Direct Access to IMesF and IMesF2 by Electrophilic Fluorination of Abnormal N-Heterocyclic Carbenes

The incorporation of fluorine atoms into the N-heterocyclic carbene (NHC) ligand backbone in a Mn(I) complex [CpMn(CO)(2)(IMes)] was achieved via a sequence of the C-H bond deprotonation and electrophilic fluorination of the anionic abnormal carbene moiety with N-fluorobenzenesulfonimide. The demetallation of resulting complexes [CpMn(CO)(2)(IMes(F1-2))] with triflic acid afforded the imidazolium salts IMes(F1-2)center dot HOTf, the precursors of the corresponding fluorinated NHCs. Interestingly, the IMes(F2) ligand was found to be more electron-donating than its well-known analogue IMes(Cl2) notwithstanding the higher electronegativity of fluorine atoms. A systematic experimental and density functional theory study of this NHC series allowed us to fully rationalize this unexpected behavior, revealing that the weaker sigma-donation of IMes(F2) is compensated by its lower pi-accepting properties due to the strong mesomeric +M effect of the fluorine substituents.