Structural studies of diaminocyclohexane-containing aza-crown ether macrocycles and their Zn(II) complexes

Abstract

A series of C2-symmetrical, diaminocyclohexane-containing macrocycles has been shown to form Zn(II) complexes of 1:1 stoichiometry in methanol. The larger, more flexible members of the series (R)-2 and (R)-3 demonstrate similar behavior in that they undergo metal ion exchange slowly on the NMR timescale and exhibit spectra reflective of C2-symmetry at all temperatures studied. NMR, X-ray crystallographic, and molecular modeling studies suggest that Zn(II) coordination significantly enhances the conformational rigidity of (R)-2 and (R)-3 by enforcing anti conformations about both N-benzyl bonds. In contrast, the Zn(II) complex of the smallest, most rigid member of the series (R)-1 shows significant exchange broadening by 1H NMR at room temperature. Variable temperature NMR studies of this complex reveal asymmetry at low temperatures (−80°C) and C2-symmetry at elevated temperatures (60°C), indicating that the complex assumes a relatively long-lived asymmetrical conformation. Additionally, the ortho-linked naphthyl-containing macrocycle (R)-4 is shown by X-ray crystallography to possess a diaxial conformation in the solid state, which is thought to be stabilized by crystal packing forces.