Imidazolidine Ring Cleavage upon Complexation with First-Row Transition Metals

The reaction of a cyclic diaminal ligand, obtained from the reaction of N, N'-bis(2-pyridylmethyl) ethane-1,2-diamine, as a secondary diamine, and isophthalaldehyde, with different first-row transition-metal ions, such as Fe-III, Zn-II, Cu-II, and Cu-I, was explored using UV/Vis kinetic studies, and cyclic voltammetry. The 3D structures of the resulting metal complexes were determined by single-crystal X-ray diffraction analysis. We demonstrate that the ring-cleavage reaction of the imidazolidine ligand upon complexation with various metal ions depends on the Lewis acidity of the metal ions, as well as on the coordinative requirements of the metal centers. With a soft acid such as Cu-I, the diaminal ligand was unmodified and stabilized tricoordinate planar cuprous ions. In contrast, in the presence of harder acids, such as Fe-III, Zn-II, and Cu-II, the diaminal ligand undergoes hydrolysis/cleavage to yield complexes of the tetradentate N, N'-bis(2-pyridylmethyl) ethane-1,2-diamine ligand. Interestingly, it was also observed that the ring-cleavage reaction in the presence of Fe-III led to the formation of Fe-II complexes. However, we demonstrate that these two events are independent from each other.