Hysteretic spin crossover in a Hofmann-type metal–organic framework constructed from a [MoIII(CN)7]4− unit†

Using a self-assembly process involving a [Mo^III(CN)₇]⁴⁻ unit, an Fe^II ion, and a 1,4-bis(4H-1,2,4-triazol-4-yl)benzene (btb) ligand, we have synthesized a novel coordination polymer, denoted as {K₂Fe^II(btb)[Mo^III(CN)₇]·5H₂O}_n (1). Single-crystal structure analysis revealed a three-dimensional (3D) Hofmann-type framework for compound 1. The [Mo^III(CN)₇]⁴⁻ unit engaged four cyanide groups to coordinate with Fe^II ions, creating an anionic two-dimensional (2D) cyano-bridged layer of {Fe^II[Mo^III(CN)₇]}_n. The btb ligands, in turn, coordinated axially with the Fe^II ions, bridging adjacent 2D layers and thus constructing a 3D framework. Magnetic studies have shown that compound 1 exhibits a hysteretic one-step spin-crossover behavior. Moreover, a ferrimagnetic ordering with a critical temperature of approximately 65 K was also observed, which may be ascribed to the compound's “core–shell” architecture: the main core is responsible for the SCO behavior, while the shell contributes to the ferrimagnetic characteristics. To the best of our knowledge, this is the first observation of spin-crossover behavior in compounds based on the [Mo^III(CN)₇]⁴⁻ unit.