Cooperative spin-crossover behaviour in polymeric 1D FeII coordination compounds: [{Fe(tba)3}X2]·nH 2O

Article abstract of DOI:10.1002/ejic.200700293

A new family of 1D cooperative spin-crossover polymers with general formula [{Fe(tba)₃}X₂]·nH₂O [tba = N-(4H-1,2,4-triazol-4-yl)benzamide; X = CF₃SO₃ ^-, n = 2 (1), n = 0 (4); BF₄^-, n = 3 (2), n = 0 (5); 4-CH₃C₆H₄SO₃^-, n = 3 (3), n = 0 (6)] has been synthesised and characterised using a series of spectroscopic methods, X-ray powder diffraction, magnetic susceptibility measurements and differential scanning calorimetry. The copper analogue of 1, [{Cu(tba)₃}(CF₃SO₃)₂]·3H ₂O (7), has also been synthesised and its crystal structure solved at 293 K. Compound 7 crystallises in the P1 space group. The bidentate N-(4H-1,2,4-triazol-4-yl)benzamide ligand bridges the copper ions through the 1,2-nitrogen positions leading to a one-dimensional infinite polymer. The EXAFS data collected at 80 K for 1 agree perfectly with a linear chain structure, as observed for 7, with [Fe(tba)₃] chromophores linked to each other through the adjacent nitrogen atoms of three N-(4H-1,2,4-triazole-4-yl)benzamide ligands. IR spectroscopic studies confirm the retention of the C _2ν symmetry for the triazole rings in 2 and 3, thereby suggesting a similar chainlike arrangement in these compounds. Compound 1 is low spin (LS) at room temperature while 2 and 3 are high spin (HS). The latter two complexes undergo cooperative thermal spin transition at T_c ↓ = 250 and T_c ↑ = 258 K in 2 and T_c ↓ = 226 and T _c ↑ = 238 K in 3. The spin transition in 2 and 3 is accompanied by a change of colour from white in the HS state to violet in the LS state. The thermodynamic parameters associated with the spin transition estimated from DSC measurements are ΔH = 17.8 (2) and 14.6 kJ mol^-1 (3), and ΔS = 67.2 (2) and 63.5 JK^-1 mol^-1 (3). In contrast, the dehydrated compounds 4, 5 and 6 show very different spin-transition behaviour. Complex 4 exhibits an abrupt spin transition centred at room temperature and is accompanied by a relatively large hysteresis loop with a width of 25 K (ΔH = 19.7 kJ mol^-1 and ΔS = 65.6 J K ^-1 mol^-1). Like 2 and 3, 4 shows a dramatic colour change upon the spin transition. Complex 5 shows a continuous and incomplete spin transition, and 6 is HS over the whole temperature range investigated (10-400 K). Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700293

FULL PAPER
DOI: 10.1002/ejic.200700293
Cooperative Spin-Crossover Behaviour in Polymeric 1D FeII Coordination
Compounds: [{Fe(tba)3}X2]·nH2O
Maksym Seredyuk,[a] Ana B. Gaspar,*[b] M. Carmen Muñoz,[c] Michel Verdaguer,[d]
Françoise Villain,[d] and Philipp Gütlich*[a]
Keywords: Spin crossover / Bistability / Cooperative effects / Iron(II) complexes / Substituted triazoles
A new family of 1D cooperative spin-crossover polymers with
low spin (LS) at room temperature while 2 and 3 are high
spin (HS). The latter two complexes undergo cooperative
thermal spin transition at Tcȇ = 250 and TcȆ = 258 K in 2
and Tcȇ = 226 and TcȆ = 238 K in 3. The spin transition in
2 and 3 is accompanied by a change of colour from white in
the HS state to violet in the LS state. The thermodynamic
parameters associated with the spin transition estimated
from DSC measurements are ∆H = 17.8 (2) and 14.6 kJmol–1
(3), and ∆S = 67.2 (2) and 63.5 JK–1 mol–1 (3). In contrast, the
dehydrated compounds 4, 5 and 6 show very different spin-
transition behaviour. Complex 4 exhibits an abrupt spin tran-
sition centred at room temperature and is accompanied by a
relatively large hysteresis loop with a width of 25 K (∆H =
19.7 kJmol–1 and ∆S = 65.6 JK–1 mol–1). Like 2 and 3, 4 shows
a dramatic colour change upon the spin transition. Complex
5 shows a continuous and incomplete spin transition, and 6
is HS over the whole temperature range investigated (10–
400 K).
general formula [{Fe(tba)3}X2]·nH2O [tba = N-(4H-1,2,4-
–
triazol-4-yl)benzamide; X = CF3SO3–, n = 2 (1), n = 0 (4); BF4
,
n = 3 (2), n = 0 (5); 4-CH3C6H4SO3–, n = 3 (3), n = 0 (6)] has
been synthesised and characterised using a series of spectro-
scopic methods, X-ray powder diffraction, magnetic suscep-
tibility measurements and differential scanning calorimetry.
The copper analogue of 1, [{Cu(tba)3}(CF3SO3)2]·3H2O (7),
has also been synthesised and its crystal structure solved at
¯
293 K. Compound 7 crystallises in the P1 space group. The
bidentate N-(4H-1,2,4-triazol-4-yl)benzamide ligand bridges
the copper ions through the 1,2-nitrogen positions leading to
a one-dimensional infinite polymer. The EXAFS data col-
lected at 80 K for 1 agree perfectly with a linear chain struc-
ture, as observed for 7, with [Fe(tba)3] chromophores linked
to each other through the adjacent nitrogen atoms of three
N-(4H-1,2,4-triazole-4-yl)benzamide ligands. IR spectro-
scopic studies confirm the retention of the C2v symmetry for
the triazole rings in 2 and 3, thereby suggesting a similar
chainlike arrangement in these compounds. Compound 1 is
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
mainly observed in pseudo-octahedral iron(II) coordination
complexes. SCO compounds have distinct magnetic and op-
tical properties, dielectric constant, colour and structure in
the HS and LS states. These properties may be altered dras-
tically in a narrow temperature range for cooperative transi-
tions.[1–4] Furthermore, hysteresis accompanies a first-order
spin transition (ST) when the structural changes are trans-
mitted cooperatively throughout the whole solid.
