A cyclic disilylated stannylene: synthesis, dimerization, and adduct formation.

Reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with [(Me(3)Si)(2)N](2)Sn led to the formation of an endocyclic distannene via the dimerization of a transient stannylene. In the presence of strong donor molecules such as PEt(3), the stannylene could be trapped as adduct. Reaction of the PEt(3) derivative with B(C(6)F(5))(3) gave rise to the formation of the stannylene B(C(6)F(5))(3) adduct.

Stannylenes were among the first reported stable group 14 ylenes.[1,2] In contrast to carbenes, they exhibit singlet ground states with a formal 5s25p2 valence electron configuration. Early examples featured heteroatom (group 15 and 16) substituents providing stabilization via interaction of the heteroatom lone pair with the vacant p orbital. If these electronegative groups are replaced by alkyl groups, the p character of the lone pair is enhanced by the inductive effect of the electropositive substituents. The exchange with silyl groups should further amplify this effect.(3) It is thus surprising that after the first examples of bis(silyl)-substituted stannylenes were reported by Klinkhammer and co-workers,[4−6] no more efforts were undertaken in this direction to modify the reactivity of stannylenes.

To avoid dissociation of the silyl groups, the current study was directed toward the introduction of a bidentate silyl ligand. By reaction of a 1,4-dipotassiotetrasilane[7,8] (1) with [(Me3Si)2N]2Sn,[9,10] a cyclic disilylated stannylene structurally related to Klinkhammer’s compound should be formed. However, instead of the expected stannylene 2, the endocyclic distannene 3 was obtained (Scheme 1).

Scheme 1

Formation of Δ9,10-Octalin-Type Distannene 3

The formation of 3 likely involved the initial formation of the cyclic stannylene 2. Dimerization of 2 generated an exocyclic distannene 4, which after two 1,2-silyl shifts via the stannylstannylene 5 formed the endocyclic distannene isomer 3. Precedent for the formation of stannylstannylenes such as 5 by dimerization of stannylenes has been given by Power and co-workers,(11) and a related reaction in silicon chemistry was recently reported by Kira and co-workers.(12) Further support for this proposed mechanism came from density functional theory (DFT) calculations at the MPW1K/SDD(Sn), 6-31G(d) level of theory.(13) The results of the computations showed that distannenes 3 and 4 and stannylstannylene 5 are all significantly lower in energy than two molecules of stannylene 2 (4 by 57.5 kJ mol−1, 5 by 35.8 kJ mol−1, and 3 by 58.7 kJ mol−1). In addition, calculations for the model compounds 2(H), 3(H), 4(H), and 5(H) indicated that the involved barriers for the 1,2 substituent shift are rather low [i.e., 29.5 kJ mol−1 for 4(H) → 5(H) and 30.5 kJ mol−1 for 5(H) → 3(H) at the MPW1K/Sn(SDD), 6-311+G(d,p) level].

As can be expected for the bicyclic structure of 3, in which the two Sn atoms are held together, a typical distannene 119Sn NMR shift of +544.5 ppm was found; this lies between the value of +630.7 ppm reported for Sekiguchi’s compound (tBu2MeSi)2SnSn(SiMetBu2)2 (6)(14) and the values of +427.3 ppm for Masmune’s tetrakis(2,4,6-triisopropylphenyl)distannene(15) and +412 ppm for Wiberg’s cyclotristannene,(16) all of which are known to retain the distannene structure in solution. This is also consistent with a UV absorption at 626 nm, which is close to the reported value of 670 nm for 6. A low-quality crystal structure of 3 (Figure 1) showed that the two Sn atoms are disordered over two positions each with very similar structural features. The resulting two Sn=Sn double bonds of 3 have lengths of 268.9(5) and 268.6(4) pm, which are among the shortest of all structurally characterized stable distannenes.(12) The sums of the bond angles around the tricoordinated Sn atoms [∑°(Sn) = 354.0(2), 352.2(2), 352.7(2), and 353.7(2)°] and the relatively large trans-bent angles (β = 29.6, 26.5, 25.8, and 28.5°) indicate a significant pyramidalization of the Sn centers. In addition, the Sn=Sn bonds in 3 are twisted by angles ε of 27.0 and 28.6°. Comparison with the structural parameters of two closely related compounds, namely, the dimeric structure [(Me3Si)3Si]2Sn (7)(4) and Sekiguchi’s compound 6,(14) reveals an amazing structural diversity of silyl-substituted stannylene dimers [for 6, d(Sn=Sn) = 266.8 pm, β = 1.2°, ε = 44.6°; for 7, d(Sn=Sn) = 282.5 pm, β = 28.6°, ε = 63.2°]. These pronounced differences suggest a high structural flexibility of the Si2Sn=SnSi2 core in distannenes 3, 6, and 7.

