Two spin-peierls-like compounds exhibiting divergent structural features, lattice compression, and expansion in the low-temperature phase

Two quasi-one-dimensional (quasi-1D) compounds, [4′-CH ₃Bz-4-RPy][Ni(mnt)₂] (mnt² = maleonitriledithi-olate), where 4′-CH₃Bz-4-RPy⁺ = 1-(4′-methylbenzyl)pyridinium (denoted as compound 1) and 1-(4′-methylbenzyl)-4-aminopyridinium (denoted as compound 2), show a spin-Peierls-like transition with T_c ≈ 182 K for 1 and T _c ≈ 155 K for 2. The enthalpy changes for the transition are estimated to be ΔH =316.6 J·mol^-1 for 1 and 1082.1 J·mol^-1 for 2. From fits to the magnetic susceptibility, the magnetic exchange constants in the gapless state are calculated to be J = 166(2) K with g = 2.020(23) for 1 versus J = 42(0) K with g = 2.056(5) for 2. In the high-temperature (HT) phase, 1 and 2 are isostructural and crystallize in the monoclinic space group P2₁/c. The nonmagnetic cations and paramagnetic anions form segregated columns with regular anionic and cationic stacks. In the low-temperature (LT) phase, the crystals of the two compounds undergo a transformation to the triclinic space group P-1, and both anionic and cationic stacks dimerize. In the transformation from the HT to LT phases, the two compounds exhibit divergent structural features, with lattice compression for 1 but lattice expansion for 2, due to intermolecular slippage. Combined with our previous studies, it is also noted that the transition temperature, T _c, is qualitatively related to the cell volume in the HT phase for the series of compounds [1-(4′-R-benzylpyridinium][Ni(mnt)₂] (where R represents the substituent). When there is a single substituent in the para position of benzene, giving a larger cell volume, the transition temperature increases.