Running in the Family: Molecular Factors controlling Spin Crossover of Iron(II) Complexes with Schiff-base like Ligands

Article abstract of DOI:10.1002/zaac.202000409

Tailoring of spin state energetics of transition metal complexes and even the correct prediction of the resulting spin state is still a challenging task, both for the experimentalist and the theoretician. Apart from the complexity in the solid state imposed by packing effects, molecular factors of the spin state ordering are required to be identified and quantified on equal rights. In this work we experimentally record the spin states and SCO energies within an eight-member substitution-series of N₄O₂ ligated iron(II) complexes both in the solid state (SQUID magnetometry and single-crystal X-ray crystallography) and in solution (VT-NMR). The experimental survey is complemented by exhaustive theoretical modelling of the molecular and electronic structure of the open-chain N₄O₂ family and its macrocyclic N₆ congeners through density-functional theory methods. Ligand topology is identified as the leading factor defining ground-state multiplicity of the corresponding iron(II) complexes. Invariably the low-spin state is sterically trapped in the macrocycles, whereas subtle substitution effects allow for a molecular fine tuning of the spin state in the open-chain ligands. Factorization of computed relative SCO energies holds promise for directed design of future SCO systems.

Full text of DOI:10.1002/zaac.202000409

Title: Running in the Family: Molecular Factors controlling Spin
Crossover of Iron(II) Complexes with Schiff-base like Ligands
Authors: Sophie Schönfeld, Wolfgang Bauer, Sebastian Thallmair,
Gerald Hörner, and Birgit Weber
This manuscript has been accepted after peer review and appears as an
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To be cited as: Z. anorg. allg. Chem. 10.1002/zaac.202000409
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ARTICLE
Running in the Family: Molecular Factors controlling Spin
Crossover of Iron(II) Complexes with Schiff-base like Ligands
Sophie Schönfeld, Wolfgang Bauer, Sebastian Thallmair,[a]# Gerald Hörner,* and Birgit Weber*
[
a]
[a]#
[a]
[a]
Dedicated to Professor Peter Klüfers on the Occasion of his 70th Birthday
Abstract: Tailoring of spin state energetics of transition metal
complexes and even the correct prediction of the resulting spin state
is still a challenging task, both for the experimentalist and the
theoretician. Apart of the complexity in the solid state imposed by
packing effects, molecular factors of the spin state ordering are
required to be identified and quantified on equal rights. In this work
we experimentally record the spin states and SCO energies within an
In spite of the vast literature on iron(II)-borne SCO, however, the
reliable prediction of the parameters that govern the phenomenon
qualitatively and quantitatively has remained a challenging task.
Particularly in the solid state, the molecule-inherent properties

can be and in fact are often overridden by packing effects.[2]
A
salient example for this complexity with a direct bearing on the
present work is provided in a recent study on SCO of iron(II)
4 2
eight-member substitution-series of N O ligated iron(II) complexes
2
-
coordination polymers. Based on an invariant N
2
O
2
planar
both in the solid state (SQUID magnetometry and single-crystal X-ray
crystallography) and in solution (VT-NMR). The experimental survey
is complemented by exhaustive theoretical modelling of the molecular
ligating platform (Scheme 1), the derived polymers
{
[Fe8(pypy)]}
n
show remarkably variable SCO in the solid state,
when the axial bridging bipyridine ligand pypy is varied.[ While
3]
4 2
and electronic structure of the open-chain N O family and its
4,4’-bipyridine as a bridge supports a LS polymer up to 400 K, the
macrocyclic N congeners through density-functional theory methods.
6
equally linear bridge 1,2-bis(4-pyridyl)ethyne traps the HS state
even at lowest temperatures. Clearly, such massive differences
cannot be associated with the very minor (if any) variations in the
ligand-field imposed by the axial ligands; they are governed by
Ligand topology is identified as the leading factor defining ground-
state multiplicity of the corresponding iron(II) complexes. Invariably
the low-spin state is sterically trapped in the macrocycles, whereas
subtle substitution effects allow for a molecular fine tuning of the spin
state in the open-chain ligands. Factorization of computed relative
SCO energies holds promise for directed design of future SCO
systems.
packing effects, which are currently beyond rational design, not to
say control.[4]
By contrast, mutual supramolecular feedback among the SCO
units can be largely ruled out, when isolated complexes are
studied in solution. In solution (predominantly) the inherent
molecular properties are recorded; it is noted that in some cases
significant solvent effects on the SCO thermodynamics have been
reported.[5] SCO is inherently a molecular property, which can be
traced to a balance of competing enthalpic and entropic
contributions. The weakened iron-donor bonds in the HS state
render both the SCO enthalpy and the entropy positive, SCOH > 0
Introduction
The electronic state of transition metal complexes from the 3d row
can be often shuttled between the low spin (LS) and high spin
4
7
(
HS) formulation of octahedral 3d d configurations. The
phenomenon coined as spin crossover (SCO) relates an abrupt
change in magnetism with similarly sharp changes in
spectroscopic markers and of the metrics of the inner coordination
sphere, reflecting the electron count in antibonding metal-borne
orbitals. Controlled turnover among spin states through external
physical stimuli has been reported in a large number of cases.
The most prominent example constitutes octahedral complexes
of iron(II), where SCO gives sharp contrast between the
diamagnetic LS and paramagnetic HS forms.[1]
and
SCOS > 0. Since both quantities largely reflect the
peculiarities of metal-ligand bonding, targeted tuning of the SCO
thermodynamics through ligand design appears to be
straightforward. In terms of ligand-field theory, for instance,
*
strong-field ligands* from the upper end of the spectrochemical
series are predicted to support large splitting among the non-
bonding t2g set and the anti-bonding e *
set. It is common sense,
g
that the stabilization of LS iron(II) complexes largely derives from
mixing of metal-borne t2g orbitals with potent acceptor states of
the ligand with * symmetry. Accordingly, solution data has been
used for the construction of structure-property relationships within
several ligand families that can be taken as a test of this
hypothesis.[6–8] These studies have led to a more differentiated
view of what actually is meant with the terminus *strong ligand
field*. Halcrow and Deeth’s blended experiment-theory scan
across the bpp ligand family (bpp = 2,6-di{pyrazol-1-yl}pyridine)
revealed opposing effects on SCO thermodynamics of terminal
pyrazol and central pyridine substitution, consistent with
differential resonance stabilization. On the other hand,
[
a]
Sophie Schönfeld, Dr. Wolfgang Bauer, Sebastian Thallmair, Dr.
Gerald Hörner, Prof. Dr. Birgit Weber
Department of Chemistry
University of Bayreuth
Universitätsstraße 30, NWI, 95440 Bayreuth, Germany
E-mail: gerald.hoerner@uni-bayreuth.de, weber@uni-bayreuth.de
The practical work was done at the Department of Chemistry, LMU
Munich, Germany.
#
Supporting information for this article is given via a link at the end of
the document.
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ARTICLE
Jakubikova’s exhaustive DFT study on 2,2’-bipy derivatives
somewhat surprisingly identified the ligands’ action as a -donor
to be as important as their established role as -acceptors. These
studies exemplarily show that the molecular factors of SCO are
not yet fully understood. Clearly the extension of molecular SCO
studies to include further ligand families is in place.
crystallization or through precipitation with excess water.
Elemental analysis is in agreement with the expected
stoichiometry. In some cases, additional pyridine or water
[Fe8(py)2]
[Fe7(py)2]
[Fe6(py)2]
[Fe5(py)2]
[Fe4(py)2]
[
Fe4(py)2]DFT
2200 2000 1800 1600 1400 1200 1000 800
Wavenumber cm-1]
Scheme 1. Synthesis of the ligand family H
2
L and the iron(II) complexes
[
FeX(py) ]; general structure with notation of the substitution pattern.
2
Figure 1. Vibrational spectra of powdered samples of [Fe4(py)
ATR crystal; room temperature); for sake of comparison, a DFT-derived
spectrum of HS-[Fe4(py) ] is shown (grey line).
2 2
] to [Fe8(py) ]
(
2
In this work we study the variation of SCO behavior across a
series of eight iron(II) complexes [FeX(py) ] deriving through
backbone substitution from a common planar directing N
2
2
-
2
O
2
molecules co-crystallized as could be supported by X-ray
crystallography (see below). While solutions of [Fe57(py) ] were
ligand of the Jäger type (Scheme 1). Keeping constant the o-
phenylenediamine bridge and axial ligation through two pyridine
molecules, the effects of substitution in the meso- and O-terminal
positions of the equatorial plane could be studied in isolation.
Magnetic response is recorded both in the solid state (SQUID
2
highly sensitive toward the presence of oxygen, solid samples
1
were less susceptible. [Fe8(py)
2
] with R = CF
3
was clearly more
inert towards oxygen also in solution. SQUID magnetometry
revealed these solid powders to be paramagnetic at room
temperature with effective magnetic moments being in agreement
with HS iron(II) centers (more details below). IR spectra obtained
of the powdered samples reveal the expected trends (Figure 1).
1
magnetometry) and in solution ( H NMR spectroscopy). The
recorded thermodynamic data are discussed in terms of
molecular structure and electronic effects, addressed through X-
ray crystallography and extended DFT modelling.
In particular, complexes with non-coordinating carbonyl moieties
2
(
R = CO
2
Me; CO
2
Et; COPh) feature prominently around 1700

