DIMENSIONALITY OF MN(II)CU(II) BIMETALLIC COMPOUNDS AND DESIGN OF MOLECULAR-BASED MAGNETS

Four compounds have been synthesized, the formulas of which are MnCu(opba)(DMSO)3 (1), MnCu-(opba).0.7DMSO (2), (NBu4(2Mn2[Cu(opba)]3.6DMSO.H2O (3), and (NBu4)2[Mn2Cu(opba)]3 (4), with opba standing for o-phenylenebis(oxamato). The crystal structure of 1 has been determined. 1 crystallizes in the monoclinic system, space group P2(1)/n. The lattice parameters are a = 16.969(7) angstrom, b = 8.367(1) angstrom, c = 18.258(7) angstrom, and Z = 4. The structure consists of well-separated Mn(II)Cu(II) zigzag chains. The copper atom is in a square pyramidal environment, and the manganese atom is in an octahedral environment with a cis coordination unprecedented for this type of compound. 2 is obtained by heating 1 under vacuum at 140-degrees-C. 3 results from our attempt to cross-link the chains with the [Cu(opba)]2- bisbidentate ligand and, therefore, to increase the dimensionality of the system. 4 is obtained by heating 3 under vacuum at 170-degrees-C. The probable structure of 3 (and 4) is that of a Mn(II)Cu(II) two-dimensional network, in which the manganese atom is surrounded by three Cu(opba) groups instead of two in the chain compound. The magnetic properties of the four compounds have been investigated in a thorough manner. 1 shows the characteristic behavior of a one-dimensional ferrimagnet, with a Mn(II)-Cu(II) interaction parameter J equal to -31.3 cm-1 (H = -J(MnCu)SIGMA(i)S(Mn,i).S(Cu,i)). 2 behaves as an amorphous magnet, with a spontaneous magnetization below T(c) = 6.5 K. 3 shows an abrupt magnetic transition at T(c) = 15 K, with a spontaneous magnetization below T(c). The magnetization versus magnetic field curve for 3 indicates that in the magnetically ordered phase all the manganese local spins are aligned along a direction and the copper local spins along the opposite direction. The saturation magnetization obtained within a field of a few tens of oersteds is 7.1 Nbeta. Finally, when the DMSO and water molecules of 3 are removed, affording 4, T(c) is shifted to 22 K. The EPR powder spectra of 1-4 have also been investigated. The line widths decrease from 1 to 3 and 4, which suggests that the three-dimensional ordering in 2-4 is dominated by the exchange interaction rather than the dipolar effect. The relationship between structures and magnetic properties for compounds 1-4 are discussed in detail.