Slow Relaxation of the Magnetization in Oximato-Bridged Heterobimetallic Copper(II)-Manganese(III) Chains

The use of the oximato-containing copper(II) complexes, [Cu(Hdeg)(2)] (H(2)deg = diethylglyoxime), [Cu(Hmeg)(2)] (H(2)meg = methylethylglyoxime) and [Cu(Hdmg)(2)] (H(2)dmg = dimethylglyoxime), as ligands toward manganese(II) acetate in methanol afforded the heterobimetallic compounds of formula [MnCu(deg)(2)(CH3COO)(H2O)(2)] (1), [MnCu(meg)(2)(CH3COO)(H2O)(2)] (2) and [MnCu(dmg)(2)(CH3COO)(H2O)(2)] (3) where the starting manganese(II) ion was oxidized to manganese(III) by air. In the lack of single crystals suitable for X-ray diffraction analysis. X-ray absorption techniques (EXAFS and XANES) at 40 K were used for the structural characterization of 1-3. The analysis of the X-ray absorption data reveals that 1-3 are oximato-bridged Cu-II-Mn-III bimetallic chains with elongated octahedral CuN4O2 and MnO6 surroundings. The study of the magnetic properties of 1-3 reveals a metamagnetic-like behavior due to the coexistence of strong intrachain ferromagnetic couplings [J = +52.5 (1), +49.1 (2) and +52.1 cm(-1) (3)] and weak interchain antiferromagnetic interactions [j = -0.070(1), -0.075 (2) and -0.078 cm(-1) (3)]. The strict orthogonality between the magnetic orbital of the copper(II) ion and those of the manganese(III) center, together with the well known ability of the oximato group to mediate magnetic interactions when acting as a bridge between two metal ions, account for the relatively strong intrachain ferromagnetic coupling in 1-3. Alternating current susceptibility measurements for 1-3 in the low temperature range under zero dc magnetic field reveal a frequency-dependence of the out-of-phase ac signal as expected for single chain magnets.