Halogenated Alkyltetrazoles for the Rational Design of FeII Spin-Crossover Materials: Fine-Tuning of the Ligand Size

Article abstract of DOI:10.1002/chem.201704656

1-(3-Halopropyl)-1H-tetrazoles and their corresponding Fe^II spin-crossover complexes have been investigated in a combined experimental and theoretical study. Halogen substitution was found to positively influence the spin transition, shifting the transition temperature about 70 K towards room temperature. Halogens located at the ω position were found to be too far away from the coordinating tetrazole moiety to have an electronic impact on the spin transition. The subtle variation of the steric demand of the ligand in a highly comparable series was found to have a comparatively large impact on the spin-transition behavior, which highlights the sensitivity of the effect to subtle structural changes.

Full text of DOI:10.1002/chem.201704656

A Journal of
Accepted Article
Title: Halogenated Alkyltetrazoles for Rational Design of Fe(II)-Spin
Crossover Materials - Fine-tuning of the Ligands Size
Authors: Danny Müller, Christian Knoll, Marco Seifried, Jan M Welch,
Gerald Giester, Michael Reissner, and Peter Weinberger
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To be cited as: Chem. Eur. J. 10.1002/chem.201704656
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Halogenated Alkyltetrazoles for Rational Design of Fe(II)-Spin
Crossover Materials – Fine-tuning of the Ligands Size
Danny Müller,*[a] Christian Knoll,[a] Marco Seifried,[a] Jan M. Welch,[b] Gerald Giester,[c] Michael
Reissner,[d] Peter Weinberger[a]
Abstract: 1-(3-halogenpropyl)-1H-tetrazoles and their corresponding
Fe(II) spin crossover complexes were investigated in a combined
experimental and theoretical study. The halogen substitution was
found to positively influence the spin transition, shifting the transition
temperature about 70 K towards room-temperature. Halogens located
in the -position were shown to be too far away from the coordinating
tetrazole-moiety to have an electronic impact on the spin transition.
The subtle variation of ligand steric demand through a highly
comparable series was found to have a comparably large impact on
the spin transition behaviour, highlighting the sensitivity of the effect
to subtle structural changes.
spin transition temperature and the abruptness of the spin
transition behaviour.[9] Whereas an abrupt change of the spin
state above room-temperature, preferably combined with a large
hysteresis, is highly desirable for technological application,[2, 10]
most of the currently known spin crossover compounds undergo
the high-spin (HS) – low-spin (LS) transition below room-
temperature in a rather gradual or even incomplete fashion.
Attempts to simplify the design of promising novel materials by
establishing a general rule of thumb for the prediction or at least
estimation of SCO behavior for new materials have been limited
to date. Except for highly isotypic series[11] the spin transition
behavior is affected by too many variables including solvent,[12]
counter ions,[11a,
H-bonding networks,[14] packing in the
12a, 13]
crystal[15] etc., to allow for a reliable assessment and rational
design.
Introduction
For
a systematic investigation of those individual factors
Switchable magneto-optical materials are considered promising
key-elements in next-generation miniaturized electro-magnetic
devices.[1] In recent years, many innovative concepts for data
storage, switching- and sensing devices based on the intrinsic
bistability of spin crossover (SCO) materials have been
suggested and discussed.[2] Therefore, currently, major emphasis
is placed on the development of novel multifunctional materials,[3]
contribution to the spin transition, Fe(II) complexes of N1-
substituted tetrazoles have proven highly suitable tools:[16]
Accessible by
a variety of different, versatile synthetic
approaches[17] terminal or bridging ligands may be obtained,
resulting in 0-3- dimensional structures.[18] The spin state of the
SCO material is easily identified by MIR / FIR spectroscopy,[18b, 19]
as the shift of the isolated tetrazolic CH-vibration is directly
correlated to the Fe-N4 bond-length and thus the irons’ actual spin
state. Based on these features, variously substituted N1-
tetrazoles were subject of choice for several groups, working on
a systematic approach for designing SCO materials with a spin
[4]
combining spin crossover behavior with conductivity,
luminescence,[5] piezoresistance,[6] paramagnetism,[7] host-guest
chemistry[8] and many other molecular features, preferably all of
which may be correlated to the spin transition. Regardless of
these combinatorial approaches, any potential technological
application is dominated by the intrinsic boundary conditions of
transition around room-temperature.
Building on Gütlich’s work on [Fe(3Tz)6](BF4)2
[16b, 20]
featuring an
abrupt, complete spin transition at 130 K with a small hysteresis,
a series of terminal and bridging alkyl-tetrazoles with various
alkyl-chain lengths were prepared. The longer tails on the
tetrazoles resulted in a kind of shock-absorber effect, reducing the
internal cooperativity of the system and resulting in only partial
spin transitions without any significant increase in T1/2. Finally,
halogenation of the 1-ethyl-1H-tetrazole[21] was found to be a
useful modification of the ligand, raising the spin transition
[a]
Dr. Danny Müller, Christian Knoll, Marco Seifried, Priv.Doz. Dr.
Peter Weinberger
Institute of Applied Synthetic Chemistry, TU Wien
Getreidemarkt 9/163-AC, 1060 Vienna, Austria
E-mail: danny.mueller@tuwien.ac.at
Dr. Jan M. Welch
Atominstitut, TU Wien
Stadionallee 2, 1020 Vienna, Austria
Ao.Univ.Prof. Dr. Gerald Giester
Department of Mineralogy and Crystallography
University of Vienna, Althanstraße 14 (UZA 2), 1090 Vienna, Austria
Ao.Univ.Prof. Dr. Michael Reissner
Institute of Solid State Physics, TU Wien
Wiedner Hauptstraße 8-10/050, 1040 Vienna, Austria
[b]
[c]
[d]
temperature T of its Fe(II)-SCO complex about 70 K as
½
compared to the alkyl-substituted equivalent.[21b] Based on the
infrared spectra and donor number calculations on the
halogenated ligands, the impact of the halogen was identified as
purely steric, since the halogen-substituent is too far away from
the coordinating N4-atom to interfere with its electronics.
