Room-temperature switching of magnetic hysteresis by reversible single-crystal-to-single-crystal solvent exchange in imidazole-inspired Fe(II) complexes

Article abstract of DOI:10.1039/c6dt01777k

The recent upsurge in molecular magnetism reflects its application in the areas of sensors and molecular switches. Thermal hysteresis is crucial to the molecular bistability and information storage, a wide hysteresis near room temperature is expected to be of practical sense for the molecular compound. In this work, spin crossover iron(ii) complexes [Fe(Liq)₂](BF₄)₂·(CH₃CH₂)₂O (1-Et₂O) and [Fe(Liq)₂](BF₄)₂·3H₂O (1-3H₂O) were prepared and structurally and magnetically analysed. The single-crystal-to-single-crystal (SCSC) solvation transformation and the influence on the crystal structures and magnetic hysteresis were investigated in an etherification-hydration cycle. At room temperature, X-ray diffraction experiments indicated a transformation from one crystal (1-Et₂O, P2₁2₁2) to another crystal (1-3H₂O, P2₁2₁2₁) upon humidity exposure and reversible recovery of its crystallinity upon exposure to ether vapor. The etherified phase 1-Et₂O exhibits room temperature spin crossover (T_1/2 = 305 K) but negligible thermal hysteresis, however the hydrated phase 1-3H₂O exhibits the apparent hysteresis loop (T_1/2↑ = 346 K, T_1/2↓ = 326 K) which expands to room temperature. This effect is associated with the change of intermolecular cooperativity in the etherification-hydration recyclability.

Full text of DOI:10.1039/c6dt01777k

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DOI: 10.1039/C6DT01777K
Journal Name
ARTICLE
Room-Temperature Switching of Magnetic Hysteresis by
Reversible Single-Crystal-to-Single-Crystal Solvent Exchange in
Imidazole-Inspired Fe(II) Complex
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
b
DOI: 10.1039/x0xx00000x
Wei Huang, Fuxing Shen, Ming Zhang, Dayu Wu, * Feifei Pan, and Osamu Sato
www.rsc.org/
The recent upsurge in molecular magnetism reflects their application in the areas of sensors and molecular switches. The
thermal hysteresis is crucial to the molecular bistability and information storage, wide hysteresis near room temperature
is expected to be of practical sense for the molecular compound. In this work, the spin crossover iron(II) complexes
[
2 4 2 3 2 2 2 2 4 2 2 2
Fe(Liq) ](BF ) ·(CH CH ) O (1-Et O) and [Fe(Liq) ](BF ) ·3H O (1-3H O) were prepared, structurally and magnetically
analysed. The single-crystal-to-single-crystal (SCSC) solvation transformation and the influence on the crystal structures
and magnetic hysteresis were investigated in etherification-hydration cycle. At room temperature, X-ray diffraction
experiments indicated a transformation from one crystal (1-Et
humidity exposure and reversible recovery of its crystallinity upon exposure to ether vapor. The etherifized phase 1-Et
exhibits room temperature spin crossover (T1/2 = 305 K) but negligible thermal hysteresis, however, the hydrated phase 1-
O exhibit the apparent hysteresis loop (T1/2↑ = 346K, T1/2↓ = 326 K) which expands to room temperature. This effect is
associated with the change of intermolecular cooperativity in the etherification-hydration recyclability.
2 1 1 2 1 1 1
O, P2 2 2) to another crystal (1-3H O, P2 2 2 ) upon
2
O
3H
2
ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (tbta)
Introduction
which displays spin crossover (SCO) with a spin transition
temperature T1/2 near room temperature. Gao et al reported
9
The spin-crossover phenomenon (SCO) is a fascinating field
that corresponds to the switching of the electronic
configuration of a transition metal compound between high
spin (HS) and low spin (LS) states as a result of an external
perturbation, such as temperature, pressure, light irradiation,
magnetic and electric fields. Considerable efforts have been
devoted to improving the performance of SCO materials in
order to realize their applications as molecular switches, nano-
sensors, data storage and display devices. However, most
SCO materials reported so far experience spin transitions (STs)
at low temperatures rather than room temperature due to the
thermal equilibrium between high spin (HS) and low spin (LS)
the solvent-sensitive spin crossover near room temperature by
10
employing the N O -type coordination donor. Hence, room
4
2
temperature (around 300 K) SCO or ST compounds have been
rarely reported so far due to the difficulty of the selecting the
11
1
suitable ligand field strength, thermal instability and etc.
