The substituent effect on the spin-crossover behaviour in a series of mononuclear Fe(II) complexes from thio-pybox ligands

The structure of organic ligands plays a pivotal role in determining the spin state of a metal ion in spin-crossover (SCO) complexes. Therefore, understanding the correlation between the ligand structure and the spin state is beneficial for the design of high performance SCO complexes. In this work, five complexes derived from [Fe(thio-pybox)(2)](2+)(ClO4-)(2) (thio-pybox = 2,6-bis(4,4-dimethyl-4,5-dihydrothiazol-2-yl)pyridine) have been constructed from the modified ligands with substituent R at the para position of pyridine (R = Cl (1), Br (2), I (3), OMe (4), and SMe (5)). Magnetic studies revealed that the substituents exert a significant influence on their spin state behaviour through finely tuning the ligand field strength. In the solid state, only the chlorine-substituted complex shows SCO, but the other four are all in a high spin state. In CD3CN solution, three halogen-substituted compounds exhibit a spin transition and the spin transition area is shifted towards lower temperatures with increasing electron-withdrawing ability of substituents. In particular, the SCO midpoint temperature (T-1/2) obtained in solution exhibits a good correlation with the substituent electronegativity. Furthermore, the experimental observation is also rationalized by DFT calculations. Our study demonstrates that the spin state behaviour can be successfully regulated through the rational modification of ligands. A deep understanding of the correlation between SCO and suitable substituent parameters will provide practical insight into the design of new SCO compounds with anticipated magnetic properties.