To ascertain whether this halogen-substitution of an alkyl-
tetrazole could provide a general tool for improving the sterics of
the ligand (leading to a positive shift of the spin transition
temperature), the 1-propyl-1H-tetrazole (3Tz) was halogenated in
the -position, allowing for investigation of the spin crossover
Supporting information for this article is given via a link at the end of
the document.
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properties of [Fe(3-X-3Tz)6](BF4)2 derivatives of the
[Fe(3Tz)6](BF4)2.
Results and Discussion
To synthesize the -halogenated 1-propyl-1H-tetrazoles,
a
divergent synthetic approach based on halogenation of the
tosylate 2 was attempted (scheme 1). Although the final
halogenation step under microwave conditions was successful,
only poor yields around 12 % could be obtained. This was
attributed to the presence of the tetrazole - probably due to its
Lewis basic properties, - which has previously proven problematic
in ligand functionalization.[22]
Figure 1. Magnetic susceptibilities of Fe(II)-complexes 7-11, including the data
of [Fe(3Tz)6](BF4)2 as reference.
Whereas all halogen-containing materials feature
a nearly
complete spin transition, the hydroxy-substituent in complex 7
causes a very gradual and incomplete transition between 50 and
200 K. Although, the hydroxy-groups in the complex where initially
expected to act as hydrogen-bonding partner, thus buffering
cooperativity and enhancing the abruptness of the spin transition,
its presence was shown to be unfavorable according to the
magnetic data. In contrast to the halogenated ethyl-
derivatives,[21b] no correlation between spin transition behavior of
complexes 8-11 and the increasing size of its halogen is observed.
The high-spin fraction remaining after transition increases
inversely with halogen atomic number. The fluorine compound (8)
shows a completely low-spin state whereas the iodine complex
(11) shows 21 % residual high-spin configuration. The T1/2 values
in the series are 175 K (Br, 10), 212 K / 160 K (Cl, 9) 194 K (F, 8)
Scheme 1. Synthesis of the halogen-substituted 1-propyl-1H-tetrazoles
Therefore, an alternative approach based on the Franke-
synthesis was selected,[16] avoiding any further functionalization.
For 3-5 the necessary halogenated propylamine is commercially
available, whereas for 6 a Finkelstein-reaction of 5 with NaI led to
the iodo-derivative (see experimental for further details). On
complexation with Fe(BF4)2∙6H2O the corresponding Fe(II)-
complexes where obtained as colorless solids, all of which
change color to bright magenta when cooled in liquid nitrogen.
and 200
K (I, 11). Compared to the parent compound
[Fe(3Tz)6](BF4)2 the spin transition temperature could be shifted
about 70 K towards room-temperature. The shape of the
magnetization curves for 8, 10 and 11 indicates the presence of
only a single magnetic site in the molecules, as the SCO occurs
as single-step transition. In contrast, for the Cl-derivative 9 at 180
K and 2.04 cm3Kmol-1 the slope of the magnetization curve
suddenly changes, decreasing faster than before. According to
the 2:1 HS-LS ratio at 180 K, the presence of three different
magnetic iron(II)-sites is deduced for 9. This could also be
confirmed by the structural characterization (see below). In 9, the
spin transition of the first iron site happens between 265 K – 180
K, whereas the two remaining iron sites undergo the spin
transition within 25 K, reaching the complete LS-state at 155 K.
While determining the magnetic susceptibility in heating mode,
the second step of the spin transition was found to show a broad
10 K hysteresis between 165 K and 190 K. Above this
temperature the heating and cooling curves are again identical,
as observed over the whole temperature regime for compounds
8, 10 and 11. The exceptional behavior of the chlorine-substituted
derivative 9 within this series was also observed in the case of
halogenated 1-ethyl-1H-tetrazoles, for which [Fe(2-Cl-
Magnetic Susceptibility
In figure 1 the temperature dependent magnetic susceptibilities of
compounds 7-11 are shown.
2Tz)6](BF4)2 shows
a
two-step transition and
[Fe(2-Cl-
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2Tz)6](ClO4)2 a two-step transition with a 17 K broad hysteresis in
the second step was reported.[21a] To investigate the impact of the
anion, [Fe(3-Cl-3Tz)6)(SbF6)2 (12) was prepared. The magnetic
data revealed, that in this case the use of a larger, octahedral
weakly coordinating anion deteriorated the SCO-properties, since
at a T1/2 of 103 K only about 50 % of the material undergoes the
LS-transition (see figure S1).
Mössbauer Spectroscopy
To further characterize the magnetic behavior of 9 temperature-
dependent 57Fe-Mössbauer spectra were recorded (see figure 2).
Due to the modest statistics at higher temperatures, all spectra
were analyzed with only two sub-spectra, one doublet for high
spin state and one for low spin state of Fe2+. Up to 120 K 80 % of
the spectra correspond to the HS state. Between 120 K and 220
K a transition mainly to the LS doublet is observed, ending with
approximately 63% LS and 37% HS at room temperature (see
figure 3). The transition observed in the Mössbauer spectra is in
good agreement with the magnetic susceptibility measurements
of 9. The two-step transition observed in the susceptibility
measurements is not visible in the Mössbauer results, as
therefore a notably higher measurement density would be
necessary. Nevertheless, a strong jump in quadrupole splitting of
the HS component QS = eQVzz/4 between 120 and 180 K
indicates a drastic change in the local surrounding of the Fe-
atoms. This observation fits very well with the data obtained from
the molecular structure, showing notable changes in the lattice
parameters for the same temperature range (see Figure 5). No
such change is found for the half width (inset figure 3). To further
evaluate the Mössbauer data regarding the three different Fe
sites, a detailed analysis with 6 sub-spectra (3 for the LS and 3
for the HS) would be necessary. Based on the insufficient
statistics of the measured spectra such detailed evaluation would
be afflicted with a high uncertainty, and was therefore avoided.
