Unusual spin gap transition observed in two new molecular magnets based on [Ni(mnt)2]− monoanion (mnt2−=maleonitriledithiolate)

Two new molecular magnets, [RBzPyN(CH3)2][Ni(mnt)2] [mnt2−=maleonitriledithiolate; [RBzPyN(CH3)2]+=1-(4′-R-benzyl)-4-dimethylaminopyridinium; R=CN(1), F(2)], with unusual magnetic properties have been prepared and characterized. Both 1 and 2 form a 3D network structure in which the [Ni(mnt)2]− anions form a 1D magnetic chain for 1 and a stepwise stack for 2 via weak π…π stacking interactions, C…C or C…N short interactions between the neighboring anions. Magnetic susceptibility measurements in the temperature range 1.8–300 K indicated that 1 and 2 show unusual spin gap transition around 30 K and 110 K, respectively. The transition for 1 and 2 is the second-order phase transition as determined by the capacity measurement.