Molecular Solid Solutions with Steric Complementary Pairing from the

Dec 16, 2008 - A 1-naphthylmethylammonium n-alkanoate formed molecular solid solutions with six or seven alkanoates due to similarity of their molecul...
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Molecular Solid Solutions with Steric Complementary Pairing from the Binary Mixtures of 1-Naphthylmethylammonium Alkanoates Katsunari Inoue,† Norimitsu Tohnai,† Mikiji Miyata,*,† Akikazu Matsumoto,‡ Takahiro Tani,§ Yuta Goto,§ Seiji Shinkai,§ and Kazuki Sada*,§

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 1072–1076

Department of Material & Life Science, DiVision of AdVanced Science and Biotechnology, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamadaoka, Suita 565-0871, Japan, Department of Applied Chemistry, Graduate School of Engineering, Osaka City UniVersity, and PRESTO, JST, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan, and Department of Chemistry & Biochemistry, Graduate School of Engineering, Kyushu UniVersity, 744 Motooka, Nishii-ku Fukuoka, 819-0395, Japan ReceiVed August 19, 2008

ABSTRACT: We report the formation of molecular solid solutions from the 1:1 binary mixtures of 1-naphthylmethylammonium n-alkanoates with different alkyl chain lengths. Systematic investigation of all the combinations from acetate to nonadecanoate concluded that a 1-naphthylmethylammonium n-alkanoate formed the molecular solid solutions with six or seven alkanoates due to similarity of their molecular structures. X-ray diffractions revealed that the molecular solid solutions had bilayered structures similar to those of pure alkanoates and that the two alkanoates formed steric complementary pairs and distributed randomly in the alkyl bilayers. When the differences of the numbers of the carbon atoms were less than three, the mixtures yielded selectively singlephased molecular solid solutions. When the differences became more than six, the mixtures yielded two-phased eutectic mixtures. Therefore, this size difference of the molecular structures (∆V ) ca. 100 Å3 for six carbon atoms) should be critical to form the molecular solid solutions. The numbers of carbon atoms in the mixed alkanoates play a crucial role in the formation of the binary solid solutions or eutectic mixtures. Introduction Solid solutions are solid-state mixtures of one or more solutes in a crystalline solvent. A typical example is metal alloys, and vast numbers of inorganic solid solutions composed of two or more kinds of atoms or ions have been prepared in the field of metallurgy and investigated in modern inorganic material sciences.1,2 The conditions for formation of the inorganic solid solutions are as follows: similar chemical properties, the same crystal structures, and similar atomic radii with a difference of 15% or less.1 On the other hand, molecular solid solutions composed of two or more kinds of organic molecules are still unpredictable,3 because the organic molecules have a wide variety of crystal structures due to irregular molecular shapes and complicated weak intermolecular interactions.4 Indeed, it is not so easy to find and predict pairs of organic molecules that share crystal structures.3,5-7 Slight modification of the molecular structures often yields different crystal structures. Thus, organic chemists mostly believe that a solution composed of two kinds of organic molecules provides a binary eutectic mixture of two different crystals that are composed of each pure component, not a mixed crystal of the two components except well-defined molecular complexes such as inclusion crystals,8 cocrystals,9 and charge-transfer complexes.10 Recently, we have found that a series of 1-naphthylmethylammonium (NMA) n-alkanoates (n, 1-19) from acetate to triacontanate were isomorphic.11a The alkanoate salts (n) are numbered in accordance with their numbers of the carbon atoms in the alkanoates. As shown in Figure 1, they were constructed by alternative stacking of three segregated layers: a hydrogenbonded (HB) layer, a naphthalene ring (Np) layer, and an alkyl (R) layer. The HB layer acted as an interface between the other * To whom correspondence should be addressed. E-mail: sadatcm@ mail.cstm.kyushu-u.ac.jp (K.S.), [email protected] (M.M.). † Osaka University. ‡ Osaka City University and JST. § Kyushu University.

Figure 1. (a) 1-Naphthylmethylammonium n-alkanoates (i and j). In the numbering scheme of the n-alkanoates, the numbers of the compounds (i and j) just represent their numbers of the carbon atoms (i and j). (b) The crystal structure of 2 and a schematic model of the bilayered structures for the single component salts. Red, blue, and gray circles represent oxygen, nitrogen, and carbon atoms, respectively. In the schematic model, the green oval represents naphthylmethylammonium. Yellow and red beads illustrate the carbonyl carbon and alkyl carbons in the n-alkanoate, respectively.

