Construction of SCO-Active Fe(II) Mononuclear Complexes from the Thio-pybox Ligand

Article abstract of DOI:10.1021/acs.inorgchem.9b03506

The development of new spin-crossover complexes provides novel promising switching materials with significant potential at the molecular level. Ter-imine-type molecules represent one of the important classes of ligands in creating SCO-active complexes. Herein we report a family of mononuclear Fe(II) SCO-active compounds constructed from a new type of ter-imine ligand named the thio-pybox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrothiazol-2-yl)pyridine, L¹). Through the variation of counteranions, some cases display complete SCO and with T_1/2 near ambient temperature. Among them, annealed [Fe^II(L¹)₂](ClO₄)₂ [1(ClO₄)] shows T_1/2↓ and T_1/2↑ as 319 and 349 K, respectively. The wide thermal hysteresis of ΔT = 30 K originated from the weak interaction between complex cations and counteranions in the crystal lattice. Impressively, its high-spin population can be increased considerably by annealing at high temperature. The metastable high-spin phase is stable in the successive magnetic measurements and would gradually relax to its initial state with high population of low-spin configuration at ambient temperature. In acetonitrile-diluted solution, 1(ClO₄) still maintains SCO with an estimated T_1/2 at 240 K. Differential scanning calorimetry discloses the structural phase at around 355 K in the first heating process and the increase in the high-spin population concomitant with annealing was also corroborated by ⁵⁷Fe M?ssbauer measurements. Additionally, the influences on SCO by counteranion and ligand structure are investigated, which show that the fine tuning of complex structures can affect the behavior of the spin state significantly. Finally, magneto-structural correlation studies were performed on the structures of 1(ClO₄) and its oxygen analogue at multiple temperatures. The analyses of some structural parameters, including terminal N···N donor separation, bite angle, patulous angle, and the root mean squared deviation indicate that the replacement of the oxygen atom with a sulfur atom can effectively improve the flexibility and release the steric strain and thus tune the SCO toward ambient temperature. Our research demonstrates the rational design of the ligand can lead to new SCO-active compounds with high performance.

Full text of DOI:10.1021/acs.inorgchem.9b03506

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Construction of SCO-Active Fe(II) Mononuclear Complexes from the
Thio-pybox Ligand
Yao Pan,§ Yin-Shan Meng,§ Qiang Liu, Wan-Qing Gao, Chun-Hua Liu, Tao Liu,* and Yuan-Yuan Zhu*
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ABSTRACT: The development of new spin-crossover complexes provides
novel promising switching materials with significant potential at the molecular
level. Ter-imine-type molecules represent one of the important classes of
ligands in creating SCO-active complexes. Herein we report a family of
mononuclear Fe(II) SCO-active compounds constructed from a new type of
ter-imine ligand named the thio-pybox ligand (2,6-bis(4,4-dimethyl-4,5-
dihydrothiazol-2-yl)pyridine, L1). Through the variation of counteranions,
some cases display complete SCO and with T1/2 near ambient temperature.
Among them, annealed [FeII(L1)2](ClO4)2 [1(ClO4)] shows T1/2↓ and T1/2↑
as 319 and 349 K, respectively. The wide thermal hysteresis of ΔT = 30 K
originated from the weak interaction between complex cations and
counteranions in the crystal lattice. Impressively, its high-spin population
can be increased considerably by annealing at high temperature. The
metastable high-spin phase is stable in the successive magnetic measurements and would gradually relax to its initial state with high
population of low-spin configuration at ambient temperature. In acetonitrile-diluted solution, 1(ClO4) still maintains SCO with an
estimated T1/2 at 240 K. Differential scanning calorimetry discloses the structural phase at around 355 K in the first heating process
57
̈
and the increase in the high-spin population concomitant with annealing was also corroborated by Fe Mossbauer measurements.
Additionally, the influences on SCO by counteranion and ligand structure are investigated, which show that the fine tuning of
complex structures can affect the behavior of the spin state significantly. Finally, magneto-structural correlation studies were
performed on the structures of 1(ClO4) and its oxygen analogue at multiple temperatures. The analyses of some structural
parameters, including terminal N···N donor separation, bite angle, patulous angle, and the root mean squared deviation indicate that
the replacement of the oxygen atom with a sulfur atom can effectively improve the flexibility and release the steric strain and thus
tune the SCO toward ambient temperature. Our research demonstrates the rational design of the ligand can lead to new SCO-active
compounds with high performance.
switches.10,11 To meet the practical applications in SCO-based
molecular devices, abrupt and symmetric hysteresis loops with
1. INTRODUCTION
Since the first discovery by Cambi et al. in 1931,1 spin-
a width of 30−50 K occurring at around room temperature are
crossover (SCO) complexes have attracted tremendous
attention because of their potential applications in information
storage, molecular switches, molecular electronics, and optical
devices.2−7 In these complexes, the rearrangement of electrons
in central metal ions, normally d4−d7 transition metal ions, can
take place between high-spin (HS) and low-spin (LS) states
under a suitable ligand field via external perturbations such as
heat, light, pressure, and magnetic fields. The change in
electron configuration can yield the maximum, intermediate,
and minimum number of unpaired electrons and thus
considerably influence their physical properties including the
magnetic moment, color, dielectric constant, and electrical
resistance.8 In some cases, there is a first-order phase transition
concomitant with the SCO process which produces an abrupt
spin transition and/or pronounced hysteresis.9 The SCO
materials possessing a stable hysteresis loop can be located in
either the high-spin state or the low-spin state at the same
temperature, which is necessary for applications in bistable
required.12
To date, quite a few mononuclear,13 polynuclear,14 and
polymeric complexes15 have been reported that show a variety
of SCO properties.3 The rational design of organic ligands and
the selection of counteranions can not only fine-tune the SCO
properties but also provide an approach to studying magneto-
structural correlations.16−20 Among them, mononuclear
complexes have always received wide investigation because
they provide several unique advantages: (1) smaller structures
Received: December 2, 2019
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benefit from the design and synthesis of the complexes; (2) the
purification and crystallization of complexes are easy to
perform, so the crystal structures can be determined
conveniently; (3) the facile incorporation of other functional
moieties into the ligands can be utilized to construct
multifunctional molecular materials in combining the SCO
with other physical properties.
