Substituent dependence on the spin crossover behaviour of mononuclear Fe(ii) complexes with asymmetric tridentate ligands

Article abstract of DOI:10.1039/c9dt00204a

Three mononuclear iron(ii) complexes of the formula [Fe^II(H₂L^1-3)₂](BF₄)₂·x(solv.) (H₂L^1-3 = 2-[5-(R-phenyl)-1H-pyrazole-3-yl] 6-benzimidazole pyridine; H₂L¹: R = 4-methylphenyl, H₂L², R = 2,4,6-trimethylphenyl, H₂L³, R = 2,3,4,5,6-pentamethylphenyl) (1, H₂L¹; 2, H₂L²; 3, H₂L³) with asymmetric tridentate ligands (H₂L^1-3) were synthesized and their structures and magnetic behaviour investigated. Significant structural distortions of the dihedral angles between phenyl and pyrazole groups were observed and found to depend on the nature of the substituent groups. Cryomagnetic studies reveal that 1 and 2 show gradual spin crossover behavior, while 3 remains in the high spin state between 1.8 and 300 K.

Full text of DOI:10.1039/c9dt00204a

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Substituent dependence on the spin crossover
behaviour of mononuclear Fe(II) complexes with
asymmetric tridentate ligands†
Cite this: DOI: 10.1039/c9dt00204a
b
Ryo Saiki,a Haruka Miyamoto,a Hajime Sagayama,b Reiji Kumai,
a
Graham N. Newton, c Takuya Shiga *a and Hiroki Oshio
*
Three mononuclear iron(II) complexes of the formula [FeII(H2L1–3)2](BF4)2·x(solv.) (H2L1–3 = 2-[5-(R-phenyl)-
1H-pyrazole-3-yl] 6-benzimidazole pyridine; H2L1: R = 4-methylphenyl, H2L2, R = 2,4,6-trimethylphenyl,
H2L3, R = 2,3,4,5,6-pentamethylphenyl) (1, H2L1; 2, H2L2; 3, H2L3) with asymmetric tridentate ligands
(H2L1–3) were synthesized and their structures and magnetic behaviour investigated. Significant structural
distortions of the dihedral angles between phenyl and pyrazole groups were observed and found to
depend on the nature of the substituent groups. Cryomagnetic studies reveal that 1 and 2 show gradual
spin crossover behavior, while 3 remains in the high spin state between 1.8 and 300 K.
Received 15th January 2019,
Accepted 29th January 2019
DOI: 10.1039/c9dt00204a
rsc.li/dalton
BF4−, PF6−, AsF6−, SbF6−), which show different SCO behaviour
depending on the nature of the interlayer elastic interactions
Introduction
Bistable molecules attract significant interest due to their mediated by the anions.4 The influence of counter ions and
potential applications as components in molecular electronic solvent molecules in cobalt SCO systems has been discussed
and nanoscale devices.1 Spin crossover (SCO) molecules are in detail by Real et al.5 However, the origin of the effects on
one such class of bistable material that lend themselves to the SCO behaviour mediated by anion and solvent molecules
potential molecular switching applications.2 SCO behaviour can be difficult to precisely define due to the complex nature
can be tuned by modifying ligand field strength, complex of supramolecular systems. In contrast, ligand substituent
nuclearity and intermolecular interactions. Many systematic groups directly affect the electron-donating nature of the
studies on the SCO behaviour of molecular species have shown ligand, and the ligand field strength can be controlled based
dependence of the bistability on anions, guest molecules, on precise molecular design. For example, tridentate 2,6-bis
solvent molecules and substituent groups.3 Anions affect the (pyrazol-1-yl)pyridine (bpp) ligands can be readily modified,
crystal packing and intermolecular electrostatic interactions, and the nature of the substituent effect on the SCO properties
while solvent molecules can interact with spin crossover com- of their complexes with iron is well understood,6 and can be
plexes through hydrogen bonds or CH–π interactions, resulting predicted by Hammett’s rule.7 Distinct differences in the SCO
in perturbation of the electronic states of SCO-active iron com- properties of [Fe(bpp)]2+ analogues with different substitution
plexes. Matsumoto et al. reported a series of two-dimensional groups were observed in solid and solution states.8 Substituent
SCO complexes, [FeIIH3LMe][FeIILMe]X (H3LMe = tris-[2-((((2-methyl- groups also exert a secondary influence on complex SCO behav-
imidazol-4-yl)methylidene)aminoethyl)amine), X−
=
ClO4
,
iour via their effect on complex topology and supramolecular
packing structure. For example, ([FeII(qsal-X)2] (qsal-X = 5-X-N-
(8-quinolyl)salicylaldimines), X = F, Cl, Br, I) complexes exhibit
different SCO behaviour, dependent on the nature of the supra-
molecular interactions of their halogen substituents.9 In order
to obtain fine-tuned SCO systems, systematic studies on substi-
tuent effects on complex structure and magnetic properties
remain a key approach. We recently developed a mononuclear
Fe(II) spin crossover system with asymmetric tridentate ligands
H2L (H2L = 2-[5-(phenyl)-1H-pyrazole-3-yl] 6-benzimidazole pyri-
dine), which shows spin transition around 260 K and found that
the electronic state of the system could be effectively tuned by
deprotonation of the pyrazole or benzimidazole moieties.10
−
aGraduate School of Pure and Applied Sciences, University of Tsukuba,
Tennodai 1-1-1, Tsukuba, Ibaraki 305-8571, Japan.
