crystals
Communication
Synthesis, Structure, and Magnetic Properties of New
Spin Crossover Fe(II) Complexes Forming Short
Hydrogen Bonds with Substituted Dicarboxylic Acids
Takumi Nakanishi and Osamu Sato *
Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motoka, Nishi-ku, Fukuoka 8190395,
Japan; [email protected]
* Correspondence: [email protected]; Tel.: +80-92-802-6208; Fax: +81-92-802-6205
Academic Editor: Helmut Cölfen
Received: 14 August 2016; Accepted: 6 October 2016; Published: 13 October 2016
Abstract: We synthesized new Fe(II) compounds consisting of [Fe(L)2 ] (HL: 2-Acetylpyridine
isonicotinoylhydrazone) and dicarboxylic acids. Single crystal analysis shows that short hydrogen
bonds are formed between [Fe(L)2 ] and dicarboxylic acids. The hydrogen bonding interactions
between metal complexes and dicarboxylic acids are complemented by π-π interactions, forming
two-dimensional network structures. Magnetic properties show that the Fe(II) compounds exhibit
gradual spin crossover behavior around room temperature and a T1/2 value is dependent on the
halogen substituent on the dicarboxylic acids.
Keywords: spin crossover; hydrazone complex; hydrogen bond
1. Introduction
Hydrogen bonding plays an important role in various biological [1], catalytic [2] and crystal
engineering systems [3]. Proton transfer or proton migration between the two components that make
up a hydrogen bonding system is an important mechanism of ferroelectricity [4–6]. Furthermore,
switching of the conduction and magnetic properties of organic conductor crystals via deuterium
transfer in a short hydrogen bond was recently reported [7]; however, although several interesting
short hydrogen bonding systems have been reported in pure organic systems, it is still a challenge to
construct short hydrogen bonding in functional coordination compounds such as in spin crossover
(SCO) and valence tautomeric complexes. There are numerous reports on SCO compounds forming
hydrogen bonding networks between anions or solvent molecules and SCO complexes, and these
investigations demonstrated that the strength and dimensionality of the hydrogen bonding network
contribute to SCO behavior, including the sharpness and hysteresis of the crossover; however, the
hydrogen bonding distances in these compounds tend to be long because the pKa values of anions
or solvent molecules tend to be much higher or lower than the pKa value at a proton acceptor site.
Therefore, for the construction of short hydrogen bonds, it is important to introduce proton donor
or acceptor molecules that have pKa values near that of the SCO complex. In this study, we have
investigated a new SCO compound comprising an Fe(II) complex and a polyprotic organic acid,
wherein a short hydrogen bond is formed between the Fe(II) complex and the polyprotic organic
acid. There are few studies that report the crystal structure of SCO compounds with the polyprotic
organic acids [8]. Here, 2-Acetylpyridine isonicotinoylhydrazone (HL) was used as a ligand for the
Fe(II) complex. The neutral Fe(II) complex bearing this ligand has a proton acceptor site at the terminal
Py ring (Scheme 1). Neutral complexes with similar ligands have been reported to exhibit SCO
behavior [9,10]. Thus, it was expected that co-crystals comprising the neutral Fe(II) complex and a
polyprotic organic acid would also exhibit SCO behavior. We focus on a group of polyprotic organic
Crystals 2016, 6, 131; doi:10.3390/cryst6100131
www.mdpi.com/journal/crystals
Crystals 2016, 6, 131
2 of 8
Crystals 2016, 6, 131
2 of 8
neutral Fe(II) complex and a polyprotic organic acid would also exhibit SCO behavior. We focus on a
group of polyprotic organic dicarboxylic acids, particularly those with halogen substituents. In the
dicarboxylic
acids,introducing
particularlyhalogen-substituted
those with halogen dicarboxylic
substituents.acids,
In thethe
SCO
compounds
SCO compounds
hydrogen
bondintroducing
distance is
halogen-substituted
dicarboxylic
acids,
the
hydrogen
bond
distance
is
expected
to
be
controlled
by
expected to be controlled by changing the halogen substituent without changing
the crystal
changing
halogenTsubstituent
withouttochanging
the crystal
structure.
T1/2 is also
structure. the
Therefore,
1/2 is also expected
be controlled
by changing
the Therefore,
halogen substituent
in
expected
to
be
controlled
by
changing
the
halogen
substituent
in
dicarboxylic
acid.
Notably,
it
has
dicarboxylic acid. Notably, it has been reported that the T1/2 value can be controlled by modifying
been
reported
that the T1/2 counter-ions
value can be controlled
by modifying[11,12].
ligands To
andconfirm
exchanging
ligands
and exchanging
in SCO compounds
this counter-ions
hypothesis,
in
SCO compounds [11,12].
2,5-dichroloterephthalic
acid were
(H2 Clchosen
2 TPA)
2,5-dichroloterephthalic
acid To
(H2confirm
Cl2TPA) this
and hypothesis,
2,5-dibromoterephthalic
acid (H2Br2TPA)
and
2,5-dibromoterephthalic
acid
(H
Br
TPA)
were
chosen
as
a
counterpart
of
the
hydrogen
bond,
2 where
2
as a counterpart of the hydrogen bond,
the carboxyl group in H2Cl2TPA and H2Br2TPA would
where
the proton
carboxyl
group in H2 Cl2 TPA and H2 Br2 TPA would act as the proton donor.
act as the
donor.
Scheme 1.
1. Schematic
Schematic of
of proton
proton accepter
accepter complex
complex [Fe(L)
[Fe(L)2]] and
and proton
proton donor
donor acids
acids H
H2Cl
Cl2TPA and
Scheme
2
2 2 TPA and
H2Br
Br2TPA. Proton acceptors on the terminal pyridine ring of 2-acetylpyridine
H
2 2 TPA. Proton acceptors on the terminal pyridine ring of 2-acetylpyridine isonicotinoylhydrazone
isonicotinoylhydrazone
are shown in blue.
are shown in blue.