py2,33 1-bpp,16,34,35 3-bpp,36−39 and trim40 were proven to be
SCO-active.
From 2015, reports from our group,41−43 Halcrow’s group,44
and Ishida’s group45,46 have shown the ability of pybox series
tridentate pincer ligands in constructing SCO-active FeII
complexes. It was also found for this system that substituents
on the ligand, counteranions, solvents, and chirality have great
impacts on their spin transition behaviors. However, the ligand
field strength for the series of [FeL2]2+ complexes (L = 2,6-
bis(4,4-dimethyl-4,5-dihydrooxazol-2-yl)pyridine) is relatively
weak and commonly results in spin transitions below 200 K.
How to increase the SCO temperature toward ambient
temperature domains with considerable hysteresis remains a
huge challenge. In this work, we propose to increase the ligand
field strength through replacing the oxygen atoms by sulfur
atoms in prototype pybox ligand. Due to the larger atomic
radius of sulfur compared to that of oxygen (1.02 vs 0.68 Å),
the replacement of a smaller oxygen atom by a larger sulfur
atom results in higher flexibility in the ligand by which the
deformation of the coordination sphere during the spin
transition can be compensated for. Furthermore, the
electron-deficient nature of sulfur compared to that of oxygen
could also produce a larger ligand-field splitting.47−49 Thus,
this change may not only drive a complete SCO but also tune
the critical temperature TC to a higher temperature region.
Herein, we report the syntheses, characterization, spin state
investigation, and magneto-structural analyses of eight
mononuclear FeII complexes constructed from thio-pybox
ligands L1−2 (Scheme 1). The magnetic study indicated that
complex [Fe(L1)2](ClO4)2 1(ClO4) exhibited pronounced
hysteretic SCO in the ambient temperature region.
Bis(ter-imine)-type mononuclear FeII/CoII complexes con-
stitute one of the important classes of SCO systems. In these
complexes, two tridentate pincer type ter-imine ligands
coordinate to the central metal ion in an approximatly
perpendicular mode, so the structure of the complex is easy
to predict. For the ligands, both electronic and structural
modifications can be carried out to tune the ligand field so as
to meet the SCO requirement. Among many families of SCO
complexes in this class, cooperative interactions can be
introduced through the incorporation of hydrogen-bonding
or π−π stacking interactions. Thus, the corresponding
complexes can display an abrupt transition associated with a
wide hysteresis loop.21 Owing to the progress in heterocycle
synthetic chemistry, numerous pincer-type ligands composed
of pyridine and other nitrogen-containing heterocycles have
been designed and synthesized (Chart 1). Some mononuclear
FeII and CoII complex families from these ligands, such as tpy
and its derivatives,22−25 phenthiazole,26 phentetrazol,27 Schiff
base ter-imines,28,29 (2-thia)2py,30,31 (4-thia)2py,32 (2-thia)-
Chart 1. Representative Tridentate Pincer-Type Ter-Imine
Ligands Employed in Constructing SCO-Active Complexes
Scheme 1. Rational Design of [FeII(thio-pybox)2]2+
Compounds in This Work
2. RESULTS AND DISCUSSION
Synthesis. In this work, two thio-pybox type ligands,
tetramethylated L1 and the parent (unmethylated) L2 (Scheme
1), were facilely prepared according to the modified literature
method.50 (See the SI for synthesis details.) The crystals of 1
and 2 series FeII complexes with ClO4 , BF4 , PF6 , and BPh4
−
−
−
−
as counteranions were grown by the slow gas evaporation of
ether into their diluted acetonitrile solution or by the direct
crystallization from their methanol solution. All crystals have a
dark-purple color. Suitable single crystals were collected to
determine their structures by single-crystal X-ray diffraction
(Figures 1 and S15−S31), except that the structural
determination of [Fe(L1)2](BF4)2 1(BF4) failed after numer-
ous attempts due to the poor quality of the crystals. In
1
addition, powdered XRD, H NMR spectra, and elemental
analyses were carried out to confirm the structure and purity
(Figures S6−S13 and S32−S35). Similar to their [Fe-
(pybox)2]2+ analogues, the cation parts in this series are all
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MeCN, and 2(PF6)·MeCN) are found in the voids of the
crystal lattice for the complex. [Fe(L1)2](ClO4)2 1(ClO4),
[Fe(L1)2](BPh4)2 [1(BPh4)], and [Fe(L2)2](BPh4)2 [2-
(BPh4)] are solvent-free complexes.
X-ray Crystal Structures. Crystals of 1(ClO4) obtained
from two different methods showed the identical structure.
