Cooperative 1D Triazole-Based Spin Crossover FeII Material with Exceptional Mechanical Resilience

Article abstract of DOI:10.1021/acs.chemmater.6b04118

A study reported the synthesis, the structural and magnetic characterizations, as well as the first accurate single crystal investigations of both high spin state (HS) and low spin state (LS) states of a new triazolebased spin crossover (SCO) Fe 1-D polymer [Fe(bntrz)₃][Pt(CN)₄]·H₂O. The bntrz ligand was prepared in two steps by reaction of triethyl orthoformate with formylhydrazine and benzylamine under nitrogen atmosphere. Single crystals of [Fe(bntrz)₃][Pt(CN)₄]·H₂O was synthesized via diffusion technique in a fine glass tube by layering an ethanolic solution of the bntrz ligand onto an aqueous solution containing both K₂[Pt(CN) ₄)]xH₂O and Fe(BF₄)₂·6H₂O salts.

Full text of DOI:10.1021/acs.chemmater.6b04118

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Communication
Cooperative 1D Triazole-Based Spin Crossover FeII
Material With Exceptional Mechanical Resilience
Narsimhulu Pittala, Franck Thetiot, Smail Triki, Kamel
Boukheddaden, Guillaume Chastanet, and Mathieu Marchivie
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04118 • Publication Date (Web): 22 Dec 2016
Downloaded from http://pubs.acs.org on December 26, 2016
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Cooperative 1D Triazole-Based Spin Crossover FeII Material With Ex-
ceptional Mechanical Resilience
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Narsimhulu Pittala,† Franck Thétiot,† Smail Triki,*,† Kamel Boukheddaden,‡ Guillaume Chastanet,§ and
Mathieu Marchivie§
†UMRꢀCNRS 6521, University de Brest (UBO), 6 Av. V. Le Gorgeu, C.S. 93837 ꢀ 29238 Brest Cedex 3 ꢀ France
‡GEMaC, UMRꢀCNRS 8635, University de ParisꢀSaclay, 45 Avenue des EtatsꢀUnis 78035 Versailles, France
§CNRS, University of Bordeaux, ICMCB, UPR 9048, 87 avenue du Dr A. Schweitzer, Fꢀ33608 Pessac, France
ABSTRACT: We report here the single crystal investigations, in both high spin (HS) and low spin (LS) states, correlated to magnetic and
heat capacity measurements, of a new cooperative and robust spin crossover triazoleꢀbased FeII 1D coordination polymer
[Fe(bntrz)3][Pt(CN)4].H2O (1) (bntrz = 4ꢀ(benzyl)ꢀ1,2,4ꢀtriazole). The compound exhibits a sharp transition at 240 K, accompanied with an
anisotropic deformation of the unit cell and a significant gliding of the chains from each other. These two features are identified as the key
parameters of the nonꢀconventional mechanical resilience of this system.
The magnetic spin change associated to the spin crossover
(SCO) phenomenon represents a paradigm of bistability at the
molecular level which is of current interest because of
potential applications in the development of new generations
of electronic devices such as nonꢀvolatil memory, molecular
sensing and displays.1ꢀ5 The SCO phenomenon is especially
observed in FeII complexes in which the paramagnetic high
spin state (HS, S = 2) can be switched reversibly to the low
spin state (LS, S = 0) by several external stimuli such as
temperature, pressure or light irradiation.6 A huge effort has
been devoted to the understanding of the transition
mechanisms using inter alia crystallographic tools.7ꢀ8 The
minimum requirements for the latter involve the determination
of the crystal structures in both LS and HS states to
comprehend the structural parameters at the molecular and
interꢀmolecular scales. While some systems were deeply
investigated due to their remarkable switching properties, the
absence of any detailed and punctilious structural informations
has precluded the definite understanding of these exciting
properties. This is particularly the case of the soꢀcalled
“triazole family”, more specifically coordination polymer
chains of general formula [Fe(Rtrz)2(trz)](X), where Rtrz is a
functionalised 1,2,4ꢀtriazole ligand and X a counterꢀanion,
which typically display bistability with wide hysteresis loops
around or above room temperature.9ꢀ11 Although known for
few decades, the major lack of high quality single crystals
with complete structural data for those materials prevent any
deep magnetoꢀstructural correlations which are of paramount
importance for (i) the understanding of the origin of the
thermal hysteresis loop, and (ii) the control of the transition
temperature. Indeed, a deep knowledge of the structural data is
required to evaluate notably the effects of the rigid triple ꢀ1,2ꢀ
trz bridges and/or the characteristics of the intramolecular
contacts on the propagation of the elastic interactions. Owing
to the great and intensifying attention given to these materials,
