Helicates as Versatile Supramolecular Complexes.

Structural Aspects of Stereodifferentiation in the Solid State.

First- and second-sphere coordination chemistry of alkali metal crown ether complexes

Multidentate Acyclic Neutral Ligands and Their Complexation

Vibrational spectroscopy and structure of polymer electrolytes, poly(ethylene oxide) complexes of alkali metal salts

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2889%2981526-1

Crystallization of proteins under microgravity

Spherical Wrapping of a Linear Polyether around a Cation—1,20‐Bis(8‐quinolyloxy)‐3,6,9,12,15,18‐hexaoxaeicosane·RbI

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.

Wrapping of Metal Cations by Linear Polyethers

α-Cyclodextrin, a torus shaped molecule with a 5 A wide central cavity, forms a number of deep green, blue and black crystals when complexed with iodine/metal iodide. In contrast, β-cyclodextrin, having a 6 A cavity produces only one type of reddish-brown crystal, no matter what metal iodide is used. The complex (β-cyclodextrin)2 ·KI7·9H2O displays space groupP21 (pseudo-C2) with cell constantsa=19.609(5),b=24.513(7),c=15.795(6)A, β=109.50(2)°,Z=4. The crystal structure was solved inC2 on the basis of 3022 absorption corrected CuKα (Ni-filter) X-ray intensities and refined by full matrix least squares toR=17%. This relatively highR-factor is due to many weak reflections (pseudo-C2) and considerable disorder exhibited by water and iodine. In the complex, β-cyclodextrin adopts a ‘round’ shape with O(2)...O(3) interglucose hydrogen bonds formed and all O(6) hydroxyls pointing away from the cavity. Two molecules are arranged head-to-head to produce a dimer, and dimers are stacked such that a slightly zigzagged cylinder with a 6 A-wide cavity is formed. In the cavity described by each dimer, an I
 7
 −
 ion composed of I2·I
 3
 −
 ·I2 units is located, with I2 and I
 3
 −
 perpendicular to each other. K+ ions and 9 H2O molecules are found in interstices between the β-cyclodextrin cylinders. This zigzag polyiodide contrasts with the linear form observed in the 5 A wide α-cyclodextrin channels. It explains differences in color of the crystals and suggests that β-cyclodextrin polyiodide is not a good model for blue starch-iodine.

Gabriela Weber

Papers

Crystallization of proteins under microgravity

Spherical Wrapping of a Linear Polyether around a Cation—1,20‐Bis(8‐quinolyloxy)‐3,6,9,12,15,18‐hexaoxaeicosane·RbI

Wrapping of Metal Cations by Linear Polyethers

(β-Cyclodextrin)2·KI7·9 H2O. Spatial fitting of a polyiodide chain to a given matrix

Versatility of the cyclic ligand 5,8,11-trioxa-2,14-dithia[15] (2,6)-pyridinophane: with copper(II) chloride