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

摘要

The lowest energy structures for the dicobaltadicarbaboranes Cp _2Co _2C _2B _(n-4)H _(n-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 Cp _2Co _2C _2B _8H _(10) 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 Cp _2Co _2C _2B _(n-4)H _(n-2) (n = 9, 10, 11, and 12) derivatives. Thus for the nine-vertex Cp _2Co _2C _2B _5H _7 system only the two lowest energy isomers are found experimentally. For the 10-vertex Cp _2Co _2C _2B _6H _8 system three of the six lowest energy isomers have been synthesized. The 11- and 12-vertex Cp _2Co _2C _2B _(n -4)H _(n-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 Cp _2Co _2C _2B _7H _9 isomers is a high-energy structure with adjacent carbon atoms lying ～26 kcal/mol above the global minimum. In addition to this high-energy isomer seven of the 12 predicted Cp _2Co _2C _2B _7H _9 isomers within 6 kcal/mol of the global minimum have been synthesized. Similarly, for the icosahedral Cp _2Co _2C _2B _8H _(10) derivatives, high-energy isomers with a Co-Co bond lying 21.8 kcal/mol above the global minimum and with a C-C bond lying 19.4 kcal/mol above the global minimum have both been synthesized as stable compounds. Pyrolysis of these high energy Cp _2Co _2C _2B _8H _(10) 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 Cp _2Co _2C _2B _8H _(10) 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.