Two ways of spin crossover in an iron(ii) coordination polymer associated with conformational changes of a bridging ligand

Article abstract of DOI:10.1039/d0dt01696a

1,4-Di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) was prepared by lithiation of 1-ethyl-1,2,3-triazole, followed by alkylation with 1,4-dibromobutane. The ligand bbtre forms a three-dimensional network with Fe(ii), [Fe(bbtre)3](ClO4)2·2CH3CN, that exhibits thermally induced spin crossover (SCO). A change of temperature or change of spin state results in various types of structural transformation, leading to different structures that are stable in strictly defined temperature ranges. As a result, there are three spin crossover transitions arranged via two different paths. Thus, cooling below 280 K involves a HT(HS) → LT(HS) (HT, high temperature structure; LT, low temperature structure; HS, high spin) phase transition (PT), which is associated with conformational changes of the bbtre molecules and with deformation of the polymeric skeleton. In the LT phase incomplete and reversible LT(HS) ? LT(HS/LS) spin crossover occurs (LS, low spin). In contrast, rapid cooling (of a sample not previously thermally treated) allows the HT(HS) → LT(HS) phase transition to be avoided, and so complete HT(HS) → HT1(LS) SCO occurs. This means that the PT plays the role of a switch, which allows a choice of one of two ways in which the SCO will proceed. After rapid cooling, further heating to 150 K and subsequent cooling results in a reversible HT1(HS) ? HT1(LS) spin crossover (T↓1/2 = 130 K, T↑1/2 = 131 K). However, raising the temperature to 170-200 K leads to formation of a modulated structure HT2(HS) exhibiting the next reversible HT2(HS) ? HT2(LS) SCO (T↓1/2 = 121 K, T↑1/2 = 123 K). Finally, heating above 200 K involves the HT2(HS) → LT(HS) PT and results in a LT(HS) structure exhibiting incomplete LT(HS) ? LT(HS/LS) spin crossover.

Full text of DOI:10.1039/d0dt01696a

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DOI: 10.1039/D0DT01696A
ARTICLE
Two ways spin crossover in iron(II) coordination polymer
associated with conformational changes of bridging ligand
Received 00th January 20xx,
Accepted 00th January 20xx
Maria Książek,a Marek Weselski,b Agnieszka Dreczko,b Vladyslav Maliuzhenko,b Marcin
Kaźmierczak,b Aleksandra Tołoczko,b Joachim Kusz,a Robert Bronisz b*
DOI: 10.1039/x0xx00000x
1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) was prepared by lithiation of 1-ethyl-1,2,3-triazole followed then by
alkylation with 1,4-dibromobutane. Bbtre forms with Fe(II) three-dimensional network [Fe(bbtre)3](ClO4)2·2CH3CN
exhibiting thermally induced spin crossover (SCO). Change of temperature or change of spin state results in various types
of structural transformations. It leads to formation of different structures which are stable in strictly defined temperature
ranges. As a result, there are three spin crossover transitions arranged in two different paths. Thusly, cooling below 280 K
involves HT(HS)LT(HS) (HT – high temperature structure; LT – low temperature structure) phase transition (PT) which is
associated with conformational changes of bbtre molecules and with deformation of the polymeric skeleton. In the LT
phase there occurs incomplete and reversible LT(HS)LT(HS/LS) spin crossover. In contrast, rapid cooling (not previously
thermally treated sample) allows to avoid HT(HS)LT(HS) phase transition and there occurs complete HT(HS)HT1(LS)
SCO. It means that PT plays a role of switch which allows to choose one of two ways in which SCO will proceed. After rapid
cooling, further heating to 150 K and subsequent cooling causes reversible HT1(HS)HT1(LS) spin crossover (T1/2= 130 K,
T1/2= 131 K). However raising the temperature to 170-200 K leads to formation of modulated structure HT2(HS) exhibiting
next reversible HT2(HS)HT2(LS) SCO (T1/2= 121 K, T1/2 =123 K). Finally heating above 200 K involves HT2(HS)LT(HS) PT
and resulted LT(HS) structure exhibits incomplete LT(HS)LT(HS/LS) spin crossover.
