Mononuclear Fe(II) Complexes Based on the Methylpyrazinyl-Diamine Ligand: Chemical-, Thermo- and Photocontrol of Their Magnetic Switchability

Article abstract of DOI:10.1021/acs.inorgchem.7b01430

Two new mononuclear Fe(II) complexes, [Fe(^2MeL_pz)(NCX)₂] (L = N,N′-dimethyl-N,N′-bis((pyrazin-2-yl)methyl)-1,2-ethanediamine and X = S (1), BH₃ (2)), have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, optical reflectivity, and photomagnetic measurements. They have similar FeN₆ coordination environments offered by the tetradentate ligand with a cis-α conformation and two NCX^- coligands in cis positions. However, 1 and 2 have different molecular arrangements and crystal packings, and are isolated in orthorhombic Pbnb and monoclinic C2/c space groups, respectively. 1 remains in a high spin state (S = 2) over all temperatures while 2 undergoes a spin transition around 168 K with a small thermal hysteresis of about 0.4 K (at a temperature scan rate of 1.3 K min^-1). This phase transition, which can also be optically detected due to the associated marked change of the sample color, occurs between two structurally characterized phases, which exhibit Fe(II) complexes in their high spin and low spin states at high and low temperatures, respectively. The reversible photoswitching between these two states has also been confirmed at low temperatures using well separated wavelength irradiations.

Full text of DOI:10.1021/acs.inorgchem.7b01430

Article
pubs.acs.org/IC
Mononuclear Fe(II) Complexes Based on the Methylpyrazinyl-
Diamine Ligand: Chemical-, Thermo- and Photocontrol of Their
Magnetic Switchability
Xue Liu,† Jian Zhou,† Xin Bao,*,† Zheng Yan,‡ Guo Peng,§ Mathieu Rouzieres,∥,⊥ Corine Mathoniere,#,¶
̀
̀
Jun-Liang Liu,∥,⊥ and Rodolphe Clerac*,∥,⊥
́
†School of Chemical Engineering, Nanjing University of Science and Technology, 210094 Nanjing, P. R. China
‡Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University,
Guangdong 515063, P. R. China
§Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, 210094 Nanjing, P. R. China
∥CNRS, CRPP, UPR 8641, 33600 Pessac, France
⊥Univ. Bordeaux, CRPP, UPR 8641, 33600 Pessac, France
#CNRS, ICMCB, UPR 9048, 33600 Pessac, France
¶Univ. Bordeaux, ICMCB, UPR 9048, 33600 Pessac, France
S
* Supporting Information
ABSTRACT: Two new mononuclear Fe(II) complexes,
[Fe(2MeLpz)(NCX)2] (L = N,N′-dimethyl-N,N′-bis((pyrazin-
2-yl)methyl)-1,2-ethanediamine and X = S (1), BH3 (2)), have
been synthesized and characterized by single-crystal X-ray
diffraction, magnetic, optical reflectivity, and photomagnetic
measurements. They have similar FeN6 coordination environ-
ments offered by the tetradentate ligand with a cis-α
conformation and two NCX− coligands in cis positions.
However, 1 and 2 have different molecular arrangements and
crystal packings, and are isolated in orthorhombic Pbnb and
monoclinic C2/c space groups, respectively. 1 remains in a
high spin state (S = 2) over all temperatures while 2 undergoes
a spin transition around 168 K with a small thermal hysteresis
of about 0.4 K (at a temperature scan rate of 1.3 K min−1). This phase transition, which can also be optically detected due to the
associated marked change of the sample color, occurs between two structurally characterized phases, which exhibit Fe(II)
complexes in their high spin and low spin states at high and low temperatures, respectively. The reversible photoswitching
between these two states has also been confirmed at low temperatures using well separated wavelength irradiations.
interactions in the crystal lattice contribute often to high
cooperativity and favor the occurrence of an ST, associated with
a large thermal hysteresis. However, good control of the
cooperativity remains a challenge for synthetic chemists who
rely mainly on the tools of crystal engineering, coordination,
and supramolecular chemistries. Many efforts have been
devoted to the design and synthesis of coordination polymers,
inspired by the promising properties of one-dimensional (1D)
triazole-based complexes3 and metallocyanate based coordina-
tion polymers.4 In this approach, the elastic contacts between
spin centers are enhanced through bridging ligands. However,
spin conversion has been frequently encountered for
coordination polymers, indicating the covalent bridge cannot
guarantee the occurrence of a spin transition. A discrete
INTRODUCTION
■
As one of the most attractive molecule-based switchable
materials, spin crossover (SCO) complexes have potential
applications as a new generation of advanced materials, such as
molecular switches, data storage materials, and data displays.1
While a spin conversion (SC) relies on a Boltzmann (i.e.,
thermal) population of the high spin (HS) excited state and
thus induces a gradual variation of the physical properties, a
first-order spin transition (ST) between a low temperature low
spin (LS) phase and a high temperature HS phase is highly
desirable to envisage practical applications.2 Although the
electronic configuration (LS versus HS) of the metal center is
determined by the magnitude of the energy gap between eg and
t2g orbitals and the spin pairing energy at the molecular scale,
the elastic interactions, also called cooperativity, between the
magnetic centers play a very important role in the bulk
magnetic properties. Short- and long-range intermolecular
Received: June 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2Me
Scheme 1. Schematic Structures of the L (left), 2MeL (middle), and
L (right) Ligands
pz
(42.5 mmol) of 2-methyl-pyrazine in 60 mL of tetrachloromethane
were added 8.10 g (60.8 mmol) of N-chlorosuccinimide and 0.51 g
(2.12 mmol) of the catalyst benzoperoxide. The mixture was refluxed
for 14 h in the dark. After the reaction was complete (monitored by
TLC), the mixture was separated by filtration and the filtrate was
concentrated in vacuo giving crude product 2-(chloromethyl)-
pyrazine.14 Then to a solution of 3.24 g (25.6 mmol) of 2-
(chloromethyl)-pyrazine in 50 mL of tetrahydrofuran were added
1.1 g (12.2 mmol) of N,N′-dimethyl-ethylenediamine and 5.1 g (52.0
mmol) of trimethylamine. The mixture was stirred for 24 h at reflux,
followed by addition of a 30 mL aqueous solution of 3 M NaOH. After
removal of the solvent in vacuum, the dark brown residue was
extracted with dichloromethane (30 mL × 3), then washed with
saturated NaCl solution (15 mL), and purified on silica gel. Elution
with ethyl acetate/methanol (ratio 10:1) followed by a methanol/
molecular system can also give rise to first-order spin transition
through supramolecular interactions. The mononuclear iron(II)
complexes based on Jager-type ligands are a classic example, as
̈
illustrated by [FeL(HIm)2] (L = Butanoic acid, 2,2′-[1,2-
phenylenebis(iminomethylidyne)]bis[3-oxo-,1,1′-diethyl ester]
and HIm = imidazole) which shows a large thermal hysteresis
of 70 K owing to a 2D network of hydrogen bonding
interactions.5 Nevertheless, the role of supramolecular inter-
actions toward SC or ST is still ambiguous and extensive
studies should be carried out to deepen the overall under-
standing of the structure−property relationship.