1
cm , typical of the intense (C=O) valence mode. Consequently,
this spectral region is silent for compounds lacking free carbonyls;
Results and Discussion
[
Fe4(py)
2
] with R2 = CN, for instance, exhibits an intense
1
resonance at 2200 cm instead, diagnostic of a nitrile moiety. The
overall appearance and the intensity pattern of the IR spectrum of
Synthesis and characterization of iron(II) complexes
[
Fe4(py)
2
] is well captured by a DFT-derived spectrum (grey in
The syntheses of the ligands H
iron(II) complexes were published previously; reviewed by Weber
and Jäger. [9,10] The proto-ligands H
5H 8 were synthesized in
2 2
1-H 4 and their bis(pyridine)
Figure 1).
2
2
Strongly broadened 1H NMR spectra of [Fe5/6(py)
support formulations as the HS species in all cases. Exemplary
spectra recorded in a mixture of pyridine-d and toluene-d (1 : 1)
at 55 °C are shown in Figure 2. In both cases, the expected
number of resonances could be identified and assigned, based
on chemical shift arguments, signal width and a comparison with
the previously published complexes. Of particular diagnostic
2
] likewise
good yields from o-phenylenediamine and the respective keto-
enol ethers, as summarized in Scheme 1.[3,11–13] The 1H NMR
resonance pattern of the proto-ligands is in agreement with a
predominant formulation as the keto-enamine tautomer.[13] It is
noted that the complementary *salophen* ligand family rather
justifies an enol-imine description. The desired complexes were
obtained in a one-pot synthesis where the proto-ligand was
agitated with iron(II) acetate in a mixture of methanol and pyridine
5
8
value is a broad resonance of [Fe5(py)
at 25 ppm (a). Similar observations have been made by Weber
and Walker with [Fe1(py)
2
] at very high field, namely
to yield complexes [Fe(58)(py)
possible to heat the corresponding methanol precursor
complexes [Fe(5‒8)(MeOH) ] in pyridine and then precipitate the
2
] in a single step. It is also
[
14]
2
] and [Fe2(py)
2
].
The latter two
complexes showed high-field resonances around  = 25 ppm as
2
they likewise share the substituent R1 = Me adjacent to the
desired pyridine complexes with water.
coordinated carbonyl moiety. In keeping with this assignment,
high-field signals are absent for [Fe6(py)
were absent also for [Fe3(py)
1
2
] with R = Ph and they
In both scenarios the products were isolated from their pyridine
solutions as intensely colored powders either through
1
[14]
2
] with R = CO
2
Et.
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ARTICLE
M
*
S
S
b
d
e
*
S
S
f
c
a
c
M
S
S
M
d
M
e
c
a
c
b
f
15
10
5
0
10
0
– 10
– 20
Figure 2. 1H NMR spectra recorded of [Fe5(py)
] (left) and [Fe6(py)
] (right) in a mixture of pyridine-d
and toluene-d (1 : 1) at T = 328 K (55 °C, 400 MHz); signal
2
2
5
8
assignment based on chemical shift and signal width; S denotes residual solvent signals pyridine and toluene, M denotes to methanol from the precursor complex
and * to an unknown contamination of the NMR solvent.
With all other resonances being unexceptional, the juxtaposition
of the phenyl-ring close to the iron(II) center in [Fe6(py) ] (f)
asymmetric unit. The asymmetric unit with the atom numbering
scheme for both temperatures is displayed in Figure 4. Notably,
2
results in substantial down-field shift of the phenyl-borne ortho-H
resonances to  = 16 ppm.
2
the cell parameters obtained of [Fe7(py) ] significantly differ at
T = 170 K and T = 220 K. In keeping with the sharply decreased
crystal quality at the lower temperature, extensive molecular and
supramolecular rearrangements can be suspected. Indeed, the
high-temperature structure reveals two HS-configured complexes,
with an average Fe‒N bond length of 2.08 Å (eq) and 2.26 Å (ax).
By contrast in the low-temperature structure one of the molecules
Solid-state structures
Crystals suitable for X-ray structure analysis of [Fe6(py)
obtained by recrystallization of the isolated powders from
mixtures of methanol and pyridine. For [Fe7(py) ] crystals suitable
for X-ray structure analysis were obtained from the filtrate. The
crystallographic data for [Fe6(py) ] were obtained at 173 K.
Fe7(py) ] was studied at two different temperatures (170 K and
20 K) that frame the first spin transition in SQUID data (see
below). The crystallographic data are summarized in Table S1.
Fe6(py) ] crystallizes in the monoclinic space group P2 /n with
the asymmetric unit containing two complex units and two
additional pyridine molecules, giving rather [Fe6(py) ]  py. Due
2
] were
2
(
1
Fe ) is found in the LS state.
2
[
2
2
[
2
1
2
to the strong disorder of the co-crystallized pyridine molecule, only
a structural motif can be discussed. The asymmetric unit with the
atom numbering scheme is displayed in Figure 3. The co-
crystallized pyridine molecules are, as mentioned before, strongly
disordered at T = 173 K. Selected bond lengths and angles within
the inner coordination sphere are summarized in Table 1. At the
measured temperature the complex is clearly in the HS state
considering the average Fe‒N bond length and of 2.12 Å
(
1
eq)/2.22 Å (ax) and an Oeq‒Fe‒Oeq angle of 113.93 and
12.80 °.[9,10] Further details on the packing of the molecules in
the crystal are given in the Supporting Information, Figure S2 and
Table S2. By contrast, [Fe7(py) ] crystallizes solvent-free in the
triclinic system, exhibiting two complex molecules in the
Figure 3. ORTEP drawing of the asymmetric unit of the motif of [Fe6(py)
2
]  py
(174 K) with the atom numbering scheme used. Co-crystallized solvent
molecules and hydrogen atoms are omitted for clarity. Thermal ellipsoids are
presented at 50% level. (for further drawings see Figure S1 and S2).
2
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ARTICLE
2 2
Table 1. Selected bond lengths (Å) and angles (°) within the inner coordination sphere of the iron(II) complexes [Fe6(py) ]  py and [Fe7(py) ] at various
temperatures
T [K]
Fe‒Neq
Fe‒Oeq
Fe‒Nax
O
eq‒Fe‒Oeq
Nax‒Fe‒Nax
[
[
[
