Preparation of Ammonium-Functionalized Polyhedral Oligomeric

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Preparation of Ammonium-Functionalized Polyhedral Oligomeric Silsesquioxanes with High Proportions of Cagelike Decamer and Their Facile Separation Kenta Imai and Yoshiro Kaneko* Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan S Supporting Information *

ABSTRACT: In this study, we found that ammonium-functionalized polyhedral oligomeric silsesquioxanes (POSSs; 2AmPOSS and Am-POSS) with high proportions of cagelike decamer (T10-POSS; 54% and 45%, estimated from 29Si NMR spectra) could be successfully prepared by the hydrolytic condensation of 3-(2-aminoethylamino)propyltrimethoxysilane and 3amimopropyltrimethoxysilane, respectively, using trifluoromethanesulfonic acid as a catalyst in hydrophobic alcohols such as 1hexanol. In addition, on the one hand, it was also found that 2Am-POSS, a mixture of cagelike octamer (T8-POSS), T10-POSS, and cagelike dodecamer (T12-POSS), could be separated into T8-POSS and a mixture of T10- and T12-POSSs by exploiting their different solubilities in 1-butanol. On the other hand, in the case of Am-POSS, T8-POSS and a mixture of T10- and T12-POSSs could be isolated by treatment with ethanol−methanol mixed solvent and 1-propanol. The XRD patterns of cast films of T8POSS showed many sharp diffraction peaks, indicating crystalline structures, whereas those of T10- and T12-POSSs showed no diffraction peaks, indicating amorphous structures, for both the case of 2Am-POSS and Am-POSS. These results suggest that the difference in crystallinities between the ammonium-functionalized T8-POSS and T10- and T12-POSSs causes their different solubilities in alcohol solvents.

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INTRODUCTION

be employed as precursors for the preparation of various functional POSS derivatives.13−15 Thus, although research into POSSs has spread to various fields, the cagelike octamer (T 8 -POSS) is the most representative structure, and research into the preferential preparation of larger POSSs, such as the cagelike decamer (T10POSS) and the cagelike dodecamer (T12-POSS), is limited. T8POSS is often a crystalline compound. Conversely, although it is known that phenyl-group-containing T10-POSS is crystalline,16 large POSSs are amorphous in some cases.17 Therefore, the development of new transparent materials is expected using larger POSSs. As a few examples of the preparation of larger POSSs, it has been reported that phenyl T10- and T12-POSSs and their derivatives can be prepared by the hydrolytic condensation method16,18 and by rearrangement reaction from T8-POSS.17,19 However, in the case of ammonium

Polyhedral oligomeric silsesquioxanes (POSSs) are organic− inorganic hybrid compounds with precisely regulated cagelike structures, which are generally prepared by the hydrolytic condensation of trifunctional silane compounds, such as organotrialkoxysilanes and organotrichlorosilanes.1−3 POSSs have attracted much attention from both academic and industrial fields,4−7 because they have various interesting characteristics, such as their thermal and chemical stabilities, derived from their siloxane-bond frameworks, and their solubility, due to their regulated cagelike structures and the presence of appropriate organic side-chain groups. In particular, POSSs containing reactive organic groups are more attractive for materials research, because the reactive groups enable covalent bonding of the POSSs to polymers, resulting in the development of polymer-related organic−inorganic hybrid materials.8−11 Among these POSS compounds, ammonium (amine)-functionalized POSSs are relatively easy to prepare from amino-group-containing organotrialkoxysilanes12 and can © 2017 American Chemical Society

