Diastereomeric ratio affecting the spin transition behaviour of three Fe4L6 tetrahedral cages in solid and solution phases

Spin crossover (SCO) in metal-organic cages (MOCs) remains underexplored despite their potential for fascinating applications in molecular switches and sensors. The interplay between the structural flexibility of MOCs and spin state switching offers a promising avenue for tuning magnetic properties in a controlled environment. Herein, we report how varying diastereomeric ratios, influenced by magnetically inequivalent protons, alter ligand field strength and result in distinct SCO profiles. Three FeII4L6 tetrahedral MOCs were synthesized via subcomponent self-assembly of benzidine, pyridine-2-carboxaldehyde, and FeX2 salt (MOCs 1–3) having anions tetrafluoroborate, perchlorate, and triflimide, respectively. Different binding motifs of the syn and anti-conformations of the diimine ligands coupled with the anion templation render a combination of different diastereomers of the MOCs, altering the structural and magnetic properties of the resulting assemblies. MOCs 1 and 2 exhibited identical and gradual SCO behaviour, whereas MOC 3 exhibited SCO above room temperature in the solid state. Variable temperature 1H NMR and UV-Visible spectroscopy predicted the spin transition (ST) temperature to be in the order: T½ (1) < T½ (2) < T½ (3), deciphering the ligand field strength and emphasizing the role of ligand conformation and anion templation on SCO behaviour.