Planar Rhombic Charge-Separated Tetrasilacyclobutadiene

The cyclobutadiene (CBD) molecule C₄H₄ deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si₄(EMind)₄ (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtai...

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Bibliographic Details
Published in:Science (American Association for the Advancement of Science) 2011-03, Vol.331 (6022), p.1306-1309
Main Authors: Suzuki, Katsunori, Matsuo, Tsukasa, Hashizume, Daisuke, Fueno, Hiroyuki, Tanaka, Kazuyoshi, Tamao, Kohei
Format: Article
Language:English
Subjects:
Alternations
Atoms
Bonding
Chemical bonding
Chemical equilibrium
Condensed matter: electronic structure, electrical, magnetic, and optical properties
Condensed matter: structure, mechanical and thermal properties
Crystals
Distortion
Electron density of states and band structure of crystalline solids
Electron states
Equivalence
Exact sciences and technology
Isomers
Jahn Teller effect
Molecular structure
Nuclear magnetic resonance
Oranges
Orbitals
Organic compounds
Physics
Polymers and organic compounds
Silicon
Spectra
Structure of solids and liquids
crystallography
Structure of specific crystalline solids
Symmetry
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Summary:The cyclobutadiene (CBD) molecule C₄H₄ deviates from a high-symmetry square geometry to compensate for its antiaromatic electronic structure. Here, we report a CBD silicon analog, Si₄(EMind)₄ (1), stabilized by the bulky 1,1,7,7-tetraethyl-3,3,5,5-tetramethyl-s-hydrindacen-4-yl (EMind) groups, obtained as air- and moisture-sensitive orange crystals by the reduction of (EMind)SiBr₃ with three equivalents of lithium naphthalenide. X-ray crystallography reveals a planar and rhombic structure of the Si₄ four-membered ring, with alternating pyramidal and planar configurations at the silicon atoms. The large ²⁹Si chemical shift differences (Δδ > 350 parts per million) in the solid-state nuclear magnetic resonance spectra suggest a contribution of an alternately charge-separated structure. The rhombic-shaped charge-separated singlet state of compound 1 thus stabilizes its cyclic 4π-electron antiaromaticity in a manner that contrasts sharply with the bond-length alternation, characterizing the rectangular distortion of carbon-based CBD.
ISSN:0036-8075
1095-9203
DOI:10.1126/science.1199906