Among various SCO complexes, those based on a neutral
tetradentate ligand and NCX− (X = S, Se, BH3) coligands allow
the straightforward formation of neutral mononuclear com-
plexes.6−9 Pyridine substituted aliphatic amine,6 Schiff base
ligands,7 and N4 embracing ligands with four aromatic donors8
have all proved their suitability to favor SCO behavior. Many
complexes of such type show structural diversity and display
polymorphism with different SCO properties, thus allowing a
systematic study of the supramolecular interaction role to
modulate the SCO behavior. Recently, we have been interested
in developing complexes based on a N,N′-bis(2-pyridylmethyl)-
1,2-ethanediamine (L) tetradentate ligand (Scheme 1). The
magnetic and photomagnetic behaviors of [Fe(L)(NCS)2] have
2Me
1
ammonia (ratio 10:1) solvent system afforded the
L ligand. H
pz
NMR (300 MHz, CDCl3) δ: 8.69 (s, 2H), 8.52 (m, 2H), 8.47 (m,
2H), 3.75 (s, 4H), 2.68 (s, 4H), 2.31 (s, 6H).
2Me
Synthesis of [Fe(2MeLpz)(NCS)2] (1). To a 20 mL solution of
L
pz
(0.027 g, 0.1 mmol) in methanol and acetonitrile (ratio 4:1) were
added 0.036 g (0.1 mmol) of Fe(ClO4)2·6H2O and 0.018 g (0.2
mmol) of potassium thiocyanide. The resulting reaction mixture was
stirred for 0.5 h, and the yellowish precipitate was filtered.
Recrystallization from hot MeCN gave orange flake crystals in 55%
yield. IR (KBr, cm−1): 3669 (w), 3083 (w), 3034 (w), 2991 (w), 2965
(w), 2932 (w), 2880 (w), 2809 (w), 2056 (s), 1590 (w), 1524 (w),
1447 (m), 1411 (m), 1366 (m), 1300 (m), 1258 (w), 1193 (w), 1151
(m), 1132 (s), 1019 (m), 966(m), 929 (w), 859 (m), 838 (m), 808
(m), 778 (m), 753 (m), 680 (w), 639 (w), 593 (w). Elemental
analysis, Calcd: C, 43.25; H, 4.54; N, 25.22. Found: C, 43.12; H, 4.38;
N, 25.17.
́
been studied comprehensively by Letard et al., showing a spin
transition between 60 and 70 K.10 In a more recent work, we
substituted the NCS− coligands by NCBH3 , aiming at
−
chemically tuning the transition temperature. Indeed, [Fe(L)-
(NCBH3)2] exhibits a one-step SC above room temperature
and can be in situ converted in air into an LS complex of
imine/CN− ligands in solution.11 We have also studied a
structurally related complex [Fe(2MeL)(NCSe)2] (2MeL = N,N′-
dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-ethanediamine) and
obtained two polymorphs.12 One of the these exhibits an
unusual hysteretic two-step ST with an ordered [HS−HS−LS]
intermediate phase, while the other polymorph remains HS
over all temperatures.
Synthesis of [Fe(2MeLpz)(NCBH3)2] (2). To a 20 mL solution of
2Me
L
(0.027 g, 0.1 mmol) in methanol and dichloromethane (ratio
pz
4:1) were added 0.036 g (0.1 mmol) of Fe(ClO4)2·6H2O and 0.0124 g
(0.2 mmol) of sodium cyanoborohydride. The resulting reaction
mixture was stirred for 0.5 h, and the yellowish precipitate was filtered.
Recrystallization from hot MeCN gave orange flake crystals in 60%
yield. IR (KBr, cm−1): 3675 (w), 3098 (w), 2974 (m), 2893 (m), 2390
(w), 2339 (s), 2314 (m), 2183 (s), 1977 (w), 1719 (w), 1479 (m),
1430 (m), 1412 (s), 1367 (s), 1302 (s), 1227 (w), 1194 (m), 1140 (s),
1119 (s), 1061 (m), 1039 (s), 1021 (m), 970 (m), 943 (w), 855 (s),
841 (s), 809 (m), 668 (w), 636 (w), 595 (m). Elemental analysis,
Calcd: C, 47.11; H, 6.42; N, 27.47. Found: C, 47.01; H, 6.31; N, 27.47.
Physical Characterization. Magnetic susceptibility measurements
of the reported Fe(II) complexes (14.68 mg for 1 and 12.12 mg for 2)
were recorded with a Quantum Design MPMS-XL SQUID magneto-
meter, operating with an applied field of 0.1 and 1.0 T at temperatures
from 1.8 to 300 K at rates from 0.01−1.3 K min−1. The photomagnetic
experiments were performed using a set of photodiodes coupled via an
optical fiber to the cavity of a MPMS-7S Quantum Design SQUID
magnetometer. Samples of 2 were maintained in a straw between two
thin layers of polyethylene films to limit orientation effects. Note that
the temperatures have been corrected to take into account the light
irradiation heating (an average +2 K has been observed with red light).
Experimental susceptibilities were corrected for sample holder and
intrinsic diamagnetic contributions. Elemental analyses were per-
formed using an Elementar Vario EL Elemental Analyzer. IR spectra
were recorded by the attenuated-total-reflectance (ATR) technique in
the range 4000−650 cm−1 using a PerkinElmer Spectrum. DSC
measurement was performed on a METTLER TOLEDO DSC1
instrument under a nitrogen atmosphere at a scan rate of 10 K min−1
in both heating and cooling modes. The surface reflectivity
In this work, we are interested to see if using a pyrazine
analogue ligand, N,N′-dimethyl-N,N′-bis(2-pyrazinylmethyl)-
1,2-ethanediamine (2MeLpz, Scheme 1), would lead to a
significant modification and thus a chemical tuning of the
SCO properties of the resulting iron(II) complexes. It is worth
noting that, in some previous reports,13 the substitution of the
pyridine group by pyrazine was found to increase the SCO
characteristic temperature (i.e., to favor the low spin state),
which was claimed to be induced by a stronger π back-bonding
effect of the pyrazine ligand. Herein, we report the syntheses,
crystal structures, and magnetic properties of two new
mononuclear complexes, [Fe(2MeLpz)(NCS)2] (1) and [Fe-
2Me
(
L )(NCBH3)2] (2).
pz
EXPERIMENTAL SECTION
■
General. All reagents obtained from commercial sources were used
without further purification. Safety Note: Perchlorate salts are potentially
explosive, and caution should be taken when dealing with such materials.