Especially, the compound with a wide thermal hysteresis at RT
12
is important for practical applications. Despite of many
attempts made to prepare one compound in previous studies,
the production of spin-crossover compounds with such a wide
2
,3
13
hysteresis at room temperature is still rare. It is still a great
challenge to design systems that can be tuned on purpose
toward around RT spin transition, especially with wide
hysteresis.
4
state with moderate field ligand. Fe(II) triazole complexes and
Hoffman-type clathrates exhibit room-temperature (RT)
hysteresis which is crucial to the application of information
Another hot topic in the SCO field is to modulate the spin-state
through host-guest sensing event, where effects of the guest
molecules are mediated by van der Waals interactions, hydrogen
5,6
storage. The nitrogen-rich chelates are known to act as
intermediate-field ligands might open an access to obtain RT-
14
7,8
bonds, π-π interactions, or structural changes.
Recent
operable switchable materials.
coworkers reported the solvent-free Fe(II) complex based on
Grimme, Sarkar and
developments in this field have aimed at the incorporation of spin-
crossover sites in 2D or 3D frameworks, which makes them
1
5
interesting candidates for potential sensing applications. In most
cases, exposure to guest solvent molecules could induce or change
a.
Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School
1
6
of Petrochemical Engineering, Changzhou University, Changzhou 213164,
People’s Republic of China.E-mail: wudy@cczu.edu.cn
Institute for Materials Chemistry and Engineering, Kyushu University, 744
the SCO properties of the host. The systematic modification of the
SCO properties, especially of the hysteresis near room temperature
in the fashion of reversible single-crystal-to-single-crystal (SCSC)
b.
Motooka, Nishi-ku, Fukuoka, 819-0395, Japan.
17
†
Footnotes relating to the title and/or authors should appear here.
guest exchange, has been rarely investigated up to now.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Here, we report the magnetostructural study on the solvate Fe(II)
complex of formula [Fe(Liq) ](BF ) ·(CH CH ) O (denoted as 1-Et O)
2
4 2
3
2 2
2
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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ARTICLE
Liq
Journal Name
(
=
3-((1H-imidazol-4-yl)methyleneamino)-2,3-dihydro-2-(1H- Magnetic Measurements
View Article Online
DOI: 10.1039/C6DT01777K
18
imidazol-4-yl)quinazolin-4(1H)-one, abbreviated as Liq, hereafter) Magnetic susceptibility measurements were performed using a
which exhibits a spin crossover with T1/2 around room temperature. Quantum Design MPMS XL-5 SQUID magnetometer. The
The solvent-exchanged species, [Fe(Liq) ](BF ) ·3H O (denoted as 1- magnetic susceptibility measurements were taken on a freshly
2
4 2
2
3
H2O), was investigated to establish the hydration induced prepared crystal sample wrapped in a polyethylene membrane
apparent hysteresis loops with ∆T = 20 K, which can be reversibly to avoid any loss of solvent for magnetic measurements while
switched on/off by the etherification-hydration recyclability.
lowering the temperature from 300 to 5 K at the sweeping rate
−
1
of 1 K min under an applied magnetic field of 2500 Oe.
Corrections for diamagnetism were applied using Pascal’s
constants.
Table 1. Structure Refinement Parameters for Complexes 1-Et
2
2
O and 1-3H O.
