Investigating novel acyclic lanthanide complexes suitable for magnetic resonance imaging
Date
2014-07-04
Authors
Padayachy, Kamentheren
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Abstract
The ligands 2-(bis(2-(octahydro-2-oxobenzo[b][1,4]oxazin-4-yl)ethyl)amino)acetic acid
trihydrochloride and bis(2-(octahydro-2-oxobenzo[b][1,4]oxazin-4-yl)ethyl)amine
trihydrochloride are synthesized for the formation of Gd(III) complexes, potentially useful as
MRI contrast agents. Speciation and hydration numbers of the Gd(III) complexes of 1,7-
bis(2-hydroxycyclohexyl)-1,4,7-tris(carboxymethyl)-1,4,7-triazaheptane (Cy2DETA.3A) and
1,7-bis(2-hydroxycyclohexyl)-1,7-bis(carboxymethyl)-1,4,7-triazaheptane (Cy2DETA.2A)
can be inferred from spectroscopic studies of the analogous Eu(III) complexes, due to their
similar ionic radii, charge and coordination geometry. The phosphorescence emission spectra
for the Eu(III) complexes are recorded as function of pH, in buffered and aqueous media.
Modulation of the relative intensities of the emission bands is observed for both Eu(III)
complexes. Spectroscopically determined pKa values are used to determine potential solution
speciation. Ligand pKa values were determined potentiometrically. Attempts were made to
correlate results from potentiometric titrations with that of lanthanide luminescent titrations
of these complexes. Hydration states (q) were determined for the Eu(III) complexes at
spectroscopically significant pH values, in buffered and aqueous media. The results confirm
the formation of a octadentate complex between Eu(III) and Cy2DETA.3A, with the inclusion of one H2O molecule in the inner sphere. The complex has high stability and is responsive towards changes in pH and analyte concentration (o-phthalate); potentially suitable as a luminescent sensor. The heptadentate complex formed between Cy2DETA.2A and Eu(III) is substantially weaker, with precipitation of Eu(OH)3 observed at neutral pH, that limits its potential application as a luminescent sensor.