Nanoporosity, Inclusion Chemistry, and Spin Crossover in Orthogonally Interlocked Two-Dimensional Metal-Organic Frameworks

Article abstract of DOI:10.1002/chem.201500310

[Fe(tvp)₂(NCS)₂] (1) (tvp=trans-(4,4′-vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two-dimensional grids. This uncommon supramolecular conformation defines square-sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest-loaded framework 1@guest displays complete thermal spin-crossover (SCO) behavior with the characteristic temperature T_1/2 dependent on the guest molecule, whereas the guest-free species 1 is paramagnetic whatever the temperature. For the benzene-guest derivatives, the characteristic SCO temperature T_1/2 decreases as the Hammet σ_p parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic-phase transition when the guest is benzonitrile or acetonitrile/H₂O.

Full text of DOI:10.1002/chem.201500310

DOI: 10.1002/chem.201500310
Full Paper
&
Host–Guest Systems
Nanoporosity, Inclusion Chemistry, and Spin Crossover in
Orthogonally Interlocked Two-Dimensional Metal–Organic
Frameworks
[a]
[b]
[c]
[a]
Tania Romero-Morcillo, Noelia De la Pinta, Lorena M. Callejo, Lucía PiÇeiro-López,
[
d]
[b]
[e]
[f]
M. Carmen MuÇoz, Gotzon Madariaga, Sacramento Ferrer, Tomasz Breczewski,
[g]
[a]
Roberto CortØs,* and JosØ A. Real*
Abstract: [Fe(tvp) (NCS) ] (1) (tvp=trans-(4,4’-vinylenedipyri-
the guest molecule, whereas the guest-free species 1 is par-
amagnetic whatever the temperature. For the benzene–
guest derivatives, the characteristic SCO temperature T1/2 de-
2
2
dine)) consists of two independent perpendicular stacks of
mutually interpenetrated two-dimensional grids. This un-
common supramolecular conformation defines square-sec-
tional nanochannels (diagonalꢀ2.2 nm) in which inclusion
molecules are located. The guest-loaded framework
creases as the Hammet s parameter increases. In general,
p
the 1@guest series shows large entropy variations associat-
ed with the SCO and conformational changes of the inter-
penetrated grids that leads to a crystallographic-phase tran-
1@guest displays complete thermal spin-crossover (SCO) be-
havior with the characteristic temperature T1/2 dependent on
sition when the guest is benzonitrile or acetonitrile/H O.
2
6
0
Introduction
(
LS) state t2g eg (S=0) in a reversible and controllable way as
response to external stimuli (usually temperature, pressure,
Much interest has been focused on materials that can switch
between two or more electronic states since they could ulti-
mately be used for the construction of sensory and memory
and light). Given the antibonding nature of the e orbitals, the
g
population–depopulation of these orbitals provokes drastic
structural changes at the coordination sphere of the SCO
iron(II) sites. In the solid state, these changes are cooperatively
transmitted from one active site to other active sites through
elastic interactions. For strong coupling between the SCO sites,
the magnetic, optical, and dielectric properties of the material
change abruptly, and, in some cases, this abrupt change is ac-
companied by thermo- and/or piezo-hysteretic behavior, thus
conferring a bistable nature to the material. In principle, strong
coupling can be achieved by “engineering” both behaviors, in-
termolecular interactions (typically hydrogen bonding and/or
p–p stacking), and coordinative covalent bonds. The former in-
teractions predominate in mononuclear systems and can be re-
placed, partially or totally, by coordination bonds in polynuc-
lear and 1–3D coordination polymers (i.e., metal–organic
[
1]
devices. Among possible switchable molecular materials, six-
coordinate iron(II) spin-crossover (SCO) complexes represent
one of the most investigated. These complexes change be-
4
2
tween the high-spin (HS) state t2g eg (S=2) and the low-spin
[
a] T. Romero-Morcillo, L. PiÇeiro-López, Prof. Dr. J. A. Real
Instituto de Ciencia Molecular (ICMol)
Universidad de Valencia, 46980 Paterna, Valencia (Spain)
E-mail: jose.a.real@uv.es
[
b] Dr. N. De la Pinta, Prof. Dr. G. Madariaga
Departamento de Física de la Materia Condensada
Universidad del Pais Vasco UPV/EHU, 48080 Bilbao (Spain)
[
c] Dr. L. M. Callejo
Unidad de Siderurgia, Fundación Tecnalia R&I, 48160 Derio (Spain)
[
2]
[
d] Prof. Dr. M. C. MuÇoz
frameworks (MOFs)).
Departamento de Física Aplicada
Universitat Polit›cnica de Val›ncia, 46022 Valencia (Spain)
Although most iron(II) SCO complexes are discrete mononu-
clear (0D) species, the last 15 years have witnessed a significant
increase in the number and diversity of nD (n=1–3) coordina-
tion polymers with SCO properties. The first reported examples
of SCO coordination polymers (SCO CPs) were 1D triazole-
[
e] Prof. Dr. S. Ferrer
Departamento de Química Inorgµnica
Universidad de Valencia, 46100 Burjassot, Valencia (Spain)
[
f] Prof.Dr. T. Breczewski
Departamento de Física Aplicada II
Universidad del Pais Vasco UPV/EHU, 48080 Bilbao (Spain)
[
3]
based complexes, such as [Fe(Htrz) (trz)](BF )
or [Fe(4-
2
4
[
4]
Rtrz) ](anion)2 (trz=1,2,4-triazole). Since then, an important
2
[
5]
[6]
[
g] Prof. Dr. R. CortØs
variety of 1–3D SCO CPs based on triazoles, tetrazoles, or
Departamento de Química Inorgµnica
Universidad del Pais Vasco UPV/EHU, 48080 Bilbao (Spain)
E-mail: roberto.cortes@ehu.es
[7]
pyridine-like bridging ligands with anionic bridges, that is, cy-
[
8]
[9]
[10]
anometallates, dicyanamide, or polynitriles,
have been
described. Among the 2D systems, those formulated
[Fe(L) (NCX) ]·guest, with L being a rodlike bis-monodentate
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500310.
