Gas-Phase Structure of the Monomeric Alkylgallium(I) Compound Ga[C(SiMe₃)₃] and the Electrochemical Behavior of Ga₄[C(SiMe₃)₃]₄ and In₄[C(SiMe₃)₃]₄ with EPR Evidence for a Ga₄R₄ Radical Anion

Abstract

The gallium(I) compound Ga4[C(SiMe3)3]4 (1) with a tetrahedral Ga4 core in the solid state gives on evaporation the monomeric Ga(I) alkyl Ga[C(SiMe3)3] with the unique structural situation of a Ga atom solely coordinated by one singly bonded carbon ligand. Its molecular structure was determined now by gas-phase electron diffraction yielding a Ga−C bond length of 206.4(17) pm, similar to that found in the solid state for the tetramer and much longer than in compounds with gallium in the normal oxidation state of +3. As shown by cyclic voltammetry, 1 and the analogous cluster In4[C(SiMe3)3]4 (2) could both be reversibly reduced at about −2 V vs Fc+/0. Whereas the reduced organoindium tetramer did not exhibit any detectable EPR signals between 4 and 300 K, the gallium analogue showed an EPR spectrum compatible with the formulation [(RGa0.75)4]•-; i.e. the unpaired electron was found to be delocalized over all four metal centers (a(69Ga) = 1.93 mT, a(71Ga) = 2.45 mT, 4 Ga).