Solvent Influence on the Magnetic Properties of Iron(II) Spin-Crossover Coordination Compounds with 4,4′-Dipyridylethyne as Linker

Article abstract of DOI:10.1002/ejic.201501175

New iron(II) coordination polymers with solvent-dependent spin-crossover behaviour have been synthesised by using an N₂O₂ Schiff base like ligand and 4,4′-dipyridylethyne as the bridging axial ligand in different solvents. The effects of different solvents and preparation methods (temperature, concentration) on the magnetic properties have been investigated. Annealing the coordination polymers prepared by the same method but in different solvents led to the same compound exhibiting the same three-step spin crossover independent of the magnetic properties of the solvated species. The X-ray structures of the axial ligand, an iron(II) dimer and one of the iron(II) coordination polymers are discussed. Powder X-ray diffraction analysis was additionally used for further comparison of the compounds.

Full text of DOI:10.1002/ejic.201501175

DOI: 10.1002/ejic.201501175
Full Paper
CLUSTER
ISSUE
Spin-Crossover Compounds
Solvent Influence on the Magnetic Properties of Iron(II) Spin-
Crossover Coordination Compounds with 4,4′-Dipyridylethyne
as Linker
[a]
[a]
[b]
[a]
Katja Dankhoff, Charles Lochenie, Florian Puchtler, and Birgit Weber*
Abstract: New iron(II) coordination polymers with solvent-de- method but in different solvents led to the same compound
pendent spin-crossover behaviour have been synthesised by exhibiting the same three-step spin crossover independent of
using an N O Schiff base like ligand and 4,4′-dipyridylethyne the magnetic properties of the solvated species. The X-ray
2
2
as the bridging axial ligand in different solvents. The effects of structures of the axial ligand, an iron(II) dimer and one of the
different solvents and preparation methods (temperature, con- iron(II) coordination polymers are discussed. Powder X-ray dif-
centration) on the magnetic properties have been investigated. fraction analysis was additionally used for further comparison
Annealing the coordination polymers prepared by the same of the compounds.
Introduction
ever, the different magnetic properties have always been associ-
ated with differences in the packing of the polymer chains. As
a consequence of this, an exchange of solvent molecules, as
already realised, for example, for porous Hoffman clathrate sys-
tems, was not possible.
In this work we present the synthesis of coordination poly-
mers with bpey (4,4′-dipyridylethyne) as a rigid linker. On the
The synthesis of functional coordination networks with switch-
able magnetic properties is a highly active area of research.
One possibility for realising such systems is the integration of
spin-crossover (SCO) centres in coordination polymers or net-
works. Such materials are known for their ability to be switched
between two different electronic states by a wide variety of
[
1,2]
[4,18,19]
one hand, rigid linkers are a pre-requisite for the observation
[
3]
physical and chemical stimuli. This switching process is ac-
companied by a change in the physical properties of the mate-
rial, for example, the magnetism or colour. The possibility of
realising guest-dependent spin transitions in porous SCO mate-
[20]
of cooperative spin transitions.
Additionally, this linker is
longer than 4,4′-bipyridine, already used as a rigid linker for the
formation of coordination polymers of the complexes investi-
gated by our group. Thus, the formation of porous structures
for the inclusion of solvent molecules is more likely and due to
the rigid structure, the chances of reversibility are higher. In-
deed, Hofmann clathrate complexes with bpey as ligand show-
ing different magnetic behaviours depending on the amount
[
2,4,5]
rials
in combination with the read-out possibilities due to
the associated changes makes this substance class highly inter-
esting for applications in the field of sensing.
[
6]
The influence of solvent molecules included in the crystal
packing on the SCO properties has already been documented
for many different systems. For the complexes investigated by
our group, this is most commonly observed for coordination
polymers, for which different solvents used in the synthesis of-
[18,19]
of solvent have also been reported.
[
7]
Results and Discussion
[
8–16]
ten lead to different magnetic properties.
changes are rare for mononuclear complexes.
In contrast, such
Synthesis
[
17]
So far, how-
The ligand bpey was synthesised as reported by Tanner and
[
21]
Ludi
by bromination of trans-4,4′-dipyridylethylene (bpee)
[
[
a] Lehrstuhl für Anorganische Chemie II, Universität Bayreuth,
Universitätsstraße 30, NW 1, 95440 Bayreuth, Germany
E-mail: weber@uni-bayreuth.de
and elimination of two molecules of HBr from 1,2-dibromo-1,2-
pyridylethane by using tBuONa in tBuOH. The purity of the
1
http://www.ac2-weber.uni-bayreuth.de
product was confirmed by elemental analysis, H NMR spectro-
b] Lehrstuhl für Anorganische Chemie I, Universität Bayreuth,
Universitätsstraße 30, NW 1, 95440 Bayreuth, Germany
http://www.ac1.uni-bayreuth.de
scopy, mass spectrometry and IR spectroscopy. The general syn-
thesis for the iron(II) complexes is based on a ligand-exchange
reaction between the axial methanol ligands of the already re-
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201501175.