1-Et
2
O
2
1-3H O
T/K
298(2)
123(2)
298(2)
123(2)
C
O
34
36
H B
2
F
8
FeN14
C
O
34
36
H B
2
F
8
FeN14
C
30
H
FeN14
32
B
2
30 32 2
C H B
F FeN14O
8 5
Formula
3
3
F
8
O
5
CCDC
FW
1430792
459.12
1430791
918.24
1430794
898.16
1430793
898.16
Crystal
system
Orthorhombi
c
Orthorhombi
c
Orthorhombic
P2
.022(15)
Orthorhombic
P2
Space group
2
1 1
2
2
1 1
2
1
P2 2
1 1
2
1
1 1 1
P2 2 2
Flack
parameter
0
0.015(9)
0.03(4)
-0.003(13)
II
a (Å)
b (Å)
c (Å)
19.425(6)
9.389(3)
10.942(4)
1995.7(11)
2
9.2819(7)
19.2352(14)
21.4352(15)
3827.0(5)
4
9.4524(19)
19.536(4)
21.598(4)
3988.5(14)
4
9.3220(19)
19.465(4)
21.148(4)
3837.4(13)
4
Scheme 1. Synthetic route to the new ligand Liq and its Fe complex reported in this
Work
3
V (Å )
Experimental
Z
Materials and General Procedures
ρcalc(g/cm)
1.528
1.594
1.496
1.555
-
1
µ (mm )
0.471
0.491
0.473
0.492
All the reagents employed were commercially available and
used without further purification. Methanol and acetonitrile
were dried using standard procedures. Perdeuterated
R
R
int
0.0603
0.0549
0.2422
0.0773
a
1
,
0.0840,
0.2182
0.1024,
0.2389
0.0666,
0.1676
0.0900,
0.1852
0.0850,
0.1979
0.2754,
0.2313
0.0707,
0.1677
0.0909,
0.1853
wR
2
[I>2σ(I)]
dimethyl sulfoxide (DMSO-d ) was commercially obtained from
6
1
R ,
Alfa Aesar Co. Ltd.
Elemental analyses (C, H, and N) were conducted with a GOOF
wR
2
[all data]
b
1.031
1.157
0.603
1.020
Perkin-Elmer 2400 analyzer. Micro-IR spectroscopy studies
a
2
2 2
2 2 1/2
R
1
=Σ||F
0
| − |F
c
||/Σ|F
0
|; wR
2
= [Σ[w(F
0
− F
c
0
) ]/Σ[w(F ) ]] .
were
performed
on
a
Nicolet
Magna-IR
750
-
1
b
2
2 2
1/2
spectrophotometer in the 4000-400 cm region (w, weak; b,
Goodness-of-fit = [Σ[w(F
0
c
− F ) ]/(Nobs − Nparams)] , based on the data I>2σ(I).
1
broad; m, medium; s, strong) by KBr disc. H NMR spectra were
Synthesis of compound 2-aminobenzhydrazide. It was
obtained from a solution in DMSO-d6 using a Bruker-500
spectrometer. (s, singlet; d, doublet; t, triplet; m, multiplet; dd,
double doublet). DSC measurement was performed on a TA
DSC Q2000 instrument under nitrogen atmosphere at a scan
synthesized according to the reported procedure with minor
19
modification.
The combined solution of ethyl 2-
aminobenzote (1.65 g, 10 mmol) and hydrazine hydrate (6.5
mL, 100 mmol) was refluxed for 6 hours. After the reaction
was finished, the solution was left standing over night and 50
mL petroleum ether was added. The crude product was
recrystallized from hot ethanol. Yields: 85%.
Synthesis of ligand Liq. A mixture methanol solution of 1H-
Imidazole-4-carbaldehyde (0.9609 g, 10.0 mmol) and 2-
aminobenzhydrazide (0.7559 g, 5.0 mmol) in presence of 0.2
mL glacial acetic acid was magnetically stirred for 4 hours at
the boiling temperature, after cooling to room temperature, a
pale yellow solid was obtained by filtration under reduced
−
1
rate of 5 K min in both heating and cooling modes. TG
analyses were recorded on NETZSCH TG209F3
a
thermoanalyzer by being filled into alumina crucibles under N2
atmosphere within the temperature range of 300−1000 K at a
−
1
heating rate of 10 K min . Cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) experiments were
undertaken in acetonitrile (0.1 M [Bu N](PF )) at 293 ± 2 K
4
6
using
a
CHI620E computer-controlled electrochemical
workstation and a standard three-electrode cell. Glassy carbon
microelectrode was used as the working electrode, whereas a
platinum mesh and Ag/AgCl electrode were used as the
counter and reference electrodes, respectively. All potentials
given in this paper are referred to the ferrocene/ferrocenium
pressure. The crude product was washed with cold ethanol
1
and dried in vacuo over P O . Yield: 91%. H NMR (500 MHz,
2
5
DMSO-d ) δ 12.42(d, J=13.1, 1H), 11.92 (s, 1H), 8.72(s, 1H),
6
0/+
7
.74(s, 1H), 7.69(dd, 1H), 7.54(s, 2H), 7.24(m, 1H), 6.82(s, 1H),
(
[FeCp2] ) reference couple under the same conditions.
