Visible-light induced photochemistry of Electron Donor-Acceptor Complexes in Perfluoroalkylation Reactions: Investigation of halogen bonding interactions through UV-Visible absorption and Raman spectroscopies combined with DFT calculations

Abstract Visible light promoted perfluoroalkylation reactions initiated via halogen bonding interactions between perfluoroalkyl iodides (Rf–I, Rf=CnF2n+1) and Lewis bases have been recognized as a powerful tool in the field of radical synthetic chemistry. In order to provide some insights into the nature, strength and role in the activation process of halogen bonding interactions, we have performed a combined UV-Visible/Raman/DFT study on a model Rf–I (C4F9I) and a series of Lewis bases (LB) frequently used for the activation of perfluoroalkylation reactions, including DBU, MTBD, TMG, TMEDA and Et3N. Raman studies conducted in acetonitrile solutions show that the band at 279 cm−1 associated with the C−I stretching normal mode of C4F9I shifts about 20 cm−1 towards lower wavenumbers upon formation of the halogen-bonded complexes. Additionally, the Raman study revealed that at concentrations typically used for perfluoroalkylation reactions, the [C4F9I-LB] complexes are the main species present in solution. UV-Vis spectroscopy revealed that complex formation is characterized by a strong increase in absorption from 225 - 350 nm. The higher the basicity of the Lewis base, the higher the increase of the UV absorbance.