2
2
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II
bridging ligand, have played an important role in the search
for novel SCO CPs. Below a certain critical length of L, the typi-
cal structure of the 2D system consists of an infinite stack of
properties of the Fe centers have been investigated so far.
Thus, we decided to complete this study by synthesizing new
inclusion derivatives of 1 by using more reliable guest mole-
cules. Therefore, we report herein the synthesis, thermal stabili-
ty, structure, and magnetic and calorimetric properties of
a series of clathrate compounds derived from the porous CP
1 with benzonitrile (PhCN), benzaldehyde (PhCHO), nitroben-
[
11]
parallel grids as described for [Fe(btr) (NCX) ]·H O (X=S,
2
2
2
[
12]
Se;
py) (NCX) ]·4CHCl
3
btr=4,4’-bis-1,2,4-triazole)
and
[Fe(4,4’-bi-
[
13]
(X=S, Se; bipy=bipyridine). For longer L
2
2
bridging ligands, the larger size of the square windows gener-
ates more intricate structures, thus affording doubly or triply
interpenetrated piles of parallel grids. Doubly interpenetrated
zene (PhNO ), dimethylacetamide (DMA), dimethylsulfoxide
2
(DMSO), acetonitrile, or tvp, formulated as 1@2PhCN,
1@2PhCHO, 1@2PhNO2, 1@2DMA·H O, 1@2DMSO·3H O,
[
14]
networks include the compounds [Fe(tvp) (NCS) ]·MeOH
2
2
2
2
[15]
(
tvp =trans-(4,4’-vinylenedipyridine)),
[Fe(azpy) (NCS) ]
1@4 MeCN·4H O, or 1@tvp·2H O and generically noted as
2 2
2
2
[
16]
[17,18]
·
guest (azpy=4,4’-azopyridine), [Fe (bpbd) (NCS) ]·guest
1@guest.
2
2
(
bpbd=2,3-bis(4’-pyridyl)-2,3-butanediol),
[Fe(bpe) (NCS) ]
2
2
[
19]
·guest
(bpe=(1,2-bis(4’-pyridyl)ethane)),
and
[Fe(b-
[
20]
ped) (NCS) ]·guest (bped=dl-1,2-bis(4’-pyridyl)-1,2-ethane- Results and Discussion
diol)). As far as we know, the system [Fe(bpb) (NCS) ]·1/
2
2
2
2
Synthesis and thermal stability
[
21]
2
MeOH (bpb=1,4-bis(pyridyl)butadiyne) is the only example
of this family of compounds that exhibits triple interpenetra-
tion (Scheme 1).
All the compounds reported herein were synthesized by
means of slow liquid–liquid diffusion, which uses the layering
method. Long parallelipedic-shaped dark-red/violet single crys-
tals grew over 1–2 months (see Figure S1 in the Supporting In-
formation). For some guests, a red microcrystalline byproduct
formulated [Fe(tvp)(NCS) ] was formed with the desired com-
2
pound. The byproduct and the Co(II) homologue have already
been reported to be metamagnetic materials composed of
linear [{M(NCS) } ] polymers connected by tvp molecules
2
n
[
22]
through the axial positions of the octahedron. It is worth
noting that this byproduct also appears as an intermediate
during the thermal decomposition of the 1@guest series (see
below). The identity of 1@guest was established by combining
thermogravimetry, elemental analysis, and single-crystal stud-
ies.
Thermogravimetric analysis (TGA) of 1@guest clathrates dis-
plays three well-defined weight-loss processes. In the first step,
the guest molecules are desorbed at 300-460 K to give the un-
Scheme 1. Bis-monodentate ligands for the construction of doubly and
triply interpenetrated 2D grids.
[
23]
loaded framework 1. In the second step, the loss of one co-
ordinated tvp ligand takes place at 460–560 K to give
Special attention has focused on the doubly interpenetrated
compounds mentioned above because they represent an un-
common example of permanent nanoporosity in molecule-
based materials. These compounds result from the orthogonal
interpenetration of 2D SCO CPs that afford a singular class of
MOF with SCO properties (SCO MOFs). The first member of this
series, [Fe(tvp) (NCS) ]·MeOH, was reported almost 20 years
[Fe(tvp)(NCS) ]. This process is well separated from the first
2
step for 1@2PhCHO, 1@4 MeCN·4H O, 1@2DMSO·3H O, and
2
2
1@2DMA·H O. However, the loss of the guest and coordinated
2
tvp partially overlap in 1@tvp·2H O. Finally, the pyrolysis of
2
the intermediate phase takes place at 535–675 K. The thermal
analyses of the derivatives are given in Table S1 and Figure S2
in the Supporting Information.
2
2
[
14]
ago. In this compound, the MeOH molecules are loosely at-
tached to the “walls of the pores” and desorb rapidly at room
temperature with the subsequent collapse of the crystals and
the SCO properties. However, detailed studies that deal with
the influence of host–guest interactions on the SCO behavior
were achieved 7 years later for the closely related porous 2D
Structure
X-ray single crystal studies were carried out for all the reported
clathrates. However, full analysis of the crystal structure of the
corresponding clathrate compounds was prevented due to the
severe disorder found with the PhCHO, PhNO , and DMA/H O
[16]
CP [Fe(azpy) (NCS) ]·guest.
