Polytopic Cation Receptors. 2. Synthesis and Selective Complex Formation of Spiro-Linked "Multiloop Crown Compounds"

Article abstract of DOI:10.1021/jo00139a019

A new type of crown compound (1-10) featuring a spiro-linked assembly of two to four individual macrorings differing in ring size, rigidity, and donor characteristics is synthesized.Syntheses were achieved by means of high-dilution techniques and blocking/deblocking procedures beginning with pentaerythritol as the basic building block.The ability of the new ligands ("multiloop crown compounds") to selectively form crystalline polyhomonuclear/heteronuclear complexes is studied.It is found that the symmetrically looped molecules (1a-c, 4a, 6, 7d) can incorporate two orthree identical metal cations well matched in their diameter (Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺).Cations which are too large to fit into the crown rings of 1a-c complex in a sandwich-like manner; those which are too small effect a high hydration in their complexes.Asymmetrical 1d and 9 allow the common uptake of different metal ions, e.g., a combination of Li⁺ and Ba²⁺ or of 2K⁺ and Co²⁺.The specific occupation of the different subrings is discussed.Ligand 1b is shown to discriminate Ca²⁺ from Na⁺; 1c and 6 preferentially complex Ba²⁺ from a Ba²⁺/K⁺ mixture.Thus, the new ligands act as manifold selective cation receptors, corresponding to the specification of their binding compartments.