Synthesis, spectroscopic, thermal and magnetic characterization of novel spin-crossover iron(II) complexes with 4-cyclopropyl-1,2,4-triazole

Article abstract of DOI:10.1016/j.ica.2014.04.017

The new bidentate ligand 4-cyclopropyl-1,2,4-triazole (4-C₃trz) has been synthesized and used for preparing 1D Fe(II) SCO compounds of the formula [Fe(4-C₃trz)₃](X)₂·nH ₂O; where: X = BF₄^-, n = 3 for compound (1) and X = ClO₄^-, n = 2 for compound (2). The new complexes (1) and (2) were characterized using elemental analyses, IR-, mass spectroscopy and thermal analysis. The thermally induced spin-crossover behavior of the complexes was inspected by temperature dependent magnetic susceptibility (SQUID) and DSC measurements. The results of magnetic susceptibility (SQUID) and DSC showed abrupt and complete spin transition with a hysteresis loop T ^?_1/2 = 230/222, T^←_1/2 = 221/208 K for complex (1) and T^?_1/2 = 218/193, T ^←_1/2 = 202/183 K for complex (2) using SQUID/DSC, respectively. The enthalpy and entropy associated with the spin transition (ST) evaluated by DSC results were Δ_transH = +21.6 and +20.4 kJ mol^-1 and Δ_transS =+ 97.5 and +105.5 J mol ^-1 K^-1for the (1) and (2), respectively.

Full text of DOI:10.1016/j.ica.2014.04.017

Accepted Manuscript
Synthesis, spectroscopic, thermal and magnetic characterization of novel spin-
crossover iron (II) complexes with 4-cyclopropyl-1,2,4-triazole
Ahmed A. Soliman, Mohamed M. Khattab, Hosam H. Abdelhady, Ivan
Šalitroš, Wolfgang Linert
PII:
S0020-1693(14)00222-9
http://dx.doi.org/10.1016/j.ica.2014.04.017
ICA 15956
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
25 January 2014
12 April 2014
17 April 2014
Please cite this article as: A.A. Soliman, M.M. Khattab, H.H. Abdelhady, I. Šalitroš, W. Linert, Synthesis,
spectroscopic, thermal and magnetic characterization of novel spin-crossover iron (II) complexes with 4-
cyclopropyl-1,2,4-triazole, Inorganica Chimica Acta (2014), doi: http://dx.doi.org/10.1016/j.ica.2014.04.017
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Synthesis, spectroscopic, thermal and magnetic characterization of novel spin-
crossover iron (II) complexes with 4-cyclopropyl-1,2,4-triazole
Ahmed A Soliman*1, Mohamed M. Khattab2, Hosam H. Abdelhady1, Ivan Šalitroš 3 and
Wolfgang Linert4
1 Department of Chemistry, Faculty of Science, Cairo University, Giza,12613 Egypt
2 Department of Chemistry, Faculty of Science, Fayoum University, Fayoum, Egypt
3 Institute of Inorganic Chemistry, Technology and Materials, Faculty of Chemical and Food Technology, Slovak
University of Technology, Bratislava 81237, Slovakia
4Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9/163-AC, A-1060
Vienna, Austria.
The new bidentate ligand 4-cyclopropyl-1,2,4-triazole (4-C3trz) has been synthesised and used
for preparing 1D Fe(II) SCO compounds of the formula [Fe(4-C3trz)3](X)2·nH2O; where:
−
−
X=BF4 , n=3 for compound (1) and X=ClO4 , n=2 for compund (2). The new complexes (1) and
(2) were characterized usingelemental analyses, IR-, mass spectroscopy and thermal analysis.
The thermally induced spin-crossover behaviour of the complexes was inspected by temperature
dependent magnetic susceptibility (SQUID) and DSC measurements.The results of magnetic
susceptibility (SQUID) and DSC showed abrupt and complete spin transition with a hysteresis
loop T↑1/2= 230/222, T↓1/2= 221/208 K for complex(1) and T↑1/2= 218/193, T↓1/2= 202/183 K for
complex (2) using SQUID/DSC; respectively. The enthalpy and entropy associated with the spin
transition (ST) evaluated by DSC results were∆transH = +21.6 and +20.4 kJ mol-1 and ∆transS =
+97.5 and+105.5 J mol-1 K-1for the(1) and (2); respectively.
Keywords: Iron (II) / Cyclopropyltriazole / Spin crossover (SCO) / SQUID / DSC
Correspondent author: Prof. Dr. Ahmed A. Soliman; ahmedsoliman61@gmail.com, Tel:
00201004817251, Fax: 0020235727556
1

