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J. Phys. Chem. B 2008, 112, 11064–11070
Multiple Equilibria in the Complexation of Dibenzylimidazolium Bromide Salts by Cyclodextrins: Toward Controlled Self-Assembly Loı¨c Leclercq and Andreea R. Schmitzer* Department of Chemistry, UniVersite´ de Montre´al, C.P. 6128 Succursale Centre-Ville, Montre´al, Que´bec, H3C 3J7, Canada ReceiVed: April 29, 2008; ReVised Manuscript ReceiVed: June 6, 2008
In aqueous solution, dibenzylimidazolium bromide salts form dimeric assemblies by T-stacking between an acidic proton of the imidazolium and the benzyl aromatic ring of another cation. This dimeric association can be disturbed by the addition of native cyclodextrins. The control of the majority species in solution can be made by the judicious choice of cyclodextrin concentration and its macrocycle size. The dimer is complexed directly by β-cyclodextrin, whereas in the presence of R-cyclodextrin, the dimer is dissociated to form a 1:1 inclusion complex; at higher concentration, this 1:1 complex can dimerize. Introduction The development of novel building blocks that are capable of self-assembly is an important topic in supramolecular chemistry today. Molecular recognition-directed self-assembly and self-organization can lead to the formation of highly complex and fascinating structures with new and interesting properties.1 In contrast to more traditional covalent systems, the extent of supramolecular aggregation and hence the properties resulting thereof can be externally controlled.2 However, these processes are still poorly understood. Various 1,3-dialkylimidazolium salts containing a wide variety of anions have been synthesized and are currently receiving a great deal of attention as novel media in organic synthesis and catalysis and in the preparation of nanostructured materials.3 In fact, imidazolium salts adopt self-organized structures mainly through H-bonds that induce structural directionality, contrary to classical salts in which the aggregates are mainly formed through electrostatic interactions.4 Further supramolecular interactions developed in imidazolium-based ionic liquids are π-stacking interactions, aliphatic interactions, and dipolarizability/polarizability.5 We have recently designed and synthesized imidazolium salts to maximize the stacking interaction by the introduction of benzyl groups on the imidazolium moiety. In this way, we demonstrated that stacking interactions can be maximized in the aggregation process of N,N′-dibenzylimidazolium bromide in aqueous solution (Figure 1).6 In this paper, we report on the influence of cyclodextrins on the dimerization process of dibenzylimidazolium bromide in aqueous solution. Results and Discussion Theoretical Consideration. N.B. For the following discussion, the dibenzylimidazolium bromide salt is represented by I. The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been extensively studied in solution.7 Evidence for direct hydrogen bonds between the imidazolium cation and the tetrafluoroborate anion was brought by NMR investigations with 1-ethyl-3-methylimidazolium and 1-butyl-3-methylimidazolium tetraborate in acetoni* E-mail:
[email protected].
trile.7 The association process of these ILs was also reported as {[(I)x(X)x-n]n+and [(I)x-n(X)x]n-} loosely bound IL aggregates that can be detected and isolated via mass-selection.8 Dibenzylimidazolium halide ILs present an increased cohesion in the solid state (X-ray diffraction analysis) and selfassemble in dimers by aromatic T-stacking of the imidazolium salts in polar and nonpolar solutions (1H NMR and HRMS).6 The imidazolium salt I presents three binding sites: one electronpoor imidazolium ring (T-stacking donor by the acidic hydrogens) and two electron-rich hydrophobic aromatic rings (Tstacking acceptor or hydrophobic binding site, Figure 2). We have shown that the complementarity between the electron-rich and the electron-poor sites allows the formation in the solid state and in solution of the dimer called I2. To our knowledge, it is the only example of IL dimerization in solution, even if C-H-π interaction between the imidazolium cation and the phenyl rings of the anions were often reported for the formation of supramolecular aggregates.9 This dimer is formed by direct C-H-π interaction between the H4 or H5 proton of the imidazolium ring and the aromatic ring of a second molecule, and no higher aggregates were detected even at high concentration. The dimerization process has been characterized in aqueous solution with a dimerization constant of 790 M-1 at 25 °C.6 At 10 mM initial concentration of I ([I]0), the majority species at 25 °C is I2 (around 64%). CDs are cyclic water-soluble oligosaccharides forming a truncated cone with hydrophilic annulus due to the primary and the secondary hydroxyl groups of the glucose units that face the exterior ends of the molecule (Figure 2).10,11 Moreover, the CDs present a cavity with a largely hydrophobic surface. This cavity provides a favorable host potential to form inclusion complexes with the free hydrophobic aromatic residues of the dibenzylimidazolium halides. Recently, the inclusion behaviors of β-cyclodextrin (β-CD) and hydrophobic 1-dodecyl-3-methyl imidazolium hexafluorophosphate (C12-mimPF6)12 and 1-butyl3-methylimidazolium (bmimPF6)13 in aqueous solution have been studied. The results indicated that β-CD and C12-mimPF6 can form inclusion complexes with molecular ratio of 1:1 and 1:2, and β-CD and bmimPF6 can form only 1:1 inclusion complexes. However, in these examples, there was no dimerization of the imidazolium salt. In the case of dibenzylimida-
10.1021/jp803760d CCC: $40.75 2008 American Chemical Society Published on Web 08/13/2008
Complexation of Dibenzylimidazolium Bromide Salts
J. Phys. Chem. B, Vol. 112, No. 35, 2008 11065
Figure 1. Structure of the dibenzylimidazolium bromide water dimer obtained by PM3 (restricted shell) in accordance with the crystalline structure (see CCDC 665763) and the conventional representation adopted in this study.6
SCHEME 3: Equilibrium Distributions for the Formation of I2 · CD2 from I2 (See Theoretical Considerations) in the Presence of (a) r-CD and (b) β-CD
Figure 2. Structure of I, cyclodextrins (CDs), used in this work and their schematic representations.
