Hydrogen-bonded self-assemblies in a polychlorotriphenylmethyl radical derivative substituted with six meta-carboxylic acid groups

Article abstract of DOI:10.1039/b508952b

The synthesis, self assembly and magnetic properties of a polychlorotriphenylmethyl radical, substituted with six meta-carboxylic groups, are reported showing that radical-radical hydrogen bonds in the solid state yield to very weak intermolecular ferroma

Full text of DOI:10.1039/b508952b

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Hydrogen-bonded self-assemblies in a polychlorotriphenylmethyl
radical derivative substituted with six meta-carboxylic acid groups{
Nans Roques,a Daniel Maspoch,a Neus Domingo,b Daniel Ruiz-Molina,a Klaus Wurst,c Javier Tejada,b
Concepcio´ Roviraa and Jaume Veciana*a
Received (in Cambridge, UK) 24th June 2005, Accepted 1st August 2005
First published as an Advance Article on the web 26th August 2005
DOI: 10.1039/b508952b
remarkable structural and magnetic properties of POROF2, and
with the idea of continuing to increase further both structural and
magnetic dimensionalities, we report herein for the first time, the
synthesis, X-ray structures and magnetic properties of a new PTM
derivative (PTMHC), that is one of the rare examples of an
organic hexacarboxylic acid ever described, and to the best of
our knowledge, the first example with electronic open-shell
characteristics.6 Compared to PTMDC and PTMTC, the increase
of the carboxylic/chlorine substituent ratio and the change in the
substitution pattern (meta vs. para) make this molecule a promising
candidate for obtaining hydrogen-bonded purely organic magnetic
material.
The synthesis, self assembly and magnetic properties of a
polychlorotriphenylmethyl radical, substituted with six meta-
carboxylic groups, are reported showing that radical–radical
hydrogen bonds in the solid state yield to very weak
intermolecular ferromagnetic interactions.
Over the last few years, there has been a considerable interest in the
design of purely organic magnetic materials. In order to fulfil one
of the major requirements to obtain magnetic properties, that is to
control the structural arrangement of the organic spin-bearing
units in the solid state, crystal engineering through hydrogen bonds
has been proved to be a powerful methodology.1 Materials
designed following this approach are mainly based on the use of
stable nitroxide radical derivatives that are able to act as hydrogen
bond acceptors. Very interesting results have been obtained when
one or several hydrogen-bond donor groups were introduced
directly on the spin bearing molecule.2 Among them, carboxylic
groups are particularly appealing since they possess both
hydrogen-bond donor and acceptor character, duality that allows
them to add to the expected radical–acid interaction the possibility
to form various supramolecular motifs.3 In a recent article, we
have shown that the formation of acid dimers was responsible of
the presence of hydrogen-bond mediated weak ferromagnetic
interactions between two polychlorotriphenylmethyl radicals sub-
stituted in para-position by a carboxylic group (PTMA).4 In order
to extend the structural and the magnetic dimensionality in such
PTM-based materials, we have designed and studied the self
assembly of PTM radicals substituted with two (PTMDC) and
three (PTMTC) para-carboxylic acid groups. In both cases,
carboxylic groups are involved in the formation of hydrogen-
bonded rings yielding two dimensional hydrogen-bonded layers.
Weak antiferromagnetic interactions are observed in the resulting
open-framework of PTMDC, while the self assembly of PTMTC
molecules led to the first example of a nanoporous purely organic
magnetic material (POROF2), exhibiting a high thermal stability
and a long range ferromagnetic ordering.5 Keeping in mind the
PTMHC was prepared following an original six step procedure,
starting from compound 1 (Scheme 1). Dichloromethyl groups
were introduced by reacting 1 with chloroform following a
Friedel–Craft reaction using AlCl3 as a catalyst (70%). These
groups were converted to carboxylic ones by a hydrolysis–
oxidation reaction using 20% oleum (75%). Treatment with
diazomethane yielded the hexamethylester 4 (80%), that was then
converted to the corresponding hexamethylester radical 5 (80%) in
a one pot reaction using subsequently tetrabutylammonium
hydroxide, as a strong base, and para-chloranil, as oxidant reagent
that converts the carbanion to radical. Removal of the methyl ester
groups was finally performed with concentrated sulfuric acid, a
methodology that allows PTMHC to be obtained in an excellent
yield (90%).
