Magnetic coupling in zero- and one-dimensional magnetic systems formed by nickel(II) and nitronyl nitroxides. Magnetic phase transition of a ferrimagnetic chain

Two novel compounds formed by nickel(I1) hexafluoroacetylacetonate and nitronyl nitroxide radicals NITR = 2-R-4,4,5,5- tetramethyl-4,5-dihydro-1H-imidazole-l-oxy3l- oxide, with R = methyl, Me, and phenyl, Ph, were synthesized. The compound Ni(hfac)2(NITPh)2 crystallizes in the monoclinic system, space group P2,/c, with a = 12.243 (4) A, b = 15.237 (4) A, c = 22.310 (8) A, fi = 100.68 (2)”, and Z = 4. The nickel ions are octahedrally coordinated to two hfac molecules and to the oxygen atoms of two different radicals. The magnetic properties indicate the presence of a strong antiferromagnetic coupling between the nickel and the radicals, which has been roughly estimated to be on the order of 400 cm-I. Even if the structure of Ni(hfac)2NITMe is not available, we suggest an infinite chain structure, with each radical bridging two Ni(hfac)* units, on the basis of analogies with other M(hfac)2NITR compounds. In agreement with the proposed structure, the magnetic susceptibility shows a divergence in the XT product at low temperature. A numerical fit, with a model previously used for Mn(hfac)2NITR ferrimagnetic chains, yields an antiferromagnetic coupling of 424 cm-I. The fast divergence of the susceptibility at very low temperature indicates the presence of a phase transition to three-dimensional order. Low-field (0-1 Oe) magnetic measurements show that the system undergoes a phase transition at ca. 5.3 K. The nature of the transition is presumably ferromagnetic and is driven by the dipolar interaction between chains, as in the analogous manganese-radical systems.