Structural and magnetic modulation of a purely organic open framework by selective guest inclusion

Article abstract of DOI:10.1002/chem.200700353

Solvent inclusion/evacuation caused variations in the structural and magnetic characteristics of the purely organic porous magnet based on the tricarboxylic-substituted PTMTC radical. Whereas no inclusion is observed for nonpolar solvents, the exposure of

Full text of DOI:10.1002/chem.200700353

FULL PAPER
DOI: 10.1002/chem.200700353
Structural and Magnetic Modulation of a Purely Organic Open Framework
by Selective Guest Inclusion
Daniel Maspoch,[a] Neus Domingo,[b] Nans Roques,[a] Klaus Wurst,[c] Javier Tejada,[b]
Concepció Rovira,[a] Daniel Ruiz-Molina,*[a] and Jaume Veciana*[a]
Abstract: Solvent inclusion/evacuation
caused variations in the structural and
magnetic characteristics of the purely
organic porous magnet based on the
tricarboxylic-substituted PTMTC radi-
cal. Whereas no inclusion is observed
for nonpolar solvents, the exposure of
crystals of the a-phase of PTMTC to
vapors of polar organic solvents with
hydrogen acceptor and/or donor func-
tionalities, such as, ethanol, benzoic al-
cohol, n-decanol, THF, and DMSO re-
sults in the inclusion of these solvents
in the highly polar tubular channels of
the a-phase. The resulting inclusion
compounds of formula PTMTC·
(guest) show several structural rear-
with the carboxylic groups of PTMTC
radicals, inducing the disruption of sev-
eral direct hydrogen bonds among
these radicals. As expected, the inter-
ruption of direct hydrogen bonds be-
tween PTMTC radicals induces large
transformations in the magnetic prop-
erties. From the ferromagnetic behav-
ior of the a-phase, predominant anti-
ferromagnetic interactions are ob-
served for the inclusion adducts. Inter-
estingly, both structural and magnetic
changes are reversible after removal of
guest solvent molecules.
x
ACHTREUNG
rangements, as confirmed by IR and
XRPD (X-ray powder diffraction)
measurements. The crystal transforma-
tions have been studied for a specific
case: the PTMTC·EtOH adduct. The
crystal structure reveals that included
guest solvent molecules participate in
the formation of new hydrogen bonds
Keywords: host–guest systems
hydrogen bonds · magnetic prop-
erties · radicals · solvatomagnetism
·
AHCTREUNG
Introduction
construction of crystalline porous molecular materials.[1] Fol-
lowing this strategy, excellent results have been obtained in
the design of zeolite-like metal-organic robust open-frame-
work structures, with a huge range of pore sizes (2–30 Š)[2]
and unprecedented topologies.[3] Even more interesting is
the possibility to add new properties to these structures be-
sides their inherent porous character, which should facilitate
multifunctionality of these systems.[1a,4] Indeed, the puzzled
construction of metal-organic open frameworks allows the
exploration of novel structural/dynamic characteristics, as
well as physical functions, such as, chirality,[5] electrical,[6] op-
tical,[7] or magnetic.[8,10–15] Among them, attainment of func-
tional magnetic and/or dynamic structural porous solids is
attracting considerable effort worldwide.[8,9] Magnetic
porous solids have the potential to produce low-density
magnetic materials or to develop multifunctional materials
thanks to their capacity to encapsulate other functional mo-
lecular systems possessing additional conducting, optical,
chiral, or nonlinear optical (NLO) properties. Thus far, fer-
romagnetic,[10] antiferromagnetic,[11] spin-frustration,[12] spin-
crossover,[13] metamagnetic,[14] or ferromagnetic[15] behavior
have been introduced in porous structures. On the other
hand, coordination polymers with dynamic pores open new
Multitopic organic building blocks with selected geometries
represent an excellent tool to resolve one of the most chal-
lenging issues facing supramolecular chemistry today, the
[a] Dr. D. Maspoch,+ Dr. N. Roques, Prof. C. Rovira,
Dr. D. Ruiz-Molina, Prof. J. Veciana
Institut de Ci›ncia de Materials de Barcelona, CSIC
Campus Universitari de Bellaterra, 08193, Bellaterra (Spain)
Fax : (+34)93-580-5729
E-mail: dani@icmab.es
vecianaj@icmab.es
[b] Dr. N. Domingo, Prof. J. Tejada
Facultat de Física, Universitat de Barcelona
Diagonal 647, 08028, Barcelona (Spain)
[c] Dr. K. Wurst
Institute für Allgemeine, Anorganische und Theoretische Chemie
Universität Innsbruck, Innrain 52a, 6020, Innsbruck (Austria)
[+] Current address: Institut Català de Nanotecnologia
Campus Universitari de Bellaterra, 08193, Bellaterra (Spain)
Supporting information (crystal structures of the a- and b-phases, and
inclusion studies (thermogravimetry and infrared spectra) of ethanol,
benzoic alcohol, and THF in the a-phase) for this article is available
on the WWW under http://www.chemeurj.org/ or from the authors.
