Recent Research at Toyota Riken
Norimichi Kojima[Study on the dynamical spin equilibrium and its induced successive
magnetic phase transition]
In general, the FeIII site coordinated by six S atoms is in the low-spin (LS) state, while
the FeIII site coordinated by six O atoms is in the high-spin (HS) state. Therefore, it isexpected that the spin state of FeIII coordinated by three S atoms and three O atoms issituated in the spin-crossover region. Actually, the spin equilibrium in which the highspin (HS) state and the low-spin (LS) state exchange have been confirmed formononuclear FeIII complexes consisting of FeIIIO3S3. However, the rapid spin equilibrium
phenomenon has not yet been observed for assembled metal complex systems. From thisviewpoint, in order to build an assembled metal complex system including FeIIIO3S3 site,
we have synthesized Ph4P[ZnIIFeIII(mto)3] (mto = monothiooxalato) consisting ofFeIIIO3S3 and ZnIIO6 octahedra and investigated the spin state of the FeIIIO3S3 site.
Ph4P[ZnIIFeIII(mto)3] has a two-dimensional honeycomb network structure.
The ESR signals corresponding to the HS state and the LS state of FeIII site wereobserved at about 300 mT (g ≈ 2.05) and at about 150 mT (g ≈ 4.25), respectively, in thetemperature range between 10 and 300 K. The ESR signal ratio of the LS state to the HS
state increases with decreasing temperature, which indicates that the spin state ofFeIIIO3S3 is the spin equilibrium between the HS and LS states, where the HS and LSstates are clearly distinguishable in the time scale (10−10 s) of ESR spectroscopy.

On the other hand, in the 57Fe Mössbauer spectra of Ph4P[ZnIIFeIII(mto)3] at 300, 77
and 10 K, a single quadrupole doublet of FeIII was observed despite of the coexistence ofthe HS and LS states in the ESR measurement between 10 and 300 K. Therefore, the 57Fe
Mössbauer spectra of Ph4P[ZnIIFeIII(mto)3] indicates that the rapid spin equilibrium
between the HS and LS states occurs in the time scale faster than 10−7 s at the FeIIIO3S3
site. Therefore, the time scale of spin equilibrium of FeIII in Ph4P[ZnIIFeIII(mto)3] isestimated at 10−10 < τ < 10−7 s from the analysis of ESR and 57Fe Mössbauer spectroscopy.
In the case of [MnIIFeIII(mto)3](mto = C2O3S) complex consisting of FeIIIO3S3 andMnIIO6 octahedra, there exists a dynamical spin equilibrium (τ < 10-7 s) between the highspin and low-spin states at the FeIIIO3S3 site, which induces the frustration of internal
magnetic field between the ferromagnetic and antiferromagnetic interactions on the MnII
site. Figure shows the schematic representation of the dynamical spin equilibrium
between the high-spin and low-spin states at the FeIIIO3S3 site and the frustration ofinternal magnetic field between the ferromagnetic and antiferromagnetic interactions on
the MnII site for [MnIIFeIII(mto)3] complex.

From this viewpoint, we have synthesized a mto (= C2O3S) bridged hetero-metal
complex system, (C6H5)4P[MnIIFeIII(mto)3] consisting of FeIIIO3S3 and MnIIO6 octahedra.

The molar magnetic susceptibility (χM) as a function of temperature has a broad maximum,
typical character of 2D Heisenberg-type antiferromagnet, around 50 K, and shows a steep
increase below 30 K with a hump around 23 K, where both of the real (χ’) and imaginary(χ”) parts in AC magnetic susceptibility exhibit steep peaks indicating a magnetic phasetransition. The field cooled magnetization shows a rapid increase below 30 K, and almost
saturares below 23 K. At 30 K, the remnant magnetization and the magnetic hysteresis
loop disappear. Therefore, it is obvious that (C6H5)4P[MnIIFeIII(mto)3] undergoes two
successive magnetic phase transitions at 30 K and 23 K. The 57Fe Mössbauer
spectroscopy for (C6H5)4P[MnIIFeIII(mto)3] implies that the spin state at the FeIII site isstill paramagnetic even at 24 K. At 23 K, both of the MnII and FeIII spins are eventuallyordered. It is considered that the successive magnetic phase transitions in (C6H5)4P[MnIIFeIII(mto)3] are induced by the rapid spin equilibrium at the FeIII site.

Figure: Successive magnetic phase transitions induced by the rapid spin equilibrium ofFeIIIO3S3 site for (C6H5)4P[MnIIFeIII(mto)3].

[pH-responsive spin-crossover complex film, [FeII(diAMsar)]-Nafion]
We have developed pH-responsive spin-crossover FeII complex films, by using
diAMsar (1,8-diaminosarcophagine) as a ligand. Sarcophagine (sar) and its derivatives
are hexadentate cage-type ligands, whose coordination compounds with FeII exhibit thespin-crossover phenomenon between the high spin state with t2g4eg2 (S = 2) and the low
spin state with t2g6 (S = 0). The spin-transition temperature and the color of[FeII(diAMsar)] in the solution depend on pH (T1/2 = 290 K for pH = 4.5; T1/2 = 370 K for
pH = 8.5) due to the protonation/deprotonation of the terminal amino groups in diAMsar.
[FeII(diAMsar)]-Nafion also has different colors depending on the proton concentration
controlled by buffer solution (pH = 4, 7 and 10). In acidic condition, the volume of[Fe(II)(diAMsar)] complex would be expanded due to the electrostatic repulsion between
the Fe(II) ion and the NH3+ groups in diAMsar derived from the protonation of terminal
amino groups, thus the strength of ligand field becomes weak, leading to the predominanthigh spin component of t2g4eg2 (5T2g, S = 2) for Fe(II). On the other hand, in basic
condition, the low spin component of t2g6 (1A1g, S = 0) is predominant because such a
volume expansion is absent due to the deprotonation of the NH3+ groups in diAMsar. ThispH-responsive spin-crossover phenomenon indicates that the spin state of[FeII(diAMsar)]-Nafion can be spatiotemporally manipulated by the gradient of proton
concentration derived from applied voltage. In this way, we can visualize the proton flowin Nafion membrane by using the pH-responsive spin-crossover complex as a color
indicator. The development of field responsive spin-crossover complex films will open alarge field of multifunctional molecular devices based on Nafion membrane.