Figure 1

Thermal ellipsoid plot for 3 drawn at the 30% probability level.

For the model distannene (H3Si)2Sn=Sn(SiH3)2 (8), DFT calculations(13) predicted a trans-bent ground-state structure having C2 molecular symmetry, in contrast to the results of previous lower-level computations.(17) The folding of the Sn=Sn bond in distannene 8 is in agreement with a significant preference of the singlet state in the constituent stannylene (H3Si)2Sn: [singlet/triplet energy difference ΔE(ST) = −99.1 kJ mol−1].

The modulus of ΔE(ST) is larger than a quarter of the modulus of the σ and π bond energy E(σ+π) of distannene 8 in its planar D2 form (|1/4E(σ+π)| = 89.3 kJ mol−1).(13) According to the CGMT model,[18,19] this results in a marked trans-bending of the double-bond system. In addition, the computations indicated the flexibility of distannene 8. That is, variation of the Sn=Sn bond length from 250 to 290 pm, the bending angle β from 0 to 60°, and the twisting angle ε from 0 to 22.5° all required less than 15 kJ mol−1 (Figure 2). For distannene 3, the computations predicted a molecular structure having C2 symmetry that closely resembles in all significant parameters the experimental structure [i.e., d(Sn=Sn) = 270.7 pm, ∑°(Sn) = 353.2°, β = 26.5°, ε = 6.0°). Natural bond order analysis(13) of the DFT density suggested multiple-bond character for the Sn=Sn bond in compound 3 on the basis of a Wiberg bond index (WBI) of 1.66. This value should be compared with the WBI values computed for 8 and the parent Sn2H4 in both their planar configurations of D2 symmetry and their trans-bent minimum structures of C2 symmetry [for D2 symmetry, WBI = 1.84 (8), 1.94 (Sn2H4); for C2 symmetry, WBI = 1.68 (8), 1.55 (Sn2H4)].

Figure 2

(a) Stretching, (b) bending, and (c) twisting potentials of the Sn=Sn bond in (H3Si)2Sn=Sn(SiH3)2 (8) calculated at the MPW1K/SDD(Sn), 6-31G(d) level.

When the reaction was repeated using the 18-crown-6 adduct of 1(8) instead of the product generated in THF, the course of the reaction was altered, and compound 9 was obtained as the main product (Scheme 2). This compound can be regarded as either the amide adduct of 2 or a stannylenoid related to Tamao’s amino-substituted silylenoids.[20−22] The formation of 9 is likely associated with the better solubility and nucleophilicity of KN(SiMe3)2 in the presence of the crown ether.(23) Relative to that of 3, the 119Sn NMR resonance of 9 was shifted markedly to higher field (−256.6 ppm), indicating sp3 hybridization. While compounds of the type Si3SnK(24) usually resonate at around −880 ppm, the signal of 9 resembles the downfield-shifted behavior typically found for 29Si NMR chemical shifts of amino-substituted silylenoids.[20−22,25] A crystal structure obtained from 9 (Figure S-2 in the Supporting Information) was of poor quality (R1 = 15.6) but nevertheless provided unambiguous proof of the assigned structure.

