Journal of Nuclear and Radiochemical Sciences, Vol. 8, No. 2, pp. 85-88, 2007

Counter Anion Effect on the Spin-crossover Behaviors of Two-dimensional Fe(II)Complexes with a Tripod Ligand Containing Three Imidazoles
S. Iijima,*,a H. Hagiwara,b H. Torigoe,b and N. MatsumotobabNational Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8566, Japan
Department of Chemistry, Kumamoto University, Kurokami, Kumamoto 860-8555, Japan

Received: October 27, 2006; In Final Form: December 18, 2006
57

Fe Mössbauer spectroscopy has provided detailed information about the low-spin/high-spin transition in twotypes of two-dimensional Fe(II) complexes, [FeII H 3L Me ][FeII L Me ]X (X– = BF4 – , ClO 4 – , PF6 – , NO3 – ) 1–4 and[FeIIH3LMe]Cl·X (X– = PF6 –, AsF6 –, SbF6 –) 5–7, where H3LMe denotes a hexadentate N6 tripod-like ligand containing
three imidazole groups, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine. The molar fraction ofhigh-spin FeII versus total FeII (γHS) was estimated from Mössbauer area intensities of each compound in different
temperatures. The γHS-T plots for 1–4 indicated that these compounds exhibit a two-step spin crossover corresponding to (LS-[FeII H3LMe] 2+ + LS-[FeII LMe] – ) ↔ (HS-[FeII H3LMe] 2+ + LS-[FeII LMe] – ) ↔ (HS-[FeII H3LMe] 2+ +HS-[FeII LMe] –) . The larger size of the counter anion separated the first and second spin crossover transitions inthe [FeIIH3LMe][FeIILMe]X system well. A variety of γHS-T curves depending on the counter anion was observed for[FeII H 3 L Me ]Cl·X. It was suggested that a smaller counter anion stabilizes the 1/2(LS-[FeII H 3 L Me ] 2+ + HS[FeIIH3LMe] 2+) state in this system.

1. Introduction–Spin-crossover (SCO) between the low-spin (LS) and highspin (HS) states is observed in some octahedral 3dn (4 ≤ n ≤ 7)
metal complexes, and SCO is a representative example ofmolecular bistability.1,2 The two spin states are interconvertible
by physical perturbations (a change in temperature, pressure,irradiation with light, etc.), and therefore, the SCO metal complexes have attracted increased attention with regard to potential applications to new electronic devices such as molecularmemories, switches, and visual displays. Matsumoto et al. andKojima et al. have extensively investigated a series of iron complexes with N6 tripod ligands containing three imidazole
groups, H3LR = tris[2-(((2-R-imidazol-4-yl)methylidene)amino)ethyl]amine.3–8 The H3LR-FeII and -FeIII complexes exhibit awide variety of SCO transitions with different features in thesteepness and multi-step nature, hysteresis, frozen-in effect,and so on. Combined with cryomagnetic measurements, 57Fe
Mössbauer spectroscopy provides important information about
SCO phenomena in iron compounds; essentially, the HS/LS
ratio of such a complex is derived directly from the Mössbauer
area intensities at different temperatures.1,2 This article reports
detailed temperature-dependences of Mössbauer spectra fortwo series of Fe(II) complexes [FeII H 3L Me ][FeII L Me ]X (X– =BF4 –, ClO4 –, PF6 –, NO3 –) 1–4 and [FeII H3LMe]Cl·X (X– = PF6 –,AsF6 – , SbF6 – ) 5–7 to understand systematically the counter
anion effect on the SCO behaviors of these two-dimensional(2D) systems, where H3LMe denotes tris[2-(((2-methylimidazol4-yl)methylidene)amino)ethyl]amine, and the chemical structures of [FeII H 3L Me ] 2+ and [FeII L Me ] – moieties are shown inFigure 1. Part of this work has been reported previously.5,8
2. Experimental
The procedures for preparing compounds 1–7 were reported
previously.5,8The 57 Fe Mössbauer spectra were recorded using a Wissel*Corresponding author. E-mail: s.iijima@aist.go.jp. Fax: +81-29861-6177.

