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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.49 n.3 Concepción set. 2004

http://dx.doi.org/10.4067/S0717-97072004000300010 

 

J. Chil. Chem. Soc., 49, N 3 (2004): 241-243

INCLUSION COMPOUNDS OF α - AND γ -CYCLODEXTRINS WITH n-ALKYL AMINE (n= 12, 18)

 

PAUL JARA, XIMENA CAÑETE, VLADIMIR LAVAYEN AND NICOLÁS YUTRONIC*

1 Departamento de Química, Facultad de Ciencias, Universidad de Chile, Santiago, Chile, E-mail: nyutroni@uchile.cl.

(Received: January 15, 2004 - Accepted: April 30, 2004)

(Dedicated to my friend and colleague Dr. David Carrillo for the occasion of this 65TH birthday)


SUMMARY

We report the synthesis of - and -ciclodextrin inclusion compounds with dodecylamine (DDA), and octadecylamine (ODA). Elemental analysis, 13C CP- MAS NMR spectroscopy, scanning electron microscopy (SEM) and powder X-ray diffraction analysis confirm the inclusion process. The basic host structure of the products is similar to that of typical cyclodextrin inclusion compounds. Depending on the guest's molecular length they could present two possible conformations. The extended linear (zig-zag) conformation occurs when the space periodicity along the prism axis, calculated from spacing of these crystallographic layer lines, is comparable to the predicted length of the guest (DDA guest). When the predicted lengths of the guest (ODA guest) are longer than the periodicity matrix layer lines, a helicoidal guest conformation is predicted.

Keywords: Cyclodextrins, Inclusion compounds, amines.


 

INTRODUCTION

-, β- or -cyclodextrins (-CD, β-CD or -CD) are cyclic oligosaccharides consisting of 6, 7 and 8 glucopyranose units, respectively. These compounds have relatively rigid doughnut-shaped structures, and constitute very useful monomolecular hosts in supramolecular chemistry.1, 2 Molecules of adequate size, shape and suitable functionality can be held within the cavity of a particular cyclodextrin. The inclusion complexation occurs when the electrostatic, van der Waals, hydrophobic interactions and hydrogen bonding3 are cooperative. The cyclodextrins and their inclusion compounds can be crystallized from water and studied by X-ray crystallography as empty molecules or as host-guest complexes. "Channel" or "cage" structures are formed depending on the size and ionic or molecular character of the substrate, in which the cyclodextrin molecules are stacked.2, 4-6

The cyclodextrin inclusion compounds, particularly those leading to supramolecular self-assemblies, continue to be a fascinating topic in modern organic chemistry, as they serve as models for understanding molecular recognition and as precursors for designing novel nanomaterials for electronic and biological applications.7

Previously we reported the synthesis and structural properties of cyclodextrin dialkylamine and bicyclic amine inclusion compounds showing channel structures with a hexagonal lattice. 13C-MAS NMR spectra show different resonance signals for the homologous carbon atom of both dialkylamine branches, providing evidence of the non-symmetric location of the amine in the cyclodextrin matrix channel.8, 9

In this paper we report the syntheses of - and -cyclodextrin solid inclusion compounds with dodecylamine (DDA) and octadecylamine (ODA).

EXPERIMENTAL

Commercially available reagents were used as received. The products were obtained from amine and saturated solutions of cyclodextrins in water at room temperature. The amine: cyclodextrin molar ratios used in the experiments were always somewhat greater than the stoichiometric ratio found for the products. Microcrystals were separated immediately, washed with hot acetone, and dried under vacuum at 50 oC. Cyclodextrin:amine ratios were determined by both elemental microanalysis (Perkin Elmer 240C microanalyzer) and 1H-NMR spectroscopy of dimethyl-d6 sulfoxide solutions. The following stoichiometries were determined: DDA-2-CD, DDA-2-CD, ODA-2-CD and ODA-2-CD. Scanning electron microscopy (SEM) was carried out in a Philips EM100 microscope. Solution 1H and 13C high resolution NMR spectra were recorded on a Bruker AMX-300. The 13C cross-polarization magic angle spinning (CP MAS) NMR spectra were recorded on a Bruker AMX-300 spectrometer at a frequency of 75.432 MHz for 13C. The number of scans varied between 4000 and 5000 with 6.2 μs 90o pulses, 1 ms cross-polarization contact time, 50 ms acquisition time during proton decoupling, and 5 s recycle delay. The polycrystalline powder samples were spun at a frequency of 3.5 kHz using a Bruker CP MAS Probe. The chemical shifts are given relative to tetramethylsilane (TMS), determined via the use of an internal standard. Powder X-ray diffractograms were recorded in the range 2o < 2θ < 30o on a Siemens D-5000 diffractometer using Cu-K radiation (40 KV, 30 mA) and a graphite monochromator (λ = 1.5418 Å). Samples were ground to a fine powder in order to reduce the likelihood of the crystallites exhibiting a preferred orientation. For all the products the diffracrtograms indicated the absence of any crystalline phases other than those of the reported inclusion compound.

