Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion

Article abstract of DOI:10.1002/anie.202015322

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)₂](BPh₄)₂?3ac (1?3 ac; FPh-terpy=4′-((3-fluorophenyl)ethynyl)-2,2′:6′,2′′-terpyridine) and is dependent on the d

Full text of DOI:10.1002/anie.202015322

Angewandte
Communications
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International Edition: doi.org/10.1002/anie.202015322
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Ferroelectrics
Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound
Induced by Polar-Ligand-Substituent Motion
Ryohei Akiyoshi, Yuki Komatsumaru, Masaki Donoshita, Shun Dekura, Yukihiro Yoshida,
Hiroshi Kitagawa, Yasutaka Kitagawa, Leonard F. Lindoy, and Shinya Hayami*
Abstract: Ferroelectric spin crossover (SCO) behavior is
demonstrated to occur in the cobalt(II) complex, [Co(FPh-
terpy) ](BPh ) ·3ac (1·3ac; FPh-terpy = 4’-((3-fluoropheny-
tions are compounds that exhibit spin crossover (SCO)
coupled with the occurrence of another phenomenon (or
[
4]
phenomena), for example, liquid crystalline behavior,
2
4 2
[5]
[6]
l)ethynyl)-2,2’:6’,2’’-terpyridine) and is dependent on the
degree of 1808 flip–flop motion of the ligandꢀs polar fluoro-
phenyl ring. Single crystal X-ray structures at several temper-
atures confirmed the flip–flop motion of fluorobenzene ring
and also gave evidence for the SCO behavior with the latter
behavior also confirmed by magnetic susceptibility measure-
ments. The molecular motion of the fluorobenzene ring was
electrical conductivity, luminescence and/or non-linear
[
7]
optical (NLO) properties. Molecular functional materials
exhibiting both SCO behavior and ferroelectric property are
of significant interest because of the potential that resulting
magnetoelectric (ME) effects might be anticipated. However,
to the best of our knowledge, there is a general lack of reports
concerned with the synthesis and investigation of such
1
9
[4f,8]
also revealed using solid-state F NMR spectroscopy. Thus the
SCO behavior is accompanied by the flip–flop motion of the
fluorobenzene ring, leading to destabilization of the low spin
cobalt(II) state; with the magnitude of rotation able to be
controlled by an electric field. This first example of spin-state
conversion being dependent on the molecular motion of
a ligand-appended fluorobenzene ring in a SCO cobalt(II)
compound provides new insight for the design of a new
category of molecule-based magnetoelectric materials.
ferroelectric SCO compounds.
Recently in a solid-state
SCO ferroelectric study it was shown that a hydrated iron(II)
complex of type [Fe(bpp) ](isonic) ·2H O (bpp = 2,6-bis-
2
2
2
(pyrazol-3-yl)pyridine; isonic = isonicotinate) was driven by
structural transformation from a non-polar to polar space
[8]
group through the removal of water. The above behavior
reflects, first, that the switching of electronic configurations
between low spin (LS) and high spin (HS) states is sensitive to
subtle ligand field strength variation reflecting the occurrence
of the structural perturbation. Secondly, the preparation of
molecular ferroelectrics should meet the strict requirement
that a compound must crystalize in a polar space group. As
a consequence, the fabrication of molecular materials com-
bining SCO and ferroelectricity has remained a formidable
challenge.
M
ultifunctional molecular materials exhibiting synergistic
coexistence of two or more properties, have received consid-
erable attention over recent years not only for their intrinsic
interest but also because of their potential applications that
include for information storage, sensors, spintronics and
[1–3]
electro-optic devices.