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8153

routes to design highly selective porous solids. For instance,
Fujita, Suh, and co-workers have recently described some
examples of porous structures with modified crystal struc-
ture after a chemical and/or physical stimulus.[16,17,18] Gener-
ally, this property comes from the structural ability to suffer
reversible structural modifications after selective sorption/
desorption of solvent guest molecules.
In view of the excellent results previously described and
to further expand the range of multifunctional porous mate-
rials, the possibility has recently been suggested to combine
both the dynamic flexibility of open-framework coordina-
tion polymers with additional magnetic properties. Dynamic
flexible open frameworks are interesting because of their ca-
pability to undergo reversible structural modifications by re-
moval/sorption of solvent guest molecules. As the magnetic
properties are closely dependent on the structure of the ma-
terial, it is expected that reversible structural readjustments
will induce a modulation of their magnetic properties.
Therefore, by controlling the presence/absence of solvent
guest within the porous structure, we may reversibly switch
the magnetic properties within two states. For example, an
“off/state” may be attained when the material is evacuated,
whereas an “on/state” may be achieved when the material is
filled (see Scheme 1). Such switchable behavior together
C
The advantages of this PTM family of radicals are mani-
fold: 1) These bulky radicals are sterically hindered due to
the presence of several chlorine atoms, which renders them
thermally and chemically stable, but also prevents close
packing and interpenetration within the final structure;[28] 2)
carboxylic-PTM radicals are open-shell species capable of
transmitting and increasing magnetic interactions through
the structure, as previously shown for PTMMC,[29] which
Scheme 1. Idealistic schematic representation of the solvatomagnetic be-
havior of a porous solid induced by the dynamic structural flexibility of
its open framework.
with the intrinsic high selective sorption properties associat-
ed with dynamic structural coordination polymers provides
the ideal scenario to build potential selective molecular-
based porous sensors for many gases or liquids.
Thus far, different examples of dynamic porous metal-or-
ganic solids with solvatomagnetic effects have been de-
scribed. In 2002, Kepert and co-workers reported a dynamic
porous coordination polymer
formed hydrogen-bonded R2 (8) dimers that promoted the
2
transmission of a weak ferromagnetic interaction; and 3)
carboxyl-substituted PTM radicals can be considered as an
expanded version of trimesic acid, in which the benzene-
1,3,5-triyl units have been replaced by an sp2-hybridized
with reversible spin-crossover
phenomena by elongation and
constriction of the pores caused
by evacuation/reabsorption of
alcohol solvent molecules.[11]
Later, Kobayashi, Kurmoo and
co-workers reported several ex-
amples of porous magnets with
very
solvatomagnetic
interesting
reversible
effects.[19]
For
example,
[Co3(OH)2- Scheme 2. Carboxylic-substituted polychlorinated triphenylmethyl radicals.
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Modulation by Selective Guest Inclusion
FULL PAPER
carbon atom decorated with three polychlorophenyl rings
substituted with carboxylic acid groups.[22]
as, elemental analysis, ESI-TOF/MS, FT-IR, and EPR spec-
troscopies.
Following this strategy, a few years ago we reported the
first example of a purely organic magnetic material with a
robust open framework based on the supramolecular ar-
rangement of an open-shell dicarboxylic perchlorinated tri-
phenylmethyl (PTMDC) radical.[26a] However, no solvato-
magnetic effects were observed for this structure. Herein,
we report the detailed synthesis of radical PTMTC and its
corresponding supramolecular packing, which will be refer-
red to from now on as a-phase or POROF-2 (POROF
stands for purely organic radical open framework). The a-
phase combines a robust open-framework structure with
magnetic ordering at low temperatures. The highly polar
void volume due to the hydrogen-bonded porous network
provides a unique environment to promote the selective in-
clusion of polar organic solvents with hydrogen-donor or
-acceptor functionalities, which strongly influence its crystal
structure and magnetic properties.[30] The structural transfor-
mations experienced by the hydrogen-bonded network of
the a-phase after inclusion of solvent guest molecules will
be studied by using a specific case, the inclusion of ethanol
molecules.
X-ray structure of the a-phase of PTMTC: Crystallization
by slow diffusion of pure n-hexane into a solution of
PTMTC in dichloromethane gave red hexagonal plate crys-
tals of the a-phase or POROF-2. Single crystals of the a-
phase suitable for single X-ray diffraction analysis were also
obtained by a slow evaporation of a solution of PTMTC in
dichloromethane and n-hexane in 2:1 ratio.
