Alkali Metal-Ion Binding by a Model Macrocycle Containing a C-I···N Halogen Bonded Network: A DFT Study of C-I···M+ and N···M+ Binding Interactions, M+ = Li+, Na+, K+, and Rb+

The complexation of an alkali metal ion by a model macrocycle is examined using the M05-2X/DGDZVP DFT method. The macrocycle is built by connecting three cyclopenta[b]pyrrole motifs with alternating acetylene and ethylene linkages. Replacing one of the C-H bonds in each motif with a C-I bond allows for the formation of three intramolecular C-I···N halogen bonds. Two distinct binding modes were found for the complexation of each metal ion. In one mode, the binding of the ion occurs solely by the iodine atoms, via I···M+ interactions, while maintaining the integrity of the halogen bonds. The complexation energies are in the range −66 to −35 kcal/mol. In the other mode, the binding of the ion includes one nitrogen atom as well, with binding energies in the range of −71 to −38 kcal/mol. In this binding mode, the halogen bond network is weakened. The presence and strength of the interactions are further examined using AIM and NBO calculations. Lastly, the geometries for the transition state structures linking the less stable to the more stable metal ion complexes were obtained, and their calculated Gibbs free energy barriers were found in the range of 1.6 to 1.9 kcal/mol.