Fe6(py)
Fe7(py)
Fe7(py)
2
2
2
] x py
Fe1
Fe2
Fe1
Fe2
Fe1
Fe2
173
2.118(3), 2.128(3)
2.101(3), 2.132(3)
1.886(4), 1.888(4)
2.087(4), 2.085(4)
2.092(2), 2.0799(19)
2.089(2), 2.0846(19)
2.042(2), 2.076(2)
2.071(2), 2.034(2)
1.941(3), 1.918(4)
2.004(4), 1.997(4)
2.0065(17), 2.0279(16)
2.0002(19), 2.0136(18)
2.241(3), 2.213(3)
2.196(3), 2.247(3)
1.984(5), 1.998(5)
2.241(4), 2.340(4)
2.287(2), 2.207(2)
2.286(2), 2.256(2)
113.93(9)
112.80(9)
88.89(15)
108.00(16)
109.21(7)
108.60(7)
167.13(11)
168.07(11)
174.85(18)
175.24(16)
169.29(7)
176.11(8)
],
],
170
220
Table 2. Coordination metrics of crystal structures in the substitution series
Fe1(py) ] to [Fe4(py) ].
[
2
2
[
Fe1(py)
2
]a
[Fe2(py)
2
]b
[Fe3(py)
2
]a
[Fe4(py)
LT
]c
2
LT/HT
LT/HT
LT
Fe-O
1.955/2.017
1.962/1.990
1.918/2.053
1.923/2.062
2.023/2.195
2.025/2.256
92.4/106.3
-
1.930/2.009
1.948/1.992
1.894/2.061
1.906/2.074
2.007/2.226
2.025/2.269
90.0/107.3
176.1/175.6
SCO
2.046
2.048
2.109
2.102
2.239
2.262
112.0
-
2.050
2.083
2.100
2.141
2.246
2.262
108.0
170.3
HS
Fe-O’
Fe-Neq
Fe-Neq
Fe-Nax
’
Fe-Nax
’
O-Fe-O’
N
ax-Fe-Nax’
SCO
HS
a: taken from Ref. [9]; b: taken from Ref. [15]; c taken from Ref. [16]
.
equatorial substitution on the coordination metrics. Therein we
cover nine (predominantly) HS structures and three
(
predominantly) LS structures as can be concluded from pertinent
metrical data summarized in Tables 1 and 2. While poor crystal
quality so far interfered with structure analysis of [Fe8(py) ], it is
noted that the molecular structure of its coordination polymer with
,4’-bipyridine, {[Fe8(bipy)]}
, could be recently resolved.[3] The
Figure 4. ORTEP drawing of the asymmetric units of crystal structures of
Fe7(py) ] obtained at T = 170 K (left) and at T = 220 K (right) with the used
atom numbering scheme. Thermal ellipsoids are presented at 50% level.
Hydrogen atoms are omitted for clarity.
[
2
2
4
n
coordination metrics were fully in accord with the LS state
structures assembled in Table 2.
The average Fe‒N bond lengths of [Fe7(py)
and 1.99 Å (ax) in the molecule with Fe clearly in a range typical
for iron(II) in the LS state whereas all values of the inner
coordination sphere around Fe indicate conserved HS
character.[9,10] At both temperatures disorder of the ester group
can be observed, which is not unusual for this kind of compound.
The packing of the molecules in the crystal and details on the
intermolecular interactions are given in the Supporting
Information, Figure S3 and Tables S3 and S4. Combined with the
2
] is with 1.89 Å (eq)
1
The low-temperature structures obtained of [Fe1(py)
2
] and
] at
[
Fe2(py) ] largely resemble molecule b obtained of [Fe7(py)
2
2
2
T = 170 K with respect to the coordination metrics. Diagnostic
markers of a low spin configured iron(II) center are both short
equatorial (d(Fe-Oeq) and d(Fe-Neq) < 1.96 Å) and axial bond
lengths (d(Fe-Nax) < 2.03 Å). Such short iron-donor distances are
possible only with a t2g6e 0
configuration. As a secondary marker,
g
the opening angle Oeq-Fe-Oeq is close to 90°, indicating a largely
undisturbed octahedral coordination. By contrast, massively
expanded bond lengths by  10 pm (d(Fe-Oeq)) and > 20 pm
previously reported crystal structures of [Fe1(py)
2
]
to
[
Fe4(py)
2
],[9,15,16] we can establish a broad inquiry in the effects of
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ARTICLE
(
d(Fe-Neq
structures of [Fe1(py)
bond expansion translates into an increased opening of
eq-Fe-Oeq > 110°. [Fe1(py) ], [Fe2(py) ] and [Fe7(py) ] at high
temperature therefore share the temperature-invariant metrics of
Fe3(py) ], [Fe4(py) ] and [Fe6(py) ] and can be readily identified
to be likewise in the t2g HS configuration with populated anti-
)
and d(Fe-Nax)) prevail in the high-temperature
this slight difference in remote substitution translates into
markedly different magnetic response. [Fe7(py) ] reveals a step-
2
2
], [Fe2(py) ] and [Fe7(py) ]. Equatorial
2
2
wise SCO, with the steps being centered at T1/2 = 217 K and 110
K. Notably, both steps are abrupt and give rise to hysteretic
splitting of the rising and falling branches by 16 K and 9 K,
respectively. With a view to the aforementioned results from X-ray
crystallography we attribute the first step to complete SCO of one
out of two iron(II) sites in the crystal lattice (see above). Finally,
O
2
2
2
[
2
2
2
4
2
e
g
bonding orbitals. These observations could be fully corroborated
by magnetic measurements of powdered samples.
[Fe8(py)
complete and abrupt SCO with T1/2 = 175 K and shows a narrow,
K wide hysteresis. In summary, at sufficiently low temperature,
the substitution series [FeX(py) ] allows for complete SCO in the
2
solid state in six out of eight cases; only [Fe3(py) ] and [Fe4(py) ]
2
]
3
with carbonyl adjacent CF -groups undergoes
3
2
2
remain in the HS state across the entire temperature range. VT-
NMR spectroscopy was therefore used to address the question
whether or not the HS character is a molecular property of the
latter two compounds.
20
15
10
5
0
CH (f)
CH2 (e)
1
.0
CH3 (d)
CH (c)
0.8
0.6
0.4
0.2
[
Fe5(py)2]
[Fe6(py)2]
0
0
1
2
3
4
5
200
250
T [K]
300
350
3
1/T [10 /K]
1
Figure 6. (left) Curie plot of H NMR chemical shifts recorded of [Fe6(py)
mixture of pyridine-d and toluene-d (1 : 1) in the temperature range between
200 K and 360 K (400 MHz); (right) averaged HS fractions of [Fe5(py) ] and
[Fe6(py) ]. Computed from chemical shift data according to equ. 1. The
2
] in a
5
8
Figure 5. Plots of the χ
M
T product vs. Temperature in the range 300‒50‒300 K
2
(scan rate 5 K/min; H = 5000 Oe).
2
complete data for both complexes are given in the Supporting Information,
Figure S5.
SQUID magnetometry
Previous work had shown that all members of the series
VT-NMR spectroscopy
[
FeX(py)
temperature.
Fe4(py) ] was conserved even at lowest temperatures, [Fe1(py)
and [Fe2(py) ] underwent complete SCO in the solid state upon
2
] with X = 1 – 4, were entirely in the HS state at room
2
