Syntheses, structures, and properties of ladder oligosilanes and ladder oligogermanes

Annelated silicon ring systems have received extensive investigation as interesting targets that provide access to physical and chemical properties that are difficult to achieve with the corresponding carbon systems. The aim of this article is to describe the results of our investigation on the chemical and physical properties of the ladder oligosilanes and the ladder oligogermanes, such as bicyclo[2.2.0]hexasilane, tricyclo[4.2.0.0(2,5)]octasilane, tetracyclo[4.4.0.0(2,5).0(7,10)]decasilane, pentacyclo[6.4.0.0(2,7).0(3,6).0(9,12)]-dodecasilane, bicyclo[2.2.0]hexagermane, and tricyclo[4.2.0.0(2,5)]octagermane systems. Such compounds can be prepared by the reductive coupling of the appropriate polychlorosilanes or polychlorogermanes with lithium or magnesium. The X-ray structural analyses of these compounds provide intriguing information about the stereochemistry of the multifused ring systems; each cyclotetrasilane ring or cyclotetragermane ring is not planar but folded, leading to a helical structure of the ladder frameworks of the anti type. The ladder oligosilanes and oligogermanes show absorption in the W-vis region and have considerably low oxidation potentials. The following examples of the reactions of the ladder compounds are described: the ring-opening chlorination, mono- and polyoxidation, and reduction with alkali metals leading to stable radical ions. In addition, chiral symmetry breaking in the anti-tricyclo[4.2.0.0(2,5)]octasilane is described. For the reactions of ladder oligosilanes, the comparison with octasilacubanes is also discussed. (C) 2000 Elsevier Science S.A. All rights reserved.