Layered ω-Substituted Alkylpyridinium Salts with Inorganic Anions: Effects of H-Bonding Patterns on the Layer Thickness Francesco Neve*,† and Oriano Francescangeli‡
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 163-166
Dipartimento di Chimica, Universita` della Calabria, I-87030 Arcavacata di Rende (CS), Italy, and Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio and Istituto Nazionale per la Fisica della Materia, Universita` Politecnica delle Marche, Via Brecce Bianche, I-60131 Ancona, Italy Received March 23, 2004
ABSTRACT: The synthesis and characterization of a carboxy-substituted alkylpyridinium cation with inorganic counterions is reported. Structural evidence points to the formation of a layered structure with mutual recognition of cations through strong O-H‚‚‚O bonding when halides X- (X ) Cl, Br) are chosen. Variation of the anionic component to complex metal ions of [MX4]2- type affords a different layer motif with weaker cation-anion O-H‚‚‚X-M interactions and interdigitation of functionalized alkyl chains. Introduction Alkylpyridinium cations are simple and easily accessible chemicals that still attract much interest due to their amphiphilic character and multiple behavior. Traditional fields of investigation on substituted pyridinium derivatives are those related to their interface activity,1 mesomorphism,2-4 and antibacterial properties,5 while more recent (or resumed) studies deal with their ability to behave as ionic liquids,6 intercalating agents for two-dimensional superconducting materials,7 or soft templates for mesostructured solids.8 It is well-known that in addition to the primary electrostatic attraction, the aggregational behavior of long-chain n-alkylpyridinium (and related cations) with inorganic anions is driven by the segregation of alkyl chains both in the crystal and in a fluid mesophase.9 It is anticipated that positioning functional polar groups at the chain terminus (ω-position) would likely promote different self-assembling properties and diverse crystalline motifs in the resulting hybrid material. As a natural extension of our previous work on the self-organization properties of lower and higher homologues of cation 1 (Scheme 1) with different anions,10-12 we have focused our attention to hydrogen-bonding functions such as carboxylic acid groups with the aim of obtaining a more versatile type of alkylpyridinium cation. Thus, COOHterminated alkyl chains could benefit from the rich structural chemistry of carboxylic acids dominated by H-bonding.13 On the other hand, the presence of a carboxyl function would offer the potential for reactive pyridinium cations,14 thus leading to new types of functionalized ionic liquids.15 Experimental Section General Methods. All solvents were dried just before use. 12-Bromo-dodecanoic acid and PdCl2 were purchased from Aldrich and used as received. Elemental analyses (C, H, N) * To whom correspondence should be addressed. Tel: (+39)0984 492060. Fax: (+39)0984 492044. E-mail:
[email protected]. † Universita ` della Calabria. ‡ Universita ` Politecnica delle Marche.
Scheme 1. Structures of the N-n-Dodecylpyridinium Cation (PY12) and Its COOH Chain-Terminated Analogue (PY11-COOH)
were carried out on a Perkin-Elmer 2400 analyzer. 1H NMR spectra were recorded on a Bruker WH300 spectrometer with tetramethylsilane as the internal standard. FT-IR spectra were recorded on a Perkin-Elmer 2000 spectrophotometer. Differential scanning calorimetry (DSC) measurements were performed with a Perkin-Elmer Pyris-1 calorimeter operating with a scan rate of 5 °C/min under a nitrogen flow. X-ray powder diffraction measurements were performed with a Bruker AXS General Area Detector Diffraction system with monochromatized Cu KR radiation. Preparation of [PY11-COOH]Br ([2]Br). A colorless solution of 12-bromo-dodecanoic acid (1 g, 3.58 mmol) in dry pyridine (10 mL) was heated to reflux under a nitrogen atmosphere for 3 h. The light brown solution was then allowed to cool to room temperature. The white microcrystalline solid which formed was removed by filtration, washed with diethyl ether, and dried in a vacuum. Yield, 1.26 g (98%). Anal. calcd for C17H28BrO2N‚0.3H2O: C, 56.14; H, 7.93; N, 3.85. Found: C, 56.70; H, 7.80; N, 3.90. 