Cooperative spin transition in a lipid layer like system

Article abstract of DOI:10.1039/c1cc12162f

A novel iron(ii) mononuclear spin transition complex [FeL(py)₂] displays an abrupt spin transition around 225 K accompanied by a very wide thermal hysteresis loop (～50 K) that spreads out over 100 K. Crystal structure analysis in both low-spin and high-spin states reveals a lipid layer-like arrangement of the complex molecules and provides insights into the spin switching mechanism.

Full text of DOI:10.1039/c1cc12162f

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Cooperative spin transition in a lipid layer like systemw
Stephan Schlamp,a Birgit Weber,*a Anil D. Naikb and Yann Garcia*b
Received 14th April 2011, Accepted 9th May 2011
DOI: 10.1039/c1cc12162f
A novel iron(II) mononuclear spin transition complex [FeL(py)2]
displays an abrupt spin transition around 225 K accompanied by a
very wide thermal hysteresis loop (B50 K) that spreads out over
100 K. Crystal structure analysis in both low-spin and high-spin
states reveals a lipid layer-like arrangement of the complex
molecules and provides insights into the spin switching mechanism.
Since the discovery of spin crossover (SCO) compounds in 1931
by Cambi et al.1 the interest in this substance class never
vanished,2 as the thermochromism associated with the spin state
change makes them potentially useful for various applications
such as display and memory device units,3 sensors4 and cold
channel control units in food and medical sectors.5 In order to
realise application it is important to explore different possibilities
for the nanostructuration of SCO materials6 and to investigate if
Fig. 1 ORTEP drawing of the asymmetric unit of 1 in the HS (top)
additional properties can be combined (e.g. liquid crystal
behaviour,7 magnetic exchange interactions8) resulting in multi-
functional SCO materials.9 In this frame, we modified Schiff base
like ligands used for the synthesis of SCO complexes10 by adding
long alkyl chains in the outer periphery by preparing [FeL(py)2]
(1) with L = (E,E)-[diethyl-2,20-[4,5-dihexadecyloxy-1,2-phenyl-
enebis(iminomethylidyne)]bis(3-oxobutanato)]. We aimed to
study the influence of this modification on the crystal packing
as well as on the SCO behaviour and investigate if additional
features could be achieved for 1.
and LS (bottom) states. Hydrogen atoms and the water molecule have
been omitted for clarity. Displacement ellipsoids are shown at the 50%
probability level.
Table 1 Selected bond lengths [A] and angles [1] within the inner
coordination sphere of 1 in the HS and LS states
b
Fe–Neq Fe–Oeq Fe–N(Lax
)
O–Fe–O Lax–Fe–Lax +Lax
HS 2.059(3) 2.001(2) 2.284(3)
2.086(2) 1.999(2) 2.280(4)a
2.288(7)a
106.10(9) 173.73(12)a
176.95(23)a 21.5a
47.5a
LS 1.897(2) 1.935(2) 2.021(2)
1.907(2) 1.947(2) 2.014(2)
88.80(7) 175.06(8)
83.6
A complete description of the synthesis of H2L and the iron
complex (1) is given in the ESI.w Single crystals suitable for an
X-ray analysis of 1ꢀ0.25H2O were obtained and the crystal
structure was determined first at 250 K and then at 125 K
a
b
Disorder. Angle between the pyridine planes.
ꢀ
(same crystal, in both cases space group P1), corresponding to
environment is given in Fig. S1 (ESIw). Selected bond lengths
and angles around the inner coordination sphere of the iron
centre are summarized in Table 1.
the high-spin (HS) and low-spin (LS) states of the complex as
seen in the magnetic measurement (Fig. 3). The crystallo-
graphic data are summarized in the ESIw, Table S1. Fig. 1
displays an ORTEP drawing of the asymmetric unit of 1 in the
HS and the LS state. An excerpt of the coordination
The average bond lengths within the first coordination sphere
of the iron(II) centres in the HS structure are 2.07 A (Fe–Neq),
2.00 A (Fe–Oeq) and 2.28 A (Fe–Lax). Those and the observed
O–Fe–O angle (1061) are in the region expected for HS complexes
of this ligand type.10,11 Upon the HS to LS transition a shortening
of the bond lengths of about 10% is observed, as observed for
