Sliding Polymeric Layers and Anion Displacement Coupled with Spin Crossover in Two‐Dimensional Networks of [Fe(hbtz)₂(CH₃CN)₂](BF₄)₂

Abstract

The abrupt high spin (HS)→low spin (LS) transition (T↓1/2=136 K) in [Fe(hbtz)2(CH3CN)2](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) is finished at 100 K and further thermal treatment influences the spin crossover. Subsequent heating involves a change of the spin state in the same way (T↑1/2=136 K) on cooling. In contrast, cooling below 100 K triggers different behavior and T↑1/2 is shifted to 170 K. The extraordinary structural changes that occurred below 100 K are responsible for the observed diversity of properties. A unique feature of the low‐temperature phase is the rebuilding of the anion network expressed by a shift of anions inside the polymeric layer at a distance of 1.2 Å as well as the relative shift of neighboring layers at over 4 Å. These structural alterations, connected with a phase transition, become the origin of the strain, which in most cases causes crystal cleaving. In a sample composed from crystals crushed as a result of the phase transition or as a result of mechanical crumbling, the hysteresis loop vanishes; however, annealing the sample allows to its partial restoration. A replacement of acetonitrile by other nitriles leads to preservation of the polymeric structure and spin crossover, but no phase transition follows.

Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand

“…Overall, the SCO character is sharpened upon further thermal cycles and the anomaly between 215–230 K in the heating curve is reduced. Changes to magnetic susceptibility traces with thermal cycling have been reported on several occasions and can occur due to sample changes over cycling (such as desolvation) or related to order/disorder phase transitions of components such as anions . Here, we confirm that the changes are not due to desolvation with cycling as the sample was contained in a sealed vessel and the reported features are reproducible over several measurements and samples.…”

Spatiotemporal Studies of the One‐Dimensional Coordination Polymer [Fe(ebtz)2(C2H5CN)2](BF4)2: Tug of War between the Nitrile Reorientation Versus Crystal Lattice as a Tool for Tuning the Spin Crossover Properties**

“…Additionally, 1,6‐di(tetrazol‐2‐yl)hexane (hbtz) shows the tendency to form topologically identical 2D polymeric systems [Fe(hbtz) 2 (RCN) 2 ](ClO 4 ) 2 (R=CH 3 , [38] C 2 H 5 , C 3 H 5 [40] ) independently of the type of coordinated nitrile molecule. The same topology also occurs for tetrafluoroborate analogs [41] . The exceptional properties are shown by the acetonitrile derivative [Fe(hbtz) 2 (CH 3 CN) 2 ](BF 4 ) 2 for which the structural phase transition is accompanied by an unprecedented large shift of the polymeric layers at distances above 4 Å as well as by relocation of anions.…”

“…[33] In contrast, application of other 1,w-di(azolyl)alkanes allowed us to preparet wo-(2D) [34,35] and three-dimensional (3D) [36] coordination polymers of different topologies. At the same time, it was found that 1,w-di(azolyl)alkane-type ligands can also form iron(II) complexes in which the first coordination sphere involves four tetrazolyl, [37][38][39][40][41] or four 1,2,3-triazolyl [30,31,42] rings, and two axially coordinated nitrile molecules. These complexes display thermally induceds pin crossover as well.…”

“…The same topology also occurs for tetrafluoroborate analogs. [41] The exceptional properties are shown by the acetonitrile derivative [Fe(hbtz) 2 (CH 3 CN) 2 ](BF 4 ) 2 for which the structural phase transition is accompaniedb ya nu nprecedented large shift of the polymericl ayers at distances above 4 as well as by relocation of anions. For the hbtz-based complexes, a change of spin state practically does not affect Fe-N-C(nitrile) angles, whicha dopt values above 1708.I ti sw orth mentioning that in other complexes containing coordinated nitrile molecules, the Fe-N-C(nitrile) geometry is nearly linear and is practically not affected by the change of spin state.…”

A Reversible Hydrogen‐Bond Isomerization Triggered by an Abrupt Spin Crossover near Room Temperature

“…Many examples of anion order‐disorder transitions showing abrupt and hysteretic SCO are known [19–22] . A special case involves anion displacement with a relative shift of cationic Fe‐based layers that leads to an extra stabilization of the LS structure [23] . Less frequent is the study of anion conformational changes.…”