Syntheses and characterization of macrocyclic ligands containing nitrogen and sulphur donor atoms




Chak, Becky

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Macrocyclic ligands containing nitrogen and sulphur donor atoms, as well as with pyridine or thiophene pendant arms were synthesized. The Co(II), Ni(II), Cu(II) and Pd(II) complexes have been characterized by crystallography, UV/Vis, NMR or ESR spectroscopy and the redox chemistry have been studied by cyclic voltammetry (CV). The Pd(II) complex of [14]aneNS3 (1,4,7-trithia-11-azacyclotetradecane), is square-planar whereas the structures of Pd(II) macrocyclic complexes with pyridine or thiophene pendent arms may be described as pseudo five-coordinate. There is considerable interaction between the apical sulphur donor atom and the palladium metal centre. In solution, the Pd(II) complex of py[14]aneNS3 (N-(2’-pyridylmethyl)- 1,4,7-trithia-11-azacyclotetradecane) exhibits fluxional behavior. There is an exchange process involving the metal-coordinated and -uncoordinated thioether atoms. By analyzing the NMR spectra obtained at different temperatures, the mechanism for the fluxional process is proposed. The cyclic voltammograms of the Pd(II) macrocyclic complexes showed irreversible reduction to Pd(I) and no oxidation to Pd(III) could be detected in the potential range studied. In the case of the Pd(II) complex with thiophene pendent arms, a quasi-reversible reduction to Pd(I) was observed. This unique behavior is rationalized as due to the proximity of a thiophene moiety in the apical position which inhibits dimerization to the Pd(I) generated, due to steric reasons. The Ni(II) complex of py[14]aneS3 is peudo-octahedral whereas the Co(Il) complex exists as two linkage isomers. In the nitrate salt, [special characters omitted], the cobalt centre is octahedral, being coordinated to only the nitrogen donors from the ligand and the remaining coordination sites are occupied by acetonitrile and oxygen atoms from two nitrate groups. In the perchlorate salt, [special characters omitted], solution studies (ESR and CV) suggest that the cobalt(II) ion is being coordinated by the nitrogen and sulphur donor atoms from the ligand. The Cu(II) complexes of these mixed donor ligands have also been studied and their spectroscopic (UV/Vis and ESR) characteristics and Cu(II)/Cu(I) reduction potential were compared to the type I Blue copper protein.



Macrocyclic compounds, Ligands