Multistability at Room Temperature in a Bent-Shaped Spin-Crossover Complex Decorated with Long Alkyl Chains

Article abstract of DOI:10.1021/jacs.7b11042

An iron(II) pyridyl-benzohydrazonate-based complex decorated with long alkyl chains is reported as a rare spin-crossover compound displaying a wide thermal hysteresis spanning room temperature. On heating, this compound exhibits a spin transition between

Full text of DOI:10.1021/jacs.7b11042

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Communication
Multistability at room temperature in a bent-shaped spin-
crossover complex decorated with long alkyl chains
Daniel Rosario-Amorin, Pierre Dechambenoit, Ahmed Bentaleb,
Mathieu Rouzieres, Corine Mathonière, and Rodolphe Clérac
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11042 • Publication Date (Web): 28 Nov 2017
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Multistability at room temperature in a bent-shaped spin-crossover
complex decorated with long alkyl chains
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,†,‡
†,‡
†,‡
†,‡
Daniel Rosario-Amorin,* Pierre Dechambenoit, Ahmed Bentaleb, Mathieu Rouzières,
§,#
,†,‡
Corine Mathonière, and Rodolphe Clérac*
†CNRS, CRPP, UPR 8641, F-33600 Pessac, France; ‡Univ. Bordeaux, CRPP, UPR 8641, F-33600 Pessac, France; §CNRS, ICMCB,
UPR 9048, F-33600 Pessac, France; #Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France
S
*
Supporting Information
gels,9 surfactant-free Langmuir Blodgett films10 and other wet-method
based self-assemblies.11 While transferring the SCO property to these
soft materials is usually successfully achieved, preserving its hysteretic
character is more challenging, owing to the reduction of size or thickness
of the self-assemblies and/or the loss of 3D structural order. Even when
occasionally these alkylated complexes crystalize and display a spin tran-
sition,12 its hysteretic character is less pronounced than in non-alkylated
analogues since the flexible alkyl chains absorb the intrinsic defor-
mations of the SCO cores without being able to efficiently propagate
them across the lattice. Nonetheless, these materials can count on other
sources of cooperativity, which rely on the ability of the flexible substit-
uents to promote phase transitions as exemplified at solid / liquid-crys-
tal13 or solid / isotropic liquid14 first order transitions. In crystalline
phases, entropy-driven conformational isomerization of alkyl chains
such as anti/gauche changes or order/disorder transitions can also affect
magnetic properties15 and promote spin-state switching as strikingly il-
lustrated in CoII/terpyridine compounds featuring "reverse" spin transi-
tion.16 Herein, we report on a novel iron(II) pyridyl-benzohydrazonate
based complex functionalized with decyl chains, [Fe(C10-pbh)2] (1;
C10-pbh = (1Z,N'E)-4-(decyloxy)-N'-(pyridin-2-ylmethylene)-benzo-
hydrazonate), displaying spin transition with a wide thermal hysteresis
around RT. Furthermore, we demonstrate for the first time that a spin
transition and the concomitant presence of a monotropic phase lead to
magnetic tristability at room temperature.
ABSTRACT: A novel iron(II) pyridyl-benzohydrazonate based com-
plex decorated with long alkyl chains is reported as a rare spin-crossover
compound displaying a wide thermal hysteresis spanning room temper-
ature. On heating mode, this compound exhibits a spin transition be-
tween a LS ground state and an ordered HS-LS phase with symmetry
breaking from monoclinic P21/n into orthorhombic P21212 space
groups. During the cooling, the compound first transits into a magneti-
cally distinguishable HS-LS phase with monoclinic P21 symmetry before
returning into the LS phase. The interconversion between the two dis-
tinct HS-LS phases is the result of subtle structural changes in the alkyl
chains and produces a second minor thermal hysteresis that superposes
to the large one. This unprecedented result shows that the combination
of a conventional cooperative spin transition and ligand-driven mag-
netic changes can promote magnetic tristability at room temperature.