One of the more attractive multi-
In recent years, the use of molecular rotators as polar-
ization rotation units has attracted increasing attention for the
development of solid-state ferroelectric materials. For
instance, Akutagawa et al. have reported that molecular
functional molecule-based materials for use in such applica-
+
ꢀ
[
*] R. Akiyoshi, Y. Komatsumaru, Prof. Dr. S. Hayami
Department of Chemistry, Graduate School of Science and Technol-
ogy, Kumamoto University
ferroelectrics of type (m-FAni )(DB[18]crown-6)[Ni(dmit) ]
2
+
(
m-FAni = m-fluoroanilinium; DB[18]crown-6 = dibenzo-
2ꢀ
[
18]-crow-6;
dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate)
2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555 (Japan)
exhibits ferroelectricity by a 1808 flip–flop motion of the m-
E-mail: hayami@kumamoto-u.ac.jp
+
[9]
FAni cation. Subsequently, several types of ferroelectrics
M. Donoshita, Dr. S. Dekura, Dr. Y. Yoshida, Prof. Dr. H. Kitagawa
Division of Chemistry, Graduate School of Science, Kyoto University
Kitashirakawa-Oiwakecho, Sakyo-ku, Kyoto 606-8502 (Japan)
[
10]
based on molecular motion have been synthesized. In these
systems, appropriate molecular design leading to a large space
for the molecular motion to occur has been of key impor-
tance. The introduction of such polarization rotation units
into SCO compounds can not only produce ferroelectricity
but can also affect SCO behavior at the metal centers, leading
to the expression of ME effects.
Dr. Y. Kitagawa
Division of Chemical Engineering, Department of Materials Engi-
neering Science, Graduate School of Engineering Science, Osaka
University
1–3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Prof. Dr. L. F. Lindoy
School of Chemistry, The University of Sydney
Sydney, NSW 2006 (Australia)
Motivated by the above studies, we aimed to construct
a new ferroelectric SCO compound whose ferroelectric
properties originated from the presence of molecular
motion. For this purpose, we synthesized a cobalt(II) com-
Prof. Dr. S. Hayami
Institute of Industrial Nanomaterials (IINa), Kumamoto University
plex, [Co(FPh-terpy) ](BPh ) (1; FPh-terpy = 4’-((3-fluoro-
2-39-1 Kurokami, Chuo-ku, Kumamoto, 860-8555 (Japan)
2
4 2
phenyl)ethynyl)-2,2’:6’,2’’-terpyridine), in which each terpyr-
idine ligand bears a fluorobenzene ring as a polarization
rotation unit (Scheme 1). It was anticipated that the use of
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202015322.
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intermolecular interaction between F1 site and neighboring
ac molecule causes the biased-population (Figure S3). Vari-
able-temperature SXRD (VT-SXRD) measurements were
performed to examine the effect on the degree of occ at the F1
and F2 sites. On increasing the temperature above 123 K, the
occ increased at the F1 site but decreased for the F2 site
accompanied by the disorder of neighboring ac molecule,
consequently reaching 1.6:1 (F1:F2) at 293 K (Table 1). These
Scheme 1. Structures of polar ligand (left) and [Co(FPh-terpy) ](BPh )
4 2
2
(1) (right).
Table 1: Changes in the occupation (occ) factor at the F1 and F2 sites of
fluorobenzene ring obtained from the VT-SXRD analyses.
T [K]
occ F1 site
occ F2 site
F1:F2
bulky BPh anions might produce a large enough space in the
lattice packing for molecular motion of fluorobenzene ring to
occur, resulting in the generation of electrically invertible
4
1
1
2
2
23
70
20
93
0.55
0.55
0.56
0.61
0.45
0.45
0.44
0.39
1.2:1
1.2:1
1.3:1
1.6:1
polarization. Related complexes of type [Co(Ph-terpy) ]-
2
(
BPh4)2 (2; Ph-terpy = 4’-(phenylethynyl)-2,2’:6’,2’’-terpyri-
dine), [Zn(FPh-terpy) ](BPh ) (3) and [Zn(Ph-terpy) ]-
2
4
2
2
(
BPh ) (4) were also prepared to compare their structures
4 2
and properties with those of 1.
