Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b00850

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H₂Bpz₂)₂(L)] (pz = pyrazolyl) were prepared, where L = 5,5′-dimethyl-2,2′-bipyridine (bipy-CH₃, 1), L = dimethyl-2,2′-bipyridyl-5,5′-dicarboxylate (MeObpydc, 2), L = diethyl-2,2′-bipyridyl-5,5′-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2′-bipyridine-5,5′-dicarboxylate (i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

Full text of DOI:10.1021/acs.inorgchem.8b00850

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Spin Crossover Behavior in a Homologous Series of Iron(II)
Complexes Based on Functionalized Bipyridyl Ligands
§
§
Shufang Xue,† Yunnan Guo,† Aurelian Rotaru⊥,‡ Helge Muller-Bunz, Grace G. Morgan,
̈
,†
Elzbieta Trzop,∥ Eric Collet,∥ Julianna Olah, and Yann Garcia*
́
†
́
Institute of Condensed Matter and Nanosciences (IMCN), Universite catholique de Louvain, Place L. Pasteur 1, 1348
Louvain-la-Neuve, Belgium
‡Department of Electrical Engineering and Computer Science and MANSiD Research Center, “Stefan cel Mare” University,
University Street, 13, Suceava 720229, Romania
§School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
∥Univ Rennes 1, CNRS, Institut de Physique de Rennes, UMR 6251, UBL, 35042 Rennes, France
⊥
́
́
Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, H-1111 Szent Gellert ter
4, Budapest, Hungary
S
* Supporting Information
ABSTRACT: A series of bulky substituted bipyridine-related iron(II)
complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L =
5,5′-dimethyl-2,2′-bipyridine (bipy-CH3, 1), L = dimethyl-2,2′-bipyridyl-5,5′-
dicarboxylate (MeObpydc, 2), L = diethyl-2,2′-bipyridyl-5,5′-dicarboxylate
(EtObpydc, 3), or L = diisopropyl-2,2′-bipyridine-5,5′-dicarboxylate (i-
PrObpydc, 4). The crystal structures of five new iron(II) complexes were
determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of
3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover
(SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo
gradual and incomplete SCO behaviors. Complexes 2 and 4 show a
completely gradual and steep SCO, respectively. Such different SCO
behaviors can be attributed to an electronic substituent effect in the bipyridyl
ligand conformation and a crystal packing effect. Importantly, the electronic
substituent effect of the isopropyl acetate group and C−H···O supramolecular
interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.
metal center as well as the control of supramolecular
interactions (crystal packing effects) between switching units,
thus justifying the use of crystal engineering concepts. Bipydine
(bipy) is considered as a classical representative of imine
ligands for SCO systems.5 The spin nature of these complexes
lies in the high σ-donor power of the imine function and the
empty, low-lying π orbitals of the ligand molecules. Therefore,
an effective strategy for tuning the ligand field strength of bipy
complexes into the crossover range consists of modifying the
σ-donor or π-acceptor character.5 From the standpoint of
organic synthesis, the substitution of the bipy ligand at the
C3−C6 positions was used to explore the impact of the
electronic substituent effect and crystal packing on the SCO
1. INTRODUCTION
Spin crossover (SCO), a spin-switching phenomenon that may
exist in 3d4−3d7 transition metal complexes, is regarded as a
fascinating field of investigation, finding uses in potential
applications as diverse as ultra-high-density memory devices,
sensors, molecular electronics and spintronics.1 Such an
outstanding and useful physical property is due to the
switching between the low-spin (LS) and high-spin (HS)
states in a reversible, detectable, and controllable fashion by
the action of external stimuli (temperature, pressure, light
irradiation, and chemical decoration). The switching can be
accompanied by drastic changes in magnetic properties
(diamagnetism−paramagnetism) and physical responses such
as changes in structural, vibrational, dielectric and optical
properties.2 Until now, a majority of SCO materials have
involved spin carriers surrounded by different organic ligands
with various morphologies. To realize potential applications,
the properties referring to an abrupt and complete signal
response3 with a hysteresis4 around room temperature are
considered as very important requirements. One efficient
approach results from fine-tuning of the ligand field around the
6
7
properties. Substituents at the C3 and C6 positions have a
strong influence on the iron spin state and steric and electronic
properties. Conversely, provided that the substituents are at
positions relatively remote from the donor atoms (C4 and C5),
no significant change occurs in the ligand field strength or on
Received: March 29, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the [FeN6]2+ core.8,9 One exception is the occurrence of “spin
equilibrium” in the FeII complex of 5,5′-diethylcarboxylate-
2,2′-bipyridine.8
electronic substituent effect and crystal packing effect, the SCO
behavior drastically differs from complex to complex: 1 and 3B
display incomplete SCO behavior, 2 shows a gradual SCO,
whereas 3 and 4B exhibit gradual and incomplete SCO
behaviors. Importantly, the combination of the electronic
substituent effect of the isopropyl acetate group and C−H···O
interactions in 4 provides relatively high cooperativity, which
leads to an abrupt SCO behavior.
As the archetype of bipy-related SCO systems, the
mononuclear complex [Fe(H2Bpz2)2(L)] (pz = pyrazolyl; L
= bipy) was found to exhibit a thermally induced SCO
transition at 160 K,10 also with pressure11 and light
irradiation.12 Since then, successive efforts have concentrated
on the modification of the bipy ligand, involving for instance a
diarylethene photoisomerizable unit for ligand-driven light-
induced spin change.13 An interdigitated aromatic donor group
to allow strong π···π interactions in the crystal lattice of
mononuclear iron(II) complexes was also introduced.14
Hydrophilic alkyl tails were also added to improve the spin-
transition temperature via nanosphere organization.15 A strong
electron-donating amino group was also recently introduced.16
Amazingly, protonation of the amino group in [Fe-
(H2Bpz2)2(bipy-NH2)] (bipy-NH2 = 4,4′-diamino-2,2′-bipyr-
idine) yields a spectacular enhancement of T1/2 from 160 to
297 K, allowing operating SCO around room temperature.16
Notwithstanding these representative examples, the inves-
tigation of structure−property relationships is at the heart of
the SCO field that aims to exert full control over the magnetic
properties. Aiming, herein, at shedding light on the SCO
influence of bipy substituents, we have designed a series of
bulky substituted bipy ligands at positions C5 and C5′ (Scheme
1) and assembled FeII complexes 1−4. We were hopeful that
2. RESULTS
2.1. Synthesis and General Characterization. The
bipy-type ligands (Scheme 1), 2,2′-bipyridine-5,5′-dicarboxylic
acid (bpydc),17 dimethyl-2,2′-bipyridyl-5,5′-dicarboxylate
(MeObpydc),18 diethyl-2,2′-bipyridyl-5,5′-dicarboxylate
(EtObpydc),19 and diisopropyl-2,2′-bipyridine-5,5′-dicarbox-
ylate (i-PrObpydc),20 were prepared following literature
procedures. Powder samples for 1−4 were prepared by similar
procedures described in the literature (Scheme 2).10
A single crystal of 1 was obtained by slow diffusion in
MeOH under Ar(g), using a single-tube glass vessel. The bipy-
CH3 in CH3OH was placed on top of an Fe(H2Bpz2)2
methanolic solution. Given the different solubility, single
crystals of 3 and 4 were obtained by slow diffusion in a
methanol/dichloromethane mixture under Ar(g) using a single-
tube glass vessel. The bipyridine dicarboxylate (MeObpydc,
EtObpydc, or i-PrObpydc) in CH2Cl2 was placed on the
bottom of a methanolic solution containing Fe(H2Bpz2)2.