properties which becomes particularly interesting for systems
exhibiting wide hysteresis loop.19,20
A key property associated with change of spin state is a variation of
Introduction
A search for systems whose spin state can be switched by an
external stimuli is one of the greatest challenges of
contemporary science.1,2 Spin crossover, observed in 3d4-3d7
octahedral complexes,3 attracts attention because of a variety
of spin state switching methods including: change of
temperature, an application of pressure,4 light irradiation5 or
high frequency pulsing magnetic field.6 Switching can be also
triggered by exploiting ligand-driven light induced spin
change,7 light induced ligand isomerisation8 as well as
chemically by a ligand exchange9 or as a result of host-guest
interactions.10 Spin state is affected by conformational
changes of ligand molecules too.11,12 Combining predefined
functionalities is an important research field2,13 and spin
crossover is a very attractive tool for creating multifunctional
materials14 in which change of spin state involves magnetic
ordering,15 occurrence of SMM16 (single-molecule magnet) or
SCM17 (single-chain magnet) behavior as well as affects electric
conductivity.18 Spin state also modulates luminescence
metal-ligand distance, particularly large in iron(II) complexes
(~0.2Å), which involves reduction of volume of [FeN6] coordination
octahedron at about 25% (~ 3 Å3).21 This perturbation can be an
origin of further structural changes leading to uncommon
properties like: a high stability of light induced high spin form,22
multiple bistability,23 multistability,24 two channel spin crossover.11
Structural alterations, including conformational flexibility of ligands,
may critically affect their properties resulting in reverse spin
crossover.24a,25 Thus, coupling between spin state switching and
structural lability is very promising because it creates conditions for
an occurrence of complex stimulus-response scenarios.10c
The aim of our research is to synthesize spin crossover systems
exhibiting structural lability associated with: conformational
flexibility of ligands,26,27 reorientation of coordinated molecules28
and changes including polymeric layers.29,30,31,32 In two-dimensional
(2D) coordination polymer [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-
triazol-1-yl)butane)33 change of spin state is accompanied by the
structural phase transition related to shift of the neighbouring
polymeric layers at about 0.7Å.29 In contrast to [Fe(bbtr)3](ClO4)2,
tetrafluoroborate analogue remains in HS form up to 10 K,
however, [Fe(bbtr)3](BF4)2 exhibits uncommon property which
depends on persistent bidirectional switching triggered by laser
light irradiation.34 Also in this case structural phase transition plays
a. Institute of Physics, University of Silesia, 75 Pułku Piechoty 1, 41-500 Chorzów,
Poland.
b. Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław,
Poland.
Electronic Supplementary Information (ESI) available: synthetic procedure of 1-
ethyl-1,2,3-triazole, Tables S1-S6, Figures S1-S7. See DOI: 10.1039/x0xx00000x
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crucial role in displacement of the polymeric layers.30 Modification
of the bbtr ligand which relies on the introduction of a second
substituent into 1,2,3-triazole ring allowed preparing 2D networks
Reactions between bbtre and iron(II) perchlorate VhieewxaArhticyledOrantlinee,
performed in acetonitrile, results in colorless crystals of
[Fe(bbtre)3](ClO4)22CH3CN (1). Crystalline samples of 1 are
unstable outside the mother liquor and therefore magnetic
and Mössbauer spectroscopy studies were carried out for
samples moistened with acetonitrile.
DOI: 10.1039/D0DT01696A
of [Fe(ebbtr)2(CH3CN)2]X24CH3CN (ebbtr
= 1,4-di(5-ethyl-1,2,3-
triazol-1-yl)butane, X = ClO4 , CF3SO3-27). In these complexes spin
- 31
crossover is governed by structural phase transitions too. In
The complex crystallizes in triclinic crystal system (P-1, Table
perchlorate
derivative
an
occurrence
of
series
of
S2) denoted as HT(HS)). At 295
K
there is one
LS(HS,LS)LSHS transitions is accompanied by reorientation of
the coordinated/noncoordinated molecules as well as deformation
and the mutual shift of the polymeric units. An exchange of
perchlorate on triflate anions results in occurrence of ‘normal’’ and
‘‘reverse’’ spin crossover which is associated with conformational
changes of the bridging ligands. In this complex spin crossover can
be accomplished in various ways: as sequence of
HS(HS/LS)HSLS transitions as well as in ‘‘normal’’ or
‘‘reverse’’ mode separately.
An occurrence of uncommon spin crossover properties, associated
with presence of the flexible building blocks, was an
encouragement to expand studies on another ligands based on
skeleton composed from 1,2,3-triazole rings tethered by butylene
spacer. Although 1,4-disubstituted-1,2,3-triazoles coordinate
monodentately, creating mono- and dinuclear iron(II) spin crossover
complexes,35 they can also act as -2N2,N3 bridging molecules.
