Pressure-Induced Hysteretic and Abrupt Spin Transition in Amine Functionalized Isoquinoline-Based Two-Dimensional Fe^II Hofmann Frameworks

Abstract

Two two-dimensional (2D) Hofmann-type coordination frameworks are synthesized by employing the square planar tetracyanometallate building blocks and an amine-functionalized isoquinoline ligand with the general formula of [Fe(L) 2 {M(CN) 4 }] (L = 5-amino isoquinoline) (M = Pt (1Pt) and Pd (1Pd)) to explore the spin-state switching behavior. The inclusion of the amine functional group in the isoquinoline ligand plays a major role in exhibiting a complete spin crossover (SCO) behavior under ambient atmospheric pressure. The effective host−host supramolecular interaction such as strong π•••π stacking and N−H•••C interactions between interlayer 2D sheets of {Fe II [Pt (CN) 4 ]} n is responsible for the abrupt hysteretic spin transition behavior. Interestingly, the applied external pressure enhances the stabilization of the low spin states revealing a one-step abrupt and hysteretic spin transition near room temperature.

Anomalous Pressure Response of Temperature-Induced Spin Transition and a Pressure-Induced Spin Transition in Two-Dimensional Hofmann Coordination Polymers

“…The pressure response of the ST behavior of compounds Isoq-Pt and Isoq-Pd is quite different from the theoretical prediction that T 1/2 is linear pressure dependent and that Δ T 1/2 decreases and disappears with pressure. A similar anomalous pressure behavior is also observed in its analogue [Fe(5-aisoq) 2 M(CN) 4 ] (5-aisoq-M) (M = Pt, Pd, 5-aisoq = 5-amino isoquinoline), where T 1/2 exhibits lower pressure dependence at the low-pressure range and higher pressure dependence at the high-pressure range, and Δ T 1/2 gradually increases with pressure throughout the pressure intervals . The reason for the abnormal pressure response and the different evolution of the hysteresis width of Isoq-M and 5-aisoq-M with pressure may be the change of the intermolecular interaction and octahedral coordination environment with pressure and the difference in its change with pressure, respectively.…”

“…A similar anomalous pressure behavior is also observed in its analogue [Fe(5-aisoq) 2 M(CN) 4 ] (5-aisoq-M) (M = Pt, Pd, 5-aisoq = 5-amino isoquinoline), where T 1/2 exhibits lower pressure dependence at the low-pressure range and higher pressure dependence at the high-pressure range, and ΔT 1/2 gradually increases with pressure throughout the pressure intervals. 37 The reason for the abnormal pressure response and the different evolution of the hysteresis width of Isoq-M and 5-aisoq-M with pressure may be the change of the intermolecular interaction and octahedral coordination environment with pressure and the difference in its change with pressure, respectively. Even more surprisingly, when the pressure returned to the ambient pressure, the ST behavior of compounds Isoq-Pt and Isoq-Pd did not return to their prepressurization behavior.…”

“…Moreover, pressure has an obvious influence on the cooperativity of the ST system by regulating the intermolecular interaction and crystal stacking, which usually leads to a change in the cooperativity, i.e., decrease in the hysteresis width (Δ T 1/2 ). However, some anomalous phenomena are found in previous studies, such as the nonlinear change of the T 1/2 and the increase or constancy of Δ T 1/2 with the pressure. − This means that there are certain deficiencies in the traditional theoretical framework, which needs to be revised and supported by a large number of experimental evidence.…”

Modulation of the spin transition in 2D Hofmann frameworks via π⋯π stacking between the axial 2,5-dipyridyl-1,3,4-oxadiazoles

“…5–11 Among them, Fe 2+ Hofmann-type SCO frameworks are one of the most notable systems. 6,10–35 These frameworks generally contain neutral bimetallic layers consisting of Fe 2+ ions and [M(CN) 4 ] 2− anions (M = Pt 2+ , Pd 2+ , Ni 2+ ) in the equatorial plane, which allow a large number of linear N-donor ligands to coordinate axially to the Fe 2+ centers in order to produce 2D/3D Hofmann SCO materials. 10 If the linear exo -bidentate ligands are used as pillars, 3D porous Hofmann SCO frameworks can be constructed, 11 while 2D Hofmann SCO networks will be built if the axial ligands are merely monodentate.…”

“…12 Since the monodentate ligand is always more readily available than the bidentate one, the 2D Hofmann SCO networks can be more easily constructed, which will provide a good opportunity for exploring the influence of intermolecular interactions (hydrogen bonding, π⋯π stacking and interdigitated effect) on the SCO behavior. 12–35 So far, most of the monodentate axial ligands used in 2D Hofmann SCO networks are substituted pyridine derivatives, owing to their cheap and easy large-scale preparation. Moreover, pyridine derivatives modified with small substituent groups (–F, –Cl, –Br, –I, –NH 2 , etc. )…”