Reversible On-Off Switching of the Hysteretic Spin Crossover in a Cobalt(II) Complex via Crystal to Crystal Transformation

Article abstract of DOI:10.1021/acs.inorgchem.9b01436

Reversible controlling and switching of magnetic bistability remains relatively difficult. Here, reversible on-off switching of a hysteretic spin transition in a Co^II complex via a single-crystal to single-crystal (SC-SC) transformation during

Full text of DOI:10.1021/acs.inorgchem.9b01436

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Reversible On−Off Switching of the Hysteretic Spin Crossover in a
Cobalt(II) Complex via Crystal to Crystal Transformation
Dong Shao,† Le Shi,† Fu-Xing Shen,† Xiao-Qin Wei,† Osamu Sato,*,‡ and Xin-Yi Wang*,†
†State Key Laboratory of Coordination Chemistry, Collaborative Innovation Center of Advanced Microstructures, School of
Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, People’s Republic of China
‡Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka Nishi-ku, 816-8580 Fukuoka, Japan
S
* Supporting Information
ABSTRACT: Reversible controlling and switching of magnetic bistability
remains relatively difficult. Here, reversible on−off switching of a hysteretic
spin transition in a CoII complex via a single-crystal to single-crystal (SC-SC)
transformation during dehydration and rehydration was reported. Upon
dehydration, a switching from a basically low spin state to an abrupt and
hysteretic spin crossover (SCO) with broad hysteresis loops was achieved.
Hysteretic and anisotropic crystal lattice expansion or contraction in the spin
transition temperature range was also observed in the dehydrated complex.
The magneto−structural relationship in this system was established on the
basis of detailed structure analyses on both the hydrated and dehydrated
examples over a wide range of temperatures. The elimination of guest internal
pressure, the tuning of the supramolecular interactions, and the strong electron−lattice coupling should be responsible for the
hysteretic SCO in the dehydrated complex.
mostly reported in FeII complexes and is usually non-
INTRODUCTION
■
hysteretic.8
Bistable materials with two switchable states in a certain range
In contrast to the overwhelming majority of FeII/III-based
of external stimuli have attracted continuous research interest
SCO complexes, CoII SCO complexes are less common and
for their potential applications in switches, sensors, and
usually exhibit gradual, incomplete, and nonhysteretic SCO
memory devices.1−4 Among these switchable materials,
behaviors.9 This is mainly attributed to the partial occupation
molecular solids showing electric or magnetic bistabilities are
of the antibonding eg orbitals in the ground state, which leads
particularly interesting with typical examples such as SCO
materials,2 single-molecule magnets (SMMs),3 and organic
to smaller structural changes arising from a spin transition.
radicals.4 Toward this end, spin crossover (SCO) materials
However, the smaller energy barriers between the potential
surfaces of the HS and LS states of CoII induce a faster spin
represent ideal candidates for their electronic conversion
transition dynamics and a high sensitivity of SCO behavior to
between low-spin (LS) and high-spin (HS) states in the
the crystalline environment or the solvents is thus
presence of an external stimulus, such as temperature, light,
anticipated.9a These characteristics offer both opportunities
and pressure.5 The development of SCO systems with a 30−50
and challenges for designing and synthesizing high-perform-
K hysteresis loop spanning the room-temperature region is
greatly desired.5c For hysteretic SCO behavior, strong
ance cobalt(II) SCO materials. Indeed, in comparison with
those for the FeII systems, the ligands for the appropriate
cooperative intermolecular interactions, such as hydrogen
bonding, π···π interactions, halogen bonding, and so on, are
considered to be of great significance. These intermolecular
ligand field for the CoII SCO complexes are much fewer,
among which terpyridine and its derivatives have probably
been the most studied.9b In all of the CoII SCO materials,
interactions can be carefully designed from the concept of
examples of a hysteretic SCO transition are relatively rare,10−12
crystal engineering. Therefore, various chemical and physical
reflecting the challenge in developing hysteretic bistable CoII
approaches have been successfully utilized to obtain the SCO
materials of hysteretic loops in various temperature regions.6,7
SCO materials.
With the above in mind, we recently designed and
Of particular interest among them are the dynamic molecular
synthesized the ligand 4′-(4-bromophenyl)-2,2′:6′,2″-terpyr-
idine (L; Scheme 1) to exploit the SCO properties of CoII
complexes. The additional phenyl ring and the bromide atom
were introduced to the parent terpy ligand, with the hope of
systems which are responsive to solvent or guest molecules
through a single-crystal to single-crystal (SC-SC) trans-
formation, especially those with reversibility.6 Notably, the
reversible switching of the thermal hysteresis loop via the SC-
SC event reported in the Hoffman-type SCO frameworks is
very attractive.6 On the other hand, the switching of SCO
behavior in a SC-SC fashion in an isolated system has been
Received: May 16, 2019
© XXXX American Chemical Society
A
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Scheme 1. Structure of Ligand L, the Previously Used
Anions [Co(NCS)4]2− and DPAS−, and the BDS2− Anion
Used to Prepare Complex 1
Figure 1. TGA curves for compounds 1·2H2O (blue), 1 (red), and 1·
2H2O-re (green).
inducing possible halogen bonding and π−π interactions.