[
22]
ported iron(II) precursor
Scheme 1). Three different methods were used for the synthe-
complex and the axial ligand bpey
©
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
(
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial-NoDerivs License, which permits use and distri-
bution in any medium, provided the original work is properly cited, the use
is non-commercial and no modifications or adaptations are made
sis of the iron(II) complexes.
Method 1: The aim of method 1 was to precipitate the com-
pound in boiling solvent. Therefore a concentrated solution of
Eur. J. Inorg. Chem. 2016, 2136–2143
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Scheme 1. Synthesis of the iron(II) complexes discussed in this work.
the precursor complex (0.2 g in 15 mL of solvent) and 2 equiva- amine, such pentacoordinate or hexacoordinate structures are
lents of the ligand bpey were heated at reflux for 1 hour and frequently observed.[23]
the precipitate was filtered off.
Method 2: The aim of method 2 was to precipitate the com-
pound at room temperature. Therefore a diluted solution of the
precursor complex (0.2 g in 60 mL of solvent) and 2 equivalents
Crystal Structure Analysis
of the ligand were heated at reflux and cooled to room temper-
Crystals suitable for X-ray structure analysis were obtained for
ature. A precipitate was obtained after several days and filtered
the ligand bpey by recrystallisation of the ligand from petro-
off.
leum ether. Even though the synthesis of the ligand bpey has
Method 3: The aim of method 3 was to obtain crystals suit-
been reported, to the best of our knowledge, its X-ray structure
able for X-ray structure analysis. Therefore a slow diffusion set-
was not available until now. The crystal data were collected at
up was prepared for a solution of the precursor complex and a
solution of the ligand.
1
33 K and are presented in Table S1 in the Supporting Informa-
tion. An ORTEP drawing of the ligand is shown in Figure 1. The
ligand bpey crystallises in the monoclinic space group C2/m.
The asymmetric unit contains one quarter of the molecule. The
bond lengths and angles are given in Table 2. The triple bond
is linear with a >C–C≡C angle of 180° and the pyridyl rings are
coplanar.
Methods 1 and 2 were used to investigate whether the tem-
perature of precipitation has an influence on the magnetic
properties. This could be the case if the complex precipitates
either in the high- or low-spin state and has already been ob-
served for other SCO complexes of this ligand type.[8,14]
Scheme 1 displays the general pathway for the synthesis of the
iron(II) complexes. The complexes all precipitated as very dark,
microcrystalline materials with varying solvent contents accord-
ing to elemental analysis, mass spectrometry, thermogravimet-
ric analysis (TGA; see Figure S1 in the Supporting Information),
and IR spectroscopy. Table 1 gives an overview of all the iron(II)
complexes prepared, the corresponding solvent used in the
synthesis and the included solvent content. Coordination poly-
mers were obtained in most cases, as expected for this type of
[
10]
compound.
However, in three cases dimers were obtained
Figure 1. ORTEP drawing of bpey. Ellipsoids are drawn at the 50 % probability
level.
with this axial ligand; atomic absorption spectroscopy was used
to determine the percentage of iron(II) and distinguish between
polymers and dimers for all the high-spin (HS) complexes. Di-
mers have not yet been reported for this class of iron(II) com-
pound with phenylenediamine-based ligands. However, for
modified ligands based on 3,4-alkoxy-substituted phenylenedi-
Table 2. Bond lengths and angles of bpey.
Bond length [Å]
Bond angle [°]
N1–C1
C1–C2
C2–C3
C3–C4
C4–C4
1.3348(17)
N1–C1–C2
C1–C2–C3
C2–C3–C2
C3–C4–C4
C2–C3–C3–C2
124.14(13)
118.77(13)
117.92(15)
180.00(19)
0.07
1.384(2)
1.3868(7)
1.439(3)
1.199(3)
Table 1. Iron(II) complexes with the solvents and methods used for their syn-
thesis.
