| Abstract
| - The solution and solid-state structures of the cationic lanthanide complexes of the new heptadentate ligand, tpaam, containing three pyridinecarboxamide arms connected to a central nitrogen are studied and compared to the structures of the lanthanide complexes of the tetradentate ligand tpa. Surprisingly, the presence of three additional amide oxygens coordinated to the metal ion does not lead to an increased stability of the tpaam complexes toward ligand dissociation in water.
- The synthesis of the potentially heptadentate ligand tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine, tpaam,containing three pyridinecarboxamide arms connected to a central nitrogen is described. Lanthanide complexes ofthis ligand are prepared and characterized. The crystallographic structure of the complexes of three lanthanideions (La, Nd, Lu) is determined. The lanthanide(III) complexes of tpaam crystallize as monomeric species (in thepresence of chloride or iodide counterions) in which the ligand tpaam acts as a N4O3 donor. The crystal structurespresented here show that the Ln−O and Ln−Npyridyl distances in the complexes of tpaam are similar to those foundfor the tpaa complexes (H3tpaa = α,α‘,α‘ ‘-nitrilotri(6-methyl-2-pyridinecarboxylic acid) despite the difference incharge. A lengthening of the Ln−Napical distance is observed in the tpaam complexes compared to the tpa (tris[(2-pyridyl)methyl]amine) complexes which is more marked for larger lanthanides than for smaller ones. The solutionstructures of the tpaam complexes were analyzed across the 4f series and compared to the solution structures ofthe lanthanide complexes of the tetradentate ligand tpa. Proton NMR studies are in agreement with the presenceof C3v symmetric solution species for both ligands. For the larger lanthanides, the cation moves away from theapical nitrogen compared to the position occupied in tpa complexes, whereas for the smaller lanthanides, the metalion is located in a similar position for the two ligands. Quite surprisingly, the formation constant of the Eu(tpaam)Cl3 complex in D2O at 298 K (log β110 = 2.34(4)) is very similar to the one reported for Eu(tpa)Cl3 (log β110 =2.49(4) at 298 K in D2O) indicating that the addition of three amide groups to the ligand tpa does not lead to anyincrease in stability of the lanthanide complexes of tpaam compared to those of tpa.
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