NHO‐Functionalized Bipyridines: Proton‐Switchable Strongly Electron‐Donating Chelating Ligands

Installing N‐heterocyclic olefins (NHOs) at the 4,4′‐positions converts 2,2′‐bipyridine into a donor‐flexible, proton‐switchable platform. A straightforward late‐stage synthesis from fluoroimidazolium salts and 4,4′‐dimethylbipyridine affords two ligands ( 3 , 4 ) that are exceptionally strong neutral σ donors by the Huynh electronic parameter (HEP2), with donor strength dictated by the NHO backbone (unsaturated > saturated). DFT places the HOMOs on the NHO units and the LUMOs on the bipyridine, accounting for cathodically shifted reductions and NHO‐centered oxidations. Protonation converts the exocyclic C═C into an azolium–CH 2 spacer, electronically resetting donor strength to that of the parent bipyridine while leaving the bipyridine coordination site available. The combination of backbone‐tunable donor power and donor flexibility positions NHO‐bipyridines as promising ligands for proton‐responsive applications, including (photo)functional materials and catalysis.