2. Results and Discussion
2. Results and Discussion
Experiment
Experiment
The new
new SCO
SCOFe(II)
Fe(II)compounds
compounds1-Cl
1-Cland
and1-Br
1-Brcontaining
containingHH22Cl
Cl22TPA
TPA and
and H
H22Br
Br22TPA,
respectively,
The
TPA, respectively,
and 2-acetylpyridine
2-acetylpyridine isonicotinoylhydrazone
isonicotinoylhydrazone as
ligand were
were synthesized
synthesized using
using the
the procedure
procedure
and
as aa ligand
described
in
the
Experimental
Section.
The
newly
synthesized
1-Cl
and
1-Br
were
characterized
using
described in the Experimental Section. The newly synthesized 1-Cl and 1-Br were characterized
X-ray X-ray
crystalcrystal
structure
analysis,
IR spectroscopy,
and magnetic
propertyproperty
measurements.
using
structure
analysis,
IR spectroscopy,
and magnetic
measurements.
The
crystal
structures
of
1-Cl
and
1-Br
were
measured
at
123
K
and
320
K.
The asymmetric
asymmetric unit
unit
The crystal structures of 1-Cl and 1-Br were measured at 123 K and 320 K. The
and
molecular
arrangement
of
1-Cl
are
displayed
in
Figure
1a,b,
respectively.
The
crystal
structures
and molecular arrangement of 1-Cl are displayed in Figure 1a,b, respectively. The crystal structures of
of 1-Cl
1-Br
isostructural.
Crystallographic
tables
of bond
distances
and angles
for
1-Cl
andand
1-Br
are are
isostructural.
Crystallographic
datadata
and and
tables
of bond
distances
and angles
for 1-Cl
1-Cl
and
1-Br
are
listed
in
Table
1.
The
Fe(II)–N
and
Fe(II)–O
coordination
distances
in
1-Cl
and
1-Br
and 1-Br are listed in Table 1. The Fe(II)–N and Fe(II)–O coordination distances in 1-Cl and 1-Br at
at 123
K were
1.947(3)–1.966(3)
Å and
1.989(2)–1.999(2)
Å, respectively,
were consistent
with
123
K were
1.947(3)–1.966(3)
Å and
1.989(2)–1.999(2)
Å, respectively,
whichwhich
were consistent
with those
those
of low-spin
Fe(II) complexes
[9].coordination
These coordination
distances
weretofound
to increase
the
of
low-spin
Fe(II) complexes
[9]. These
distances
were found
increase
with the with
increase
increase
in
temperature,
indicating
that
both
1-Cl
and
1-Br
exhibit
SCO.
However,
the
distances
in temperature, indicating that both 1-Cl and 1-Br exhibit SCO. However, the distances observed at
observed
320equal
K were
not equal
to those
expectedby
to high-spin
be achieved
bycomplexes;
high-spin Fe(II)
complexes;
320
K wereatnot
to those
expected
to be achieved
Fe(II)
therefore,
SCO in
therefore,
SCO
in
both
1-Cl
and
1-Br
was
not
completed
at
320
K.
The
crystal
structural
both 1-Cl and 1-Br was not completed at 320 K. The crystal structural data measured at 320 K wasdata
not
measuredtoatdetermine
320 K wasthe
not
sufficient
to determine
the position
hydrogen
atom
involved in
sufficient
position
of the
hydrogen atom
involvedofinthe
hydrogen
bond
determination
hydrogen
bond determination
the difference
Fouriercoordination
map. The Fe(II)–N
Fe(II)–O
from
the difference
Fourier map.from
The Fe(II)–N
and Fe(II)–O
distances and
in 1-Cl
were
coordination
distances
in
1-Cl
were
observed
to
be
larger
than
those
in
1-Br
at
320
K
because
1-Cl
observed to be larger than those in 1-Br at 320 K because 1-Cl has a higher rate of high-spin iron(II) than
has
a
higher
rate
of
high-spin
iron(II)
than
that
of
1-Br
at
this
temperature.
As
shown
in
Figure
1a,
that of 1-Br at this temperature. As shown in Figure 1a, the asymmetric unit of 1-Cl comprises one Fe(II)
the asymmetric
1-Cl comprises one Fe(II) complex and one H2Cl2TPA molecule. The
complex
and one unit
H2 Clof
2 TPA molecule. The molecular arrangement shows that a zig-zag chain was
molecular
arrangement
shows
that a zig-zag
chain
was formed
alternating arrangement of
formed with an alternating arrangement
of Fe(II)
complexes
and with
H2 Clan
2 TPA molecules via hydrogen
Fe(II) complexes
and Hthe
2Cl2TPA molecules via hydrogen bonds formed between the N atom in the
bonds
formed between
N atom in the terminal Py ring in the Fe(II) complex and a carboxyl group
terminal
Py
ring
in
the
Fe(II)
a carboxyl
in H
2TPA (Figure 1b). The N4∙∙∙O3
in H2 Cl2 TPA (Figure 1b). Thecomplex
N4···O3 and
hydrogen
bondgroup
distance
in2Cl
the
terminal pyridine ring was
hydrogen
bond
distance
in
the
terminal
pyridine
ring
was
relatively
long
(N4∙∙∙O3was
= 2.663
Å), whereas
relatively long (N4···O3 = 2.663 Å), whereas the N8···O5 hydrogen bond distance
relatively
short
the ···
N8∙∙∙O5
hydrogen
was
(N8∙∙∙O5 bond
= 2.552distances
Å) at 123inK.
Thewere
N4∙∙∙O3
and
(N8
O5 = 2.552
Å) at bond
123 K.distance
The N4···
O3relatively
and N8···short
O5 hydrogen
1-Br
slightly
N8∙∙∙O5
hydrogen
bond
distances
in
1-Br
were
slightly
longer
(N4∙∙∙O3
=
2.681
Å,
N8∙∙∙O5
=
2.560
Å)
than
longer (N4···O3 = 2.681 Å, N8···O5 = 2.560 Å) than those in 1-Cl at 123 K. The N8···O5 bond distances
those
1-Cl
at were
123 K.
Thefor
N8∙∙∙O5
bond distances
1-Clbut
and
1-Br longer
were short
for a commonly
N∙∙∙O-type
in
1-Clinand
1-Br
short
a N···O-type
hydrogen in
bond
slightly
than those
hydrogeninbond
butcompounds
slightly longer
than those
observed
organic compounds
that
observed
organic
that exhibit
protoncommonly
migration induced
byin
temperature
change [13–15].
exhibit
proton
migration
induced
by
temperature
change
[13–15].