Pink-violet single crystals for 1 and dark green bulk crystals for
3 and 4, namely, 3B and 4B, respectively, formed after one
week. Of particular interest was the fact that needle crystals of
3 and 4 were observed overnight. Attempts to crystallize 2
failed because of the poor solubility of the MeObpydc ligand,
yielding systematically a solid crystalline product despite
numerous varied reaction conditions. All these complexes
were successfully characterized by elemental analysis, mass
spectral analysis (MS), thermogravimetric analysis (TGA),
Fourier-transform infrared spectroscopy (FTIR), differential
Scheme 1. FeII Complexes Based on a Series of Bulky
Substituted bipy Ligands at Positions C5 and C5′
thermal analysis (DSC), single-crystal X-ray diffraction,
57
̈
magnetic susceptibility measurements, and Fe Mossbauer
spectroscopy. The crystal structures of all the complexes except
2 were determined by single-crystal X-ray diffraction at variable
temperatures. Details for the structure solution and refinement
the central skeleton of FeN6 would be found to be similar to
the original crystal arrangement around the FeII center,10
which would allow meaningful comparisons. Because of the
Scheme 2. Synthetic Procedure for the Bulky Substituted bipy Ligands (bipy-CH3, MeObpydc, EtObpydc, and i-PrObpydc)
and the Iron(II) Complexes (1−4)
B
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
are summarized in Table S1, and selected bond distances and
angles are listed in Table S2. Importantly, no lattice solvent
molecules were detected by crystallography or TGA (Figure
1). The thermal stability of complexes 1−4 was found to be
is identified, which matches that of reference complex
[Fe(H2Bpz2)2(bipy)] (5),9 a comparison of the bond distances
in 1 and 5 (Figure S2) shows considerable differences in ligand
field strength, thus leading to different magnetic behaviors
(vide infra). The crystal packing also shows some differences
with weaker π···π interactions between the adjacent bipy
ligands in 1 than in 5 (Figure S3).
Careful inspection of the packing arrangement (Figure S3)
reveals a two-dimensional (2D) supramolecular structure that
is organized via weak C−H···π intermolecular interactions
(C3−H3···centroid and C11−H11B···centroid distances of
2.513 and 2.828 Å, respectively) from the bipy-CH3 ligand to a
pyrazolyl moiety. Attempts to collect data at 20 K after
detecting a spin state crossover below 90 K from magnetic
measurements (see Figure 12) reveal the Fe−N bond length is
still larger than 2.1 Å (Table S2). In this case, X-ray
crystallography is limited for spin detection given the small
amount of spin carriers involved in the SCO process [∼10% as
57
̈
found by Fe Mossbauer spectroscopy (vide infra)]. The HS
state could also result from flash cooling of the crystals from
room temperature by our He cryostream at 20 K, which may
trap the HS state, as observed in another example by X-ray
diffraction.21
2.2.2. Crystal Structures of 3 and 4. To prevent the
occurrence of transesterification (vide infra), crystal growth
was performed overnight with the corresponding ligand-
containing dichloromethane solution in a methanolic solution
of Fe(H2Bpz2)2 leading to X-ray quality crystals of 3 and 4 of
[Fe(H2Bpz2)2(EtObpydc)] and [Fe(H2Bpz2)2(i-PrObpydc)],
respectively (Figures 3 and 4).
Figure 1. TGA profile for powder samples 1−4 performed under
N2(g)
.
X-ray crystallographic analysis at both high (296 K) and low
(92 and 20 K) temperatures reveals that 3 shows a significant
disorder of the boron-containing pyrazolyl ligand, denoted as
Part Δ and Part Λ, which are enantiomers (Figure 3 and
Figure S4). The disorder can be viewed as being generated by
applying a mirror plane along the EtObpydc ligand or
perpendicular to it. All FeII ions are in a distorted FeN6
octahedral coordination environment derived from one
EtObpydc ligand and two (H2Bpz2)− anions. Comparison of
structural data at different temperatures reveals that the
average Fe−Npz bond length variation (Δr) is ∼0.135 Å in Part
Δ whereas the mean Δr values is <0.055 Å in Part Λ (Table
S2). Such considerable reorganization of molecular geometry
corresponds to a temperature-driven spin transition in Part Δ.
Careful inspection of the packing arrangement reveals a one-
dimensional supramolecular chain generated by short inter-
ligand C−H···O contacts [C28−H28···O1*, symmetry code:
*, −1 + x, y, 1 + z (Table S3)] within the complex units
(Figure 5).
very high, >500 K for 1 and 4 or slightly lower for 2 and 3.
FTIR analysis revealed that all the complexes have a similar
coordination mode irrespective of the bipy-type ligand around
the iron(II) center (Figure S1).
2.2. X-ray Crystallography. 2.2.1. Crystal Structure of 1.
Complex 1 crystallizes in the monoclinic P21/c space group
with the formula [Fe(H2Bpz2)2(bipy-CH3)]. A perspective
view of the molecular structure of 1 at 95 K is represented in
Figure 2. The whole molecule consists of one bidentate bipy-
Needle-shaped crystals of 4 (Figure 5) crystallize in the
orthorhombic Pbca space group with the expected formula of
[Fe(H2B(pz)2)(i-PrObpydc)]. The central skeleton is iso-
morphous to 1, thus revealing a mononuclear complex (Figure
2). The crystal structure was determined at both 297 and 90 K
to study the reorganization of the coordination geometry
expected in the case of a HS ↔ LS transition. The mean value
of the Fe−N variation is found to be 0.194 Å, which lies within
the expected range for S = 2 ↔ S = 0 transitions in FeIIN6 SCO
systems.10,13a,22 A detailed structural analysis revealed that a
larger decrease is observed for Fe−Npy distances (0.25 Å) than
for Fe−Npz distances (0.17 Å) after a spin transition that might
be mainly ascribed to the fact that such a bipy-type ligand acts
as a better π-electron acceptor than pyrazolyl ligands.10 Short
Figure 2. Perspective view of the molecular structure of 1 at 95 K.
Displacement ellipsoids are drawn at the 30% probability level, and H
atoms were omitted for the sake of clarity.
CH3 group and two (H2Bpz2)− anions coordinated to FeII in
cis mode. The Fe−N bond lengths are in the range of
2.150(2)−2.240(2) Å, which is consistent with HS FeII. The cis
angles of the iron(II) coordination sphere range from
74.62(9)° to 98.38(9)°, and the octahedral distortion
parameter (Σ) is found to be 50.7°. Although an FeN6 core
C
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 3. Perspective view of the molecular structures of enantiomers in 3 at 296 K. Displacement ellipsoids are drawn at the 10% probability level,
and the H atoms have been omitted for the sake of clarity.
and Fe2 (Figure 7). The Fe1 site is composed of one bidentate
ligand EtObpydc and two (H2Bpz2)− anions, leading to an
FeN6 coordination sphere, similar to that found in 1. The
situation is dramatically different for the Fe2 site, which reveals
one terminal ethyl acetate group in a disordered state with an
occupancy factor of 21%, whereas an unexpected methyl
acetate group is detected with an occupancy factor of 79%
(Figure 7). It is worth noting that the disorder of the terminal
acetate group has a significant effect on the FeII environment,
especially on the Fe−Npy bond length when considering its
temperature dependence, which was recorded down to 100 K.
