Selective Imidazolidine Ring Opening during Complex Formation of Iron(III), Copper(II), and Zinc(II) with a Multidentate Ligand Obtained from 2-Pyridinecarboxaldehyde N-Oxide and Triethylenetetramine

The condensation of 2-pyridinecarboxaldehyde N-oxide and triethylenetetramine yields a product with two imidazolidine rings, as proven by a solid-state X-ray structure analysis as well as by NMR solution spectra. This ligand, L¹, undergoes a ring-opening reaction on complex formation with Cu(II), yielding [CuL²]²⁺ where L² functions as a pentadentate ligand, containing only one imidazolidine ring. On complexation with Zn(II) and Fe(III), both rings are opened and the complexes [ZnL³]²⁺ and [FeL³]³⁺ with a hexadentate L³ ligand are formed. The recrystallization of [ZnL³]²⁺ from DMSO solution results in the complex [ZnL¹(DMSO)₂]²⁺ in which L¹ behaves as a tetradentate ligand. Thus L¹, L², and L³ are structural isomers with two, one, or no imidazolidine rings, as confirmed by X-ray structure analyses. The intramolecular ring formation is the result of the nucleophilic addition of the N(amino) group to the electrophilic sp²-hybridized −HC^δ+N site. Owing to the absence of the chelate effect on the sp³-hybridized carbon atom belonging to the imidazolidine ring, the ring opening is facilitated and readily observed upon complex formation with Cu(II), Zn(II), and Fe(III).