<i>exo</i>‐<i>endo</i> Isomerism of Carboranes: Unusual Geometries of C₂B₅<i>X</i>₇ (<i>X</i>=Cl, Br) and C₂B₇Cl₉ with <i>exo</i>‐Skeletal BCl₂ Groups on Carbon Revealed by Joint Spectroscopic/Computational Studies

Reexamination of the co-pyrolysis reactions B2Cl4 with C2Cl4 at 350 °C and B2Br4 CBr4 300 in vacuo confirmed carboranes C2B5Cl7 (1), C2B7Cl9 (2), C2B5Br7 (3) as low-yield products. While 1 only could be concentrated by repeated vacuum fractionation, 2 3 now isolated from conglomerate mixtures for a full spectroscopic characterization compounds were verified their geometries detailed DFT computations. Surprisingly, perhalogenated do not adopt expected “all-endo”-geometries cluster sizes derived sum n boron two carbon atoms (n+2) known syntheses parent closo-carboranes C2BnHn+2. Instead, DFT/GIAO(ZORA)/NMR (GIAO X=Cl, ZORA X=Br) computations revealed that favor structures BX2 groups exo-skeletal ligands attached to both cage-carbon yielding five-vertex closo-1,5-(CBX2)2B3X3 (1: X=Cl; 3: seven-vertex closo-2,4-(CBCl2)2B5Cl5 2. In contrast these carboranes, analogous on hydrogen substituted C2BnHn+2, silaboranes Si2BnHn+2 Si2BnXn+2 (n=5, 7) show all cases thermodynamic favorization where formula are endo-skeletally incorporated into frameworks.