This document is the accepted manuscript version of a published work that appeared in final form in Dalton Trans., 2018, 47,
11895, DOI: 10.1039/c8dt01339j.

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Tuning the spin crossover behavior of the
polyanion [(H2O)6Fe3(µ-L)6]6–: the case of
the cesium salt
Andrea Moneo-Corcuera, David Nieto-Castro, Cristina Sáenz de Pipaón,
Verónica Gómez, Pilar Maldonado-Illescas and
Jose Ramon Galan-Mascaros
The magnetic behavior of the polyanion [Fe3(µ-L)6(H2O)6]6– (L2– = (1,2,4-triazol-4-yl)ethanedisulfonate)
as the corresponding dimethylammonium salt shows memory effect above room temperature, with a
dynamic thermal hysteresis cycle over 90 K and temperature-induced excited spin state trapping (TIESST)
phenomena at the highest temperatures reported. Taking advantage of the polyanionic nature of this trimetallic complex, we were able to substitute the dimethylammonium cations by the monovalent heavy
alkali metal cesium. This methathesis yielded the salt Cs6[Fe3(µ-L)6(H2O)6], with different molecular
packing that increases the number and strength of cation–anion interactions, including a more robust
H-bonded network. In this phase, the spin transition still occurs above room temperature, but it is more
abrupt and narrow (≈50 K wide hysteresis). Despite these differences, TIESST is observed with almost
identical characteristic temperature (TTIESST = 240 K) than in the parent compound, which is an additional
experimental evidence supporting the molecular origin of the TIESST behavior in these materials.

Introduction
In the last few decades, molecular materials have been proposed as plausible alternatives for the miniaturization of components for information storage devices.1–5 Molecules represent the well-defined minimum size limit,6–10 along with
additional advantages when compared with the more common
top-down approach: solid state materials typically lose their
bulk properties at the nanoscale.11–13 Molecule-based alternatives to optical,14,15 electric16,17 or magnetic18,19 materials have
been studied with excellent perspectives, including multifunctional materials exhibiting tailor-made combinations of the
abovementioned materials.20–23 This trend is particularly true
in magnetic materials, which are able to mimic and even
surpass the performance of solid state materials. Some unique
a

Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of
Science and Technology (BIST), Av. Països Catalans, 16, 43007 Tarragona, Spain.
E-mail: jrgalan@iciq.es; Fax: +34 977 920 224; Tel: +34 977 920 808
b
ICREA, Passeig Lluïs Companys 23, 08010 Barcelona, Spain
† Dedicated to Prof. Kim R. Dunbar on occasion of her 60th birthday, with deep
gratitude.
‡ Electronic supplementary information (ESI) available: TGA, and crystallographic data. CCDC 1832927. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c8dt01339j
§ These authors contributed equally to this work.

examples include light-weight room temperature magnets,24,25
single molecule magnets26–33 and photoactive magnets.34–37
Successful attempts to incorporate such materials into singlemolecule based nanostructured devices have been already
reported following this trend.38–45
In this field, spin crossover (SCO) compounds 46,47 are some
of the most extraordinary magnetic materials.48–52 They exhibit
switching between the low spin (LS) and the high spin (HS)
metastable excited state
controlled by external
stimuli.
Additionally, in the solid state, a thermal hysteresis cycle may
appear, conferring on them true memory effect that can be
tuned at and well above room temperature via preparation
and/or processing.53–57 Because of these
reasons, SCO
materials are promising candidates for the actual implementation of molecules into molecular electronics, display devices,
data storage, etc.58–64
Although several transition metal centers are prone to
exhibiting SCO phenomena,65,66 most studies have been
focused on Fe(II) compounds, where the switching between
LS and HS states brings some additional changes including
spectroscopic properties and even molecular volume/chemical pressure.67,68 Fe(II) complexes, including coordination
polymers, have shown wide thermal hysteresis,69 ultrafast
switching, 70–72 and electromechanical features,73,74 bridging
them closer to practical applications. In the miniaturization

