Molecular Alloying of Three Different Organic Ligands within a Spin‐Transitional and SHG‐Active Fe^II‐W^IV(CN)₈ 3‐D Coordination Framework

Abstract The effect of ligand‐based molecular alloying on spin crossover (SCO) materials has been studied using various polydentate organic ligands; however, this effect using small monodentate ligands has not been explored. To design a proper structure that can accommodate a variety of small ligands, we employed the structural matrix of a three‐dimensional cyanido‐bridged coordination framework, [Fe II (L mix ) 4 ] 2 [W IV (CN) 8 ] ( FeW ; L mix =a mixture of different ligands). We found that three different ligands of 4‐chloropyridine (4‐Clpy), 4‐iodopyridine (4‐Ipy), and pyrazole (pz) were able to be alloyed at the L mix site of the Fe II ion. Our obtained FeW framework had a 4‐Clpy:4‐Ipy:pz alloying ratio of 1.32 : 1.56 : 1.12 per Fe II ion, and it demonstrated a gradual SCO phenomenon in response to temperature change. Furthermore, the chiral structure of FeW allows the observation of second‐harmonic generation (SHG) signal; however, the SHG signal was quite weak owing to the racemic twinning of the mirrored enantiomers. This work demonstrates that the Fe II –W IV (CN) 8 structural matrix provides a viable approach to investigating the molecular alloying effect of various types of small organic ligands on magneto‐optical properties.