Synthesis of a series of octaalkoxy-substituted cage silsesquioxanes catalyzed by zinc acetate†

Naoki Watanabe^abc, Hiroaki Imoto^ab and Kensuke Naka*^ab
aFaculty of Molecular Chemistry and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Kyoto 606-8585, Japan. E-mail: kenaka@kit.ac.jp
^bMaterials Innovation Lab, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
^cJNC Petrochemical Corporation, 5-1, Goikaigan, Ichihara, Chiba 290-8551, Japan

First published on 27th May 2024

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Introduction

Alkoxysilanes are essential materials for constructing organic–inorganic hybrid chemical structures and provide various silicone resins through hydrolysis and condensation reactions.^1,2 Silicone resins have unique properties such as thermal stability, oxidation resistance, water repellency, high gas permeability, and excellent electrical insulation. These characteristics are also utilized in making composite materials such as organic–inorganic hybrid materials. The structures and properties of silicone resins are strongly affected by the structures of the alkoxysilane monomers as starting materials.^3–6

Polyhedral oligomeric silsesquioxane (POSS) is an effective compound for synthesizing structure-controlled molecular-based hybrid materials and is a well-known framework structure used as a nano-building block. A typical POSS molecule possesses a cubic rigid structure represented by the formula R₈Si₈O₁₂, where R represents organic substituents such as alkyl and aryl groups, and the various substituents are linked by Si–C bonds to each vertex of the cage structure. They are applied to nanomaterials, organic–inorganic hybrid materials, electronic materials, and biomaterials.^7–23 Octaalkoxy-substituted POSS (8RO-POSS), which has reactive alkoxy silyl groups at the eight vertices, is considered an attractive starting material for the production of silicone resins because sol–gel products from 8RO-POSS have a high surface area, excellent heat resistance, and durability.^24–27 These properties are expected to be applicable in various fields, such as catalysis, adsorption, and electronics. However, the sol–gel process of 8RO-POSS has seldom been reported, and the sol–gel reaction has not yet been studied in detail. This was due to the difficulty in synthesizing 8RO-POSS.

Millar et al. reported that the synthetic routes to 8RO-POSS require multiple steps or toxic reagents, and few studies have focused on their properties and reactivities. Specifically, octahydride-POSS (8H-POSS) was converted to octachloride-POSS (8Cl-POSS) using toxic chlorine (Cl₂), which was further reacted with methyl nitrite (CH₃ONO) to produce octamethoxy-POSS (8MeO-POSS) (Scheme 1a).²⁸ Ruben et al. synthesized 8MeO-POSS in one step from 8H-POSS by using an alkoxy-tin compound, (CH₃)₃SnOCH₃, which requires multiple steps for preparation (Scheme 1b).²⁵ Noltemeyer et al. synthesized octaisopropoxy-POSS (8iPrO-POSS) from 8H-POSS using toxic and pyrophoric dicobalt octacarbony (Co₂(CO)₈) as a catalyst, and the substrate scope was quite limited (Scheme 1c).²⁹ A practical synthetic method using diethyl hydroxylamine (Et₂NOH) as a catalyst was proposed to prepare 8RO-POSS from 8H-POSS with alcohol (Scheme 1d).^30–32 However, this reaction requires a basic catalyst, which promotes hydrolysis of Si–O bonds in the POSS framework. Therefore, this reaction requires low-polarity solvents, such as benzene, and is inapplicable to the highly reactive 8Me-POSS. Hence, it is important to develop an alternative synthesis method that suppresses the hydrolytic condensation reactions, even when using a polar solvent.

