Study on the spin-crossover transition in [Fe(cis-/trans-stpy)4(X)2] (stpy: styrylpyridine, X: NCS, NCBH3) under high pressure toward ligand-driven light-induced spin change

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy)4(X)2] (stpy = styrylpyridine, X = NCS, NCBH3) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe(trans-stpy)4(NCS)2], while [Fe(cis-stpy)4(NCS)2] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH3, due to the stronger ligand field of NCBH3, the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe(trans-stpy)4(NCBH3)2]. In the case of [Fe(cis-stpy)4(NCBH3)2], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65–1.09 GPa where [Fe(stpy)4(NCBH3)2] undergoes LD-LISC at room temperature.

Graphical abstractThe spin transition temperature increases to 360 K with increasing applied pressure up to 0.50 GPa for [Fe(trans-stpy)4(NCBH3)2], while in the case of [Fe(cis-stpy)4(NCBH3)2], the spin state is the high-spin state under this pressure around 300 K. Therefore, the complexes have a possibility of LD-LISC around room temperature.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We applied high pressure to [Fe(stpy)4(X)2] (X = NCS, NCBH3). ►T1/2 increase up to 360 K (0.5 GPa) for [Fe(trans-stpy)4(NCBH3)2]. ► [Fe(cis-stpy)4(NCBH3)2] retain the high-spin state around 300 K under high pressure. ► T1/2 increase up to 175 K (0.7 GPa) for [Fe(cis-stpy)4(NCBH3)2]. ► [Fe(stpy)4(NCBH3)2] have a possibility of LD-LISC around 300 K under pressure.