M and P double helical complexes of copper(I) with bis-imino bis-quinoline enantiomerically pure chiral ligands

The enantiomerically pure bis-imino bis-quinoline ligands R,R-ImQ and S,S-ImQ have been prepared by Schiff condensation of 2-quinoline carboxyaldehyde with the pure R,R and S,S enantiomers of trans-1,2-diaminocyclohexane; Both ligands form 2:2 helical complexes with Cu-I perchlorate, and the crystal and molecular structure of [Cu-2(R,R-ImQ)(2)]ClO4.H2O have been determined by X-ray diffraction methods: the [Cu-2(R,R-ImQ)(2)](2+) molecular cation is a chiral double helix of M handedness, in which the two ligands are entertwined in such an arrangement that half of each ligand is not equivalent to the other half of the same ligand. Coupled circular dichroism and H-1 NMR studies reveal that in CH3CN solution a rearrangement takes place toward a more symmetric helical structure (in which the two halves of the same ligand become equivalent), which mantains the same handedness found in the solid state and is a pure M isomer. Solid state and CH3CN solution CD experiments confirm that [Cu-2(S,S-ImQ)(2)]ClO4.H2O, both in solution and in the solid state, is a purl double helix of P handedness, i.e., the enantiomer of the species containing the RR ligand.