Previous studies had observed that NMR spectra of [Fe3(py) ]
with R = R = CO Et undergo continuous evolution upon cooling
2
which is fully in agreement with Curie behavior of an enduring
[
10]
2
While the HS character of [Fe3(py) ] and
1
2
[
2
2
]
2
paramagnet down to T = 180 K. On the other hand, chemical shifts
cooling with T1/2 = 220 K (gradual) and 190 K (narrow hysteresis),
respectively. The extension of our inquiry in the magnetic
behavior with the new members of the series with X = 5 – 8 led to
further diversification of the SCO phenomenology, illustrated in
Figure 5. Firstly, [Fe5(py)
1
2 2
recorded for [Fe1(py) ] and [Fe2(py) ] strongly deviated from the
[
14]
linear 1/T dependence implied by Curie’s law. These deviations
rather signaled spin-pairing to partially form the LS state, a notion
which was supported by Evans NMR. Deviations of NMR
parameters from Curie’s law have been associated with SCO in
several previous cases.[ Clearly, the nature of substituents R1
2
] undergoes an abrupt SCO at T1/2
=
05 K, giving rise to a narrow hysteresis with a width of 2 K. By
17]
2
contrast, [Fe6(py) ] shows a rather gradual SCO, centered at
2
and R serves to significantly modulate the SCO energies. This
T
1/2 = 200 K. The gradual transition in the bulk material is most
probably due to the bulky phenyl rings. Please note that the single
crystals with the composition [Fe6(py) ]  py remain in the HS
notion is underpinned by the results in the present study obtained
in VT-NMR scans of [Fe5(py) ] and [Fe6(py) ] in toluene/pyridine
2 2
mixtures. An exemplary Curie plot of the temperature
dependence of chemical shifts for various proton sites in
2
state across the entire temperature range, as shown in the
Supporting Information, Figure S4. Solution-phase NMR studies
[
[
Fe6(py)
Fe5(py)
2
] is shown in Figure 6, the corresponding results for
] and all further details for the determination of the HS
later confirmed SCO also for isolated molecules of [Fe5(py)
Fe6(py) ]. Remarkable is the complex behavior of [Fe7(py)
2
2
] and
], the
2
[
2
fraction HS are given in the Supporting Information, Figure S5.
Between 370 K > T > 270 K the plots are approximately linear, but
tend to massively deviate from linearity at even lower temperature.
structure of which differs only marginally from parent [Fe1(py)
the nature of the substituent R ; that is, R = CO
for [Fe1(py) ] and [Fe7(py)
2
] in
Me
2
], respectively. As is shown in Figure 5,
2
2
2 2
Et and CO
2
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Zeitschrift für anorganische und allgemeine Chemie
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ARTICLE
From the recorded chemical shifts, obs, HS was calculated as
discussed in the SI according to equation 1.
coverage of the sigmoidal curves, the extracted enthalpy and
entropy carry a significant uncertainty. Suffice to say here, that
there is only minor scatter among the thermodynamics of the
obs = HS  HS (T) + (1  HS)  LS
(1)
complexes [Fe1(py)
2
], [Fe2(py)
2
], [Fe5(py)
2
] and [Fe6(py)
2
],
1
pointing to a likewise minor influence of the substituents R and
Therein the chemical shifts for the LS species LS were taken as
invariant with temperature and equal to the chemical shift of the
free ligand, whereas the temperature dependence of the HS
chemical shifts HS (T) were extrapolated along Curie’s law from
the linear branch of the high-temperature data.
2
R on the SCO. By contrast, the tendency of [Fe3(py)
2
], [Fe4(py)
2
]
not to undergo SCO in solution (and in the solid) indicates
disfavored thermodynamics. The largely conserved molecular
structure across the entire series of complexes renders large
fluctuations in SCO entropy unlikely. Therefore, the influence of
the substitution pattern on the enthalpic factors of SCO was
studied through DFT methods.
The resulting computed HS fractions are given in the SI, Figure
S5, for both complexes. Upon decrease of temperature, the HS
fraction massively decreases, reaching fractions of HS  0.4 at the
lowest available temperature of 200 K. T1/2 where HS = LS can be
estimated to be around 205 K (Figure 6, right). It should be
compared with the locked-HS state of the single crystals of
DFT structure optimization
The impact of remote substitution on the coordination metrics was
addressed with DFT methods across the entire complex series
given in Scheme 1; both the LS (S = 0) and the HS state (S = 2)
were considered. Optimization with the BP86 functional and large
triple- basis sets was calibrated against experimental metrics
whenever accessible (for Computational Details, see the
Experimental Section). Harmonic frequencies were computed for
the HS states of all complexes; the absence of imaginary modes
identified the minima as stationary points on the energy
[
Fe6(py)
2
]  py. Interestingly, a very similar plot of HS(T) prevails
], indicating an only marginal difference in SCO
for [Fe5(py)
2
thermodynamics of both complexes. This notion is corroborated
by Van’t Hoff analysis of the entire temperature scans of HS(T).
The corresponding thermodynamic parameters, SCOH and SCOS,
are summarized in Table 3. For the sake of comparison, the data
2 2
] and [Fe2(py)
] are re-plotted from Ref.[14] It is noted
of [Fe1(py)
that, due to the incomplete
2
hypersurface (a computed IR spectrum of HS-[Fe4(py) ] is shown
in Fig. 1). Pertinent metrical data are summarized in Table 4.
Comparison with the experimental data (Tables 1 and 2; see
above) reveals an excellent match with respect to metal-ligand
bond lengths and core angles.
Table 3. Thermodynamic parameters of solution phase SCO as obtained
from H NMR temperature scans.
1
]a
]a
[
Fe1(py)
2
[Fe2(py)
25.5
2
[Fe5(py)
20.2
98
2
]
[Fe6(py)
2
]
-
1
As was apparent from the experimental data in Tables 1-2 also,
the N O planar platform largely predefines the coordination
2 2
metrics, both in the HS and the LS form. In particular, the bond
lengths of the enaminate nitrogen atoms very narrowly cluster at

