Insights on the Structure in Solution of Paramagnetic LnIII/GaIII 12-Metallacrown-4 Complexes Using 1D 1H NMR and Model Structures

The solution structure of Ln^IIINa^I(OBz)₄[12-MC_GaIII(N)Shi-4] complexes was studied through paramagnetic ¹H NMR and DFT models. Although isostructural in the solid state, their ¹H NMR spectra in DMSO-d₆ are extremely different from one another due to the magnetic anisotropy of the lanthanide(III) ions. NMR data were analyzed by the "all-lanthanide" method that were compared to X-ray structures and model structures, allowing to establish the extent of the structural changes that occur from the solid state to the solution phase. Major structural changes involve the phenyl groups of the benzoate ions that, quite surprisingly, in solution present preferential orientations lowering the symmetry of the complex contrary to what observed in the solid state. Overall, DFT methods and 1D NMR data allowed us to clarify aspects related to molecular rearrangement processes in solution that could not be predicted by a simple look at the X-ray structures of these complexes.