Introduction
Iron(II) 4-substituted- 1,2,4-triazole polymeric chain compounds have attracted particular
attention, as their abrupt ST is generally associated with both hysteretic and thermochromic
effects [1]. Indeed, such bidentate ligand molecules are able to propagate elastic [2] and
magnetic interactions [3] between transition metal ions in polynuclear complexes, which are of
fundamental importance in molecular magnetism[4]. Several one-dimensional (1D) spin
crossover (SCO) polymers of formula [Fe(4-R-1,2,4-triazole)3](anion)2 have been prepared and
published[5]. These coordination polymers display a spin transition (ST) with a hysteresis
widthin temperature ranges, which is in some cases centered near room temperature [2,5,6].
Their associated thermochromism make them potentially useful for various applications such as
display and memory devices units [7], sensors [8], and cold channel control units in food and
medicals storage [9]. The origin of the hysteresis loop of these materials and their thermal extent
are not yet fully understood [10].It is recognized that strengthening of intramolecular interactions
by triply 1,2,4-triazole bridges connecting spin changing moleculescan promote cooperative
effects, resulting in the observation of hysteresis width ranging between2 and 20 K [4]. In fact,
1D chains, where the Fe(II) ions are linked by loose bridges, all displaygradual SCO behaviour
without any hysteresis effect. This is the case when triply bis-tetrazolesbridges with flexible
spacers are employed [11] or when single bridges are used (e.g., 1,2-bis(4-pyridyl)etha(e)ne [12],
4,4`-bipyridine [13], dicyanamide[14]). Intermolecular interactions are believed to also
contribute to the ST cooperative mechanism, thus resulting in larger hysteresis loops [2,15,16].
This is the case for [Fe(Htrz)2trz]BF4 (Htrz = 4-H-1,2,4-triazole, trz = triazolato)that displays a
ST above room temperature with a hysteresis width of ~40 K [15], and for[Fe(hyptrz)3](4-
chloro-3-nitrophenylsulfonate)2.H2O (hyptrz = 4-(3`-hydroxypropyl)-1,2,4-triazole) that displays
a ST with an hysteresis width of ~50 K below room temperature[16]. In previous work we
reported synthesis, spectroscopic and magnetic behaviour of the iron(II)-cyclopropyltetrazole
[Fe(C3tz)6](BF4)2 [17] with the aim of increasing the abruptness of the spin transition through
blocking the shock absorption effect via decreasing the mobility of the alkyl chain through the
introduction of a more rigid cyclic moiety. In the previous work we reported synthesis,
spectroscopic and magnetic behavior of the iron(II)-cyclopropyltetrazole [Fe(C3tz)6](BF4)2[17]
with the aim of increasing the abruptness ofthe spin transition through blocking the shock
2

absorption effect via decreasing the mobility ofthe alkyl chain through the introduction of a more
rigid cyclic moiety. Extending our work bypreparation and study of spincrossover complexes of
FeN6systems, we report about new resultsconcerning the synthesis, spectroscopic and magnetic
behaviour of iron(II)-4-cyclopropyl-1,2,4-triazole, [Fe(4C3trz)3](BF4)2·3H2O (1) and [Fe(4-
C3trz)3](ClO4)2·2H2O with the aim of not onlyimproving the abruptness of the spin transition
through blocking the shock absorption effect, but also shifting the transition (T1/2) to higher
temperature.
2. Experimental Section
2.1. Chemicals
Triethyl orthoformate 99% (Sigma-Aldrich), monoformylhydrazine (Sigma-Aldrich)
cyclopropylamine (Sigma-Aldrich) and iron(II)tetrafluoroborate hexahydrate 97% (Aldrich)
were used as supplied. All solvents used were obtained from Sigma-Aldrich.
2.2. Synthesis
2.2.1. Synthesis of 4-cyclopropyl-1,2,4-triazole (4-C3trz)
The ligand 4-cyclopropyl-1,2,4-triazole was synthesized using the method described by Soliman
et al [17] (Scheme 1). 24.15 g trimethylorthoformate (26.83 cm3; 0.163 mol) were mixed with a
solution of 9.13 g (0.152 mol) monoformylhydrazine in 200 cm3 methanol. The solution was
refluxed for 4 hours, then cooled to 40oC and 4.0 g (4.88 cm3; 0.07 mol) cyclopropylamine were
added. The mixture was further refluxed for 20 hours. Then the solvent was removed with a
rotary evaporator. The ligand was purified by vacuum distillation at 130 °C and 0.9 mbar,
o
yielding a colorless solid, 6.84 g (89.4 % yield) and melting point of 84-88 C. The ligand
(C5H7N3; Mol. Wt.: 109.13)(Anal. Calc. C, 55.03; H, 6.47; N, 38.50; Found C, 54.94 ; H, 6.41;
1
N, 38.47%), H-NMR, δ (ppm): 0.98 (d, 2H, CH2cyclopropyl); 3.51 (m, 1H, CHcyclopropyl); 8.54 (s,
2H, CH-triazole).
2.2.2. Synthesis of [Fe(4-C3trz)3](BF4)2·3H2O (1)
3

