Incorporating Methyl and Phenyl Substituted Stannylene Units into Oligosilanes. The Influence on Optical Absorption Properties

Molecules containing catenated heavy group 14 atoms are known to exhibit the interesting property of σ-bond electron delocalization. While this is well studied for oligo- and polysilanes the current paper addresses the UV-absorption properties of small tin containing oligosilanes in order to evaluate the effects of Sn–Si and Sn–Sn bonds as well as the results of substituent exchange from methyl to phenyl groups. The new stannasilanes were compared to previously investigated oligosilanes of equal chain lengths and substituent pattern. Replacing the central SiMe2 group in a pentasilane by a SnMe2 unit caused a bathochromic shift of the low-energy band (λmax = 260 nm) of 14 nm in the UV spectrum. If, instead of a SnMe2, a SnPh2 unit is incorporated, the bathochromic shift of 33 nm is substantially larger. Keeping the SnMe2 unit and replacing the two central silicon with tin atoms causes shift of the respective band (λ = 286 nm) some 26 nm to the red. A similar approach for hexasilanes where the model oligosilane [(Me3Si)3Si]2(SiMe2)2 (λmax = 253 nm) was modified in a way that the central tetramethyldisilanylene unit was exchanged for a tetraphenyldistannanylene caused a 50 nm bathochromic shift to a low-energy band with λmax = 303 nm.


Introduction
It is well known that oligo-and polymeric chains of silicon, germanium, and tin atoms possess the unusual feature of σ-bond electron delocalization [1]. This property reflects the smaller HOMO-LUMO gaps of higher elements and in a molecular way resembles the semiconducting nature of the higher group 14 elements and is best known for polysilanes [2].
Despite the fact that σ-bond electron delocalization seems to be an exotic property it is actually not so much different from the much more familiar concept of π-bond electron delocalization. Both types of delocalization rely on two important prerequisites. The first requirement is an electronic one and it says that HOMO-LUMO gaps need to be reasonably small. This requirement is not met for alkanes which is the reason that σ-bond electron delocalization is very much restricted to compounds containing σ-bonds between heavier elements. The second prerequisite is a spatial one and it requires the involved orbitals to be in plane to allow extended orbital overlap over the involved atoms along the chain. For the π-bond electron delocalization this means all delocalized bonds are in one plane. A similar overlap for σ-bond electron delocalization would require an all-trans-conformation of the involved chain. As catenated heavier elements usually bear comparatively large substituents there are steric 1,2-and 1,3-interactions between these substituent, thereby causing a deviation from all-transto a transoid conformation.
Our group and others have shown that oligosilanes with large end groups exhibit a preference to acquire a transoid conformation as long as the end groups are not too far apart from each other [14][15][16][17][18][19][20]. In a recent study we reported on the influence of the formal replacement of silicon atoms in oligosilanes by germanium atoms on the σ-electron delocalization properties, which was found to be rather negligible [21]. However, related studies by Weinert and co-workers on the optical absorption properties on oligogermanes [22][23][24][25][26][27][28][29][30][31] concentrated mainly on phenylated molecules which were found to exhibit bathochromic shifted low-energy bands compared to our methylated oligosilanes of the same chain lengths. This is not unexpected, as the introduction of more electronegative substituents is known to cause absorption bands of oligosilanes to shift to longer wavelengths.
The current account is concerned with the influence of the formal exchange of silicon by tin atoms and of methyl groups by phenyl substituents on the optical absorption properties, which reflect the extent of σ-bond electron delocalization.

Synthesis
Over the last 20 years, our group has prepared many different oligosilanes. The key synthetic advancement that allowed for the preparation of structurally reasonably complex molecules was the discovery of the facile silanide formation accomplished by silyl group abstraction with potassium alkoxides [32]. The potassium silanides thus obtained proved to be ideal building blocks, which could be reacted with silyl halides of triflates to a large variety of oligosilanes. Subsequent application of the silyl group abstraction method to silylated germanes [33] and stannanes [34] provides access to silylated germanides and stannides. Synthesis of organostannanes with attached oligosilanyl groups as required for the current study therefore could be achieved in a fairly straightforward way.
