A crystalline phase transition and optical properties in CoIICuII oxamato-bridged ferrimagnetic chain

The compound [CoCu(opba)(DMSO)(3)] (1) [opba = orthophenylenebis(oxamato)] has been synthesized and characterized. Its crystal structure has been analyzed by X-ray diffraction techniques at 100 and 298 K. A structural phase-transition has been detected at around 150 K. An orthorhombic crystalline system is found at both temperatures, with very similar unit-cell dimensions. At room temperature 1 crystallizes in the Pnam space group (alpha-1 phase), with a = 7.6712(2), b = 14.8003(3), c = 21.0028(5) angstrom, and Z = 4, whereas at low temperature it crystallizes in the Pna2(1) space group (beta phase), with a = 7.3530(2), b = 14.5928(4), c = 21.0510(7) angstrom, and Z = 4. Both crystalline phases consist of linearly ordered bimetallic chains with the [Cu(opba)](2-) units tied by Co-II ions to form a one-dimensional system. The DMSO molecules in a-1, which are coordinated to either Cu-II or Co-II, are disordered. At low temperature, a small reorganization of the Cu-II and Coli environments is observed. The origin of this phase transition, which is completely reversible, is the modification of the crystalline packing with the temperature. Linear birefringence measurements were done on single crystals in the 100-300 K temperature range. Around 150 K, the linear birefringence curve shows an inflexion that is interpreted as being related to the conversion of alpha-1 into beta-1. Both dc and ac magnetic measurements were performed on the polycrystalline sample. The results reveal a one-dimensional ferrimagnetic behavior. Single crystal optical characterization at room temperature shows that 1 presents a very strong dichroism superposed on the linear birefringence. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).