More recently related studies
2
2
2
2
[17,18]
[19]
[24]
have been extended to the bridging ligands bpbd,
bpe,
guest molecules. Nevertheless, preliminary single-crystal X-
ray analysis confirmed the porous structure of the mentioned
[
20]
and bped.
In these materials, the SCO properties can be
tuned by playing with the nature of the adsorbed solvents.
derivatives.
The
crystal
structures
of
1@2PhCN,
Although [Fe(tvp) (NCS) ]·MeOH (1@MeOH) represents the
1@2DMSO·3H O, 1@4MeCN·4H O, and 1@tvp·2H O were an-
2
2
2
2
2
first reported porous SCO MOF of this series, no further studies
on inclusion chemistry and its influence on the electronic
alyzed at 120 and 295, 110 and 250, 120 and 260, and 173 and
293 K, respectively. The structures of 1@2PhCN,
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1
@2DMSO·3H O, and 1@4MeCN·4H O found at high temper-
and in 1@4MeCN·4H O is disordered in two close positions.
2
2
2
À
ature correspond to the tetragonal P4/ncc space group, where-
The FeÀN(CS) bond is strictly collinear with the NCS ion for
as 1@tvp·2H O displays the orthorhombic Pnn2 space group.
1@2PhCN, 1@2DMSO·3H O, and 1@4MeCN·4H O. In con-
2
2
2
The structures of 1@2PhCN and 1@4MeCN·4H O at low tem-
trast, a slight off-axis tilt of the FeÀN(CS) bond system is ob-
2
perature show the occurrence of a crystallographic-phase tran-
sition that adopts the orthorhombic Pccn space group, where-
as no change of space group was observed for
served for 1@tvp·2H O (ca. 7.5 (5.6)8).
2
The
low-temperature
structure
of
1@2PhCN,
1@2DMSO·3H O, 1@4MeCN·4H O, and 1@tvp·2H O shows
2
2
2
1
@2DMSO·3H O and 1@tvp·2H O. The relevant crystallo-
a remarkable decrease of the FeÀN bond lengths: 1) FeÀ
2
2
graphic data and a selection of significant bond lengths and
angles are collected in Tables S2 and S3 in the Supporting In-
formation.
N (pyridine)=1.990(5), 1.995(4), 2.004(4), and 1.96(3)
eq
(2.00(3)) Š; 2) FeÀN (CS)=1.936(4), 1.936(5), 1.948(4), and
ax
1.98(2) (1.912(17)) Š; and 3)<FeÀN> =1.972(5), 1.975(4),
av
1
.985(4), and 1.97(2) (1.97(2)) Š, respectively. This shortening of
the FeÀN bond lengths makes the octahedrons more regular
and there is a small decrease of in the S value to 9.08, 9.76,
Coordination site
9
.80, and 16.2 (11.08) for 1@2PhCN, 1@2DMSO·3H O,
2
II
The Fe ion defines a compressed [FeN ] octahedron in the
À
6
1
@4MeCN·4H O, and 1@tvp·2H O, respectively. The NCS ion
2
2
high-temperature structure of 1@2PhCN, 1@2DMSO·3H O,
2
is now tilted with respect to the FeÀN(CN) bond by 6.9 and
and 1@4MeCN·4H O. The long FeÀN equatorial positions are
2
4
.58 in 1@2PhCN and in 1@4MeCN·4H O, respectively. These
2
occupied by four pyridine moieties that belong to four tvp li-
II
results are consistent with the LS state of the Fe ion. The dif-
gands. The pyridine units are arranged in a propeller fashion
ference in the average FeÀN bond lengths between the HS
II
around the Fe ion with FeÀN (pyridine) bond lengths of
SCO
HS
eq
and
LS
states
(i.e.,
<FeÀN>
= <FeÀN>
À
av
av
2
.212(2), 2.208(2), and 2.216(2) Š, respectively. The short FeÀN
LS
av
<
FeÀN> =0.204(8), 0.197(8), 0.198(8), and 0.24(3) (0.23) Š)
À
axial positions are occupied by two NCS ions, and the FeÀ
are in agreement with a complete LS–HS SCO transition of the
N (CS) bond lengths are 2.094(4), 2.100(4), and 2.117(4) Š, re-
II
ax
Fe ion.
spectively (Figure 1a). Compound 1@tvp·2H O displays two
2
crystallographically independent iron centers (i.e., Fe1 and Fe2)
that feature noticeably less regular [FeN ] octahedrons. Never-
6
Porous metal–organic framework
theless, both [FeN ] sites can also be described as compressed
6
II
octahedrons with Fe1(2)ÀN (pyridine) and Fe1(2)ÀN (CS)
The tvp ligand connects the two Fe ions that define the
eq
ax
bond lengths of 2.216(6) (2.212(6)) and 2.097(6) (2.092(6)) Š, re-
spectively.
edges of [Fe(tvp)] rhombuses (Figure 1b). The separation be-
4
II
tween these Fe ions through the tvp ligand (Fe-tvp-Fe) de-
II
The average <FeÀN> bond length of the [FeN ] octahe-
pends primarily on the spin state of the Fe ion, but also, to
av
6
dron is equal to 2.176(3), 2.172(4), 2.183(4), 2.176(6) (2.172(6)),
a lesser extent, on the guests located in the pores (see below).
For the HS state, this distance ranges from 13.683 to 13.718 Š.
