Stack pattern of the countercation-modulating magnetic property of low-dimensional [Pt(mnt)₂]^- monoanion spin systems

Three [1-N-(4′-R-benzyl)-4-aminopyridinium][Pt(mnt)₂] compounds were structurally and magnetically characterized, where the substituent was attached to the para-position of the phenyl ring (R = CN (1), Cl (2), and H (3); mnt^2- = maleonitriledithiolate). 1 and 2 crystallized in the monoclinic space group P2(1)/c, with the cations and anions forming segregated columnar stacks. Their structural differences involved two aspects: (1) both anion and cation stacks were regular in 1 and irregular in 2; (2) the neighboring cations were arranged in the boat-type pattern in 1, whereas these cations were in the chair-type pattern in 2 within the cation stack. 3 belonged to the triclinic space group P1̄, where the anions were assembled into the stack with a tetrameric [Pt(mnt)₂]^- subunit, but the cations did not form the columnar stack. Magnetic measurements disclosed that a spin-Peierls-type transition occurred around 240 K for 1, whereas a long-range, antiferromagnetic ordering took place at about 5.8 K, and a metamagnetic phenomenon was observed with H_C ≈ 1000 Oe for 2; 3 showed very strong antiferromagnetic interactions with diamagnetism in the temperature range 5-300 K. Combined with our previous studies, the correlation between the stacking pattern of benzylpyridinium derivatives in a cation stack and the spin-Peierls-type transition is discussed for the series of quasi-1-D [M(mnt)₂]^- (M = Ni, Pd and Pt) compounds.