trend, SCO materials have also shown to retain the memory
effect down to the nanoscale.75–82 However, since the spin
transition is governed by lattice dynamics,83–85 it is still
uncertain if memory effect (hysteretic behavior) exists at the
single molecule level.86
Our group is interested in anionic SCO complexes, which
are less common than the cationic or neutral counterparts.87
One example is the polyanion [Fe3(µ-L6)(H2O)6]6– (1, L = 4(1,2,4-triazol-4-yl)ethanedisulfonate).88,89 Its molecular structure consists of a linear array of three octahedral Fe(II) centers,
which are bridged by two triple 1,2,4-triazole bridges. The two
terminal Fe(II) centers complete their octahedral coordination
with three water molecules. In the solid state, as the corresponding dimethylammonium (Me2NH2) salt, a thermally
induced spin transition above room temperature was found for
the central Fe(II), with a hysteresis cycle of 85 K and notably
slow dynamics. In addition, the excited state (HS) of this compound can be easily quenched in a temperature-induced
excited spin-state trapping (TIESST) phenomena90–93 when it is
cooled at a rate of at least 5 K min−1 or faster. A characteristic
TTIESST of 250 K was determined, which is still the highest
TTIESST reported up to date.
Since spin transition phenomenon is very sensitive to
changes in crystal structure and packing, we realized that substitution of the dimethylammonium cations by other cationic
types could influence the bistability and switchability of these
trimeric units. By analogy, in the most common case of cationic SCO complexes, changes in anionic content and solvation may drastically change the magnetic features from complete and cooperative transition to stabilization of the LS or HS
states.94–99 Herein, we describe how simple metathesis in
aqueous solution of 1 with excess Cs salt yields single crystals
of the corresponding Cs+ salt, displaying distinct magnetic features. Indeed, the nature of the cation induces a different
crystal structure and packing, severely affecting the SCO behavior. These results show an easy strategy to tune the magnetic
features of this intriguing SCO complex.

Experimental
Materials and instrumentation
All reagents were obtained from commercial sources and used
without further purification. The ligand 4-(1,2,4-triazol-4-yl)
ethanedisulfonate (L) and
the
dimethylammonium salt
(Me2NH2)6[Fe3(µ-L)6(H2O)6] (1) were prepared according to the
literature procedures.88,100 Thermogravimetric analyses were
performed using a TGA/SDTA851 Mettler Toledo with a
MT1 microbalance. Differential Scanning Calorimetry analyses
(DSC) were performed using a Mettler Toledo/DSC822e instrument with a heating rate of 1 °C min−1 in a nitrogen stream.
Magnetic measurements were carried out on grained single
crystals with a Quantum Design MPMS-XL SQUID magnetometer (Quantum Design, Inc., San Diego, CA, USA). Magnetic
measurements were carried out under an applied field of 1000
Oe at different temperature scan rates.

Synthesis
Cs6 [Fe3 (µ-L)6 (H2 O)6 ]·13H2 O (2). This compound was
obtained by metathesis. Initially, 180 mg (0.082 mmol, 1 eq.)
of 1 and 275.6 mg (1.64 mmol, 20 eq.) of CsCl were separately
dissolved in 5 mL of distilled water. Ascorbic acid (≈2 mg) was
added to both solutions to avoid the oxidation of Fe(II) to
Fe(III). Both solutions were mixed for 10 minutes under stirring. Subsequently, the solution was filtered off and ethanol
vapor was slowly diffused into the solution to promote the
growth of single crystals. After two days, single crystals of
2 were collected, filtered, washed with ethanol and dried in
air. No proper yield was estimated since only single crystals
were used for further characterization to assure complete
purity of the samples.
Single crystal X-ray diffraction
Single crystal X-ray diffraction measurements were performed
on a Bruker-Nonius diffractometer with an APPEX 2 4 K CCD
area detector at 100 K. Crystal structure solution and refinement were performed using SHELXTL Version 6.10. Data collection and refinement parameters are shown in Table 1.

