Cyclobutadiene Dianions Consisting of Heavier Group 14 Elements:  Synthesis and Characterization

Abstract

The first "heavy" cyclobutadiene dianions (CBD2-), tetrakis(di-tert-butylmethylsilyl)-1,2-disila-3,4-digermacyclobutadiene dianion 22- and tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene dianion 42-, have been synthesized by the reductive dehalogenation of the corresponding precursors with potassium graphite in THF. Structural characterization of [K+(thf)2]2.22- and [K+(thf)2]2.42- has been achieved by X-ray crystallography, showing that the central four-membered ring is significantly folded and has two eta2-1,3-coordinated potassium cations accommodated above and below the ring. The "heavy" CBD2-'s 22- and 42- were found to be nonaromatic compounds, a conclusion that was supported by nucleus-independent chemical shift (NICS) calculations showing a lack of diatropic ring current effects.

A Stable Derivative of the Global Minimum on the Si6H6 Potential Energy Surface

“…[18] At first glance the low-field resonance of 3 at d = 174.6 ppm seemed to be at odds with the solid-state structure, suggesting a formally sp 2 -hybridized silicon atom such as in tetrasilyl disilenes [2] or silylium cations (e.g. Mes 3 Si + B(C 6 F 5 ) 4 À in [D 6 ]benzene: d = 225.5 ppm). [22] On the basis of the 2D- 29 Si/ 1 H correlation of 3, however, this signal is assigned to one of the SiTip 2 units and hence to a tetracoordinate silicon atom.…”

Formation of a Donor-Stabilized Tetrasilacyclobutadiene Dication by a Lewis Acid Assisted Reaction of an N-Heterocyclic Chloro Silylene

“…In contrast to dications I, the tetrasilacyclobutadiene dianion II has been successfully synthesized by Sekiguchi and coworkers (Scheme 1). 7 Interestingly, II has a puckered dianionic Si 4 ring and lacks a diatropic ring current. Very recently, Tamao and co-workers reported the isolation of neutral tetrasilacyclobutadiene III with a ylide-like Si 4 ring, which disfavors 4π-electron delocalization and thus no ring current can be obtained (Scheme 1).…”

Functional Disilenes in Synthesis

“…The electronegative chalcogen atoms cause a diagnostic 29 Si NMR low‐field shift of the unsaturated silicon atoms bonded to the chalcogens atoms: δ =95.1 ppm (Si−Si=Si) and 195.0 ppm (Si=Si−S) for 75 a , δ =100.8 ppm (Si−Si=Si) and 191.1 ppm (Si=Si−Se) for 75 b . The four membered rings 75 a and 75 b are perfectly planar and UV/Vis absorption bands corresponding to π→π* electronic transitions are observed in the typical range for cyclic disilenes at λ max =454 and 463 nm . The thermal isomerization of 73 a gives cyclotrisilene 76 , representing the first cyclotrisilene with a heteroatom substituent (Scheme ) .…”