scarce crystallographic investigations have been recently
attempted. Among them, the structural characterizations of the
HS and LS states of the [Fe(Htrz)2(trz)](BF4) derivative, using
distinctively highꢀresolution synchrotron Xꢀray powder
diffraction combined with Raman spectroscopy12 or Xꢀray
diffraction data from highꢀquality crystalline powder,13 have
been reported. While the 1D chain structure was proven for
both HS and LS states, the rather limited quality of the
diffraction patterns could not provide highly precise structural
data suitable for deep magnetoꢀstructural correlations. In the
same time, a single crystal of the parent CuII compound
[Cu(NH2trz)3](NO3)2.3H2O has been finely investigated by
Garcia et al. to parallel with the crystal structure of the
[Fe(NH2trz)3](NO3)2 for which only nanoꢀsized thin crystallite
needles (120ꢀ330 nm) could be prepared. However, the
powder diffraction pattern of the latter did not fully fit to the
diffraction pattern observed for the single crystals of the CuII
compound.10 Besides, the lone single crystal structural study
proving
the
presumably
1D
character
of
[Fe(NH2trz)3](NO3)2.2H2O was only reported in 2011 by
Guionneau et al..14 However, as clearly claimed by the
authors, the very low diffraction pattern induced essentially by
the subꢀmicrometric size (> 20x1x1 ꢀm3) of the crystals and
the damages due to the Xꢀray beam during the data collection
led to a rather low quality of the structural data which did not
allow an accurate detailed discussion of the polymeric chain
structure. In addition, the study, performed at 120 K,
concerned only the LS state of the sample since the damages
caused on the crystals when warming over room temperature
have hindered data collection for the HS state. Ultimately,
those recent examples clearly expose the challenge to access
refined structural data in both spin states in order to fully
apprehend and rationalize the conspicuous SCO properties in
the FeII/trz systems.
In the last few years, some FeII systems based on functionꢀ
alized triazole ligands (Rtrz) and different counterꢀanions have
been reported.11 However, the majority of the involved counꢀ
terꢀions are rather standard monoanions, while the functional
groups (R) of the triazole molecules are essentially narrowed
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to linear or branched alkyl or alkoxylated chains.11 Recently,
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we have extended this synthetic approach to combinations of
triazole ligands and anions which were not used so far within
the “triazole family”; among those combinations, triazole
ligands involving rigid aryl groups with an alkyl spacer assurꢀ
ing the electronic integrity of the triazole motif, coupled with
more sophisticated anions such as polycyanometallates or
organic cyanocarbanions which display very diverse sizes,
geometries and charges, have been initially considered.15ꢀ16 In
this ongoing work, we have realized at an early stage that this
chemical approach involving these unusual building blocks
tends to improve unprecedentedly the size, the quality and the
robustness of the single crystals of the resulting original
FeII/Rꢀtrz systems. Ultimately, this synthetic strategy should
allow the access to single crystals of novel SCO cooperative
materials, which is essential to understand the origin of the
unparalleled cooperativity observed in those 1D coordination
polymers. Thus, the present work reports the synthesis, the
structural and magnetic characterizations, as well as the first
accurate single crystal investigations of both HS and LS states
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Figure 1. Thermal dependence of the χmT product recorded at 0.4
K.mnꢀ1 in a settle mode, DSC study recorded at 2 K.mnꢀ1 (inset),
and crystal images in the LS and HS states showing the clear
thermochromism from purple to colourless, respectively.
of
a
new triazoleꢀbased SCO FeII 1ꢀD polymer
[Fe(bntrz)3][Pt(CN)4].H2O (1).
The bntrz ligand was prepared in two steps by reaction of
triethyl orthoformate with formylhydrazine and benzylamine
under nitrogen atmosphere (SI). Single crystals of
[Fe(bntrz)3][Pt(CN)4].H2O (1) have been synthesized via
diffusion technique in a fine glass tube (3.0 mm diameter) by
layering an ethanolic solution of the bntrz ligand onto an
aqueous solution containing both K2[Pt(CN)4)].xH2O and
Fe(BF4)2.6H2O salts (SI). The magnetic susceptibility (
was measured over the 2ꢀ300 K temperature (T) range on a set
of single crystals. The mT versus T plot is displayed in Figure
1. In the highꢀtemperature region, the
mT value (3.45 cm3.
