Structure and magnetic properties of ferromagnetic alternating spin chains

A novel compound formed by copper(I1) and a nitronyl nitroxide stable radical of formula Cu(hfac),NITiPr (hfac = hexafluoroacetylacetonate; NITiPr = 2-isopropyl-4,4,5,5-tetramethyl-4,5-dihydroI-H -imidazolyl-1 -oxy1 3-oxide) has been synthesized. It crystallizes in the monoclinic system, space group P2,/c with u = 13.807 (7) A, b = 10.966 (2) A, c = 19.037 (8) A, 0 = 102.42 (4)O, and Z = 4. The structure consists of chains in which the copper ions are bridged by the nitronyl nitroxides, which coordinate to two different metal ions by the two equivalent oxygen atoms. The magnetic susceptibility was measured in the temperature range 1-300 K, and the value of xT diverges on lowering the temperature. This behavior is typical of an infinite array of spins ferromagnetically coupled, and the coupling constant has been estimated to be J = -21.6 cm-I. The single-crystal EPR spectra have shown a behavior typical of one-dimensional magnetic systems with line widths that follow a ((3 cos2 0) - 1)" angular dependence. Also g shifts have been observed at low temperature. The low-temperature magnetic data have suggested the presence of a very weak 3-D ferromagnetic interaction. We report for comparison also the magnetic data of the analogous Cu(hfac),NITMe compound (NITMe = 2,4,4,5,5-pentamethyI-4,5-dihydr1oH-- imidazolyl-I-oxy13 -oxide) in the range 1-4 K, which have shown the presence of a stronger 3-D antiferromagnetic interaction. The difference in the low-temperature behavior of these two compounds with that of already reported ferrimagnetic chains containing manganese(I1) and nickel(I1) has been rationalized by using a simple model that takes into account only the dipolar interaction between chains.