Indeed, the average Fe−Npy bond length for the Fe1 molecule
(∼2.194 Å) was found to be slightly shorter than that of the
Fe2 molecule (∼2.243 Å) at 100 K. Also, the Fe−Npy bond
length in the Fe1 molecule is found to decrease with
temperature contrary to the case for the Fe2 molecule (Figure
8). This behavior calls for a partial thermally induced SCO for
one iron(II) site, which should lead to an incomplete SCO, a
result that will be confirmed by magnetic measurements (vide
infra).
Figure 4. Perspective view of the molecular structures of 4 at 297 K.
Displacement ellipsoids are drawn at the 30% probability level, and
the H atoms have been omitted for the sake of clarity.
The crystal structure of 4B was determined at 175 K in
monoclinic space group P21/c. Two crystallographically
independent units, Fe1 and Fe2, are identified (Figure 9).
Both Fe1 and Fe2 molecules consist of an Fe atom surrounded
by one bidentate bipy-type ligand and two (H2Bpz2)− anions
with an Fe−N bond distance of 2.139(2)−2.251(2) Å,
indicative of a HS FeII state (Figure 9).
To our surprise, a closer look at the FeN6 environment
reveals that the original i-PrObpydc ligand has been
completely changed into a new asymmetrical ligand L′,
where L′ is 5-isopropyl 5′-methyl-2,2′-bipyridine-5,5′-dicar-
boxylate (Scheme 3). Because the EtObpydc and i-PrObpydc
ligands used for the synthesis of 3 and 4, respectively, were free
of any impurities, we thought that a transesterification,23 could
interligand contacts involve two types of CH···O interactions,
C11−H11···O43* and C35−H35···O37# [symmetry code: *,
#
1.5 − x, 0.5 + y, z; , −1.5 + x, 0.5 − y, 1 − z (Table S4)]
(Figure 6, top, and Figure S5), that give rise to a 2D
supramolecular framework (Figure 6, bottom).
2.2.3. Unexpected Crystal Structures of 3B and 4B.
Increasing crystallization time afforded translucent prism
crystals, dark green at room temperature, that were formed
after one week. The crystal structure determination revealed
however for both complexes, denoted 3B and 4B, respectively,
an unexpected ligand impurity, as shown in Figures 7 and 9,
which will be discussed below.
Complex 3B crystallizes in monoclinic space group P21/c
with two crystallographically independent sites denoted Fe1
Figure 5. Crystal packing for 3 showing intermolecular C−H···O interactions (C28−H28B···O1*, symmetry code: *, −1 + x, y, 1 + z, blue dashed
line). The molecules in part Λ have been omitted for the sake of clarity.
D
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. Fe−Npy bond lengths vs temperature for 3B considering
Fe1 and Fe2 sites. Whereas no dependence is detected for the Fe2
site, a smooth decrease in the bond lengths is found for the Fe1 site.
occur during the crystallization process. This reaction would be
catalyzed by an FeIII impurity provided by the FeII salt used
during the synthesis, which would be partially oxidized into
FeIII. This impurity would be favored in case a slight excess of
Fe(ClO4)2 would be present in the reaction medium, so that
the residual of FeIII would run as a catalyst to induce the
transesterification from the original isopropyl group into a
methyl group via the MeOH solvent. This process would be
more probable in the case of a slow reaction, such as a
crystallization by slow evaporation.
2.3. Spectroscopic and Magnetochemical Studies.
2.3.1. Ultraviolet−Visible Diffuse Reflectance Spectroscopy
(DRS). In the solid state, powder samples 1−4 are strongly
thermochromic. At room temperature, complex 1 is pink while
the three other complexes are green. When they cool, the color
changes to deep red for 1 and deep green for 2−4. For
instance, 4 exhibits reversible pronounced thermochromism
from green to dark green when the sample is quenched in
liquid nitrogen for several seconds (Figure 10). The visible
change in color is due essentially to the usual shift of the
charge transfer transition during the SCO to a lower energy at
low temperatures.
Figure 6. Packing diagram of 4 showing (top) two types of
intermolecular C−H···O interactions [C11−H11···O43* and C35−
H35···O37# (symmetry code: *, 1.5 − x, 0.5 + y, z; #, −1.5 + x, 0.5 −
y, 1 − z; blue dashed line)] and (bottom) the 2D supramolecular
framework.
Figure 7. Crystal structure of 3B at 150 K. Displacement ellipsoids are drawn at the 30% probability level, and the H atoms have been omitted for
the sake of clarity. The red circle shows the terminal ethyl acetate group in a disordered state with an occupancy factor of 21%, whereas an
unexpected methyl acetate group is detected with an occupancy factor of 79%.
E
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 9. Crystal structure of 4B at 175 K. Displacement ellipsoids are drawn at the 30% probability level, and H atoms have been omitted for the
sake of clarity. The red circle shows the unexpected methyl acetate group.
Scheme 3. Suggested Mechanism of Formation of 4B
Figure 10. Illustration of thermochromism of powder sample 4 after quench cooling the sample to liquid nitrogen from dark green to green within
several seconds.
As observed in Figure 11, a broad band centered at ∼11800
cm−1 for 1 can be assigned to a metal-to-ligand charge transfer
(MLCT) adsorption from the metal dπ orbitals into π* orbitals
of the ligands.7,24 For the other three complexes, such ligand
field absorption is shifted to a higher energy and further split
into the principal component occurring around 14000 cm−1
with a shoulder at 12000 cm−1. The splitting arises presumably
from Jahn−Teller and/or low-symmetry effects associated with
the T2g → Eg transition. Additionally, the position of the
MLCT band changes from the methyl group to the alkyl
acetate series. This indicates that the introduction of an
electron-withdrawing alkyl acetate group leads to an increase in
the π-accepting ability of the bipy-type ligand,16 thus
influencing the SCO behavior.
7
5
5
2.3.2. Magnetic Properties. The solid state magnetic
behavior of all the complexes was probed by variable-
temperature direct current (dc) susceptibility measurements
on polycrystalline samples under an applied field of 1 T. The
room-temperature χMT values of all complexes except 2 and
4B ranging around 3.5 cm3 K mol−1 are consistent with pure
non-interacting HS FeII species. Upon cooling, however,
different thermal evolutions are recorded as shown in Figure
12.
Complex 1 shows rather unusual magnetic properties. Upon
cooling, no dramatic variation is observed down to ∼150 K,
after which a decrease to ∼77 K is noticed and then a plateau
below 55 K. The sharper decrease observed below 25 K is
likely due to zero-field splitting of a large fraction of HS FeII
ions. On warming, no hysteresis loop is detected but a small
decrease in the χMT product on warming is noticed, just before
Figure 11. Ultraviolet−visible DRS of powdered samples 1−4
recorded at room temperature.
F
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 12. Temperature-dependent χMT plots for all the complexes. The red line for 1, 3B, and 4B shows the heating mode. The red solid curve
corresponds to data fitting using the ideal solution model25 for 2 and 3 and the Slichter and Drickamer model26 for 4.
the transition temperature of 77 K. This behavior presumably
value nicely corresponds to the recorded transition temper-
ature.
calls for a freezing-in effect,27 which hinders further the spin
57
̈
conversion. Fe Mossbauer spectroscopy (vide infra),
The temperature dependence of the magnetic properties of
4B resembles that of 3 with a gradual spin crossover profile
performed on the same sample batch, confirms the incomplete
character of the SCO behavior with ∼9.5% of the LS FeII ions
being detected at 78 K. The magnetic properties of 3B
resemble those of 1 with a gradual-type transition involving a
that plateaus at 1.73 cm3 K mol−1 over the range of 20−60 K.