Figure 2. 57Fe-Mössbauer spectra recorded for 9 at different temperatures.
Figure 3. Temperature dependence of the intensity (left) and quadrupole
splitting QS (right) for the LS component in the Mössbauer spectra. Inset:
temperature dependence of the half width of LS component.
Structural Characterization
For 3-(1H-tetrazol-1-yl)propan-1-ol (1), the only solid ligand in this
study, single crystals were grown by slow evaporation of a
̅
CH2Cl2-solution. 1 crystallizes in the triclinic space group P1 (N°
2) as crystallographically unique molecule on a general position
and two formula units per cell (see figure 4).
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Figure 4. Molecular structure of 3-(1H-tetrazol-1-yl)propan-1-ol (1), view along
the crystallographic b-axis at 200 K. Details on the structure determination are
found in table S1.
Many attempts and various techniques were used to grow single
crystals suitable for structural characterization of compounds 8-
12. Finally, only for [Fe(3-Cl-3Tz)6](BF4)2 very small, hexagonal
platelets could be obtained by diffusion of Et2O into an MeCN
solution, combined with slow evaporation of the solvents. The
structure was determined at three different temperatures,
covering the most interesting temperatures of the magnetic
measurement. The reason for the two-step spin-transition was
found in a splitting of the iron positions, corresponding to a
crystallographic phase-transition, resulting in expansion of the
unit cell. In figure 5 the temperature dependent variation of cell
parameters and angles are compared with the magnetic
susceptibility of 9. Accompanying the onset of the spin transition,
a
structural transformation and symmetry loss occurs,
transforming the structure from the former trigonal space group
Figure 5. Magnetic susceptibility of 9 correlated to a) temperature dependent
variation of the cell-parameters b) temperature dependent variation of the
angles
̅
̅
R3 (N° 148) of the high-spin state to the triclinic space group P1
(N° 2) for the mixed HS-LS state. Upon reaching the complete
low-spin state, lower symmetry is retained, concomitant with a
further distortion of the unit cell. In table S1 and S2-S4 the
corresponding crystallographic data, as well as representative
angles and bond-lengths are listed.
At room-temperature the structure of 9 possesses a magnetically
unique iron position with the iron atom coordinated by 6 1-(3-
chloropropyl)-1H-tetrazoles with a Fe-N4 distance of 2.194 Å
characteristic for an Fe(II) HS-state. The large thermal ellipsoids
for the -carbon of the propyl-chain (C4) and the Cl-atom seen in
figure 6 are caused by a free thermal motion in the solid/ local
disorder of the 3-chloropropyl chains. In the unit cell three different
iron locations on the Wyckoff position 3b, having the site
-
symmetry group -3 are found. The BF4 anions are disordered,
relating two different orientations of the tetrahedron by a pseudo
mirror-symmetry with the mirror-plane parallel to (001). The
anions establish a network of H∙∙∙∙F bonds between the anions
and the H-atom of the tetrazole rings, which is observed for all
three measured structures.
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step (see figure 1). View along the crystallographic c-axis. For a list of bond-
lengths and bond-angles see table S3
At 150 K a further structural transformation occurs involving only
the a-axis (shortened to 10.698 Å) and the unit cellangles. The
former three magnetically different Fe(II)-positions are now
reduced to two distinguishable positions, having Fe-N4 distances
of 1.993 Å (Fe1) and 1.991 Å (Fe2), as shown in figure 8. As for
the 200 K structure, the bond-lengths and angles around the iron
atoms vary slightly. Both iron positions are still located on the
Wyckoff position 18f with symmetry 1. The disorder of the
chloropropyl-chains is now limited to one ligand coordinated to the
Fe2, which could be refined with a split-position -carbon of the
propyl-chain (C4) and the Cl-atom, both with a weighting scheme
of 0.5
Figure 6. Coordination environment around the Fe-center of 9 in the HS-state
at 300 K, view along the crystallographic c-axis. For a list of bond-lengths and
bond-angles see table S2.
Cooling to 235 K a crystallographic phase transition is observed,
splitting the former unique iron position to one position for the LS-
state and two positions for the remaining HS-state, both on the
Wyckoff position 18f with symmetry 1. Accompanying the change
to the triclinic system, the c-axis is shortened from 36.116 Å to
23.929 Å, whereas the a- and b-axis are elongated from 10.900
Å
to 18.787 Å. The Fe-N4 distance for the LS-iron is
approximately 2.020 Å at 200 K (figure 7), which agrees well with
literature known bond-lengths for Fe(II) in the LS-state. Due to the
thermal contraction the Fe-N4 bond-length for the HS-state is
shortened to around 2.170 Å. Unlike the structure at 300 K, the
six Fe-N4 distances and N-Fe-N angles are no longer equal and
the bond-lengths mentioned here are representative examples.
For a list of bond-lengths and bond-angles see table S3.
Nonetheless, for both the -carbon of the propyl-chain (C4) and
the Cl-atom large thermal ellipsoids are observed.
Figure 8. Asymmetric unit of 9 at 120 K, both iron sites in the LS-state, view
along the crystallographic c-axis. For a list of bond-lengths and bond-angles see
table S4.
