Kinetic versus thermodynamic isomers of the deltahedral dicobaltadicarbaboranes having nine to 12 vertices

Abstract

The lowest energy structures for the dicobaltadicarbaboranes Cp2Co2C2Bn−4Hn−2 (n=9, 10, and 11) are found by density functional theory to be the most spherical borane deltahedra with the carbon atoms at degree 4 vertices and the cobalt atoms at degree 5 or 6 vertices. For the icosahedral 12-vertex dicobaltadicarbaboranes Cp2Co2C2B8H10 with only degree 5 vertices, the lowest energy structures are those without Co–Co or C–C edges. These theoretical predictions agree well with experimental data on the numerous known Cp2Co2C2Bn−4Hn−2 (n=9, 10, 11, and 12) derivatives. Thus for the nine-vertex Cp2Co2C2B5H7 system only the two lowest energy isomers are found experimentally. For the 10-vertex Cp2Co2C2B6H8 system three of the six lowest energy isomers have been synthesized. The 11- and 12-vertex Cp2Co2C2Bn−4Hn−2 systems provide examples of stable high energy isomers with direct Co–Co or C–C bonds arising from the synthetic methods used. Thus one of the experimentally known 11-vertex Cp2Co2C2B7H9 isomers is a high-energy structure with adjacent carbon atoms lying ∼26kcal/mol above the global minimum. In addition to this high-energy isomer seven of the 12 predicted Cp2Co2C2B7H9 isomers within 6kcal/mol of the global minimum have been synthesized. Similarly, for the icosahedral Cp2Co2C2B8H10 derivatives, high-energy isomers with a Co–Co bond lying 21.8kcal/mol above the global minimum and with a C–C bond lying 19.4kcal/mol above the global minimum have both been synthesized as stable compounds. Pyrolysis of these high energy Cp2Co2C2B8H10 isomers at temperatures up to 340°C gives a lower energy isomer with neither a Co–Co nor a C–C bond. Further pyrolysis of this Cp2Co2C2B8H10 isomer at the incredibly high temperature of 650°C for an organometallic reaction gives a complicated mixture of seven of the 12 possible lowest energy isomers, namely those with neither Co–Co nor C–C bonds.