cis,cis-1,3,5-Trihydroxynonamethylcyclohexasilane - ACS Publications

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cis,cis-1,3,5-Trihydroxynonamethylcyclohexasilane: A Cyclopolysilane with Unusual Properties Harald Stueger,* Joerg Albering, Michaela Flock, Gottfried Fuerpass, and Thomas Mitterfellner Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria

bS Supporting Information ABSTRACT: cis,cis-Trihydroxynonamethylcyclohexasilane (2) is easily accessible by the controlled hydrolysis of Cl3Si6Me9 in the presence of Et3N as an auxiliary base. The crystal structure of 2, as determined by single-crystal X-ray crystallography, exhibits “barrel-type” face-to-face dimeric aggregates held together by six intermolecular hydrogen bonds. NMR and IR spectra suggest that these OH-bonded aggregates are also present in nonpolar solutions. Full geometry optimization (B3LYP/TZVP) of the gas-phase structure of 2 further reveals unusually high energy differences between different conformers and a considerable stabilization of the molecule upon dimerization due to O
H hydrogen bonding. Time-dependent DFT B3LYP/TZVP calculations allow for a detailed interpretation of the UV absorption spectra of 2. The electronic transitions occur between occupied molecular orbitals with predominant σ(Si
Si) character and a small contribution of the oxygen lone pairs, and virtual MOs with contributions from σ*(Si
C), σ*(Si
O), and σ*(Si
Si) type orbitals. If 2 is reacted with MeSiCl3/Et3N, the adamantane-like cage MeSi(O3Si6Me9) (4) is obtained without the formation of considerable amounts of polymeric material, which is most likely a consequence of the preferred conformation of 2 with three adjacent OH groups in axial positions.

’ INTRODUCTION Siloxene, a two-dimensional layered polymer with the empirical formula Si6H6
n(OH)n consisting of stacked Si planes terminated by
H,
OH, or
O groups, is strongly photoluminescent and, therefore, is considered as an alternative material to crystalline silicon for the manufacture of Si-based luminescent devices.1 The physical and structural characterization of siloxene is, however, strongly impeded by its insolubility in organic solvents. In a preceding study we were able to prepare polysiloxane polymers with cyclosilane subunits, which exhibit improved solubility and properties closely resembling those of siloxene with photoemission maxima between 400 and 550 nm.2 Since only polymers derived from cyclic starting materials exhibit color and fluorescence, we assume the polysilane ring to be the chromophore responsible for the optical properties observed. Moreover, we found that even monomeric permethylcyclohexasilanes with siloxy substituents, Si6Me12
n(OSiR3)n (n = 1
3), display room-temperature photoluminescence with fluorescence intensities remarkably enhanced and UV absorption maxima bathochromically shifted in comparison to the unsubstituted Si6Me12.3 This unexpected observation strongly supports earlier assumptions that the luminescence of siloxene is a molecular rather than a solid-state property.4 Thus, we consider cyclohexasilanes of the general formula Si6Me12
n(OR)n (R = H, alkyl, silyl) to be excellent model substances to explore and elucidate the outstanding optical properties of siloxene. r 2011 American Chemical Society

Structural and UV absorption characteristics of undecamethylcyclohexasilanol, HOSi6Me11 (1), have recently been investigated using a combined experimental and theoretical approach.5 It has been found that the substituent effects apparent in the UV absorption spectra are primarily due to the contribution of substituent lone pair orbitals and σ*(Si
O) orbitals to the occupied and unoccupied molecular orbitals, respectively. Furthermore, computational results suggest that, for cyclic oligosilanes, several conformers of similar energies contribute to the UV absorption spectra, just as was found earlier for their open-chained counterparts.6 In this paper an analogous approach is applied to examine selected spectral and structural properties of the previously unknown compound 1,3,5-trihydroxynonamethylcyclohexasilane (2), which has been prepared by the controlled hydrolysis of 1,3,5-trichlorononamethylcyclohexasilane.

’ RESULTS AND DISCUSSION Synthesis and Characterization. Hengge et al. demonstrated earlier that one, two, or three of the methyl groups present in (Me2Si)6 (3) can be replaced by chlorine with SbCl5 under very mild conditions.7 On the basis of this general method, we discovered a convenient route to 2 (compare Scheme 1). When 3 was reacted with 2 equiv of SbCl5, a mixture of 1,3,5-Cl3Si6Me9 Received: January 12, 2011 Published: March 17, 2011 2531