E-mail: shiga@chem.tsukuba.ac.jp, oshio@chem.tsukuba.ac.jp
bPhoton Factory and Condensed Matter Research Center, Institute of Materials
Structure Science, High Energy Accelerator Research Organization (KEK), Oho 1-1,
Tsukuba, Ibaraki 305-0801, Japan
cGSK Carbon Neutral Laboratories for Sustainable Chemistry,
The University of Nottingham, Nottingham, NG7 2TU, UK
†Electronic supplementary information (ESI) available: Molecular structure of
1′–3′, Mössbauer spectra of 1, and LIESST behaviour of 1. CCDC
1889870–1889875 for 1′–3′, respectively. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c9dt00204a
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0.10 mmol) was added to a solution of H2L3 (1.0 mg,
0.20 mmol) in methanol (10 mL). The resulting red solution
was filtered and allowed to diffuse with i-Pr2O. After a few
days, red plates of [FeII(H2L3)2](BF4)2·2(i-Pr2O) (3′) were
obtained. The crystals were collected by suction and air-dried,
affording
[FeII(H2L3)2](BF4)2·2.5(H2O)·(i-Pr2O)
(3).
Yield
33.4 mg (32%). Elemental analysis anal. (calc.) for
C52H55N10O2.5B2F8Fe1: C, 57.51 (57.32); H, 5.20 (5.09); N, 12.55
(12.86)%. IR (KBr): 3280.8 (s, νN–H), 1057.0 (s, νB–F) cm−1
.
Physical measurements
Physical measurements were performed using air-dried
samples 1–3, except for the single crystal X-ray measurements,
in which fresh samples 1′–3′ were used. Infrared (IR) spectra
were recorded (400–4000 cm−1) on a SHIMADZU IRAffinity-1
spectrometer using KBr pellets. Direct current magnetic sus-
ceptibility measurements of polycrystalline samples for 1–3
were measured in the temperature range of 1.8–300 K with a
Quantum Design MPMS-5XL SQUID magnetometer under an
applied magnetic field of 500 Oe or 1 T. Data were corrected
for the diamagnetic contribution calculated from Pascal’s con-
stants including the contribution of the sample holder.
Data collections for single crystal X-ray diffraction for 1′–3′
were performed on a Bruker SMART APEX II for all complexes,
with a CCD area detector with graphite monochromated Mo-Kα
(λ = 0.71073 Å) radiation. All structures were solved by direct
methods and refined by full-matrix least-squares methods
based on F2 using the SHELXL software. Non-hydrogen atoms
Scheme 1 Three asymmetric tridentate ligands L1–L3.
In this work, new asymmetric tridentate ligands H2L1–3
(H2L1–3
= 2-[5-(R-phenyl)-1H-pyrazole-3-yl] 6-benzimidazole
pyridine; H2L1: R = 4-methylphenyl, H2L2: R = 2,4,6-trimethyl-
phenyl, H2L3: R = 2,3,4,5,6-pentamethylphenyl, Scheme 1) were
designed as supports for SCO materials. Three bis-chelate type
iron complexes, [FeII(H2L1–3)2](BF4)2 (1, H2L1; 2, H2L2; 3, H2L3),
were synthesized, and their structures and magnetic properties
were investigated.
