Lanthanide(III) Complexes of Novel Mixed Carboxylic‐Phosphorus Acid Derivatives of Diethylenetriamine: A Step towards More Efficient MRI Contrast Agents

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Abstract
Three novel phosphorus‐containing analogues of H5DTPA (DTPA = diethylenetriaminepentaacetate) were synthesised (H6L1, H5L2, H5L3). These compounds have a ‐CH2‐P(O)(OH)‐R function (R = OH, Ph, CH2NBn2) attached to the central nitrogen atom of the diethylenetriamine backbone. An NMR study reveals that these ligands bind to lanthanide(III) ions in an octadentate fashion through the three nitrogen atoms, a PO oxygen atom and four carboxylate oxygen atoms. The complexed ligand occurs in several enantiomeric forms due to the chirality of the central nitrogen atom and the phosphorus atom upon coordination. All lanthanide complexes studied have one coordinated water molecule. The residence times (τ${{298\hfill \atop {\rm M}\hfill}}$) of the coordinated water molecules in the gadolinium(III) complexes of H6L1 and H5L2 are 88 and 92 ns, respectively, which are close to the optimum. This is particularly important upon covalent and noncovalent attachment of these Gd3+ chelates to polymers. The relaxivity of the complexes studied is further enhanced by the presence of at least two water molecules in the second coordination sphere of the Gd3+ ion, which are probably bound to the phosphonate/phosphinate moiety by hydrogen bonds. The complex [Gd(L3)(H2O)]2− shows strong binding ability to HSA, and the adduct has a relaxivity comparable to MS‐325 (40 s−1 mM−1 at 40 MHz, 37 °C) even though it has a less favourable τM value (685 ns). Transmetallation experiments with Zn2+ indicate that the complexes have a kinetic stability that is comparable to—or better than—those of [Gd(dtpa)(H2O)]2− and [Gd(dtpa‐bma)(H2O)].

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