Inclusion of methano[60]fullerene derivatives in cavitand-basedcoordination cages

Laura Pirondini,a Davide Bonifazi,b,* Barbara Cantadori,a Paolo Braiuca,b Mara Campagnolo,c

Rita De Zorzi,c Silvano Geremia,c Francois Diederichd,* and Enrico Dalcanalea,*

aDipartimento di Chimica Organica e Industriale and INSTM UdR Parma, Universita degli Studi di Parma, 43100 Parma, ItalybDipartimento di Scienze Farmaceutiche, Universita degli Studi di Trieste, 34127 Trieste, Italy

cDipartimento di Scienze Chimiche, Universita degli Studi di Trieste, 34127 Trieste, ItalydLaboratorium fur Organische Chemie, ETH-Honggerberg, CH-8093 Zurich, Switzerland

Received 24 May 2005; accepted 7 June 2005

Available online 28 November 2005

Abstract—The X-ray study of self-assembled coordination cage 1, constituted of two tetrapyridyl-substituted resorcin[4]arene cavitandscoupled through four square-planar palladium complexes is reported. The coordination cage, embracing an internal cavity of ca. 840 A3,reveals to have the right size for the inclusion of large molecules such as fullerenes. Cage 1 forms 1:1 complexes with methano[60]fullerenederivatives 3 and 4 bearing a dimethyl and a diethyl malonate addend, respectively. Evidence for inclusion complexation was provided by 1HNMR spectroscopic studies and ESI-MS investigations, which unambiguously showed the formation of 1:1 fullerene–cage complexes. Theassociation constants (Ka) were experimentally determined to be ca. 150 MK1 at 298 K in CD2Cl2. In both complexes 1$3 and 1$4, themalonate residue is threaded through one of the four lateral portals, as clearly shown by docking simulations.q 2005 Elsevier Ltd. All rights reserved.

1. Introduction

Current materials-oriented science is characterized by thevigorous exploration of new efficient chemical methods forthe development of new nano-sized architectures in whichfunctional molecules possessing unique optical and electro-chemical properties can be selectively hosted.1–3 The abilityof the supramolecular approach to confine and stabilizemolecular species by specific weak intermolecular inter-actions in both solid and liquid phase makes it particularlyattractive for engineering many potential molecule-basedapplications ranging from sensors to nanotechnology.4,5

Metal-directed self-assembly has been widely employed toengineer two- and three-dimensional structures showinginternal cavities capable of trapping molecules.6–10 In thisrespect, cavitand-based coordination cages11–16 are increas-ingly attracting attention as one of the most promisingclasses of molecular hosts due to the versatile properties ofthe cavitand scaffold in terms of preorganization, geometry,and synthetic modularity.17

0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.tet.2005.06.122

Keywords: Fullerenes; Cavitands; Self-assembly; Inclusion; Coordinationcages.* Corresponding authors. Fax: C39 521 905472 (E.D.); C39 040 52572

(D.B.); C41 1 632 1109 (F.D.); e-mail addresses: dbonifazi@units.it;diederich@org.chem.ethz.ch; enrico.dalcanale@unipr.it

p-Conjugated molecules such as fullerenes with tunableelectronic properties are appealing building blocks for theconstruction of functional materials possessing exceptionalelectrochemical and photophysical properties. With theavailability of macroscopic quantities of these all-carboncompounds, large efforts have been geared towarddiscovering rapid and efficient methods for their purifi-cation18 and subsequent inclusion in photosynthetic,19,20

photonic,21,22 and molecular electronics devices.23,24

Numerous examples of fullerene-containing complexeshave appeared in the literature, in which the host moleculesconsist mainly of macrocyclic molecules, such as calix-arenes,25–28 resorcinarenes,29,30 cyclotriveratrylenes,31

cyclodextrins,32,33 porphyrins,34 and cyclic [6]paraphenylene-acetylene,35 which spontaneously form complexes in thepresence of C60 or C70. Consequently, the preparation ofspecific preorganized36 receptors containing two or morecovalently-linked macrocycles, exploiting a high selectivitytoward fullerenes binding, were also explored.37 In thisrespect, Fukazawa and co-workers prepared the firstcovalently-linked biscalix[5]arene fullerene receptor, withassociation constants ranging from 76 to 163!103 MK1 at298 K in toluene.38 Recently, Saigo, Aida, and co-workersreported a series of cyclophane-type metal-porphyrindimers that encapsulate fullerenes in organic solvents(with fast host–guest exchange on the NMR time scale)