Experimental
Materials
Unless otherwise stated, all starting materials and solvents were refined anisotropically. All hydrogen atoms were posi-
were reagent grade and were purchased from Wako or Tokyo tioned geometrically and refined with isotropic displacement
Chemical Industry Co. Ltd and used without further parameters according to the riding model. All geometrical cal-
purification.
culations were performed using the SHELXL software. X-ray
Synthesis of [FeII(H2L1)2](BF4)2·0.5(i-Pr2O)·2(H2O)·2(CH3OH) diffraction experiments at 20 K for complex 1′ after light
(1). Methanol solution (5 mL) of Fe(BF4)2·6H2O (30.0 mg, irradiation using green laser (532 nm) were performed by
0.09 mmol) was added to a solution of H2L1 (63.2 mg, using the synchrotron radiation source (λ = 1.0 Å) at the
0.18 mmol) in methanol (10 mL). The resulting red solution Photon Factory BL-8A at the High Energy Accelerator Research
was filtered and allowed to diffuse with i-Pr2O. After a few days, Organization (KEK), Japan. The diffraction data were collected
red plates of [FeII(H2L1)2](BF4)2·(i-Pr2O)·2(CH3OH) (1′) had on the Imaging plate system (Rigaku). The structures were
formed. The crystals were collected by suction and solved by direct methods and refined by full-matrix least-
air-dried,
affording
[FeII(H2L1)2](BF4)2·0.5(i-Pr2O)·2(H2O)·2 square techniques on F2 using SHELXTL. The A-level alerts in
(CH3OH) (1). Yield 55.9 mg (66.4%). Elemental analysis anal. checkcif for the data of Complex 1′ at KEK are due to the
(calc.) for C49H53N10O4.6B2F8Fe1: C, 54.41 (54.32); H, 4.76 (4.93); experiments using the KEK synchrotron. The detector is
N, 13.18 (12.93)%. IR (KBr): 3277.1 (s, νN–H), 1084 (s, νB–F) cm−1
.
cylindrical and therefore has a huge diffraction angle range
Synthesis
of
[FeII(H2L2)2](BF4)2·0.5(AcOEt)·0.5(H2O)·1.5 but can only rotate the crystal along one axis. Therefore, all
(CH3OH) (2). Methanol solution (5 mL) of Fe(BF4)2·6H2O possible reflections cannot be obtained. The A-level alert in
(34 mg, 0.10 mmol) was added to a solution of H2L2 (75.8 mg, checkcif for 2′ is due to the disorder of the BF4− anion.
0.20 mmol) in methanol (10 mL). The resulting red solution
Variable-temperature Mössbauer experiments were carried
was filtered and allowed to diffuse with AcOEt. After a few out using a 57Co/Rh source in a constant acceleration trans-
days, red plates of [FeII(H2L2)2](BF4)2·(AcOEt)·1.5(CH3OH) (2′) mission spectrometer (Topologic Systems) equipped with an
were obtained. The crystals were collected by suction and Iwatani HE05/CW404 Cryostat. The spectra were recorded in
air-dried, affording [FeII(H2L2)2](BF4)2·0.5(AcOEt)·0.5(H2O)·1.5 the temperature range of 20–300 K. The spectrometer was cali-
(CH3OH) (2). Yield 55.4 mg (56.1%). Elemental analysis anal. brated using standard α-Fe foil.
(calc.) for C49.5H49N10O2B2F8Fe1: C, 56.51 (56.87); H, 5.01 (4.72);
Syntheses
N, 13.30 (13.40)%. IR (KBr): 3280.9 (s, νN–H), 1051.2 (s, νB–F) cm−1
.
Synthesis
of
[FeII(H2L3)2](BF4)2·2.5(H2O)·(i-Pr2O)
(3). The mononuclear bis-chelate iron(II) complexes (1–3) with
Methanol solution (5 mL) of Fe(BF4)2·6H2O (34 mg, asymmetric tridentate ligands H2L1–3 was synthesized. All com-
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plexes were synthesized in methanolic solution, but crystalliza-
tions for single crystal X-ray crystallography were performed
using different antisolvents. Fresh single crystals tend to lose
lattice solvent to the air. Therefore, physical measurements
were performed using air-dried samples.