two and produced a set of these layers with the order of NpHB-R. Head-to-head/tail-to-tail stacking (....//Np-HB-R//R-HBNp//...) yielded the alkyl bilayers sandwiched between the others. The two-dimensional hydrogen bond network between NH and O in the HB layer and π-π/CH-π interaction between the naphthalene rings in the Np layer should contribute to robustness of the segregated layer structure. Elongation of the alkyl groups in the carboxylate anions did not destroy these attractive intermolecular interactions and the robust bilayered structures. It only increased the interlayer distances, d-spacings of the bilayer structures in the X-ray diffraction (XRD) patterns. Therefore, the isomorphic structures of the wide range of NMA salts prompted us to investigate formation of the molecular solid solutions from the binary salts of NMA (i and j in Figure 1). Formation of the solid solutions could be detected easily by similarity of the XRD patterns, and shifts of the d-spacings, and the linear correlation should provide the possible bilayered

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Figure 3. (a) The crystal structure of the solid solution from a 1:1 mixture of 7 and 9 ([7, 9]). (b) The crystal structure and XRD pattern of the solid solutions [7, 9] with [7], [8], and [9] as the references. Red, blue, and gray circles represent oxygen, nitrogen, and carbon atoms, respectively.

Figure 2. XRD patterns of solid solutions. (a) A homogeneous solid solution ([4, 8]) from a mixture of 4 and 8 (1:1), together with the reference single component salts 4, 6, and 8, (b) a eutectic mixture ([6] + [18]) from the mixture of 6 and 18 (1:1), together with the reference single component salts of 6, 12, and 18, (c) a eutectic mixture from the mixture of 10 and 14 (1:1), together with the reference single component salts 10, 12, and 14.

structures of the resulting solid solutions. In this report, we demonstrate systematic investigation of formation of molecular solid solutions from the binary mixtures of NMA salts. Results and Discussion Formation of Binary Solid Solution. We systematically investigated the formation of molecular solid solutions of the binary mixtures of the NMA salts. They were prepared by mixing equimolar mixtures of two aliphatic acids (i, j) and two equivalent NMA in methanol. Theoretically, the solid mixtures from a binary solution mixture are classified into three typical types: (1) a binary solid solution composed of the both components, (2) a eutectic mixture of two single-component salts, mixtures of crystals crystallized separately, and (3) a eutectic mixture of a binary solid solution and two single component salts. X-ray diffraction indicated that many binary salts of NMA form these three types dependent on the length of the alkyl chains. Figure 2a indicates the binary solid solution from the mixture of 4 and 8. The resulting mixture had an XRD pattern similar to the pure salts. In the region of 002 diffraction, a sole peak was observed between those of 4 and 8, not identical to their sum. The d-spacing of the mixed salt (d002 ) 15.0 ( 0.3 Å) at the 002 diffraction was just intermediate between 4 ([4]:12 d002(4) ) 13.2 ( 0.3 Å) and 8 ([8]: d002(8) ) 17.2 ( 0.2 Å). This d-spacing was quite similar to that of 6 ([6]: d002(6) ) 15.2 ( 0.2 Å), indicating that the resulting mixture is the solid solution of 4 and 8 ([4, 8]12). Similarity of the XRD patterns clearly indicated that the solid solution of 4 and 8 had the same

bilayered structures as the single-component salts. The intermediate d-spacing of [4, 8] suggested that the two alkanoates were faced to each other in the alkyl bilayers to form steric complementary pairs. As the second type, an equimolar mixture of 6 and 18 yielded a eutectic mixture, as shown in Figure 2b. The two d002 peaks at 15.1 ( 0.3 Å and 25.6 ( 0.6 Å were assigned to each pure salt ([6]: d002(6) ) 15.2 ( 0.2 Å and [18]: d002(18) ) 26.3 ( 0.7 Å). No other peaks were observed in the same region, indicating that the resulting mixture is simple sum of each component, that is, the eutectic mixture of 6 and 18 ([6] + [18]12). As the last one, the mixed salt from 10 and 14 yielded a mixture of the solid solution ([10, 14]) and the eutectic mixture ([10] + [14]), as shown in Figure 2c. It yielded three diffraction peaks at d002 ) 22.3 ( 0.2 Å, 20.8 ( 0.5 Å, and 19.3 ( 0.3 Å, which were assigned to the pure salt of 14 ([14]: d002(14) )22.5 ( 0.5 Å), the solid solution [10, 14] similar to 12 ([12]: d002(12) ) 20.8 ( 0.4 Å), and the pure salt of 10 ([10]: d002(10) ) 19.0 ( 0.3 Å), respectively. In order to verify the similarity of the crystal structures of the molecular solid solutions and the single-component salts, we tried to prepare single crystals of the solid solution. Figure 3 shows the crystal structure from a 1:1 mixture of 7 and 9 and the XRD pattern together with those of the corresponding singlecomponent salts ([7], [8], and [9]). The d002-spacing of the resulting solid (d002 ) 17.2 ( 0.5 Å) in the XRD pattern agreed with half of the crystallographic c-axis (c/2 ) 16.9 Å), and was intermediate between [7] (d002(7) ) 16.3 ( 0.5 Å) and [9] (d002(9) ) 17.9 ( 0.5 Å). Thus, the single crystal is likely a solid solution of 7 and 9. The crystal structure showed the bilayered structure was identical to that of the single-component salts.11a The 1-naphthylmethylammonium cation, the carboxylate group, and the methylene groups near the carboxylate group could be found in the Fourier map, but we could not produce the model for the terminal methyl and a few adjacent methylene groups. The terminal of the alkyl group would be severely disordered, and this supported a random arrangement of the alkyl groups. The d-spacing thus far suggests that the alkyl groups formed heteropair (7-9) rather than homopairs, 7-7 and 9-9 of the two different carboxylic acids in the alkyl bilayer by steric