Introduction
Spin crossover (SCO) has turned out to be a particularly
appealing phenomenon as it offers one of the best examples
of molecular bistability. This reversible change between
low-spin (LS) and high-spin (HS) states driven by variation
of temperature and/or pressure or also by light irradiation is
SCO materials are potentially useful for rewritable op-
tical, thermal or pressure memories at a nanometric scale
due to their switching properties.[5–7] The necessary condi-
[a] Institut für Anorganische und Analytische Chemie, Johannes-
Gutenberg-Universität,
Staudinger-Weg 9, 55099 Mainz, Germany
[b] Institut de Ciència Molecular/Departament de Química Inorgà-
nica, Universitat de València, Edifici de Instituts de Paterna, tions that an SCO material must fulfil for the construction
Apartat de Correus 22085,
46071 València, Spain
of devices for thermal displays or recording data include
abruptness of spin transition with hysteresis, room-tempera-
ture operation, change of colour and chemical stability.
Prototypes of thermal displays based on the 1D SCO poly-
meric systems [Fe(4-Rtrz)3](ClO4)2, [Fe(4-R1trz)3–3x(4-
[c] Departament de Física Aplicada, Universitat Politécnica de
València,
Camí de Vera s/n, 46022, València, Spain
[d] Laboratoire de Chimie Inorganique et Matériaux Moléculaires,
UMR-CNRS 7071, FR-CNRS 2769, Université Pierre et Marie
Curie,
R2trz)3x](A)2·nH2O and [Fe(4-Rtrz)3](A11–xA2 ) ·nH2O (4-
x 2
4 place Jussieu, case courrier 42, 75252 Paris Cedex 05, France
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Rtrz = 4-substituted-1,2,4-triazole), where x Յ 1 and n Ն 0,
have been described.[6,7] These salts, and their closely related
Eur. J. Inorg. Chem. 2007, 4481–4491
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A. B. Gaspar, P. Gütlich et al.
FULL PAPER
analogs [Fe(Htrz)3](A)2, have been subjected to intense
studies because of their interesting properties.[8] Similarly,
4-substituted triazole ligands like 4-amino-1,2,4-triazole,[9]
4-(2Ј-hydroxyethyl)-1,2,4-triazole (hyetrz),[10] 4-n-alkyl-
1,2,4-triazoles[11] and 4-(3Ј-hydroxypropyl)-1,2,4-triazole
(hyptrz)[12] have also afforded interesting 1D SCO polymers.
This family of polymeric compounds has recently been re-
viewed.[13] Several of the polymers belonging to this family
undergo cooperative SCO at close to room temperature
with large hysteresis loops. Furthermore, these materials
display dramatic changes in colour from purple to white
upon spin transition LS↔HS, which makes them particu-
larly attractive for memory and sensor devices.[6,7] Other
groups linked to the 4-substituted triazole ligands have
yielded 1D SCO polymers exhibiting liquid-crystal proper-
Scheme 1. Ligand N-(4H-1,2,4-triazol-4-yl)benzamide (tba).
Results and Discussion
Crystal Structure of [{Cu(tba)3}(CF3SO3)2]·3H2O (7)
As discussed in the Experimental Section, all attempts to
ties,[14] Langmuir–Blodgett films showing spin transition obtain single crystals of the [{Fe(tba)3}X2]·nH2O com-
and SCO gels.[15b,15c] Two more kinds of one-dimensional pounds were unsuccessful. However, it proved possible to
(1D) FeII SCO compounds can be distinguished depending obtain single crystals of the copper analog [{Cu(tba)3}-
on the nature of the N-donating groups involved in the (CF3SO3)2]·3H2O (7). X-ray diffraction data were collected
bridges, namely those bearing tetrazole[13,16,17] or pyridine at 293 K and the structure was solved by direct methods
groups.[18–21]
using SHELXS-97[22] and refined by full-matrix least
In our search for new SCO systems, and in light of the squares on F2 using SHELXL-97. Due to the medium qual-
suitability of triazole-based ligands for achieving coopera- ity of the single crystals it was not possible to decrease the
tive spin transitions, we chose the ligand N-(4H-1,2,4-tria- R factor of the structure below 0.1749. The powder diffrac-
zol-4-yl)benzamide (tba). Herein we present the synthesis tion pattern of 7 was collected at 293 K, where it matches
and physical characterisation by means of magnetic suscep- exactly that calculated from the single crystal data (Fig-
tibility measurements, Mössbauer spectroscopy, differential ure S1, Supporting Information). Compound 7 crystallises
¯
scanning calorimetry, EXAFS and XRD analysis of a new in the triclinic space group P1. The crystallographic data
family of 1D spin crossover polymers with general formula are provided in the Experimental Section and a selection of
–
[{Fe(tba)3}X2]·nH2O [Scheme 1; X = CF3SO3 , n = 2 (1), n bond lengths and angles is given in Table 1. Figure 1 dis-
–
–
= 0 (4); BF4 , n = 3 (2), n = 0 (5); 4-CH3C6H4SO3 , n = 3 plays a fragment of the 1D polymer 7 together with the
(3), n = 0 (6)]. The copper analogue [{Cu(tba)3}(CF3SO3)2]· corresponding atom-numbering scheme. The unit cell con-
3H2O (7) has also been synthesised and its crystal structure tains four crystallographically independent copper ions
solved at 293 K.
with an occupancy factor of 0.5 for Cu1 and Cu4, nine tba
Table 1. Selected bond length [Å] and angles [°] for 7 at 293 K.
