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 FeCF3+ and CoCF3+ 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 CF3I, both react with alkanes larger than ethane and with small alkenes primarily by CF2 displacement reactions. Hydride abstraction is also observed in some cases. Collision-induced dissociation and ion-molecule reactions indicate that the structure of MCF3+ (M = Fe, Co) is an ion-dipole complex, FM+... F2C, involving C-F activation. A good fit of pseudo-first-order kinetics is obtained for the reactions of FeCF3+ and CoCF3+ with the above selected hydrocarbons. The reaction rates of FeCF3+ and CoCF3+ With the alkanes increase dramatically with the length of the alkane chain. The C-F activation mechanism of Fe+ or Co+ with the CF3 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+-CF3, the inserted fluoro-difluorocarbene isomer, F2C-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 CF2 unit to the final FC ... Fe+-F structure with an activation barrier of 2.4 kcal/mol. Three similar minima are also detected for CoCF3+.