Spin-crossover (SCO), which involves an interconversion between
the low-spin (LS) and high-spin (HS) states of a metal ion, is considered
as one of the most appealing phenomena for the development of mole-
cule-based switchable materials.1 Remarkably, the spin-state switching
can be triggered by a large panel of means (e.g. temperature, pressure,
light, magnetic/electric fields or chemical stimuli) and in the meantime
affords a broad variety of detectable changes associated with magnetic,
optical, dielectric and mechanical properties. Experimentally, the SCO
process can be gradual in temperature, following Boltzmann population
of the excited HS state (i.e. a thermal spin conversion), or it can involve
a first order phase transition (i.e. a spin transition) that can be abrupt,
stepwise or hysteretic, providing ideal conditions for potential applica-
tions in either sensors, switches or memories.2 Controlling the nature of
the SCO behavior and its characteristic temperature through molecular
design and crystal engineering is one of the main focuses in molecule-
based magnetism, and in this frame, materials displaying wide thermal
hysteresis (> 30 K) centered at room temperature (RT) are the most
wanted.3 Spin transitions have been successfully promoted by creating
covalent linkage between SCO centers in coordination polymers4 or
through supramolecular approaches5 by generating for instance efficient
hydrogen bonding networks. However, SCO systems fulfilling both
large hysteresis and RT transition criteria still remain extremely uncom-
mon.2a,5,6 In other hand, major research efforts have been recently dedi-
cated to nanostructuring and processing SCO compounds as fundamen-
tal steps toward their possible implementation into technological de-
vices.7 In this respect, modification of ligands or counter-ions with long
alkyl chains appears as one of the best and straightforward options since
the low melting temperature, high solubility and amphiphilic character
of resulting materials provide opportunities to produce liquid-crystals,8
Complex 1 was prepared by combining stoichiometric amounts of
FeII(ClO4)2 and C10-pbhH ligand in hot methanol in presence of a base
(Et3N). In a few hours, black prismatic crystals form in good yield
(~80%; Supporting Information). At 315 K, 1 crystallizes in the ortho-
rhombic P21212 space group (Tables S1-S3) for which the asymmetric
unit consists of two crystallographically unique halves of [Fe(C10-pbh)2]
(Figure 1), located on a 2-fold rotation axis that crosses the correspond-
ing Fe site along c-axis. Both Fe sites feature a distorted octahedral N4O2
coordination sphere provided by two tridentate ligands in mer configu-
ration. The Fe-ligand distances for Fe1 (dFe1-O = 2.08 and <dFe1-N> = 2.16
Å) and Fe2 (dFe2-O = 1.98 and <dFe2-N> = 1.91 Å) are in fair agreement
with values previously reported for Fe(II)/pbh analogues with HS and
LS configurations,14,17 respectively, indicating that 1 forms an ordered
HS-LS phase. The coexistence of HS and LS species is also supported by
the octahedral distortion parameter (S),18 which is much larger for Fe1
(154.1°) than Fe2 (79.2°).17 The long alkyl chains attached to the aryl
moieties afford a bent geometry to the complexes, which self-assemble
in a bilayer lamellar organization along the b-axis with an upside-down
arrangement between two subsequent layers; the interlamellar distance,
corresponding to b/2, is 21.05 Å (Figures 2, S2-S4). The bending angles
d, defined by the metal center and the two terminal carbons of the alkyl
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Figure 2. Crystal packing of 1 at 315 K viewed along the a-axis. FeII
LS
and FeIIHS are shown as orange and green octahedra, respectively.
