Three Distinct Spin-Crossover Pathways in Halogen-Appended 2D Hofmann Frameworks.

We probe, here, a family of 2D Hofmann-type frameworks, [Fe(Pd(CN))(bztrzX)]·HO [·HO; X = F, Cl, Br; = 1 (X = Cl, Br) and 3 (X = F); bztrzX = ()-1-(2-Xphen-1-yl)--(4-1,2,4-triazol-4-yl)methanimine], with halogen-appended ligands. In all cases, there are two crystallographically distinct Fe sites, ({Fe1-Fe2}), driven by the presence of a range of host-host and host-guest interactions. We find that lattice modification through X variation influences the elastic coupling between the Fe sites, the emergence of ferroelastic or antiferroelastic interactions between these sites, and the relative spin-state stabilization/destabilization at each site. In ·HO, the Fe sites show strong elastic coupling, as evidenced by both Fe sites undergoing a spin transition in a single cooperative step, as driven by the volume strain over the high-spin (HS)-to-low-spin (LS) transition. The Fe sites in ·3HO are also elastically coupled; however, the change of the X atom characteristics and increased guest molecules in the pores result in an antiferroelastic interaction characteristic between Fe1 and Fe2 and a resultant two-step spin-state transition. The change of the X atom to Br in ·HO results in the Fe sites being decoupled due to halogen atom steric bulk, resulting in the independent spin-state transition of Fe1 and Fe2 sites and a two-step spin-state transition pathway. Uniquely, all three possible spin-state transition pathways of a two-site switching system are observed in this family [(1) {HS-HS} ↔ {HS-LS} ↔ {LS-LS} for ·HO, (2) {HS-HS} ↔ {LS-HS} ↔ {LS-LS} for ·3HO, and (3) {HS-HS} ↔ {LS-LS} for ·HO for {Fe1-Fe2}]. Overall, these findings broadly support recent theoretical models but highlight that additional structural and topological complexities are needed to form a holistic picture of the drivers of elastic frustration.