Synergistic Effect of a Low-Valent Cobalt Complex and a Trimethylphosphine Ligand on Selective C-F Bond Activation of Perfluorinated Toluene

The aryne cobalt complex [Co(4-CF3-eta(2)-C6F3)(PMe3)(3)] (1) was formed from the reaction of [CO(PMe3)(4)] and perfluorinatcd toluene through selective activation of two C-F bonds of perfluorotoluene. A mechanism for the formation of complex 1 is proposed and in most parts experimentally verified. Following this mechanism, a synergistic effect of all electron-rich cobalt(0) center and one of its trimethylphosphine ligands is responsible for the C-F activation of two carbon-fluorine bonds of perfluoro toluene. The detection of difluorotrimethylphoshphorane as the sole byproduct provides strong evidence for this mechanism. Complex [Co(4-CF3-C6F4)(PMe3)(3)] (4), an intermediate of the proposed mechanism to the aryne complex, was also isolated and structurally characterized. Complex 4 transforms to complex 1 via activation of a second C-F bond of a perfluorotolyl ligand only in the presence of trimethylphosphine in the reaction mixture. Complex 4 reacts With CO under atmospheric pressure and room temperature to give [Co(4-CF3-C6F4)(CO)2(PMe3) (6) and with bromobenzene via one-electron oxidative addition of the C-Br bond to give the cobalt(II) bromide [CoBr(4-CF3-C6F4)(PMe3)(3)] (8) and a C-C-coupling product, 4-phenytheptafluorotoluene (7). The structures of complexes 1, 4, and 8 were determined by X-ray crystallography.