Insights on the Structure in Solution of Paramagnetic Ln^III/Ga^III 12-Metallacrown-4 Complexes Using 1D ¹H NMR and Model Structures

The structure in solution of a seriesof lanthanide metallacrownswas studied through paramagnetic H-1 NMR and DFT models.The analysis of NMR data (treated with the "all-lanthanide"method), X-ray structures, and DFT models allowed to establish thenature of dynamic equilibrium processes in solution otherwise notnoticeable from the X-ray data alone. Thesolution structure of Ln(III)Na(I)(OBz)(4)[12-MCGaIII(N)Shi-4] complexes was studied throughparamagnetic H-1 NMR and DFT models. Although isostructuralin the solid state, their H-1 NMR spectra in DMSO-d (6) are extremely different from one another dueto the magnetic anisotropy of the lanthanide(III) ions. NMR data wereanalyzed by the "all-lanthanide" method that were comparedto X-ray structures and model structures, allowing to establish theextent of the structural changes that occur from the solid state tothe solution phase. Major structural changes involve the phenyl groupsof the benzoate ions that, quite surprisingly, in solution presentpreferential orientations lowering the symmetry of the complex contraryto what observed in the solid state. Overall, DFT methods and 1D NMRdata allowed us to clarify aspects related to molecular rearrangementprocesses in solution that could not be predicted by a simple lookat the X-ray structures of these complexes.