Cation coordination of bisamidopyridine-derived receptors as investigated in the solid-state and in solution

A bisamidopyridine-type receptor, N,N'-bis(6-methyl-2-pyridyl)pyridine-2,6-dicarboxamide (1), and its Co-III complex were prepared and their X-ray structures were compared to those of N,N'-diphenylpyridine-2,6-dicarboxamide (2) and Co-III(2)(2). Introduction of the two additional coordinative groups resulted in second-order interactions between the central ion and the nitrogen atoms of the terminal pyridine moieties in the crystalline state. Solution studies in acetonitrile revealed the importance of these interactions for the ligand's metal ion recognition ability. Whereas 2 only binds to Pb-II and Cu-II, 1 yields complexes with a majority of the heavy and transition metal ions studied, Co-II, Ni-II, Cu-II, Zn-II, Fe-III, Fe-II, Hg-II, and Pb-II, respectively. The cation binding properties in solution were investigated by absorption spectroscopy and in the case of 1-M-II/III, the formation of two spectroscopically distinguishable types of complexes was found. Protonation experiments and theoretical considerations helped to gain further insight into possible modes of coordination in solution.