2
| J. Name., 2012, 00, 1-3
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ARTICLE
1
3
6
(
.77(d, J=7.5, 1H), 6.76(t, 1H), 6.23(s, 1H) ppm. C NMR
View Article Online
DOI: 10.1039/C6DT01777K
500MHz, DMSO-d ): δ 161.45, 160.25, 147.58, 146.00, 139.53, Results and discussion
6
1
1
6
8
1
1
36.75, 134.89, 133.04, 127.51, 117.89, 116.70, 115.42, Recrystallization of the pinkish solid obtained from 1:2
-1
14.53, 113.57, 67.84 ppm. IR(KBr pallet cm ): 517(w), 542(w), reactions of Fe(BF ) ·6H O with Liq ligand in MeCN by vapor
07 (w), 620 (m), 700 (w), 720 (w), 748 (m), 782 (w), 822(w), diffusion of ether gave rise to dark-red single crystals of
52(w), 933 (w), 955 (w), 987 (m), 1083(w), 1147(w), 1255(m), [Fe(Liq) ](BF ) ·(CH CH ) O -Et O). The crystal -Et O
296(w), 1325(w), 1345(w), 1405(m), 1436(m), 1516(w), crystallized in orthorhombic chiral space group P2 2 2 at 298 K.
538(w), 1585(m), 1624(s), 2861(w), 3195(s). Anal. Calcd for (Table 1) The unit cell consists of half of the symmetry-related
4
2
2
(
1
1
2
4 2
3
2 2
2
2
1
1
2+
−
C H N O: H, 4.26; C, 58.62; N, 31.91. Found: H, 4.19; C, 58.71; [Fe(Liq)2] cations and one BF counteranion (Figure. 1). Each
15
13
7
4
N, 31.83.
mer-tridentate Liq ligand coordinates to Fe(II) center
Preparation of compound [Fe(Liq) ](BF ) ·(CH CH ) O (1- equatorially via two imidazol donors and axially via the imine N
2
4 2
3
2 2
Et O): A solution of Fe(BF ) · 6H O (0.05 mmol, 16.85 mg) in atoms completing the coordination sphere. The value of trans
2
4 2
2
MeCN (3 mL) was added to a solution of Liq (0.1 mmol, 30.71 N-(imidazol)−Fe−N(imidazol) angle [166.82°] together with
mg) in MeCN (5 mL). The resulting dark-red mixture was another trans Nazo−Fe−Nazo angles [172.45°] reflect the steric
stirred under nitrogen atmosphere for 1 hour. The suspension constraints imposed by the rigidity of the tridentate imidazol
2
2
was then filtered and the filtrate was layered below diethyl ligand. The distortion parameter Σ equals 58.9(3)°. The axial
ether in a tiny tube. Dark red block-shaped single crystals bond with the Fe−Nazomethine lengths of 2.064(5) Å is
suitable for X-Ray diffraction analysis were obtained after comparable to the equatorial Fe−Nimidazol averaged 2.050(9)
-1
several days. Yield: 51% (based on Fe). IR(KBr pallet cm ): and 2.078(5) Å (Table 2). It is worthy to note the bonding
5
1
1
40(w), 616(w), 668(w), 751(m), 840.30(w), 993 (m), 1089 (s), parameter in 1-Et O, while smaller than typical Fe-N bond of
2
II
159 (m), 1236(m), 1267(w), 1338(w), 1395(m), 1446(w), 2.15 Å for high-spin Fe , are nevertheless larger than those
II
510(m), 1606(s), 1644(w), 2927(w), 3125(m), 3412(s). Anal. expected for low-spin Fe centers, indicative of the
Calcd (%) for [Fe(C H N O) ](BF ) ·(CH CH ) O: H, 3.95; C, intermediate spin state between HS and LS.
15
13
7
2
4 2
3
2 2
4
4.48; N, 21.36. Found(%): H, 3.88; C, 44.25; N, 21.03.
