DFT study on the ability of calix[2]furano[2]pyrrole to form host-guest type complexes with alkali and alkaline-earth metal ions

A DFT based methodology was used to study the complexation of the alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) and alkaline-earth metal cations (Be2+, Mg2+, Ca2+, Sr2+ and Ba2+) by the calix[2]furano[2]pyrrole. This compound, which is a promising selective metal extracting agent, has revealed large flexibility, with 4.4 kcal mol−1 for the maximum amplitude of conformational energy. The most stable conformation, 1,3-alternate, is the one adopted by the ligand to host both series of metal ions. Except for the case of lithium, all the metal ions are held at the center of the ligand cavity, with the vertical position depending on the ion radius; small metal ions are found in inner positions and bigger ones are shifted progressively to the upper rim of the cavity. The coordination to the central ion is made by the oxygen atoms and the delocalized electronic density of the pyrrole rings. The binding strength of the ions to the calix[2]furano[2]pyrrole was found to increase with charge and decrease with their size. The case of beryllium complex is noteworthy, not only because of its high binding energy but mainly for the particular geometrical features. The small size of the ion enables the distortion of the ligand in order to produce a firm square planar coordination using the oxygen and nitrogen atoms. Interestingly the pyramidalization observed in the nitrogen atoms of this complex, which is an indication of the increased electronic density in these atoms and a break of delocalization in the pyrrole rings.