The Conformation of Free and Complexed Oligoethers

Abstract

Recent X-ray and BC NMR results on macrocyclic compounds containing oxyethylene units (1,4 dioxa groupings) are reviewed. When such "saturated" crown ethers and related compounds form complexes with cations and uncharged protonacidic molecules, they adopt conformations which contain chiefly ag "a units or g ±g±a units (genuine comers) and are thus of the types encountered, or conceived for corresponding cycloalkanes. Uncomplexed or incompletely complexed crown ethers, on the other hand, adopt as a rule unusual conformations never encountered in cycloalkanes. These latter crown ethers contain g±g'a units (pseudo corners) characterized by close 1,5 CH· .. 0 interactions. Such conformational units are also found in uncomplexed cryptands. In open chains, there is no evidence of this type of chain folding, except in the gas phase for 1,2-dimethoxyethane. It is concluded that pseudo-corner formation is resorted to only when solvation of ether oxygen is excluded, such as in the gas phase or in the inaccessible interior of macrocyclic compounds.

Oligoether complexes of alkaline-earth metal ions. I. Structures of 2,5,8,11,14-pentaoxapentadecane (`tetraglyme`) complexed with calcium, strontium and barium thiocyanates

“…The sequence ag+-a with alternating signs observed here is also typical of the familiar conformation of 18-crown-6 when 'equatorially' coordinated to a central cation (Dale, 1980). It is also observed in the complex of tetraethyleneglycol dimethyl ether with HgC12 (Iwamoto, 1973) and in part in 2,5,8,11,14,17,20-heptaoxahenicosane-barium thiocyanate (Weber, Hirayama, Saenger & Sheldrick, 1984).…”

“…This explains the abnormally short distances in these parts of the molecules. The wrapping mode of linear polyether ligands around cations is known to depend on their lengths (V6gtle & Weber, 1979;Dale, 1980). Ligands with five heteroatoms fold in an equatorial plane around the cation, while a helical arrangement is observed with longer polyethers.…”

{Ca(OH2)3[Ca(DOTA)].7.7H2O}n

“…Preorganization is one characteristic of the DOTA ligand that may prove beneficial to the overall stability of its metal complexes. As previously reported, the tetraazacyclododecane ring, which is the backbone of the DOTA ligand, has been shown to display the [3333] or square conformation (Reibenspies, 1992;Reibenspies & Anderson, 1991;Dale, 1980). Such a conformation is ideal for positioning the pendant carboxylate arms of the DOTA ligand on the same side of the tetraazacyclododecane ring, thus preorganizing the ligand for complexation.…”

A Macrobicyclic Thiophosphonamide Polyether Ligand

“…The succession of intracyclic torsion angles (N6--P31--N2-IC3--C4---C5--N6) is 23,41,16,respectively,for (I),and 36,25,35,respectively,for (II). The conformation of the oligoether chain can be described qualitatively, according to Dale (1980), as a succession of groups of three torsion angles in each oxyethylene unit. Molecules (I) and (IIA) are characterized by exactly the same (ag-a)(ag+g-)(ag-a)(ag-a) arrangement.…”