Therefore, taking into account exclusively monodentate
coordination fashion of 1,5-disubstituted-1,2,3-triazoles, we have
focused on synthesis of novel ligands in which 1,5-disubstituted
rings are, in contrast, joined through carbon atoms. An application
of 1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) resulted in
structurally labile 3D network [Fe(bbtre)3](ClO4)22CH3CN (1) in
which complex spin crossover is governed by conformational
changes of bbtre molecules. However, 1 exhibits completely
different properties in comparison with ebbtr based systems
because, instead of reverse spin crossover, there occurs three spin
crossover arranged in two paths.
crystallographically independent iron(II) ion in the crystal
lattice. 1 represents unique example of the complex based on
1,5-disubstituted-1,2,3-triazole type ligands in which the first
coordination sphere is composed exclusively
from six azole rings (Fig. 2). In comparison with mono 1-
substituted 1,2,3-triazoles, which form homo- and heteroleptic
complexes, among all known examples based on 1,5-
disubstituted-1,2,3-triazoles, only heteroleptic systems were
isolated to date. In 1 ligand molecules are connected with
iron(II) through exo positioned nitrogen atoms N3.
a)
Results and discussion
1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane was prepared by
lithiation of 1-ethyl-1,2,3-triazole followed then by alkylation
with 1,4-dibromobutane. The ligand was isolated with 34%
yield as colorless crystalline solid, soluble in common solvents
like: acetonitrile, methanol or dichloromethane. The crystal
structure determined for bbtre confirmed localization of the
1,4-butylene spacer on C5 carbon atoms of the triazole rings
(Fig. 1, Table S1).
b)
Figure 2. Fragment of the three-dimensional network together with coordination
environment of iron(II) in 1 at 295 K (HT(HS) structure) showing two neighboring layers
(colored, thick lines and grey, thin lines) which are crosslinked by bbtre molecules
adopting gtg conformation (colored, thick lines) (a) and arrangement of anions and
acetonitrile molecules between layers (b). Hydrogen atoms are omitted for clarity.
Motif of single layer (marked by thick lines) was exploited on Figure 4 to present
conformational changes which occurs only within the layer.
The Fe1-N(triazole) bond lengths are equal to 2.162(5),
2.167(5) and 2.201(5) Å and are characteristic for high spin
form of the complex (Table S3). The range of the N-Fe-N angles
is from 88.8(2) to 92.1(2). Thus, the geometry of the
Figure 1. Molecular structure of bbtre (CH bonds are marked by thin lines).
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coordination octahedron is very similar to the one found in the over two positions (occupancy factor = 0.5). Also in this case
View Article Online
-
-
family of [Fe(bbtr)3]X2 (X=ClO4 , BF4 ) complexes as well as in reduction of the distance between bridDgOedI: 1i0r.o10n3(9II/)Di0oDnTs01f6r9o6mA
the complexes based on 1-substituted tetrazoles.36 Inspection 13.8 to 12.6 Å takes place. In comparison, separations
of torsion angles indicates that bbtre molecules adopt two between iron(II) ions still bridged by ttt conformers is only
types of conformations: trans-trans-trans (ttt) and gauche- slightly reduced of about 0.05 Å.
trans-gauche (gtg). Molecules adopting ttt conformation link
iron(II) ions arranged in [-110] and [10-1] directions which
results in two-dimensional polymeric layer (Fig. 2a). Iron(II)
ions from neighbouring layers are linked in [001] direction by
ligand molecules adopting gtg conformation which leads to
formation of the three-dimensional (3D) network of -Po
topology (Fig. 2b, Fig. S1). Iron(II) ions linked in [-110], [10-1],
and [001] directions are separated at distances 13.7, 13.8 and
12.6 Å, respectively. Ethyl substituents of ligand molecules,
bridging metal ions in [-110] and [001] directions, are
disordered. Perchlorates form intermolecular contacts with
disordered butylene chains and with triazole rings.
a)
Noncoordinated acetonitrile molecules are disordered, too.
The path from HT(HS) to LT(LS).
Results of magnetic studies show that in the temperature
range 150-300 K the complex 1 remains in the HS form.
