Pyridinophane Platform for Stable Lanthanide(III) Complexation

A detailed investigation of the solid state and solution structures of lanthanide(III) complexes with the macrocyclic ligand 2,11,20-triaza[3.3.3](2,6) pyridinophane (TPP) is reported. The solid state structures of 14 different Ln3+ complexes have been determined using X-ray crystallography. The ligand is coordinating to the Ln3+ ion by using its six nitrogen atoms, while nitrate or triflate anions and water molecules complete the metal coordination environments. The structure of the complexes in solution has been investigated by 1H and 13C NMR spectroscopy, as well as by DFT calculations (TPSSh model) performed in aqueous solution. The structures obtained from these calculations for the complexes with the lightest Ln 3+ ions (La-Sm) are in very good agreement with those determined by the analysis of the Ln3+-induced paramagnetic shifts. A structural change occurs across the lanthanide series at Sm3+; the complexes of the large Ln3+ ions (La-Nd) are chiral due to the nonplanar conformation of the macrocycle, and present effective C3v symmetries in solution as a consequence of a fast interconversion of two enantiomeric forms with C3 symmetry. The activation free energy for this enantiomerization process, as estimated by using DFT calculations, amounts to 33.0 kJ·mol-1. The TPP ligand in the complexes of the heaviest Ln3+ ions (Eu-Lu) presents a half-chair conformation, which results in Cs symmetries in solution. © 2013 American Chemical Society.