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Aqueous coordination chemistry of aluminum and gallium with 3-hydroxy-4-pyridinone ligands Nelson, William Otto


Aluminum(III) and gallium(III) complexes with the following synthesized: 3-hydroxy-2-methyl-4(1H)-pyridinone (Hmpp), the 1-methyl (Hdpp), (Hmepp), and 1-hexyl (Hmhpp) derivatives of Hmpp, and β-[N-3-hydroxy-4-pyridinone)]-α-aminopropionic acid (mimosine). The 3-hydroxy-4-pyridinone employed in this study (except mimosine) were prepared by the heterocycle, 3-hydroxy-2-methyl-4-pyrone, to the corresponding nitrogen heterocycle reaction with a primary arnine. These bidentate ligands contain an α-hydroxyketone moiety and their conjugate bases form neutral complexes with trivalent metals. The ligands and the metal complexes were fully characterized by mass spectrometry and and ultraviolet spectroscopy. The structures of several ligands and metal complexes were determined by X-ray diffraction. Hmpp, Hdpp, and Hmepp crystallize as centrosymmetric O-H…O=C hydrogen bonded dimeric units. The facial geometric isomers Al- and Ga(dpp)₃ crystallize as the dodecahydrate in which the water molecules are associated in hexagonal rings similar in structure to that of ice I[sub h]. The oxygen atoms of the metal complexes are hydrogen bonded to bridging waters so that the water rings and metal complexes are interconnected in a three-dimensional array. An analogous water network is found in the structures of Al- and Ga(mepp)3. The proton NMR spectra in CD₃OD and D₂O indicate the metal complexes are fluxional above -30°C. Variable-temperature proton NMR experiments identified the exchange process as facial to meridional geometric isomerization. Ligand exchange experiments using proton NMR indicated the isomerization follows an intermolecular rather than intramolecular pathway in CD₃OD. Variable-pH ²⁷Al NMR experiments show the tris-ligand aluminum complexes to be resistant to hydrolysis from pH 4-9. The formation constants of the metal-ligand complexes were determined by potentiometric titrations and this study indicates the gallium complexes have a similar pH region of hydrolytic stability. The overall formation constants for the tris-ligand aluminum and gallium complexes were all greater than 10³⁰, indicating that these ligands could compete for aluminum and gallium in blood plasma models. Water solubilities and octanol/water partition coefficients of the ligands and metal complexes were measured and they indicate the complexes for ⁶⁷Ga animal biodistribution experiments. The results of the biodistribution study show that under conditions of ligand excess ⁶⁷Ga is redirected from transferring; however, the ⁶⁷Ga-ligand complexes do not localize in any organs. It appears the ligands greatly enhance the removal of the radionuclidefrom the body.

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