Silanetriols as Powerful Starting Materials for Selective Condensation to Bulky POSS Cages.

Controlled condensation reactions of tertiary silanetriols CH3(CH2) n (CH3)2CSi(OH)3 (1b-f; n = 1-5) in the presence of trifluoroacetic acid and the hydrolysis of CH3(CH2)6(CH3)2CSiCl3 (3) lead to the selective formation of the corresponding disiloxane tetrols [CH3(CH2) n (CH3)2CSi(OH)2]2O (2b-g; n = 1-6) in good yields. The TBAF-driven condensation reactions of the silanetriols CH3(CH2) n (CH3)2CSi(OH)3 (1a-c; n = 0-2) as well as of the disiloxane-1,1,3,3-tetrol 2d (n = 3) yield in the selective formation of the first T8 cages bearing tertiary carbon substituents, CH3(CH2) n (CH3)2C (4a-d; n = 0-3), which was not possible via the condensation of their alkoxysilane counterparts so far. The resulting compounds 2b-g and 4a-d have been characterized by multinuclear NMR, MS, and single-crystal X-ray diffraction.

Introduction

Octasilsesquioxanes T8 (T = RSi(O−)3 with R = inorganic or organic substituent, H) belong to the class of POSS compounds (POSS = polyhedral oligomeric silsesquioxanes) which have attracted much attention in recent years.[1,2] As hybrid materials with properties of both SiO2 and siloxanes,[3] they enjoy widespread applications for example in catalysis,[4] as model systems for silica surfaces,[5,6] in the design of superoleophobic surfaces,[7] ionic liquids,[8] and biocompatible materials,[9] and in polymer chemistry.[2] The synthetic approach toward such octasilsesquioxanes is based on the hydrolytic condensation of trifunctional silanes RSiX3, where R is a stable organic substituent and X a reactive moiety (i.e. X = Cl, OMe, etc.) and mostly accompanied by low selectivity, low yields, and long reaction times strongly dependent on many parameters such as the nature of the substituent R, the catalyst, the amount of water, etc.[2,3] The synthesis of T8 was revolutionized by Bassindale et al., who treated trialkoxysilanes with tetrabutylammonium fluoride (TBAF) in THF, which in many cases led to the selective formation of T8 cages in yields of up to 95%.[10−12] The precise mechanism of POSS formation is complex and is still under investigation. The intermediate formation of the primary hydrolysis products RSi(OH)3 has been postulated starting from trichlorosilanes.[13] The synthesis of T6 cages from their corresponding silanetriol counterparts has already been described by Unno et al. and in our laboratories using multistep procedures.[14−16] Therefore, we wanted to investigate whether a one-pot synthesis starting from silanetriols works equally well as the classical TBAF procedure established by Bassindale et al., which starts from alkoxysilanes. In addition, this investigation should also shed light on the question of whether silanetriols really do occur as intermediates in the classical TBAF procedure.

Results and Discussion

Recently we have established a multistep synthesis toward stable amphiphilic silanetriols bearing tertiary carbon substituents, H3C(CH2)(CH3)2CSi(OH)3 (1b–f; n = 1–5).[17] Owing to their great stability in the solid state as well as in solution these compounds are potential starting materials for controlled condensation reactions toward discrete siloxane species.[18,19] First we wanted to investigate whether the condensation of such compounds can be stopped at its primary stage: the disiloxane-1,1,3,3-tetrols. Despite their interesting properties and possible applications, e.g. as anion receptors[20] or in optically active materials,[21] examples of such disiloxanetetrols are rather rare. Only a few publications have dealt with the targeted synthesis of such compounds via the hydrolysis of the corresponding trichlorosilane precursors.[22−26] We have already been able to show that treatment of t-BuSi(OH)3 (1a) with trifluoroacetic acid (TFA) yielded the exclusive formation of the corresponding primary condensation product [t-BuSi(OH)2]2O (2a) as a crystalline material.[16] Following this protocol, mixing the silanetriols 1b–f with TFA in THF solution resulted in the formation of the corresponding disiloxane-1,1,3,3-tetrols [H3C(CH2)(CH3)2CSi(OH)2]2O (2b–f; n = 1–5) in pure form and high yields (Scheme 1). In addition to this, the hydrolysis of (1,1-dimethyloctyl)trichlorosilane, H3C(CH2)6(CH3)2CSiCl3 (3), in the presence of NEt3 gave the disiloxane-1,1,3,3-tetrol 2g exclusively instead of the corresponding silanetriol (Scheme 1). The 29Si NMR data of compounds 2b−g are in agreement with the chemical shift of 2a known in the literature and range between −49.9 and −51.9 ppm.[16]

Scheme 1

Synthesis of the Disiloxane-1,1,3,3-tetrols 2b–g

Isolated yields are quoted in parentheses.

