Molecular Alloying of Three Different Organic Ligands within a Spin-Transitional and SHG-Active FeII-WIV(CN)8 3-D Coordination Framework

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.