SCOH [kJ mol ]
18.2
88
25.1

SCOS [J mol-1 K-1]
121
121
T1/2 [K]
207
211
206
207
1.90 ± 0.01 Å and 2.08 ± 0.01 Å in the LS and HS states,
a
taken from Ref. [Walker, Weber]
respectively.
2 2
Table 4. Coordination metrics of DFT-optimized structures of the substitution series [Fe1(py) ] to [Fe8(py) ] for S = 0 (LS) and S = 2 (HS); italicized data
correspond to structures for which comparison is possible with experimental data in Tables 1-2.
[
Fe1(py)
2
]
[Fe2(py)
2
]
[Fe3(py)
2
]
[Fe4(py)
2
]
[Fe5(py)
2
]
[Fe6(py)
2
]
[Fe7(py)
2
]
2
[Fe8(py) ]
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
LS
HS
Fe-O
1.937
2.013
2.011
2.080
2.078
2.171
2.260
110.5
178.3
1.937
1.934
1.892
1.892
1.953
1.982
87.4
2.011
2.020
2.076
2.083
2.170
2.253
111.5
178.0
1.966
1.959
1.895
1.898
1.949
1.968
87.4
2.053
2.055
2.085
2.086
2.153
2.243
109.7
176.8
1.973
1.973
1.903
1.902
1.954
1.970
85.9
2.065
2.067
2.089
2.090
2.153
2.238
107.4
176.4
1.933
1.944
1.891
1.894
1.955
1.977
87.1
2.004
2.022
2.077
2.080
2.174
2.290
109.3
178.6
1.936
1.936
1.902
1.902
1.949
1.984
86.2
2.014
2.014
2.084
2.087
2.177
2.247
109.2
177.7
1.939
1.937
1.893
1.893
1.951
1.982
87.1
2.014
2.011
2.079
2.078
2.171
2.260
110.6
178.3
1.935
1.933
1.897
1.897
1.959
1.990
86.8
2.015
2.022
2.077
2.079
2.178
2.238
109.9
177.3
1
.938
1.893
.893
1.950
.982
Fe-Neq
Fe-Nax
O-Fe-O
1
1
87.1
Nax-Fe-
178.2
178.2
177.8
177.8
178.3
178.1
178.3
178.1
N
ax
Table 5. Fitting results of the linear regressions for complexes [FeX(py)
2
] in Scheme 1; energies in kJ mol1..
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Zeitschrift für anorganische und allgemeine Chemie
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ARTICLE
X =
1
2
3
4
5
6
7
8

SCO
E
629(7)
108(1)
0.9995
633(7)
108(1)
0.9995
635(9)
98(1)
624(9)
94(1)
626(8)
104(1)
0.9994
622(6)
107(1)
0.9996
628(7)
108(1)
0.9995
610(6)
102(1)
0.9996

SCOE0
R2
0.9992
0.9992
0

SCOE = SCOE + (SCOE × a
0
)
(2)
The common slope of the linear fits largely supports the notion of
[FeX(py) ] being a family of complexes. It is only [Fe8(py) ] where
1
20
2
2
the slope is significantly smaller, indicating electronically slightly
deviant behavior. More instructive is the vertical ordering of the
lines. While most lines (almost) coincide, the apparent SCO
8
4
0
0
0
2 2
energies of [Fe3(py) ] and [Fe4(py) ] are significantly smaller
1
across the entire Fock-exchange scan by > 10 kJ mol . With a
view to the limited accuracy of computed absolute SCO energies
SCOE, we go on with discussion being based on relative SCO
energies SCOE. The latter measure the deviation from a pre-
selected standard complex, in the following SCOE = SCOE 
[
Fe3(py)2]
Fe4(py)2]
−1

2
SCOE(ref) (with ref =[Fe2(py) ]; T1/2 = 211 K; ΔSCOS = 121 J K
[
-40
−
1
−1
mol ; ΔSCOH = 25.5 kJ mol ; Table 3). Internal referencing within
the complex family gives most effective cancellation of DFT-
inherent systematic errors, as has been shown previously.[17,20]
The averaged values of the relative SCO energies are shown in
0
0.05
0.1
0.15
0.2
a0
Figure 7. DFT-derived (B3LYP-D/TZVP) apparent SCO energies of complexes
FeX(py) ] as a function of the amount of exact Fock-exchange a
−
1
Figure 8 (standard error ≤ 1.5 kJ mol ). Added to the plot is a
[
2
0
.
number of related systems.
These values closely match the experimental data. More diversity
prevails in the Fe-Oeq bond lengths, likely due to the enhanced
flexibility of the open end of the binding pocket. Here we find two
clusters of values, both in the HS and the LS state. On the one
hand, values of 1.94 ± 0.01 Å and 2.01 ± 0.01 Å in the LS and HS
states, respectively, cover the majority of the complex series. It is
only for [Fe3(py) ] and [Fe4(py) ] on the other hand that we find
2 2
the Fe-Oeq bonds consistently expanded by 4-5 pm, irrespective
of the spin state. It is noted, that this feature is apparent in the

SCOE [kJ mol1]
pp
LS
80
pe
ee
6
1
60
0
8
5
T1/2 = 207 ± 2 K
40
5
20
SCO
HS
10
15
3
2 2
experimental data of [Fe3(py) ] and [Fe4(py) ] also. Notably,
4
these two outliers are temperature-invariant HS complexes, both
in the crystal and in solution. This coincidence indicates a
molecular origin of the HS preference, which must be associated
20
en2

20
1
with the nature of the substituent, R = OEt.
Computation of SCO energies
Figure 8. Relative SCO energies SCOE of the substitution series [FeX(py)
2
] (X
]; additional data of the
2
] and of macrocyclic systems pp,
=
1, 2…, 8); energies are given relative to [Fe2(py)
2
The impact of the substitution pattern on the SCO energies was
addressed with DFT methods. Improved precision was obtained
through a Fock-exchange scan via variation of the exact
ethylenediamine-derived system [Feen2(py)
pe, and ee featuring phenylenediamine and ethylenediamine bridges; colors
denote the experimentally observed SCO behavior.
exchange a
apparent SCO energies against a
0
in the B3LYP functional. Linear plots of the derived
show slopes that reflect the
0
extent of SCO-related metrical changes (equ. 2).[18,19] That is, the
slopes of the plots will be structure-independent, if belonging to
the same family of complexes. By contrast, the offset of the plots
will reflect the structure-bias of the SCO energies. The resulting
plots are shown in Figure 7, the results of the linear regression
can be found in Table 5.
2 2
For instance, in [Feen2(py) ] the phenylene bridge of [Fe2(py) ] is
replaced by ethylene, whereas pp, pe, and ee denote macrocyclic
ligands with an N 2