Yellow prismatic single-crystals, suitable for X-ray diffraction,
of the solvate [PTMHC?(THF)6] were grown from a mixture of
THF and n-hexane. The radical crystallizes in a trigonal R-3 space
group with six molecules of PTMHC and thirtysix molecules of
aInstitut de Cie`ncia de Materials de Barcelona (CSIC), Campus
Universitari, 08193, Bellaterra, Catalonia, Spain.
E-mail: vecianaj@icmab.es; Fax: 34 93 580 5729; Tel: 34 93 580 1853
bFacultat de Fisica, Universitat de Barcelona, Diagonal 647, 08208,
Barcelona, Spain
cInstitut fu¨r Allgemeine Anorganische und Theoretische Chemie,
Universita¨t Innsbruck, A-6020, Innrain 52a, Austria
{ Electronic supplementary information (ESI) available: crystallographic
data of [PTMHC?(THF)6] and [PTMHC?(Et2O)3] solvates and crystal
structure of diamagnetic [3?(THF)3] solvate (PDF, CIF). See http://
dx.doi.org/10.1039/b508952b
Scheme 1
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THF packed in the unit cell showing a structural disorder.{ The
high molecular C3 symmetry is reflected by the presence of a
crystallographic three-fold rotation axis that passes through the
central carbon atom (C3) of the molecule that adopts a three-
bladed propeller-like conformation. Thus, the three polychlori-
nated aromatic rings are symmetry related. Owing to the steric
hindrance of the chlorine atoms in ortho positions with respect the
carboxylic groups, these groups are twisted by 84u and 87u with
respect to the plane of the phenyl group to which they are bonded,
with the two acidic hydrogen atoms lying above and below the
plane of the plane of the phenyl ring, respectively (Fig. 1).
As occurs with other polycarboxylic acid derivatives,7 the
PTMHC radical forms hydrogen bonds with disordered THF
molecules. However, the [PTMHC?(THF)6] solvate presents the
particularity that all its six carboxylic acid groups are involved in
the formation of hydrogen bonds with one THF. Consequently,
the resulting ‘‘supramolecular radical–THF clusters’’ (Fig. 2a)
avoid direct hydrogen bonds between radical molecules pushing
avoid the immediate loss of crystallinity that is observed when the
crystals are removed from the mother liquor, as confirmed by
XRPD experiments.
In order to increase the structural dimensionality and to
disfavour the formation of hydrogen bonds between PTMHC
and solvent molecules, several attempts of crystallisation were
performed using various solvent mixtures and crystallisation
techniques. All these tries failed, mainly because of the lack of
solubility of the highly polar radical PTMHC in non oxygenated
solvents. We finally succeed in obtaining
a new solvate,
[PTMHC?(Et2O)3], as orange prismatic crystals, using a mixture
of diethyl ether and n-hexane. Like in the previous solvate, the
radical crystallizes in a trigonal R-3 space group with six molecules
of PTMHC and eighteen molecules of solvent in the unit cell.§ This
solvate also exhibits a three-fold rotation axis passing through the
central carbon atom of the radical molecule with dihedral angles
between the phenyl rings and the carboxylic groups of 83u and 85u,
respectively.
…
them far away. Thus, the shortest C3 C3 distance, where the
˚
most part of the spin density is localized, is of 7.51A. Close packing
The most striking difference of the [PTMHC?(Et2O)3] solvate
with respect to that with THF is the presence of several direct
hydrogen bonds between PTMHC molecules that results in the
formation of two-dimensional hydrogen bonded corrugated layers
in the ab plane. The main repeating unit consists of an unusual
of the such supramolecular radical–THF clusters take place
3
…
through several weak Csp –H O hydrogen bonds between
neighboring THF molecules leading to a PTMHC-templated
honeycomb arrangement of the solvent molecules (Fig. 2b).8
Interactions between solvent molecules are not strong enough to
6
R6 (48) hydrogen-bonded motif formed by six PTMHC molecules
with alternating plus and minus helicities in their three-bladed
propeller-like conformations (Fig. 3a). In this motif, each radical is
hydrogen bonded to two neighbouring radicals through one
…
˚
carboxylic group, with a bond length of 1.86A (O1 H–O4) and a
bond angle of 164u (O1–H–O4). The other carboxylic group
localized in the same phenyl ring is involved in one hydrogen bond
˚
with a diethyl ether molecule, with a bond length of 1.83A
…
(O5 H–O2) and a bond angle of 153u (O5–H–02).
