Cubane Variations:  Syntheses, Structures, and Magnetic Property Analyses of Lanthanide(III)−Copper(II) Architectures with Controlled Nuclearities^‖

Starting from a molecular cubane [Cu₄L₄] (1, with LH₂ = 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 [Cu₄L₄] (1, [Cu₄]) together with original yttrium(III) and gadolinium(III) heterometallic derivatives:  [YCu₃L₃(hfac)₃]^- (2, [YCu₃]); [GdCu₃L₃(hfac)₃]^- (3, [GdCu₃]); [Y₃Cu₆L₆(OH)₆(MeOH)₆(H₂O)₆]³⁺ (4, [Y₃Cu₆]); [Gd₃Cu₆L₆(OH)₆(MeOH)₆(H₂O)₆]³⁺ (5, [Gd₃Cu₆]). 1 crystallizes in the P2₁/c monoclinic space group with a cubane-like structure and shows ferromagnetic behavior. 2 and 3 crystallize in the P1̄ 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 ∼ −30 cm^-1). 4 and 5 crystallize in the C2/c monoclinic space group and result from the condensation of three {Ln₂Cu₂} 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.