Synthetic Procedures. Synthesis of N,N′-Dimethyl-N,N′-bis(2-
pyrazinylmethyl)-1,2-ethanediamine (2MeLpz). To a solution of 4 g
B
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Crystal Data and Refinement Details for 1 and 2
[Fe(2MeLpz)(NCS)2]
[Fe(2MeLpz)(NCBH3)2]
(1)
(2)
temperature/K
empirical formula
formula weight/g mol−1
crystal system
space group
a/Å
298
123
298
C16H20FeN8S2
444.37
C16H26B2FeN8
407.92
C16H26B2FeN8
407.92
orthorhombic
Pbnb
monoclinic
C2/c
monoclinic
C2/c
8.1117(13)
14.678(2)
17.567(3)
90
19.5442(15)
7.5846(7)
15.6160(12)
124.268(2)
1913.0(3)
4
20.386(2)
7.6711(11)
15.6307(16)
123.929(3)
2028.2(4)
4
b/Å
c/Å
β/deg
volume/Å3
2091.6 (6)
4
Z
ρcalc/mg mm−3
μ/mm−1
1.411
1.416
1.336
0.938
0.807
0.761
F(000)
920.0
856.0
856.0
reflections collected
independent reflections
Rint
8928
6561
5294
2375
1738
1815
0.1068
0.0856
0.0781
goodness-of-fit on F2
1.042
1.008
1.026
a
Final R indexes
R1 = 0.0673
wR2 = 0.1460
R1 = 0.1571
R1 = 0.0434
wR2 = 0.0739
R1 = 0.0694
wR2 = 0.0815
R1 = 0.0469
wR2 = 0.0835
R1 = 0.0844
wR2 = 0.0943
[I ≥ 2σ(I)]
Final R indexes
[all data]
wR2 = 0.1942
a
2
2
R1 = Σ||F0| − |Fc||/Σ|F0|. wR2 = (Σ[w(F0 − Fc2)2]/Σ[w(F0 )2])1/2
.
Table 2. Selected Bond Lengths and Structural Parameters for 1 and 2 (X being S or BH3)
1
2
T/K
298
123
298
HS
spin state
HS
LS
Fe−NNCX/Å
Fe−Npz/Å
2.092(5)
2.223(5)
2.272(4)
2.19
1.951(3)
1.975(2)
2.051(2)
1.99
2.113(3)
2.198(2)
Fe−Namine/Å
Fe−Naverage/Å
cis N−Fe−N/deg
trans N−Fe−N/deg
ΣFe/deg
2.255(2)
2.18
76.02(16)−103.9(3)
163.00(17)−171.4(2)
78.4(8)
80.69(9)−98.77(9)
170.39(9)−178.27 (16)
60.1(7)
75.4(9)−106.3(14)
161.33(9)−173.98(13)
85.7(5)
ΘFe/deg
N−C−X/deg
Fe−N−CNCX/deg
214.2(16)
179.0(6)
166.2(5)
180.4(11)
175.0(3)
167.3(2)
254.8(10)
177.3(4)
162.8(3)
measurements have been performed with a home-built system,
operating between 10 and 300 K and in the spectrometric range
400−1000 nm. A halogen-tungsten light source (Leica CLS 150 XD
tungsten halogen source adjustable from 0.05 mW cm−2 to 1 W cm−2)
was used as the spectroscopic light. The measurements were calibrated
by a NIST traceable standard for reflectance (sphereOptics, ref
SG3054). As the samples are potentially very photosensitive, the light
exposure time was minimized during the experiments keeping the
samples in the dark except during the spectra measurements when
white light is shined on the sample surface (P = 1 mW cm−2). For all
the excitation/de-excitation experiments performed at 10 K, the
sample was initially placed at this temperature keeping the sample in
the dark to avoid any excitation. Heating and cooling measurements
were carried out at 4 K min−1. For white light irradiation, the source
described above was used, but in a continuous manner with a power of
1 mW/cm2. Light Emitting Diodes (LEDs) operating between 365
and 1050 nm (from Thorlabs) were used for excitation experiments.
Single Crystal X-ray Diffraction. Single-crystal X-ray data were
collected on a Bruker D8 Quest diffractometer or a Rigaku R-AXIS
SPIDER IP diffractometer using graphite monochromated Mo Kα
radiation (λ = 0.71073 Å). A multiscan absorption correction was
performed. The structures were solved using direct methods and
refined by full-matrix least-squares on F2 using SHELXS and
SHELXL15 and the graphical user interface Olex2.16 Non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were placed in
calculated positions refined using idealized geometries (riding model)
and assigned fixed isotropic displacement parameters. CCDC
1553897−1553899 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
RESULTS AND DISCUSSION
■
2Me
Synthesis and Characterization. Reaction of
L ,
pz
Fe(ClO4)2·6H2O and KNCS or NaNCBH3 in a mixture
solvent of MeOH and CH2Cl2/MeCN results in fast
precipitation of 1 and 2, respectively. Similar to our previously
reported analogues,11,12 1 and 2 have low solubility at room
temperature in MeOH and EtOH. Therefore, single crystals
were obtained by recrystallization of those precipitates from hot
MeCN.
C
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. View of the molecular structures of the Fe(II) complex in 1 (a) and 2 (b) at 298 K. Thermal ellipsoids are presented at 50% probability.
Color code: Fe, red; C, gray; H, white; S, yellow; B, green; N, blue.
Figure 2. Overlay of the molecular structures of the Fe(II) complex in (a) 1 (orange) and 2 (yellow) at 298 K; (b) 2 at 123 K (red) and 298 K
2Me
(yellow), showing different arrangements of both
L
and NCX− ligands (X being S or BH3).
pz
Figure 3. Packing diagrams of 1 showing the 1D supramolecular arrangement (a and b) and the 2D corrugated layer packed in the ac plane (c).
Color code: C, light gray; H, gray; Fe, orange; N, blue; S, yellow. Short interactions are indicated by dotted lines.
Solid-state infrared (IR) spectra were recorded for 1 and 2 at
room temperature (Figure S1). For complexes which contain
NCS− anions, the region of the CN stretch frequency is
particularly useful in characterizing the spin state of the iron(II)
center. It has been demonstrated that the N-bound HS complex
normally shows absorption between 2020 and 2080 cm−1,
whereas a band at ∼2100 cm−1 denotes an LS ground state.17
For 1, the stretching mode at 2056 cm−1 falls within the range
(NCBH3)2] is an LS complex at room temperature and shows a
band at 2203 cm−1.11 In complex 2, the corresponding band
observed at 2183 cm−1 is in good agreement with HS
complexes. The multibands around 2310 cm−1 are ascribed to
B−H stretches.
Crystal Structures. The crystal structures were determined
at 298 K (1 and 2) and 123 K (1). The crystallographic data
and refinement parameters are listed in Tables 1 and 2.
Complex 1 crystallizes in the orthorhombic Pbnb space group
while the monoclinic C2/c space group is observed for 2. The
asymmetric unit contains half of an Fe(II) complex centered on
a 2-fold axis. As shown in Figures 1, S2, and S3, the tetradentate
ligand wraps an iron(II) metal ion in a cis-α conformation,
−
for HS species. Fe(II) complexes containing NCBH3 are
relatively rare, and most of them are HS at room temperature,
showing CN stretch around 2180−2190 cm−1.17b,18 One
example in an HS/LS mixed state shows a stretch at 2192
cm−1.19 In our previous report, we showed that [Fe(L)-
D
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Packing diagrams of 2 showing (a) the flat layer packed in the ab plane and (b) the 1D supramolecular arrangement connected by pyrazine
π···π interactions. Color code: C, light gray; H, gray; Fe, orange; N, blue; B, green. Short interactions are indicated by dotted lines. (c) Overlay of
crystal packing in the ab plane at 123 K (red) and 298 K (yellow).
leaving two cis positions for NCX− coligands. As imposed by
the tetradentate ligand, the Fe(II) center can adopt either Δ or
Λ chirality leading to racemic compounds.
at 298 and 123 K (Figure 2b) shows not only the shrinkage of
Fe−N bond lengths but also rearrangement of the tetradentate
and NCX− ligands. Compared with aliphatic amine, the
pyrazine ring moves closer to the metal ion center, as reflected
by a larger variation in Fe−Npyrazine (0.22 Å) bond length than
Fe−Namine (0.20 Å). The two NCX− groups also move closer
upon thermal spin crossover from HS to LS, with the angle
defined by the two linear NCX− ligands of 138.2° at 298 K to
128.2° at 123 K, respectively. Accordingly, the Fe−NC angle
at 123 K deviates from the ideal 180° to a smaller extent
(162.8(3)° versus 167.3(2)° at 298 and 123 K respectively).