Radical PTMTC crystallizes in the a-phase in the trigonal
¯
P3c1 group with Z=4 (Table 1). An ORTEP representation
Table 1. Crystallographic data for the a-phase and b-phase of PTMTC.
a-phase
b-phase
formula
Fw
C22H3Cl12O6
788.64
C24H11Cl12O8
852.73
crystal system
space group
a[Š]
b[Š]
c[Š]
trigonal
P3c1
15.9283(7)
15.9283(7)
13.8886(11)
90.00
90.00
120.00
3051.6(3)
4
triclinic
P1
¯
¯
8.7737(4)
13.3814(8)
14.4784(8)
68.519(3)
79.629(3)
88.026(3)
1554.99(11)
2
a[8]
b[8]
g[8]
V[Š3]
Z
T[K]
R1
233(2)
0.0585
233(2)
0.0512
Results and Discussion
wR2
0.1418
0.1154
GOF
1.133
1.035
Synthesis: The preparation of radical PTMTC was carried
out following a four-step procedure outlined in Scheme 3.
Tris(2,3,4,5-tetrachlorophenyl)methane (1) was synthesized
according to the well-documented method found in the liter-
ature.[31] Three dichloromethyl groups were introduced into
1 by reacting it with anhydrous chloroform following a Frie-
del–Crafts reaction under high pressure at 1608C for 8 h.
The resulting molecule 2 was treated with fuming sulfuric
acid at 1508C for 12 h. At high temperatures, 20% fuming
sulfuric acid hydrolyzes and subsequently oxidizes the di-
chloromethyl groups of 2 to carboxylic groups to furnish tri-
acid 3. Finally, treatment of the hydrocarbon precursor 3
with sodium hydroxide in DMSO gave a red solution of the
tricarboxylate carbanion, which was later oxidized to the tri-
carboxylate radical in the presence of iodine and finally
acidified with HCl to obtain radical PTMTC. The PTMTC
radical is a stable species both in solution and in the solid
state, and it was characterized by different techniques, such
of the molecular conformation is shown in Figure 1. As can
be observed, it adopts the typical propeller-like conforma-
tion usually found in this family of radicals, which is defined
by the dihedral angles, with values in the range of 44–568,
between the mean planes of each of the three polychlorinat-
ed aromatic rings and the reference plane, formed by the
three bridgehead carbon atoms and the methyl one.[32] The
high molecular symmetry on the crystal lattice is reflected
by the presence of a C3 symmetry axis that crosses through
the central carbon (C8) of PTMTC. Thus, the three poly-
chlorinated aromatic rings are identical with a dihedral
angle of 508. Carboxylic groups are found to present a syn–
planar conformation.[33] As was previously observed for
PTMMC, the plane of the carboxylic groups lies orthogonal
to that of the aromatic ring (878) due to the great steric hin-
Scheme 3. Synthesis of the PTMTC radical.
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J. Veciana, D. Ruiz-Molina et al.
of 1698 (for more details of the geometry, see Figure 2).
Such a motif is a very rare case among the hydrogen-
bonded supramolecular structures based on carboxylic
groups, and probably it is associated with the rigidity and
bulkiness of these PTM radicals. Indeed, one of the most
frequent and dominant hydrogen-bonded motifs is the syn–
syn centrosymmetric R2 (8) dimer.[33,35] However, other pat-
2
terns in the form of infinite noncyclic hydrogen-bonded cat-
emer motifs and even rarer cyclic trimeric and tetrameric
motifs have also been found.[36] In fact, a crystallographic
search done by us by using the Cambridge Structural Data-
base (CSD, version 5.28, November 2006) confirmed that
the largest hydrogen-bonded cyclic motif found, exclusively
formed by carboxylic groups, is the unusual tetrameric
R4 (16) motif.[37] For the sake of clarity, a representation of
4
the different hydrogen-bonded motifs found for carboxylic
groups is shown in Figure 2b.
Figure 1. ORTEP plots at 50% of probability of PTMTC radical in the
a-phase.
The participation of each PTMTC radical in the formation
of three identical hydrogen-bonded R6 (24) hexameric
6
drance of the chlorine atoms in ortho positions with respect
to the carboxylic group.[34]
motifs leads to the formation of a two-dimensional hydro-
gen-bonded plane, in which each hexameric unit is hydro-
gen-bonded to six identical motifs along the ab plane (Fig-
ure 3a). The proper pillared packing of these hydrogen-
bonded layers creates an open-framework structure. ABAB
alternation of the layers along the c axis generates a three-
dimensional structure that exhibits tubular channels, where
a sphere 5.2 Š in diameter can fit inside them (Figure 3b).
Indeed, the columnar disposition of hydrogen-bonded hex-
americ units creates highly hy-
As far as the supramolecular packing is concerned, the
PTMTC radical arranges in a large hexameric R6 (24) motif
6
(Figure 2a), similar to that observed for the dicarboxylic
PTMDC radical.[26a] This R6 (24) centrosymmetric cyclic
6
motif adopts a nearly planar arrangement formed by six mu-
tually hydrogen-bonded carboxylic groups of PTMTC radi-
ꢀ
cals with O1···O2 distances of 2.657 Š and O H···O angles
drophilic tubular channels with
solvent-accessible voids that
3
amount up to 15% (450 Š per
unit cell) of the total volume.
Further weaker supramolecular
interactions stabilize the overall
structure. Among them, several
Cl···Cl contacts (six per mole-
cule) stabilize each hydrogen-
bonded sheet, whereas six addi-
tional Cl···Cl contacts per mole-
cule link adjacent layers.