donor set and a substitution pattern deriving
4
from 2. In experimental studies [Feen2(py)
2
] had been identified as
a HS system,[ whereas the macrocyclic complexes pp, pe, and
9]
ee preferred the LS state.[21] Structure plots of these systems are
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202000409
ARTICLE
given in Figure S6, SI. The computed relative SCO energies give
systems, apparent from experiment, is likewise matched by DFT
computed SCO energy. These extreme cases may be taken to
define the Jäger-system overall variability in SCO energy that
1
an energy ladder spanning a range of ca. 100 kJ mol , with
Fe2(py) ] defining zero energy. The energy ordering found with
[
2
1
DFT gives three separate regimes. Firstly, according to DFT, the
three macrocycles possess strongly favored LS states. Relative
amounts to SCOE  100 kJ mol .
energies exceed SCOE > 60 kJ mol- in all cases. If a common
1
2
Admittedly, the energy span of [FeX(py) ], directly addressable
entropy ΔSCOS = 100 J K−1 mol−1 is assumed for all complexes,
through remote ligand substitution, is much smaller at present,
1
amounting to ca. 15 kJ mol . This value should be compared with
the related energy span of ca. 20 kJ mol and ca. 25 kJ mol1
computed by Deeth and Halcrow et al. and Jakubikova et al. for
derivatives of 2,6-di{pyrazol-1-yl}pyridine and 2,2’-bipyridine,
respectively.[6,7] With a view to the narrow energy span in
the SCO energies translate into transition temperatures well
beyond T1/2 = 800 K for ee, pe and pp. The lower end of the
energy ladder constitutes of the ethylenediamine-derived
1
complex [Feen2(py)
Fe3(py)
2
] and the previously discussed complexes
1
[
2
] and [Fe4(py)
2
], both equipped with R = OEt. The three
complexes reside at SCOE < 10 kJ mol1 translating into
2
[FeX(py) ], it is highly motivating to note that our DFT routines
were able to correctly assign the solution SCO behavior across
the entire substitution series. Through this we could further
specify the origin of enduring HS character of a group of
complexes in the solid-state. Molecule-inherent HS complexes
which reflect a diminished ligand-field can be safely differentiated
from supra-molecularly-trapped HS complexes.
molecular transition temperatures below T1/2 = 100 K. In both
cases discussed, therefore the computed energies fully match the
experimental phenomenology symbolized by the colors; that is,
red for LS and green for HS. Finally, the remaining members of
1
the family narrowly crowd around SCOE  0 kJ mol , with a
maximum deviation of 3 kJ mol1 (blue in Figure 8). Computation
thus suggests narrowly crowding transition temperatures also,
with T1/2 < 30 K, relative to [Fe2(py)
SCO energies has been recently reported by us for a number of
-extended congeners of [Fe7(py) ], indicating an only minor role
of -modulation.[22] Comparison with the NMR-derived
experimental transition temperatures of T1/2 = 207 ± 2 K
emphasizes the highly satisfying quality of the prediction.
2
]. A very similar range of
Experimental Section

2
Synthesis
Iron(II) acetate,[24] [Fe5(MeOH) 5,[11] 6,[12] 7[13] and H 8[3] were
]/H H H
2 2 2 2 2
synthesized as described in literature. Methanol was purified as described
in literature. Pyridine was purchased from Acros organics (99.5 %, extra
dry over molecular sieve, nitrogen flushed) and used as supplied. All air
sensitive syntheses were carried out under argon 5.0 using Schlenk tube
techniques.
Conclusion
In the period following the landmark reviews collected by Gütlich
and Goodwin in 2004,[23] spin crossover research took a clear
conceptual realignment towards application. That is, material
science with a focus on device-design makes up the vast majority
of spin crossover studies. As an inevitable side-effect of this
development which emphasizes solid materials, the inherent
molecule character of the phenomenon is biased and often
overridden by packing effects. Structure-function relationships
therefore are a blend of molecular and supramolecular properties.
It has been only very recently that the molecular factors defining
and modulating the energy spacing among spin states have been
rigorously addressed in concerted theory-experiment efforts; the
reader is reminded of the insightful studies by, among others,
Deeth and Halcrow et al. that made use of spectroscopic studies
of isolated actors supplemented by DFT computation.[6] While this
approach allowed for some surprising turns, to date only a limited
number of complex families has been addressed in such detail.
X-ray Structure Analysis
2
The intensity data of [Fe6(py) ] were collected with an Oxford XCalibur
diffractometer using graphite-monochromated MoKα radiation. The data
was corrected for Lorentz and polarization effects. The structures were
solved by direct methods using SIR97[25] and refined by full-matrix least-
2
(SHELXL-97).[26] The hydrogen atoms were
square techniques against F
0
included at calculated positions with fixed displacement parameters. All
non-hydrogen atoms were refined anisotropically. The intensity data of
[
Fe7(py)
2
] were collected with a Stoe StadiVari diffractometer using
graphite-monochromated Mo-K
α
radiation at 220 K and 170 K. The data
were corrected for Lorentz and polarization effects. The structures were
solved by direct methods (SIR-2019)[27] and refined by full-matrix least-
2
(SHELXL-2018).[28] All hydrogen atoms
square techniques against F
o
2-F
c
In this work we have moved the focus to a family of octahedral
iron(II) complexes of planar-directing Schiff-base like ligands of
the Jäger type with invariant axial ligation and invariant o-
phenylene bridge. Accompanied by a DFT inquiry in the SCO
energetics and structural changes, eight members of the family
were addressed experimentally in the solid state (X-ray diffraction
and SQUID magnetometry) and in solution (VT-NMR
spectroscopy). The theoretical section additionally covered
topological mutants with an ethylene bridge (en) or a number of
were calculated in idealized positions with fixed displacement parameters.
ORTEP-III[29] was used for the structure representation and Mercury
2020.1[30] for the representation of the molecule packing
Magnetic measurements on the bulk materials were carried out using a
SQUID MPMS-XL5 from Quantum Design with an applied field of 5000 G
in the temperature range from 300 to 50 K in the settle mode. The samples
were prepared in a gelatin capsule and held in a plastic straw. The raw
data were corrected for the diamagnetic part of the sample holder and the
diamagnetism of the organic ligand using tabulated Pascal’s constants.[31]
2