Complex
Solvent
Method
1
1
1
1
1
1
1
a
b
c
d
e
f
μ-(bpey)-[FeL1(MeOH)]
μ-(bpey)-[FeL1(MeOH)]
2
MeOH
MeOH
EtOH
EtOH
dmf
2
1
2
1
1
1
3
2
Crystals suitable for X-ray structure analysis of the dimer 1g
were obtained by slow diffusion of the iron(II) precursor and
bpey in EtOH at room temperature. The crystal data were col-
lected at 133 K and are presented in Table S1 in the Supporting
Information. An ORTEP drawing of the complex is shown in
{[FeL1(bpey)]·0.5EtOH}
[FeL1(bpey)]
{[FeL1(bpey)]·dmf}
{[FeL1(bpey)]·1.25toluene}
μ-(bpey)-[FeL1(EtOH)]
n
n
n
n
toluene
EtOH
g
2
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Figure 2. The compound crystallises in the triclinic space group the dimer. Selected bond lengths and angles are given in
P 1¯ . The asymmetric unit contains one and a half molecules of Table 3. The sixth coordination site of the iron centre is occu-
pied by an ethanol molecule, the methyl group of which is
disordered over more than two positions (probably rotating).
This disorder could not be solved.
The iron(II) centres of 1g have an octahedral N O coordina-
3
3
tion sphere, with the ligand bpey linking the two iron(II) cen-
tres. The average bond lengths are 2.08 (Fe–N ), 2.00 (Fe–O ),
eq
eq
2
.26 (Fe–N ) and 2.24 Å (Fe–O ). The average O –Fe–O an-
ax ax eq eq
gle of 108° is clearly in the range typical of the HS state for
[
17,24]
iron(II) complexes of this type.
The average L –Fe–L an-
ax ax
gle of 175.1° does not differ significantly from the expected
angle of 180° for an ideal octahedron. The >C–C≡C angle has
an average value of 176.7°, which indicates a slight bending of
the ligand. The torsion angle between the pyridyl rings of the
ligand bpey is small with an average value of 1.2°. The OH
groups of the coordinating ethanol molecules in the asymmet-
ric unit are donors for hydrogen bonds with a carbonyl oxygen
of a neighbouring molecule (O17–H17A···O60, O57–H57A···O55
and O64–H64···O15). Through these hydrogen bonds, the di-
mers form zig-zag chains along the vector [1–11]. The details
of the hydrogen bonds are summarised in Table 4 and the mo-
lecular packing of the compound in the crystal is shown in
Figure 3.
Table 4. Overview of the intermolecular hydrogen bonds in 1g and 1e.
D–H···A
D–H [Å]
H···A [Å]
D···A [Å]
D–H···A [°]
1
g
e
O17–H17A···O60a
0.84
0.84
0.84
0.95
0.95
2.01
1.97
1.92
2.38
2.43
2.729(12)
2.748(12)
2.714(11)
3.010(3)
3.039(3)
143
154
158
122
122
b
O57–H57A···O55
a
O64–H64···O15
c
1
C24–H24···O7
c
C25–H25···O7
a: 2 – x, 1 – y, 2 – z, b: 1 – x, 1 – y, 1 – z, c: 2 – x, –y, 2 – z
Crystals of the coordination polymer 1e were obtained di-
rectly from the synthesis. The crystal data were collected at
1
33 K and are presented in Table S1 in the Supporting Informa-
tion. An ORTEP drawing of the asymmetric unit is given in Fig-
ure 2. The coordination polymer crystallises in the monoclinic
space group P2 /c. The asymmetric unit contains the mono-
1
meric unit of the coordination polymer [FeL1(bpey)] and one
n
non-coordinating dmf molecule. Selected bond lengths and an-
gles are given in Table 3. The iron(II) centre has an octahedral
N O coordination sphere. The average bond lengths are 1.90
4 2
Figure 2. ORTEP drawings of 1g (top) and 1e (bottom). Hydrogen atoms have
been omitted for clarity. Ellipsoids are drawn at the 50 % probability level.
(Fe–N ), 1.94 (Fe–O ) and 1.99 (Fe–N ). The O –Fe–O angle
eq eq ax eq eq
Table 3. Spin state, selected bond lengths and angles within the inner coordination sphere of the iron(II) complexes discussed in this work, and angles and
[
a]
torsion angles of the triple bond of bpey.
S
Fe–Neq [Å]
2.071(10)
Fe–Oeq [Å]
2.007(8)
Fe–Nax [Å]
2.275(9)
2.258(9)
2.250(9)
Fe–Oax [Å]
O
eq–Fe–Oeq [°]
L
ax–Fe–Lax [°]
bpey >C–C≡C [°]
Torsion bpey [°]
1
g
2
2.225(8)
2.247(8)
2.236(8)
107.7(3)
107.4(3)
107.3(3)
174.7(3)
175.0(3)
175.5(3)
178.3(12)
177.1(14)
174.7(14)
0.48
1.60
1.45
2
2
2
2
2
.081(10)
1.989(8)
2.015(8)
1.994(8)
1.988(8)
2.012(8)
1.9427(14)
1.9472(13)
.075(10)
.070(10)
.095(9)
.086(10)
1
e
0
1.9004(17)
.9052(17)
1.9865(15)
1.9833(15)
89.49(6)
175.54(7)
179.3(2)
176.4(2)
88.76
88.52
1
[
a] Crystal data determined at 133 K.
