DRAMATIC MODIFICATIONS OF MAGNETIC-PROPERTIES THROUGH DEHYDRATION-REHYDRATION PROCESSES OF THE MOLECULAR MAGNETIC SPONGES COCU(OBBZ)(H2O)(4)CENTER-DOT-2H(2)O AND COCU(OBZE)(H2O)(4)CENTER-DOT-2H(2)O, WITH OBBZ = N,N'-BIS(2-CARBOXYPHENYL)OXAMIDO AND OBZE = N-(2-CARBOXYPHENYL)-N'-(CARBOXYMETHYL)OXAMIDO

The two compounds CoCu(obbz)(H2O)(4) . 2H(2)O (I) and CoCu(obze)(H2O)( 4) . 2H(2)O (II) have been synthesized, and their crystal structures h ave been determined at room temperature. Both compounds crystallize in the monoclinic system, space group P2(1)/n. The lattice parameters ar e a = 7,8190(6) Angstrom, b = 12.534(2) Angstrom, c = 20.2770(10) Angs trom, beta = 97.306(6)degrees, and Z = 4 for CoCu(obbz)(H2O)(4) . 2H(2 )O and a = 17.966(5) Angstrom, b = 6.907(1) Angstrom, c = 13.678(3) An gstrom,beta = 97.69(1)degrees and Z = 4 for CoCu(obze)(H2O)(4) . 2H(2) O. For both compounds, the structure consists of isolated Co2+CU2+ pai rs (in which the metal ions are bridged by an oxamide group) and nonco ordinated water molecules. A thermogravimetric analysis has been carri ed out. Three water molecules are removed at a temperature T-2 (96 deg rees C for I and 120 degrees C for II), and five water molecules are r emoved at a temperature T-3 (205 degrees C for I and 190 degrees C for II). When cooling down, the two compounds reabsorb water slowly and t hen return to the initial state. The dehydration-rehydration process i s totally reversible, provided that the temperature does not exceed 27 5 degrees C. The magnetic properties of the starting materials, as wel l as the materials containing three water molecules and then one, have been investigated, both in the de and ac modes. The two starting mate rials, CoCu(obbz)(H2O)(4) . 2H(2)O and CoCu(obze)(H2O)(4) . 2H(2)O, po ssess a nonmagnetic ground state. The compounds CoCu(obbz)(H2O)(3) and CoCu(obze)(H2O)(3) behave as one-dimensional ferrimagnets without lon g-range magnetic ordering down to 2 K. The compounds CoCu(obbz)(H2O) a nd CoCu(obze)(H2O), finally, also behave as ferrimagnets, but with a l ong-range magnetic ordering occurring at 25 and 20 K, respectively. Up on rehydration, the compounds return to the initial state with a nonma gnetic ground state. Infrared and Raman vibrational spectroscopies hav e been used to obtain further insights into the structural modificatio ns accompanying the removal and the uptake of water molecules.