The complex (MW: 610.14 g/mol) was synthesized in dry nitrogen atmosphere. 0.328 g (3
mmol) of 4-cyclopropyl-1,2,4-triazole (4-C3trz) were dissolved in 15 cm3 purged ethanol and
0.255 g (1 mmol) of Fe(BF4)2·6H2O was dissolved in 15 cm3 purged ethanol( few drops of water
were added to ensure the solubility of the salt). Both solutions were purged again with dry
nitrogen to make certain of the removal of any dissolved oxygen. Under stirring and continuous
purging with nitrogen the solution of iron(II) tetrafluoroborate hexahydrate was slowly added to
the solution of the ligand within 1 h.[17]. The system was sealed and left in the fridge for several
days to precipitate a fine yellowish-white precipitate of [Fe(4-C3trz)3](BF4)2·3H2O. The
separated complex was filtered off, washed twice with cold ethanol followed by cold diethyl
ether and then dried carefully in vacuo yielding 65% of the theoretical amount. The complex was
subjected to elemental analyses. (Anal. Calc. for C15H27B2F8FeN9O3 : C, 29.49; H, 4.45; N,
20.64. Found: C, 29.52; H, 4.43; N, 20.66 %.)
2.2.3. Synthesis of [Fe(4-C3trz)3](ClO4)2·2H2O (2)
The complex (MW: 618.13 g/mol) was synthesized in a dry nitrogen atmosphere. 0.328 g (3
mmol) of 1-cyclopropyltriazole (C3trz) were dissolved in 15 cm3 purged ethanol and 0.363 g (1
mmol) of Fe(ClO4)2·6H2O was dissolved in 15 ml purged ethanol (few drops of water were
added to ensure the solubility of the salt). Both solutions were purged again with dry nitrogen to
ensure of the removal of any dissolved oxygen. Under stirring and continuous purging with
nitrogen the solution of iron(II) tetrafluoroborate hexahydrate was slowly added to the solution
of the ligand within 1 h[17]. The left in the fridge for several days to precipitate fine yellowish
white precipitate of [Fe(4-C3trz)3](ClO4)2.2H2O. The separated complex was filtered off, washed
twice with cold ethanol followed by cold diethylether and then dried carefully in vacuo yielding
85% of the theoretical amount. The complex was subjected to elemental analyses. (Anal. Calc.
for C15H25Cl2FeN9O10: C,29.14; H, 4.08; N, 20.39, Found: C,29.22 ; H, 4.12; N, 20.25 %..
2.3. Measurements
Elemental analyses (C, H, N) were performed in the Microanalytical Center, Cairo University.
IR spectra were recorded on both NEXUS 470 FT-IR spectrometer, Thermo Nicolet running
under OMNIC software package in wave number region 4000–400 cm-1.DSC thermal analysis
were carried out on a DSC (Polymer Laboratory, England). DSC was calibrated with high-purity
4

indium, and α-Al2O3 was used as the reference material. Cooling and heating sweeps were
performed at a scanning rate of 10 K/min covering a temperature range from 120 K to 300 K and
the temperature difference between the sample and reference is measured as a function of
temperature. The thermal analysis of the complexes were carried out using a Shimadzu
thermogravimetric analyzer TGA-50H in a dynamic nitrogen atmosphere (flow rate 20 ml min−1)
with a heating rate of K min−1. The % weight loss was measured from the ambient temperature
◦
up to 900 C. Mass spectrometry measurements were carried out using Shimadzu GCMS-
QP1000EX. The herein reported magnetic measurements were performed on a SQUID
magnetometer (Quantum Design, model MPMSXL5). In all cases, the temperature dependence
of the magnetic moment was recorded at B = 0.1 T as an external magnetic field. The
temperature sweeping rate was 1 K min-1 and was the same for cooling and heating mode.
Gelatine capsules as sample containers for the measurement in the temperature range 5 ↔ 300 K
were used. The very small diamagnetic contribution of the gelatine capsule had a negligible
contribution to the overall magnetization dominated by the sample. The diamagnetic corrections
of the molar magnetic susceptibilities were applied using Pascal’s constant.
3. Results and Discussion
3.1. Spectroscopic characterizations
The most prominent absorption peaks in the mid-range IR of [Fe(4-C3trz)3](BF4)2.3H2O
(1) and [Fe(4-C3trz)3](ClO4)2.2H2O (2) complexes are given in Table(1). The broad bands at
3428 cm-1 and 3433 cm-1 are assigned for the stretching vibration of the associated water
molecule(s) for complexes (1) and (2); respectively. These water molecules are predicted from
the elemental analyses and ascertained by both the DSC and the TGA/DTA studies, vide infra.
This is also familiar in the linear polynuclear Fe(II) spin-crossover compounds of general
−
formula [Fe(NH2trz)3](anion)2·xH2O (NH2trz = 4-amino-1,2,4-triazole; anion = NO3 [4a,18-20],
−
−
, BF4 [7a,19], I− [21], Br− [7a,19], and CH3SO3 [22].
The strong bands found at 3105 cm-1 assigned for the triazole C-H stretching vibration of
the free ligand were shifted to 3138 cm-1 and 3117 cm-1 in the spectra of the complexes (1) and
(2); respectively. The triazole C=N stretching vibrations found at 1555 cm-1 and 1553 cm-1for
the complexes (1) and (2); respectively, were found to be shifted from that of the free triazole
(1529 cm-1) [23]. The bands at 1111 cm-1 and 1074 cm-1 for the complexes (1) and (2);
5

