Tetrasilacyclobutadiene (^tBu₂MeSi)₄Si₄:  A New Ligand for Transition-Metal Complexes

The anionic complex of [tetrakis(di-tert-butylmethylsilyl) tetrasilacyclobutadiene]dicarbonylcobalt, [(R₄Si₄)Co(CO)₂]^-·K⁺ (R = SiMe^tBu₂) 2^-·K⁺, was synthesized by the reaction of tetrasilacyclobutadiene dianion dipotassium salt [R₄Si₄]^2-·2K⁺ 1 with an excess of CpCo(CO)₂ in THF. X-ray analysis of 2^-·[K⁺(diglyme)₂(THF)] showed an almost planar Si₄ ring of rectangular shape with an in-plane arrangement of the silyl substituents. 2^- was also prepared as a free anion with the [K⁺[2.2.2]cryptand] counterion by complexation with [2.2.2]cryptand and as a dimer {2^-·[K⁺(THF)₃]}₂ without complexing reagents in THF. Such a tetrasilacyclobutadiene fragment represents a new type of ligand for Co complexes, being the first example of a cyclobutadiene containing only heavier group 14 elements.