Reductions of 1,3,5-triboracyclohexanes and related 1,3,5-triboraalkanes with alkali metals to trishomoaromatic aggregates - Tetrameric lithium and sodium, and polymeric sodium and potassium trishomoaromatic cyclotriborate compounds

Hexamethyl-1,3,5-triboracyclohexane can be reduced by alkali metals in dono r solvents. Two electrons are added to the combination of p-orbitals at the boron centers forming trisho-moaromatic dianions. X-ray structure analyses show a variety of arrangements, depending on the alkali metal: Tetrameric aggregates with lithium, a polymeric, meander-like chain or a complex tetra mer with sodium, and a zigzag-chain polymer with potassium, The set of dian ions show characteristic NMR chemical shifts in the high-field region, but do not obey a monotonous order with the counterions Li+ to Cs+. Lithium is out of line because of its exceptionally low electronegativity, IGLO chemic al shift calculations agree very well with the experimentally observed data . The computed Nucleus- Independent Chemical Shift (NICS) values prove the strong homoaromatic character of the reduced triboraheterocycles. Compared with isoelectronic analogues the reduced triboraheterocycle shows a remarka ble stabilization energy towards the corresponding, non-aromatic species.