The terpyridine ligand derivatives, FPh-terpy and Ph-
terpy, were synthesized by minor modification of previously
reported methods (Scheme S1).
synthesized by reaction of CoCl ·6H O (or ZnCl ), FPh-
results clearly demonstrate that raising the temperature
increases the 1808 flip–flop motion of the fluorobenzene
ring. The average CoꢀN bond lengths (2.034 ꢀ) at 293 K also
[
11]
The complexes were
indicates the presence of thermally-induced SCO behavior
(Table S2). Above temperature-dependent structural varia-
tion that includes F1:F2 occ ratio and CoꢀN bond length is
2
2
2
terpy (or Ph-terpy) and NaBPh in MeOH; in each case the
4
initial product was recrystallized from acetone to yield single
crystals of 1·3ac, 2·3ac, 3·3ac and 4·3ac (where ac = acetone).
Single crystal X-ray structures of each product were deter-
mined at 123 K. Crystallographic data are presented in
Table S1. Compound 1·3ac crystallized in the low crystal
fully reversible in the temperature range 123 to 293 K. The
crystal structures of 2·3ac, 3·3ac and 4·3ac, determined at
123 K, are isostructural with that of 1·3ac (Figure S4, S5 and
S6). For 3·3ac, the fluorine group disorder was observed with
an F1:F2 occ ratio of 1.2:1, similar to that observed for 1·3ac.
The lattice ac molecules were gradually removed at 298 K,
and the structures were maintained up to ca. 350 K as
confirmed by thermogravimetric analysis (TGA) and varia-
ble-temperature powder X-ray diffraction (VT-PXRD) pat-
terns (Figure S7 and S8). Hence, all temperature-dependent
measurements were carried out below 350 K.
symmetry of P2 /c (centrosymmetric space group). As shown
1
in Figure 1, two FPh-terpy ligands coordinate to the cobalt(II)
In order to further investigate the molecular motion of
fluorobenzene ring, we measured the temperature-depen-
1
9
dent, solid-state
F
nuclear magnetic resonance (SS-
F NMR) spectra for 3·3ac (Figure 2a). As presented in
Figure 2b, two signals were present at ca. ꢀ83.1 and
50.0 ppm. These signals are assigned to the F1, F2 and F3
1
9
Figure 1. Crystal structure of 1·3ac showing the F1/F2 disorder.
ꢀ
Counter anions, H atoms and solvent molecules are omitted for clarity.
Color code: Co; magenta, C; grey, N; blue, F; green.
atoms as observed by SXRD analyses, with the ratio of F1 and
F2 being 1.2:1 at 150 K. On heating from 150 K, the signal
intensity increases for F1 while decreasing for F2 and reaches
the ratio of 1.6:1 at 298 K (Figure 2c). Such behavior is in full
accord with the presence of thermally-induced molecular
motion of the fluorobenzene ring in each complex, and is also
consistent with the change in degree of occ occurring at the F1
and F2 sites obtained from the VT-SXRD analyses of 1·3ac
(Figure S9).
[
15]
center meridionally to yield a N donor set. The CoꢀN bond
6
lengths are 1.999(3) ꢀ, 1.858(3) ꢀ, 2.013(3) ꢀ, 2.128(3) ꢀ,
1
.905(3) ꢀ and 2.122(3), in accord with 1·3ac being in its LS
state (Table S2). One fluorobenzene moiety is distorted with
respect to the terpyridine plane due to a CH-p interaction
with a neighboring ligand, with a H25···C43 distance of
2
.825 ꢀ, while the second (bound) FPh-terpy ligand has
The molecular motion of phenyl ring was revealed by
means of SS- H NMR spectroscopy for 4·3ac and SS-
1
a planar structure due to the absence of such an intermolec-
ular interaction (Figure S1). The bulky tetraphenylborate
anions surround the fluorobenzene ring to produce an open
space that is sufficient to allow molecular motion of the
fluorobenzene ring (Figure S2). The fluorine substituent is
disordered over two sites, F1 and F2. The occupation (occ)
ratio between F1 and F2 is 1.2:1 at 123 K, where the
2
H NMR spectroscopy for deuterated 4·3ac on phenyl
1
moiety. Temperature-dependent SS- H NMR spectra for
4·3ac is shown in Figure S10. At 180 K, two signals centered
at ca. 6.0 ppm and ca. 1.0 ppm were evident. When the
temperature varied from 180 to 250 K, new sharp signal
centered at ca. 5.0 ppm starts to be appearance at 200 K and
2
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Figure 2. a) F1, F2 and F3 sites in 3·3ac. b) Temperature-dependent
