Silicon–silicon π single bond

A carbon–carbon double bond consists of a σ bond and a π bond. Recently, the concept of a π single bond, where a π bond is not accompanied by a σ bond, has been proposed in diradicals containing carbon and heteroatom radical centers. Here we report a closed-shell compound having a silicon–silicon π single bond. 1,2,2,3,4,4-Hexa-tert-butylbicyclo[1.1.0]tetrasilane has a silicon−silicon π single bond between the bridgehead silicon atoms. The X-ray crystallographic analysis shows that the silicon−silicon π single bond (2.853(1) Å) is far longer than the longest silicon−silicon bond so far reported. In spite of this unusually long bond length, the electrons of the 3p orbitals are paired, which is confirmed by measurement of electron paramagnetic resonance, and magnetic susceptibility and natural bond orbital analysis. The properties of the silicon−silicon π single bond are studied by UV/Vis and 29Si NMR spectroscopy, and theoretical calculations.

T he concept of a carbon-carbon double bond is one of the fundamentals in organic chemistry. A carbon-carbon double bond is formed by overlapping of the sp 2 orbitals (σ bond) and 2p orbitals (π bond). In this double bonding, two sp 2 carbon atoms are faced in the direction of the sp 2 orbital. Another possible bond is a π single bond, where two sp 2 carbon atoms are faced in the bisecting direction between two sp 2 orbitals (Fig. 1a). This new π single bond has recently been reported to present in singlet diradicals with π single bond character and stabilization of such singlet diradicals have been challenged [1][2][3] . On the other hand, studies on singlet diradicals consisting of heteroatoms have recently been developed [4][5][6] . For example, the singlet diradical [B(t-Bu)] 2 [P(i-Pr) 2 ] 2 was isolated as a stable singlet diradical and its structure and properties have been reported 7 .
A question comes from this planar structure: can the electron in the 3p orbital of the bridgehead silicon atom interact with each other in spite of this unusually long distance? To clarify this point, we measured EPR spectra and magnetic susceptibility.  (4). d Temperature-dependent magnetic susceptibility of 2. The data were measured 12 times at each temperature. The black dots denote average values and the error bars show SDs. from 4 to 300 K (Fig. 1d). As no temperature dependence was observed, the magnetic susceptibility does not obey the Curie law. These results clearly show that 2 is not a paramagnetic compound such as a triplet silyl biradical but a diamagnetic compound with a singlet state.
The electronic interaction between the bridgehead silicon atoms was studied by theoretical calculations. The optimized structure of 2 well reproduces the planar structure (Supplementary Tables 1 and 2). The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are shown in Fig. 2a. In the HOMO, the 3p orbitals of the bridgehead silicon atoms overlap to form a π bond. Four silicon-carbon σ orbitals contribute to the HOMO, showing the σ-π conjugation between the π-orbital and the four silicon-carbon σ orbitals. The LUMO is a π* orbital with a nodal plane. The NBO analysis was also carried out (Supplementary Data 1-3). The calculation showed that the hybrid orbital of the bridgehead silicon atom used for the bonds to the neighboring silicon and carbon atoms has 33.57% (Si) and 32.42% (C) of s-characters and 66.06% (Si) and 66.87% (C) of pcharacter, indicating sp 2 hybrid orbitals. The orbital used for the silicon-silicon π single bond has 0.32% of s-character and 99.46% of p-character. Wiberg bond index of the silicon-silicon π single bond is 0.67. These results show that 2 is a compound with a silicon-silicon π-single bond rather than a singlet diradical.
In the 29 Si NMR spectrum, 2 shows the signal of the bridgehead silicon atoms at δ 117.4 ppm (Supplementary Fig. 3).