Our group and others have shown that oligosilanes with large end groups exhibit a preference to acquire a transoid conformation as long as the end groups are not too far apart from each other [14][15][16][17][18][19][20]. In a recent study we reported on the influence of the formal replacement of silicon atoms in oligosilanes by germanium atoms on the σ-electron delocalization properties, which was found to be rather negligible [21]. However, related studies by Weinert and co-workers on the optical absorption properties on oligogermanes [22][23][24][25][26][27][28][29][30][31] concentrated mainly on phenylated molecules which were found to exhibit bathochromic shifted low-energy bands compared to our methylated oligosilanes of the same chain lengths. This is not unexpected, as the introduction of more electronegative substituents is known to cause absorption bands of oligosilanes to shift to longer wavelengths.
The current account is concerned with the influence of the formal exchange of silicon by tin atoms and of methyl groups by phenyl substituents on the optical absorption properties, which reflect the extent of σ-bond electron delocalization.

Synthesis
Over the last 20 years, our group has prepared many different oligosilanes. The key synthetic advancement that allowed for the preparation of structurally reasonably complex molecules was the discovery of the facile silanide formation accomplished by silyl group abstraction with potassium alkoxides [32]. The potassium silanides thus obtained proved to be ideal building blocks, which could be reacted with silyl halides of triflates to a large variety of oligosilanes. Subsequent application of the silyl group abstraction method to silylated germanes [33] and stannanes [34] provides access to silylated germanides and stannides. Synthesis of organostannanes with attached oligosilanyl groups as required for the current study therefore could be achieved in a fairly straightforward way.
Reactions of tris(trimethylsilyl)silyl potassium [35,36] with dimethyldichlorostannane or diphenyldichlorostannane provided bis[tris(trimethylsilyl)silyl]dimethylstannane (1) [37] and bis[tris(trimethylsilyl)silyl]diphenylstannane (2) in 86% and 61% yield, respectively (Scheme 1). The NMR spectroscopic properties observed, and in particular the 29 Si and 119 Sn chemical shifts of 1 [37] and 2 (Table 1), are quite similar and completely within the range of what to expect [38,39]. The NMR spectroscopic properties observed, and in particular the 29 Si and 119 Sn chemical shifts of 1 [37] and 2 (Table 1), are quite similar and completely within the range of what to expect [38,39]. Single crystals of compound 2 were subjected to XRD analysis ( Figure 1) showing 2 to crystallize in the monoclinic space group P2 1 /n. Structural parameters include Sn-Si bond distances of 2.6071(7) and 2.6099(8) Å and an Si-Sn-Si bond angle of 132.507 (17) • . While a number of tin compounds with two attached tris(trimethylsilyl)silyl groups are known, most of the structurally characterized examples feature divalent tin, where the Sn-Si bonds are significantly elongated to values above 2.65 Å and the Si-Sn-Si bond angles are smaller than 115 • [40][41][42][43]. Nevertheless dichlorostannane [(Me 3 Si) 3 Si] 2 SnCl 2 [37] was found to exhibit quite similar Si-Sn distances of 2.604(3) Å and the Si-Sn-Si angle of 142.5(1) • was even somewhat narrower than in 2. For the structurally related cyclic compound (Me 2 Si) 2 [(Si(SiMe 3 ) 2 ] 2 SnPh 2 [44] the Si-Sn distances amount to 2.594(4) and 2.620(4) Å. For the related compounds (Me 3 Si) 3 SiSn i Pr 3 with only one tris(trimethylsilyl)silyl group and three isopropyl groups on tin and hexakis(trimethylsilyl)distannane even shorter Si-Sn distance of 2.573(4) and 2.5707(17) Å, respectively, were reported [45]. Of particular interest for the σ-bond delocalization are the conformational properties, which are reflected by the dihedral angles. As listed in Table 2, the two relevant dihedral angles of 2 are exhibiting a deviant conformational preference, which is likely caused by steric interactions of the phenyl groups with trimethylsilyl groups.  