Portal de Periódicos da CAPES

Spin-Transition and Ferromagnetic Interactions in Copper(II) Complexes of a 3-Pyridyl-Substituted Imino Nitroxide. Dependence of the Magnetic Properties upon Crystal Packing

1996; American Chemical Society; Volume: 35; Issue: 12 Linguagem: Inglês

10.1021/ic951295y

1520-510X

Fabrice Lanfranc de Panthou, Dominique Luneau, R. N. Musin, Lars Öhrström, André Grand, Philippe Turek, Paul Rey,

Metal-Catalyzed Oxygenation Mechanisms

2-(3-Pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy, IM-3Py (1), reacts with copper(II) bis(hexafluoroacetylacetonate), Cu(hfac)2, to give three complexes. One of them is a centrosymmetric three-spin species, Cu(hfac)2(IM-3Py)2 (4), where two nitroxide ligands are pyridyl bound to the metal and the spins independent from 300 to 5 K. The two other complexes, [Cu(hfac)2]4(IM-3Py)2 (α-phases, 5, and β-phases, 6) are six-spin clusters comprising four copper(II) ions linked by two tridentate IM-3Py units. Their molecular structures are almost identical, but they crystallize in different space groups. For both complexes, two octahedral endo metal ions are involved in a cyclic structure and the two others are exocyclic in a trigonal-bipyramidal environment. On the one hand, the magnetic properties of these two clusters are similar since, for both, the high-temperature behavior can only be accounted for by large ferromagnetic interactions; on the other hand they are dramatically different because the α-phase has a S = 3 ground spin state, while the β-phase exhibits two transitions resulting in two independent spins below 70 K. The transition temperatures are TC1↑ = 211 K, TC1↓ = 152 K, TC2↑ = 80 K, and TC2↓ = 70 K. With the help of ab initio calculations on model fragments, the presence of a ferromagnetic interaction through the pyridyl ring has been tentatively rationalized invoking McConnell's electron transfer mechanism. The high-temperature transition has been assigned to a change of the N-imino coordination from equatorial in a trigonal-pyramidal coordination to axial in a square-planar pyramidal environment at the exocyclic copper center. The low-temperature transition corresponds to a switch from axial to equatorial of the nitroxyl coordination at the endocyclic octahedral metal ion. Relevant crystallographic parameters are as follows: 4, triclinic, P1̄, a (Å) = 6.955(2), b (Å) = 11.485(2), c (Å) = 14.012(3), α (deg) = 112.55(1), β (deg) = 98.79(1), γ (deg) = 90.28(1), Z = 1; 5, monoclinic, C2/c, a (Å) = 25.319(4), b (Å) = 17.653(3), c (Å) = 19.973(4), β (deg) =96.60(2), Z = 4; 6, triclinic, P1̄, a (Å) = 12.200(2), b (Å) = 13.389(3), c (Å) = 15.457(3), α (deg) = 102.54(2), β (deg) = 110.16(2), γ (deg) = 92.57(2), Z = 1.