Consistently, the two diagonals of the rhombuses are in the
ranges 22.721–22.403 and 15.398–15.838 Š, and the corre-
sponding angles are 109.48–111.968 and 68.038–70.528, respec-
tively. As mentioned above, the change to the LS structures in-
volves an average decrease of the FeÀN bond length of about
0.2 Š, and hence, a decrease in the Fe-tvp-Fe distance by ap-
proximately 0.4 Š. Accordingly, the larger angle of the rhombus
and 2.207(4) (2.206(4)) Š for 1@2PhCN, 1@2DMSO·3H O,
2
1
@4MeCN·4H O, and 1@tvp·2H O, respectively. These values
2
2
II
are consistent with the HS state of the Fe ion. The angles of
the [FeN ] sites deviate slightly from those expected for a regu-
6
lar octahedron. The sum of the deviation from 908 of the 12 N-
Fe-N angles of the [FeN ] octahedron S=(jqÀ90j) is 11.74,
6
1
6.1, 13.62, and 17.50 (16.928), respectively. As expected, the
À
NCS ligands are linear; however, the sulfur atom in 1@2PhCN
Figure 1. a) Representative coordination site obtained for the structure of 1@2BzCN at 120 K (thermal ellipsoids at the 50% probability level). b) Layer com-
posed of edge-shared [Fetvp] rhombuses. c) Superposition of four consecutive layers ([110] view). The hydrogen atoms were omitted for simplicity.
4
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increases by 1.67–3.518, whereas the small angle decreases by
about 1.98–3.738. Simultaneously, the larger diagonal shortens
by approximately 1 Š; in contrast, the modification of the
shorter diagonal is rather small (see Table S4 in the Supporting
Information).
1@2PhCN and 1@tvp·2H O because the guest molecules
2
show relatively low disorder. In the latter compound, each
crystallographically unequivalent nitrogen atom of the uncoor-
dinated tvp ligands establishes a strong hydrogen-bonding in-
teraction with a crystallographically distinct molecule of water,
thus defining [HOH···N(tvp)N···HOH] units (d[N···O]=2.681(14)
and 2.927(12) Š). Furthermore, these units are arranged in
The edge-shared rhombuses define an infinite set of 2D
layers (Figure 1b), which stack in the [110] direction. The layers
II
are displaced in such a way that the Fe centers of a layer are
[HOH···N(tvp)N···HOH] pairs that display short p–p contacts in
2
right above or below the center of the [Fe(tvp)] windows of
the range 3.573(16)–3.694(16) Š. Two contiguous pairs are
twisted in such a way that the terminal water molecules define
a distorted square motive held by strong HOH···HOH hydrogen
bonds (d[O···O]=2.822(11) and 2.983(11) Š), thereby defining
infinite chains that run along the c direction (see Figure 3b
and Table S5 in the Supporting Information). Only one short
C···C contact, smaller than the sum of the van der Waals radius
(3.7 Š), has been observed between the host framework and
the guest tvp molecules.
4
the adjacent layers (Figure 1c).
This arrangement, the appropriate separation of the layers,
and the length of the tvp ligand enables interpenetration of
an orthogonal set of equivalent layers in the [À110] direction,
thereby defining large square-sectional channels that run
along the c axis, where the guest molecules are located
(
Figure 2). This orthogonality, imposed by symmetry for all the
derivatives in the HS state, persists for 1@2DMSO·3H O and
2
1
@tvp·2H O in the LS state. However, the interpenetration be-
As far as 1@2PhCN is concerned, the PhCN guest molecules
2
comes oblique in the LS state for 1@2PhCN and
@4MeCN·4H O due to the crystallographic-phase transition
mentioned above. The angle defined by two interpenetrated
layers is 84.22(2) and 84.46(2)8 for the benzonitrile and aceto-
nitrile derivatives, respectively (Figure 3, right).
form discrete [PhCN] pairs that display p–p interactions, with
2
1
C···C contacts in the range 3.470(14)–3.696(12) Š at 120 K (see
Figure 3c), although the intensity of these contacts significant-
ly decreases at 295 K (see Table S5 in the Supporting Informa-
tion). In addition, there are host–guest interactions, with C···C
contacts in the range 3.551(12)–3.679(12) Š, which remain es-
sentially constant at 120 and 295K (see Figure 3d and Table S5
in the Supporting Information).
2
Guest–guest and host–guest intermolecular interactions
As mentioned above, the highly disordered molecules of
DMSO, MeCN, and H O in 1@2DMSO·3H O and
Figure 3 displays a perspective view of the porous framework
2
2
1
@tvp·2H O, which shows the channels loaded with tvp and
1@4MeCN·4H O have prevented us from giving a detailed de-
2
2
H O guest molecules. Equivalent pictures of 1@2DMSO·3H O,
scription of the guest–guest and host–guest contacts in the
pores. Still, it can be inferred from the data that hydrogen
2
2
1
@4MeCN·4H O, and 1@2PhCN are displayed in Figure S4 in
2
the Supporting Information. In general, the guest molecules in-
teract with each other and/or the walls of the host framework.
These interactions can be reasonably well described for
bonds between DMSO or MeCN and H O molecules are
2
formed.
Figure 2. Space-filling representation of 1@2PhCN at 120 K. a) Orthogonal interpenetration of two layers. b) Perspective view of a fragment of the MOF that
illustrates the square-sectional channels. The guest molecules were omitted for simplicity.
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Figure 3. a) Perspective view of a fragment of the 1@tvp·2H
2
2
O MOF that shows the square-sectional channels loaded with guest molecules of tvp and H O.
