Polynuclear Iron(II) Complexes with 2,6-Bis(pyrazol-1-yl)pyridine-anthracene Ligands Exhibiting Highly Distorted High-Spin Centers

Article abstract of DOI:10.1021/acs.inorgchem.8b03432

Two bis-tridentate ligands L1 and L2 that contain 2,6-bis(pyrazol-1-yl) pyridine N-donor embraces introduced on a anthracene-acetylene backbone were used for the synthesis of a tetranuclear compound [Fe₄(L1)₄](CF₃SO₃)₈·7CH₃CN (1) and a hexanuclear compound [Fe₆(L2)₆](CF₃SO₃)₁₂·18CH₃NO₂·9H₂O (2). The polynuclear structures of both complexes were confirmed by X-ray diffraction studies, which revealed a [2 + 2] grid-like complex cation for 1 and a closed-ring hexagonal molecular architecture for the complex cation in 2. Although both compounds contain anthracene moieties arranged in a face-to-face manner, attempts at [4 + 4] photocyclization remain unsuccessful, which can be explained either by steric restraints or by inhibition of the photo-cycloaddition. Magnetic studies identified gradual and half-complete thermal spin crossover in the tetranuclear grid 1, where 50% of ferrous atoms exhibit thermal as well as photoinduced spin state switching and the remaining half of iron(II) centers are permanently blocked in their high-spin state. On the contrary, the hexanuclear compound 2 exhibits complete blocking in a high-spin state. Analysis of the magnetic data reveals the zero-field splitting parameter |D| ≈ 6-8 cm^-1 with a large rhombicity for all high-spin iron(II) atoms in 1 or 2. The electronic structures and the magnetic anisotropies were also investigated by the multireference CASSCF/NEVPT2 method, and intramolecular exchange interactions were calculated by density functional theory methods.

Full text of DOI:10.1021/acs.inorgchem.8b03432

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Polynuclear Iron(II) Complexes with 2,6-Bis(pyrazol-1-yl)pyridine-
anthracene Ligands Exhibiting Highly Distorted High-Spin Centers
,†,‡
⊥
,§,#
Radovan Herchel,‡ Olaf Fuhr,§,∥ Rodrigo Gonzalez-Prieto, and Mario Ruben*
̌
̌
́
Ivan Salitros,*
†Department of Inorganic Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava,
Bratislava SK-81237, Slovakia
‡
́
Department of Inorganic Chemistry, Faculty of Science, Palacky University, 17. listopadu 12, 771 46 Olomouc, Czech Republic
§Institut fu
Karlsruhe 76021, Germany
̈
r Nanotechnologie and Karlsruhe Nano Micro Facility (KNMF), Karlsruher Institut fu
̈
r Technologie, Postfach 3640,
∥
⊥
́
Departamento de Química Inorganica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria,
E-28040 Madrid, Spain
#
́
́
Institute de Physique et Chimie de Materiaux de Strasbourg (IPCMS), CNRS-Universite de Strasbourg, 23, rue du Loess, BP 43
67034 Strasbourg cedex 2, France
S
* Supporting Information
ABSTRACT: Two bis-tridentate ligands L1 and L2 that contain
2,6-bis(pyrazol-1-yl) pyridine N-donor embraces introduced on a
anthracene-acetylene backbone were used for the synthesis of a
tetranuclear compound [Fe4(L1)4](CF3SO3)8·7CH3CN (1) and
a hexanuclear compound [Fe6(L2)6](CF3SO3)12·18CH3NO2·
9H2O (2). The polynuclear structures of both complexes were
confirmed by X-ray diffraction studies, which revealed a [2 + 2]
grid-like complex cation for 1 and a closed-ring hexagonal
molecular architecture for the complex cation in 2. Although
both compounds contain anthracene moieties arranged in a face-
to-face manner, attempts at [4 + 4] photocyclization remain
unsuccessful, which can be explained either by steric restraints or
by inhibition of the photo-cycloaddition. Magnetic studies
identified gradual and half-complete thermal spin crossover in
the tetranuclear grid 1, where 50% of ferrous atoms exhibit thermal as well as photoinduced spin state switching and the
remaining half of iron(II) centers are permanently blocked in their high-spin state. On the contrary, the hexanuclear compound
2 exhibits complete blocking in a high-spin state. Analysis of the magnetic data reveals the zero-field splitting parameter |D| ≈
6−8 cm−1 with a large rhombicity for all high-spin iron(II) atoms in 1 or 2. The electronic structures and the magnetic
anisotropies were also investigated by the multireference CASSCF/NEVPT2 method, and intramolecular exchange interactions
were calculated by density functional theory methods.