Single crystals of compound 2 were subjected to XRD analysis ( Figure 1) showing 2 to crystallize in the monoclinic space group P21/n. Structural parameters include Sn-Si bond distances of 2.6071(7) and 2.6099(8) Å and an Si-Sn-Si bond angle of 132.507(17)°. While a number of tin compounds with two attached tris(trimethylsilyl)silyl groups are known, most of the structurally characterized examples feature divalent tin, where the Sn-Si bonds are significantly elongated to values above 2.65 Å and the Si-Sn-Si bond angles are smaller than 115° [40][41][42][43]. Nevertheless dichlorostannane [(Me3Si)3Si]2SnCl2 [37] was found to exhibit quite similar Si-Sn distances of 2.604(3) Å and the Si-Sn-Si angle of 142.5(1)° was even somewhat narrower than in 2. For the structurally related cyclic compound (Me2Si)2[(Si(SiMe3)2]2SnPh2 [44] the Si-Sn distances amount to 2.594(4) and 2.620(4) Å. For the related compounds (Me3Si)3SiSn i Pr3 with only one tris(trimethylsilyl)silyl group and three isopropyl groups on tin and hexakis(trimethylsilyl)distannane even shorter Si-Sn distance of 2.573(4) and 2.5707(17) Å, respectively, were reported [45]. Of particular interest for the σ-bond delocalization are the conformational properties, which are reflected by the dihedral angles. As listed in Table 2, the two relevant dihedral angles of 2 are exhibiting a deviant conformational preference, which is likely caused by steric interactions of the phenyl groups with trimethylsilyl groups. Starting out from 1,2-diphenylated distannanes 3 [46,47] and 4, the respective triflates were obtained by treatment with two equiv triflic acid and treated further on with two equiv tris(trimethylsilyl)silyl potassium. Tris(trimethylsilyl)silylated distannanes 5 and 6 were formed in 48% and 33% yields (Scheme 2). In addition to distannane 6 a few crystals of a tris(trimethylsilyl)silylated tristannane (6a) were obtained. Although it was not possible to subject 6a Starting out from 1,2-diphenylated distannanes 3 [46,47] and 4, the respective triflates were obtained by treatment with two equiv triflic acid and treated further on with two equiv tris(trimethylsilyl)silyl potassium. Tris(trimethylsilyl)silylated distannanes 5 and 6 were formed in 48% and 33% yields (Scheme 2). In addition to distannane 6 a few crystals of a tris(trimethylsilyl)silylated tristannane (6a) were obtained. Although it was not possible to subject 6a to NMR spectroscopic characterization, the few crystals obtained permitted crystallographic analysis (vide infra). Table 2. Dihedral angles of the Si 1-2 -(Sn) 1-3 -Si 1-2 framework of compounds 2, 5, 6, 6a, and 8.

Dihedral Angles ω [ • ] Determined by X-ray Crystallography
Compound to NMR spectroscopic characterization, the few crystals obtained permitted crystallographic analysis (vide infra). The molecular structure of distannane 5 in the solid state ( Figure 2) is isostructural to that of the respective 1,2-bis[tris(trimethylsilyl)silyl]tetramethyldisilane [48,49] with slightly extended cell parameters. Both compounds crystallize in the triclinic space group P-1 and feature perfect transalignment along the Si-E-E-Si bond arrangement (E = Si, Sn) ( Table 2). For the all-silicon molecule we have shown that the conformational preference in the solid state is maintained in solution so that σ-electron conjugation along a chain of six heavy atoms is facile [14]. Sn-Si distances of 5 are with 2.579(3) Å shorter than in 2 reflecting the fact that only one bulky tris(trimethylsilyl)silyl group is attached to a tin atom. The Sn-Sn bond distance of 2.7950(13) is close to that of 2.7930(12) observed for (Me3Si)3SnSn(SiMe3)3 [45]. The Si-Sn-Sn bond angle of 119.939(12)° is somewhat increased in comparison to that of the all-silicon analog. The molecular structure of distannane 5 in the solid state ( Figure 2) is isostructural to that of the respective 1,2-bis[tris(trimethylsilyl)silyl]tetramethyldisilane [48,49] with slightly extended cell parameters. Both compounds crystallize in the triclinic space group P-1 and feature perfect