Multi-step spin-crossover in a 2D Hofmann-type clathrate with bisubstituted pyrazine.

Multi-step spin-crossover (SCO) materials, which exhibit multiple distinct spin states, are considered promising candidates for high-order data storage and multiple switches. However, such materials remain rare, necessitating the development of innovative design strategies. Here, we chose an inexpensive asymmetric ligand, 2-chloro-6-methylpyrazine (ClMepz), and synthesized two-dimensional (2D) Hofmann-type clathrates [Fe(ClMepz){M(CN)}]·ClMepz (M = Au (1), Ag (2)), in which ClMepz serves as a monodentate ligand and a guest molecule. Unlike the two-step SCO behavior in 2, compound 1 possesses more spin states and exhibits multi-step SCO behavior. The magneto-structural analysis reveals that the more linear [Au(CN)] unit exerts greater stress on the Hofmann layer, leading to the antiferro-elastic interactions. Additionally, the asymmetric host-guest interactions contribute to the stepwise SCO properties. Thus, the incorporation of asymmetric bisubstituted ligands into 2D Hofmann-type systems represents a viable and cost-effective strategy for the development of multi-step SCO materials.