CO2-Induced Spin-State Switching at Room Temperature in a Monomeric Cobalt(II) Complex with the Porous Nature

Article abstract of DOI:10.1002/anie.202003811

CO₂-responsive spin-state conversion between high-spin (HS) and low-spin (LS) states at room temperature was achieved in a monomeric cobalt(II) complex. A neutral cobalt(II) complex, [Co^II(COO-terpy)₂]?4 H₂O (1?4 H₂O), stably formed cavities generated via π–π stacking motifs and hydrogen bond networks, resulting in the accommodation of four water molecules. Crystalline 1?4 H₂O transformed to solvent-free 1 without loss of porosity by heating to 420 K. Compound 1 exhibited a selective CO₂ adsorption via a gate-open type of the structural modification. Furthermore, the HS/LS transition temperature (T_1/2) was able to be tuned by the CO₂ pressure over a wide temperature range. Unlike 1 exhibits the HS state at 290 K, the CO₂-accomodated form 1?CO₂ (P (Formula presented.) =110 kPa) was stabilized in the LS state at 290 K, probably caused by a chemical pressure effect by CO₂ accommodation, which provides reversible spin-state conversion by introducing/evacuating CO₂ gas into/from 1.

Full text of DOI:10.1002/anie.202003811

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: CO2-induced spin state switching at room temperature in a
monomeric cobalt(II) complex with the porous nature
Authors: Manabu Nakaya, Wataru Kosaka, Hitoshi Miyasaka, Yuki
Komatsumaru, Shogo Kawaguchi, Kunihisa Sugimoto, Yingjie
Zhang, Masaaki Nakamura, Leonard F. Lindoy, and Shinya
Hayami
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003811
Angew. Chem. 10.1002/ange.202003811
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10.1002/anie.202003811
Angewandte Chemie International Edition
RESEARCH ARTICLE
CO2-induced spin state switching at room temperature in a
monomeric cobalt(II) complex with the porous nature
Manabu Nakaya,[a] Wataru Kosaka,[b] Hitoshi Miyasaka,*[b] Yuki Komatsumaru,[a] Shogo Kawaguchi,[c]
Kunihisa Sugimoto,[c] Yingjie Zhang,[d] Masaaki Nakamura,[a] Leonard F. Lindoy[e] and Shinya
Hayami*[a],[f]
Dedication ((optional))
Abstract: The first example of CO2-responsive spin state conversion
Introduction
between high-spin (HS) and low-spin (LS) states at room temperature
was achieved in a monomeric cobalt(II) complex. A neutral cobalt(II)
Metal complexes that are capable of accommodating guest
molecules, such as particular porous metal-organic frameworks
(MOFs) and coordination polymers (PCPs), have attracted much
complex, [CoII(COO-terpy)2]·4H2O (1·4H2O), stably formed cavities
generated via π–π stacking motifs and hydrogen bond networks,
resulted in the accommodation of four water molecules. A crystalline
attention for the development of functional materials for use in a
1·4H2O transformed to the solvent-free 1 without loss of the porosity
range of applications.[1] Many materials of this type also exhibit
by heating up to 420 K. Compound 1 exhibited a selective carbon
guest-induced changes of their intrinsic properties[2,3] including
dioxide (CO2) adsorption via a gate-open type of the structural
amongst others in their emission,[4] magnetic,[5-7] conductance
modification. Furthermore, the HS/LS transition temperature (T1/2
)
properties,[8] and so on.[9] Also, many of these display selective
guest uptake giving considerable potential for the development of
new chemo-sensing materials and devices.[10]
was able to be tuned by the CO2-pressure over a wide temperature
range. Unlike 1 exhibits the HS state at 290 K, the CO2-accomodated
form 1 CO2 (PCO2 = 110 kPa) was stabilized in the LS state at 290 K
⊃
The spin-crossover (SCO) phenomenon represents a stimulus-
driven spin state change between the low-spin (LS) and high-spin
(HS) states of a metal center and is quite sensitive to structural
changes that can also act as ‘triggers’ for inducing the SCO
behavior.[11-13] The introduction of guests into a porous metal
complex can act as such a trigger. Motivated by this, such porous
materials have been seen as potential guest-detectable SCO
materials.[14-21] The uptake/release of organic solvents has been
used in many instances to induce guest-dependent spin
