CF4defluorination by Cp2Ln–H: a DFT study

Abstract

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-η²-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.