A mixed-valence Co(II)/Co(III) heptanuclear wheel [Co(II)3Co(III)4(L)6(MeO)6] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin-orbit coupling effects within the SO-RASSI approach. The four Co(III) ions were found to be diamagnetic and to give a significant temperature-independent paramagnetic contribution to the susceptibility. The spin-orbit coupling on the three Co(II) sites leads to separations of approximately 200 cm(-1) between the ground and excited Kramers doublets, placing the Co7 wheel into a weak-exchange limit in which the lowest electronic states are adequately described by the anisotropic exchange interaction between the lowest Kramers doublets on Co(II) sites. Simulation of the exchange interaction was done within the Lines model, keeping the fully ab initio treatment of magnetic anisotropy effects on individual cobalt fragments using a recently developed methodology. A good description of the susceptibility and magnetization was obtained for nearest-neighbor (J1) and next-nearest-neighbor (J2) exchange parameters (1.5 and 5.5 cm(-1), respectively). The strong ferromagnetic interaction between distant cobalt ions arises as a result of low electron-promotion energies in the exchange bridges containing Co(III) ions. The calculations showed a large value of the magnetization along the main magnetic axis (10.1 mu(B)), which is a combined effect of the ferromagnetic exchange interaction and negative magnetic anisotropy on the two marginal Co(II) sites. The lack of single-molecule magnet behavior in [Co(II)3Co(III)4(L)6(MeO)6] is explained by relatively large matrix elements of transverse magnetic moments between states of maximal magnetization of the ground Kramers doublet, evidenced by ab initio calculations, and the associated large tunneling rates between these states in the presence of dipolar transverse magnetic fields in the crystal.

The discovery that some metal coordination clusters may behave as single-molecule magnets (SMMs) [1][2][3][4] is currently stimulating abundant research in relation to potential applications in information processing and storage.[5] Indeed, SMMs are molecules that can be magnetized in a magnetic field and retain the magnetization when it is switched off. As a consequence, they may show hysteresis loops reminiscent of magnets.[2] In metal clusters, such behavior results from a strong magnetic ground state with large negative axial anisotropy (D < 0) [6,7] and induces two possible orientations (up and down), between which the magnetization can fluctuate. The fluctuation rate, also named relaxation, depends on the energy barrier U that separates the orientations. In the case of an ideal ground spin state S well separated from the excited states, U is equal to ÀD S 2 for an integer spin and ÀD(S 2 À 1 = 4 ) for a half-integer spin (D < 0). Therefore, the larger the D and S values are, the higher the barrier is and the longer the magnetization is retained. This barrier can be thermally overcome or shortcut by quantum tunneling of magnetization (QTM). [8] This tunneling trough the barrier contributes to accelerating the overall relaxation process. In practice, coexistence of the two processes leads to an experimental effective barrier U eff defined by an Arrhenius law: t = t 0 exp(U eff /k T).[9] One of the main goals of current research is to achieve long relaxation times t, which are crucial for information storage applications. [10,11] In this context, the use of lanthanide ions, such as Dy III and Tb III , has many advantages. Indeed, their large spins and pronounced spin-orbit coupling result in strong Ising-type magnetic anisotropy.[12] Recent reports have shown that even some of their mononuclear complexes may behave as SMMs. [13,14] During this work, a Dy III 3 trinuclear cluster was also reported to exhibit slow relaxation despite its near diamagnetic ground state.[15] Moreover, the combination of 3d and 4f transition-metal ions may increase the ground spin state through d-f magnetic interactions. [16][17][18][19][20] Lanthanides have high coordination numbers and geometries, which may be useful for engineering large polynuclear clusters, and their potential optical properties are of interest to prospective multifunctional materials. [21,22] With this in mind, and as part of our work on polynuclear metal complexes, [23,24] we chose the Schiff base 1,1,1-trifluoro-7-hydroxy-4-methyl-5-azahept-3-en-2-one (LH 2 , Scheme 1 (Figure 1 c) (Figure 1 b). This behavior affords distorted {Cu 2 L 2 Dy 2 (OH) 2 } cubane-like moieties in a similar way to homometallic cubane-like compounds. [25,26] The cationic entity can also be described as resulting from condensation of three distorted {Dy 2 Cu 2 O 4 } cubane-like moieties that share the Dy III ions in a triangular fashion. The structural features of the {Cu 2 L 2 Dy 2 (OH) 2 } moieties (Figure 1 c)