Syntheses, crystal structures, ferroelectrics and magnetic properties of [R-BzPy]₂[Cu(mnt)₂] complexes ([R-BzPy]⁺ = 1-(4′-R-benzyl)-pyridinium, R = NO₂ or Br; mnt^2- = maleonitriledithiolate)

The structures of two ion-pair complexes, which consist of 1-(4′-R-benzyl)pyridinium (R = NO₂ (1), Br (2)) and bis(maleonitriledithiolato)cuprate(II), have been determined by X-ray single crystal analyses, and the results show the stacking patterns in 1 and 2 are different from each other although the difference between the molecular structures of these complexes is only the substituent of the benzene ring, which may arise from the difference of interactions between cations and anions in these two complexes. Complex 1 crystallizes in the non-centrosymmetric space group P1, and dielectric measurements shows ferroelectric behavior with a small hysteresis loop. Quantum chemistry calculation further indicate the electric dipole in a unit cell is 56.5509 Debye, and DSC analysis reveal a T_c of 377 K, from the paraelectric to the ferroelectric phase. The magnetic susceptibilities of these two complexes measured in the temperature range 1.8-260 K show an antiferromagnetic coupling feature between neighboring Cu(II) ions with 0= -0.580 K for 1 and -2.167 K for 2. The polycrystalline EPR spectra of the two complexes at room temperature are different from each other, characterized axial g tensors are g_|| of 2.09, g_⊥ of 2.02 for 1, and g_|| = 2.05, g_⊥ = 2.03, A_|| = 40.6 × 10^-4 cm^-1 for 2.