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The complexation characteristics of mixed [2 + 2'] calix[4]aryl derivatives (3 and 4) with alkali metal cations were investigated by the mPW1PW91 (hybrid HF-DF) calculation method. The total electronic and Gibbs free energies of the various complexes (cone, partial-cone, 1,2-alternate, and 1,3-alternate) of sodium and potassium cations with 3 and 4 were analyzed and compared. The structures of the endo- or exo-complexes of the alkali metal cation with the host 3 were optimized using the mPW1PW91/6-31G(d) method, followed by mPW1PW91/6-311+G(d) calculations. The structures of the endo- or exo-complexes of the alkali metal cation with the host 4 were optimized using the mPW1PW91/6-31G(d,p) method. The mPW1PW91 calculated relative energies of the various conformations of the free hosts (3 and 4) suggest that the cone conformers of 3 and 4 are the most stable. The mPW1PW91calculations also suggest that the complexation efficiencies of the sodium ion with hosts 3 and 4 are about 24 and 27 kcal/mol better than those of the potassium ion, respectively. These trends are in good agreement with the experimental results. The exo-complexation efficiencies of the sodium ion toward the conformers of hosts 3 and 4 are roughly 14 and 17 kcal/mol better than those for the endo--complexes of 3 and 4, respectively. The exo-complexation of the cone isomer of 3 with cation could be confirmed by the differences of the diagnostic C=O bands in the free host and its complex's IR spectra.

mPW1PW91 Study of Complexation of Alkali Metal Ions with Mixed [2 + 2'] Calix[4]aryl Derivatives