Structural rigidity of the a-
phase: As stated above, the
single-crystal X-ray diffraction
study of the a-phase showed
the lack of guest solvent mole-
cules within the tubular chan-
nels of the open-framework
structure. This fact has been
confirmed by elemental analysis
and thermogravimetric (TG)
studies.[38] As can be seen in
Figure 4,
thermogravimetric
analysis of a few single crystals
of the a-phase showed no
weight loss in the temperature
range of 25 to 2508C. More-
Figure 2. a) Geometrical characteristics of the hexameric R6 (24) ring formed by intermolecular hydrogen
bonds of six PTMTC radicals. Distances are in Š. b) Cyclic hydrogen-bonded carboxylic patterns found for
other organic carboxylic acids.
6
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Modulation by Selective Guest Inclusion
FULL PAPER
In addition to the highly robust hydrogen-bonded net-
work, the large number of chlorine atoms present in the
skeleton of the tricarboxylic PTMTC radicals has significant
implications for such structural stability. Indeed, both stabili-
zation of the crystal structure through the formation of mul-
tiple Cl···Cl contacts in the solid state and the remarkable
steric congestions due to bulky chlorine atoms, may be the
main reason for obtaining noncatenated crystal packing[39] in
spite of the Kitaigorodski principle of close packing.
Inclusion properties of the a-phase: The absence of sol-
vent guest molecules within the tubular pores provides an
approximate free pore volume of around 15% of the total
volume cell. This highly polar potentially accessible void
volume combined with its high thermal stability up to 2508C
are well known optimum conditions to consider a-phase as
a good candidate to selectively include certain solvent guest
molecules and, therefore, to behave as a purely organic
“zeolite-like” material. In such a context, to study the guest-
exchange properties of the a-phase, we performed a series
of solid–gas experiments in which crystals of as-synthesized
a-phase were introduced into a sealed vessel containing the
liquid solvent and placed in equilibrium with its vapor for
48 h.
Interestingly, the a-phase does not exhibit guest-exchange
behavior in contact with vapor of “nonpolar” solvents, such
as n-hexane, dichloromethane, chloroform, benzene, or tolu-
ene. Similar results were obtained by dipping crystals of a-
phase directly into liquid solutions of n-hexane, chloroform,
toluene, and carbon tetrachloride for 48 h, which do not
show any sign of sorption. This lack of affinity for nonpolar
solvents is most likely due to the highly polar environment
character of the channels in the a-phase, and agrees with its
solvent-free structure. Accordingly, exposure to solvent
vapors of polar organic solvents with hydrogen acceptor
and/or donor functionalities, such as ethanol (EtOH), ben-
zoic alcohol (Bz-CH2-OH), n-decanol (C10H22O), tetrahy-
drofuran (THF), and dimethyl sulfoxide (DMSO) leads to
the formation of different adducts or inclusion compounds
Figure 3. a) Stepwise growth of the two-dimensional hydrogen-bonded
sheets in a-phase; b) space-filling representation showing the tubular
channels of the a-phase.
of the formula PTMTC·x(guest), as evidenced by combined
G
elemental analysis, IR spectroscopy, and TG studies. In this
case, a direct contact with the liquid was not possible be-
cause the PTMTC radical is very soluble in these solvents at
room temperature. A summary of the results obtained with
different polar solvents is shown in Table 2.
Figure 4. Thermal behavior of the as-synthesized a-phase and (inset)
comparison of the XRPD patterns for the as-synthesized and heated ma-
terials.
As can be seen in Table 2, elemental analysis and FT-IR
spectra of all inclusion compounds or adducts confirmed the
presence of solvent molecules with different host–guest stoi-
chiometries. For instance, the FT-IR spectra of an as-synthe-
sized sample of a-phase and a few selected adducts are
shown in the Supporting Information. Close examination of
the FT-IR spectra shows the characteristic bands of both the
PTMTC host radical and solvent guest bands as indicated
below: 1) EtOH bands at 2979, 1455, and 1377 cmꢀ1 for
PTMTC·EtOH; 2) benzyl alcohol bands at 3030, 1454, 1207,
and 700 cmꢀ1 for PTMTC·0.5BzCH2OH; 3) n-decanol bands
at 2926, 1466, and 1301 cmꢀ1 for PTMTC·0.6C10H22O; 4)
THF bands at 2974, 1462, 1292, 915, and 877 cmꢀ1 for
over, XRPD experiments over such a temperature range,
shown in Figure 4, also showed that positions and intensities
of all lines remain unchanged when compared with the
XRPD pattern of an as-synthesized sample, confirming not
only the lack of guest solvent molecules, but also that the a-
phase has a high structural rigidity because it does not col-
lapse when heated up to 2508C. Finally, a further increase of
the temperature above 2858C led to the decomposition of
the PTMTC, as confirmed by combined XRPD and FT-IR
characterization.
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J. Veciana, D. Ruiz-Molina et al.
