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Inclusion complex formation of diferrocenyldimethylsilanewith b-cyclodextrin

Jose A. Fernandes a , Sergio Lima a , Susana S. Braga a , Paulo Ribeiro-Claro a, *,Jose E. Rodriguez-Borges b , Catia Teixeira b , Martyn Pillinger a ,

Jose J.C. Teixeira-Dias a , Isabel S. Goncalves a, *a Department of Chemistry, CICECO, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal

b CIQ, Department of Chemistry, University of Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received 28 April 2005; received in revised form 19 July 2005; accepted 20 July 2005

Available online 22 September 2005

Abstract

An inclusion compound comprising b-cyclodextrin ( b-CD) and diferrocenyldimethylsilane, FcSiMe 2 Fc [Fc = ( g 5-C5H5)Fe(g 5-C5H4)], has been prepared and characterized in the solid state by powder X-ray diffraction (XRD), thermogravimetric analysis(TGA) and magic angle spinning (MAS) NMR spectroscopy ( 13 C, 29Si). Elemental analysis indicated that the host:guest molar ratioin the product was approximately 1.5. Ab initio calculations in vacuo were carried out in order to investigate the possible inclusionmodes. The rst structure, designated form 2a , was determined assuming a barrel-type conformation, with parallel head-to-headcyclodextrins. A second V-shaped complex ( 2b) was also found by allowing variation of the angle between the cyclodextrins. Form2a was assumed to be the best approximation to the real structure, since this geometrical arrangement favors the formation of achannel-type packing motif. 2005 Elsevier B.V. All rights reserved.

Keywords: b-Cyclodextrin; Inclusion compounds; Hostguest systems; Oligo(ferrocenes); Ferrocene; Ab initio calculations

1. Introduction

Cyclodextrins (CDs) are cyclic oligosaccharides com-prising six (a -CD), seven (b-CD), eight ( c-CD) and moreD-glucose units linked by a -(1 ! 4) glycosidic bonds [1 3]. Their shape is like a hollow truncated cone. A widerange of organic molecules, inorganic ions and metal-

lo-organic species form inclusion complexes with CDs[35]. Suitable guests include transition metal complexesand organometallic compounds bearing hydrophobicligands such as cyclopentadienyl (Cp = g 5-C5H 5) andg 6-arene groups [57]. With these ligands, the weakercategories of non-covalent bonding, such as van der

Waals and charge transfer interactions, assume consid-erable importance. Encapsulated metallo-organic com-plexes often exhibit markedly different physical andchemical characteristics compared to the bulk material[8], for example, in their non-linear optical [9], photo-physical [10] and electrochemical properties [11], andligand substitution/insertion reactions [12]. Cyclodex-

trins are known to bind ferrocene and its derivatives[11b,11c,11d,11e,13] , metallocene dihalides [14], aro-matic ruthenium complexes [15], mixed sandwich com-plexes such as [(g 5-C5H 5)Fe(g 6-C6H6)](PF6) [16], andvarious half-sandwich metal carbonyl and cyanocomplexes [17]. Inclusion compounds containing 4-ferro-cenylpyridine methyltrioxorhenium and ferrocenyldi-imine metal carbonyl complexes have also beenreported [18]. The difficulty in obtaining high qualitysingle crystals suitable for X-ray diffraction (XRD) has

0022-328X/$ - see front matter 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.jorganchem.2005.07.073

* Corresponding authors. Tel.: +351 234 378190; fax: +351 234370084.

E-mail address: igoncalves@dq.ua.pt (I.S. Goncalves).

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meant that only a few solid-state structures have beenfully determined for CD inclusion compounds contain-ing metallo-organic complexes. Nevertheless, full char-acterization using other methods such as powder XRDand NMR can provide important structural information[1921]. Molecular modeling calculations also help to

investigate hostguest interactions and predict possiblebinding modes [2127]. Herein, we present an experi-mental and theoretical study of the encapsulation of diferrocenyldimethylsilane by b-cyclodextrin. Diferroce-nyldimethylsilane is the rst member of a series of linearoligo(ferrocenyldimethylsilanes) with the general for-mula FcSiMe 2[(g 5-C5H4)Fe(g 5-C5H 4SiMe2)]n Fc [Fc =(g 5-C5H5)Fe(g 5-C5H4), n = 07], which can be preparedby the anionic ring-opening oligomerization of thesilicon-bridged [1]ferrocenophane Fe( g 5-C5H4)2SiMe2[28]. These oligo(ferrocenes) have attracted attentionwith respect to their electrochemical, electronic andmagnetic properties, and they are also models for thecorresponding poly(ferrocenylsilane) high polymers [29].