conversions and, for example, a wide range of organic solvents
has been reported to influence on SCO behaviors in PCPs.[22-33]
Such solvent guest uptake/release has also been well
demonstrated to influence on SCO behaviors in discrete
molecular complexes via structural changes in the associated
crystal lattices.[34-42] Similarly, non-solvent guest molecules such
as organic molecules,[43-49] halogens,[50,51] and common gas
molecules such as nitrogen (N2), oxygen (O2), carbon monoxide
(CO), carbon dioxide (CO2), and sulfur dioxide (SO2), have all
been shown to affect SCO behavior in a range of metal complex
types.[52-57] However, the development of SCO materials that
show a strong response to the ubiquitous gases, N2, O2, and CO2,
remains challenging because these gas molecules will normally
undergo relatively weak physisorption to porous hosts and thus
will tend to be less effective in triggering SCO behavior relative to,
for example, solvent molecules and other larger neutral or
charged guests.
probably caused by “chemical pressure” effect by CO2
accommodation, which provides reversible spin-state conversion by
introducing/evacuating CO2 gas into/from 1. This result represents an
important advance providing new insights for the development of new
gas-responsive functional magnetic materials in the future.
[a]
Dr. M. Nakaya, Y. Komatsumaru, Dr. M. Nakamura, Prof. Dr. S.
Hayami
Department of Chemistry
Graduate School of Science and Technology
Kumamoto University, 2-39-1, Kurokami, Chuo-ku, Kumamoto 860-
8555 (Japan)
E-mail: hayami@kumamoto-u.ac.jp
Dr. W. Kosaka, Prof. Dr. H. Miyasaka
Institute for Materials Research
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577
(Japan)
E-mail: miyasaka@imr.tohoku.ac.jp
Dr. S. Kawaguchi, Dr. K. Sugimoto
Diffraction & Scattering Division
[b]
[c]
Japan Synchrotron Radiation Research Institute（JASRI)
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198 （Japan）
Dr. Y. Zhang
Australian Nuclear Science and Technology Organization
Locked Bag 2001, Kirrawee DC, NSW 2232 (Australia)
Dr. Y. Zhang
School of Chemistry, The University of Sydney
NSW 2006 (Australia)
Prof. Dr. S. Hayami
[d]
[e]
[f]
One approach towards overcoming the above difficulty is to
employ a CoII SCO system which typically requires a lower
entropy change, ΔSspin (based on the pure spin-multiplicity
change), (ΔSspin for CoII = 5.8 kJ/mol) for achieving SCO rather
than, for example, employing an FeII system where the entropy
Institute of Pulsed Power Science (IPPS)
Kumamoto University, 2-39-1 Kurokami, Chuo-ku
Kumamoto 860-8555 (Japan)
Supporting information for this article is given via a link at the end of
the document
change is typically higher (ΔSspin for FeII = 13.4 kJ/mol).[58]
further strategy is to employ flexible structures; for example, low-
A
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10.1002/anie.202003811
Angewandte Chemie International Edition
RESEARCH ARTICLE
dimensional coordination polymers or assembled frameworks
consisting of discrete units that display structural flexibility. Such
systems also need to exhibit gate-open type gas adsorption, as
recently reported for other gas-responsive magnetic materials.[6]
Here, we report a unique example of LS/HS-conversion
behavior that is highly responsive to CO2 adsorption and is
associated with a wide transition temperature (T1/2) range of from
180 K (PCO2 = 0 kPa) to 300 K (PCO2 = 110 kPa). The study
employed the discrete CoII complex, [Co(COO-terpy)2] (1; COOH-
terpy = 4’-calboxyl-2,2’:6’,2”-terpyridine), that assembles to form
a porous pseudo three-dimensional framework stabilized by
intermolecular π–π stacking and CH···π interacting motifs. The
framework-like assembly in 1 was found to be suitable for
selective CO2 sorption and gave rise to CO2-responsive spin-state
switching, with the HS-state occurring for the guest-free form (1)
while the LS-state occurred for the CO2-included form (1 CO2),
⊃
with the T1/2 temperature able to be tuned by varying the degree
of CO2 uptake – that is, by varying the CO2 pressure. The spin-
state switching behavior was found to occur in the room
temperature region for a pressure (PCO2) of 110 kPa.