1
9
SS- F NMR spectra of 3·3ac. c) Fitting results for spectra at 150 K and
98 K, respectively (blue: F1, magenta: F2, green: F3, red: fitting line).
2
the original signal at ca. 1.0 ppm gradually become broader
over 230 K. Although we cannot assign each signal, it is
thought that the observed spectral change between 200 and
2
50 K relates to the thermally-induced molecular motion of
2
phenyl ring in 4·3ac. The SS- H NMR spectra for deuterated
·3ac on phenyl moiety, [Zn(Ph-d -terpy) ](BPh ) , also
4
5
2
4 2
supported the presence of molecular motion of phenyl ring.
The spectra vary progressively with increasing temperature
and are successfully simulated by taking into account of two-
site 1808 flip–flop motion of phenyl ring with the aid of
[
12]
software NMR WEBLAB (Figure 3a).
At 190 K, the
spectrum was reproduced well with the flipping rate (k)
3
being 1.0 ꢁ 10 Hz, indicating slow flip–flop motion of the
phenyl ring. On heating from 190 K, the spectral shape starts
2
Figure 3. a) Temperature-dependent SS- H NMR spectra for deuterated
4
to change at ca. 230 K (k = 2.0 ꢁ 10 Hz) as indicated by
4
·3ac on phenyl ring moiety (left: experimental, right: simulation).
6
arrows and reaches k value of 7.9 ꢁ 10 Hz at 306 K. The
ꢀ1
b) Arrhenius plot of ln(k) vs. 1000T
.
temperature at which the spectral change starts to emerge in
2
SS- H NMR spectra is consistent with the temperature of the
fluorine-group occ change resulted from SXRD and SS-
F NMR spectra, thus suggesting that 4·3ac (which has no
fluorine group) exhibits the similar thermally-activated
of calculation, two potential energy barriers were observed
1
9
centered at f = ꢀ134 and 338, with the DE being 254.3 and
ꢀ
1
457.9 kJmol , respectively (red plot in Figure 4). The respec-
tive DE maxima are attributed to the intermolecular contacts
with neighboring acetone molecule and [Co(Ph-terpy)2]
cation (Figure S12). The value of k T (k = Bolzmann con-
molecular motion to 3·3ac (which has fluorine group). The
ꢀ1
k vs. 1000T plot is shown in Figure 3b. From this linier plot,
ꢀ
1
the activation energy (E ) was estimated to be 36.2 kJmol
a
B
B
ꢀ1
ꢀ1
ꢀ1
by using the Arrhenius equation, t ¼ t0 expðꢀE =k TÞ.
stant) is 2.8 kJmol at 298 K, which is significantly smaller
than the DE maxima observed (for 1·3ac). Undoubtedly, this
indicates that the rotation of the fluorobenzene will be almost
suppressed at 298 K. Hence, 1·3ac is expected to display
a remnant polarization after removing the electric field.
Compound 2·3ac also gave a double potential curve (blue plot
in Figure 4). Since the two symmetric potential energy
a
B
This value is quite similar to that of 1808 flip–flop motion of
[
13]
phenyl ring in molecular rotor based on [18]crown-6.