The structural analyses were performed at four temperatures,
400, 380, 300, and 153 K, to investigate the possible SCO
behavior. Under the variation of measured temperatures, it is
preserved to crystallize in the same orthorhombic space group
Pbca with Z = 8 (Figure 1(a)). The asymmetric unit contains a
unique [Fe(L1 )]2+ cation and two perchlorate anions. No
2
solvent molecules were found in the lattice by the TGA trace
(Figure S36). The average Fe−N bond lengths of 2.178 Å at
400 K and 2.175 Å at 380 K denote that the FeII center is in
the HS state. When the temperature is lowered, the Fe−Nav
length decreases to 2.140 Å at 300 K and 2.022 Å at 153 K,
implying that thermally driven SCO takes place in this
temperature region. Accordingly, the S(Oh) value that shows
that the deviation from ideal Oh symmetry for the FeN6 sphere
continuously decreases from 4.681 at 400 K to 2.774 at 153 K
(Table 1 and S3). The overlay of the HS (380 K, blue) and LS
(153 K, red) structures of the cation part indicates that it
undergoes a significant structural contract and deformation
upon cooling (Figure 1(b) and S16). Although 1(ClO4) is a
mononuclear complex, a two-dimensional network is formed
by two C−H···O hydrogen bonds and two S···O contact
Figure 1. (a) Crystal structure of 1·ClO4 at 153 K, hydrogen
bondings (purple), and S···O interactions (yellow) are illustrated by
dashed lines. (b) Overlay of the HS at 380 K (blue) and LS at 153 K
(red) structures of its cation part, showing the considerable
contraction of the molecule and the more serious disorder of one
thiazole ring after SCO. (c) View of the packing diagram of 1(ClO4)
at 153 K along the b axis, hydrogen bondings, and S···O interactions
between the complex cation and perchlorate anion stabilizing the
solid structure and forming a two-dimensional network.
interactions51 between the perchlorate anion and [Fe(L1 )]2+
2
cation at 153 K (Figure 1(c)). Nevertheless, the S···O contact
interactions disappear and hydrogen bondings become longer
when the measured temperature is above 300 K. It can be
attributed to the rotation of the perchlorate anion (Figure
S31), so the structural changes in 1(ClO4) between high and
low temperatures are mainly contributed by the deformation of
the complex cation and the reorientation of the counteranion.
The positional disorder of one carbon atom in the
dihydrothiazole ring exists in 1(ClO4) and 1(PF6)·MeCN.
However, no disorder is found in their oxygen-replaced
analogues.41
composed of a compressed octahedral FeII center coordinated
along the Npy−Fe−Npy axis with two approximately
perpendicular plane ligands. In every compound, the Fe−Npy
bond lengths are shorter than the Fe−Nav bond length (Tables
S2 and S3). The counteranions and solvent (MeCN and/or
H2O) in [Fe(L1)2(PF6)2·3MeCN], 1(PF6)·MeCN, [Fe-
(L2)2(ClO4)2·MeCN·H2O], 2(ClO4)·MeCN·H2O, [Fe-
(L2)2(BF4)2·MeCN], 2(BF4)·MeCN, [Fe(L2)2](PF6)2·
In order to gain detailed insight into the phase transition
process, the crystal structure of 1(ClO4) before and after
annealing was investigated by single-crystal and powder X-ray
Table 1. Comparison of the Key Structural Parameters and Spin State for 1(ClO4) and Its Oxygen-Replaced Analogue,
a
3(ClO4)
entry
T (K)
S(Oh)
μrms (Å)
ρav (Å)
θav (deg)
ϕav (deg)
spin state
T1/2 (K)
d
d
1(ClO4)
1(ClO4)
1(ClO4)
1(ClO4)
1(ClO4)
3(ClO4)
3(ClO4)
3(ClO4), Fe1
3(ClO4), Fe2
3(ClO4)
400
380
300
300
153
250
180
173
173
100
100
4.681
4.633
4.214
4.235
2.774
4.756
4.674
3.653
4.735
2.354
5.045
0.229
0.231
0.242
0.241
0.243
0.199
0.200
0.209
0.176
0.199
0.209
4.306
4.298
4.2435
4.249
4.0565
4.265
4.2425
4.148
4.238
3.948
4.287
150.12
110.11
109.89
HS
319 (T1/2↓
)
150.255
151.715
151.635
157.275
149.795
150.095
153.605
149.995
159.105
149.305
SCO
SCO
SCO
LS
HS
HS
SCO
HS
LS
b
109.185
109.235
106.225
108.875
108.595
106.74
108.89
103.645
108.61
349 (T1/2↑)
c
173
3(ClO4)
HS
a
S(Oh): the result of CShMs calculation denotes the deviation value of ideal Oh symmetry.53 Terminal N···N donor separation (ρ/Å): the
interatomic separation of the two terminal N donor atoms (as shown in purple).60 Bite angle (θ/deg): the angle between the two bond vectors
from the two terminal N donor atoms to the central FeII ion (as shown in blue). Patulous angle (ϕ/deg) represents the angle between the two line
b
c
vectors connected from the two terminal N donor atoms to the central N donor, respectively. Before annealing and measured at 300 K. After
annealing at 400 K for 2 h and measured at 300 K. The annealing sample of 1(ClO4).
d
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Figure 2. (a) Temperature dependence of χMT for 1(ClO4) upon two repeated scannings and its LIESST effect in the low-temperature region. (b)
Plot of dχMT/dT vs T for 1(ClO4), which indicates the inconstant spin transitions of cooling and heating cycles and the second small spin
transition at 139 K and temperature dependence of χMT for (c) 1(BF4), (d) 1(PF6)·MeCN, (e) 1(BPh4), and (f) 2(ClO4)·MeCN·H2O.
diffraction measurements. After annealing, the PXRD patterns
at 300 K showed changes in some peaks’ shape, but no shift in
the peaks can be observed (Figure S32). However, the single-
crystal data at 300 K before and after annealing show very
small discernible changes in the complex skeleton and its
counteranions (Figure S17). Their structural parameters were
almost identical (Tables 1, S3, and S5). A slight rotation in the
perchlorate anion after annealing took place, but it did not
affect the intermolecular S−O interactions. The increased HS
population under annealing is carried out in a slow and gradual
process so that the structural change may be distinguished.