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gand. The two pyridyl rings of bpey are almost perpendicular
to each other with an average torsion angle of 88.64°. In addi-
tion to the monomeric unit of the complex, the asymmetric
unit contains one molecule of dmf connected through
hydrogen bonds to one pyridyl ring of the axial ligand. Details
of the hydrogen bonds are given in Table 4; two aromatic CH
groups of the axial ligand bpey (C24–H24 and C25–H25) are the
donor groups and the carbonyl oxygen (O7) of the dmf is the
acceptor group. These hydrogen bonds are the reason for the
bending of the axial ligand. The packing of the molecules in
the crystal with the hydrogen bonds is shown in Figure 3.
Magnetism
Magnetic susceptibility measurements were performed at an
applied field of 5000 G by using a SQUID magnetometer over
a range of temperatures to follow the spin transition (ST).
All the coordination polymers 1c–f show solvent-dependent
Figure 3. Molecular packing of compounds 1g (top: A: along [010]; B: along
111]) and 1e (bottom: C: along [001]; D: along [100]) in the crystal. Hydrogen
atoms not involved in hydrogen bonds have been omitted for clarity.
Hydrogen bonds are drawn as dashed lines.
[
SCO behaviour. The ꢀ T products for the dimeric complexes 1a,
M
is 89.49(6)°, clearly in the range for LS iron(II).[17,24] This is in 1b and 1g are in the range expected for dimeric HS iron(II)
agreement with the results of the magnetic measurements, complexes (S = S2 = 2; expected spin-only value: ꢀ T =
1
–1
M
3
which show that the full HS state is only reached above room 6 cm K mol ) with a significant orbital momentum contribu-
temperature, therefore it was not possible to determine the tion. The room temperature values are 7.56 (1a), 7.16 (1b) and
3
–1
structure of the HS state. The L –Fe–L angle of 175.54(7)° 7.60 cm K mol (1g). The plots of ꢀ T versus T in the ranges
ax
ax
M
does not differ significantly from the expected angle of 180° for 300–50 K (1a and 1g) and 300–4 K (1b) for these complexes
a perfect octahedron. The bidentate ligand bpey links the are presented in Figure S2 in the Supporting Information. In
iron(II) centres in infinite chains along [001]. The average all cases, the ꢀ T product does not change significantly with
M
>C–C≡C angle of 177.85° indicates a slight bending of the li- temperature, which indicates the absence of any strong ex-
M
Table 5. Overview of the magnetic properties, T1/2 values, values of the ꢀ T product at the corresponding temperatures, and HS residues of the synthesised
compounds.
T (HS) [cm K mol–1]
3
ꢀ
T (LS) [cm K mol–1]
3
γ
SCO
T
1/2 [K]
ꢀ
M
M
HS
1
1
1
a
b
c
HS
HS
–
–
7.56 (300 K)
7.16 (300 K)
3.37 (300 K)
1.87 (145 K)
–
–
1
1
stepwise, abrupt with hysteresis
↓ 170 ↑ 190
110 ↑ 135
1 (300 K)
↓
0.55 (145 K)
0.13 (50 K)
1 (300 K)
0.48 (160 K)
0.25 (50 K)
1 (400 K)
0.57 (250 K)
0.34 (175 K)
0.05 (50 K)
1 (400 K)
0.53 (250 K)
0.29 (175 K)
0.04 (50 K)
1 (400 K)
0.02 (200 K)
1 (400 K)
0.56 (250 K)
0.33 (175 K)
0.10 (50 K)
1 (200 K)
0.46 (50 K)
1 (400 K)
0.54 (250 K)
0.30 (175 K)
0.06 (50 K)
0
.43 (50 K)
.85 (50 K)
incomplete, abrupt with hysteresis[a]
multi-step, gradual, abrupt
↓ 175 ↑ 185
3.37 (300 K)
1
.63 (160 K)
0
1
d
320
10
40
3.34 (400 K)
1.92 (250 K)
1.12 (175 K)
2
1
0
.18 (50 K)
multi-step, gradual, abrupt[a]
320
10
40
3.34 (400 K)
1.77 (250 K)
0.96 (175 K)
2
1
0
.12 (50 K)
1
1
e
f
gradual
305
3.65 (400 K)
0
.06 (200 K)
multi-step, gradual, abrupt[a]
320
10
40
3.65 (400 K)
2.05 (250 K)
1.20 (175 K)
2
1
0
.38 (50 K)
.39 (50 K)
gradual
125
2.99 (200 K)
1
multi-step, gradual, abrupt[a]
320
10
40
3.22 (400 K)
1.74 (250 K)
0.97 (175 K)
2
1
0
.19 (50 K)
[
a] After annealing.
