Imidazolidine Ring Cleavage upon Complexation with First Row Transition Metals

The reaction of a cyclic di-aminal 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-visible kinetic studies, and cyclic voltammetry. The 3D structure of the resulting metal complexes were determined by 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 di-aminal ligand was unmodified and stabilized tricoordinated planar cuprous ions. In contrast, in the presence of harder acids, such as Fe(III), Zn(II) and Cu(II), the di-aminal 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 presence of Fe(III) led to the formation of Fe(II) complexes. However, we demonstrate that these two events are independent from each other.