The other two 1 series complexes bearing different
counteranions, named 1(PF6)·MeCN and 1(BPh4), crystallize
in the monoclinic system with space groups of P21/c for
1(PF6)·MeCN and P21/n for 1(BPh4), respectively. Weak C−
H···F interactions exist in 1(PF6), but no secondary interaction
can be found in 1(BPh4) (Figures S18, S20, and S24−S25). At
153 K, the average Fe−N bond lengths of 2.001(4) Å for
1(PF6)·MeCN and 1.980(4) Å for 1(BPh4) show that the two
compounds approach the LS state at the measured temper-
ature (Table S4).
possess similar coordination models, but the minor differences
in the shape and strength of the ligand field may bring about
distinctive spin-state behaviors.
Because 1(ClO4) and its oxygen analogue (named 3(ClO4)
in this article) are isostructures, their single-crystal X-ray
structures were collected at multiple temperatures.41 There-
fore, their structures were compared in detail to disclose the
structural changes caused by the heteroatom replacement in
the pybox ligand. Some structural parameters of Fe(II) centers
in 1(ClO4) and 3(ClO4) at multiple temperatures are
summarized in Table S3.41 Distortion parameter Σ is defined
as the sum of the deviations from 90° of the 12 cis angles in the
coordination sphere.13,52 Angle α is the dihedral angle between
the last-squares planes of the tridentate ligands, and angle β
refer to the trans-N[pyridine]−Fe−N[pyridine] angle.34 The
distance Fe−Nav is the average value of six Fe−N bond lengths
in one FeN6 coordination sphere. S(Oh) is the CShMs
calculation result, which is the deviation value of ideal Oh
symmetry.53
The high- and low-spin states of 1(ClO4) show discernible
changes in structural parameters such as Σ, Fe−Nav, α, β, and
S(Oh) due to the contraction and deformation of the FeN6
coordination sphere after SCO. In contrast to the almost
complete transition in 1(ClO4) with T1/2↓ and T1/2↑ as 319 and
349 K, only half of the Fe(II) centers in 3(ClO4) undergo
SCO at ∼173 K. The analyses of the structural parameters for
their high- and low-spin species show that 1(ClO4) series
species undergo smaller changes in contrast to its 3(ClO4)
analogue. It indicates that the deformation of the FeN6 sphere
from the HS configuration to the LS configuration is
compensated for by increased flexibility of S-ligand L1.
Normally, the flexible ligand can effectively compensate for
the possible deformation through the release of steric strain.
We consider that, in the S-pybox ligand, the large-radius sulfur
atom increases the flexibility and releases the strain in the
ligand so that less deformation in the FeN6 sphere is needed
between its HS and LS forms.
Structural analyses for the four L2 series structures show the
average Fe−N bond lengths of their FeII species are all below
2.0 Å, indicating that all FeII centers in the solid state are
located in the complete LS state at the measured low
temperatures (Table S4). For example, the structures of
1(PF6)·MeCN and 2(PF6)·MeCN at 153 K are both located
in the complete LS state. Their overlay cation parts are
illustrated in Figure S19. The FeN6 coordination sphere of
2(PF6)·MeCN shows a discernible contraction compared to
that of 1(PF6)·MeCN. It quantitatively shows that the average
terminal N···N donor separations (ρ/Å) are 4.033 Å for
1(PF6)·MeCN and 3.854 Å for 2(PF6)·MeCN, respectively. In
the solid state, the methyl group also produces a larger
distortion of the ligand in 1(PF6)·MeCN. The steric hindrance
by four methyl groups weakens the coordination so that it
generates a weaker ligand field. Overall, 1 and 2 series FeII
complexes in this work and their oxygen-replaced analogues
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Solid-State Magnetic Studies. Temperature-dependent
magnetic measurements for these compounds in the solid state
were conducted to investigate their possible SCO behaviors.
The χMT vs T plots of 1(ClO4) were recorded in the
temperature range of 400−2 K for several cooling/heating
cycles (Figure 2(a)). At the beginning, the χMT value was 1.24
cm3 K mol−1 at 300 K, indicating a considerably incomplete
HS state. Upon cooling, the χMT value decreased to 0.14 cm3
K mol−1 at 105 K, where it reached the complete LS state. The
following heating cycle coincided completely with the cooling
process in the temperature range of 300−2 K. Above 300 K,
the χMT value increased abruptly to 3.69 cm3 K mol−1 at 367 K
and then increased slowly to 3.90 cm3 K mol−1 at 400 K. In the
subsequent cycles, a thermal hysteresis loop appeared in the
temperature range of 280−360 K with a temperature-scan rate
of 3 K min−1. The T1/2 for the cooling cycle (T1/2↓) is 319 K,
and that for the heating cycle (T1/2↑) is 349 K, giving a wide
hysteresis loop of ΔT = 30 K. In addition to the spin transition
around ambient temperature, the dχMT/dT vs T curves also
revealed a second-step transition centered at 139 K (Figure
2(b)). Continuous measurement of the third to fifth cycles
reproduced the loops with the identical shape and width
(Figure S44(a)). The difference in χMT curves between the
first cycle and following cycles may be attributed to the
possible phase transition when staying at higher temperatures.
