Triflic Acid Rearrangement of T8 into T10 Amino POSS Salts

The publication titled “Novel organic–inorganic hybrids based on T₈ and T₁₀ silsesquioxanes: synthesis, cage-rearrangement and properties,” published in RSC Advances in 2015, presents a controlled route to amino- and amido-functionalized cage silsesquioxanes together with a detailed account of T₁₀ silsesquioxane cage rearrangement promoted by trifluoromethanesulfonic acid. Treatment of the octameric hydrochloride salt with twelve equivalents of this Brønsted superacid in dimethyl sulfoxide delivered the decameric triflate in 44% yield, and the species formed along the reaction path were isolated and identified. The two long-chain amido derivatives obtained from these salts decompose at 471 and 451 °C and form optically transparent films whose water contact angles reach 104° and 110°.

For a general introduction to decameric cages, their D_5h geometry, and isolation methods, see T10 Decameric Silsesquioxanes: Structure, Symmetry, and Isolation.

Amino-Functionalized T8 and T10 Silsesquioxane Salt Synthesis

Polyhedral oligomeric silsesquioxanes (POSS) of general formula (RSiO_1.5)_n combine a rigid siloxane framework with organic side chains that can be tailored almost at will, and the hexahedral T₈ cage built on an Si₈O₁₂ core has attracted by far the largest share of synthetic attention because it dissolves readily in common organic solvents and is straightforward to purify. Higher homologues such as T₁₀ and T₁₂ arise through spontaneous reorganization of the octameric framework toward thermodynamically more stable cages, yet reports of pure, isolated decameric species remain scarce. The obstacle is practical rather than conceptual, because the solubility profiles of the three cage sizes overlap and the products resist separation. The study discussed here addresses that gap directly and complements earlier structural work on cage-to-ladder rearrangement in phenylsilsesquioxanes.

Three crystalline amino-functionalized salts define the entry point of the synthesis. Hydrolytic condensation of (3-aminopropyl)triethoxysilane (APTES) in methanol in the presence of concentrated hydrochloric acid delivered octa(3-aminopropyl)silsesquioxane hydrochloride, compound 1, in 45% yield after four weeks at ambient temperature. Performing the same condensation in 0.5 M trifluoromethanesulfonic acid at a 1:2 molar ratio instead produced a crude mixture of the octameric triflate 2 and the decameric triflate 3. Washing that solid with acetone left analytically pure 2 in 92% yield, while evaporation of the acetone filtrate afforded 3 as a white powder in 3% yield. Acylation of the amino salts with decanoyl chloride and triethylamine in dimethylformamide gave octa(3-decanamidopropyl)silsesquioxane 4 in 70% yield from 1 and in 86% yield from 2, and deca(3-decanamidopropyl)silsesquioxane 5 in 81% yield from 3. Flash column chromatography with diethyl ether and hexane (1:1, v/v) separated the two amido cages, which elute with R_f values of 0.56 and 0.40.

Triflic Acid-Promoted T8 to T10 Cage Rearrangement Mechanism

Scheme 1. Synthesis of compounds 1–5 from (3-aminopropyl)trialkoxysilane. Conditions: (a) H₂O, HCl, (b) H₂O, CF₃SO₃H, (c) acetone, precipitate, (d) acetone solution, (e) decanoyl chloride, dimethylformamide, NEt₃. The hexahedral salts 1 and 2 both furnish the octameric amide 4, whereas the heptahedral triflate 3 furnishes the decameric amide 5. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

The reorganization of the siloxane core was examined first on the NMR scale. Addition of fourteen equivalents of trifluoromethanesulfonic acid to a solution of 1 in DMSO-d₆, eight of which serve for anion exchange and six for reaction with the cage itself, provoked rapid gas evolution and produced the silicon resonance expected for a decameric species. On a preparative scale the same transformation carried out at a 1:12 molar ratio in dimethyl sulfoxide for two hours at 40 °C, followed by extraction with acetone and recrystallization from methanol, furnished 3 in 44% yield, a decisive improvement over the 3% obtained from direct condensation. Reactions run at 1:1, 1:4, 1:8, 1:12 and 1:16 molar ratios and monitored by silicon-29 NMR showed that a single equivalent of acid is already sufficient to attach a triflate group to silicon, which fixes the first step of the sequence.