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change interactions through the bpey ligand; the linker bpey is
After heating the sample to 400 K and keeping it at this
probably too long to allow an effective super-exchange path- temperature for 1 hour to remove the included solvent mol-
way between the iron(II) centres. All measurements for the co- ecules, the SCO behaviour changed. Now an incomplete spin
ordination polymers were taken in settle mode, but to show transition with hysteresis is observed. The first step is similar to
that there is no significant dependence on scan rate, the plots the first cooling and heating cycle with a hysteresis width of
of ꢀ T versus T for the sweep measurements of all complexes 10 K (T ↓ = 175 K and T ↑ = 185 K). After the first step the ꢀ T
M
1
1
–1
M
3
are presented in Figure S2 in the Supporting Information, as it value is 1.63 cm K mol at 160 K. On further cooling it gradu-
has been shown that the scan rate can significantly influence ally decreases until it reaches 0.85 cm K mol at 50 K. To rule
the width of the hysteresis. An overview of all the ꢀ T values, out different iron sites (e.g., due to short polymer chains with
3
–1
[
25]
M
the fraction of the HS state, and the SCO behaviour is presented EtOH coordination at the chain ends) as a reason for the re-
in Table 5. The plots of ꢀ T versus T in the temperature ranges maining HS fraction, a Mössbauer spectrum was recorded at
M
of interest for the ST of complexes 1c–1f are displayed in Fig- room temperature (see Figure S3 and Table S2 in the Support-
ure 4. Plots of ꢀ T versus T in the full temperature range of 50– ing Information), which shows only one HS iron(II) site. Thus,
M
400 K can be found in Figure S2 in the Supporting Information. other reasons are responsible for the incomplete spin transition.
Complex 1c exhibits almost complete solvent-dependent SCO Complex 1d shows a multi-step ST over the complete tem-
behaviour. The first cooling and heating cycle reveals a stepwise perature range studied (50–400 K). The first, rather gradual step
ST with hysteresis. The first step has a hysteresis width of 20 K takes place at 320 K with a third of the iron(II) centres undergo-
with T ↓ = 170 K and T ↑ = 190 K. The second step has a ing the ST. The value of the ꢀ T product decreases from
1
1
M
3
–1
3
–1
hysteresis width of 25 K with T ↓ = 110 K and T ↑ = 135 K, and 3.34 cm K mol at 400 K to 1.92 cm K mol at 250 K. The
2
2
the complex is almost totally LS at 85 K. The ꢀ T product is second, more abrupt step takes place at 210 K. Another third
M
3
–1
3
.37 cm K mol at 300 K and is typical of HS iron(II). It de- of the iron(II) centres undergo the ST with a value of the ꢀ T
creases to 1.87 cm K mol at 145 K (plateau), and after the product of 1.12 cm K mol at 175 K. The last, abrupt step in
M
3
–1
3
–1
3
–1
second step it reaches 0.43 cm K mol at 50 K.
the transition takes place at 140 K. The remaining iron(II) cen-
Figure 4. Plots of the ꢀ
M
T products versus T for compounds 1c (left, top), 1d (left, bottom), 1e (right, top) and 1f (right, bottom). Black squares represent the
first cooling and heating cycle, red circles represent the cooling and heating cycle after annealing.
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tres change their spin state to LS. The final ꢀ T product is ruled out. To further compare compounds 1d–f, the powder
M
3
–1
0
.18 cm K mol at 50 K. After heating to 400 K for 1 hour the patterns of 1e and 1f were recorded at room temperature after
SCO properties do not change significantly, the only change annealing. It can be seen that all the complexes are isostruc-
that can be observed is a small decrease in the value of the tural.
ꢀMT product after each step in the ST (see Table 5).