The compounds 2b,d,g could be obtained as single crystals suitable for X-ray diffraction. The solid-state structures of all compounds show extensive networks of intermolecular hydrogen bonding similar to that for 2a.[16,24] One molecule interacts with six adjacent molecules, resulting in sheetlike structures (Figures 1–3). Whereas compounds 2b,g arrange in parallel sheet structures, compound 2d shows a herringbone pattern (Figures 2 and 3). The increasing alkyl chains show a near-linear zigzag orientation (Figure 4).

Figure 1

Exemplary H bonding between one molecule and six neighbors in 2b. The carbon substituents are omitted for clarity. The ellipsoids are drawn at the 30% probability level.

Figure 3

Herringbone pattern of compound 2d. The ellipsoids are drawn at the 30% probability level.

Figure 2

Exemplary layer structure in 2b. The ellipsoids are drawn at the 30% probability level.

Figure 4

Crystal structures of disiloxanes 2b,d,g. The hydrogen atoms of the carbon substituents are omitted for clarity. The ellipsoids are drawn at the 30% probability level. In 2b the carbon atoms of the ethyl group and one methyl group are disordered over two sites. In 2b,g the H atoms of the OH group are disordered over two sites (occupancy 0.5).

The O···O and H···O distances are comparable to those of the two known modifications of [t-BuSi(OH)2]2O, 2a and 2a′ (Table 1).[16,24] On the basis of their geometric parameters the hydrogen bond interactions can be classified as moderate to strong.[27] While the observed Si–O–Si angle of compound 2b is 180° due to symmetry reasons (monoclinic, C2/m) and therefore comparable to Lickiss’ modification of 2a′, the angles of compounds 2d,g are 175.0(4) and 173.8°, respectively, and lie between the different observed Si–O–Si angles (165.6° (2a)[16] and 180° (2a′)[24]) of [t-BuSi(OH)2]2O. These angles, however, are in agreement with the observed strong intermolecular interactions via hydrogen bonding. The Si–Osiloxane distances are slightly shortened in comparison to the Si–OH distance (Table 2). The Si–C distances range between 1.8553(19) and 1.881(6) Å. The data found in 2b,d,g are in good agreement with values known in the literature for other disiloxane-1,1,3,3-tetrols.[16,24]

Table 1

Hydrogen Bonds of 2b,d,ga

	D–H···A	d(D···A), Å	∠(D–H···A), deg
2a(16)	O–H···O	2.668–2.674	152–176
2á(24)	O–H···O	2.678–2.690	N/A
2b	O–H···Oi,ii	2.658–2.662	176–177
2d	O–H···Oiii.iv,v	2.658–2.706	142–162
2g	O–H···Ovi	2.662–2.695	145–161

Symmetry transformations used to generate equivalent atoms: (i) x – 1, y, 2 – z; (ii) 3/2 – x, 3/2 – y, 2 – z; (iii) x, −y – 1, −z; (iv) x + 1/2, −y – 1/2, −z; (v) x – 1/2, −y – 1/2,–z; (vi) x – 1, y – 1, z.

Table 2

Selected Bond Lengths (Å) and Angles (deg) of 2b,d,g

	2b	2d	2g
Si–OH	1.6340(8)	1.647(4)	1.6104(8)-1.636(2)
Si–Osiloxane	1.6050(4)	1.6234(14)	1.610(8)
Si–C	1.8553(19)	1.881(6)	1.860(3)
Si–O–Si	180.0	175.0(4)	173.8(2)
O(H)–Si–O(H)	105.99(6)	105.7(2)	105.44(11)
O(H)–Si–Osiloxane	105.99(6)–108.79(3)	105.7(2)–108.9(2)	109.03(12)–109.07(13)
O(H)–Si–C	111.56(5)	111.3(2)–111.5(2)	111.56(15)–111.94(15)
Osiloxane–Si–C	110.03(7)	110.57(19)	109.67(11)
Si–C–C–C	–170.9(5)	171.44(12)	–172.4(3)
C–C–C–C(H3)		174.08(17)	–176.5(4)