Infrared Spectroscopy
The characteristic vibrations of the tetrazole-ring are found in the
mid infrared around 3130 cm-1 (the stretching vibration of the
tetrazolic C-H) and 1480-1495 cm-1 (the N2=N3 stretching
vibration) and are listed for the ligands 1 and 3-6 in table 1. As the
energies of the bands in the MIR are very sensitive to the
electronic environment, the lack of a systematic shift for the
various halogen-substituents confirms their purely steric impact,
leaving the electronics of the coordinating tetrazole ring
unaffected.
Figure 7. Asymmetric unit of 9 at 200 K, including three different Fe(II)-sites.
Fe1 and Fe2 are in the HS-state, Fe3 transformed to the LS-state in the first
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and or around 420 cm-1 for the corresponding LS-state. The
temperature-dependent FIR spectra for the series are shown in
figure S8-S13.
Table 1. Comparison of characteristic tetrazolic bands of the ligands
OH (1)
F (3)
[cm-1]
Cl (4)
[cm-1]
Br (5)
[cm-1]
I (6)
[cm-1]
[cm-1]
CH
3130
1495
3136
1486
3135
1485
3134
1485
3131
1484
N=N
Upon coordination the tetrazolic vibrations are affected due to
formation of the Fe-N4 bond, resulting in an increase in energy of
about 11 – 17 cm-1 in complexes 7-11 (see table 2).
Table 2. Comparison of characteristic tetrazolic bands for the complexes 7-
11 in the HS and LS state
OH (7)
[cm-1]
F (8)
[cm-1]
Cl (9)
[cm-1]
Br (10)
[cm-1]
I (11)
[cm-1]

CH (HS)
CH (LS)
N=N (HS)
N=N (LS)
3147
3151
1505
1511
3147
3147
1506
1512
3145
3151
1506
1513
3145
3149
1505
1511
3139
3148
1505
1510

Figure 9. Variable temperature MIR spectra of 7 at 298 K (red) and 100 K (blue)
with labelled characteristic tetrazolic bands shifted due to the spin transition.


Temperature dependent MIR / FIR spectroscopy was used for
complementary characterization of the spin-states in complexes
7-12, as on the HS-LS transition, the bond-strength of the Fe-N4
bond is drastically changed. This results in a shift of the
neighboring CH and N=N to higher wavenumbers in the LS-state.
Again, as seen by comparison of the vibrational energies in table
2 the shifts and positions are relatively constant over the whole
series, excluding any electronic impact of the terminal halogen on
the spin transition. In figure 9 representative MIR spectra for
[Fe(3-OH-3Tz)6)(BF4)2 are compared for the HS and LS state, for
8-12 the spectra are found in figure S3-S7.
In the case of the two-step spin transition observed for [Fe(3-Cl-
3Tz)6)(BF4)2 the mixed HS-LS state concomitant with the
crystallographic phase transition can be monitored in the MIR.
Starting at 3144 cm-1 at 298 K in the high-spin state, a shoulder
appears for the CH-vibration on cooling and splitting to one LS
and two HS iron positions. This occurs accompanied by a slight
shift to 3146 cm-1. Upon further cooling a more pronounced shift
at 155 K to 3152 cm-1 indicates the complete transformation to the
LS state. A three-dimensional representation of these effects is
given in figure 10. Characteristically for complexes with the BF4-
anion are broad, intense B-F stretching vibrations with a
maximum at 1030 cm-1. In the case of 12 a comparable vibration,
attributed to the SbF6-anion is found at 660 cm-1.
Figure 10. Stepwise shift of the tetrazolic CH during the two-step spin transition
of 9. For a better visibility the spectra are represented in absorption.
UV-VIS / NIR Spectroscopy
The different spin states of an Fe(II)-complex correlate to a
change in the associated electronic transitions. Solid state UV-
VIS / NIR spectroscopy in diffuse reflection was used to obtain the
ligand-field spectra of the Fe(II)-complexes 7-12 in both spin-
states (see figure S13-S18). For Fe(II)-complexes octahedrally
coordinated by tetrazoles, in the HS-state at room-temperature
Another prominent feature of hexacoordinated Fe(II)-tetrazole
complexes is the spin-state dependent vibration of the Fe atom in
the N6-coordination octahedron in the N-N-N planes.[18b] This
more or less decoupled vibrational mode yields either an
absorption band around 220 cm-1 for an Fe(II) in high-spin state
5
5
the characteristic T2 → E transition occurs around 850 cm-1 in
the near infrared, thus invisible for the human eye. In contrast, the
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1
1A1 →1T1 transition, as well as the A1 →1T2 transition are both
observed in the visible region of the electromagnetic spectrum
around 550 cm1 and 380 cm-1 and are responsible for the bright
magenta color of the LS-state. From the absorption maxima
observed in the HS- and LS-spectra the octahedral ligand field
strength 10 Dq of the different compounds may be estimated, as
for octahedral complexes with neutral ligands the ligand field
correlates with the dipole-moment of the ligand µ and the distance
from metal to ligand r (Eq. 1).[23]
10퐷푞 ≈ µ/푟6
(1)
This leads to Eq. 2
6
퐿푆
10 퐷푞
10 퐷푞
= (푟
)
(2)
퐿푆
퐻푆
푟
퐻푆
Whereas 10 DqHS is directly obtained from the ligand field
spectrum in form of the 5T2 5E transition, 10 DqLS may be
approximated according to Eq. 3.
→
1
1
10 퐷푞퐿푆 = 퐸( 1푇 ) − 퐸( 1퐴1) + (퐸( 푇 )−퐸( 푇 )
)
(3)
2
1
1
4
The values for the ligand field strengths of compounds 8-11 listed
in table 3. 7 and 12 were omitted, as the spin transitions of both
complexes are incomplete.