Received: January 16, 2017 Published: March 16, 2017 4133

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

Article

Inorganic Chemistry (amine)-functionalized POSSs, although it was recently reported that T10-POSS was preferentially prepared by rearrangement reaction from T8-POSS,20 there have been no reports of the preferential preparation of larger POSSs, such as T10- and T12-POSSs, by hydrolytic condensation of aminefunctionalized organotrialkoxysilanes as starting materials. As a representative example of the preparation of ammoniumfunctionalized POSS, the hydrolytic condensation of 3aminopropyltriethoxysilane using concentrated hydrochloric acid as a catalyst in methanol has been demonstrated;12 however, the compound obtained by this method was only T8POSS. Meanwhile, we have also reported the facile preparation of ammonium-functionalized POSS compounds in higher yield with shorter reaction time by the hydrolytic condensation of 3amimopropyltrimethoxysilane (APTMS),21 3-(2aminoethylamino)propyltrimethoxysilane (AEAPTMS),22 and an APTMS/AEAPTMS mixture22 using the superacid trifluoromethanesulfonic acid (HOTf) as a catalyst and water as the solvent. Incidentally, soluble ladderlike polysilsesquioxanes could be obtained by same procedure using aqueous strong acids, such as hydrochloric acid and nitric acid, as catalysts, while use of an aqueous NaOH gave an insoluble randomstructured polysilsesquioxane.23−26 The aforementioned method using a superacid catalyst could be applied to other types of cationic POSSs.27−30 However, the main product obtained by this method is T8-POSS in all cases. During the aforementioned studies, we continuously investigated the hydrolytic condensation using the same starting materials (APTMS and AEAPTMS) and catalyst (HOTf) with various alcohols as the reaction solvents. We found, coincidentally, that when hydrophobic alcohols such as 1-hexanol were used as solvents, ammonium-functionalized POSSs with high proportions of T10-POSS were obtained. In addition, it was also found that mixtures of ammoniumfunctionalized T8-, T10-, and T12-POSS compounds could be separated into T8-POSS and T10- and T12-POSSs by exploiting their different solubilities in alcohols. Herein, we report the preparation method of these ammonium-functionalized POSSs with high proportions of T10-POSS and methods for their separation into T8-POSS and T10- and T12-POSSs.

Scheme 1. Preparation of 2Am-POSS by Hydrolytic Condensation of AEAPTMS Using HOTf and Water in Various Solvents

substituent groups in 2Am-POSS, 1H NMR measurements were performed. The 1H NMR spectra of 2Am-POSS in D2O showed only four signals, which were assigned to the corresponding side-chain groups in all cases (Figure S1), indicating that the AEAPTMS reagent was not present in the products. To investigate the correlation between the molar ratio of T8POSS/T10-POSS/T12-POSS and the type of solvent, 29Si NMR measurements of 2Am-POSS were performed. We first confirmed the formation of POSS structures. All products showed only sharp signals in the T3 regions at ca. −66.7 to −67.1 ppm for T8-POSS, −68.6 to −69.0 ppm for T10-POSS, and −67.4 to −68.2 ppm and −71.0 to −71.3 ppm for T12POSS (Figure 1), indicating the absence of silanol groups and the formation of POSS structures. In 2Am-POSS obtained using water as the reaction solvent, the molar ratio of T8POSS/T10-POSS/T12-POSS was 74:26:0, which was calculated from the integrated ratio of the signals; that is, T8-POSS was the main product (Figure 1a, Run 1 in Table 1).22 Conversely, when alcohols were used as reaction solvents, the ratio of T10POSS increased. In particular, the use of 1-hexanol resulted in the highest ratio of T10-POSS (T8-POSS/T10-POSS/T12-POSS = 21:54:25) of all the products (Figure 1f, Run 6 in Table 1). Plausible formation mechanisms of T8-POSS (in water) and T10-POSS (in hydrophobic alcohols) as the main products can be suggested as follows. Protonation of the amino groups of AEAPTMS by superacid HOTf in water occurs easily, because dissociation of protons from the superacid is facile, and the generated ammonium cations are remarkably stable due to solvation by water (hydration). Electrostatic repulsion occurs between these ammonium cations during the condensation reaction, resulting in the formation of a compound with long distances between side-chain groups, namely, T8-POSS. Incidentally, the use of such electrostatic repulsion between the cationic ammonium side-chain groups could also be

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RESULTS AND DISCUSSION Preparation and Characterization of POSSs Containing Two Ammonium Groups in Each Repeating Unit. POSSs containing two ammonium groups in each repeating unit (2Am-POSS) were successfully prepared by the hydrolytic condensation of AEAPTMS under the following conditions (Scheme 1). Mixtures of AEAPTMS, HOTf, and water (molar ratio 1:6:5) were stirred in various solvents, namely, water, methanol, ethanol, 1-propanol, 1-butanol, 1-hexanol, and 1octanol, at room temperature for 2 h. The resulting mixtures were heated at ca. 50 °C in open containers, until the solvent completely evaporated. The resulting crude products were continuously maintained at 100 °C for 2 h and then washed with acetone−chloroform mixed solvent (1:1 v/v) at room temperature to obtain various forms of 2Am-POSS. The 2AmPOSSs obtained in all reaction solvents were soluble in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetonitrile, methanol, ethanol, acetone, 1-propanol, 2-propanol, and water (i.e., the obtained solutions were homogeneous and transparent). To confirm the progress of the hydrolytic condensation of AEAPTMS and the structures of the 4134