more attractive because light is a cheap and convenient trigger
that is capable of alternating between LS and HS states by
selective wavelengths with short response times and low power
dissipation.3 Two main concepts must be considered in this
regard: (i) the reversible spin state photoconversion based on
the direct irradiation of the coordination center, called a light
induced excited spin state trapping (LIESST) effect4,5 and (ii)
the change of ligand field strength by light-induced isomer-
ization of photosensitive substituents introduced on the ligand
skeleton, called ligand driven light induced spin change (LD
LISC).6 While the lifetime of metastable HS or LS complexes
formed by the direct LIESST mechanism is limited to
cryogenic temperatures (usually T < 100 K), the photo-
INTRODUCTION
■
Spin crossover (SCO) coordination compounds exhibiting a
competition between low-spin (LS) and high-spin (HS) states
are one of the most exciting examples of molecular bistability.1
Among the transition metal complexes showing the SCO
effect, the most intense variation of physical properties (i.e.,
change in magnetic moment, color, conductivity, or other
physicochemical properties) has often been found in ferrous
complexes with N-donor ligands.2 SCO events in this class of
complexes can be activated by different external stimuli (T, p,
B, hν), and their switching characteristics make them of
interest in a variety of applications, including sensors, memory,
and solid-state electronic devices. However, in practical
applications, SCO should be ideally operated at room
temperature and atmospheric pressure.1c,2 Moreover, com-
pared with thermally activated SCO, photoinduced SCO is
Received: December 10, 2018
© XXXX American Chemical Society
A
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Figure 1. Molecular structures of bis-tridentate anthracene-bpp ligands L1 and L2 and their corresponding iron(II) polynuclear complexes 1 and 2.
Figure 2. Representations of the complex cation [Fe4(L1)4]8+ of 1 (a) and the complex cation [Fe6(L2)6]12+ of 2 (b) in the crystal structures.
Visualization of the coordination of two L2 ligands with iron(II) coordination centers in the crystal structure of 2 (c) (orange − Fe, blue − N, red
− O, gray − C).
isomerization of ligand moieties may lead to reproducible
isothermal LS ↔ HS alternation even at ambient temperatures.
The efforts of the last three decades in this field have therefore
been focused on the preparation of SCO complexes with
ligands containing light-sensitive stilbenes,7 azabenzenes,8 or
diarylethenes moieties.9 Surprisingly, there is a lack of SCO
systems with anthracene photoisomerizable groups, despite
their potential to carry out visible light [4 + 4] photo-
cyclization, UV light (or thermal) cycloreversion, and the high
thermal stability of the corresponding photoisomers (Figure
S2, see Supporting Information).10−12 To the best of our
knowledge, there is only one report of an dinuclear copper(II)
complex, in which light-induced reversible photoisomerization
was employed for controlling magnetic properties.13
Our group has recently designed three novel bis-tridentate
2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands that consist of two
parallel 1,8-diethynylanthracene-bpp shoulders oriented to
each other in a face-to-face fashion and connected to one of
the central aromatic moieties: anthracene, 10-methoxyantra-
cene, or anthraquinone.14 Successful intramolecular photo-
B
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a
Table 1. Bond Lengths, Angles, and Distortion Parameters of the Reported Complexes
1
Fe1
Fe2
Fe3
Fe4
2
Fe−Npy1
Fe−Npz1
Fe−Npz2
Fe−Npy2
Fe−Npz3
Fe−Npz4
Σ /°
1.919(6)
1.968(7)
1.980(7)
1.911(6)
1.982(7)
1.967(8)
84.5
1.896(6)
1.959(6)
1.948(7)
1.877(6)
1.952(6)
1.965(7)
86.1
2.134(6)
2.188(7)
2.181(7)
2.120(6)
2.174(7)
2.145(6)
157.3
2.168(6)
2.169(7)
2.191(7)
2.150(6)
2.150(6)
2.199(9)
173.0
2.157(5)
2.182(7)
2.192(8)
2.163(4)
2.171(6)
2.178(5)
194.2
Θ /°
290.4
276.2
491.9
517.9
587.3
ϕ /°
θ /°
170.8(3)
88.48
171.7(3)
88.11
159.9(2)
78.21
151.5(3)
74.72
148.2(3)
66.33
α /°
φ /°
S(OC-6)
S(TPR-6)
79.85(3)
159.5(3)
2.420
80.35(3)
160.5(3)
2.172
73.25(2)
145.5(2)
7.289
72.75(3)
143.5(2)
8.871
71.93(2)
139.7(2)
11.447
9.731
10.199
7.447
5.951
5.079
a
pz-pyrazole; py-pyridine; see Supporting Information, Table S2 and Figure S5 and S6 for an explanation of all the listed parameters.
cyclization of the peripheral anthracenes and thermally
activated decyclization have been observed in two earlier
examples. However, the presence of anthraquinone in the
molecular structure of the third ligand (Figure 1, L2) caused
the inhibition of [4 + 4] photo-cycloaddition. Nevertheless, the
topological molecular architecture of these ligands, aiming two
parallel tridentate N-donor embraces oriented in the same
direction, offers the formation of oligonuclear iron(II)
complexes rather than coordination polymers.15 In addition,
the well-known family of iron(II)-bpp complexes often exhibits
abrupt and room temperature thermal SCO accompanied by
thermal hysteresis loops as well as photoinduced LS ↔ HS
transitions (LIESST effect).16 Thus, following the idea about
the phototunable oligomeric SCO systems with regular
molecular architectures, we report on the synthesis and the
structural and magnetic characterization of [2 + 2] grid-like
tetranuclear and ring-shaped hexanuclear ferrous coordination
compounds based on bis-tridentate anthracene-bpp ligands
(Figure 1). The magnetic studies reveal half-complete and
gradual SCO for the tetranuclear compound [Fe4(L1)4]-
(CF3SO3)8·7CH3CN (1) and complete HS paramagnetic
behavior for the hexanuclear compound [Fe6(L2)6]-
(CF3SO3)12·18CH3NO2·9H2O (2). The Ising-like model was
utilized to analyze the SCO behavior in 1, and the zero-field
splitting parameters of the HS Fe(II) atoms in 1 and 2 were
extracted. The study is corroborated also by CASSCF/
NEVPT2 calculations and density functional theory (DFT)
calculations.
