Enantiomerically Pure Pentagonal-Bipyramidal Metal Complexes with Predetermined Helicity in the Solid and Solution States

New metal complexes with pentagonal-bipyramidal geometry have been synthesized with the chiral, pentadentate
bis(oxazoline) ligand (R,R)-1, including the metal ions magnesium(II), iron(II), and cadmium(II). In the solid state,
a complete transfer of chirality from the ligand is observed to exclusively yield enantiomerically pure P-helical,
isostructural pentagonal bipyramidal complexes, as determined by X-ray analysis of four compounds. This uncommon
coordination geometry is likely to be driven by π−π-stacking of the terminal phenyl groups of the linear ligands.
The complex cations in [Fe((R,R)-1)(H2O)2](ClO4)2 (3), [Cd((R,R)-1)(H2O)2](ClO4)2 (4), and [Mg((R,R)-1)(H2O)2](ClO4)2
(5) are mononuclear with the two apical positions of the pentagonal bipyramide occupied by two water molecules.
In contrast, the structure in dinuclear [Cd((R,R)-1)(MeOH)(μ-I)(CdI3)] (2c) can be described as pentagonal-bipyramidal
around cadmium with MeOH and distorted-tetrahedral CdI4 (via one bridging iodo ligand) completing the coordination
sphere in axial positions. The crystal packing of 3−5 shows a highly ordered orientation of the mononuclear helical
cations into one-dimensional chains along the crystallographic axis a, stabilized by intermolecular π−π-stacking. In
contrast, the dinuclear helices in 2c are tilted relative to one another, and consequently, directed, one-dimensional
helicity in the solid state is not observed. Studies using a combination of mass spectrometry and NMR and CD
spectroscopy indicate the presence of only one C2-symmetrical, mononuclear species in acetonitrile for each case,
suggesting the formation of diastereo- and enantiomerically pure complexes also in the solution state. All compounds
exhibit a very characteristic and almost identical CD pattern between 200 nm and 300 nm. This signal can be
attributed to the P-helical, pentagonal arrangement of the ligand.