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Scheme 1

and 1,4-Cl2Si6Me10 was obtained, which afforded the corresponding silanols by addition of water in the presence of Et3N as an auxiliary base. Isolation of pure 2 was easily accomplished, because cis,cis-2 precipitated directly from the crude reaction mixture due to its low solubility in organic solvents. The 1H NMR spectrum of the isolated product features three methylsilyl signals at 0.56, 0.51, and
0.007 ppm, which allows us to conclude that only three magnetically nonequivalent methyl groups, HOSiCH3, Si(CH3)2 axial, and Si(CH3)2 equatorial, are present in the molecule. In accordance with the formation of pure cis,cis-2, only two signals of equal intensity at 12.41 (SiMeOH) and
47.78 ppm (SiMe2), are detected in the 29Si NMR spectrum. Solid-State Structure of 2. Crystals of 2 suitable for an X-ray diffraction study were grown from a THF solution by slowly evaporating the solvent at room temperature. A drawing of the molecular structure is given in Figure 1, along with key bond lengths and angles. 2 crystallizes in the monoclinic space group C2/c. The core of the structure is the cyclohexasilanyl ring in a chair conformation. All OH groups are oriented in the same direction with respect to the ring plane and occupy axial positions. The average Si
Si distance of 2.352 Å is unexceptional. The mean Si
O bond length of 1.675 Å is close to the average Si
OH distance for silanols of 1.64 Å,8 which is markedly shorter than the Si
O bond length of 1.77 Å estimated from the sum of the covalent radii of silicon and oxygen. Due to the impact of the electronegative OH substituents the Si(O)
C bonds are significantly shortened by 0.012 Å as compared to the average bond distance of 1.890 Å observed for the residual Si
C bonds. The geometry around the silicon atoms of 2 is approximately tetrahedral, with Si
Si
Si bond angles close to the respective angles found in the similar cyclohexasilanol 15 or 1,3- and 1,4-(HO)2Si6Me10.9 In the

Figure 1. Molecular structure of 2 in the crystal (50% thermal probability ellipsoids). The structure consists of two molecules linked by six intermolecular hydrogen bridges. Each HO proton is disordered over two positions (H10/H11, H12/H13, and H14/H15, respectively). CH3 hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) with estimated standard deviations: Si(1)
O(1) 1.6751(5), Si(3)
O(2) = 1.6764(5), Si(5)
O(3) = 1.6727(5), Si(1)
Si(2) = 2.3521(2), Si(2)
Si(3) = 2.3517(2), Si(3)
Si(4) = 2.3511(2), Si(4)
Si(5) = 2.3471(2), Si(5)
Si(6) = 2.3518(2), Si(1)
Si(6) = 2.3473(2), Si(1)
C(1) = 1.8786(6), Si(2)
C(2) = 1.8887(7), Si(2)
C(3) = 1.8905(7), Si(3)
C(4) = 1.8773(6), Si(4)
C(6) = 1.8902(7), Si(4)
C(5) = 1.8908(7), Si(5)
C(7) = 1.8792(7), Si(6)
C(8) = 1.8904(7), Si(6)
C(9) = 1.8889(7); Si(6)
Si(1)
Si(2) = 113.011(9), Si(3)
Si(2)
Si(1) = 111.676(9), Si(4)
Si(3)
Si(2) = 112.468(9), Si(5)
Si(4)
Si(3) = 110.847(9), Si(4)
Si(5)
Si(6) = 112.606(9), Si(1)
Si(6)
Si(5) = 108.954(8), O
Si
Si(mean) = 108.6, O
Si
C(mean) = 108.7, C
Si
C(mean) = 108.5, C
Si
Si(mean) = 109.3.

crystal packing two molecules are bridged by six intermolecular hydrogen bonds, forming “barrel-type” dimers just as were observed for the isostructural six-membered rings [RSi(OH)O]3 and [RSi(OH)NH]3 (R = (Me3Si)2CH) or by hydroxy-substituted oligosilane dendrimers MeSi[SiOHMeSiMe(SiMe3)2]3 recently reported in the literature.8,10 It was possible to locate the three hydroxyl protons in the difference Fourier map. Each of them turned out to be disordered over two positions. The average O
H 3 3 3 O distance was determined as 1.95 Å, with an almost linear O
H
O angle of 169.2°. The dimers are packed in a columnlike fashion (compare Figure 2) comprising double layers of molecules joined in a faceto-face manner reminiscent of the structure of c-C6H11Si(OH)3.11 The methyl groups and the OH groups thus form alternating hydrophobic and hydrophilic double sheets with a highly hydrophobic outer surface which prevents further interaction between the sheets. Structure of 2 in Solution. According to DFT (B3LYP/ TZVP) full geometry optimizations, 2 exhibits four minima on the potential energy surface corresponding to the cis,cis all-axial chair conformer 2a, the twist-boat structure 2b, the all-equatorial chair 2c, and the twist conformer 2d with relative energies of 0.0, 13.79, 20.29, and 22.91 kJ mol
1, respectively (compare Figure 3).12 Furthermore, our calculations clearly demonstrate a considerable stabilization of 2a upon dimerization. On the basis of eq 1, 2532

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Figure 4. UV absorption spectra of 1
3 (n-hexane solution, c = 10
4 M).