II
̈
Mossbauer spectroscopy reveals that ∼48% of HS Fe ions are
populated at 78 K (vide infra). Thus, a gradual half-spin
conversion was described. It is noteworthy that a knot was
detected at 195 K that exactly corresponds to the T1/2
observed in 4 (Figure S6). This feature suggests that the
transesterification is not complete with a residual amount of 4
when the crystallization time is increased to one week.
̈
few spin carriers. Mossbauer spectroscopy shows that 83.3% of
the HS FeII ions are populated at 78 K (vide infra).
Complex 2 exhibits a gradual and almost complete SCO
behavior. Indeed, the thermal evolution of χMT continuously
decreases from ambient temperature to 170 K, drops to 0.59
cm3 K mol−1 at 80 K, and reaches 0.33 cm3 K mol−1 at 10 K.
The SCO profile can be fitted well using the ideal solution
model,25 leading to a T1/2 of 140.4(2) K, an enthalpy ΔHHL of
8.18(9) kJ mol−1, and an entropy ΔSHL of 58.4(5) J K−1 mol−1.
For 3, the χMT value exhibits a nearly constant evolution at
3.36 cm3 K mol−1 in the temperature range of 300−210 K,
after which it decreases gradually, disclosing a HS → LS
conversion with a T1/2 of 113 K and reaching a second plateau
at 1.28 cm3 K mol−1 over the range of 20−60 K, indicative a
nearly half-spin transition. No change was observed while again
recording the magnetic properties over the range of 20−300 K.
Fitting the magnetic data using the ideal solution model25
provides the following thermodynamic parameters associated
with the SCO: ΔHHL = 5.90(6) kJ mol−1, ΔSHL = 52.2(4) J
K−1 mol−1, and T1/2 = 113.4(2) K.
2.3.3. Differential Scanning Calorimetry (DSC). Given the
sharp spin transition identified for 4, this complex was also
investigated by differential scanning calorimetry (DSC) in both
warming and cooling modes at a scan rate of 2 K min−1.
Corresponding heat capacity temperature profiles, Cp, are
displayed in Figure 13. One exothermic peak, characteristic of
↓
a first-order phase transition, is identified on cooling at a Tmax
of 195.5 K. On warming, an endothermic peak is observed at a
↑
Tmax of 195.5 K, thus delineating no hysteresis effect. The
variations of enthalpy (ΔHHL) and entropy (ΔSHL) have been
determined to be 11.9 kJ mol−1 and 60.7 J K−1 mol−1,
respectively. These values are within the experimental range for
FeII SCO systems.28 The entropy gain is found to be much
larger than the electronic contribution to the entropy, R ×
ln(5) = 13.4 J K−1 mol−1, for FeII SCO complexes.29 This is
due to the large extent of the vibrational entropy that was
identified as 47.3 J K−1 mol−1.
For 4, the χMT value slowly decreases from 3.60 cm3 K
mol−1 at 300 K to 3.04 cm3 K mol−1 at 200 K and then drops
steeply at a T1/2 of 195 K to reach to 0.01 cm3 K mol−1 at 15 K.
Such behavior indicates an abrupt spin transition from a HS
state (S = 2) at high temperatures to a LS ground state (S = 0)
below 195 K. Fitting the magnetic data using the Slitcher and
Drickamer model26 provides the following thermodynamic
parameters: ΔHHL = 12.5(1) kJ mol−1, ΔSHL = 63.6(4) J K−1
mol−1, Γ = 4.16(4) kJ mol−1, and T1/2 = 195.9(1) K. The latter
2.3.4. 57Fe Mossbauer Spectroscopy. Temperature-depend-
̈
57
̈
ent zero-field Fe Mossbauer spectroscopy was applied to
investigate the stability of our materials (oxidation of iron) and
to determine the nature of spin states at low and high
temperatures (Figure 14 and Figure S7). As shown in Table 1,
all the complexes display an isomer shift (δ) of ∼1 mm s−1 at
room temperature. An identical room-temperature isomer shift
G
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
started from the single-crystal X-ray structure data using the
BP86 functional in combination with the def2-SVP basis set on
all atoms with the exception of Fe, for which the triple-ζ def2-
TZVP basis set was used. The HS state for 1 can be attributed
to the relatively small energy difference (Δ) between the
2
highest dπ orbital and the dz orbital, while the series of 2−4
bearing different ester groups turned out to reveal a relatively
large energy splitting of Δ and stronger π-acceptor properties,
which stabilized the LS state16 (Figures S8−S12). Within the
series of 2−4, however, the difference between the ligand field
strengths of the various acetate ligands is negligible (Figure
S8), which indicates that crystal packing plays an important
role in switching the SCO14 (vide infra).
3. DISCUSSION
Figure 13. Heat capacity thermal profile on cooling and warming
modes of 4 recorded at a scan rate of 2 K min−1.
In our system, the SCO behavior is drastically different from
complex to complex, which is caused by subtle but crucial
structural differences between the respective FeIIN6 skeletons.
It is well-known that a prototype of the [Fe(H2Bpz2)2(bipy)]
complex (5) has been reported to show a thermally induced
spin transition.10 However, the introduction of different
aromatic directing groups into the bipy ligand can affect the
π-acceptor character, leading to the distinction of ligand field
strength and diversity of SCO properties.16 As typical electron-
withdrawing groups, ester moieties in 2−4 are good candidates
for supporting the bipy ligands acting as a better π-electron
acceptor. This point has been reflected by the shorter Fe−Npy
distances [1.968(1) and 1.964(1) Å] observed for 4 in the LS
state compared to that of 5 [2.013(2) Å]. The BP86
calculations for the discussed complexes also revealed that
the energy difference (Δ) between the highest dπ orbital and
was found for the mononuclear SCO complex [Fe(H2B-
(pz)2)2phen*], where phen* is a diarylethene-derived
phenanthroline ligand.13a The listed Mossbauer parameters
̈
point unambiguously to the presence of HS FeIIN6 species at
room temperature. For 2, LS Fe(II) ions with an isomer shift
(δ) of 0.49 mm s−1 are also identified, whereas for 4B, Fe(III)
species reflected by an isomer shift (δ) of 0.33 mm s−1 are
revealed at room temperature (Figure S7), presumably caused
by air oxidation. Upon cooling to 78 K, the isomer shift
increases to 1.12 mm s−1 because of the expected second-order
Doppler shift,30 and quadrupole splitting ΔEQ becomes as
large as 3.11 mm s−1 for 2. More importantly, a new
quadrupole doublet appears for all the complexes with a
relative intensity of a full population for 4, which is
characterized by a smaller isomer shift around 0.50 mm s−1
and a much smaller quadrupole splitting of <1.0 mm s−1. This
later signal is characteristic of a LS FeII ion in a distorted
pseudooctahedral environment. Incomplete SCO behavior is
thus identified for 1−3, 3B, and 4B but not for 4.
2
the dz orbital of 2.70 eV for 2−4 is slightly larger than that of
2.67 eV for 1 (Figure S8). Therefore, the introduction of
electron-withdrawing substituents leads to increased ligand
field strengths for members of that ester series. On the other
hand, a crystal packing effect has been shown to influence the
magnetic properties of FeII SCO complexes, illustrating
explicitly the importance of the crystalline arrangement for
realizing special cooperative behavior. Closer inspection of the
crystal structures of 3 and 4 reveals the main differences that
can be summarized as follows:
2.3.5. Density Functional Theory (DFT) Calculations. To
probe the influence of electronic substituents on SCO, we
performed density functional calculations for complexes 1−4
using ORCA version 4.0.0.31 Geometry optimizations were
57
̈
Figure 14. Fe Mossbauer spectra of polycrystalline samples for all the complexes recorded at 298 K (top) and 78 K (bottom). Red and blue
correspond to the FeII HS and LS doublets, respectively.