The mutually interpenetrated orthogonal sets of layers are highlighted in orange and violet. b) Arrangement of tvp and H
2
O molecules along the channels
showing p–p guest–guest contacts and N···HOH and HOH···OH hydrogen bonds. c,d) Arrangement of PhCN molecules along the channels that show p–p
2
guest–guest PhCN···PhCN (c) and host–guest tvp···PhCN (d) interactions for 1@2PhCN. C=black, N=blue, S=yellow.
Magnetic and calorimetric properties
that remains constant till 50 K. The small decrease of c T value
M
The plots of c T versus T, where c is the molar magnetic sus-
at temperatures lower than 50 K corresponds to the zero-field
M
M
ceptibility and T is the temperature, are displayed in Figure 4
splitting of the S=2 ground state. This behavior is expected
II
for all the derivatives, including the fully desorbed compound
for the Fe ion in the HS state. Similarly, the c T value ranges
M
3
À1
3
À1
1
. The c T product for 1 is 3.39 cm Kmol at 290 K, a value
from 3.31 to 3.68 cm Kmol at 290 K for the whole series of
M
Figure 4. Magnetic and calorimetric properties of 1 and 1@guest. Down and up triangles represent the cooling and heating modes, respectively.
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clathrates, thus indicating that these compounds are in the HS
Discussion
state at 290 K. Upon cooling, the c T value decreases to 0.17–
M
3
À1
0
.28 cm Kmol
at 100 K for 1@2PhCN, 1@2PhCHO,
This work was undertaken to investigate the host–guest
chemistry and its influence on the SCO properties of 1@guest,
one of the scarce bifunctional molecular materials of the type
[Fe(L) (NCX) ]·guest that combine nanoporosity and SCO prop-
1
@2PhNO , 1@2DMA·H O, and 1@2DMSO·3H O, which im-
2
2
2
plies an occurrence of an almost complete SCO transition from
the HS state to the LS state with residual HS molar fractions of
about 6%. The SCO behavior is less complete for
2
2
erties. Despite the structural similarity between the tvp and
azpy ligands and their isoelectronic character, the results re-
ported herein confirm that the equilibrium temperature T1/2 is
on average approximately 90 K higher for the tvp than for the
azpy derivatives. This stronger stabilization of the LS state in
1@guest can be ascribed to the lower electronegative charac-
ter of the tvp ligand. Furthermore, the orthogonal interpene-
tration observed in the tvp derivative that defines equivalent
square-shaped pores contrasts with the oblique interpenetra-
tion showed by the azpy structure, in which half of the pores
are available for the guest molecules. This fact is consistent
with the approximate 50% spin conversion observed for the
clathrates of the azpy derivatives. However, thermal quenching
of the SCO at temperatures below 100 K could be an alterna-
tive explanation.
1@4MeCN·4H O and 1@tvp·2H O, with c T values at 100 K of
2 2 M
3
À1
0
.59 and 0.66 cm Kmol , respectively, thus denoting a residual
HS molar fraction of about 17%. The heating mode evidences
a quite narrow hysteresis (ca. 5 K) for 1@2PhCN, 1@2PhCHO,
1@2PhNO , and 1@tvp·2H O. No appreciable hysteresis was
2
2
observed for the remaining derivatives.
All the c T versus T curves show a more or less marked char-
M
acteristic change in the slope, which takes place at about 50%
of SCO for 1@2PhNO . However, the turning for 1@2PhCHO,
2
1@2DMSO·3H O, and 1@2DMA·H O occurs in the low-tem-
2
2
perature interval of the SCO and becomes more perceptible in
plots of @(c T)/@T versus T (see Figure S4 in the Supporting In-
M
formation). For 1@2PhCN, the inflection is produced in a differ-
ent way, that is, the onset of a SCO (interrupted at 224 K)
occurs in the 250–224 K interval and is followed by a drastic
A complete loss of the guest molecules in the interpenetrat-
ed frameworks [Fe(L) (NCS) ]·guest when L=tvp and azpy is
drop of the c T product, thus revealing the existence of signifi-
M
2
2
cant cooperativity. For the whole series, the equilibrium tem-
accompanied by stabilization of the HS state at any tempera-
ture. However, this behavior is not generic for this type of
framework; for example, the SCO properties remain after re-
moving the guest molecules in the orthogonally interpenetrat-
perature T , at which the molar fractions of the HS and LS
1/2
species are equal to 50% takes place, happens in the interval
10–169 K. The transition temperatures T for the different
2
c
[
17,18]
slopes obtained from @(c T)/@T versus T are given in Table 1.
ed frameworks [Fe(bpbd) (NCS) ]·guest
and [Fe(b-
M
2
2
[
20]
Quantitative differential scanning calorimetry (DSC) measure-
ped) (NCS) ]·3EtOH. In general, these interpenetrated com-
2
2
ments were carried out for 1@guest. The anomalous variation
pounds display gradual SCO behavior, a fact attributed to the
flexibility of the bridging ligand and the lack of relevant host–
guest contacts. Two important exceptions have been reported
for the bpbd-based framework, namely, when the guest mole-
of the molar specific heat DC versus T for the cooling mode is
p
displayed together with the c T versus T plot in Figure 4. The
M
DC versus T plots show the presence of two or three peaks
p
[
18]
that overlap to a greater or lesser extent and reflect the
cules are MeOH or EtOH.
