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Keywords:

  • Proton sponge;
  • Phosphanes;
  • Mass spectrometry;
  • Organocatalysis

Abstract

Proton sponge phosphanes are unusual ligands in that they possess two basic sites with very different functions: the phosphorus binds to a metal center, while the 1,8-bis(dimethylamino)naphthalene binds protons strongly. Both groups are selective for their target, and the binding of the proton iseasily reversible by controlling the pH of the solution. Proton sponge phosphanes therefore make useful chargeable ligands for the purposes of electrospray ionization mass spectrometric analysis of the complexes in which they are bound. However, where the 1,8-bis(dimethylamino)naphthalene is functionalized with the phosphane is critical, and the 1-dimethylamino group can become involved in binding to the metal when diphenylphosphane is in the 2-position. This behavior is undesirable from the point of view of keeping proton and metal binding segregated, and we introduce an efficient synthesis of the 4-substituted proton sponge phosphane in which the two functional groups are on opposite sides of the naphthalene ring. This precaution solves the problem of the molecule acting as a bidentate ligand, but caution is still needed when using the ligand, because ionization pathways such as halide dissociation to form [M – X]+ ions and oxidation of electron-rich metal complexes to form [M]·+ can be competitive with protonation.