpy-entropy compensation incomplexes involving cyclodextrins is a general phenomenon.

�‡H ◦ value for inversion of the cis-decalin ring in the com-plex 2@1 is significantly larger than the corresponding valuein neat 2. On the other hand, the results reported by Müller[9]

show even lower �‡H ◦ values inside the complex 2@thioureathan in liquid 2. The differences between the values obtained forthe complexes of 2 with 1 and thiourea could be partly due tothe differences in the crystal structure of the complexes, namely,2@thiourea forms a channel structure, while 2@1 is a fixed 1:1inclusion complex.[17] Thus, an increase in the �‡H ◦ value inthe complex 2@1 is tentatively ascribed to the crystal packing.

It is worth mentioning that the accuracy of the temperaturedetermination in an NMR rotor under MAS conditions is ratherlow, especially for high and low temperatures. The data pre-sented in Fig. 1c show real distributions of temperatures insidethe sample in thermal equilibrium and a perfect stability of tem-perature measured outside the rotor (±0.2 K). This suggests thatthe accuracy of the temperature determination of 1 K claimed byMüller[9] for the 2@thiourea complex is probably considerablyunderestimated. As a consequence, the accuracy of the thermo-dynamic parameters from the latter work given in Table 1 seemsalso to be underestimated.

To summarize, contrary to the common wisdom, cis-decalinburied inside the β-CD cavity executes large magnitude move-ment inverting in the solid state even at room temperature.

Accessory Publication

Experimental details, 13C CPMAS NMR spectra of the free 1 andthe 2@1 complex (β-CD portion) at room and low temperatureand a quantitative 13C MAS NMR spectrum of the 2@1 complexare provided in the Accessory Publication and is available on theJournal’s website.

AcknowledgementsWe are grateful to Wacker, GmbH for the gift of β-cyclodextrin, toDr M. Asztemborska for carrying out the complex crystallization, to Pro-fessor J. Lipkowski for allowing us to cite their preliminary results, and toProfessor S. Szymanski for his remarks on the manuscript.

References[1] H. Dodziuk, Introduction to Supramolecular Chemistry 2002

(Kluwer: Dordrecht).[2] Cyclodextrins and Their Complexes Chemistry. Analytical Methods,

Applications (Ed. H. Dodziuk) 2006 (Wiley-VCH: Weinheim).[3] D. J. Cram, M. E. Tanner, R. Thomas, Angew. Chem. Int. Ed. Engl.

1991, 30, 1024. doi:10.1002/ANIE.199110241

[4] J. L. Dye, in Encyclopedia of Supramolecular Chemistry (Eds J. L.Atwood, J. W. Steed) 2004 (CRC Press, Taylor and Francis Group:Boca Raton, FL).

[5] K. Lips, M. Waiblinger, B. Pietzak, A. Weidinger, Mol. Mater. 2000,13, 217.

[6] E. L. Eliel, S. H. Willen, Stereochemistry of Organic Compounds 1994(Wiley-Interscience: New York, NY).

[7] H. Dodziuk, Modern ConformationalAnalysis. Novel Exciting Molec-ular Structures 1995, p. 31 (VCH Publishers: New York, NY).

[8] S. Nishikiori, C. I. Ratcliffe, J. A. Ripmeester, J. Phys. Chem. 1990,94, 8098. doi:10.1021/J100384A022

[9] K. Mueller, J. Phys. Chem. 1992, 96, 5733. doi:10.1021/J100193A015[10] Y. Inoue, Y. Takahashi, R. Chujo, Carbohydr. Res. 1985, 144, C9.

doi:10.1016/S0008-6215(00)90685-2[11] (a) H. Dodziuk, in Cyclodextrins and Their Complexes. Chemistry,

