Aromatic Silicon Ring Debuts

MAIN-GROUP CHEMISTRY

The first stable all-silicon aromatic ring compound, the cyclotrisilenylium cation, has been synthesized by a group in Japan after years of theoretical speculation and failed attempts. The molecule breaks new ground in understanding structure and bonding and could be a stepping-stone toward larger all-silicon aromatic rings.

Chemists have been intrigued with the possibility of preparing silicon, germanium, tin, and lead analogs of all types of carbon compounds. In such analogs, these heavier group-14 elements must be stabilized by bulky substituents. That strategy has proved generally successful, except with pure aromatic compounds. The new cation is analogous to the cyclopropenylium cation, the simplest and smallest aromatic compound with two electrons, first reported by Ronald Breslow of Columbia University in 1957.

Three-membered saturated rings of silicon, germanium, and tin have been known since the 1980s, and these compounds have been reduced to form cyclopropene analogs that could be transformed into aromatic rings. For example, Akira Sekiguchi of the University of Tsukuba, in Japan, oxidized a cyclotrigermene to form the aromatic cyclotrigermenylium cation in 1997. His group's similar attempts to make the silicon analog weren't successful, until now.

This time, Sekiguchi's group replaced the di-tert-butylmethylsilyl substituents on the cyclotrisilene ring's double-bonded silicon atoms with even bulkier tri-tert-butylsilyl groups. The change flattened the structure to a nearly planar geometry, in contrast to the trans-bent orientation of the double bond in the original compound. The structure was altered enough so that the cyclotrisilene was readily oxidized (J. Am. Chem. Soc. 2005, 127, 9978). The aromatic ring forms a nearly perfect equilateral triangle with an average Si-Si bond length of 2.217 .

"Isolation and characterization of such a unique molecule is worthy of praise as a milestone not only in organosilicon chemistry but also in physical organic chemistry," notes Norihiro Tokitoh of Kyoto University, a leader in the synthesis of silicon and germanium aromatic heterocycles.