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

• Published in: Chemistry : a European journal 2020-11, Vol.26 (63), p.14419-14434
• Main Authors: Książek, Maria, Weselski, Marek, Kaźmierczak, Marcin, Tołoczko, Aleksandra, Siczek, Miłosz, Durlak, Piotr, Wolny, Juliusz A., Schünemann, Volker, Kusz, Joachim, Bronisz, Robert
• Format: Article
• Language: English
• Subjects: Chemistry; Coordination polymers; Coupling; Coupling (molecular); Crystal lattices; Crystals; Cyanides; Density functional theory; Ethane; Hysteresis; Hysteresis loops; Iron; Mixed crystals; Nitriles; Polymers; Propionitrile; spatiotemporal effects; spin crossover
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202002460

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. Nitrile reorientation versus crystal lattice: The transition from the thermally frozen high‐spin (HS) form of the 1D system [Fe(ebtz)2(C2H5CN)2](BF4)2 (ebtz=1,2‐di(tetrazol‐2‐yl)ethane) to the low‐spin (LS) one is accompanied by complex transformation associated with temporal decoupling between electronic and structural properties. Expanded studies on mixed crystal systems [Fe(ebtz)2(C2H5CN/C3H7CN)2](BF4)2 confirmed the importance of reorientation of propionitrile in relation to the coordination octahedron, which was supported by DFT calculations of various nitrile‐based models.