Dismutational and Global-Minimum Isomers of Heavier 1,4-Dimetallatetrasilabenzenes of Group 14

Jana, A.; Huch, V.; Repisky, M.; Berger, R. J. F.; Scheschkewitz, D. Angew. Chem. Int. Ed., 2014. Early View

DOI: 10.1002/anie.201310475

Aromatic species with heavier Group 14 elements show remarkable differences in terms of stability, structure, and reactivity. Herein we report our experimental and theoretical investigations regarding isomers of germanium- and tincontaining benzene analogues E₂Si₄R₆ (E=Ge, Sn). The germanium-substituted dismutational isomer with a tricyclic six-membered scaffold is isolable, but unlike the homonuclear Si₆ analogue slowly rearranges even at room temperature to give the propellane-type global minimum isomer. In case of E=Sn the dismutational isomer may be an intermediate on the pathway to the propellane-type species obtained, but cannot be detected even at low temperature. Unprecedentedly large chemical shift anisotropies in the ²⁹Si NMR spectra that increase from the Si₆ species through Ge₂Si₄ to Sn₂Si₄ are rationalized by progressively larger paramagnetic-term contributions to the chemical shift tensor as a result of diminishing HOMO–LUMO gaps, which are also reflected in the absorption spectra, as well as by appearance and symmetry of these frontier orbitals.

Due to the poor stability of heavier group 14 elements with sp²-hybridization, the synthesis of the heavier analogues of simple compounds in carbon chemistry is not always simple. For instance benzene (C₆H₆) is an air- & moisture- stable aromatic compound. However, when the heavy atoms are incorporated into the ring system, the chemical property becomes totally different from that of C₆H₆. In this paper, Scheschkewitz group reports that the global minimum isomers of Ge₂Si₄ and Sn₂Si₄compounds show non-planar, propellane-type structures. For the Ge₂Si₄ compound, they also could identify another isomer which has a dismutational aromatic nature. Scheschkewitz describes that these chemistry will be extended to the application such as staffane-type materials.^[1]

References

[1] “Persilastaffanes: Design, Synthesis, Structure, and Conjugation between Silicon Cages”

Iwamoto, T.; Tsushima, D.; Kwon, E.; Ishida, I.; Isobe, H. Angew. Chem. Int. Ed., 2012. 51, 2340 – 2344. DOI: 10.1002/anie.201106422

By stepwise catenation of bicyclo[1.1.1]pentasilane units persila[n]staffanes (n=1, 2, and 3) were synthesized as air-stable colorless crystals. A remarkable red-shift of the UV/Vis absorption bands with increasing number of bicyclo[1.1.1]pentasilane units suggests remarkable interactions between bridgehead SiSi σ-orbitals and between cage SiSi σ-orbitals