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

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. Spin and phase transitions: Two‐dimensional coordination network [Fe(hbtz)2(CH3CN)3](BF4)2 (hbtz=1,6‐di(tetrazol‐2‐yl)hexane) exhibits a high spin (HS)⇄low spin (LS) transition, which is accompanied by significant structural changes, in particular, involving the shift of neighboring polymeric layers at distances above 4 Å. Occurrence of a structural phase transition as well as hysteresis loop is blocked by crystal cleavage, however, sample annealing restores hysteretic behavior (see scheme). [ABSTRACT FROM AUTHOR]