Cu1–N14
Cu1–N71
Cu1–N24
Cu2–N16
Cu2–N11
Cu2–N13
2.011(9)
2.151(10)
2.242(10)
2.044(10)
2.159(10)
2.259(10)
180.0(5)
91.0(4)
89.0(4)
180.0(5)
90.0(4)
180.0(4)
93.5(4)
86.5(4)
180.0(4)
88.3(4)
91.7(4)
180.0(0)
90.4(4)
89.6(4)
93.1(4)
86.9(4)
180.0(0)
Cu3–N18
Cu3–N19
Cu3–N15
Cu3–N21
Cu3–N22
Cu3–N7
N18–Cu3–N19
N18–Cu3–N15
N19–Cu3–N15
N18–Cu3–N21
N19–Cu3–N21
N15–Cu3–N21
N18–Cu3–N22
N19–Cu3–N22
N15–Cu3–N22
N21–Cu3–N22
N18–Cu3–N7
N19–Cu3–N7
N15–Cu3–N7
N21–Cu3–N7
N22–Cu3–N7
2.016(9)
2.029(10)
2.043(11)
2.080(10)
2.329(10)
2.365(10)
176.7(4)
89.1(4)
92.3(4)
90.5(4)
88.0(4)
179.6(4)
92.1(4)
90.8(4)
92.8(4)
87.0(4)
90.2(4)
86.9(4)
89.3(4)
90.9(4)
176.9(3)
Cu4–N30
Cu4–N17
Cu4–N25
Cu4–N8
Cu4–N23
Cu4–N20
N30–Cu4–N17
N30–Cu4–N25
N17–Cu4–N25
N30–Cu4–N8
N17–Cu4–N8
N25–Cu4–N8
N30–Cu4–N23
N17–Cu4–N23
N25–Cu4–N23
N8–Cu4–N23
N30–Cu4–N20
N17–Cu4–N20
N25–Cu4–N20
N8–Cu4–N20
N23–Cu4–N20
2.008(10)
2.036(10)
2.035(11)
2.053(9)
2.329(10)
2.351(10)
177.7(4)
88.9(4)
91.4(4)
91.3(4)
88.4(4)
179.5(4)
90.7(4)
91.6(4)
91.9(4)
87.7(4)
90.1(4)
87.6(4)
89.1(4)
91.3(3)
178.7(4)
N14–Cu1–N14[a]
N14–Cu1–N24
N14–Cu1–N24[a]
N24–Cu1–N24[a]
N14–Cu1–N71
N71–Cu1–N71[a]
N71–Cu1–N24
N71[a]–Cu1–N24
N16–Cu2–N16[a]
N16–Cu2–N11
N16–Cu2–N11[a]
N11–Cu2–N11[a]
N16–Cu2–N13
N16–Cu2–N13[a]
N11–Cu2–N13
N11–Cu2–N13[a]
N13–Cu2–N13[a]
[a] i = 1 – x, 1 – y, –z
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Cooperative Spin-Crossover in FeII Complexes
–
ligands, six CF3SO3 anions and nine water molecules. The bond length is 2.134, 2.154, 2.143 and 2.135 Å for Cu1,
bidentate N-(4H-1,2,4-triazol-4-yl)benzamide ligand brid- Cu2, Cu3 and Cu4, respectively. The Cu–Cu distance
ges the copper ions through the 1- and 2-nitrogen atoms to through the tba bridge is 3.8915(12) Å. These structural pa-
form a one-dimensional infinite polymer. Each CuII ion is rameters are similar to those observed previously for other
coordinated to six N atoms belonging to six tba ligands CuII-triazole systems.[23]
with pseudo-octahedral symmetry, as indicated by the bond
Figure 2 illustrates the crystal packing of 7 in the xy
lengths and angles (Table 1). The axial bond lengths are plane. The molecular crystal packing of 7 can be described
notably larger than the equatorial ones due to the Jahn– as an infinite hexagonal array of [Cu(tba)3]2+ linear chains
–
Teller distortion, which are in the range 2.242(10)– along the z direction, with water molecules and CF3SO3
2.351(10) Å for the four CuII ions. The average CuII–N counterions in the vacancies. The distance between the CuII
linear chains is 17.816(2) Å. There are no intermolecular
interactions between the chains through the phenyl group
of the tba ligands or between the counterions, the water
molecules and the tba ligands. The shortest distance be-
tween the carbon atoms of two phenyl groups belonging to
adjacent chains is 5.219(2) Å. The water molecules are lo-
cated around the CuII linear chain at a distance to the CuII
ion between 4.509(2) and 4.951(2) Å.
X-ray Powder Diffraction of Compounds 1–3 and 7
X-ray powder patterns of 1–3 and 7 were recorded at
293 K (Figure 3). Only two compounds have similar pro-
files, namely compounds 1 and 7, where small discrepancies
between weak peaks can be interpreted as being due to the
hydration numbers of both compounds. The profiles of
compounds 2 and 3 are not similar and therefore they are
not isostructural with 1. Only two intense peaks located at
about the same values (5–6° and 23–24°) were found to be
common to all the patterns. This, together with the fact that
similar patterns are only found for the triflate complexes of
iron(II) and copper(II), shows that, although a chain struc-
ture seems to be present in all of these compounds, the dif-
Figure 1. Fragment of the 1D polymer 7 together with the corre-
sponding atom numbering scheme.
–
Figure 2. View of the crystal packing of compound 7 in the xy plane depicting the packing of the CF3SO3 anions (light grey polyhedra)
and water molecules (black spheres) between the cationic linear chains.
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ferent packing and lattice arrangements caused by the dif- spond to torsion vibrations of the triazole rings coordinated
ferent nature of the anions yield completely different crys- in a bridging fashion through N(1) and N(2) (Scheme 1) to
tallographic patterns.[24]
neighbouring FeII or CuII ions.[25a] The band at 1207 cm–1
assigned to the exocyclic N–N bond stretching in uncoordi-
nated tba ligand should be shifted to higher frequency upon
bidentate coordination. Indeed, the value of this band in
the linear coordination polymers is typically 15–30 cm–1
higher than in the free ligand[25b] (1228, 1222, 1223 and
1227 cm–1 for compounds 1–3 and 7, respectively). These
findings confirm the one-dimensional polymeric organis-
ation of 1 and 7 and strongly suggest the same for com-
pounds 2 and 3.
IR Spectroscopic Studies of 1–3 and 7
Information about the coordination modes of tba in
complexes 1–3 and 7 can be derived directly from the room-
temperature IR spectra. The intense peaks located at 627,
625, 625 and 628 cm–1 for 1–3 and 7, respectively, corre-
EXAFS and XANES Characterisation of Compound 1
The normalised XANES spectrum of 1 recorded at 80 K
is typical for an octahedral complex of iron(II) (Figure 4a).