Analytical Methods, Applications (Ed. H. Dodziuk) 2006, Ch. 1.3(Wiley-VCH: Weinheim).(b) H. Dodziuk, J. Mol. Struct. 2002, 614, 33. doi:10.1016/S0022-2860(02)00236-3(c) M. G. Usha, R. J. Wittebord, J. Am. Chem. Soc. 1992, 114, 1541.doi:10.1021/JA00031A001(d) Y. Inoue, T. Okuda, R. Chujo, Carbohydr. Res. 1985, 141, 179.doi:10.1016/S0008-6215(00)90450-6(e) Y. Yamamoto, M. Onda, Y. Takahashi, Y. Inoue, R. Chujo, Carbo-hydr. Res. 1988, 182, 41. doi:10.1016/0008-6215(88)84090-4(f)Y. Inoue, F. H. Kuan,Y.Takahashi, R. Chujo, Carbohydr. Res. 1985,135, C12. doi:10.1016/S0008-6215(00)90790-0(g) Y. Inoue, F. H. Kuan, R. Chujo, Carbohydr. Res. 1987, 159, 1.doi:10.1016/S0008-6215(00)90001-6

[12] (a) H. Dodziuk, in Cyclodextrins and Their Complexes. Chemistry,Analytical Methods, Applications (Ed. H. Dodziuk) 2006, Chapter 13(Wiley-VCH: Weinheim).(b) H. Dodziuk, K. Nowiñski, J. Mol. Struct. THEOCHEM 1994, 304,61. doi:10.1016/0166-1280(94)80009-X

[13] T. Koscielski, D. Sybilska, J. Lipkowski, A. Mediokritskaja, J.Chromat. 1986, 351, 512. doi:10.1016/S0021-9673(01)83530-5

[14] H. Dodziuk, J. Sitkowski, L. Stefaniak, J. Jurczak, D. Sybilska, J.Chem. Soc. Chem. Commun. 1992, 207. doi:10.1039/C39920000207

[15] 1H NMR spectra of guest molecules change only slightly after com-plexation by β-CD. A. Ejchart, W. Kozminski, in Cyclodextrins andTheir Complexes. Chemistry, Analytical Methods, Applications (Ed.H. Dodziuk) 2006, Ch. 9 (Wiley-VCH: Weinheim). Completely dif-ferent guest spectra in free and complexed states, as in Ref. 16, areuncommon.

[16] H. Dodziuk, J. Sitkowski, L. Stefaniak, J. Jurczak, D. Sybilska,Supramol. Chem. 1993, 3, 79. doi:10.1080/10610279308029842

[17] J. Lipkowski, M. Asztemborska, A. Bielejewska, D. Swierczynski,in Book of Abstracts of 12th International Seminar on InclusionCompounds, Stellenbosch, South Africa, April 4–9, 2009.

[18] (a) G. Neue, C. Dybowski, Solid State Nucl. Magn. Reson. 1997, 7,333. doi:10.1016/S0926-2040(96)01291-X(b) T. Takahashi, H. Kawashima, H. Sugisawa, T. Baba, Solid StateNucl. Magn. Reson. 1999, 15, 119. doi:10.1016/S0926-2040(99)00039-9(c) P. A. Beckmann, C. Dybowski, J. Magn. Reson. 2000, 146, 379.doi:10.1006/JMRE.2000.2171

[19] P. H. M. Budzelaar, gNMR, v. 5.0 2002 (IvorySoft).[20] (a) G. Binsch, J. Am. Chem. Soc. 1969, 91, 1304. doi:10.1021/

JA01034A007(b) D. S. Stephenson, G. Binsch, J. Magn. Reson. 1978, 30, 625.

[21] Molecular Reaction Dynamics in Physical Chemistry, 7th edn (Eds P.W. Atkins, J. de Paula) 2002 (Oxford University Press: Oxford).

[22] D. M. Grant, D. K. Dalling, L. F. Johnson, J. Am. Chem. Soc. 1971,93, 3678. doi:10.1021/JA00744A021

[23] M. V. Rekharsky, Y. Inoue, in Cyclodextrins and Their Complexes.Chemistry, Analytical Methods, Applications (Ed. H. Dodziuk) 2006,Ch. 8 (Wiley-VCH: Weinheim).