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

• There are preferential chlorination sites in metallacarboranes.
• There is no thermodynamic chlorination preference sites in metallacarboranes.
• The preferential chlorination sites in metallacarborane anions are of kinetic origin.
• The mechanism of the chlorination reaction in anionic boron clusters is radicalary.

Preferential chlorination sites resulting from sequential substitution reactions in the representative metallacarborane [3,3′-Co(1,2-C2B9H11)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 Δy and πy 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.

The radicallary chlorination reaction in metallacarborane anions, whose origin is kinetic, has preferential chorination sites. The substitution reaction rates have been rationalised by considering 2a-NPA on the starting metallacarborane, [3,3'-Co(1,2-C2B9H11)2]−, unperturbed by chlorine substituents. The 2a-NPA value is defined as the sum of the NPA charges of the two bonded atoms.Figure optionsDownload as PowerPoint slide