CROWN ETHER COMPLEXES BY DESIGN: IS IT POSSIBLE TO REACH A PERFECT FIT OF THE Cs ATOM INSIDE THE CROWN ETHER CAVITY ?

J.A. Manskaya¹, Phil Squattrit², J.Sieler³, V.N.Kokozay¹, V.V.Ponomareva¹, K.V.Domasevitch¹

¹ Department of Chemistry, Kiev Shevchenko University, Vladimirskaya St. 64 252033 KIEV UKRAINE
² Central Michigan University, Department of Chemistry, 356 Dow Science Complex,
³WB Anorganische Chemie, Universität Leipzig, Linnéstraße 3, D-04103 Leipzig, Deutschland,

Keywords: 18-crown-6, cesium, crystal engineering

The geometry of the [M(18-crown-6)]⁺ moieties in the crystal structures of alkali metal, ammonium and thallium(I) complexes depends of the size cation. Recently we have reported the unique examples for perfect fit of the large Rb and Tl(I) atoms inside the crown cavity, and have shown that the centrosymmetric structure of [M(18-crown-6)]⁺ may be stabilized by means of equally effective coordination interactions of the metal atom with centrosymmetric counter anions at both axial sides of an 18-crown-6 molecule simultaneously [1, 2]. This raises the question about possibility of centrosymmetric encapsulation for the largest monocation Cs in the complex of the type Cs(crown ether)_X, where X are centrosymmetric bridging counter anions of linear (Ag(CN)₂), planar (AuCl₄^-) or octahedral (SbCl₆) structure.

In the complex of centrosymmetric SbCl₆^- the macrocyclic fragment Cs(18-crown-6)⁺ likewise has a centrosymmetric structure with the metal ion equally disordered above and below the crown ether plane. The large Cs⁺ cation is thus located slightly off the center of the crown ether cavity, despite the requirements as imposed by the centrosymmetric structure of the macrocyclic ligand (Cs-O 2.939(3)-3.094(3) Å). It is important, however, that the deviation of the caesium atom from the mean plane of the oxygen atoms is found to be only 0.936(2) Å. The latter is significantly less than the values for the Cs(18-crown-6) complexes observed earlier (1.40 to 2.17 Å).

The same type linear polymeric array demonstrates the structure of Cs(dibenzo-24-crown-8)Ag(CN)₂ complex, with the dicyanoargentate groups as bridges between two macrocyclic cations (Cs-N 3.21(1), 3.44(1) Å). The latter, however, adopt a "wrap-on" encapsulation mode for the caesium atom, the characteristic feature of complexes with higher crown ligands. The Cs(18-crown-6)Ag(CN)₂ complex shows a different organization of the components with the typical "sunrise" coordination in the Cs(18-crown-6) fragment. The Cs atom deviates by 1.570(3) Å from the mean plane of the macrocycle. The 18-crown-6 molecules are bridges between the two metal atoms.

Thus, we may conclude that the adaptibility of the Cs⁺ - 18-crown-6 geometry to the nature of the anion may result in the formation of a genuine centrosymmetric [Cs(18-crown-6)]⁺ moiety. This offers interesting possibilities for syntheses in the field of macrocyclic coordination chemistry, especially for large cations coordinated to small ligand cavities and vice versa [1].

[1] K.V. Domasevitch, V.V. Ponomareva, E.B. Rusanov, T. Gelbrich, J. Sieler, V.V. Skopenko, Inorg. Chim. Acta 268/1, 93 (1998).
[2] K.V. Domasevitch, V.V. Skopenko, J. Sieler, Inorg. Chim. Acta 249/2, 151 (1996).