Complexes 1e and 1f show a gradual, complete (1e) or in-
complete (1f) ST with T1/2 = 305 K for 1e and T1/2 = 125 K for
3
–1
1
f. The value of the ꢀ T product of 1e is 0.06 cm K mol at
M
2
00 K, and therefore clearly in the range for LS iron(II). After the
3
–1
ST it has a value of 3.65 cm K mol at 400 K, which is typical
for HS iron(II). Complex 1f has a ꢀ T product value of
M
3
–1
2
.99 cm K mol at 200 K and after the ST the value of the ꢀ T
product is 1.39 cm K mol at 50 K. After heating at 400 K for
hour, both complexes show the same ST as complex 1d with
M
3
–1
1
just small differences in the values of the ꢀ T product, which
M
indicates that these complexes have the same “dry” phase.
Powder Diffraction Analysis
All the samples were subjected to powder X-ray diffraction anal-
ysis at relevant temperatures for the magnetic properties of the
complexes. Two regions of the powder diffraction patterns are
of special interest: the iron(II)–iron(II) distance is often observed
between 7 and 10° and the inter-chain distance can be ob-
served between 24 and 27°. These regions were determined by
Figure 5. Powder X-ray diffraction patterns of 1c–f.
Discussion
using Bragg's law (sin θ = λ/2d) and the results obtained for Of the seven synthesised complexes, three are dimers (1a, 1b
similar 1D chain compounds have already demonstrated that and 1g) and therefore are HS over the complete temperature
even changes in the Fe–Fe distances in the chain due to the range studied. The remaining four synthesised coordination
spin transition can be explained (larger θ → smaller d and vice polymers show different SCO behaviour when solvated, also the
[
15]
versa).
The strong diffraction peaks in the region between 7 powder X-ray diffraction patterns are different for these com-
and 10° most likely arise from iron centres with a high electron plexes. Once dried, the three complexes synthesised by the
density. Additionally, for most of the 1D coordination polymers same method (for 1d–f) but in different solvents show the
synthesised by our group, the polymer chains are parallel, sup- same three-step ST, which indicates that annealing leads to a
porting a diffraction pattern dominated by inter-chain iron–iron similar “dry phase”. This can be seen in the powder X-ray dif-
distances. The powder X-ray diffraction patterns of the dimeric fraction patterns of the annealed complexes, which are all the
complexes 1a and 1g are similar and are shown in Figure S4 in same. The complexes have a different structure when solvated.
the Supporting Information. The recorded and calculated spec- Therefore the exchange of solvents is not possible. The complex
tra of the complexes 1c–1f are shown in Figure 5. The spectra synthesised by a different method (for 1c) shows a different ST
of each of 1c and 1f are the same at all the temperatures meas- to the others, and also a different powder X-ray diffraction pat-
ured; the small shifts are due to the different temperatures at tern. This indicates that the temperature of precipitation has a
which the spectra were recorded. The powder diffraction spec- strong influence on the magnetic properties. The greater
tra of 1d at three different temperatures (100, 175 K and room amount of solvent used in the synthesis caused a slower precip-
temp.) look very similar. The disappearance of the peak at 8.46° itation of the complex and therefore a different packing in the
(
100 and 175 K) and the appearance of the peak at 8.35° (175 K crystal. The solvent dependence of SCO behaviour has already
and room temp., denoted by * in Figure 5) can be correlated to been described for this type of compound but with N-(4-pyrid-
the change in the spin state of the iron(II) centre from LS (100 K) yl)isonicotinamide as the axial ligand. In contrast to this linker,
to almost HS (room temp.). The powder diffraction spectra of which has an amide group that can easily form hydrogen bonds
1
e are drastically different at the two measured temperatures with solvent molecules or neighbouring complexes,[15] bpey is
(
calculated at 133 K, and measured at room temp.). For the a ligand that has no strong hydrogen-bond donor or acceptor
complexes 1d–f, a change in the position of the peak represent- atoms. Similar complexes with 4,4′-bipyridine as axial ligand
ative of the iron–iron distance can be observed depending on show hysteretic SCO behaviour, but have no solvent included
[
14,26]
the temperature of the measurement. This change can be ex- in the crystal packing.
As the ligand bpey is longer it leads
plained by the change in the spin state of the iron(II) centre. In to more space between the iron(II) centres and therefore there
the HS state, the peak has a position at a smaller 2θ angle is a higher possibility that it will trap solvent molecules. The
(
larger distance) than in the LS state. This is due to a volume trapped solvent molecules form hydrogen bonds with the li-
change upon ST. Based on this data, a new structural phase gand bpey, as shown in the structure of 1e, and significantly
corresponding to the plateau in the magnetic data cannot be influence the SCO properties of the material. However, as only
Eur. J. Inorg. Chem. 2016, 2136–2143
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Full Paper
1
D coordination polymers are formed, the pores in the structure in ethanol (60 mL) was heated at reflux for 1 h. After cooling and
leaving to stand at room temperature for 1 d, the black precipitate
are not stable enough to allow the reversible binding of the
solvent molecules.
was filtered off, washed twice with ethanol (3 mL) and dried in
vacuo to give 1c, yield 0.16 g (63 %). IR: ν˜ = 1683 (CO, s), 1567 (CO,
–
1
+
s) cm . MS (EI, +): m/z (%) = 442 (66) [C H FeN O ] , 180 (100)
[
20
22
2 6
+
C H N ] . C H FeN O ·0.5EtOH (645.49): calcd. C 61.40, H 5.15,
12 8 2 32 30 4 6
Conclusions
N 8.68; found C 61.20, H 5.15, N 8.82.