Regarding
the
temperature
Regarding the temperature dependency of hydrogen bond distances, studies have reported that the
dependency
of hydrogen
bond distances,
studies
have reported
the hydrogen
bondofdistance
hydrogen
bond
distance between
the Py ring
in isonicotinic
acid that
hydrazide,
a precursor
L, and
Crystals 2016, 6, 131
Crystals 2016, 6, 131
3 of 8
3 of 8
between the Py ring in isonicotinic acid hydrazide, a precursor of L, and the carboxyl group tends to
increase
withgroup
increasing
[16–18].
The increment
of N4∙∙∙O3
and
N8∙∙∙O5
bondofdistances
the
carboxyl
tendstemperature
to increase with
increasing
temperature
[16–18].
The
increment
N4···O3
fromN8
123
K tobond
320 K
was observed
in 1-Cl
1-Br.
addition,inthe
difference
in In
hydrogen
bond
and
···O5
distances
from 123
K to and
320 K
wasInobserved
1-Cl
and 1-Br.
addition,
the
distances at
K and 300
K in
1-Cl was
be300
larger
1-Br, suggesting
spin
difference
in123
hydrogen
bond
distances
atfound
123 K to
and
K inthan
1-Clthat
wasinfound
to be largerthat
thanthe
that
in
transition
from low
to high
spin contributes
to the
elongation
of hydrogen
bond
distance of
in
1-Br,
suggesting
thatspin
the spin
transition
from low spin
to high
spin contributes
to the
elongation
these compounds.
The O–H
bond
distances inThe
theO–H
hydrogen
bonds show
that
the hydrogen
hydrogen
bond distance
in these
compounds.
bond distances
in the
hydrogen
bondsatoms
show
involved
in both the
short
and long
hydrogen
bonds
closer
to were
the Hlocated
2Cl2TPA
molecules
that
the hydrogen
atoms
involved
in both
the short
andwere
long located
hydrogen
bonds
closer
to the
at2123
K. The
C–O bond
distances
in thebond
carboxyl
groups
andcarboxyl
the C–N–C
angles
terminal
Py
H
Cl2 TPA
molecules
at 123
K. The C–O
distances
in the
groups
and in
thethe
C–N–C
angles
ring
also
support
the
proposition
that
the
hydrogen
atoms
in
the
hydrogen
bonds
were
located
in the terminal Py ring also support the proposition that the hydrogen atoms in the hydrogen bonds
closerlocated
to the H
2Cl2TPA
molecules.
123 K, theAt
C34–O3
andC34–O3
C27–O5and
bond
distances
1.318(4)
Å
were
closer
to the
H2 Cl2 TPAAt
molecules.
123 K, the
C27–O5
bondwere
distances
were
and 1.305(4)
respectively,
demonstrating
that theythat
arethey
single
C–O.
theOn
other
1.318(4)
Å andÅ,1.305(4)
Å, respectively,
demonstrating
arebonds,
single i.e.,
bonds,
i.e.,On
C–O.
thehand,
other
the C34–O4
and C27–O6
bond distances
were 1.210(4)
Å andÅ1.217(4)
Å, respectively,
indicating
that
hand,
the C34–O4
and C27–O6
bond distances
were 1.210(4)
and 1.217(4)
Å, respectively,
indicating
thesethese
are double
bonds,
i.e.,i.e.,
C=O.
TheThe
C–O
bond
distances
that
are double
bonds,
C=O.
C–O
bond
distanceswere
wereconsistent
consistentwith
withthose
those found
found in
117.6(3)°◦
carboxylic acids [19]. Furthermore, the C–N–C angles in the terminal Py rings at 123 K were 117.6(3)
◦
118.7(3)° for
for C13–N4–C11
C13–N4–C11and
andC24–N8–C26,
C24–N8–C26,respectively,
respectively,
which
were
in good
agreement
and 118.7(3)
which
were
in good
agreement
withwith
the
the angle
characteristics
of a non-protonated
ring
[19].
The
same parameters
metrical parameters
are also
angle
characteristics
of a non-protonated
Py ringPy
[19].
The
same
metrical
are also present
in
present
in
1-Br.
These
results
were
consistent
with
the
hydrogen
atom
positions
determined
from
1-Br. These results were consistent with the hydrogen atom positions determined from the difference
the difference
map.
In addition,
similar
tendencies
of C=O and
distances
C–N–C
Fourier
map. InFourier
addition,
similar
tendencies
of C=O
and C–O distances
andC–O
C–N–C
angles and
as observed
angles
123were
K infound
1-Cl and
1-Br
found that
at 320
suggesting
thatpositions
the hydrogen
atom
at
123 Kasinobserved
1-Cl andat
1-Br
at 320
K,were
suggesting
theK,hydrogen
atom
involved
in
positionsthe
involved
in bond
forming
the significantly
hydrogen bond
do according
not significantly
according
SCO;
forming
hydrogen
do not
change
to SCO; change
substantial
proton to
transfer
substantial
proton group
transfer
thering
carboxyl
group
to the
Py ring
not occur.
Thus,
1-Cl and
from
the carboxyl
tofrom
the Py
does not
occur.
Thus,
1-Cl does
and 1-Br
represent
a co-crystal
1-Br represent
a co-crystal
comprising
complex
a neutral
acid
rather than the
an
comprising
a neutral
Fe(II) complex
andaa neutral
neutral Fe(II)
acid rather
thanand
an ionic
crystal.
Furthermore,
ionic crystal.