Using a multicomponent strategy, we obtained the ion pair
bicomponent system [CoII(L)2][CoII(NCS)4]·2MeCN with
the coexistence of SMM and SCO behaviors.13a Furthermore,
we realized the reversible on−off switching between SCO and
SMM behaviors via a SC-SC event in the mononuclear CoII
complex [CoII(L)2](DPAS)2·DMF·2H2O (DPAS− = 4-
(phenylamino)benzenesulfonate; Scheme 1).13b In this system,
the weak supramolecular interactions and local coordination
environment of the CoII centers were effectively and reversibly
modified during dehydration−rehydration processes. These
results indicate that anions and solvent molecules play
significant roles in the magnetic behaviors of the CoII SCO
complexes, which inspired us to further develop dynamic
molecular CoII materials through the modification of the
anions and guest solvent molecules.
dehydrated form of 1 can be prepared by heating the sample of
1·2H2O at 400 K, in the X-ray diffractometer (for single-crystal
and powder XRD measurements), in the SQUID machine (for
the magnetic measurement), or under a dynamic vacuum (for
other measurements, see details in the Experimental Section).
Furthermore, 1 resorbs water and returns to the rehydrated
form (1·2H2O-re) upon exposure to air for several hours. TGA
was also performed on the dehydrated form 1 and the
rehydrated form 1·2H2O-re. For 1, the experimental weight
loss is ca. 0.03%, which indicates the completeness of the
dehydration of 1·2H2O to 1. For 1·2H2O-re, the TGA curve is
almost identical with that of 1·2H2O with a 3.36% weight loss,
suggesting the reversibility of the dehydration and rehydration
processes. This reversible process was also confirmed by single-
crystal X-ray diffraction (SCXRD), powder XRD (Figure S1),
elemental analyses, and magnetic measurements (vide infra).
Interestingly, the dehydration can occur in a SC-SC fashion
by heating a single crystal of 1·2H2O in the X-ray
diffractometer at 400 K for 1/2 h. SCXRD analyses were
conducted on both 1·2H2O and the in situ generated single
crystal of 1 at various temperatures (300 → 120 → 350 K for
1·2H2O and 400 → 120 → 350 K for 1). In addition, the
SCXRD was also performed on a rehydrated single crystal of 1·
2H2O-re at 300 K to confirm the complete reversibility of the
dehydration and rehydration even in a SC-SC fashion. The
data collection and structure refinement parameters for 1·
2H2O, 1, and 1·2H2O-re are given in Tables S1−S4 in the
Supporting Information, and the selected bond lengths and
bond angles at several specific temperatures are given in Tables
1 and 2 and Tables S5 and S6. Given in Tables 1 and 2 are also
some more specific structural parameters, including the
averaged Co−N bonds, the continuous shape measure values
(CShMs, calculated using SHAPE14), and so on (vide infra).
Herein, we present a CoII complex where the hysteretic SCO
can be reversibly switched on and off. Through a reversible
dehydration/rehydration process between [CoII(L)2][BDS]·
2H2O (1·2H2O) and [CoII(L)2][BDS] (1, H2BDS = benzene-
1,3-disulfonic acid; Scheme 1) occurring in a SC-SC fashion,
the magnetic behavior transfers from basically LS in 1·2H2O to
a cooperative SCO with a hysteresis loop in 1. Moreover,
hysteretic and anisotropic lattice expansion or contraction in
the SCO temperature range was observed in 1. Detailed single-
crystal structure analyses over a wide range of temperatures
revealed that the elimination of guest internal pressure, the
significant modification of the intermolecular interactions, and
the strong electron−lattice coupling are responsible for the
observed spin bistability in the dehydrated form. Notably, the
electron−lattice coupling herein refers to the correlation and
synergetic changes of the spin state of the metal center and the
crystal lattice.
̅
Both 1·2H2O and 1 crystallize in the triclinic space group P1 at
RESULTS AND DISCUSSION
all studied temperatures. For both forms, the asymmetric units
contain one Co2+ ion, two unique L ligands, and one BDS2−
anion (Figure 2). The CoII ion is coordinated by six N atoms
from two L ligands in a bis-meridional fashion, forming an
axially compressed CoN6 octahedron where the Co−Nequatorial
bonds are longer than the Co−Naxial bonds. The CShM values
relative to an ideal octahedron increase slightly from 2.413 at
200 K to 2.546 at 300 K for 1·2H2O and decrease remarkably
from 3.747 at 300 K to 2.422 at 200 K for 1 (Tables 1 and 2).
These values are consistent with a large distortion from the
ideal octahedron of the local CoII geometry for all structures.
