Synthesis, Structures, and Magnetic and Optical Properties of a Series of Europium(III) and Gadolinium(III) Complexes with Chelating Nitronyl and Imino Nitroxide Free Radicals

This paper reports the synthesis, structures, and magnetic and optical properties of a series of gadolinium(III)
(1a−4a) and europium(III) (1b−4b) complexes with nitronyl or imino nitroxide radicals. The crystal structures of
compounds 1a and 1b consist of [LnIII(radical)2(NO3)3] entities in which the gadolinium(III) (1a) or europium(III) ion
(1b) is 10-coordinated to two nitronyl nitroxide radicals and three nitrato ligands. The crystal structures of compounds
2a−4a and 2b−4b consist of [LnIII(hfac)3(radical)] entities in which the gadolinium(III) (2a−4a) or europium(III) ion
(2b−4b) is 8-coordinated to one nitronyl (2a and 2b) or one imino (3a, 4a and 3b, 4b) nitroxide radical and three
hexafluoroacetylacetonato ligands. The gadolinium(III) complexes (1a−4a) are isostructural with their europium(III)
analogues (1b−4b). The magnetic properties of the gadolinium complexes were studied. Along the series 1a−4a
only compound 2a exhibits a ferromagnetic GdIII−radical coupling (JGd-rad = +1.7 cm-1), while for the others this
coupling is antiferromagnetic (1a: JGd-rad1 = −4.05 cm-1 and JGd-rad2 = −0.80 cm-1; 3a: JGd-rad = −2.6 cm-1; 4a:
JGd-rad = −1.9 cm-1). The first full luminescence spectra of lanthanide complexes with free radical ligands are
reported between 650 and 1200 nm. The rich vibronic structure in luminescence and absorption spectra indicates
that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be
established between magnetic ground state properties and the energies and fine structure of the title compounds.