Dehydrogenative alkoxylation of Si–H groups using alcohol with transition metal catalysts, sodium hydroxide, and boron catalysts has been developed.^33–41 However, these catalysts have disadvantage such as requiring multistep in preparation and being unstable in air. Shimada et al. reported that alkoxylation of trimethoxysilane in the presence of zinc acetate (Zn(OAc)₂) as a catalyst with various alcohols proceeded in high yields under mild condition (1 h reaction time at room temperature) (Scheme 1e).⁴² Zn(OAc)₂ catalyst is easily soluble in many organic solvents and can be easily handled due to its low toxicity and high stability in air. The hydrolysis and condensation of alkoxysilanes are suppressed under neutral or weakly acidic conditions by pH control. Therefore, we focused on Zn(OAc)₂ because of its weak acidity and expected Zn(OAc)₂ to suppress the hydrolysis and condensation of 8RO-POSS, even in polar solvents such as tetrahydrofuran (THF). In this study, Zn(OAc)₂ was investigated as a catalyst for the synthesis of a series of 8RO-POSS from 8H-POSS (Scheme 1f). In addition, the sol–gel process and the products of the obtained 8RO-POSS derivatives were investigated.

Results and discussion

Heptaisobutylmonohydride-POSS (7iBu1H-POSS) was selected as a model compound to optimize the reaction conditions for the alkokylation of the SiH group in the POSS cage with methanol (MeOH) using Zn(OAc)₂ as a catalyst (Table 1). Hydrogen gas generation was observed during the reaction, similar to previous reports by Gunji et al. and Shimada et al.^31,42 The methoxylation conversion was analyzed using the ¹H NMR spectra of the crude products. First, the methoxylation of 7iBu1H-POSS was carried out under the same conditions as those reported by Shimada (entry 1).⁴² However, the reaction did not proceed, and only 7iBu1H-POSS was recovered. Increasing the amount of Zn(OAc)₂ to 0.5 and 3.0 mol% improved reaction yield to 55% and 90%, respectively (entries 2 and 3). We assume that the steric bulkiness of the POSS cage and the isobutyl groups prevented the hydrogen in 7iBu1H-POSS from abstraction by Zn(OAc)₂. Increasing collision frequency between 7iBu1H-POSS and Zn(OAc)₂ by increasing the concentration of Zn(OAc)₂ and applying the reaction at 40 °C for 3 h and 25 °C for 18 h further improved product yields to 96% (entry 4, Fig. 1). Hydrogen gas generation almost stopped after the reaction at 40 °C for 3 h. However, only the reaction at 40 °C for 3 h showed a 58% yield (entry 5). Decreasing the amount of MeOH from 100 mol ratio to 4 and 10 mol ratios drastically decreased the product yields to 50% and 68%, respectively (entries 6 and 7). Excessive amounts of MeOH were required to obtain a high yield owing to the steric hindrance of the POSS cage. The structure of purified heptaisobutylmonomethoxy-POSS (7iBu1MeO-POSS) was confirmed by ¹H and ²⁹Si NMR spectra (Fig. 1, S1†).

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The methoxylation of 8H-POSS was performed under the optimized reaction conditions for the model reaction of entry 4 in Table 1. Unlike the case of 7iBu1H-POSS, hydrogen gas generation was lower and continue to occur even after the reaction at 40 °C for 3 h. Therefore, we increased the reaction temperature to 60 °C and found hydrogen gas generation almost stopped after the reaction at 60 °C for 3 h. After applying the reaction at 60 °C for 3 h and 25 °C for 18 h, however, the product obtained after drying the reaction mixture under reduced pressure hardly dissolved in any organic solvent. The FT-IR spectrum of the insoluble product showed no signals corresponding to the SiH (2100 cm⁻¹) group. Based on these observations, we assumed that the hydrolysis of the resulting alkoxysilane proceeded to give silanols and that some of the silanols in the POSS compound progressed to a silicate structure (Fig. S6†). To suppress hydrolysis and condensation of the resulting product, acetic acid (0.7 mol ratio to the amount of 8H-POSS) was added to the reaction mixture after stirring at 60 °C for 3 h and 25 °C for 18 h. After drying under vacuum, the colorless solid was purified by extraction with chloroform (CHCl₃) and filtration. A solid product was obtained after the solvent was removed under reduced pressure and subsequently dried under vacuum pressure at 40 °C for 18 h (Table S1†). The resulting product was soluble in n-hexane, toluene, THF, CHCl₃, and acetonitrile. These results suggest that the addition of acetic acid makes the solution slightly acidic, suppressing the hydrolysis and condensation of alkoxysilanes.