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

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Inorganic Chemistry

Table 1. Molar Ratios of T8/T10/T12 in 2Am-POSS and AmPOSS molar ratio run

POSS

reaction solvent

T8/T10/T12

1 2 3 4 5 6 7 8 9 10 11 12 13 14

2Am-POSS 2Am-POSS 2Am-POSS 2Am-POSS 2Am-POSS 2Am-POSS 2Am-POSS Am-POSS Am-POSS Am-POSS Am-POSS Am-POSS Am-POSS Am-POSS

water methanol ethanol 1-propanol 1-butanol 1-hexanol 1-octanol water methanol ethanol 1-propanol 1-butanol 1-hexanol 1-octanol

74:26:0 58:34:8 48:39:13 37:47:16 29:48:23 21:54:25 40:37:23 82:18:0 67:26:7 63:30:7 48:38:14 46:42:12 44:45:11 67:33:0

Scheme 2. Preparation of Am-POSS by Hydrolytic Condensation of APTMS Using HOTf and Water in Various Solvents

Figure 1. 29Si NMR spectra in DMSO-d6 at 40 °C of 2Am-POSS obtained using (a) water, (b) methanol, (c) ethanol, (d) 1-propanol, (e) 1-butanol, (f) 1-hexanol, and (g) 1-octanol as the reaction solvents. Chemical shifts were referenced to TMS (δ 0.0).

applicable to the formation of single-structured cyclic tetrasiloxane.31 Conversely, we assume that the stability of the ammonium cations is slightly reduced in hydrophobic alcohols compared with that in water because of weaker solvation, resulting in lower electrostatic repulsion. Consequently, a compound containing a structure with slightly shorter distances between the side-chain groups is preferentially formed, which is probably T10-POSS. Detailed studies of the formation mechanisms are now in progress. Preparation and Characterization of POSSs Containing One Ammonium Group in Each Repeating Unit. POSSs containing one ammonium group in each repeating unit (Am-POSS) were also successfully prepared using almost the same procedure as that used for 2Am-POSS but using APTMS as the starting material (feed molar ratio of APTMS/HOTf/ water = 1:3:5; Scheme 2). The Am-POSSs obtained in all reaction solvents were soluble in DMSO, DMF, methanol, and water (i.e., the obtained solutions were homogeneous and transparent). The 1H NMR spectra of the Am-POSSs indicated

that the APTMS reagent was not present in any of the products (Figure S2). From the 29Si NMR spectra of Am-POSS, we confirmed the formation of POSS structures, that is, all products showed only sharp signals in the T3 regions at ca. −66.3 to −66.7 ppm for T8-POSS, −68.2 to −68.7 ppm for T10-POSS, and −67.5 to −67.9 ppm and −70.5 to −71.0 ppm for T12-POSS (Figure 2). In addition, it was also found that the correlation between the molar ratio of T8-POSS/T10-POSS/T12-POSS and the type of solvent was similar to that of 2Am-POSS; that is, the molar ratio of T8-POSS/T10-POSS/T12-POSS in the case of AmPOSS obtained using water as the reaction solvent was 82:18:0 (T8-POSS was the main product; Figure 2a, Run 8 in Table 1),21 whereas that of Am-POSS obtained using 1-hexanol as the reaction solvent was 44:45:11 (the ratio of T10-POSS was 4135

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

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Inorganic Chemistry

In the case of 2Am-POSS, it was easy to separate a mixture of T8-, T10-, and T12-POSSs into T8-POSS and a mixture of T10and T12-POSSs by exploiting their different solubilities in 1butanol. The detailed procedure was as follows (Chart 1). 1Chart 1. Separation of 2Am-POSS by Exploiting the Different Solubilities of Its Components in 1-Butanol

Butanol was added to the 2Am-POSS obtained using 1-hexanol as the reaction solvent (a mixture of T8-, T10-, and T12-POSSs: Figure 1f, Run 6 in Table 1), and the resulting suspension was stirred at room temperature overnight. Next, it was separated into insoluble (Fraction A in Chart 1) and soluble (Fraction B in Chart 1) parts in 1-butanol by suction filtration. The 29Si NMR spectrum of Fraction A in Chart 1 (1-butanol-insoluble part) showed only signals assignable to T8-POSS (Figure 3a),