complexes were prepared by a reaction between the
corresponding ligand and the iron(II) triflate in an equimolar
ratio. The synthesis was carried out in a nitromethane/
acetonitrile solvent mixture (1:1) and resulted in the self-
assembled formation of tetranuclear and hexanuclear com-
pounds 1 and 2, respectively. Thus, as emphasized in the
introduction, bis-tridentate ligands with two parallel tridentate
N-donor embraces indeed offer the formation of oligomeric
structures when coordinated with transition metals. In line
with previous reports on tetranuclear ferrous complexes with
miscellaneous bis-tridentate ligands,15 compound 1 forms the
tetranuclear [2 + 2] grid complex cations (vide infra, Figure
2a). On the other hand, the self-assembly of L2 with iron(II)
metal ions results in the formation of the closed ring-shaped
hexanuclear complex cation (Figure 2b), which most probably
shows higher thermodynamical stability than a possible
tetranuclear grid architecture, as observed in 1. Single crystals
were obtained by the slow diffusion of diisopropyl ether vapor
into the respective solution of these compounds at room
temperature. The UV−vis absorption spectra of both
polynuclear complexes show a series of vibrationally spaced
bands at wavelengths of λ = 360−480 nm, which are typically
attributed to π−π* absorptions of the anthracene systems.
Irradiation into these bands has been reported to lead to [4 +
4] photo-cyclization (Figure S2, see Supporting Information)
of the anthracene rings.11,12 However, in both cases, blue light
irradiation (λ = 470 nm 50 nm) in an acetonitrile solution of
1 and 2 did not afford the formation of photoisomers, which
can tentatively be explained by steric issues in the earlier case
and by inhibition of [4 + 4] photocyclization in the latter case.
From structural investigations, it is evident (vide infra) that the
distances between opposite carbon atoms (9′···9′′ and 10′···
10′′) of two parallel anthracenes of 1 are more than 7 Å long,
which makes the intramolecular [4 + 4] photo-cycloaddition
impossible, even though both anthracenes are arranged in a
face-to-face manner. Therefore, the steric problems concerning
the realization of photoisomerization in the solution originate
from the structure of the self-assembled tetranuclear complex
cation itself. On the other hand, the distance between the two
peripheral face-to-face arranged anthracenes in 2 are ideal
(∼3.5 Å; vide infra) in terms of intramolecular photo-
cyclization. However, as previously reported in the case of
uncoordinated L2,14 photoisomerization is most probably
RESULTS AND DISCUSSION
■
Synthesis and Structural Investigation. Ligand L1 was
prepared by a three-step synthetic protocol (see Supporting
Information) starting from 1,8-dichloroantraquinone through
the reduction to 1,8-dichloroanthracene, the formation of 1,8-
diethynylanthracene by a Grignard reaction, and the
consecutive deprotection of TMS groups. Finally, the
introduction of 2,6-bis(pyrazol-1-yl)pyridine moieties was
achieved by Pd-catalyzed Sonogashira coupling. The total
yield exceeds 40%, which can be considered as reasonable
given the complexity of the three-step synthesis, and it
furnishes the ligand L1 on a rather useful scale of hundreds of
milligrams. Ligand L2 was synthesized according to a
previously reported synthetic procedure.14 The reported
C
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Article
inhibited by the presence of anthraquinone in the molecular
structure of 2.
dihedral angle between the least-squares planes of the two bpp
moieties coordinating the same metal center θ and the rotation
of one bpp moiety with respect to the opposite one about the
iron(II) atom ϕ (Npy−Fe−Npy)19a (Figure S6, see Supporting
Information) would acquire 90° and 180°, respectively, when
the coordination polyhedron {FeN6} exhibits ideal octahedral
symmetry.20 Since the LS sites are less distorted than the HS
sites due to the absence of Jahn−Teller distortion, their values
are closer to the regular octahedron angles (Table 1), while the
HS centers exhibit those angles significantly lower (Δϕ(HS-
LS) ≈ 16°; Δθ(HS-LS) ≈ 7°). A similar trend has also been
observed in the case of the bite angle α (Npz−Fe−Npy) and
clamp angle φ (Npz−Fe−Npz), which vary in the suggested
ranges α = 72−74°, φ = 150−140° for HS centers, and α =
79−81°, φ = 155−163° for LS centers, respectively.19a The
best way of classifying the geometry of the coordination
polyhedron in 1 is to use the continues shape measures
methodology proposed by Alvarez et al.21 The results of the
calculations (see Supporting Information, Table S2, Figure S5)
show that the lowest deviations in both LS centers are for a
regular octahedron (S(OC-6) = 2.4 and 2.2, respectively) and
the second lowest for a trigonal prism shape (S(TPR-6) = 9.7
and 10.2, respectively). The HS coordination center Fe3
exhibits a comparable level of deviation from an octahedral and
from a trigonal prism geometry (S(OC-6) = 7.3; S(TPR-6) =
7.4). Interestingly, the HS polyhedron of Fe4 with the highest
degree of angular distortion (i.e., Σ, Θ) shows the lowest
deviation from a trigonal prism geometry (S(TPR-6) = 6.0)
and the second lowest from an octahedral symmetry (S(OC-6)
= 8.9). Neighboring tetranuclear complex cations are
interconnected via weak π−π interactions, which are
responsible for the formation of the one-dimensional (1D)
chain along the a−c plane (see Supporting Information, Figure
S7). Four anthracene moieties of two [Fe4(L1)4]8+ cations are
arranged in a head-to-tail manner, with a distance of 3.8 Å
between 9′ and 10′′ carbon atoms. According to this motive,
two peripheral and two inner anthracenes, respectively, are
almost linear to each other, and the angle between the
peripheral and inner couple is around 11.1° (Figure S7, see
Supporting Information). Such arrangement of anthracene
units of neighboring complex cations of 1 is promising with
respect to intermolecular [4 + 4] photo-cycloaddition;
however, the blue light irradiation of the single crystals did
not show any signature of photocyclization within a two-
month period of irradiation (see Supporting Information).