H
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
57
̈
Table 1. Fe Mossbauer Parameters for All the Complexes
HS FeII (mm/s)
FeII LS (mm/s)
FeIII (mm/s)
complex
T (K)
A
HS/Atot (%)
δ
ΔEQ
Γ/2
δ
ΔEQ
Γ/2
δ
ΔEQ
Γ/2
b
1
298
78
298
78
298
78
298
78
298
78
298
78
100
1.01(4)
1.12(4)
1.01(1)
1.12(1)
1.01(3)
1.12(6)
0.99(5)
1.84(8)
2.53(8)
2.41(3)
3.11(3)
2.00(5)
2.76(1)
1.80(1)
0.15(6)
0.15(8)
0.25(3)
0.20(3)
0.23(4)
0.16(9)
0.16(8)
90.5(3)
88.3(5)
21(2)
100
46.9(2)
100
0
100
83.3(1)
63.4(5)
47.8(6)
0.50(1)
0.49(2)
0.48(3)
0.33(2)
0.71(1)
0.65(5)
0.23(2)
0.19(1)
0.17(6)
c
2
b
3
0.50(5)
0.48(3)
0.50(1)
0.49(2)
0.55(9)
0.66(5)
0.57(2)
0.71(3)
0.16(7)
0.16(3)
0.16(2)
0.23(2)
b
4
b
3B
1.02(8)
1.12(2)
1.00(1)
1.12(3)
1.99(2)
2.68(4)
2.21(3)
2.64(7)
0.23(1)
0.17(3)
0.19(2)
0.30(5)
b
4B
0.33(4)
0.65(7)
0.26(5)
a
b
δ, isomer shift (with respect to α-Fe at 298 K); ΔEQ, quadrupole splitting; Γ/2, half -width at half-maximum. Crystals that had been ground.
c
Powder sample.
Figure 15. Illustration of the crystal packing for the Δ isomer with the ordered H2Bpz2 ligands (top), the Λ isomer with the ordered H2Bpz2 ligands
(middle), and the disordered H2Bpz2 ligands in 3 (bottom).
3.1. Supramolecular Interactions. Crystal packing of
both 3 and 4 is dominated by C−H···O interactions but with
different supramolecular structures (Figures 5 and 6). 3
displays a Csp3−H···O interaction between the carbonyl oxygen
and ethyl β-carbon atom, which is located far from the iron(II)
centers. 4, in turn, undergoes two kinds of C−H···O
interactions, forming a 2D supramolecular structure. More
importantly, both types of proton donors in 4 originate from
py and pz rings, which are directly coordinated to the FeII ion.
Therefore, these C−H···O interactions, as a catalog of weak H-
bonds,32 can shift in electron density that accompanies the
magnitudes of the various components of the interaction
energy to influence cooperative SCO behavior.
3.2. Structural Disorder. It is the disordered H2Bpz2
ligands in 3 that clearly affect the SCO properties between
two different enantiomers. As shown in Figure 15, in ordered
Δ isomers, there is a B···B short contact from the adjacent
H2Bpz2 ligands at a distance of 3.626 Å. This B···B contact was
strongly contracted to a length as short as 3.475 Å at 92 K and
even to 3.452 Å at 20 K. Such contraction is observed for the
first time for [Fe(H2Bpz2)2(L)]-related complexes. In addition,
I
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
5.1.2.1. [Fe(H2Bpz2)2(bipy-CH3)] (1). To a solution of K[H2B(pz)2]
(160 mg, 0.88 mmol) in methanol (5 mL) was added a solution of
Fe(ClO4)2·6H2O (160 mg, 0.44 mmol) in methanol (5 mL). The
formed KClO4 precipitate was removed by filtration, affording a
yellow solution. A solution of bipy-CH3 (81 mg, 0.44 mmol) in
methanol (10 mL) was then added dropwise to the solution, causing
an immediate color change to dark pink. After the solution was stirred
for 2 h at room temperature, a pink precipitate was collected, washed
with methanol, and dried under a stream of N2(g). Yield: 158 mg
(67%). Anal. Calcd for 1 (C24H28N10B2Fe): C, 53.98; H, 5.28; N,
26.23. Found: C, 53.06; H, 5.15; N, 26.33. MS (FTMS + pESI): m/z
535.21 [M+]. Single crystals of 1 were obtained by slow diffusion in
methanol under Ar(g), using a single-tube glass vessel. The bipy-CH3
in CH3OH was placed on the top of an Fe(H2B(pz)2)2 methanolic
solution. Pink-violet single crystals of [Fe(H2Bpz2)2(bipy-CH3)] (1),
suitable for X-ray diffraction analysis, formed after one week.
5.1.2.2. [Fe(H2Bpz2)2(MeObpydc)] (2). The same method that was
used for 1 was followed using MeObpydc (120 mg, 0.44 mmol),
which yielded an olive-green precipitate of 2. Yield: 165 mg (60%).
Anal. Calcd for 2 (C26H28N10B2O4Fe): C, 50.20; H, 4.54; N, 22.52.
Found: C, 51.73; H, 4.53; N, 20.71. MS (FTMS + pESI): m/z 623.19
[M+]. Attempts to crystallize complex 2 failed because of the poor
solubility of MeObpydc.
5.1.2.3. [Fe(H2Bpz2)2(EtObpydc)] (3). The same method that was
used for 1 was followed using EtObpydc (130 mg, 0.44 mmol), which
yielded an olive-green precipitate. Yield: 166 mg (56%). Anal. Calcd
for 3 (C28H32N10B2O4Fe): C, 51.73; H, 4.96; N, 21.55. Found: C,
50.99; H, 4.75; N, 21.83. MS (FTMS + pESI): m/z 651.22 [M+].
Single crystals were obtained by slow diffusion in a methanol/
dichloromethane mixture under Ar using a single-tube glass vessel.
The EtObpydc in CH2Cl2 was placed at the bottom of an
Fe(H2B(pz)2)2-containing methanol solution overnight, affording
green needle crystals, namely 3, suitable for X-ray diffraction analysis.
Increasing the crystallization time to one week afforded prism-shaped
crystals, which were identified as 3B.
5.1.2.4. [Fe(H2Bpz2)2(i-PrObpydc)] (4). The same method that was
used for 1 was followed using i-PrObpydc (140 mg, 0.44 mmol),
which yielded an olive-green precipitate. Yield: 168 mg (54%). Anal.
Calcd for 4 (C30H36N10B2O4Fe): C, 53.13; H, 5.35; N, 20.65. Found:
C, 51.96; H, 5.27; N, 20.48. MS (FTMS + pESI): m/z 679.25 [M+].
The same crystallization method that was used for 3 afforded single
crystals of 4 using i-PrObpydc instead of EtObpydc. Increasing the
crystallization time to one week afforded crystals of another
morphology, namely 4B.
