Ionic Pair Complexes with Well-Separated Columnar Stack Structure Based on [Pt(mnt)₂]^- Ions Showing Unusual Magnetic Transition: Syntheses, Crystal Structures, and Magnetic Properties

Three ion pair complexes, [4-R-benzylpyridinium][bis(maleodinitriledithiolato)platinum(III)] (abbreviated as [RBzPy]-[Pt(mnt)₂]; R = Cl (1), Br (2), or NO₂ (3)), have been synthesized. The cations and anions stack into well-separated columns in the solid state, and the Pt(III) ions form a 1-D zigzag chain within a [Pt(mnt)₂]^- column through Pt⋯S, S⋯S, and Pt⋯S⋯Pt interactions. The chain is uniform in 1 and 2, while it alternates in 3. Unusual magnetic phase transitions from paramagnetism to diamagnetism were observed in these three complexes at ∼275 K for 1, ∼269 K for 2, and ∼184 K for 3. These phase transitions were also found in DSC measurements for 1 and 2. The overall magnetic behaviors for 1-3 indicate the presence of antiferromagnetic exchange interactions in the high-temperature phase and spin-gapped systems in the low-temperature phase. Below 50 K, 2 exhibits weak ferromagnetism. The spontaneous moments are nearly repressed by a field of 1.0 T. The crystal structure of 2 at 173 K reveals that there are two crystallographically independent [Pt(mnt)₂] ^- entries in an asymmetric unit. These two crystallographically independent [Pt(mnt)₂]^- entries satisfy the spin-canting condition, and the EPR spectra measured at room temperature exhibit anisotropic character. Therefore, the weak ferromagnetic behavior in the low-temperature region for 2 can be attributed to the spin-canting phenomenon.