Two Spin-Peierls-like Compounds Exhibiting Divergent Structural Features, Lattice Compression, and Expansion in the Low-Temperature Phase

Two quasi-one-dimensional (quasi-ID) compounds, [4'-CH(3)Bz-4-RPy][Ni(mnt)(2)] (mnt(2-) = maleonitriledithiolate), where 4'-CH(3)Bz-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 approximate to 182 K for 1 and T-c approximate to 155 K for 2. The enthalpy changes for the transition are estimated to be Delta H = 316.6 J center dot mol(-1) for 1 and 1082.1 J center dot 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 P2(1)/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 I 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.