1P035

A Neutral Fe(III) Compound Exhibiting A Two-Step Spin
Transition and Dielectric Anomalies
中性鉄(III)錯体の二段階スピン転移と誘電特性(Kyushu University, Institute for Materials Chemistry and Engineering) Zhao-Yang Li,Jing-Wei Dai, Osamu Sato*, Kim R. Dunbar*(九州大学，先導物質化学研究所) 李朝陽，代婧伟，佐藤治*，Kim R. Dunbar*
Introduction
Spin-crossover (SCO) compounds have been the subjects of intense research efforts for many
decades, activity that is motivated by their potential applications in molecular-level sensors,
switches and memory devices.1 Large numbers of polymeric and polynuclear compoundswith two-step spin transitions have been reported, behaviour that is associated with relatively
strong covalent bonds from the ligands that bridge the metal centres. In the case ofmononuclear SCO compounds, the cooperative interactions can be derived from hydrogenbonds, which are often mediated through anions or solvent molecules. The first two-step Fe(II)spin-crossover compound, [FeII(2-pic)3]Cl2·EtOH (pic = picolylamine), in which hydrogenbonds play an important role in the spin transition was discovered by Gütlich and co-workers
in 1985.2 In terms of relevance to the present work, we note that Morgan and co-workers
reported an example of a concerted two-step SCO and symmetry breaking in a d5 ion for theFe(III) salt [FeIII(L)]ClO4 but that neutral mononuclear ferric compounds with wide hysteresis,
two-step spin transitions have not been intensively studied.3
The aim of the present study was to design a mononuclear ferric compound that exhibits
dielectric, spin-crossover and photochemical properties. Multifunctional SCO materials thatsimultaneously respond to electric and magnetic stimuli have attracted increasing attention inrecent years. Herein we report the photo-responsive compound, [FeIII(H-5-Cl-thsa-Me)(5-Clthsa-Me)]·H2O (H2-5-Cl-thsa-Me = 5-chlorosalicylaldehyde methylthiosemicarbazone) (1),that exhibits a two-step spin transition and dielectric anomalies. For the H2-5-Cl-thsa-Me
ligand, we note that an anionic SCO compound based on the H2-5-Br-thsa ligand has beenreported by Boillot and coworkers and characterized using powder X-ray diffraction.
Experiments
The Schiff base ligand 5-Cl-thsa-Me (2 mmol, 0.548 g) and DMA (4 mmol, 0.45 g in MeOH)
were reacted in 50 mL of methanol and 5 mL of water at 80°C and 1 mmol Fe(NO3)3·9H2O
was slowly added to the refluxing mixture. After 8 h, the reaction was cooled to room
temperature and left to stand overnight. The resulting black precipitate was collected andwashed with acetone/water and then dried under vacuum for 4 h. Single crystals suitable for
X-ray diffraction had deposited in the bottom of flask. Yield is ~ 92%. Elemental analysis (%)
for compound 1: Calc. (found) for C18H19O3N6S2Cl2Fe: C 38.73% (38.45%), H 3.43%(3.33%), N 15.05% (15.00%).
Results and Discussion
As depicted in Fig. 1a, the asymmetric unit of 1 consists of one Fe(III) centre, monodeprotonated H-5-Cl-thsa-Me and di-deprotonated 5-Cl-thsa-Me ligands, and an
uncoordinated water molecule. The Fe(III) metal centre is six-coordinate and at 110 K the Fe–
N and Fe–S bond distances are 1.933(5)/1.948(5) Å and 2.2389(15)/2.2566(16) Å,
respectively, indicating a low spin state for Fe(III). At room temperature the Fe–N and Fe–Sbond distances are elongated, namely they are 2.111(10)/2.124(9) Å and 2.391(4)/2.453(4) Å,respectively, which is indicative of a high spin state for Fe(III). The structure of thisintermediate state was further probed at 260 K using both synchrotron and Mo(Kα) X-ray
radiation; for Mo(Kα) diffraction the Fe–N and Fe–S bond distances are 1.999(8)/2.016(9) Åand 2.308(4)/2.338(4) Å, respectively whereas for the synchrotron diffraction data, thedistances are 2.034(4)/2.030(4) Å and 2.3051(16)/2.3465(16) Å, respectively. These results
are clear evidence of the structural transformation between high-spin (HS) and low-spin (LS)states but the structure is averaged so that the independent ½HS/½LS coexistence could notbe observed.
Magnetic susceptibility data for compound 1 acquired on a sample obtained from a bulk
reaction revealed that χT (3.89 cm3 K mol−1) is slightly lower than that expected for one highspin Fe(III) metal centre (4.375 cm3 K mol−1) at 300 K if one assumes that g = 2. Withdecreasing temperature, χT is nearly constant down to 275 K and then undergoes a two-steptransition as shown in Fig. 1b. The transition temperatures of the two steps were calculated as
δ(χT)/δT; Tc(↓) to be 270 K and 245 K in the cooling mode and Tc(↑) = 249 K and 278 K
in the heating mode.
The hysteresis widths are 4 and 8 K for the two-step spin transition. It is important to notethat the purity and crystallinity of the bulk samples were confirmed using powder X-ray
diffraction analysis.

(a)
(b)
(c)
Fig. 1 (a) Unit cell structure of compound 1 at 110 K. (b) Magnetic susceptibility of compound 1 from 5 to 300
K. (c) Dielectric constant vs T plot of compound 1 under different frequencies.

Temperature-dependent dielectric constant measurements of compound 1 revealed
dielectric anomalies at approximately 270 and 240 K upon cooling at high frequencies, whichis consistent with that of the DSC anomalies and the spin transition temperatures. As shownin Fig. 1c, an increase of dielectric constant with increasing temperature was observed. The
first anomaly temperature shifts to higher temperature with an increase in frequency. From thedielectric losses plot, two-step anomalies are observed at 1 MHz. The dielectric hysteresis
was not obtained for compound 1 because of the relatively weak cooperativity showing asmall magnetic hysteresis loop.
References
1 (a) P. Gütlich, Nucl. Instrum. Meth. B, 1993, 76, 387. (b) M. A. Halcrow, Spin-Crossover Materials, WILEYVCH, 2013. (c) K. S. Murray, H. Oshio and J. A. Real, Special Issue: Spin-Crossover Complexes (ClusterIssue) Eur. J. Inorg. Chem., 2013, 5-6, 574-1067.
2 S. Decurtins, P. Gütlich, K. M. Hasselbach, A. Hauser and H. Spiering, Inorg. Chem., 1985, 24, 2174.
3 M. Griffin, S. Shakespeare, H. J. Shepherd, C. J. Harding, J. F. Létard, C. Desplanches, A. E. Goeta, J. A. K.
Howard, A. K. Powell, V. Mereacre, Y. Garcia, A. D. Naik, H. Müller-Bunz and G. G. Morgan, Angew.
Chem. Int. Ed., 2011, 50, 896.