Figure 2. 29Si NMR spectra in DMSO-d6 at 40 °C of Am-POSS obtained using (a) water, (b) methanol, (c) ethanol, (d) 1-propanol, (e) 1-butanol, (f) 1-hexanol, and (g) 1-octanol as the reaction solvents. Chemical shifts were referenced to TMS (δ 0.0).

increased; Figure 2f, Run 13 in Table 1). Although the detailed formation mechanism is not yet clear, it is assumed that T10POSS easily forms in hydrophobic alcohols for the same reason as for 2Am-POSS. Separation of Ammonium-Functionalized POSSs. As described above, we clarified the correlation between the molar ratio of T8-POSS/T10-POSS/T12-POSS and the type of reaction solvent, and found that ammonium-functionalized POSSs with high proportions of T10-POSS were formed using hydrophobic alcohols as the reaction solvent. However, the selective preparation of single-structured POSSs could not be achieved by the aforementioned method. Therefore, we investigated the facile separation of the resulting ammoniumfunctionalized POSSs (mixtures of T8-, T10-, and T12-POSSs) by exploiting their different solubilities in various solvents. Incidentally, separation by the column chromatography using appropriate eluents of T8-, T10-, and T12-POSSs containing other side-chain groups has already been reported.32

Figure 3. 29Si NMR spectra in DMSO-d6 at 40 °C of (a) Fraction A (1-butanol-insoluble part) and (b) Fraction B (1-butanol-soluble part) obtained by the operations shown in Chart 1. Chemical shifts were referenced to TMS (δ 0.0).

whereas that of Fraction B in Chart 1 (1-butanol-soluble part) showed signals due to T10- and T12-POSSs (Figure 3b). At this time, a solvent capable of separating T10-POSS and T12-POSS has not yet been found. For the XRD measurements, cast films of Fraction A (1butanol-insoluble part: T8-POSS) and Fraction B (1-butanolsoluble part: T10- and T12-POSSs) in Chart 1 were obtained by drying the corresponding methanol solutions of these POSS compounds spread on glass substrates at room temperature. The XRD pattern of the cast film of T8-POSS showed many sharp diffraction peaks (Figure 4a), indicating a crystalline structure. Conversely, that of T10- and T12-POSSs showed no 4136

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

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Inorganic Chemistry

Chart 2. Separation of Am-POSS by Exploiting the Different Solubilities of Its Components in Ethanol−Methanol Mixed Solvent (3:1 v/v) and 1-Propanol

Figure 4. Powder XRD patterns of films prepared by drop-casting methanol solutions of (a) Fraction A (1-butanol-insoluble part: T8POSS) and (b) Fraction B (1-butanol-soluble part: T10- and T12POSS) obtained by the operations shown in Chart 1. The amount of each product on the glass was ca. 1.0 mg cm−2.

diffraction peaks (Figure 4b), indicating amorphous structures. We presume that the difference in crystallinities between T8POSS and T10- and T12-POSSs causes the difference in their solubilities in 1-butanol. However, the separation of Am-POSS was slightly more complicated. It has recently been reported that a mixture of similar ammonium-functionalized T8- and T10-POSSs (T8POSS is a main product) can be separated by exploiting the different solubilities of its components in acetone.20 However, our sample (Figure 2f, Run 13 in Table 1) could not be separated using acetone probably because of the difference of T8/T10 ratio in the used POSS mixture. Therefore, another procedure for the separation of Am-POSS was investigated (Chart 2). After ethanol−methanol mixed solvent (3:1 v/v) was added to Am-POSS obtained using 1-hexanol as the reaction solvent (a mixture of T8-, T10-, and T12-POSSs: Figure 2f, Run 13 in Table 1) and the resulting suspension was stirred at room temperature for 10 min, it was separated into insoluble and soluble parts in this mixed solvent by suction filtration. The insoluble part in this solvent (Fraction C in Chart 2) contained only T8-POSS (Figure 5a), whereas the soluble part (Fraction D in Chart 2) was a mixture of all components (Figure 5b), which were confirmed by the 29Si NMR spectra. Next, 1propanol was added to the soluble part (Fraction D in Chart 2), and the resulting suspension was separated into insoluble and soluble parts by suction filtration. The 29Si NMR spectrum of the 1-propanol-insoluble part (Fraction E in Chart 2) showed signals arising from T8- and T10-POSSs (Figure 5c), whereas the 1-propanol-soluble part (Fraction F in Chart 2) showed signals arising from T10- and T12-POSSs and no signal due to T8-POSS (Figure 5d). Although it is a two-step operation, the product could be separated to yield T8-POSS as the insoluble part in ethanol−methanol mixed solvent (Fraction C) and T10and T12-POSSs as the soluble part in the mixed solvent and 1propanol (Fraction F). As was the case for 2Am-POSS, separation of T10-POSS and T12-POSS in Am-POSS could not be achieved by using their different solubilities in any solvents at this time. In Am-POSS, the cast films of Fraction C (insoluble part in ethanol−methanol mixed solvent: T8-POSS) and Fraction F (soluble part in ethanol-methanol mixed solvent and 1propanol: T10- and T12-POSSs) from Chart 2 were also prepared from the corresponding methanol solutions, respectively, and XRD measurements of the cast films were performed. Consequently, it was confirmed that T8-POSS had