respectively, are assigned for the triazole C-N stretching being shifted to lower wave number
from that of the free traizole (1184 cm-1). The bands at 1235 cm-1 in both complexes (1) and (2)
are assigned as the 1,2-bi-coordinating triazole C-H bending [23]. The peaks at 1071 cm-1 and
910 cm-1, found in complex (1) are due to the asymmetric and the symmetric stretching
-
vibrations of the BF4 anion. The umbrella-like bond deformation of the BF4 counter anion has
-
been found as medium strong absorption at 527 cm-1. Similarly the bands at 1022 and 620 cm-1
-
found in the spectrum of complex (2) are assigned to the ClO4 anion stretching [2, 23]. In
addition, the stretching vibration of the metal-traizole nitrogen (νM-N) were found at 473 cm-1 and
463 cm-1 for the complexes (1) and (2); respectively.
The mass spectral data of the complexes (1) and (2) showed the major mass fragmentation peaks.
The 1D chain of the complexes was decomposed showing the fragmentation peaks of the
mononuclear species, the ligand and iron oxide and iron isotopes. The mononuclear species of
complex (1) and complex (2) were found at m/z =608.10 (3.18%) and m/z = 616.10 (1.1%). The
mass fragmentation peak of the 4-C3trz ligand (C5H7N3) was found at m/z = 109.10 (100%) and
109.10(85.86%) for complexes (1) and (2); respectively. The fragment of the metal oxides as
metallic residues observed in the complexes thermograms (vide infra) was observed at m/z =
72.05. It is interesting to highlight the contribution of the different iron isotopes to the molecular
mass of their complexes (1) and (2). This is clear from the mass fragmentation peaks at m/z =
54.05(100%), 56.05(46.87%) and 58.05(0.04%) for complex (1) and m/z = 54.05(100%),
56.05(4.19%), 57.10 (0.70%) and 58.00(0.74%) for complex (2) and m/z = 58.00(3.71%) for
complex (3).
3.2. Differential scanning calorimetry (DSC)
Calorimetry is a very sensitive and powerful tool to detect and study phase transitions in
Spin-crossover (SCO) solids, in particular systems of Fe(II), where the primary driving force
governing the spin conversion is the increase of entropy arising from differences in spin
multiplicity and density of vibrational states between LS and HS states. Sorai and Seki [24]
proved the presence of strong cooperative intermolecular interactions, proceeding via significant
coupling between the electronic state and the phonon system, which can be seen as the origin of
sharp heat capacity peaks observed. The strength of these intermolecular interactions
6

