Experimental and Theoretical Studies of MCF₃⁺ (M = Fe and Co):  Reactivities, Structures, and Potential Energy Surface for C−F Activation

The gas-phase reactions of FeCF 3 + and CoCF 3 + with a series of small alkanes and alkenes were studied by Fourier transform ion cyclotron resonance (FTICR) mass spectrometry. These ions, which are generated from the reactions of the bare metal ions with CF 3 I, both react with alkanes larger than ethane and with small alkenes primarily by CF 2 displacement reactions. Hydride abstraction is also observed in some cases. Collision-induced dissociation and ion−molecule reactions indicate that the structure of MCF 3 + ( M = Fe, Co) is an ion−dipole complex, FM + ···F 2 C, involving C−F activation. A good fit of pseudo-first-order kinetics is obtained for the reactions of FeCF 3 + and CoCF 3 + with the above selected hydrocarbons. The reaction rates of FeCF 3 + and CoCF 3 + with the alkanes increase dramatically with the length of the alkane chain. The C−F activation mechanism of Fe + or Co + with the CF 3 ligand was also investigated theoretically. The potential energy surface (PES) of the [Fe, C, F 3 ] + system and the local minima of the [Co, C, F 3 ] + system are examined by density functional calculations. For the iron system, three local minima are detected including the intact trifluoromethyl isomer, Fe + −CF 3, the inserted fluoro−difluorocarbene isomer, F 2 C−Fe + −F, and the ion−dipole complex, FCF···Fe + −F. Two transition-state structures connecting the above minima are also found. The PES of [Fe, C, F 3 ] + indicates a unique mechanism in which C−F insertion takes place first with a large activation barrier of 6.4 kcal/mol, followed by rotation of the CF 2 unit to the final FCF···Fe + −F structure with an activation barrier of 2.4 kcal/mol. Three similar minima are also detected for CoCF 3 + .