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Reaction of 1,2‐di(tetrazol‐2‐yl)ethane (ebtz) with Fe(BF4)2⋅6 H2O in different nitriles yields one‐dimensional coordination polymers [Fe(ebtz)2(RCN)2](BF4)2⋅nRCN (n=2 for R=CH3 (1) and n=0 for R=C2H5 (2) C3H7 (3), C3H5 (4), CH2Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3–5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T1/2↓=78 K, T1/2↑=123 K) and proceeds extremely slowly. Isothermal (80 K) time‐resolved single‐crystal X‐ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes VHS−VLS) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe–nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)2(C2H5CN/C3H7CN)2](BF4)2 mixed crystals (2 a, 2 b), showing a shift of T1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1–5 suggest a particular possibility of 2 to adopt a low (140–145°) value of its Fe‐N‐C(propionitrile) angle. [ABSTRACT FROM AUTHOR]

In [Fe(ebbtr)2(CH3CN)2](CF3SO3)2·4CH3CN spin crossover is associated with the occurrence of “normal” and “reverse” hysteresis loops separated by a region of stable HS form. This results from trans–trans→gauche–trans conformational changes of ebbtr molecules and anion reorientation, which occur in different ways during cooling and during heating. [ABSTRACT FROM AUTHOR]

New ligand (Z)-2-(1-methyl-2-(pyridine-2-ylmethylene)hydrazinyl)benzoxazole ( L ) reacts with Fe(II) ions what results in formation of mononuclear complexes of 2:1 stoichiometry (complexes 1 and 2 ), which crystallize in the rare space group Ia–3d (No. 230) with solvent–acetonitrile–molecules in the crystal structure. Obtained complexes were studied in terms of their properties in the solid state concerning thermal stability and magnetism. N 3 -donor Schiff base type L is a strong crystal field ligand, therefore both obtained Fe(II) complexes are diamagnetic at room temperature what has been confirmed by magnetic susceptibility measurements and Mössbauer spectroscopy. However, the reversible LS ( 1 A 1 ) ↔ HS ( 5 T 2 ) spin-transitions may be observed starting above room temperature, remaining incomplete even up to 400 K. The thermal stability and electronic reflectance spectra of complexes were determined as well. [ABSTRACT FROM AUTHOR]

A bitopic ligand 2-hydroxy-1-(tetrazol-1-yI)-3-(tetrazol-2-yl)propane (12pbtzOH) was synthesized and reacted with Fe(ClO4)2 · 6H2O, giving a 1D coordination polymer {(Fe(12pbtzOH)2(CH3CN)2](ClO4)2 · 2CH3CN}∞ that exhibits a high-spin to low-spin transition (T1/2↓ = T1/2↑ ≅ 104 K). This is an unprecedented example of an iron(II) complex containing Fe(tetrazolyl)4(MeCN)2 cores. [ABSTRACT FROM AUTHOR]