It includes absorption in the pre-edge region P assigned to
the symmetry-forbidden electronic dipolar 1sǞ3d transi-
tion.[26] The first strong absorption (peak A) is located at
7130 eV and can be attributed to a metal 1sǞ4p dipole-
allowed transition.[26] Further intense shoulders B and M
with energies of 7142 and 7154 eV, respectively, are caused
by multiple electron-scattering processes.[27] Finally, absorp-
Figure 3. XRD powder diffraction patterns of complexes 1–3 and
7 at 293 K.
Figure 4. (a) XANES spectrum of complex 1 at 80 K. P: pre-edge; A: top of the edge; B and M: shoulders; E: EXAFS oscillations. (b)
k3χ(k) spectrum of complex 1. (c) Fourier transforms of the EXAFS spectrum of complex 1 at 80 K including the modulus and imaginary
parts. The arrow indicates the peak at about 7 Å that corresponds to the multiple scattering path Fe–Fe–Fe. (d) Comparison of the
experimental and calculated kχ(k) values for the first filtered shell of complex 1 (solid line: experimental; dashed line: fitting curve).
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Cooperative Spin-Crossover in FeII Complexes
tion E is at the frontier of the XANES and EXAFS regions the temperature, are shown in Figure 5 for compounds 1–
and, for such molecular systems, is mainly determined by 6. The temperature dependence of the magnetic suscep-
scattering processes inside the first coordination shell. The tibility was recorded at a rate of 2 Kmin–1. Compound 1 is
EXAFS oscillation positioned at relatively high energy in the LS configuration at 290 K, as shown by the low χMT
(7205 eV) confirms the low-spin character of the iron cen- value. Upon heating, χMT increases abruptly, reaching a
tre.
value of 3.29 cm3 Kmol–1 at 400 K. TGA experiments have
The EXAFS spectrum of 1 measured at 80 K is shown shown that dehydration takes place in the same temperature
in Figure 4b, with its Fourier transform displayed in Fig- region where the change of spin state occurs (Figure S2,
ure 4c. The EXAFS signal provides direct information Supporting Information). The dehydration and change of
about the radial distribution of the backscattering atoms spin state take place in two distinct steps. A synergy be-
around the absorbing atom. Similar materials have been in- tween dehydration and spin-state change, as observed in
vestigated by means of EXAFS analysis in previous studies this material, has been reported in several other cases.[1,6,14]
and their linear local structure deduced.[28–30] The complex
The χMT value for compound
2
at 330 K is
shown in Scheme 2 is composed of triazole moieties and 3.41 cm3 Kmol–1, which falls into the range of values ex-
iron(II) ions arranged in a one-dimensional fashion, with pected for an FeII compound in the high-spin configuration.
the N–N groups of the triazoles bridging adjacent metal This value decreases abruptly at around 260 K as a conse-
centres. The spectrum at 80 K consists of five main peaks
(Figure 4c). The first intense peak at 1.53 Å can be attrib-
uted to the first coordination shell N6(1) of the iron ion.
The peak value is located at a short distance, thus clearly
indicating the LS state. Several peaks are distinguishable in
the region between 2 and 4 Å. These correspond to several
kinds of distances such as Fe–N(2), C(5), Fe–N(3), C(4)
(Scheme 1), and Fe with its two nearest Fe neighbours.[28]
According to a previous study,[29] the peak at about 7.0 Å
for the LS triazole-based complexes is ascribed to the Fe–
Fe–Fe alignment, the intensity of which is strongly en-
hanced due to the multiple-scattering focusing effect of the
collinearly arranged Fe chains.[28] The presence of this peak
provides confirmation of the polymeric structure of 1.
Scheme 2. Drawing of the linear chain structure of [Fe(R-trz)3]2+
.
ϱ
A curve-fitting analysis was performed for the first coor-
dination sphere of iron (Figure 4d). The fitting variables
were the Fe–N distance R, the edge energy shift ∆E0 and
the Debye–Waller factor σ; the coordination number N was
fixed to 6. The obtained values were R = 1.96 Å, ∆E0 =
1.8 eV, σ = 0.068 Å with goodness of fit ρ = 1.2%. The
value of the Fe–N contact distance is in good agreement
with previously reported data for the same type of com-
pounds.[28,30]
Magnetic Properties of Compounds 1–6
The magnetic data expressed in the form of χMT vs. T
plots, where χM is the molar magnetic susceptibility and T χMT vs. T plots.
Figure 5. Magnetic properties of compounds 1–6 in the form of
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A. B. Gaspar, P. Gütlich et al.
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quence of
a
spin transition, reaching
a
value of since a similar behaviour has been found in the triazole-
0.43 cm3 Kmol–1 at 230 K (Tcȇ = 250 K). The transition is based copper analogue 7 (Figure S3, Supporting Infor-
complete at this temperature as the magnetic susceptibility mation) as well as in another copper-based triazole poly-
decreases only very gradually down to 0.07cm3 Kmol–1 at mer.[23a]
10 K, thus indicating that almost all the FeII ions are in the
LS state. In the warming mode, the measurement reveals a
hysteresis width of 8 K (TcȆ = 258 K).
Mössbauer Spectroscopy of Compounds 1–4
Like 2, compound 3 undergoes an abrupt spin transition,
although at lower temperatures (Tcȇ = 226 and TcȆ =
238 K). The χMT value for 3 at 290 K is 3.30 cm3 Kmol–1,
which diminishes slightly on lowering the temperature to
250 K (3.07 cm3 Kmol–1). χMT falls to 0.65 cm3 Kmol–1
upon further cooling as a consequence of a change of spin
state of approximately 90% of the FeII ions. The χMT value
at 145 K is 0.19 cm3 Kmol–1, which corresponds to about
95% LS FeII.
The magnetic properties of the dehydrated complexes 4–
6 were also investigated (Figure 5). Dehydration of these
complexes is complete at 400 K (Figure S2, Supporting In-
formation). The samples were therefore heated from 300 to
400 K and then kept at this temperature for 15 min in the
SQUID magnetometer. Subsequently, the magnetic suscep-
tibility of the dehydrated samples was recorded from 400 K
down to 10 K and again up to 400 K. Compound 4 exhibits
an abrupt spin transition centred at room temperature. The
χMT value at 400 K is 3.28 cm3 Kmol–1, which remains
practically constant down to the vicinity of Tcȇ. χMT di-
Table 2 collects the Mössbauer parameter values and per-
centages of populations in the HS and LS states for com-
pounds 1–4 recorded at different temperatures. The Möss-
bauer spectrum of compound 1 was recorded at 80 K (Fig-
ure 6a). This spectrum clearly indicates the presence of only
one slightly asymmetrical doublet characterised by an iso-
mer shift and a quadrupole splitting typical of LS iron(II),
which matches the data from the magnetic measurements
of 1. The Mössbauer spectrum of 4 recorded at 80 K shows
a doublet with parameters typical for HS iron(II) (Table 2),
the intensity of which increases dramatically upon heating
up to 330 K. The spectrum also reveals the presence of LS
FeII ions, thereby confirming the incomplete character of
the spin transition.
