Ⅲ-2. Atomic & Solid State Physics (Muon)

RIKEN Accel. Prog. Rep. 47 (2014)

Study on static and dynamic spin-crossover tripyrazolylmethane iron(II)complexes by using µSR spectroscopy
N. Kojima,*1 H. Kobayashi,*1 A. Okazawa, *1 I. Kawasaki, *2 and I. Watanabe*2
Transition-metal complexes have attracted muchattention from the viewpoints of magnetic, redox, andoptical properties originating from d spins. In particular,
complexes with a d4–d7 configuration have the possibility ofundergoing spin crossover transitions between low-spin(LS) and high-spin (HS) states, showing bistability withcolor and magnetic susceptibility changes. Spin-crossoverphenomena can be classified into two types according to thetime window of a measurement, (i) static spin crossover,which is often observed with a thermal hysteresis, and (ii)dynamic spin crossover, which sometimes shows anequilibrium obeying the van’t Hoff equation (i.e., spinequilibrium).
A large number of spin-crossover iron(II) complexes
have been developed and their spin dynamics has been
investigated by means of Mössbauer spectroscopy, nuclearmagnetic resonance, neutron scattering, etc. Muon spinrelaxation (μSR) spectroscopy, which has the unique time
range (10–5 ~ 10–11 s) to observe magnetic fluctuations, isuseful for the investigation of spin-crossover phenomena.
However, the μSR spectroscopy has scarcely been applied
to the study of dynamic spin-crossover systems. To
investigate the rapid spin equilibrium in detail, we selectediron(II) complexes containing tripyrazolylmethane ligands(Fig. 1), [Fe{(pz)3CH}2](BF4)2 (1; pz = 1-pyrazolyl) and[Fe{(pz)3CH}{(3,5-Me2pz)3CH}](BF4)2 (2), which show
dynamic and static spin crossover, respectively, on 57Fe
Mössbauer spectroscopy.1,2) Similar molecular structures of
1 and 2 facilitate μSR study. Thus, we can expect that
positive muons would be trapped at the same sites in theircompounds.
Polycrystalline samples of 1 and 2 were wrapped in silver
foil and stuck to a silver plate. We used He-flow cryostats inthe temperature range between 50 and 475 K for 1 andbetween 50 and 300 K for 2. μSR time spectra wereobtained in the zero field (ZF) and longitudinal field (LF)applied along the direction of the initial muon-spin
polarization. LF-μSR spectra were analyzed using eq (1):A(t) = afexp(-λft) + asexp(-λst) + abg,

*2

Graduate School of Arts and Sciences, The University of Tokyo
RIKEN Nishina Centerand [Fe{(pz)3CH}{(3,5-Me2pz)3CH}](BF4)2 (2).

decoupled by a field of 1000 Oe. The behavior indicates theformation of paramagnetic muonium species in pyrazolylrings. The fluctuation can be completely decoupled at 3000
Oe.
On the other hand, the initial asymmetry in the HS states(1: 475 K; 2: 300 K) is lower, even above 1000 Oe,
compared to those in the LS states, suggesting the existenceof other strong fluctuations. The time spectra of 1 and 2 in
LF = 3000 Oe drastically changed with a change oftemperature, and the relaxation rates (λf) derived fromstrong fluctuations increased on heating. We found that thetemperature dependences of λf strongly correlate with

χmolT vs T profiles (Fig. 2). Thus, the results clarified
that μSR spectroscopy using a high LF can detect spintransitions in both dynamic and static spin-crossovercomplexes.
In the ZF-μSR spectra, there is an apparent difference
between temperature dependences of the initial asymmetriesof 1 and 2. The initial asymmetry of 1 decreased around thespin transition, although that of 2 was constant over theentire temperature range. Such a decay is presumablycaused by the spin fluctuation of the equilibrium between
the HS and LS. The detailed analysis is now in progress.

(1)

where af and as are initial asymmetries, λf and λs are themuon-spin relaxation rates for the fast and slow relaxation
components, respectively. For ZF-μSR, we analyzed thespectra considering a single relaxation process (as = 0).
The LF-μSR spectra of both 1 and 2 show a similartendency. In the spectra at 100 K corresponding to LS states,
fast relaxations were observed for 1 and 2, which were
*1

Fig. 1. Structural formula of [Fe{(pz)3CH}2](BF4)2 (1)Fig. 2. Temperature dependences of relaxation rates (λf)
under LF = 3000 Oe for (a) 1 and (b) 2. The λf plots aresuperimposed on χmolT vs T plots.
References

1) D. L. Reger et al.: Inorg. Chem. 40, 1508 (2001).
2) B. Moubaraki et al.: Dalton Trans. 4413 (2007).

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