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

Abstract

The aryne cobalt complex [Co(4-CF 3 -η 2 -C 6 F 3 )(PMe 3 ) 3 ] ( 1 ) was formed from the reaction of [Co(PMe 3 ) 4 ] and perfluorinated 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 an electron-rich cobalt(0) center and one of its trimethylphosphine ligands is responsible for the C−F activation of two carbon−fluorine bonds of perfluorotoluene. The detection of difluorotrimethylphoshphorane as the sole byproduct provides strong evidence for this mechanism. Complex [Co(4-CF 3 -C 6 F 4 )(PMe 3 ) 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-CF 3 -C 6 F 4 )(CO) 2 (PMe 3 ) 2 ] ( 6 ) and with bromobenzene via one-electron oxidative addition of the C−Br bond to give the cobalt(II) bromide [CoBr(4-CF 3 -C 6 F 4 )(PMe 3 ) 3 ] ( 8 ) and a C−C-coupling product, 4-phenylheptafluorotoluene ( 7 ). The structures of complexes 1, 4, and 8 were determined by X-ray crystallography.