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

Abstract

We probe, here, a family of 2D Hofmann-type frameworks, [Fe II (Pd(CN) 4 )(bztrzX) 2 ]· n H 2 O [ X · n H 2 O; X = F, Cl, Br; n = 1 (X = Cl, Br) and 3 (X = F); bztrzX = ( E )-1-(2-Xphen-1-yl)- N -(4 H -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 ·H 2 O, 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 ·3H 2 O 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 ·H 2 O 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 ·H 2 O, (2) {HS–HS} ↔ {LS–HS} ↔ {LS–LS} for F ·3H 2 O, and (3) {HS–HS} ↔ {LS–LS} for Cl ·H 2 O 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.

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