Table 2. Mössbauer parameters (IS and ∆EQ) for 1–4 at different
temperatures.
T [K] Spin state IS [mms–1] ∆EQ [mms–1] A [%]
minishes suddenly at 300 K, reaching
a
value of
1
2
80
80
d, LS
d, LS
d, HS
d, LS
d, HS
d, LS
d, HS
d, LS
d, HS
d, LS
d, HS
d, LS
d, HS
0.49(0)
0.51(0)
1.22(0)
0.29(0)
1.04(0)
0.52(1)
1.15(1)
0.40(1)
1.05(0)
0.51(0)
1.17(1)
0.32(0)
1.02(0)
0.23(1)
0.27(0)
3.40(0)
0.15(0)
2.80(1)
0.28(1)
3.35(0)
0.14(0)
2.85(1)
0.25(1)
3.29(0)
0.20(0)
2.71(1)
100
97
3
0.39 cm3 Kmol–1 at 250 K and 0.090 cm3 Kmol–1 at 10 K.
The critical temperatures in the cooling and warming mode
(Tcȇ and TcȆ) are 284 and 309 K, respectively. It is worth-
while mentioning the relatively large hysteresis width of
25 K that accompanies the spin transition in 4. The repro-
ducibility of the thermal hysteresis loops in 2–4 was con-
firmed, and they are perfectly reproducible after two or
three cooling/warming cycles. Moreover, the spin transition
in 2–4 is accompanied by a change of colour from white
(HS state) to violet (LS state).
290
145
290
80
3
97
95
5
3
4
8
92
95
5
14
86
330
Compound 5 exhibits a very gradual and incomplete spin
transition. The χMT value at around 400 K shows that all
molecules are in the HS state (3.41 cm3 Kmol–1). This value
The Mössbauer spectra of 2 were recorded above and
remains practically constant down to 250 K, where the below Tc (Figure 7a). The spectrum at 290 K shows mainly
HSǞLS transition sets in. A further decrease of the tem- a doublet characteristic of the HS state of FeII. The Möss-
perature gives rise to a progressive decrease of the magnetic bauer spectrum recorded at 80 K clearly evidences the
susceptibility due to a spin transition of approximately 50% change of spin state. After cooling, the HS doublet almost
of the FeII ions. This spin transition is kinetically blocked disappears and the LS fraction increases significantly. Like
below 70 K, and the decrease of χMT below this tempera- the magnetic measurements, the Mössbauer analysis proves
ture is due most probably to zero-field splitting (ZFS) of that almost 100% of the FeII ions undergo a spin transition
the 50% of FeII ions that remain in the HS state (S = 2). in 2.
The temperature dependence of the magnetic susceptibility
The Mössbauer spectra of 3 were recorded at 290 and
of compound 6 shows a high spin behaviour over the whole 145 K (Figure 7b). Two doublets are observed at 290 K cor-
temperature range under study (10–400 K). The χMT value responding to the HS and LS states of the FeII ions. The
at 400 K is 3.38 cm3 Kmol–1 and remains constant down to analysed spectrum at 290 K yields a relative population of
100 K, where it diminishes most probably due to ZFS.
95% HS and 5% LS states. There is a clear inversion of
A weak antiferromagnetic coupling between high-spin the population distribution at 145 K due to spin transition:
iron(II) ions for 5 and 6 cannot be excluded from this expla- practically all the FeII ions are in the LS state at this tem-
nation. This could, in part, also be responsible for the de- perature (92%). The Mössbauer spectral analyses of 1–4
crease of the susceptibility on lowering the temperature agree well with the above reported magnetic properties.
4486
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Eur. J. Inorg. Chem. 2007, 4481–4491

Cooperative Spin-Crossover in FeII Complexes
Figure 8. The anomalies in the heat flow for compound 2
appear in the cooling mode at Tcȇ = 240 and TcȆ = 252 K
in the warming mode, thus indicating the occurrence of a
hysteresis with a width of 12 K. These values agree reason-
ably well with those observed from the χMT vs. T plot.
Compounds 3 and 4 exhibit anomalies in the heat flow at
Tcȇ = 226 and TcȆ = 238 K and Tcȇ = 290 and TcȆ =
310 K, respectively, which match reasonably well with the
reported magnetic data. The small differences in critical
temperatures between the DSC and magnetic data are due
to the different heating/cooling rates used in the experi-
ments. Table 3 summarises the derived thermodynamic pa-
Figure 6. (a) Mössbauer spectrum of 1 at 80 K. (b) Mössbauer
spectra of 4 recorded at 80 and 330 K. Dark grey corresponds to
the LS doublet; light grey corresponds to the HS doublet.
Figure 7. (a) Mössbauer spectra of 2 at 80 and 290 K. (b) Möss-
bauer spectra of 3 recorded at 145 and 290 K. Dark grey corre-
sponds to the LS doublet; light grey corresponds to the HS doub-
let.
Figure 8. DSC data of complexes 2–4 in the warming and cooling
modes recorded at a rate of 7 Kmin–1.
Table 3. Thermodynamic parameters (∆H and ∆S) for compounds
2–4.