The superposition of the Fe(II) complex in 1 and 2 (Figure 2a)
at 298 K shows main differences arising from different
coordination orientations of the NCX− ligand and pyrazine
ring. The angle between the two NCX− groups in 1 (106.5°) is
much smaller than that in 2 (138.2°).
The average value of the six Fe−N bond lengths determined
at 298 K (2.19 Å for 1 and 2.18 Å for 2) falls within the range
expected for HS Fe(II) complexes with octahedral geometry, in
agreement with IR spectroscopy data. Upon cooling to 123 K,
the average Fe−N bond length of 2 decreases dramatically to
1.99 Å due to the spin state change from HS to LS state. The
octahedral distortion parameters Σ and Θ (Σ is the sum of
deviations from 90° of the 12 cis N−Fe−N angles; Θ is the sum
of the deviations from 60° of the 24 possible octahedron twist
angles; i.e., Σ = 0 and Θ = 0 are corresponding to a perfect
octahedral site)20 at 298 K indicate that the Fe(II) site in 2 (Σ
= 85.7(5)° and Θ = 254.8(10)°) has a more distorted N6
coordination sphere than in 1 (Σ = 78.4(8)° and Θ =
214.2(16)°). At 123 K, the Σ and Θ values for 2 decrease
significantly to 60.1(7)° and 180.4(11)°, respectively, corre-
sponding to a more regular octahedral geometry in the LS state.
Overlay of the molecular structures of the Fe(II) complex in 2
As shown in Figure 3a and 3b, each [Fe2MeLpz(NCS)2] unit
embraces a second molecule by its two NCS− arms in 1,
forming a 1D supramolecular organization along the b axis with
short van der Waals interactions, S1···H6A (2.9178(23) Å),
E
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
C8···H6A (2.7684(59) Å), and C6···C8 (3.3498(77) Å), which
were found between adjacent molecules. Besides, S1···H3
(2.9368(24) Å) and S1···C3 (3.4545(72) Å) interactions
between lateral molecules also link them into a supramolecular
chain. The Fe(II) complexes with Δ and Λ chirality propagate
alternatively along the above two supramolecular chains. A
homochiral layer parallel to the ac plane is formed by N2···H5B
(2.5941(57) Å) intermolecular interactions emphasizing the
key role of the pyrazine group in the establishment of a dense
supramolecular network in 1 (Figure 3c).
As shown in Figure 4, the crystal packing of 2 is quite
different. Homochiral Fe(II) complexes pack in the ab plane via
multiple intermolecular interactions involving pyrazine rings,
−
ethyl groups, and NCBH3 coligands. The overlay of crystal
packings at 298 and 123 K in this crystallographic plane shows
clearly a large rearrangement of the lattice associated with the
spin transition, which has not been observed in our previously
reported [Fe2MeL(NCSe)2] and [FeL(NCBH3)2] ana-
logues.11,12 The reduction of the Fe(II) coordination sphere
volume at the transition induces a significant modification of
intermolecular interactions. Upon cooling from 298 to 123 K,
the intermolecular C8···H6B, B1···H6B, B1···H7B, B1···H3,
and H1C···H3 short interactions decrease notably, with
corresponding values of 2.875(35)/2.5912(33), 3.6607(71)/
3.1975(51), 3.1388(52)/2.9638(39), 2.8885(77)/2.8405(54),
and 2.5220(2)/2.3946(1) Å at 298/123 K, respectively.
Additionally, the pyrazine groups form strong π···π interactions
between adjacent complexes leading to a 1D supramolecular
organization with alternating chirality (Figure 4b). The centroid
distance between the two pyrazine rings does not change
significantly upon spin state change with values of 3.6140(4)/
3.6681(2) Å at 298/123 K, respectively. The dense crystal
structure in 2 involving numerous supramolecular interactions,
favored in part by the presence of the pyrazine groups, plays an
important role in enhancing the elastic interactions (i.e.,
cooperativity) between the switchable Fe(II) sites and thus
naturally favor the occurrence of a first-order phase transition in
agreement with the structural data and the absence of
symmetry breaking (see Table 1).
The thermal stability of the two complexes were analyzed on
crystalline samples by thermogravimetric analyses (TGA) from
30 to 800 °C at a rate of 10 K min−1 under a N2 atmosphere
(Figure S4). The curves show that they are stable up to 250 and
300 °C, respectively. No weight loss was observed before
decomposition, confirming the absence of interstitial solvent
molecules in both compounds. The impressive thermal stability
of this family of mononuclear complexes is likely the result of
the strong chelating effect of the tetradentate 2MeLpz ligand and
the dense supramolecular network.
Figure 5. Top: Temperature dependence of the χT product (χ being
the magnetic susceptibility defined as M/H per mole of complex) at
0.1 T for 1 and 1 T for 2; the temperature scan rate is indicated in the
figure. Bottom: differential scanning calorimetry (DSC) thermograms
for 2 at 10 K min−1.
value drops sharply to 0.2 cm3 K mol−1 at 166 K, and slightly
further to 0.1 cm3 K mol−1 in the 145−10 K range. This
behavior is associated with an almost complete one-step spin
transition from an HS (S = 2) to an LS (S = 0) phase as the low
temperature χT value corresponds to less than 3% of residual
HS Fe(II) centers. Below 10 K, the small decrease of the χT
product is probably due to the zero-field-splitting of residual
HS Fe(II). The magnetic measurements performed in the
subsequent heating mode revealed a narrow thermal hysteresis
of 0.4 K at a scan rate of 1.3 K min−1, associated with the
observed first-order transition. The experimental χT versus T
curve can be fitted to the domain model (see Figure S12 for
details)22 with a number of interacting Fe(II) complex per
domain of 51.0, underlying the strong cooperativity present in
this system. The limit temperatures of this hysteresis
(determined as the temperatures for which 50% of the spins
are converted) are T1/2 ↓ = 168.2 K, T1/2 ↑ = 168.6 K in the
cooling and warming modes, respectively. Moreover, it is worth
noting that the width of thermal hysteresis is small and thus not
very sensitive to the scan rate between 1.3 K min−1 (0.4 K) and
0.01 K min−1 (0.2 K) (Figure S5).
The study of complex 2 by differential scanning calorimetry
(DSC; bottom part of Figure 5) confirms the presence of a
first-order phase transition associated with the SCO process, in
good agreement with the magnetic measurements and the
numerous supramolecular interactions (vide supra). Upon
cooling/heating at 10 K min−1, exothermic/endothermic peaks
were observed at 166.5/169.8 K. The thermal hysteresis
observed by DSC (3.3 K) is wider than by magnetic
measurement as a result of the faster temperature scan rate.
