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

Abstract

A binary reversible switch between low-temperature multi-step spin crossover (SCO), through the evolution of the population <i>γ</i> _HS(<i>T</i>) with high-spin (HS)-low-spin (LS) sequence: HS₁LS₀ (state 1) ↔ HS_2/3LS_1/3 (state 2) ↔ HS_1/2LS_1/2 (state 3) ↔ HS_1/3LS_2/3 (state 4) ↔ HS₀LS₁ (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}·<i>x</i>PhNO₂ (3,8phen = 3,8-phenanthroline, PhNO₂ = nitrobenzene), made up of two identical interpenetrated pcu-type frameworks. The included PhNO₂ guest (<i>x</i> = 1, <b>1·PhNO2</b>) acts as a molecular wedge between the interpenetrated 3D frameworks <i>via</i> PhNO₂-3,8phen 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 <i>γ</i> _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 <i>γ</i> _HS ≈ 2/3) and 4 (with <i>γ</i> _HS ≈ 1/3), respectively. Below <i>c.a.</i> 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 (<i>x</i> = 0, <b>1</b>) 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.