Synthesis and Reduction of Octasilyl[4]radialene. Structure and dynamic Study of [4]Radialene Dianion with Eight-Center Ten-Electron π-System

摘要

The intramolecular reaction of 3,3,5,5,8,8,10,10,13,13,15,15,18,18,20,20-hexadecamethyl-3,5,8,10,13,15,18,20-octasilacycloicosa-1,6,11,16-tetrayne (1) with three molar amounts of [Mn(Co)_3(Me-Cp)] in THF under photochemical and refluxing conditions produced octasilyl[4]radialene derivative (2). The reaction of 2 with alkali metals (Li, Na, and K) in THF gave dialkali metal salts of the corresponding dianion (4) with an eight-center ten-electron π-system. The molecular structure of dilithium salt of octasilyl[4]radialene dianion (4a) has been established by X-ray crystallography. The two lithium atoms are located above and below and are bonded to the atoms of the radialene framework to live a bis-CIP structure. The structural parameters of 4a are discussed in comparison to those of 2. The structure of 4a in solution has also been discussed on the basis of NMR spectroscopic data. The two Li~+ ions of 4a are not fixed to the π-skeleton in toluene-d_8, but are fluxional, giving a symmetric structure (bis-CIP) on the NMR time scale. In a solvating medium such as THF-d_8 one of the Li~+ ions dissociates to yield an ion pair (CIP and SSIP). Some evidence for the Li~+ ion walk on the π-skeleton is demonstrated.