Results and discussion
Synthesis and structure
Single crystals of 2 were obtained by ethanol slow vapor
diffusion into an aqueous solution of 1 with 20-fold excess of
cesium chloride. These single crystals contain the same trinuclear polyanion, [Fe3(µ-L)6(H2O)6]6– (Fig. 1), where all dimethylammonium cations have been substituted by Cs+ cations. The
molecular structure of the trimer is analogous to that
described before.87 The iron ions occupy two crystallographic

Table 1 Crystallographic data for compound 2

Chemical formula
Formula weigh
T (K)
Crystal system
Space group
Crystal size
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (g cm−3 )
µ (mm−1)
F (000)
θ range (°)
Index ranges
Data/restr/parameters
Goodnesss-of-fit on F2
R1 (I > 2σ(I))
wR2 (I > 2σ(I))

C24H68Cs6Fe3N18O55S12
2838.69
100(2)
Monoclinic
C2/c
0.01 × 0.02 × 0.4
34.4273(12)
11.4107(4)
26.1316(9)
90
128.2290(8)
90
8064.0(5)
4
2.338
3.631
5512
1.583–28.010
–45 ≤ h ≤ 44
–14 ≤ k ≤ 12
–32 ≤ l ≤ 34
9659/921/585
1.028
0.0684
0.1988

sulfonate groups and from coordinated water molecules of
adjacent trimers. The sulfonate groups that do not participate
in this intralayer H-bonded network point towards the cationic
layers.
The cationic layers are formed by two planes of Cs+ cations,
separated by 2.1 ± 0.1 Å (Fig. 3b and c). Each Cs+ center has
short contacts with both adjacent anionic layers, connecting
them with Cs+⋯O3S distances in the 2.9–3.4 Å range. The
three different Cs+ crystallographic positions exhibit different
coordination modes. Cs1 is heptacoordinated, surrounded by
three SO3 groups in monodentate coordination mode and four
water molecules. Cs2 is octacoordinated by six SO3 groups (one
acting as bidentate ligand) and one water molecule. Finally,
Cs3 is heptacoordinated to four SO3 groups (one acting as
bidentate ligand) and two water molecules. This complex
H-bonded network connects crystallographic trimers between
layers with direct O⋯Cs+⋯O contacts.
Magnetic measurements
Fig. 1 Molecular structure of [Fe3(µ-L)6 (H2O)6]6– (top) and labelled
ORTEP representation of the core trimer (bottom), showing the triple triazole-bridges and terminal water molecules. (H atoms have been
omitted for clarity.)

positions (Fe1 and Fe2), forming a linear array of octahedral
Fe(II) linked by triazole bridge. The outer irons (Fe1) complete
their hexacoordination with three water molecules in fac conformation. The metal to ligand distances obtained at 100 K
reveal the spin state of the SCO centers to be high spin (HS)
for Fe1 (average Fe1–N distance: 2.19 Å) and low spin (LS) for
Fe2 (average Fe2–N distance: 1.97 Å).
Regarding the crystallographic packing, cations and anions
form segregated layers along the a axis, following ABAB stacking pattern (Fig. 2). The structure of the anionic layers is
pseudo-hexagonal, with each trimer surrounded by six nearest
neighbors (Fig. 3a). There are multiple intermolecular H-bond
intralayer interactions, with very short H-bond contacts in the
range Ow⋯OSO2 = 2.69–2.95 Å between oxygen atoms from the

