Chemical/Physical Pressure Tunable Spin-Transition Temperature and Hysteresis in a Two-Step Spin Crossover Porous Coordination Framework

A two-dimensional (2D) square-grid type porous coordination polymer [Fe(bdpt)(2)].guest (1.g, Hbdpt = 3-(5-bromo-2-pyridy1)-5-(4-pyridy1)-1,2,4-triazole) with isolated small cavities was designed and constructed as a spin-crossover (SCO) material based on octahedral (FeN6)-N-II units and an all-nitrogen ligand. Three guest-inclusion forms were successfully prepared for 1.g (1.EtOH for g = ethanol, 1.MeOH for g = methanol, 1 for g = Null), in which the guest molecules interact with the framework as hydrogen-bonding donors. Magnetic susceptibility measurements showed that 1.g exhibited two-step SCO behavior with different transition temperatures (1-EtOH < 1.MeOH < 1) and hysteresis widths (1.EtOH > 1.MeOH > 1 approximate to 0). Such guest modulation of two-step spin crossover temperature and hysteresis without changing two-step state in a porous coordination framework is unprecedented. X-ray single-crystal structural analyses revealed that all two-step SCO processes were accompanied with interesting symmetry-breaking phase transitions from space group of P2(1)/n for all high-spin Fe(II), to P (1) over bar for ordered half high-spin and half low-spin Fe(II), and back to P2(1)/n for all low-spin Fe(II) again by lowering temperature. The different SCO behaviors of 1.g were elucidated by the steric mechanism and guest-host hydrogen-bonding interactions. The SCO behavior of 1.g can be also controlled by external physical pressure.