The corresponding ΔH and ΔS values were estimated to be
6.85/6.85 kJ mol−1 and 41.11/40.34 J K−1 mol−1 respectively.
These values are within the experimental range generally
observed for analogue Fe(II) SCO complexes.23
Magnetic Susceptibility Studies. Magnetic susceptibility
data were recorded on polycrystalline samples between 1.8 and
300 K in both cooling and heating modes. The results are
shown in Figure 5 in the form of χT versus T plots (where χ is
molar magnetic susceptibility and T is temperature). The χT
value of 1 at 300 K is 3.8 cm3 K mol−1, lying in the range
typically observed for Fe(II) ion in the HS state. This value
remains relatively constant until 50 K. Below this temperature,
the χT value decreases rapidly to 1.1 cm3 K mol−1 at 1.85 K,
probably due to the magnetic anisotropy of the Fe(II) ion and/
or intermolecular antiferromagnetic interactions.
Optical Reflectivity and Photomagnetic Studies.
Optical reflectivity and photomagnetic studies were carried
out on complex 2. Changes in the reflectivity spectra measured
as a function of temperature were used to monitor the relative
amounts of HS and LS iron(II) species. As shown in Figure 6,
the absorption band intensity with a maximum around 850 nm
The χT product of 2 is 3.6 cm3 K mol−1 at 300 K and
decreases slightly to 3.4 cm3 K mol−1 at 171 K, indicating the
presence of HS Fe(II) ion sites. Upon further cooling, the χT
F
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 6. Left: Selected reflectivity spectra emphasizing the reversible temperature dependence of the optical properties of 2. Right: Thermal
evolution of reflectivity signal at 850 nm between 10 and 270 K. A white halogen light was used as a spectroscopic source with a power of 0.8 mW
cm−2, and the temperature scan rate was 4 K min−1.
Figure 7. Left: Comparison of the reflectivity spectra at 250 K (black), at 10 K before irradiation (red) and after 30 min of excitation with a 625-nm
LED (10 mW cm−2, green). Middle: Thermal variation of the reflectivity signal recorded at 850 nm increasing the temperature in dark at 4 K min−1
after irradiation by a 625-nm LED at 10 K for 30 min (with different light power from 0.1 to 10 mW cm−2). Right: Time evolution of the absolute
−2
●
●
reflectivity recorded at 850 nm under successive irradiations at 10 K (at 4 mW cm ) by 625-nm (red ) and 850-nm (black ) LEDs showing the
good reversibility of the photo induced spin state switching.
5
5
(attributed to T2 → E d−d transition of the HS Fe(II))
decreases markedly upon cooling and, concomitantly, the
absorption band near 650 nm (tentatively assigned to both 1A1
→ 1T1 d−d and MLCT transitions of the Fe(II) complex in its
LS state) increases. These spectral changes are reversible in the
subsequent warming/cooling cycles. Thermal variation of the
absolute reflectivity recorded at 850 nm clearly confirms a spin
transition with T1/2 ↓ = 168 K, T1/2 ↑ = 171 K, in good
agreement with magnetic studies (Figures 6 and S6).
reversibly switched on and off by 625- and 850-nm irradiations
over many cycles (right part of Figure 7).
The photoswitching between the LS and HS phases for 2 was
also monitored by photomagnetic measurements (Figure 8).
First, the sample was cooled in the dark down to 10 at 0.3 K
Irradiation with white light (0.8 mW cm−2) at 10 K did not
change the spectrum significantly, indicating that the complex is
not very photosensitive to broad white light exposure (Figure
S7).24 Hence different wavelengths from 365 to 1050 nm were
tested as excitation light and those between 505 and 660 nm
were found to be very efficient to undergo LS to HS
photoconversion (Figure S8). A 625-nm LED was used in
the following measurements, as it possesses the highest
irradiation power in our setup. As seen in Figure 7, after 30
min of irradiation at 10 K with a power of 10 mW cm−2 (the
highest available power that is also the most efficient one), the
spectrum of the HS phase is completely recovered. In the
subsequent warming of the sample at 4 K min−1, the
photoinduced excited HS phase relaxes fully at 60 K.
Interestingly, this complex also presents reverse photoswitch-
ability. In the 365−1050 nm range, the 850-nm LED is the
most efficient source to de-excite the photoinduced state
(Figure S9). Remarkably, the photoexcited HS state can be
Figure 8. Temperature dependence of the χT product (χ being the
magnetic susceptibility defined as M/H per mole of complex) at 1 T
for 2 in the dark (blue dotted line) and after photoexcitation of a 650
nm LED (at 1.5 mW cm−2) with a temperature scan rate of 0.3 K
min−1 (red dotted line). Inset: Time evolution of the χT product at 10
K during the photoexcitation with a 650 nm LED (at 1.5 mW cm−2).
G
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
min−1. Upon red light irradiation (650 nm, 1.5 mW cm−2), the
χT product increases with time and reaches, after 260 min, 1.42
cm3 K mol−1, a value that is still far from the expected
saturation for the HS phase. So the photoconversion that is
quantitatively seen by optical reflectivity is efficiently occurring
at the surface and is rather much less efficient at the bulk level.
After the irradiation was turned off, the sample was warmed up
at 0.3 K min−1. The initial increase of the χT product is
associated with the zero-field splitting effect of the photo-
induced HS Fe(II) centers which fully relax around 40 K.
Discussion of the Magnetic Property in Relation with
the Molecular and Crystal Structures. Including the two
new complexes discussed here, the crystal structure of six Fe(II)
complexes of this family with a related tetradentate ligand (see
Scheme 1) and NCX− coligands have been reported: (i)
[FeL(NCS)2] that crystallizes in the P21/n space group and
shows a spin transition between 60 and 70 K;10 (ii)
[FeL(NCBH3)2]11 and (iii) [Fe2MeL(NCSe)2]12 (polymorph
a), which are isostructural and crystallize in the Aba2 space
group, with the former complex showing a spin conversion
above room temperature, while the latter exhibits a thermal
hysteresis with a two-step spin transition below 130 K; and
finally (iv) [Fe2MeL(NCSe)2] (polymorph b) that is isostruc-
tural to 2 but stays in its HS state above 1.8 K.12 A careful
comparison of those structures shows that the monodentate
NCX− coligands are flexible and can adopt different
coordination orientations, while the observed geometry of
tetradentate ligands around the Fe(II) site is really consistent
along this family of complexes as expected due to the
geometrical restriction imposed in the chelating mode.