The hexanuclear compound 2 with the formula [Fe6(L2)6]-
(CF3SO3)12·18(CH3NO2)·9(H2O) crystallizes in a trigonal R3
symmetry, and the asymmetric unit consists of one ligand L2,
one iron(II) central atom, two triflate anions, and the
corresponding solvent molecules 3 CH3NO2 and 3/2 H2O.
In the complex cation, the six ligands L2 coordinate six
iron(II) centers and form a hexagonal closed ring shape of
[Fe6(L2)6]12+ composition (Figure 2b). The intramolecular
distance between the neighboring coordination centers is 8.26
Å, and each of them is surrounded by six nitrogen atoms of two
bpp moieties originating from two opposite ligands L2(Figure
2c). Each coordinated ligand contains two anthracene rings
packed in a face-to-face fashion with distances of 3.7 and 3.5 Å
for 9′···9′′ and 10′···10′′ carbon atoms, respectively, and with
the angle between their planes about 19.5°. Again, such
packing of anthracene units usually supports the [4 + 4] photo-
cycloaddition. However, in line with our previous results
related to the photoisomerization attempts with the pure
The crystallographic investigation confirmed the polynuclear
structures of both compounds 1 and 2. The single-crystal X-ray
diffraction of 1 revealed a monoclinic P21/c space group at 100
K, and selected crystallographic parameters are given in Table
S1 (see Supporting Information). The asymmetric unit
consists of one cation [Fe4(L1)4]8+, eight triflate anions, and
seven acetonitrile solvent molecules. The shape of the complex
cation [Fe4(L1)4]8+ is like a [2 + 2] grid, in which four L1 act
as bis-tridentate ligands and coordinate four iron(II) metal
centers (Figure 2a). The intramolecular distances between the
iron(II) centers range from 8.05 to 8.67 Å. The anthracene
moieties of the two neighboring parallel ligands are arranged in
a face-to-face fashion at a distance of about 7.3 Å from each
other and with the angles between the anthracene planes of
14.2° and 11.0°, respectively. Each iron(II) center is
surrounded by six N donor atoms of two bpp moieties of
opposite ligands L1. The Fe−N bond distances clearly
distinguish two different coordination sites at 100 K (Table
1): Fe1 and Fe2 with average Fe−N distances of 1.94 Å are LS,
while the bond lengths of the second couple (Fe3 and Fe4) are
with 2.16 Å Fe−N indicative of the HS state. Selected angles of
the coordination polyhedra (Table 2; Figure S6, see
Table 2. CASSCF/NEVPT2 Calculated Parameters for HS
Iron Atoms in 1 and 2
a
a
Fe3 in 1
Fe4 in 1
Fe1 in 2
ZFS and g parameters based on CASSCF/NEVPT2 calculations with
CAS(6,5)
D (cm−1
E/D
gx
)
−17.0
0.212
2.063
1.984
2.346
590
−13.6
0.196
2.051
1.995
2.288
704
7.08
0.216
2.067
2.160
2.011
1374
gy
gz
b
ΔLFT (cm−1
)
ZFS and g parameters based on CASSCF/NEVPT2 calculations with
CAS(6,10)
D (cm−1
E/D
gx
)
−15.9
0.227
2.065
1.979
2.336
575
−13.1
0.207
2.049
1.989
2.286
665
6.94
0.216
2.064
2.159
2.008
1278
gy
gz
b
ΔLFT (cm−1
)
ZFS and g parameters based on CASSCF/NEVPT2 calculations with
CAS(10,12)
D (cm−1
E/D
gx
)
−17.1
0.216
2.061
1.976
2.358
523
−14.4
0.189
2.044
1.985
2.312
586
7.15
0.241
2.062
2.171
2.008
1172
gy
gz
b
ΔLFT (cm−1
)
a
The first excited state is relatively close to the ground state; thus, the
b
spin Hamiltonian parameters must not be valid. The energy
separation of the first excited ligand-field term.
Supporting Information) together with Σ17 and Θ18
parameters, which are sensitive to the degree on Jahn−Teller
distortion existing only in the HS iron(II) centers, therefore
being excellent indicators of the spin state.19 Furthermore, it is
obvious that angular distortion parameters Σ and Θ acquire
double values for the HS iron(II) sites compared to the LS
ones (ΔΣ(HS-LS)avg ≈ 80°; ΔΘ(HS-LS)avg ≈ 222°). The
D
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Figure 3. Magnetic data for compound 1. The temperature dependence of the effective magnetic moment measured at B = 0.1 T with a zoom-in
into the low temperature region shown in the inset (left) and the reduced isothermal magnetizations measured at T = 1.8, 3, 4, and 5 K (right). The
empty circles represent the experimental data points, and the black full line represents the best fit calculated with eq 1 using D = +6.8 cm−1, E/D =
0.28, g = 2.35, zj = 0.068 cm−1. The blue full line represents the best fit calculated with eqs 4−5 using gHS = 2.38, ΘHS = −1.48 K, Δ = 704 K, reff
14, and γ = 48 K.