Having established a synthetic protocol toward successively CH2 elongated disiloxane-1,1,3,3-tetrols, we wanted to investigate whether a targeted approach to higher condensed oligomers such as POSS cages is possible starting from these silanetriols. Traditionally POSS cages are prepared by controlled hydrolysis of suitable trichlorosilanes or trialkoxysilanes. A general method has been established by Bassindale and co-workers, employing TBAF as a catalyst to transform alkoxysilanes into T8-POSS cages in the presence of water.[10,12] The templating effect of the fluoride ion has been nicely illustrated by its encapsulation into a T8 cage.[11] Despite the great success of this method, the yields can be quite low with certain substituents. For tert-butyl for instance, the corresponding T8 cage could not be obtained.[10] On the other hand, we recently reported the syntheses and crystal structures of tert-butyl-substituted POSS cages T6 and T7(OH)3 starting from the corresponding silanetriol, which confirms that the formation of tert-butyl-substituted POSS cages is possible.[16] Therefore, we combined both approaches and explored the formation of POSS cages starting from silanetriols 1a–c in the presence of TBAF as catalyst. Following this protocol, we obtained several T8-POSS cages 4a–c, which were previously unknown (Scheme 2). In addition to the high potential of silanetriols as starting materials in POSS synthesis, also the treatment of its primary condensation product, i.e. disiloxane-1,1,3,3-tetrol 2d, with TBAF has been explored, which results in the selective formation of the corresponding T84d as well. The latter results provide experimental proof in support of the hypothesis that POSS formation proceeds via the corresponding disiloxanes.

Scheme 2

Synthesis of the Compounds 4a–d

In addition, the tert-butyl-substituted T8 cage 4a has been obtained by starting from silanetriol 1a, which was not available starting from the corresponding trialkoxysilane.[10] The identity of compound 4a could be established on the basis of X-ray crystallography, solid-state NMR, and mass spectrometry. In agreement with Bassindale’s report[10] dealing with t-BuSi(OR)3, the treatment of 1a with TBAF also resulted in a white insoluble powder which can hardly be characterized with solution-based techniques except for proton NMR. We assume that tert-butyl-substituted T cages have also been formed in Bassindale’s approach but high substituent rigidity resulting in low solubility precluded further characterization. The solubility issue can be overcome by introducing the elongated and more flexible tert-butyl derivatives CH3(CH2)(CH3)2C– (n = 1–3). The 29Si chemical shifts of compounds 4a–d range between −67.1 and −68.4 ppm and are comparable to data known in the literature for T8 cages.[10,12] Compounds 4a–d could be obtained as single crystals suitable for X-ray diffraction. Compound 4a crystallizes in the triclinic space group P1̅ and is the first T8 cage bearing a tertiary carbon substituent (Figure 5).

Figure 5

Crystal structures of compounds 4a–d. The hydrogen atoms have been omitted for clarity. The ellipsoids are drawn at the 30% probability level.

Compound 4b crystallizes in the trigonal space group R3̅c. The 1,1-dimethylpropyl groups are disordered over three orientations, and the almost spherical molecules arrange in a slightly distorted cubic closed packing (Figure 6). Compound 4c crystallizes in the triclinic space group P1̅ (Figure 5). Although the conformations of the four independent 1,1-dimethylbutyl substituents are quite different, the C atoms at the ends of the chains (C14, C24, C34, C44) lie in a plane that is almost coplanar with the best plane through the Si atoms Si1, Si2, Si3, and Si4 (angle of 2.78(1)° between the planes). Compound 4d crystallizes in the monoclinic space group P21/n (Figure 5). Due to the unrefinable displacement parameters the probability ellipsoids of the 1,1′-dimethylpentyl moieties are significantly large. Nevertheless, the structure refinements reported here yielded R1 values in the range of 2.64–4.54% for compounds 4a–c, remarkably low in comparison to those for other reported T8 solid-state structures. The residual electron densities for all T8 cages reported here give no indication for any unallocated atoms. Therefore, these results corroborate that the Si8O12 cages of compounds are empty voids.

