Modulation of the spin transition in 2D Hofmann frameworks via π⋯π stacking between the axial 2,5-dipyridyl-1,3,4-oxadiazoles

The π⋯π interactions in spin crossover (SCO) complexes can be used as an important means to fine-tune the spin transition behaviors. In this paper, three novel isostructural 2D Hofmann-type frameworks, [Fe(ppo)₂M(CN)₄] (1, M = Pt²⁺; 2, M = Pd²⁺; 3, M = Ni²⁺) have been successfully synthesized by judiciously choosing a large conjugated molecule, 2-(2-pyridyl)-5-(4-pyridyl)-1,3,4-oxadiazole (ppo) as the axial ligand. Single crystal X-ray crystallography analysis showed that there are different π⋯π interactions between the axial ppo ligands in 1-3. Variable temperature magnetic susceptibility studies revealed that 1 and 3 displayed incomplete and gradual SCO behaviors with T_1/2 = 94 and 78 K, respectively, while 2 stayed in a high-spin state in the measured temperature range. The stronger π⋯π interactions between the conjugated ppo ligands observed in 1 and 3 than those in 2 may result in their different magnetic behaviors.