Furthermore,
the terminal
ring with
atom in [Fe(L)
2] forms
interactions
terminal
Py ring
with N4 atom
in [Fe(L)Py
π-π N4
interactions
with the
H2 Clπ-π
and H2 Br2with
TPA
2 ] forms
2 TPA
the H2Cl2TPA
2Br2TPA are
molecules
distances
are 3.363
Å and
3.299 Å,1-Cl
respectively
molecules
and and
theseHdistances
3.363 Åand
and these
3.299 Å,
respectively
(Figure
2). Overall,
and 1-Br
(Figure
2). Overall, 1-Clnetwork
and 1-Br
form aone-dimensional
two-dimensionalhydrogen
networkbond
structure;
form
a two-dimensional
structure;
chainsone-dimensional
interact through
hydrogen
bond chains
interact
through π-π interactions,
π-π
interactions,
forming
a two-dimensional
structure. forming a two-dimensional structure.
1-Cl; (b)
(b) Molecular
Molecular arrangements
arrangements of
of[Fe(L)
[Fe(L)22] and
and H
H22Cl22TPA
zig-zag
Figure 1. (a) Asymmetric unit of 1-Cl;
TPA in zig-zag
hydrogen bonded
chain.
bonded chain.
Crystals 2016, 6, 131
4 of 8
Table 1. Crystallographic data and coordination distances and angles around Fe(II) and the proton
donor and acceptor sites, and the hydrogen bond geometry for 1-Cl and 1-Br at 123 K and 320 K.
Crystallographic Data
Formula
Formula weight
Crystal system
T (K)
Space group
a (Å)
b (Å)
c (Å)
α (◦ )
β (◦ )
γ (◦ )
V (Å3 )
Z
Dcal (g/cm3 )
F (000)
Data collected
Unique data
R(int)
GOF on F2
R1 [I > 2σ(I)]
1-Cl
1-Bt
C34H26Cl2FeN8O6
769.38
Tricrinic
123
320
P-1
P-1
8.336(3)
8.1849(16)
8.606(3)
8.6570(17)
23.279(8)
23.026(5)
99.212(9)
97.818(5)
97.935(8)
98.149(10)
91.648(7)
91.536(6)
1630.60(10)
1667.7(6)
2
2
1.563
1.528
855
784
27257
28574
7389
7590
0.0499
0.0500
1.131
1.160
0.0600
0.0733
C34H26Br2FeN8O6
858.28
Tricrinic
123
320
P-1
P-1
8.3141(12)
8.2936(18)
8.7502(14)
8.7689(15)
23.465(4)
24.150(4)
77.589(8)
77.177(11)
82.085(10)
81.980(10)
87.203(10)
87.835(13)
1651.0(4)
1695.7(5)
2
2
1.726
1.681
860
860
29119
29629
7545
7715
0.0672
0.0687
1.084
1.146
0.0463
0.0738
Bond Lengths (Å) and Angles (◦ )
Fe1–N1
Fe1–N2
Fe1–O1
Fe1–N5
Fe1–N6
Fe1–O2
C34–O3
C34–O4
C27–O5
C27–O6
C13–N4–C11
C24–N8–C26
1-Cl (123 K)
1-Cl (320 K)
1-Br (123 K)
1-Br (320 K)
1.966(3)
1.878(2)
1.999(2)
1.947(3)
1.879(2)
1.989(2)
1.318(4)
1.210(4)
1.305(4)
1.217(4)
117.6(3)
118.7(3)
2.077(4)
1.989(3)
2.047(3)
2.089(3)
1.988(4)
2.043(3)
1.315(4)
1.187(6)
1.299(5)
1.208(5)
116.6(4)
118.3(3)
1.959(3)
1.873(2)
1.991(2)
1.943(3)
1.869(3)
1.991(2)
1.316(4)
1.204(4)
1.302(4)
1.212(4)
117.3(3)
117.9(3)
2.011(4)
1.923(4)
2.010(3)
1.974(4)
1.916(4)
2.016(4)
1.306(7)
1.173(6)
1.312(6)
1.199(6)
115.8(5)
117.7(5)
Hydrogen Bond Geometries (Å, ◦ )
N4–O3
N8–O5
O3–H25
N4–H25
O5–H26
N8–H26
N4–H25–O3
N8–H26–O5
2.663(4)
2.552(4)
1.03(7)
1.64(7)
1.09(6)
1.47(6)
174(6)
175(6)
2.692(5)
2.574(4)
–
–
–
–
–
–
2.681(4)
2.560(4)
0.82(4)
1.86(4)
1.04(7)
1.52(7)
177(4)
173(6)
2.694(4)
2.578(4)
–
–
–
–
–
–
Crystals 2016, 6, 131
Crystals 2016, 6, 131
5 of 8
5 of 8
Crystals 2016, 6, 131
5 of 8
Figure
Figure2.2.The
Theπ-π
π-πinteraction
interactionbetween
between[Fe(L)
[Fe(L)2 2]]and
andH
H22Cl
Cl22TPA
TPA in
in 1-Cl.
1-Cl.
The IR
IR spectra
spectra of
of 1-Cl
1-Cl and
and 1-Br
1-Bralso
alsosupport
supportthe
theproposal
proposalthat
thatboth
bothcompounds
compounds are
areco-crystals
co-crystals
The
(Figure3a,b).
3a,b). ItIt is
is known
known that
that
the
broad
peaksderived
derivedfrom
from
O–H∙∙∙N
stretching appear
appear around
around 1950
1950
(Figure
the
broad
peaks
O–H
···N stretching
Figure
2. The
π-π interaction
between [Fe(L)
2] and H2Cl2TPA in 1-Cl.
−1 when strong hydrogen bonds are present in the crystal [20]. However, the peak
−
1
and
2450
cm
and 2450 cm when strong hydrogen bonds are present in the crystal [20]. However, the peak around
IR spectra
of 1-Cl2150
1-Br
also
support
proposal
that both
compounds
are
co-crystals
−1 shifts
−1
1 shifts
−1 if
around
1950
cmThe
toward
cm
ifproton
the
proton
is located
onNthe
N of
side
ofhydrogen
the hydrogen
bond,
1950
cm−
toward
2150
cmand
the
isthelocated
on
the
side
the
bond,
i.e.,
(Figure 3a,b). It is known that the broad peaks derived from O–H∙∙∙N stretching appear around 1950
i.e.,
O∙∙∙H–N
[21].