Clearly, the above spin conversion behavior correlates with the
denser (lower porosity) packing in the hydrated pseudo-
polymorph species, [Co(COO-terpy)2]·4H2O (1·4H2O), with its
similar framework to 1, which also stabilizes the LS state, whereas
the looser packing (higher porosity) in the dehydrated form favors
the HS state. To the best of our knowledge, the present study is
the first to demonstrate that CO2-uptake can act as a crucial
trigger for inducing spin-state crossover in a metal complex at
room temperature.
Fig. 1. (a) Single crystal structure of 1·4H2O at 120 K. Hydrogen atoms and
water molecules are omitted for clarity. Color code: C, grey; N, blue; O, red; Co,
Magenta. (b) Comparison of the variation between the [CoN6] coordination
bonds for 1·4H2O at 120 K (blue) and 1 at 420 K (pink).
The [Co(COO-terpy)2] molecules pack to form a quasi-2D
assembled layer in the ab plane via π–π stacking using the
terminal pyridine (py) rings of the terpy ligands, with a py···py
midpoint-to-midpoint distance of 3.637 Å [orange dashed line in
Fig. 2(a)] and CH–π contacts involving the terminal pyridine rings
with CH···py distances of 2.728 – 3.078 Å [blue dashed line in Fig.
2(a), Table S3]. The quasi-2D assembled layers were,
furthermore, stacked in the c-axis direction with C–H···O
hydrogen bonds between pyridine rings and carboxy groups with
intermolecular hydrogen bond distances of 2.530 Å and 2.652 Å
(green dashed line in Fig. 2b, Table S3). These layer-stacked
assemblies result in 1-D voids that are ordered orthogonally along
the a and b axes [indicated as red and green arrows, respectively,
in Fig. 2(b)], with the lattice water molecules being located in the
resulting cavities. The water molecules aligned in the a-axis-
direction cavity form a hydrogen bonded chain-like network
(O···O = 2.748 Å and 2.760 Å; blue dashed line in Fig. 2a) that is
supported by branched hydrogen bonds with carboxy groups from
the COO-terpy– ligands (Ow···OCO2 = 2.767 Å; red dashed line in
Fig. 2b), while no close contacts occur along the b axis direction.
As seen in the thermogravimetric analysis (TGA) plot (Fig. S1),
on heating of 1·4H2O from room temperature to ~420 K, all four
water molecules were lost to produce 1. SC-XRD measurements
for 1·4H2O were carried out at several temperatures between 120
K and 420 K. The structures determined between 120 K to 300 K
were shown to be the fully hydrated form (1·4H2O) while those
determined at T ≥ 375 K showed that dehydration had occurred
Results and Discussion
The solvated complex 1·4H2O was prepared as dark orange
prisms by reaction Co(CH3COO)2·6H2O with COOH-terpy[59] in
the presence of base (NaOH) using a hydrothermal procedure
that involved heating the reaction mixture at 160 °C for 4 days
(see Experimental Section in supporting information (SI)). The
presence of four water molecules per formula unit as lattice
solvents was confirmed by elemental analysis and
thermogravimetric analysis (TGA) (Fig. S1). Compound 1·4H2O
was also characterized by single-crystal X-ray diffraction (SC-
XRD) measurements at several temperatures (Table S1). The
initial SC-XRD measurement was performed at 120 K, at which
temperature 1·4H2O was present in the tetragonal space group I–
42d (#122) (Table S1). Two (deprotonated) COO-terpy– ligands
were shown to be meridionally coordinated to a CoII center via
each of the terpy tridentate coordinating sites to form a slightly
distorted octahedral N6-donor coordination geometry, with the
COO– group not coordinated (Fig. 1a). The geometry is in accord
with the presence of a (compressed) tetragonal Jahn–Teller
distortion (Fig. 1b and Table S2) as expected for a low-spin CoII
ion; the Co–N1 (A and B) bonds correspond to the short axis,
directed to the middle-pyridine groups of the COO-terpy– ligands,
and the Co–N2 (N2A~D) bonds define a quasi-equatorial plane.
