Field-Induced SMM Behaviour of Tetranuclear Coordination Clusters with a Cubane [Ni4O4] Core

Two tetranuclear Ni(II) complexes: [Ni-4(HL1)(4)]center dot H2O (1) and [Ni-4(HL2)(4)]center dot 1.5 dmf (2) where dmf=dimethylformamide, H3L1=4-(tert-butyl)-2-(((2-hydroxy-5-nitrophenyl)imino)methyl)-6-(hydroxymethyl)phenol and H3L2=4-(tert-butyl)-2-(hydroxymethyl)-6-(((2-hydroxyphenyl)-imino)methyl)phenol, have been prepared and characterized by single crystal X-Ray diffraction, elemental analysis and FT-IR spectroscopy. The solid-state structures reveal the formation of highly symmetric and asymmetric [Ni4O4] cubane cores in complexes 1 and 2, respectively. Extensive magnetic studies show that both complexes present ferromagnetic exchange interactions between the Ni(II) ions within the cubane core with g=2.113(3), J(1)=-7.89(8) cm(-1), J(2)=13.3(1) cm(-1) and vertical bar D vertical bar=11.3(4) cm(-1) (for 1) and g=2.206(4), J(1)=1.0(1) cm(-1), J(2)=7.8(1) cm(-1) and vertical bar D vertical bar=8.7(2) cm(-1) (for 2). The large anisotropy, high ground spin state (arising from the ferromagnetic coupling) and the good isolation of the clusters provided by the Schiff base ligands, give rise to the first examples of field-induced single-molecule magnets (FI-SMM) in Ni4O4 clusters and to the highest energy barrier reported to date in a Ni4O4 cluster.