Clarifying the electronic effect of the tetradentate ligand
modifications (pyrazine versus pyridine, bis-methylated or not)
by comparison of these complexes is not an easy task as other
variations in coligand, molecular arrangement, and crystal
packing (vide infra) have also an impact on the overall
magnetic properties of the materials. Nevertheless, in order to
probe the local influence of the ligand modifications (i.e.,
between NCX− and tetradentate ligands are recurrent
characteristics of these SCO systems. It is also worth noting
that π···π interactions are observed in three of the four highly
cooperative SCO complexes (which display a spin transition),
emphasizing their key role in increasing elastic interactions
between Fe(II) molecules and thus the cooperativity.
CONCLUSIONS
■
In the present report, two mononuclear Fe(II) complexes based
on the methylpyrazinyl-diamine tetradentate ligand have been
synthesized and characterized. NCS− and NCBH3 coligands
−
were used to adjust the ligand field strength around the Fe(II)
center. Although 1 and 2 have quite similar compositions, they
crystallize in different space groups and have different
molecular arrangements and crystal packings. Complex 1 is
stabilized in its HS state over the entire temperature range in
agreement with the presence of NCS− coligands, which impose
−
a weaker ligand field strength to the metal ion than NCBH3
(see Figure S11). In contrast, complex 2 undergoes a one-step
spin transition around 168 K with an extremely small thermal
hysteresis. As shown by the structural analysis of 2, the dense
network of supramolecular contacts, favored by the presence of
pyrazine groups, is creating strong elastic interactions (i.e.,
cooperativity) between the switchable Fe(II) complexes in the
solid. As a consequence, the resulting phase transition is
strongly of first-order nature as illustrated by the large
rearrangements of the molecular structure and the crystal
packing, and the remarkably abrupt and one-step switching of
the magnetic and optical properties, which is unprecedented in
this family of SCO complexes. The spin state switching of the
complex 2 can also be photocontrolled and cycled back and
forth by using selected and well separated wavelength
irradiation in the red (625−650 nm) and near-infrared (850
nm) regions. The present work together with our previous
reports11,12 confirms that this pyrazine-based tetradentate N4
ligand (Scheme 1) is a good candidate to design new thermally
and photoswitchable SCO complexes but that the choice of the
NCX− coligand to adjust the ligand field strength is also critical.
pyrazine versus pyridine and NCS− versus NCBH3 ) on the
−
ligand field strength, the optical spectra for the [NiL(NCX)2]
analogous complexes (L = 2MeL or 2MeLpz and X = S or BH3; in
CH3CN) have been compared in the range of the 3A2g → 3T2g
transition allowing direct access to the ligand field splitting
(10Dq).21 From these spectra (Figure S11), the 10Dq values
for [Ni(2MeL)(NCS)2], [Ni(2MeLpz)(NCS)2], [Ni(2MeL)-
(NCBH3)2], and [Ni(2MeLpz)(NCBH3)2] complexes are found
to be 10735(70), 10760(120), 11182(140), and 11187(60)
In addition, it should be emphasized that the pyrazine group of
2Me
the
L
ligand favors remarkably dense supramolecular
pz
interactions and thus first-order spin transition allowing a
“sharp” switching of the magnetic and optical properties.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01430.
cm−1, respectively, suggesting that the substitution of the
2Me
pyridine group in 2MeL by a pyrazine one in
L does not
pz
significantly modify the ligand field strength. In contrast, the
IR spectra, view of molecular structures, TG curves,
additional magnetic and reflectivity spectra of 2 (PDF)
replacement of NCS− by NCBH3 results in a blue shift of
−
about 430 cm−1, confirming that NCS− coligands impose a
weaker ligand field to the metal ion and thus favor the
stabilization of the high spin state in 1. Again it must be
emphasized that the conclusions from these solution measure-
ments on analogous complexes might not be conserved in the
solid state, as the crystal packing and intermolecular
interactions are well-known to significantly alter the metal ion
coordination sphere and consequently the magnetic properties.
By extending the discussion to the crystal packing, it is very
clear that these complexes are involved in different sets of
supramolecular interactions and organizations. Nevertheless, in
all cases, a very dense crystal packing without interstitial solvent
molecules and with multiple intermolecular interactions
Accession Codes
CCDC 1553897−1553899 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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: baox199@126.com (X.B.).
*E-mail: clerac@crpp-bordeaux.cnrs.fr (R.C.).
■
H
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1170−1172. (e) Wei, R. J.; Tao, J.; Huang, R. B.; Zheng, L. S.
Reversible and Irreversible Vapor-Induced Guest Molecule Exchange
in Spin-Crossover Compounds. Inorg. Chem. 2011, 50, 8553−8564.
ORCID
Xin Bao: 0000-0002-2725-0195
Jun-Liang Liu: 0000-0002-5811-6300
Notes
(7) (a) Bref
́
uel, N.; Shova, S.; Lipkowski, J.; Tuchagues, J. P. FeII Bi-
Stable Materials Based on Dissymmetrical Ligands: N4 Schiff Bases
Including 2-Pyridyl and 5-Methylimidazol-4-yl Rings Yield Various FeII
Spin-Crossover Phenomena around 300 K. Chem. Mater. 2006, 18,
The authors declare no competing financial interest.
5467−5479. (b) Bref
́
uel, N.; Shova, S.; Tuchagues, J. P. FeII Bistable
ACKNOWLEDGMENTS
■
Materials with Dissymmetrical Ligands: Synthesis, Crystal Structure,
Magnetic and Mossbauer Properties of FeII Complexes Based on N
Schiff Bases Possessing 2-Pyridyl and 1-R-Imidazol-2-yl Rings. Eur. J.
This work was supported by the National Natural Science
Foundation of China (21401104), the Natural Science
Foundation of Jiangsu Province (BK20140774), the Funda-
mental Research Funds for the Central Universities
(30920140111002), and financial support from State Key
Laboratory of Coordination Chemistry of Nanjing University.
Z.Y. is thankful for the financial support from the NSFC
(21501067) and the Zhejiang Provincial Natural Science
Foundation of China (Grant LQ15B010002). R.C., C.M., and
M.R. thank the University of Bordeaux, the CNRS, the
̈
4
́
Inorg. Chem. 2007, 4326−4334. (c) Brefuel, N.; Vang, I.; Shova, S.;
Dahan, F.; Costes, J. P.; Tuchagues, J. P. FeII Spin Crossover Materials
Based on Dissymmetrical N4 Schiff Bases Including 2-Pyridyl and 2R-
imidazol-4-yl Rings: Synthesis, Crystal Structure and Magnetic and
Mossbauer Properties. Polyhedron 2007, 26, 1745−1757. (d) Hagiwara,
̈
H.; Okada, S. A Polymorphism-dependent T1/2 Shift of 100 K in a
Hysteretic Spin-Crossover Complex Related to Differences in
Intermolecular Weak CH···X Hydrogen Bonds (X = S vs. S and N).
Chem. Commun. 2016, 52, 815−818.
Nouvelle Aquitaine Reg
CA15128, and the GdR MCM-2: Magnet
Moleculaires.
́
ion, the MOLSPIN COST action
(8) (a) Costa, J. S.; Lappalainen, K.; de Ruiter, G.; Quesada, M.;
́
isme et Commutation
Tang, J. K.; Mutikainen, I.; Turpeinen, U.; Grunert, C. M.; Gutlich, P.;
̈
́
́
Lazar, H. Z.; Letard, J. F.; Gamez, P.; Reedijk, J. Remarkable Steric
Effects and Influence of Monodentate Axial Ligands L on the Spin-
Crossover Properties of trans-[FeII(N4 ligand)L] Complexes. Inorg.