The FT-IR spectrum of the obtained product exhibited peaks derived from methoxy groups at 2950, 2850, and 1460 cm⁻¹ (Fig. S7†), and no peak was derived from the SiH group at 2100 cm⁻¹, suggesting the quantitative methoxylation. The ¹H- and ²⁹Si-NMR spectra of the resulting product showed the peaks due to the SiOMe unit at 3.4 ppm and −101 ppm, respectively (Fig. 2 and 3), which agreed with those of the previous report.²⁸ The signals due to the SiH group disappeared in the ¹H NMR (4.2 ppm) and ²⁹Si NMR (−85 ppm) spectra,⁷ suggesting that the reaction quantitatively proceeded. The ²⁹Si NMR spectrum showed a weak signal at −92.7 ppm derived from (SiO)₂Si(OR)₂ (R = Me or H) unit assignable to a cage-opened byproduct.²⁸ The integration ratio of the peak derived from (SiO)₃SiOMe to the peak derived from (SiO)₂Si(OR)₂ was 17:1, suggesting that 8MeO-POSS was a main component. The MALDI-TOF-MS spectrum of the obtained product exhibited a molecular ion peak at 685.9, corresponding to the exact mass of 8MeO-POSS (Fig. 4(a)). 8MeO-POSS stored in air for 1 week could not solve in acetonitrile and chloroform, but octaethoxy-POSS (8EtO-POSS) and 8iPrO-POSS stored same condition could solve in chloroform.

8EtO-POSS and 8iPrO-POSS were synthesized from 8H-POSS with EtOH and i-PrOH, respectively, under the optimized conditions for 8MeO-POSS. 8EtO-POSS and 8iPrO-POSS were obtained as white products in high yields (NMR yield >99%; isolated yield = 99%). ¹H NMR spectrum of 8EtO-POSS showed the ethoxy unit at 1.24 and 3.89 ppm (Fig. 2(b)). The ²⁹Si NMR spectrum exhibited the (SiO)₃SiOEt group at −102.7 ppm (Fig. 3(b)). These peaks agreed well with the previous report.³¹ The ¹H and ²⁹Si NMR spectra of 8iPrO-POSS were also confirmed the ideal structure.^{31 29}Si NMR spectra of 8EtO-POSS and 8iPrO-POSS exhibited no peaks at −85.0 ppm corresponding to the (SiO)₃SiH group, −90 to −95 ppm from (SiO)₂Si(OR)₂ (R = alkyl or H), and −100 ppm from (SiO)₃SiOH (Fig. 3(c)). The FT-IR spectra of 8EtO-POSS and 8iPrO-POSS showed peaks at 2900 and 1390 cm⁻¹ derived from the ethoxy (EtO) group (Fig. S8†) and peaks at 2970 and 1370 cm⁻¹ derived from the isopropoxy (iPrO) group (Fig. S9†); the peak derived from the SiH group at 2100 cm⁻¹ was not detected, which supported the quantitative alkoxylation. The MALDI-TOF-MS spectra of 8EtO-POSS and 8iPrO-POSS exhibited molecular ion peaks at 799.5 and 911.7, which correspond to the exact masses of 8EtO-POSS and 8iPrO-POSS (Fig. 4(b) and (c)).

Furthermore, alkoxylation of 8H-POSS using t-BuOH was performed. Synthesis and purification were performed under the same conditions as those for 8EtO-POSS or 8iPrO-POSS. The degree of alkoxylation was 55%, as calculated from the integrated ratio of the peaks derived from the (SiO)₃SiOtBu group (1.36 ppm) and the (SiO)₃SiH group (4.25 ppm) in the ¹H NMR spectrum (Fig. S5†). Increasing the reaction time for 1 d at 25 °C did not improve the degree of the alkoxylation. This result was attributed to the steric hindrance of t-BuOH.

