SYNTHESIS AND REDUCTION OF OCTASILYL[4]RADIALENE - STRUCTURE AND DYNAMIC STUDY OF [4]RADIALENE DIANION WITH 8-CENTER 10-ELECTRON PI-SYSTEM

The intramolecular reaction of ,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]radi alene derivative (2). The reaction of 2 with alkali metals (Li, Na, an d K) in THF gave dialkali metal salts of the corresponding dianion (4) with an eight-center ten-electron pi-system. The molecular structure of dilithium salt of octasilyl[4]radialene dianion (4a) has been estab lished by X-ray crystallography. The two lithium atoms are located abo ve and below and are bonded to the atoms of the radialene framework to give a bis-CIP structure. The structural parameters of 4a are discuss ed in comparison to those of 2. The structure of 4a in solution has al so been discussed on the basis of NMR spectroscopic data. The two Liions of 4a are not fixed to the pi-skeleton in toluene-d(8), but are f luxional, 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 dissocia tes to yield an ion pair (CIP and SSIP). Some evidence for the Li+ ion walk on the pi-skeleton is demonstrated.