Synthesis, Structure and Magnetic Differences of Two a-Nitronyl Nitroxide Isomers

Abstract

The two isomeric organic nitronyl nitroxide free radicals 2-(2¿-hydroxyl-3¿-methoxy-phenyl)-4, 4, 5, 5-tetramethyl-4, 5-dihydro- 1H-imidazoyl-1-oxyl-3-oxide, 1, and 2-(3¿-hydroxyl-4¿-methoxyphenyl)- 4, 4, 5, 5-tetramethyl-4, 5-dihydro-1H-imidazoyl-1-oxyl-3- oxide, 2, have been synthesized and their structures determined by X-ray diffraction. Structure 1 has three dimensional, strong, intramolecular hydrogen bonds in addition to intermolecular interactions favoring sheets. The structure of radical 2 also shows strong hydrogen bonds both intra- and intermolecular. Some of these bonds are with a lattice water molecule, yielding a three-dimensional crystal structure. An important structural difference between both radicals is that in compound 1, one of the nitrogen atoms of the radical shows smaller angles than the other atom in the same radical and than both nitrogen atoms in compound 2. This suggests that in 1, one of the nitrogen atoms has a higher sp3 character than the other. This proposal is supported by the N-O bond length which is significantly longer in the sp3 nitrogen side. Both isomers show quite different magnetic behavior. In 1 there are both ferromagnetic (JF/k = + 0.11 ± 0.01 K) and antiferromagnetic (JAF/k = - 0.81 ± 0.10 K) interactions, whereas 2 only shows antiferromagnetism (JAF/k = - 1.60 ± 0.02 K) with a phase transition at Tc = 2.48 K.