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and in the solid state, showing exceptionally highassociation constants (ca. 2.4!107 MK1 for C60 in benzeneat 298 K) as a consequence of the strong charge-transferinteraction between the two electron-rich tetrapyrrolicmacrocycles and the electron-deficient fullerene sphere.Parallel studies were also reported by Reed, Boyd, andco-workers, who engineered fashionable jaw-like hostsas fullerene binders.39,40 Consecutive to these works,several other papers have been reported for the constructionof porphyrin-based receptors for fullerenes showing avariety of stimulating features. Thus a dendritic porphyrinreceptor with a pronounced allosteric effect in the binding ofthree C60 molecules41 and a bisporphyrin receptor that canbe reversibly switched by complexation with metals weredescribed by Shinkai and co-workers.42 However, morerarely the encapsulation of fullerenes in self-assembledcages has been employed.43–45 Shinkai et al., reported forthe first time a 1:1 complex between a C60 molecule and ahomooxocalix[3]arene dimer cross-linked by three Pd(II)complexes.43,46 Very recently, Claessens and Torresdescribed the encapsulation of C60 in an isomeric mixtureof C3-symmetric coordination cages constituted by twopyridine-bearing subphtalocyanine ligands connected bythree metal complexes.47

In this work, we describe the self-assembly, crystalstructure, and fullerene inclusion properties of cavitand-based coordination cage 1 composed of two tetrapyridyl-substituted cavitands connected through four square-planarpalladium complexes.

2. Results and discussion

Self-assembly of cavitand ligand 2 into molecular cage 1was carried out as outlined in Scheme 1, following theprotocol reported in the literature.48 The typical procedurefor the cage preparation is based on the mixing of cavitandligand 2 with a metal precursor, such as [Pd(dppp)(CF3SO3)2],in a 1:2 molar ratio at room temperature in chlorinatedsolvents or acetone, affording cage 1 in quantitative yield.The 1H NMR spectrum showed the formation of a new set ofproton resonances, indicative of the presence of a singlecompound. The downfield shift of the pyridine protons

Scheme 1. Schematic representation of the coordination cage formation and the1,1 0-bis(diphenylphosphino)propane.

ortho to the nitrogen is indicative of coordination to the Pdions, whereas the upfield shift of the OCHO protons,pointing inside the cavity, is diagnostic of the cageformation.48 The high symmetry of compound 1 in solutionwas also confirmed by the 31P NMR spectra, which featureda sharp singlet indicating the equivalence of the eightphosphorous atoms.

A small crystal of coordination cage 1, suitable for X-raydiffraction, was obtained by slow evaporation of a solutionof 1 in acetone. The electron density maps and therefinement of the structure revealed the presence of oneocta-cationic coordination cage in the triclinic unit cell,intramolecularly related by a crystallographic symmetrycenter. The molecular structure, depicted in Figure 1, nicelyreveals the presence of four square-planar Pd(II) complexesin the equatorial region connecting two cavitand units(equatorial distance between two successive Pd(II) ions is12.3 A). The coordination cage shows a pseudo C2h

symmetry in the solid state, which increases to D2h if theterminal (3-phenyl)propyl substituents are excluded.