The rotational potential energy in 1·3ac was calculated by
using the B3LYP-D3BJ/(F:6–31 + G* others:6–31G*) level of
theory and atomic coordinates at 123 K. Computational
details used for the calculation are shown in Table S3. The
single point energy for the forward-backward rotation of
fluorobenzene ring was plotted against the dihedral angle C9-
C8-C18-C19 (f) with taking into account of intermolecular
contacts with surrounding molecules (Figure S11). The f
value of the initial structure obtained from SXRD analysis is
ꢀ1
barriers (157.0 and 155.2 kJmol ) observed in 2·3ac are
lower than occur for 1·3ac, the intermolecular contacts
involving the fluorine group clearly lead to increased
potential energy barriers. The calculated potential energy
barrier for 2·3ac is 4.3 times as large as the E value obtained
a
ꢀ
1
2
1
42.38, where the DE value is defined as 0 kJmol . As a result
from SS- H NMR spectroscopy. It should be emphasized that
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nating from the molecular rotation were analyzed for the
pellet sample using polarization vs. electric field (P-E) curves
with the aid of a TF analyzer1000 (T= 298 K, f = 100 Hz).
Compound 1·3ac displayed a distinct ferroelectric hysteresis
loop, with the remnant polarization being 0.24 mC—a typical
value for ferroelectric compounds (red line in Figure 5b). The
distinct frequency dependence was found in ferroelectric
hysteresis loop, which is typical nature of ferroelectric
compound (Figure S13). In the temperature-dependent P–E
curves, no ferroelectric hysteresis loop was evident below
2
00 K because of the small-degree of biased-population (1.2:1
F1:F2) or frozen molecular motion at low temperature
Figure S14). This result is undoubtedly consistent with
ꢁ
(
temperature dependence of dielectric constants. In general,
ferroelectric behavior stems from non-centrosymmetric
group. In the present case, such behavior, however, occurred
Figure 4. Calculated potential energy curves and dihedral angle C9-C8-
C18-C19 (f) for the fluorobenzene (phenyl) ring in a) 1·3ac and
b) 2·3ac. The single point energies for the model structures (Fig-
ure S11) were calculated for every 308 rotation.
in the centrosymmetric compound (P2 /c). This situation is
1
quite similar to ferroelectric molecular rotor reported by
[
9]
Akutagawa et al. Thus, it is likely that the biased occ factor
(1.6:1) of fluorine group in the initial structure transforms to
the more biased population after applying an electric field
(Figure S15). As expected, no ferroelectric hysteresis loop
resulting in ferroelectricity was observed for 2·3ac (blue line
in Figure 5b). Collectively, these results clearly indicate that
the invertible polarization necessary for ferroelectricity is
generated via the rotational motion of the fluorobenzene ring
in 1·3ac.
the calculated potential energy barriers are overestimated to
the experimental data because atomic coordinates are fixed to
the SXRD structure measured at 123 K except for the
dihedral angle, and no structural relaxation due to the
rotation is considered.
The 1808 flip–flop motion of the fluorobenzene ring has
the potential to induce ferroelectric properties. The temper-
ature dependences of the dielectric constants (e ) for 1·3ac
r
and 2·3ac were measured on a pellet sample in the frequency
range 0.1 kHz to 1 MHz in order to investigate any phase
transition/electric field effects. With increasing temperature,
the dielectric constant for 1·3ac is nearly constant between
SCO behaviors of 1·3ac and 2·3ac were investigated
employing a Superconducting Quantum Interference Device
ꢀ
1
(SQUID) in the temperature range 5 to 350 K at 5 Kmin
(Figure 6). In general, an octahedral cobalt(II) complex
6
1
2
00 and 240 K, but then gradually increases in accord with the
shows spin conversion between S = 1/2 (t e ) and S = 3/2
2g g
5
2
fluorine-group occ change taking place (above plot in Fig-
ure 5a). The dielectric constant for 1 at higher frequency is
significantly smaller than that observed at 0.1 kHz, undoubt-
edly indicating that the rotation of the fluorobenzene ring is
unable to fully respond to the electric field owing to the
latterꢂs higher frequency. For 2·3ac (which has no fluorine
group), no elevation of the dielectric constant was evident
between 200 and 350 K due to the absence of dipole moment
(t2g e ) with the c T for the LS state being around
g m
3
ꢀ1
0.5 cm Kmol and for the HS state being in the range 1.7–
3
ꢀ1
3.0 cm Kmol (reflecting the contribution of orbit angular
momentum). Compounds 1·3ac and 2·3ac each undergo
(
plot below in Figure 5a). The ferroelectric behaviors origi-
Figure 5. a) Temperature- and frequency-dependent dielectric con-
stants (e ) of 1·3ac and 2·3ac using pellet samples (red: 0.1 kHz, blue:
r
1
kHz, green: 10 kHz, yellow: 100 kHz, black: 1000 kHz). b) P–E
Figure 6. Temperature-dependent magnetic susceptibility data for
1·3ac collected in the range 5–350 K under an applied field of 0.5 T.