For the bond angles, the axial N2−Co−N5 angle in 1·2H2O is
■
Syntheses and Structures. Dark red single crystals of 1·
2H2O were prepared by a slow evaporation method from
[Co(L)2](ClO4)2·3DMF (see details in the Experimental
Section). Thermal gravimetric analysis (TGA) of 1·2H2O
revealed a one-step dehydration process with ca. 3.30% weight
loss at 20−150 °C under a dry N2 flow, corresponding to the
release of two lattice H2O molecules (calculated weight loss
3.25%, Figure 1). After loss of the two lattice water molecules,
the TGA curve of 1·2H2O exhibits a negligible mass loss up to
ca. 400 °C, suggesting the high thermal stability of the
dehydrated form of 1. On the basis of this TGA result, the
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Table 1. Selected Bond Lengths (Å) and Structural
Parameters for 1·2H2O
Table 2. Selected Bond Lengths (Å) and Structural
Parameters for 1
T (K)
T (K)
300
260
230
200
300
260
230
200
cooling →
cooling →
Co(1)−N(1)
Co(1)−N(3)
Co(1)−N(4)
Co(1)−N(6)
Co−Nequatorial
Co(1)−N(2)
Co(1)−N(5)
Co−Naxial
2.027(4)
2.039(4)
2.132(4)
2.120(4)
2.079
1.887(4)
1.915(4)
1.901
2.020(3)
2.028(3)
2.140(2)
2.127(2)
2.079
1.883(2)
1.917(2)
1.900
2.012(3)
2.021(3)
2.143(3)
2.129(3)
2.076
1.874(2)
1.915(2)
1.894
2.003(3)
2.018(3)
2.147(3)
2.129(3)
2.0745
1.874(3)
1.915(3)
1.894
Co(1)−N(1)
Co(1)−N(3)
Co(1)−N(4)
Co(1)−N(6)
Co−Nequatorial
Co(1)−N(2)
Co(1)−N(5)
Co−Naxial
2.098(5)
2.132(5)
2.134(5)
2.134(4)
2.124
1.987(4)
1.981(5)
1.984
2.093(5)
2.127(5)
2.113(5)
2.130(5)
2.120
1.957(5)
1.950(5)
1.954
2.004(5)
2.012(6)
2.125(5)
2.136(5)
2.069
1.858(5)
1.907(5)
1.883
2.000(3)
2.012(3)
2.143(3)
2.140(3)
2.068
1.871(3)
1.925(3)
1.898
Co−Nav
CShMs
2.020
2.546
88.039
90.57
2.019
2.460
88.114
88.3
2015
2.435
88.237
87.51
2.014
2.413
88.551
86.76
Co−Nav
CShMs
2.078
3.747
84.686
110.4
2.055
3.394
85.018
107.8
2.006
2.404
89.491
88.03
2.015
2.422
89.526
86.87
a
a
b
b
dihedral angle (deg)
dihedral angle (deg)
c
c
∑
∑
T (K)
T (K)
200
230
260
300
200
230
260
300
heating →
heating →
Co(1)−N(1)
Co(1)−N(3)
Co(1)−N(4)
Co(1)−N(6)
Co−Nequatorial
Co(1)−N(2)
Co(1)−N(5)
Co−Naxial
2.009(2)
2.015(2)
2.141(2)
2.130(2)
2.073
1.870(6)
1.915(0)
1.892
2.014(2)
2.018(2)
2.141(2)
2.126(2)
2.074
1.874(6)
1.915(4)
1.895
2.023(3)
2.028(3)
2.140(3)
2.125(2)
2.079
1.880(2)
1.917(2)
1.898
2.030(3)
2.043(3)
2.133(3)
2.122(3)
2.082
1.896(2)
1.926(3)
1.911
Co(1)−N(1)
Co(1)−N(3)
Co(1)−N(4)
Co(1)−N(6)
Co−Nequatorial
Co(1)−N(2)
Co(1)−N(5)
Co−Naxial
2.004(4)
2.007(5)
2.131(4)
2.139(4)
2.070
1.857(4)
1.915(4)
1.886
2.004(5)
2.016(5)
2.124(5)
2.138(5)
2.071
1.856(5)
1.907(5)
1.882
2.060(3)
2.010(3)
2.100(2)
2.100(2)
2.070
1.870(2)
1.880(2)
1.870
2.115(4)
2.152(4)
2.132(4)
2.135(3)
2.133
1.995(4)
1.980(4)
1.987
Co−Nav
CShMs
2.013
2.436
88.367
87.45
2.014
2.436
88.285
87.54
2.019
2.473
88.098
88.87
2.025
2.560
88.193
91.32
Co−Nav
CShMs
2.009
2.451
89.570
87.45
2.008
2.452
89.496
87.41
2.05
2.586
88.7125
93.4
2.085
3.984
82.339
117.54
a
a
b
b
dihedral angle (deg)
dihedral angle (deg)
c
c
∑
∑
a
b
a
b
CShMs: continuous shape measure values. Value of the dihedral
CShMs: continuous shape measure values. Value of the dihedral
angle between the least-squares planes defined by the coordinated
pyridyl rings of the two Brphterpy ligands. ∑ is the sum of the
deviation from 90° of the 12 cis angles of the CoN6 octahedron.
angle between the least-squares planes defined by the coordinated
pyridyl rings of the two Brphterpy ligands. ∑ is the sum of the
deviation from 90° of the 12 cis angles of the CoN6 octahedron.