All Pd(II) metal ions form tetra-coordinated species, withthe N(Py)–Pd–N(Py) and P(dppp)–Pd–P(dppp) anglesranging from 85.9 to 87.58 and from 91.1 to 91.98,respectively, indicating a small distortion in the square-planar coordination geometry at the Pd(II) metal centers.The positive charges of the four Pd(II) metal centers arecounterbalanced by eight CF3SO3

K anions, of which four arecrystallographically independent. All CF3SO3

K ions aredisordered at least along two directions. Six CF3SO3

K

counterions closely surround the cage (Fig. 2a), of whichfour are in close contact with the Pd(II) metal ions (Pd–Odistance 3.0 A) and two hosted within the two hinderedportals between two Pd(II) centers (shortest Pd–O dis-tanceZ4.3 A). Of the two remaining CF3SO3

K molecules,one is located outside the cage in a crystallographic centerof symmetry and one is confined within the capsule cavity.Both sites are at half occupancy. The latter observationcontrasts with the 19F NMR measurements obtained insolution, which clearly showed no permanent inclusion ofCF3SO3

K anions inside the cage cavity.48 The lateral portalsare wide enough to allow a fast exchange of the counterionswith the bulk solvent.49 Eleven acetone molecules (5.5

complexation of 1 with methano[60]fullerene derivatives 3 and 4. dppp:

Figure 1. ORTEP representation of cavitand-based cage 1. Eleven acetone solvent molecules and eight CF3SO3K anions in the crystal are omitted for clarity.

Atomic displacement parameters obtained at 100 K are drawn at the 30% probability level. Atom colors: blue N, red O, green Pd, okra P, grey C.

Figure 2. (a) Top view and (b) side view of the crystal structure of coordination cage 1 with eight CF3SO3K anions and four acetone molecules, respectively,

showing that three guest molecules have been confined within the cage cavity. Only the acetone molecules close to the cage have been displayed. For the sakeof clarity, only one position of the disordered CF3SO3

K and acetone molecules is depicted. All H-atoms have been omitted for clarity. Atom colors: blue N, redO, green Pd, okra P, sky-blue F, yellow S, grey C.

L. Pirondini et al. / Tetrahedron 62 (2006) 2008–20152010

molecule per asymmetric unit) were also detected percoordination cage. Among all, only two acetone moleculesare confined within the cage cavity disordered along twodirections, while the rest is located outside the cavity(Fig. 2b).

Several encapsulation experiments with suitably sizedguests such as adamantane, 1,2,4,5-tetramethylbenzene,anthracene, pyrene, sodium picrate, and fullerenes invarious solvents were attempted. The complexation

experiments, monitored by 1H NMR, were performed at298 K by adding an excess of guest to a solution of cavitand2 in CD2Cl2, followed by addition of 2 equiv of the metalprecursor [Pd(dppp)(CF3SO3)2] in order to form the cage inthe presence of the guest. Adamantane, 1,2,4,5-tetramethyl-benzene, anthracene, pyrene, and sodium picrate were notpermanently encapsulated in the cage probably due to therapid exchange with the solvent molecules through the largeequatorial portals. On the other hand, the encapsulationexperiments using methano[60]fullerene 3 as guest showed

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a slow host–guest exchange on the 1H NMR time scale(298 K). Coordination cage 1 self-assembled in the presenceof methano[60]fullerene 3 (c1Z6.5!10K1 mM and c3Z5.4 mM) in CD2Cl2 displaying three sets of resonances,which correspond to the two free components (1 and 3) andthe 1:1 complex 1$3, respectively (Fig. 3). The singlets atdZ4.10 and 3.57 ppm corresponds to the OCH3 protons(Fig. 3) of free and complexed methano[60]fullerene 3,respectively. The upfield shift of the latter signal is a goodindication for the formation of the cage–fullerene complex1$3. Integration of the resonance signals for the free andcomplexed methano[60]fullerene species affords the asso-ciation constant Kaz150 MK1. The complexation betweencavitand 2 and methano[60]fullerene 3 in the absence of themetal precursor [Pd(dppp)(CF3SO3)2] was also investi-gated. As expected, the 1H NMR spectrum of a 1:2 mixtureof fullerene 3 and cavitand 2 shows no changes in thefullerene- and cavitand-centered proton resonances, thusindicating that tetrapyridyl-substituted molecule 2 alonedoes not form any supramolecular complex with themethano[60]fullerene guest.