other iron(II) SCO complexes.2,11 The average bond lengths in the
LS-structure are 1.90 A (Fe–Neq), 1.94 A (Fe–Oeq) and 2.02 A
(Fe–Lax) with an O–Fe–O angle of 891. In the HS state, a disorder
is observed of one pyridine and at the end of one of the C16 alkyl
chains. The pyridine ring including N4 is contorted in two
directions in a relative ratio of 60 : 40, the same ratio is observed
for the ethyl endgroup (C35 and C36) in the alkyl chain bound by
O7. The planes spanned by the two axial pyridine rings are
a Inorganic Chemistry II, Universitat Bayreuth, Universitatsstrabe 30,
¨
¨
NW 1, 95440 Bayreuth, Germany. E-mail: weber@uni-bayreuth.de;
Fax: +49-92155-2157; Tel: +49-92155-2555
b Institute of Condensed Matter and Nanosciences,
´
MOST-Inorganic Chemistry, Universite Catholique de Louvain,
Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium.
E-mail: yann.garcia@uclouvain.be; Fax: +32 10472831;
Tel: +32 10472826
w Electronic supplementary information (ESI) available: Experimental
details and spectroscopic analysis. CCDC 821515 and 821516. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc12162f
c
7152 Chem. Commun., 2011, 47, 7152–7154
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an iron(II) complex in the LS state. Upon heating the ST occurs in
two steps with about 30% of the molecules involved in the first
(Tc(30)m = 229 K) and about 70% of the molecules involved in the
second step (Tc(70)m = 245 K). The width of the thermal hysteresis
loop is 23 K for the first and 47 K for the second step. The second
and third thermal cycles reveal the absence of the second gradual
step and the remaining wMT product is 0.3 cm3 K molꢁ1 at 100 K
indicating an iron(II) complex almost completely in the LS state.
The sample has been studied by differential scanning
calorimetry (DSC) over the temperature range 300–98 K in
cooling and warming modes, at 10 K minꢁ1, in order to extract
thermodynamical parameters associated with the spin transition,
and probe the order of the phase transitions. On warming from
98 K, two major endothermic peaks corresponding to a first order
Fig. 2 Packing of the molecules of 1ꢀ0.25 H2O in the crystal projected
along [1 0 0] in the HS state.
staggered in the HS state. Upon cooling the disordered pyridine
ring changes its orientation resulting in a nearly perpendicular
arrangement in the LS state. An additional water molecule is
observed in the crystal packing with an approximate occupation
number of 0.25. Several hydrogen bonds and short contacts
are observed between the complex molecules and the water
(Table S2, ESIw).
In the crystal the molecules are packed in a lipid layer like
arrangement as illustrated in Fig. 2 with the layers running
along the a–b-plane. Within one layer, the alkyl chains of the
Schiff base like ligand are packed in the middle and the SCO
centres are on the outer sides.
phase transition proceeding in two steps were detected (Fig. 4).
(1)m
The first one is observed at Tmax
(2)m
= 235(1) K and a broader
peak is found at Tmax
= 250(1) K. These data match the
transition temperatures derived from SQUID measurements (see
Fig. 3). A very less intense peak, whose shape indicates a
continuous or weakly first-order phase transition, is observed at
T(3) = 240 K corresponding to the plateau region between the two
phase transitions. On cooling from room temperature, broader
exothermic peaks arise at different temperatures, confirming
the hysteretic character of the ST process. A peak is found at
Tmax(2)k = 226(1) K, in agreement with the first ST branch, which
is followed by a second peak at Tmax(1)k = 207(1) K, which only
corresponds to the onset of the plateau region of the magnetic
curve. The tiny second step in the cooling mode of the first cycle of
the SQUID measurements around 182 K is not seen by DSC.
Such a DSC profile was confirmed by two successive cooling and
heating cycles. Interestingly, two peaks are still observed in the
warming and cooling process after the first cycling. One is clearly