In this report we have described the influence of solvent on the [FeL1(bpey)]n (1d): A dark red-brown solution of [FeL1(MeOH)2]
SCO properties of an iron(II) coordination polymer with a Schiff (0.2 g, 0.315 mmol) and bpey (0.14 g, 0.777 mmol) in ethanol
base like equatorial ligand and 4,4′-dipyridylethyne (bpey) as (15 mL) was heated at reflux for 1 h. After cooling to room tempera-
[
8–16]
ture, the black precipitate was filtered off, washed twice with eth-
axial ligand. As has been shown previously,
the solvent has
anol (5 mL) and dried in vacuo to give 1d, yield 0.17 g (64 %). IR:
a significant influence on the SCO behaviour of this type of
compound. Depending on the solvent used and preparation
method, different iron(II) complexes were obtained. The first
iron(II) dimers of this type of compound were obtained, and are
HS over the complete temperature range studied (50–400 K).
The iron(II) coordination polymers all exhibit SCO behaviour. For
those synthesised by the same method but in different solvents,
different spin transitions can be observed when solvated. Once
fully dried, the complexes show the same, three-step spin tran-
sition over almost the entire temperature range studied (50–
–
1
ν˜ = 1685 (CO, s), 1559 (CO, s) cm . MS (EI, +): m/z (%) = 442 (66)
+
+
[
C H FeN O ] , 180 (100) [C H N ] . C H FeN O (622.46):
20 22 2 6 12 8 2 32 30 4 6
calcd. C 61.75, H 4.86, N 9.00; found C 61.32, H 5.12, N 8.93.
[FeL1(bpey)]·dmf}n (1e): dark red-brown solution of
FeL1(MeOH) ] (0.2 g, 0.315 mmol) and bpey (0.14 g, 0.777 mmol)
{
A
[
2
in dmf (15 mL) was heated at reflux for 20 min. After cooling and
leaving to stand at room temperature for 5 d, the black crystals
were filtered off and dried in vacuo to give 1e, yield 0.1 g (46 %).
–1
IR: ν˜ = 1676 (CO, s), 1561 (CO, s) cm . MS (EI, +): m/z (%) = 442 (66)
+
+
[
C H FeN O ] , 180 (100) [C H N ] . C H FeN O ·dmf (695.55):
20 22 2 6 12 8 2 32 30 4 6
4
00 K). The powder X-ray diffraction patterns of the solvated calcd. C 60.44, H 5.36, N 10.07; found C 60.35, H 5.37, N 10.03.
complexes are different for each complex, but the same when
fully dried. The iron(II) coordination polymer synthesised by a
different method shows incomplete, but hysteretic SCO behav-
iour. The synthesis and characterisation of other complexes
{
[FeL1(bpey)]·1.25toluene}n (1f): A dark red-brown solution of
[FeL1(MeOH) ] (0.2 g, 0.315 mmol) and bpey (0.14 g, 0.777 mmol)
2
in toluene (15 mL) was heated at reflux for 1 h. After cooling and
leaving to stand at room temperature for 1 d, the dark-purple pre-
with bpey as axial ligand are currently ongoing, for example, cipitate was filtered off, washed twice with toluene (5 mL) and dried
[
27]
with alkoxy derivatives of our Schiff base like ligands.
in vacuo to give 1f, yield 0.26 g (89 %). IR: ν˜ = 1686 (CO, s), 1564
–
1
+
(
CO, s) cm . MS (EI, +): m/z (%) = 442 (66) [C H FeN O ] , 180
20 22 2 6
+
(
100) [C H N ] . C H FeN O ·1.25toluene (737.64): calcd. C 66.35,
12
8
2
32 30
4 6
H 5.47, N 7.60; found C 66.20, H 5.58, N 7.50.
Experimental Section
μ-(bpey)-[FeL1(EtOH)]2 (1g): Black crystals of 1g were obtained
by slow diffusion in a home-made Schlenk apparatus containing
Materials and Methods: All syntheses involving iron(II) were car-
ried out under argon using Schlenk tube techniques. The solvents
used were of analytical grade and degassed with argon. The precur-
[
FeL1(MeOH) ] (0.1 g, 0.198 mmol) and bpey (0.071 g, 0.395 mmol)
2
in ethanol solution for 1 month.