macrocyclic N
4
ligands. As a matter of fact, the HS character of
the en-derivative and the LS character of the macrocycle-derived
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202000409
ARTICLE
IR spectra were recorded with a 520 FT-IR (NICOLET). Mass spectra were
recorded with a Finnigan MAT 8500 with a data system MASPEC II. For
CHN analysis a Unicube (ELEMENTAR) was used.
Single crystals with the composition {[Fe6(py) ]  py} were obtained from
2
a pyridine solution after addition of water.
2
[Fe7(py) ]
T-dependent NMR Spectroscopy: Pyridine-d
5 8
(D, 99,5%) and toluene-d
H
2
7 (0.5 g, 1.38 mmol, 1 eq.) and iron(II) acetate (0.38 g, 2.22 mmol,
(D, 99.6%) were purchased from Euriso-top. The solvents were degassed
1
.6 eq) were dissolved in 10 mL pyridine (9.88 g, 124.8 mmol, 150 eq).
0 ml of MeOH were added and the mixture was heated to reflux for 1 h.
with argon and stored over molecular sieves. The NMR samples were
prepared under argon by using Schlenk techniques and locally made
sealing equipment. Saturated solutions of the iron(II) complexes were
1
After cooling to room temperature 7 mL water were added. After storage
at -22°C for 24 h brown needles crystallized from the reaction mixture.
They were filtered, washed two times with methanol (2 mL) and dried in
vacuum. Yield: 0.24 g(572.40 g/mol, 30.4 %) Elemental analysis
5 8
prepared in pyridine-d / toluene-d (50/50 v/v) mixtures and stored in
sealed or air-tight 5-mm NMR tubes. The NMR spectra were recorded on
a JEOL EX 400e spectrometer operating at 400.182 MHz equipped with a
variable-temperature unit over the temperature range –75 to +85 °C (198–
(
C
H28FeN O
28 4 6
, %) calcd: C 58.75 H 4.93 N 9.79; found: C 58.66 H 4.79
N 9.85. MS ((+)-DEI), 70 eV): m/z (%) 414 (100) [[Fe7]], 383 (24)
]+, 340 (40) [C15
], 309 (35) [C14 FeN ], 79
O]. IR: ṽ =3072 (w, ν[=C-H]),
945 (w, ν[-C-H]), 1684 (s, ν[C=O]), 1562 (s, ν[C=C]).
3
58 K).
Fe4(py)
4 (0.5 g, 1.41 mmol, 1eq.) and iron(II) acetate (0.39 g, 2.26 mmol,
[
C
H15FeN O
17 2 5
2
H12FeN O
4
H
9
2 3
O
·+
[
2
]
(38) [C H N], 52 (45) [C H ] , 44 (47) [C H
5 5 4 4 2 4
2
H
2
1
.6 eq) were dissolved in 11.4 mL pyridine (11.16 g, 141.1 mmol, 100 eq).
2 ml of MeOH were added and the mixture was heated to reflux for 1 h.
[Fe8(py)
2
]
1
H
2
8 (0.2 g, 0.40 mmol, 1 eq.) and iron(II) acetate (0.11 g, 0.64 mmol,
After cooling to room temperature and storage at 8°C for 24 h red-brown
needles precipitated. The material was filtered off and washed two times
with MeOH (2 mL). The precipitate was dried in vacuum. Yield: 0.37 g
1.6 eq) were dissolved in 3.2 mL pyridine (3.21 g, 40.2 mmol, 100 eq).
0 ml of MeOH were added and the mixture was heated to reflux for 1 h.
1
After cooling to room temperature 5 mL water were added. After storage
at -22°C for 24 h black needles crystallized from the mixture. They were
filtered, twice washed with methanol (2 mL) and dried in vacuum. Yield:
(
566.40 g/mol, 46.3 %). Elemental analysis (C28
6 4
H32FeN O , %) calcd: C
5
9.38 H 4.63 N 14.84; found: C 59.97 H 4.63 N 14.91. MS ((+)-DEI), 70
]+, 352 (52)
eV): m/z (%) 408 (100) [[Fe4]], 380 (35) [C17H18FeN O
3 4
+
0.18 g (708.40 g/mol, 63.2 %) Elemental analysis (C30
calcd: C 50.87 H 3.70 N 7.91 found: C 51.00 H 3.82 N 7.93. MS ((+)-DEI),
0 eV): m/z (%) 550 (45) [[Fe8]], 408 (21) [C16 FeN
]2+, 363 (42)
FeN N], 52 (79) [C
]3+, 79 (100) [C ]·+. IR: ṽ = 2987 (w,
]), 2919 (w, ν[CH ]), 1698 (s, ν[C=O]), 1680 (s, ν[C=C]), 1573 (s,
ν[C=C]).
26 6 4 6
H F FeN O , %)
[
C
14
H
8
FeN
4
O
4
], 44 (84) [C
2
H
6
N] . IR: ṽ = 2982 (w, ν[-C-H]), 2188 (s, ν[-
C≡N]), 1610 (s, ν[C=C]), 1532 (s, ν[C=C]).
7
H
11
F
3
2 4
O
{
[Fe5(py) ]·0.55 H O}
2
2
[C14
H
6
F
3
O
2 3
H
5 5
4 4
H
ν[CH
3
2
[
Fe5(MeOH)
85,8 mmol, 357 eq) and heated to reflux for 1 h. After cooling to room
temperature 10 mL water were added. The reaction mixture was stored at
22°C for 24 h. A fine brown crystalline solid precipitated. It was filtered,
twice washed with methanol (2 mL) and dried in vacuum. Yield: 0.22 g
674.45 g/mol, 62.7 %). Elemental analysis (C38 · 0.55H O, %)
calcd: C 67.66 H 4.95 N 8.31; found: C 67.57 H 4.83 N 8.26. MS ((+)-DEI),
2
] (0.3 g, 0.52 mmol, 1eq.) was dissolved in pyridine (14.7 g,
1
Computational Details
-
DFT calculations were performed using ORCA2.9.1.[32] Large TZVP basis
sets[33] were used throughout. The structures of the iron(II) complexes
were optimized with the GGA functional BP86;[34] the absence of imaginary
modes in numerical frequency calculations proved the optimized
structures to be stationary points. Complexes were optimized in both their
LS and HS states. Cartesian coordinates of all optimized structures are
compiled in the Supporting Information (see Table S5-S28 in the SI). In
(
H32FeN O
4 4
2
+
·
7
0 eV): m/z (%) 506 (98) [[Fe5]], 464 (62) [C26H20FeN O ] , 105 (100)
2 3
]+, 77 (98) [C
H
6 5
]+, 43 (49) [C
O] . IR: ṽ = 3071 (w, ν[=C-H]), 2946
+
[
C
8
H
9
2 3
H
(
w, ν[-C-H]), 2946 (w, ν[=C-H]), 1684 (m, ν[C=O]), 1559 (s, ν[C=C]).
order to assess the SCO energies, we used five derivatives of the well-
established hybrid functional B3LYP[19,35] in single-point calculations. In
[Fe6(py)
2
]
0
these derivative functionals the amount of exact exchange a has been
2
H 6 (0.5 g, 0.97 mmol, 1eq.) and iron(II) acetate (0.27 g, 1.56 mmol,
varied stepwise from 0.20 (native B3LYP) to 0.00. Dispersion contributions
were approximated using Grimme’s DFT-D3 atom pairwise dispersion
corrections of the parent B3LYP functional.[36] Solvent effects were
1
.6 eq) were dissolved in pyridine (7.58 g, 97.5 mmol, 100 eq). 8 ml of
MeOH were added and the mixture was heated to reflux for 1 h. After
cooling to room temperature 5 mL of water were added. After storage at -
