Cyclobutadiene Dianion and Tetrasilabicyclo1.1.0butane Dianion Alkaline Earth Metal Salts: Synthesis and Structure

摘要

Cyclobutadiene dianion (CBD~(2-)) is one of the most attractive charged 6π-electron systems particularly from the viewpoint of its aromaticity. Although Huckel's rule predicted CBD~(2-) to be aromatic, Coulombic repulsion of the two additional electrons was thought to offset dlocalization energy of 6π-electron in four-membered ring. Up to now, we have succeeded in the synthesis of cyclobutadiene dianion dilithium salt [(Me3Si)4C4]~(2-)·2Li~+ A and tetrasilacyclobutadiene dianion dipotassium salt [(~tBu2MeSi)4Si4]~(2-)·2K~+ B, and revealed that carbon CBD~(2-) is aromatic, whereas its silicon analogue is nonaromatic. However, the chemistry of half sandwich CBD~(2-) is quite unexplored until now both experimentally and theoretically. Half-sandwich CBD~(2-) is of particular interest due to expected unique crystal structure, electronic structure and reactivity. We report here the synthesis and structure of half sandwich cyclobutadiene dianion and its heavy valence isomer, tetrasilabicyclo[1.1.0]butane dinaion al kaline earth metal salts.