STRUCTURAL AND MAGNETIC VERSATILITY OF THE MN(II)/[CU(X4OBBZ)]2- SYSTEM (X4OBBZ = OXAMIDO-N,N'-BIS(3,5-DIHALOGENOBENZOATO, X = CL, BR) - CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF MNCU(CL4OBBZ)(H2O)5 AND MNCU(BR4OBBZ)(H2O)3.2.5H2O

The two compounds MnCu(Cl4obbz)(H2O)5 (1) and MnCu(Br4obbz)(H2O)3.2.5H2O (2) have been synthesized. Cl4obbz stands for oxamido-N,N'-bis(3,5-dichlorobenzoato) and Br4obbz for oxamido-N,N'-bis(3,5-dibromobenzoato). Their crystal structures have been determined. The tetrachloro derivative crystallizes in the monoclinic system, space group P2(1)/n, with a = 10.351 (4) angstrom, b = 19.130 (4) angstrom, c = 11.225 (2) angstrom, beta = 97.82 (2)-degrees with Z = 4 MnCu units. The structure consists of binuclear units with Mn(II) and Cu(II) ions bridged by a carboxylate group (Mn...Cu = 5.195 (1) angstrom). The bimetallic units are connected through Cu...Cl contacts of 2.930 and 3.422 angstrom. The configuration around the carboxylate bridge is of the anti-syn type. The tetrabromo derivative crystallizes in the monoclinic system, space group P2(1)/c with a = 9.294 (2) angstrom, b = 17.981 (4) angstrom, c = 15.021 (2) angstrom, beta = 100.00 (2)-degrees, and Z = 4 MnCu units. The structure consists of alternating bimetallic chains with Mn(II) and Cu(II) ions bridged by oxamide (Mn...Cu = 5.341 (2) angstrom) and carboxylate (Mn...Cu = 5.394 (2) angstrom) groups. The chains are connected through Cu...Br contacts of 2.971 angstrom. The configuration around the carboxylate bridge is again of the anti-syn type. The magnetic properties of both compounds have been investigated in the 2-300 K temperature range. For 1, the molar magnetic susceptibility chi-M follows the Curie law expected for uncoupled Mn(II) and Cu(II) ions. For 2, chi-M varies as expected for a Mn(II)-Cu(II) pair with a quintet-septet energy gap equal to 3J = 87.0 cm-1. This rather strong antiferromagnetic interaction occurs through the oxamide bridge. For both compounds, the interaction through the carboxylate bridge is negligible owing to the anti-syn configuration around this bridge. The EPR spectra reveal long-range interactions not detectable through magnetic susceptibility measurements. The structural and magnetic differences between the two compounds are discussed.