trans-alignment along the Si-E-E-Si bond arrangement (E = Si, Sn) ( Table 2). For the all-silicon molecule we have shown that the conformational preference in the solid state is maintained in solution so that σ-electron conjugation along a chain of six heavy atoms is facile [14]. Sn-Si distances of 5 are with 2.579(3) Å shorter than in 2 reflecting the fact that only one bulky tris(trimethylsilyl)silyl group is attached to a tin atom. The Sn-Sn bond distance of 2.7950(13) is close to that of 2.7930(12) observed for (Me 3 Si) 3 SnSn(SiMe 3 ) 3 [45]. The Si-Sn-Sn bond angle of 119.939 (12) • is somewhat increased in comparison to that of the all-silicon analog. to NMR spectroscopic characterization, the few crystals obtained permitted crystallographic analysis (vide infra). The molecular structure of distannane 5 in the solid state ( Figure 2) is isostructural to that of the respective 1,2-bis[tris(trimethylsilyl)silyl]tetramethyldisilane [48,49] with slightly extended cell parameters. Both compounds crystallize in the triclinic space group P-1 and feature perfect transalignment along the Si-E-E-Si bond arrangement (E = Si, Sn) ( Table 2). For the all-silicon molecule we have shown that the conformational preference in the solid state is maintained in solution so that σ-electron conjugation along a chain of six heavy atoms is facile [14]. Sn-Si distances of 5 are with 2.579(3) Å shorter than in 2 reflecting the fact that only one bulky tris(trimethylsilyl)silyl group is attached to a tin atom. The Sn-Sn bond distance of 2.7950(13) is close to that of 2.7930(12) observed for (Me3Si)3SnSn(SiMe3)3 [45]. The Si-Sn-Sn bond angle of 119.939(12)° is somewhat increased in comparison to that of the all-silicon analog.  Distannane 6 was found to crystallize with one additional benzene molecule in the monoclinic space group P2 1 /n ( Figure 3). Also the structure of 6 contains an inversion center and thus exhibits trans-alignment along the Si-Sn-Sn-Si unit ( Table 2). Compared to 5 the Sn-Si distances of 6 are slightly elongated to a value of 2.5988(6) Å caused by the increased steric demand of the Ph 2 Sn unit compared to Me 2 Sn. This is also responsible for the elongated Sn-Sn bond distance of 2.8217(7) observed for 6. Distannane 6 was found to crystallize with one additional benzene molecule in the monoclinic space group P21/n ( Figure 3). Also the structure of 6 contains an inversion center and thus exhibits trans-alignment along the Si-Sn-Sn-Si unit ( Table 2). Compared to 5 the Sn-Si distances of 6 are slightly elongated to a value of 2.5988(6) Å caused by the increased steric demand of the Ph2Sn unit compared to Me2Sn. This is also responsible for the elongated Sn-Sn bond distance of 2.8217(7) observed for 6. The molecular structure of 6a (Figure 4) in the solid state is asymmetric and a comparably small dihedral angle (148°) on one side of the molecule is observed ( Table 2). As expected Sn-Sn bond lengths of 6a are somewhat shorter than in 6 (2.8022(15) and 2.8161(14) Å) reflecting the diminished interaction between phenyl and trimethylsilyl groups.  The molecular structure of 6a (Figure 4) in the solid state is asymmetric and a comparably small dihedral angle (148 • ) on one side of the molecule is observed ( Table 2). As expected Sn-Sn bond lengths of 6a are somewhat shorter than in 6 (2.8022(15) and 2.8161(14) Å) reflecting the diminished interaction between phenyl and trimethylsilyl groups. Distannane 6 was found to crystallize with one additional benzene molecule in the monoclinic space group P21/n (Figure 3). Also the structure of 6 contains an inversion center and thus exhibits trans-alignment along the Si-Sn-Sn-Si unit ( Table 2). Compared to 5 the Sn-Si distances of 6 are slightly elongated to a value of 2.5988(6) Å caused by the increased steric demand of the Ph2Sn unit compared to Me2Sn. This is also responsible for the elongated Sn-Sn bond distance of 2.8217(7) observed for 6. The molecular structure of 6a (Figure 4) in the solid state is asymmetric and a comparably small dihedral angle (148°) on one side of the molecule is observed ( Table 2). As expected Sn-Sn bond lengths of 6a are somewhat shorter than in 6 (2.8022(15) and 2.8161(14) Å) reflecting the diminished interaction between phenyl and trimethylsilyl groups.  