Observation of Magnetic Bistability in Polymorphs of the [Ni(dmit)₂]⁻ Complexes

Four ion-pair complexes of [Ni(dmit)2]− with [NO2bzql]+ have been obtained, which belong to two kinds of polymorph forms, [NO2bzql][Ni(dmit)2] (1α and 1β) and [NO2bzql][Ni(dmit)2]·CH3COCH3 (2α and 2β) (where dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate and [NO2bzql]+ = 1-(4-nitrobenzyl)quinolinium). Though 1α, 2α, and 2β all show anionic dimerization structures at room temperature, they have different anionic and cationic arrangement fashions, which give rise to different magnetic behaviors for these polymorphs or pseudo-polymorphs. Compounds 1α, 1β, and 2α exhibit magnetic bistabilities. In particular, 1α has a hysteretic loop at ∼55 K, while 2β does not display a spin transition in the 2−300 K range. On the basis of the crystal structure data of 2α in high- and low-temperature phases, the magnetic coupling feature within the [Ni(dmit)2]− spin dimer was explored with the broken-symmetry approach at the UBPW91/LANL2DZ level; combined with the experimental data and theoretical analyses, the relationship between the magnetic coupling nature and the stacking pattern of [Ni(dmit)2]− anions as well as the origin of the phase interconversion are discussed.