Notably, 1·4H2O is a neutral species without outer counter anions
(Fig. 1a).
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10.1002/anie.202003811
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layer assembly in the ab plane is effectively maintained in both
structures. However, reflecting the absence of water molecules in
1, each carboxylate group is rotated by ca. 20°, giving rise to an
additional anion–π interaction[60] with the pyridine ring of a
neighboring [Co(COO-terpy)2] molecule (with interaction
separations of 3.302 Å and 3.335 Å) (Fig. 3c, Table S3). Unlike
1·4H2O which has hydrogen bonds between the lattice water
molecules and carboxyl groups and which partially stabilize the
assemblies along the c axis, the anion–π interactions in 1 assist
the stabilization of the layered assemblies along the c axis.
Further, the dihedral angle between two ‘outside’ pyridine rings
(py-1 and py-2 in Fig. 3c) changed from 8.39° (in 1·4H2O at 120
K) to 9.9° (in 1 at 420 K). Associated with this, the distortion
parameter Σ for the [CoN6] octahedral core changed from 89.16°
(at 120 K) to 120.52° (at 420 K) (Table S2), with these values
corresponding to the LS and HS states of [Co(COO-terpy)2]
respectively (vide infra).[61] The solvent accessible void volume
(viz. water occupied space) in 1·4H2O was calculated to be 515
Å3 (17.2 %) and 515 Å3 (16.9 %) at 120 K and 300 K, respectively,
whereas that for 1 at 420 K was 490 Å3 (16.6 %) (Table S4). Thus,
the void volume was not changed upon cooling.
to give 1 (Fig. 3, Table S1 and S2); that is, dehydration was
confirmed for temperatures between 300 K and 375 K. The
temperature variation of the average of the Co–N lengths is
shown in Fig. 4. On increasing the temperature from 120 K to 420
K, the Co–N distances all increase but remain in the typical range
for octahedral CoII complexes. However, the observed change
between 300 K to 375 K was much more abrupt than occurs
outside this temperature range [with a variation of Δ(Co–Nave) =
0.07 Å], reflecting the structural change on transforming 1·4H2O
to 1.
The temperature (T)-variable magnetic susceptibility (χm)
measurement for 1·4H2O was conducted using
a
Superconducting Quantum Interference Device (SQUID) in a
sweep mode with a scan rate of 2 K min−1 (Fig. 5). A freshly
prepared crystal sample of 1·4H2O was measured from 300 K to
10 K; the χmT value of 1.08 cm3 K mol−1 at 300 K was observed
to gradually decrease to 0.43 cm3 K mol−1 at 10 K, which is
consistent with the formation of the LS state of octahedral CoII ion.
On reaching 10 K, heating was then employed to bring the
temperature to 400 K, with χmT completely following the first
cooling process plot until 300 K then gradually increased with an
inflection point at ca.370 K, finally reaching χmT = 1.98 cm3
K
Fig. 2. Crystal packing diagram for 1·4H2O at 120 K (a) in the ab plane and (b)
along the c axis. Dashed lines in (a) indicate the intermolecular π–π interaction
(orange), CH–π stacking (aqua blue), hydrogen bond between water molecules
(blue) and hydrogen bond between water and carboxy group (red), respectively.