Lowering of temperature below 150 K (MT = 3.45 cm3Kmol-1)
involves gradual reduction of MT indicating change of the spin
state of iron(II) ions (Fig. 3a). Spin crossover is finished at 70 K
however it is incomplete and about 70% of iron(II) ions still
remains in HS form (MT = 2.5 cm3Kmol-1). In the heating mode
the MT(T) dependence is practically the same as during
cooling.
Mössbauer spectrum of 1 recorded at 260 K indicates presence
of one doublet with isomer shift and quadrupole splitting
parameters of values  = 1.16 mm/s and EQ= 1.70 mm/s,
respectively, characteristic for HS form of the octahedral
iron(II) complex. Below 160 K a single line with  = 0.55 mm/s
corresponding to low spin form of the complex appears (Fig.
b)
3a inset). At 70 K the relative area of HS component AHS/(AHS
+
ALS) (AHS – peak area of HS doublet, ALS – peak area of LS
singlet) equals to 0.72 which remains in agreement with
results of magnetic studies. During heating the temperature
dependence of relative area of HS form is practically the same
as it was found during cooling (Fig. 3a).
Multitemperature single crystal X-ray diffraction studies
revealed occurrence of the crystallographic phase transition
below 280 K. At 200 K the Fe-N distances are in the range from
2.192(4) to 2.207(4) Å, thus they are characteristic for HS form
(Table S4). Formation of LT(HS) structure is accompanied by
the change of the ligand conformation. The changes occurs
exclusively within the layers (Fig. 4, Fig. S2). In LT(HS) structure
(Fig. 4b), in bridging direction corresponding to [-110] in
HT(HS) structure, a half of bbtre molecules change the
conformation from ttt to gtg. It results in significant shortening
of distance between bridged iron(II) ions from 13.8 to 12.6 Å.
In the second bridging direction (corresponding to [10-1] in
HT(HS) structure) every second molecule changes its
conformation from ttt to gtg with simultaneous disordering
c)
Figure 3. Temperature dependences of MT (1K/min, filled triangles) together with
relative areas of HS form component (AHS/(AHS+ALS)) derived from Mössbauer spectra
(open triangles) recorded in cooling and heating mode (a), series of selected MT(T)
dependences (heating/cooling cycles) recorded after initial rapid cooling of sample at
10 K (b), average Fe-N distances determined during cooling/heating (first crystal, filled
triangles) as well as in the heating mode after rapid cooling (second, previously not
thermally treated crystal, open triangles) (c). Cooling mode is marked by blue and
heating by red colour (figures a, b and c).ii - denotes second heating/cooling cycle
carried out in the same temperature range as previous one. Magnetic and Mössbauer
spectroscopy studies were carried out for samples (the same synthetic batch)
moistened with acetonitrile.
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Nonequivalence of ironiron separations causes the polymeric The path from HT(HS) to HT1(LS).
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DOI: 10.1039/D0DT01696A
grid to deform (Fig. 5a) and resulted strains entail corrugation
Recent studies of the complexes based on 1,4-di(5-ethyl-1,2,3-
of the layer - iron atoms deviate from least-squares plane at
about 0.7-0.8 Å (Fig. 5b, Fig. S2). Ligands adopting gtg
conformation in HT(HS) remain unchanged in LT(HS) structure
and distance between bridged iron(II) ions still equals 12.6 Å.
Further cooling leads to gradual and incomplete
LT(HS)LT(HS/LS) spin crossover (Fig. 3c). At 80 K Fe-N
distances are in the range 2.130(5) - 2.148(5) Å. Taking into
account shortening of Fe-N distance at ca. 0.2 Å, expectable
for complete spin crossover, it can be estimated that about
70% iron(II) ions remain in HS form. Distances between
bridged iron(II) ions in LT(HS/LS) structure are shorter at 0.1 Å
in comparison with LT(HS) structure. Spin crossover does not
involve significant structural changes of bridging ligands (Fig.
4c, Table S4) and polymeric layers remain corrugated.
Heating of the crystal from 80 to 280 K does not lead to
restoration of initial HT(HS) structure and the complex exists in
LT(HS) structure. It is worth adding that cooling (not previously
thermally treated crystal) from 300 to 240 K, that is below
temperature of structural phase transition, followed then by
its heating again to 280 K, does not allow to restore HT(HS)
structure, too. Hence, structural phase transition is not
reversible in the investigated temperature range.
triazol-1-yl)butane27,31
and
tris(n-butyl-imidazol(2-
ethylamino))amine11,37 revealed that phase transitions, in
particular associated with structural changes of ligands, may
play a role of steering switch to various phases which exhibit
different spin crossover properties. Therefore, in order to test
such possibility, we have tried to suppress occurrence of
HT(HS)LT(HS/HS) transition by rapid cooling (sample was
placed in the chamber of SQUID magnetometer cooled to 10
K). It is well known that usually fast cooling can lead to partial
or complete quenching of the HSLS transition.38 In contrast,
rapid cooling of 1 at 10 K results surprisingly in complete
switching to low spin form (denoted further as HT1(LS)).