In those cases, the observed
changes in the slope observed in the c T versus T plots. The
abrupt SCO behavior with thermal hysteresis has been ascribed
to remarkable OH···S(CN) host–guest interactions. All previous
studies on [Fe(L) (NCS) ]·guest frameworks with orthogonal or
M
critical temperatures obtained from these DSC data agree rea-
sonably well with the data deduced from the magnetism
measurements. The average enthalpy DH and entropy DS var-
iations associated with the cooling and heating modes are
gathered in Table 1.
2
2
oblique interpenetration of 2D [Fe(L) (NCS) ] layers have es-
2
2 1
sentially been focused on the clathration of hydroxylic sol-
vents. In the present work, we have investigated a heterogene-
ous series of guests, including aromatic molecules (PhCN,
PhCHO, PhNO , and tvp) or aprotic solvents (DMSO, DMA and
c p
Table 1. Transition T and equilibrium T1/2 temperatures, average DH and DS values of the SCO for the title compounds, and dielectric e and Hammett s
constants for some guest molecules.
[
a]
Compound
First slope
T
c
[K]
T
[K]
1/2
DH
[kJmol
DS
[Jkmol ]
e
s
p
[b]
À1
À1
Second slope
]
1
1
1
1
1
1
1
@tvp·2H
@2PhCHO
@2PhCN
@2PhNO
@2DMA·H
@2DMSO·3H
@4MeCN·4H
2
O
221.0
194.0
224.0
226.0
186.7
195.0
223.0
201.0(À), 203.5 (›)
167.7(À), 175.7 (›)
197.7 (À), 202.0 (›)
191.0 (À), 197.0 (›)
145.0
208.5
189.6
200.2
209.4
171.3
182.8
214.9
17.1
15.8
20.6
18.1
16.8
15.3
23.4
82
–, 80
17
26
–
82.5
100.1
86.0
98.0
81
0.42
0.67
0.77
–
–
–
2
35
2
O
38, 80
47, 80
37, 80
2
O
O
162.7
203.5
2
106.5
[
[
a] T
(c
c
values were taken from @(c
M
T)/@T versus T plots. [b] T1/2 values were estimated from the temperature at which the experimental c
M
T value is equal to
M
M M
T)maxÀ(c T)min]/2+(c T)min.
Chem. Eur. J. 2015, 21, 12112 – 12120
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
MeCN). The aromatic clathrates display appreciable hysteretic
behavior, thus indicating the occurrence of some cooperativity
lographic phase transition and the SCO. For the other mem-
bers of the 1@guest series, the DS and DH values are slightly
smaller, but large in relation to those values obtained for other
(
Table 1). On the basis of the crystal structures of 1@tvp·2H O
2
II
and 1@2PhCN, this cooperativity can be attributed to guest···
Fe SCO compounds with similar SCO behaviors. Although no
guest and host···guest intermolecular interactions, which in-
crystallographic-phase transition was observed for the remain-
ing members of the series, we believe that subtle structural
changes related to orthogonal$oblique interpenetration labili-
ty of the layers should play an important role during the SCO
and represent an important source of entropy for these com-
pounds. This speculative idea is supported by the change in
the slope and the presence of several maxima in the cMT
versus T and DCp versus T plots, respectively.
II
crease in number and intensity as the population of the LS Fe
ion increases (see Table S5 in the Supporting Information). The
latter consideration can also be extended to 1@2PhCHO and
1
@2PhNO given that these compounds are strictly isostruc-
2
tural to 1@2PhCN at 300 K. The crystal analysis of
@2DMSO·3H O and 1@4 MeCN·4H O shows the existence of
1
2
2
hydrogen bonds between the H O molecules and the guests.
2
The strong disorder, however, prevented us from confirming
the presence of significant host···guest contacts. This effect
No direct correlation was found between T1/2 and the nature
(i.e., size and/or shape) of the guest molecule for the whole re-
ported series. A similar situation was noted for the clathrates
[Fe(bpbd) (NCS) ]·guest (guest=CH CN, MeOH (CH ) CO, EtOH,
should also be applicable to the isostructural 1@2DMA·1H O
2
derivative. Despite the lack of appreciable hysteresis for the
three latter compounds, the SCO behavior could be considered
to be moderately cooperative.
2
2
3
3 2
and PrOH). However, a general stabilization of the LS state (in-
crease of T ) with decreasing the dielectric constant e of the
1/2
[
18]
A common feature of the 1@guest derivatives, including the
previously reported 1@MeOH, is the occurrence of a more or
guest was found for the latter system. In the present case,
we observed correlation for 1@2PhCN,
a
clear eÀT
1/2
less marked change in the slope in the c T versus T curves,
1@2PhCHO, and 1@2PhNO , which feature comparable guest
M
2
clearly visible in the @(c T)/@T versus T plots. This change in
molecules and loading (Table 1). In contrast to the bpbd
M
the slope could reflect some structural change produced
during the SCO, which is characteristic of this family of com-
pounds and could be responsible for the crystallographic-
system, this relationship suggests that high e values result in
a LS-state stabilization. The eÀT1 correlation could be ex-
/2
plained by considering the noncovalent host–guest interac-
tions between the tvp and PhX aromatic centers in terms of
phase transition observed for 1@2PhCN and 1@4MeCN·4H O.
