Enantiomerically pure pentagonal-bipyramidal metal complexes with predetermined helicity in the solid and the solution state

Abstract

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.