decideswhether the SCO system is of the “abrupt type”, that is, with the spin state conversion
occurring within a narrow temperature range (typically < ~10 K), or of the “gradual type”, that
is, with the spin state conversion taking place over a much wider range. In the latter case, no
sharp heatcapacity peak is observed. Moreover, larger excess enthalpies and entropies due to the
spintransition were found in abrupt type systems (8-10 kJ mol-1 and 50-70 J mol-1 K-1,
respectively), compared to gradual-type transitions (4-8 kJ mol-1 and 25-40 J mol-1 K-1,
respectively) [24-26].
Differential scanning calorimetric measurements (DSC) of compounds (1) and (2) were
carried out at a scan rate of 10 K/min over the temperature range 120-300 K during heating and
cooling. The molar heat capacities at constant pressure, Cp, of compounds (1) and (2) deduced
from DSC experiments are given in Figures 1-2, displayed endothermic peaks on warming at
T↑ = 222, 193 K and exothermic peaks, on cooling, at T↓ = 208, 183 K, thus revealing a
1/2
1/2
hysteresis loop of width 14, 10 K for (1) and (2); respectively. This behavior is characteristic of a
first-order phase transition and can be related to the ST detected by magnetic measurements
studies. The deduced calorimetric figures associated with the SCO ∆transH (integration of ∆Cp
over T) and ∆transS (integration of ∆Cp over ln T) and the observed difference in sharpness and
are gathered in Table 2. Compounds (1) and (2) (see Figures 1 and 2) exhibit very sharp heat
capacity anomalies. The enthalpy and entropy variation have been evaluated, after correction for
the presence of residual HS at low temperature, as ∆H = 21.6(1) kJ mol-1, ∆S = 97.5(1) J mol-1K-
1; ∆H = 20.4(1) kJ mol-1, ∆S = 105.5(1) J mol-1K-1for (1) and (2); respectively. The vibrational
contribution to the entropy variation is evaluated as ∆Svib= 84.1 and 92.1 J mol-1K-1for (1) and
(2);
respectively,
the
electronic
contribution
alone
is
being
equal
to
(∆S=Rln[(2S+1)HS/(2S+1)LS] = R ln5= 13.4 Jmol-1K-1for SHS= 2 and SLS= 0) for Fe(II)
SCOcompounds [27].
The excess enthalpies are higher than 10 kJ mol-1 and the vibrational entropies higher
than 35 J mol-1K-1 are considered here somewhat larger than usually observed for Fe(II)
mononuclear SCO compounds [28], but conforms well with the strong cooperative nature of the
ST for 1D coordination polymer [9,15]. Such high figures of excess enthalpy and entropy are
supporting the strong cooperative behavior of these materials and thought to be a consequence of
7

both strong intra-chain interactions, arising from direct rigid chemical link between the Fe(II)
centers and the triple N1,N2-triazole bridges (Scheme 2)[29] and inter-chain intermolecular
-
-
interactions e.g., the strong hydrogen bonding between the BF4 and ClO4 anions and the
triazoleligands. On the other hand, it is clear that the presence and the nature of the substituent
cyclopropyl on the triazole rings will trigger the intermolecular interactions between the
polymeric chains.
3.3 Magnetic characterization
The spin-crossover phenomenon between the high spin and low spin states for the
polycrystalline samples of [Fe (4-C3trz)3](BF4)2.3H2O (1) and [Fe(4-C3trz)3](ClO4)2.2H2O (2)
have been recorded at 0.1 T in the temperature range 5−300 K. The (χMT) versus T plots for (1)
and (2) are given in Figures (3-4) where (χMT: molar susceptibility; T: temperature). In the
heating mode, the course of the χMT(T) dependency in (1) and (2) is accompanied by a very
abrupt LS→HS transition with T↑1/2= 230 and 218 K for (1) and (2); respectively. The χMTfor
compounds (1) and (2)at room temperature are found to equal to 3.16, 3.20 cm3K/mol;
respectively correspond to the expected value for a HS iron(II) ion. On cooling, similar trend to
that observed in the heating cycle, the lowering of temperature involves in both complexes, a
very abrupt HS→LS transition with T↓1/2= 221 K for (1) and 202 K for (2). The sharpness in
-
-
transition in case of BF4 (1) and ClO4 (2) is in a good agreement with the literature [28-29]. On
further lowering of temperature, the χMT went to much lower values but it does not reach zero,
this value of χMT at low temperatures is indicative of the amount of residual HS centers, which is
ascribed to the polymeric chain ends that bear coordination water molecules and/or
monocoordinated triazole ligands. The presence of high spin terminal {LFe(H2O)3} coordination
sides in polymeric chain was proven by i.e. Rajadurai et al [30] and indeed, the presence of water
molecules in herein reported complexes is supported by FTIR spectroscopy and TG/DTA (vide
supra). Therefore, the residual paramagnetism at lower temperatures supposes to be a measure of
actual length of polymeric chains. However, the estimation of residual HS fraction is very
difficult, since positive values of χMT product in the low spin state temperatures might be
polluted with small remnant high spin fraction (incomplete spin crossover) or by small amount of
paramagnetic Fe(II)/Fe(III) impurities. The abruptness of these transition curves is usually
8