c
c
close to 175° and is almost temperature independent from 120
to 350 K, while it is temperature dependent in 1, ranging
between 175.0 and 169.1° from 120 to 400 K (Tables S5 and
S6 in the Supporting Information). In addition, the average
Co−N bond length (Co−Nav) increases slowly from 2.013 Å
at 200 K to 2.025 Å at 300 K in 1·2H2O (Δ = 0.012 Å), while
it increases abruptly from 2.009 Å at 200 K to 2.085 Å at 300 K
in 1 (Δ = 0.076 Å) (Tables 1 and 2). These bond lengths are
in accordance with the reported values for terpy-based CoII
SCO complexes.10,15,16
To further estimate the structural distortions, the distortion
parameters ∑ (the sum of the deviation from 90° of the 12 cis
N−Co−N angles) were calculated for both 1·2H2O and 1
(Tables 1 and 2). The ∑ values for 1·2H2O vary in the range
of 86.76−91.32 with |Δ∑| = 3.81 (upon cooling from 300 to
200 K) and 3.87 (upon heating from 200 to 300 K). These
values are very close to those for the LS CoII complexes and
suggest a basically LS state of 1·2H2O.10,11 For 1, the ∑ values
at 200 and 230 K are around 87, indicating a LS state. Upon an
increase in temperature, the ∑ values become obviously larger
with |Δ∑| = 23.53 (cooling from 300 to 200 K) and 30.09
(heating from 300 to 200 K), reflecting a large structural
distortion during the spin transition. The relatively large |Δ∑|
values are comparable with those of reported hysteretic CoII
SCO complexes.10,15 For example, Hayami and co-workers
Figure 2. Asymmetric units of 1·2H2O (left) and 1 (right). The red
and blue arrows represent the dehydration and rehydration process,
respectively. The dotted lines indicate the O−H···O and C−H···O
hydrogen bonds, which disappear in the dehydrated form. The H
atoms have been omitted for the sake of clarity.
recently reported a complex, [Co(C14-terpy)2](BF4)2·MeOH,
in which the ∑ values were 91.6 (LS) and 117.8 (HS) with the
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|Δ∑| value being 26.2.10b In addition, the ∑(LS), ∑(HS),
and |Δ∑| values were 76.1, 110.8, and 34.7, respectively, in a
nonterpy-based CoII SCO complex, [Co(dpzca)2], reported by
Brooker et al.11a All of these above structural parameters
clearly indicate that 1 undergoes a hysteretic spin transition,
while 1·2H2O is a basically LS complex.
phase transition (Figure 4. The PXRD patterns of 1·2H2O
were recorded from 300 to 350 K. Then, the sample was
Differential Scanning Calorimetry Measurements. To
probe the expected SCO transition in 1·2H2O and 1,
differential scanning calorimetry (DSC) experiments were
performed on the ground crystal samples of 1·2H2O and 1. For
1·2H2O, the DSC curve shows no peaks below 300 K,
consistent with the basically LS state of 1·2H2O (Figure S2 in
the Supporting Information). A broad endothermic peak
observed at 375 K is consistent with the loss of water
molecules. For 1 (Figure 3), in the first cycle, sharp exothermic
Figure 4. Experimental variable-temperature powder X-ray diffraction
patterns (PXRD) from 1·2H2O to 1, together with the simulated
PXRD patterns calculated from the single-crystal structures measured
at 300 K. The diffraction data were recorded on a powder sample of 1·
2H2O following the sequential heating−cooling−heating processes
(300 → 400 → 200 → 350 K).
further heated to 400 K to generate 1 in situ. As we can see, the
PXRD patterns at 300 and 350 K of 1·2H2O closely matched
the simulated pattern from the single-crystal data, indicating its
phase purity. At 400 K, the PXRD pattern differs significantly
from those at 300 and 350 K and closely matches the
calculated pattern for the dehydrated sample 1, confirming the
occurrence of a structural transformation due to dehydration.
After that, the PXRD patterns of 1 were then collected on
subsequent cooling and heating cycles from 350 to 200 and
back to 350 K. Upon cooling from 350 to 250 K, no obvious
change in the positions of the peaks was detected. At these HS
phases of 1, the powder patterns showed four strong reflections
at 2θ = 7.4, 9.7, 12.8°, and 17°. Upon cooling to 230 K, the
spin transition took place and thus a new LS phase of 1
appeared. While the peak at 12.8° splits into two peaks at 13.1
and 13.5°, the other three strong reflections exhibit
considerable shifts. These variations reflect a remarkable
moving of the crystal lattice. Upon further cooling to 200 K
and subsequent heating to 250 K, the powder patterns were
almost the same as that at 200 K, suggesting the LS state in this
temperature region. Moreover, the pattern of the HS phase was
recovered when the complex was warmed to 300 K, confirming
the full reversibility of the structural transformation of 1.
