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

Abstract

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 [Ln(III)(radical)(2)(NO(3))(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 [Ln(III)(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 Gd(III)-radical coupling (J(Gd-rad) = +1.7 cm(-1)), while for the others this coupling is antiferromagnetic (1a: J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1); 3a: J(Gd-rad) = -2.6 cm(-1); 4a: J(Gd-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.

Excited States and Optical Spectroscopy of Nitronyl Nitroxides and their Lanthanide and Transition Metal Complexes

“…2 shows that transitions to at least two excited states form the observed absorption band system. This experimental conclusion is based on the luminescence and absorption spectra discussed in the preceding sections [27,28]. Electronic structure calculations, as illustrated for NITBzImH in Fig.…”

“…Most nitronyl nitroxides and their metal complexes show absorption spectra with resolved patterns as illustrated in Figs. 2, 3, and 7, consisting of a sequence of four to six peaks [26,27,30]. The energy differences between these maxima are constant within experimental precision, with the exception of the interval between the two lowest-energy peaks labeled a and b in the absorption spectra.…”

“…2a, where the intensity of the first peak, labeled a, is lower by at least an order of magnitude than the intensity of the following bands, labeled b, c and d. To the best of our knowledge, the results in Fig. 2 are the only published comparisons of full luminescence and absorption spectra for uncoordinated nitronyl nitroxides and their lanthanide complexes [27,28]. Their vibronic structure has been analyzed quantitatively for uncoordinated radicals and reveals offsets of the excited-state potential energy surfaces along several Raman-active modes [28].…”

“…1. The ligator atoms of the radicals are marked by asterisks, based on the crystallographic structures of the complexes [26,27,33].…”

Two New Nitronyl Nitroxide Radicals and Their Complexes with M(hfac)2 [M=Co(II), Ni(II), Mn(II)]: Syntheses, Crystal Structures, and Magnetic Characterizations

“…This is particularly true if there are potential coordination sites available in the R-group. [4][5][6][7][8][9][25][26][27][28][29][30][31][32][33][34][35] Therefore, modification of the R-group would tune the coordination mode between the nitronyl nitroxides and metal ions and thus alter the magnetic interaction between the nitronyl nitroxides and metal ions. New nitronyl nitroxides with coordination ligands such as pyridyl [27][28][29][30][31] and bipyridyl 9,32,34 have been synthesized, and accordingly a great deal of paramagnetic metal complexes of the nitronyl nitroxides with interesting structures have been reported.…”