Phosphazenyl Phosphines: The Most Electron-Rich Uncharged Phosphorus Brønsted and Lewis Bases.

It was discovered that phosphazenyl phosphines (PAPs) can be stronger P-superbases than the corresponding Schwesinger type phosphazene N-superbases. A simple synthetic access to this class of PR3 derivatives including their homologization is described. XRD structures, proton affinities (PA), and gas-phase basicities (GB) as well as calculated and experimental pK BH + values in THF are presented. In contrast to their N-basic counterparts, PAPs are also privileged ligands in transition metal chemistry. In fact, they are currently the strongest uncharged P-donors known, exceeding classical and more recently discovered ligands such as PtBu3 and imidazolin-2-ylidenaminophosphines (IAPs) with respect to their low Tolman electronic parameters (TEPs) and large cone angles.