Synthesis, spectroscopic and magnetic properties of lanthanide(III) complexes with a chelated imino nitroxide radical

The treatment of [Ln(hfac)₃(H₂O)₂] (Ln(III)= Y, Nd-Lu) with IM-2py (2-(2-pyridyl)-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxyl) readily gave a series of novel rare earth or lanthanide radical complexes [Ln(hfac)₃(IM-2py)]. The molecular structures of the Sm, Gd, Dy, Er and Yb complexes have been determined to be bicapped trigonal prismatic (TP RS) by X-ray diffraction. The magnetic susceptibility data for [Gd(hfac)₃(IM-2py)] show that the Gd-IM-2py interaction is weakly antiferromagnetic with an exchange coupling constant J = -3.00 cm^-1 in contrast to the ferromagnetic interaction in the Gd(III)-nitronyl nitroxide complexes. The n-π* transition of IM-2py is found to shift to lower frequency associated with intensity enhancement and vibronic structure. The luminescence measurement of the Eu(III) and Tb(III) complexes demonstrated energy transfers for IM-2py (SOMO π*) → Ln(III) (4f) as a result of emission quenching; this depends on the energy gap between the excited levels in Ln(III) and IM-2py. These results reveal that the metal-radical interactions are very weak magnetically but are relatively strong with respect to energy transfer.