Preferential chlorination vertices in cobaltabisdicarbollide anions. Substitution rate correlation with site charges computed by the two atoms natural population analysis method (2a-NPA)

Preferential chlorination sites resulting from sequential substitution reactions in the representative metallacarborane [3,3′-Co(1,2-C ₂B₉H₁₁)2]^- have been studied by combining experimental and computational methods. Similarly to aromatic substitution, the preferential chlorination sites in metallacarborane and carborane anions are of kinetic origin. There is no site preference in thermodynamic terms; however, there are sites on which the reaction takes place more rapidly. Preferential chlorination numbers have been proven experimentally by mixing the substrate with incremental ratios of N-chlorosuccinimide at 194 ± 6 C and analyzing the resulting samples by negative MALDI-TOF-MS. Two parameters Δ_y and π_y have been developed which are indicators of the ease of synthesis of species with y chlorine substituents, relative to the species with y + 1 substituents, and the persistence of the y species, relative to that with y + 1, in excess of chlorinating agent. These parameters have been used to account for the rates of the reaction. They explain the largely known stability of the hexachlorinated cobaltabisdicarbollide anion along with the mono, and dichlorinated species, but strongly support the stability of the octachlorinated cobaltabisdicarbollide anion, a species never previously reported. An excellent correlation for the relative reaction rates has been found with the 2a-NPA charges on the non-chlorinated starting material. The 2a-NPA value for a bond is defined as the sum of the NPA charges of the two bonded atoms (e.g. B-H or C-H). 2a-NPA charges match the order of attack on the bonds.