Thermal- and Guest-Induced Spin Transition. Varia-
ble-temperature magnetic susceptibilities were first recorded
on a collection of small crystals of 1·2H2O under 1 kOe up to
350 K (Figure 5a). For 1·2H2O, the χMT values remain almost
constant at 0.45 cm3 mol−1 K from 2 to 240 K, which is in
accordance with literature values for LS terpy-based Co2+
complexes.16 Upon further heating, the χMT value gradually
increases to 1.1 cm3 mol−1 K at 350 K, reflecting the basically
LS state of 1·2H2O. Further heating to 400 K leads to an
abrupt increase to 2.4 cm3 mol−1 K at 400 K (Figure S3 in the
Supporting Information). According to the TGA and DSC
data, this final increase should be caused by the dehydration of
the sample under dynamic vacuum during the magnetic
measurement. Then, the sample was further kept at 400 K for 1
h to obtain the entirely dehydrated phase of 1 and the
Figure 3. Differential scanning calorimetry (DSC) results of 1,
recorded in three cooling and warming cycles with different scan rates
of 2 (blue curve), 5 (red curve) and 10 K min−1 (green curve).
and endothermic peaks were observed at 240 K upon cooling
and 271 K upon heating, respectively, at a scan rate of 2 K
min−1. In the second and third cycles with faster scan rates (5
and 10 K min−1), the peaks did not change positions in the
heating mode but appeared at a slightly higher temperature
(246 K) in the cooling mode, generating a stable hysteretic
effect of about 25 K. The sharp peaks and the presence of a
thermal hysteretic effect indicate the occurrence of a first-order
phase transition in 1. The integrated enthalpy and entropy
values are ΔH = −8.52, −7.79, and −10.16 kJ mol−1 and ΔS =
−35.50, −31.66, and −41.30 J mol−1 K−1 for the three cycles
for the HS to LS transition (cooling). On the other hand for
the LS to HS transition, ΔH = 12.79, 12.05, and 13.96 kJ
mol−1 and ΔS = 47.19, 44.46, and 51.51 J mol−1 K−1 for the
three cycles. The values are similar to those obtained for other
CoII SCO complexes with an abrupt spin transition.10a It was
noted that the enthalpy changes in the heating mode are larger
than those in the cooling mode. This observation might reflect
a larger structural change during the SCO in the same
temperature range, which is consistent with the larger Δ∑
value during heating from 200 to 300 K (30.09) in comparison
with that (23.53) during cooling. Furthermore, the large
entropy change (compared with the ΔS = R ln[(2S + 1)HS/(2S
+ 1)LS] = 5.8 J mol−1 K−1 for a pure SCO transition from S =
1/2 to 3/2 for a CoII center) clearly reveals a considerable
contribution from the structure transformation and lattice
vibration during the phase transition, which is consistent with
the structure analyses.
Powder XRD Experiments. Variable-temperature PXRD
experiments were further performed to monitor the structural
D
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Figure 5. (a−c) Variable-temperature magnetic susceptibility data of 1·2H2O and 1 measured on the microcrystals (a), powders from the well-
ground single crystals (b), and a single crystal (c). The red and green data points were obtained by using a settle mode, while the blue and violet
data points were recorded by using sweep modes at 2 and 5 K min−1 scan rates, respectively. (d) Reversible variation of the unit cell volume during
the dehydration−rehydration processes and SCO transitions.
Figure 6. Temperature dependence of the crystal lattice parameters of 1·2H2O (circles) and 1 (squares): (a) a axis; (b) b axis; (c) c axis; (d) unit
cell volume.
magnetic data were recorded further during heating and
cooling processes for four cycles with both settle (cycles 1 and
2) and sweep modes (cycles 3 and 4 at 2 and 5 K min−1 scan
rates, respectively).
After dehydration, a singular SCO behavior with a wide
thermal hysteresis loop was observed for 1 (Figure 5a). At 300
K, the χMT value of 1 is 2.2 cm3 mol−1 K and decreases slowly
to 2.0 cm3 mol−1 K at 250 K. Below this temperature, χMT
shows an abrupt and complete SCO transition centered at
T1/2↓ = 235 K in the first cooling mode. Upon warming in the
heating mode, χMT increases steeply with T1/2↑ = 267 K, giving
a 35 K apparent thermal hysteresis loop. Notably, this abrupt
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SCO with a significant hysteresis loop is close to room
temperature. To check the stability of the hysteresis, the
cooling−heating cycles were repeated three more times using
settle and sweep modes with different scan rates (Figure 5a).
In the following cycles, the SCO takes place at T1/2↓ = 250 K,
15 K higher than that observed in the first cycle, while the
T1/2↑ values stay almost the same for all cycles. This observed
shrinking of the loop is not uncommon in SCO systems and
could be attributed to the change in the size of the crystalline
sample during the initial abrupt transition.12 After the first
cycle, the characteristics of the loops were independent of the
scan rates, giving a stable hysteresis loop of 20 K width.
In addition, the well-ground sample and one large single
crystal (8.90 mg) of 1·2H2O were also measured using the
same procedure. For the well-ground sample (Figure 5b),
T1/2↓ shifts to higher temperatures (T1/2↓ = 249, 251, and 251
K for the three cycles), while T1/2↑ decreases slightly to a lower
temperature of 263 K, giving a smaller hysteresis of only 12 K
width. For the large single crystal (Figure 5c), an apparent 53
K loop with T1/2↓ = 207 and T1/2↑ = 260 K was observed in
and 1 are in good agreement with their SCO behaviors,
indicating the strong coupling of the SCO transition and lattice
expansion or contraction.
