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

摘要

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: [(FeLn')-L-II](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. (c) 2005 Elsevier B.V. All rights reserved.