Differential Scanning Calorimetry of Compounds 2–4
∆H [kJmol–1]
17.8
∆S [JK–1 mol–1]
The differential scanning calorimetric measurements
were carried out in the 175–400 K temperature range at a
rate of 7 Kmin–1. The temperature dependence of the heat
flow in the heating and cooling modes for 2–4 is shown in
2
3
4
67.2
63.5
65.6
14.6
19.7
Eur. J. Inorg. Chem. 2007, 4481–4491
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4487

A. B. Gaspar, P. Gütlich et al.
FULL PAPER
rameters ∆HSCO and ∆SSCO for the compounds 2–4; the
Compounds 2 and 3 undergo abrupt spin transitions at
values obtained are in the range of values expected for FeII 258 and 238 K, respectively, both with a hysteresis width of
SCO compounds.[1,31,32]
about 10 K. A similar cooperative spin transition has also
been observed for [Fe(fatrz)3](X)2·nH2O[24a] and [Fe(4-n-
alkyl-trz)3](X)2·nH2O.[11] The estimated average chain size
in these compounds is 7 nm.[24] Mössbauer spectra recorded
for compounds 2 and 3 at liquid-nitrogen temperature indi-
Discussion
The linear chain structure of compound 1 has been con- cate that almost all FeII ions are in the LS state, with a
vincingly derived from EXAFS data, thereby confirming small residual HS fraction. A very different temperature-
the collinear alignment of the metal ions within the poly- dependent magnetic behaviour is observed for the corre-
meric aggregates formed by alternating triazole bridges. sponding dehydrated compounds 5 and 6. Thus, complex 5
Moreover, the crystal structure of the analog [{Cu(tba)3}- presents a continuous and incomplete spin transition at
(CF3SO3)2]·3H2O (7) confirms the structural features of 1 T1/2 ≈ 100 K, while 6 shows a paramagnetic behaviour over
due to the almost identical powder XRD profiles of these the whole temperature range studied (10–400 K). These
two complexes. The fact that the powder X-ray patterns of results demonstrate that the water contained in the lattices
2 and 3 are not similar to those of 1 and 7 does not rule of these compounds plays a dominant role in the SCO
out a similar structure for these complexes as different behaviour.
packing and lattice arrangements would yield different crys-
tallographic patterns even for similar linear chain struc-
Conclusion
tures. The final evidence for the similar structural organis-
ation in 1–3 and 7 is provided by IR spectroscopy, namely
by the absorption bands assigned to the vibrations of the
bridging triazole rings featuring C2v symmetry.
The present family of polymeric 4-R-substituted triazole
FeII compounds adds new examples to the richness of spin-
transition properties of triazole-based polymers. These ma-
terials have been suggested to have possible applications as
optical sensors by virtue of their thermochromic properties.
Indeed, compound 1 represents a new example of a material
with potential applications as a temperature threshold op-
tical indicator. As compound 4 is stable at ambient condi-
tions, however, 1 can only be used as a “single-shot” indi-
cator. On the other hand, compound 4 could be used for
thermal displays or recording data as it fulfils the necessary
conditions (abruptness, hysteresis, room-temperature opera-
tion, change of colour and chemical stability). Compounds
2 and 3 could be useful in another temperature range of
operation, namely the freezing-water region.
The new family of chainlike 4-substituted-1,2,4-triazole
FeII compounds [{Fe(tba)3}X2]·nH2O presents similar SCO
characteristics to other complexes reported previously.[6–13]
Three main influences have been reported to play a crucial
role in the spin transition and its characteristics in these
compounds: (i) the water content in the crystal lattice,[4,10]
(ii) the chemical nature and size of the 4-R substituents[11]
and the counterions,[12] and (iii) the length of the polymeric
chains.[24a] Stabilisation of the low-spin state by interaction
with lattice water molecules has frequently been observed
for FeII SCO compounds, particularly for polymers based
on a triazole bridge.[1,13] This is also the case for compound
1, which loses its lattice water upon heating and undergoes
an abrupt change from the LS to the HS state. When the
dehydrated material 4 is cooled, an abrupt HSǞLS transi-
tion occurs at Tcȇ = 284 K (TcȆ = 309 K). Subsequent
reheating reveals a hysteresis loop of 25 K width centred at
room temperature. The length of the chains has been found
to be critical for the observation of cooperative and com-
plete spin transition in such triazole polymers.[6–13] The
longer the chains are the lower is the residual HS fraction.
The Mössbauer spectrum of compound 4 at 80 K clearly
denotes the completeness of the spin transition in this com-
plex, with almost all iron(II) ions in the LS state. For com-
Experimental Section
Synthesis of Materials: Starting reagents and solvents were ob-
tained commercially from Aldrich and used as received.
Measurements: FTIR spectra (KBr pellets) were recorded with a
Bruker Tensor 27 spectrometer in the range of 400–4000 cm–1. Ele-
mental analyses were performed with a Vario EL Mikro Elementar-
analysator. NMR spectroscopic measurements were performed
with a Bruker DRX 400 spectrometer.
N-(4H-1,2,4-Triazol-4-yl)benzamide (tba):
A suspension of 4-
pound 1, the Mössbauer spectrum recorded at 80 K shows amino-4H-1,2,4-triazole (4.2 g, 50 mmol) and benzoyl chloride
(7.0 g, 50 mmol) in dry pyridine (100 mL) was refluxed for 24 h.
The solvents were then removed in vacuo to provide a cream-col-
oured solid, which was dissolved in hot water (100 mL) and neu-
tralised with sodium hydrogen carbonate until no further efferves-
cence was observed. The white precipitate obtained on cooling was
filtered, washed with water and finally recrystallised from ethanol
to give 6.48 g of tba (69%). 1H NMR (400 MHz, [D6]DMSO): δ =
12.1 (br. s, 1 H, NH), 8.73 (s, 2 H, CH of trz), 7.89 (dd, J = 1.36,
7.20 Hz, 2 H, phH), 7.61 (tt, J = 1.36, 7.20 Hz, 1 H, phH), 7.52 (t,
that all iron(II) ions are in the LS state. These facts, to-
gether with the observation that all water molecules are re-
moved below 400 K (lattice water), allow us to rule out the
possibility of having water coordinated to the outer iron(II)
ions of the polymeric chains in 1 and 4. It is therefore likely
that the end groups are monocoordinated tba ligands. Tak-
ing into account that the spin-transition properties and
thermodynamic parameters of 4 are similar to those ob-
served in [Fe(Htrz)trz](BF4), one could suggest a similar
J = 7.2 Hz, 2 H, phH) ppm. FT-IR (KBr): ν = 3113 [ν(CH of trz)],
˜
average chain size for both compounds (around 13 nm).[24a] 2000–1750 [ν(Ph)], 1674 [ν(C=O)] cm–1. C9H8N4O (188.19): calcd.