Figure 3. Excerpt of the 300-MHz 1H NMR spectrum of cage–fullerenecomplex 1$3 (CD2Cl2, 298 K).

Figure 4. Excerpt of the 300-MHz 1H NMR spectrum of cage–fullerenecomplex 1$4 (CD2Cl2, 298 K).

Table 1. m/z Values and relative intensities of the peaks observed in massspectra from samples (solvent: CH2Cl2/MeOH 9:1) containing cage–fullerene complexes 1$3 or 1$4.

Observed ion Experimentalmass (m/z)

Calculatedmass (m/z)

Relativeintensity (%)

Complex 1$3[1$3–2!CF3SO3]2C 3172.0 3171.80 5[1–2!CF3SO3]2C 2746.5 2746.42 70[1$3–3!CF3SO3]3C 2064.9 2064.84 35[1–3!CF3SO3]3C 1780.5 1781.26 100

Complex 1$4[1–2!CF3SO3]2C 2745.7 2746.42 20[1$4–3!CF3SO3]3C 2075.2 2074.20 20[1$4–4!CF3SO3]4C 1518.0 1518.38 15[1–4!CF3SO3]4C 1298.4 1298.68 55

Next, the complexation of methano[60]fullerene derivative 4with capsule 1 was also investigated. The 1H NMR spectrum(300 MHz, CD2Cl2, 298 K) of cage 1 self-assembled in thepresence of fullerene 4 (c1Z6.5!10K1 mM and c4Z5.4 mM)shows two sets of resonances, one for the free and one for theencapsulated fullerene 4, in agreement with slow host–guestexchange. The 1H NMR spectrum resembles the one recordedfor the mixture of 1 and 3, displaying the diagnostic signals forthe OCH2CH3 and OCH2CH3 protons of methano[60]-fullerene derivative 4 as quadruplet (Fig. 4) and triplet,respectively, in their free and encapsulated forms. Themultiplets centered at dZ4.56 and 1.49 ppm correspond tothe OCH2CH3 proton resonances of free guest, whereas thoseat dZ4.58 and 1.51 ppm are assigned to the methano[60]ful-lerene species embraced within the cage. The Ka value for theformed 1:1 cage–fullerene complex 1$4 in CD2Cl2 revealed tobe, within the experimental errors, the same as that reported forcomplex 1$3. Encapsulation experiments with pristine C60

in CD3C6D5 were also performed (c1Z6.5!10K1 mM and

cC60Z5.4 mM). However, no fullerene-centered resonanceswere observed for the complexed species in the 13C NMRspectrum, probably because the anisotropic ring current effectsof the interior of the cage do hardly affect the 13C NMRresonances of the carbon sphere. Similarly, complexation of 3and 4 could not be detected by 13C NMR spectroscopy.

The formation of complexes 1$3 and 1$4 was alsoevidenced by electrospray ionization (ESI) mass spec-trometry in the positive ion mode. The mass spectrumrecorded from a 1:8 mixture of coordination cage 1 andmethano[60]fullerene 3 (solvent: CH2Cl2/MeOH 9:1)displayed a set of peaks belonging to the M2C and M3C

ions of both 1 and 1$3 species (Table 1). The base peakdetected at m/z 1780.5 corresponds to the triply-charged ion[1–3!CF3SO3]3C, while the corresponding [1$3–3!CF3SO3]3C ion is observed at m/z 2064.9. Under the sameconditions, the M3C and M4C ions of complex 1$4 werealso detected, albeit with lower abundance (Table 1).

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Docking simulations (carried out using the MMFF94!force field as implemented in MOE, see Section 4)performed on cage 1 encapsulating C60 or methano[60]ful-lerenes 3 and 4, indicate that all fullerenes species are wellaccommodated within the cage cavity (Fig. 5). The energyminimized geometry of complex 1$C60 (not shown) showed

Figure 5. Energy minimized structures (MMFF94!-based docking calculation a1$4. Top (a) and side (b) view. All H-atoms have been omitted for clarity. Atom

that the fullerene sphere is positioned in the geometricalcenter of the cavity with the distance between the fullerenesphere and the pyridine ring planes ranging from 3.0 to3.7 A, suggesting fullerene–pyridine p-stacking inter-actions. Although complexes 1$3 and 1$4 maintain thesame geometry as compound 1$C60, the fullerene sphere is

s implemented in MOE) of cage–fullerene complexes (left) 1$3 and (right)colors: blue N, red O, green Pd, okra P, grey C.