The X-ray diffraction (XRD) pattern of 8MeO-POSS showed a broad peak centered at 2θ value of 25°, indicating an amorphous state (Fig. 5(a)). This was due to the contamination of the cage-opened byproducts. The XRD patterns of 8EtO-POSS and 8iPrO-POSS showed several diffraction peaks, indicating a crystalline state (Fig. 5(b) and (c)). Strong diffraction peaks at a 2θ value of approximately 8° are corresponded to the cage silsesquioxane units. Comparing the main diffraction peaks of 8EtO-POSS and 8iPrO-POSS, 8iPrO-POSS (2θ = 8.58°, d = 1.03 nm) showed wider lattice constant than that of 8EtO-POSS (2θ = 9.01°, d = 0.98 nm). A larger iPrO group volume than that of the EtO group increased the lattice constant of the crystal structure.

8MeO-POSS, 8EtO-POSS, and 8iPrO-POSS showed 5% weight losses (T_d5) at 250, 210, and 240 °C, respectively, under air (Fig. 6). The decomposition temperature of 8iPrO-POSS was higher than that of 8EtO-POSS, despite its larger organic segment content. The terminal bulkiness of the iPrO group may contribute to thermal stabilization of the compound compared to linear substituents. Sharper diffraction peaks were observed for 8iPrO-POSS than 8EtO-POSS in the XRD analysis. The higher crystallinity of 8iPrO-POSS compared with that of 8EtO-POSS contributes to its thermal stability. The contribution of terminal bulkiness to the higher thermal stability was also observed in the case of the thermal stability relationship between octa(n-propyl)-POSS and octaisobutyl-POSS.¹⁶ The onset of weight loss in octa(n-propyl)-POSS is 166 °C, which is lower than that in octaisobutyl-POSS. 8MeO-POSS exhibited a higher T_d5 than the others, owing to the contamination of SiOH groups in the sample.

Reactivities of the alkoxy groups in 8MeO-POSS, 8EtO-POSS and 8iPrO-POSS were investigated in toluene with a trace amount of pure water, followed by adding hydrochloric acid (HCl) aqueous solution (0.1 mol L⁻¹) and stirred for 1 h at 60 °C and for 1 h at 100 °C (Scheme 2). After the volatile materials were removed under reduced pressure, the structures of the residues were analyzed by FT-IR (Fig. 7, 8, and 9). The degree of hydrolysis of the MeO, EtO, and iPrO groups under acid treatment was estimated by decreasing the ratios of the corresponding adsorption at 2850, 2900, and 2980 cm⁻¹, respectively, to those of the Si–O bond (Si–O–Si: 1090 cm⁻¹) before and after the reaction. The results are summarized in Table 2. After acid treatment of 8MeO-POSS, 8EtO-POSS, and 8iPrO-POSS, the peak intensities of the alkoxy groups decreased to 8%, 64%, and 55%, respectively. 8EtO-POSS and 8iPrO-POSS showed higher stability against the acid-treated hydrolysis reaction than 8MeO-POSS, similar to the trend in the reactivity between TMOS and tetraethoxysilane (TEOS). The hydrolysis reaction of TMOS proceeded faster than TEOS in the first step hydrolysis.³ 8iPrO-POSS exhibited a relatively higher hydrolysis ratio than 8EtO-POSS. This may be due to the higher steric repulsion between the iPrO groups in the POSS framework. The current study suggests that the steric hindrance between the alkoxy groups on the cage structure affects the reactivity in hydrolysis. We observed that 8MeO-POSS was decomposed to insoluble products under storage in air for 1 week, while 8EtO-POSS and 8iPrO-POSS were stable under the same condition.