Moreover, subsequent measurement performed in the heating
mode revealed an occurrence of abrupt HT1(LS)HT1(HS) spin
crossover (Fig. 3b).
X-ray diffraction studies on single crystals placed directly into
the stream of nitrogen at 80 K were carried out to understand
structural processes related to rapid cooling. In contrast to
gradual and incomplete LT(HS)LT(HS/LS) spin crossover,
rapid cooling is accompanied by significant shortening of Fe-N
distances to 2.002(2), 2.002(2) and 2.011(2) Å (Table S5). It
results in formation of new HT1(LS) phase, which means
suppression of HT(HS)LT(HS) transition. In HT1(LS) structure
bbtre molecules, linking iron(II) ions in [10-1] direction change
their conformation from ttt to tgt (trans-gauche-trans) (Fig.
4d) and become disordered in two positions (o.f. = 0.5).
Conformation of ligand molecules linking iron(II) ions in [-110]
and [001] directions remain unchanged. Formation of HT1(LS)
structure is accompanied by shortening of ironiron distances
in all bridging directions at 0.2-0.3 Å. In comparison to
HT(HS)LT(HS) transition, rapid cooling does not involve
deformation of the polymeric layers. Noncoordinated
acetonitrile molecules as well as perchlorate anions are
disordered.
a)
b)
Figure 4. Scheme showing occurred phase transitions (PT) and spin crossover (SCO) in
1 together with conformations (t = trans, g = gauche) of bbtre molecules (ethyl
substituents were removed for clarity) in: HT(HS) structure at 295 K (a), LT(HS) at 200K
(b), LT(HS/LS) at 80K (c), HT1(LS) at 80K, (d) HT1(HS) at 150 K (e), HT2(HS) at 170 K (f),
HT2(LS) at 80 K (g). T1/2 values for reversible SCO are placed together with suitable
arrows.
Figure 5. Schematic presentation of LT(HS) structure showing deformation of the layer
grid (a) and corrugation of the polymeric layer (b) as a result of HT(HS)LT(HS)
structural phase transition in 1. Orange lines mark ligand molecules linking iron(II) ions
(green balls).
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An elevation of temperature to 150 K is accompanied by Thus, HT2(HS)HT2(LS) is not associated with coVniefworAmrticaletiOonnlinael
DOI: 10.1039/D0DT01696A
increasing the Fe-N distances to values characteristic for HS changes of bridging ligands. Inspection of the Fe-N distances in
form of the complex (Table S5). Formation of HT1(HS) the average structure shows complete spin crossover (Table
structure (Fig. 4e) is reversible and cooling to 80 K leads to S6). Re-heating to 170-200 K again results in modulated
HT1(LS) structure (Fig. 4d).
HT2(HS) structure which confirms reversibility of
It was found that heating of the crystal to temperature 170- HT2(HS)HT2(LS) spin crossover.
200 K is accompanied by appearance of very weak satellite It is worth noticing that cell volumes (calculated for one
reflections with a modulation vector q=(0.303, 0.300, 0.406) molecule of the complex) decreases for LT(HS/LS) (1457.9 Å3),
leading to ca. seven times enlarged volume of the unit cell for HT1(LS) (1452.5 Å3) and HT2(LS) (1426.2 Å3) structures
the superstructure (at 80K unit cell parameters for the average determined at 80 K. It corresponds to increase of abruptness
structure are a=10.4994(2)Å, b=12.2450(4)Å, c=12.1324(3)Å, of SCO and can indicate an importance of crystal packing
α=84.628(2)⁰, β=73.4236(19)⁰, γ=72.557(2)⁰, V=1426.18(6)Å3, effects.