=
ligand L2,14 the blue light irradiation of single crystals of 2 has
neither brought any evidence for a solid-state intramolecular
photocyclization. At 180 K, the Fe−N bond distances acquire
an average value of around 2.17 Å, typical for the HS state.
Angular distortion parameters Σ and Θ as well as the selected
angles of the coordination polyhedra (Table 2; Figure S6, see
Supporting Information) indicate a pronounced Jahn−Teller
effect, and according to Halcrow and co-workers, their
extremely high values situate compound 2 to the group of
permanently HS iron(II)-bpp complexes where SCO is
disfavored or prohibited.19 Continues shape measures (Table
S2, see Supporting Information) indicate severe deviation from
all considered symmetries. Surprisingly, the coordination
polyhedra of all six crystallographically equivalent iron(II)
centers are like a trigonal prismatic shape (S(TPR-6) = 5.1),
while the second lowest symmetry measure parameters for the
expected octahedral symmetry (S(OC-6) = 11.4) indicate
already pronounced divergence. This can be attributed to the
double axial distortion quantified by an extremely low clamp
angle φ.21d Molecules of crystal water create a hydrogen-
bonded network (d(O···O) = 2.79 Å) in the central cavity of
the hexanuclear wheel [Fe6(L2)6]12+, and some other weak
contacts between lattice solvent molecules and triflate anions
are present as well. Careful investigation of supramolecular
packing of the neighboring complex cations has not revealed
any significant noncovalent interactions between them.
one tetranuclear complex unit at 100 K. Further cooling down
to 2 K causes a decrease in the product function of up to 5.0
cm3 K mol−1, which can be ascribed to the zero-field splitting
of HS iron(II) atoms and eventually to weak exchange
interactions. The magnetic anisotropy is also evident from the
isothermal magnetization measurements undertaken at the
lowest temperatures, where Mmol/NAμB reaches a value of 6.8
at B = 7 T and T = 1.8 K, much lower than theoretical limit
M
mol/NAμB = 2 × g × S = 8. Compound 1 also exhibits
photoinduced SCO upon the red-light irradiation (637 nm),
causing an increase in the χT product from 7.3 cm3 K mol−1 to
8.8 cm3 K mol−1 at 10 K. The χT product of the photoexcited
metastable HS fraction increases with further temperature-
sweeping in the dark and reaches the maximum 9.5 cm3 K
mol−1 at 35 K, corresponding to about 70% of HS iron(II)
centers. Subsequent increase of the temperature leads to
thermal relaxation back to the LS state, which is completed
above 80 K. The rather gradual character of the thermal HS →
LS relaxation made the calculation of the T(LIESST)
temperature impossible.
To estimate the magnetic anisotropy of these HS iron(II)
atoms, the low temperature data below 30 K, unaffected by
SCO, were fitted with the following spin Hamiltonian
2
2
2
2
̂
̂
̂
̂
̂
̂
̂
̂
H = D(Sz − S /3) + E(Sx − Sy ) + μ BgS − zj⟨S ⟩S
a
a
a
B
(1)
Magnetic Properties. The temperature and field-depend-
ent magnetic data on the tetranuclear compound 1 are shown
in Figure 3. The value of the χT measured at 400 K is 14.1 cm3
K mol−1, close to the theoretical value of 12.0 cm3 K mol−1
(with g = 2.0 for four non-interacting HS iron(II) atoms
showing a S = 2 ground spin state). Upon cooling, the χT
product starts to decrease at ca. 330 K, reaching a plateau of
8.4 cm3 K mol−1 at ca. 100 K and manifesting the presence of
the SCO phenomenon in 1. This experimental value
corresponds to the two iron(II) atoms in HS state and the
two iron(II) atoms in LS state for which the theoretical χT is
6.0 cm3 K mol−1. The presence of half-complete SCO in 1 with
a transition temperature determined at T1/2 = 236 K agrees
with the results from the structural investigations (vide supra),
where two LS and two HS iron(II) centers were established in
where the single-ion zero-field splitting parameters D and E,
the isotropic g-factor, and the molecular field correction zj
parameters are present. The ⟨Sa⟩ is the thermal average of the
molecular spin projection in the a direction of the magnetic
field, defined as Ba = B·(sin θ cos ϕ, sin θ sin ϕ, cos θ), with
help from the spherical coordinate system. The molar
magnetization in a-direction of magnetic field can then be
numerically calculated as
∑ ∑ ∑ C+(Z ) C exp(−ε /kT)
(
)
ik a kl li
a,i
i
k
l
Ma = −N
A
∑ exp(−ε /kT)
(2)
ai
i
where Za is the matrix element of the Zeeman term for the a
direction of the magnetic field and C refers to the eigenvectors
resulting from the diagonalization of the complete spin
E
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Figure 4. Magnetic data for compound 2. Temperature dependence of the effective magnetic moment measured at B = 0.1 T (left) and reduced
isothermal magnetizations measured at T = 1.8, 3, 4, and 5 K (right). The empty circles represent the experimental data points. The black full line
represents the best fit calculated by eq 1 using D = +7.8 cm−1, E/D = 0.31, g = 2.14, zj = 0.14 cm−1, χTIP = 20 × 10−9 m3 mol−1.