Figure 2. Columnlike arrangement of OH-bonded dimers in crystalline 2 forming alternating hydrophobic and hydrophilic double sheets (methyl groups are omitted for clarity). d(O
O)intramol. = 4.00 ( 0.12 Å, d(O
O)intermol. = 2.72 ( 0.02 Å.

Figure 3. DFT (B3LYP/TZVP)-optimized geometries and relative energies of monomeric and dimeric 2..

where Emonomer and Edimer are the calculated B3LYP/TZVP zeropoint vibration-corrected energies, the dimerization energy of 2a in the gas phase is estimated to be
89.06 kJ molup>
1 (
44.53 kJ mol
1 per monomeric unit). This result strongly suggests the presence of dimeric aggregates rather than isolated monomers, at least in nonpolar solvents. ΔEdimerization ¼ Edimer
2Emonomer

ð1Þ

As a matter of fact, there is pronounced spectroscopic evidence for the presence of OH-bonded aggregates of 2 in nonpolar solutions. Both in the solid state and in CCl4 solution, the stretching

mode of the OH group appeared as a broad intensive band centered at about 3240 cm
1 in the IR spectra, which is characteristic of hydrogen-bonded silanols.13 Furthermore, 2 exhibits a 1H NMR signal for the OH proton remarkably shifted to lower field at δ(1H) 6.53 ppm in C6D6 solution. This finding is reproduced quite nicely by DFT (B3LYP/IGLO-III) calculations,14 predicting δ(1H) 7.1 ppm for the OH-bonded dimer d-2a but only 3.1 ppm for the monomeric species. Both results are in agreement with literature data recently published for related systems. On the basis of the results of temperature-dependent IR and 1H NMR measurements Krempner et al. suggest dynamic monomer/dimer equilibria for OH-containing oligosilane dendrimers MeSi[SiOHMeSiMe(SiMe3)2]3 in solution and hydrogen-bonded dimeric structures at room temperature.10c For trisilanols of the type R7Si7O9(OH)3 (POSS-triol) Unno et al. observed a dynamic equilibrium between a single molecule and an intermolecular hydrogenbonded dimer in C6D6 or CDCl3 solution by 1H NMR dilution experiments.15 With decreasing concentration the POSS-triol dimer was shown to gradually dissociate to form monomeric POSS-triol, which is indicated by an upfield shift of the OH signal from 6.3 ppm by 0.9 ppm upon dilution from 10
2 to 6.25  10
4 M. In contrast, dilution of C6D6 solutions of 2 (10
3
10
4 M) caused virtually no shift (Δδ < 0.1 ppm), which suggests that dimeric 2 is even more stable in solution than dimeric POSS-triol and dissociation is negligible in nonpolar solvents. In the coordinating solvent THF, however, the OH signal of 2 is shifted strongly upfield to 4.16 ppm, which is very likely caused by extensive dissociation of dimeric 2 under formation of monomeric species stabilized by specific interactions with the solvent. UV Absorption Spectra and Computational Assignment. Cyclohexasilane derivatives usually show only small energy differences between different conformers because of the high flexibility of the polysilane framework.16 In 1, for instance, the equatorial chair has been calculated to be only 0.9 kJ mol
1 higher in energy than the axial chair conformer.5 Due to the proximity of the three OH groups in 2a, however, intramolecular hydrogen bonding is much more effective than in the other conformers, which leads to a stabilization of 2a by nearly 14 kJ mol
1 in comparison to 2b (compare Figure 3). As a consequence, Boltzmann statistics predict that conformers 2b
d 2533

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Table 1. Experimental Absorption Data for 2 and TDDFT B3LYP/TZVP Calculated Excitation Wavelengths and Oscillator Strengths for 2a and d-2a d-2a λ (nm)

f (au)

282.5 0.00

exptl λmax (ε)a

2a λ (nm)

f (au)

n-hexane

292.6 2.00  10

-5

THF

280 sh (700) 291 (670)

274.8 5.95  10
4 260.6 2.00  10
6 258.0 1.72  10-2 258.5 3.17  10-2 256 (4400)

255 sh (2600)

255.4 0,00 247.3 5.32  10-4 237.5 6.05  10-3 246 sh (3800) 244.6 0.00 238.6 1.44  10
5 238.1 0.00 231.3 0.00 230.1 1.27  10-2 224.0 2.16  10-2 220 sh (7100) 227.1 3.71  10
5 217.1 6.83  10
3 225.8 3.12  10
3 211.7 2.26  10
3 225.7 0.00 224.9 0.00 223.2 1.38  10
2

λmax values are given in nm and ε values in L mol
1 cm
1; spectra were recorded in solution with c = 10
4 M. a

Figure 5. Contour plots and energies of DFT B3LYP/TZVP calculated HOMOs and LUMOs of 3 (a), 1 (b) and 2a (c).20 The HOMOs of 1 and 2a show rather small contributions from the oxygen lone pairs, which are indicated by two lobes on each side of the OH groups. The respective LUMOs are distinctly influenced by antibonding σ*(Si
O) orbitals, with a smaller lobe on the oxygen atom and a larger lobe underneath the silicon atom.

