Podands with Aromatic Donor End Groups

7 Open-Chain Polyethers Wrapping of Metal Ions 7.1 Podands with Aromatic Donor End Groups 

When going from the cyclic [18]crown-6 to its open-chain analog pentaglyme (Fig. 40) as a ligand for K, the complex stability decreases by factor as high as j 0 324) although both these ligands offer the same number of donor atoms. The 

The short ligand 1 (five heteroatoms) wraps about the Rb ion in a circular arrangement with all the torsion angles in the usual range While the cation is shielded 

Interestingly, the Rb. .. O distances to the aromatic oxygen atoms are significantly longer (3.07 and 3.09 A) than those to sp oxygens. This is due to the diminished basicity of the former probably associated with an unfavorable orientation of the electron lone pairs with respect to the metal ion. 

The interaction between Rb and the aromatic nitrogens (distances 3.04 and 3.11 A) is weaker compared to the quinoline complex (2.93 and 2.% A). This is compensated for by additional coordination to the iodide anion and by the energy gained from stacking interactions. 

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