Dramatic Modifications of Magnetic Properties through Dehydration−Rehydration Processes of the Molecular Magnetic Sponges CoCu(obbz)(H₂O)₄·2H₂O and CoCu(obze)(H₂O)₄·2H₂O, with obbz = <i>N</i>,<i>N</i>‘-Bis(2-carboxyphenyl)oxamido and obze = <i>N</i>-(2-Carboxyphenyl)-<i>N</i>‘-(carboxymethyl)oxamido

Abstract

The two compounds CoCu(obbz)(H2O)4·2H2O (I) and CoCu(obze)(H2O)4·2H2O (II) have been synthesized, and their crystal structures have been determined at room temperature. Both compounds crystallize in the monoclinic system, space group P21/n. The lattice parameters are a = 7.8190(6) Å, b = 12.534(2) Å, c = 20.2770(10) Å, β = 97.306(6)°, and Z = 4 for CoCu(obbz)(H2O)4·2H2O and a = 17.966(5) Å, b = 6.907(1) Å, c = 13.678(3) Å, β = 97.69(1)°, and Z = 4 for CoCu(obze)(H2O)4·2H2O. For both compounds, the structure consists of isolated Co2+Cu2+ pairs (in which the metal ions are bridged by an oxamide group) and noncoordinated water molecules. A thermogravimetric analysis has been carried out. Three water molecules are removed at a temperature T2 (96 °C for I and 120 °C for II), and five water molecules are removed at a temperature T3 (205 °C for I and 190 °C for II). When cooling down, the two compounds reabsorb water slowly and then return to the initial state. The dehydration−rehydration process is totally reversible, provided that the temperature does not exceed 275 °C. The magnetic properties of the starting materials, as well as the materials containing three water molecules and then one, have been investigated, both in the dc and ac modes. The two starting materials, CoCu(obbz)(H2O)4·2H2O and CoCu(obze)(H2O)4·2H2O, possess a nonmagnetic ground state. The compounds CoCu(obbz)(H2O)3 and CoCu(obze)(H2O)3 behave as one-dimensional ferrimagnets without long-range magnetic ordering down to 2 K. The compounds CoCu(obbz)(H2O) and CoCu(obze)(H2O), finally, also behave as ferrimagnets, but with a long-range magnetic ordering occurring at 25 and 20 K, respectively. Upon rehydration, the compounds return to the initial state with a nonmagnetic ground state. Infrared and Raman vibrational spectroscopies have been used to obtain further insights into the stuctural modifications accompanying the removal and the uptake of water molecules.