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Table 1. Formation of Binary Solid Solution from 2 to 19a

a

S denotes solid solutions, E is eutectic crystals, S+E is the mixture of solid solutions and eutectic crystals.

mixtures. The other four intermediate salts 3, 4, 14, and 15 yielded the mixtures of the solid solutions and the eutectic mixtures of the single component salts.

Figure 4. Three types of the crystalline mixtures from the 1:1 binary mixtures of NMA alkanoates (i and j) (a) a molecular solid solution, (b) a eutectic mixture, and (c) a mixture of eutectic mixture and the solid solution.

complementary. The steric complementary pairing provided the closest packing of the alkyl bilayered, and made the bilayer structure flat to contribute to both the two-dimensional hydrogen bond network among the ion-pairs and CH-π/π-π interactions between the naphthalene rings.11 All the binary mixtures from acetic acids (2) to nonadecanoic acids (19) could be classified as the above three types as shown in Table 1. Classification of the resulting mixtures is summarized in Figure 4. As an example, when the salt 10 was used as one of the two components (i ) 10), the molecular solid solutions [10, j] formed in the mixtures of eight salts 5, 6, 7, 8, 9, 11, 12, and 13, and the completely eutectic mixtures ([10 + j]) formed in the mixture of five salts, 2, 16, 17, 18, and 19. The longer alkanoates than 19 were expected to form the latter segregated

Increase of the number of carbon atoms (i) of one component i did not expand the ranges of the other components (j) that form the solid solutions, and only shifted them to longer alkyl chains. Thus, the crystallization behaviors were controlled by the difference of the carbon numbers of the mixed components (∆ ) j - i), as shown in Figure 4. When the differences in the length between the two alkanoates were less than three (∆ e 3), the mixtures yielded solid solutions with single phase (type 1). During evaporation, they did not crystallize separately from the solution mixture. When they were more than six (∆ g 6), they yielded eutectic mixtures with two phases (type 2). Otherwise, they formed mixtures of the eutectic crystals and the solid solutions with three phases (type 3). Thus, the critical difference for forming the solid solutions was three or four carbon atoms between the mixed salts, and that for forming the eutectic mixtures was five or six. Thus the alkanotes could form solid solutions with neighboring six or seven alkanoates. Since these binary solid solutions and all the single-component salts have similar bilayered structures with an identical twodimensional hydrogen bond network between NH and O and π-π/CH-π interactions, their lattice energies depend only on two-dimensional packing of the alkyl groups in the alkyl bilayers. The similar alkyl chain lengths should have similar lattice energies, which enables them to form the solid solutions that are composed of two randomly arranged alkyl groups due to the gain of mixing entropy. On the other hand, a difference in alkyl chain length should have differences in the lattice energies over the mixing entropy, which induces the segregated crystallization to form eutectic mixtures. The volume differences of about ∆V ) ca. 100 Å3 for six carbon atoms should be critical

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Figure 6. Changes of XRD patterns from the mixed NMA salts as a function of mole fractions. (a) Mixtures of 3 and 7, x shows the molar fraction of 7 as a function of 3. (b) Mixtures of 3 and 13, x shows the molar fraction of 13 as a function of 3.

Figure 5. Formation of the molecular solid solutions from the mixtures of 6 and 8, as a function of mole fractions; (a) 002 diffraction peaks in XRD patterns, (b) the plots for d002 spacings versus molar fraction of 8, and (c) melting points in the DSC.