considered to be related to thecooperativity exhibited by the compound upon the spin transition
and can be evaluated by the temperature range ∆T80in which 80% of the transiting Fe(II) ions
undergo their spin transition(see Table 2). In addition, the hysteresis cycle obtained from both
heating and cooling runs indicates a hysteresis loop ~ 9 and 16 K for (1) and (2), respectively
and suggests a strongly pronounced cooperativity of the spin transition and it is in agreement
with the results obtainedfrom the DSC measurements. The much more rigid cyclic moiety (i.e.
the interlocking of the cyclopropyl substituent of neighboring complexes) may play a crucial rule
in the abruptness ofthis spin transition[31]. All the transition parameters for complexes (1) and
(2) both on coolingand heating are listed in Table 2.
The temperature dependence of magnetic susceptibilities for complex (3) is shown in Fig. 6. The
heating and the cooling modes are showing a very gradual transition accompanied by a
narrow hysteresis loop of width of about 7 K (T↑1/2= 150, T↓1/2= 143 K).
3.4. Thermal analyses
A deeper insight on the thermal stability of the investigated complexes (1), (2) and (3)
was undertaken using thermogravimetry (TG) and differential thermal analysis (DTA)
techniques, Figures 5-6.
[Fe(4-C3trz)3](BF4)2.3H2O complex (1) (m/z = 608.10; 3.18% as M-2H) decomposed in
two thermal steps (Figure 5) covering the temperature 50-420oC. The first step is an
o
endothermic step (DTA curve) occurred at 50-172 C is the dehydration of the complex with
removal of three water molecules with a weight loss of 5.85 % (5.90% calc.) (mass loss = 35.69).
The second decomposition peak is an exothermic one occurred at 172-420 oC with a weight loss
of 81.53 % (82.12 % calc.) which could be attributed to the total loss of three triazoles species
(3C5H7N3; mass = 109.13; m/z =109.06; 100%) and2HBF4 species (mass loss = 87.81; m/z =
87.10; 0.27 %) leaving FeO (11.80%) as the metallic residue (mass loss = 72.00, m/z = 72.05 ;
0.14% )
The thermogram of [Fe(4-C3trz)3](ClO4)2.2H2O (2) (Figure 6) showed similar trend to
complex (1) and displayed two successive decomposition steps; The first decomposition peak is
an endothermic step (DTA curve) occurred at 50-194 oC with weight loss of 2.90 % (2.91% calc.)
(mass loss = 17.93) was assumed to the loss of two molecule of water of hydration. The second is
9

o
an exothermic one (DTA curve) existed over a temperature range 194-250 C, with a net mass
loss of 86.02 % (85.14% calc.) which could be attributed to the total loss of three triazoles
species (3C5H7N3; mass = 109.13; m/z =109.10; 85.86%) and2HClO4species (mass loss = 87.81;
m/z = 87.10; 2.32%). leaving FeO (11.80%) as the metallic residue (mass loss = 72.00, m/z =
72.05 ; 2.07% )
3.5. Water Content and Anion
The role of the noncoordinated solvent molecules in most Fe(II) spin crossover
complexes is well understood where the incorporation of the solvent molecules into the crystal
lattice mostly influences the interlinking of the spin crossover centers and leads to higher
transition temperatures and abruptness of the spin transition. The explanation of the hydrogen
bonding effect and its influence on the parameter T1/2 implies formation of the strong hydrogen
bonds N-H….OH2, where the nitrogen atom belongs to the heterocycle or amine group directly
bonded to the Fe(II) ion. This interaction results in strengthening the metal-ligand interaction and
then favoring the low-spin state at higher temperatures [32,33].In the present work, the nitrogen
atom of triazole ring seems to be not involved in the hydrogen bonding with the water molecule.
Therefore, the observed change in the magnetic properties on the decrease in the number of
water molecules from compound (1) to (2) cannot be attributed to an electronic effect induced by
the kinds of hydrogen bonding discussed above.
In the particular case of the [Fe(Rtrz)3](A)2compounds, the role of water is assumed to
be important as stated in numerous reports [6a,34,35,15,36]. However, to the best of our
knowledge the involvement of water molecules into the crystal lattice of the triazole-based Fe(II)
polymers was never analyzed despite available crystal structures of the copper(II) analogues.
According to the crystal data, in [Cu(NH2-trz)3](BF4)2·H2O [3b] the water molecules bridge CH
groups of the neighboring triazole units reinforcing the polymeric chain by weak double
hydrogen bonds CH….Ow….HC [3b,35,37]. In the case where the number of water molecules per
monomeric unit is less than three, the anion is involved in the double hydrogen bonding with the
CH groups of the triazole rings [37,3b,37]. In this work, the spin transition temperature was
found to increase along the series (1) > (2). It seems to be the case for compounds (1) and (2)
10

