Coupling of Disilane and Trisilane Segments Through Zero, One, Two

Article pubs.acs.org/Organometallics

Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes Andreas Wallner,†,§ Rikard Emanuelsson,†,§ Judith Baumgartner,‡ Christoph Marschner,‡ and Henrik Ottosson*,† †

Department of Chemistry-BMC, Uppsala University, Box 576, SE-75123 Uppsala, Sweden Institut für Anorganische Chemie, Technische Universität Graz, A-8010 Graz, Austria

‡

S Supporting Information *

ABSTRACT: Several six-membered cyclic and [2.2.2]bicyclic organosilanes with varying proportions of silicon atoms in the bridges have been prepared following a stepwise approach that exploits dianionic polysilanes. Focus in our analysis was placed on the bicyclic compounds which all have silicon atoms at the bridgehead positions. Quantum chemical calculations of these compounds revealed the possibility to enhance the coupling through a single cisoid tetrasilane cage segment by replacing one or two of the other −SiMe2SiMe2− bridges with −CH2CH2− bridges. UV absorption spectroscopy revealed a red shift in the lowest visible transitions when going from a bicyclo[2.2.2]octane with three −SiMe2SiMe2− bridges to those with two or one such bridge. However, these red shifts are deceptive, as the lowest vertically excited singlet states, which are dark according to TD-DFT calculations, do not display the same trend. Still, since these compounds have (i) excellent structural rigidity, (ii) provide potentials for functionalization through their exocyclic trimethylsilyl groups, and (iii) display electronic structure variations with the number of −SiMe2SiMe2− bridges, they could be interesting for further studies: e.g., in single-molecule electronics.

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upon increasing the chain length, a finding which can be rationalized as σ−σ* transitions of the polysilane chain. These transitions are strongly dependent on the silicon backbone conformation, and it is widely accepted that the anti (Si−Si− Si−Si dihedral angle ω ≈ 180°) and transoid (ω ≈ ±165°) conformers of a linear tetrasilane segment effectively extend σ conjugation while gauche (ω ≈ ±65°) and cisoid (ω ≈ ±40°) conformers disrupt it.17−27 Furthermore, similar to the case for linear oligo- and polysilanes, cyclic and bicyclic oligosilanes also display σ conjugation.5,6,28,29 For partially trimethylsilyl substituted methylcyclosilanes the UV absorption behavior differs from that of permethylated cyclosilanes. In this case the permethylated silanylene segments between the trimethylsilyl attachment points behave, to some extent, like acyclic transoid oriented chains.30,31 Recently, silicon has attracted interest in the field of singlemolecule electronics, as the conductance through several different permethylated oligosilanes was measured.32 Indeed, oligosilanes display a very slow decay of conductance with length, comparable to that of polyacetylene fragments, and this finding experimentally represents the entry of σ conjugation and silicon chemistry into single-molecule electronics. However, it also points to a strong conformational dependence of

INTRODUCTION The first cyclopolysilanes were prepared by Kipping in the early 1920s,1,2 and they are thus among the oldest known compounds with silicon−silicon bonds. The cyclopolysilanes of Kipping were perphenyl substituted, and three decades later the first permethylated cyclohexasilane was described.3 This was followed by further systematic studies on the synthesis and properties of cyclic polysilanes.4−7 The initial synthetic approach was the Wurtz-type coupling of dihalosilanes with alkali metals, and this is still a commonly applied procedure, even though it has considerable drawbacks. These disadvantages include the harsh conditions employed, limitation to certain ring sizes, and dependence of the steric properties of the substituents. Thus, alternative synthesis methods were needed in order to further expand the scope of cyclic polysilanes. Accordingly, the groups of Marschner8 and Kira9 independently developed a stepwise construction approach using dianionic polysilanes.10,11 This method allows for the simple and welldirected synthesis of cyclopolysilanes with different ring sizes, and it also represents a new route to bicyclic polysilanes. Importantly, it facilitates the derivatization of the generated cyclopolysilanes, which thereby can be incorporated into more complex molecular structures. Since the 1960s it is known that oligosilanes absorb nearultraviolet light as a result of delocalization of Si−Si σ bonds (σ conjugation).12−16 Their absorption maxima are red-shifted © XXXX American Chemical Society

Received: July 17, 2012

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dx.doi.org/10.1021/om3006678 | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Overview of the Synthetic Pathways to the Investigated Compoundsa

a

The silicon atoms and the Si−Si bonds which become parts of the oligosilane segments of the final compounds are displayed in red.

tetrakis(trimethylsilyl)octamethylcyclohexasilane (1a) and 1,4bis(trimethylsilyl)dodecamethylbicyclo[2.2.2]octasilane (2a) (Scheme 1). To rationalize the experimentally acquired data obtained by UV absorption spectroscopy and X-ray crystallography, the bicyclic compounds were also investigated by computational means. Similar bicyclic structural segments have been attractive targets, primarily as spacer units, since the birth of molecular electronics in the 1970s.34 Studies on electron transfer have exploited conformationally constricted all-carbon cyclic and bicyclic structures as a way to achieve structural rigidity and to investigate, for example, the distance dependence of electron transfer.35,36 The compounds reported here should thus be of clear interest, as they connect to the all-carbon bicyclo[2.2.2]octane used regularly in charge transfer and transport studies. Indeed, fluorescence from an intramolecular charge transfer state has earlier been reported for 1-phenyltridecamethylbicyclo[2.2.2]octasilane, revealing the involvement of the bicyclo[2.2.2]octasilane cage in a charge transfer process.40 An improved understanding of the electronic structures of silicon-containing bicyclo[2.2.2]octanes should be important and could open up possibilities for rational tuning of the

the conductance, as the investigated pentasilane chain was somewhat less conducting than extrapolated from the decay constant β of the shorter chains, a fact that was attributed to a slightly twisted oligosilane backbone. This is in accordance with results obtained through nonequilibrium Green’s function (NEGF) calculations of conductance through a 1,6-diaminohexasilane, for which conductance variations of up to 3 orders of magnitude, depending on conformation, were found, with the all-syn (ω ≈ 0°) conformation having the lowest conductance and the all-anti having the highest.33 Considering that a gauche, cisoid, or syn conformation of a tetrasilane segment, which is part of a longer oligosilane chain, functions as an insulating unit for σ conjugation, one may ask if the coupling through two or three such segments in parallel is better than the poor coupling through one single segment, or if it is even worse? We were therefore interested in exploring how the fundamental electronic structure properties differ among compounds with varying numbers of parallel tetrasilane segments with small dihedral angles (gauche, cisoid, and syn). For this reason we synthesized six-membered cyclic and bicyclic organosilane compounds with different numbers of silicon atoms in the backbone, starting from the all-silicon 1,1,4,4B