L. Pirondini et al. / Tetrahedron 62 (2006) 2008–2015 2013

slightly shifted toward one portal by ca. 0.1–0.2 A withrespect to the cavity centers because of the presence of themalonate addend. Specifically, in both 1$3 and 1$4complexes, the malonate residue is threaded through oneof the four lateral portals (Fig. 5a). Notably, under thegeometrical constrictions applied for these calculations,only two of the four lateral portals are sterically suitable foraccommodating the malonate substituents (calculateddifference in energy for malonate placement in favorablevs unfavorable portals: DDEz6.6 kcal molK1). The volumeof the internal cavity of 1 has been estimated in 840 A3 andthe four lateral portals have an approximate diameter of 6 Aat the van-der-Waals limits.48 The van-der-Waals volume ofC60 has been calculated in the range 450–549 A3 with anintermediate value of about 500 A3.50 The resulting packingcoefficient of C60 within the cavity of 1 is ca. 0.59, which isvery close to the optimal value of 0.55 predicted foreffective volume-optimized cavity inclusion.51 Althoughthe geometrical requisites of the preorganized cavity of cage1 are ideal for the inclusion of C60 and its derivatives, nospecific interactions, that is, charge-transfer, accounting forhigh association constants have been identified.

3. Conclusion

Cavitand-based coordination cage 1 features a molecularrecognition geometry suitable for the construction of novelsupramolecular architectures hosting spherically sizedguests, such as fullerenes, as determined by the crystalstructure. In chlorinated solvents, the coordination cageforms a 1:1 complex with methano[60]fullerene derivatives3 and 4. The inclusion of fullerene derivatives is not relatedto specific host–guest interactions, but is driven bydispersion forces and p-stacking contacts between theguest and the inner surface of the host. For this reason, thecorrect match between cavity size, guest shape, and volumeis pivotal for effective inclusion, both in terms of packingcoefficient and guest confinement (i.e., no guest escape fromthe lateral portals). The kinetic stability of cavitand-basedcoordination cages,52 is sufficient to observe fullereneinclusion via NMR under slow exchange conditions and inperspective, to exploit some of the peculiar properties offullerenes in confined environment. Besides, the surface-

Table 2. Crystal and refinement data

Formula (C276H250N8O16P4Pd4)8!(CF3SO3) 11!(C3H6O)

l/2

Fw 6429.5 ReT (K) 100 (2) Rel (A) 1.000 PaCryst system, space group Triclinic, P-1 Rea, b, c (A) 21.5179(8) Re

21.7484(8)22.6496(7)

a, b, g (deg) 93.854(2) Rm

116.314(2)106.929(2)

V (A3) 8848.1(5) GoZ,Dcalcd 1, 1.199 mg/m3 Rm (mmK1) 0.843 RF(000) 3320

directed self-assembly of this class of coordination cages ongold provides an opportunity for the direct formation ofthese supramolecular complexes on surfaces.53

4. Experimental

4.1. General

Reagents and solvents were purchased reagent grade andused without further purification. Cavitand 248 andmethano[60]fullerene derivatives 354 and 454 were syn-thesized according to the protocols reported in literature. 1HNMR spectra were recorded on a Brucker Avance 300 MHzspectrometer at 300 K, and all chemical shifts were reportedin ppm relative to the proton resonances resulting fromincomplete deuteration of the NMR solvents. ESI-MSexperiments were performed on a Waters ZMD spec-trometer equipped with an electrospray interface.

