N-BONDED COPPER(II) IMINO NITROXIDE COMPLEXES EXHIBITING LARGE FERROMAGNETIC INTERACTIONS

The magnetic interactions between copper(II) and imino nitroxide free radicals have been investigated in two novel adducts of copper(II)-bis(hexafluoroacetylacetonato): bis(mu-1,3-(2-phenyl-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy))hexakis(hexafluoroacetylacetonato)tricopper(II) (1) and (2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy)bis(hexafluoroacetylacetonato)copper(II) (2). The crystal and molecular structures of both compounds have been determined. In 1, the three metal ions are linked by two mu-1,3 bridging nitroxide ligands, the two external copper ions are in trigonal-bipyramidal environments with the imino nitroxide nitrogen atom coordinated in the basal plane, while the central metal ion is axially coordinated by the two nitroxide oxygen atoms. In 2, the imino nitroxide and the imino pyridyl nitrogen atoms are both equatorially bound to the octahedral copper ion; in addition, the molecules interact in pairs in the crystal lattice. In both cases, the high-temperature magnetic behavior is governed by a large ferromagnetic interaction, J > 300 cm-1 (H = JS(i)S(j)), which develops within the imino coordinated metal ion and the free radical. By contrast, the low-temperature magnetic properties strongly depend on weak additional interactions corresponding to the binding of the nitroxide oxygen to the central metal ion in 1, and to the intermolecular short contact between the uncoordinated NO groups in 2. The ferromagnetic coupling of spins is rationalized as arising from the exchange interaction of unpaired electrons in orthogonal magnetic orbitals. This situation is the consequence of the binding geometry of the imino nitroxide nitrogen atom to the copper(II) ion. Crystal data. 1: a = 12.740 (4) angstrom, b = 16.269 (5) angstrom, c = 19.470 (6) angstrom, alpha = 78.89 (1)-degrees, beta = 78.30 (1)-degrees, gamma = 81.00 (1)-degrees, triclinic, PlBAR. 2: a = 9.317 (4) angstrom, b = 12.922 (5) angstrom, c = 13.525 (6) angstrom, alpha = 95.24 (1)-degrees, beta = 109.71 (1)-degrees, gamma = 110.92 (1)-degrees, triclinic, PlBAR. This study shows that large ferromagnetic interactions between metal ions and nitroxide free radicals are possible, it opens new perspectives for the design of molecular high-spin species.