A cryptand containing three diiminopyridyl units; aromatic-aromatic interaction and the stability of the complexes

The solution properties of a hexaimino cryptand containing 2,6-diiminopyridyl units have been investigated by NMR, UV-VIS absorption and luminescence spectroscopy. The difference in the NMR and UV spectra of the Ba²⁺ complex and the cryptand is attributed to the conformational change of the diiminopyridyl group. The luminescence spectra of the compounds show near-UV bands and unexpected low-energy bands. The former is attributed to the normal π-π* transition, while the origin of the low-energy bands is discussed in terms of inter- and infra-molecular aromaticaromatic interactions. The inter-molecular interaction is important in the free ligand, however, intra-molecular interaction is suggested in the Ba²⁺ complex. Further, the specific conformational change of the Ba²⁺ complex in DMSO-acetonitrile solution is investigated by the measurements of the stability constant of the Ba²⁺ complex in various DMSO concentrations. The large decrease over 4.0 in logK is found only for the Ba¹⁺ complex. The behavior is explained by the factors brought about the nature of DMSO and the interactions among the diiminopyridyl units, DMSO and the Ba²⁺ ion.