1H-NMR study of the stoichiometry and stability of the Ba2+, Sr2+, Hg2+, Pb2+, K+, Ag+, and Tl+ complexes with a new macrocyclic diamide in acetonitrile-nitrobenzene solvent mixture

Proton NMR technique was utilized to study the complexation reactions of a new macrocycle diamide with Ba2+, Sr2+, Hg2+, Pb2+, K+, Ag+, and Tl+ ions in % 50 (w/w) acetonitrile-nitrobenzene binary mixture. In all the cases, the chemical shifts of the single NMR signals due to the -O-CH2-CH2-O- and Ph-O-CH2- groups of the ligand were determined at 300 K. The exchange between free form of the macrocycle (I) and its complexes was fast on the NMR time scale and just a single population average H-1 signal was observed. Formation constants of the resulting 1:1 complexes were determined by computer fitting of the chemical shift-mole ratio data. The stabilities of the resulting 1:1 complexes varying in the order Ba2+ > Sr2+ > Hg2+ > Pb2+ > Ag+ > K+ > Tl+ were determined. For two singlet groups of ligand (I) (Ph-O-CH2- and -O-CH2-CH2-O-), the values of the formation constants which were obtained from data fitting were found to be similar, but shift of the signal for Ph-O-CH2- group was larger than for -O-CH2-CH2-O- group. The theoretical studies on the structures and the energetic data of the five conformers of ligand macrocycle (I) and the complexes with Ba2+, Sr2+, Hg2+, Pb2+, Ag+, K+, and Tl+ have been performed by DFT-B3LYP/6-31G* method for ligand (I) and QM-PM6 method for the complexes. The comparison of the experimental and theoretical data with respect to the formation constants and the structures of the complexes have shown good agreement.