STRUCTURE OF THE LINEAR TRINUCLEAR COMPLEX MNII3(CH3CO2)6(BPY)2 - DETERMINATION OF THE J ELECTRON-EXCHANGE PARAMETER THROUGH MAGNETIC-SUSCEPTIBILITY AND HIGH-FIELD MAGNETIZATION MEASUREMENTS

The new trinuclear cluster Mn(II)3(CH3CO2)6(bpy)2 has been synthesized, and its crystal structure was determined. It crystallizes in the triclinic system, space group P1BAR, with a = 12.849 (4) angstrom, b = 9.790 (3) angstrom, c = 8.187 (2) angstrom, alpha = 108.50 (2)-degrees, beta = 96.69 (2)-degrees, gamma = 106.41 (3)-degrees, Z = 1, and V = 912.5 (5) angstrom 3. The structure was solved and refined by using 2071 reflections with I greater-than-or-equal-to 2.5-sigma(I) collected in the range 2-degrees less-than-or-equal-to theta-less-than-or-equal-to 25-degrees. Final values of conventional indices R and R(w) were 5.1% and 4.8%, respectively. The structure consists of linear trinuclear molecules; each pair of manganese atoms are bridged by three acetato groups, two of them acting as bidentate bridges and a third one bridging by one oxygen atom only. The central manganese has a coordination sphere made of six oxygen atoms from six different acetato groups, and the terminal ones have a distorted environment of four oxygen atoms and two nitrogen atoms that come from a bipyridine molecule. The molar magnetic susceptibility chi-M has been investigated as a function of temperature T. The chi-M(T) product is equal to 11.87 cm3 mol-1 K at 285 K and decreases to 4.32 cm3 mol-1 K at 4.2 K. The data have been fitted with an expression for the magnetic susceptibility derived from the Van Vleck formula by using the solutions of the spin Hamiltonian H = -J(S1.S2 + S2.S3) - J13S1.S3. Taking J13 = 0, we found J = -4.4 cm-1. The ground state was found to be the \S2 = 5/2, S13 = 5, S = 5/2> state. Confirmation of the nature of the ground state and the energy ordering of the lowest excited states was obtained by measuring the magnetization as a function of the magnetic field up to 20 T at 4.2 K. The crossover between the \S2 = 5/2, S13 = 5, S = 5/2> and the \S2 = 5/2, S13 = 5, S = 7/2> states was observed. Those measurements constitute an independent determination of the J exchange parameter: the J value was found to be in perfect agreement with the one deduced from magnetic susceptibility measurements. A preliminary EPR study at room and helium temperatures is reported.