Photon Factory Activity Report 2014 #32 (2015) BBL9C*

Temperature-dependent EXAFS study for spin crossover complex:Fe(pyridine)2Ni(CN)4Jun Okabayashi,1* Shoutaro Ueno,2 Yuki Wakisaka,3 and Takafumi Kitazawa2
Research Center for Spectrochemistry, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan2
Department of Chemistry, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan
3
Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
1

Bimetallic assemblies with transition-metalcomplexes bridged by ligand molecules have attracted
much attention for their cooperative interactions
caused by transitions between high-spin (HS) andlow-spin (LS) states, which result in interesting
physical and chemical phenomena. The controlling ofligand field strength via external perturbations thatchange the coordinating ligand polymers is also one of
the most fascinating subjects in molecularmagnetochemistry.
Thecomplex
compoundFe(pyridine)2Ni(CN)4, which was synthesized by
Kitazawa et al. in 1996, has been regarded as one ofthe prototype spin crossover (SCO) complexes [4].
The
Hofmann-type
structurespossessing
two-dimensional networks bridged by cyanides withFe2+ (3d 6) SCO compounds are interesting in thesynthesis of self-assemblies of new kinds of SCO
molecules. However, the element-specific electronicstructures, including the information about the local
structural environments around Fe or Ni sites, have notbeen investigated explicitly because of the small sizeof single crystals and poly-crystalline samples. In thisstudy, we aim to examine the changes in electronic andstructural properties across the SCO transition inFe(pyridine)2Ni(CN)4 by using temperature-dependent
XAFS and EXAFS.
wereSamplesofFe(pyridine)2Ni(CN)4synthesized by the vapor diffusion method using theprecursors of Mohl’s salt, Fe[(NH4 )2(SO4)26H2O] andK2[Ni(CN)4], with additional water. The pH of thesolutions was adjusted to 7.5 by adding the citric acid.
Most parts of the sample were obtained aspoly-crystalline shapes. Temperature-dependent
XAFS and EXAFS were performed at BL-9C, Photon
Factory. The sample used for XAFS measurements
was diluted by BN powder formed into pellets. Thesample was cooled by the conventional He cryostat
method from room temperature to 15 K.
The inset of Fig. 1 shows the sample structure
determined by the single-crystal XRD. The
magnetization data in Fig. 1 reveals the SCO transition
at 200 K for decreasing temperature and 223 K for
increasing temperature.
Figure 2 shows the temperature-dependent XAFS
spectra of Fe(pyridine)2Ni(CN)4, for Fe and Ni
K-edges. In the Fe K-edge XAFS spectra shown in Fig.
2 (a), the shifts in the main peaks originating from 1sto 4p electric dipole transitions can be clearly observed.
The main peak positions correspond to the Fe2+ states.
At temperatures less than 200 K, the peak positions
shift to higher photon energies, which can be

explained by the decreased nearest-neighbor distanceand the shift of unoccupied states toward higher
energies through the increase of the crystal fieldstrength, which induces the transition to the LS states.
Figure 1 (b) displays the Ni K-edge XAFS spectraof HS and LS states. Contrary to those of Fe, the peak
positions of the Ni K-edges remain unchanged acrossthe SCO transition. These results demonstrate thewell-known property that the SCO occurs only at Fe2+sites. This behavior suggests that the total volume inthis compound remains unchanged across the SCO
transition and the local distortion only near the Fe sites
brings the SCO phenomena [2].

Fe(pyridine)2Ni(CN)4Fig. 1, Magnetic properties of Fe(py)2Ni(CN)4 measured by
SQUID. Inset shows the crystal structure.
(a)

(b)

Fig. 2, XAFS spectra of Fe(py)2Ni(CN)4 at varying
temperatures, for (a) the Fe K-edge and (b) the Ni K-edge.
[1] T. Kitazawa et al., J. Mate. Chem. 6, 119 (1996).
[2] J. Okabayashi et al., Inorg. Chimi. Acta 426, 142(2015).

*e-mail: jun@chem.s.u-tokyo.ac.jp