Preparation of compound [Fe(Liq) ](BF ) ·3H O (1-3H O): The
2
4 2
2
2
sample
1
-3H O was obtained by exposure of
1
-Et O in
2
2
moisture under ambient condition for 2 weeks, in such case,
the saturated amount of water in lattice, namely, three water
-
1
molecules per host were at most adsorbed. IR(KBr pallet cm ):
33(w), 616(w), 689(w), 754(m), 785(w), 986(w), 1084(s), 1124
m), 1154(m), 1232(w), 1274(w), 1342(m), 1388(w), 1506(m),
611(s), 1652(m), 2927(w), 3122 (m), 3419(s). Anal. Calcd (%)
for [Fe(C H N O) ](BF ) ·3H O: H, 3.59; C, 40.12; N, 21.83.
5
(
1
Figure 2. Reversible SCSC transformations between
induced by guest exchange.
2 2
1-Et O, 1-3H O (bottom)
1
5
13
7
2
4 2
2
Found(%): H, 3.53; C, 40.72; N, 21.39. Since the single
crystallinity was retained during the solvent exchange process
in air, we were able to obtain the crystals suitable for X-ray
The unit cell measurements on an individual single crystal of 1-
Et O in the cooling mode from 298 K to 120 K were undertaken
2
to observe the thermal-induced structure phase change. When
the temperature was lowered to 120 K, at which the spin
diffraction analysis and refine the structure data of
successfully.
1-3H O
2
crossover is complete as indicated later, compound
1-Et O
2
The crystal 1'- Et O can be recovered by exposure of 1-3H O in
2
2
crystallizes in another orthorhombic space group P2 2 2 with
1
1 1
ether for several hours, which is evidenced by the element
inverse a and b cell lengths and a doubling of the c axis and cell
volume (Table 1). The asymmetric unit contains one entire
molecule due to loss of the inversion symmetric element,
revealing that the crystallographic phase change is possibly
associated with spin-crossover. The variation of Fe–N and
octahedral distortion parameter Σ are effective indicators for
the occurrence of SCO (Table 2). Upon cooling to 120 K, the
average axial/equatorial Fe–N bonds become obviously short
and reach 1.969(6)/1.982(5) Å, characteristic of the low-spin
analysis (1'- Et O, found(%): H, 3.54; C, 44.12; N, 20.54) and
magnetic susceptibility comparison with the data of as-
synthesized 1-Et O.
2
2
o
Fe(II) site. In addition, ΣFe decreases to 44.2(2) , in accordance
with smaller Σ in the LS state and larger Σ in the HS state. The
abundant hydrogen bonding interactions are involved
between counteranions and imidazole nitrogen atoms (ESI,
Figure S3, S5). The presence of N−H···F(anion) interactions in 1-
2+
Et O will assist in the stabilization of a 2-dimensional
2
Figure 1. Crystal structure of the cation [Fe(Liq)
2
]
2
in 1-Et O at 298 K with the selected
atom label. The anions and solvent molecules are omitted for clarity.
supramolecular network along the [ab] plane. The neighbor
supramolecular layer is further linked through the interlayer
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ARTICLE
Journal Name
hydrogen bond association along the crystallographic c axis. In water molecules favors the stabilization of low-spin state. Two
View Article Online
DOI: 10.1039/C6DT01777K
addition, the O atom of solvent molecule (Et O) strongly water molecules, i.e, O2w and O4w, link the neighboring Fe(II)
2
interacts with nitrogen atom on dihydroquinazoline group cations into infinite 1-D chain along the crystallographic c axis
through the N−H…O hydrogen bonding with D…A distance of through the dihydroquinazoline nitrogen and carboxylate
3
.022/3.150 Å, which will aid to stabilize the ether molecule at oxygen in N…O…O H-bonding series with donor…accept
high temperature (Figure 2). The presence of off-center distance of 3.154 (3.149) Å for N…O and 3.706 (3.802) Å for
parallel π−π stacking interactions along the a-axis between the O…O in complex -3H O. It is interesting that the hydrated
dihydroquinazoline rings of two Liq ligands from two different crystal -3H O is able to re-exchange guest molecules and
1
2
1
2
2+
[
Fe(Liq)2] units with a N3···N12 distance of 3.215 Å, as well as recover to the etherified form, 1-Et O, by exposure to ambient
2
3
.707 Å between C5 and C26, gave rise to the short Fe···Fe ether vapor for several hours, indicative of reversible SCSC
distance of 9.282 Å at 120 K.
transformations along with guest exchange between ether and
water molecules.