merely account for less than 0.5% of an equilibrium mixture of 2 at room temperature. Thus, discussions of spectral properties of 2 can be confined solely to the most stable conformer 2a. Note that this is in contrast to the situation encountered with 1 and 3 or with open-chain permethylpolysilanes, where contributions from several conformers very close in energy need to be considered in order to achieve satisfactory agreement between calculated and experimental UV absorption data.6

Figure 4 shows the experimental UV absorption spectrum of 2 together with the spectra of 1 and 3. To allow for the interpretation of the low-energy UV absorption bands, the electronic transitions of 2 were calculated using time-dependent DFT (B3LYP/TZVP). Experimental and calculated absorption wavelengths and intensities of monomeric and dimeric 2a can be found in Table 1. The low-energy absorption bands visible in the UV spectra of permethylcyclopolysilanes (SiMe2)n are generally assigned to σ f σ* type excitations of highly delocalized Si
Si σ electrons within the Si
Si backbone.17 Upon introduction of the
OH groups the first absorption maximum observed in hydrocarbon solution is red-shifted from 256 nm for 318 to 270 nm for 1 and 282 nm for 2. The bathochromic substituent effect as apparent in the absorption spectra of 1
3 is consistent with earlier observations in UV
vis spectra of heterosubstituted polysilanes.19 Our calculations enable us to shed light on the nature of the perturbation the substituent orbitals exercise on the electronic structure of the Si
Si backbone. Figure 5 depicts the shape and the energy of the calculated frontier orbitals involved in the longest wavelength electron transitions. In all cases an electron is excited from occupied orbitals with predominant σ(Si
Si) contribution, which are delocalized over the cyclohexasilanyl framework, to strongly delocalized unoccupied orbitals with a high σ*(Si
Si) and σ*(Si
C) character. The corresponding HOMOs of 1 and 2 additionally exhibit small contributions of oxygen lone pairs, which leads to a slight destabilization, while the LUMOs have a notable portion of antibonding σ*(Si
O) type character, which stabilizes them to a greater extent. These combined effects result in a significant reduction of the HOMO
LUMO gap from 5.80 eV in 3 to 5.58 eV in 1 and 5.15 eV in 2. Similar conclusions were drawn by Krempner et al., who investigated the UV absorption properties of open-chained oxygen-containing oligosilanes.19g Moreover, there is excellent agreement between the position of the absorption bands appearing in the experimental spectra of 2 with the values calculated for 2a and d-2a, respectively (compare Table 1). Both for the monomer and for the dimer the longest wavelength absorption band is computationally assigned to the HOMO f LUMO transition, which is symmetry forbidden and therefore appears in the experimental spectra only with rather low intensity. For d-2a the position of this first absorption maximum is predicted at 282.5 nm, which excellently coincides with the experimental value of λmax 280 nm observed for hydrocarbon solutions of 2 containing OH-bonded dimeric aggregates rather than isolated monomers. In THF a red shift of the first absorption maximum to 291 nm is observed, which is reproduced quite nicely by the value calculated for monomeric 2a. In addition to NMR spectroscopy, UV absorption spectra thus provide further evidence for the dissociation of dimeric 2 in coordinating solvents, which yields monomeric species stabilized by specific interactions with the given solvent. Chemical Properties of 2. Intermolecular hydrogen bonding is also reflected by selected chemical properties of 2. Compound 2 turned out to be unusually stable toward self-condensation in the crystalline state and can be stored at 25 °C for months without any noticeable change. In THF solution containing monomeric rather than dimeric species as outlined above, however, 2 isomerizes and decomposes further within several days at room temperature (Scheme 2). As depicted in Figure 6, after several days at room temperature additional signals appear in the 1H NMR spectrum of cis,cis-2 dissolved in THF, which indicated the formation of increasing amounts of the cis,trans isomer. After prolonged storage 2534

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Figure 6. 1H NMR spectrum (Si
methyl region) of THF solutions of 2: (a) as prepared; (b) after 10 days; (c) after 4 weeks at 25 °C.