4.2. Determination of the association constants

The association constants (Ka) were obtained by means ofsingle-point measurements of the concentration of thecomplexed and uncomplexed species from the 1H NMRspectra at 298 K by the relationship KaZ[cage$fullerene]/[cage][fullerene].

4.3. Geometry optimazation

The geometries of supramolecular complexes 1$C60, 1$3,and 1$4 have been calculated by molecular dockingsimulations. The spatial coordinates of cage 1 have beenderived from the X-ray crystallographic structure, whilethose for C60 and methano[60]fullerenes 3 and 4 havebeen obtained from their energy minimized structure(calculated via a truncated Newton algorithm55 andMMFF94x force field56 as implemented in MOE). Theforce field method has been applied for 50 runs of a6-cycles simulation annealing protocol, by means of thefunction DOCK as implemented in the molecularoperating environment (MOE) package (Chemical Com-puting Inc., Montreal, 2004) version 2004.03, startingfrom 50 random conformations/positions and from a

sin qmax ( A) 0.94

flns measured 29390flns unique 18707rams 1934straints 1494flns IO2.0s(I) 17329

erge 0.026 (0.077, in shell 0.97–0.94 A)

f 1.81[IO2s(I)] R1Z0.116, wR2Z0.354(all data) R1Z0.119, wR2Z0.361

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simulation temperature of 1000 K. All calculations havebeen performed on an Intel Xeon 3.0 GHz bi-processorworkstation, running Red Hat 9 Linux OS.

4.4. X-ray crystal structure determination

Crystals of coordination cage 1 were grown by slowevaporation of a solution of 1 in acetone. The X-raystructure of a small crystal was obtained using synchrotronradiation under cryogenic control. Data collection wasperformed at the X-ray diffraction beamline of ElettraSynchrotron, Trieste (Italy) (monochromatic wavelengthlZ1.0000 A) using a Mar CCD detector with the rotatingcrystal method. The crystal soaked with an aqueous solutionof PEG 1500 (50% g/ml) was mounted in a loop and flashfrozen to 100 K with a nitrogen stream. The diffraction datawere indexed and integrated using DENZO57 and scaledwith SCALEPACK.57 The structure was solved by directmethods using SHELXS58 and Fourier analyses and refinedby the full-matrix least-squares based on F2 usingSHELXL-97.59 The refinement was performed usinggeometrical constrains for the CF3SO3

K ions and the acetonemolecules. To further optimize the content of the cavities,the SQUEEZE function of the program PLATON60 wasused. A residual electron density of 23 electrons/cell wasfound in the remaining voids (11% of cell volume). Arefinement using reflections modified by the SQUEEZEprogram fit well and the R-factor was substantially reducedfrom 0.123 to 0.119. The 23-electrons count per cell was notincluded in the calculated density and in F(000). In the finalrefinement, all non-hydrogen atoms (except those of thedisordered CF3SO3

K anions and the acetone molecules)were treated anisotropically (using the card SIMU torestrain the anisotropic thermal parameters) and thehydrogen atoms were included at calculated positions withisotropic U factorsZ1.2Ueq. Essential crystal and refine-ment data are given in Table 2. Crystallographic data(excluding structure factors) for the structures reported inthis paper have been deposited with the CambridgeCrystallographic Data Centre (CCDC) as supplementarypublication no. CCDC 267407. Copies of the data can beobtained, free of charge, on application to the CCDC, 12union Road, Cambridge CB2 1EZ UK (fax: C44 1223 336033; e-mail:deposit@ccdc.cam.ac.uk).

Acknowledgements

F.D. and D.B. acknowledge the financial support by the SwissNational Science Foundation and the NCCR ‘Nanoscience’.E.D. and L.P. acknowledge the financial support by MURSTthrough FIRB ‘Nanoorganization of materials with magneticand optical properties’. Use was made of instrumentalfacilities at the Centro Interfacolta di Misure G. Casnati ofthe University of Parma. S.G. thanks Italian MIUR, FIRBRBAU01HAAA, for generous financial support.

Supplementary data

Supplementary data associated with this article can be found,in the online version, at doi:10.1016/j.tet.2005.06.122.

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