A 7-SPIN CLUSTER FORMED BY AN ALKYL NITRONYL NITROXIDE BIRADICAL AND COPPER(II) - CRYSTAL-STRUCTURE AND MAGNETIC-PROPERTIES

The crystal structure and the magnetic properties of [Cu(hfac)2]3[NITnPr(3-NIT)]2, a novel adduct of Cu(hfac)2 (hfac=hexafluoroacetylacetonate) and the alkyl nitronyl nitroxide biradical NITnPr(3-NIT) = 1,3-bis(1'-oxyl-3'-oxide-4',4',5',5'-tetramethyl-4,5-dihydro-1H-imidazol-2'-y l)propane, are reported. The compound crystallizes in the triclinic space group P1BAR, with a = 11.740(3), b = 14.316(3), c = 14.737(2) angstrom, alpha = 106.5(l), beta = 103.3(l), gamma = 111.5(2)-degrees and Z = 2. In the complex one copper(II) ion is hexacoordinated, with two NO donor groups of two biradicals in the axial positions, while the other two copper(II) ions have a square-pyramidal geometry, with an NO group in the apical position. A cluster of seven spins magnetically interacting is the result. The magnetic susceptibility measurements in the temperature range 3-280 K show an increase of the chi(m)T product on lowering the temperature, followed by a decrease for temperatures lower than 8.0 K. The susceptibility data have been interpreted with the help of magneto-structural correlations. They are consistent with strong antiferromagnetic couplings (J'' > 500 cm-1) between the square-pyramidal copper(II) and the coordinating radical spins, and two ferromagnetic couplings (J almost-equal-to -37 cm-1) between the hexacoordinated copper(II) and the two spins of the coordinating radicals.