Figure 6

Packing in the crystal structure of compound 4b. The hydrogen atoms have been omitted for clarity. The ellipsoids are drawn at the 30% probability level.

In 4a–d all interatomic distances and angles are quite uniform, indicating a comparable steric situation for this continuous series of tertiary alkyl substituted POSS cages which is not significantly affected by the chain extension (Table 3). Moreover, the observed data fit well with those available from the literature.[10,12] The internal Si···Si distances reflect the geometry of the Si8O12 cage (Figure 7). Although the data vary only marginally among 4a–d, the lower limit of the p-Si···Si distances decreases constantly with increasing chain length (Table 3).

Table 3

Selected Bond Lengths (Å) and Angles (deg) of 4a–d

	4a	4b	4c	4d
Si–O	1.6217(16)–1.6308(15)	1.617(13)–1.6203(12)	1.6215(6)–1.6269(7)	1.616(3)–1.626(3)
Si–C	1.868(2)–1.872(2)	1.865(4)–1.865(2)	1.8634(8)–1.8727(8)	1.861(4)–1.873(4)
o-Si···Si (A)	3.12–3.16	3.12	3.11–3.14	3.12–3.14
m-Si···Si (B)	4.42–4.45	4.42	4.40–4.43	4.42–4.44
p-Si···Si (C)	5.43–5.44	5.41	5.40–5.44	5.39–5.43
Si–O–Si	146.76(11)–152.81(11)	148.96(8)–149.53(9)	146.06(4)–151.20(4)	147.84(18)–151.62(18)
O–Si–O	108.83(9)–109.75(10)	108.78(7)–109.17(7)	108.25(3)–109.43(3)	108.30(14)–109.47(14)
O–Si–C	109.19(10)–110.83(10)	109.33(8)–110.35(7)	108.14(4)–112.48(4)	108.61(16)–111.37(16)

Figure 7

Interatomic Si···Si distances in T8 POSS cages.[12]

Summary and Conclusion

In summary, we have shown that the treatment of silanetriols H3C(CH2)(CH3)2CSi(OH)3 (n = 1–5) with trifluoroacetic acid results in the exclusive formation of the corresponding primary condensation products [H3C(CH2)(CH3)2CSi(OH)2]2O. The range of known disiloxane-1,1,3,3-tetrols could be extended by a series of successively CH2 elongated alkyl-substituted congeners which may have interesting properties as gemini surfactants, a class of compounds which has attracted much attention in recent years.[28] These properties will be investigated in the future. In addition, a synthetic access toward the first T8 POSS cages bearing tertiary carbon substituents was possible. In a variation of Bassindale’s TBAF protocol the corresponding T8 cages could be isolated in pure form in good yields starting from the aforementioned silanetriols H3C(CH2)(CH3)2CSi(OH)3 (n = 0–2) and disiloxane-1,1,3,3-tetrol [H3C(CH2)(CH3)2CSi(OH)2]2O (n = 3). High-quality X-ray diffraction data confirm the presence of an empty void within the Si8O12 cages and therefore encourage future investigation of the encapsulation of small molecules and neutral atoms or ions.[11,29−32] The fact that POSS cages are accessible from silanetriols which are not accessible from the corresponding trialkoxysilanes under comparable conditions indicate that discrete silanetriols may not necessarily occur in protocols based on alkoxysilanes, even at low pH values where hydrolysis is expected to outpace condensation.

Experimental Section

All manipulations were carried out under an inert argon atmosphere using standard Schlenk techniques. All solvents were dried and freshly distilled over Na/K alloy (Et2O, THF, pentane, heptane) or CaH2 (NEt3, CH2Cl2, CDCl3). Compound 1a was prepared according to literature procedures.[33] The preparation of the silanetriols 1b–f and trichlorosilane 3 has been described elsewhere.[17] TBAF (1 M in THF) and trifluoroacetic acid have been purchased and used without further purification.

1H and 13C NMR data have been recorded on a Bruker Avance III 300 MHz spectrometer (operating at 300 and 75.4 MHz) or a Varian MR-400 MHz spectrometer (operating at 400 and 100.5 MHz). 29Si NMR data have been recorded on a Bruker Avance III 300 MHz spectrometer (operating at 59.6 MHz) or a Varian VNMRS 500 MHz spectrometer (operating at 99.3 MHz). All measurements have been performed at room temperature using TMS as external standard.

