Isolation and characterization of the tetralithium salt of [5]radialene tetraanion stabilized by silyl groups

Abstract

Persilyl-substituted [5]radialene 3 was reacted with lithium metal in THF to yield dark red crystals of the tetralithium salt of tetraanion 4 with a novel ten-center, fourteen π-electron system stabilized by silyl groups.

References

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5. Persilylated [5]radialene 3 was obtained by the intramolecular reaction of hexadecamethyl-3,6,8,11,14,16,19,21-octasilacycloicosa-1,4,9,12,17-pentayne with an excess of [Mn(CO)₃(C₅H₄Me)] by irradiation (λ > 300 nm) in refluxing THF, see: T. Matsuo, H. Fure and A. Sekiguchi, Chem. Lett., 1998, 1101.
6. PM3 calculation was performed with geometry optimization. The geometry of 3 by X-ray diffraction was successfully reproduced by PM3 calculation.
7. T. Matsuo, A. Sekiguchi, M. Ichinohe, K. Ebata and H. Sakurai, Organometallics, 1998, 17, 3143; T. Matsuo, A. Sekiguchi and H. Sakurai, Bull. Chem. Soc. Jpn., 1999, 72, 1115.
8. As the temperature was lowered, the ⁶Li NMR signal broadened and eventually yielded two very sharp signals at δ–0.55 and 0.08 with the same intensity at 187 K. This can be explained by assuming that the two Li⁺ ions (Li1 and Li2) are fixed to the seven-embered rings (A and B), whereas the other two Li⁺ ions (Li3 and Li4) are still fluxional over two five-membered rings (D and E) and one seven-membered ring (C). Since the latter three rings are almost coplanar, the fluxional behavior of the two Li⁺ ions (Li3 and Li4) is not hindered even at 187 K. The possibility of producing a solvent-separated ion pair (SSIP) in toluene-d₈ is unlikely. The ⁶Li NMR in THF-d₈ gave broad and complex signals.