2. Experimental

2.1. Materials and methods

All air-sensitive reactions and manipulations wereperformed using standard Schlenk techniques underan oxygen-free and water-free argon atmosphere. Sol-vents were dried by standard procedures ( n-hexaneover Na/benzophenone ketyl and CH 2Cl2 over

CaH 2), distilled under argon and kept over 4 A molec-ular sieves. b-CD (Fluka) and ferrocene (Aldrich) wereobtained from commercial sources and used as re-ceived. Monolithioferrocene was prepared accordingto the literature procedure [30].

Microanalyses for CHN were performed at theITQB, Oeiras, Portugal (by C. Almeida), and Fe wasdetermined by ICP-OES at the Central Laboratoryfor Analysis, University of Aveiro (by E. Soares). Pow-der XRD data were collected on a Philips X pert dif-fractometer using Cu K a radiation ltered by Ni(k = 1.5418 A ). TGA studies were performed using aMettler TA3000 system at a heating rate of 5 K min 1

under a static atmosphere of air. Infrared spectra wererecorded on a Unican Mattson Model 7000 FT-IRspectrophotometer using KBr pellets. 1H NMR spectrawere measured in solution using a Bruker CXP 300spectrometer. Solid-state 13 C and 29 Si CP MAS NMRspectra were recorded at 100.62 and 79.49 MHz,respectively, on a (9.4 T) Bruker Avance MSL 400Pspectrometer. 13 C CP MAS NMR spectra were re-corded with 4.5 l s 1H 90 pulses, 1.0 ms contact time,a spinning rate of 5.0 kHz and 4 s recycle delays. 29 SiCP MAS NMR spectra were recorded with 5 l s 1H 40pulses, 5.0 ms contact time, a spinning rate of 5.0 kHz

and 5 s recycle delays. Chemical shifts are quoted inparts per million from TMS.

2.2. Preparation of (1,1 0-ferrocenediyl)dimethylsilane

A mixture of ferrocene (20 g, 108 mmol), n-butyl-lith-

ium (99 mL, 2.5 M), n-hexane (135 mL) and tetrameth-ylethylenediamine (20 mL) was stirred at 0 C for 16 h.The resulting yellow solid was isolated by ltration andwashed several times with anhydrous n-hexane (31 g,90%). This product (31 g, ca. 97 mmol) was mixed withCl2SiMe2 (10.25 mL) and n-hexane (365 mL), and leftstirring overnight at 0 C. The solvent and the unreacteddichlorodimethylsilane were removed under vacuum toobtain an orange solid. This product was washed severaltimes with anhydrous n-hexane to isolate (1,1 0-ferro-cenediyl)dimethylsilane (15.4 g, 66%).

2.3. Diferrocenyldimethylsilane ( 1 )

Following the literature method [28,31], a mixture of ferrocenylsilane oligomers was obtained from the reac-tion of (1,1 0-ferrocenediyl)dimethylsilane with monolith-ioferrocene. The mixture was fractionated using analumina chromatographic column (90 active, acidic[Activity I] Merck 0.0630.200 mm, 70230 meshASTM) and a 9:1 hexane/dichloromethane mixture asthe mobile phase. Fractions of 250 mL were collected,the rst six containing pure ferrocene. Fractions 719,containing a mixture of ferrocene and the dimer 1, weresubmitted to a second chromatography using neutral

alumina (Typ. 507C Brockmann I, STD grade, Approx.150 Mesh, 58 A) and the same mobile phase to yield2.6 g of pure diferrocenyldimethylsilane ( 1). Fractions2039 of the rst chromatography contained ferrocene,compound 1 and the trimer 1,1 0-bis(ferrocenyldimethyl-silyl)ferrocene, and were separated in another chroma-tography with neutral alumina to yield 0.33 g of 1.The total amount of compound 1 obtained was there-fore 2.93 g.

