The Diamond Within a Silicon Analog of Cyclobutadiene

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Science  11 Mar 2011:
Vol. 331, Issue 6022, pp. 1277-1278
DOI: 10.1126/science.1202157

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Organic chemists recognized the distinctive stability and reactivity of aromatic molecules long before chemical bonding was explained in terms of quantum mechanical interactions of electrons. Early quantum mechanics studies by Hückel showed that molecules containing rings with 4n + 2 delocalized π-electrons (such as benzene, an n = 1 case) will gain extra stability [see, e.g., (1)]. Later studies showed that rings with 4n π-electrons, such as cyclobutadiene (C4H4), will be destabilized and highly reactive and they were termed antiaromatic (2). Understanding antiaromaticity in C4H4, an apparently simple molecule, has presented a challenge. The consensus view of its ground-state structure (1, 3) was upset by recent studies of 1,3-dimethylcyclobutadiene (47). Additional insights can be gained from studies of silicon (Si) analogs of C4H4, which are challenging to prepare because of the weakness and high reactivity of Si π-bonds [see (8) and references therein]. On page 1306 of this issue, Suzuki et al. (9) describe the synthesis and characterization of a derivative of the silicon analog of C4H4 (compound 1 in the first figure, panel A) that has two conjugated Si=Si double bonds within a four-member ring. Their study provides an understanding of how the structure of 1 accommodates antiaromatic π-interactions.