Figure 5. 29Si NMR spectra in DMSO-d6 at 40 °C of (a) Fraction C, (b) Fraction D, (c) Fraction E, and (d) Fraction F obtained by the operations shown in Chart 2. Chemical shifts were referenced to TMS (δ 0.0).

a crystalline structure (Figure 6a) and that T10- and T12-POSSs had amorphous structures (Figure 6b). These results show a similar tendency to those of 2Am-POSS.

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CONCLUSION We found that ammonium-functionalized POSSs, 2Am-POSS and Am-POSS, with high proportions of T10-POSS (54% and 45%, respectively) could be successfully prepared by the hydrolytic condensation of AEAPTMS and APTMS, respectively, using HOTf as the catalyst in hydrophobic alcohols such 4137

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

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Inorganic Chemistry

1530.8; found m/z 1531.4 [M + H]+ and m/z 1681.3 [M + HOTf + H]+. Calcd for [C60H156N24O18Si12] assigned to T12-POSS 1836.9; found m/z 1837.0 [M + H]+ and m/z 1987.7 [M + HOTf + H]+. Anal. Calcd for C7H15N2O7.5F6Si1S2 (one repeating unit): C, 18.54; H, 3.33; N, 6.18. Found: C, 18.16; H, 3.19; N, 5.81%. Preparation of Am-POSS. A typical experimental procedure for the preparation of POSS using 1-hexanol as the solvent was as follows (Figure 2f, Run 13 in Table 1). A mixture of a 1-hexanol solution (0.5 mol/L, 12 mL) of HOTf (purity: 98%, 0.919 g, 6.0 mmol) and water (0.180 g, 10.0 mmol) was added to APTMS (purity: 96%, 0.374 g, 2.0 mmol) while stirring at room temperature. The subsequent procedure was the same as described above for the preparation of 2Am-POSS, and 0.472 g of a white powder, Am-POSS, was obtained (yield: ca. 91%; the ideal chemical formula of one unit of this product [SiO1.5(CH2)3NH3+·CF3SO3−, FW = 260.3] was used for determination of the yield). 1H NMR (400 MHz, D2O): δ 3.07−2.86 (br, 2H, −CH2NH3), δ 1.83−1.67 (br, 2H, −SiCH2CH2CH2NH3), δ 0.85− 0.67 (br, 2H, −SiCH2−). 29Si NMR (79.5 MHz, DMSO-d6): δ −66.5 (T8-POSS), δ −68.4 (T10-POSS), δ −67.3 and −70.8 (T12-POSS). MALDI-TOF MS analysis of Am-POSS using DHB as the matrix: Calcd for [C24H64N8O12Si8] assigned to T8-POSS 880.3; found m/z 881.7 [M + H]+. Calcd for [C30H80N10O15Si10] assigned to T10-POSS 1100.4; found m/z 1101.7 [M + H]+. Calcd for [C36H96N12O18Si12] assigned to T12-POSS 1320.4; found m/z 1321.7 [M + H]+. Anal. Calcd for C4H9N1O4.5F3Si1S1 (one repeating unit): C, 18.46; H, 3.49; N, 5.38. Found: C, 18.26; H, 3.46; N, 5.07%. Separation of 2Am-POSS. The experimental procedure for the separation of T8-POSS and a mixture of T10- and T12-POSSs exploiting their different solubilities in 1-butanol was as follows. 1-Butanol (ca. 40 mL) was added to 1.757 g of 2Am-POSS as a mixture of T8-, T10-, and T12-POSSs obtained using 1-hexanol as a reaction solvent (Figure 1f, Run 6 in Table 1), and the resulting suspension was stirred at room temperature overnight (ca. 15 h). The mixture was then separated into insoluble and soluble parts in 1-butanol by suction filtration, and the insoluble part was washed with 1-butanol (ca. 40 mL × 3) and acetone−chloroform mixed solvent (1:1 v/v, ca. 20 mL × 1) and then dried under reduced pressure at room temperature to yield 0.321 g of a white powder, T8-POSS (yield: ca. 18%, Fraction A in Chart 1). 