2.4. Reaction of b-CD with diferrocenyldimethylsilane( b-CD/ 1 )

A solution of diferrocenyldimethylsilane ( 1) (20.0 mg,0.047 mmol) in ethanol (2.0 mL) was added to a solutionof b-CD (122.8 mg, 0.094 mmol) in water (3.0 mL) at60 C. A yellow suspension formed immediately, whichturned paler after stirring for 4 h. The pale yellow solidwas isolated by centrifugation and washed with water(2 5 mL). Yield: 100 mg. Anal. Calc. for 2(C 42 H70 -O35 ) 1.3(C22 SiH24 Fe 2) 17H2O (3132.9): C, 43.17; H,6.60; Fe, 4.63. Found: C, 43.27; H, 7.22; Fe, 4.64. IR(KBr, cm1): m= 3371 s, 2929 m, 1643 m, 1422 m,1371 m, 1334 m, 1304 m, 1248 m, 1203 m, 1157 s,1099 sh, 1080 s, 1053 sh, 1028 vs, 1002 s, 938 m, 861

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m, 842 w, 819 m, 797 m, 756 m, 704 m, 682 w, 668 s, 653w, 608 m, 577 m, 529 m, 503 w, 483 w, 451 m, 356 w.13 CCP MAS NMR: d = 104.0 (b-CD, C 1), 82.3, 80.6 (b-CD, C 4), 72.8 (b-CD, C 2,3,5 ), 68.9 (Cp), 60.8 (b-CD,C6), 1.0, 0.2, 1.0, 1.6 (SiMe2) ppm. 29 Si CP MASNMR: d = 7.0 ppm (SiMe 2).

2.5. Ab initio calculations

Ab initio calculations were carried out using theGaussian 03W program package, running on a personalcomputer [32]. Compound 1 was fully optimized at theB3LYP/LanL2DZ level. Several conformers are possi-ble due to rotation about the C (ring) Si bonds. In addi-tion, the internal rotation of cyclopentadienyl ringsabout the principal axes of the ferrocene units allowsthe existence of eclipsed and staggered conformations.In ferrocene, the eclipsed (D 5h ) form was found to bemore stable than the staggered (D 5d ) form by 2.78 kJmol1 [33]. Molecular mechanics (MM) studies of thedimer 1 were reported by Barlow et al. [34]. For the sakeof comparison, we will use their terminology for the twodihedral angles FeC (ring) SiC(ring) , / and w, which de-ne the relative disposition of the two ferrocene units.The MM calculations performed by Barlow et al. gavethe lowest energy global minimum at / = 36 ,w = 162 and the next minimum 4.2 kJ mol 1 higherat / = w = 70 . The third minimum, 16.3 kJ mol 1

higher than the global minimum, was found at / =w = 167 . The ab initio calculations carried out in thepresent work do not agree with these MM results, con-

cerning both the geometry and the energy of the min-ima. Thus, three different isomers of 1 were found,with an energetic difference of only 1.5 kJ mol 1 and arotational barrier of 16 kJ mol 1 between them(Fig. 1). The pair of conformers with / = 66 andw = 179 correspond to the lowest energy, but theirgeometry is not suitable for inclusion in head-to-headb-CD dimers (which, on the basis of the characterizationdata for the sample b-CD/ 1, are the likely species pres-ent in the inclusion compound). The other conformer,with / = w = 179 (i.e., close to the idealized / =w = 180 conformation) was chosen as the guest forinclusion, given its higher symmetry and low energy dif-ference relative to the absolute minimum (the in-vacuumvalue of ca. 1.5 kJ mol 1 can easily be overcome byintermolecular interactions in the solid).

The models of the inclusion complexes were obtainedby single point scanning calculations, using the two-layerapproximation of Morokuma and co-workers [35], withthe organometallic guest treated at high layer (B3LYP/LanL2DZ) and the cyclodextrin host set as low layer(HF/CEP-4G). The host molecule was created usingthe crystallographic data for a b-CD p-hydroxybenzal-dehyde inclusion complex [36]. Two different b-CD dif-errocenyldimethylsilane geometries were calculated,

based on a head-to-head dimer conguration with a 2:1(host-to-guest) stoichiometry. The optimization throughsingle point calculations was used due to the failure of the built-in optimization procedures to converge to a sta-ble geometry under the ONIOM conditions. The scan-ning was performed on a 30 pm/5 grid for the r, Dh, h,

u and c coordinates (see Fig. 2 for the denition of thesecoordinates). The minimum energy structures weredetermined from quadratic interpolation, using the low-est energy points in the grid. A word of caution isrequired concerning the calculated energy values, as theyare not corrected for basis set superposition error (BSSE)or for zero point vibrational energy. The BSSE correc-tion is not available for ONIOM calculations andfrequency calculations only apply to fully optimizedstructures.