The new phase possessing hysteresis was stable in the
continuous measurement cycles and returned to its initial
phase at ambient temperature for a long time (Figure S46b).54
Changing the temperature-scan rate from 1 to 5 K·min−1 did
not significantly influence the hysteresis width, indicating that
the bistable states are well maintained (Figure S45).55 The
dilution experiment is a favorable tool for investigating the
relationship between the elastic interaction of FeII centers and
the thermal hysteresis. As expected, Zn analogue 1(ClO4)-Zn
adopts the same space group as for 1(ClO4) so that the diluted
sample 1(ClO4)-Zn/Fe with a Zn/Fe feed of 19:1 was
prepared (Figures S30 and S35). The actual Zn/Fe mole ratio
was determined to be 19.3:1 by inductively coupled plasma-
mass spectrometry (ICP-MS). This diluted sample showed a
similar two-step SCO process whereby the hysteresis loop
disappeared (Figure S48). These results demonstrate that the
hysteresis is mainly caused by the cooperative interactions of
neighboring SCO centers.
limitation (Figure 2(e)). Four L2 series compounds 2(ClO4)·
MeCN·H2O, 2(BF4)·MeCN, 2(PF6)·MeCN, and 2(BPh4) are
within the LS state. Taking 2(ClO4)·MeCN·H2O as an
example, its solvated and desolvated forms were both located
in the LS state at all temperatures (Figure 2(f)).
Solution-State Magnetic Studies. At ambient temper-
ature, four L1 series complexes were dissolved in CD3CN and
1
subjected to H NMR measurement. The spectra showed that
three proton signals in the ligand downshifted to 59.9, 24.9,
and 13.5 ppm but one proton signal upshifted to −8.2 ppm,
which is caused by the strong shielding and deshielding effects
from the paramagnetic FeII ion, suggesting the HS character-
istic of the FeII center at ambient temperature in solution
(Figures S6−S9). Taking 1(ClO4) as an example, its magnetic
susceptibility determination under different temperatures in
solution was carried out using Evans’ 1H NMR method.56,57 In
CD3CN, it appeared to undergo gradual incomplete SCO in
the measured temperature range of 243−313 K (Figure 3). At
Figure 3. Solution-phase temperature dependence of χMT for
1(ClO4). The experimental data were collected between 243 and
313 K. The red line corresponds to the best fit found for each
compound using the regular solution model (eq 1).
283 K, it reached the complete HS state with a χMT value of
3.81 cm3 K mol−1. Upon cooling to 243 K, the χMT value
1
gradually decreased to 2.24 cm3 K mol−1. In the stacked H
Additionally, the light-induced excited spin state trapping
(LIESST) effect was investigated at low temperature. When
532 nm light was applied, the χMT value increased to 0.64 cm3
K mol−1 at 28 K, suggesting that about 16% of LS state FeII
species were excited to the metastable HS state (Figure 2(a)).
Subsequently, the influence of counteranions and substituent
on the spin state of the Fe(II) center in this series complexes
was investigated. 1(BF4) exhibited behavior similar to that of
1(ClO4). After annealing at 400 K, it changed to a new
metastable phase and underwent complete and one-step SCO
with T1/2 = 313 K. However, no hysteresis can be observed
(Figure 2(c)). 1(PF6)·MeCN underwent an abrupt increase in
the χMT value at the end of the first heating cycle due to
solvent loss. After the solvent is completely removed at high
temperature, a complete and gradual SCO took place with T1/2
= 310 K, which was similar to that of 1(BF4) (Figure 2(d)).
1(BPh4) exhibited a gradual and incomplete SCO with no
hysteresis loop in all temperatures. At 400 K, its χMT value
reached only 2.14 cm3 K mol−1, implying the complete HS
state located at a temperature higher than our measurement
NMR spectra, the continuous shift of proton signals
concomitant with the increase in the HS population could
be observed (Figure S50). Due to the limitation of the melting
point of acetonitrile at 228.15 K, the measurement toward the
lower-temperature region could not be performed. Therefore,
the data were modeled as a gradual and complete SCO (see
red line in Figure 3) using the following regular solution model
(eq 1)
χ T
1 + e
(max)
M
χMT =
(ΔH/RT−ΔS/R)
(1)
where χMT(max) is the maximum χMT value that reaches the
complete HS state. In 1(ClO4), 3.9 cm3 K mol−1 was used in
eq 1. The good fitting gave ΔH, ΔS, and the least-squares
values of 44.2 kJ mol−1, 184 J mol−1 K−1, and 0.99, respectively.
The literature values of ΔH and ΔS for solution-phase studies
of SCO-active FeII complexes are in the ranges of ΔH = 4−41
kJ mol−1 and ΔS = 22−146 kJ mol−1 K−1.16,18,44,58 The values
for 1(ClO4) are slightly larger than these. Then we calculated
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that T1/2 in acetonitrile solution is 240 K according to T1/2
=
relatively small, suggesting their minor thermal changes
accompanied by the SCO process. Overall, the changes in
enthalpy and entropy are relatively smaller compared to that
obtained from the fitting of NMR data by the regular solution
model (Table S7) possibly because the SCO of the solid-state
sample in the high-temperature region is still incomplete.