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from pentane/acetone also resulted in decomposition of the target molecule, and it seems that extended exposure to acetone is detrimental to the product. However, by recrystallization from diethyl ether, it was possible to obtain crystals from the cyclic component 1c. Washing of the crystals with ice-cold acetone and collecting the liquid after removal of the crystalline material yielded enriched 2c after evaporation of the solvent. By repetition of this process for eight more times we obtained a 3:1 mixture of 2c and 1c. To complete the series, we started from bis[tris(trimethylsilyl)silyl]ethane (4) with the aim of synthesizing the cyclic compounds 1b,c. Starting from 1c, we wanted to first explore an alternative route to the problematic molecule 2c and, furthermore, possibly construct the all-carbon-bridged 2d. This was achieved by allowing 4 to react with potassium tertbutoxide to form the dianionic species, which subsequently was reacted with 1,2-dichlorotetramethyldisilane (path A). It is noteworthy that this latter route to 1b gave a slightly higher yield than the method starting from 3 (path B). Furthermore, usage of 1,2-bis[(p-tolylsulfonyl)oxy]ethane provided pure compound 1c as colorless crystals after removal of polymeric material via column chromatography and subsequent Kugelrohr distillation. Using the same methodology, we were, however, not able to form the dianionic species from 1c, and this blocked further progress along this route to 2c,d. Crystal Structure Analyses. Single-crystal structure analyses could be carried out for 1b,c and 2b. Compound 1b (Figure 1 and Table 1) crystallizes in the monoclinic space

electronic properties of rigid silicon-based compounds for potential applications in single-molecule electronics.

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RESULTS AND DISCUSSION The discussion on the cyclic (1a−c) and bicyclic structures (2a−d) will follow the order (i) synthesis, (ii) analyses of the crystal structures, (iii) computed geometric and electronic structures, and finally, (iv) UV absorption spectroscopic investigations. Synthesis. Influenced by the previously reported route to construct bis(trimethylsilyl)dodecamethylbicyclo[2.2.2]octasilane (2a),8 we decided to synthesize the analogues by replacing one (2b) and two (2c) −SiMe2SiMe2− bridges by −CH2CH2− bridges. This route involved usage of 2 equiv of potassium tert-butoxide and 18-crown-6 to form the dianionic polysilane species (Scheme 1). Starting from 1,1,1,4,4,4hexakis(trimethylsilyl)-2,2,3,3-tetramethyltetrasilane (3), the dianion that formed was allowed to react with 1,2dichlorotetramethyldisilane (path A) to give 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane (1a). By repeated usage of KOtBu and 18-crown-6 it was further possible to synthesize the cyclic dianion, which again reacted with 1,2dichlorotetramethyldisilane to yield the all-silicon bicyclic cage 2a. We then reasoned that 2b,c could be synthesized in a similar manner through usage of 1,2-bis[(p-tolylsulfonyl)oxy]ethane (path B) instead of 1,2-dichlorotetramethyldisilane. The first compound, 2b, with one −CH2CH2− bridge and two disilane bridges, was prepared by the mixing of 1a with 2 equiv of potassium tert-butoxide and 18-crown-6 in toluene to afford the corresponding 1,4-dipotassium compound. Subsequent reaction with 1,2-bis[(p-tolylsulfonyl)oxy]ethane gave 2b (path B). In this reaction the addition of 1,2-bis[(ptolylsulfonyl)oxy]ethane was performed in one portion, leading to a markedly exothermic reaction. Experiments on cooling the reaction mixture, or on slower addition of the ditosylate, produced a higher amount of polymeric material, and in some cases no product could be detected at all. After aqueous workup, column chromatography was required for removal of polymeric material. The material was then recrystallized from pentane/acetone to yield colorless crystals of 2b. Encouraged by these results, we attempted to synthesize 1,1,4,4-tetrakis(trimethylsilyl)-2,2,3,3-tetramethyl-1,2,3,4-tetrasilacyclohexane (1b) using the same methodology (path B) starting from 3 and allowing the resulting dianionic compound to react with the ditosylate. Again, the addition of 1,2-bis[(ptolylsulfonyl)oxy]ethane to the dianion led to a strongly exothermic reaction. After removal of the polymeric material and recrystallization from pentane/acetone we could obtain 1b as colorless crystals in 45% yield. This Si/C mixed cyclic compound acted as a precursor for the subsequent reactions to give 2b, with one −CH2CH2− bridge, and to 2c, with two −CH2CH2− bridges. The reaction of the dianionic species of 1b with 1,2-dichlorotetramethyldisilane (path A) by slow addition gave the desired compound 2b in low yield (4%). On the other hand, addition of the ditosylate to the dianion of 1b (path B) gave a 1:1 mixture of the expected bicyclic compound 2c and the monocyclic 1c, the all-carbon-bridged analogue of 1a,b. However, these two compounds were almost impossible to separate from each other. They exhibit the same Rf value (Rf = 0.7 in pentane) with the choice of eluents limited to nonpolar solvents, making column chromatography futile. Purification by sublimation was attempted but resulted in decomposition of 2c at only slightly elevated temperatures. Slow recrystallization

Figure 1. Molecular structure and numbering of 1b with 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) with standard deviations: Si(1)−C(1) = 1.9167(16), Si(1)−Si(5) = 2.3526(10), Si(1)−Si(2) = 2.3562(8), Si(1)−Si(6) = 2.3575(7), Si(2)−Si(3) = 2.3490(9), Si(3)−Si(4) = 2.3462(7), Si(4)−C(2) = 1.9128(18), Si(4)−Si(7) = 2.3489(10), Si(4)−Si(8) = 2.3565(8), C(1)−C(2) = 1.547(2); C(1)−Si(1)−Si(5) = 103.89(6), C(1)−Si(1)−Si(2) = 112.27(6), Si(5)−Si(1)−Si(2) = 111.68(3), C(1)−Si(1)−Si(6) = 108.42(6), Si(5)−Si(1)−Si(6) = 108.92(3), Si(3)−Si(2)−Si(1) = 107.75(3), Si(4)−Si(3)−Si(2) = 108.41(2), C(2)−Si(4)−Si(3) = 110.43(6), Si(7)−Si(4)−Si(8) = 109.10(3), C(2)−C(1)−Si(1) = 117.24(12), C(1)−C(2)−Si(4) = 115.61(12).