To study the influence of the exchange of silicon for tin atoms we decided to prepare a structure isostructural to compound 1 but with tris(trimethylsilyl)stannyl end groups. As depicted in Scheme 3 this goal can be achieved by reacting dimethyldichlorostannane with tris(trimethylsilyl)stannyl potassium (obtained easily from tetrakis(trimethylsilyl)stannane (7)  To study the influence of the exchange of silicon for tin atoms we decided to prepare a structure isostructural to compound 1 but with tris(trimethylsilyl)stannyl end groups. As depicted in Scheme 3 this goal can be achieved by reacting dimethyldichlorostannane with tris(trimethylsilyl)stannyl potassium (obtained easily from tetrakis(trimethylsilyl)stannane (7) [34]) yielding compound 8 in 66% yield.

UV-Vis Spectroscopy
From our studies on the σ-bond electron delocalization of oligosilanes and germaoligosilanes [14][15][16][17][18][19][20][21] we have learned that that tris(trimethylsilyl)silyl terminated methyloligosilanes exibit a strong preference for transoid conformations in the solid state and in solution. Increasing the lengths of the main chain results in bathochromic shifts of the low-energy (σ→σ*) transition indicating extension of the delocalized system. Exchange of selected silicon atoms from the main chain for germanium was found to change the spectroscopic properties only slightly. For a similar exchange of silicon with tin atoms a significantly stronger effect is expected.
This expectation is already fulfilled for compound 1. The low-energy band of its UV spectrum is located at 260 nm ( Figure 6) which is 14 nm red shifted compared to the analogous all-silicon compound [(Me3Si)3Si]2SiMe2. To study the influence of the exchange of silicon for tin atoms we decided to prepare a structure isostructural to compound 1 but with tris(trimethylsilyl)stannyl end groups. As depicted in Scheme 3 this goal can be achieved by reacting dimethyldichlorostannane with tris(trimethylsilyl)stannyl potassium (obtained easily from tetrakis(trimethylsilyl)stannane (7) [34]) yielding compound 8 in 66% yield.

UV-Vis Spectroscopy
From our studies on the σ-bond electron delocalization of oligosilanes and germaoligosilanes [14][15][16][17][18][19][20][21] we have learned that that tris(trimethylsilyl)silyl terminated methyloligosilanes exibit a strong preference for transoid conformations in the solid state and in solution. Increasing the lengths of the main chain results in bathochromic shifts of the low-energy (σ→σ*) transition indicating extension of the delocalized system. Exchange of selected silicon atoms from the main chain for germanium was found to change the spectroscopic properties only slightly. For a similar exchange of silicon with tin atoms a significantly stronger effect is expected.
This expectation is already fulfilled for compound 1. The low-energy band of its UV spectrum is located at 260 nm ( Figure 6) which is 14 nm red shifted compared to the analogous all-silicon compound [(Me3Si)3Si]2SiMe2.

UV-Vis Spectroscopy
From our studies on the σ-bond electron delocalization of oligosilanes and germaoligosilanes [14][15][16][17][18][19][20][21] we have learned that that tris(trimethylsilyl)silyl terminated methyloligosilanes exibit a strong preference for transoid conformations in the solid state and in solution. Increasing the lengths of the main chain results in bathochromic shifts of the low-energy (σ→σ*) transition indicating extension of the delocalized system. Exchange of selected silicon atoms from the main chain for germanium was found to change the spectroscopic properties only slightly. For a similar exchange of silicon with tin atoms a significantly stronger effect is expected. This expectation is already fulfilled for compound 1. The low-energy band of its UV spectrum is located at 260 nm ( Figure 6) which is 14 nm red shifted compared to the analogous all-silicon compound [(Me 3 Si) 3 Si] 2 SiMe 2 .