Table 2. Results of inclusion studies on crystals of the a-phase of PTMTC and the resulting characterization data of PTMTC adducts.
Adducts
Elemental analysis
Weight loss in TG
IR ]
[cmꢀ1
A
found: C 34.39%, H 0.88%
calcd: C 34.54%, H 1.08%
found: C 36.57%, H 1.01%
calcd: C 36.32%, H 0.83%
found: C 37.82%, H 1.66%
calcd: C 38.06%, H 1.83%
found: C 36.49%, H 0.82%
calcd: C 36.28%, H 1.28%
found: C 33.39%, H 1.04%
calcd: C 33.48%, H 1.61%
found: 5.0% (from 50 to 1758C)
calcd: 5.5%
found: 5.8% (from 50 to 1158C)
calcd: 6.5%
found: 10.8% (from 90 to 1208C)
calcd: 10.7%
found: 9.0% (from 75 to 1308C)
calcd: 8.4%
PTMTC·EtOH
PTMTC·0.5BzCH2OH
2979, 1455, 1377
3030, 1454, 1207, 700
2926, 1466, 1301
PTMTC·0.6C10H22O
PTMTC·THF
2974, 1462, 1292, 915, 877
2996, 1435, 1014, 947, 671
found: 16.8% (from 75 to 2258C)
calcd: 15.4%
PTMTC·2DMSO
PTMTC·THF, and; 5) DMSO bands at 2996, 1435, 1014,
947, and 671 cmꢀ1 for PTMTC·2DMSO. At this point, it is
also worth mentioning that in addition to the presence of
the solvent bands, significant changes in the bands around
1750–1650 cmꢀ1, attributed to the asymmetric and symmetric
C=O stretching vibrations of the carboxylic groups, were ob-
served. For instance, FT-IR spectra of the as-synthesized a-
phase show the two bands of the carboxylic groups at 1722
and 1678 cmꢀ1. However, when solvent guest molecules are
included, the first band increases in intensity and shifts to
higher values (1739–1727 cmꢀ1), whereas the second band
shifts to values around 1660–1693 cmꢀ1. This fact may indi-
cate that the hydrogen-bonded network of the a-phase ex-
periences conformational readjustments with the inclusion
of guest solvent molecules. Such conformational readjust-
ments were confirmed by XRPD experiments. Indeed, the
XRPD pattern for the solvent adducts exhibited different
peaks from those observed for an as-synthesized sample of
a-phase (Figure 5).
Even more interesting was the reversible removal of the
guest solvent molecules and the recovery of the initial crys-
talline structure of the a-phase. Indeed, as can be seen in
Table 2 and in the Supporting Information, thermogravimet-
ric studies performed on the resulting polycrystalline
PTMTC·x(guest) adducts revealed that all of solvent guest
A
molecules can be evacuated from the channels at tempera-
tures not higher than 2508C. Moreover, the crystal structure
of the a-phase is mostly recovered after removal of included
solvent guest molecules as confirmed by: 1) the resulting
material for all the evacuated crystals can be elucidated by
elemental analysis as PTMTC; 2) their XRPD patterns are
in perfect agreement with that of an as-synthesized sample
of guest-free a-phase (see Figure 5); and 3) FT-IR spectra
for evacuated samples reveal the C=O bands characteristic
of a-phase at 1722 and 1667–1668 cmꢀ1 are re-established
(see Supporting Information). The reversibility of this pro-
cess was further confirmed by recycling the evacuated a-
phase for additional solvent inclusion experiments.
Structural readjustments in PTMTC·EtOH adduct: the
new b-phase of PTMTC: To confirm the structural transfor-
mations experienced by the porous network of the a-phase
after inclusion of solvent molecules, we first attempted the
crystal structure resolution of some of the inclusion
Figure 5. XRPD studies of the reversible structural rearrangements expe-
rienced within the hydrogen-bonded network of the a-phase with incor-
porated: a) ethanol, b) benzyl alcohol, and c) THF. In these studies, the
bottom, medium, and top spectra correspond to the initial guest-free as-
synthesized a-phase material, vapor-exposed a-phase, and evacuated a-
phase, respectively.
PTMTC·x(guest) compounds. However, the poor quality of
A
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Modulation by Selective Guest Inclusion
FULL PAPER
the resulting inclusion crystals
prevented us from solving any
of those crystal structures
except in the case of ethanol.
We then focused our attention
on the study of the inclusion of
ethanol molecules by perform-
ing a direct crystallization of an
as-synthesized sample of the a-
phase in ethanol. The idea was
the preparation of a new crys-
talline phase with a structure
similar
to
that
of
the
PTMTC·EtOH adduct. There-
fore, a microcrystalline as-syn-
thesized sample of the a-phase
was first dissolved in ethanol,
and the resulting solution was
slowly evaporated for seven
days to give red prismatic crys-
tals of a new crystalline b-phase
(Table 1). The XRPD pattern
obtained for the PTMTC·EtOH
adduct was then compared with
that obtained for the b-phase.