group P21/n and exhibits a six-membered ring in a somewhat twisted chair conformation with ring bond angles between 107° and 118°. The Si−Si and C−C bond lengths exhibit ordinary values between 2.34 and 2.36 Å and 1.55 Å, respectively, whereas the Si−C bond lengths inside the ring are elongated to 1.92 Å. The distance to the axially and equatorially positioned trimethylsilyl groups show nearly identical values of 2.36 and 2.35 Å, respectively. The tetrasilane dihedral angle within the ring (ωring) of 1b exhibits a relatively small value of −24.98°, C


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Table 1. Crystallographic Data for Compounds 1b,c and 2b empirical formula Mw temp (K) size (mm) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (g cm−3) abs coeff (mm−1) F(000) θ range no. of collected/ unique rflns completeness to θ (%) no. of data/ restraints/ params goodness of fit on F2 final R indices (I > 2σ(I)) R indices (all data) largest diff peak/ hole (e/Å3)

1b

1c

2b

Si8C18H52 493.32 100(2) 0.44 × 0.32 × 0.27 monoclinic P21/n 9.2189(18) 30.807(6) 12.014(2) 90 101.35(3) 90 3345.3(12) 4 0.979 0.325 1088 1.32 < θ < 26.33 22159/6654

Si6C16H44 405.05 100(2) 0.36 × 0.29 × 0.22 triclinic P1̅ 6.6479(13) 12.825(3) 16.735(3) 107.92(3) 90.17(3) 101.70(3) 1326.1(5) 2 1.014 0.312 448 1.28 < θ < 26.37 10375/5326

Si8C16H46 463.25 100(2) 0.34 × 0.28 × 0.26 orthorhombic Pbcn 10.954(3) 12.750(3) 20.977(4) 90 90 90 2929.7(10) 4 1.050 0.368 1016 1.94 < θ < 26.30 21741/2968

97.8

98.0

99.9

6654/0/251

5326/0/211

2968/0/116

1.107

1.057

1.113

R1 = 0.0376, wR2 = 0.0919 R1 = 0.0412, wR2 = 0.0938 0.434/−0.218

R1 = 0.0486, wR2 = 0.1209 R1 = 0.0575, wR2 = 0.1252 0.772/−0.301

R1 = 0.0288, wR2 = 0.0751 R1 = 0.0308, wR2 = 0.0763 0.385/−0.137

Figure 2. Molecular structure and numbering of 1c with 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) with standard deviations: Si(1)−C(2) = 1.902(2), Si(1)−C(1) = 1.909(2), Si(1)− Si(3) = 2.3518(11), Si(1)−Si(2) = 2.3522(9), C(1)−C(2A) = 1.550(3); C(2)−Si(1)−C(1) = 107.43(9), C(2)−Si(1)−Si(3) = 110.72(8), C(1)−Si(1)−Si(3) = 111.70(7), C(2)−Si(1)−Si(2) = 110.11(7), C(1)−Si(1)−Si(2) = 106.02(7), Si(3)−Si(1)−Si(2) = 110.71(4), C(2a)−C(1)−Si(1) = 113.99(14), C(1A)−C(2)−Si(1) = 113.49(14).

regular, whereas the Si−C bonds of the ring (1.90 and 1.91 Å) are slightly elongated. The bond lengths for the Si−Si bonds to the axially and equatorially positioned trimethylsilyl groups are both 2.35 Å. The bicyclic compound 2b (Figure 3 and Table 1), with one −CH2CH2− bridging unit, crystallizes in the orthorhombic

which is most appropriately labeled as a cisoid bond conformer. This should be compared to the all-silicon species 1a with ωring angles between −26.97 and 61.63°.41 The Si−Si−Si−Si dihedral angles involving the exocyclic trimethylsilyl groups (ωexo) of 1b possess values in the range −83.74° up to 154.82°, close to the values found for the corresponding angles in 1a (−90.06° up to 154.27°) (Table 2). Figure 3. Molecular structure and numbering of 2b with 50% probability thermal ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and bond angles (deg) with standard deviations: Si(1)−C(1) = 1.9207(14), Si(1)−Si(4) = 2.3433(7), Si(1)−Si(3A) = 2.3495(6), Si(1)−Si(2) = 2.3521(6), Si(2)−Si(3) = 2.3577(7), C(1)−C(1A) = 1.547(3); C(1)−Si(1)−Si(4) = 109.22(4), C(1)−Si(1)−Si(3A) = 109.11(4), Si(4)−Si(1)−Si(3A) = 114.40(2), C(1)−Si(1)−Si(2) = 105.84(5), Si(4)−Si(1)−Si(2) = 111.33(2), Si(3A)−Si(1)−Si(2) = 106.56(2), Si(1)−Si(2)−Si(3) = 102.743(19), Si(1A)−Si(3)−Si(2) = 104.188(19), C(1A)−C(1)−Si(1) = 117.54(6).

Table 2. Summary of Measured Si−Si−Si−Si Dihedral Angles ω (deg) of Cyclic Species 1a,b 1a

1b

ωexo

ωring

ωexo

ωexo

ωring

ωexo

93.80 91.05 150.34 152.16

26.97 28.62 28.99 30.98 59.60 61.63

90.06 93.61 149.65 154.27

90.18 147.82

24.98

83.75 154.82

space group Pbcn. All bond lengths, Si−Si (2.34−2.36 Å), C−C (1.55 Å), and Si−C (1.92 Å), exhibit ordinary values. In comparison to the all-silicon bicyclus 2a,42 which possesses Si− Si−Si angles of 108.50−109.00° within the cage, compound 2b as a result of the −CH2CH2− bridge being shorter than the −SiMe2SiMe2− bridge has distinctly smaller Si−Si−Si angles (102.74 and 104.19°) and a shorter Si···Si through-space distance of 3.55 Å: i.e., a 0.40 Å difference from that of 2a (3.95 Å). However, the observed dihedral angles of the tetrasilane

Compound 1c (Figure 2 and Table 1) crystallizes in the triclinic space group P1̅ with two molecules in the asymmetric unit and with an inversion center in the middle of each sixmembered ring. In contrast to the all-silicon cycle 1a, which exhibits a twist conformation,41 the six-membered ring of 1c shows a chair conformation with ring bond angles between 107 and 114°. Moreover, the C−C bond lengths (1.55 Å) are D


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Table 3. Summary of Measured and Calculated Si−Si−Si−Si Dihedral Angles ω (deg) of Bicyclic Species 2a−c 2a measd ω (deg)

2b calcd ω (deg)

measd ω (deg)

2c calcd ω (deg)

calcd ω (deg)