while for the superstucture are a=13.5837(5)Å, b=24.1794(8)Å, An occurrence of HT1(HS)HT2(HS) structural phase
c=31.1525(11)Å, α=77.484(3)⁰, β=88.255(3)⁰, γ=87.785(3)⁰, transition has been confirmed by magnetic studies carried out
V=9978.7(6)Å3). The characteristic feature of novel HT2(HS) on a sample rapidly frozen at 10 K. Raising the temperature, in
structure (Fig. 4f) is long range ordering of ligand molecules. In steps by 5 or 10 K on each consecutive heating/cooling cycle,
the modulated structure bbtre molecules adopt alternately tgt revealed two sets of spin crossover dependences (Fig. 3b, Fig.
and ttt conformations in the bridging direction corresponding S3). Heating to 160 K results in abrupt and complete spin
to [10-1] in HT(HS) structure. In HT2(HS) structure iron(II) ions crossover with T1/2131 K and T1/2130 K. Further elevation
occupy common plane and corrugation of the polymeric layer of the heating limit to 180 K involves abrupt lowering of SCO
is not observed. However, we have determined only the temperatures to values of T1/2123 K and T1/2121 K.
tentative model of the modulated HT2(HS) structure because A dependence of T1/2 vs. maximum heating temperature Tmax
satellite reflections are very weak. In the average structure (Fig. 6) shows a discontinuity (165-175 K) which corresponds to
(Table S6, Fig. S2) a presence of two bbtre conformers is visible results of diffraction studies and indicates that HT1 and HT2
as a ligand disorder occurring in [10-1] direction. It should be structures have different spin crossover properties. It is worth
noticed that similar behavior depending on a long range noticing that differential scanning calorimetry (DSC)
ordering of ligand molecules, triggered by the change of measurements revealed, in accordance with results of
temperature or spin crossover, occurs in another three- magnetic studies, shift of peaks to lower temperatures after
dimensional coordination polymer [Fe(qbtr)3](ClO4)2 (qbtr=1,5- reaching 180 K (Fig. S4).
di(1,2,3-triazol-1-yl)pentane).26b It is worth to add that in 1D Finally, raising the temperature above 200 K results in
systems based on ligands possesing expanded spacer like 1,3- conformational changes of bbtre molecules leading to
bis((1H-tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane
or
m- reconstruction of LT(HS) structure (Fig. 4b). It explains why
xylylenebis(tetrazole) ligand disorder is not observed.39 After after rapid cooling, followed then by heating to 300 K, only
lowering of temperature from 170 to 80 K satellite reflections gradual and incomplete spin crossover occurs during next
in 1 are still present indicating the existence of a long range cooling/heating cycles. Experiments depending on annealing
ordering of bbtre molecules (Fig. 4g).
sample at 330 K followed then by rapid cooling to 10 K did not
allow to obtain abrupt and complete spin crossover, thus
formation of LT(HS) structure is irreversible.
What is important the uncommon behavior, presented here, is
not associated with the change of the chemical composition.40
Summary
In this report we have presented synthesis of 1,4-di(1-ethyl-
1,2,3-triazol-5-yl)butane (bbtre) and its application to
preparation of iron(II) spin crossover systems. A presence of
1,4-butylene spacer together with additional alkyl substituent
was dictated by need to increase ligands flexibility, thus,
facilitating occurrence of structural lability of the complexes.
This approach has been successfully realized in the case of
Figure 6. Dependence of T1/2 (blue) and T1/2 (red) versus highest temperature Tmax
three-dimensional
coordination
polymer


reached during heating. Filled symbols denote T1/2 and T1/2 values obtained as a
result of second heating/cooling cycle carried out in the same temperature range
immediately after reaching Tmax value for the first one. The points on the plot were
determined on the basis of the measurements which are shown on Figure S4 (it
contains full experimental protocol).
[Fe(bbtre)3](ClO4)22CH3CN prepared in the reaction of bbtre
with iron(II) perchlorate. In this complex, conformational
changes tttgtg (t = trans, g = gauche), associated with
HT(HS)LT(HS) structural phase transition upon cooling below
280 K, play a role of the switch which allows to perform spin
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crossover in two ways. Thus, further cooling involves below cm-1. Temperature dependent measurements of the magnetic
View Article Online
DOI: 10.1039/D0DT01696A
150 K reversible, gradual and incomplete LT(HS)LT(HS/LS) susceptibility of complexes were carried out with a Quantum
spin crossover. In contrast, rapid cooling triggers complete Design SQUID magnetometer under 1T applied magnetic field.
