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

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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.

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