Realizing the Multi-Step Spin Crossover in Two-Dimensional Cyano-Bridged {W^VFe^II} Assemblies

Spin crossover (SCO) materials constructed by polycyanidometallate building units have garnered significant attention in high-density information storage and multiswitch applications, owing to their flexible molecular geometry and diverse physical properties. However, it is challenging to obtain two-dimensional (2D) complexes with multi-step spin transitions by using octacyanometallic building units. In this work, a pair of 2D cyano-bridged {WVFeII} assemblies, namely {[FeII(Pdpi)4][WV(CN)8]} (1) and {[FeII(Pepi)4][WV(CN)8]} (2) (Pdpi = 1-[4-(2-phenyldiazenyl)phenyl]-1H-imidazole, Pepi = 1-[4-(2-phenylethenyl)phenyl]-1H-imidazole), was obtained by combining octacyanotungstate building blocks with FeII ions in the presence of imidazole derivative ancillary ligands. Variable-temperature crystal structures and magnetic measurements reveal that both complexes exhibit incomplete two-step SCO behaviors with distinct transition ratios, as confirmed by temperature-dependent UV–vis spectra. Furthermore, both complexes at low temperatures possess shaped hysteresis and light-induced excited spin state trapping (LIESST) upon 808 nm irradiation. Thus, employing polycyanidometallate building units can be an effective strategy to synthesize 2D layered materials with cooperative multi-step transitions.