Synthesis and characterization of an extremely hindered tetraaryl-substituted digermene and its unique properties in the solid state and in solution

An extremely hindered digermene (E)-Tbt(Mes)Ge-Ge(Mes)Tbt (1; Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl; Mes = mesityl) was synthesized. X-ray crystallographic analysis of the hexane solvated single crystal [1(.)0.5hexane] revealed that I has an extremely long germanium-germanium double bond [2.416(2) A] as that of a carbon-substituted digermene. The temperature-dependent change of UV-Vis absorption of digermene I in solution indicated the quantitative interconversion between 1 and the corresponding germylene Tbt(Mes)Ge: (3). The thermodynamic parameters (DeltaH = 14.7 +/- 0.2 kcal mol(-1) and AS = 42.4 +/- 0.8 cal mol(-1) deg(-1)) for the dissociation of digermene I to germylene 3 were obtained from temperature dependence of the absorption of 1. Since the reactivity of germylene 3 is much higher than that of digermene 1, almost all the intermolecular reactions of I in solution proceeded via dissociated 3. For instance, the reaction of 1 with oxygen in solution resulted in a non-stereospecific formation of the cis- and trans-1,3,2,4-dioxadigermetanes 11 and 7 via the initial formation of germanone 12 derived from oxygenation of the dissociated germylene 3. In case of the oxidation in the solid state, however, digermene 1 reacted with O-2 without dissociation to give the corresponding trans-substituted 1,3,2,4-dioxadigermetanes stereospecifically. The reaction of digermene 1 with W(CO)(5)(THF) was also examined to give the corresponding terminal tungsten complex of the dissociated germylene 3, i.e. Tbt(Mes)Ge=W(CO)(5) (23), as a marginally stable orange yellow paste. (C) 2002 Published by Elsevier Science Ltd.