accounted for in
parametrized for MeCN.
a
dielectric continuum approach (COSMO),[37]
22°C for 24 h a red-brown fine crystalline powder precipitated. The
material was filtered off and washed two times with MeOH (2 mL) and dried
in vacuum. Yield: 0.25 g (724.60 g/mol, 35.6 %). Elemental analysis
(
C
40
H
36FeN
4
O
6
, %) calcd: C 66.30 H 5.01 N 7.73; found: C 66.36 H 5.04
N 7.80. MS ((+)-DEI), 70 eV): m/z (%) 566 (62) [[Fe6]], 422 (31)
]2+, 105 (88) [C ]+ 77 (100) [C ]+. IR: ṽ = 2976 (w,
]), 2923 (w, ν[CH ]), 1671 (s, ν[C=O]), 1656 (s, ν[C=C]), 1557 (s,
ν[C=C]).
Acknowledgements
[
C
24
H18FeN
2
O
2
H
8 9
6 5
H
ν[CH
3
2
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Zeitschrift für anorganische und allgemeine Chemie
10.1002/zaac.202000409
ARTICLE
[
[
19] M. Reiher, O. Salomon, B. Artur Hess, Theor. Chem. Acc. 2001, 107,
Financial supports from the German Research foundation
48–55.
(
SFB840; project A10), the BayNAT program, and the University
20] a) P. Stock, D. Wiedemann, H. Petzold, G. Hörner, Inorganics 2017, 5,
of Bayreuth are gratefully acknowledged.
60; b) H. Petzold, G. Hörner, L. Schnaubelt, T. Rüffer, Dalton Trans. 2018,
47, 17257–17265.
Keywords: Schiff base • iron • Spin crossover • Density
[21] B. Weber, I. Käpplinger, H. Görls, E.-G. Jäger, Eur. J. Inorg. Chem. 2005,
005, 2794–2811.
2
functional theory
[
22] a) H. Kurz, G. Hörner, B. Weber, Z. Anorg. Allg. Chem. 2020, submitted;
b) S. Schönfeld, K. Dankhoff, D. Baabe, M.-K. Zaretzke, M. Bröring, K.
Schötz, A. Köhler, G. Hörner, B. Weber, Inorg. Chem. 2020, 59, 8320–
[
1]
a) M. A. Halcrow (Ed.) Spin-Crossover Materials, John Wiley & Sons Ltd,
Chichester 2013; b) K. Senthil Kumar, M. Ruben, Coord. Chem. Rev.
8333.
2017, 346, 176–205; c) E. Coronado, Nature Rev. Mater. 2020, 5, 87–
[
[
[
23] P. Gütlich, H. Goodwin (Eds.) Topics in Current Chemistry, 233-235,
Springer Berlin / Heidelberg 2004.
104; d) M. A. Halcrow, Dalton Trans. 2020; e) K. Senthil Kumar, Y. Bayeh,
T. Gebretsadik, F. Elemo, M. Gebrezgiabher, M. Thomas, M. Ruben,
24] B. Weber, R. Betz, W. Bauer, S. Schlamp, Z. Anorg. Allg. Chem. 2011,
Dalton Trans. 2019, 48, 15321–15337; f) S. Brooker, Chem. Soc. Rev.
637, 102–107.
2015, 44, 2880–2892.
25] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giacovazzo,
[
2]
M. A. Halcrow, I. Capel Berdiell, C. M. Pask, R. Kulmaczewski, Inorg.
Chem. 2019, 58, 9811–9821.
A. Guagliardi, A. G. G. Moliterni, G. Polidori, R. Spagna, J. Appl. Cryst.
1999, 32, 115–119.
[
[
3]
4]
H. Kurz, J. Sander, B. Weber, Z. Anorg. Allg. Chem. 2020, 646, 800–807.
M. Hostettler, K. W. Törnroos, D. Chernyshov, B. Vangdal, H.-B. Bürgi,
Angew. Chem. Int. Ed. 2004, 43, 4589–4594.
[
[
26] a) G. Sheldrick, Acta Cryst. A 2008, 64, 112–122; b) G. Sheldrick,
SHELXL-97, University of Göttingen, Göttingen, Germany 1997.
27] M. C. Burla, R. Caliandro, B. Carrozzini, G. L. Cascarano, C. Cuocci, C.
Giacovazzo, M. Mallamo, A. Mazzone, G. Polidori, J. Appl. Cryst 2015,
[
[
5]
6]
S. A. Barrett, C. A. Kilner, M. A. Halcrow, Dalton Trans. 2011, 40, 12021–
12024.
48, 306–309.
Kershaw Cook, Laurence J., R. Kulmaczewski, R. Mohammed, S.
Dudley, S. A. Barrett, M. A. Little, R. J. Deeth, M. A. Halcrow, Angew.
Chem. Int. Ed. 2016, 55, 4327–4331.
[
[
28] G. M. Sheldrick, Acta Cryst. C 2015, 71, 3–8.
29] a) C. K. Johnson, M. N. Burnett, ORTEP-III, Oak-Ridge National
Laboratory, Oak-Ridge, TN 1996; b) L. Farrugia, J. Appl. Cryst. 1997, 30,
[
[
7]
8]
D. C. Ashley, E. Jakubikova, Inorg. Chem. 2018, 57, 9907–9917.
S. C. C. van der Lubbe, P. Vermeeren, C. Fonseca Guerra, F. M.
Bickelhaupt, Chem. Eur. J. 2020.
565.
[
30] C. F. Macrae, I. Sovago, S. J. Cottrell, P. T. A. Galek, P. McCabe, E.
Pidcock, M. Platings, G. P. Shields, J. S. Stevens, M. Towler et al., J.
Appl. Cryst. 2020, 53, 226–235.
[
[
[
[
9]
B. Weber, E.-G. Jäger, Eur. J. Inorg. Chem. 2009, 2009, 465–477.
10] B. Weber, Coord. Chem. Rev. 2009, 253, 2432–2449.
[
[
[
[
[
31] O. Kahn, Molecular Magnetism, VCH, New York, N.Y 1993.
32] F. Neese, WIREs Comput. Mol. Sci. 2012, 2, 73–78.
11] S. Thallmair, W. Bauer, B. Weber, Polyhedron 2009, 28, 1796–1801.
12] T. M. Pfaffeneder, S. Thallmair, W. Bauer, B. Weber, New J. Chem. 2011,
33] A. Schäfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571–2577.
34] Becke, Phys. Rev. A 1988, 38, 3098–3100.
3
5, 691–700.
13] W. Bauer, T. Ossiander, B. Weber, Z. Naturforsch. B 2010, 2010, 323–
28.
[
35] a) R. M. Dickson, A. D. Becke, J. Chem. Phys. 1993, 99, 3898–3905; b)
B. Miehlich, A. Savin, H. Stoll, H. Preuss, Chem. Phys. Letters 1989, 157,
3
[
[
14] B. Weber, F. A. Walker, Inorg. Chem. 2007, 46, 6794–6803.
15] B. Weber, E. Kaps, J. Weigand, C. Carbonera, J.-F. Létard, K.
Achterhold, F. G. Parak, Inorg. Chem. 2008, 47, 487–496.
2
00–206.
36] S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 2010, 132,
54104.
37] A. Klamt, G. Schüürmann, J. Chem. Soc., Perkin Trans. 2 1993, 799–
05.
[
[
1
[
[
16] H. Görls, E.-G. Jäger, Cryst. Res. Technol. 1991, 26, 349–355.
17] H. Petzold, P. Djomgoue, G. Hörner, C. Lochenie, B. Weber, T. Rüffer,
Dalton Trans. 2018, 47, 491–506.
8
[
18] D. N. Bowman, E. Jakubikova, Inorg. Chem. 2012, 51, 6011–6019.
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Sophie Schönfeld,[a] Wolfgang Bauer,[a]#
Sebastian Thallmair,[a]# Gerald
Hörner,*[a] and Birgit Weber*[a]
Page No. – Page No.
Running in the Family:
Molecular Factors controlling
Spin Crossover of Iron(II)
Complexes with Schiff-base
like Ligands
Additional Author information for the electronic version of the article.
Birgit Weber: ORCID identifier: 0000-0002-9861-9447
Gerald Hörner: ORCID identifier: 0000-0002-3883-2879
This article is protected by copyright. All rights reserved.