RESUMEN
The tetranuclear [Ni4(pypentO)(pym)(mu 3-OH)2(mu-Oac)2(NCS)2(OH2)] cubane-like complex has been prepared, and its structure and magnetic properties have been studied (pypentO and pym are the deprotonated forms of 1,5-bis[(2-pyridylmethyl)amino]pentane-3-ol and 2-pyridylmethanol, respectively). The X-ray diffraction analysis of this novel nickel complex (C61H74N14O25.5S4Ni8, monoclinic, P2(1), a = 13.9375(14) A, b = 20.6604(18) A, c = 16.6684(19) A, beta = 110.619(12) degrees, Z = 2) showed a Ni4O4 cubane arrangement of four nickel atoms, four mu 3-O bridging ligands (one pypentO, one pym, and two OH-), two syn-syn bridging acetates, and three terminal monodentate ligands (two NCS- and one OH2). In this low-symmetry elongated cubane, the four Ni-Ni long distances (3.18 A) correspond to the faces of the cube including two mu 3-OR bridges, and the two Ni-Ni short distances (2.94 A) correspond to the faces including two mu 3-OR and one acetate bridges. The temperature dependence of the magnetic susceptibility was fitted with J1 = -3.09 cm-1, J2 = 15.0 cm-1, J3 = 6.72 cm-1, and g = 2.27. The differences in sign among the J1, J2, and J3 superexchange interactions is in good agreement with the different types of faces present in this Ni4O4 cubane core. The two faces of the cube, including two mu 3-OR bridges associated with one acetate bridge, exhibit ferromagnetic interactions, while the four faces which include only mu 3-OR bridges exhibit antiferromagnetic interactions. The very small zero field splitting may be attributed to the fact that the ground state is diamagnetic. The nature of the ground state is confirmed by the good simulation of the magnetization curves at 2 and 5 K (diagonalization of the full matrix taking into account all energy levels obtained with the parameter set resulting from the fit of the susceptibility curve). The large differences in J values resulting from small differences in Ni-O-Ni angles in this Ni4O4 core of very low symmetry reflect a quite strong magnetostructural correlation.