Comparative study of the influence of the metal centres: Fe(III), Cu(II) and Zn(II), on the ring opening and oxidative dehydrogenation reactions occurring in a coordinated imidazolidine ligand

Condensation of 2-pyridinecarboxaldehyde and 1,9-bis-(2′-pyridyl)-2,5,8-triazanonane, L1, yields 1-[3-aza-4-(2-pyridyl)butyl]-2-(2-pyridyl)-3-[(2-pyridyl)methyl]imidazolidine, L2, as proven by NMR solution spectra. When L2 is reacted with Fe(III) in different alcohols, an imidazolidine ring opening and an oxidative dehydrogenation reaction occur resulting in new complexes of the type: [FeIILn′]2+. Compound 1 with a coordinated L3′ ligand was obtained in n-propanol as a solvent. Compounds 2, 3 and 4 were obtained with L4′, L5′ and L6′ when iso-propanol, n-butanol and iso-butanol were used as solvent, respectively. The structures for 1, 2, 3 and 4 were determined by NMR solution spectra and additionally by X-ray crystallography in the case of the n-butoxy derivative 3. When Cu(II) was used, the hexadentate ligand L2 undergoes also an imidazolidine ring opening reaction on complex formation, however, now generating the well-known pentadentate ligand L1 that is coordinated to the metal ion, 7. Evidence is again provided by the corresponding X-ray structure. With Zn(II) the initial structure of L2 is maintained and in this case L2 functions as a tetradentate, 5, or bis-tridentate ligand, 6, depending on whether the stoichiometric ratio M:L was 1:1 or 2:1, respectively. This has been proven by a solid-state X-ray structure analysis as well as by NMR solution spectra. The ring opening reaction in the presence of Fe(III) can be explained as a result of a higher Lewis acidity of this metal centre, which decreases the electronic density on the nitrogen atom of the imidazolidinic cycle, thus weakening the nitrogen–carbon bond, favouring the nucleophilic attack on the carbon atom by alcohols and producing a more stable hexacoordinated species. Electrochemical evidence is provided in order to support this reaction mechanism.

When the new cyclic imidazolidine ligand (L2) is reacted with Fe(III), Cu(II), and Zn(II), complexes with different ligand structures are obtained. In the presence of the redox-active Fe(III) centre, L2 suffers a ring opening and an oxidative dehydrogenation reaction and the nature of the products depends on the employed alcohol. When Cu(II) was used, L2 is decomposed into the pentadentate ligand L1, which was used in L2 synthesis. With Zn(II), mono and dinuclear complexes were obtained, the cyclic imidazolidine moiety is conserved in the coordinated ligand.Figure optionsDownload as PowerPoint slide