Table 3. Ligand field strengths of complexes 8-11
1A1 →1T1
(cm-1)[a]
1A1 →1T2
(cm-1) [b]
10 DqHS
(cm-1)
10 DqLS
(cm-1)
10 DqLS
/
10 DqHS
27027
27027
27027
27027
18450
18349
18281
18215
11416
11834
11442
12255
20594
20519
20468
20418
1.80
8
9
1.73
1.79
10
11
1.67
퐸( 1푇2)
퐸( 1푇1)
[a]
[b]
To compare the spectroscopically obtained values for the ligand
field strength with the ones obtained from structural parameters,
the experimental Fe-N4 distances of 9 in the HS- and LS state
were used. According to Eq.1 this results in a value of 1.78 for 10
Dq, which is in reasonable agreement with the values calculated
in table 3. Interestingly, for 9 and for 11 a literature value for the
ligand field strength of 1.72, obtained from averaged Fe-N4
distances is in better accordance than the current experimental
one.
Figure 11. Calculated total electron density (left) and electro static potential
(right) for the unsubstituted 1-propyl-1H-tetrazole and the halogenated
derivatives 3-6, virtual plane through the tetrazole ring.
Comparing these representations, the only significant difference
in the electronics of the various ligands is in the electron density
around the halogen. At the tetrazole, and especially the
coordinating N4, no significant impact of halogen substitution on
the charge distribution is observed. Although, figure 11 represents
the potential in a virtual plane parallel to the tetrazole, a similar
picture is obtained when a virtual plane parallel to the halogen is
used (see figure S19). From the theoretically calculated
characteristic vibrations of the halogenated ligands (table S5) and
the calculated Mulliken charges of the compounds (table S6),
again, no significant differences throughout the series are
observed. These theoretical results serve to confirm the
experimentally observed behavior of the series thus
substantiating the conclusion that the impact of the halogen is
purely steric. This small impact on the molecular alignment and
Quantum Chemical Calculations
Quantum chemical calculations were used to theoretically model
the impact of the halogens on ligand electronics. In figure 11 the
calculated total electron density and the electro static potential for
the halogenated propyl-1H-tetrazoles 3-6 and the unsubstituted
1-propyl-1H-tetrazole (3Tz) as reference are shown.
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were commercially obtained and used without further purification. All NMR
spectra were recorded in dry, deuterated solvents as indicated on a Bruker
Avance UltraShield 400 MHz. Chemical shifts are reported in ppm; 1H and
13C shifts are referenced against the residual solvent resonance. The
magnetic moment of the Fe(II)-complexes was measured using a Physical
Property Measurement System (PPMS®) by Quantum Design. The
packing in the crystal is sufficient to cause the observed variation
in the spin transition behavior.
Calculated structures and spectra of the complexes 8-11 as Fe2+-
cations were used to support the interpretation of the spectra. The
calculated structure parameters are in good agreement with
experimental findings. In table S7 and S8 the Mulliken charges of
selected atoms are compared in both spin-states, again
comparable within the series.
experimental setup consisted of
a vibrating sample magnetometer
attachment (VSM), bearing a brass-sample holder with a quartz-glass
powder container. The moment was determined in an external field of 1 T
in the range of 10 K to 300 K. 57Fe-Mössbauer measurements were
performed with
a standard constant acceleration spectrometer in
transmission geometry. The 57CoRh source was mounted on the driving
system and kept at room temperature. Calibration of velocity scale was
carried out with α-Fe foil. For temperature variation between 4.2 K and
room temperature a continuous flow cryostat was used in which the
sample is kept in He exchange gas. Temperature stability was ±0.5 K at
temperatures above 77 K and ±0.2 K below. The spectra were analysed
by solving the full Hamiltonian with both magnetic and electrostatic
interaction. Variable temperature solid state UV-Vis-NIR spectra were
recorded on a Perkin-Elmer Lambda 900 spectrophotometer between 320
and 1400 nm in diffuse reflectance against BaSO4 as background. A
Harrick coolable/heatable powder sample holder in “Praying Mantis”
configuration was used. Variable temperature mid-range (4000 - 450 cm-1)
and far-range (700-130 cm-1) infrared spectra were recorded in ATR
Conclusions
In the current study a series of -halogenated 1-propyl-1H-
tetrazoles and their Fe(II) spin crossover complexes were
investigated. Compared to an earlier study on halogenated 1-
ethyl-1H-tetrazole, the sharp and complete spin transition of
[Fe(3Tz)6](BF4)2 was used as initial point to verify the hypothesis,
halogenation of an alkyl-tetrazole ligand could be used as
versatile synthetic tool for in an increase of the spin transition
temperature.
For the complete series of OH (7), F (8), Cl (9), Br(10), I(11)
substituted 1-propyl-1H-tetrazoles spin crossover active Fe(II)-
complexes were obtained, featuring a higher spin transition
temperature than the parent compound with a maximal T1/2 of 200
K for the iodo-complex 11. All materials except the Cl-derivative
led to a single-step transition. Similarly to the 1-(2-chloroethyl)-
1H-tetrazole from literature, the 1-(3-chloropropyl)-1H-tetrazole
also yields a material showing a two-step spin transition, for which
the second step shows a 10 K hysteresis. The magnetic transition
is concomitant with a crystallographic phase transition at 235 K,
for which the structure with a single iron position is transformed
from the trigonal system to triclinic with three distinguishable iron
positions (2 HS, 1 LS). This process was monitored by
temperature dependent crystallography, as well as infrared
spectroscopy, both clearly resolving the different spin-states on
the iron centers.