Magneto−Structural Correlation. Careful examination
of the intermolecular interactions in both 1·2H2O and 1
revealed the structure−function relationship of the observed
properties and the switching of the SCO properties upon
dehydration. As depicted in Figures 2 and 7a, the two water
the first cycle, while the stable loop has a width of 34 K (T1/2
↓
= 226 and T1/2↑ = 260 K). The widths of these loops are very
large and unusual for the CoII SCO complexes with clear
structures.10,17 Moreover, the on−off switching of the
hysteretic SCO bistability is fully reversible, as proved by the
magnetic measurements on the rehydrated sample (Figure S4
in the Supporting Information). Clearly, the χMT vs T curve of
this rehydrated sample is almost the same as that of the original
1·2H2O. Two successive cooling and warming cycles give two
hysteresis loops with different widths of 39 and 19 K.
In addition to this reversible magnetic bistability, the
endurance of the reversibility of this compound was further
confirmed by the single-crystal X-ray data. Comparing their
structures, we noticed that the dehydration triggers a structural
transformation from 1·2H2O to 1 with a ∼32 Å3 decrease in
the unit cell volume, while the SCO transition in 1 induces a
phase transition with a further larger decrease in the unit cell
volume of ca. 53 Å3. These distinct changes in the unit cell
volume allow us to monitor the dynamic structural trans-
formation via SCXRD (Figure 5d). The reversibility of the
structural transformation and SCO transition can be repeatedly
detected for at least four times in a single crystal with no
apparent loss in crystal integrity. It is noteworthy that other
examples showing guest-dependent multicycle reversibility of
the spin state have also been reported,6f,8b,c,17a,18 such as the
guest-dependent SCO in a FeII-based metal−organic frame-
work reported by Kepert and co-workers,18a and the reversible
quantitative guest sensing in a nonporous FeII complex
reported by Brooker et al.18d
Figure 7. (a, b) Change in the hydrogen-bonding interactions
between two BDS2− anions and four water molecules in 1·2H2O,
which can be broken (b) and reconstructed (a) by dehydration and
rehydration. (c, d) Intermolecular π···π interactions between pyridines
(type A, green; type B, red) and benzene rings (type C, blue) in 1·
2H2O and 1, which can be reversibly modified during dehydration−
rehydration.
Furthermore, the lattice parameters of 1·2H2O and 1
determined from variable-temperature SCXRD experiments on
cooling and heating are plotted in Figure 6 (see details in
Tables S7 and S8). For both complexes, the a and c axes have a
positive thermal expansion, while the b axes have a negative
thermal expansion. From 200 to 300 K, the a (c) axis expands
∼2% (2%) for 1·2H2O and ∼6% (3%) for 1, while the b axis
shrinks ∼3% for 1·2H2O and ∼6% for 1. In addition to the
anisotropy of the thermal expansion, the most remarkable
difference in the lattice expansion between 1·2H2O and 1 is
the appearance of the thermal hysteresis loops for the lattice
parameters of 1. The width of the hysteresis loops for all three
axes are ca. 30 K with T1/2↓ = 250 K and T1/2↑ = 280 K
(Figure 6). These anisotropic lattice changes of both 1·2H2O
molecules of 1·2H2O have rich hydrogen bonds (HBs)
between themselves (O7···O8 = 2.862 Å, O7···O8′ = 2.993
Å) and their neighbors, including the O atoms of BDS2− (O6···
O7 = 2.826 Å, O4···O8′ = 2.951 Å) and the CH groups of the
L ligand (O7···H30 = 2.762 Å, O7···H33 = 2.629 Å, O7···H39
= 2.579 Å). Connected by the OH···O HBs, two BDS2− anions
are linked together by four H2O molecules in 1·2H2O (Figure
7a). These strong HBs in 1·2H2O form a rigid environment of
the complex cations and generate the possible internal
pressure, which could prevent the expansion of the CoII
coordination sphere, lock the spin state of 1·2H2O in the LS
state, and hinder its spin state switching. Removal of the lattice
water unlocked these strong HB interactions in 1 (Figure 7b),
F
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Article
which leads to a decrease in the internal pressure and stabilizes
the HS state. In fact, lattice water molecules are usually able to
stabilize the LS state in some FeII/III-based SCO complexes.5b
For example, lattice water molecules stabilize the LS states via
HB to the ligands in the [Fe(3-bpp)2]2+ hydrates19 and
[Fe(H2bip)3]2+ hydrates.20 Dehydration of these complexes
leads to the stabilization of the HS states and SCO
behavior.19,20 Furthermore, other small molecules (i.e.,
MeCN, acetone, and so on) can also stabilize the LS forms,
and HS or hysteretic SCO can be observed after desolvation
via SC-SC transformation.8c,21
In addition, abundant π···π interactions (offset face to face
mode) between the complex cations were found in both 1·
2H2O and 1 (Figure 7c,d and Tables S9 and S10). The
dehydration leads to significant shortening of the π···π
distances (Figure 7d), which could be favorable for a
cooperative spin transition. At 300 K, the crystal packing
structure of 1·2H2O shows that the neighboring [Co(L)2]2+
units form rather weak intermolecular π···π interactions with
parallel-displaced distances of 3.459 Å (type A), 3.521 Å (type
B), and 3.771 Å (type C) (Figure 7c and Table S9).
