Crystal and molecular structures of the nido-carborane anions, 7,9- and 2,9-C₂B₉H₁₂^-

Deboronation of meta-carborane, closo-1,7-C₂B₁₀H₁₂, with refluxing piperidine results in the quantitative formation of [(C₅H₁₀NH)₂H⁺](nido-7,9-C ₂B₉H₁₂^-) whose structure was determined by X-ray diffraction. Heating under vacuum liberates piperidine to give (C₅H₁₀NH₂⁺)(nido-7,9-C ₂B₉H₁₂^-). The reaction of meta-carborane with KOH in refluxing ethanol yields K⁺ nido-7,9-C₂B₉H₁₂^- and the cage substituted anion 10-EtO-7,9-C₂B₉H₁₁^-. The protonated Proton Sponge salt (PSH⁺)(nido-7,9-C₂B₉H₁₂ ^-) (PS = 1,8-N,N,N′,N′-tetramethylnaphthalenediamine) was obtained by metathesis from the potassium salt, an X-ray crystallographic study located the bridging hydrogen in the anion on the open face between the two adjacent boron atoms. Anaerobic deboronation of para-carborane, closo-1,12-C₂B₁₀H₁₂, with KOH in refluxing tetraglyme-toluene gives the potassium salt of nido-2,9-C₂B₉H₁₂^- from which 2,9-C₂B₉H₁₃ was isolated in 45% yield by protonation. The salt (PSH⁺)(nido-2,9-C₂B₉H₁₂ ^-), obtained from nido-2,9-C₂B₉H₁₃ and PS, was shown by single crystal X-ray crystallography to contain a hydrogen bridging two boron atoms opposite the carbon atom on the open face. Aerobic deboronation of para-carborane with KOH forms closo-1,10-C₂B₈H₁₀ and 2-HO-closo-1,10-C₂B₈H₉ in low yields. Fully assigned multinuclear NMR (¹¹B, ¹³C and ¹H) data are reported for the isomeric 7,8-, 7,9- and 2,9-nido-C₂B₉H₁₂^- anions and neutral 7,8- and 2,9-C₂B₉H₁₃, and compared with computed shifts (at the GIAO B3LYP/6-311G* level) from optimized (at MP2/6-31G*) geometries.