Structural characterisation of [Cu 2(bptd) (H 2O) Cl 4] and [Ni 2(bptd) 2(H 2O) 4] Cl 4·3H 2O; evidence of null intramolecular exchanges by EPR and magnetic measurements

Abstract

Using the 2,5-bis(2-pyridyl)-1,3,4-thiadiazole (bptd), we recently prepared [Cu 2(bptd) (H 2O) Cl 4] and [Ni 2(bptd) 2 (H 2O) 4] Cl 4, 3H 2O in which the magnetic centres are connected through one diazine+one chloro and two diazine ligand bridges, respectively. These two compounds are the first examples that show null intramolecular magnetic interactions despite M–M distances close to 3.7 Å within perfectly planar edifices: Down to T t =125 K , [Cu 2(bptd)Cl 4(H 2O)] is paramagnetic while, below T t, half of the Cu 2+ions interact, leading to residual paramagnetism of one Cu 2+/f.u. Magnetic susceptibility measurements, EPR and pulsed EPR study indicate the original intermolecular nature of AF exchanges. [Ni 2(bptd) 2(H 2O) 4]Cl 4·3H 2O susceptibility obeys a Curie-law involving pure paramagnetism. Moreover, its EPR spectrum can be interpreted on the basis of virtual S=1 monomers. Below 70 K, Zero Field Splitting ( D∼275 G) due to dipolar interactions without magnetic exchanges could be responsible for the LT spectra splitting. For both compounds, the thia role is suggested as partially responsible for the null-in-plane magnetic exchanges.