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)

The structures of 1-Me-2-COOH-1,2-closo-C₂B₁₀H ₁₀ (1), 1-Ph-2-COOH-1,2-closo-C₂B₁₀H ₁₀ (2) and 1-Ph-2-COPh-1,2-closo-C₂B₁₀H ₁₀ (3) have been determined by X-ray crystallography. In 1 the orientation of the COOH group is defined by ?_COOH=65.0(2) _syn°, and this group H-bonds to that in a second molecule via a centrosymmetric eight-membered ring, R₂²(8). DFT calculations on the parent species 1-COOH-1,2-closo-C₂B ₁₀H₁₁ reveal that the structure with ? _COOH=90_syn°is preferred, with a barrier to COOH rotation of ˜17 kJmol^-1. Similar calculations on 1 yield a lower rotational barrier, ˜7 kJmol^-1, because internal H-bonding is now denied. In compound 2 the COOH group is twisted to lower ? values [5.0(3)°and 39.7(3)°] but both crystallographically independent molecules exist as dimers in the solid state by virtue of R₂²(8) rings. Compound 2 crystallises from wet solvent as a monohydrate, the structure of which reveals a non-planar centrosymmetric R ₄⁴(12) ring and near-orthogonal Ph and COOH substituents [?_Ph=6.7(2) and ?_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. © 2003 Elsevier Ltd. All rights reserved.