Table 2. Selected Interatomic Distances(Å) and Angles(deg) for 1-Et
2
2
O and 1-3H O.
2.5
0.3
hν
2
1-Et O
1-3H
123
2
O
0.2
2.0
T(K)
123
298
298
0.1
1
.983(5)
.998(5)
2.052(6)
2.052(6)
2.052(6)
2.078(5)
2.078(5)
2.078(5)
2.064(5)
2.064(5)
2.064(5)
58.9(3)
1.987(5)
1.998(5)
1.992(5)
1.973(5)
1.982(5)
1.978(0)
1.987(5)
1.998(5)
1.993(0)
42.0 (3)
1.973(4)
1.997(4)
1.985(4)
1.977(4)
2.012(4)
1.995(4)
1.962(5)
2.038(4)
2.000(5)
48.3(17)
1
.5
0.0
Fe-Nim1
0
20
40
T / K
60
80
100
1
1.0
Fe-Nim, avg
Fe-Nim2
1.991(0)
1
.981(5)
.983(5)
as-synthesized 1-Et O
χ
0.5
0.0
2
1
st cycle
1
2nd cycle
Fe-Nim, avg
Fe-Nazom
Fe-Nazom, avg
1.982(5)
1
.967(5)
.972(5)
2
00
250
300
350
400
1
T / K
1.969(6)
44.2(2)
Figure 3. Variable-temperature magnetic susceptibility data of as
Et and photo-induced magnetization (inset).
-
synthesized 1-
a
2
O
∑
a
Thermogravimetry shows that desolvation is difficult to occur
The distortion parameter Σ is defined as the sum of the deviation from 90° of the
2 cis angles of the FeN octahedron.
o
1
6
upon heating
1-Et O. The mass loss with heating to ca. 108 C
2
accounts for the loss of all ether molecules (experimental:
To investigate structures associated with different SCO phases 8.4%, calculated 8.1 %). The de-solvated material is stable to
o
identified by the magnetic studies (see below) before and after 240 C, beyond which temperature it decomposes (ESI, Figure
guest-induced SCSC transformation, single-crystal X-ray S7). Magnetic susceptibility measurements have been
diffraction data were collected in situ on one crystal of
1
-Et O performed on
a
freshly as-synthesized sample of
2
in a successive exposure to ambient air/ether cycle. Upon [Fe(Liq) ](BF ) ·(CH CH ) O (
1
-Et O). The etherified form -Et O
1
2
4 2
3
2 2
2
2
standing in air for two weeks, the ether guest molecules were were rapidly cooled to 100 K in SQUID apparatus and then
removed and surprisingly SCSC transformation occurred from measured upon at least two heating/cooling cycles. It is worth
1
-Et O to its hydration form, denoted as 1-3H O, as shown in noting that no significant thermal hysteresis or irreversibility
2
2
Figure 2. At room temperature, the relatively high R and wR
was observed when the temperature is cycled below 350 K
1
2
values may be caused by the heavily disordered solvents but despite partial ether molecules are removed from the solid in
the diffraction data is sufficient to confirm connectivity. the SQUID magnetometer based on the TGA data. (The data up
Fortunately, the high-quality single-crystal X-ray diffraction to 400 K, see supporting information, Fig S12) At lower
data were obtained when lowering the temperature to 120 K. temperature, the molecules correspond to low spin species,
3
−1
1
-3H O crystallizes in a chiral space group P2 2 2 , the average however, the χ T value abruptly increased to 1.04 cm K mol
2
1
1
1
M
Fe-N bond lengths shrink from 2.062(3) Å in
Å in -3H O, and Σ value also changed from 58.9(3) to 48.3(17)° which is associated with phase transition. (Figure. 3) Upon
for Fe1, suggesting that Fe ion is in a full LS state. When this further warming, χ T values gently increase to 2.28 cm K
1-Et O to 1.993(3) at 300 K and showed discontinuity in both cycles at ca. 275 K,
2
1
2
II
3
M
−
1
sample was cooled to 120 K, unlike compound
1-Et O, no mol at 350 K and SCO is still incomplete. In the subsequent
2
crystallographic phase change was observed for
average Fe-N bond lengths and Σ values for Fe in
1
-3H O. The cooling mode, χ T vs T curve is perfectly reproducible and no
2
M
II
1
-3H O thermal hysteresis is observed. The second heating/cooling
2
(Table 2) varied only slightly from the corresponding values at cycle produced a small thermal hysteresis loop, which is
room temperature, thus suggesting that the exchange of guest possibly related with the tiny solvent loss. The continuous
4
| J. Name., 2012, 00, 1-3
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Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Journal Name
heating-cooling cycles up to 400 K gave rise to the different state of the Fe complex
ARTICLE
1
-3H O was further probed by
2
View Article Online
DOI: 10.1039/C6DT01777K
magnetic hysteresis loops, which was perhaps associated with Mössbauer spectroscopy (ESI, Figure S9), which shows an
−
1
the solvent-free phase in the SQUID holder under He isomer shift of 0.30 mm s and quadrupole splitting of 0.235
-
1
atmosphere.(Figure S8, ESI) Thus, the etherfied sample mm s at 300 K, indicating the exclusive existence of a LS Fe(II)
undergoes a reversible SCO behavior with T value of ca. 305 center at that temperature, a result which is consistent with
1/2
23, 24
28
K.