5.2. Characterization Techniques. Elemental analyses for C, H
and N were performed at Medac. Nuclear magnetic resonance
(NMR) spectra were recorded at room temperature with a Bruker
Avance II 300 MHz instrument. Chemical shifts (δ) are reported in
parts per million from CDCl3 (δ = 7.27) or DMSO-d6 (δ = 2.50) for
1H NMR. Mass spectra (MS) were recorded using a Q-Exactive
ThermoFisher spectrometer. Infrared spectra were recorded on a
Shimadzu FTIR-8400S instrument with KBr pellets. Diffuse
reflectance spectra (DRS) were obtained with a PerkinElmer Lambda
9 UV/vis/NIR spectrophotometer equipped with a 60 mm integrating
sphere and converted into absorption spectra by using the Kubelka−
Munk function, using BaSO4 as a reference. Thermogravimetric
analyses (TGAs) were performed in N2(g) (100 mL min−1) at a
heating rate of 10 K min−1 from 298 to 873 K using a Mettler Toledo
TGA/SDTA 851e analyzer. Magnetic susceptibilities were measured
on a Quantum design MPMS-5s SQUID magnetometer. The
magnetic data were corrected for the sample holder and diamagnetic
contributions. The crystal sample was quickly loaded into a gelatin
capsule and immediately inserted within the SQUID cavity. 57Fe
to the best of our knowledge, this distance is the shortest B···B
contact so far found for a
stacking motif, based on a CCDC search (Figure S13). The
B···B distances at low temperatures were much shorter than
the van der Waals radii of the B−B contact (3.6−4.1 Å),33
indicating crowded stacking for the Δ configuration to a
certain extent (Figure 15, top). However, this crowded
situation can be avoided by changing the Δ configuration to
the Λ one as shown in the middle panel of Figure 15.
Therefore, the disorder in 3 can be viewed as the insertion of
Λ molecules into the ordered Δ lattices, breaking down the
boron contact of the H2Bpz2 chain to a short-range one
(Figure 15, bottom). The LS molecules are conducted within
these “crowded” short chains, while the HS molecules are kept
in these disordered Λ species, because of their “roomy”
surroundings. FeII compounds showing incomplete or
intermediate SCO have been reported,34 based on the
hypothesis that SCO sites strongly depend on cooperativity
(hydrogen bonds, π−π stacking, and van der Waals
interactions within the crystal lattice), leading to a specific
stoichiometric combination of HS and LS FeII molecules. The
cooperativity in the disordered structure of 3 presented here
demonstrates that the control of chiral enantiomers within
crystal packing can become an outlet for the development of
new molecular devices based on SCO materials
4. CONCLUSION
The foregoing results demonstrate that subtle, remote variation
of the ligand field can lead to significant modulation of the
transition temperature in [Fe(H2Bpz2)2(bipy-type)] SCO
complexes with electronic substituent-decorated 5,5′-posi-
tioned bipy ligands. Importantly, the electronic substituent
effect isopropyl acetate group and C−H···O supramolecular
interactions in 4 contribute to efficient elastic interactions
associated with the spin crossover behavior, leading to an
abrupt thermally induced spin transition. This example thus
pinpoints the importance of supramolecular interactions, to
design highly cooperative SCO complexes,35 which is quite
relevant for mononuclear complexes where no important
cooperative effects are expected.36 The addition of electro-
negative atoms on the bipy ligands increases the low-lying π-
acceptor character, which is suitably oriented for interaction
with the filled dπ orbitals of the metal atom and therefore for
strengthening the metal−ligand interaction. This methodology,
which can provide a tool for the further design and fabrication
of SCO material switching at higher temperatures, is currently
under consideration in our laboratory. In addition, their higher
thermal stability is an important asset for these materials in
view of their future nanostructural iteration as thin films, e.g.,
using CVD processes.
5. EXPERIMENTAL SECTION
5.1. Syntheses. All chemicals were used as commercially obtained
without further purification.
̈
Mossbauer spectra were recorded in transmission geometry mode
5.1.1. Synthesis of Bipyridine Derivatives. 2,2′-Bipyridine-5,5′-
dicarboxylic acid (bpydc),17 dimethyl-2,2′-bipyridyl-5,5′-dicarboxy-
late (MeObpydc),18 diethyl-2,2′-bipyridyl-5,5′-dicarboxylate (EtOb-
pydc),19 and diisopropyl-2,2′-bipyridine-5,5′-dicarboxylate (i-PrOb-
pydc)20 were prepared using literature procedures.
with a constant acceleration mode conventional spectrometer
equipped with a 50 mCi 57Co(Rh) source and a Reuter Stokes
proportional counter. The powdered samples were sealed in
aluminum foil, and spectra were recorded at 298 and 78 K. All
samples were ground because spectra of fresh crystals systematically
afforded line dissymmetry due to texture. The spectra were fitted
5.1.2. Synthesis of Complexes 1−4. Syntheses were performed
under Ar(g) using Schlenk techniques.
J
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
37
̈
using Recoil 1.05 Mossbauer Analysis software. The isomer shift
values are given with respect to α-Fe at 298 K.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yann.garcia@uclouvain.be.
ORCID
Aurelian Rotaru: 0000-0002-8782-7988
Eric Collet: 0000-0003-0810-7411
Yann Garcia: 0000-0002-3105-0735
■
5.3. Single-Crystal X-ray Analyses. Suitable single crystals were
selected for single-crystal X-ray diffraction analysis. Data at 20 K for
complexes 1 and 3 were collected with an Oxford Diffraction
Xcalibur3 diffractometer, using monochromated Mo Kα radiation (λ
= 0.71073 Å). The diffractometer was fitted with a liquid helium low-
temperature device, a Helijet Oxford Diffraction Cryostat. Crystallo-
graphic data at other temperatures were collected on a MAR345
image plate using Mo Kα radiation (λ = 0.71073 Å). The crystals
were selected, mounted in inert oil, and transferred to the cold gas
stream for flash cooling. Data were integrated by CrysalisAlisPro
(Agilent Technologies UK Ltd., Oxford, U.K., Xcalibur/SuperNova
CCD system, CrysAlisPro Software system, versions 1.171.37.35 and
1.171.38.41). Absorption correction was applied using the integrated
multiscan absorption algorithm. The structures were determined by
direct methods (SHELXS) and refined by full-matrix least squares on
F2 using SHELXL2014.38 The location of the Fe atom was easily
determined, and O, N and C atoms were subsequently located in the
difference Fourier maps. The non-hydrogen atoms were refined
anisotropically. The H atoms were introduced in calculated positions
and refined with fixed geometry with respect to their carrier atoms.
DFIX, FLAT, DELU, SAME, ISOR, and EADP constraints were
applied in the refinement of the disordered boron-containing pyrazoyl
group and alkyl ester substituents. CCDC 1829354 (1_95 K),
1832091 (2_20 K), 1829355 (3_296 K), 1829358 (3_92 K),
1832092 (3_20 K), 1829356 (4_297 K), 1829357 (4_90 K),
1832660 (3B_150 K), 1829660 (3B_100 K), and 1829661 (4B_175
K) are the supplementary crystallographic data for this paper. They
can be obtained free of charge from the Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
5.4. DFT Calculations. All calculations were performed using
ORCA version 4.0.0.31 Geometry optimizations were started from the
X-ray structure data using the BP86 functional in combination with
the def2-SVP basis set on all atoms with the exception of Fe, for which
the triple-ζ def2-TZVP basis set was used. These calculations made
use of the RI density fitting approximations implemented in ORCA,
using the def2/J auxiliary basis set.39 On the basis of the optimized
geometries, single-point energy calculations were performed with the
B3LYP functional in conjunction with the def2-TZVP basis set. All
calculations employed the relativistic recontracted version of the def2
basis sets,40 the zeroth-order relativistic approximation (ZORA),41
and D3 dispersion correction with the Becke−Johnson damping
function [D3(BJ)].42 Second-derivative calculations were performed
for all structures to ensure that they are minima on the potential
energy surface. The DFT calculations predicted a very stable LS
ground state for all the complexes using BP86, while B3LYP predicted
that the HS state was below the LS state by ∼3 kcal/mol.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge financial support from FNRS (PDR
T.0102.15), and FNRS fellowships of charge de Recherches
́
allocated to S. XUE (24918505) and Y. GUO (28109061), the
Romanian National Authority for Scientific Research, CNCS-
UEFISCDI, Project PN-II-RU-TE-2014-4-2695, FNRS-Aca-
demie Roumaine, WBI Roumanie, and COST Actions
CM1305 and CA15128. The authors thank Prof. J. Wouters
for the use of a diffuse reflectance spectrometer. The Bolyai
Janos Research Fellowship is acknowledged and NKFIH Grant
PD115503 is given by the “National Research, Development
and Innovation Office” in Hungary.
