Novel iron(II) microporous spin-crossover coordination polymers with enhanced pore size

Article abstract of DOI:10.1021/ic301639r

In this Communication, we report the synthesis and characterization of novel Hofmann-like spin-crossover porous coordination polymers of composition {Fe(L)[M(CN)₄]}·G [L = 1,4-bis(4-pyridylethynyl)benzene and M^II = Ni, Pd, and Pt]. The spin-crossover properties of the framework are closely related to the number and nature of the guest molecules included in the pores.

Full text of DOI:10.1021/ic301639r

Communication
pubs.acs.org/IC
Novel Iron(II) Microporous Spin-Crossover Coordination Polymers
with Enhanced Pore Size
†
†
†
‡
§
Francisco J. Mun
̃
oz-Lara, Ana B. Gaspar, M. Carmen Munoz, Vadim Ksenofontov,
̃
,
and Jose
́
Antonio Real*
†
Institut de Cien
̀
cia Molecular (ICMol)/Departament de Química Inorgan
cia, Spain
Departamento de Física Aplicada, Universitat Politec
̀ ̀ ́ ́ ́
ica, Universitat de Valencia, C/ Catedratico Jose Beltran
Martínez 2, 46980 Paterna, Valen
̀
‡
̀
nica de Valen
̀
cia, Camino de Vera s/n, 46022 Valen
̀
cia, Spain
§
Institut fu
̈
r Anorganische und Analytische Chemie, Johannes-Gutenberg-Universitat
̈
, Staudinger-Weg 9, D-55099 Mainz, Germany
*
S Supporting Information
8
isomer) and dates back to 1995. Almost a decade later was
published a detailed investigation of the guest influence on the
SCO properties in analogous frameworks with composition
ABSTRACT: In this Communication, we report the
synthesis and characterization of novel Hofmann-like
spin-crossover porous coordination polymers of composi-
[
Fe(L)(NCS) ]·G [L = 4,4-azopyridine (azpy) and DL-1,2-
2
9
tion {Fe(L)[M(CN) ]}·G [L = 1,4-bis(4-pyridylethynyl)-
4
bis(4′-pyridyl)-1,2-ethanediol (bped)]. In these SCO-PCPs,
the adsorption of guest molecules induces some stabilization of
the framework in the LS state. Typically, they show a gradual
SCO in the temperature region 10−200 K.
II
benzene and M = Ni, Pd, and Pt]. The spin-crossover
properties of the framework are closely related to the
number and nature of the guest molecules included in the
pores.
More recently, a drastic influence of the guest inclusion on Tc
and cooperativity has been reported for the three-dimensional
Hofmann-like SCO-PCPs {Fe(L)[M(CN) ]}·G [L = pyrazine
4
1
10−12
13
14
orous coordination polymers (PCPs), where the frame-
(pz);
azpy, 4,4′-bis(pyridyl)acetylene (bpac), and dpe
2
15
II
P
work exhibits solid-state properties like magnetism,
(trans isomer) and M = Ni, Pd, and Pt]. These types of
frameworks provide two guest interactive sites, one between the
organic bridges (site A) and another between the four-
coordinate M centers (site B). Particularly interesting are the
3
4
5
luminiscence, conductivity, or charge transport, which
change concomitantly with the sorption/desorption of guest
molecules, represent a new class of multifunctional materials.
This new generation of PCPs acts by expressing the host−guest
interactions in a sensory way.
Among PCP porous and magnetic properties, those porous
polymers made up of bistable iron(II) building blocks are
noteworthy. Iron(II) spin-crossover (SCO) building blocks
present labile electronic configurations switchable between the
high-spin (HS) and low-spin (LS) states in response to external
stimuli (temperature, pressure, light, or absorption/desorption
of an analyte). In the HS and LS states, the iron(II) SCO
centers reveal differences in magnetism, optical properties,
dielectric constant, color, and structure. For cooperative spin
transitions (STs), this switch can be performed within the
hysteresis loop based on the first-order hysteretic ST, which
confers to the material a memory function. Thus, guest
inclusion in the SCO framework can provoke stabilization of
the Fe ions in the HS or LS state or has no effect. The
resulting effect relies on the chemical nature and size of the
guest molecules that occupy the pores. Detection of the
sorption/desorption of guest molecules in the framework is
performed by analysis of the magnetic and optical outputs (the
temperature dependence of the magnetic susceptibility and/or
by the change of color at a given temperature). In principle, a
fingerprint in the form of a magnetic response pattern may be
attainable for distinct analytes.
results obtained with the SCO-PCP {Fe(pz)[M(CN) ]}. In
4
fact, it represents a new generation of functional materials
responding to their environment because the framework can
allow reversible control of the magnetic and optical outputs
1
0,11
through the chemical response at room temperature.
Further development of these polymers points at increasing
the pore size and inner pore functionality. In this
Communication, we report the synthesis and characterization
of a novel family of iron(II) Hoffman-like three-dimensional
c o o r d i n a t i o n p o l y m e r s { F e ( b p e b e n ) [ M -
6
(CN)
]}·nH
O·0.5bpeben [M = Pt, n = 1.5H
O (1) and n =
2
4
2
0 (2); M = Pd, n = 1.5H
O (3) and n = 0 (4); M = Ni, n = 2
2
(5) and n = 0 (6)], based on the organic pillar ligand 1,4-bis(4-
7
16
pyridylethynyl)benzene (bpeben). These novel iron(II)
coordination polymers exhibit SCO and enhanced porous
properties.
The coordination polymers precipitate instantaneously as
yellow-orange microcrystalline powders when a water solution
II
of K M(CN) ·nH O is added to a continuously stirring
2
4
2
methanolic solution containing Fe(BF ) ·6H O and the ligand
4 2
2
bpeben in equimolar amounts. Because of the great insolubility
of these compounds, we were able to get single crystals only for
1
. For the same reason, the quality of these crystals only
enabled us to complete the crystallographic analysis at 120 K.
Examples of iron(II) SCO-PCPs reported up to now are still
scarce. The first report on a microporous iron(II) polymer was
based on the ligand 1,2-di(4-pyridyl)ethylene (dpe; trans
Received: July 26, 2012
Published: December 21, 2012
©
2012 American Chemical Society
3
dx.doi.org/10.1021/ic301639r | Inorg. Chem. 2013, 52, 3−5

