Anomalous diamagnetic behaviour of the ¹H NMR spectra of a coordinated imino nitroxide radical in diamagnetic and paramagnetic lanthanide(III) complexes

Diamagnetic lanthanide(III) complexes with an imino nitroxide radical IM2py [2-(2′-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl], [Ln(hfac)₃(IM2py)] (Ln = Y and Lu; hfac = hexafluoroacetylacetonate), exhibit ¹H NMR chemical shifts for the pyridyl protons of the coordinated IM2py suggesting it behaves as if it were diamagnetic. To a series of paramagnetic lanthanide complexes of this type, the structural analyses of the lanthanide induced shifts (LIS) of the hfac 3-proton and pyridyl protons in IM2py were successfully applied, showing not only the diamagnetic behaviour of the pyridyl protons in the IM2py group, but also the dominant dipolar contribution and isostructural nature with apparent or effective axial symmetry in solution irrespective of the asymmetry in the solid state.