[FeIIH3LMe]2+[FeIILMe]–

Figure 1. Molecular structures of [FeIIH3LMe] 2+ and [FeIILMe] –.

1200 spectrometer and a proportional counter. 57Co(Rh) moving in a constant acceleration mode was used as radioactive
source. The calculated lines were obtained by least-squares
fitting to Lorentzian peaks. The isomer shifts are reported relative to metallic iron foil. The sample temperature was controlled by a Heli-tran liquid transfer refrigerator (Air Productsand Chemicals, Inc.) within an accuracy of ±0.5 K.
3. Results and Discussion[FeIIH3LMe][FeII LMe]X complexes 1–4 assume a homochiral
2D sheet structure, in which the capped tripodlike ligands[FeII H3LMe] 2+ and [FeII LMe] – are alternately arrayed in an upand-down mode and are linked by the imidazole-imidazolatehydrogen bonds.5 The counter anions are located between the2D sheets. The 57 Fe Mössbauer spectra were investigated inthe 78–298 K range for 1–3 and in the 4.2–298 K range for 4.
The sample was initially rapidly cooled to 78 K for 1–3 and 4.2K for 4, (within 3 min from room temperature), and the spectra
were measured at selected temperatures in the warming mode,and then cooling mode.
The representative spectra of 1 (X– = BF4 –) in the warming
mode are shown in Figure 2. At 78 K (Figure 2(d)), the spectrum consisted of a single quadrupole doublet assignable to LS-© 2007 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 10/19/2007

86Iijima

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

Figure 3. Temperature dependences of the molar fraction of HS versus total FeII, γHS (▲), and χMT ( ) for [FeIIH3LMe][FeIILMe]BF4 (1) inthe warming mode. The γHS value was obtained by deconvolution
analysis of the Mössbauer spectrum at each temperature; χM = molar
magnetic susceptibility.

Figure 2. Selected 57Fe Mössbauer spectra of [FeIIH3LMe][FeIILMe]BF4(1) at (a) 298 K, (b) 200 K, (c) 160 K, and (d) 78 K in the warmingmode.

TABLE 1: Mösbauer parameters for [FeIIH3LMe][FeIILMe]
BF4 (1) in the warming mode
T / K δ a / mm⋅s–1 ∆EQ / mm s–1 Γ b / mm s–1 Relative area / %
298
275
250
220
200
180
170
160
140
120
100
78
ab

0.98
1.01
1.02
0.47
1.02
0.46
1.06
0.45
1.05
0.44
1.07
0.45
1.07
0.45
1.06
0.47
1.02
0.47
0.48
0.48

1.60
1.63
1.66
0.20
1.74
0.20
1.76
0.20
1.81
0.21
1.81
0.21
1.84
0.21
1.87
0.20
2.16
0.20
0.21
0.21

0.31
0.31
0.31
0.25
0.29
0.25
0.27
0.25
0.26
0.26
0.26
0.28
0.28
0.26
0.26
0.26
0.29
0.28
0.26
0.28

100
100
93
782
18
63
37
46
54
42
58
38
62
17
83
595
100
100

Isomer shift data are reported to iron foil.
Full width at half-height.

FeII (isomer shift δ = 0.48 mm/s and quadrupole splitting ∆E Q= 0.21 m m / s) . A lt hough two t ypes of i ron species,[FeIIH3LMe] 2+ and [FeIILMe] –, would be expected, the Mössbauer
spectra did not distinguish them. A large extent of charge delocalization between [FeIIH3LMe] 2+ and [FeIILMe] – units probably
arises from the extended imidazole-imidazolate hydrogen bond
network, and it would yield the undistinguishable quadrupole
doublets. The Mössbauer spectrum of 1 at 298 K exhibited asingle doublet with large δ (0.98 mm/s) and ∆E Q (1.60 mm/s),
which are characteristic of HS-FeII (Figure 2(a)). Between 120

