Coordination Chemistry of a Molecular Pentafoil Knot.

The binding of Zn(II) cations to a pentafoil (51) knotted ligand allows the synthesis of otherwise inaccessible metallated molecular pentafoil knots via transmetallation, affording the corresponding 'first sphere' coordination Co(II), Ni(II) and Cu(II) pentanuclear knots in good yields (≥85 %). Each of the knot complexes was characterized by mass spectrometry, the diamagnetic (zinc) knot complex was characterized by 1H and 13C NMR spectroscopy, and the zinc, cobalt and nickel pentafoil knots afforded single crystals whose structures were determined by X-ray crystallography. Lehn-type circular helicates generally only form with tris-bipy ligand strands and Fe(II) (and, in some cases, Ni(II) and Zn(II)) salts, so such architectures become accessible for other metal cations only through the use of knotted ligands. The different metallated knots all exhibit 'second sphere' coordination of a single chloride ion within the central cavity of the knot through CH…Cl- hydrogen bonding and electrostatic interactions. The chloride binding affinities were determined in MeCN by isothermal titration calorimetry and the strength of binding shown to vary over three orders of magnitude for the different metal-ion-knotted-ligand second sphere coordination complexes.