Furthermore,
C=O
stretching
in
the
carboxyl
group
has
been
utilized
to
O
···H–N
[21].
Furthermore,
C=O
stretching
in
the
carboxyl
group
has
been
utilized
to
differentiate
−1
and 2450 cm when strong hydrogen bonds are present in the crystal [20]. However, the peak
−1 shifts
−1
differentiate
between
the
protonated
and
deprotonated
state
of
the
carboxyl
group;
for
example,
in
around
1950
cm
toward
2150
cm
if
the
proton
is
located
on
the
N
side
of
the
hydrogen
bond,
between the protonated and deprotonated state of the carboxyl group; for example, in terephthalic acid,
−1, carboxyl
i.e.,
O∙∙∙H–N
stretching
incm
the
group
has been utilized
to
−1 ,C=O
terephthalic
acid,
C=O[21].
stretching
at 1692
whereas
in disodium
terephthalate,
C=O
C=O
stretching
appeared
at Furthermore,
1692 cmappeared
whereas
in disodium
terephthalate,
C=O stretching
appeared
differentiate
between
the
protonated
and
deprotonated
state
of
the
carboxyl
group;
for
example,
in
−1
−1 . The IR
−1 ,
stretching
at
1567
cm
.
The
IR
spectra
of
1-Cl
and
1-Br
show
broad
peaks
at
around
1950
and
at
1567 cmappeared
spectra
of
1-Cl
and
1-Br
show
broad
peaks
at
around
1950
and
2450
cm
terephthalic acid, C=O stretching appeared at 1692 cm−1, whereas in disodium terephthalate, C=O
−1, and strong peaks−
−1 and
−1, similar to those observed for H2Cl2TPA (Figure
1
−
1
−1
2450
cm
at
1713
cm
1717
cm
stretching
appeared
. Thecm
IR spectra
of 1-Cl to
andthose
1-Br show
broad peaks
1950(Figure
and
and strong peaks
at 1713
cm at 1567
andcm1717
, similar
observed
for at
Haround
3a, gray
2 Cl2 TPA
cm−1,H
and
strong
peaks
at 1713
cm−1 and
1717
cm−1, similar
to IR
those
observed
for
Hdata
2Cl2TPA
(Figurecompounds
3a, gray
2Br
2TPA
(Figure
3b,
gray
line),
respectively.
The
IR
indicate
that
these
line)
and line)
H22450
Brand
TPA
(Figure
3b,
gray
line),
respectively.
The
data
indicate
that
these
2
3a, gray line) and H2Br2TPA (Figure 3b, gray line), respectively. The IR data indicate that these
compounds
contain
a
neutral
Fe(II)
complex
and
neutral
H
2
Cl
2
TPA
and
H
2
Br
2
TPA,
respectively.
contain a neutral Fe(II) complex and neutral H2 Cl2 TPA and H2 Br2 TPA, respectively.
compounds contain a neutral Fe(II) complex and neutral H2Cl2TPA and H2Br2TPA, respectively.
(a)
(b)
Figure 3. (a) IR spectra for 1-Cl (green line) and H2Cl2TPA (gray line); (b) IR spectra for 1-Br (brown line)
Figure 3. (a)and
IR H
spectra
for 1-Cl (green line) and H2 Cl2 TPA (gray line); (b) IR spectra for 1-Br (brown
2Br2TPA (gray line).
line) and H2 Br2 TPA (gray
(a) line).
(b)
Magnetic susceptibility measurements for 1-Cl and1-Br were performed over the temperature
range of 10–400 K at a sweep rate of 1 K∙min−1 with a magnetic field of 5 kOe. The χmT-T plots of 1-Cl
Figure 3. (a) IR spectra for 1-Cl (green line) and H2Cl2TPA (gray line); (b) IR spectra for 1-Br (brown line)
andsusceptibility
1-Br in Figure 4 show
that both compounds
exhibited
gradual
spinperformed
crossover. In the
low-spin
Magnetic
measurements
for 1-Cl
and 1-Br
were
over
the temperature
and H2Br
2TPA (gray line).
−
1
state,
the
χ
mT values in 1-Cl and 1-Br were
nearly
zero,
which
was
in
good
agreement
with
theplots of 1-Cl
range of 10–400 K at a sweep rate of 1 K·min with a magnetic field of 5 kOe. The χm T-T
expected value for low-spin Fe(II) (S = 0). Upon warming, the χmT value of 1-Cl gradually increased
and 1-Br in Figure
4 show
both2.49
compounds
exhibited
gradual
crossover.
In
the low-spin state,
−1 at
above
210
K andthat
reached
cm3∙K∙molfor
400 K,
whereas
the spin
χmT
value
of 1-Br
gradually
Magnetic
susceptibility
measurements
1-Cl
and1-Br
were
performed
over
the temperature
3
−1
the χm T values
in
1-Cl
and
1-Br
were
nearly
zero,
which
was
in
good
agreement
with
the expected
increased above 250 K and reached 1.90 −1cm ∙K∙mol at 400 K. These values were less than the
range of 10–400 K at a sweep rate of 1 K∙min with a magnetic field of 5 kOe. The χmT-T plots of 1-Cl
expected
value
for
high-spin
Fe(II)
(S
=
2)
because
the
spin
crossover
was
not
complete
even
at
400
K.
value for low-spin Fe(II) (S = 0). Upon warming, the χm T value of 1-Cl gradually increased above
and 1-Br in The
Figure
4 show
that
compounds
exhibited
gradual
spin−1,crossover.
