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 T${1 \over 2}$ = 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 T${1 \over 2}$ 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. The spin‐transition behavior of a dinuclear FeII compound is studied to explore how increased pressure favors the stabilization of the system through a transition from the [HS–HS] to the [HS–LS] state with no evidence of the [LS–LS] state even at the highest applied pressure (HS = high spin, LS = low spin). The origin of the spin transition lies with the rigidness of the bridging ligand.