Pressure induced separation of phase-transition-triggered-abrupt vs. gradual components of spin crossover

The application of pressure on [Co(II)(dpzca)2], which at ambient pressure undergoes abrupt spin crossover (SCO) with thermal hysteresis, gives unique insights into SCO. It reversibly separates the crystallographic phase transition (I41/a↔P21/c) and associated abrupt SCO from the underlying gradual SCO, as shown by detailed room temperature (RT) X-ray crystallography and temperature dependent magnetic susceptibility studies, both under a range of 10 different pressures. The pressure effects are shown to be reversible. The crystal structure of the pressure-induced low-spin state is determined at RT at 0.42(2) and 1.78(9) GPa. At the highest pressure [1.78(9) GPa] the Co-N bond lengths are consistent with the complex being fully LS, and the conjugated terdentate ligands are significantly distorted out of plane. The abrupt SCO event can be shifted up to RT by application of a hydrostatic pressure of ∼0.4 GPa. These magnetic susceptibility (vs. temperature) and X-ray crystallography (at RT) studies, under a range of pressures, show that the SCO can be tuned over a wide range of temperature and pressure space, including RT SCO.