Low‐Spin State Structure of [Fe(chloroethyltetrazole)₆](BF₄)₂ Obtained from Synchrotron Powder Diffraction Data

Abstract

The complex [Fe(teec)6](BF4)2 (teec = chloroethyltetrazole) shows a two-step complete spin-crossover transition in the temperature range 300-90 K. Time-resolved synchrotron powder diffraction experiments have been carried out in this temperature range, and crystal structure models have been obtained from the powder patterns by using the parallel tempering technique. Of these models, the low-spin state structure at 90 K has been refined completely with Rietveld refinement. Its structural characteristics are discussed in relation to the high-spin state model and other spin-crossover compounds. The complex shows a remarkable anisotropic unit-cell parameter contraction that is dependent on the applied cooling rate. In addition, the possible important implications for the interpretation of spin-crossover behavior in terms of structural changes are discussed.

High‐Spin‐ and Low‐Spin‐State Structures of [Fe(chloroethyltetrazole)6](ClO4)2 from Synchrotron Powder Diffraction Data

“…The N···HÀC bonds (Table 6) are rather long compared to those in the tetrafluoroborate com- plex. [24] From the N···H distances it can be concluded that, just as in the case of the tetrafluoroborate complex, at 300 K more N···HÀC hydrogen-bond stabilization exists than at 90 K. Actually, at 90 K two of the three N···H distances are too long to be considered as hydrogen bonds.…”

“…[22][23][24] Table 1 lists unit-cell data while the unit-cell volume as a function of temperature is shown in Figure 3 together with the Imaging-Plate data of the scan measurements and the magnetic susceptibility versus temperature (T› = 160-100 K and Tfl = 100-298 K).…”

“…Interlayer-short-contacts interactions exist between Cl and hydrogen atoms (Table 5) and this interaction seems to be stronger at 90 K, a similar observation as for the tetrafluoroborate complex. [24] In Table 6 possible intermolecular hydrogen bonds are listed. According to PLATON, three Cl···HÀC bonds at 300 K are short enough to be considered as hydrogen bonds, one of them being an intralayer hydrogen bond (Cl1···H3a2ÀC3a) and almost parallel to the c axis, and the other two being interlayer hydrogen bonds.…”

“…As it turns out, unlike the cases of [FeA C H T U N G T R E N N U N G (teeX) 6 ]A C H T U N G T R E N N U N G (BF 4 ) 2 (X = Cl, Br, I), [23,24,35] …”

“…These differences can be attributed to the relaxation of the structure during the long-term measurements, a conclusion drawn also in the case of [FeA C H T U N G T R E N N U N G (teec) 6 ]A C H T U N G T R E N N U N G (BF 4 ) 2 . [24] Indeed, inspection of the long-term diffraction patterns at 155-90 K does not show a significant amount of HS state. [23,24] therefore the incomplete SCO cannot be attributed to the existence of two Fe II ions behaving differently, one of which does not undergo SCO, as in the case of [FeA C H T U N G T R E N N U N G (mtz) 6 ]X 2 , X = BF 4 À , ClO 4 À .…”