Solid-state NMR spectra at 9.4 T (ν0 = 79.5 MHz (29Si) or 100.6 MHz (13C)) were recorded at ambient temperature (296 K) on a Bruker Avance 400 spectrometer equipped with a 4 mm MAS probe. All spectra were acquired using magic angle spinning (MAS) at spinning speeds between 9 and 14 kHz, and high power 1H decoupling was applied during acquisition.

EI-mass spectra have been recorded on a Agilent Technologies 5975C inert XL MSD with SIS Direct Insertion Probe. ESI and APCI mass spectra have been recorded on a Finnigan LCQ Deca (ThermoQuest). HR-ESI-MS spectra have been recorded using a micrOTOF (Bruker Daltonics) and an Apollo “Ion Funnel” ESI ion source. IR spectra have been recorded using a Bruker Alpha Platinum ATR spectrometer. Elemental analysis data have been recorded on a Euro EA Elementaranalysator of HEKAtech. X-ray diffraction measurements were performed on a Stoe IPDS2 or a Bruker-AXS SMART APEX 2 CCD diffractometer using graphite-monochromated Mo Kα radiation. The structures were solved using direct methods (SHELXL-97) and refined by full-matrix least-squares techniques against F2 (SHELXL-97). Details of the structure determinations and refinement for 2b,d,g and 4a–d are summarized in Table S1 (Supporting Information). Supplementary crystallographic data for this paper can be obtained free of charge by quoting CCDC 1031585–1031591 from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Synthesis of the 1,3-Bis(1,1-dimethyl-n-alkyl)disiloxane-1,1,3,3-tetrols 2b–f

Trifluoroacetic acid (0.15 mL, 0.22 g, 1.9 mmol) was added to a solution of 0.3 mmol of the corresponding tertiary silanetriol 1b–f in 2 mL of THF at room temperature. After partial evaporation of the volatiles the compounds 2b–f were obtained as pale crystalline solids which were washed with pentane and dried in vacuo. The yields ranged between 60 and 84%.

1,3-Bis(1,1-dimethylpropyl)disiloxane-1,1,3,3-tetrol (2b)

Yield: 83%. 1H NMR (300 MHz, THF-d8): δ (ppm) 4.87 (b, 4H, −Si–OH), 1.40 (m, 4H, −CH2−), 0.94 (s, 12H, −C(CH3)2), 0.90 (t, 3JHH = 7.5 Hz, 6H, −CH3). 13C NMR (75.4 MHz, THF-d8): δ (ppm) 32.7 (CH2), 23.7 (Cq), 21.7 (CH3), 9.5 (CH3). 29Si NMR (59.6 MHz, THF-d8): δ (ppm) −49.9. IR: 3126 (b), 2946 (s), 2865 (w), 1463 (w), 1109 (s), 847 (s), 704 (s), 624 (w), 423 (s) cm–1. MS/EI: m/z (%) 282 (5) [M]+, 71 (20) [C5H11]+, 57 (100) [C4H9]+. MSAPCI(+): m/z (%) 586 (100) [2 M + Na]+, 868(37) [3 M + Na]+, 305 [M + Na]+. HRMS/ESI(+): m/z 305.1222 [M + Na]+, calcd for [C10H26O5Si2Na]+ 305.1216. Anal. Calcd for C10H26O5Si2: C, 42.45; H, 9.26. Found: C, 42.34; H, 9.37.

1,3-Bis(1,1-dimethylbutyl)disiloxane-1,1,3,3-tetrol (2c)

Yield: 84%. 1H NMR (300 MHz, THF-d8): δ (ppm) 4.88 (b, 4H, −Si–OH), 1.45–1.28 (m, 8H, −CH2–CH2−), 0.96 (s, 12H, −C(CH3)2), 0.86 (m, 6H, −CH3). 13C NMR (75.4 MHz, THF-d8): δ (ppm) 43.5 (CH2), 24.3 (CH2), 21.7 (Cq), 18.7 (CH3), 15.5 (CH3). 29Si NMR (59.6 MHz, THF-d8): δ (ppm) −51.9. IR: 3103 (b), 2954 (s), 2865 (w), 1463 (w), 1113 (s), 848 (s), 717 (w), 650 (w), 431 (s) cm–1. HRMS/ESI(+): m/z 333.1529 [M + Na]+, calcd for [C12H30O5Si2Na]+ 333.1529. Anal. Calcd for C12H30O5Si2: C, 46.41; H, 9.74. Found: C, 46.66; H, 9.63.

