Pressure Effect Studies on the Spin-Transition Behavior of a Dinuclear Iron(II) Compound



Magnetic studies into the effect of different hydrostatic pressures between ambient and 1.03 GPa on the high-spin (HS) ⇄ low-spin (LS) transition behavior of the dinuclear iron(II) compound [FeII2(PMAT)2](BF4)4·DMF (1, PMAT = 4-amino-3,5-bis{[(2-pyridylmethyl)amino]methyl}-4H-1,2,4-triazole, DMF = N,N-dimethylformamide) have been carried out at 2–300 K. Under ambient pressure, the sample studied exhibits a [HS–HS] to [HS–LS] half spin transition (ST) at Tequation image = 208 K without any thermal hysteresis. Increasing the pressure above 0.2 GPa causes an increase (initially rapid but above 0.5 GPa more gradual) of Tequation image as well as a matching reduction in the residual high-spin fraction at room temperature. This paper probes in detail how the increased pressure favors the stabilization of the system through a transition from the [HS–HS] state to the [HS–LS] state, although there is no evidence of the [LS–LS] state even under a pressure of 1.03 GPa and down to 2 K. This work includes magnetic measurements, a calorimetric study of the ST behavior, and an estimation of the entropy change for such a half-ST process. The origin of [HS–HS] ⇄ [HS–LS] transition behavior, which likely lies with the rigidness of the bridging ligand, is explained in greater detail. This is consistent with significant stabilization of the [HS–LS] form by the two very rigid bridging ligands between the two FeII centers. The role of intermolecular interactions in the stabilization of the dinuclear lattice system is established.