for formation of molecular solid solutions. Generally, such differences should provoke the change in the crystal structures themselves. Effect of Mole Fractions of Mixed Salts for the Formation of Solid Solution. We next investigated effects of mole fractions for the formation of binary solid solutions. Inorganic solid solutions are well-known to depend on Vegard’s law.1-3,13 Figure 5a shows the change in the XRD patterns of the mixture of 6 and 8 as a function of their mole fractions, and Figure 5b shows the plot of the d002 as a function of the mole fraction of 8. All the XRD patterns were similar to each other and the diffraction peaks shifted continually from the lower angle, and the d-spacings increased gradually with increasing the mole fractions. The relationship between them exhibited a good linear correlation (R2 ) 0.985), and followed Vegard’s rule. Moreover, differential scanning calorimetry (DSC) measurements in Figure 5c showed the change in the melting point as a function of the molar fractions, which showed typical thermal behavior for the substituted alloy.2 These results indicate that all the mixtures of 6 and 8 with variable ratios formed continuous solid solutions. Other binary mixtures such as 5/7 and 7/8 showed similar behavior. It is noteworthy that all the binary mixtures from the salts with even-even, odd-odd and even-odd numbers yielded continuous solid solutions. This result is in good contrast with the results for solid solutions of n-alkanes.5 On the other hand, XRD patterns from a mixture of 3 and 7 with varying mole fractions are shown in Figure 6a. When the mole fraction of 3 was lower than 0.5, the XRD patterns had two peaks assigned to 7 and the solid solution of [3, 7]. Increasing the 3 fraction increased the intensity of the peak of the latter solid solution and decreased that of 7. At a nearly 1:1 ratio, the solid solution [3, 7] formed predominantly. After the mole fraction of 3 became larger than 0.5, the diffraction peaks were gradually shifted to higher angles and became broader. This indicates the formation of the continuous solid solution and a decrease in crystallinity of the resulting mixture. Moreover, in the case of a mixture of 3 and 13 (Figure 6b), the positions of all the shaped peaks assigned to [3] and [13] remained unchanged and the relative intensities only changed as a change in the mole fraction, which indicates the eutectic mixture of 3 and 13. Therefore, the formation of solid

solutions depends on the differences in the number of carbon atoms between the mixed salts. Conclusion We have thus demonstrated formation of molecular solid solutions from the binary mixtures of the 1-NMA salts. The formation of the solid solutions should be controlled by the similarity of molecular structures within four or five carbon atoms, although they all have similar crystal structures with identical hydrogen-bond networks and π-π/CH-π interaction. We believe that this difference in the steric dimensions is critical to form the solid solutions. This observation agrees with the fact that the crystal structures are often changed by the small modification of molecular structures. Experimental Section General. All chemicals and solvents used in this study were commercially available and were used without purification. Powder X-ray diffraction (XRD) data were collected on a Rigaku RINT-1100 using graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å) at room temperature. DSC data were collected on a Rigaku instruments DSC 8230 under a N2 flow. Preparation of Solid Solutions. Mixtures of two or more alkanoic acids were treated with 1-naphthylmethylamine in methanol and the removal of the solvent by an evaporator yielded a white solid. X-ray Crystallographic study. X-ray diffraction data were collected on a Rigaku R-AXIS RAPID diffractometer with a 2D area detector using graphite-monochromatized Cu KR radiation (λ ) 1.54178 Å). Lattice parameters were obtained by a least-squares analysis from the reflections for three oscillation images. Direct methods (SIR92) were used for the structure solutions. The structures were refined by a full matrix least-squares procedure using all the observed reflections based on F2. All non-hydrogen atoms were refined with anisotropic displacement parameters. All calculations were performed using the TEXSAN14 crystallographic software package. The crystal data for the solid solution from 7 and 9 was as follows: C38N2O4H54, M ) 602.86, triclinic, a ) 4.922(1) Å, b ) 10.735(3) Å, c ) 33.888(9) Å, R ) 87,80(3)°, β ) 85.85(2)°, γ ) 89.97(3)°, V ) 1784.7(9) Å3, temperature 300 K, space group P1j (no. 2), Z ) 2, µ(Cu KR) ) 5.63 cm-1, Dc ) 1.122 g cm-3. There were 5823 unique reflections, and 5823 observed reflections with |F0|2 > 3σ|F0| were used for further calculations after Lorenz and polarization corrections. The final R1 and Rw values were 0.139 and 0.376, respectively. The crystal data were deposited at the Cambridge Crystallographic Data Center, and the deposition number is CCDC 2883836.

Acknowledgment. Financial support for this research was provided by the Ministry of Education, Culture, Sports, Science and Technology of Japan for K.S. and S.S. K.I. expresses his special thanks for the JSPS Research Fellowships for Young Scientists and the 21COE project “Creation of Integrated

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EcoChemistry” of Osaka University. T.T. and S.S. thank the 21COE project “Functional Innovation of Molecular Informatics” of Kyushu University for final support. A part of this work was supported by the “Nanotechnology Support Project” of MEXT. Supporting Information Available: Crystallographic information file (CIF) for the solid solution of 7 and 9 and 002 diffraction peaks from the mixtures of 5 and 7, and 7 and 8 as a function of mole fractions. This material is available free of charge via the Internet at http://pubs.acs.org.

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