Stable mononuclear radical anions of heavier group 13 elements: [(tBu2MeSi)3E.-].[K+(2.2.2-Cryptand)] (E = Al, Ga)

The one-electron reduction of tris(di-tert-butylmethylsilyl)aluminum and -gallium with alkali metals (Li, Na, K) results in the formation of the corresponding radical anions (tBu2MeSi)3Al*-] (3) and (tBu2MeSi)3Ga]*- (4), which were isolated in the form of the potassium salt as extremely air- and moisture-sensitive deep red crystals, representing the first isolable mononuclear radical anions of heavier group 13 elements. The molecular structures of both 3.K+(2.2.2-cryptand)] and 4.K+(2.2.2-cryptand)] were established by X-ray crystallography, which showed a nearly planar geometry around the radical centers. The EPR spectra of 3 and 4 showed strong characteristic signals with g-values of 2.005 for 3 and 2.015 for 4 with hyperfine coupling constants of a(27Al) = 6.2 mT for 3, a(69Ga) = 12.3 mT, and a(71Ga) = 15.7 mT for 4, corresponding to a planar geometry of the radical center.