In contrast to the literature-known halogenated ethyl-tetrazoles for
the propyl-series no clear correlation between increasing spin
transition temperature and halogen size / atomic number was
found. By a combined experimental and quantum chemical
approach it was confirmed, that the halogen substitution has only
an impact on the steric demand of the ligand, but does not affect
the electronics of the coordinating tetrazole. The approach of a
fine-tuning of the spin transition temperature by modification of
(N1-substituted alkyltetrazole)-ligands was found successful,
demonstrating, how sensitive SCO-systems are to subtle
modifications.
technique with
a
Perkin-Elmer Spectrum 400, fitted with
a
coolable/heatable PIKE Gladi ATR-Unit.[25] Single crystals were attached
to a glass fiber by using perfluorinated oil and were mounted on a Bruker
KAPPA APEX II diffractometer equipped with a CCD detector with Mo Kα
radiation (Incoatec Microfocus Source IµS: 30 W, multilayer mirror,
λ=0.71073 Å). Data of 9 were collected in a dry stream of nitrogen
(Cryostream 800, Oxford Cryosystems) at 300 K, for 1 200 K were used.
Redundant data sets up to 2θ = 55° were collected. For the measurements
of 9 at 200 K and 120 K a Nonius KappaCCD diffractometer (graphite
monochromatized Mo Kα-radiation, λ = 0.71073 Å) equipped with a 0.3 mm
monocapillary optics collimator was used. For all measurements data were
reduced to intensity values by using SAINT-Plus[26], and an absorption
correction was applied by using the multi-scan method implemented by
SADABS.[26] For the iron(II) complexes, protons were placed at calculated
positions and refined as riding on the parent C atoms. All non-H atoms
were refined with anisotropic displacement parameters. For
thermogravimetric analyses / decomposition of the Fe(II)-complexes a
Netzsch Jupiter STA-system with a heating rate of 10 K/min under N2
atmosphere was used between 25 °C and 450 °C. The powder X ray
diffraction measurements were carried out on a PANalytical X'Pert
diffractometer in Bragg Brentano geometry using Cu Kα1,2 radiation, an
X’Celerator linear detector with a Ni filter, sample spinning with zero
background sample holders and 2θ = 4-70°, T = 298 K. A background
correction and a Kα2 strip were performed using the PANalytical program
suite HighScorePlus v3.0d. The computational results presented were
achieved using Gaussian 09 Rev.D01 [27] and visualized using GaussView
5.0.8. The structures of the ligands were drawn using GaussView followed
by a molecular mechanics structure optimization implemented in the
software package. For the complexes, the molecular structures were
imported from the single crystal X-ray analysis to GaussView. The
calculations were run using density functional theory with the hybrid
[28]
functional B3LYP
for structure optimization to a ground state and
vibrational modes to prove the discovery of a local minimum. Ligands were
[29]
optimized with the 6-311++g(d,p) valence triple zeta basis set
with
polarization and diffuse functions on all atoms for all elements except
halogen atoms where the LanL2DZ [30] basis set was used for refining the
structure. Complexes were optimized using the CEP-121G [31] basis set as
cations, high spin and low spin states were calculated applying the
according spin multiplicity.
Synthesis
Experimental Section
Tetrazoles and their derivatives are potentially shock sensitive or
explosive compounds and should therefore be handled with great
care and under appropriate safety precautions!
Methodology
If not otherwise stated, all operations involving iron(II) were carried out
under inert gas atmosphere (argon 5.0). The glassware used were oven
dried at 120 °C before use for at least 2 hours. All solvents for the
complexation reactions were dried before use and stored over molecular
sieve 3 Å under argon.[24] Unless otherwise stated, all starting materials
3-(1H-tetrazol-1-yl)propan-1-ol (1): 3-Aminopropan-1-ol (25 g, 0.33 mol,
1 eq.) was suspended together with NaN3l (32.5 g, 0.5 mol, 1.5 eq.) in a
mixture of triethylorthoformate (86 ml, 0.52 mol, 1.55 eq.) and 100 ml
This article is protected by copyright. All rights reserved.

10.1002/chem.201704656
Chemistry - A European Journal
FULL PAPER
acetic acid for 18 h at 95 °C. After cooling, the precipitated salts were
separated by filtration and the solvents were removed by evaporation. The
residual yellow oil was refluxed for 5 h in 50 ml concentrated hydrochloric
acid to decompose the formed 3-(1H-tetrazol-1-yl)propyl acetate. On
completion, the hydrochloric acid was evaporated and the resulting oil
purified by column chromatography on silica gel (MeCN : CH2Cl2 = 3:2), to
give 1 (15.9 g, 37.4 %) as colourless oil, which crystallized on standing.
Mp: 60.8 °C; νCH(Tz) cm–1: 3130; 1H NMR (400 MHz, CDCl3 , δ): 1.96 – 2.49
(m, 2H), 2.50 (s, 1H), 3.66 (t, J = 5.7 Hz, 2H), 4.63 (t, J = 6.7 Hz, 2H), 8.72
(s, 1H) ppm; 13C{1H } NMR (101 MHz, CDCl3, δ): 31.92, 45.21, 58.34,
143.23 ppm; EA Calc. for C4H8N4O: C 37.49, H 6.29, N 43.73, O 12.49;
found: C 37.43, H 6.44, N 43.83, O 13.41.
1-(3-iodopropyl)-1H-tetrazole (6): 5 (1.5 g, 7.9 mmol, 1 eq.) was
dissolved in 50 ml dry acetone. After addition of NaI (1.77 g, 11.8 mmol,
1.5 eq.) the yellow suspension was refluxed for 18 h. All volatiles were
removed and the yellow residue extracted with CH2Cl2. The crude product
was purified by column chromatography on silica gel (MeCN : CH2Cl2 =
3:2) resulting 6 (1.2 g, 64.2 %) as slightly yellow oil.
