Tuning Spin Crossover Properties in Hofmann-Type Framework by Guest-Adaptive Deformation

Three three-dimensional Hofmann-type metal-organic frameworks (MOFs) [Fe(bpn){Ag(CN)₂}₂]·Ph₂S (1·Ph₂S, bpn = 1,4-di(pyridin-4-yl)naphthalene, Ph₂S = diphenylsulfide), [Fe(bpn){Ag(CN)₂}₂]·Ph₂SO (1·Ph₂SO, Ph₂SO = diphenylsulfoxide) and [Fe(bpn){Ag(CN)₂}₂]·Ph₂SO₂ (1·Ph₂SO₂, Ph₂SO₂ = diphenylsulfone) were synthesized by employing sulfur-containing aromatic guests varying in oxidation states. 1·Ph₂S performed a complete four-step spin crossover (SCO) behavior with the sequence of HS↔~LS_1/3HS_2/3↔~LS_1/2HS_1/2↔ ~LS_2/3HS_1/3↔LS, while an incomplete two-step SCO profile with the sequence of HS↔~LS_1/3HS_2/3↔~LS_2/3HS_1/3 and a faint SCO behavior at low temperature for 1·Ph₂SO and 1·Ph₂SO₂. Photomagnetic experiments indicate the light-induced excited spin-state trapping (LIESST) effect in 1·Ph₂S and the bi-directional LIESST effect for 1·Ph₂SO and 1·Ph₂SO₂. Variable-temperature structural analyses reveal the evolution of host-guest synergy and highlight the mechanism of adaptive deformation of guests mediated by phenyl rotation amid spin transition. As the oxidation state of sulfur-containing guests increases, the host-guest cooperation within the lattice is limited by the steric effect, which stabilizes the high-spin state and consequently diminishes the SCO capability in this system. These results demonstrated herein open a new perspective on host-guest chemistry within SCO frameworks.