Complexation of alkali metal cations by crown-ether type podands with applications in solvent extraction: insights from quantum chemical calculations.

The complexation behavior of nine polyether type podands with a varying number of oxygen donor atoms (4-10) towards the alkali metal cations Li(+), Na(+) and K(+) was studied by quantum chemical methods at the DFT-B3LYP level of theory using the all-electron split-valence 6-311++G(d,p) basis set. The optimized structures of the complexes show a regular increase in the mean cation-oxygen distance with the coordination number. OC-CO dihedral angles of the podand arms were also found to increase with the coordination number and with the size of the cation. Maximum values for the number of strong cation-oxygen interactions (effective coordination numbers) were found for each cation (six for Li(+), seven for Na(+) and eight for K(+)). The calculated values for thermodynamic parameters relative to the binding of free and solvated cations to the podands allowed the assessment of binding constants in vacuum, in water and in dichloromethane. The estimated cation extraction constants mimic the experimental extraction trends, but their values are much larger than experimental values. Scale factors were determined to correct the values effectively. For each podand the ratios between the calculated extraction constants of Li(+) (or Na(+)) and the corresponding ones for K(+) (seen as extraction selectivities) compare acceptably with the corresponding experimental values.