1H NMR (CDCl3-CD3OD): δ 9.05 (d, 2 H, py), 8.56 (t, 1 H, py), 8.14 (t, 2 H, py), 4.71 (t, 2 H, CH2N), 2.29 (t, 2 H, CH2COOH), 2.03 (m, 2 H, CH2CH2N), 1.61 (m, 2 H, CH2CH2COOH), 1.40-1.20 (m, 14 H, (CH2)7). A singlet at δ 2.12 can be attributed to the residual water of crystallization. Relative signal integration gives ca. 0.3 water molecules per formula unit. Preparation of [PY11-COOH]Cl ([2]Cl). Exchange of bromide for chloride in [2]Br was obtained by means of freshly prepared AgCl following a published procedure.10 Yield g 92%. Anal. calcd for C17H28ClO2N: C, 65.05; H, 8.99; N, 4.46. Found: C, 64.60; H, 8.89; N, 5.02. NMR features are identical to those of [PY11-COOH]Br, except for the absent water signal. Preparation of [2]2[PdCl4]. A solution of PdCl2 (0.05 g, 0.29 mmol) in CH3CN (15 mL) was added to a solution of [2]Cl (0.18 g, 0.59 mmol) in CH3CN/H2O (15 mL, 12:3 v/v). The resulting brown solution was heated to reflux for 30 min and
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then cooled to room temperature. After a small amount of black byproduct was removed by filtration, the brown filtrate was evaporated to dryness under reduced pressure. The addition of a small amount of CH3CN to the solid residue, followed by filtration and repeated washings with ethanol and diethyl ether, afforded the product as a light brown powdered solid in 73% yield (0.17 g). Anal. calcd for C34H56Cl4N2O4Pd: C, 50.73; H, 7.01; N, 3.48. Found: C, 50.97; H, 7.09; N, 3.66. Preparation of [2]2[PdBr4]. A solution of KBr (0.35 g, 2.9 mmol) in H2O (2 mL) and a suspension of PdCl2 (0.05 g, 0.29 mmol) in CH3CN/H2O (22 mL, 20:2 v/v) were combined, and the resulting red-brown mixture was heated gently for a few minutes. The subsequent addition of [2]Br (0.21 g, 0.58 mmol) then afforded a clear solution, which was heated to reflux for 30 min. After a tiny black byproduct was filtered, slow evaporation in air of the filtrate gave the desired product as a microcrystalline red-brown solid in 83% yield (0.24 g). Anal. calcd for C34H56Br4N2O4Pd: C, 41.55; H, 5.74; N, 2.85. Found: C, 41.96; H, 5.92; N, 2.66.
Neve and Francescangeli
Figure 1. Powder XRD patterns for (a) [2]Cl and (b) [2]Br. (00l) reflections for [2]Cl as assigned by peak indexing.
Results and Discussion Halides of PY11-COOH (2) were prepared by heating dry pyridine and 12-bromododecanoic acid and processing the resulting bromide salt. Both chloride and bromide salts are white, moisture stable microcrystalline solids that dissolve in water or protic solvents. Appreciable solubility is also observed in chlorinated solvents. The halide salts of 2 were investigated for polymorphic behavior through DSC. Both halides revealed no DSC transitions other than the melting to an isotropic liquid. [2]Cl melted at 186 °C (∆H ) 66.6 kJ mol-1), while the melting of [2]Br occurred at a lower temperature (169 °C) with a similar enthalpy value (∆H ) 60.1 kJ mol-1). It is interesting to note that both the corresponding halides of the nonfunctionalized cation 1 are thermotropic liquid crystals,16 revealing a smectic A phase in the temperature ranges of 67-154 ([1]Cl) and 49-124 ([1]Br) °C.17 The introduction of single or multiple H-bonding functions into an amphiphilic ionic liquid crystal has no easily predictable consequences.18 Apart from the present situation (in 2, the H-bonding group and the charged headgroup are far apart), when both the H-bonding and the ionic interactions operate “side by side” in the polar sublayer of lamellar structures (thus concurring to define the interface between polar and apolar regions), the thermotropic behavior may be either stabilized or destabilized (leading to less disordered phases/higher transition temperatures) with respect to comparable only ionic systems.19-21 In our opinion, a comparison with nonionic H-bonded counterparts is much less significant and it cannot be raised for an explanation of the thermal behavior of [2]X. Further characterization of the solid phase of [2]Cl and [2]Br was achieved through powder X-ray diffraction (XRD). Figure 1 shows that a lamellar organization is apparent with (00l) reflections in evidence. For [2]Cl, it was possible to assign (00l) reflections up to the sixth order (Figure 1 and Supporting Information). For this salt, the indexing procedure converged to a monoclinic solution with unit cell parameters a ) 9.521(8) Å, b ) 4.851(3) Å, c ) 22.730(9) Å, β ) 85.196(61)°, and V ) 1046.1 Å-3. Attempts to derive similar parameters for [2]Br gave unreliable multiple solutions. Nevertheless, the main result of the diffractometric characterization of [2]X is that for both solids the observed layer
Figure 2. Powder XRD patterns for (a) [2]2[PdCl4] and (b) [2]2[PdBr4].
