The First Kinetically Stabilized Germanethiones and Germaneselones:  Syntheses, Structures, and Reactivities

The first kinetically stabilized germanethiones (germanium−sulfur double-bond species) and germaneselones (germanium−selenium double-bond species) were synthesized. Dechalcogenation of the novel 1,2,3,4,5-tetrachalcogenagermolane Tbt(Tip)GeX4 (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Tip = 2,4,6-triisopropylphenyl, X = S or Se) with 3 molar equiv of triphenylphosphine gave diaryl-substituted germanethione Tbt(Tip)GeS 1a and germaneselone Tbt(Tip)GeSe 2a as orange−yellow crystals and red crystals, respectively, which were unstable in air but thermally quite stable. Chalcogenation of germylene Tbt(Dis)Ge:  resulted in the isolation of the corresponding germanethione Tbt(Dis)GeS 1b and germaneselone Tbt(Dis)GeSe 2b (Dis = bis(trimethylsilyl)methyl). The molecular structures of germanethione 1a and germaneselones 2a,b determined by X-ray crystallographic analysis indicate that they have trigonal planar geometies around the germanium atom and a remarkably shorter Ge−S (2.049(3) Å for 1a) or Ge−Se bond lengths (2.180(2) Å for 2a and 2.173(3) Å for 2b), compared with that of typical single bonds. The double-bond characters are also reflected in their spectral data such as UV/vis (1a, 450 nm; 1b, 432 nm; 2a, 519 nm; 2b, 492 nm), Raman (1a, 521 cm-1; 1b, 512 cm-1; 2a, 381 cm-1; 2b, 386 cm-1), and 77Se NMR (2a, 941 ppm; 2b, 872 ppm). The germanethiones and germaneselones underwent [2 + 2] (with phenyl isothiocyanate), [3 + 2] (with mesitonitrile oxide), and [4 + 2] cycloadditions (with 1,3-dienes) to give the corresponding adducts in high yields. The regioselectivities of the [4 + 2] cycloadditions were examined using unsymmetrical dienes such as isoprene and interpreted in terms of the interaction between LUMO of the GeX unit and HOMO of the diene part.