As can be seen in Figure 6, the
b-phase of PTMTC shows an
Figure 7. Comparison between crystal structures of a- and b-phases, showing the structural rearrangement ex-
perienced by the hydrogen-bonded network of the a-phase with incorporated ethanol solvent molecules.
the repetitive cyclic R6 (24) hexameric motif is divided into
6
two hydrogen-bonded trimers formed by three carboxylic
groups linked through two hydrogen bonds (O1···O4 and
O3···O6) with bond lengths of 2.591 and 2.75 Š, respectively.
Besides the disruption and enlargement of each R6 (24)
6
hexamer, these new trimers are also slightly displaced along
one direction (see Figure 7 top). The disruption of direct hy-
drogen bonds between PTMTC radicals has also an effect
on the long-range hydrogen-bonded assembly of these radi-
cals. Indeed, as shown in Figure 7 bottom, the two-dimen-
Figure 6. Comparison of the simulated XRPD pattern of b-phase
(bottom) and experimental XRPD of PTMTC·EtOH adduct (top).
sional layers of hydrogen-bonded PTMTC radicals in the a-
phase are reduced to one-dimensional chains that run along
the [110] axis. Now, these chains are connected to each
other by solvent molecules to form a two-dimensional hy-
drogen-bonded sheet along a plane with the vectors [110]
and [101].
Magnetic properties: As we have already described for
the monocarboxylic PTMMC radical,[29] hydrogen bonds be-
tween carboxylic groups are capable of transmitting weak
ferromagnetic interactions. However, when these supra-
molecular interactions are broken, ferromagnetic exchange
interactions are also disrupted. As state above, the a-phase
has the ability to incorporate solvent molecules by inducing
large structural changes on its crystal structure. In the case
of the PTMTC·EtOH adduct, the reversible solvent-induced
dynamic structural changes involve the disturbance of some
direct hydrogen bonds between PTMTC radicals and, there-
fore, changes in the magnetic properties should be expected.
These expectations prompted us to study in detail the mag-
identical XRPD pattern to that of a polycrystalline sample
of the inclusion PTMTC·EtOH adduct, and confirms that
both crystalline samples are isostructural. This fact, together
with the chemical shifts observed in the IR spectra for the
band characteristics of the carboxylic groups, led us to be-
lieve that the structural rearrangements originating from the
inclusion of ethanol guest molecules in the a-phase could be
understood by simply comparing both a- and b-phases.
If one compares both crystal structures, the major differ-
ence is the presence of solvent guest molecules inside the
channels that interact through hydrogen bonds with the car-
boxylic groups of PTMTC radicals forming the inner walls
of these channels (Figure 7). From these interactions, direct
hydrogen bonds between PTMTC radicals are broken, and
Chem. Eur. J. 2007, 13, 8153 – 8163
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8159

J. Veciana, D. Ruiz-Molina et al.
netic properties of a-phase and its related adduct to observe
possible solvatomagnetic effects.
The transition to a ferromagnetic ordered state at very
low temperatures was also confirmed by isothermal magnet-
ization curves measured above and below the critical tem-
perature (Figure 9). Whereas at 1.35 K, the a-phase remains
Magnetic properties of a-phase: Temperature dependence
of magnetic susceptibility of a polycrystalline sample of a-
phase was measured from 300 to 1.8 K in a SQUID magne-
tometer (Figure 8a). The temperature was further decreased
to 70 mK by using a dilution cryostat (Figure 8b).
Figure 9. a) Isothermal field dependence of the magnetization for a-
*
*
phase at 1.35 K ( ) and 80 mK ( ); b) the hysteresis cycle at 80 mK.
in the paramagnetic region and therefore the magnetization
curve has a slight gradient, the curve at 80 mK, even though
it is very close to the critical temperature, traces a hysteresis
loop characteristic of a soft ferromagnet, with a sharp in-
crease of the magnetization with the applied magnetic field
that is almost saturated at about 400 Oe. The hysteresis loop
for the a-phase at 80 mK has a coercive field of the order of
50 Oe, and a remnant magnetization at zero field of about
35% of the saturation value.
Figure 8. a) Temperature dependence of c·T product for a polycrystalline
sample of the a-phase in the range of 0.1 to 300 K, measured under an
applied magnetic field of 1000 Oe; b) temperature dependence of the c·T
product at very low temperatures, measured under different applied mag-
~
*
netic fields: H=200 Oe ( ), 500 Oe (&), 1000 Oe ( ).
As can be seen in Figure 8a, the magnetization of this hy-
drogen-bonded network of the a-phase follows a typical
Magnetic properties of PTMTC·EtOH adduct: The tem-
perature dependence of the magnetic susceptibility of a
polycrystalline sample of PTMTC·EtOH is shown in
Figure 10. As for the a-phase, this new crystalline phase
shows paramagnetic behavior in the temperature range from
50 to 300 K, with a c·T product value of 0.38 emuKmolꢀ1 at
300 K. However, on decreasing the temperature, the c·T
value slightly decreases according to the presence of very
weak antiferromagnetic interactions. Experimental magnetic
data was fit to the Curie–Weiss law with C=0.38 emuK
molꢀ1 and a Weiss constant of q=ꢀ0.4 K. As expected,
magnetic measurements performed on the isostructural b-
phase showed identical predominant antiferromagnetic in-
teractions.