ωexo

ωcage

ωexo

ωcage

ωexo

ωcage

ωexo

ωcage

ωexo

ωcage

167.29 167.81 167.29

19.04 19.04 22.74

168.44 168.44 168.44

19.77 19.77 19.77

153.07 177.99

20.96 20.96

153.62 179.89

19.40 19.40

167.02

18.25

Sn, Pb) related a gradual increase in E−C−C−E dihedral angles when element E is changed from C to Pb to a successive shift of the E−C bond orbitals toward the C atoms.44 Computed Electronic Structures. Drawings of the highest occupied molecular orbitals (HOMO) of the three cage compounds 2a−c show that these orbitals are exclusively localized at the silicon segments and that they all are of σSiSi character (Figure 4). However, the shapes of the HOMOs still

segments of 2b inside the cage (ωcage = 20.96°) display nearly the same values as found in compound 2a (19.04−22.74°). With regard to the Si−Si−Si−Si dihedral angles involving the Si atoms of the exocyclic trimethylsilyl groups, these display nearly identical values in 2a (167.29 and 167.81°, respectively) but distinctly different values in 2b (153.07 and 177.99°, respectively). Computed Geometric Structures. The calculated structures of 2a,b, for which crystal structures are available, show bond angles and dihedral angles in close agreement with those of the crystal structures42 (Table 3). The two compounds are rigid, and only small changes would be expected in solution and in the gas phase in comparison to the solid state. There is a slight overestimation of the Si−Si bond lengths in 2a, similar to what has been observed for calculated bond lengths to Si with hybrid DFT methods.43 As a result, the distance between the bridgehead atoms is calculated as 4.03 Å, to be compared to 3.95 Å in the crystal structure. However, the calculated bond angles within the ring are in good agreement with those of the crystal structure. In 2b, the distance between the bridgehead Si atoms is calculated to be 3.57 Å, in line with the crystal structure value of 3.55 Å. Compound 2c, for which a crystal structure could not be obtained, displays a calculated Si···Si bridgehead distance of 3.27 Å, and the corresponding distance in 2d is 3.05 Å. Thus, the reductions of the through-space distances when going from 2a to 2b−d are approximately 0.4, 0.7, and 0.9 Å, respectively. The main structural differences between the cages when going from 2a to 2c are therefore the shortened through-space distances between the silicon bridgehead atoms, enforced by the change from −SiMe2SiMe2-−to −CH2CH2− bridges. This also affects the Si−Si−Si bond angles within the cage tetrasilane segments, which decrease and potentially influence the degree of σ conjugation. In 2a these angles are 109.5°, in 2b they are 102.5°, and in 2c they are 99.9°. A variation in the computed Si−Si−Si−Si dihedral angles (ωexo) that involve the exocyclic trimethylsilyl group and the ring system is also found, similar to that observed from the X-ray crystal structure determination. In 2a,c these tetrasilane segments adopt computed dihedral angles within a very narrow interval (ωexo = 167−168°), while the dihedral angles of the two corresponding tetrasilane segments in 2b have dissimilar values (154 and 180°, respectively). The dihedral angles of the tetrasilane segments within the bicyclic moieties of 2a−c are found in the narrow range ωcage = 18−23° in measured as well as calculated structures. Another interesting property is the Si−C−C−Si dihedral angle, which is reduced from 42.7° in 2b to 37.4 and 29.1° in 2c,d, respectively. This change primarily reflects the structural requirements imposed by the proportions of −SiMe2SiMe2− versus −CH2CH2− bridges in a particular cage compound because the latter bridge adapts to longer Si···Si bridgehead− bridgehead distances through larger Si−C−C−Si dihedral angles. It is worth noting that a previous computational study of 1,4-ditetrelbicyclo[2.2.2]octanes (tetrel element E = C, Si,

Figure 4. The lowest unoccupied and the three highest occupied molecular orbitals and orbital energies of 2a (left), 2b (middle), and 2c (right) calculated at the PBE0/6-311+G(2d,p)//PBE0/6-311G(d) level. It should be noted that only one orbital in each of the two degenerate orbital pairs which represent HOMO-1 and HOMO-2 of 2a is displayed. The complete HOMO-1 and HOMO-2 orbital pairs as well as the LUMO+1 and LUMO+2 orbital pairs are shown in the Supporting Information.

vary significantly as one goes from the all-silicon-bridged 2a to the mixed silicon/carbon-bridged 2c, because there are contributions from the radial Si−Si bonds to the HOMO of the latter species but not to the former. With regard to the LUMOs of 2a−c they are all of π character in the Si−Si bonds of the cage, yet again, there is a marked difference between 2a and 2b,c because the LUMO of 2a has a large lobe in the interior of the cage. E


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orbital plots of n-Si6Me14 see the Supporting Information). The latter hexasilane conformer has its HOMO at −6.14 eV, HOMO-1 at −6.65 eV, and HOMO-2 at −7.36 eV, respectively. That is, the energies of the HOMOs of 2c and n-Si6Me14 are similar but the energies of the lower orbitals differ remarkably, an effect which, in part, should stem from the more acute Si−Si−Si bond angles in 2c. In general, the highest few occupied orbitals of 2a−c are clustered at energies closer to the EF of gold in comparison to the orbitals of the a+g+a+ conformer of n-Si6Me14. The cage compounds should thus provide more channels for charge transport than the ordinary linear permethylated oligosilanes, and one may expect this to be reflected in improved charge transport characteristics of the former class of compounds at low bias voltage. UV Spectroscopic Investigations. It has earlier been shown that UV absorptions of oligosilanes constrained to anticisoid-anti conformations of the silicon backbone display absorption characteristics which are connected to the anti or transoid segments: i.e., the cisoid conformers do not extend the conjugation.23 This feature is particularly useful in interpreting the UV spectra of compounds which contain an acyclic oligosilane segment, e.g., 1b and 2c. An important value is therefore the UV absorption maximum of the transoid conformer of n-Si4Me10, which has been reported at 228 nm (43 900 cm−1).47 As can be expected, compound 1c did not absorb in the investigated spectroscopic region (Figure 5), showing that the