Green dashed line in (b) indicates CH···O interaction between pyridine ring and
carboxy group. Connolly surfaces are also depicted in (b), which show the
orthogonal 1-D cavities for the guest water molecules assembling between the
2-D layers of [Co(COO-terpy)2] molecules. Hydrogen atoms are omitted for
clarity. Color code: C, grey; N, blue; O, red; Co, Magenta.
mol−1 at 400 K. The significantly larger χmT value at 400 K than at
300 K is in accord with a spin conversion from the LS state to the
HS state having occurred, even though the value is still less than
the pure CoII HS state as for 1 (vide infra). The inflection point in
the χmT plot is at the same temperature as that observed in the
TGA for the abrupt weight decrease due to the loss of the water
molecules (Fig. S1), confirming that the LS→HS transition
corresponds to the 1·4H2O→1 conversion. In fact, the χmT value
at 400 K was found to gradually increase to 2.48 cm3 K mol–1
when the sample was purged at 400 K (over 2 h) in situ in the
SQUID system in order to completely remove any remaining
lattice water. Next, a second cooling process (from 400 K to 10 K)
under a vacuum condition was carried out. Interestingly, the χmT
versus T plots did not follow the same pathways for those
obtained for the first cooling and the heating processes. While χmT
gradually decreased, a higher value was maintained until ca. 100
K (0.71 cm3 K mol−1) but this was followed by reaching the same
value of 0.47 cm3 K mol−1 at 10 K to that obtained in the first
cooling process for 1·4H2O at this temperature. This χmT variation
as a function of temperature is due to a gradual spin crossover
(SCO) of the CoII center in 1. The observed behavior clearly
demonstrates that: i) the existence of lattice water molecules in
1·4H2O stabilizes the LS state even in the higher temperature
The packing structure observed for 1 (at 420 K) is essentially
the same as that found for 1·4H2O (Fig. 3a). Furthermore, the
structure of the coordination sphere in 1, showing slightly
elongated Co–N distances at 420 K, was largely maintained when
this compound was cooled to 200 K in vacuo. Thus, the bond
elongation in observed at 200 K, following heating to 420 K,
shows that it is clearly different from the initial form of 1·4H2O
obtained at 200 K. A pseudo-polymorph change from 1·4H2O to
1 is revealed, although the packing structures of both forms are
similar (Fig. 3a, Table S1 and S2.)
The local packing motif for 1·4H2O measured at 120 K followed
by the motif for 1 at 420 K is displayed in Fig. 3(b). The packing
motifs are essentially the same with each showing similar π–π
and CH–π interaction patterns (Table S3). Thus, the quasi-2-D
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10.1002/anie.202003811
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RESEARCH ARTICLE
Fig. 3. (a) Changes in the crystal packing structure of 1 with changes in temperature. (b) Projection of the local packing motif along the b axis before/after dehydration
showing intermolecular interactions. (c) Rotation of the terminal carboxy group over the temperature range 120 K to 420 K, and the anion-π interaction that results
in the distortion of the [CoN6] core following dehydration.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
region above 100 K, ii) the LS to HS state conversion gradually
occurs on the removal of the water molecules at temperatures
higher than room temperature, and iii) 1 undergoes a LS/HS spin
conversion, even though the structures of 1·4H2O and 1 are very
similar with the exception that lattice waters are absence in the
latter case. Importantly, the water molecules in 1·4H2O inhibit the
anion-π interaction mentioned earlier, leading to an inhibition of
the [CoN6] distortion and resulting in stabilization of the LS state,
even at higher temperatures. On the other hand, removal of the
water molecules generated a larger distortion in the [CoN6] core,
leading to the HS state.
1st cooling
1st heating
2nd cooling (dry)
ꢀ
Incubated
over 2 h
ꢃ
ꢁ
ꢂ
0
100
200
T / K
300
400
2.14
2.12
2.10
2.08
2.06
2.04
2.02
2.00
Fig. 5. The χmT vs T plots for 1·4H2O for the first cooling process (blue) and first
heating process (red); the green plots are for the second cooling process for 1
which was annealed at 400 K over 2 h in the SQUID system immediately after
the first heating process.
