Asymmetric 1,8/13,2,x-M2C2B10 14-vertex metallacarboranes by direct electrophilic insertion reactions; the VCD and BHD methods in critical analysis of cage C atom positions

Abstract

The isolation of six isomeric, low-symmetry, dicobaltacarboranes with bicapped hexagonal antiprismatic cage structures, always in low yield, is described from reactions in which 13-vertex cobaltacarborane anions and sources of cobalt-containing cations were present. The vertex-to-centroid distance (VCD) and boron–H distance (BHD) methods are used to locate the correct C atom positions in the cages, thus allowing the compounds to be identified as 1,13-Cp₂-1,13,2,10-closo-Co₂C₂B₁₀H₁₂ (1), 1,8-Cp₂-3-OEt-1,8,2,10-closo-Co₂C₂B₁₀H₁₁ (2), 1,13-Cp₂-1,13,2,9-closo-Co₂C₂B₁₀H₁₂ (3), 1,8-Cp₂-1,8,2,4-closo-Co₂C₂B₁₀H₁₂ (4), 1,13-Cp₂-1,13,2,4-closo-Co₂C₂B₁₀H₁₂ (5) and 1,8-Cp₂-1,8,2,5-closo-Co₂C₂B₁₀H₁₂ (6). It is shown that a common alternative method of cage C atom identification, using refined (as B) U_eq values, does not work well, at least in these cases. Having identified the correct isomeric forms of the six dicobaltacarboranes, their syntheses are tentatively rationalised in terms of the direct electrophilic insertion of a {CpCo⁺} fragment into [CpCoC₂B₁₀]⁻ anions and it is demonstrated that compounds 1, 4, 5 and 6 can be successfully prepared by deliberately performing such reactions.