The synthesis and molecular and crystal structures of 1 -methyl-2-carboxy-1,2-dicarba-closo-dodecaborane(12), 1-phenyl-2-carboxy-1,2-dicarba-closo-dodecaborane(12) and 1 -phenyl-2-benzoyl-1,2-dicarba-closo-dodecaborane(12)

Abstract The structures of 1-Me-2-COOH-1,2- closo -C 2 B 10 H 10 ( 1 ), 1-Ph-2-COOH-1,2- closo -C 2 B 10 H 10 ( 2 ) and 1-Ph-2-COPh-1,2- closo -C 2 B 10 H 10 ( 3 ) have been determined by X-ray crystallography. In 1 the orientation of the COOH group is defined by θ COOH =65.0(2) syn °, and this group H-bonds to that in a second molecule via a centrosymmetric eight-membered ring, R 2 2 (8). DFT calculations on the parent species 1-COOH-1,2- closo -C 2 B 10 H 11 reveal that the structure with θ COOH =90 syn ° is preferred, with a barrier to COOH rotation of ≈17 kJ mol −1 . Similar calculations on 1 yield a lower rotational barrier, ≈7 kJ mol −1 , because internal H-bonding is now denied. In compound 2 the COOH group is twisted to lower θ values [5.0(3)° and 39.7(3)°] but both crystallographically independent molecules exist as dimers in the solid state by virtue of R 2 2 (8) rings. Compound 2 crystallises from wet solvent as a monohydrate, the structure of which reveals a non-planar centrosymmetric R 4 4 (12) ring and near-orthogonal Ph and COOH substituents [ θ Ph =6.7(2) and θ COOH =78.0(2) syn °]. In compound 3 molecular association by H-bonding is blocked, so 3 serves as a useful comparative structure for 2 .