Molecules based on M(V) (M = Mo, W) and Ni(II) ions: a new class of trigonal bipyramidal cluster and confirmation of SMM behavior for the pentadecanuclear molecule {Ni-II[Ni-II(tmphen)(MeOH)](6)[Ni(H2O)(3)](2)[mu-CN](30)[W-V(CN)(3)](6)}
Résumé
The preparation, single crystal X-ray crystallography, and magnetic properties are reported for four new clusters based on [M'(V)(CN)(8)](3-) octacyanometallates (M' = Mo, W). Reactions of [M-'V(CN)(8)](3-) with mononuclear Ni-II ions in the presence of the tmphen blocking ligand (tmphen = 3,4,7,8-tetramethyl-1, 10-phenanthroline) in a 2 : 3 : 6 ratio, respectively, lead to the formation of the trigonal bipyramidal clusters [Ni-II(tmphen)(2)](3)[M'(V)(CN)(8)](2). Analogous reactions with the same starting materials performed in a 2 : 3 : 2 ratio, respectively, produce pentadecanuclear clusters of the type {Ni-II[Ni-II(tmphen)-(MeOH)](6)[Ni(H2O)(3)](2)[mu-CN](30)[W-V(CN)(3)](6)}. The W2Ni3 (1) and Mo2Ni3(2) pentanuclear clusters and the W6Ni9 (3) and Mo6Ni9 (4) pentadecanuclear molecules are isostructural to each other and crystallize in the space groups P2(1)/c and R (3) over bar respectively. Magnetic measurements indicate that the ground states for the trigonal bipyamidal clusters are S = 4 as a consequence of ferromagnetic coupling with J(W-Ni) = 9.5 cm(-1), J(Mo-Ni) = 10 cm(-1). The pentadecanuclear clusters exhibit ferromagnetic coupling as well, which leads to S = 12 ground states (J(W-Ni) = 12 cm(-1), J(Mo-Ni) = 12.2 cm(-1)). Reduced magnetization studies on the W-Ni analogues support the conclusion that they exhibit a negative axial anisotropy term; the fits give D values of -0.24 cm(-1) for the W2Ni3 cluster and D = -0.04 cm(-1) for the W6Ni9 cluster. AC susceptibility measurements indicate the beginning of an out-of-phase signal for the W2Ni3 and the W6Ni9 compounds, but detailed low temperature studies on small crystals by the microSQUID technique indicate that only the pentadecanuclear cluster exhibits hysteresis in accord with SMM behavior. Neither Mo cluster reveals any evidence for slow paramagnetic relaxation at low temperatures.