Replacement of two additional silicon atoms leads to compound 8 with a central tristannanylene unit. The low-energy band of the UV spectrum of 8 ( Figure 6) is located at 286 nm indicating another red shift of 24 nm. Overall, the replacement of three silicon atoms with tin atoms in linear pentasilane unit caused a bathochromic shift of 38 nm.
[(Me 3 Si) 3 Si] 2 (SiMe 2 ) 2 , which is the all-silicon analog of compound 5, exhibits a low-energy band at 253 nm. In contrast to this, the UV spectrum of 5 ( Figure 6) with the two central SnMe 2 units featured the respective band at 280 nm. The formal replacement of two silicon atoms with tin atoms here amounts to a bathochromic shift of 27 nm.
Previous attempts to attach tris(trimethylsilyl)silyl groups to perphenylated silanylene units were not successful, but the stannaoligosilanes of this study allow the study of the influence of attached phenyl groups on the σ-electron delocalization of the oligomers. This can be achieved by comparison of compounds 1 with 2 and compounds 5 with 6. In addition to the σ→σ* transition bands, the UV spectra of 2 and 6 ( Figure 6) also show bands which we assign to π→π* transitions with vibrational fine structure.
Compound 2, where compared to 1 the two methyl groups on the central tin atom are replaced by phenyl groups, shows a further bathochromic shift of 19 nm for a low-energy band at 279 nm ( Figure 6). Still, molecule 2 contains a linear arrangement of five connected heavy atoms where the σ-electron delocalization is supposed to occur. The overall formal replacement of a SiMe 2 unit in [(Me 3 Si) 3 Si] 2 SiMe 2 with a SnPh 2 entity causes a bathochromic shift of the low-energy band of 36 nm. A similar relationship exists between is [(Me 3 Si) 3 Si] 2 (SiMe 2 ) 2 (λ max = 253 nm) and compound 6 which extends its low-energy band to 303 nm. The formal exchange of a tetramethyldisilanylene unit with a tetraphenyldistannanylene in this case causes a 50 nm bathochromic shift. Replacement of two additional silicon atoms leads to compound 8 with a central tristannanylene unit. The low-energy band of the UV spectrum of 8 ( Figure 6) is located at 286 nm indicating another red shift of 24 nm. Overall, the replacement of three silicon atoms with tin atoms in linear pentasilane unit caused a bathochromic shift of 38 nm.
[(Me3Si)3Si]2(SiMe2)2, which is the all-silicon analog of compound 5, exhibits a low-energy band at 253 nm. In contrast to this, the UV spectrum of 5 ( Figure 6) with the two central SnMe2 units featured the respective band at 280 nm. The formal replacement of two silicon atoms with tin atoms here amounts to a bathochromic shift of 27 nm.
Previous attempts to attach tris(trimethylsilyl)silyl groups to perphenylated silanylene units were not successful, but the stannaoligosilanes of this study allow the study of the influence of attached phenyl groups on the σ-electron delocalization of the oligomers. This can be achieved by comparison of compounds 1 with 2 and compounds 5 with 6. In addition to the σ→σ* transition bands, the UV spectra of 2 and 6 ( Figure 6) also show bands which we assign to π→π* transitions with vibrational fine structure.
Compound 2, where compared to 1 the two methyl groups on the central tin atom are replaced by phenyl groups, shows a further bathochromic shift of 19 nm for a low-energy band at 279 nm ( Figure 6). Still, molecule 2 contains a linear arrangement of five connected heavy atoms where the σ-electron delocalization is supposed to occur. The overall formal replacement of a SiMe2 unit in [(Me3Si)3Si]2SiMe2 with a SnPh2 entity causes a bathochromic shift of the low-energy band of 36 nm. A similar relationship exists between is [(Me3Si)3Si]2(SiMe2)2 (λmax = 253 nm) and compound 6 which extends its low-energy band to 303 nm. The formal exchange of a tetramethyldisilanylene unit with a tetraphenyldistannanylene in this case causes a 50 nm bathochromic shift.

