Ag Complexes of NHC Ligands Bearing Polyfluoroalkyl and/or Polyfluoropolyalkoxy Ponytails. Why Are Polyethers More Fluorous Than Alkyls?

A series of fluorous silver complexes bearing two fluorous NHC ligands was synthesized from bis(polyfluoroallcylated) or bis(polyfluoropolyoxaalkylated) imidazolium salts and silver oxide in acetonitrile. The starting salts were prepared either under conventional conditions in two steps via polyfluoroalkylated imidazoles or preferably directly from imidazole and the respective polyfluoroalkyl triflates or polyfluoropolyoxaalkyl nonaflates bearing a nonfluorinated ethylene or methylene spacer. Surprisingly, striking differences in fluorophilicity were observed for both starting imidazolium salts and target Ag-NHC complexes depending on the type of polyfluorinated chains. While the complexes bearing a polyfluoroalkyl ponytail displayed moderate fluorophilicities f(i), in the range of 0.9-1.8, the presence of fluorinated polyether chains resulted in much higher fluorophilicity reaching for unbranched polyethers values as high as 3.2 with excellent solubility in perfluorinated solvents. For the explanation, DFT calculations on the model imidazolium salts showed that, in contrast to polyfluoroalkyl ponytails pointing out of the imidazolium rings, dominant conformation of the polyfluoropolyether chain is able to shield fluorophobic counteranions against the perfluorinated environment, minimizing thus fluorophobic interactions. We also employed DFT calculations for the confirmation of the higher flexibility of perfluoropolyether chains compared with perfluoroalkyl chains, scanning the energy content of two model compounds, perfluorohexane and perfluoro-3-oxahexane, on their C3-C4 or O-C3 torsion angle.