Inorganic Chemistry
Communication
As for the analogous polymers with pz, dpe, azpy, and bpac
The water contents of 1, 3, and 5 have been established by
thermogravimetric analyses (TGA). The compounds show
pillar ligands, the crystal structure of 1 was solved in the P4/
10−15
mmm space group.
STable1 in the Supporting Information
similar TGA profiles. A smooth release of H O molecules
2
(
SI) gathers the relevant crystallographic data as well as bond
occurs between room temperature and 400 K, followed by the
lengths and angles. The structure is constituted of alternate
decomposition process of the {Fe(bpeben)[M(CN) ]} frame-
4
II
octahedral [Fe N ] cationic centers and square-planar [Pt-
work and the bpeben guest molecule, which takes place as a
multiple-step process in the interval 500−800 K.
6
CN)4]2 anions (Figure 1). The equatorial positions are
−
(
The thermal dependences of χ T for 1−6 (χ stands for the
M
M
molar magnetic susceptibility and T for the temperature) are
shown in Figure 3 and SFigure 3 in the SI. At 300 K, the χ T
M
3
−1
value of 1, 1.2 cm K mol , denotes a relevant amount of the
II
57
Fe centers in the LS state, in agreement with the Fe
Mossbauer spectra recorded at 295 K (Figure 2). The
̈
Figure 1. View of the crystal strutucture of 1 along the [010] direction.
Color code: N, green; C, gray and blue; Fe, orange; Pt, violet;
O(H O), blue.
2
2
−
occupied by four N atoms of four equivalent [Pt(CN) ]
4
II
groups, which act as bridges linking four Fe atoms, generating
̈
Figure 2. Mossbauer spectra of 1 measured at 85 and 295 K.
two-dimensional {Fe[Pt(CN)4] } layers. The layers are
∞
pillared by the ligand bpeben along the [001] direction,
II
which occupies the axial positions of the Fe ions, thereby
Mo
̈
ssbauer spectra consist of two doublets. The characteristic
generating an open three-dimensional framework. The axial and
equatorial Fe−N bond distances are consistent with Fe in the
LS state, with Fe−N(1) = 1.99(2) Å and Fe−N(2) = 1.918(16)
Å. The pores running parallel to the [010] and [100] directions
are defined by the Fe−Fe distances through the pillar ligand
bpeben and the anionic linkers [Pt(CN) ] with a gate size in
the LS state of 20.49(3) Å × 7.14(3) Å. Inside the pores, there
are half of a bpeben molecule and 1.5 H O molecules per Fe
atom. Several host−guest weak intermolecular contacts
between pillar and clathrated bpeben ligands are present
C···N [3.60(2)−3.865(9) Å] and C···C [3.64(2)−3.796(8) Å]
STable2 in the SI). The solvent is heavily disordered and has
isomer shift (δ, relative to α-iron) and quadrupole splitting
II
−1
(
ΔE ) parameters are 0.680(91) and 1.56(19) mm s for the
−1
Q
HS doublet and 0.413(16) and 0.244(31) mm s for the LS
doublet. The populations in the HS and LS states deduced
from the least-squares fit routine analysis of the spectra are 35
and 65%, respectively. Upon lowering of the temperature to 85
K, the population of the LS doublet increases by 10% at the
expense of the HS doublet. The parameters δ and ΔEQ
practically remain constant for the LS doublet, while they
increase for the HS doublet (STable 3 in the SI).
2−
4
II
2
Upon warming from 300 to 400 K, the χ T value for 1
experiences an abrupt increase up to 3.65 cm K mol , a value
that falls into the range of values expected for 100% of the Fe
centers in the HS state. This change in the magnetic
susceptibility is not due to a thermal SCO of 1. Dehydration
of 1 and concomitant transformation into 2 take place in this
temperature interval.
[
(
M
3
−1
II
been modeled as such. There is also disorder observed in the
clathrated bpeben ligand. However, while these issues reduce
the quality of the structural model, none of them affect the
conclusions drawn as to the atomic connectivity or network
topology. The accessible volume of the framework withou3t
Figure 3 illustrates the χ T versus T curve for 2 in the
M
considering the guest molecules was determined to be 511 Å
3
cooling and warming modes. Compound 2 undergoes a
continuous and incomplete thermal SCO behavior in the
temperature interval of 300−100 K. Below this temperature,
(
198.9 Å considering the bpeben and H O guest molecules),
which corresponds to 48.9% of the unit cell at 120 K
1044.7(2) Å ] and is 2 times the order of magnitudes found
2
3
[
3
−1
χ T drops to 1.0 cm K mol at 5 K because of the zero-field
10−15
M
for azpy, dpe, and bpac homologues.
II
splitting (ZFS) of the Fe centers that remain in the HS state.
The palladium analogues, compounds 3 and 4, exhibit magnetic
behavior identical with described for 1 and 2, respectively.
In contrast to 1 and 2, the hydrated nickel derivative,
compound 5, presents at 300 K a χ T value of 3.0 cm K
mol . The magnetic susceptibility decreases progressively to
1.5 cm K mol (100 K) because of the thermal SCO of
approximately 50% of the Fe centers. A further lowering of
Powder X-ray diffraction (PXRD) patterns for 2−6 were
recorded at 293 K (see the SI). Compounds 2−6 have very
similar profiles, with most of the significant peaks located at 2θ
4.2°, 8.5°, 12.75°, 17.20°, 18.04°, 19.50°, 21.5°, and 24.9°. In
particular, the peak at 4.2° corresponds to the separation
between {Fe[Pt(CN) ] } sheets. From these PXRD profiles
and taking into account the analytical data [IR, energy-
dispersive X-ray microanalysis, and CHN analysis] of
compounds 2−6, it is reasonable to propose that they are
isostructural to 1.
=
3
M
−1
3
−1
4
∞
II
II
χ T is ascribed to the ZFS of the paramagnetic Fe ions.
M
Compound 6 behaves similarly to 5, but the temperature at
which the thermal SCO takes place is shifted 50 K downward.
4
dx.doi.org/10.1021/ic301639r | Inorg. Chem. 2013, 52, 3−5