Chem. 2007, 46, 4079−4089. (b) Bonnet, S.; Siegler, M. A.; Costa, J.
REFERENCES
■
(1) Kahn, O.; Martinez, C. J. Spin-Transition Polymers: From
Molecular Materials Toward Memory Devices. Science 1998, 279, 44−
́
S.; Molnar, G.; Bousseksou, A.; Spek, A. L.; Gamez, P.; Reedijk, J. A
Two-Step Spin Crossover Mononuclear Iron(II) Complex with a
[HS−LS−LS] Intermediate Phase. Chem. Commun. 2008, 5619−5621.
(c) Castíllo, Z. A.; Zheng, S.; Siegler, M. A.; Roubeau, O.; Bedoui, S.;
Bonnet, S. Tuning the Transition Temperature and Cooperativity of
bapbpy-Based Mononuclear Spin-Crossover Compounds: Interplay
between Molecular and Crystal Engineering. Chem. - Eur. J. 2011, 17,
14826−14836. (d) Zheng, S.; Siegler, M. A.; Roubeau, O.; Bonnet, S.
Influence of Selenocyanate Ligands on the Transition Temperature
and Cooperativity of bapbpy-Based Fe(II) Spin-Crossover Com-
pounds. Inorg. Chem. 2014, 53, 13162−13173.
48.
(2) (a) Gutlich, P.; Garcia, Y.; Woike, T. Photoswitchable
̈
Coordination Compounds. Coord. Chem. Rev. 2001, 219, 839−879.
́
(b) Bousseksou, A.; Molnar, G.; Matouzenko, G. Switching of
Molecular Spin States in Inorganic Complexes by Temperature,
Pressure, Magnetic Field and Light: Towards Molecular Devices. Eur.
J. Inorg. Chem. 2004, 4353−4369. (c) Gutlich, P.; Ksenofontov, V.;
̈
Gaspar, A. B. Pressure effect studies on spin crossover systems. Coord.
Chem. Rev. 2005, 249, 1811−1829. (d) Real, J. A.; Gaspar, A. B.;
Munoz, M. C. Thermal, Pressure and Light Switchable Spin-Crossover
̃
(9) (a) Lennartson, A.; Bond, A. D.; Piligkos, S.; McKenzie, C. J.
Four-Site Cooperative Spin Crossover in a Mononuclear FeII
Complex. Angew. Chem., Int. Ed. 2012, 51, 11049−11052. (b) Arroyave,
A.; Lennartson, A.; Dragulescu-Andrasi, A.; Pedersen, K. S.; Piligkos,
S.; Stoian, S. A.; Greer, S. M.; Pak, C.; Hietsoi, O.; Phan, H.; Hill, S.;
McKenzie, C. J.; Shatruk, M. Spin Crossover in Fe(II) Complexes with
N4S2 Coordination. Inorg. Chem. 2016, 55, 5904−5913.
Materials. Dalton Trans. 2005, 2062−2079. (e) Sato, O.; Tao, J.;
Zhang, Y. Z. Control of Magnetic Properties through External Stimuli.
Angew. Chem., Int. Ed. 2007, 46, 2152−2187.
(3) (a) Kitchen, J. A.; Brooker, S. Spin Crossover in Iron(II)
Complexes of 3,5-di(2-pyridyl)-1,2,4-triazoles and 3,5-di(2-pyridyl)-
1,2,4-triazolates. Coord. Chem. Rev. 2008, 252, 2072−2092. (b) Aromí,
G.; Barrios, L. A.; Roubeau, O.; Gamez, P. Triazoles and Tetrazoles:
Prime Ligands to Generate Remarkable Coordination Materials.
Coord. Chem. Rev. 2011, 255, 485−546.
́
(10) Letard, J. F.; Asthana, S.; Shepherd, H. J.; Guionneau, P.; Goeta,
A. E.; Suemura, N.; Ishikawa, R.; Kaizaki, S. Photomagnetism of a sym-
cis-Dithiocyanato Iron(II) Complex with a Tetradentate N,N′-Bis(2-
pyridylmethyl)1,2-ethanediamine Ligand. Chem. - Eur. J. 2012, 18,
5924−5934.
(4) Munoz, M. C.; Real, J. A. Thermo-, Piezo-, Photo- and Chemo-
switchable Spin Crossover Iron(II)-Metallocyanate Based Coordina-
̃
tion Polymers. Coord. Chem. Rev. 2011, 255, 2068−2093.
(11) Zhou, J.; Zhu, B. W.; Luan, J.; Liu, Z.; Fang, J. K.; Bao, X.; Peng,
G.; Tucek, J.; Bao, S. S.; Zheng, L. M. In Air a Spin Crossover Active
Iron(II) Complex of Amine/NCBH3− Ligands is Converted to a Low
Spin Complex of Imine/CN− Ligands. Dalton Trans. 2015, 44,
20551−20561.
(12) Luan, J.; Zhou, J.; Liu, Z.; Zhu, B. W.; Wang, H. S.; Bao, X.; Liu,
W.; Tong, M. L.; Peng, G.; Peng, H.; Salmon, L.; Bousseksou, A.
Polymorphism-Dependent Spin-Crossover: Hysteretic Two-Step Spin
Transition with an Ordered [HS−HS−LS] Intermediate Phase. Inorg.
Chem. 2015, 54, 5145−5147.
(13) (a) Mohammed, R.; Chastanet, G.; Tuna, F.; Malkin, T. L.;
Barrett, S. A.; Kilner, C. A.; Letard, J.-F.; Halcrow, M. A. Synthesis of
́
(5) Weber, B.; Bauer, W.; Obel, J. An Iron(II) Spin-Crossover
Complex with a 70 K Wide Thermal Hysteresis Loop. Angew. Chem.,
Int. Ed. 2008, 47, 10098−10101.
(6) (a) Matouzenko, G. S.; Bousseksou, A.; Lecocq, S.; van
Koningsbruggen, P. J.; Perrin, M.; Kahn, O.; Collet, A. Spin Transition
in [Fe(DPEA)(NCS)2], a Compound with the New Tetradentate
Ligand (2-Aminoethyl)bis(2-pyridylmethyl)amine (DPEA): Crystal
Structure, Magnetic Properties, and Mossbauer Spectroscopy. Inorg.
̈
Chem. 1997, 36, 2975−2981. (b) Matouzenko, G. S.; Bousseksou, A.;
Lecocq, S.; van Koningsbruggen, P. J.; Perrin, M.; Kahn, O.; Collet, A.
Polymorphism in Spin Transition Systems. Crystal Structure, Magnetic
Properties, and Mossbauer Spectroscopy of Three Polymorphic
̈
2,6-Di(pyrazol-1-yl)pyrazine Derivatives and the Spin-State Behavior
of Their Iron(II) Complexes. Eur. J. Inorg. Chem. 2013, 819−831.
