Observation of a magnetic phase transition but absence of an electrical response in a new two-dimensional mixed-valence nickel-bis-dithiolene molecular crystal

A new mixed-valence molecular crystal, [C₃-Apy][Ni(dmit)₂]₃(1) (dmit^2- = 2-thioxo-1,3-dithiole-4,5-dithiolate and C₃-Apy⁺ = 4-amino-1-propylpyridinium) was synthesized utilizing a facile solution process and a small size of counter-cation. This is distinct from the general strategy to obtain mixed-valence of [Ni(dmit)₂] molecular crystal in which the large size counter-cation is desirable and the electrocrystallization method was used. The mixed-valence molecular crystal shows a magnetic phase transition around 77 K with a ca. 6 K thermal hysteresis loop. The crystal structure analysis at ambient temperature, the electronic band structure calculation based on the crystal structure, the variable temperature infrared spectra in 6-293 K, and the conductance measurements in 4-295 K were further performed, and the results revealed that 1 is a semiconductor and the magnetic phase transition is probably related to the electronic structure change in the {[Ni(dmit)₂]₃}^1-.