hysteresis curves for 1·3ac (red) and 2·3ac (blue) at 298 K. The
frequency employed for generating the data for the P–E curves was
fixed at 100 Hz and was obtained using pellet sample.
Inset graph displays c T and F1/F2 vs. T plots (red: SXRD for 1·3ac,
m
1
9
blue: SS- F NMR for 3·3ac).
4
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incomplete SCO behavior between 5 and 350 K. For 1·3ac,
Keywords: cobalt(II) complexes · ferroelectrics ·
molecular rotors · spin crossover
3
ꢀ1
the c T value of 0.41 cm Kmol at 5 K, clearly indicates the
m
LS state for cobalt(II). Upon heating from 5 K, the c T value
m
remains essentially constant between 5 and ꢂ 200 K. Sub-
3
ꢀ1
sequently, the c T value gradually rises to 0.71 cm Kmol at
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ꢀ1
3
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Similar SCO behavior was observed for 2·3ac (Figure S16).
3
ꢀ1
The c T value at 5 K is 0.44 cm Kmol and starts to increase
m
3
ꢀ1
at ꢂ 200 K to reach 0.89 cm Kmol at 350 K. The temper-
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m
2
·3ac are in accordance with the thermally-induced 1808 flip–
flop motion obtained from SS-NMR spectra for 3·3ac, 4·3ac
and deuterated 4·3ac (Figure 6 inset). Thus, in each case the
behavior is in keeping with structural changes associated with
the molecular motion of the respective aromatic rings. In
general, SCO is an entropy-driven phenomenon, in which the
LS state is the ground state at low temperature whereas the
HS state becomes the ground state at high temperature. The
difference in energy is thermally compensated by the higher
entropy of the HS state, which mainly stems from electronic
and vibrational contributions. In the present case, the change
in rotation velocity of the fluorobenzene ring may contribute
considerably to this entropy variation, consequently influenc-
ing the SCO process. Thus, LS state is favorable at low
temperature due to the restricted mobility of fluorobenzene
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(
phenyl) ring while showing HS state at high temperature
owing to the acquirement of entropy occurring from the
mobility of the fluorobenzene (phenyl) ring. A similar
example of SCO behavior being triggered by the degree of
rotation velocity has also been reported by Rodrꢃguez-
[
14]
Velamazꢄn et al.
[
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SXRD analyses and SS- F NMR spectra confirmed the
presence (as well as the effect) of the 1808 flip–flop motion
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provides new insight for the design and synthesis of further
ME materials in the future.
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This work was supported by KAKENHI Grant-in-Aid for
Scientific Research (A) JP17H01200. R.A. is grateful to JSPS
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Conflict of interest
The authors declare no conflict of interest.
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Manuscript received: November 18, 2020
Revised manuscript received: March 12, 2021
Accepted manuscript online: March 12, 2021
Version of record online: && &&, &&&&
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Communications
Ferroelectrics
Inspired by molecular ferroelectrics
induced by molecular rotors, multifunc-
tional compound exhibiting both ferroe-
lectricity and spin crossover behavior was
constructed. The flip–flop motion of
polarization unit results in the expression
of ferroelectricity and spin crossover
behavior.
R. Akiyoshi, Y. Komatsumaru,
M. Donoshita, S. Dekura, Y. Yoshida,
H. Kitagawa, Y. Kitagawa, L. F. Lindoy,
S. Hayami*
&&&& — &&&&
Ferroelectric and Spin Crossover Behavior
in a Cobalt(II) Compound Induced by
Polar-Ligand-Substituent Motion
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
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