Tetrasilacyclobutadiene

The PES of tetrasilacyclobutadiene, 144, the full silicon analogue of cyclobutadiene, was studied extensively using relatively high levels of theory which include the contributions of polarization functions and of electron correlation52 163-165. The calculations revealed a very complex SizjlI4 PES. 

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In contrast to 69 Li, which was formulated above as the bji-electron aromatic compound, other representatives of this class of cyclic compounds, the heavy analogs of cyclobutadiene dianion, were found to be nonaromatic. The two compounds of this type, disiladigermacyclobutadiene dianion 71 K2 and tetrasilacyclobutadiene dianion 72 K2, were synthesized by the reductive dehalogenation of the corresponding precursors 73 and 74 " with KCg (Scheme 2.61)... 

Cyclobutadiene dication, 61, is a 2jr-aromatic cation. In the carbon case, tetramethyl- and tetraphenyl-substituted derivatives of this dication were characterized in superacidic media already in 1976 [66]. A related persila-analogue, the tetrasilacyclobutadiene dication 62, was obtained only recently in an unexpected reaction between donor-stabilized silylene 63 and Cp ZrCl3. in 29% yield (Scheme 20) [67]. As an intermediate, a silicon(ll) cation, [LSi]", resulting from... 

Fig. 7 Molecular structures of tetrasilacyclobutadiene dication 62 as determined by XRD in the salt 62[Zr2Cp 2Cl7]2- (Ellipsoid presentation. H-atoms are omitted for clarity, thermal ellipsoids at 50% probability level, color code Si pink, C black, N blue. Cl green.) Important bond lengths [pm] and dihedral angles [ jiSi -Si = 232.1 Si -Si = 233.1 Si. ..Si = 302.6 Si. .Si = 353.4 Si -SP-Si = 81.15 Si -Si -Si = 98.85 [67]...

Takanashi K, Lee VY, Ichinohe M, Sekiguchi A (2006) A (tetrasilacyclobutadiene) tricarbonyliron complex [ ri -(tBu2MeSi)4Si4 Fe(CO)3] the silicon cousin of Pettit s (cyclobutadiene)tricarbonyliron complex [(ti -H4C4)Fe(CO)3]. Angew Chem Int Ed 45 3269... 

Lee VY, Takanashi K, Sekiguchi A (2010) A two-and-a-half-layer sandwich potassium salt of anionic (ti -tetrasilacyclobutadiene)(ti -cyclopentadienyl)ruthenium. Dalton Trans... 

Takanashi K, Lee VY, Matsuno T, Ichinohe M, Sekiguchi A (2005) Tetrasilacyclobutadiene (tBu2MeSi)4Si4 a new ligand for transition-metal complexes. J Am Chem Soc 127 5768... 

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