which have three and two molecules of water in the pristine form as shown from TGA data;
respectively. In both cases, because both types of hydrogen bonding, either between water
molecules or anions, can be involved with the triazole units, a larger electrostatic pressure
created and leads finally to the higher transition temperatures of these two compounds
[6a,15,37a]. The IR data confirm the decrease of the symmetry of the tetrafluoroborate and
perchlorate anions because of their interaction with the lattice surrounding that might indicate
their involvement in the hydrogen bonding.
Conclusions
Because the spin transitionof [Fe(NH2trz)3](NO3)2 exhibits a wide hysteresis loop occurring
around and even above room temperature, this material could be useful for optical data recording
or thermalsensing around room temperature. The special characteristics for this material
hasencouraged our research group to perform further investigations and to a continuation of
research along this ligand’s family,especially taking into account gain in rigidity of the bridging
ligand (blocking the shock absorption effect) and also the interesting relations between the
special fitting of other anions into the coordination network structure and their SCO properties.
As earlier work showed a favorable spin transition behavior using a cycloalkyl substituent on the
tetrazole ring [27] it seemed to us a promising synthetic path is to establish a series of new
triazole ligand with cycloalkyl-substituent of different ring sizes. Using the new ligand, 4-
cyclopropyl-1,2,4-triazole (4-C3trz), we successfully isolated three novel 1D coordination
polymers; [Fe(4-C3trz)3](BF4)2·3H2O (1) and [Fe(4-C3trz)3](ClO4)2·2H2O (2). The investigations
on the spin transition behavior of compounds (1) and (2) yielded a complete and abrupt spin
crossover transitions.
Acknowledgments
Grant agencies (Egypt: TA Fund from Faculty of Science, Cairo University; Austria: FWF
project 19335-N17; Slovakia: VEGA 1/0233/12 and VEGA 1/0522/14, APVV-0014-11) are
thankfully acknowledged for their financial support.
11

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14

4
3.5
3
T1/2↓ =208(1) K
2.5
2
1.5
1
0.5
0
T1/2↑ =222(1) K
-0.5
100
120
140
160
200
220
240
260
18T0 (K)
Fig.1. DSC curves of [Fe(4-C3trz)3](BF4)2.3H2O complex.
15

3.5
3
T1/2↓ =183(1) K
2.5
2
1.5
1
T1/2↑ =193(1) K
0.5
0
-0.5
100
120
140
160
180
200
220
240
260
T(K)
Fig. 2. DSC curves of [Fe(4-C3trz)3](ClO4)2.2H2O complex.
16

3.5
3
2.5
2
T1/2 ↑= 230 K
T1/2 ↓= 221 K
1.5
1
0.5
0
0
50
100
150
200
250
300
350
T (K)
Fig. 3. Temperature-dependent magnetic susceptibility of the [Fe(4-C3trz)6](BF4)2.3H2O complex in
an external field of 0.1 Tesla.
17

3.5
3
2.5
2
T
↑ = 218 K
½
T
↓ = 202 K
½
1.5
1
0.5
0
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300
T (K)
Fig. 4. Temperature-dependence magnetic susceptibility of the [Fe(4-C3trz)6](ClO4)2.2H2O complex
in an external field of 0.1 Tesla.
18

120
100
80
60
40
20
0
30
25
20
15
10
5
TGA
DTA
0
o
Temperature, C
Fig. 5. TGA and DTA curves of the [Fe(4-C3trz)3](BF4)2.3H2O complex.
19

30
25
20
15
10
5
100
80
60
40
20
0
TGA
DTA
0
-5
26
215
396
577
754
o
Temperature; C
Fig. 6. TGA and DTA curves of the [Fe(4-C3trz)3](ClO4)2.2H2O complex.
20

O
OC2H5
OC2H5
OC2H5
C2H5O
HC
O
H
N
-C2H5OH
CH
NH
H
+
H2N
C
H
N
monoformylhydrazine
triethylorthoformate
NH2
cyclopropylamine
R
N
N
C2H5O
CH
NH
+ H2O
CH
-C2H5OH
N
N
N
4-cyclopropyl-1,2,4-triazole
Scheme 1. Synthesis of 4-cyclopropyl-1,2,4-triazole (4-C3trz)
R
R
N
R
R
R
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
R
N
R
N
R
N
R
N
R
N
N
N
N
N
N
N
N
N
N
Fe
Fe
Fe
Fe
Fe
Fe
N
N
N
N
N
N
N
N
N
N
N
R
N
R
N
R
N
R
N
R
Scheme 2. Representation of the [Fe(4-R-trz)3]2+ chain
The structure of Fe(II) complexes with 1,2,4-triazoles is known to be generated by iron ions connected by bridging
triazole ligands to form 1D-coordination polymer chains [32]. The coordination environment of the metal centers in
these compounds and the possible interaction between the chains cause an ability of the iron ions to undergo
cooperative spin transition which can occur near room temperature.
21

Table 1.
IR spectral data of the 4-cyclopropyl-1,2,4-triazole and its complexes.
[b]


