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The variation of the number and position of OH substituents a t the phenyl ring of a-phenyl nitronyl nitroxide radicals yields different hydrogen-bonded molecular self-assem blies with distinct dimensionalities; the mono-ortho-su bstituted isomer is most remarkable since it shows a three-dimensional network of weak hydrogen bonds exhibiting a bulk ferromagnetic transition at 0.45 K.

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 T C1 ↑ = 211 K, T C1 ↓ = 152 K, T C2 ↑ = 80 K, and T C2 ↓ = 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.

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On the basis of the 25-year studies on the synthesis of metal complexes with nitroxides, it was believed that nitroxides were kinetically stable during complex formation only if all substituents in the -position to the nitroxyl group were hydrocarbons.* 12 This concept was extended to the recent extensive investigations in the design of molecular ferromagnets based on metal complexes with stable nitroxides. Since in molecular ferromagnets the hydrocarbon substituents should be as small as possible, as is the case with methyl groups, the number of stable nitroxides employed at present in the design of molecular ferromagnets is extremely limited. In fact, it includes only the 2,2,5,5-and 4,4,5,5-tetramethyl derivatives of 3and 2-imidazoline, respectively.2,3 According to current concepts, nitroxides with one of the carbon atoms being a to the nitroxyl group and carrying an H atom are kinetically unstable and disproportionate to diamagnetic compounds.4Recently, terf-butyl diphenylmethyl nitroxide, L, has been reported as a representative of a new family of such a-Hcontaining nitroxides which are stable in the solid state as well as in solution.5 In these spin carriers the extensive functionalization of the carbon framework is possible and since the '-O group is less sterically hindered than that in fully -substituted analogs, the use of such spin carriers as building blocks in the design of exchange-coupled species is antici-* Author to whom correspondence should be addressed.(1) (a) International Tomography Center, (b) Centre d'Etudes Nucleates,

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