Crystal structure of 1′
Complex 1′ crystallizes in the monoclinic space group C2/c
(Fig. 1). The cationic part of the complex consists of two H2L1
ligands and one Fe(II) ion, forming a bis-chelate type mono-
nuclear structure. The Fe(II) ion exists in an octahedral coordi-
nation environment, coordinated by six nitrogen atoms from
two H2L1 ligands. At 100 K, the coordinated benzimidazole
group was solved in two positions, the occupancies of which
were equal (Fig. S1†). The average Fe–N distances are 2.088 Å
and 2.104 Å, and the ∑ values are 105.8° and 134.4° for low
and high spin parts, respectively. From the data collected at
20 K with the synchrotron X-ray source, a similarly disordered
structure was determined. The average Fe–N distances are
2.063 Å and 2.097 Å, and the ∑ values are 110.6° and 138.3°
for low and high spin parts, respectively. The average dihedral
angle between the pyrazole and trimethylphenyl moieties is
12.30° (Fig. 2a) at 100 K.
Fig. 2 Dihedral angles between pyrazole and phenyl groups of (a) 1’, (b)
2’ and (c) 3’ at 100 K. For 2’, one orientation of the disordered trimethyl-
phenyl group was shown. Angles were indicated green letter, and the
value of disordered group was shown in parentheses.
In this structure, hydrogen bonded interactions were
observed between the pyrazole and benzimidazole moieties.
Two pyrazole N atoms of the iron complex interact with BF4
Fig. 3 One-dimensional chain network structure of 1’. Carbon atoms
of phenyl and pyridyl rings were omitted for clarity. Lattice solvents and
non-interacting BF4− anions have been omitted. Colour code: C, grey; N,
light blue; Fe(II), brown.
−
anions, forming a one-dimensional chain network along the b
axis (Fig. 3). Two benzimidazole N atoms interact with metha-
nol molecules. There are π stacking interactions between pairs
of 4-methylphenyl and benzimidazole groups, resulting a tetra-
mer of iron complexes.
Crystal structure of 2′
ˉ
Complex 2′ crystallizes in the triclinic space group P1
(Fig. S2†). The structure of 2′ is similar to 1′, but the solvent
molecules and intermolecular interactions were different. At
Fig. 4 One-dimensional chain network structure of 2’. Carbon atoms
of the phenyl and pyridyl rings were omitted for clarity. Lattice solvents
−
and non-interacting BF4 anions have been omitted. Colour code: C,
grey; N, light blue; Fe(II), brown.
100 K, the average Fe–N distance is 2.129 Å, and the ∑ value is
136.5°, indicating a mixture of low and high spin complexes.
The lattice contains disordered trimethyl phenyl groups and
−
BF4 anions. The average dihedral angle between the pyrazole
and trimethylphenyl moieties is 70.39° (Fig. 2b). Hydrogen
bonded interactions are operative between two pyrazole groups
and methanol molecules and two benzimidazole groups and
−
BF4 anions respectively, forming a one-dimensional chain
Fig. 1 Molecular structure of 1’. Lattice solvents and anions have been
omitted for clarity. Colour code: C, grey; N, light blue; Fe(II), brown. One
benzimidazole moiety of the disordered ligand has been omitted for
clarity.
structure along the b axis (Fig. 4). There are π stacking inter-
actions between benzimidazole moieties, forming a dimer of
iron complexes.
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Crystal structure of 3′
Complex 3′ crystallizes in the monoclinic space group P21/n
(Fig. S3†). The structure of 3′ is also similar to that of 1′, but
again, the solvent molecules and interactions with counter
anions are different. The Fe(II) ion exists in an octahedral
coordination environment, coordinated by six nitrogen atoms
from two H2L3 ligands. At 100 K, the average Fe–N distance is
2.178 Å, and the ∑ value is 142.3°, indicative of an iron(II) ion
in the high spin state. The average dihedral angle between pyr-
azole and trimethylphenyl moieties is 73.61° (Fig. 2c).
Disordered BF4− anions are present in the lattice. One pyrazole
moiety interacts with a solvent iPr2O molecule, while the other
pyrazole group interacts with a BF4− anion. Two benzimidazole
−
−
groups interact with BF4 anions. In the lattice, BF4 anions
link mononuclear iron moieties, forming a one-dimensional
network structure (Fig. 5). There are two kinds of π stacking
interactions; one between pentamethylphenyl and benzimida-
zole groups, and the other between two benzimidazole groups.
Distinct distortions of the dihedral angles between the pyra-
zole and phenyl groups were observed depending on the differ-
ences between the substituent groups. The trimethyl and pen-
tamethyl groups are likely to cause significant steric hindrance
of the neighbouring pyrazole moieties, resulting in large di-
hedral angles. This twisting of the molecules affects the inter-
molecular interactions such as π stacking and hydrogen
bonds.