REFERENCES
■
(1) (a) Gu
̈
tlich, P.; Gaspar, A. B.; Garcia, Y. Spin state switching in
iron coordination compounds. Beilstein J. Org. Chem. 2013, 9, 342−
́
391. (b) Molnar, G.; Rat, S.; Salmon, L.; Nicolazzi, W.; Bousseksou,
A. Spin Crossover Nanomaterials: From Fundamental Concepts to
Devices. Adv. Mater. 2018, 30, 1703862.
(2) Zhao, T.; Boldog, I.; Spasojevic, V.; Rotaru, A.; Garcia, Y.;
Janiak, C. Solvent-triggered relaxative spin state switching of
[Fe(HB(pz)3)2] in a closed nano-confinement of NH2-MIL-
101(Al). J. Mater. Chem. C 2016, 4, 6588−6601.
(3) Murray, K. S.; Kepert, C. J. Cooperativity in spin crossover
systems: memory, magnetism and microporosity. Top. Curr. Chem.
2004, 233, 195−228.
(4) Brooker, S. Spin crossover with thermal hysteresis: practicalities
and lessons learnt. Chem. Soc. Rev. 2015, 44, 2880−2892.
(5) Goodwin, H. A. Spin crossover in iron(II) tris(diimine) and
bis(terimine) systems. Top. Curr. Chem. 2004, 233, 59−90.
(6) Onggo, D.; Goodwin, H. Steric Effects of the Spin State of
Iron(II) in Complexes of Substituted Bipyridine Derivatives. Aust. J.
Chem. 1991, 44, 1539−1551.
(7) Onggo, D.; Hook, J. M.; Rae, A. D.; Goodwin, H. A. The
influence of steric effects in substituted 2,2′-bipyridine on the spin
state of iron(II) in [FeN6]2+ systems. Inorg. Chim. Acta 1990, 173,
19−30.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00850.
■
S
(8) James, B. R.; Parris, M.; Williams, R. J. P. Spectrophotometric
and thermodynamic properties of some copper and iron complexes. J.
Chem. Soc. 1961, 4630−4637.
(9) Buldt, L. A.; Prescimone, A.; Neuburger, M.; Wenger, O. S.
̈
Overview of IR, structure, and crystallographic data,
magnetic data for 4B, Mossbauer spectra of 3B and 4B,
and computed frontier molecular orbitals for 1−4
(PDF)
Photoredox Properties of Homoleptic d6Metal Complexes with the
Electron-Rich 4,4′,5,5′-Tetramethoxy-2,2′-bipyridine Ligand. Eur. J.
Inorg. Chem. 2015, 2015, 4666−4677.
̈
(10) Real, J. A.; Munoz, M. C.; Faus, J.; Solans, X. Spin Crossover in
̃
Novel Dihydrobis(1-pyrazolyl)borate [H2B(pz)2]-Containing Iron-
(II) Complexes. Synthesis, X-ray Structure, and Magnetic Properties
of [FeL{H2B(pz)2}2] (L = 1,10-Phenanthroline and 2,2′-Bipyridine).
Inorg. Chem. 1997, 36, 3008−3013.
Accession Codes
CCDC 1829354−1829358, 1829660−1829661, 1832091−
1832092, and 1832660 contain the supplementary crystallo-
graphic 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.
(11) Galet, A.; Gaspar, A. B.; Agusti, G.; Munoz, M. C.; Levchenko,
̃
G.; Real, J. A. Pressure Effect Investigations on the Spin Crossover
Systems {Fe[H2B(pz)2]2(bipy)} and {Fe[H2B(pz)2]2(phen)}. Eur. J.
Inorg. Chem. 2006, 2006, 3571−3573.
́
(12) Moliner, N.; Salmon, L.; Capes, L.; Munoz, M. C.; Letard, J.-F.;
̃
Bousseksou, A.; Tuchagues, J.-P.; McGarvey, J. J.; Dennis, A. C.;
K
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Castro, M.; Burriel, R.; Real, J. A. Thermal and Optical Switching of
Molecular Spin States in the {[FeL[H2B(pz)2]2} Spin-Crossover
System (L = bpy, phen). J. Phys. Chem. B 2002, 106, 4276−4283.
(13) (a) Milek, M.; Heinemann, F. W.; Khusniyarov, M. M. Spin
Crossover Meets Diarylethenes: Efficient Photoswitching of Magnetic
Properties in Solution at Room Temperature. Inorg. Chem. 2013, 52,
1,2,4-Triazole Derivative, Operating at, below, and above Room
Temperature. Inorg. Chem. 2016, 55, 4278−4295.
(30) Gutlich, P.; Bill, E.; Trautwein, A. X. Mossbauer Spectroscopy
̈
and Transition Metal Chemistry. Fundamentals and Applications;
Springer-Verlag: Berlin, 2011.
(31) Neese, F. WIREs Comput. Mol. Sci. 2018, 8, e1327.
(32) Gu, Y.; Kar, T.; Scheiner, S. Fundamental Properties of the
CH···O Interaction: Is It a True Hydrogen Bond? J. Am. Chem. Soc.
1999, 121, 9411−9422.
̈
11585−11592. (b) Rosner, B.; Milek, M.; Witt, A.; Gobaut, B.;
Torelli, P.; Fink, R. H.; Khusniyarov, M. M. Reversible Photo-
switching of a Spin-Crossover Molecular Complex in the Solid State
at Room Temperature. Angew. Chem., Int. Ed. 2015, 54, 12976−
12980. (c) Moertel, M.; Witt, A.; Heinemann, F. W.; Bochmann, S.;
Bachmann, J.; Khusniyarov, M. M. Synthesis, Characterization, and
Properties of Iron(II) Spin-Crossover Molecular Photoswitches
Functioning at Room Temperature. Inorg. Chem. 2017, 56, 13174−
13186.
(33) Batsanov, S. S. Van der Waals Radii of Elements. Inorg. Mater.
2001, 37, 871−885.
(34) (a) Gru
Brooker, S.; Gu
Dinuclear Spin-Transition Complex: Confirmation by Variable-
̈
nert, G. C.; Reiman, S.; Spiering, H.; Kitchen, J. A.;
̈
tlich, P. Mixed Spin-State [HS-LS] Pairs in a
57
̈
Temperature Fe Mossbauer Spectroscopy. Angew. Chem., Int. Ed.
2008, 47, 2997−2999. (b) Griffin, M.; Shakespeare, S.; Shepherd, H.
J.; Harding, C. J.; Letard, J.-F.; Desplanches, C.; Goeta, A. E.;
Howard, J. A. K.; Powell, A. K.; Mereacre, V.; Garcia, Y.; Naik, A. D.;
Mueller-Bunz, H.; Morgan, G. G. A Symmetry-Breaking Spin-State
Transition in Iron(III). Angew. Chem., Int. Ed. 2011, 50, 896−900.
