DESIGN OF METAL-COMPLEX MAGNETS - SYNTHESES AND MAGNETIC-PROPERTIES OF MIXED-METAL ASSEMBLIES (NBU4[MCR(OX)3])X (NBU4+=TETRA(NORMAL-BUTYL)AMMONIUM ION OX(2-)= OXALATE ION M=MN2+,FE2+,CO2+,NI2+,CU2+,ZN2+)

The reaction of K3[Cr(ox)3].3H2O, a metal(II) salt, and tetra(n-butyl)ammonium bromide in the molar ratio of 1:1:1.5 in water at room temperature afforded a series of mixed-metal assemblies with the formula {NBU4[MCr(OX)3]}x (M = Mn2+ (1), Fe2+ (2), Co2+ (3), Ni2+ (4), Cu2+ (5), Zn2+ (6)). These compounds are designed so as to assume a three-dimensional structure consisting of alternately arrayed Cr(III) and M(II) ions due to the D3 symmetry of the building block [Cr(ox)3]3-. The magnetic susceptibility data of 1-5 above ca. 20 K obey the Curie-Weiss law with positive Weiss constants, and upon lowering the temperature, the effective magnetic moments increase gradually in the 300-20 K range and abruptly below ca. 20 K. The temperature dependences of magnetizations studied under a weak applied field (1 G) showed a magnetic phase transition at T(c) = 6, 12, 10, 14, and 7 K for 1, 2, 3, 4, and 5, respectively. Field dependences of the magnetization up to 7.5 T have been studied at 4.2 K using a pulsed magnet and the saturation magnetization values per formula M(S) were compatible with M(S) = Ng-beta(S(Cr) + S(M)) (M(S)/N-beta = 7.74, 6.71, 5.94, 4.94, 3.83, and 2.79 for 1, 2, 3, 4, 5, and 6, respectively). Hysteresis loops have been obtained at 5 K, where the remnant magnetizations are 1500, 4100, 3000, 1200, and 1500 cm3 mol-1 G and the coercive fields are 20, 320, 80, 160, and 30 G for 1, 2, 3, 4, and 5, respectively.