Measurements were carried out with rate 1 K/min (cooling,
heating). The only exception is generation of HT1(HS) phase
depending on rapid cooling below phase transition. All
magnetic measurements were carried out for samples
moistened with acetonitrile and placed in the sealed glass
tube. Magnetic data were corrected for signal of glass tube
and for the diamagnetic contributions of the sample, which
were estimated from Pascal’s constants. DSC measurements
(Fig. S4) were carried out with Mettler Toledo DSC 3 at 2 K min-
1. Mössbauer spectra were recorded using 57Co/Rh source (ca.
10mCi) on POLON spectrometer working at constant
acceleration mode. The sample (ca. 100 mg) moistened with
acetonitrile was placed in the sealed plastic container and
mounted on a cold finger of helium cryostat (ARS DMX-20
cooling system). The isomer shift  and quadrupole splitting
EQ were calculated in relation to -Fe (295 K).
HT(HS)HT1(LS) spin crossover which is accompanied by
ttttgt transformations of bbtre molecules. In the heating
mode spin crossover behaviour is complex in nature. Raising of
temperature to 150-160 K triggers abrupt and reversible

HT1(LS)HT1(HS) spin crossover with T1/2= 130 K and T1/2
=131 K. An elevation of temperature to 170-200 K is associated
with formation of modulated HT2(HS) structure. Also in this
case it is possible to carry out reversible and complete spin

crossover HT2(HS)LT2(LS) (T1/2= 121 K and T1/2 =123 K).
Modulation of the crystal structure HT2 in the low spin form of
the complex is preserved and hence a return to HT1 structure
is not possible. Finally, heating above 200 K leads to
reconstruction of LT(HS) structure, thus, gradual and
incomplete LT(HS)LT(HS/LS) spin crossover in the next
cooling/heating cycles can be realized. HT(HS)LT(HS) and
HT2(HS)LT(HS) transitions are irreversible, therefore,
accomplishment of LT(HS) phase does not allow to repeat
(after next rapid cooling) abrupt and complete spin crossover
and LT(HS)LT(HS/LS) spin crossover can be carried out.
Results, presented in this paper, show importance of
implementation of potentially structurally labile building
blocks as functionalities increasing an inclination to occur
structural phase transitions governing properties of the
material. It is clearly visible that assembling in one material of
spin crossover and conformational flexibility remarkably
increases complexity of stimulus-response but, what is
important, allowing behave control over the state of the
system. It support recent studies on spin crossover systems
based on 1,4-di(5-ethyl-1,2,3-triazol-1-yl)butane27,31 and tris(n-
butyl-imidazol(2-ethylamino))amine11 in which structural
phase transitions allow to switch properties by choice of
desired spin crossover active phase.
Caution! Even though no problems were encountered it is
worth to mention that complexes containing perchlorates are
potentially explosive and should be synthesized in milligram
scale and handled with care.
X-ray data collection and structure determination. The crystals of
complexes suitable for X-ray measurements were obtained
directly from syntheses of macroscopic samples and
preselected using the stereoscopic Zeiss microscope under
nitrogen atmosphere. For the X-ray measurements single
crystals of the complexes were placed on the loops in the drop
of oil. The measurement of single crystal of bbtre was
performed with a four-circle Xcalibur X-ray diffractometer
equipped with Sapphire3 CCD detector and Mo-Kα (λ =
0.71073 Å) radiation. The controlling of the measurement
procedure and data reduction were performed by CrysAlisPro
software.42 Structural studies of complex 1 were performed
with
a four-circle SuperNova X-ray diffractometer with
microfocus X-ray tube, optimized multi-layer optics for Mo-Kα
(λ = 0.71073 Å) or Cu-Kα (λ = 1.54184 Å) radiation and an Atlas
CCD detector. In these cases the controlling of the
measurement procedure and data reduction were performed
also by CrysAlisPro software.43 The same program was used to
determine and refine the lattice parameters. Low
temperatures were achieved with a stream of cold nitrogen
gas (Oxford Cryosystem cooling device); the temperature
stability was 0.1 K.
