Investigating novel acyclic lanthanide complexes suitable for magnetic resonance imaging

Padayachy, Kamentheren
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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.