3 K∙mol−1 and
χmT values
in 1-Cl
andboth
1-Br
320 K are 1.60 cm
0.53 cm3 K∙mol
respectively. In
Thisthe low-spin
3 ·K
−1at at
210 K and reached
2.4953%
cmand
·and
mol
400
K, changed
whereas
the χm TFe(II),
value
of 1-Br in
gradually
increased
that in
of 1-Br
low-spin
Fe(II)nearly
to high-spin
respectively,
1-Cl and
state, the χmindicates
T values
1-Cl 18%
were
zero,
which
was
in
good
agreement
with the
3
−
1
above 250 K1-Br
andatreached
cm ·ofKthe
·
mol
atof400
K. These
were
less
expected value
320 K. The1.90
difference
fraction
high-spin
Fe(II) values
between 1-Cl
and
1-Brthan
at 320the
K could
expected value for low-spin Fe(II) (S = 0). Upon warming, the χmT value of 1-Cl gradually increased
be followed
the result
the single-crystal
X-raynot
measurement
showed
the K.
Fe(II)–N
for high-spinalso
Fe(II)
(S = 2)from
because
the inspin
crossover was
completethat
even
at 400
The χm T values
above 210 K and reached 2.49 cm3∙K∙mol−1 at 400 K, whereas
the χmT value of 1-Br gradually
in 1-Cl and 1-Br at 320 K are 1.60 cm3 K·mol−31 and 0.53
cm3 K·mol−1 , respectively. This indicates
−1
increased above 250 K and reached 1.90 cm ∙K∙mol at 400 K. These values were less than the
that 53% and 18% of low-spin Fe(II) changed to high-spin Fe(II), respectively, in 1-Cl and 1-Br at
expected value for high-spin Fe(II) (S = 2) because the spin crossover was not complete even at 400 K.
320
difference
ofand
the 1-Br
fraction
of high-spin
between
1-Cl
at −1
320
K could alsoThis
be
−1 and
TheK.
χmThe
T values
in 1-Cl
at 320
K are 1.60 Fe(II)
cm3 K∙mol
0.53and
cm31-Br
K∙mol
, respectively.
followed
from
the
result
in
the
single-crystal
X-ray
measurement
that
showed
the
Fe(II)–N
and
Fe(II)–O
indicates that 53% and 18% of low-spin Fe(II) changed to high-spin Fe(II), respectively, in 1-Cl and
coordination
for 1-Cl
than
for 1-Br
at 320
K. The
difference
inat
T1/2
1-Br at 320 K.distances
The difference
ofare
thelarger
fraction
of those
high-spin
Fe(II)
between
1-Cl
and 1-Br
320between
K could
also be followed from the result in the single-crystal X-ray measurement that showed the Fe(II)–N
Crystals 2016, 6, 131
6 of 8
Crystals 2016, 6, 131
6 of 8
1-Cl (314 K) and
1-Br (376
K) indicates
changing
substituent
the dicarboxylic
acids
and Fe(II)–O
coordination
distancesthat
for 1-Cl
are larger the
than halogen
those for 1-Br
at 320 K. Theon
difference
in T1/2
between
1-Cl (314 K)bond
and 1-Br
(376 K)and
indicates
that changing
the without
halogen substituent
on the
the molecular
can influence
the hydrogen
distance
the SCO
behavior
changing
dicarboxylic
acids
can influence
the hydrogen
bondexhibit
distance abrupt
and the SCO
behavior
without
arrangement;
however,
these
compounds
do not
SCO
despite
thechanging
presence of short
the molecular arrangement; however, these compounds do not exhibit abrupt SCO despite the presence
hydrogen bonds,
caused
by
the
low
dimensionality
of
the
hydrogen
bonding
network.
of short hydrogen bonds, caused by the low dimensionality of the hydrogen bonding network.
Figure 4. χmT-T plots for 1-Cl (blue dots) and 1-Br (brown dots).
Figure 4. χm T-T plots for 1-Cl (blue dots) and 1-Br (brown dots).
3. Experimental Section
3. Experimental Section
3.1. Synthesis
3.1. Synthesis3.1.1. Synthesis of Ligand HL
HL was synthesized according to the method described by Musuc et al. [22].
3.1.1. Synthesis of Ligand HL
3.1.2. Synthesis of [Fe(L)2](H2Cl2TPA) (1-Cl)
HL was synthesized according to the method described by Musuc et al. [22].
HL (48 mg, 0.20 mmol) was dissolved in methanol (30 mL), and FeSO4∙7H2O (27 mg, 0.10 mmol)
was added to the solution. The mixture was stirred for 30 min. Following this, H2Cl2TPA (23.5 mg, 0.10
3.1.2. Synthesis
of [Fe(L)2 ](H
Cl TPA) (1-Cl)
mmol) was added
and2 the2 mixture was stirred for 30 sec. Subsequently, the solution was evaporated
using a N2 gas flow over several days. Plate shape crystals were obtained (47 mg, 61%). Elemental
HL (48 mg,
0.20 mmol) was dissolved in methanol (30 mL), and FeSO4 ·7H2 O (27 mg, 0.10 mmol)
analysis calcd for C34H24Cl2FeN8O6 (769.38); C 53.08, H 3.41, N 14.56; found. C 52.66, H 3.54, 14.50.
was added to the solution. The mixture was stirred for 30 min. Following this, H2 Cl2 TPA (23.5 mg,
[Fe(L)
2](Hmixture
2Br2TPA) (1-Br)
0.10 mmol) 3.1.3.
wasSynthesis
added ofand
the
was stirred for 30 sec. Subsequently, the solution was
Thea synthetic
forseveral
1-Br is similar
that for
1-Cl crystals
except for were
the useobtained
of H2Br2TPA
evaporated using
N2 gas procedure
flow over
days.toPlate
shape
(47 mg, 61%).
shape
crystals
were
obtained byC
evaporation
of3.41,
the solution
containing
instead ofcalcd
H2Cl2TPA.
Elemental analysis
for Plate
C34 H
Cl
FeN
O
(769.38);
53.08,
H
N
14.56;
found.
C 52.66,
24 2
8 6
HL, FeSO4∙7H2O and H2Br2TPA using a N2 gas flow over several days (65 mg, 76%). Elemental
H 3.54, 14.50.analysis calcd for C34H24Br2FeN8O6 (858.28); C 47.58, H 3.05, N 13.06; found. C 47.66, H 3.10, 12.85.
