Access to a Stable Si2N2 Four-Membered Ring with Non-Kekule Singlet Biradical Character from a Disilyne

The reactions of 1,1,4,4-tetrakis[bis(trimethylsilyl)methyl]-1,4-diisopropyltetrasila-2-yne 1 with an equivalent amount of trans- and cis-3,3',5,5'-tetramethylazobenzenes produced a Si2N2 four-membered ring biradicaloid [RSi(mu-NAr)(2)SiR] 2 (R = (SiPr)-Pr-i[CH-(SiMe3)(2)](2), Ar = 3,5-Me2C6H3), which was isolated as air- and moisture-sensitive dark purple crystals. Compound 2 displays no EPR signal, and the molecular structure of 2 was characterized by NMR spectroscopy and X-ray crystallography, revealing that 2 has a planar centrosymmetric Si2N2 four-membered ring. The Sil-Sil' distance is 2.63380(9) angstrom, and there is no bond interaction between the Sil and Sil' atoms of 2. The reactions of 2 with methanol and carbon tetrachloride show that 2 has both dosed-shell and radical-type reactivity.