[22]
sor complex [FeL1(MeOH) ]
and the axial ligand 4,4′-dipyridyl-
2
ethyne[ were synthesized according to literature procedures. CHN Single-Crystal X-ray Diffraction: The X-ray crystal analysis of 1e
21]
analyses were performed with a Vario El III instrument from Elemen-
tar AnalysenSysteme. Mass spectra were recorded with a Finnigan
MAT 8500 spectrometer with a MASPEC II data system.
and 1g was performed with a Stoe StadiVari diffractometer using
graphite-monochromated Mo-K radiation. The X-ray analysis of
bpey was performed with a Stoe IPDS II diffractometer using graph-
α
ite-monochromated Mo-Kα radiation. The data were corrected for
Lorentzian and polarization effects. The structures were solved by
μ-(bpey)-[FeL1(MeOH)]2 (1a):
A dark red-brown solution of
[
FeL1(MeOH) ] (0.2 g, 0.395 mmol) and bpey (0.14 g, 0.777 mmol)
2
[28]
direct methods (SIR-97)
and refined by full-matrix least-squares
in methanol (60 mL) was heated at reflux for 1 h. After cooling and
leaving to stand at room temperature for 1 d, the black precipitate
was filtered off, washed twice with methanol (3 mL) and dried in
vacuo to give 1a, yield 0.16 g (36 %). IR (neat): ν˜ = 1686 (CO, s),
techniques against Fo2 – F 2 (SHELXL-97).
[29]
All hydrogen atoms
were calculated in idealised positions with fixed displacement pa-
c
[
30]
rameters. ORTEP-III
was used for the structure representations,
to illustrate molecular packing.
[
31]
SCHAKAL-99
–
1
+
1
1
5
564 (CO, s) cm . MS (EI, +): m/z (%) = 442 (66) [C H FeN O ] ,
20 22 2 6
+
80 (100) [C H N ] . C H Fe N O (1128.78): calcd. C 57.46, H CCDC 1426886 (for bpey), 1426887 (for 1e), and 1426888 (for 1g)
12
8
2
54 60
2 6 14
.36, N 7.45, Fe 9.89; found C 57.55, H 5.33, N 7.51, Fe 10.22.
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
μ-(bpey)-[FeL1(MeOH)]2 (1b): A dark red-brown solution of
FeL1(MeOH) ] (0.2 g, 0.315 mmol) and bpey (0.14 g, 0.777 mmol)
[
2
in methanol (15 mL) was heated at reflux for 1 h. After cooling and
leaving to stand at room temperature for 1 d, the black precipitate
was filtered off, washed with methanol (5 mL) and dried in vacuo
X-ray Powder Diffraction: Powder diffractograms were recorded
with a STOE StadiP Powder Diffractometer (STOE, Darmstadt) using
Cu-K 1 radiation with a Ge Monochromator and a Mythen 1K Strip-
α
–
1
to give 1b, yield 0.17 g (38 %). IR: ν˜ = 1686 (CO, s), 1564 (CO, s) cm . detector in transmission geometry.
+
+
MS (EI, +): m/z (%) = 442 (66) [C H FeN O ] , 180 (100) [C H N ] .
2
0
22
2
6
12 8 2
Magnetic Measurements: Magnetic measurements were carried
out by using a SQUID MPMS-XL5 instrument from Quantum Design
at an applied field of 5000 G and in the temperature range of 400–
50 K in settle mode. The sample was prepared in a gelatine capsule
held in a plastic straw. The raw data were corrected for the diamag-
C H Fe N O (1128.78): calcd. C 57.46, H 5.36, N 7.45, Fe 9.89;
54
60
2 6 14
found C 57.27, H 5.36, N 7.44, Fe 10.89.
[FeL1(bpey)]·0.5 EtOH}n (1c): A dark red-brown solution of
FeL1(MeOH) ] (0.2 g, 0.315 mmol) and bpey (0.14 g, 0.777 mmol)
{
[
2
Eur. J. Inorg. Chem. 2016, 2136–2143
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2142
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Full Paper
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is acknowledged. The EU-COST Network (CM1305) on spin-state
reactivity is acknowledged for support.
[
[
2
2
Keywords: Spin crossover · Coordination compounds ·
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Received: October 15, 2015
Published Online: January 15, 2016
Eur. J. Inorg. Chem. 2016, 2136–2143
www.eurjic.org
2143
© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim