Spatiotemporal studies of the one dimensional coordination polymer [Fe(ebtz)2(C2H5CN)2](BF4)2. Tug of war ‐ nitrile reorientation versus crystal lattice as tool for tuning of spin crossover properties

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF4)2·6H2O 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 slow. Isothermal (80K) time resolved single crystal X-ray diffraction studies revealed a complex nature of the HS→LS transition in 2 . Initial, slow stage is associated with shrinkage of polymeric chains and with reduction of cell volume at 77% (in relation to difference between cell volumes VHS -VLS ) whereas only 16% of iron(II) ions changed spin state. In the second stage an abrupt SCO commences, associated with breathing of crystal lattice along direction of 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 (2a, 2b), showing a shift of T1/2 to higher values and narrowing of the hysteresis loop occurring together with an increase of fraction of butyronitrile. This increase reduces the capability of nitriles 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.