Wurster's Thiacrown Ethers: Synthesis, Properties, and Pt(II)-Coordination Chemistry

Two isomeric redox-responsive azathiacrown ethers, based on p-phenylenediamine, have been synthesized in
traditional crown (L1) and crownophane (L2) architectures. Each of these “Wurster's crowns” was designed to
target the encapsulation of transition or heavy metal ions. The solid-state structures of these ligands show binding
cavities defined by three exocyclic sulfur atoms and either a N donor atom (L1) or the electron-rich π face of the
phenylenediamine subunit (L2). The ability of these ligands to form complexes with platinum(II) was investigated by
various techniques including 1H NMR spectroscopy, electrospray mass spectrometry, cyclic voltammetry, and single-crystal X-ray analysis. The traditional crown geometry proved to be better at forming stable endocyclic complexes
with Pt(II) than the crownophane geometry. The square-planar Pt(II) crown complex includes direct bonding to the
redox center (Pt1−N1 = 2.125 Å and Pt1−Sav = 2.278 Å) with concomitant polarization of the phenylenediamine
moiety. This results in the crown complex oxidizing 916 mV more anodically than the free ligand. In contrast,
modest shifts in the oxidation potential of the crownophane isomer indicate poor interaction between the redox
center and complexed Pt(II) ion.