Unusual Magnetic Properties of One-Dimensional Molecule-Based Magnets Associated with a Structural Phase Transition

Three ion-pair complexes, [RbzPy]+[Ni(mnt)2]- (mnt2- = maleonitriledithiolate; [RbzPy]+ = 4-R-benzylpyridinium;
R = Br (1), Cl (2), and NO2 (3)), with unusual magnetic properties have been synthesized and characterized. The
crystal structures of 1 and 2 have been solved. The two complexes belong to the P21/c space group with Z = 4
and C20H11BrN5NiS4, a = 12.0744(17) Å, b = 26.369(4) Å, c = 7.440(3) Å, and β = 102.63(3)° for 1 and C20H11ClN5NiS4, a = 12.105(2) Å, b = 26.218(4) Å, c = 7.374(2) Å, and β = 102.55(2)° for 2, respectively. The
[Ni(mnt)2]- anions in 1−3 form uniformly spaced one-dimensional (1-D) magnetic chains of s = 1/2 at room
temperature. The temperature dependences of the susceptibility for 1−3 show that they undergo phase transitions.
All three complexes are paramagnetic in their high-temperature (abbreviation HT) phase and diamagnetic in the
low-temperature (abbreviation LT) phase because of strong dimerization along the stacking direction. The results
of thermal analysis (DSC) further confirm that the phase transition for 1 and 2 is first-order but maybe second-order for 3. The phenomena observed in this study are similar to those of the 1-D radical systems.