Wurster's crowns: a comparative study of ortho- and para-phenylenediamine-containing macrocyclic receptors.

Two series of isomeric, redox-responsive azacrown ethers based on ortho- and para-phenylenediamine (Wurster's crowns) have been synthesized and their properties explored through 13C NMR spectroscopy, electrospray ionization mass spectrometry, cyclic voltammetry, and X-ray crystallography. These crowns display strong affinity for alkali metal cations while maintaining comparable selectivity profiles to the parent crown ethers from which they are derived. Like Wurster's reagent (N,N,N',N'-tetramethyl-p-phenylenediamine or para-TMPD), the para-Wurster's crowns undergo two reversible one-electron oxidations. The integrity of the alkali metal ion complexes is maintained in the neutral and singly oxidized ligand states but not after removal of two electrons. In contrast, the oxidation of ortho-Wurster's crowns is scan rate dependent, occurring at potentials substantially higher than their para counterparts, with their complexes oxidizing irreversibly. X-ray crystal structures of representative complexes show, in all cases, participation of the redox-active phenylenediamine subunits in complex formation via direct bonding to the guest cation.

Electron transfer vs coordination chemistry: isomer-specific binding of HgII by an ortho-Wurster's thiacrown ether.

Isomeric p- and o-phenylenediamine-containing macrocyclic hosts (Wurster's thiacrown ethers L1 and L2, respectively) were prepared and studied as Hg(II) ionophores. The distinct electrochemical properties of the two hosts allowed for the formation of a coordination compound with the ortho-Wurster's thiacrown ether but not the para isomer. In the latter case, the Hg(II) ion served as an oxidant in an electron-transfer reaction with the host. Solutions of the Hg(II) complex of L2 were studied by 13C NMR spectroscopy and cyclic voltammetry and revealed a strong interaction between the redox-active phenylenediamine subunit and the bound metal cation. An X-ray analysis confirmed the participation of the three macrocyclic S atoms and both phenylenediamine N atoms in the stabilization of the complex.

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 pi 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 angstroms and Pt1-S(av) = 2.278 angstroms) 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.

"ortho-Wurster's crowns": synthesis and properties of a novel phenylenediamine-based redox-active macrocyclic ligand.

A hybrid, macrocyclic structure based on o-phenylenediamine and a crown ether promotes an intimate mutual interaction with a bound potassium ion in the form of chelation by the redox-active moiety. A general synthetic method and properties are described for the first member of a new class of redox-active, lariat-type macrocycles called the "o-Wurster's crowns".