CF4 defluorination by Cp2Ln-H: a DFT study

The reaction of CF4 with Cp2Ln-H has been studied with DFT(B3PW91) calculations for the entire family of lanthanide elements. The reaction paths for H/F exchange (formation of CF3H and Cp2Ln-F) and alkylation (formation of Cp2Ln-CF3 and HF) have been determined. Even though a transition state for formation of Cp2Ln-CF3 has been located, Cp2Ln-CF3 reacts with no energy barrier with HF to give Cp2Ln-F and CF3H. The products of the reactions of H/F exchange and alkylation are thus identical. The former reaction is found to be kinetically preferred although the energy barrier is high (>30 kcal mol-1) which suggests that CF4 would not react with Cp2Ln-H derivatives. These reactions contrast with that of CH4 and Cp2Ln-H for which the energy barrier for the alkylation reaction is lower. The difference in the energy barriers is attributed to an unfavourable charge distribution in the 3c-4e transition state. The structure of Cp2Ln-CF3 differs from that of Cp2Ln-CH3. Because of the high affinity of Ln for F, CF3 is dihapto-(eta)^2-C-F bonded. The Ln...F interaction is strong and Cp2Ln-CF3 can be viewed as an F bridged Ln-CF2 complex. The presence of a nascent carbene CF2 group in this complex rationalizes its reactivity with HF.