The magnetic properties of grained single crystals of 2 were
studied in the 200–400 K range. At room temperature, the χmT
product is 6.0 cm3 mol−1 K, which corresponds to two high
spin iron centers per trimer, suggesting a HS–LS–HS configuration for the ground state as in the parent compound 1. At
very slow temperature scan (Fig. 4), this value remains constant in the 200–350 K range. Above 350 K, χmT shows a sharp
increase, indicating the onset of the spin transition. This transition to the HS–HS–HS state shows two different regimes: first
a sharp increase reaching 8.1 cm3 mol−1 K at 354 K with T1/2(↑)
= 352 K and second, a gradual increase reaching 9.2 cm3 mol−1
K at 400 K. This second regime shows slow kinetics, and at
400 K (the highest T available in our set-up), about 140 min
are needed for χmT to saturate (see Fig. 4 inset). According to
these values, about 2/3 of the centers participate in the abrupt
process, but all anions reach the thermally induced HS–HS–HS
state after the gradual transition is over. When temperature is
decreased at the same scan rate (0.3 K min−1), a thermal hysteresis cycle opens for both regimes. The gradual transition

Fig. 2 Projection of the crystal structure of 2 on the ac (a) and the ab (b) planes, showing the stacking of alternating layers along the a axis.
(Solvation water molecules and H atoms have been omitted for clarity.)

Fig. 3 Representation of the H-bonded networks in the crystal structure of 2. (a) Top view of the anionic layer, showing the pseudo-hexagonal
arrangement of the trinuclear complexes and their closest contacts as dotted lines. (b, c) Side and top views of the cationic layer, showing the contacts between cations, anions and water molecules as dotted lines. Cs atoms are represented in two colors to highlight the two planes in the cationic
layers. (H atoms omitted for clarity.)

We studied the effect of the scan rate in the thermal hysteresis cycle of 2 (Fig. 5). The hysteresis loop is indeed very sensitive to scan rate.101 Dynamic hysteresis cycles observed at 1
and 2 K min−1 are wider, with loops of 57 and 65 K, respectively. The effect upon the cooling branch is particularly significant, and the signature of temperature induced excited spin
state trapping (TIESST) is already apparent at 2 K min−1. The
compound is not able to relax to the ground state, reaching a
minimum low temperature value of 6.25 cm3 K mol−1. This
indicates that about 8% of the molecules are trapped in the
HS–HS–HS state. In the heating branch, these molecules are
able to relax to the ground state above 230 K. At faster cooling
Fig. 4 χmT vs. T plot for 2 during the heating and cooling processes, at
−1
rates, the fraction of HS–HS–HS trapped molecules is also
a 0.3 K min
scan rate. Inset: The sample was kept at 400 K for
higher, reaching 40% at 5 K min−1, and 52% at 10 K min−1
140 minutes to reach saturation.
(Fig. 5). A characteristic TTIESST = 240 K was estimated following the method established by Letard et al.102 (Fig. 6). This
closes just below 300 K, where the abrupt transition drives the value is very close to that reported for 1 (250 K).
compound back to the ground state with T1/2(↓) = 305 K. This
It is important to note that all these cycles were robust and
creates a thermal hysteresis cycle of 47 K for the abrupt reproducible since they correspond to the second and succesregime.

Fig. 5
rates.

Thermal hysteresis cycles for compound 2 at different scan

Fig. 6 χmT vs. T plot for the heating branch (at 0.3 K min−1) after trapping the metastable HS–HS–HS of a sample of 2 via fast cooling at 10 K
min−1. (Inset: The derivate of the heating data, defining TTIESST).

sive cycles once the sample was dehydrated in situ during the
first heating process. We are not reporting the magnetic data
from the very first cycle, where dehydration may also play a
role, as confirmed by thermogravimetric analyses (TGA).
Interstitial solvent molecules are lost upon heating below
130 °C, and the bound water molecules begin to release above
160 °C (Fig. S1‡). The gravimetric plateau above 220 °C, which
is stable at least up to 300 °C, corresponds to a total weight
loss of 8.5%, indicating that not all water molecules are lost
even at such high temperatures. Thus, this material shows very
high thermal stability, with no decomposition of the organic
matter in this temperature range.
Differential Scanning
Calorimetry (DSC) detected two dehydration processes during
the first heating cycle (see Fig. S2‡). However, successive cycles
did not show any feature that could be assigned to a phase
transition associated with the spin transition. In the same
direction, powder XRD data from the same sample used for
multiple thermal hysteresis measurements ( four cycles in the
200–400 K range) confirms that the crystal structure is robust
and preserved after the thermal treatment (Fig. S3‡) and after
partial dehydration, supporting that no major structural
change occurs.

