SPIN EXCHANGE COUPLING IN ASYMMETRIC HETERODINUCLEAR COMPLEXES CONTAINING THE MU-OXO-BIS(MU-ACETATO)DIMETAL CORE

Synthesis of asymmetric heterodinuclear complexes [LM1(mu-O)(mu-CH3CO2)2M2L']2+, where L represents 1,4,7-triazacyclononane, L' is 1,4,7-trimethyl-1,4,7-triazacyclononane, and M1 and M2 are two different trivalent first-row transition metals, has been achieved by the hydrolytic reaction of an equimolar mixture of LM1Cl3 and L'M2Cl3 in aqueous sodium acetate solution. The asymmetric homodinuclear species [LM(mu-O)(mu-CH3CO2)2ML']2+ (M = Mn(III) (1), Fe(III) (2)) have also been prepared. The following heterodinuclear species have been isolated as PF6- or ClO4- salts and characterized: FeIIIMnIII (3), FeIIICrIII (4), CrIIIMnIII (5), and CrIIIVIII (7). 4 is protonated in acidic solution affording the mu-hydroxo bridged complex [L'Cr(mu-OH)(mu-CH3CO2)2FeL](ClO4)3.2H2O (4H). Oxidation of 3 and 5 in aqueous solution with Na2S2Og gave [L'FeIII(mu-O)(mu-CH3CO2)2MnIVL](ClO4)3.H2O (8) and [L'CrIII(mu-O)(mu-CH3CO2)2MnIVL] (ClO4)3 (6). The crystal structures of the ClO4- salts of 3, 5, and 6 and the PF6 salt of 4 have been determined by single crystal X-ray crystallography: 3, monoclinic C2/c, a 23.929 (6) angstrom, b 19.036 (5) angstrom, c = 16.701 (3) angstrom, beta = 119.52 (1)-degrees, Z = 8; 4, orthorhombic Pbca, a = 18.951 (4) angstrom, b = 14.160 (5) angstrom, c = 24.619 (5) angstrom, Z = 8; 5, monoclinic P2(1)/c, a = 11.146 (2) angstrom, b = 20.975 (4) angstrom, c = 13.563 (3) angstrom, beta = 99.71 (2)-degrees, Z = 4. 6, monoclinic P2(1)/m, a = 11.332 (3) angstrom, b = 9.715 (1) angstrom, c = 16.180 (4) angstrom, beta = 110.16 (1)-degrees, Z = 2. Bulk magnetic properties of all compounds have been studied in the temperature range 4.2-298 K. It has been found that the spins of the two paramagnetic metal ions are either intramolecularly antiferromagnetically (2, 3, 4, 4H, 6, 8) or ferromagnetically coupled (1, 5, 7). An analysis of the interacting magnetic orbitals in complexes containing the mu-oxo-bis(mu-carboxylato)dimetal core is presented.