1,3-Bis(1,1-dimethylpentyl)disiloxane-1,1,3,3-tetrol (2d)

Yield: 79%. 1H NMR (300 MHz, THF-d8): δ (ppm) 4.89 (b, 4H, −Si–OH), 1.37–1.21 (m, 12H, −CH2–CH2–CH2−), 0.95 (s, 12H, −C(CH3)2), 0.89 (m, 6H, −CH3). 13C NMR (75.4 MHz, THF-d8): δ (ppm) 40.5 (CH2), 27.6 (CH2), 24.6 (CH2), 24.2 (Cq), 21.4 (CH3), 14.4 (CH3). 29Si NMR (59.6 MHz, THF-d8): δ (ppm) −51.7. IR: 3125 (b), 2952 (s), 2862 (w), 1463 (w), 1115 (s), 854 (s), 721 (s), 668 (s), 450 (s) cm–1. MS/EI: m/z (%) = 338 (2) [M]+, 239 (40) [M - C7H15]+, 155 (100), 99 (5) [C7H15]+, 57 (30) [C4H9]+. MS/ESI(+): m/z (%) 361 (100) [M + Na]+, 699 (2) [2 M + Na]+. HRMS/ESI(+): m/z 361.1842 [M + Na]+, calcd for [C14H34O5Si2Na]+ 361.1837. Anal. Calcd for C14H34O5Si2: C, 49.66; H, 10.12. Found: C, 49.78; H, 10.09.

1,3-Bis(1,1-dimethylhexyl)disiloxane-1,1,3,3-tetrol (2e)

Yield: 60%. 1H NMR (300 MHz, THF-d8): δ (ppm) 4.88 (b, 4H, −Si–OH), 1.40–1.23 (m, 16H, −CH2–CH2–CH2–CH2−), 0.96 (s, 12H, −C(CH3)2), 0.88 (m, 6H, −CH3). 13C NMR (75.4 MHz, THF-d8): δ (ppm) 40.7 (CH2), 34.1 (CH2), 25.0 (CH2), 24.2 (CH2), 23.5 (Cq), 21.5 (CH3), 14.4 (CH3). 29Si NMR (59.6 MHz, THF-d8): δ (ppm) −49.9. IR: 3096 (b), 2950 (s), 2860 (w), 1464 (w), 1116 (s), 849 (s), 722 (s), 669 (s), 427 (s) cm–1. MS/EI: m/z (%) 366 (2) [M]+, 253 (40) [M – C8H17]+, 155 (100), 113 (5) [C8H17]+, 57 (30) [C4H9]+. HRMS/ESI(+): m/z 389.2138 [M + Na]+, calcd for [C16H38O5Si2Na]+ 389.2155. Anal. Calcd for C16H38O5Si2: C, 52.41; H, 10.45. Found: C, 52.07; H, 10.56.

1,3-Bis(1,1-dimethylheptyl)disiloxane-1,1,3,3-tetrol (2f)

Yield: 63%. 1H NMR (400 MHz, THF-d8): δ (ppm) 5.02 (s, 4H, −Si–OH), 1.40–1.24 (m, 20H, −CH2–CH2–CH2–CH2–CH2−), 0.95 (s, 12H, −C(CH3)2), 0.88 (m, 6H, −CH3). 13C NMR (100.5 MHz, THF-d8): δ (ppm) 41.0 (CH2), 33.0 (CH2), 31.7 (CH2), 25.3 (CH2), 24.4 (CH2), 23.7 (Cq), 21.6 (CH3), 14.5 (CH2). 29Si NMR (99.3 MHz, THF-d8): δ (ppm) −49.9. IR: 3127 (b), 2926 (s), 2857 (s), 1465 (w), 1080 (b), 856 (s), 722 (w), 670 (w), 429 (w) cm–1. MS/ESI(+): m/z (%) 417 (100) [M + Na]+. HRMS/ESI(+): m/z 417.2455 [M + Na]+, calcd for [C18H42O5Si2Na]+ 417.2468. Anal. Calcd for C18H42O5Si2: C, 54.77; H, 10.73. Found: C, 54.57; H, 10.99.