SCHEME 1: Possible Equilibrium Involved in the Complexation of I by CDsa
a
Kass represents the association constant of I species.
SCHEME 2: Possible Equilibrium Involved in the Complexation of I by CDsa
dim a Kass and Kass represent, respectively, the association constant of I and the association constant of I2 species with CDs; Kdim represents the dimerization constant relatively to I species.
zolium halides, more complex equilibria occur, because the dimerization can interfere with the inclusion process. From a theoretical point of view, if we consider that only the monomer of I exists in water, CD can form two complexes with one or both hydrophobic binding sites (I · CD and/or I · CD2). In this case, the complexation pathway involves two equilibria: (i) between the first binding site of I and a CD and (ii) between the free binding site of I · CD with a second CD, where Kass1 and Kass2 represent, respectively, the first and the second association constants with I monomer (Scheme 1). However, I and I2 coexist in aqueous solution because of the dimerization of I (where Kdim1 represents the dimerization constant). and both I and I2 can be complexed by the CDs (where Kass and dim Kass represent, respectively, the association constant of I and association constant of I2 dimer). In this case, multiple equilibrium exists for the formation of inclusion complexes between CDs and dim dim I, where Kass1, Kass2, Kass1 , and Kass2 represent the association dim2 dim3 constants and K and K represent the dimerization constants of I · CD to form the intermediate I2 · CD complex and I2 · CD2, respectively (Scheme 2).
Because of the cyclic nature of these equilibriums, all constants (except Kass2) depend on five others; that is, Kdim1 dim dim depends on Kass1, Kass1 , Kass2 , Kdim2, and Kdim3, and similar derivations could be made for other constants. It is important to notice that the direct dimerization of I · CD2 is impossible because the recognition sites required for the dimerization process (i.e., the aromatic residues) are hidden in the presence of CDs. All species are in equilibrium, and each species can exist in an aqueous solution of I and CD. The behavior of such a complex system is difficult to predict because of the various equilibria. The influence of r- or β-CDs on the dimerization process of I was studied. The theoretical dependence of the equilibrium constants could be predicted but with difficulty; therefore, an experimental approach was considered: (i) a study using ESI/ HRMS to identify the majority complexes at equilibrium for various [CD]0/[I]0 ratio, (ii) a study in aqueous solution by 1H NMR and UV spectroscopy for various [CD]0/[I]0 mixtures, and (iii) a determination of the geometries of the complexes by 2D NOESY NMR experiments and semiempirical PM3 calculations. The possibility of control in the majority species in aqueous solution will be presented as the conclusion of the present study. The high-resolution mass spectrometry (HRMS) study was carried out in order to confirm the presence of the various supramolecular complexes foreseen by theoretical considerations. HRMS was used with electrospray ionization (ESI), in positive mode, in order to fish loosely bonded supramolecules of I and I2 with CDs in solution and transfer them to a mass spectrometer to investigate their assemblies. ESI-HRMS has become the principal technique for studying salt cluster ion formation and properties.14 The positive ions are formed in solution and then transferred by ESI directly to the gas phase. ESI is characterized by the gentleness by which the gaseous ions are formed, being able to transfer to the gas phase very labile, loosely bonded supramolecules. Previously, this technique has already been used to investigate the formation of I2 in aqueous solution.6 Analyses were performed at various [CD]0/ [I]0 ratio. For each CD investigated here, the concentration of
11066 J. Phys. Chem. B, Vol. 112, No. 35, 2008
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TABLE 1: Comparison between the Observed m/z values (m/zobs) and the Calculated m/z (m/zcalc) for three [r-CD]0/[I]0 Ratios (Obtained by ESI/HRMS) [r-CD]0/[I]0 ratio 1/3
1 Int.a
attributed species
m/zcalc
m/zobs
(I · r-CD-H-Br)+ (I · r-CD2-H-Br)+ (I2 · r-CD-2H-2Br)2+ (I2 · r-CD2-2H-2Br)2+
1102.4639 2074.7805 676.3054 1162.9654
1102.4912 n.d. b n.d. b n.d. b
+ -
2
m/zobs
Int.a
m/zobs
Int.a
1102.4217 2074.7367 n.d b 1163.0234
++ + +
1102.4371 n.d.b n.d.b 1162.6742
+ ++
a The observed intensity is reported in the following relative convention: -, species not detected (relative intensity