1
νCH(Tz) cm–1: 3131; H NMR (400 MHz, CDCl3 , δ): 2.39-2.46 (m, J = 6.6
Hz, 2H), 3.10 (t, J = 6.4 Hz, 2H), 4.58 (t, J = 6.7 Hz, 2H), 8.73 (s, 1H) ppm;
13C{1H } NMR (101 MHz, CDCl3, δ): 32.44, 48.47, 60.67, 143.11 ppm; EA
Calc. for C4H7N4I: C 20.18, H 2.96, N 23.54, I 53.31; found: C 20.86, H
3.39, N 24.36.
[Fe(3-OH-3Tz)6](BF4)2 (7): Fe(BF4)2∙6H2O (216.4 mg, 0.6 mmol, 1 eq.)
and 1 (500 mg, 3.91 mmol, 6.1 eq.) were dissolved in 10 ml MeCN and
stirred at 50 °C for 18 h. After evaporation of the solvent the residual oil
was triturated in 10 ml CH2Cl2 for 1 h to remove the excess of ligand. The
CH2Cl2-phase was removed and the oily residue dissolved in 5 ml MeCN.
By slow addition of Et2O until the solution became opaque and subsequent
cooling at – 30 °C for 12 h 7 was obtained as white solid (214 mg, 33.4 %).
νCH(Tz) cm–1: 3147; EA Calc. for C24H48B2F8FeN24O6: C 28.88, H 4.85, B
2.17, F 15.23, Fe 5.59, N 33.68, O 9.62; found: C 27.57, H 4.94, N 32.04.
Decomposition: see Figure S20; P-XRD: see Figure S26
3-(1H-tetrazol-1-yl)propyl 4-methylbenzenesulfonate (2): 1 (5 g, 39.0
mmol, 1 eq.) was dissolved with 4-toluenesulfonyl chloride (8.1 g, 39.4
mmol, 1 eq.) in 100 ml dry CH2Cl2. Then a tiny amount of DMAP was added
and the reaction mixture cooled to 0 °C. Triethylamine (10.9 ml, 78.0 mmol,
2 eq.) was added slowly and on complete addition, the mixture was
refluxed for 18 h. After removal of all volatiles, the sticky residue was
redissolved in CH2Cl2 and extracted four times with 100 ml H2O. After
drying over MgSO4, the solvent was removed and the beige solid
recrystallized from CHCl3 / CCl4, to give 2 (8.6 g, 78.2 %) as fluffy white
needles.
Mp: 76.3 °C; νCH(Tz) cm–1: 3137; 1H NMR (400 MHz, CDCl3 , δ): 2.21 – 2.35
(m, 2H), 2.39 (s, 3H), 3.68 – 4.08 (m, 2H), 4.49 (t, J = 6.7 Hz, 2H), 7.49
(dd, J = 156.9, 8.0 Hz, 4H), 8.57 (s, 1H) ppm; 13C{1H } NMR (101 MHz,
CDCl3, δ): 21.69, 29.07, 44.27, 65.94, 127.88, 130.13, 132.21, 143.09,
145.54 ppm; EA Calc. for C11H14N4O3S: C 46.80, H 5.00, N 19.85, O 17.00,
S 11.36; found: C 45.78, H 5.10, N 17.84, O 18.12, S 11.23.
[Fe(3-F-3Tz)6](BF4)2 (8): Fe(BF4)2∙6H2O (20.5 mg, 61 µmol, 1 eq.) and 3
(48 mg, 0.37 mmol, 6.1 eq.) were dissolved in 2 ml MeCN and stirred at
50 °C for 18 h. After evaporation of the solvent the residual solid was
triturated in 2 ml CH2Cl2, resulting 8 as white solid (17.4 mg, 28.0 %).
νCH(Tz) cm–1: 3147; EA Calc. for C24H42B2F14FeN24: C 28.54, H 4.19, B 2.14,
F
26.33, Fe 5.53,
N 33.28; found: C 28.41, H 3.96, N 31.53.
Decomposition: see Figure S21; P-XRD: see Figure S27
1-(3-fluoropropyl)-1H-tetrazole (3): Same procedure as for 1, 3-Fluoro-
propylamine hydrochloride (1 g, 8.8 mmol) was used as starting material.
Purification of the crude material by column chromatography on silica gel
(MeCN : CH2Cl2 = 3:2) resulted 3 (160 mg, 14 %) as colourless oil.
νCH(Tz) cm–1: 3136; 1H NMR (400 MHz, CDCl3 , δ): 2.37 (dtt, J = 26.4, 6.8,
5.4 Hz, 2H), 4.48 (dt, J = 46.9, 5.5 Hz, 2H), 4.62 (t, J = 6.8 Hz, 2H), 8.64
(s, 1H) ppm; 13C{1H } NMR (101 MHz, CDCl3, δ): 30.52 (d, J = 20.0 Hz),
44.57 (d, J = 4.7 Hz), 79.98 (d, J = 166.9 Hz), 143.01 ppm; EA Calc. for
C4H7N4F: C 36.92, H 5.42, N 43.06, F 14.6; found: C 36.54, H 5.39, N
40.93.
[Fe(3-Cl-3Tz)6](BF4)2 (9): Fe(BF4)2∙6H2O (188.7 mg, 0.56 mmol, 1 eq.)
and 4 (500 mg, 3.4 mmol, 6.1 eq.) were dissolved in 10 ml MeCN and
stirred at 50 °C for 18 h. After evaporation of the solvent the residual solid
was triturated in 10 ml CH2Cl2, resulting 9 as white solid (214 mg, 34.5 %).