Curie–Weiss behavior in the range from 300 to 10 K. In fact,
C
ðTꢀqÞꢁ
the data was successfully fitted by using the law cM =
Nb2g2
with C=0.38 emuKmolꢀ1 and a Weiss constant of q=
2kBðTꢀq
+0.2 K. The c·T product had a value of 0.38 emuKmolꢀ1 at
300 K, as expected for uncorrelated S=1/2 spins, which also
confirms the purity of this sample. Below 10 K, magnetic
measurements performed with the a-phase show that cT
value increases on decreasing the temperature up to a maxi-
mum around 110 mK, followed by a plateau. This peak indi-
cates the long-range ordering of the magnetic moments.[40]
Moreover, the intensity of the peak decreases on increasing
the external applied magnetic field as shown in Figure 8b.
For instance, for an applied magnetic field of 200 Oe a value
of 2.2 emuKmolꢀ1 was obtained, whereas for an external
field of 500 Oe the value reduced to 1.4 emuKmolꢀ1; thus,
obeying a saturation of the magnetization on increasing the
applied magnetic field.
It should be noted that interruption of the hydrogen-
bonded network of the a-phase, through the inclusion of
guest solvent molecules, inverts the magnetic properties.
The predominant ferromagnetic interactions found for the
a-phase have been transformed into very weak antiferro-
8160
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Chem. Eur. J. 2007, 13, 8153 – 8163

Modulation by Selective Guest Inclusion
FULL PAPER
netic interactions between the unpaired spins of the radicals.
At very low temperatures, the PTMTC a-phase behaves as
a soft magnet showing long-range magnetic ordering.
On the basis of its supramolecular character and high
thermal stability, the hydrogen-bonded open framework of
the a-phase allows organic solvents with hydrogen-acceptor
and/or -donor functionalities to be included that interact
with the PTMTC radicals and, thereby, modify the overall
magnetic properties. As a result, we observed a selective in-
clusion of polar organic solvents in the highly polar tubular
channels, which induces structural rearrangements in the hy-
drogen-bonded network. The crystal transformations oc-
curred so that solvent guest molecules located at the chan-
nels were hydrogen-bonded to the carboxylic groups of the
PTMTC radicals, disrupting some direct hydrogen bonds be-
tween radicals. As expected, the interruption of direct bonds
between PTMTC radicals induces large changes in the mag-
netic properties of the solid, yielding dominant ferromagnet-
ic interactions in the a-phase and predominant antiferro-
magnetic interactions in the corresponding adducts. The in-
vestigation of this solvatomagnetic effect is particularly rele-
vant in the context of the newly emerging field of multifunc-
tional porous materials.
Figure 10. Comparison of the temperature dependence of c·T product of
*
)
a
polycrystalline sample of porous a-phase
(
and PTMTC·EtOH
*
adduct ( ) in the range of 1.8 to 150 K, measured with an applied mag-
netic field of 1000 Oe.
magnetic interactions for PTMTC·EtOH. Although the
exact nature of these antiferromagnetic interactions has not
been established, it is clear that the disruption of R6 (24)
6
hexameric motifs by breaking two direct hydrogen bonds
between PTMTC radicals prevents the propagation of ferro-
magnetic interactions along the planes, leading to the obser-
vation of predominant antiferromagnetic interactions.
The larger size compared to ethanol molecules and the
ability to form hydrogen bonds of all other studied solvents
prompted us to consider that inclusion of these molecules in
the a-phase may also induce the disruption of direct hydro-
gen bonds between PTMTC radicals and, therefore, a simi-
lar inversion on their magnetic properties. These expecta-
tions were confirmed by studying the magnetic properties of
Experimental Section
All solvents were reagent grade from SDS and were used as received and
distilled unless otherwise indicated. All reagents, organic and inorganic,
were of high-purity grade and were obtained from E. Merck, Fluka
Chemie, and Aldrich Chemical Co. Thin-layer chromatography (TLC)
was performed on aluminum plates coated with Merck Silica gel 60. Ele-
mental analyses were obtained from the Servei de Anàlisi de la Universi-
tat Autònoma de Barcelona. The 1H NMR spectra were recorded on a
Bruker ARX 250 spectrometer. FT-IR spectra were performed on a
Perkin–Elmer Spectrum One spectrometer. Thermogravimetric analyses
where performed under argon with a Perkin–Elmer TGA7 balance. X-
ray powder diffraction (XRPD) experiments were recorded by using a
diffractometer (INEL CPS-120) with Debye–Scherrer geometry.
