Cage-closing reactions of the nido-carborane anion 7,9-C2B9H12− and derivatives; formation of neutral 11-vertex carboranes by acidification

Abstract

Reactions of the nido-carborane salts (Bu₄N)(nido-7,9-R,R′-7,9-C₂B₉H₁₀) (R,R′ = H; R,R′ = Me; R,R′ = Ph; R = Ph, R′ = Me) with H₂SO₄ at ambient temperature give the 11-vertex closo-carboranes 2,3-R,R′-2,3-C₂B₉H₉ in high yields (R,R′ = H; R,R′ = Me; R,R′ = Ph; R = Ph, R′ = Me). Halogenated derivatives of closo-carboranes, 10-X-2,3-C₂B₉H₁₀ (X = Cl, I), are formed from reactions of salts containing 1(6)-X-7,9-C₂B₉H₁₁⁻ anions with H₂SO₄, and 4-F-2,3-(p-FC₆H₄)₂-2,3-C₂B₉H₈ is formed from (Bu₄N)(3-F-7,9-(p-FC₆H₄)₂-7,9-C₂B₉H₉) and F₃CSO₃H. Elimination of the ethoxide and fluoride substituents from the cage occurs during acidification of salts containing 10-OEt-7,9-C₂B₉H₁₁⁻ and 10-F-7,9-(p-FC₆H₄)₂-7,9-C₂B₉H₉⁻ by F₃CSO₃H, resulting in the closo-carboranes 2,3-C₂B₉H₁₁ and 2,3-(p-FC₆H₄)₂-2,3-C₂B₉H₉ as major products respectively. A mixture of B-Me-2,3-C₂B₉H₁₀ isomers is obtained from salts containing the 8-Me-7,9-C₂B₉H₁₁⁻ and 10-Me-7,9-C₂B₉H₁₁⁻ anions with H₂SO₄. B-Me-2,3-C₂B₉H₁₀ is suggested to be fluxional in solution with 4-Me-2,3-C₂B₉H₁₀ as the major component. Protonation of the fluxional anion 10(11)-endo-Me-7,9-C₂B₉H₁₁⁻ with acetic acid gives a neutral nido-carborane 11-Me-2,8-C₂B₉H₁₂ rather than closo-B-Me-2,3-C₂B₉H₁₀. Molecular geometries of 11-vertex closo-carboranes and B-methyl-nido-carboranes are determined by the combined ab initio/GIAO/NMR method at the GIAO-B3LYP/6-311G*//MP2/6-31G* level of theory.