Experimental Section
All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glove box. Solvents were dried using a column solvent purification system [51]. Potassium tert-butanolate was purchased exclusively from MERCK. Dichlorodiphenylstannane, dichlorodimethylstannane, hexaphenyldistannane (4) and all other chemicals were obtained from different suppliers and were used without further purification. Tris(trimethylsilyl)silyl potassium [35,36], and tetrakis(trimethylsilyl)stannane (7) [52] have been prepared following published procedures.

Experimental Section
All reactions involving air-sensitive compounds were carried out under an atmosphere of dry nitrogen or argon using either Schlenk techniques or a glove box. Solvents were dried using a column solvent purification system [51]. Potassium tert-butanolate was purchased exclusively from MERCK. Dichlorodiphenylstannane, dichlorodimethylstannane, hexaphenyldistannane (4) and all other chemicals were obtained from different suppliers and were used without further purification. Tris(trimethylsilyl)silyl potassium [35,36], and tetrakis(trimethylsilyl)stannane (7) [52] have been prepared following published procedures. 1 H-(300 MHz), 13 [55]. X-ray structure determination: For X-ray structure analyses the crystals were mounted onto the tip of glass fibers, and data collection was performed with a BRUKER-AXS SMART APEX CCD diffractometer (Bruker-AXS, Madison, WI, USA) using graphite-monochromated Mo Kα radiation (0.71073 Å). The data were reduced to F 2 0 and corrected for absorption effects with SAINT [56] and SADABS [57,58], respectively. Structures were solved by direct methods and refined by full-matrix least-squares method (SHELXL97 and SHELX2013) [59,60]. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in calculated positions to correspond to standard bond lengths and angles.
Dimethyldiphenylstannane. A solution of PhMgBr in Et 2 O (prepared from Mg (466 mg, 19.2 mmol) and bromobenzene (2986 mg, 19.2 mmol) was slowly added to dichlorodimethylstannane (2057 mg, 9.4 mmol) in THF (20 mL) at 0 • C. The mixture was stirred for 12 h before saturated NH 4 Cl solution (30 mL) was added. The organic phase was separated, reduced in volume, and then passed through a pad of silica gel. After evaporation of solvent dimethyldiphenylstannane (1829 mg, 64%) was obtained as a colorless oil. NMR spectroscopic data in accordance with published values [63]: 119 Sn-NMR (δ in ppm): −60.
Chlorodimethylphenylstannane. To a solution of dimethyldiphenylstannane (1829 mg, 5.3 mmol) in Et 2 O (25 mL) HCl (1.2 equiv., 1 M solution in diethyl ether) was added at 0 • C. The mixture was allowed to come to r.t. and then stirred for 12 h before saturated NH 4 Cl solution (10 mL) was added. The layers were separated, the organic layer dried over anhydrous Na 2 SO 4 and the solvent removed. Chlorodimethylphenylstannane (851 mg, 61%) was obtained as a colorless oil and was used without further purification. NMR spectroscopic data in accordance with published values [63]: 119 Sn-NMR (δ in ppm): 91.

Conclusions
With the intention to study the influence of incorporated heavier group 14 element atoms, a number of stannaoligosilanes were synthesized. The UV absorption properties of these were compared to those of previously investigated oligosilanes of equal chain lengths and substituent pattern. Replacing one dimethylsilylene group in a pentasilane by a dimethylstannylene unit was found to cause a bathochromic shift of the low-energy band (corresponding to HOMO-LUMO transition) (λ max = 260 nm) of 14 nm. By introduction of a diphenylstannylene group the bathochromic shift is raised to 33 nm. Keeping the dimethylstannylene unit and exchanging the two adjacent silicon atoms by tin shifts the respective band (λ = 286 nm) some 26 nm to the red.
These results clearly show that by deliberate exchange of silicon for tin and switching to phenyl substituents, the absorption properties and the degree of σ-bond electron delocalization can be tuned over a large spectral range.