Structure and magnetic properties of a chain compound formed by copper(II) and a tridentate nitronyl nitroxide radical

A novel compound formed by copper(II) and a nitronyl nitroxide radical of formula [Cu(hfac)2]3(NITpPy)2 (where hfac = hexafluoroacetylacetonato and NITpPy = 2-(4-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide) was synthesized. It crystallizes in the monoclinic system, space group C2/c, with a 36.540 (9) angstrom, b = 23.484 (7) angstrom, c = 21.292 (6) angstrom, beta = 118.90 (2)-degrees, and Z = 8. The structure consists of chains in which dimeric units [Cu(hfac)2NITpPy]2 are bridged by Cu(hfac)2 molecules; this is the first compound in which a nitronyl nitroxide radical binds to three metal ions. The magnetic susceptibility was measured in the temperature range 4.2-300 K, and the product chi-T value as a function of T reaches a broad maximum at about 60 K, due to the predominance of the ferromagnetic exchange interactions between copper(II) and the radical through the short Cu-ON bonds (J = -80 cm-1) rather than the antiferromagnetic ones through the p-pyridyl nitrogen (J' = 15 cm-1). A maximum also occurs in the X versus T curve, at about 13 K. The decrease of chi observed down to the lowest temperature indicates that the ground state is nonmagnetic: this is due to the weak ferromagnetic exchange interaction between copper(II) and the radical through the long Cu-ON bonds. The compound can therefore be considered as a one-dimensional antiferromagnet, and this is also confirmed by single-crystal EPR spectra.