and 250 K, both of the LS-FeII and the HS-FeII absorptions
were observed in the spectra (Figures 2(b) and 2(c)). A deconvolution analysis of the spectrum at each temperature was carried out to determine the molar fraction of the HS-FeII versus
total FeII (γHS). The resulting Mössbauer parameters are summarized in Table 1. The γ HS derived from Mössbauer area
intensities are plotted against T in Figure 3, along with themagnetic susceptibility (χM) data reported.5 The temperaturedependence of γHS was essentially in agreement with that ofχMT for 1. The γHS-T curve had a plateau region at ca. 160–180
K with the γHS value close to 0.5, indicating a two-step SCO
process; ( LS- [Fe I I H 3 L M e ] 2 + + LS- [Fe I I L M e ] – ) ↔ (HS[FeII H 3L Me ] 2+ + LS-[FeII L Me ] – ) ↔ (HS-[FeII H 3L Me ] 2+ + HS[FeIILMe] –) . It is probable that the spin transition temperature(T1/2) of [FeII H3LMe] 2+ is lower than that of [FeII LMe] –, because
the deprotonated L Me ligand would supply a stronger ligand
field to FeII compared to the neutral H3LMe ligand. From theγ HS -T curve, T1/2 values were estimated to be ca. 150 K for[FeIIH3LMe] 2+ and 210 K for [FeIILMe] –, the temperatures whereγHS = 0.25 and 0.75, respectively. The Mössbauer spectra of 1
in the warming mode and in the cooling mode were almost thesame at each temperature.
The γHS values of 2–4 derived from their Mössbauer spectrain the warming mode are plotted against T in Figure 4, together
with the γ HS data of 1 for comparison. Compounds 2 (X– =ClO4 – ) and 3 (X– = PF6 – ) gave γHS-T curves closely similar tothat of 1, showing a two-step SCO. However, the intervals ofthe T1/2 values for [FeIIH3LMe] 2+ and [FeIILMe] – (∆T1/2), were different to each other; ∆T1/2 ≈ 60 K, 70 K, 90 K for 1, 2, 3, respectively. A clear plateau region is observable in the γHS-T curveof 3 between ca. 180–220 K, due to the large ∆T1/2 value of thiscompound. The molecular volumes, V, of the counter anions
were reported by using quantum chemical calculation as follows: 53.4 Å3 for BF4 –, 54.4 Å3 for ClO4 –, and 73.0 Å3 for PF6 –.8
It is suggested that the ∆T1/2 depends on the size of the counteranion. The γHS-T curve of 4 (X– = NO3–) showed a small hump
below ca. 50 K, which is ascribable to frozen-in effect; such ahump was not observed in the γHS-T curve obtained by the cooling mode measurements (data not shown), while a somewhat
large γHS (0.24–0.26) still remained at 4.2–50 K for 4.
The general crystal structure of [FeIIH3LMe]Cl·X complexes
5–7 consists of a 2D extended network constructed by NH···Cl–hydrogen bonds between Cl– and imidazole NH groups of threeneighboring [FeII H 3L Me ] 2+ ions.8 The anion X occupies theCounter Anion Effect on the Spin-crossover Behaviors ofFigure 4. Molar fraction of HS versus total Fe I I , γ H S -T, for[FeII H 3L Me][FeII L Me]BF4 (1) (●), [FeII H 3L Me][FeII L Me]ClO 4 (2) ( ),[FeIIH3LMe][FeIILMe]PF6 (3) ( ), and [FeIIH3LMe][FeIILMe]NO3 (4) ( ■ ),obtained by deconvolution analysis of the Mössbauer spectra in thewarming mode.

Figure 5. Selected 57Fe Mössbauer spectra of [FeIIH3LMe]Cl·PF6 (5)at (a) 160 K, (b) 125 K, (c) 120 K, (d) 78 K, and (e) 4.2 K in thewarming mode.

