Structural and spectroscopic studies of tripodal [MgL] 2+ chelates containing only nitrogen donor atoms: alkaline earth metal complexes as potential drug delivery agents

Several tripodal diimine ligands, tris(2-(2-thiazolyl)methyliminoethyl)amine, 2-Tatren, tris(2-(4-(5-methyl)imidazolyl)methyliminoethyl)amine, 5-Me-4-Imtren, tris(2-(4-imidazolyl)methyliminoethyl)amine, 4-Imtren, tris(2-(2-imidazolyl)methyliminoethyl)amine, 2-Imtren, and their Mg(2+) complexes were prepared and characterized. X-ray diffraction studies show that the Mg(2+) ions are six-coordinate, with three acyclic imine N atoms and three imidazolyl or thiazolyl N atoms coordinated with the general formula [Mg(L)](ClO(4))(2) (L=4-Imtren (1), 2-Imtren (2), 2-Tatren (3), and 5-Me-4-Imtren (4)). These complexes are chiral with both Delta and Lambda isomers present in the unit cell. (1)H NMR titrations reveal that complexes also form in solution and that the chirality is maintained. Variable temperature (1)H NMR reveals that the Delta and Lambda isomers interconvert in the intermediate to slow time scale. The interconversion rate slows with increasing pK(a) of the ligand heterocycle, suggesting that interconversion proceeds through a partially dissociated state. These complexes undergo trans-metallation by Zn(2+), indicating that their ligands can be released in a kinetically facile manner to form more stable metal ion complexes.