Scheme 2

Stannylene Amide Adduct Formation

To obtain a neutral stannylene base adduct, the reaction of 1 with [(Me3Si)2N]2Sn was repeated in the presence of triethylphosphane.(26) In the absence of crown ether, PEt3 added to stannylene 2, affording stannylene adduct 10 (Scheme 3). The NMR spectra of 10 showed its electronic similarity to 9: the 119Sn NMR resonance of −224.4 ppm was in the same region, and the 29Si signals for the attached silicon atoms were also very close [−137.9 ppm (10) vs −139.6 ppm (9)]. A large 1J119SnP coupling constant of 2200 Hz was observed. Two different resonances for SiMe3 groups were observed for 10 but only one for 9, indicating configurational stability at Sn for 10.

Scheme 3

Stannylene Phosphane Adduct Formation

The only structurally characterized compound containing a 1-stannacyclopentasilane unit known to date is 3,3,4,4-tetramethyl-1,1-diphenyl-2,2,5,5-tetrakis(trimethylsilyl)-1-stannacyclopentasilane,(8) which was obtained from the reaction of 1 with dichlorodiphenylstannane. The Si−Sn bond lengths in this compound are 262.0(4) and 259.4(4) pm, and the five-membered ring shows a twisted half-chair conformation in which the two SiMe2 groups lie ∼8° below or above the ring plane. In comparison with this, the picture is different for 10 (Figure 3), where the ring adopts an envelope conformation with one of the Si(SiMe3)2 groups on the flap. The Si−Sn bond lengths are elongated to 264.8(3) and 265.3(3) pm, and the Si−Sn−Si bond angle, which has a value of 105.2(1)° in the diphenyl compound, decreases to 98.17(9)° in 10. The Sn−P bond distance of 260.8(3) pm is slightly shorter than in a comparable stannylene [266.3(2) pm].(27)

Figure 3

Thermal ellipsoid plot for 10 drawn at the 30% probability level.

With 10 in hand, it was possible to test the Lewis base properties of 2. Reaction of 10 with 2 equiv of the strong Lewis acid B(C6F5)3 proceeded smoothly, leading to the corresponding borane stannylene adduct[28,29]11 accompanied by 1 equiv of the borane−phosphane adduct (F5C6)3B·PEt3(30) (Scheme 4).

Scheme 4

Stannylene B(C6F5)3 Adduct Formation

It should be noted that in the solid state, B(C6F5)3 serves not only as a Lewis acid but also as a Lewis base. A fluorine atom in the ortho position of one of the C6F5 groups donates electron density into the empty p orbital of the stannylene(31) (Figure 4). This interaction is also observed in solution in the 19F spectrum, where the ortho-F signal displays 117/119Sn satellites with coupling constants of 113/123 Hz. As only three signals for the respective ortho, meta, and para positions were observed in the 19F and 13C NMR spectra, rotation around the Sn−B and B−C bonds is fast at ambient temperature. The 119Sn resonance of 11, which was downfield-shifted to +68.1 ppm was very broad, as a result of the interaction with the quadrupole boron nuclei. Therefore the coupling to the fluorine atom thus could not be detected. How weak the interaction between B(C6F5)3 and the stannylene is can be estimated from the fact that a change of solvent from benzene to THF led to the fast formation of 3.

Figure 4

Thermal ellipsoid plot of 11 drawn at the 30% probability level.

Similar to 9, compound 11 (Figure 4) exhibits an envelope ring conformation with a SiMe2 group on the flap. The Si−Sn bond lengths of 259.7(1) and 260.9(1) pm are close to those in the published stannacyclopentasilane,(8) as is the Si−Sn−Si bond angle of 104.10(4)°. The Sn−B bond length of 235.9(5) pm is in the normal range, and the dative character of the Sn−F interaction is clearly shown by the elongation to 248.7(2) pm from a typical Sn−F value of 208 pm.