J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 2007

87

Figure 6. Molar fraction of HS versus total Fe I I , γ H S -T, for[Fe I I H 3 L M e ] Cl·PF 6 (5) ( ), [Fe I I H 3 L M e ] Cl·AsF 6 (6) (▲), and[FeIIH3LMe][FeIILMe]SbF6 (7) ( ) obtained by deconvolution analysisof the Mössbauer spectra in the warming mode.

space between the 2D sheets. The 57Fe Mössbauer spectra of
5–7 were measured in the 4.2–298 K range in the warming
mode after rapid cooling to 4.2 K, and then in the coolingmode. A variety of SCO behaviors depending on the counter
anion was revealed in this 2D system.
Selected Mössbauer spectra of 5 (X– = PF6 –) obtained in thewarming mode are shown in Figure 5. Between 4.2 K and 110
K, both of LS-FeII (δ = 0.47 mm/s and ∆E Q = 0.23 mm/s at 78K) and HS-FeII (δ = 1.10 mm/s and ∆E Q = 2.16 mm/s at 78 K)
absorptions were observed in the area intensity ratio of about
1:1 (Figures 5(d) and 5(e)). On elevating the temperature
above 120 K, the HS-FeII absorption increased at the expenseof the LS-FeII absorption (Figures 5(b) and 5(c)), and the LSFeII component disappeared at 160 K (Figure 5(a)). The γHSvalues derived from Mössbauer area intensities for 5 in thewarming mode are plotted against T in Figure 6, along with theγHS data for 6 and 7 mentioned later. The γHS-T curve of 5 indicates that 1/2(LS-[FeII H 3L Me ] 2+ + HS-[FeII H 3L Me ] 2+) state isstable for 5 in the lower temperature region. The Mössbauer
spectra of 5 in the warming mode and in the cooling mode
were nearly the same at each temperature.
The γ HS -T curve of 6 (X– = AsF6 – ) in the warming mode,shown in Figure 6, exhibited a plateau region at ca. 100–120 K
with the γHS value close to 0.5, indicating a two-step SCO process; LS- [Fe I I H 3 L M e ] 2 + ↔ 1/2 (LS- [Fe I I H 3 L M e ] 2 + + HS[FeIIH3LMe] 2+) ↔ HS-[FeIIH3LMe] 2+. The hump below 50 K in
the γHS -T curve is ascribable to frozen-in effect, because thehump disappeared in the cooling mode measurements. On theother hand, compound 7 (X– = SbF6 – ) afforded a γHS -T curve
cor responding to a one-step LS- [Fe I I H 3 L M e ] 2 + ↔ HS[FeIIH3LMe] 2+ transition with T1/2 = 130 K, as shown in Figure 6.
A small HS-FeII absorption (γ HS = 0.08–0.12) was observed
between 4.2 K and 78 K for 7 both in the warming and coolingmodes. The above SCO behaviors of 5–7 suggests that theincrease in the size of the counter anion in the series PF6 – ,AsF6 –, and SbF6 – (V = 73.0 Å 3, 78.5 Å 3, and 88.7 Å 3, respectively) 8 leads a decrease in the stabilization of 1/2(LS[FeIIH3LMe] 2+ + HS-[FeIIH3LMe] 2+) state. There is the possibility
that the optimum size of the counter anion to stabilize theintermediate spin transition states is around that of PF6 –, commonly in these 2D systems [Fe I I H 3 L M e ][Fe I I L M e ]X and[FeIIH3LMe]Cl·X. For clarifying this point, it would be needed
to investigate a further variety of [FeII H3LMe][FeII LMe]X and[FeII H3LMe]Cl·X salts by the use of 57 Fe Mössbauer spectroscopy.

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J. Nucl. Radiochem. Sci., Vol. 8, No. 2, 200757

By applying Fe Mössbauer spectroscopy, this paper has
revealed that the counter anion size is one of the important factors governing the SCO behavior for two types of 2D iron complexes, [Fe I I H 3 L Me ][Fe I I L Me ]X and [Fe I I H 3 L Me ]Cl·X. It isprobable that the anion size affects the interaction between the2D sheets in the crystal lattice of each type of complexes,
which controls the stabilities of the intermediate spin transitionstates, (HS-[Fe I I H 3 L Me ] 2 + + LS-[FeI I L Me ] – ) in [FeI I H 3 L Me ][FeII LMe]X and 1/2(LS-[FeII H3LMe] 2+ + HS-[FeII H3LMe] 2+) in[FeII H 3L Me ]Cl·X. The results obtained in the present study
would be useful for experimental and theoretical investigationof the origin of multistep spin transition.1,9–12
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