INTRAMOLECULAR FERROMAGNETISM IN A NOVEL HEXANUCLEAR (MU-HYDROXO)(MU-CARBONATO)COPPER(II) BIPYRIDINE COMPLEX - STRUCTURE OF [CU-6(BPY)(10)(MU-CO3)(2)(MU-OH)(2)](CLO4)(6)CENTER-DOT-4H(2)O AND OF A DINUCLEAR MU-CARBONATO COMPLEX, [CU-2(BPY)(4)(MU-CO3)](PF6)(2)CENTER-DOT-2DMF

A novel hexanuclear copper(II) complex, [Cu-6(bpy) (10)(mu-CO3)(2)(mu-OH)(2)] (ClO4)(6) . 4H(2)O, (1, bpy = 2,2'-bipyridine), has the following crystallographic data: triclinic, space group P ($) over bar 1, a 18.330(5) Angstrom, b = 13.497(3) Angstrom, c = 13.790(8) Angstrom, alpha = 119.26(2)degrees, beta = 98.96(4)degrees, gamma = 104.19(2)degrees, Z = 1, V = 2728(2) Angstrom(3). The structure of 1 is made up of discrete centrosymmetric hexanuclear units in which a central planar bis(mu-hydroxo)copper(II) bipyridine moiety contains two cis-Cu(bpy)(2) fragments attached via carbonate bridging groups, trans-axially, to the central Cu atoms. Viewed along the central plane, the bpy rings are arranged in a parallel fashion due to pi-pi interactions. Detailed magnetic studies on 1, over the temperature range 2-300 K and field range 0.1-3.0 T, show that ferromagnetic coupling of the spins on each Cu(II) center occurs, leading to a S = 3 ground state. Application of an appropriate Heisenberg spin-coupling model led to a good fit of the susceptibility and isofield magnetization data with a J value for the central Cu-2(OH)(2) moiety of +24 cm(-1) and J values, for the three identifiable Cu-2-(mu-CO3) pathways, in the range +2.75 to +3.3 cm(-1). The corresponding zero-field spin-ladder of energy levels show that the low-lying S levels are within a few wavenumbers of the ground S = 3 level. The consequent M(S) Zeeman level population leads to a maximum in the moment versus temperature plot, at ca. 4 K, which varies in size and shape as a function of applied field. A second complex, 2, was formed on recrystallization of the PF6- salt of 1 from dimethylformamide. It is binuclear, with formula (CU2(bpy)(4)(mu-CO3)](PF6)(2) . 2DMF and has the following crystallographic data: 2, triclinic, space group P ($) over bar 1, a = 9.461(1) Angstrom, b = 11.282(2) Angstrom, c = 12.742(2) Angstrom, alpha = 99.78(2)degrees, beta = 96.83(1)degrees, gamma = 97.19(1)degrees, Z = 1, V = 1316.1(4) Angstrom(3). The structure of 2 reveals a unique anti-anti mode of carbonate bridging and a separation of 5.26 Angstrom between the two square-pyramidally coordinated Cu atoms. The bridging CO32- group and the DMF solvate molecules show structural disorder. In contrast to 1, this complex displays antiferromagnetic coupling with a J value of -70.2 cm(-1) (i.e., singlet-triplet splitting of 140.4 cm(-1)).