νCH(Tz) cm–1: 3145; EA Calc. for C24H42B2Cl6F8FeN24: C 26.00, H 3.82, B
1.95, Cl 19.18, F 13.71, Fe 5.04, N 30.31; found: C 25.91, H 3.78, N 29.93.
Decomposition: see Figure S22; P-XRD: see Figure S28
[Fe(3-Br-3Tz)6](BF4)2 (10): Fe(BF4)2∙6H2O (145.1 mg, 0.43 mmol, 1 eq.)
and 5 (501 mg, 2.6 mmol, 6.1 eq.) were dissolved in 10 ml MeCN and
stirred at 50 °C for 18 h. After evaporation of the solvent the residual solid
was triturated in 10 ml CH2Cl2, resulting 10 as white solid (185 mg, 31.3 %).
Decomposition: see Figure S22
νCH(Tz) cm–1: 3145; EA Calc. for C24H42B2Br6F8FeN24: C 20.95, H 3.08, B
1.57, Br 34.85, F 11.05, Fe 4.06, N 24.44; found: C 20.33, H 3.07, N 21.76.
Decomposition: see Figure S23; P-XRD: see Figure S29
1-(3-chloropropyl)-1H-tetrazole (4): Same procedure as for 1, 3-Chloro-
propylamine hydrochloride (25 g, 170.6 mmol) was used as starting
material. Purification of the crude material by column chromatography on
silica gel (MeCN : CH2Cl2 = 3:2) resulted 4 (13.2 g, 46.7 %) as colourless
oil.
1
νCH(Tz) cm–1: 3135; H NMR (400 MHz, CDCl3 , δ): 2.31-2.38 (m, J = 6.4
Hz, 2H), 3.47 (t, J = 6.1 Hz, 2H), 4.60 (t, J = 6.8 Hz, 2H), 8.82 (s, 1H) ppm;
13C{1H } NMR (101 MHz, CDCl3, δ): 31.77, 40.83, 45.20, 143.24 ppm; EA
Calc. for C4H7N4Cl: C 32.78, H 4.81, N 38.22, Cl 24.19; found: C 33.42, H
5.24, N 32.15.
[Fe(3-I-3Tz)6](BF4)2 (11): Fe(BF4)2∙6H2O (64.6 mg, 0.19 mmol, 1 eq.) and
6 (278 mg, 1.2 mmol, 6.1 eq.) were dissolved in 10 ml MeCN and stirred
at 50 °C for 18 h. After evaporation of the solvent the residual solid was
triturated in 10 ml CH2Cl2, resulting 11 as white solid (135 mg, 42.5 %).
νCH(Tz) cm–1: 3139; EA Calc. for C24H42B2I6F8FeN24: C 17.39, H 2.55, B 1.30,
F 9.17, Fe 3.37, I 45.93, N 20.28; found: C 18.85, H 2.78, N 20.98.
Decomposition: see Figure S24; P-XRD: see Figure S30
1-(3-bromopropyl)-1H-tetrazole (5): Same procedure as for 1, 3-Bromo-
propylamine hydrobromide (25 g, 114.2 mmol) was used as starting
material. Purification of the crude material by column chromatography on
silica gel (MeCN : CH2Cl2 = 3:2) resulted 5 (9.2 g, 42.1 %) as colourless
oil.
νCH(Tz) cm–1: 3134; 1H NMR (400 MHz, CDCl3 , δ): 2.37 – 2.64 (m, 2H),
3.34 (t, J = 6.1 Hz, 2H), 4.63 (t, J = 6.7 Hz, 2H), 8.75 (s, 1H) ppm; 13C{1H }
NMR (101 MHz, CDCl3, δ): 31.85, 46.30, 60.65, 143.17 ppm; EA Calc. for
C4H7N4Br: C 25.15, H 3.69, N 29.33, Br 41.83; found: C 25.23, H 3.53, N
31.82.
[Fe(3-Cl-3Tz)6](SbF6)2 (12): FeCl2∙4H2O (113 mg, 0.57 mmol, 1 eq.) and
AgSbF6 (390.7 mg, 1.14 mmol, 2 eq.) were stirred in 2 ml MeCN for 1 h.
The precipitated AgCl was separated and the clear solution added to 4
(500 mg, 3.4 mmol, 6.1 eq.), dissolved in 10 ml MeCN. After stirring at
50 °C for 18 h the solvent was evaporated and the residue triturated in 10
ml CH2Cl2, resulting 12 as white solid (384 mg, 48.0 %).
This article is protected by copyright. All rights reserved.

10.1002/chem.201704656
Chemistry - A European Journal
FULL PAPER
νCH(Tz) cm–1: 3160; EA Calc. for C24H42Cl6F12FeN24Sb2: C 20.49, H 3.01, Cl
15.12, F 16.21, Fe 3.97, N 23.90, Sb 17.31; found: C 19.18, H 2.66,
N 21.11. Decomposition: see Figure S25; P-XRD: see Figure S31
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Acknowledgements
We acknowledge financial support of the Austrian Science Fund
(FWF Der Wissenschaftsfond) project P24955-N28. The
computational results presented have been achieved using the
Vienna Scientific Cluster, elemental analysis was performed at
the Microanalytical Laboratory, Vienna University. Special thanks
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10.1002/chem.201704656
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A
halogen substitution of 1-
Danny Müller,* Christian Knoll,
(propyl)-1H-tetrazole was found to
shift the spin transition temperature
Marco Seifried Jan M. Welch, Gerald
Giester, Michael Reissner, Peter
Weinberger
about 70
K
towards room-
temperature. The reason for this
impact was found in the steric fine-
tuning of the ligands backbone.
Page No. – Page No.
Halogenated Alkyltetrazoles for
Rational Design of Fe(II)-Spin
Crossover Materials – Fine-tuning
of the Ligands Size
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