STERIC AND STEREOELECTRONIC EFFECTS IN AZA-CROWN ETHER COMPLEXES

Stability constants and enthalpy changes determined by calorimetric ti trations and supported by selected NMR titrations are reported for the complexation of sodium and potassium cations with 18 different crown ethers containing nitrogen atoms with different number, location and s ubstitution pattern. The data, measured in methanol mostly with potass ium salts, are compared to literature data; they show striking differe nces between all-oxygen analogs and the macrocycles with NH groups. In contrast, affinities with aza crown ethers bearing alkyl groups at th e nitrogen as well as with the cryptand [2.2.2] come closer to the com plexation free energies predicted from the number and electron donatin g capacity of the ligand heteroatoms. This is rationalised on the basi s of molecular mechanics calculations, showing that a NH-containing cr own predominates in conformations with axial N lone pairs, due to thei r repulsive electrostatic interactions with the ring oxygen atoms. Rep lacement of the hydrogen by alkyl groups forces the lone pairs to an e quatorial position, thus enabling better complex formation, as borne o ut by experiment. In Line with these arguments the IgK differences are with some exceptions more due to Delta H than to T Delta S difference s. The calorimetric data show linear isoequilibrium correlations betwe en T Delta S and Delta H, with slopes between those observed with othe r crown ether and cryptand complexes. Preliminary investigations of so me synthetic macrocyclic amide precursors yield appreciable complexati on only, if the two carbonyl oxygens can come in close contact with th e guest cation. Computer aided molecular modelling shows that this is possible in a small 15C5-derivative, in which the polyethylenglycol cy cle only serves as ring template without binding contributions from th e ether oxygen atoms.