Decreasing the temperature to 200 K does not change these
weak intermolecular interactions significantly with a change in
the π···π distances of around 0.03 Å (Table S9). However,
upon dehydration, these distances in 1 at 300 K decrease
significantly to 3.438, 3.269, and 3.651 Å, respectively (Figure
7d and Table S10). These more effective π···π interactions
should favor the cooperative spin transition in 1. In addition,
these distances increase upon cooling (Table S10), reaching
3.398 Å (type A), 3.395 Å (type B), and 3.585 Å (type C) at
200 K. In comparison with the 0.03 Å in 1·2H2O, the change
in π···π distances in 1 between 200 and 300 K is quite large
(0.08 Å), reflecting the larger structural modification of the
structure of 1 due to the hysteretic spin transition.
The observation of the hysteresis loops of both the SCO and
lattice parameters in the same temperature region reflects the
strong electron−lattice cooperativity and deserves more
discussion. Recently, the changes in the lattice parameters
and even the shape of macro crystals have been of significant
interest. The rotation of the molecular component of the
complexes is one of the driving forces. For examples, Sato et al.
has demonstrated that the singular anisotropic shape change of
the mononuclear NiII and CoII complexes can be induced by
the rotation of the oxalate molecules and n-butyl group,
respectivety.22 By inspecting the crystal structure studies, we
attempted to rationalize the possible underlying mechanism of
the anisotropic lattice expansion and contraction in 1. Usually,
the increases in the molecular volume and bond length of the
metal centers are isotropic during a SCO transition. Thus, the
anisotropic thermal expansion of the lattice should have some
specific reason. As depicted in Figure 8a, we noticed that the
N1−Co−-N5 bond angle experiences a pronounced decrease
from 112.05 to 100.63° during the spin transition from HS to
LS in 1, causing the rotation and bending of one of the ligands
(Figure 8). Consequently, this SCO-driven ligand rotation and
bending leads to the anisotropic expansion and contraction
along the b and c axes, as schematically shown in Figure 8b.
However, for 1·2H2O, the decrease in the N1−Co−N5 bond
angle is very small (101.97 to 100.69° from 300 to 120 K,
Table S5), which leads to the quite small variations of its lattice
parameters.
Figure 8. (a) Change in the N−Co−N bond angles and the rotation
and bending of the ligands during the SCO transition of 1. (b)
Schematic drawing of the SCO-driven rotation and bending of the
ligands, which leads to the anisotropic thermal expansion.
CONCLUSION
■
In summary, we have presented a reversible on−off switching
of the hysteretic spin transition in a mononuclear CoII complex
through crystal to crystal transformation during dehydration−
rehydration. Detailed experiments revealed that the switching
of the spin bistability with broad thermal hysteresis loops is
realized by a collaborative effect of the elimination of guest
internal pressure, the modification of supramolecular inter-
actions, and the strong electron−lattice coupling. This result
highlights the significant role of the guest molecules on the
turning or switching of the spin bistability.
EXPERIMENTAL SECTION
■
Materials and Synthesis. All reagents were commercially
available and were used as received without further purification.
Caution! Although our samples never exploded during handling,
perchlorate metal complexes are potentially explosive. Only a small
amount of material should be prepared, and it should be handled with
great care.
Synthesis of the Ligand L. The powder of the starting material
4′-(4-bromophenyl)-2,2’:6′,2″-terpyridine (L) was synthesized by the
reaction of 4-bromobenzaldehyde (0.01 mol, 1.85 g), 2-acetylpyridine
(0.02 mol, 2.42 g), KOH (0.055 mol, 3.08 g), and ammonia (60 mL,
25%) in ethanol (100 mL) under reflux for 1 day. The crude white
product has poor solubility in common organic solvents and was
purified by Soxhlet extraction into chloroform (200 mL). Yield: 1.7 g,
ca. 45%. Anal. Calcd for C21H14BrN3: C, 64.96; H, 3.63; N, 10.82.
1
Found: C, 64.91, H, 3.71; N, 10.78. H NMR spectra: Figure S5 in
the Supporting Information.
Synthesis of [Co(L)2](ClO4)2·3DMF. The L ligand (380 mg, 1
mmol) was dissolved in 30 mL of DMF and heated to boiling. A 10
mL portion of a water solution of Co(ClO4)2·6H2O (180 mg, 0.5
mmol) was added to the above boiling solution. After heating for 10
G
DOI: 10.1021/acs.inorgchem.9b01436
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Article
min, the mixed dark red solution was filtered into a 50 mL beaker and
kept in the dark. Dark red single crystals were obtained after slow
evaporation for 2 weeks. Anal. Calcd for C51H49Br2Cl2CoN9O11: C,
48.86; H, 3.94; N, 10.05. Found: C, 48.81, H, 3.41; N, 10.08.
