Cationic lanthanide complexes of neutral tripodal N,O ligands: enthalpy versus entropy-driven podate formation in water

Abstract

The cationic lanthanide complexes of two neutral tripodal N,O ligands, tpa (tris[(2-pyridyl)methyl]amine) and tpaam (tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine) are studied in water. The analysis of the proton lanthanide induced NMR shifts indicate that there is no abrupt structural change in the middle of the rare-earth series. Unexpectedly, the formation constant values of the lanthanide podates of tpaam and tpa in D2O at 298 K are similar, suggesting that the addition of the three amide groups to the ligand tpa does not lead to any increase in stability of the lanthanide complexes of tpaam in respect to tpa, even though the amide groups are coordinated to the metal in aqueous solution. The measurement of the enthalpy and entropy changes of the complexation reactions shows that the two similar ligands tpa and tpaam have different driving forces for lanthanide complexation. Indeed, the formation of tpa podates benefits from an exothermic enthalpy change associated with a small entropy change, whereas the complexation reaction with tpaam is clearly entropy-driven though opposed by a positive enthalpy change. The hydration states of the europium complexes were measured by luminescence and show the coordination of 4-5 water ligands in [Eu(tpa)]3+ whereas there are only 2 in [Eu(tpaam)]3+. Therefore the heptadentate ligand tpaam releases the translational entropy of more water molecules than does the tetradentate ligand tpa.

Enthalpy- vs Entropy-Driven Complexation of Homoallylic Alcohols by Rhodium(I) Complexes

“…22 The lanthanide complexes of neutral tripodal ligands also showed different driving forces where the formation of the complexes of tetradentate versus heptadentate ligands was enthalpy and entropy-driven, respectively. 23 …”

“…For bisphosphonate complexes of the FPPS enzyme, the binding of bisphosphonates with neutral side chains is enthalpy-driven, whereas bisphosphonates with charged side chains bind in an entropy-driven manner . The lanthanide complexes of neutral tripodal ligands also showed different driving forces, where the formation of the complexes of tetradentate vs heptadentate ligands was enthalpy- and entropy-driven, respectively …”

Lanthanide-binding peptides with two pendant aminodiacetate arms: Impact of the sequence on chelation

“…the carbonyl group of Ada 1 (2)-does not stabilize the mononuclear Tb 3+ complex. This may be explained by the metal dehydration contributions associated to the coordination of a neutral amide donor group 32 and also by the absence of the H-bond which is present in the β-turn of TbP 22 . Interestingly at physiological pH, TbP 11 and TbP 12 complexes display higher stabilities than TbP 22 due to their lower overall basicity thanks to the electron-withdrawing effect of the peptide chain.…”

Development of Highly Selective Ligands for Separations of Actinides from Lanthanides in the Nuclear Fuel Cycle

“…Whereas complexation of tris(2-pyridylmethyl)amine ( 51) is largely enthalpy driven, complexation with 53 is completely driven by entropy. 55 The additional ligating sites present in 53 displace more water molecules from the hydrated lanthanide ion during complex formation than 51. The results of EXAFS on An(III) and Ln(III) complexes of tris(pyrazin-2-ylmethyl)amine (52) showed no obvious structural differences between the two sets of complexes, 56 although relatively short uranium(III)-nitrogen bond distances were observed, suggesting a higher degree of covalent character.…”