two
of
these
adducts;
the
PTMTC·THF and
PTMTC·2DMSO adducts. Magnetic measurements per-
formed on both adducts showed them to exhibit very similar
magnetic properties, with essentially paramagnetic behavior
in the whole temperature range and the presence of very
weak antiferromagnetic interactions at very low tempera-
tures. Again, experimental magnetic data of both adducts
were fit to the Curie–Weiss law with C=0.374 and
0.380 emuKmolꢀ1 and a Weiss constant of q=ꢀ0.3 and
ꢀ0.1 K, respectively.
aH-(octadecachloro-4,4’,4’’-trimethyltriphenyl)methane (2): A mixture of
tris(2,3,5,6-tetrachlorophenyl)methane (1) (1.70 g, 2.58 mmol), chloro-
form (30 mL), and aluminum chloride (0.40 g, 3.00 mmol) was heated at
1608C for 8 h in a glass pressure vessel. The mixture was then poured on
to ice/1 n hydrochloric acid (50 mL) and was extracted with chloroform
(2”50 mL). The organic mixture was washed with aqueous sodium hy-
drogen carbonate (100 mL) and water (100 mL). Immediately, the organ-
ic layer was filtered and the collected solid was washed with water and
Conclusion
pentane to give
2
(2.22 g, 95% yield) as white powder. 1H NMR
(250 MHz, CDCl3): d=7.1 (s, 1H; CH), 7.7 ppm (s, 3H; CHCl2); IR
(KBr): n˜ =3040, 1540, 1390, 1368, 1350, 1303, 1271, 1139, 775, 713, 692,
485 cmꢀ1; elemental analysis calcd (%) for C22H4Cl18: C 29.15, H 0.44;
found: C 29.34, H 0.28.
This work demonstrates that a network based only on car-
boxylic–carboxylic hydrogen bonds can behave as a reversi-
ble dynamic open framework capable of showing solvato-
magnetic effects. Thus, the proper self-assembly of stable
polycarboxylic organic radicals can generate magnetic
porous molecular materials with a purely organic character
in which the carboxylic groups interact through hydrogen
aH-(dodedecachlorotriphenyl)methane-4,4’,4’’-tricarboxylic acid (3):
A
mixture of 2 (0.55 g, 0.61 mmol) and 20% oleum (100 mL) was heated at
1508C for 12 h. The final deep blue solution was cooled and poured onto
crushed ice, and the collected solid was washed with water, dissolved in
diethyl ether, and extracted with aqueous sodium hydrogen carbonate.
The aqueous layer was acidified and extracted with diethyl ether. The ex-
tracted solid was recrystallized from Et2O/n-pentane to give triacid 3
(0.44 g, 92% yield) as a white powder. 1H NMR (250 MHz, CDCl3): d=
4.2 (broad band, COOH), 7.1 ppm (s, 1H; CH); IR (KBr): n˜ =3500–
2500, 1721, 1556, 1331, 1300, 1274, 824, 795, 715, 664, 571, 472 cmꢀ1; ele-
bonds generating a novel cyclic hexameric R6 (24) motif.
6
The packing of such entities in the a-phase defines the walls
of the robust accessible tubular channels of 5 Š in diameter
and serves as the pathway for the transmission of ferromag-
Chem. Eur. J. 2007, 13, 8153 – 8163
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8161

J. Veciana, D. Ruiz-Molina et al.
mental analysis calcd (%) for C22H4O6Cl12: C 33.46, H 0.51; found: C
33.13, H 0.63.
grateful to the Ministerio de Educación y Ciencia (Spain) for the predoc-
toral grant No. AP2000–1842 of the FPU program. We thank Dr. Xavier
AlcobØ of the Unitat de Difracció de Raigs-X of Servei Científico-T›cnic
of Universitat de Barcelona for X-ray powder diffraction measurements.
(Dodecachloro-4,4’,4’’-tricarboxytriphenyl)methyl radical (PTMTC):
A
mixture of triacid 3 (0.25 g, 0.317 mmol) and powdered NaOH (0.7 g)
was shaken with DMSO (30 mL) for 72 h in the dark. The mixture was
filtered and iodine (0.093 g, 0.36 mmol) was added immediately into the
resulting solution. The solution was left undisturbed in the dark for
30 min, after which it was washed with an aqueous solution of sodium hy-
drogen sulfite and then treated with Et2O. Radical PTMTC was extracted
with aqueous sodium hydrogen carbonate, and the aqueous layer was
acidified and extracted with Et2O. The solid extracted was recrystallized
from Et2O/n-pentane to give radical PTMTC (0.21 g, 85% yield) as red
powder. IR (KBr): n˜ =3500–2500, 1740, 1694, 1662, 1537, 1441, 1408,
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Acknowledgements
This work was supported by Dirección General de Investigación (Spain),
under project CONSOLIDER-C EMOCIONa (contract CTQ2006–
06333/BQU) and Generalitat de Catalunya (2005 SGR-00591). We also
thank the European Commission for the Marie Curie Research Training
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and the NoE MAGMANet (contract NMP3-CT-2005–515767) and also
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Modulation by Selective Guest Inclusion
FULL PAPER
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Received: March 2, 2007
Published online: July 16, 2007
Chem. Eur. J. 2007, 13, 8153 – 8163
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