4488 Eur. J. Inorg. Chem. 2007, 4481–4491
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Cooperative Spin-Crossover in FeII Complexes
C 57.44, H 4.28, N 29.77; found C 57.41, H 4.24, N 29.81. EI MS:
with the Netzsch Proteus software (NETZSCH-Geraetebau
GMBH; http://www.e-thermal.com/proteus.htm).
m/z (%) = 188.2 (2) [M]+, 105.0 (100) [M – trzNH]+.
Cu(CF3SO3)2·6H2O: Copper(II) hydroxide, obtained by the me-
tathesis reaction of copper(II) sulfate with sodium hydroxide, was
dissolved in a hot aqueous solution of trifluoromethanesulfonic
acid. The blue crystals that formed upon cooling were filtered off
Magnetic Susceptibility Measurements: The variable-temperature
magnetic susceptibility measurements were performed with a
Quantum Design MPMS2 SQUID susceptometer equipped with a
5.5 T magnet at 1 T and 1.8–400 K. Experimental data were cor-
rected for diamagnetism using Pascal’s constants.[33]
and air-dried to provide the analytically pure salt in quantitative
–
yield. FT-IR (KBr): ν = 3473 [br., ν(O–H)], 1255 [s, ν (SO )], 1032
˜
a
3
Mössbauer Spectroscopy: Mössbauer spectra were recorded in
transmission geometry with a 57Co/Rh source kept at room tem-
perature and a conventional spectrometer operating in the con-
stant-acceleration mode. The samples were sealed in a Plexiglass
sample holder and mounted in a nitrogen-bath cryostat. The Recoil
1.03a Mössbauer Analysis Software (Dr. E. Lagarec; http://www.is-
apps.ca/recoil/) was used to fit the experimental spectra.
[s, ν(C–F)] cm–1. C2H12CuF6O12S2 (469.76): calcd. C 5.11, H 2.57,
S 13.65; found C 5.07, H 2.67, S 13.92.
General Procedure for the Synthesis of the Iron(II) Salts: Iron pow-
der was dissolved in an aqueous solution containing a stoichiomet-
ric amount of the corresponding acid by heating. The greenish crys-
talline precipitate that formed upon cooling was filtered off and
air-dried to provide the analytically pure salt in quantitative yield.
X-ray Single-Crystal Crystallographic Study: Diffraction data for
complex 7 were collected with a Nonius Kappa-CCD single crystal
diffractometer using Mo-Kα radiation (λ = 0.71073 Å). A multi-
scan absorption correction was found to have no significant effect
on the refinement results. The structure was solved by direct meth-
ods using SHELXS-97 and refined by full-matrix least squares on
F2 using SHELXL-97.[22] All non-hydrogen atoms were refined an-
isotropically. Further details can be found in Table 4. CCDC-
636097 contains the supplementary crystallographic data for com-
pound 7. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[Fe(CF SO ) ]·6H O: FT-IR (KBr): ν = 3473 [br., ν(O–H)], 1188
˜
3
3 2
2
–
[νa(SO3 )], 1035 [s, ν(C–F)] cm–1. C2H12F6FeO12S2 (462.06): calcd.
C 5.20, H 2.62, S 13.88; found C 5.25, H 2.60, S 13.89.
[Fe{4-CH (C H )SO } ]·6H O: FT-IR (KBr): ν = 3407 [br., ν(O–
˜
3
6
4
3 2
2
–
H)], 1669 (s), 1496 (s), 813 [s, ν(Ph)], 1190 [s, νa(SO3 )], 3407 [br.,
ν(O–H)] cm–1. C14H26FeO12S2 (506.32): calcd. C 33.21, H 5.18, S
12.67; found C 33.24, H 4.93, S 13.05.
General Procedure for the Synthesis of Compounds 1–3 and 7: A
methanolic solution of the MA2 salt (0.53 mmol; M = Cu or Fe; A
–
–
–
= 4-CH3C6H4SO3 , CF3SO3 , BF4 ) was added to a hot methanolic
solution of N-(4H-1,2,4-triazol-4-yl)benzamide (0.3 g, 1.6 mmol).
The mixture was refluxed for 5 min, then concentrated by concen-
trating and cooled to room temperature. The resulting precipitate
was filtered off and dried in air. The yields of complexes varied in
the range 70–85%.
Table 4. Crystal data for 7 at 293 K.
Empirical formula
Formula mass
Space group
a [Å]
b [Å]
c [Å]
C29H30CuF6N12O12S2
980.31
P1
17.8020(3)
17.8160(4)
23.2650(5)
89.2120(10)
60.3130(10)
6371.8(2)
6
1.533
2994
0.709
0.02ϫ0.02ϫ0.05
293(2)
¯
[{Fe(tba) }(CF SO ) ]·2H O (1): FT-IR (KBr): ν = 3483 [br., ν(O–
˜
3
3
3 2
2
H)], 2000–1750 [ν(Ph)], 1694 [ν(C=O)], 1269 [νas(SO3 )] cm–1.
C29H28F6FeN12O11S2 (954.57): calcd. C 36.49, H 2.96, N 17.61, S
6.72; found C 36.10, H 2.93, N 17.21, S 6.95.
–
α [°]
γ [°]
V [Å3]
[{Fe(tba) }(BF ) ]·3H O (2): FT-IR (KBr): ν = 3491 [br., ν(O–H)],
˜
3
4 2
2
3127 [νas(C–H)], 2000–1750 [ν(Ph)], 1688 [ν(C=O)], 1093 [ν(B–F)].
C27H30B2F8FeN12O6 (848.06): calcd. C 38.24, H 3.57, N 19.82;
found C 38.02, H 3.56, N 19.87.
Z
Dcalcd. [mgcm–3]
F(000)
µ (Mo-Kα) [mm–1]
Crystal size [mm]
Temperature [K]
Total reflections
Reflections [I Ͼ 2σ(I)]
[{Fe(tba) }(4-CH C H SO ) ]·3H O (3): FT-IR (KBr): ν = 3483
˜
3
3
6
4
3 2
2
–
[br., ν(O–H)], 2000–1750 [ν(Ph)], 1694 [ν(C=O)], 1168 [νas(SO3 )]
cm–1. C41H44FeN12O12S2 (1016.84): calcd. C 48.43, H 4.36, N
16.53, S 6.31; found C 47.61, H 4.50, N 16.20, S 6.65.
