Syntheses and characterization of macrocyclic ligands containing nitrogen and sulphur donor atoms
Date
2018-07-09
Authors
Chak, Becky
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Abstract
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.
Description
Keywords
Macrocyclic compounds, Ligands