1,3-Bis(1,1-dimethyloctyl)disiloxane-1,1,3,3-tetrol (2g)

A solution of 2.71 g (9.8 mmol; 1 equiv) of (1,1-dimethyloctyl)trichlorosilane (3) in Et2O was slowly added to a solution of 0.53 g (29.4 mmol; 3 equiv) of H2O and 2.97 g (29.4 mmol; 3 equiv) of NEt3 in Et2O at 0 °C. After the addition was complete, the reaction mixture was slowly warmed to room temperature and stirred for further 3 h. The white precipitate was removed by filtration, and all volatiles were removed in vacuo. The remaining solid was washed with pentane and recrystallized from Et2O/heptane (1/1). Compound 2g could be obtained in 72% yield (1.5 g, 3.5 mmol) as a pale crystalline material.

1H NMR (300 MHz, THF-d8): δ (ppm) 4.89 (s, 3H, −Si–OH), 1.40–1.23 (m, 16H, −CH2–CH2–CH2–CH2−), 0.96 (s, 12H, −C(CH3)2), 0.88 (m, 6H, −CH3). 13C NMR (75.4 MHz, THF-d8): δ (ppm) 41.0 (CH2), 33.0 (CH2), 31.9 (CH2), 30.7 (CH2), 30.5 (CH2), 24.6 (CH2), 23.6 (Cq), 21.6 (CH3), 14.5 (CH3). 29Si NMR (59.6 MHz, THF-d8): δ (ppm) −49.9. IR: 3080 (b), 2925 (s), 2856 (s), 1463 (w), 1119 (s), 857 (s), 722 (s), 670 (s), 433 (s) cm–1. MS/EI: m/z (%) 422 (2) [M]+, 281 (70) [M – C10H21]+, 219 (10) [M – C10H23O2Si]+, 203 (5) [M – C10H23O3Si]+, 141 (70) [C10H21]+, 57 (100) [C4H9]+. MS/ESI(+): m/z (%) 699 (2) [2 M + Na]+, 445 (100) [M + Na]+. HRMS/ESI(+): m/z 445.2763 [M + Na]+, calcd for [C20H46O5Si2Na]+ 445.2781. Anal. Calcd for C20H46O5Si2: C, 56.82; H, 10.97. Found: C, 57.31; H, 10.99.

Synthesis of the Octakis(1,1-dimethyl-n-alkyl)octasilsesquioxanes 4a–c

One equivalent of TBAF (1 M in THF) was added to a solution of 2 equiv of the corresponding tertiary silanetriol 1a–c in THF at room temperature. After the mixture was stirred for 24 h, all volatiles were removed in vacuo and the yellow oil was mixed with acetone. The white precipitate was filtered off and recrystallized from acetone/dichloromethane. The compounds 4b,c could be obtained as pale crystalline materials in yields of 40–44%. Compound 4a was obtained in 60% yield as a pale, poorly soluble powder which was washed several times with acetone and dried in vacuo.

Octakis(tert-butyl)octasilsesquioxane (4a)

Yield: 60%. 1H NMR (250 MHz, acetone-d6): δ (ppm) 1.04 (s, −C(CH3)3). 13C NMR (CP-MAS): δ (ppm) 55.6 (Cq), 24.2 (CH3). 29Si NMR (CP-MAS): δ (ppm) −68.4. IR: 2957 (w), 2935 (w), 2859 (w), 1474 (w), 1234 (w), 1095 (s), 837 (w), 532 (s), 467 (s), 418 (s) cm–1. MS/EI: m/z (%) 872 (10) [M]+, 815 (100) [C28H63Si8O12]+, 57 (45) [C4H9]+. MS/ESI(+): m/z 895.29 (17) [M + Na]+. HRMS/ESI(+): m/z 895.3070 [M + Na]+, calcd for [C32H72O12Si8Na]+ 895.3076. Anal. Calcd for C32H72O12Si8: C, 44.00; H, 8.31. Found: C, 43.98; H, 8.40.