Inorganic Chemistry
Communication
2
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̃
Figure 3. Magnetic properties of 1 and 2 in the form of χ T versus T.
M
̈
̃
(
́
In these coordination polymers, the hydration−dehydration
process is totally reversible and the ST properties are recovered
at will. The hydrated compounds are dark yellow, while the
dehydrated ones are light yellow. In general, the inclusion of
(
̃
́
H O molecules in the framework leads to stabilization of the
́
̃
2
II
Fe ions in the LS state. It is well-known that solvent molecules
(
may provoke dramatic changes in the ST behavior via subtle
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11−15
for the azpy, dpe, and bpac homologues.
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in terms of host−guest interactions and more particularly in the
(
̃
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̃
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́
ard, J.
(
́
̃
Chem., Int. Ed. 2009, 48, 8944.
ASSOCIATED CONTENT
Supporting Information
X-ray crystallographic data in CIF format, experimental details,
synthetic pathways, additional crystal data, TGA, PXRD,
■
(12) Ohtani, R.; Yoneda, K.; Furukawa, S.; Horike, N.; Kitagawa, S.;
*
S
Gaspar, A. B.; Mun
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̃
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́
, G.; Ould Moussa,
magnetic and Mo
material is available free of charge via the Internet at http://
pubs.acs.org.
̈
ssbauer properties, and IR spectra. This
́
́
̃
(
̃
AUTHOR INFORMATION
Corresponding Author
E-mail: jose.a.real@uv.es.
■
(
̃
̃
oz, M. C.; Arai, M.;
*
(
̈
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Spanish Ministerio de Ciencia
■
e Innovacion
́
(MICINN) and FEDER funds (CTQ2010-
1
8414) and the Generalitat Valenciana (ACOMP2012/233 and
PROMETEO2012/049). We acknowledge Prof. P. Gutlich for
helpful discussions and for providing Mossbauer spectroscopy
̈
̈
facilities. F.J.M.-L. thanks the MICINN for a predoctoral (FPI)
fellowship.
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dx.doi.org/10.1021/ic301639r | Inorg. Chem. 2013, 52, 3−5