3.2. X-ray
Structure
Determination
3.1.3. Synthesis
of [Fe(L)
2 ](H
2 Br2 TPA) (1-Br)
Diffraction
data of 1-Cl
1-Br
123 K andto320
K was
on a Rigaku
The synthetic
procedure
for and
1-Br
isatsimilar
that
forcollected
1-Cl except
forcharge-coupled
the use of H2 Br2 TPA
device (CCD) diffractometer (Rigaku, Tokyo, Japan). A crystal was glued onto a nylon loop and
instead of H2 Cl2 TPA. Plate shape crystals were obtained by evaporation of the solution containing HL,
enveloped in a temperature-controlled steam (Rigaku, Tokyo, Japan) of dry N2 gas during data
FeSO4 ·7H2 Ocollection.
and H2The
Br2 TPA
using
N2 gas
several
dayswith
(65 full-matrix
mg, 76%).
Elemental analysis
structures
werea solved
by flow
direct over
methods
and refined
least-squares
using
the
program
SHELXS-97
[23].
Hydrogen
atoms
involved
in
hydrogen
bonds
were
calcd for C34procedures
H24 Br2 FeN
O
(858.28);
C
47.58,
H
3.05,
N
13.06;
found.
C
47.66,
H
3.10,
12.85.
8 6
determined from Fourier Difference Map at 123 K. Other hydrogen atoms were determined by
calculation
and refined using the riding model. All non-hydrogen atoms were refined anisotropically.
3.2. X-ray Structure
Determination
Diffraction data of 1-Cl and 1-Br at 123 K and 320 K was collected on a Rigaku charge-coupled
device (CCD) diffractometer (Rigaku, Tokyo, Japan). A crystal was glued onto a nylon loop and
enveloped in a temperature-controlled steam (Rigaku, Tokyo, Japan) of dry N2 gas during data
collection. The structures were solved by direct methods and refined with full-matrix least-squares
procedures using the program SHELXS-97 [23]. Hydrogen atoms involved in hydrogen bonds were
determined from Fourier Difference Map at 123 K. Other hydrogen atoms were determined by
calculation and refined using the riding model. All non-hydrogen atoms were refined anisotropically.
CCDC 1487718 (1-Cl 123 K), 1506004 (1-Cl 320 K), 1497560 (1-Br 123 K) and 1506005 (1-Br 320 K)
contains the supplementary crystallographic data for this paper. These data can be obtained free of
Crystals 2016, 6, 131
7 of 8
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected]).
3.3. IR Spectra
IR spectra were recorded with KBr pellet sample by a JASCO FT/IR-660 Plus spectrometer
(JASCO, Tokyo, Japan) in the 600–4000 cm−1 region.
3.4. Magnetic Property Measurement
Magnetic measurements were performed on a Quantum Design MPMS-5S SQUID (superconducting
quantum-interference device) magnetometer (Quantum Design, San Diego, CA, USA), and data were
corrected for the diamagnetic contribution, calculated from Pascal’s constants.
4. Conclusions
In summary, we have synthesized a new SCO complex [Fe(L)2 ] that can form short hydrogen
bonds with a dicarboxylic acid. The crystal structures of 1-Cl and 1-Br showed that a zig-zag
hydrogen-bonded chain was formed between the terminal Py ring in the Fe complex and the carboxyl
group in H2 Cl2 TPA and H2 Br2 TPA. Two different hydrogen bond distances were observed; one was
long, whereas the other was short, and these distances in 1-Br are slightly longer than in 1-Cl. Both
short hydrogen bond distances are attributed to N···O-type hydrogen bonds and are slightly longer
than those for organic compounds reporting the proton migration phenomenon. The dependence of
hydrogen bond distances on temperature was observed to be similar to that in organic compounds
comprising isonicotinic acid hydrazide and dicarboxylic acids. Furthermore, the spin transition from
low-spin to high-spin state was stated to contribute to the elongation of hydrogen bond distances
in 1-Cl and 1-Br. O–H and C–O bond distances in the carboxyl group and the C–N–C angle in the
terminal Py rings suggested that the hydrogen atoms involved in the hydrogen bonds are localized
on the carboxyl group in H2 Cl2 TPA and H2 Br2 TPA. The IR spectra of 1-Cl and 1-Br, in particular the
presence of an O–H···N stretching mode, also supported the fact that the hydrogen atoms are located
on the carboxyl group. This indicated that 1-Cl and 1-Br are co-crystals comprising neutral [Fe(L)2 ]
and H2 Cl2 TPA and H2 Br2 TPA. Magnetic measurements for 1-Cl and 1-Br showed that a gradual SCO
occurred around room temperature, and it was confirmed that the T1/2 value in SCO compounds
comprising halogen-substituted dicarboxylic acids can be influenced by only changing the halogen
substituent without changing the molecular arrangement.
Author Contributions: Takumi Nakanishi and Osamu Sato have designed the compounds and performed any
experiment and analysis.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
Yamaguchi, S.; Kamikubo, H.; Kurihara, K.; Kuroki, R.; Niimura, N.; Shimizu, N.; Yamazaki, Y.; Kataoka, M.
Low-barrier hydrogen bond in photoactive yellow protein. Proc. Natl. Acad. Sci. USA 2009, 106, 440–444.
[CrossRef] [PubMed]
Ohshima, T.; Shibuguchi, T.; Fukuta, Y.; Shibasaki, M. Catalytic asymmetric phase-transfer reactions using
tartrate-derived asymmetric two-center organocatalysts. Tetrahedron 2004, 60, 7743–7754. [CrossRef]
Shan, N.; Bond, A.D.; Jones, W. Crystal engineering using 4,40 -bipyridyl with di- and tricarboxylic acids.
Cryst. Eng. 2002, 5, 9–24. [CrossRef]
Horiuchi, S.; Ishii, F.; Kumai, R.; Okimoto, Y.; Tachibana, H.; Nagaosa, N.; Tokura, Y. Ferroelectricity near
room temperature in co-crystals of nonpolar organic molecules. Nat. Mater. 2005, 4, 163–166. [CrossRef]
[PubMed]
Horiuchi, S.; Kumai, R.; Tokura, Y. A supramolecular ferroelectric realized by collective proton transfer.