d-spacing is around 2.3 nm, a value similar to that found for the monohydrated form of [1]Cl.22 Designing synthetic hybrid materials based on organic cations and inorganic anions may require the chemical and structural variation of both components. In addition to modifications on the organic cation, the availability of an entire library of counterions (from simple halide ions to more synthetically and sterically demanding ones) paves the way for fine-tuning of physicochemical properties (and potential applications) of the hybrid structures. In recent years, ammonium,23-26 imidazolium,27-30 and pyridinium8,10-12,16 salts of metalcontaining [MXm]n- anions have become a widespread subject in this field. Thus, testing the ability of cation 2 to give lamellar structures with [PdX4]2- anions (X ) Cl, Br) was the next step of our study. Salts of [2]2[PdX4] stoichiometry were obtained as microcrystalline solids following a well-established procedure.10 As in the case of 1:1 halides, DSC measurements revealed that these salts are nonmesomorphic, showing a phase diagram with a single, although less stabilized, crystal phase. Melting of [2]2[PdCl4] was observed at 159 °C (∆H ) 86.6 kJ mol-1), while [2]2[PdBr4] melted at 137 °C (∆H ) 77.0 kJ mol-1). XRD data on powder samples were collected at room temperature. Figure 2 shows the XRD patterns of both solids. An intense low-angle reflection is a common feature of the two patterns that can be described as characteristic of a lamellar solid. The pattern of [2]2-
Alkylpyridinium Salts with Inorganic Anions Table 1. Infrared Spectral Dataa of Inorganic Derivatives of Cation 2
a
compound
ν(CdO) (cm-1)
ν(O-H) (cm-1)
[2]Cl [2]Br [2]2[PdCl4] [2]2[PdBr4]
1698s 1708s 1741s 1742s
2900-2300vbr 2850br 3196m 3254m
Crystal Growth & Design, Vol. 5, No. 1, 2005 165 Scheme 2. Schematic Model Structure for the Proposed Mutual Arrangements of Cations and Anions in the Lamellar Solid Phase of Salts Based on Cation 2 and Spherical (X-) or Planar ([PdX4]2-) Anions
KBr pellets.
[PdCl4] has a better resolution, which is reflected in the more accurate assignment of the unit cell parameters. The indexing procedure assigned both salts to the triclinic crystal system with similar cell dimensions (Supporting Information). Noteworthy, with respect to the halides, [2]2[PdX4] salts exhibit a much shorter interlayer distance, with d-spacings being 1.67 nm for [2]2[PdCl4] and 1.63 nm for [2]2[PdBr4]. Because we were not able to grow single crystals of neither the halides nor the metal-containing salts, further structural evidence was sought. Because of the presence of carboxy-terminated chains, the infrared vibrational modes of the COOH group of 2 were used as effective tools to probe the local order within the organic-inorganic solids. In the case of halide salts [2]X, the FT-IR measurements (Table 1) pointed to the presence of strong H-bonding involving both OH and CdO groups of each carboxyl function. This is consistent with the formation of carboxylic acid dimers (through intermolecular O-H‚‚‚O contacts),31 leading to a fair lowering of both ν(CdO) and ν(O-H) frequencies with respect to a free COOH group.32 On the other hand, when the [PdX4]2- anion is present (and irrespective of the M-X halide type), both the Cd O and the O-H stretching vibrations move to higher energies with respect to the simple halide salts (Table 1). In the 2:1 salts, experimental observations agree with free CdO groups and OH groups engaged in weaker H-bonds of the type O-H‚‚‚X.33,34 This suggestion seems to work rather well, as higher O-H stretching frequencies correspond to weaker O-H‚‚‚Br contacts (Table 1). On the ground of the structural evidence gathered, we can therefore attempt to draw a simple model that can likely account for the supramolecular organization of the 1:1 and 2:1 salts of cation 2 with different inorganic anions. In our hypothesis of the crystal packing of [2]X and [2]2[PdX4] salts (X ) Cl, Br), we will refer to a general lamellar structure with alternation of ionic and paraffinic regions along the layer normal. In this picture, the ionic region will comprise both anions and cation headgroups. Within the layer structure of [2]X salts, neighboring cations are laterally packed in a head-to-head arrangement with parallel alkyl chains. Although it is hard to foresee the degree of canting of alkyl chains (and the presence of kinks in the linear chain conformation), chain termini are supposed to mutually recognize along the layer normal through H-bonding with formation of acid dimers. This would result in a nonintercalated layer structure (Scheme 2). Changing the anion type to a palladate [PdX4]2- anion, and therefore imposing a different cation:anion ratio, does affect the layer type through chain relocation. As a result, an intercalated type structure is formed (Scheme 2), wherein the layer thickness is considerably reduced. This layer shrinking
does allow the carboxy-terminated chains of 2 to closely approach the [PdX4]2- anions. In addition to the expected interionic Pd-X‚‚‚H-C interactions,35 the supposed formation of additional Pd-X‚‚‚H-O bonds is perfectly in line with the recognized ability of M-X bonds to participate in moderately strong M-X‚‚‚H bonds.36 Conclusion In conclusion, we have prepared a novel example of alkylpyridinium cation with an H-bonding function in the ω-position. While preparing a new bolaamphiphile that self-assembles according to the characteristics of the anionic counterpart,37 at the same time, we have introduced a remote function that may allow binding of the cation through covalent or noncovalent interactions to ions, molecules, substrates, or surfaces.38 Following the same principle, other types of active functions may be introduced in the organic component and we are planning to explore this opportunity. Acknowledgment. We are indebeted to Prof. Alessandra Crispini for indexing the powder XRD patterns. Supporting Information Available: Indexing parameters of the diffraction patterns (Tables S1-S3) and FT-IR spectra of the reported species. This material is available free of charge via the Internet at http://pubs.acs.org.
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