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

We probe, here, a family of 2D Hofmann-type frameworks, [Fe^II(Pd(CN)₄)(bztrzX)₂]·nH₂O [X·nH₂O; X = F, Cl, Br; n = 1 (X = Cl, Br) and 3 (X = F); bztrzX = (E)-1-(2-Xphen-1-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine], with halogen-appended ligands. In all cases, there are two crystallographically distinct Fe^II 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^II sites, the emergence of ferroelastic or antiferroelastic interactions between these sites, and the relative spin-state stabilization/destabilization at each site. In Cl·H₂O, the Fe^II sites show strong elastic coupling, as evidenced by both Fe^II 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^II sites in F·3H₂O 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 Br·H₂O results in the Fe^II 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 Br·H₂O, (2) {HS-HS} ↔ {LS-HS} ↔ {LS-LS} for F·3H₂O, and (3) {HS-HS} ↔ {LS-LS} for Cl·H₂O 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.