Wrapping of Metal Cations by Linear Polyethers

The wrapping of linear polyethyleneglycol ligands around metal cations is dependent on their lengths. Short ligands (containing 5 heteroatoms) form circular complexes; extension by one to five heteroatoms leads to helical structures, and ligands containing 11 heteroatoms are able to bind two cations in an S‐like arrangement. It is remarkable (1) that the longer achiral ligands form chiral complexes, (2) that in general all the heteroatoms are coordinated to the cation, (3) that the cation … heteroatom distance is dependent on the electronegativity of this atom, (4) that the C–C–O–C and C–O–C–C torsion angles are trans while the O–C–C–O are gauche , (5) that while the O–C–C–O torsion angles are always gauche the C–C–O–C and C–O–C–C are more flexible and can adopt a non‐ trans form, and (6) that the ligands wrap preferentially in an “equatorial” zone around the cation while the “polar” zones are occupied by one or both ends of the ligands depending on their lengths. In this study we describe the molecular structures of the following complexes as determined by X‐ray crystallographic methods: the RbI‐complexes of α, ω ‐bis(8‐quinolyloxy)‐3‐oxapentane, and of the homologous – 3, 6, 9‐trioxaundecane and ‐3, 6, 9, 12, 15, 18‐hexaoxaeicosane; the RbI‐complex of 1, 11‐bis(8‐quinaldyloxy)‐3, 6, 9‐trioxaundecane; the NaSCN‐complex of 1, 8‐bis(2‐methoxyphenoxy)‐3, 6‐dioxaoctane; and the 2 · KSCN‐complex of 1, 5‐bis{2‐[5‐(2‐nitrophenoxy)‐2‐oxypentyloxy]‐phenoxy}‐3‐oxapentane.