Pentameric Circular Iron(II) Double Helicates and a Molecular Pentafoil Knot

We report on the synthesis of 11 pentameric cyclic helicates
formed
by imine condensation of alkyl monoamines with a common bis­(formylpyridine)­bipyridyl-derived
building block and iron­(II) and chloride ions. The cyclic double-stranded
helicates were characterized by NMR spectroscopy, mass spectrometry,
and in the case of a 2,4-dimethoxybenzylamine-derived pentameric cyclic
helicate, X-ray crystallography. The factors influencing the assembly
process (reactant stoichiometry, concentration, solvent, nature and
amount of anion) were studied in detail: the role of chloride in the
assembly process appears not to be limited to that of a simple template,
and larger circular helicates observed with related tris­(bipyridine)
ligands with different iron salts are not produced with the imine
ligands. Using certain chiral amines, pentameric cyclic helices of
single handedness could be isolated and the stereochemistry of the
helix determined by circular dichroism. By employing a particular
diamine, a closed-loop molecular pentafoil knot was prepared. The
pentafoil knot was characterized by NMR spectroscopy, mass spectrometry,
and X-ray crystallography, confirming the topology and providing insights
into the reasons for its formation.