The CO2 gas adsorption and desorption isotherms for 1
measured at several temperatures are shown in Fig. 6(a). Typical
gate-opening (GO) adsorption behavior was observed for CO2,
while N2 and O2 were not adsorbed (Fig. S2). The adsorption
isotherm for CO2 at 220 K shows a gradual increase at lower
pressures, and then a steep rise began at PCO2 = 0.3 kPa,
indicating GO, to reach an adsorption amount of 72 mL (stp) g−1
(2.0 mol per formula unit) at 97 kPa. At 195 K, smaller CO2
adsorption amount of 46 mL (stp) g−1 (~1.2 mol per formula unit)
at 96 kPa is observed, indicating non-completion of the adsorption
100
200
300
400
T / K
Fig. 4. Variation in the average Co–N bond length. Red and blue plots indicate
heating and cooling processes, respectively.
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10.1002/anie.202003811
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equilibrium due to the rigid structural feature and the consequent
slow adsorption kinetics. On increasing the temperature from 220
K, the CO2 adsorption amount at the highest pressure decreased
with increasing the gate-opening pressure, and finally, the gate-
opening adsorption behavior was not observed below a CO2
pressure of 100 kPa at high temperatures above 300 K (Fig. S3).
The temperature dependence of the gate-opening pressure
obeys well the Clausius–Clapeyron relationship (Fig. 6b),62
d(ln(P))/d(T−1) = ΔHTrans/Rg, where ΔHTrans is the variation in the
transition enthalpy and Rg is the gas constant. Least-square fitting
gave ΔHTrans = −39.8 kJ mol−1 (lnP = –4784.5/T + 20.76). This
relationship suggested that the gate-opening pressure at 298 K
could be 110 kPa. Notably, the desorption isotherms, except at
195 K, followed the profile of the adsorption isotherms, i.e., there
is no pressure hysteresis present in the gate-opening/gate-
closing behavior.
sealed cell that enables accurate control of the CO2 pressure
(CO2 was loaded at 400 K). The χmT versus T plot for the cooling
process with a temperature sweep rate of 0.5 K min−1 taken under
several CO2 gas pressures in the range of 0–110 kPa (those for
the heating processes are given in Fig. S4) is shown in Fig. 7(a),
where as PCO2 = 0 kPa, a helium (He) gas with PHe = 110 kPa
(atmosphere pressure) was filled in the sealed cell to enhance the
thermal conductivity rather than using a vacuum. It should be
noted that the He gas brings no influence in the magnetic
properties of 1 as realized from a comparison of data between the
blue plot in Fig. 7(a) and the green plot in Fig. 5. Upon introducing
CO2 gas after evacuating the He gas, the magnetic properties of
1 were found to change. At lower pressure (~5 kPa) an abrupt,
but stepwise change in the χmT versus T plot occurred, while on
increasing the pressure to PCO2 = 110 kPa, a one-step abrupt
change with a shift in the transition temperature (Tm) to a higher
value was observed, indicating that the LS state is stabilized upon
CO2 adsorption. The Tm, defined by the peak of the dχmT /dT−dT
plot [shown at the bottom in Fig. 7(a) and Fig. S4], differs for
cooling (Tm↓) and heating (Tm↑) under the same CO2 pressure
(Table S5), indicating the presence of thermal hysteresis (ΔTm)
with a temperature spread of 9−21 K [Fig. (7b)]. The average
temperatures [Tm/2 = (Tm↓ + Tm↑)/2] with respect to the applied CO2
pressures were also plotted [red circles in the lnP–T plot
presented in Fig. 6(b)]. The observed Tm/2 variation correlates with
the result obtained from the adsorption study, which indicates that
(a)
80
220 K
2.0
250 K
60
280 K
1.5
195 K
300 K
350 K
40
20
0
1.0
0.5
0.0
the abrupt SCO behavior observed for
associated with the gate-opening CO2 adsorption process.
Nevertheless, the material is clearly a mixture of 1 and 1 CO2 at
1 CO2 is mainly
⊃
⊃
lower CO2 pressures as is apparent from the nature of the χmT
plots (stepwise increase) at PCO2 = 1.1, 3.5, and 11 kPa.