Synthesis of [Co(L)2][BDS]·2H2O (1·2H2O). A solution of
[Co(L)2](ClO4)2·3DMF (200 mg) in 60 mL of MeCN was mixed
with a 20 mL water solution of Na2BDS (2 mmol, 564 mg, H2BDS =
benzene-1,3-disulfonic acid). After heating for 5 min, the dark red
solution was cooled to room temperature and filtered. Dark red single
crystals of 1·2H2O were obtained after slow evaporation for 4 days.
Yield: 150 mg (ca. 65%). Anal. Calcd for C48H36Br2CoN6O8S2: C,
52.04; H, 3.27; N, 7.58. Found: C, 51.96, H, 3.48; N, 7.68.
Synthesis of [Co(L)2][BDS] (1). For single-crystal X-ray
diffraction measurements, a single crystal of 1 was obtained by
heating a single crystal of 1·2H2O in the diffractometer under a dry
N2 flow from 300 to 400 K at a heating rate of 3 K min−1 and further
annealing at 400 K for 1 h. For magnetic measurement, the well-
ground crystals of 1·2H2O were heated at 400 K in the dynamic
vacuum chamber of a SQUID VSM instrument for 2 h. For other
physical measurements, a fresh sample of 1 was prepared by heating
the sample of 1·2H2O at 400 K under a dynamic vacuum for 2 h.
Anal. Calcd for C48H32Br2CoN6O6S2: C, 53.79; H, 3.01; N, 7.84.
Found: C, 53.61, H, 3.18; N, 7.74.
data for 1·2H2O at 200, 230, 260, 300, 320, and 350 K
in heating order, X-ray crystallographic data for 1 at 400,
350, 300, 260, 230, 200, aand 120 K in cooling order,
and X-ray crystallographic data for 1 at 200, 230, 260,
300, and 350 K in heating order (PDF)
Accession Codes
CCDC 1889167−1889174, 1889176−1889181, 1889184,
1889186−1889188, and 1910380−1910383 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for O.S.: sato@cm.kyushu-u.ac.jp.
*E-mail for X.-Y.W.: wangxy66@nju.edu.cn.
■
ORCID
Synthesis of the Rehydrated [Co(L)2][BDS]·2H2O (1·2H2O-
re). The samples of 1 were exposed at room temperature to air
atmosphere for 1 day. Anal. Calcd for C48H36Br2CoN6O8S2: C, 52.04;
H, 3.27; N, 7.58. Found: C, 51.91, H, 3.34; N, 7.62.
Dong Shao: 0000-0002-3253-2680
Le Shi: 0000-0002-8830-883X
Xiao-Qin Wei: 0000-0003-0877-0328
Osamu Sato: 0000-0003-3663-5991
Xin-Yi Wang: 0000-0002-9256-1862
Physical Measurements. Elemental analyses for C, H, and N
were measured on an Elementar Vario MICRO analyzer. PXRD
measurements were recorded at various temperatures on a Bruker D8
Advance diffractometer with Cu Kα X-ray source (λ = 1.54056 Å)
operated at 40 kV and 40 mA. TGA was measured in Al2O3 crucibles
using a PerkinElmer Thermal Gravimetric Analysis instrument in the
temperature range of 20−700 °C under N2 flow at a heating rate of 20
°C/min. DSC was performed using a Mettler Toledo DSC1
differential scanning calorimeter. Magnetic measurements were
measured under a dc field of 1 kOe with a Quantum Design VSM
magnetometer. The experimental data were corrected for the
diamagnetic contribution to the magnetic susceptibility by means of
Pascal’s constants.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the financial support of the National Key
R&D Program of China (2018YFA0306002), the NSFC
(91622110, 21522103), and the China Postdoctoral Science
Foundation (2018M640473). O.S. thanks the MEXT
KAKENHI (Grant Number JP17H01197).
X-ray Crystallography. SCXRD data were collected on a Bruker
D8 Venture diffractometer with a CCD area detector (Mo Kα
radiation, λ = 0.71073 Å) at various temperatures. The APEXII
program was used to determine the unit cell parameters and for
data collection. The data were integrated and corrected for Lorentz
and polarization effects using SAINT.23 Absorption corrections were
applied with SADABS.24 The structures were solved by direct
methods and refined by full-matrix least-squares methoda on F2 using
the SHELXTL crystallographic software package.25 All of the non-
hydrogen atoms were refined anisotropically. Hydrogen atoms of the
organic ligands were refined as riding on the corresponding non-
hydrogen atoms. Additional details of the data collections and
structural refinement parameters are provided in Tables S1−S4.
CCDC 1889167−1889174, 1889176−1889181, 1889184, 1889186−
1889188, and 1910380−1910383 are the supplementary variable-
temperature crystallographic data for 1·2H2O and 1, respectively.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
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* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01436.
■
S
Crystallographic data and additional figures and tables
X-ray crystallographic data for 1·2H2O at 300, 260, 230,
200, and 120 K in cooling order, X-ray crystallographic
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