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Several groups reported that the absorption derived from the Si₈O₁₂ framework (SiO_ring) in octahydroxy-POSS (8HO-POSS) appeared at 577 cm⁻¹.^19,43,44 In this study, 8MeO-POSS and 8EtO-POSS exhibited absorption at 580 cm⁻¹, while 8iPrO-POSS exhibited absorption at 590 cm⁻¹, which is assignable to SiO_ring. These absorptions were expected to disappear as condensation proceeded, owing to the constraint of the molecular vibration of the monomer state Si₈O₁₂ framework by the formation of the silicate network. Therefore, the progress of the condensation reaction can be monitored by the reduction of the peaks at 580 and 590 cm⁻¹ assignable to the ring vibration of SiO_ring in the POSS monomer, against the peak at 1090 cm⁻¹ corresponding to asymmetric Si–O–Si stretching. The reduction of the peak at 580 cm⁻¹ was 49% in the acid-treated 8MeO-POSS. By contrast, the peak intensity was negligibly reduced for 8EtO-POSS, indicating that condensation did not proceed and that the silanol groups generated by hydrolysis were maintained. The remaining 64% of the EtO groups effectively suppress condensation. The acid-treated 8iPrO-POSS proceeded with 23% condensation owing to the smaller amounts of remaining alkoxy groups in 8iPrO-POSS than in 8EtO-POSS.

The reactivities of 8MeO-POSS, 8EtO-POSS, and 8iPrO-POSS were studied using triethylamine (Et₃N) instead of HCl. Hydrolysis was nearly complete for 8MeO-POSS, 8EtO-POSS, and 8iPrO-POSS under basic conditions, according to the decreased ratios of the absorption peaks corresponding to the alkoxy groups in the FT-IR spectra (Table 2(c)). The nearly complete disappearance of the 580 and 590 cm⁻¹ peaks due to the SiO_ring in 8EtO-POSS and 8iPrO-POSS indicates that the condensation proceeded efficiently. By contrast, the condensation of 8MeO-POSS was less efficient, probably because 8MeO-POSS contained broken cages, which prevented condensation. When considering contraction at the eight vertices, the molecules of POSS with the reaction group should be close and dense. When all the molecules were cube-shaped, the POSS molecules were assembled closely together. However, if molecules with broken cage structures are present, this assembly collapses, and as a result, condensation does not progress efficiently.

Compared with the random silicate obtained by base-treated tetramethoxysilane (TMOS), base-treated 8EtO-POSS and base-treated 8iPrO-POSS showed negligible intensity peaks in the wavenumber range of 1200–1300 cm⁻¹ (Fig. 7, 8, and 9). The absorption centered at 1100 cm⁻¹ is assignable to the cage structures, as reported by Scotti et al.,⁴⁵ while the broad absorption peak around 1200–1300 cm⁻¹ is derived from random silicate, as reported by Maschmeyer et al.⁴⁶ The present observations suggest that the cage structures were maintained after the formation of the silicate structures via the condensation reaction from 8EtO-POSS and 8iPrO-POSS under the base treatment. By contrast, condensation product from 8MeO-POSS exhibited a broad absorption peak between 1100–1300 cm⁻¹, suggesting the formation of random silicate structures. The partial collapse of the cage structure of 8MeO-POSS during synthesis indicated that the steric hindrance of the MeO group was small, and the cage structure of 8MeO-POSS was easily attacked by the base. By contrast, the EtO and iPrO groups prevented the base from attacking and maintained the cage structure.