●
■
▲
Fig. 6 Magnetic properties of 1 ( ), 2 ( ), and 3 ( ).
densities of the crystals are 1.347, 1.385, and 1.324 g cm−3 for
complexes 1′, 2′ and 3′ respectively. One can also speculate
that this may influence the magnetic behaviour and that the
lower density exhibited by 3′ may stabilise a high spin state in
the lower temperature region. A detailed consideration of
packing effects will require estimation of precise thermo-
dynamic parameters and DFT calculations.11
In order to elucidate this partial spin transition, Mössbauer
spectra were measured for complex 1 at 20 K and 300 K
(Fig. S7†). At 20 K, the data can be analysed as a mixture of
high- and low spin iron(II) species. On the other hand, the data
collected at 300 K reveals only high spin iron(II) species. These
facts indicate the occurrence of partial spin crossover in 1. The
obtained fitting parameters for Fe(II) high- or low-spin species
are reasonable values for low symmetry SCO complexes (Tables
S3 and S4†).12 Structural analysis of 1′ at 270 K was also per-
formed, and confirmed that the iron ion has a high spin state
based on coordination bond length (2.173 Å) and ∑ value
(140.0°). These data are consistent with partial spin crossover
behaviour.
LIESST experiments were conducted on complex 1, and the
magnetic data is shown in Fig. S8.† An increase in χmT values
was observed after light irradiation with a green. Structural
analyses after light irradiation were also performed at the KEK
synchrotron and the high spin spin state of iron(II) ion was
confirmed based on average coordination bond lengths
(2.127 Å) and ∑ values (129.1°).
Magnetic properties
The magnetic susceptibilities of 1–3 were studied by SQUID
magnetometry. The χmT vs. T plots for all complexes are shown
in Fig. 6. Complexes 1 and 2 show gradual SCO behaviour,
while complex 3 has a high spin state (S = 2) in the whole
temperature range. The χmT values for 1 and 2 at room temp-
erature are 3.11 and 3.37 emu mol−1 K, values consistent with
magnetically isolated high-spin Fe(II) ions (S = 2). Spin cross-
over temperatures for 1 and 2 were determined by maxima on
the dχ/dT plots, affording 200 K and 250 K, respectively. Below
100 K, the χmT values for 1 and 2 were almost constant (1.46
and 1.59 emu mol−1 K, respectively). The paramagnetic state
indicates partial spin transition of the iron ions, in agreement
with the structural data discussed above. Complexes 1–3
exhibit different magnetic behaviour originating from the
steric and packing effects of the ligands. We also note that the
Conclusions
Three iron(II) complexes of the general formula [FeII(H2L1–3)2]
(BF4)2·x(solv.), with different substituent groups, were syn-
thesized and their electronic states were investigated.
Complexes 1 and 2 show spin crossover behaviour, while 3 has
a high-spin state in the temperature range of 1.8–300 K.
Fig. 5 One-dimensional chain network structure of 3’. Carbon atoms
of phenyl and pyridyl rings were omitted for clarity. Lattice solvents and
non-interacting BF4− anions have been omitted. Colour code: C, grey; N,
light blue; Fe(II), brown.
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Asymmetric ligands with benzimidazole and pyrazole coordi-
nation moieties are found to provide iron(II) centres with
ligand field strengths of the right order to allow spin crossover
phenomena. The substituent groups affect the supramolecular
packing of the molecular species. These structural pertur-
bations are passed on to the coordination geometries of the
iron ions, significantly influencing the spin states of the com-
plexes. This insight into the design of modified asymmetric
mononuclear SCO complexes will aid in the future fine design
of bistable molecular systems.
L. J. K. Cook, R. Kulmaczewski, R. Mohammed, S. Dudley,
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Conflicts of interest
There are no conflicts to declare.
4 M. Yamada, M. Ooidemizu, Y. Ikuta, S. Osa, N. Matsumoto,
S. Iijima, M. Kojima, F. Dahan and J.-P. Tuchagues, Inorg.
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Acknowledgements
5 A. Galet, A. B. Gaspar, M. C. Munoz and J. A. Real, Inorg.
Chem., 2006, 45, 4413–4422.
This work was supported by a Grant-in-Aid for Scientific
Research (C) (no. 17K05800) and Grant-in-Aid for Scientific
Research on Innovative Areas ‘Coordination Asymmetry’ (no.
JP16H06523) from the Japan Society for the Promotion of
Science (JSPS). This work was performed under the approval of
the Photon Factory Program Advisory Committee (Proposal No.
2018G102).
6 SCO complex with bpp ligand: L. J. K. Cook,
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