2
[
25]
Both derivatives reversibly change from the tetragonal P4/ncc
space group to the orthorhombic Pccn space group when
moving from the HS to the LS state. The change rends the in-
terpenetration of the layers less symmetrical, thus turning from
strictly orthogonal (HS) to slightly oblique (LS). Interestingly,
the same phenomenology was described for the [Fe-
Hammett parameters. The correlation between the Hammett
sp constant of the three aromatic PhX guests and the T1/2 value
of their inclusion compounds is similar to that observed in this
study for eÀT1 (Table 1), thereby suggesting that T , and
/2
1/2
II
hence the ligand field felt by the Fe centers, increases as the
electron-withdrawing nature of substituent X increases. Since
[
18]
(
bpbd) (NCS) ]·guest (guest=MeOH and EtOH) systems.
the location of the PhCHO and PhNO guests in the channels
2
2
2
Concerning the DSC measurements, the DCp versus T curves
must be essentially the same as that described for the PhCN in
1@2PhCN, it could be considered that the interaction between
the guest PhX and the tvp bridging ligand enhances the p-ac-
ceptor character of the latter. In turn, this effect increases with
the electron-withdrawing nature of X in PhX. It is worth men-
tioning that, despite the different nature of the system, a simi-
lar correlation between T1/2 and the Hammett constant was re-
show the occurrence of two or three peaks reminiscent of
those peaks detected in the @(c T)/@T versus T plots, which
M
correspond to the changes in the slope of the c T versus T
M
curves mentioned above. The DH and DS values are in the
À1
À1
ranges 15–23 kJmol
Table 1). Unfortunately, the lack of calorimetric data for the
previously investigated SCO MOFs of the [Fe(L) (NCS) ]·guest
and 81–107 Jkmol , respectively
(
II
ported for the series of dinuclear Fe complexes [Fe(X-pyri-
2
2
series prevents any comparison. Still, these values are in gener-
dine)(NCY)] (m-bpypz) (X=4-Me, 3-Me, H, 3-Cl, 3Br; Y=S,
2
II
[26]
al larger than those typically observed for Fe SCO complexes
BH3).
À1
(
50–60 Jkmol ). The DS value takes into account the changes
in the electronic and vibrational degeneracies of the system
DS +DS ) upon spin-state change, where DS is composed
Conclusion
(
el
vibr
el
orb
spin
of orbital and spin parts (DSel +DSel ). In a regular octahe-
We have investigated a series of inclusion compounds derived
from [Fe(tvp) (NCS) ], a rare nanoporous MOF constituted by
5
1
dron, the HS and LS states correspond to the T and A
2
g
1g
2
2
orb
II
terms, and consequently DSel =Rln(3/1). However, real Fe HS
two orthogonally interpenetrated identical stacks of 2D CPs.
The inclusion of aromatic molecules, such as benzonitrile, ni-
trobenzene, benzaldehyde, and tvp, or of aprotic solvents,
such as DMSO and DMA, among others, has been investigated
in this study for the first time with this type of porous material.
Magnetic and DSC analysis have revealed the occurrence of
complete SCO behavior, characterized, at least, by two re-
gimes, as clearly denoted by the presence of different slopes
sites are usually quite distorted and the actual HS state is also
orb
spin
an orbital singlet; consequently, DSel ꢀ0. DS
is Rln(5/1)=
el
À1
1
3.45 JKmol , a value much smaller than that experimentally
À1
observed. The excess of entropy DSvibr (36–47 JKmol ) stems
from the change in the vibrational degeneracy (molecular and
lattice) upon HS $ LS spin-state change. The much larger DH
and DS values observed for 1@2PhCN and 1@4MeCN·4H O
2
must be ascribed to the coexistence of the mentioned crystal-
and maxima in the thermal variation of c T and DCp, respec-
M
Chem. Eur. J. 2015, 21, 12112 – 12120
www.chemeurj.org
12118
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
tively. X-ray crystal analysis corroborates the porous nature of
the host framework and the occurrence of complete SCO be-
havior when the guest molecules fill the pores, a necessary
condition to observe SCO behavior in the investigated series.
Physical measurements
Variable-temperature magnetic susceptibility data were recorded
on a Quantum Design MPMS2 SQUID susceptometer equipped
with a 7 T magnet, operating at 1 T, and at temperatures 2–400 K.
Experimental susceptibilities were corrected from diamagnetism of
the constituent atoms by the use of the Pascal constants. TGA
measurements were performed on a Mettler Toledo TGA/SDTA
For 1@2PhCN and 1@4 MeCN·4H O, the SCO is concomi-
2
tant with a crystallographic-phase transition, which essentially
involves a change in the angle defined by the interpenetrated
layers. This concomitance accounts for the high DH and
DS values observed for these compounds. On the basis of
comparable magnetic and calorimetric properties, and even
though no crystallographic-phase transition was detected, we
suggest the occurrence of similar structural changes upon SCO
in the remaining members of the series.
8
51e in the temperature range 290–800 K in a nitrogen atmos-
À1
phere at a rate of 10 Kmin . DSC measurements were performed
on dry samples of 1@guest on a Mettler Toledo DSC 821e DSC cal-
orimeter. Low temperatures were obtained by using an aluminum
block attached to the sample holder, refrigerated with a flow of
liquid nitrogen, and stabilized at a temperature of 110 K. The
sample holder was kept in a drybox under a flow of dry nitrogen
to avoid water condensation. The measurements were carried out
on crystalline samples (ca. 10 mg) sealed in aluminum pans with
a mechanical crimp. An overall accuracy of 0.2 K and 2% for the
temperature and heat capacity was estimated. The uncertainty in-
creases for the determination of the anomalous enthalpy and en-
tropy due to the subtraction of an unknown baseline. Microanaly-
ses were performed on a LECO CHNS-932 analyzer.
Experimental Section
Synthesis
All the samples were exclusively composed of single crystals pre-
pared by using the slow liquid–liquid diffusion technique (layering)
II
in standard test tubes. All manipulations involving solutions of Fe
Single-crystal X-ray diffraction
were performed in an argon atmosphere. Furthermore, very small
quantities of ascorbic acid were added to these solutions to pre-
vent oxidation of the Fe species.