Scheme 2

Figure 7. Molecular structure of 4 in the crystal (50% probability thermal ellipsoids). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg) with estimated standard deviations: Si(1)
O(1) = 1.6730(15), Si(3)
O(2) = 1.6738(10), Si(5)
O(1) = 1.6335(15), Si(5)
O(2) = 1.6335(10), Si(1)
Si(2) = 2.3610(6), Si(2)
Si(3) = 2.3612(9), Si(3)
Si(4) = 2.3616(2); Si(2)#1
Si(1)
Si(2) = 111.18(3), Si(3)
Si(2)
Si(1) = 99.08(2), Si(4)
Si(3)
Si(2) = 110.83(2), Si(3)
Si(4)
Si(3)#1 = 98.92(3), C
Si
Si(mean) = 113.6, Si(5)
O(1)
Si(1) = 129.11(9), Si(5)
O(2)
Si(3) = 129.66(6).

oligomerization finally occurs due to self-condensation of the less stable cis,trans isomer as shown by broad, poorly resolved signal groups in the Si
Me region. The 29Si NMR spectrum of the final product, furthermore, exhibits signal groups typical of polymers or oligomers containing SiOSi (3 to
10 ppm) and endocyclic SiMe2 (
42 to
49 ppm) moieties. If 2 is reacted with MeSiCl3 in the presence of Et3N as an auxiliary base, the adamantane-like cage 4 is obtained without the formation of considerable amounts of polymeric material (Scheme 3). This reaction course is most likely related to the preferred cis,cis conformation of 2 with its three OH groups occupying axial positions of the cyclohexasilanyl ring. 4 is obtained in >90% yield as a white and air-stable crystalline solid and has been fully characterized by spectroscopic means and elemental analysis. Analytical data (compare Experimental Section) are consistent with the proposed structure. The molecular structure of 4, as determined by X-ray crystallography, is depicted in Figure 7 and shows the adamantanoid structure of the molecule with nearly identical Si
Si bond lengths of 2.36 Å and Si
O distances of 1.63
1.67 Å.

Scheme 3

Compound 4 crystallizes in the monoclinic space group C2/m with a mirror plane including Si1, Si4, Si5, and O1. Bond and torsion angles within the cyclohexasilane chair turn out to be considerably distorted, due to the presence of the bridging SiO3 unit. Thus, the mean bond angle Si(O)
Si
Si(O) is significantly decreased to 99.0° as compared to the value of 110.5° measured for 2. The mean endocyclic Si
Si
Si
Si dihedral angle of 69.9° (54.3° for 2) is unusually high. The mean axial C
Si
Si angle is somewhat widened to 114.3°, while the average Si
O
Si angle of 129.4° is small for compounds containing the Si
O
Si moiety, due to the cagelike structure of 4. For cyclohexasilane derivatives bearing
OSiR3 side groups, for instance, Si
O
Si bond angles in the range of 151.8
163.9° were observed,3 which are in line with literature data for disiloxanes of the general formula X3SiOSiX3 (X = H, halogen, Me, Ph) exhibiting Si
O
Si bond angles between 142.2 and 180°.21 2535

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’ CONCLUSIONS In summary, we have investigated selected physical and chemical properties of cis,cis-trihydroxynonamethylcyclohexasilane (2) experimentally and theoretically. First, it has been found that O
H hydrogen bonding considerably determines the molecular structure observed and the resulting spectroscopic properties of 2 in the solid state as well as in solution. Furthermore, our results offer computational evidence of the fact that the substituent effects apparent in the UV absorption spectra of permethylcyclohexasilanes bearing hydroxy side groups are caused by the perturbation of the electronic structure of the Si
Si backbone by the oxygen lone pairs in the HOMO and by σ*(Si
O) type orbitals in the LUMO, respectively. Finally, 2 turned out to be a valuable precursor for the preparation of polysilane cages with adamantane-type structures. Further studies to broaden the scope of this unprecedented synthetic route are currently underway.

ARTICLE

All experiments were performed under a nitrogen atmosphere using standard Schlenk techniques. Solvents were dried using a column solvent purification system.22 SbCl5 (99%, Aldrich) was used as purchased, Et3N (99%, Sigma-Aldrich) was dried by distillation from solid KOH, and Me3SiCl (donated by Wacker GmbH) was distilled prior to use. Si6Me1223 and 1,3,5-Cl3Si6Me97b were synthesized as previously reported. 1H (299.95 MHz), 29Si (59.59 MHz), and 13C (75.43 MHz) NMR spectra were recorded on a Varian INOVA 300 spectrometer in C6D6 solution and referenced versus TMS using the internal 2H-lock signal of the solvent. Mass spectra were run on a HP 5971/A/5890-II GC/MS coupling (HP 1 capillary column, length 25 m, diameter 0.2 mm, 0.33 μm poly(dimethylsiloxane)). Infrared spectra were obtained using a KBr pellet and in CCl4 solution with a 2.5 cm path length, respectively, on a Perkin-Elmer 883 spectrometer. UV
visible spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer. Melting points were determined using a Buechi 535 apparatus and are uncorrected. Elemental analyses were carried out on a Hanau VARIO ELEMENTAR EL apparatus. cis,cis-1,3,5-Trihydroxynonamethylcyclohexasilane (2). A mixture of 1,3,5-trichlorononamethylcyclohexasilane with 1,4-dichlorodecamethylcyclohexasilane (prepared by the chlorination of 100 g of Si6Me12 (0.287 mol) with 2 equivalents of SbCl57b) was dissolved in 250 mL of pentanes and added to a mixture of 150 mL of H2O and 150 mL of pentanes simultaneously with a solution of 114.4 g (1.14 mol) of Et3N in 100 mL of pentanes at 0 °C over a period of 1 h. A white precipitate was formed immediately. After addition of the reactants, the ice bath was removed and the solution was stirred for another 45 min. Filtration and washing of the precipitate with diethyl ether and pentanes afforded, after drying in vacuo, 8.3 g (8.2%) of pure, white and powdery cis,cis-2. Mp: 190
191 °C dec. Anal. Found: C, 30.55; H, 8.39. Calcd for C9H30O3Si6: C, 30.46; H, 8.52. IR (KBr pellet): ν(O
H) 3240 cm
1 (s, br). 29Si NMR (C6D6, TMS, ppm): 12.41 (SiMeOH);
47.78 (SiMe2). 1 H NMR (C6D6, TMS, ppm, relative intensity): 6.53 (b, 3H) (OH); 0.558 (s, 9H), 0.512 (s, 9H),
0.007 (s, 9H) (SiCH3). 13C NMR (C6D6, TMS, ppm):
1.13 (HOSiCH3);
5.94,
7.53 (Si(CH3)2).

