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Recent Research Topic from Coordination Chemistry Laboratory: Iron(II) complex showing scan rate dependent spin-crossover behavior

Posted on : January 28, 2019 | Modified on : February 6, 2019 [NEW]

To the Coordination Chemistry Laboratory webpage 

Unpaired electrons do not stably exist on typical elements except for special cases, but unpaired electrons can stably exist on transition metal ions. Among them, in the hexacoordinate octahedral metal complexes of manganese, iron and cobalt, there are two states, a high-spin state where the number of unpaired electrons is maximized and a low-spin state where it is minimized depending on the conditions. Under suitable conditions, these two spin states can be reversibly exchanged by external stimuli such as temperature, pressure, light, and so on. This is called a spin-crossover phenomenon.

The spin-crossover phenomenon is mainly confirmed by measuring the temperature dependence of the magnetic susceptibility, but in the case of the iron (II) complex, the Mössbauer spectroscopy is also used because the ratio between the high- and the low-spin states can be obtained by this measurement. When the spin-crossover complex is rapidly cooled from room temperature to an extremely low temperature of about 5 to 10 K, the high-spin state at room temperature is maintained to some extent after cooling. This phenomenon is known as a frozen-in effect. Therefore, magnetic susceptibility measurements of the spin-crossover complexes are usually carried out at a relatively slow temperature scan rate of 1 K min‒1 or so. At such slow scan rate, the measurement results at different scan rates usually correspond to one another. On the other hand, the Mössbauer measurement at every temperature spends the long measuring time. Therefore, the Mössbauer result can be regarded as the result at the infinite slow scan rate and usually agrees well with the magnetic susceptibility result at temperature scan rates around 1 K min-1.

Recently, however, complexes exhibiting scan rate dependent spin-crossover behavior have been found at the above-mentioned temperature scan rate. [1] In our laboratory also in collaboration with Kumamoto University, the spin-crossover iron (II) complex [FeII(HLn-Pr)3]Cl PF6 with the ligand HLn-Pr (Figure A) was prepaered. This complex has two polymorphs. One is obtained from methanol [2] and the other from ethanol. [3] They have the same chemical composition but different crystal structure. In addition, both polymorphs show the temperature scan rate dependent spin crossover behavior (Fig. B). As a result of careful determination of the crystal structure of these polymorphs at various temperatures, the movement of the propyl group on the ligand accompanying the spin crossover behavior in the crystal (FIG. C) was observed in both polymorphs. We concluded that scan rate dependent spin-crossover behavior was observed because the movement of the molecule accompanying the spin-crossover behavior in solid state occurs slower than the temperature scan rate of the magnetic susceptibility measurement for the usual spin-crossover complex.

Figure. A) Molecular structure of ligand HLn-Pr, B) The temperature scan rate dependent spin crossover behavior of [FeII(HLn-Pr)3]Cl PF6 from methanol, C) Molecular structure of [FeII(HLn-Pr)3]Cl PF6 from methanol in 110 K (left) and 95 K (right).

[1] “Spin crossover with thermal hysteresis: practicalities and lessons learnt”, S. Brooker, Chem. Soc. Rev.2015, 44, 2880−2892.

[2] “Scan Rate Dependent Spin Crossover Iron(II) Complex with Two Different Relaxations and Thermal Hysteresis fac-[FeII(HLn-Pr)3]Cl PF6 (HLn-Pr = 2‑Methylimidazol-4-yl-methylideneamino‑n‑propyl)”, T. Fujinami, K. Nishi, D. Hamada, K. Murakami, N. Matsumoto, S. Iijima, M. Kojima, and Y. Sunatsuki, Inorg. Chem. 2015, 54, 7291−7300.

[3] “Polymorphs of spin-crossover iron(II) complex fac-[FeII(HLn-Pr)3]Cl PF6 (HLn-Pr = 2-methylimidazol-4-yl-methylideneamino-n-propyl):Assembly structures and scan rate dependent spin-crossover properties with thermal hysteresis”, T. Ueno, Y. Ii, T. Fujinami, N. Matsumoto, S. Iijima, Y. Sunatsuki, Polyhedron 2017, 136, 13–22.

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