The structures were solved by direct methods and
subsequently completed by the difference Fourier. All the non-
hydrogen atoms were refined using full-matrix, least-squares
technique. The hydrogen atoms were treated as riding on their
parent carbon atoms with isotropic displacement parameters
Experimental
Materials and methods. HPLC grade THF (Across) was dried for
two weeks over molecular sieves 3A activated at 160C.41 1,4-
dibromobutane purchased from Aldrich was distilled and
stored also over molecular sieves 3A. Dichloromethane (POCh)
was distilled prior to use. Hexane solution of n-BuLi (2.5M,
Aldrich), acetonitrile (HPLC grade, Across), sodium sulfate
anhydrous, silica gel (Merck, K60, 230-400 mesh) and iron(II)
perchlorate hexahydrate were used as delivered without
further purification. Acetonitrile used to synthesis of the
complexes was distilled over the calcium hydride.
Elemental analyses for carbon, hydrogen and nitrogen were
performed on Perkin Elmer 240C analyzer. H NMR and 13C
NMR spectra were recorded on AMX Bruker NMR 500 MHz
spectrometer at room temperature in CDCl3 solutions. IR
spectra of bbtre (KBr pellet) and 1 (nujol mull) were recorded
with the Jasco FT/IR-4700 spectrometer in the range 400-4000
1
(equal 1.2 or 1.5 times the value of
C displacement
parameters). The structures were severely disordered,
therefore various constraints and restraints were used during
refinement (EADP, EXYZ, DFIX, DANG, SAME). SHELXS-2013
6 | J. Name., 2012, 00, 1-3
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ARTICLE
and SHELXL-2018/3 programs were used for the calculations.44
CCDC 1885384 (compound bbtre) and 1953184 - 1953190
(compound 1) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
13CNMR (CDCl3) : 135.82, 132.14, 42.84, 27.87,V2ie3w.0Ar2tic,le1O5n.l5in2e
DOI: 10.1039/D0DT01696A
ppm. See Fig. S6 for IR spectrum.
Synthesis of [Fe(bbtre)3](ClO4)22CH3CN (1). Syntheses of the
complex were carried out under nitrogen atmosphere using
the standard Schlenk technique. To the solution of bbtre (0.20
mmol, 49.6 mg). in acetonitrile (5 mL) the solution of
Fe(ClO4)26H2O (0.10 mmol, 36.3 mg) in acetonitrile (5mL) was
added. The obtained colorless solution was left in the closed
Schlenk flask. After 2 days, colorless crystals were filtered off.
Yield 29.0 mg. Anal. found: C, 44.2; H, 6.2; N, 26.2%. Calc. for
FeC40H66N20Cl2O8 (Mw=1081.83): C, 44.4; H, 6.1; N, 25.9 %. See
Fig. S7 for IR spectrum.
Synthesis of 1-ethyl-1,2,3-triazole.45 In 1000 mL round bottom flask
1,2,3-triazole (69.06 g, 1.0 mol) and water (50 mL) was placed and
then solution of NaOH (39.99 g, 1.0 mol) in water (150 mL) was
added in few portions. Water was removed under reduced pressure
using rotary evaporator. The white solid was dried by aseotropic
distillation with acetonitrile. CH2Cl2 (400 mL) was added to resulted
product. Then diethyl sulfate (128.5 mL, 1.0 mol) was dropped over
the time 80 minutes to ice cooled mixture. Reaction mixture was
stirred at room temperature for seven days and then the white
solid was filtered off and solvent was removed under reduced
pressure. 2-ethyl-1,2,3-triazole (2etr) was isolated by fractional
distillation under atmospheric pressure collecting fraction boiling at
temperature range 113-115C. 1-ethyl-1,2,3-triazole (1etr) was
isolated by distillation under reduced pressure (8 mmHg). The
fraction boiled at 86-87C was collected. Yield 36.8g (38%) for 1etr.
1HNMR (CDCl3) : 7.68 (s, 1H, H-tr), 7.54 (s, 1H, H-tr), 4.43 (q, 2H,
3J=7.5 Hz, tr-CH2-CH3), 1.54 (t, 3H, 3J=7.4 Hz, tr-CH2-CH3) ppm.
13CNMR (CDCl3) : 133.9, 122.7, 45.2, 15.6 ppm. See Fig. S5a for
FTIR spectrum. Yield 17.1g (18%) for 2etr. 1HNMR (CDCl3) : 7.56 (s,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Polish National Sciences
Centre Grant No. DEC-2014/15/B/ST5/04771. We are grateful
to Professor T. Lis for the consultation and valuable remarks.
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Structural phase transition in [Fe(bbtre)3](ClO4)22CH3CN (bbtre=1,4-di(1-ethyl-1,2,3-triazol-5-
yl)butane) playes a role of switch allowing to carry out spin crossover in two ways.