Ether carbaboranes. Synthesis and characterization of mono- and di-ether carbaboranes with 1,2-closo-C2B10 structures, and their mono- and di-anionic nido-derivatives

A series of new closo ether carbaboranes 1-R¹-2-R²-1,2-closo-C₂B₁₀H₁₀ [1; R¹ = R² = (CH₂)OCH₃: 2; R¹ = (CH₂)OCH₃, R² = H: 3; R¹ = (CH₂)₂OCH₃, R² = H: 4; R¹ = (CH₂)₃OCH₃, R² = H] have been prepared and characterized. Trends in ¹¹B NMR chemical shifts of 1, 2 and 1,2-closo-C₂B₁₀H₁₂ are discussed in terms of the electron-withdrawing properties of the CH₂OCH₃ group. A single crystal X-ray diffraction study on 2 reveals dimer formation in the solid state via reciprocal C_cageH···O bonding. Partial cage degradation of 1 and 2 affords derivatives of [7,8-nido-C₂B₉H₁₂]^- and of [7,8-nido-C₂B₉H₁₁]^2- (compounds 5-10). Crystallographic study of one of these, [BTMA][7-(CH₂OCH₃)-7,8-nido-C₂B₉H₁₁] (6), shows that the conformation of the pendant ether group is such that its oxygen atom lies to maximum elevation above the open C₂B₃ polyhedral face, and this conformation is shown to be close to the minimum energy one by the results of molecular orbital calculations at the extended Huckel level.