RESULTS AND DISCUSSION

Analytical as well as further characterization of the products clearly shows that primary amines CH3(CH2)nNH2 with n = 11, 17 can be accommodated by , and -cyclodextrin matrices to form stable inclusion compounds with channel structures at room temperature which are similar to those obtained from the inclusion of the other guest.8, 9 Fig. 1 shows a schematic representation of the inclusion of ODA in cyclodextrins. Crystal inspection by scanning electron microscopy (SEM) shows thin hexagonal needles. This morphology is consistent with a hexagonal crystalline system (Fig. 2). All peaks in the diffractograms can be indexed on the basis of a hexagonal lattice with parameter values close to a = b 27 ≈, c 16 ≈, = β =90° and = 120 for -cyclodextrin and a = b ≈ 37 Å, c ≈ 16 Å, = β =90 and = 120° for -cyclodextrin8. A typical indexed diffractogram for the ODA-2-CD inclusion compound is shown in Fig. 3. In all inclusion compounds the amine guest occupies two cyclodextrin units. Depending on the guest's molecular length, they could present two possible conformations. The extended linear (zig-zag) conformation occurs when the space periodicity along the prism axis, calculated from the spacing of these crystallographic layer lines, is comparable to the predicted length of the guest (16.3 Å for DDA in extended linear conformation). When the predicted length of the guest is longer than the periodicity matrix layer lines (24.0 Å for ODA in the extended linear conformation) a helicoidal guest conformation is predicted.

Fig. 1.- Schematic representation of the ODA-2CD inclusion compound.


Fig. 2.- Crystal inspection by scanning electron microscopy (SEM) showing the thin hexagonal needles in a) ODA-2gCD and b) g-cyclodextrin.


Fig. 3.- A typical indexed diffractogram for the ODA-2CD inclusion compound.

The cyclodextrin molecules take on the shape of a cone with C2 and C3 hydroxyls groups located around the larger opening and the more reactive C6 hydroxyl aligned around the smaller opening.1-3 The arrangement of the C6 hydroxyl opposite to the hydrogen-bonded C2 and C3 hydroxyls forces the oxygen bonds into close proximity within the cavity, leading to an electron rich, hydrophobic interior. In cyclodextrin inclusion compounds the matrix channel is formed by these cone units which interact through van der Waals forces and are ordered to meet the similar end of neighbouring units (larger opening-larger opening, smaller opening-smaller opening). In the inclusion of primary amines the ­NH2- group of the guest could be located at the extreme boundary of a cyclodextrin unit, in the electron rich space density with the alkyl chain located in the apolar and low electron density zone of the cyclodextrins cavities. The 13C-chemical shifts of the amine carbon atoms inside the cavity of the cyclodextrins are higher than the corresponding ones for the amine in dilute CDCl3 solution, which is consistent with a weaker interaction with the medium in the inclusion compound than in the amine dissolved in a relatively inert solvent (Table I). Similar results have been obtained with urea-secondary alkylamine systems, where the 13C-chemical shifts of the amines inserted in the urea matrix are similar to the corresponding ones for a free amine, or at least for an amine in a weak interaction medium, like an amine dissolved in an inert solvent.10, 11 The highest 13C-chemical shifts correspond to the C11-C12 carbon atoms, probably because the terminal carbon atoms are located between the outer cones of the cyclodextrins.

From the results discussed above it can be concluded that, in presence of primary amines, cyclodextrin forms solid inclusion compounds accommodating the guest in channels that possibly present different conformations depending on the guest's molecular length.

ACKNOWLEDGEMENTS

Research financed by FONDECYT 1010891 and 1040581

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