29Si NMR (79.5 MHz, DMSO-d6): δ −66.7 (T8-POSS). The remaining 1butanol-soluble part was heated at ca. 50 °C in an open container until the solvent completely evaporated. The resulting viscous product was washed with acetone−chloroform mixed solvent (1:1 v/v, ca. 20 mL × 4), isolated by decantation, and then dried under reduced pressure at room temperature to yield 1.250 g of a brown solid, a mixture of T10and T12-POSSs (yield: ca. 71%, Fraction B in Chart 1). 29Si NMR (79.5 MHz, DMSO-d6): δ−68.9 (T10-POSS), δ −68.0 and −71.3 (T12POSS). Separation of Am-POSS. The experimental procedure for the separation of T8-POSS and a mixture of T10- and T12-POSSs exploiting their different solubilities in ethanol−methanol mixed solvent and 1propanol was as follows. Ethanol−methanol mixed solvent (3:1 v/v, ca. 40 mL) was added to 1.448 g of Am-POSS as a mixture of T8-, T10-, and T12-POSSs obtained using 1-hexanol as the reaction solvent (Figure 2f, Run 13 in Table 1), and the resulting suspension was stirred at room temperature for ca. 10 min. The insoluble and soluble parts in the mixed solvent were separated by suction filtration, and the insoluble part was washed with ethanol−methanol mixed solvent (ca. 10 mL × 3) and then dried under reduced pressure at room temperature to yield 0.576 g of a white solid, T8-POSS (yield: ca. 40%, Fraction C in Chart 2). 29Si NMR (79.5 MHz, DMSO-d6): δ −66.4 (T8-POSS). The remaining mixed-solvent-soluble part was heated at ca. 50 °C in an open container until the solvent completely evaporated to yield 0.622 g of a white solid, a mixture of T8-, T10-, and T12-POSSs (yield: ca. 43%, Fraction D in Chart 2). 29Si NMR (79.5 MHz, DMSOd6): δ −66.3 (T8-POSS), δ −68.2 (T10-POSS), δ −67.5 and −70.6 (T12-POSS). Next, after 1-propanol (ca. 40 mL) was added to Fraction D (0.508 g) and the resulting suspension was stirred at room temperature for ca. 10 min, the resulting suspension was separated into insoluble and soluble parts by suction filtration. The insoluble part was washed with 1-propanol (ca. 10 mL × 3) and then dried under

Figure 6. Powder XRD patterns of films prepared by drop-casting methanol solutions of (a) Fraction C (insoluble part in ethanol− methanol mixed solvent: T8-POSS) and (b) Fraction F (soluble part in ethanol−methanol mixed solvent and 1-propanol: T10- and T12-POSS) obtained by the operations shown in Chart 2. The amount of each product on the glass was ca. 1.0 mg cm−2.

as 1-hexanol. In addition, on the one hand, it was also found that 2Am-POSS as a mixture of T8-, T10-, and T12-POSSs could be separated into T8-POSS and a mixture of T10- and T12POSSs by exploiting their different solubilities in 1-butanol. On the other hand, in the case of Am-POSS, T8-POSS and a mixture of T10- and T12-POSSs could be isolated by treatment with ethanol−methanol mixed solvent and 1-propanol. The XRD patterns of the cast films of T8-POSS showed many sharp diffraction peaks, indicating crystalline structures, whereas those of T10- and T12-POSSs showed no diffraction peaks, indicating amorphous structures, for both the case of 2Am-POSS and Am-POSS. These results suggest that the difference in crystallinities between ammonium-functionalized T8-POSS and T10- and T12-POSSs causes their different solubilities in alcohol solvents.