3. Results and discussion

3.1. Synthesis and characterization of theb-CD diferrocenyldimethylsilane inclusion compound

A solution of diferrocenyldimethylsilane ( 1) in etha-nol was added to an aqueous solution of b-CD to forma thin yellow suspension that became paler as the inclu-sion compound was formed. The product ( b-CD/ 1) wasisolated by centrifugation and washed with water.Elemental analysis indicated that the b-CD:difer-rocenyldimethylsilane molar ratio in the product wasapproximately 1.5. Since powder XRD, TGA and 29 Si

Fig. 1. (a) The lowest (/ = 66 and w = 179 ) and (b) second lowest(/ = w = 179 ) energy conformations of 1, as determined by ab initiocalculations.

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MAS NMR data did not reveal the presence of bulk 1 inthe product (see below), the elemental analysis can beexplained by the formation of a 2:1 (host:guest) complex(hereafter referred to as 2) mixed with a small amount of a 1:1 complex, or perhaps even a 2:1 complex with addi-

tional diferrocenyldimethylsilane molecules located atinterstitial positions between the macrocycles. Fig. 3shows the powder XRD patterns for pristine b-CDhydrate, the oligo(ferrocenylsilane) 1 and the sampleb-CD/ 1. The powder XRD pattern of the sampleb-CD/ 1 does not contain peaks corresponding to pris-tine b-CD hydrate or compound 1, and in fact showsseveral new peaks which indicate the formation of anew crystalline phase. This is an initial indication forthe formation of a true inclusion complex [4]. Fig. 3 alsoshows a diffraction pattern calculated using the crystalstructure data for the 1:1 b-CD inclusion compound of ethyl 4-aminobenzoate (benzocaine) [37]. This com-pound exhibits the typical channel-type structure con-sisting of head-to-head dimers of b-CD moleculesstacked along the crystallographic c-axis. b-CD s fre-quently crystallize as face-to-face dimers when includingmoderate-to-large size molecules. Such b-CD dimershave been observed to pack in up to six different modes[1,2,38,39], the most common of which is where the b-CD molecules are stacked along the crystallographic c-axis, in an alternating head-to-head and tail-to-tailchannel mode. Caira [19] showed that, in general, withinan isostructural series of CD inclusion complexes, thegross features of the powder diffraction patterns are

constant, regardless of the nature of the included guest.Although there are substantial differences between theexperimental and simulated patterns shown in Fig. 3,it is evident that both patterns have peaks centered

around 6.1 , 7.2 , 9.9 , 12.0 , 17.6 and 18.7 2h, andwe may therefore tentatively assume channel-type pack-ing for the CD molecules in b-CD/ 1.

TGA was performed on pure b-CD hydrate, the sam-ple b-CD/ 1 and a 2:1 physical mixture of b-CD and dif-errocenyldimethylsilane ( 1) (Fig. 4). The physicalmixture exhibits two separate weight losses which corre-spond to the zones where compound 1 sublimes (160 265 C), and b-CD melts and decomposes (280340 C). The trace for the sample b-CD/ 1 is very differentin that decomposition only begins at about 260 C.There is no identiable step below this temperaturecharacteristic of non-included 1. These results indicatethat the thermal behavior of the greater part of the orga-nometallic species present has been modied by inclu-sion in b-CD [17e]. This conclusion is also supportedby the low temperature thermal behavior. Thus, TGAof b-CD shows a well-dened step from room tempera-ture up to about 120 C, assigned to removal of watermolecules located in the b -CD cavities, and also in theinterstices between the macrocycles (14.2%, 1011 watermolecules per b-CD molecule). The corresponding dehy-dration prole for the sample b-CD/ 1 is different and ex-tends over a much wider range (25150 C), althoughthe weight loss is less (8.5%). The lower water content

Fig. 2. Coordinate system used in the single point scanningcalculations.

Fig. 3. Experimental (ac) and simulated (d) powder XRD patterns of plain b-CD hydrate (a), compound 1 (b), the sample b-CD/ 1 (c), andthe 1:1 complex of b-CD with benzocaine [37](guest molecules omittedfrom the simulation for simplicity).

