Guest induced reversible on–off switching of elastic frustration in a 3D spin crossover coordination polymer with room temperature hysteretic behaviour

A binary reversible switch between low-temperature multi-step spin crossover (SCO), through the evolution of the population γ(HS)(T) with high-spin (HS)-low-spin (LS) sequence: HS(1)LS(0) (state 1) ↔ HS(2/3)LS(1/3) (state 2) ↔ HS(1/2)LS(1/2) (state 3) ↔ HS(1/3)LS(2/3) (state 4) ↔ HS(0)LS(1) (state 5), and complete one step hysteretic spin transition featuring 20 K wide thermal hysteresis centred at 290 K occurs in the three-dimensional (3D) Hofmann-type porous coordination polymer {Fe(II)(3,8phen)[Au(CN)(2)](2)}·xPhNO(2) (3,8phen = 3,8-phenanthroline, PhNO(2) = nitrobenzene), made up of two identical interpenetrated pcu-type frameworks. The included PhNO(2) guest (x = 1, 1·PhNO(2)) acts as a molecular wedge between the interpenetrated 3D frameworks via PhNO(2)-3,8phen intermolecular recognition and is the source of the strong elastic frustration responsible for the multi-step regime. Detailed X-ray single crystal analysis reflects competition between spatial periodicities of structurally inequivalent HS and LS SCO centres featuring: (i) symmetry breaking (state 3) with ⋯HS–LS⋯ ordering with γ(HS) = 1/2; and (ii) occurrence of spatial modulation of the structure providing evidence for stabilization of local or aperiodic ordered mixed spin states for states 2 and 4 (with γ(HS) ≈ 2/3) and 4 (with γ(HS) ≈ 1/3), respectively. Below c.a. 20 K, structural and magnetic analyses show the photogeneration of a metastable HS*, state 6. The room-temperature single-step hysteretic regime appears with release of the guest (x = 0, 1) and the elastic frustration, and reversibly switches back to the original four-step behaviour upon guest re-adsorption. Both uncommon relevant SCO events meeting in the same material represent a rare opportunity to compare them in the frame of antiferro- and ferro-elastic transitions.