Toward Optimized High-Relaxivity MRI Agents: The Effect of Ligand Basicity on the Thermodynamic Stability of Hexadentate Hydroxypyridonate/Catecholate Gadolinium(III) Complexes

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Abstract
The thermodynamic stabilities of the GdIII complexes of five hexadentate ligands, which incorporate the 2,3-dihydroxyterephthalamide and 2,3-hydroxypyridonate chelating moieties, have been determined by potentiometric and spectrophotometric titration. The ligands were chosen to span a range of basicities while maintaining a similar tripodal structural motif, facilitating a study of the effect of ligand basicity on the thermodynamic stability of the GdIII complexes. The relative stability of the five complexes is found to be highly pH dependent, with the most acidic ligands forming the most stable complexes at low pH and more basic ligands forming more stable complexes at high pH. The most stable GdIII complex at a physiological pH of 7.4 is formed with a ligand of intermediate basicity and is of stability comparable to that of GdIII complexes that feature eight-coordinate amino−carboxylate ligands and are currently used as magnetic resonance imaging contrast agents in diagnostic medicine. A single-crystal X-ray structure of the intermediate compound 3-hydroxy-6-methyl-2-oxo-1,2-dihydropyridine-4-carboxylic acid ethyl ester is described: This compound crystallizes in the triclinic space group P1̄ with a = 7.4801(3) Å, b = 8.0671(3) Å, c = 8.3457(4) Å, α = 72.242(2)°, β = 80.693(2)°, γ = 69.943(3)°, V = 449.60(3) Å3, Z = 2, and R = 0.042.