Indeed, second-order perturbation theory analyses of donor− acceptor orbital interactions based on the natural bond orbitals (NBO) localized to individual bonds reveal that there is a larger interaction between an exocyclic−bridgehead σSiSi bond orbital and a bridgehead−bridge σSiSi* bond orbital in 2c (stabilization 1.29 kcal/mol) in comparison to 2b (0.97 kcal/mol) and 2a (0.72 kcal/mol). The same trend is seen in the coupling of an exocyclic−bridgehead σSiSi* bond orbital and a bridgeheadbridge σSiSi bond orbital, as these interaction energies are 1.13 (2c), 0.90 (2b), and 0.78 kcal/mol (2a). On the other hand, the couplings between the bridgehead−bridge Si−Si bonds and the bridge−bridge Si−Si bonds are similar in the three compounds because the strength of the coupling between a bridgehead−bridge σSiSi bond orbital and a bridge−bridge σSiSi* orbital merely decreases from 1.10 kcal/mol in 2c to 1.03 kcal/ mol in 2a. The inverse coupling between a bridgehead−bridge σSiSi* orbital with a bridge−bridge σSiSi orbital is 0.99 kcal/mol in each of the three compounds. Thus, the orbital coupling of an exocyclic Si−Si bond with one single tetrasilane cage segment is strongest in the case of 2c (2.42 kcal/mol) and weakest in the case of 2a (1.50 kcal/mol). Still, the cumulative interaction between the natural bond orbitals of the two exocyclic Si−Si bonds and those of all bridgehead−bridge Si−Si bonds of a particular cage is strongest in 2a (9.00 kcal/mol) and weakest in 2c (4.84 kcal/mol). For single-molecule electronics applications the molecular orbitals closest to the Fermi level of the electrode are typically of interest, as these provide channels for charge transport by the nonresonant tunneling mechanism.45 The electrode material most commonly applied is gold with the Fermi level (EF) at −5.31 eV.46 For the cage compounds reported here, the highest occupied molecular orbitals are therefore of interest, and for 2a−c the HOMO energies are found at nearly constant levels (2a, −6.02 eV; 2b, −6.02 eV; 2c, −6.06 eV; with PBE0/ 6-311+G(2d,p)//PBE0/6-311G(d)). The lowest unoccupied molecular orbitals (LUMO), in contrast, are far from the EF of gold (2a, −0.32 eV; 2b, −0.49 eV; 2c, −0.40 eV). The transport properties of these compounds should thus be dominated by hole tunneling through the highest occupied MOs. However, despite the fact that the HOMOs of 2a−c are essentially isoenergetic, the differences in interaction strength between localized σ/σ* orbitals of the Si−Si bonds in the compounds could provide for variations in the charge transport. This assumption must, however, be substantiated through nonequilibrium Green’s function (NEGF) calculations of current−voltage characteristics and, potentially, through measurements. On the other hand, considering the earlier results of George, Ratner, and Lambert on the conformational dependence of the conductance of 1,6-diaminohexasilane,33 one may argue that none of the species examined here will be overly conductive, as they all contain cisoid tetrasilane segments. Still, the orbital properties of 2a−c are different from that of the open-chain permethylated hexasilane in its a+g+a+ conformer: i.e., the hexasilane conformer with the structure of the Si backbone closest to that of 2c. With regard to the orbitals of 2a−2c one finds larger energy variations for the HOMO-1 and HOMO-2 among the three compounds than for HOMO because the energies for HOMO-1 are −6.49 (2a), −6.33 (2b), and −6.35 eV (2c), and for HOMO-2 they are −6.70 (2a), −6.46 (2b), and −6.71 eV (2c), respectively. With regard to the orbitals and orbital energies of 2c it is notable that these are distinctly different from those of the a+g+a+ n-Si6Me14 (for

Figure 5. UV absorption spectra of 1a−c.

two trisilane units are isolated from each other by the −CH2CH2− bridges. In contrast, 1b, with one −SiMe2SiMe2− bridge and one −CH2CH2− bridge, displays a shoulder at 253 nm (39 500 cm−1) and a very broad maximum centered at 241 nm (41 500 cm−1, ε = 2.6 × 104 M−1 cm−1). This reveals a σdelocalized path (Table 4) which incorporates the two trisilane units in a branched octasilane, with the longest linear segment being a hexasilane segment. As noted above, two of the four tetrasilane segments of 1b which incorporates the exocyclic SiMe3 groups have large dihedral angles (ωexo = 148 and 155°), Table 4. Summary of Measured UV Absorption Data for the Cyclic Species 1a,b 1a

1b

253 nm (39 500 cm−1) (sh) 264 nm (37 300 cm−1) (sh) −1 4 246 nm, (40 700 cm , ε = 2.6 × 10 241 nm (41 500 cm−1, ε = 2.6 × 104 M−1 cm−1) M−1 cm−1) F


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Table 5. Summary of the Measured UV Absorptions of the Bicyclic Species 2a−d and the Few Highest Visible Transitions Calculated at the TD-PBE0/6-311+G(2d,p)//PBE0/6-311G(d) level 2a measd

2b calcd

242 nm (41 300 240.6 nm (41 560 cm−1) (sh) cm−1) ( f = 0.04)

measd

2c calcd

246 nm (40 700 245.9 nm (40 670 cm−1) (sh) cm−1) ( f = 0.05) 238.9 nm (41 860 cm−1) ( f = 0.05)

2d

measd

calcd

measd

calcd

252 nm (39 683 cm−1) (sh)

251.7 nm (39 730 cm−1) ( f = 0.07) 236.3 nm (42 320 cm−1) ( f = 0.16)

n/a

209.7 nm (47 690 cm−1) ( f = 0.06)

237 nm (42 200 cm−1) (ε = 2.8 × 104 M−1 cm−1)

comparison to that of 2d reveal gradually stronger coupling when the distance between the two (formally) isolated Si−Si bonds becomes shorter. The previously reported all-silicon bicyclic species 2a was studied for comparison to the carbon-containing bicyclic cages 2b,c, and its UV spectrum displays a shoulder at 242 nm (41 300 cm−1) in line with previous experiments (Figure 6 and