K.molꢀ1) is consistent with a HS (S = 2) configuration of the
hexacoordinated FeII ions. Upon cooling, the
mT remains
χm) of 1
χ
Figure 2. Views of the cationic chain in 1: (a) linear chain runꢀ
ning along the a axis; (b) projection of the chain along the [100]
direction. (a) 1ꢀx, ꢀy, 1ꢀz; (b) = ꢀx, ꢀy, 1ꢀz.
χ
χ
constant down to 240 K, then sharply decreases to ca. 0.0 cm3.
K.molꢀ1, indicating the presence of a complete sharp HS to LS
firstꢀorder spin transition (T1/2 = 242 K). However, no
significant thermal hysteresis effects were detected after the
warming mode, at the value of the temperature scan rate of 0.4
K.mnꢀ1. The DSC measurements for 1, performed in the
temperature range 200ꢀ260 K with a temperature scan rate of 2
K.mnꢀ1, reveal exoꢀ and endoꢀthermic transitions with maxima
at 241.1 K and 243.5 K, respectively (Inset Figure 1).
Furthermore, the phase transition has been found to occur with
an enthalpy and entropy changes of ꢀH = 16.87 kJ.molꢀ1 and
ꢀS = 69.61 J.Kꢀ1.molꢀ1, respectively. These values are in the
range of those reported in the literature for similar SCO
compounds.17 The presence of a weak hysteresis in the
calorimetry data while it was absent in magnetic
measurements can be reasonably attributed to the difference of
temperature scan rates.
The three bntrz ligands act as ꢀ2ꢀbridging mode (ꢀ1,2ꢀbntrz
bridges) leading to a regular chain structure running along the
[100] direction (Figure 2). The average FeꢀN distances and
distortion parameters for both spin states are listed in Table 1.
The selected FeꢀN bond lengths and NꢀFeꢀN bond angles, at
296 K and 120 K, are gathered in Table S2. The two crystalloꢀ
graphically distinct FeII ions (Fe1 and Fe2) adopt similar and
very regular FeN6 octahedral geometries, as demonstrated by
the unusually low values of Σ and Θ parameters (Table 1).18
Thus, it is worth noting that the distortion of the octahedrons is
higher in the LS state than in the HS state, conversely to what
is generally observed. While more pronounced for Fe1 comꢀ
pared to Fe2, this unusual effect confirms that 1D triazole
systems tend to adopt rigid and regular coordination spheres,13
but also indicates that the FeII environment is more constrained
in the LS state. The average values of the FeꢀN distances, at
296 K (2.190(3), 2.189(3) Å) and 120 K (1.991(4), 1.998(4)
Å), are in agreement with the presence of HS and LS states,
respectively. The latter observation is consistent with the
presence of a complete HS/LS SCO transition, as revealed by
the magnetic data.
Based on the magnetic and calorimetric measurements, the
crystal structure of 1, defined in the triclinic space group, has
been determined at 296 K (HS state, colourless crystal), and at
120 K (LS state, purple crystal) (Table S1). An inꢀdepth examꢀ
ination of the unit cell parameters, at these two temperatures,
indicates the absence of any symmetry breaking structural
transition within the studied temperature range. The structure
of 1 is built from two crystallographically independent FeII
sites, Fe1 and Fe2, and two [Pt(CN)4]2ꢀ anions located on
inversion centers, and three bntrz ligands and one solvent
water molecule located on general positions.
Table 1. Average FeꢀN distances and distortion parameters in 1.
T (K)
296
120
FeII
Fe1
2.190(3)
10/23
HS
Fe2
2.189(3)
10/26
HS
Fe1
1.991(4)
16/40
LS
Fe2
1.998(4)
13/33
LS
〈dFeꢀN〉 / Å
Σ
/
Θ
/ °
Spin state
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Figure 4. Intermolecular hydrogen bonding contacts in 1 (a)
along the a direction. Shortest contacts (Å): N11…O1 = 2.944,
O1…N12 = 2.995 at 296 K; N11…O1 = 2.888, O1…N12 = 2.883
at 120 K.
Figure 3. Thermal evolution of the lattice parameters for 1 showꢀ
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ing the anisotropic changes upon SCO transition.