When further cooling, a second transition occurs around 270 K as
shown by the brutal change of lattice parameters (Figure S1, Tables S1,
S3). At 220 K, a centrosymmetric P21/n space group is found revealing
a second crystallographic symmetry breaking. In this low temperature
phase, the asymmetric unit contains a single Fe(II) site for which the co-
ordination bond distances reveal unambiguously its LS state (Figure 1,
Table S2). Remarkably, the geometry of the molecule is strongly modi-
fied with a bending angle of 87.5° (Figure S2). This d angle is close to
the ideal value for an octahedral center and contrasts with the larger val-
ues (98.6-107.6°) found for the HS-LS phases due to the strong octahe-
dral distortion of the HS complexes. This structural change leads to an
impressive shrinkage (about 11 %) of the interlamellar distance from
20.99 Å (290 K) to 18.80 Å (220 K). While shorter Fe-ligand bonds and
the 2D organization of the molecule rigid cores (within ac plan) should
favor a concerted contraction of the network, an increase of the first
neighbor Fe···Fe distance from 6.49 Å (290 K) to 7.19 Å (220 K) and
an expansion of the ac lattice surface (about 7 %) are observed. This
counter-intuitive effect can be easily rationalized considering the strong
decrease of the bending angle and the resulting shortening of the inter-
lamellar distance, which mechanically implies an expansion within the
2D metal array in order to preserve an optimum interdigitation of alkyl
chains. These observations support that the overall deformation of the
crystal packing is driven by the "breathing" of the alkyl chains relaying
the octahedral distortion of the Fe coordination sphere. As a result, im-
portant rearrangements and a weakening of the supramolecular interac-
tions are seen within the metal ion layers (Table S4, Figure S4). Recip-
rocally in the P21 and P21212 phases, stronger intermolecular constraints
resulting from ac plan contraction is believed to be responsible for the
partial spin transition and thus the stabilization of a HS-LS state.
Figure 1. Balls and sticks view of crystallographically unique com-
plex(es) 1 in the P21212 (315 K), P21 (290 K) and P21/n (220 K) phases
viewed along the c-axis. Color scheme: FeII green, FeII orange, N
HS
LS
blue, O red, C grey, H Pink.
chains (dHS = 107.6°, dLS = 101.8°), are similar despite a much stronger
octahedral distortion of the HS Fe(II) site, allowing therefore a near
ideal lipid-like assembly with an optimal interdigitation. The rigid-core
sheets consist of alternating HS/LS moieties where the Fe(II) centers
are perfectly co-planar (ac plan). The cohesion within Fe(II) layers is
essentially assured through C-H···X (X = N, O, p) short contacts.
Among those, C-H···O interactions, which range between 2.38 and 2.67
Å for oxygen atoms belonging to the Fe(II) coordination spheres, ap-
pear as the most relevant to explain cooperative effects within the 2D
metal ion array (Figure S4, Table S4).
Upon cooling to 290 K, unit-cell parameters of 1 remain nearly con-
stant except for b that increases to 91.3°, leading to a change of symmetry
from orthorhombic P21212 into monoclinic P21 space group. This phase
transition is not related to a SCO phenomenon since no significant evo-
lution of the Fe(II) coordination sphere and distortions (S, Q) was de-
tected for the two independent complexes (Table S2). A close examina-
tion of structure suggests that the symmetry change is due to an entropy-
driven transition,15,16,19 involving a conformational isomerization occur-
ring in decyloxy chains of the HS molecules. Indeed, the oxy-pentylene
fragments adopt a gauche-anti-gauche (gag) conformation at 315 K (the
rest of the chain being all anti), while the comparative fragments at 290
K embrace an energetically more stable anti-anti-anti (aaa) geometry
(Figure 1).15,19b Although this transition does not provoke spin-state
switching, the twisting in the ac plan and loss of C2 symmetry elements
modify the intermolecular interactions within the 2D metal layers. Espe-
cially, the HS Fe(II) ion establishes shorter contacts with a pyridyl ring
of a neighboring LS molecule (C-H···Fe 3.20 Å), while noticeable
changes in C-H···O interactions involving Fe(II) coordination spheres
are also observed (Figure S4 and Table S4). As revealed by the magnetic
properties below, such local changes, which slightly affect the coordina-
tion geometry of metal ion and its resulting g factor, induce a substantial
alteration of the magnetic susceptibility, giving rise to non-SCO mag-
netic bistability.15,19a
As in many other switchable materials, the strong deformations inher-
ent to the present first order phase transitions unfortunately affect the
quality of the single-crystals, preventing further single-crystal X-ray dif-
fraction experiments on heating mode. Thus, to probe the reversibility
of the thermal behavior and eventual hysteretic effects, differential scan-
ning calorimetry (DSC) measurements were performed. DSC data re-
vealed a perfectly reproducible thermal behavior over cycling, while no
significant effect of the sweeping rates from 1 to 10 K/min was evi-
denced (Figures 3 and S7). During the cooling, two exothermic peaks
are found at 296 and 267 K confirming the two successive P21212 → P21
and P21 → P21/n phase transitions, respectively (Figure S1). The en-
thalpy and entropy changes associated to spin transition at 267 K, DH =
-13.9 kJ mol-1 and DS = -52.1 J K-1 mol-1, are relatively high considering
that only one of the two complexes experiences spin-state switching.
Nevertheless, these values can be rationalized considering that strong
deformations of the overall crystal packing (vide supra) are associated to
the spin transition. The enthalpy and entropy of high-temperature tran-
sition (296 K), DH = -10.1 kJ mol-1 and DS = -34.1 J K-1 mol-1, fall in the
expected range for anti/gauche conformational isomerizations taking
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into account that four of them are present in the HS complex.15 Hence,
P21212 → P21 transition between HS-LS phases detected by DSC and X-
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the gag/aaa isomerization appears unambiguously the primary cause be-
hind this crystallographic phase transition. During the warming (Figures
3 and S7), only one sharp endothermic peak is found at 301 K with DH
= 22.6 kJ mol-1 and DS = 75.1 J K-1 mol-1. These values, which roughly
correspond to the DH and DS sums obtained from the two exothermic
peaks, and the high symmetry of the peak, suggest that both spin transi-
tion and isomerization of alkyl chains occur in a concerted fashion
(P21/n → P21212). As a consequence, the P21 phase is unequivocally a
monotropic polymorph, which appears only in a metastable domain be-
tween the thermodynamic P21/n (LS) phase and the entropic P21212
(HS-LS) phase. When warming from the P21 phase, the reverse P21 →
P21212 transition is observed at 299 K (DH = 10.2 kJ mol-1 and DS = 34.1
J K-1 mol-1), revealing a hysteresis of about 3 K. Hence, between 296 and
299 K, three distinct phases can be observed depending on the thermal
history of 1, conferring to this material an unprecedented tristability.
ray diffraction techniques. Therefore, the variation of the susceptibility
is undoubtedly induced by the slender modification of supramolecular
contacts around the HS iron(II) site (vide supra).
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Figure 4. Variable-temperature PXRD patterns of 1 recorded between
273 and 383 K on heating-cooling-heating modes, showing the P21/n →
P21212 (left), P21212 → P21 (center), P21 → P21212 (right) transitions.
Figure 3. Differential scanning calorimetry (DSC) traces for 1 in the
range 350-230 K (black) and 350-285 K (red), showing the monotropic
thermal behavior (10 K/min).
This intriguing thermal behavior was also monitored by powder X-ray
diffraction (PXRD) experiments. The sample was first cooled to 77 K to
reach the LS phase and then PXRD patterns were collected on warming
mode between 273 and 383 K (Figure 4 left, Figure S5a). The diffracto-
grams below and above 303 K are in good agreement with simulated
powder patterns for the P21/n and P21212 phases (Figure S6), respec-
tively. No evidence for the P21 phase was detected including during the
transition, confirming DSC data and the monotropic nature of this
phase. On the subsequent cooling/heating cycle between 383 and 273
K (Figure 4, center and right), the P21212 → P21 transition is detected
somewhere between 298 and 293 K, while the reverse transition is found
between 298 and 303 revealing a slightly hysteretic behavior. It is note-
worthy that the monotropic phase remains stable down to 273 K, show-
ing a bistable P21/n - P21 domain of at least 25 K. The tristability of 1 is
illustrated by the subsequent collections at 298 K (central pink diffrac-
togram in Figure 4), which can be assigned to the three distinct phases,
P21/n, P21212 and P21, from left to right parts of Figure 4.
Figure 5. Temperature dependence of the cT product at 1 T for 1 over
the 1.8-400 K range (0.2 K/min). Inset: expanded view between 292
and 306 K emphasizing the P21 / P21212 transition (0.05 K/min).