0
20
40
60
80
100
Pressure / kPa
Temperature / K
250
(b)
300
200
100
4
2
lnP = -4784.5/T +20.76
10
1
0
from adsorption
from SCO behaviors
-2
0.1
3.0
3.5
4.0
4.5
5.0
1000T-1 / K-1
Fig. 6. (a) CO2 gas adsorption (closed) and desorption (open) isotherms for 1
measured at several temperatures. (b) Log-scale plots of the CO2 pressure vs
the inverse of the gate-opening temperature for 1 (blue circles). Averaged
transition temperatures for CO2 obtained from the respective SCO behaviors on
cooling and heating processes under variable CO2 pressure (red circles).
The magnetic variation upon CO2 adsorption was monitored by
in situ magnetic measurements using a custom built, atmosphere-
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10.1002/anie.202003811
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RESEARCH ARTICLE
words, CO2 accommodation into 1 under these CO2 pressure
partially stabilize the LS state of 1. The mixture ratio between
these two components can be expressed as an exponential
function (Fig. 8) as seen in the GO/GC plot given in Fig. 6(b).
(a)
3.0
110 kPa
35 kPa
11 kPa
3.5 kPa
1.1 kPa
0 kPa
2.5
2.0
1.5
1.0
0.5
0.0
100
80
60
40
20
0
0
20
40
60
80
100
120
CO2 pressure / kPa
0
100
200
T / K
300
400
Fig. 8. The ratio of 1 CO2 under respective CO2 pressure. Black dashed line
⊃
indicates the exponential fit.
(b)
This result means that the HS/LS conversion would be almost
completed by only the CO2 treatments in the temperature range
of 275 K (PCO2 = 35 kPa) − 297 K (PCO2 = 110 kPa) (Fig. 7b).
The spin state of 1 can be reversibly switched between the LS
and HS states by introducing/evacuating CO2 gas into/from 1 (Fig.
9); at 290 K, the conversion from HS to LS is complete after ca.15
min under the CO2 atmosphere (115 kPa), and the conversion
from LS to HS is complete within 3 min under a vacuum. Similar
switching behaviors were also observed at 280 K and 300 K, but
not at 250 K, for which slow LS–HS conversion took ca.100 min
for one cycle, due to a slow CO2 diffusion into/from 1 (Fig. S5).
110 kPa
35 kPa
11 kPa
3.5 kPa
1.1 kPa
0 kPa
2.5
2.0
1.5
1.0
0.5
120
100
80
60
40
20
0
0
100
200
300
400
T / K
Fig. 7. (a) Magnetic behavior of 1 CO2 at various pressures (0–110 kPa) of
⊃
CO2 for the cooling process, where as PCO2 = 0 kPa, a He gas with PHe ≈ 110
kPa (atmosphere pressure) was filled in the sealed cell. The dχmT/dT–dT plots
⊃
are displayed at the bottom. (b) Thermal hysteresis behaviors of 1 CO2 at
various pressures (0–110 kPa) of CO2. Red and blue plots indicate the heating
and cooling processes, respectively.
0
20
40
60
80
We evaluated the ratio of 1 CO2 in the heating process
⊃
Time / min
because its stepwise feature was conspicuously observed with
lower CO2 pressure (see Fig. S4). At higher CO2 pressure (above
35 kPa), no stepwise behavior was observed, and the both
behaviors are almost overlapped before the CO2 release. Thus,
under higher CO2 pressures (above 35 kPa), LS state of 1 is
completely induced, indicating the pseudo-polymorph change
Fig. 9. LS/HS conversion triggered by introducing/evacuating CO2 gas into/from
1. Time-course of the χmT (red circles) while the CO2 pressure (blue line)
alternated between <0.1 and 115 kPa every 15 min under a 1000 Oe magnetic
field at 290 K.