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EXPERIMENTAL SECTION

Materials. All reagents and solvents were commercially available and used without further purification. Preparation of 2Am-POSS. A typical experimental procedure for the preparation of POSS using 1-hexanol as the reaction solvent was as follows (Scheme 1f, Run 6 in Table 1). A mixture of a 1-hexanol solution (0.5 mol/L, 24 mL) of HOTf (purity: 98%, 1.838 g, 12.0 mmol) and water (0.180 g, 10.0 mmol) was added to AEAPTMS (purity: 95%, 0.468 g, 2.0 mmol) while stirring at room temperature. The resulting solution was further stirred at room temperature for 2 h, and then heated at ca. 50 °C in an open container until the solvent completely evaporated. Subsequently, the crude product was maintained at 100 °C for 2 h, and then an acetone−chloroform mixed solvent (1:1 v/v, ca. 30 mL) was added to the product at room temperature. The insoluble part was isolated by decantation, washed with the mixed solvent (ca. 20 mL × 4), and then dried under reduced pressure at room temperature to yield 0.831 g of a white, slightly sticky solid, 2Am-POSS (yield: ca. 92%; the ideal chemical formula of the repeating unit of this product [SiO1.5(CH2)3NH2+(CH2)2NH3+· 2(CF3SO3−), FW = 453.4] was used for determination of the yield). 1 H NMR (400 MHz, D2O): δ 3.41−3.36 (br, 4H, −NH2CH2CH2NH3), δ 3.09 (t, J = 7.33 Hz, 2H, −SiCH2CH2CH2NH2−), δ 1.84−1.64 (br, 2H, −SiCH2CH2CH2NH2−), δ 0.82−0.65 (br, 2H, −SiCH2−). 29Si NMR (79.5 MHz, DMSO-d6): δ −66.8 (T8-POSS), δ −68.7 (T10POSS), δ −67.9 and −71.1 (T12-POSS). Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDITOF MS) analysis of 2Am-POSS using 2,5-dihydroxybenzoic acid (DHB) as the matrix: Calcd for [C40H104N16O12Si8] assigned to T8POSS 1224.6; found m/z 1225.6 [M + H]+ and m/z 1375.3 [M + HOTf + H]+. Calcd for [C50H130N20O15Si10] assigned to T10-POSS 4138

DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

Article

Inorganic Chemistry

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reduced pressure at room temperature to yield 0.229 g of a white solid, a mixture of T8- and T10-POSSs (yield: ca. 45%, Fraction E in Chart 2). 29Si NMR (79.5 MHz, DMSO-d6): δ −66.4 (T8-POSS) δ −68.3 (T10-POSS). The remaining 1-propanol-soluble part was heated at ca. 50 °C in an open container until the solvent completely evaporated to yield 0.268 g of a white solid, a mixture of T10- and T12-POSSs (yield: ca. 53%, Fraction F in Chart 2). 29Si NMR (79.5 MHz, DMSO-d6): δ −68.3 (T10-POSS), δ −67.6 and −70.7 (T12-POSS). Measurements. 1H and 29Si NMR spectra were recorded using a JEOL ECX-400 spectrometer. Chemical shifts were referenced to tetramethylsilane (TMS). The XRD measurements were performed at a 2θ scanning speed of 1.8° min−1 using an X’Pert Pro diffractometer (PANalytical) with Ni-filtered Cu Kα radiation (λ = 0.154 18 nm). The MALDI-TOF MS measurements were performed using a Shimadzu Voyager Biospectrometry Workstation Ver. 5.1 in positiveion mode with DHB as the matrix. The elemental analyses were performed using a PerkinElmer 2400 Series II CHN elemental analyzer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00131. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yoshiro Kaneko: 0000-0001-6386-9166 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank Prof. Y. Suda and Dr. M. Wakao of the Graduate School of Science and Engineering, Kagoshima Univ. (Japan), for support in MALDI-TOF MS measurements. This work was supported by JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research) No. 15K13711.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140

Article

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DOI: 10.1021/acs.inorgchem.7b00131 Inorg. Chem. 2017, 56, 4133−4140