Condensation of a nickel tetranuclear cubane into a heptanuclear single-molecule magnet.

A tetranuclear complex, [Ni(4)], with a cubane-like structure synthesized from hexafluoroacetylacetone gives, after drying at high temperature and treatment with pyridine, a heptanuclear nickel(II) complex, [Ni(7)]. The crystal structures of both compounds have been determined by single-crystal X-ray diffraction. Their magnetic properties have been studied by SQUID and µ-SQUID magnetometry as well as by high-frequency EPR spectroscopy (HF-EPR). For [Ni(4)], the temperature dependence of the magnetic susceptibility can be fitted by taking into account strong Ni···Ni ferromagnetic interactions which lead to an S = 4 ground-state spin, in good agreement with the HF-EPR study. For [Ni(7)], the temperature dependence of the magnetic susceptibility shows that the Ni···Ni ferromagnetic interactions are kept within the metal core. However, it was not possible to fit this with a clear set of parameters, and the ground-state spin was undetermined. The field dependence of the magnetization indicates an S = 7 ground-state spin at high field. In contrast, the temperature dependence of the magnetic susceptibility indicates a ground-state spin of S = 6 or even S = 5. These results agree with complicated high-frequency EPR spectra which have been ascribed to the superposition of signals from the ground spin multiplet and from an excited spin multiplet very close in energy, with the excited state having a larger S value than the ground state. Very low temperature studies show that only the heptanuclear complex behaves as a single-molecule magnet.

Structure, magnetic properties, polarized neutron diffraction, and theoretical study of a copper(II) cubane.

The paper reports the synthesis, X-ray and neutron diffraction crystal structures, magnetic properties, high field-high frequency EPR (HF-EPR), spin density and theoretical description of the tetranuclear CuII complex [Cu4L4] with cubane-like structure (LH2=1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one). The simulation of the magnetic behavior gives a predominant ferromagnetic interaction J1 (+30.5 cm(-1)) and a weak antiferromagnetic interaction J2 (-5.5 cm(-1)), which correspond to short and long Cu-Cu distances, respectively, as evidence from the crystal structure [see formulate in text]. It is in agreement with DFT calculations and with the saturation magnetization value of an S=2 ground spin state. HF-EPR measurements at low temperatures (5 to 30 K) provide evidence for a negative axial zero-field splitting parameter D (-0.25+/-0.01 cm(-1)) plus a small rhombic term E (0.025+/-0.001 cm(-1), E/D = 0.1). The experimental spin distribution from polarized neutron diffraction is mainly located in the basal plane of the CuII ion with a distortion of yz-type for one CuII ion. Delocalization on the ligand (L) is observed but to a smaller extent than expected from DFT calculations.

Cubane variations: syntheses, structures, and magnetic property analyses of lanthanide(III)-copper(II) architectures with controlled nuclearities.

Starting from a molecular cubane [Cu(4)L(4)] (1, with LH(2) = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one), we successfully replaced one and then two copper(II) ions of the cubane core by lanthanide ions to elaborate new families of 3d-4f complexes. Here, we report the syntheses, crystal structures, magnetic properties, and theoretical description of the tetranuclear copper(II) complex [Cu(4)L(4)] (1, [Cu(4)]) together with original yttrium(III) and gadolinium(III) heterometallic derivatives: [YCu(3)L(3)(hfac)(3)](-) (2, [YCu(3)]); [GdCu(3)L(3)(hfac)(3)](-) (3, [GdCu(3)]); [Y(3)Cu(6)L(6)(OH)(6)(MeOH)(6)(H(2)O)(6)](3+) (4, [Y(3)Cu(6)]); [Gd(3)Cu(6)L(6)(OH)(6)(MeOH)(6)(H(2)O)(6)](3+) (5, [Gd(3)Cu(6)]). 1 crystallizes in the P2(1)/c monoclinic space group with a cubane-like structure and shows ferromagnetic behavior. 2 and 3 crystallize in the P triclinic space group and exhibit also cubane-like structures in which one copper(II) ion of the cubane core is substituted by one lanthanide ion. The magneto-structural correlations carried out on the yttrium(III) derivative reveal a spin frustration between the copper(II) ions that is retained in the gadolinium(III) analog (J approximately -30 cm(-1)). 4 and 5 crystallize in the C2/c monoclinic space group and result from the condensation of three {Ln(2)Cu(2)} cubane-like moieties giving rise to nonanuclear architectures. On the basis of the theoretical investigations, it is suggested that the electronic distribution on the yttrium(III) ion may influence the magnetic interaction between the copper(II) pairs. Indeed, the sign and magnitude of the Cu-Cu interaction extracted from 4 do not seem to be retained in 5. Thus, the introduction of lanthanide ions is likely to influence the nature of the Cu-Cu magnetic interactions in addition to their magnetic contribution. This work should contribute to improve the SMM synthesis strategy on the basis of the association of 3d and 4f ions.