whereas the other two correspond to ortho conformers (84 and 90°). The tetrasilane segment within the ring has a small dihedral angle (ωring = 25°) and should effectively split the branched octasilane into two branched pentasilane chromophores, where the two Si atoms of the bridge are part of both chromophores. The earlier reported cyclohexasilane 1a,42 with an all-silicon ring, in turn shows a bathochromic shift in comparison to 1b, as it has a broad shoulder at 264 nm (37 300 cm−1) and another broad maximum at 246 nm (40 700 cm−1, ε = 2.6 × 104 M−1 cm−1). Although the exocyclic dihedral angles for 1a,b both exhibit similar values, the all-silicon cyclic species 1a shows significantly larger values for the dihedral angles inside the ring, with values up to 62° as compared to 25° in 1b (Table 2). However, it is still not fully clear to what extent gauche conformers imposed by the cyclic structure influence the degree of σ conjugation within the ring. The UV absorption spectra are rather featureless. Therefore, in addition to the experimentally recorded UV spectra we also performed time-dependent DFT (TD-DFT) calculations at TD-PBE0/6-311+G(2d,p)//PBE0/6-311G(d) level (Table 5) to examine the four cages 2a−d, as this allows for a more thorough understanding of the electronic transitions. Overall, good agreements between the experimentally obtained UV absorption characteristics for 2a−c and those obtained from the computations were observed (see the Supporting Information for details on the calculated transitions as well as simulated spectra based on the computations). However, due to the featureless character of the spectra we also calculated the transitions at the TD-M062X/6-311+G(2d,p) and TD-B3LYP/ 6-311+G(2d,p) levels, using the PBE0/6-311G(d) geometries, in order to obtain wider support for our spectral interpretations. Indeed, we found close similarities between the data calculated by the three methods. With regard to the TD-PBE0 results they are found at values between those of the two other methods (see Table S1 in the Supporting Information for full comparison of the various transitions calculated with the three methods). With regard to computed electronic transitions, it can first be noted that the all-carbon-bridged compound 2d, which was not studied experimentally, displays its first visible absorption at 209.7 nm (47 690 cm−1) and its first dark state at 211.8 nm (47 210 cm−1), suggesting a very weak interaction between the two isolated exocyclic disilane segments. As shown earlier, the distance between these two silane segments can be shortened by replacements of the ethylene tethers with methylene tethers, giving 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptane and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexane with Si···Si through-space distances of 2.732 and 2.499 Å at the MP2/cc-pVTZ level.48 Calculations of the vertical excitations at the MS-CASPT2 level performed on these geometries revealed lowest excitations at 225.0 and 260.5 nm (44 444 and 38 394 cm−1), respectively. These red shifts in the lowest excitations in

Figure 6. UV absorption spectra of 2a−c.

Table 5).42 The TD-DFT calculation of 2a gave the first visible transition at 240.6 nm (41 560 cm−1, f = 0.04), in good agreement with the experimentally obtained value. Considering that the Si backbones of the tetrasilane segments of 2a involving the exocyclic trimethylsilyl group adopt near-perfect transoid bond conformers (ωexo = 167−168°), it is tempting to interpret the transition in terms of a transition predominantly localized at the six transoid tetrasilane segments. However, the first transition is red-shifted by ∼2000 cm−1 in comparison to that of n-Si4Me10 (reported λmax of transoid n-Si4Me10 is 228 nm47 = 43 900 cm−1). Moreover, the TD-DFT computations describe the first visible transition of 2a to 78% as a doubly degenerate HOMO → LUMO+2 transition where the LUMO +2 orbital pair has π symmetry (for the LUMO+1 and LUMO +2 orbitals of 2a−c, see the Supporting Information). Thus, the transition is of σπ* character, in contrast to n-Si4Me10, for which the first visible transition is of σσ* character. The TDDFT calculation also indicates that there are four singlet excited states below the first visible state. The lowest vertically excited singlet state of 2a is calculated at 258.2 nm (38 730 cm−1) and it is to 89% described as the HOMO → LUMO+1 transition. The HOMO → LUMO excitation corresponds to the second vertically excited state and is computed at 252.1 nm (39 660 cm−1). In comparison to 2a, the bicyclic compound 2b, containing two −SiMe2SiMe2− bridges and one −CH2CH2− bridge, revealed a slight bathochromic shift in the lowest visible transition, as its UV spectrum has a shoulder at 246 nm (40 650 cm−1). As noted above, the four tetrasilane segments of 2b involving the two exocyclic SiMe3 groups adopt two different G


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to 3.05 Å. The tetrasilane segments within the cages of 2a−c display Si−Si−Si−Si dihedral angles in the range 18−23°; however, the Si−Si−Si bond angles become gradually more acute on going from 2a to 2c. With regard to the electronic structure it was found that the frontier orbitals differ significantly among the three compounds with oligosilane cages. For example, despite the fact that the HOMOs of 2a−c are all of σSiSi orbital character and nearly isoenergetic, the distributions of the orbitals over the Si−Si bonds vary markedly. Furthermore, the orbitals immediately below the HOMO of 2a−c are energetically clustered close to the HOMO, a distinct difference from the highest few occupied orbitals of the a+g+a+ conformer of n-Si6Me14: i.e., the linear permethylated oligosilane which is most closely related to the linear oligosilane segments found in 2a−c. These orbital properties could potentially lead to charge transport characteristics which are different from those of linear oligosilanes, which were recently studied.32,33 The UV absorption spectra also reveal differences among the three [2.2.2]bicyclic oligosilanes because there is a red shift in the first visible transition when going from 2a, having three −SiMe2SiMe2− bridges, to 2c, with only one such bridge. However, the description of the transitions of these compounds in terms of σσ* transitions localized to anti or transoid tetrasilane segments is deceptive, as the transitions according to TD-DFT computations should be described as σπ* transitions.

bond conformers (ωexo = 178.0 and 153.1°, respectively). One may thus propose that the slightly red shifted lowest visible transition, in comparison to 2a, has a significant contribution from the two anti tetrasilane segments while the transition with a predominant contribution from the two deviant tetrasilane segments should be blue shifted (ω ≈ 150° for deviant conformation). Indeed, the TD-DFT calculations reveal the two lowest allowed transitions at 245.9 and 238.9 nm, respectively, in line with this argumentation. However, this view is deceptive, as the transition at 245.9 nm to 86% is described as a combined HOMO-1 → LUMO+1 and HOMO → LUMO+2 transition: i.e., a transition from orbitals of σ character (HOMO and HOMO-1) to orbitals of π character (LUMO+1 and LUMO+2). To evaluate transitions of oligosilanes with branched oligosilane paths in terms of σσ* excitations of linear anti or transoid tetrasilane segments could therefore be ambiguous. For 2b the lowest calculated vertically excited singlet state is the HOMO → LUMO excited state, but it is a dark state found at 272.2 nm (36 730 cm−1). In total, there are four states below the first visible transition of 2b. Interestingly, it was found that 2c, having two −CH2CH2− bridges and only one −SiMe2SiMe2− bridge, shows a further shift in its first visible transition to a longer wavelength at 252 nm (39 700 cm−1) in comparison to the observed values of 2a,b (note that the UV spectrum of compound 2c was recorded on a mixture of 2c and 1c, where the latter compound does not absorb above λ 240 nm, as seen in Figure 5). Compound 2c also displays a broad maximum at 237 nm (42 200 cm−1, ε = 2.8 × 104 M−1 cm−1), and both transitions are excellently described by the TD-DFT calculations as transitions at 251.7 and 236.3 nm, respectively. The first of these transitions is to 97% described as the HOMO → LUMO excitation: i.e., again a transition from an orbital of σ character to an orbital of π character. Interestingly, in 2c there is only one dark state below the first visible excitation, and this state is found at 252.9 nm (39 530 cm−1) according to the TD-DFT results. It is described as a HOMO → LUMO+2 transition, a transition from an orbital of σ character to one with π character. Clearly, the visible transitions in the three cage compounds 2a−c are of σπ* character, and the bathochromic shift on going from 2a to 2c is more complex than can be explained by the model based on linear tetrasilane chromophores. First, the structural symmetries of the compounds will have a significant impact on which transitions are visible and which ones are not. In addition, the smaller Si−Si−Si bond angles have an impact on the relative order of the frontier orbitals, and branched oligosilanes clearly have different orbital patterns than linear oligosilanes, as seen for both the HOMOs and LUMOs of 2a− c (Figure 4).