(b) Hogue, R. W.; Miller, R. G.; White, N. G.; Feltham, H. L. C.;
Jameson, G. N. L.; Brooker, S. Hysteretic Spin Crossover in Iron(II)
Complexes of a New Pyridine−Triazole−Pyrazine Ligand is Tuned by
Choice of NCE co-Ligand. Chem. Commun. 2014, 50, 1435−1437.
(14) Ali, K. A.; Elsayed, M. A.; Ragab, E. A. Catalyst Free Synthesis of
Pyridine-2, 6-Bis (2-bromo-propane-1,3-dione) and Pyridine-2,6-
Modifications of [Fe(DPPA)(NCS)2] [DPPA = (3-Aminopropyl)bis-
(2-pyridylmethyl)amine]. Inorg. Chem. 1997, 36, 5869−5879. (c) Li,
B.; Wei, R. J.; Tao, J.; Huang, R. B.; Zheng, L. S.; Zheng, Z. Solvent-
Induced Transformation of Single Crystals of a Spin-Crossover (SCO)
Compound to Single Crystals with Two Distinct SCO Centers. J. Am.
Chem. Soc. 2010, 132, 1558−1566. (d) Wei, R. J.; Li, B.; Tao, J.;
Huang, R. B.; Zheng, L. S.; Zheng, Z. Making Spin-Crossover Crystals
by Successive Polymorphic Transformations. Inorg. Chem. 2011, 50,
I
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
bis(N-arylthiazoline-2-thiones). Green Sustainable Chem. 2015, 5, 39−
45.
(15) Sheldrick, G. M. A Short History of SHELX. Acta Crystallogr.,
Sect. A: Found. Crystallogr. 2008, 64, 112−122.
(16) Dolomanov, O.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.;
Puschmann, H. OLEX2: a Complete Structure Solution, Refinement
and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339−341.
5
(17) (a) Konig, E.; Madeja, K. T -1A Equilibriums in Some Iron
̈
2
1
(II)-Bis (1, 10-Phenanthroline) Complexes. Inorg. Chem. 1967, 6, 48−
55. (b) Gobeze, W. A.; Milway, V. A.; Olguín, J.; Jameson, G. N. L.;
Brooker, S. Nine Diiron(II) Complexes of Three Bis-tetradentate
Pyrimidine Based Ligands with NCE (E = S, Se, BH3) Coligands.
Inorg. Chem. 2012, 51, 9056−9065.
(18) (a) Schneider, C. J.; Cashion, J. D.; Chilton, N. F.; Etrillard, C.;
Fuentealba, M.; Howard, J. A. K.; Letard, J.-F.; Milsmann, C.;
́
Moubaraki, B.; Sparkes, H. A.; Batten, S. R.; Murray, K. S. Spin
Crossover in a 3,5-Bis(2-pyridyl)-1,2,4-triazolate-Bridged Dinuclear
Iron(II) Complex [{Fe(NCBH3)(py)}2(μ-L1)2] − Powder versus
Single Crystal Study. Eur. J. Inorg. Chem. 2013, 2013, 850−864. (b) de
Gaetano, Y.; Jeanneau, E.; Verat, A.; Yu; Rechignat, L.; Bousseksou, A.;
Matouzenko, G. S. Ligand-Induced Distortions and Magneto-
Structural Correlations in a Family of Dinuclear Spin Crossover
Compounds with Bipyridyl-Like Bridging Ligands. Eur. J. Inorg. Chem.
2013, 1015−1023. (c) Scott, H. S.; Ross, T. M.; Phonsri, W.;
́
Moubaraki, B.; Chastanet, G.; Letard, J.-F.; Batten, S. R.; Murray, K. S.
Discrete FeII Spin-Crossover Complexes of 2,2′-Dipyridylamino-
Substituted s-Triazine Ligands with Phenoxo, Cyanophenoxo and
Dibenzylamino Functionalities. Eur. J. Inorg. Chem. 2015, 2015, 763−
777. (d) Wu, X.-R.; Shi, H.-Y.; Wei, R.-J.; Li, J.; Zheng, L.-S.; Tao, J.
Coligand and Solvent Effects on the Architectures and Spin-Crossover
Properties of (4,4)-Connected Iron(II) Coordination Polymers. Inorg.
Chem. 2015, 54, 3773−3780.
(19) Klingele, J.; Kaase, D.; Klingele, M. H.; Lach, J. Synthesis and
Mononuclear Complexes of the Bis-Bidentate Ligand 2,5-Di(2-
Pyridyl)-1,3,4-Thiadiazole (dptd): Spin Crossover in
[FeII(dptd)2(NCSe)2] and [FeII(dptd)2(NCBH3)2]·H2O. Dalton
Trans. 2012, 41, 1397−1406.
́
(20) (a) Guionneau, P.; Marchivie, M.; Bravic, G.; Letard, J.-F.;
Chasseau, D. Co(II) Molecular Complexes as a Reference for the Spin
Crossover in Fe (II) Analogues. J. Mater. Chem. 2002, 12, 2546−2551.
́
(b) Guionneau, P.; Marchivie, M.; Bravic, G.; Letard, J. F.; Chasseau,
D. Structural Aspects of Spin Crossover. Example of the
[FeIILn(NCS)2] Complexes. Top. Curr. Chem. 2004, 234, 97−128.
́
(c) Marchivie, M.; Guionneau, P.; Letard, J.-F.; Chasseau, D. Photo-
Induced Spin-Transition: The Role of the iron(II) Environment
Distortion. Acta Crystallogr., Sect. B: Struct. Sci. 2005, 61 (1), 25−28.
(21) Phan, H. V.; Chakraborty, P.; Chen, M.; Calm, Y. M.; Kovnir,
K.; Keniley, L. K., Jr.; Hoyt, J. M.; Knowles, E. S.; Besnard, C.; Meisel,
M. W.; Hauser, A.; Achim, C.; Shatruk, M. Heteroleptic FeII
Complexes of 2,2′-Biimidazole and Its Alkylated Derivatives: Spin-
Crossover and Photomagnetic Behavior. Chem. - Eur. J. 2012, 18,
15805−15815.
1
(22) Sorai, M.; Seki, S. Phonon Coupled Cooperative Low-Spin A1
high-Spin 5T2 Transition in [Fe(phen)2(NCS)2] and [Fe-
(phen)2(NCSe)2] Crystals. J. Phys. Chem. Solids 1974, 35 (4), 555−
570.
̌
̌ ̌
(23) Salitros, I.; Madhu, N. T.; Boca, R.; Pavlik, P.; Ruben, M. Room-
Temperature Spin-Transition Iron Compounds. Monatsh. Chem. 2009,
140, 695−733.
(24) Thompson, J. R.; Archer, R. J.; Hawes, C. S.; Ferguson, A.;
̀ ́
Wattiaux, A.; Mathoniere, C.; Clerac, R.; Kruger, P. E. Thermally and
Photo-Induced Spin Crossover Behaviour in an Fe(II) Imidazolyli-
mine Complex: [FeL3](ClO4)2. Dalton Trans. 2012, 41, 12720−
12725.
J
DOI: 10.1021/acs.inorgchem.7b01430
Inorg. Chem. XXXX, XXX, XXX−XXX