Discussion
When compared with the parent dimethylammonium salt (1),
we can highlight some significant differences in the crystal
packing. The crystal structure of 1 showed high crystallographic disorder in the position of cations and solvent water
molecules even when having lower solvent content (5 vs. 13
crystallization water molecules). The disorder affects the intermolecular interactions network. In particular, the connectivity
among trimers increases from four Ow⋯OSO2 contacts in 1 up
to six nearest neighbors in 2. Additionally, the strong and
directional interlayer interactions mediated by the Cs+ cations
were also absent in 1. In general, the crystal structure of 2
increases the connectivity between trimers in all crystallographic directions.
This phenomenon is expected to increase the cooperativity of the spin transition in this new salt, which is indeed
confirmed from the magnetic data, with the appearance of
an abrupt transition. In addition, the transition moves to
lower temperatures and is complete at 400 K. Apart from this
observation, the increase in cooperativity is typically associated with wider thermal hysteresis. However, the thermal
hysteresis shrinks from 90 K (1) down to 50 K (2). Even more
surprisingly, the higher cooperativity does not affect the
TIESST effect, showing freezing of the metastable state at
slower cooling rates ( just above 2 K min−1). In case of the
parent compound, these combinations are counterintuitive
to the common magneto-structural correlations established
for spin crossover materials. The completeness of the spin
transition in this new compound below 400 K, as confirmed
by magnetic measurements, has also allowed us to study the
DCS data. No features indicative of a phase transition were

found after the first cycle showed some events due to dehydration. The second and successive cycles were featureless. This result supports the primarily molecular origin of
the magnetic features and switching kinetics of this
polyanion.

Conclusions
A cesium salt of the polyanionic trimer [Fe3(µ-L6)(H2O)6]6– was
isolated by completely substituting the dimethylammonium
cations of its soluble salt by excess Cs+ salt and its crystal
structure was determined. This compound displays SCO behavior, showing a two-step spin transition of the central Fe(II)
atom in each trimer from the combination of an abrupt step
above 350 K and a gradual transition up to completeness at
400 K. The difference between heating and cooling branches
showed a quasi-static 47 K wide hysteresis cycle. This finding
is in contrast with that for the parent compound, which
showed wider hysteresis (>85 K) but a gradual transition.
Furthermore,
cooperativity increases while
hysteresis
decreases, which is a counterintuitive observation. The appearance of SCO hysteresis should be associated with crystallographic transition, that is, cooperativity. In addition to the
abrupt hysteresis, there is a second regime that still showed
gradual transition. We assign this non-cooperative contribution to defects in the crystal and surface effects, arising
from molecules loosely participating in the H-bonded
network.
These results show that simple cation exchange can modify
the SCO behavior of this complex since substitution of dimethylammonium by the heavy cesium monocation has
changed the gradual SCO transition into an abrupt process,
which we related to stronger intermolecular connectivity.
Nevertheless, the almost identical TIESST phenomenon
suggests that single-molecule events may be at the origin of
the slow dynamics and features of the compounds containing
this polyanionic trimer. Further studies to confirm if memorylike effects may arise in this polyanionic complex at the single
molecule level are under way.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgements
We would like to acknowledge financial support from the
Spanish Ministerio de Economía y Competitividad (MINECO)
through project CTQ2015-71287-R and the Severo Ochoa
Excellence Accreditation 2014–2018 SEV-2013-0319 and the
Generalitat de Catalunya (2017-SGR-1406 and the CERCA
Programme). AMC thanks MINECO for a pre-doctoral FPI
fellowship.

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