Pressure-Induced Unexpected Stabilization of the High-Spin State of Iron(II) in a Metal–Organic Framework

The effect of hydrostatic pressure on the spin state of the metal organic framework {Fe(R-pbpy⁺)₂[μ₂-M(CN)₄]₂·2H₂O} (pbpy = phenylbipyridinium, R = CH₃, M = Pd) was investigated by Raman spectroscopy and single crystal X-ray diffraction (SC-XRD). As expected, the application of a hydrostatic pressure of 1.2 GPa fully transforms the high-spin (<i>S</i> = 2) ferrous ions into the low-spin (<i>S</i> = 0) form. Surprisingly, further increase of the pressure to 2.0 GPa induces a pressure-driven return to the <i>S</i> = 2 state, which can be switched again to the <i>S</i> = 0 state at 2.5 GPa. This unusual sequence of pressure-driven spin-state transitions is completely reversible and is also observable in two analogous compounds with M = Pt and R = Br. High-pressure X-ray crystal structure analysis reveals concurrent pressure-driven structural changes, assigned to an electron transfer (ET) process between the redox-active ligands. These processes cause a simultaneous elongation of the metal-ligand bond lengths and a contraction of the lattice volume, stabilizing, thus, the high-spin state in the pressure range of ca. 1.5-2.3 GPa, opposite to classical thermodynamics of spin crossover.