ΔH/ΔS. Compared with the χMT curve in the solid state in
which the annealed sample shows T1/2↓ and T1/2↑ as 319 and
349 K (Figure 2(a)), the spin transition downshifts to low
temperature, which is due to the lack of intermolecular
interaction and the introduced effect of solvent interaction in
the diluted solution.
̈
Mossbauer Measurement. The electronic state of
According to the 1H NMR spectra for four L2 series
complexes, all of them show normal chemical shifts of proton
signals, indicating that their FeII centers maintain the LS state
at ambient temperature in CD3CN solution (Figure S10−S13),
suggesting that L2 provides a stronger ligand field than does L1.
In combination with the aforementioned structural analysis
and solid-state magnetic studies, it further supports the fact
that the steric effect of the methyl substituent overwhelms its
electron-donating contribution, providing a weaker ligand field
of L1 compared to that of L2.
1(ClO4) during the SCO process was further elucidated by
57
̈
Fe Mossbauer spectroscopy. For the pristine sample at 50 K,
only one quadrupole doublet was observed with parameters of
δ (isomer shift) = 0.275 mm s−1 and ΔEQ (quadrupole
splitting) = 1.07 mm s−1, characteristic of the LS FeII species.
Upon warming to 300 K, the quadrupole doublet originating
from the LS FeII species still existed (δ = 0.208 mm s−1 and
ΔEQ = 1.13 mm s−1), but no HS FeII species can be detected
(Figure 5(a)). After the same sample was annealed at 400 K
DSC Study. The SCO behavior of 1(ClO4) was further
corroborated by differential scanning calorimetry (DSC)
measurements for five successive cycles (Figures 4 and S49).
̈
Figure 5. Mossbauer spectra of 1(ClO4) recorded at 50 and 300 K:
(a) the pristine sample and (b) the sample after annealing at 400 K
for 2 h.
for 2 h, the measurement at 300 K showed the emergence of a
new quadrupole doublet with δ = 0.538 mm s−1 and ΔEQ =
2.11 mm s−1 corresponding to the HS FeII species. According
to the peak area ratio, the HS to LS FeII species is 0.41:0.59
(Figure 5(b)). The metastable HS species could gradually relax
to its previous LS state. After staying at ambient temperature
Figure 4. Differential scanning calorimetry profile of 1(ClO4) in
successive three cycles. The scan rate in the first to third cycles is 3 K
min−1.
In the first cycle, it revealed an endothermic peak at 355 K
(ΔH = 6.18 kJ mol−1, ΔS = 17.39 J mol−1 K−1) upon heating
and an exothermic peak at 320 K (ΔH = 1.74 kJ mol−1, ΔS =
5.44 J mol−1 K−1) at a scan rate of 3 K min−1, delineating a
hysteresis loop of ∼35 K. But in the second cycle, the
endothermic peak shifted to 350 K while the exothermic peak
was still located at around 320 K. The main difference is that
the ΔH of the endothermic peak greatly decreased to 1.57 kJ
mol−1. The relative larger ΔH value in the first heating
semicycle indicates the occurrence of an irreversible phase
transition in the first heating process, consistent with the
observed irreversible magnetic change in the first cycle (Figure
2(a)). The consecutive measurements reproduced the
reversible thermal change even when the scan rate was
reduced to 1 K min−1 in the fourth and fifth cycles (Figure
S49). The broadened exothermic peak implied that the
cooperativity of the structural transition in the cooling process
was weaker than that in the heating process. When the scan
rate was reduced from 3 to 1 K min−1, the hysteresis loop
slightly decreased from 32 to 26 K. Compared with its oxygen-
replaced analogue, 3(ClO4), their ΔH and ΔS values are all
for 2 days, the ratio of HS to LS FeII species reduced to
57
̈
0.23:0.77 (Figure S51). Generally, the Fe Mossbauer results
were consistent with the magnetic data and further confirmed
that the new metastable phase with a higher HS fraction was
formed by annealing at 400 K and the unstable HS state would
relax to the LS state slowly.
Variable Temperature UV−Vis Spectra. Solid-state
temperature-dependent UV−vis absorption spectra of 1(ClO4)
were recorded using surface-reflectivity mode (Figure 6(a)).
Upon cooling from 390 to 130 K, two absorption bands
centered at ∼480 and ∼730 nm gradually increased, which
II
1
1
could be ascribed to the A1 → T1 transition of the FeLS
ion.59 In the near-infrared region of 900−1500 nm, a
considerably broad absorption band decreased, accompanied
by temperature lowering. This absorption band originated
5
5
II
from the T2 → E transition of the FeHS ion. The change in
absorption intensity at 730 nm agreed well with the trend in
the χMT vs T curve (Figure 6(b)), implying the synergistic
transformation of the absorption and the spin state during the
SCO process.
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Figure 6. (a) Solid-state temperature-dependent absorption spectra of
1(ClO4) in cooling mode from 390 to 130 K at an interval of 20 K.
(b) Cooling χMT curve (gray) and the change in absorption intensity
(red) at 730 nm.
Figure 8. Definition of three structural parameters: terminal N···N
donor separation (ρ/Å), bite angle (θ/deg), and patulous angle (ϕ/
deg), respectively.
