Magnetic properties of lanthanide complexes with nitronyl nitroxides.

Abstract

1H-imidazolyl-1-oxy1 3-oxide, NITR; R = phenyl, ethyl). The magnetic properties of Gd(hfac),(NITPh), were investigated in the range 1.1-300 K, while all the other compounds were studied in the range 4-300 K. The gadolinium(II1) ions were found to be weakly coupled in a ferromagnetic fashion to the radical, while the radicals are antiferromagnetically coupled to each other. The possible mechanisms responsible for this behavior are discussed. Many efforts have currently been devoted to the synthesis of materials with expected magnetic properties. In order to arrive at such a result, chemists must have available different building blocks with which it will be possible to arrange complex architectures. Therefore, it is of fundamental interest to know the conditions under which, for instance, ferro- or antiferromagnetic coupling will be developed between different spins, and simple molecular clusters need to be synthesized and investigated in order to test theories. This procedure is now fairly well established for transition-metal ions:*3 but much less is known for lanthanide ions. Indeed, although lanthanides have long been known to develop interesting magnetic properties when used to dope transition-metal ionic the study of discrete magnetically coupled molecular systems has been largely overlooked up to the present. Some of us have recently reported the magnetic properties of trinuclear complexes containing one gadolinium(II1) and two copper(I1) ions magnetically coupled in such a fashion that a smooth increase of XT on lowering T was observed.e12 These data were explained with an isotropic ferromagnetic exchange between the gadolinium and the copper ions. The coupling must be determined by superexchange through the oxygen bridges, but no attempt was made to use an orbital model to justify the observed coupling. This purpose in principle might be achieved more easily by studying the interaction of lanthanide ions with stable organic radicals such as nitroxides, because in this case the magnetic interaction would be of the direct exchange type, and the observed coupling might be directly connected with the nature of the bond interaction between the x* orbital on the ligand and the f orbitals of the lanthanides. In other words, the nitroxides, which are widely used as spin labels and spin probes, in this case might be used as bond probes,13 in the sense that an analysis of their interaction with the metal ions should provide first-hand information on the metal-ligand bond and on the nature of the magnetic orbitals. With these considerations in mind, we synthesized a series of stable adducts of lanthanides with nitronyl nitroxides of general formula 0