the information obtained from X-ray diffraction analysis.
Furthermore, magnetic data comparison revealed the sample
-3H O can be reversibly transformed to etherifized sample '-
Et O. (ESI, Figure S10). On the basis of the single-crystal
1
1
2
2
structures and magnetic data of 1-Et O and 1-3H O, the single-
2 2
crystal-to-single-crystal transformation gives rise to the
hysteresis “off-on” switching behavior due to the exchange of
lattice solvent molecules.
0
.30
3
.5
.0
.5
.0
.5
.0
.5
.0
0.25
3
hν
0.20
2
0.15
0.10
2
0.05
0
10
20
30
T / K
40
50
60
1
1
χ
0
2
nd cycle
1st cycle
0
200
250
300
350
400
T / K
Figure 5. Variable-temperature magnetic susceptibility data of
2
1-3H O, inset
shows the photo-induced magnetization. The arrows indicate the heating or
cooling mode in the measurements.
Figure 4. Variable temperature powder XRD patterns of the fresh ether sample
-Et O (a) and -3H O (b). The top line of (a) corresponds to the Et O-reabsorbed
sample 1'-Et O.
1
2
1
2
2
Photo-induced spin transition was further investigated on
1-
2
Et O and -3H O, respectively, to differentiate the light
1
2
2
induced excited spin state trapping (LIESST) effect.(inset of
Figure 3 and Figure 5) After illumination with 532 nm light for
The hydrated sample can be obtained by exposing 1-Et O in air
2
for two weeks to allow full exchange of guest molecules.
Element analysis is agreement with the single crystal X-ray
diffraction data, which revealed three water molecules can be
3
-1
3
0 min at 5 K, the χ T product increased to 0.32 cm K mol
M
3
-1
and to 0.24 cm K mol to achieve almost saturation value for
-Et O and -3H O, respectively, featuring spin transition from
26
1
1
adsorbed per host at most. After
1
-3H O sample was
2
2
2
diamagnetic low spin to metastable high spin species. The
efficiency of transition is far from being quantitative for -Et O
10%) and hydrate analogue, -3H O (8%). The low transition
exposed to ether vapor and allowed to reabsorb Et O in the
lattice, the unit cell recovered to the one corresponding to the
2
1
2
(
1
parent sample
1
-Et O. The data of
1
-3H O and
1
-Et O were
2
2
2
2
efficiency is frequently associated with the short lifetime of
LIESST state, in agreement with the original observations by
also checked by the powder XRD in Figure 4 to establish the
structural integrity upon heating the samples despite the
potential loss of partial solvent molecules.