Octakis(1,1-dimethylpropyl)octasilsesquioxane (4b)

Yield: 44%. 1H NMR (400 MHz, CDCl3): δ (ppm) 1.40 (q, 3JHH = 7.5 Hz, 16H, −CH2−), 0.97 (s, 48H, −C(CH3)2), 0.92 (t, 3JHH = 7.5 Hz, 24H, −CH3). 13C NMR (100.5 MHz, CDCl3): δ (ppm) 32.1 (Cq), 23.1 (CH3), 20.9 (CH2), 9.5 (CH3). 29Si NMR (99.3 MHz, CDCl3): δ (ppm) −67.2; IR: 2963 (w), 2860 (w), 1460 (w), 1260 (w), 1083 (s), 836 (w), 798 (s), 548 (s), 482 (s), 416 (s) cm–1. MS/EI: m/z (%) 984 (10) [M]+, 914 (10) [C35H78O12Si8]+, 844 (30) [C30H68O12Si8]+, 774 (10) [C25H58O12Si8]+, 704 (5) [C20H48O12Si8]+, 71 (100) [C5H11]+. HRMS/ESI(+): m/z 1023.4061 [M + K]+, calcd for [C40H88O12Si8K]+ 1023.4061. Anal. Calcd for C40H88O12Si8: C, 48.73; H, 9.00. Found: C, 48.51; H, 8.89.

Octakis(1,1-dimethylbutyl)octasilsesquioxane (4c)

Yield: 40%. 1H NMR (300 MHz, CDCl3): δ (ppm) 1.45–1.26 (m, 32H, −CH2–CH2−), 0.98 (s, 48H, −C(CH3)2), 0.87 (t, 3JHH = 6.8 Hz, 24H, −CH3). 13C NMR (75.4 MHz, CDCl3): δ (ppm) 42.9 (Cq), 23.6 (CH3), 20.7 (CH2), 18.8 (CH2), 15.0 (CH3). 29Si NMR (59.6 MHz, CDCl3): δ (ppm) −67.4. IR: 2956 (w), 2870 (w), 1469 (w), 1259 (w), 1067 (s), 838 (s), 798 (s), 545 (s), 447 (s) cm–1. MS/EI: m/z (%) 1096 (10) [M]+, 928 (10) [C36H80O12Si8]+, 844 (5) [C30H68O12Si8]+, 760 (2) [C24H56O12Si8]+, 591 (5) [C12H32O12Si8]+, 507 (5) [C6H20O12Si8]+, 85 (100) [C6H13]+. HRMS/ESI(+): m/z 1119.5561 [M + Na]+, calcd for [C48H104O12Si8Na]+ 1119.5574. Anal. Calcd for C48H104O12Si8: C, 52.51; H, 9.55. Found: C, 52.73; H, 9.59.

Octakis(1,1-dimethylpentyl)octasilsesquioxane (4d)

One equivalent of TBAF (1 M in THF) was added to a solution of 2 equiv of the corresponding disiloxane-1,1,3,3-tetrol 2d in THF at room temperature. After the mixture was stirred for 24 h, all volatiles were removed in vacuo and the yellow oil was mixed with acetone. The white precipitate was filtered off and recrystallized from acetone/dichloromethane. Compound 4d could be obtained as a pale crystalline material in 10% yield.

Yield: 10%. 1H NMR (300 MHz, CDCl3): δ (ppm) 3.31–3.26 (m, 16H, −CH2−), 1.63–1.55 (m, 16H, −CH2−), 1.42–1.33 (m, 16H, −CH2−), 1.18 (s, 48H, −C(CH3)2), 0.93 (t, 3JHH = 7.1 Hz, 24H, −CH3). 13C NMR (75.4 MHz, CDCl3): δ (ppm) 58.8 (CH2), 31.8 (Cq), 29.3 (CH3), 24.1 (CH2), 19.8 (CH2), 13.7 (CH3). 29Si NMR (59.6 MHz, CDCl3): δ (ppm) −66.9. HRMS/ESI(+): m/z 1231.6850 [M + Na]+, calcd for [C56H120O12Si8Na]+ 1231.6826. Anal. Calcd for 2(C56H120O12Si8)·C16H36FN: C, 57.32; H, 10.37. Found: C, 57.69; H, 10.51.