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(o pba)(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-pheny lenebis(oxamato). The crystal structure of 1 has been determined. 1 cr ystallizes in the monoclinic system, space group P2(1)/n. The lattice parameters are a = 16.969(7) angstrom, b = 8.367(1) angstrom, c = 18.2 58(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 env ironment, and the manganese atom is in an octahedral environment with a cis coordination unprecedented for this type of compound. 2 is obtai ned by heating 1 under vacuum at 140-degrees-C. 3 results from our att empt to cross-link the chains with the [Cu(opba)]2- bisbidentate ligan d and, therefore, to increase the dimensionality of the system. 4 is o btained by heating 3 under vacuum at 170-degrees-C. The probable struc ture of 3 (and 4) is that of a Mn(II)Cu(II) two-dimensional network, i n which the manganese atom is surrounded by three Cu(opba) groups inst ead of two in the chain compound. The magnetic properties of the four compounds have been investigated in a thorough manner. 1 shows the cha racteristic behavior of a one-dimensional ferrimagnet, with a Mn(II)-C u(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 spontaneou s magnetization below T(c) = 6.5 K. 3 shows an abrupt magnetic transit ion at T(c) = 15 K, with a spontaneous magnetization below T(c). The m agnetization versus magnetic field curve for 3 indicates that in the m agnetically ordered phase all the manganese local spins are aligned al ong a direction and the copper local spins along the opposite directio n. The saturation magnetization obtained within a field of a few tens of oersteds is 7.1 Nbeta. Finally, when the DMSO and water molecules o f 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 f rom 1 to 3 and 4, which suggests that the three-dimensional ordering i n 2-4 is dominated by the exchange interaction rather than the dipolar effect. The relationship between structures and magnetic properties f or compounds 1-4 are discussed in detail.