Scheme 2. Direct synthesis of the heptahedral decamer 3 using the superacid CF₃SO₃H. Hydrolytic condensation of the (3-aminopropyl)trialkoxysilane in hydrochloric acid gives the octameric salt 1 carrying eight chloride counterions, whereas treatment of either the silane itself or the isolated cage 1 with trifluoromethanesulfonic acid delivers the ten-vertex framework of 3 with ten trifluoromethanesulfonate counterions. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

The pathway proposed on the basis of those experiments begins with attack of the acid on a siloxane Si−O−Si linkage of the octameric salt A. A triflate ester forms at silicon and one edge of the cage opens to give the intermediate B, and the inversion observed at that center matches the behavior expected when a leaving group is displaced by a soft nucleophile. Continued acid attack cleaves B further into the incompletely condensed species T₆(OH)₄, denoted C, together with a triflate-rich siloxane dimer D. Because the reaction proceeds under aqueous conditions, hydrolysis of B also delivers the tetrahydroxylated cage T₈(OH)₄ as E. Condensation of D with E then abstracts the triflate anion and closes the framework, and the product of that closure is the heptahedral decamer F. Every intermediate along this sequence was isolated and examined by high-resolution mass spectrometry and silicon-29 NMR, which places the mechanism on experimental rather than purely inferential footing.

Fig. 1. Proposed mechanism of the T₈ → T₁₀ cage rearrangement, drawn for R = CH₂CH₂CH₂NH₃Cl and OTf = CF₃SO₃. Trifluoromethanesulfonic acid opens one edge of the closed cage A to give the triflate ester B, further attack releases the incompletely condensed T₆(OH)₄ species C together with the siloxane dimer D, and hydrolysis of B supplies T₈(OH)₄ as E. Condensation of D with E proceeds with loss of CF₃SO₃H and closes the ten-vertex framework F. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

Molecular mechanics calculations underline how unusual this outcome is. The total MM2 energy of the octameric triflate 2 amounts to 11.72 kcal/mol against 60.92 kcal/mol for the decameric salt 3, and the corresponding amides give 31.10 and 102.35 kcal/mol for 4 and 5. The heptahedral cage is therefore distinctly the less favorable product in energetic terms, and its formation is driven instead by the assembly of a new Si₄O₄ ring from two considerably less stable precursors. The geometric consequence is apparent in the optimized structures. The eight-vertex core is compact and highly symmetrical, whereas the ten-vertex framework adopts the lower D_5h symmetry that later governs its packing behavior in the solid state.

Silicon-29 and Carbon-13 NMR Evidence for Decameric Cage Formation

Fig. 2. Molecular mechanics models of the hexahedral octamer 2 (left) and the heptahedral decamer 3 (right), with silicon shown in violet, oxygen in red, nitrogen in blue, carbon in gray and hydrogen in white. The eight-vertex Si₈O₁₂ core of 2 is fully symmetrical, whereas the ten-vertex core of 3 adopts the lower D_5h symmetry. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

Distinguishing the two cages spectroscopically proved less trivial than their difference in composition suggests. The proton spectra of 2 and 3 are indistinguishable, with resonances for the ammonium, methylene and silicon-bound methylene groups at 7.52, 2.73, 1.49 and 0.59 ppm. Silicon-29 NMR separates them cleanly, giving singlets at −66.5 and −68.6 ppm, both within the range expected for alkyl-substituted cages. Diffusion ordered spectroscopy provided the decisive discrimination in solution, returning diffusion coefficients of 6.18×10⁻⁷ and 6.80×10⁻⁷ cm² s⁻¹ for 2 and 3 in DMSO-d₆. The same experiment applied to the amido derivatives in chloroform gave 2.96×10⁻⁶ and 5.76×10⁻⁶ cm² s⁻¹ for 4 and 5, whose silicon resonances at −66.3 and −68.5 ppm are separated by 2.21 ppm.

Fig. 3. Silicon-29 NMR spectra recorded during the cage rearrangement, shown from bottom to top for the octameric hydrochloride 1, for 1 after addition of CF₃SO₃H, for the octameric triflate 2, for 2 after addition of the same acid, and for the isolated decameric triflate 3. The single resonance at −66.5 ppm characteristic of the T₈ cage is progressively replaced by the signal at −68.6 ppm belonging to the T₁₀ cage, with tetramethylsilane at 0.0 ppm as internal reference. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

Carbon-13 NMR turns out to be the more practical everyday diagnostic, and the study makes that point explicitly. For the octameric triflate 2 the carbon resonances appear at 122.5 ppm for the triflate anion and at 41.2, 20.7 and 8.6 ppm for the three carbon atoms of the aminopropyl arm, whereas the decameric analogue 3 gives 122.5, 41.1, 20.6 and 8.2 ppm. The upfield displacement is systematic and reaches its maximum for the carbon atom bonded directly to silicon, which is precisely the position most sensitive to the geometry of the siloxane core. The same relationship reappears in the amido pair 4 and 5. Since carbon spectra require far shorter acquisition times than silicon spectra and no relaxation agent, this observation offers a genuinely useful shortcut for anyone working with mixtures of cage sizes.