Magnetostructural Correlation Studies. Our current
work reveals that the replacement of oxygen by sulfur in the
pybox ligand creates a unique behavior in the spin transition
which is caused by the increasd flexibility in the ligand due to
the larger radius of the sulfur atom. To obtain more solid
evidence, four key structural parameters were introduced to
investigate the increased flexibility by the sulfur replacement.
They are the root-mean-squared (rms) deviation from the
coordination plane (μrms/Å), terminal N···N donor separation
(ρ/Å), bite angle (θ/deg), and patulous angle (ϕ/deg),
respectively (Figures 7a and 8).
ring are (Figures 7(a) and S37−S40). The μrms value of each
FeII complex cation is calculated from the projection distances
of 20 atoms in the 4 dihydrothiazole/dihydrooxxazole rings to
the coordination plane constructed through the central Fe2+
ion and 3 N atoms. The μrms vs T plots are depicted in Figures
7(b), S41(a), S42(a), and S43(a), and they show that the μrms
values of cation species in 1(ClO4) at both HS and LS
configurations are comparatively larger than that of 3(ClO4)
with an increased average difference of ∼0.056 Å. Furthermore,
the plots of μrms vs T show a better linear correlation for
1(ClO4) (solid red line, R2 = 0.704) than for 3(ClO4) (dashed
red line, R2 = 0.047), suggesting that enhanced flexibility can
lead to less deformation of the ligand during the spin
transition. Due to the greater flexibility of the ligand in
1(ClO4), the structural contraction of the coordination sphere
is compensated for by the release of steric strain in the
dihydrothiazole ring so that less deformation occurs.
The rms deviation parameter, μrms, is employed to
quantitatively describe how far out of the FeN3 plane the
atoms in the five-membered dihydrothiazole/dihydrooxxazole
Similarly, other key structural parameters of the ligand in the
series of structures 1(ClO4) and 3(ClO4) are calculated and
compared as well. Three parametrizations of ligand geometric
structure are illustrated in Figure 8. The terminal N···N donor
separation (ρ/Å) is the interatomic separation of the two
terminal N donor atoms (as shown in purple).60 The bite angle
(θ/deg) denotes the angle between the two bond vectors from
the two terminal N donor atoms to the central FeII ion (as
shown in red). The patulous angle (ϕ/deg) represents the
angle between the two line vectors connected by the two
terminal N donor atoms to the central N donor (as shown in
blue), respectively. These parameters are sensitive to the fine
geometric structure of the FeN6 coordination sphere, so the
degree of deformation undergoing SCO for different
complexes can be quantitively compared. The average values
of three parameters were calculated with respect to the
structure of each cationic species (Table S5). These values at
multiple temperatures were plotted in Figure 9. It can be found
that the three parameters for complete HS configuration are
roughly unchanged at different temperatures. When the species
are changed to the LS configuration, these parameters
experience a discernible variation in value. The differences
between HS and LS configurations for 1(ClO4) are 0.249 Å for
ρ, 7.155° for θ, and 3.883° for ϕ, respectively. The values of
Δρ, Δθ, and Δϕ for 3(ClO4) are 0.321 Å, 9.562°, and 4.971°,
respectively. The smaller changes in 1(ClO4) imply that less
deformation in its FeN6 sphere takes place during SCO, further
supporting the enhanced flexibility in the ligand that originatde
from the sulfur replacement. In addition, the combination of all
data in series 1 and 2 in this work and series 3 in previous
results is illustrated in Figures S41 and S42. It shows that good
Figure 7. (a) Definition of and equation for the μrms value for
deviation from coordination planarity, which was measured by the
root mean square of the distances (μ, Å) of atoms from five-
membered ring to the mean plane constructed by the central FeII ion
and three N atoms (shown in sky blue). (b) μrms vs T plots for species
in 1(ClO4) (solid triangle) and 3(ClO4) (hollow triangle) at multiple
temperatures. Plots of the FeII center at different spin states are
distinguished by color: HS, black; SCO, blue; and LS, pink.
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possesses robust thermal hysteresis with a width of 30 K.
The temperature-dependent cooperative weak interactions
within the crystal lattice were found to play an important
role in the thermal hysteresis and phase transition during the
first heating process. Structural phase changes concomitant
with the increase in the HS fraction in 1(ClO4) were
discovered, as further confirmed by single-crystal X-ray
̈
diffraction, PXRD, DSC, Mossbauer spectra, and absorption
spectra in detail. Similar to our reported oxygen pybox systems,
the SCO properties in this series are sensitive to some
structural feature such as counteranions, substituents, and
solvents. Larger and steric counters tend to move T1/2 to
higher temperature. The removal of solvent in 1(PF6)·MeCN
contributes to complete SCO and increases T1/2. L1 possess a
weaker ligand field strength mainly due to the steric effect of
four methyl substituents. So L2 series FeII compounds are all
located in LS configuration at measured temperature region
whereas all L1 complexes are SCO-active between 2 and 400 K.
This work not only enriches the SCO family but also
demonstrates a new candidate of optimizing SCO properties
through fine-tuning of ligand structure. Due to the easy
modification in synthesis, these SCO complexes will be served
as a suitable platform to incorporate other functional units.
Through this strategy, fluorescence, redox, conductivity, and
other physical properties can be combined and thus applied to
multifunctional molecular magnets. We anticipate the obser-
vation of the possible synergistic effect between interesting
physical properties and SCO in our designed molecules.
Further studies are ongoing and will be reported in the near
future.