Figure 5. Molecular fragment of 2, comprising an Fe1 atom derived from the experimental X-ray geometry used for CASSCF/NEVPT2’s
calculation, also showing the principal axes of the D-tensor with the arrows (x-axis is green, y-axis is violet, and z-axis is red).
Hamiltonian matrix. The inclusion of zj means that an iterative
procedure needs to be applied.22 Then, the averaged molar
magnetization of the powder sample can be calculated as the
integral (orientational) average
temperature dependence of the mole fraction of the HS state
χHS, which is the next step used to calculate the molar
susceptibility of 1 as
χmol = 2χHS + 2xHSχHS + 2(1 − xHS)χ
(5)
LS
2π
Mmol = 1/4π
π Masin θ dθdφ
∫ ∫
(3)
0
0
taking into account that two iron atoms remain in an HS state
and the next two iron atoms undergo spin crossover. The
molar susceptibility of the HS state χHS was calculated by the
Curie−Weiss equation, and χLS equals zero. Finally, the
parameters Δ γ, reff, gHS, and ΘHS (per one iron atom) were
varied during the fitting procedure. The best fit was found with
Both temperature and field dependent data were fitted
simultaneously, which resulted in D = +6.8 cm−1, E/D =
0.28, g = 2.35, zj = 0.068 cm−1, and similarly, a good fit was
also found for the negative D-parameter: D = −5.9 cm−1, E/D
= 0.33, g = 2.35, zj = −0.019 cm−1. The results suggest the axial
type of the magnetic anisotropy and the presence of weak
exchange or dipolar interactions.
these parameters: gHS = 2.38, ΘHS = −1.48 K, Δ = 704 K, reff
=
14 and γ = 48 K (Figure 3). The fitted g-factor is close to that
found by the spin Hamiltonian in eq 1, and the negative Weiss
constants mostly reflect the zero-field splitting of the HS
iron(II) atoms. The microscopic parameters of the Ising-like
model were then utilized to calculate the enthalpy and the
entropy of the spin transition as ΔH = NAΔ = 5.85 kJ mol−1
and ΔS = R ln reff = 21.9 J K−1 mol−1, with a transition
temperature of T1/2 = ΔH/ΔS = 266 K. These parameters
adopt typical values for iron(II) SCO compounds.
The spin crossover behavior of 1 is quantitatively described
by an Ising-like model based on the following Hamiltonian23
Δ
2
̂
H =
σ − γ⟨σ⟩σ
̂ ̂
(4)
where Δ is the energy difference between HS and LS states, γ
is the cooperativeness, and reff is the effective degeneracy ratio
between HS and LS states. This model enables to calculate the
F
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Figure 6. Graphical output of the CASSCF/NEVPT2 calculations with CAS(6,5) for the mononuclear molecular fragments of 1 and 2. Plot of the
d-orbitals splitting calculated by ab initio ligand field theory (AILFT) (left), low-lying ligand-field terms (middle), and ligand-field multiplets
(right). The singlet, triplet, and quintet spin states are shown in red, green, and blue, respectively.
The temperature and field-dependent magnetic data of the
hexanuclear compound 2 are depicted in Figure 4. The value of
the χT product measured at 300 K is 23.4 cm3 K mol−1, which
is slightly higher than the theoretical value of 18.0 cm3 K mol−1
(with g = 2.0 for six noninteracting HS iron(II) atoms with S =
2 ground spin state). The monotonical decrease of χT down to
ca. 30 K can probably be ascribed to the temperature-
independent paramagnetism (TIP), and the drop of χT to 14.0
cm3 K mol−1 at T = 2 K is due to the zero-field splitting and
weak exchange interactions among the paramagnetic atoms.
This finding is also apparent from the isothermal magnet-
ization measurements undertaken at the lowest temperatures,
where Mmol/NAμB adopts values of 17.6 at B = 7 T and T = 1.8
K, which is much lower than the theoretical limit Mmol/NAμB =
6 × g × S = 24.
Therefore, the magnetic data of 2 were treated with a spin
Hamiltonian in eq 1, and as a result, two parameter sets are
obtained: (i) D = +7.8 cm−1, E/D = 0.31, g = 2.14, zj = 0.14
cm−1, χTIP = 20 × 10−9 m3 mol−1, and (ii) D = −7.6 cm−1, E/D
= 0.33, g = 2.15, zj = 0.11 cm−1, χTIP = 19 × 10−9 m3 mol−1.
Obviously, both sets are able to satisfactorily describe the
experimental data with |D| ≈ 8 cm−1 and limiting rhombic
anisotropy E/D ≈ 1/3; thus, the determination of the sign of
the D-parameter from the experimental data is inconceivable.
