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

Abstract

Condensation of 2-pyridinecarboxaldehyde and 1,9-bis-(2′-pyridyl)-2,5,8-triazanonane, L 1, yields 1-[3-aza-4-(2-pyridyl)butyl]-2-(2-pyridyl)-3-[(2-pyridyl)methyl]imidazolidine, L 2, as proven by NMR solution spectra. When L 2 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: [Fe IIL n′ ] 2+. Compound 1 with a coordinated L 3′ ligand was obtained in n-propanol as a solvent. Compounds 2, 3 and 4 were obtained with L 4′, L 5′ and L 6′ 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 L 2 undergoes also an imidazolidine ring opening reaction on complex formation, however, now generating the well-known pentadentate ligand L 1 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 L 2 is maintained and in this case L 2 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.