Figure 9. ρ vs T plots (a, b), θ vs T plots (c, d), and ϕ vs T plots (e, f)
of the species in 1(ClO4) and 3(ClO4) at multiple temperatures. The
red lines are produced by linear fitting of HS plots at different
temperature or as a constant if only one HS plot is measured, which
benefits the estimation of the possible values of the HS configuration
at a specific temperature.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03506.
correlation can be obtained from the unique series bearing the
same ligand. The collection of four structural parameters for
series 1, 2, and 3 compounds was plotted at different spin
states as well. It was found that the terminal N···N donor
separation (ρ/Å), bite angle (θ/deg), and patulous angle (ϕ/
deg) were well correlated with their spin states (Figure S43).
The above structural analyses confirm our initial goal of
increasing flexibility and releasing the steric strain in the ligand
by sulfur replacement. As expected, after the replacement of an
oxygen atom by a sulfur atom, the SCO temperature move to a
high-temperature region and the spin transition takes place
completely. It is attributed to the increased ligand field
strength of the thio-pybox ligand compared to that of their
pybox analogue.
Synthesis, characterization, and NMR spectra of
intermediates, ligands and complexes, X-ray single-
crystal structures, and PXRD patterns; structural analysis
of complexes; and temperature-dependent paramagnetic
1H NMR spectra and supplementary magnetic measure-
ments (PDF)
Accession Codes
CCDC 1969316−1969326 contain the supplementary crys-
tallographic data for this article. 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.
3. CONCLUSIONS
A new type of thio-pybox tridentate nitrogen ligand was
designed and synthesized. Compared with our reported
archetype pybox analogue, this sulfur-replaced ligand can
provide a relatively stronger ligand field due to its enhanced
flexibility that releases steric strain, which is evidenced by the
comparison and analyses of some structural parameters
including the terminal N···N donor separation, bite angle,
patulous angle, and root-mean-squared deviation. Therefore,
the spin-transition temperatures of thio-pybox-type SCO
complexes shift to the ambient temperature region compared
with that of their pybox analogues. Solvent-free compound
1(ClO4) exhibits SCO in solution and the solid state which
takes place around ambient temperature. In addition, it
AUTHOR INFORMATION
■
Corresponding Authors
Yuan-Yuan Zhu − School of Chemistry and Chemical
Engineering, Hefei University of Technology and Anhui Key
Laboratory of Advanced Catalytic Materials and Reaction
Engineering, Hefei 230009, China; State Key Laboratory of
Fine Chemicals, Dalian University of Technology, Dalian
116024, China; orcid.org/0000-0002-3142-0396;
Email: yyzhu@hfut.edu.cn
Tao Liu − State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, China; orcid.org/
0000-0003-2891-603X; Email: liutao@dlut.edu.cn
H
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Authors
Yao Pan − School of Chemistry and Chemical Engineering, Hefei
University of Technology and Anhui Key Laboratory of
Advanced Catalytic Materials and Reaction Engineering, Hefei
230009, China
Yin-Shan Meng − State Key Laboratory of Fine Chemicals,
Dalian University of Technology, Dalian 116024, China
Qiang Liu − State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, China
Wan-Qing Gao − School of Chemistry and Chemical
Engineering, Hefei University of Technology and Anhui Key
Laboratory of Advanced Catalytic Materials and Reaction
Engineering, Hefei 230009, China
Chun-Hua Liu − School of Chemistry and Chemical
Engineering, Hefei University of Technology and Anhui Key
Laboratory of Advanced Catalytic Materials and Reaction
Engineering, Hefei 230009, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03506
Author Contributions
§Y.P. and Y.-S.M. contributed equally.
Funding
(17) Steinert, M.; Schneider, B.; Dechert, S.; Demeshko, S.; Meyer,
F. Spin-State Versatility in a Series of Fe-4 2 × 2 Grid Complexes:
Effects of Counteranions, Lattice Solvent, and Intramolecular
Cooperativity. Inorg. Chem. 2016, 55 (5), 2363−2373.
This work is partially supported by the National Natural
Science Foundation of China (nos. 21771049, 21421005,
91422302, 21429201, 21871039, and 21801037), the State
Key Laboratory of Fine Chemicals at Dalian University of
Technology (KF 1607), and the Fundamental Research Funds
for the Central Universities of China (nos. PA2019GDPK0055
and PA2020GDJQ0028).
́
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Brooker, S. A Simple Method of Predicting Spin State in Solution. J.
Am. Chem. Soc. 2017, 139 (50), 18392−18396.
(19) Yergeshbayeva, S.; Hrudka, J. J.; Lengyel, J.; Erkasov, R.; Stoian,
S. A.; Dragulescu-Andrasi, A.; Shatruk, M. Heteroleptic Fe(II)
Complexes with N4S2 Coordination as a Platform for Designing
Spin-Crossover Materials. Inorg. Chem. 2017, 56 (18), 11096−11103.
(20) Shiga, T.; Sato, Y.; Tachibana, M.; Sato, H.; Matsumoto, T.;
Sagayama, H.; Kumai, R.; Murakami, Y.; Newton, G. N.; Oshio, H.
Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for
Tunable Switching and Hybrid Electrode Fabrication. Inorg. Chem.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Dr. Liang Zhao and Mr. Hai-Lang Zhu (Dalian
University of Technology) for experimental assistance, Ms. Lin
Wang (Beijing Institute of Technology) for help with X-ray
single-crystal measurements, and Prof. Wei-Xiong Zhang (Sun
Yat-Sen University). Y-Y.Z. thanks Prof. Sally Brooker and Dr.
Raf Kulmaczewski (University of Otago) for constructive
discussions.
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