Angew. Chem. Int. Ed. Engl. 2007, 46, 3497–3501. [CrossRef] [PubMed]
Crystals 2016, 6, 131
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
8 of 8
Sato, O. Dynamic molecular crystals with switchable physical properties. Nat. Chem. 2016, 8, 644–656.
[CrossRef] [PubMed]
Ueda, A.; Yamada, S.; Isono, T.; Kamo, H.; Nakao, A.; Kumai, R.; Nakao, H.; Murakami, Y.; Yamamoto, K.;
Nishio, Y.; et al. Hydrogen-bond-dynamics-based switching of conductivity and magnetism: A phase
transition caused by deuterium and electron transfer in a hydrogen-bonded purely organic conductor crystal.
J. Am. Chem. Soc. 2014, 136, 12184–12192. [CrossRef] [PubMed]
Coronado, E.; Gimenez-Lopez, M.C.; Gimenez-Saiz, C.; Romero, F.M. Spin crossover complexes as building
units of hydrogen-bonded nanoporous structures. CrystEngComm 2009, 11, 2198–2203. [CrossRef]
Hill, S.; Datta, S.; Liu, J.; Inglis, R.; Milios, C.J.; Feng, P.L.; Henderson, J.J.; del Barco, E.; Brechin, E.K.;
Hendrickson, D.N. Magnetic quantum tunneling: Insights from simple molecule-based magnets.
Dalton Trans. 2010, 39, 4693–4707. [CrossRef] [PubMed]
Zhang, L.; Xu, G.C.; Xu, H.B.; Zhang, T.; Wang, Z.M.; Yuan, M.; Gao, S. Abrupt spin transition around room
temperature and light induced properties in Fe(II) complexes with N4 O2 coordination sphere. Chem. Commun.
2010, 46, 2554–2556. [CrossRef] [PubMed]
Paulsen, H.; Duelund, L.; Zimmermann, A.; Averseng, F.; Gerdan, M.; Winkler, H.; Toftlund, H.;
Trautwein, A.X. Substituent effects on the spin-transition temperature in complexes with tris(pyrazolyl)
ligands. Monatsh. Chem. 2003, 134, 295–306. [CrossRef]
Lemercier, G.; Brefuel, N.; Shova, S.; Wolny, J.A.; Dahan, F.; Verelst, M.; Paulsen, H.; Trautwein, A.X.;
Tuchagues, J.P. A range of spin-crossover temperature T1/2 > 300 k results from out-of-sphere anion exchange
in a series of ferrous materials based on the 4-(4-imidazolylmethyl)-2-(2-imidazolylmethyl)imidazole (trim)
ligand, [Fe(trim)2]X2 (X = F, Cl, Br, I): Comparison of experimental results with those derived from density
functional theory calculations. Chem. Eur. J. 2006, 12, 7421–7432. [PubMed]
Cowan, J.A.; Howard, J.A.; McIntyre, G.J.; Lo, S.M.; Williams, I.D. Variable-temperature neutron diffraction
studies of the short, strong hydrogen bonds in the crystal structure of pyridine-3,5-dicarboxylic acid.
Acta Crystallogr. B 2005, 61, 724–730. [CrossRef] [PubMed]
Steiner, T.; Majerz, I.; Wilson, C.C. First O–H–N hydrogen fond with a centered proton obtained by thermally
induced proton migration. Angew. Chem. Int. Ed. 2001, 40, 2651–2654. [CrossRef]
Cowan, J.A.; Howard, J.A.K.; McIntyre, G.J.; Lo, S.M.F.; Williams, I.D. Variable-temperature neutron
diffraction studies of the short, strong n center dot center dot center dot o hydrogen bonds in the 1:2 co-crystal
of benzene-1,2,4,5-tetracarboxylic acid and 4,40 -bipyridyl. Acta Crystallogr. Sect. B-Struct. Sci. 2003, 59,
794–801. [CrossRef]
Grobelny, P.; Mukherjee, A.; Desiraju, G.R. Drug-drug co-crystals: Temperature-dependent proton mobility
in the molecular complex of isoniazid with 4-aminosalicylic acid. CrystEngComm 2011, 13, 4358–4364.
[CrossRef]
Cherukuvada, S.; Nangia, A. Fast dissolving eutectic compositions of two anti-tubercular drugs.
CrystEngComm 2012, 14, 2579–2588. [CrossRef]
Aitipamula, S.; Wong, A.B.H.; Chow, P.S.; Tan, R.B.H. Novel solid forms of the anti-tuberculosis drug,
isoniazid: Ternary and polymorphic cocrystals. CrystEngComm 2013, 15, 5877–5887. [CrossRef]
Bis, J.A.; Zaworotko, M.J. The 2-aminopyridinium-carboxylate supramolecular heterosynthon: A robust
motif for generation of multiple-component crystals. Cryst. Growth Des. 2005, 5, 1169–1179. [CrossRef]
Bhunia, M.K.; Das, S.K.; Bhaumik, A. Temperature induced proton transfer in a hydrogen bonded
supramolecule. Chem. Phys. Lett. 2010, 498, 145–150. [CrossRef]
Aakeröy, C.B.; Hussain, I.; Forbes, S.; Desper, J. Exploring the hydrogen-bond preference of N–H moieties
in co-crystals assembled via O–H(acid)···N(py) intermolecular interactions. CrystEngComm 2007, 9, 46–54.
[CrossRef]
Ababei, L.V.; Kriza, A.; Andronescu, C.; Musuc, A.M. Synthesis and characterization of new complexes of
some divalent transition metals with 2-acetyl-pyridyl-isonicotinoylhydrazone. J. Therm. Anal. Calorim. 2011,
107, 573–584. [CrossRef]
Sheldrick, G.M. A short history of SHELX. Acta Cryst. A 2008, A64, 112–122. [CrossRef] [PubMed]
© 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC-BY) license (http://creativecommons.org/licenses/by/4.0/).