related to the spin transition (2) whereas the other peak (1) relates
to a thermal anomaly that plays a role in the spin state during the
first cycle but not later, still existing but not affecting it. Enthalpy
and entropy associated with these peaks are given in Table S3
(ESIw).
In the LS state a network of hydrogen bonds is formed
between the SCO centres between the layers and within the
layer, the additional water molecule being also involved, as
illustrated in Fig. S2 (ESIw). In the HS state the rearrangement
of the pyridine ring and the alkyl chain results in changes for
the short contacts. Now, short contacts are only observed
between the molecules of one lipid like layer but not between
the layers as illustrated in Fig. S2 and S3 (ESIw).
Magnetic susceptibility data of 1ꢀ0.25 H2O were recorded on
cooling and warming over the temperature range 300–5 K (Fig. 3).
At room temperature the wMT product is 3.0 cm3 K molꢁ1 which
is in the range expected for an iron(II) complex in the HS state. In
the first cycle (squares) upon cooling the magnetic moment
remains constant until 229 K where an abrupt spin transition
(ST) takes place with about 60% of the molecules and Tc(60)k
=
This unprecedented ST behaviour can be explained thanks
to the results from X-ray structure analysis. Step-wise spin
transitions are often related to the presence of two or more
inequivalent iron centres.12 In our case, the disorder of the
pyridine ring in the HS state could be responsible for inequivalent
iron centres and thus the steps in the transition curve. Such a
222 K. In the temperature range between 214 K and 185 K the
magnetic moment decreases gradually from wMT (214 K) =
1.1 cm3 K molꢁ1 to wMT (185 K) = 0.6 cm3 K molꢁ1. Below
185 K a second abrupt step is observed involving the remaining
20% of the molecules Tc(20)k = 182 K. At 100 K the remaining
wMT product is 0.1 cm3 K molꢁ1 which is in the range expected for
Fig. 4 Heat capacity vs. T of 1 over the temperature range 261–180 K
at a scanning rate of 10 K minꢁ1 in the cooling (’) and warming (-)
modes.
Fig. 3 wMT vs. T plot of 1 of the first cycle (squares) and all the following
cycles (open circles). Displayed in the temperature range 300–100 K.
c
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Chem. Commun., 2011, 47, 7152–7154 7153

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situation was recently discussed by Matouzenko et al.13 with the
same symmetries in the HS and LS states and a symmetry
breaking on the plateau.14 As different intermolecular interactions
are observed for the disordered parts of the HS state (Fig. S2,
ESIw) the differences in the hysteresis width can be explained with
differences in the H-bond network. It should be noted that the
space group does not change upon ST and thus the observed
hysteresis cannot be related to a structural phase transition but
must be related to other cooperative effects. Due to the long alkyl
chains in the outer periphery of the complex the observed change
in the cell volume (DV/V = 2.9%) is very small, especially when
the contribution from the thermal contraction is considered. Thus
the hysteresis cannot solely be related to elastic interactions.
The changes in the hydrogen bond network are one possible
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A very
interesting feature concerns the second peak in the DSC measure-
ments that is related to the gradual part in the ST curve. This
thermal anomaly could result from an order–disorder transition of
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orders in the LS, as detected by X-ray diffraction. Such types of
transitions are known to be able to control the course of a ST
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crystallise in a lipid layer like structure and show a cooperative
spin transition with a wide thermal hysteresis loop (up to
B50 K). The X-ray structure solved in the HS and LS states
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This journal is The Royal Society of Chemistry 2011