Conclusions

We attempted to use Zn(OAc)₂ as a catalyst for synthesizing a series of 8RO-POSS from 8H-POSS and several alcohols. Zn(OAc)₂ is weakly acidic with low toxicity and is stable in air. The alkoxylation progresses even when a polar solvent is used. The reaction conditions for the alkoxylation were optimized using 7iBu1H-POSS as the model compound, and 7iBu1MeO-POSS was obtained at a high reaction rate of 96%. The alkoxylation of 8H-POSS with MeOH progressed with a high conversion under the modified conditions. After the isolation of the product, the formation of 8MeO-POSS was confirmed as the main product, but some byproducts were also contaminated. Alkoxylation of 8H-POSS with EtOH and i-PrOH provided 8EtO-POSS and 8iPrO-POSS in >99% conversion and >99% isolated yields. The structures of 8EtO-POSS and 8iPrO-POSS were confirmed using FT-IR, ¹H- and ²⁹Si-NMR, and MALDI-TOF-MS analyses. The stability of 8RO-POSS under acidic conditions depends on the structure of the alkoxyl groups. The cage structures of 8EtO-POSS and 8iPrO-POSS were maintained after the formation of the silicate structure under base treatment. The cage structures in the resulting silicates are expected to alter their physical or electrical properties compared to traditional random structures. Because less stable 8MeO-POSS is not suitable for long term storage, 8EtO-POSS and 8iPrO-POSS are expected to be applied as excellent precursors for organic–inorganic materials. The synthetic route discovered in this study can significantly contribute to the development of various 8RO-POSS. The synthetic route discovered in this study can contribute significantly to synthesizing various alkoxy-substituted POSS with different properties.

Experimental

Materials

Trisilanol isobutyl-POSS (7iBu3OH-POSS) was obtained from Hybrid Plastics Inc. Heptaisobutylmonohydride-POSS (7iBu1H-POSS) was synthesized from 7iBu3OH-POSS and trichlorosilane (HSiCl₃) by a previously reported procedure.¹⁷ Octahydride-POSS (8H-POSS) was synthesized from HSiCl₃ by a previously reported procedure.⁷ Methanol (MeOH), ethanol (EtOH), isopropyl alcohol (i-PrOH), tert-butyl alcohol (t-BuOH), and chloroform (CHCl₃) were purchased from Nacalai Tesque (Kyoto, Japan). Tris(2,4-pentanedionato) chromium(III) was purchased from Tokyo Chemical Industry (Tokyo, Japan). Toluene, tetrahydrofuran (THF) (super dehydrated), chloroform-d (CDCl₃, D: 99.8%), and acetonitrile-d₃ (D: 99.8%) were purchased from Kanto Chemical (Tokyo, Japan). Hydrochloric acid (HCl), triethylamine (Et₃N), and Zn(OAc)₂ were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan).

Measurements

¹H (400 MHz) and ²⁹Si (80 MHz) nuclear magnetic resonance (NMR) spectra were obtained using a BRUKER AVANCEIII400 (Bruker Biospin, Rheinstetten, Germany). For quantitative comparison, tris(2,4-pentanedionato)chromium(III) was added to the solutions as a relaxation agent for NMR measurements. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS) was recorded on a Bruker Autoflex III instrument (Bruker Daltonics, Billerica, MA, USA): Dithranol matrix (20 mg mL⁻¹ in THF) and sodium trifluoroacetate cationizing agents (1 mg mL⁻¹ in THF). Fourier transform infrared (FT-IR) spectra were obtained using a JASCO FT/IR-4600 spectrometer (JASCO, Tokyo, Japan). X-ray diffraction (XRD) was performed using a Rigaku MiniFlex600 X-ray diffractometer using X-rays from 600 W X-ray tubes (CuKα, λ = 1.54 Å) and a D/teX Ultra2 semiconductor detector. Thermogravimetric analysis (TGA) was performed using a Shimadzu TGA-50H thermogravimetric analyzer (Shimadzu, Kyoto, Japan) at a heating rate of 10 °C min⁻¹ under airflow.

Synthesis

Author contributions

N. Watanabe: synthesis, structural analysis, data curation, writing – original draft; H. Imoto: conceptualization, investigation, writing – original draft, writing – review and editing, funding acquisition project administration, supervision; K. Naka: conceptualization, investigation, writing – review and editing, funding acquisition, project administration, supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Mr Takumi Sato of JNC Petrochemical Corporation for performing MALDI-TOF-MS.

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Footnote

† Electronic supplementary information (ESI) available: Synethesis, NMR spectra, FT-IR spectra, MALDI-TOF-MS spectra, effect of acetic acid addition, solubility. See DOI: https://doi.org/10.1039/d4dt01008f

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