(c) 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. (d) Murnaghan, K.
D.; Carbonera, C.; Toupet, L.; Griffin, M.; Dîrtu, M. M.; Desplanches,
C.; Garcia, Y.; Collet, E.; Letard, J.-F.; Morgan, G. G. Spin-State
Ordering on One Sub-lattice of a Mononuclear Iron(III) Spin
Crossover Complex Exhibiting LIESST and TIESST. Chem. - Eur. J.
2014, 20, 5613−5618. (e) Fitzpatrick, A. J.; Trzop, E.; Muller-Bunz,
H.; Dîrtu, M. M.; Garcia, Y.; Collet, E.; Morgan, G. G. Electronic vs.
structural ordering in a manganese(III) spin crossover complex. Chem.
Commun. 2015, 51, 17540−17543.
(14) Kulmaczewski, R.; Shepherd, H. J.; Cespedes, O.; Halcrow, M.
A. A Homologous Series of [Fe(H2Bpz2)2(L)] Spin-Crossover
Complexes with Annelated Bipyridyl Co-Ligands. Inorg. Chem.
2014, 53, 9809−9817.
(15) Luo, Y.-H.; Liu, Q.-L.; Yang, L.-J.; Sun, Y.; Wang, J.-W.; You,
C.-Q.; Sun, B.-W. Magnetic observation of above room-temperature
spin transition in vesicular nano-spheres. J. Mater. Chem. C 2016, 4,
8061−8069.
(16) Luo, Y.-H.; Nihei, M.; Wen, G.-J.; Sun, B.-W.; Oshio, H.
Ambient-Temperature Spin-State Switching Achieved by Protonation
of the Amino Group in [Fe(H2Bpz2)2(bipy-NH2)]. Inorg. Chem.
2016, 55, 8147−8152.
(17) Venema, F.; Nelissen, H. F. M.; Berthault, P.; Birlirakis, N.;
Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Synthesis, Conformation,
and Binding Properties of Cyclodextrin Homo- and Heterodimers
Connected through Their Secondary Sides. Chem. - Eur. J. 1998, 4,
2237−2250.
(18) Case, F. H. The Synthesis of Certain Substituted 2,2′-
Bipyridyls. J. Am. Chem. Soc. 1946, 68, 2574−2577.
̈ ̈
(19) Gunyar, A.; Betz, D.; Drees, M.; Herdtweck, E.; Kuhn, F. E.
(35) Garcia, Y.; Robert, F.; Naik, A. D.; Zhou, G.; Tinant, B.;
Robeyns, K.; Michotte, S.; Piraux, L. Spin Transition Charted in a
Fluorophore-Tagged Thermochromic Dinuclear Iron(II) Complex. J.
Am. Chem. Soc. 2011, 133, 15850−15853.
(36) Zhong, Z. J.; Tao, J.-Q.; Yu, Z.; Dun, C.-Y.; Liu, Y.-J.; You, X.-
Z. A stacking spin-crossover iron(II) compound with a large
hysteresis. J. Chem. Soc., Dalton Trans. 1998, 327−328.
Highly soluble dichloro, dibromo and dimethyl dioxomolybdenum-
(VI)-bipyridine complexes as catalysts for the epoxidation of olefins. J.
Mol. Catal. A: Chem. 2010, 331, 117−124.
̈
(37) Lagarec, K.; Rancourt, D. G. Recoil, Mossbauer Spectral Analysis
(20) Delaive, P. J.; Lee, J. T.; Abruna, H.; Sprintschnik, H. W.;
̃
software for Windows 1.0; Department of Physics, University of
Ottawa: Ottawa, ON, 1998.
Meyer, T. J.; Whitten, D. G. Light-Induced Electron Transfer
Reactions of Hydrophobic Analogs of Ru(bipy)32+. In Inorganic and
Organometallic Photochemistry; American Chemical Society: Wash-
ington, DC, 1978; Vol. 168, pp 28−43.
(38) Sheldrick, G. A short history of SHELX. Acta Crystallogr., Sect.
A: Found. Crystallogr. 2008, 64, 112−122.
(39) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn.
Phys. Chem. Chem. Phys. 2006, 8, 1057−1065.
́
(21) Marchivie, M.; Guionneau, P.; Letard, J. F.; Chasseau, D.;
Howard, J. A. K. Thermal trapped iron(II) high spin state investigated
by X-ray diffraction. J. Phys. Chem. Solids 2004, 65, 17−23.
(22) Konig, E.; Ritter, G.; Kulshreshtha, S. K. The nature of spin-
state transitions in solid complexes of iron(II) and the interpretation
of some associated phenomena. Chem. Rev. 1985, 85, 219−234.
(23) Kumar, B.; Chopra, H.; Parmar, A. Iron(III) Perchlorate: A
Reagent for Transesterification. Indian J. Chem., Sect. B: Org. Chem.
Incl. Med. Chem. 1993, 32, 292−293.
(40) Pantazis, D. A.; Chen, X.-Y.; Landis, C. R.; Neese, F. All-
Electron Scalar Relativistic Basis Sets for Third-Row Transition Metal
Atoms. J. Chem. Theory Comput. 2008, 4, 908−919.
(41) van Wullen, C. Molecular density functional calculations in the
̈
regular relativistic approximation: Method, application to coinage
metal diatomics, hydrides, fluorides and chlorides, and comparison
with first-order relativistic calculations. J. Chem. Phys. 1998, 109,
392−399.
(24) Ferrere, S.; Gregg, B. A. Photosensitization of TiO2 by
[FeII(2,2′-bipyridine-4,4′-dicarboxylic acid)2(CN)2]: Band Selective
Electron Injection from Ultra-Short-Lived Excited States. J. Am. Chem.
Soc. 1998, 120, 843−844.
(42) (a) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping
function in dispersion corrected density functional theory. J. Comput.
Chem. 2011, 32, 1456−1465. (b) Grimme, S.; Antony, J.; Ehrlich, S.;
Krieg, H. A consistent and accurate ab initio parametrization of
density functional dispersion correction (DFT-D) for the 94 elements
H-Pu. J. Chem. Phys. 2010, 132, 154104.
(25) Gutlich, P.; Garcia, Y.; Spiering, H. Spin transition phenomena.
̈
In Magnetism: Molecules to Materials IV: Nanosized Magnetic Materials;
Joel, S., Miller, M. D., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA,
2003; pp 271−344.
(26) Slichter, C. P.; Drickamer, H. G. Pressure-Induced Electronic
Changes in Compounds of Iron. J. Chem. Phys. 1972, 56, 2142−2160.
(27) Mishra, V.; Mukherjee, R.; Linares, J.; Codjovi, E.; Varret, F.;
Lawson-Daku, M. Spin-transition in nearly cubic site in [FeII(L)3]-
[PF6]2. Hyperfine Interact. 2009, 188, 71−78.
(28) Gutlich, P.; Garcia, Y.; Goodwin, H. A. Spin crossover
̈
phenomena in Fe(II) complexes. Chem. Soc. Rev. 2000, 29, 419−427.
(29) Dîrtu, M. M.; Naik, A. D.; Rotaru, A.; Spinu, L.; Poelman, D.;
Garcia, Y. FeII Spin Transition Materials Including an Amino−Ester
L
DOI: 10.1021/acs.inorgchem.8b00850
Inorg. Chem. XXXX, XXX, XXX−XXX