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-pyridyl)-5-(4-pyridyl)-1,2,4-triazole) with isolated
small cavities was designed and constructed as a spin-crossover (SCO)
material based on octahedral FeIIN6 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 ≈
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 P21/n for all high-spin Fe­(II), to P1̅ for ordered half high-spin and half low-spin Fe­(II),
and back to P21/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.