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EXPERIMENTAL SECTION

Computational Methods. All computations were performed with the Gaussian 09 program package, revision A.1.49 The structures were optimized at the PBE0/6-311G(d) hybrid density functional theory level,50,51 and frequency calculations were performed at the same level to verify that stationary points correspond to minima. Time-dependent density functional theory (TD-DFT)52 calculations were performed as implemented in Gaussian 09 using the PBE0/6-311+G(2d,p) basis set on the optimized geometries, and the 10 lowest excitations were calculated.53,54 The natural bond orbital (NBO) calculations were performed using the NBO 3.1 program55 as implemented in the Gaussian 09 package at the above level in order to understand various second-order interactions between filled localized orbitals of one molecular segment and vacant orbitals of another segment, which is a measure of the intramolecular interaction or hyperconjugation. General Remarks and Experiments. All reactions involving airsensitive compounds were carried out under an atmosphere of dry argon using Schlenk techniques. All reaction solvents in addition to C6D6 were purified via a solvent purification system before usage. Commercially available reagents were used without further purification. 1,1,1,4,4,4-Hexakis(trimethylsilyl)-2,2,3,3-tetramethyltetrasilane (3),56 1,1,4,4-tetrakis(trimethylsilyl)octamethylcyclohexasilane (1a),8 1,4-bis(trimethylsilyl)dodecamethylbicyclo[2.2.2]octasilane (2a),8 1,2bis[tris(trimethylsilyl)silyl]ethane (4),57 and 1,2-bis[(p-tolylsulfonyl)oxy]ethane58 have been prepared according to literature procedures. Aqueous workup was performed by pouring the reaction mixture into 1 M H2SO4 and separation of the layers; subsequently the aqueous layer was extracted twice with the solvent given in the text. The combined organic layers were washed with saturated NaCl solution and dried over magnesium sulfate, which was filtered off afterward. NMR spectra were recorded on a Varian Unity Inova 400 (1H at 399.97 MHz, 13C at 100.57 MHz, 29Si at 79.46 MHz) spectrometer at 25 °C. Chemical shifts are reported in ppm referenced to tetramethylsilane via the residual solvent (C6D6, 1H at 7.16 and 13C at 128.0 ppm). To compensate for the low isotopic abundance of 29Si, the INEPT pulse sequence was used for amplification of the signal.59,60 UV absorption spectra were recorded in cyclohexane solution with a quartz cell of 1.0 cm length. For X-ray structure analysis the crystals were mounted onto the tip of glass fibers, and data collection was performed

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CONCLUSIONS A set of cyclic and bicyclic organosilanes with six-membered rings and exocyclic trisilane or disilane segments were studied by both experimental and computational means. The compounds contain zero, one, two, or three −SiMe2SiMe2− bridges between the two exocyclic di- or trisilane segments. The aim of the study was to reveal the geometric, electronic, and optical properties of the compounds in dependence on the number of −SiMe2SiMe2− bridges. Focus was placed on the [2.2.2]bicyclic oligosilanes 2a−d. The most apparent structural change when going from 2a, having three −SiMe2SiMe2− bridges, to 2d, which instead has three −CH2CH2− bridges, is the shortening in the Si···Si through-space distance from 3.95 H


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with a Bruker-AXS SMART APEX CCD diffractometer using graphitemonochromated Mo Kα radiation (0.71073 Å). The data were reduced to Fo2 and corrected for absorption effects with SAINT61 and SADABS,62 respectively. The structures were solved by direct methods and refined by full-matrix least-squares methods (SHELXL97).63 All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located at calculated positions to correspond to standard bond lengths and angles. All structural diagrams were drawn as 50% probability thermal displacement ellipsoids, and all hydrogen atoms were omitted for clarity. Crystallographic data (excluding structure factors) for the structures of 1b,c and 2b reported in this paper have been deposited with the Cambridge Crystallographic Data Center as supplementary publications CCDC 835815 (1b), CCDC 835813 (1c), and CCDC 835814 (2b). Copies of the data can be obtained free of charge on application to the CCDC at www.ccdc.cam.ac.uk/products/csd/request. 1,1,4,4-Tetrakis(trimethylsilyl)-2,2,3,3-tetramethyl-1,2,3,4tetrasilacyclohexane (1b). Method A. A solution of 3 (3.90 g, 6.37 mmol) in toluene (100 mL) was treated with potassium tert-butoxide (1.57 g, 14.0 mmol) and 18-crown-6 (3.88 g, 14.7 mmol). After the mixture was stirred for 5 h at room temperature, 1,2-bis[(ptolylsulfonyl)oxy]ethane (2.36 g, 6.37 mmol) was added to the mixture, causing an exothermic reaction. Stirring was continued for 2 h at room temperature, and then the reaction mixture was subjected to an aqueous workup with diethyl ether/1 M H2SO4. After removal of the solvent the product was purified in the first place via column chromatography with pentane as eluent. Subsequent recrystallization from pentane/acetone gave 1.42 g (45%) of pure product 1b as colorless crystals. Method B. A solution of 4 (0.484g, 0.927 mmol) in toluene (20 mL) was treated with potassium tert-butoxide (0.213 g, 1.90 mmol) and 18-crown-6 (0.515 g, 1.95 mmol). After it was stirred for 2 h at room temperature, this solution was added dropwise to a solution of 0.174 g (0.929 mmol) of dichlorotetramethyldisilane in toluene (60 mL). The solution was stirred overnight and was then subjected to an aqueous workup with diethyl ether/1 M H2SO4. After removal of the solvent the product was purified in the first place via column chromatography with pentane as eluent. Subsequent recrystallization from pentane/acetone gave 0.242 g (53%) of pure product 1b as colorless crystals. Mp: 127−128 °C. 1H NMR (399.97 MHz, C6D6): δ 1.15 (s, 4H, CH2), 0.39 (s, 12H, Me2Si), 0.27 (s, 36H, Me3Si). 13C NMR (100.57 MHz, C6D6): δ 5.6, 1.4, −2.7. 29Si NMR (79.46 MHz, C6D6): δ −12.8, −45.7, −76.2. EI-MS: m/z (%) 492 (M+, 100), 403 (40), 345 (78), 271 (69), 73 (Me3Si+, 51). UV absorption: λ1 241 nm (ε1 = 2.6 × 104 mol−1 cm−1), absorption shoulder at 253 nm. HRMS (DI-EI): calcd for C18H52Si8 492.2223, found 492.2229. 1,1,4,4-Tetrakis(trimethylsilyl)-1,4-disilacyclohexane (1c). To a solution of 4 (8.68 g, 16.6 mmol) in toluene (160 mL) were added potassium tert-butoxide (3.86 g, 34.4 mmol) and 18-crown-6 (9.67 g, 36.6 mmol) to yield a yellow solution. After the mixture was stirred for 4 h at room temperature, 1,2-bis[(p-tolylsulfonyl)oxy]ethane (6.15 g, 16.6 mmol) was added to the mixture in one portion, causing an exothermic reaction. Stirring was continued for 3 h at room temperature, and then the reaction mixture was subjected to an aqueous workup with diethyl ether/1 M H2SO4. The solvent was removed, and the product was purified via column chromatography with pentane as eluent. After Kugelrohr distillation 2.09 g (31%) of 1c was obtained as colorless crystals. Mp: 109−110 °C. 1H NMR (499.94 MHz, C6D6): δ 1.16 (s, 8H, CH2), 0.22 (s, 36H, SiMe3). 13C NMR (125.7 MHz, C6D6): δ 5.7, 0.0. 29Si NMR (79.46 MHz, C6D6): δ −17.1, −44.4. EI-MS: m/z (%) 404 (M+, 34), 331 (M+ − SiMe3, 30), 315 (83), 257 (100), 73 (Me3Si+, 8). UV absorption: no maxima or shoulder in the observed region. Anal. Calcd for C16H44Si6: C, 47.45; H, 10.95. Found: C, 47.07; H, 10.53. 1,4-Bis(trimethylsilyl)-2,2,3,3,5,5,6,6-octamethyl-1,2,3,4,5,6hexasilabicyclo[2.2.2]octane (2b). Method A. To a solution of 1a (1.00 g, 1.72 mmol) in toluene (60 mL) were added potassium tertbutoxide (0.42 g, 3.78 mmol) and 18-crown-6 (1.04 g, 3.95 mmol) to yield a yellow solution. After the mixture was stirred for 7 h at room temperature, 1,2-bis[(p-tolylsulfonyl)oxy]ethane (0.65 g, 1.74 mmol)

