Cyclogallanes and Metalloaromaticity. Synthesis and Molecular Structure of Dipotassium Tris((2,6-dimesitylphenyl)cyclogallene), K₂[(Mes₂C₆H₃)Ga]₃ (Mes = 2,4,6-Me₃C₆H₂): A Structural and Theoretical Examination

Abstract

The cyclogallane dipotassium tris((2,6-dimesitylphenyl)cyclogallene), K2[(Mes2C6H3)Ga]3 (Mes = 2,4,6-Me3C6H2), has been prepared by either of two distinct synthetic routes. Disproportionation of gallium(II) chloride, Ga2Cl4, in the presence of (2,6-dimesitylphenyl)lithium, (Mes2C6H3)Li, followed by potassium metal reduction in Et2O affords the title compound as large ruby red crystals. Alternatively, K2[(Mes2C6H3)Ga]3 may readily be approached by direct alkali metal reduction of (Mes2C6H3)GaCl2. The title compound was characterized by 1H and 13C NMR, elemental analyses, IR spectroscopy, and single-crystal X-ray diffraction. The cyclogallane resides about the planar Ga3 triangle constituted by extremely short Ga−Ga bonds: 2.4260(5), 2.4317(5), and 2.4187(5) Å. Theoretical examination of the Ga32- anionic core in [GaH]32-, Na2[GaH]3, and K2[GaH]3 suggests a well-defined π-molecular orbital. Both experimental and theoretical results strongly support the cyclogallene dianion, [(Mes2C6H3)Ga]32-, as a well-defined metalloaromatic systema metallic ring system exhibiting aromatic behavior.