29
Decurtins et al. The relaxation temperature for
is obviously higher than that for -3H O (40 K), which reveals
that the etherifized phase -Et O should more stabilize the
1-Et O at 70 K
2
1
The magnetic comparison revealed
1
-3H O SCO property is
2
2
1
significantly different from that of
1
-Et O, the magnetic
2
2
excited high spin state than hydration phase, consistent with
the magnetic data at high temperature.
susceptibility data revealed the diamagnetic character at room
temperature (Figure. 5). The χ T value increases gradually to
M
3
−1
3
−1
The solvent effect on the spin crossover property that may
trigger the intermolecular cooperativity mainly includes the
0
3
.4 cm mol K at 320 K, then abruptly to 2.79 cm mol K at
70 K, and then increases gradually to a final value of 3.22 cm
3
3
0
−
1
hydrogen bonding interaction, the steric effect of the
mol K at 400 K. This curve indicates an SCO transition with
T1/2↑ = 346 K. The subsequent cooling mode provides
evidence for a 20 K hysteresis loop. Although the spin
transition is accompanied by partial removal of solvent
molecules from TG analysis, an additional heating/cooling
cycle shows almost reproducibility and didn’t significantly
31
solvent, and crystal symmetry of the solvate of complex.
However, it seems obscure how the H-bonds influence the SCO
transition temperatures. In some cases, authors suggest that
stronger H-bonds can obviously weaken the ligand field, in
favor of the HS states and lower transition temperature
32
27
(
T1/2). Conversely, the H-bonds in some cases can slightly
change the thermal hysteresis loop. The spin and oxidation
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Ple aDs ea l dt oo nn To rt a and sj ua sc tt imo na sr gins
Page 6 of 8
ARTICLE
Journal Name
increase the electron density on the amine/imine nitrogen of cooperativity difference. Magnetic studies indicated that
1-
View Article Online
DOI: 10.1039/C6DT01777K
33
the ligand, leading to the high transition temperature. In our Et O exhibit room temperature SCO behavior (T = 305 K).
2
1/2
case, the transition temperature is lifted with the increasing The hydration moved the transition temperature and switched
hydrogen bonding. As for the influence of the H-bonds on the on an apparent thermal hysteresis. This finding emphasizes the
cooperativity, it was established that H bonds between the importance of studying the solvation sensitive SCO magnetic
SCO active centers through the solvent molecules should properties of such materials and the interesting switching
efficiently transmit cooperativity and lead to more abrupt property of hysteresis around room temperature by reversible
3
4
and/or wide hysteresis loops. As for hydrated sample
its SCO transition temperature is shifted higher than that of
Et O. More interestingly, apparent hysteresis loop (20 K) was
1-3H O, SCSC transformation, which is crucial to magnetic sensor and
2
1
-
molecular switch.
2
observed in the hydrated sample 1-3H O, despite of the partial
2
Acknowledgements
dehydration. The modification of H-bond interactions might be
responsible for the amplification of thermal hysteresis.
Through structure comparison between them, it was found
that the water molecule as bridge smooth the strong
We are thankful for financial support by the Priority Academic
Program Development of Jiangsu Higher Education
Institutions. This experimental work is financially funded by
the NSFC program (Grants 21471023) and sponsored by
Jiangsu Provincial QingLan Project. Wu thanks Prof. Y. Einaga’s
group in Keio University, Japan, for the data of Mössbauer
spectra.
intermolecular interaction. However, in complex 1-Et O, this
2
type of N…O…O hydrogen bonding series is blocked due to the
weak donor ability of oxygen atom on ether guest by the
distant O…O separation of 4.376 and 4.572 Å, respectively,
which will weaken the intermolecular cooperativity.
Notes and references
1
2
3
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II
stabilizes the Fe oxidation state. From the observed
electrochemical behavior, it can be asserted that the reduced
species is very stable. Taken together, the voltammetric data
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↔Fe(III) reversible redox process,
II
which potentially switch the molecule spin between S = 0 (Fe )
ls
III 35
and S = 1/2 (Fe ls).
Conclusions
II
In conclusion, different solvate Fe complexes from the
imidazole-bearing dihydroquinazoline tridentate donor
providing strong ligand field were synthesized, structurally and
magnetically characterized. X-ray diffraction revealed a
C. Cook, F. Habib, T. Aharen, R. Clérac, A. Hu and M.
Murugesu, Inorg. Chem., 2013, 52, 1825-1831.
reversible transformation between etherifized form
1-Et O and
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hydrate form -3H O, which helps to understand the
1
2
6
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DOI: 10.1039/C6DT01777K
Table of Contents:
Molecule switch ON/OFF magnetic hysteresis around room temperature can be achieved by reversible
single-crystal-to-single-crystal (SCSC) solvent exchange in imidazole-inspired dihydroquinazoline Fe(II) complex.