Single-crystal X-ray data of 1@tvp·2H O were collected on an
Oxford Diffraction KM4 CCD and a STOE StadiVari Pilatus-100K
single-crystal diffractometer. Single-crystal X-ray data of 1@2PhCN,
2
II
Synthesis of 1@2PhCN, 1@2PhCHO, and 1@2PhNO : A solution
2
1
@2DMSO·3H O, and 1@4MeCN·4H O were collected on an
2
2
of tvp (98.3 mg, 0.54 mmol) in BzX (X=CN, NO , or CHO; 5 mL)
2
Oxford Diffraction Supernova. In all cases, MoKa radiation (l=
.71073 Š) was used. Data scaling and empirical or multiscan ab-
was poured into a test tube (bottom layer). A mixture of BzX/
MeOH (1:1; 10 mL) was slowly added (middle layer). Finally, a fil-
0
sorption correction was performed. The structures were solved by
using direct methods with SHELXS-97 and refined by using full-
II
À
tered colorless solution of Fe /NCS (1:2) in methanol (ca. 5 mL)
was prepared by mixing FeSO ·7H O (75 mg, 0.27 mmol) and KNCS
4
2
[27]
matrix least squares on F2 with SHELXL-97. Non-hydrogen atoms
(
52.3 mg, 0.54 mmol) and was then slowly poured on the middle
layer.
Synthesis of 1@2DMA·H O and 1@2DMSO·3H O: The layering se-
were refined anisotropically, and hydrogen atoms were placed in
calculated positions refined by using idealized geometries (riding
model) and assigned fixed isotropic displacement parameters.
CCDC 1042610, 1042611, 1042612, 1042613, 1042614, 1042615,
2
2
quence was modified for DMSO and DMA due to their higher den-
sities. Bottom layer: a mixture of tvp (98.3 mg, 0.54 mmol) and
KSCN (52.3 mg, 0.54 mmol) dissolved in DMSO or DMF; middle
1
042616, and 1042617 contain the supplementary crystallographic
data for this paper. These data are provided free of charge by The
Cambridge Crystallographic Data Centre.
layer: mixture of DMSO or DMA/H O (1:1; 10 mL); top layer: aque-
2
ous solution (5 mL) of FeSO ·7H O (75 mg, 0.27 mmol).
4
2
Synthesis of 1@4 MeCN·4H O: Another modified layering se-
2
quence was used. Bottom layer: an aqueous solution (5 mL) of Acknowledgements
FeSO ·7H O (75 mg, 0.27 mmol) and KNCS (52.3 mg, 0.54 mmol);
4
2
middle layer: a mixture of H O/MeCN (1:1, 10 mL); top layer: a solu-
tion of tvp (98.3 mg, 0.54 mmol) in MeCN (5 mL).
The research reported herein was supported by the Spanish
Ministerio de Economía y Competitividad (MINECO) and FEDER
funds (CTQ2013-46275-P; MAT2012-34740), the Basque Gov-
ernment (project IT-779-13), and Generalitat Valenciana (PROM-
ETEO/2012/049). T.R.M. thanks the Spanish Ministerio de Econo-
mía y Competitividad (MINECO) for a predoctoral grant (FPI.).
L.P.-L. thanks the Generalitat Valenciana for a predoctoral fel-
lowship in the frame of the project PROMETEO/2012/049.
2
Synthesis of 1@tvp·2H O: For this derivative only two layers were
2
used. Bottom layer: mixture of KNCS (52.3 mg, 0.54 mmol) and tvp
(
(
184 mg, 1 mmol) in DMSO (5 mL); top layer: an aqueous solution
5 mL) of FeCl ·4H O (54 mg, 0.27 mmol) was poured slowly on top
2
2
of the first layer.
The tubes were sealed and left in a quiet place for at least
month. Single crystals of the clathrates appeared as red/purple
1
or red/black long parallelepipeds. The yield was about 40–50% for
all the derivatives. Elemental analysis (%): 1@2PhCN: C 64.69, H
Keywords: inclusion compounds · interpenetration · iron ·
metal–organic frameworks · spin crossover
4
.07, N 15.09; found: C 64.05, H 3.96, N 14.81; 1@2PhNO : C 58.32,
2
H 3.86, N 14.32; found: C 57.84, H 3.82, N 14.05; 1@2PhCHO: C
4.17, 4.31, 11.23; found: 63.62, 4.20, 11.13;
@2DMA·H O: C 56.04, H 5.53, N 15.38; found: C 56.61, H 5.45, N
6
H
N
C
H
N
[
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1
2
1
5.51: 1@2DMSO·3H O: C 48.25, H 5.13, N 11.25; found: C 48.67,
2
H 5.01, N 11.45; 1@4 MeCN·4H O: C 52.85, H 5.22, N 18.30; found:
2
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[15] Ligand tvp is also referred in other reports as 1,2-di(4-pyridyl)ethylene
(dpe) and more frequently as 1,2-bis(4-pyridyl)ethylene (bpe); however,
we consider that these abbreviations may be misleading because bpe
is also usually employed for the closely related 1,2-bis(4-pyridyl)ethane.
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1
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Received: January 23, 2015
Revised: May 14, 2015
lows, D. J. Price, G. J. Halder, K. W. Chapman, B. Moubaraki, K. S. Murray,
J. F. LØtard, C. J. Kepert, J. Am. Chem. Soc. 2009, 131, 10998; j) G. Agustí,
Published online on July 14, 2015
Chem. Eur. J. 2015, 21, 12112 – 12120
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