16963
10839
0.1749
[a]
R1 [I Ͼ 2σ(I)]
[Fe(tba)3](CF3SO3)2 (4), [Fe(tba)3][BF4]2 (5) and [Fe(tba)3](4-
CH3C6H4SO3)2 (6): Compounds 4–6 were obtained by heating the
hydrated compounds 1–3 in an oven at 400 K for 30 min.
wR
S
0.4591
1.396
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|; wR = {Σ[w(Fo – Fc ) ]/Σ[w(Fo2)2]}1/2; w
2
2 2
= 1/[σ2(Fo2) + (mP)2 + nP] where P = (Fo + 2Fc )/3, m = 0.2000
and n = 0.0000.
2
2
[{Cu(tba)3}(CF3SO3)2]·3H2O (7): This complex was recrystallised
from hot water by slow cooling in a thermostat overnight to pro-
vide blue, needle-like crystals. FT-IR (KBr): ν = 3480 [br., ν(O–
˜
–
H)], 2000–1750 [ν(Ph)], 1697 [ν(C=O)], 1267 [νas(SO3 )] cm–1.
X-ray Powder Diffraction: X-ray measurements were obtained with
a STOE STADI 2 diffractometer equipped with a linear position-
sensitive detector (STOE mini PSD). Monochromatic Cu-Kα radia-
tion was obtained by using a curved germanium detector (111
plane).
C29H32CuF6N12O13S2 (998.30): calcd. C 34.89, H 3.23, N 16.84, S
6.42; found C 35.03, H 3.11, N 16.55, S 6.24.
Thermogravimetric Analysis: TGA measurements were performed
with a Mettler Toledo TGA/SDTA 851 in tandem with a mass spec-
trometer in the 300–680 K temperature range under nitrogen with
a heating rate of 10 Kmin–1.
X-ray Absorption Data Recording and Processing: The Fe K-edge
XANES (X-ray absorption near-edge structures) and EXAFS (ex-
tended X-ray absorption fine structures) data were taken in the
conventional transmission mode on beamline A1 at the German
Electron Synchrotron (DESY), Hamburg. The spectrum was re-
corded from 6970 to 7997 eV. The energy scale at the iron K-edge
Differential Scanning Calorimetry: DSC measurements were per-
formed with a Mettler model DSC 822e calibrated with metallic
indium and zinc. The samples were located in sealed sample pans.
DSC profiles were recorded at the rate of 7 Kmin–1 and analysed
Eur. J. Inorg. Chem. 2007, 4481–4491
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4489

A. B. Gaspar, P. Gütlich et al.
FULL PAPER
was calibrated with reference to the strong absorption peak of ele-
mental iron at 7111.2 eV. A water-cooled Si(111) channel-cut crys-
tal was employed as a monochromator. The intensities of the inci-
dent and transmitted X-rays were recorded using ionisation cham-
bers. The mass of the sample was calculated to obtain a product
aϫd of about 2.5 for energies just above the absorption K-edge of
iron (a is the linear absorption coefficient and d is the thickness of
the pellet). The calculated amount of sample was ground, then
mixed with crystalline cellulose and pressed into a pellet of 13 mm
diameter and about 1 mm thickness. The temperature of the experi-
ment was 80 K, which was maintained with a closed-cycle He cryo-
stat. The temperature was measured with an Si diode placed close
to the sample. The acquisition time for each data point was 1 s.
EXAFS data analysis was performed with the program
EXAFS98.[34] This standard analysis includes linear pre-edge back-
ground removal, a smoothing spline atomic absorption energy-de-
pendent procedure, called Lengeler–Eisenberg EXAFS spectrum
normalisation, and reduction from the absorption data µ(E) to the
EXAFS spectrum χ(k) with
thanks the Spanish MEC for a Ramón y Cajal research contract
and the Alexander von Humboldt Foundation for a fellowship.
A. B. G. and M. C. M. thank the Spanish MEC (project CTQ2004-
03456-BQU) and the Generalitat Valenciana (ACOMP07/110). We
thank Dr. V. Ksenofontov for helpful discussions. We also acknow-
ledge the support of Dr. E. Welter from HASYLab, Dr. H. Ehren-
berg and Dominic Stürmer from TU Darmstadt for the EXAFS
measurements at beamline A1, the technicians E. Muth and P.
Räder from MPI Polymerforschung Mainz for DSC and TGA mea-
surements and A. Michalowicz for providing us with his XAS
analysis package.[35]
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by Fourier transformation of k3w(k)χ(k) in the 2–14 Å–1 range; w(k)
is a Kaiser–Bessel apodisation window with smoothness coefficient
τ = 3. After Fourier filtering, the first single-shell Fe–N6 was fitted
in the 4–12 Å–1 range to the standard EXAFS formula, without
multiple scattering:
[7] Y. G a r c ía, V. Ksenofontov, P. Gütlich, Hyperfine Interact. 2002,
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where S02 is the inelastic reduction factor, N is the number of nitro-
gen atoms at a distance R from the iron centre, λ is the mean free
path of the photoelectron, σ is the Debye–Waller coefficient, char-
acteristic of the width of the Fe–N distance distribution, Ѩ1(k) is
the central atom phase shift, and |(Fπ,k)| and ψ(k) the amplitude
and phase of the nitrogen back-scattering factor, respectively. The
curve-fitting analysis for the first coordination sphere of the iron
atom was performed with the Round Midnight[35] code after Fou-
rier filtering in the 0.7–2.0 Å range of the EXAFS spectrum with
spherical-wave theoretical amplitudes and phase shifts calculated
by the McKale[36] code. Since theoretical phase shifts were used, it
was necessary to fit the energy threshold E0 by adding an extra
fitting parameter, ∆E0. The goodness of fit was given by:
[11] O. Roubeau, J. M. Alcazar-Gómez, E. Balskus, J. J. A. Kolnaar,
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Acknowledgments
We acknowledge financial help from the Deutsche Forschungsge-
meinschaft (Priority Program 1137 “Molecular Magnetism”), the
Fonds der Chemischen Industrie and the Materialwissen-
schaftliches Forschungszentrum der Universität Mainz. A. B. G.
4490
www.eurjic.org
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