tendency since the interꢀchain distances along the b direction
(14.4975(4) Å) are longer than those along the c axis
(14.2832(4) Å), leading finally to very similar distances in the
LS and HS states, but inverted with respect to b and c parameꢀ
ters (Figures S5ꢀS6). These observations indicate a significant
motion of the chains with respect to each other in the bc plane
that coꢀoccurs with a slight gliding along the a direction as the
chains displacements increase during the LS to HS transition
(c.a. 0.35 Å). Additionally, the LS to HS transition is accomꢀ
panied by a rearrangement of the side chain of the triazole
ligand, leading to a much better accommodation of the modifiꢀ
cation of the iron coordination sphere volume in 1 than in the
parent compound [Fe(Htrz)2(trz)](BF4).13 Indeed, while the
octahedron volume (Vp) increases in similar proportion for
both compounds (Vp increase of 32 % for 1 and 36 % for
[Fe(Htrz)2(trz)](BF4)), the molecular volume remains almost
the same for 1 (+0.8 %) but increases appreciably for
[Fe(Htrz)2(trz)](BF4) (+4.7 %, Table S3). This structural rearꢀ
rangement of the chains and the related accommodation of the
molecular volume to the transition might be responsible for
the smaller unit cell volume change at the transition when
compared to the parent compound [Fe(Htrz)2(trz)](BF4) (4.5 %
vs 11.5 %).13 Such capability of structural breathing was not
observed in [Fe(Htrz)2(trz)](BF4) and may explain the (i)
unusual crystal quality of compound 1 and its (ii) exceptional
reliability and mechanical resilience to repeated LS to HS
transition. The latter has been clearly evidenced by crystal
mosaïcity studies and optical microscopy investigations perꢀ
formed over 20 SCO cycles. Thus, (i) the crystal mosaïcity
was found to be nearly constant around 0.49(2) between the
LS and HS states (Figure S7), and (ii) no damage or alteration
at the resolution of the optical microscopy (~0.3 micron) was
detected within the single crystal (Figure S8). In addition, an
elongation of 5.3 % of the crystal upon LS to HS transition
was observed in optical microscopy studies. This value is in
excellent agreement with Xꢀrays data which indicates an elonꢀ
gation of 5.32 % of the a parameter during the LS/HS thermal
transition.
In order to understand how the crystal structure is affected by
the SCO transition, the temperature dependence of the lattice
parameters of a single crystal of 1 was measured in the range
200ꢀ280 K. As clearly depicted in Figure 3, the evolution of
the unit cell parameters (a, b, c) reveals significant anisotropic
changes at the transition temperature (ca. 242 K), more preꢀ
cisely a contraction upon heating for the c parameter paralꢀ
leled with positive expansions occurring simultaneously in the
ab plane. The amplitude of the anisotropic expansion is much
more pronounced along the a axis, proving that the major
structural change accompanying the SCO transition in the
overall structure arises along the 1D chain. In comparison with
the parent 1D compound [Fe(Htrz)2(trz)](BF4),13 the careful
examination of the shortest interchain contacts in 1 reveals the
absence of direct Hꢀbonding interchain interactions while
strong direct Hꢀbonding interchain interactions through the Nꢀ
H
group of the triazole have been identified in
[Fe(Htrz)2(trz)](BF4) (Figures S1ꢀS2). However, the presence
of significant indirect Hꢀbonding, occurring via the [Pt(CN)4]2ꢀ
anions, links the chains in the b and c directions, leading to an
overall 3D packing (Figures 4 and S3). These interactions in 1
are more pronounced in the LS state than in the HS state; this
observation may explain the more constrained iron geometry
along the chain in the LS state. The intraꢀchain Fe⋅⋅⋅Fe distancꢀ
es in
1 are almost similar to the values than in
[Fe(Htrz)2(trz)](BF4), in both spin states, while the interꢀchain
Fe⋅⋅⋅Fe ones are significantly longer than those observed in
other parent 1D triazoleꢀbased derivatives that have been
structurally characterized (see Table S3). Moreover, the
Fe⋅⋅⋅Fe intraꢀchain distances in 1 appear also longer in both HS
and LS states than the corresponding distances observed in
discrete trinuclear complexes based on triazole ligands,19ꢀ22
although this latter observation refers only to the few comꢀ
pounds that display ‘full’ HS or LS states i.e. involving all the
iron centers. Due to the triclinic symmetry of the crystal latꢀ
tice, the interꢀchain distances do not exactly match the Fe⋅⋅⋅Fe
intermolecular distances, but are shorter because the chains are
slightly shifted from each other in the a direction (Figure S4).