pentane, respectively. From the combined extracts 461 mg (97%) of pure 4 was obtained as a colorless, waxy solid, which could be crystallized from pentane at
80 °C. Mp: 176
179 °C. Anal. Found: C, 30.70; H, 7.69. Calcd for C10H30O3Si7: C, 30.41; H, 7.66. 29Si NMR (C6D6, TMS, ppm): 13.96 (SiMeO);
45.66 (SiMe3);
47.52 (SiMe2). 1H NMR (C6D6, TMS, ppm, relative intensity): 0.571 (s, 9H), 0.397 (s, 9H), 0.181 (s, 3H), 0.156 (s, 9H) (SiCH3). 13C NMR (C6D6, TMS, ppm):
0.82 (OSiCH3);
4.28 (O3SiCH3);
6.41,
8.08 (Si(CH3)2). MS (m/e (relative intensity)): 394 (100%, Mþ). X-ray Crystallography. Suitable crystals of 2 and 4 were grown by slow evaporation of THF and C6D6, respectively, at room temperature. Data collection for compound 2 was performed on a Bruker AXS Smart Apex CCD diffractometer at 150 K and for compound 4 on a Bruker Kappa Apex II CCD diffractometer at 100 K, respectively, using graphite-monochromated Mo KR radiation in both cases. Details of the crystal data and structure refinement are provided as Supporting Information. Absorption corrections were performed by SADABS.24 The structures were solved by direct methods and refined by fullmatrix least-squares methods (SHELXL97).25 All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms H(1), H(2), and H(3) belonging to the hydroxyl groups of 2 could be located as the six highest peaks in the difference Fourier map: i.e., each proton is disordered over two positions. H11, H13, and H15 and H10, H12, and H14, respectively, were grouped into two domains using the PART command. Subsequent refinement with a restrained O
H distance of 0.84 Å (SHELXL-97) afforded a site occupancy of 53(1)% for the larger domain (i.e., 47% for the other, dependent domain). All other hydrogen atoms were calculated to correspond to standard bond lengths and angles (riding model). Crystallographic data (excluding structure factors) for the structures of 2 and 4 have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC-803455 and CCDC-803375, respectively. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax (internat.) p44-1223/336-033; e-mail [email protected]). Computational Methods. Structures were optimized at the DFT B3LYP/TZVP level using the Turbomole 6.0 program package.26 Analytical frequency calculations ensured the nature of the stationary points (no imaginary frequencies). For the electronic absorption spectra time-dependent DFT B3LYP/TZVP (singlet excitations) calculations were performed. Relative energies given include zeropoint vibrational energy (ZPVE) corrections. For the NMR calculations, all molecules (including TMS) were optimized at the DFT B3LYP/6-311G(d,p) level of theory using the Gaussian03 program package.27 Subsequently, NMR GIAO B3LYP/IGLO-III calculations were performed. Each of the resulting isotropic nuclear shielding values was referenced against the respective value of the TMS molecule to get the δ values comparable with the experiment. The IGLO-III basis set28 was obtained from the EMSL basis set exchange Web site.29 All molecular graphics (including crystal structures) were created using USCF Chimera.30 For the calculation of the relative energy of d-2a, its B3LYP/TZVP SCF energy was corrected for the basis set superposition error by applying the counterpoise correction routine implemented in Turbomole 6.0.31 The ZPVE was taken from the uncorrected frequency calculation.