-
-
-
-
[a] (H2O)
C-H
C=N
C-N
C-H
ClO4
--
ClO4
BF4
BF4 AsF6
AsF6
M-N
--
Compounds
(triazole)
(triazole ) (triazole) (triazole)
4-C3trz
--
3105s
3138w
3117m
1529s
1184s
1111s
1074s
--
--
--
--
--
--
--
--
[Fe(4-C3trz)3](BF4)2.3H2O
[Fe(4-C3trz)3](ClO4)2.2H2O
3428b
3433b
1555m
1553m
1235w
1235w
--
--
1071s 527m --
-- -- --
473w
463w
1022w
620m
[a] stands for stretching mode of vibration. [b] : stands for bending mode of vibration
Table 2.
Comparison of transition parameters of the spin transition in compounds1-3 measured by SQUID and DSC.




T1/2
T1/2
∆T1/2 ΔT80 HS resd. T1/2
T1/2
∆T1/2

transH

transS
ΔSvib
Compound
(K)[a] (K)[a] (K)[a] (K)[a] (%)
(K)[b] (K)[b] (K)[b] (kJ mol-1)[c] (J mol-1K-1)[c] J mol-1 K-1
[Fe(4-C3trz)3](BF4)2.3H2O
[Fe(4-C3trz)3](ClO4)2.2H2O 218
230
221
202
9
16
20
19
4
3
222
193
208
183
14
10
21.6(1)
20.4(1)
97.5(1)
105.5(1)
84.1
92.1
T1/2 stands for the temperature at which half of the SCO centres have changed their spin; ΔT80 the temperature range in which 80% of
the transiting Fe(II) ions undergo their spin transition; [a] from magnetic data(SQUID). [b] from DSC. [c] Excess enthalpies and
entropies are given per mole of transition Fe(II) ions: they are corrected of the residual HS fraction γHS deduced from the magnetic
data using the relation [γHS = (χT - χTLS)/(χTHS - χTLS)], where χTLS and χTHS stand respectively for the values of χT in the LS
and HS states.

Graphical Abstract
New 4-cyclopropyl-1,2,4-triazole (4-C3trz) was synthesised and used for the preparation of 1D
−
Fe(II) SCO compounds of the formula [Fe(4-C3trz)3](X)2·nH2O, (compound (1): X=ClO4 , n=3;
−
compound (2): X=BF4 , n=2). The results of magnetic susceptibility (SQUID) and DSC showed
1/2
abrupt and complete spin transition with a hysteresis loop T↑ = 230/222, T↓1/2= 221/208 K for
tetrafluoroborate and T↑ = 218/193, T↓1/2= 202/183 K for the perchlorate analogue using
(SQUID and DSC); respectively. The enthalpy and entropy associated to the ST evaluated by
1/2
-
DSC results as ∆transH = +21.6, +20.4 kJ mol-1 and ∆transS = +97.5, +105.5 J mol-1 K-1 for the BF4
and ClO4 analogues; respectively.
-
3.5
3
2.5
2
T1/2 ↑= 230 K
T1/2 ↓= 221 K
1.5
1
0.5
0
0
50
100
150
200
250
300
350
T (K)
Temperature-dependent magnetic susceptibility of the [Fe(4-C3trz)6](BF4)2.3H2O complex in an
external field of 0.1 Tesla.
23

Graphical Abstract
New 4-cyclopropyl-1,2,4-triazole (4-C3trz) was synthesised and used for the preparation of 1D
−
Fe(II) SCO compounds of the formula [Fe(4-C3trz)3](X)2·nH2O, (compound (1): X=ClO4 , n=3;
−
compound (2): X=BF4 , n=2). The results of magnetic susceptibility (SQUID) and DSC showed
1/2
abrupt and complete spin transition with a hysteresis loop T↑ = 230/222, T↓1/2= 221/208 K for
tetrafluoroborate and T↑ = 218/193, T↓1/2= 202/183 K for the perchlorate analogue using
(SQUID and DSC); respectively. The enthalpy and entropy associated to the ST evaluated by
1/2
-
DSC results as ∆transH = +21.6, +20.4 kJ mol-1 and ∆transS = +97.5, +105.5 J mol-1 K-1 for the BF4
and ClO4 analogues; respectively.
-
24

Research Highlights
• A new 4-cyclopropyl-1,2,4-triazole was synthesized.
• 1D [Fe(4-C3trz)3](BF4)2.3H2O and [Fe(4-C3trz)3](ClO4)2.2H2O chains were prepared.
• The complexes were characterized by analytical methods.
• SCO was inspected by magnetic susceptibility (SQUID) and DSC measurements.
• The improved cooperativity was attributed to hydrogen bonding interactions.
25