It follows that the interꢀchain distances are different along the
b and c directions. Thus, in the LS state, they are shorter in the
b direction (14.2708(4) Å) compared to the c direction
(14.5072(4) Å), while in the HS state, we observe the opposite
In conclusion, we have prepared and characterized a new
triazoleꢀbased [Fe(bntrz)3][Pt(CN)4].H2O (1) salt exhibiting an
abrupt spin transition with a transition temperature of ca. 242
K. The cationic [Fe(bntrz)3]2+ complex displays a polymeric
chain, similar to that reported for the FeII triazoleꢀbased sysꢀ
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tems exhibiting magnetic bistability around or above room materials, but also to rationalize the tuning of the SCO properꢀ
temperature.11ꢀ14 The present study clearly evidences the starꢀ
ring roles of the ligand substitution and counterion changes to
prevail over the implicit crystallogenesis issue in the FeII/trz
systems, with the association of bntrz ligand and [Pt(CN)4)]2ꢀ
anion leading for the first time to robust single crystals of high
quality and relatively considerable size (up to 0.45 × 0.04 ×
0.04 mm3) that withstand the LS/HS firstꢀorder spin transition
upon repeated SCO cycles. Distinctively from other 1D FeII/trz
systems for which the origin of the strong cooperativity reꢀ
mains strictly indefinite because of the lack of structural inꢀ
formation, the characteristics of complex 1 led us to single
crystal Xꢀray data collections of highꢀquality, allowing highly
refined structural characterizations of both high and low spin
states, and the thermal evolution of the lattice parameters. The
polymeric chain structure of 1, crystallographically deterꢀ
mined at 296 K and 120 K, is built from two different FeII
centers for which the average values of the FeꢀN distances are
in agreement with the complete HS/LS spin transition revealed
by the magnetic data. The crystal structures of the LS and HS
states definitely confirm the perfect linearity of the chains of
such polymeric triazole based SCO compounds, even in the
HS state conversely to what was suggested from the first inꢀ
vestigations based on EXAFS and WAXS studies.23,24 The
thermal variation of the lattice parameters shows anisotropic
changes at the transition temperature, and proves that the most
pronounced structural changes occur along the 1D covalent
chain. The intermolecular interꢀchain contacts occur through
hydrogen bonding via the [Pt(CN)4]2ꢀ anions leading to an
overall 3D arrangement. Although displaying cooperativity,
the absence of a wide hysteresis loop in the SCO of 1 reveals
that the strong shortꢀrange ″FeIIꢀ(Rtrz)3ꢀFeII″ interactions along
the chain are not sufficient to drive the existence of long lifeꢀ
time metastable states at the origin of thermal hysteresis; it
also confirms that the overall longꢀrange interactions, with an
emphasis on the direct contacts between the chains, should
play a crucial role to promote significant elastic strains which
stabilizes the spin state changes. The exceptional resilience of
the present crystals upon repeated switching cycles is most
likely due to the accommodation of the molecular volume and
the anisotropic expansion/contraction of the unit cell upon
SCO transition, which allows the existence of a mismatch free
HS/LS interface resulting from the capability of the “FeIIꢀ
(Rtrz)” chains to glide from each other and accommodate the
structural modifications despite the sharp transition. This
observation is similar to that of the spinꢀcrossover single crysꢀ
tals [{Fe(NCSe)(py)2}2(mꢀbpypz)] (with py = pyridine and
bpypz = 3,5ꢀbis(2ꢀpyriꢀdyl)pyrazolate) recently reported by K.
Boukhedadden et al..25 In this context, we are currently invesꢀ
tigating by optical microscopy the response of a single crystal
of 1 upon thermal SCO transition in order to visualize the
nucleation, the growth, and the propagation of HS and LS
domains accompanying the firstꢀorder transition.26 The obserꢀ
vation of the orientation and the dynamics of the HS/LS interꢀ
face should allow for a better understanding of the role of the
anisotropic changes of the lattice parameters on the exceptionꢀ
al resilience of this material. On the other hand, we are also
currently exploring other systems exhibiting similar chain, but
with different crystal packing to enhance/control the interꢀ
chain interactions and produce hysteretic features. In fine, the
aim is to establish thorough and systematic magnetoꢀstructural
correlations which are essential to understand the physicoꢀ
chemical origin of the strong cooperativity in such striking
ties.
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ASSOCIATED CONTENT
Experimental details, syntheses of the ligands, the complex 1 and
characterizations, Xꢀray crystallographic data in CIF format
(CCDC 1492971 and 1492972), and additional structural data
(Tables S1ꢀS3, Figures S1ꢀS8). This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
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* S. Triki. Eꢀmail: Smail.Triki@univꢀbrest.fr.
Author Contributions
All authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the CNRS, the Universities of
Brest, ParisꢀSaclay and Bordeaux, the "Agence Nationale de la
Recherche" (ANR project BISTAꢀMAT: ANRꢀ12ꢀBS07ꢀ0030ꢀ
01), the European community (FP7 MarieꢀCurie project:
PCIGꢀGAꢀ2011ꢀ304193 NANOCOORD), and the “Région
Bretagne” for the funding of this work. We thank M. Sy and
G. Bouchez for the optical microscopy measurements.
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