1,3,4,4,5,6,6,7,10,10-Decamethyl-2,8,9-trioxatricyclo[3,3,1,13,7]decasilane (4). To a suspension of 414 mg (1.17 mmol)

’ ASSOCIATED CONTENT

’ EXPERIMENTAL SECTION

of finely powdered 2 in 30 mL of THF containing 678 mg (6.70 mmol) of NEt3 was added 172 mg (1.15 mmol) of MeSiCl3 dissolved in 10 mL of THF over a period of 5 min. After the resulting mixture was stirred for 60 h at room temperature, all volatiles were removed in vacuo, leaving a colorless solid which was extracted two times with 25 and 10 mL of

bS

Supporting Information. A table and CIF files giving crystal, collection, and refinement data for the structures of compounds 2 (CCDC-803455) and 4 (CCDC-803375), a table of DFT B3LYP/TZVP calculated structures (Cartesian

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Organometallics coordinates) and absolute ZPVE corrected energies of 2a
d and d-2a, and a table of NMR GIAO B3LYP/IGLO-III calculated NMR chemical shifts of 2a,c and d-2a. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: þ43/316/873-32111. Fax: þ43/316-32102. E-mail: [email protected].

’ ACKNOWLEDGMENT We thank the FWF (Wien, Austria) for financial support (project number P 17654-N19) and the Wacker Chemie GmbH (Burghausen, Germany) for the donation of silane precursors. ’ REFERENCES (1) For chemical and physical properties of siloxene see: (a) Hengge, E. Fortschr. Chem. Forsch. 1967, 9, 145. (b) Hengge, E. Top. Curr. Chem. 1974, 51, 1. (c) Gmelin Handbook of Inorganic Chemistry; Springer: Berlin, 1982; Silicon, Vol. B1, p 252f. (d) Dettlaf-Weglikowska, U.; H€onle, W.; Molassioti-Dohms, A.; Finkbeiner, S.; Weber, J. Phys. Rev. B: Condens. Matter 1997, 56, 13132. (e) Brandt, M. S.; Puchert, T.; Stutzmann, M. In Tailor-Made Silicon-Oxygen Compounds; Corriu, R., Jutzi, P., Eds.; Vieweg: Wiesbaden, Germany, 1994; p 117. (f) Weiss, A.; Beil, G.; Meyer, H. Z. Naturforsch. 1979, 34b, 25. (g) Brandt, M. S.; Ready, S. E.; Boyce, J. B. Appl. Phys. Lett. 1997, 70, 188. (h) Kurmaev, E. Z.; Shamin, S. Z.; Ederer, D. L.; Dettlaf-Weglikowska, U.; Weber, J. J. Mater. Res. 1999, 14, 1235. (2) (a) Kleewein, A.; Stueger, H. Monatsh. Chem. 1999, 130, 69. (b) Stueger, H. In Silicon Chemistry, From the Atom to Extended Systems; Jutzi, P., Schubert, U., Eds.; Wiley-VCH: Weinheim, Germany, 2003; p 214. (3) Stueger, H.; Fuerpass, G.; Renger, K. Organometallics 2005, 24, 6374. (4) Fuchs, H. D.; Stutzmann, M.; Brandt, M. S.; Rosenbauer, M.; Weber, J.; Breitschwerdt, A.; Deak, P.; Cordona, M. Phys. Rev. B 1993, 48, 8172. (5) Stueger, H.; Fuerpass, G.; Baumgartner, J.; Mitterfellner, T.; Flock, M. Z. Naturforsch. 2009, 64b, 1598. (6) (a) Rooklin, D. W.; Schepers, T.; Raymond-Johansson, M. K.; Michl, J. Photochem. Photobiol. Sci. 2003, 2, 511. (b) Albinsson, B.; Teramae, H.; Downing, J. W.; Michl, J. Chem. Eur. J. 1996, 2, 529. (7) (a) Hengge, E.; Eibl, M. J. Organomet. Chem. 1992, 428, 335. (b) Eibl, M.; Katzenbeisser, U.; Hengge, E. J. Organomet. Chem. 1993, 444, 29. (8) Chandrasekhar, V.; Boomishankar, R.; Nagendran, S. Chem. Rev. 2004, 104, 5847. (9) (a) Spielberger, A.; Gspaltl, P.; Siegl, H.; Hengge, E. J. Organomet. Chem. 1995, 499, 241. (b) Korlyukov, A. A.; Larkin, D. Y.; Chernyavskaya, N. A.; Antipin, M. Y.; Chernyavskii, A. I. Mendeleev Commun. 2001, 195. (c) Larkin, D. Y.; Korlyukov, A. A.; Matukhina, E. V.; Buzin, M. I.; Chernyavskaya, N. A.; Antipin, M. Y.; Chernyavskii, A. I. Russ. Chem. Bull. Int. Ed. 2005, 54, 1612. (10) (a) Ackerhans, C.; Roesky, H. W.; Labahn, T.; Magull, J. Organometallics 2002, 21, 3671. (b) Ackerhans, C.; R€ake, B.; Kr€atzner, R.; M€uller, P.; Roesky, H. W.; Uson, I. Eur. J. Inorg. Chem. 2000, 827. (c) J€ager-Fiedler, U.; K€ockerling, M.; Ludwig, R.; Wulf, A.; Krempner, C. Angew. Chem., Int. Ed. 2006, 45, 6755. (11) Ishida, H.; Koenig, J. L.; Gardner, K. C. J. Chem. Phys. 1982, 77, 5748. (12) Tables of DFT B3LYP/TZVP calculated structures and absolute ZPVE corrected energies can be found in the Supporting Information. (13) Chandrasekhar, V.; Nagendran, S.; Butcher, R. J. Organometallics 1999, 18, 4488.