Upon further cooling, the transition into the diamagnetic P21/n phase is
observed at 268 K, revealing a thermal hysteresis of about 35 K, which is
reproducible over cycling and almost scan rate independent (0.2 to 12
K/min; Figure S9). It should be emphasized that the hysteresis is much
wider than in any pbh based analogues,17 making of this compound one
of the rare examples displaying an improvement of the cooperativity
upon functionalization with alkyl chains.11a,16 Remarkably, 1 is also one
of the very few SCO compounds displaying this type of properties in the
RT region, which is considered as a prerequisite for numerous practical
applications. In agreement with calorimetric and X-ray diffraction exper-
iments, the warming from the HS-LS P21 phase reveals a weak hysteresis
loop (DT = 1.2 K; 0.05 K/min) centered at 298.8 K associated with the
P21 / P21212 transition (Figures 5 inset, S10). Although the shape and
small width of the hysteresis loop prevent from reaching the maximal
amplitude of the magnetic signal at the same temperature, three distinct
cT values (e.g. at 298.8 K: 0.14, 1.51 and 1.57 cm3 K mol-1) can be meas-
ured depending on the thermal history of the material. It is noteworthy,
that a similar small jump of the magnetic moment occurring in a spin
transition hysteresis loop was previously described in a Co(II) terpyri-
dine complex, however the crystal structures on both sides of the transi-
tion, as well as its potential hysteretic character, were not reported.16a
Magnetic susceptibility measurements recorded between 1.8 and 400
K are shown as cT vs. T plot in Figure 5. Below 300 K, the cT value is
close to zero (at 100 K: 0.08 cm3 K mol-1) as expected for a diamagnetic
material like 1 in its LS ground state. Above 300 K, a first-order spin tran-
sition centered at 303 K is observed. At 320 K, the cT product (normal-
ized per Fe(II) complex) reaches 1.55 cm3 K mol-1 in agreement with a
HS-LS phase containing a 1:1 mixture of two isolated Fe(II) magnetic
centers, one being LS (S = 0) and one HS (S = 2). Up to 400 K, no fur-
ther marked evolution of the cT product is observed confirming the in-
complete SCO behavior. When cooling, the cT product decreases
slowly from 1.67 cm3 K mol-1 at 400 K to 1.49 cm3 K mol-1 at 300 K, upon
which its value jumps to 1.58 cm3 K mol-1 (295 K), reaching a plateau
down to about 273 K. This magnetic anomaly coincides with the
In summary, we have reported the synthesis, structural and magnetic
characterizations of a new alkyl-chain-functionalized Fe(II) SCO com-
plex exhibiting a cooperative spin transition at room temperature. The
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favorable conditions for these cooperative properties. The single-crystal
X-ray diffraction investigations revealed that the alkyl groups magnify
the coordination sphere distortion provoking a huge deformation of the
overall crystal packing that is manifested by a crystallographic symmetry
breaking. Furthermore, thanks to the intrinsic conformational changes
of the flexible alkyl chains, which impact the magnetic behavior at a tem-
perature concomitant to the spin transition, we demonstrate for the first
time that the association of these two phenomena can be used to pro-
mote magnetic tristability at room temperature.
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n
ASSOCIATED CONTENT
S
*
Supporting Information
X-ray crystallographic files (CIFs), experimental details, additional crys-
tallographic, DSC, PXRD and magnetic data. The Supporting Infor-
mation is available free of charge on the ACS Publications website.
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AUTHOR INFORMATION
Corresponding Authors
rosario@crpp-bordeaux.cnrs.fr; clerac@crpp-bordeaux.cnrs.fr
ORCID
Rodolphe Clérac: 0000-0001-5429-7418
Notes
The authors declare no competing financial interests.
n
ACKNOWLEDGMENTS
This work was supported by the CNRS, the ANR (PDOC-13-003201,
MagnLC project), the University of Bordeaux, the Région Nouvelle Aq-
uitaine, the GdR MCM-2 and the MOLSPIN COST action CA15128.
We thank Dr. Yun-Nan Guo for his contribution in related studies.
n
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