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is in reasonable agreement with elemental analysis, sug-gesting about 8 water molecules per b-CD molecule. Thereduction in the number of water molecules per b-CDmolecule is consistent with at least partial occupation

of the b-CD cavity by the organometallic guest.Fig. 5 shows the solid-state 13 C CP MAS NMR spec-tra of b-CD hydrate, diferrocenyldimethylsilane ( 1) andthe sample b-CD/ 1. The spectrum of b-CD hydrate issimilar to that previously reported and exhibits multipleresonances for each type of carbon atom [40]. This hasbeen mainly correlated with different torsion anglesabout the (1 ! 4) linkages for C 1 and C 4 [40a,40b], andwith torsion angles describing the orientation of the hy-droxyl groups [40c]. The different carbon resonancesare assigned to C 1 (101104 ppm), C 4 (7884 ppm),C2,3,5 (7176 ppm) and C 6 (5765 ppm). The spectrumof compound 1 shows single peaks for the Cp ( g 5-C5H5) and methyl groups at about 69 and 1 ppm, respec-tively. Several weaker signals in the range 7077 ppm areassigned to the carbon atoms of substituted cyclopenta-dienyl rings. The multiplicities in the resonances for theb-CD carbon atoms are reduced in the spectrum of b-CD/ 1, giving broader signals with much less structure,if any, thus suggesting a symmetry increase for the b-CDmacrocycle upon inclusion. In contrast, in the region of the methyl group resonances, several overlapping peaksare observed between 2 and 0 ppm. These are attrib-uted to encapsulated diferrocenylsilane molecules thatare in different environments. A single peak is present

at about 69 ppm for the Cp carbon atoms. 29 Si CPMAS NMR spectra were also recorded for the dimer 1and b-CD/ 1 (not shown). Crystalline 1 exhibits one peakfor the bridging dimethylsilyl group at about 5.3 ppm.A single slightly broader peak was observed for b-CD/ 1at 7.0 ppm. The shift to higher eld is probably due todifferences in the bulk susceptibility of the compounds 1and b-CD/ 1. The absence of a peak at 5.3 ppm in thespectrum of b-CD/ 1 conrms that the sample did notcontain non-included 1.

3.2. Ab initio calculations

Two different inclusion modes were calculated for theadduct 2 comprising two b-CD molecules and one difer-rocenyldimethylsilane molecule ( Table 1 , Fig. 6). One of these, form 2a , was determined assuming a barrel-typeconformation, with parallel cyclodextrin hosts. The V-shaped form 2b was determined by allowing variation

Fig. 5. Solid-state 13C CP MAS NMR spectra of (a) plain b-CDhydrate, (b) diferrocenyldimethylsilane ( 1), and (c) the sample b-CD/ 1.Spinning sidebands are denoted by asterisks.

Fig. 4. Thermogravimetric proles of plain b-CD hydrate ( ),a 2:1 physical mixture of b-CD and diferrocenyldimethylsilane ( 1)( ) and the sample b-CD/ 1 ().

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of the angle between the cyclodextrins. The calculatedinclusion energies are 100 kJ mol

1 for 2b and 81kJ mol 1 for 2a . These values are comparable with theinclusion energy of ca. 70 kJ mol

1 obtained for the1:1 b-CD ferrocene adduct, using the same calculationscheme (see computational details).

The reliability of the inclusion geometries obtained atdifferent computational levels has recently been assessedby Casadesu s et al. [41], based on the occurrence of unrealistic H H hostguest contacts. Both of the struc-tures 2a and 2b present two H H contacts that are inthe 188200 pm range. Nevertheless, according to Casa-desus et al. [41], we may assume that the calculatedstructures are plausible, since only a few distances arebelow 220 pm. On the other hand, the presence of suchshort contacts shows that a signicantly deeper inclusionof the guest in the CD cavity is not possible.

The presence of intermolecular hydrogen bonds,hostguest, hosthost or guestguest, is known to playan important role in the stabilization of CD inclusioncomplexes [23a,23b,42]. Both of the calculated struc-tures 2a and 2b present some short contact interactionsbetween the hydrogen atoms of the guest cyclopentadie-nyl groups and the oxygen atoms of b-CD. These shortcontacts, which are within the 224235 pm range for 2a ,and within the 266298 pm range for 2b , fall in the ex-pected range for CH O interactions, and are likelyto contribute to the stability of the complex [42].

A different situation arises concerning the intermolec-ular bonds between the two b-CD rims, which are ex-pected to contribute to the stabilization of b-CD

dimers [23a,23b]. For a direct interaction, the O O dis-tance between the two cyclodextrins OCD OCD 0mustbe less than about 350 pm [43]. Both of the structures 2aand 2b present O CD OCD 0 distances above this mini-mum value. The shortest O CD OCD 0 distances arefound for structure 2b 380 pm < OCD OCD 0