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(14) A detailed table of chemical shift values calculated for 2a and d2a can be found in the Supporting Information (15) Liu, H.; Kondo, S.; Tanaka, R.; Oku, H.; Unno, M. J. Organomet. Chem. 2008, 693, 1301. (16) (a) Hoelbling, M.; Flock, M.; Hassler, K. ChemPhysChem 2007, 8, 735. (b) Tekautz, G.; Binter, A.; Hassler, K.; Flock, M. ChemPhysChem. 2006, 7, 421. (17) West, R. In Comprehensive Organometallic Chemistry 2; Abel, E., Ed.; Pergamon Press: London, 1995; Vol. 2, p 77 and references therein. (18) Brough, L. F.; West, R. J. Am. Chem. Soc. 1972, 94, 9285. (19) (a) Hsiao, Y. L.; Waymouth, R. M. J. Am. Chem. Soc. 1994, 116, 9779. (b) Takeda, K.; Shiraishi, K. Solid State Commun. 1993, 85, 301. (c) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (d) Stueger, H.; Hengge, E. Monatsh. Chem. 1988, 119, 873.(e) Pitt, C. G. In Homoatomic Rings, Chains, and Macromolecules of Main-Group Elements; Rheingold, A. L., Ed.; Elsevier: Amsterdam, 1977; p 203. (f) J€agerFiedler, U.; K€ockerling, M.; Reinke, H.; Krempner, C. Chem. Commun. 2010, 46, 4535. (g) Krempner, C.; Ludwig, R.; Flemming, A.; Miethchen, R.; K€ockerling, M. Chem. Commun. 2007, 1810. (20) Data for 1 and 3 were taken from ref 5. (21) For an excellent discussion on that topic see: Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; p227. (22) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (23) Carberry, E.; West, R. J. Am. Chem. Soc. 1969, 91, 5440. (24) (a) Sheldrick, G. M. SADABS, Version 2.10, Siemens Area Detector Correction; Universit€at G€ottingen, G€ottingen, Germany, 2003. (b) Blessing, R. H. Acta Crystallogr. Sect. A 1995, 51, 33. (25) (a) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112. (b) Sheldrick, G. M. SHELXTL, Version 6.1; Bruker AXS, Inc., Madison, WI, 2002. (c) Sheldrick, G. M. SHELXS97 and SHELXL97; Universit€at G€ottingen, G€ottingen, Germany, 2002. (26) (a) Ahlrichs, R.; B€ar, M.; H€aser, M.; Horn, H.; K€olmel, C. Chem. Phys. Lett. 1989, 162, 165. (b) Sch€afer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829. (c) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454. (d) Bauernschmitt, R.; H€aser, M.; Treutler, O.; Ahlrichs, R. Chem. Phys. Lett. 1997, 264, 573. (27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J. ; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Rev. C.02; Gaussian, Inc., Wallingford, CT, 2004. (28) Kutzelnigg, W.; Fleischer, U.; Schindler, M. The IGLOMethod: Ab Initio Calculation and Interpretation of NMR Chemical Shifts and Magnetic Susceptibilities; Springer Verlag: Heidelberg, Germany, 1990. (29) Schuchardt, K.; Didier, B.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. J. Chem. Inf. Model. 2007, 47, 1045. (30) Pettersen, E. F.; Goddard, T. D.; Huang, C. C.; Couch, G. S.; Greenblatt, D. M.; Meng, E. C.; Ferrin, T. E. J. Comput. Chem. 2004, 25, 1605. (31) Boys, S. F.; Bernardi, F. Mol. Phys. 1972, 19, 553. 2537

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’ NOTE ADDED AFTER ASAP PUBLICATION After this paper was published on the Web on March 17, 2011, the authors realized that some important references to related work published by Krempner et al. were inadvertently omitted from the manuscript. As a consequence the following changes have been made as of April 1, 2011. Studies on the solid-state and solution structure of a hydrogen-bonded hydroxy-substituted oligosilane now are mentioned in the text and cited in ref 10c. Additional citations concerning the UV absorption properties of oxygen containing oligosilanes were added to ref 19. Furthermore, the fact that similar conclusions concerning the nature of the perturbation the oxygen substituent orbitals exercise on the electronic structure of the Si
Si backbone were drawn by Krempner in ref 19g and by us now is briefly mentioned in the text.

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