In situ synchrotron powder X-ray diffraction (PXRD)
measurements (Fig. S6 and S7) under several CO2 pressures
⊃
were undertaken to investigate the LS structure of 1 CO2. The
from 1 to 1 CO2. From the evaluation for PCO2 = 1.1, 3.5 and 11
⊃
kPa, ~49%, ~72% and ~92% of LS state, respectively, were
induced by CO2 accommodation (Fig. 8 and Table S6); in other
This article is protected by copyright. All rights reserved.

10.1002/anie.202003811
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RESEARCH ARTICLE
cell volumes for 1 at 300 K was optimized to be 2947.5 Å3, using
the Le Bail refinement. This is consistent with the value obtained
from the SC-XRD studies at the same temperatures (2933.6 Å3).
Meanwhile, the change of PXRD patterns was clearly observed
even at a very low CO2 pressure of PCO2 = 1 kPa, and was
completed at PCO2 = 20 kPa (Fig. S6). Le Bail analyses provided
reasonable lattice constants with a monoclinic system in the CO2
accommodated phase, which has an approximate relationship
This work was supported by KAKENHI Grant-in-Aid for Scientific
Research (Grant No. JP17H01200 (S.H.), 16H02269 (H.M.),
18K19050 (H.M.), 15H05208 (H.M.), 18K05055 (W.K.)) from the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan, and a Grand-in-Aid for Scientific Research on Innovative
Areas (‘π-System Figuration’ Area 2601, No. 17H05137 (H.M.))
from the Japan Society for the Promotion of Science, and the E-
IMR project. This work was also supported by the Cooperative
Research Program of ‘Network Joint Research Centre for
Materials and Devices’. The single crystal data were collected on
the MX2 beamline at the Australian Synchrotron, a part of ANSTO.
This work was performed under the Inter-University Cooperative
Research Program of the Institute for Materials Research, Tohoku
University (Proposal No. 19K0091).
with those of 1 under vacuum as aCO2 ~ bCO2 ~ Ö2avac and cCO2
cvac/2. This fact indicates that the symmetry reduction from the
tetragonal system in 1 to a monoclinic system in 1 CO2 was
~
⊃
taken place upon CO2 adsorption (Fig. S7b). Unfortunately, the
construction of structural model for the monoclinic system of
⊃
1 CO2 was not successful due to severe preferred orientation
effects. The optimized cell volumes at 200 K under PCO2 = 40 kPa
increased to 2967.6 Å3 compared to the value obtained from the
SC-XRD at 200 K (2920.1 Å3), in strong agreement with CO2
being accommodated in the pores. The change in cell volume (ΔV
= 47.5 Å3) is much smaller than the conventional volume expected
for CO2 gas occupation calculated from solid-state CO2 (d = 1.56
g cm–3), providing that the estimated volume required for CO2
accommodation is 374.6 Å3, which can be rationalized by the fact
that 1 has void volumes of 493.3 Å3, 490.1 Å3, and 492.9 Å3 at
200 K, 375 K, and 420 K, respectively. The observed behavior is
similar to that for the presence of H2O; that is, the LS state is
stabilized by the water accommodation in 1·4H2O. These
observations with CO2 adsorption indicate that the pores in 1
readily accommodate guest inclusion until they are full, with
further addition of guest exerting a “chemical pressure” on the CoII
center, which results in stabilization of the LS state.
Keywords: Carbon dioxide • spin state change • cobalt(II) ion
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The first example of CO2-responsive
spin state switching between high-spin
(HS) and low-spin (LS) states at room
Manabu Nakaya, Wataru Kosaka,
Hitoshi Miyasaka,* Yuki Komatsumaru,
Shogo Kawaguchi, Kunihisa Sugimoto,
Yingjie Zhang, Masaaki Nakamura,
Leonard F. Lindoy and Shinya Hayami*
temperature was achieved in
a
monomeric cobalt(II) complex forming
cavities generated via π–π stacking
motifs and hydrogen bond networks.
Page No. – Page No.
CO2-induced spin state switching at
room temperature in a monomeric
cobalt(II) complex with the porous
nature
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Author(s), Corresponding Author(s)*
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