Theoretical Calculations. The multireference ab initio
calculations based on the state-averaged complete active space
self-consistent field method (SA-CASSCF) were carried out to
inspect the electronic structure of the iron atoms in
compounds 1 and 2. Therefore, the ORCA software24 was
used in the CASSCF calculation with the active space defined
for six electrons in five d-orbitals, CAS(6,5). Additionally,
dynamic electronic correlation was handled by using the
NEVPT2 method. These calculations were undertaken on the
mononuclear molecular fragments extracted from the exper-
imental X-ray structures of 1 and 2 by replacing the anthracene
units with methyl groups as shown in Figure 5. Figure 6
summarizes the obtained results for 1 and 2. The atomic
positions of the hydrogen atoms were optimized using the
B97-3c method.25
energies of the three former t2g and two former eg orbitals, the
estimates of 10Dq were calculated as 17 405 cm−1 for Fe1 of 1,
18 956 cm−1 for Fe2 of 1, 7480 cm−1 for Fe3 of 1, 6768 cm−1
for Fe4 of 1 and 6248 cm−1 for Fe1 of 2. Thus, the ratio for the
10DqLS/10DqHS is approximately 2.3−2.8 in 1. Subsequently,
the CASSCF/NEVPT2 ligand field terms are shown in Figure
6 (middle), where the large energy splitting of the d-orbitals
results in the singlet ground state of the Fe1 and Fe2 atoms in
1 originating from the 1A1 state, and the Fe3 and Fe4 atoms in
1 and the Fe1 in 2 have the three lowest quintet states
originating from the splitting of 5T2 state in ideal Oh symmetry.
The first excited quintet terms have energy separations of 590
cm−1 in Fe3 of 1, 704 cm−1 in Fe4 of 1, and 1374 cm−1 in Fe1
of 2. In the case of the Fe1 and Fe2 atoms in 1, the first excited
singlet terms suitable for the LIESST effect have energies of
16881 and 18135 cm−1, respectively, in reasonable agreement
with the energy of 637 nm (15700 cm−1) used for the
photoexcitation in 1.3b After the spin−orbit coupling was
incorporated, the ligand-field terms (LFT) are split into the
ligand-field multiplets (LFM), as plotted in Figure 6 (right).
The application of the zero-field splitting spin Hamiltonian to
the quintet ground states enabled us to extract the ZFS D and
E parameters, and g-tensor parameters are summarized in
Table 2. The utilization of the effective Hamiltonian theory
resulted in relatively large and negative D-parameters for Fe3
and Fe4 in 1 with large rhombicity, E/D ≈ 0.2. However, the
presence of close lying excited states means that the spin
Hamiltonian approach is no longer valid, and more elaborated
theoretical models incorporating the orbital angular momen-
tum should be utilized. This could also explain the large
difference between ab initio calculated as D = −13.6/−17.0
cm−1 and the fitted averaged D-parameter, D = 6.8/-5.9 cm−1.
On the contrary, the ab initio calculations for Fe1 in 2 show a
good agreement between the calculated (D = 7.08 cm−1).
Furthermore, the fitted (D = 7.8/−7.6 cm−1) ZFS parameters
and the large energy of the first excited state also validate the
utilization of the spin Hamiltonian. The calculated principal
axes of the D-tensor are visualized in Figure 5. Furthermore,
the double-shell effect, which is supposed to be important for
late 3D-metals, was accounted for by enlarging the active space
to 10 d-orbitals (3d + 4d). Remarkably, this approach
(CAS(6,5) vs CAS(6,10)) has only a minor impact on the
derived D and E ZFS parameters and g-factors (Table 2). In
line with previous studies,26 we extended the active space with
two σ-type orbitals to CAS(10,12), and again, there were only
The ab initio ligand field theory (AILFT) was used to
calculate the energy of the d-orbitals as depicted in Figure 6
(left). In perfect Oh symmetry, the d-orbitals split into two sets
of t2g and eg orbitals separated by the energy of 10Dq, but due
to the lower symmetry of the coordination polyhedron, the
degeneracy of these orbitals is removed. By averaging the
G
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Figure 7. Molecular fragment [Fe6(L2)6]12+ of 2 and B3LYP calculated spin isodensity surface for the HS state (left) and the broken-symmetry
spin state (right) with a cutoff value of 0.005 e·bohr−3. The positive and negative spin densities are represented by the yellow and cyan surfaces,
respectively. The hydrogen atoms are omitted for clarity.
minor changes in the ZFS parameters, as listed in Table 2. A
comparison of all ligand-field terms and multiplets for all three
active space choices is plotted in Figure S8 (see Supporting
Information).
Finally, the DFT calculations were applied to 2 to evaluate
the superexchange interactions among the Fe atoms, because
the fitting of the experimental magnetic data suggests non-
negligible ferromagnetic interactions (zj = 0.14 cm−1).
Therefore, the broken symmetry approach was used for the
proposed spin Hamiltonian
those iron(II) sites, which are permanently retained in the HS
state. Even higher angular distortion was found to be present in
the hexanuclear complex 2. We can conclude that the
pronounced structural deviation of the coordination polyhedra
stabilizes the HS state of iron(II) centers. An analysis of the
magnetic properties revealed the zero-field splitting parameter |
D| ≈ 6−8 cm−1 with large rhombicity (E/D ≈ 0.28−0.33) for
all HS iron(II) atoms in 1 or 2, indicating the axial type of the
magnetic anisotropy. The Ising-like model was applied to
analyze the SCO behavior of 1, providing estimates in spin
transition enthalpy and entropy, with the spin transition
temperature being T1/2 = 266 K. The electronic structure and
magnetic anisotropy were also investigated by means of
multireference CASSCF/NEVPT2 methods, confirming the
ground spin state properties for all iron atoms in 1 and 2 and
also revealing the substantial magnetic anisotropy of the
involved HS iron(II) atoms. The non-negligible weak
ferromagnetic exchange in 2 was also substantiated by the
DFT level using well-established B3LYP functional.