was added to the mixture in one portion, causing an exothermic reaction. Stirring was continued for 2 h at room temperature, and then the reaction mixture was subjected to an aqueous workup with diethyl ether/1 M H2SO4. The solvent was removed, and the product was purified via column chromatography with pentane as eluent. After recrystallization from pentane/acetone 0.15 g (19%) of 2b was obtained as colorless crystals. Method B. To a solution of 1b (0.25 g, 0.50 mmol) in toluene (40 mL) were added potassium tert-butoxide (0.12 g, 1.10 mmol) and 18crown-6 (0.30 g, 1.15 mmol) to yield a yellow solution. After it was stirred for 7 h at room temperature, this solution was added dropwise to a solution of dichlorotetramethyldisilane (0.10 g, 0.53 mmol) in toluene (60 mL). Stirring was continued for 2 h at room temperature, and then the reaction mixture was subjected to an aqueous workup with diethyl ether/1 M H2SO4. The solvent was removed, and the product was purified via column chromatography with pentane as eluent, yielding 0.010 g (4%) of 2b. Mp: 137−138 °C. 1H NMR (399.97 MHz, C6D6): δ 1.18 (s, 4H, CH2), 0.37 (s, 12H, Me2Si), 0.33 (s, 12H, Me2Si), 0.23 (s, 18H, Me3Si). 13C NMR (100.57 MHz, C6D6): δ 2.8, 1.2, −3.1, −3.8, 29Si NMR (79.46 MHz, C6D6): δ −10.9, −41.6, −83.1. EI-MS: m/z (%) 462 (M+, 64), 361 (M+ − Si2Me3, 43), 287 (25), 73 (Me3Si+, 100). UV absorption: shoulder 246 nm. HRMS (DIEI): calcd for C16H46Si8 462.1754, found 462.1766. 1,4-Bis(trimethylsilyl)-2,2,3,3-tetramethyl-1,2,3,4tetrasilabicyclo[2.2.2]octane (2c). To a solution of 1b (3.50 g, 7.10 mmol) in toluene (100 mL) were added potassium tert-butoxide (1.75 g, 15.6 mmol) and 18-crown-6 (4.31 g, 16.3 mmol). The red reaction mixture was stirred for 7 h at room temperature. Thereafter 1,2-bis[(ptolylsulfonyl)oxy]ethane (2.76 g, 7.45 mmol) was added to the mixture, causing an exothermic reaction. Stirring was continued for 2 h at room temperature, and then the reaction mixture was subjected to an aqueous workup with diethyl ether/1 M H2SO4. After removal of the solvent the product was purified via column chromatography with pentane as eluent, resulting in a 0.6 g mixture of 1c and product 2c. Repeated recrystallization (9 times) from diethyl ether yielded an enriched product sample (3:1). 1H NMR (399.97 MHz, C6D6): δ 1.21 (m, 4H, CH2); 0.98 (m, 4H, CH2); 0.32 (s, 12H, Me2Si), 0.16 (s, 18H, SiMe3). 13C NMR (100.57 MHz, C6D6): δ 2.7, −0.8, −5.6. 29Si NMR (79.46 MHz, C6D6): δ −13.2, −16.2, −48.9. EI-MS: m/z (%) 374 (M+, 100), 285 (25), 227 (6), 73 (Me3Si+, 5). UV absorption: λ1 237 nm (ε1 = 2.8 × 104 mol−1 cm−1), shoulder 252 nm.

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ASSOCIATED CONTENT

* Supporting Information S

Figures, text, tables, NMR spectra for compounds 1b,c and 2b,c, the complete ref 48, absolute electronic energies, Cartesian coordinates, TD-DFT results, together with X-ray crystallographic information and CIF files for 1b,c and 2b. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from Uppsala University for the U3MEC KoF07 initiative on molecular electronics, from the Swedish Research Council (Vetenskapsrådet, Grant No. 2008-3710), and from the Wenner-Gren Foundations and the Göran Gustafsson Foundation for postdoctoral fellowships to A.W. The National Supercomputer Center (NSC) in LinI


Organometallics

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Köping, Sweden, is acknowledged for generous allotment of computer time.

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