Effect of Axial Ligation on the Magnetic and Electronic Properties of Lanthanide Complexes of Octadentate Ligands

A detailed investigation of the solution structure of two Yb(III) complexes of C₄ symmetric chiral tetraamide ligands based on cyclen is performed in different solvents with the combined use of ¹H NMR, NIR-CD, and luminescence techniques. Pseudocontact chemical shifts and NOE measurements allow one to assess the relative positions of the ring and the side chains; NIR-CD spectra provide a description of the local stereochemistry around the metal ion; and finally, the axial exchange dynamics can be quantitatively characterized by means of the luminescence emission data. The two complexes can be described by a Λ(δδδδ) (i.e., square antiprismatic) coordination polyhedra, capped by a ninth axial solvent molecule. On changing the solvent composition (water, methanol, acetonitrile, dimethylsulfoxide), equilibria involving the ring or the sidearms are not detected, and the macrocycle conformation stays the same: the relevant changes in the observed spectra can thus be related to different axial coordinations. The susceptibility anisotropy factor D in the ¹H NMR spectra becomes progressively smaller as the strength of the axial ligand increases; correspondingly, the different crystal field caused by the varied contribution of this further ligand shapes differently the CD and the emission patterns of the central cation. The present study is the first attempt to rationalize a correspondence between the magnetic and optical parameters used in the description of the behavior of lanthanide ions in solution.