We also tried to control the magnetic and spin state
properties by light. Ferrous complexes with potential SCO
properties containing photoactive anthracene moieties offer
the possibility of [4 + 4] photoisomerization. However, due to
steric problems in the case of tetranuclear compound or the
inhibition of the photo-cycloaddition mechanism caused by the
presence of anthraquinone in the hexanuclear compound, both
complexes exhibit innocent behavior independently upon the
wavelength selected for irradiation. Thus, further careful
molecular design is necessary for the synthesis and successful
photo-isomerization of the next iron(II) SCO complexes
containing the anthracene photoisomerizable units.
̂
H = −J(S1·S2 + S2·S3 + S3·S4 + S4·S5 + S5·S6 + S1·S6)
(6)
where J is the isotropic exchange parameter among the
adjacent iron atoms. Then, Ruiz’s approach was employed to
derive the following relationship
J = Δ/90
(7)
where Δ is the energy difference between the broken-
symmetry spin state and the HS spin state, Δ = εBS − εHS.
Herein, the well-established B3LYP functional was used to
calculate these spin states, for which the spin densities are
shown in Figure 7, which resulted in Δ = 2.984 cm−1 and
consequently, J = 0.033 cm−1. Thus, these calculations support
the weak ferromagnetic interactions mediated by the
anthracene-bpp L2 ligand in 2.
CONCLUSION
■
In conclusion, we have successfully prepared two new
polynuclear iron(II) complexes with bis-tridentate ligands
containing two 2,6-bis(pyrazol-1-yl)pyridine N-donor moieties
introduced on an acetylene-anthracene skeleton. Magnetic
investigations revealed the presence of a half-complete SCO in
the tetranuclear compound and HS behavior in the
hexanuclear compound. Structural investigations confirmed
the polynuclear structures of both compounds. In line with
magnetic properties, X-ray diffraction studies reveal two
iron(II) centers in HS state and another two in LS state in
the tetranuclear [2 + 2] grid-like complex. Inspection of the
symmetry in the coordination polyhedra revealed severe
angular deviation from the expected octahedral shape for
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03432.
Synthesis and structural, magnetic, and computational
details (PDF)
H
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Light-Induced Spin Change at Ambient Temperatures in Bis-
(dipyrazolylstyrylpyridine)iron(II) Complexes. Inorg. Chem. 2012,
51 (9), 5188−5198.
Accession Codes
CCDC 1862242−1862243 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(8) Hasegawa, Y.; Kume, S.; Nishihara, H. Reversible light-induced
magnetization change in an azobenzene-attached pyridylbenzimida-
zole complex of iron(II) at room temperature. Dalton Trans. 2009,
280−284.
̈
(9) Rosner, B.; Milek, M.; Witt, A.; Gobaut, B.; Torelli, P.; Fink, R.
H.; Khusniyarov, M. M. Reversible Photoswitching of a Spin
Crossover Molecular Complex in the Solid State at Room
Temperature. Angew. Chem., Int. Ed. 2015, 54, 12976−12980.
(10) Becker, H.-D. Unimolecular photochemistry of anthracenes.
Chem. Rev. 1993, 93, 145−172.
AUTHOR INFORMATION
Corresponding Authors
*(M.R.) E-mail: mario.ruben@kit.edu.
*(I.S.) E-mail: ivan.salitros@stuba.sk.
ORCID
■
(11) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade,
R. Photodimerization of anthracenes in fluid solution:structural
aspects. Chem. Soc. Rev. 2000, 29, 43−55.
̌
̌
Ivan Salitros: 0000-0003-3856-0399
Radovan Herchel: 0000-0001-8262-4666
(12) Bouas-Laurent, H.; Castellan, A.; Desvergne, J.-P.; Lapouyade,
R. Photodimerization of anthracenes in fluid solutions: (part 2)
mechanistic aspects of the photocycloaddition and of the photo-
chemical and thermal cleavage. Chem. Soc. Rev. 2001, 30, 248−263.
(13) Castellano, M.; Ferrando-Soria, J.; Pardo, E.; Julve, M.; Lloret,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
́
F.; Mathoniere, C.; Pasan, J.; Ruiz-Perez, C.; Canadillas-Delgado, L.;
Ruiz-García, R.; Cano, J. Photoswitching of the antiferromagnetic
coupling in an oxamato-based dicopper(II) anthracenophane. Chem.
Commun. 2011, 47, 11035−10983.
Grant agencies (Slovakia: APVV-14-0078, APVV-14-0073,
VEGA 1/0125/18); Czech republic: Authors acknowledge
the financial support from institutional sources of the
Department of Inorganic Chemistry, Palacky University
Olomouc, Czech Republic, and the Transregio Project TM
88 “3Met”and project A8 are acknowledged for the financial
support. I.S. and R.H. acknowledge financial support from the
Grant Agency of the Czech Republic (GACR 17-08992S).
̌
̌
́
̌
(14) Salitros, I.; Fuhr, O.; Gal, M.; Valasek, M.; Ruben, M.
Photoisomerization of Bis(tridentate) 2,6 Bis(1H pyrazol 1 yl)-
pyridine Ligands Exhibiting a Multi anthracene Skeleton. Chem. - Eur.
J. 2017, 23, 10100−10109.
(15) (a) Ruben, M.; Breuning, E.; Lehn, J.-M.; Ksenofontov, V.;
Renz, F.; Gutlich, P.; Vaughan, G. B. M. Supramolecular Spintronic
̈
Devices: Spin Transitions and Magnetostructural Correlations in
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