Fig. 4. Carbon-13 NMR spectra of the octameric triflate 2 (top) and the decameric triflate 3 (bottom) recorded in DMSO-d₆ at 300 K. The quartet centered near 122.5 ppm arises from the trifluoromethanesulfonate anion, while the three aliphatic resonances at 41.2, 20.7 and 8.6 ppm for 2 shift upfield to 41.1, 20.6 and 8.2 ppm for 3. The largest difference is found for the carbon atom bound to silicon. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

Amido-POSS Derivatives and Hydrophobic Film Contact Angle

In the solid state the two cage sizes behave very differently. Powder X-ray diffraction of 2 produces sharp, well-resolved reflections consistent with a highly symmetrical and closely packed crystalline solid, whereas 3 yields only broad features that indicate irregular packing, a consequence of the lower symmetry of the decameric core. Infrared spectroscopy reinforces the same conclusion through the shape of the siloxane stretching region. The asymmetric Si−O−Si band is narrow and intense at 1116 and 1138 cm⁻¹ for 1 and 2, while for 3 it broadens across 1148 to 1085 cm⁻¹. In the amido derivatives the bands at 3278, 1636, 1558 and 1383 cm⁻¹ confirm formation of the amide linkage, and the complete disappearance of ammonium and triflate absorptions establishes that every side chain has been acylated.

Thermal analysis quantifies the advantage that the siloxane core confers on these materials. The amino salts 1–3 decompose fully between 314 and 428 °C in air, while the amido derivatives withstand considerably more, decomposing at 471 °C for the octamer 4 and 451 °C for the decamer 5. The residues match the values calculated for quantitative conversion into silica, at 22.75% against 22.72% for 4. The purely organic reference compound N-propyldecanamide melts at 42 °C and reaches its maximum rate of decomposition at 265 °C, roughly two hundred degrees below the hybrids, which places the benefit of the inorganic core beyond dispute. Differential scanning calorimetry locates the melting points of 4 and 5 at 190 and 193 °C, and reveals for 4 an additional reversible transition near 150 °C that survives repeated heating and cooling cycles and is assigned to rotation and minor reordering of the alkyl chain ends. That transition is absent for 5, whose crystallinity is too low for the effect to be detected. Related thermal behavior has been described for other POSS-based inorganic and organic hybrid materials.

The practical payoff appears at the surface. Because the long amido-functionalized side chains lower molecular symmetry and suppress crystallization, films of 4 and 5 cast on glass at approximately 3.0 mg cm⁻² are optically transparent throughout the visible region, as confirmed by ultraviolet and visible spectroscopy and supported by powder diffraction and transmission electron microscopy. Those same lamellar-like chains render the surfaces hydrophobic, with water contact angles of 104° for 4 and 110° for 5, whereas the water-soluble amino salts 1–3 could not be measured at all. The difference of approximately 10 °C between crystallization and melting temperatures indicates the degree of supercooling required to initiate ordering within lamellar microdomains, which points to genuine long-range organization on the glass surface. Comparable surface-directed behavior has been reported for amphiphilic azobenzene-functionalized silsesquioxanes.

Fig. 5. Water droplets resting on glass plates coated with (b) the octameric amide 4 and (c) the decameric amide 5, each deposited at approximately 3.0 mg cm⁻². The printed text beneath the plates remains legible through both films, which demonstrates their transparency in the visible region, while the droplet profiles correspond to contact angles of 104° and 110° respectively. Reproduced from RSC Advances 2015, 5, 72340 (Royal Society of Chemistry).

Implications for Controlled Silsesquioxane Cage-Size Chemistry

Taken together, the results establish that the T₈ to T₁₀ transformation is a controllable synthetic operation rather than an unwanted side reaction, that its intermediates can be trapped and characterized, and that carbon-13 NMR deserves a far more prominent place in the analysis of cage mixtures than it has traditionally received. Equally significant is what the resulting coatings do not require. They contain no fluorine, they are single well-defined molecules rather than composite blends, and they are deposited from ordinary solution without specialized equipment, yet they combine transparency, hydrophobicity and thermal stability above 450 °C. For anyone designing protective or self-cleaning layers where fluorinated reagents are undesirable, that combination marks out amido-functionalized decameric silsesquioxanes as a class worth revisiting.

Full Citation “Novel organic–inorganic hybrids based on T₈ and T₁₀ silsesquioxanes: synthesis, cage-rearrangement and properties.” RSC Advances, 2015, 5, 72340–72351.
DOI: 10.1039/c5ra10136k
Full text: RSC Advances → Publisher

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This post has its own persistent identifier. Please use the DOI below when citing it.

Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2023). Triflic Acid Rearrangement of T8 into T10 Amino POSS Salts. https://doi.org/10.59350/ev8ms-m7736