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

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 Ga₃^2- anionic core in [GaH]₃^2-, 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]₃^2-, as a well-defined metalloaromatic systema metallic ring system exhibiting aromatic behavior.