In the case of disilenes, the signals of sp 2 silicon atoms of most tetraalkyl-and tetraaryldisilenes have been reported to be observed at 50-100 ppm, while those of tetrasilyldisilenes show a large downfield shift (δ 140-170 ppm) 12 Analysis of the planar structure. The planar structure of 2 is interesting from the following viewpoints. Table 1 shows the Xray structural parameters of 2 and related cyclotetrasilanes 1, 3, and 4 23,33 . These cyclotetrasilanes have been reported to have planar cyclotetrasilane rings and, therefore, the steric repulsion between neighboring tert-butyl groups is large due to the eclipsed conformation. When the van der Waals radius of the substituents on the 1,3-silicon atoms (i.e., H, Br, and K) becomes smaller, the geometry of the 1,3-silicon atoms change from pyramidal to trigonal monopyramidal structures as shown by the Σ (Si) values. This is caused by the reduction of the steric repulsion between neighboring tert-butyl groups. When the hydrogen atoms on the silicon atoms of 3 are removed, the silicon atoms of 2 become nearly planar. Table 1 also shows the Si-Si-Si bond angles. As the van der Waals radius of the substituents on the 1,3-silicon atoms becomes smaller, the θ 1 value becomes slightly larger and the θ 4 value becomes slightly smaller, because of the Thorpe-Ingold effect of the SiX(t-Bu) and Si(t-Bu) 2 moieties [34][35][36] ; finally, the cyclotetrasilane ring becomes rhombohedral in 2 37 . The increment of the θ 1 value from 3 (93.7(1)°) to 2 (103.65(4)°) is significantly large in this series. This structural change corresponds to the formation of the transannular silicon-silicon π single bond.
The structure of 2 is also interesting from the viewpoint of the bond-stretch isomers of bicyclo[1.1.0]tetrasilane. It was predicted by theoretical calculations that bicyclo[1.1.0]tetrasilane has two bond-stretch isomers, i.e., a short-bond isomer and a long-bond isomer [38][39][40][41][42][43][44][45] . The short-bond isomer has a small bridgehead Si-Si bond length (r), a small fold angle between two three-membered rings (ϕ), and a large H−Si−Si bond angle (θ). On the other hand, the long-bond isomer has large r, large ϕ, and small θ values (Supplementary Tables 1 and 2 Tables 1 and 2). When methyl groups are introduced, the energy curve is greatly affected. The long-bond isomer of 6 is considerably destabilized because of the steric repulsion between the bridgehead methyl groups. On the other hand, the short-bond isomer of 7 is destabilized due to the steric hindrance between the faced tert-butyl groups on the folded two cyclotrisilane rings. In the case of 8, both destabilization effects work, and as a result, the energy curve resembles that of 5 to some extent.
We also calculated the energy curves of 9, 10, and 2 with tertbutyl groups (Fig. 3 and Supplementary Tables 1 and 2). The energy curves of 9 and 10 show more pronounced features of 6 and 7: the long-bond isomer of 9 is more destabilized than its short-bond isomer and the short-bond isomer of 10 is more destabilized than its long-bond isomer. This is due to the large steric effect of the tert-butyl group. When six tert-butyl groups are attached (2), both short-bond and long-bond isomers are highly destabilized. Instead, the planar structure becomes the energy minimum. In the planar structure, the steric repulsion among the bridgehead tert-butyl groups and those at the 2,2,4,4-positions can be avoided. Therefore, the planar isomer may be added to the bond-stretch isomers as a different isomer.
In conclusion, bicyclo[1.1.0]tetrasilane having a silicon-silicon π single bond was synthesized by introduction of six tert-butyl groups. As this compound was isolated as stable crystals, synthesis of related compounds may be possible. Such studies are now in progress.  (2). A mixture of 1 (41.1 mg, 6.68 × 10 −2 mmol), potassium (5.5 mg, 0.14 mmol), benzene (6 mL), and tetrahydrofuran (THF) (4 mL) was stirred at 50°C for 2 h in a glovebox. Insoluble materials were removed by filtration through a glass filter. The filtrate was concentrated by slow evaporation of the solvents to give 2 (15.1 mg, 50%) as orange crystals.   (8) X-ray crystallographic analysis. Orange crystals of 2 were obtained from a benzene-THF (6 : 4) solution by slow evaporation under an argon atmosphere in a glovebox. A crystal specimen was mounted in a loop and was used for data collection on a Rigaku R-AXIS IV ++ imaging plate diffractometer using graphitemonochromated Mo Kα radiation. The data were corrected for Lorentz and polarization effects. An empirical absorption correction based on multi-scan was also applied. The structure was solved by a direct method using SHELXS-97 49 . Nonhydrogen atoms were refined anisotropically by the full-matrix least-squares method on F 2 for all reflections using SHELXL-2014/7 49,50 . All hydrogen atoms were generated by AFIX instructions and were not refined. All calculations were carried out using Yadokari-XG 2009 51

Data availability
The data supporting this study are available within this paper, Supplementary Information, and Supplementary Data. CCDC 1997577 (2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.