Exchange interactions in bis(hexafluoroacetylacetonato)(4-hydroxy-2,2,6,6-tetramethylpiperidinyl-N-oxy)copper(II): a nitroxyl radical complex of copper(II)

The single-crystal EPR spectra and the temperature dependence of the magnetic susceptibility of bis(hexafluor0- acetylacetonato)(4-hydroxy-2,2,6,6-tetramethylpiperidinyl-~-oxy)copper( 11) (Cu(hfac),(TEMPOL)) have been measured. The compound has a linear chain structure, with a TEMPOL ligand bridging two C ~ ( h f a cm) ~oi eties, but the EPR spectra are typical of a triplet state, showing that magnetically the compound is a chain of weakly coupled metal-radical pairs. The magnetic coupling between the two spins is ferromagnetic, with a singlet-triplet separation of -13 f 5 cm-I. The single-crystal EPR spectra show that g and D are practically parallel to each other. The principal values are the following: g, = 2.057, g,, = 2.013, g, = 2.157, D, = 88 X lo4 cm-I, D,, = -1140 X lo4 cm-I, D, = 1052 X IO4 cm-I, with z roughly parallel to the Cu-O(TEMP0L) bond direction. The g tensor has been decomposed into the copper and radical contributions, yielding information on the nature of the magnetic orbital localized on copper. The zero-field splitting tensor has been decomposed into the sum of a dipolar and an exchange component.