Since every radical unit contains six carboxylic groups (three are
involved in hydrogen bonds with PTMHC molecules and three in
hydrogen bonds with solvent molecules) each radical molecule
participates in the construction of three identical hexameric units
resulting in open polar windows that propagate along the ab plane
(Fig. 3b). Three diethyl ether molecules are localized on each side
of each polar window contributing to isolate one layer from the
Fig. 1 ORTEP view of the [PTMHC?(THF)6] solvate (thermal ellipsoids
set at 20% of probability; THF molecules are omitted for clarity).
…
two neighbouring ones (the shortest C3 C3 interlayer distance is
6
Fig. 3 Crystal structure of solvate [PTMHC?(Et2O)3]. a) R6 (48)
2
hexamer, surrounded by six secondary R2 (24) dimers, and b) Two
Fig. 2 Crystal structure of the solvate [PTMHC?(THF)6]. a) View of the
supramolecular PTMHC?(THF)6 cluster. View, along the c axis of
PTMHC-templated honeycomb arrangement of solvent THF molecules
(THF molecules are represented by orange spheres for clarity).
dimensional hydrogen-bonded layer, with Et2O molecules lying above and
below the polar windows originated by the free carboxylic acid groups
(Et2O molecules are represented as blue spheres for clarity).
4802 | Chem. Commun., 2005, 4801–4803
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ferromagnetic ordering at lower temperatures in the order of few
millikelvin. Further work to confirm this behaviour is undergoing
together with new crystallisation tries in order to increase both
structural and magnetic dimensionalities.
This work has been supported by the EU under a Marie Curie
Research Training Network (contract ‘‘QuEMolNa’’ number
MRTN-CT-2003-504880) and by DGI (Spain) under project
MAT2003-04699.
Notes and references
{ Crystal data for a-PTMHC [(C25H6Cl9O12?6(C4H8O)]: trigonal, R-3,
˚
a 5 24.9133(6), b 5 24.9133(8), c 5 15.0874(8) A, a 5 b 5 90, c 5 120u,
3
Fig. 4 Magnetic characterization of solvates [PTMHC?(THF)6] (%) and
[PTMHC?(Et2O)3] (#) in the presence of their mother liquors. Product of
the magnetic susceptibility with the temperature as a function of the
temperature in the lower limit.
˚
˚
V 5 8109.7(5) A , Z 5 6, l(Mo–Ka) 5 0.71073 A, 13298 reflections
collected, 1761 independent reflections with I . 2s(I), R1 5 0.0678,
wR2 5 0.1884, GOF 5 1.044. Both kind of THF-molecule present
disorder. For C10–C13–O5, the disorder was not solved. For C14–C17–06
a 1 : 1 disorder model with partial overlying position (C15 5 C15A and
C17 5 C17A) and bond restraints was used. Hydrogen atoms of both
THF-molecules were neither found nor calculated. CCDC 276959. See
http://dx.doi.org/10.1039/b508952b for crystallographic data in CIF or
other electronic format.
˚
6.89A). Interdigitation of diethyl ether molecules, promoted by
3
…
˚
weak PTMHC–Et2O O1 H–Csp hydrogen bonds (2.71A, 166u),
belonging to neighbouring layers avoid interlayer chlorine–chlorine
short contacts and/or hydrogen bonding between PTMHC
molecules, leading to an ABC arrangement of the layers along
the c axis (Fig. 3c). As in the first THF solvate, crystallinity is
immediately lost when the crystals are removed from their mother
liquor.
§ Crystal data for b-PTMHC [(C25H6Cl9O12?3(C4H10O)]: trigonal, R-3,
˚
a 5 15.6488(8), b 5 15.6448(6), c 5 33.8402(7) A, a 5 b 5 90, c 5 120u,
3
˚
˚
V 5 7.1767(5) A , Z 5 6, l(Mo–Ka) 5 0.71073 A, 12097 reflections
collected, 1880 independent reflections with I . 2s (I), R1 5 0.0588,
wR2 5 0.1624, GOF 5 1.037. Data were measured on a Nonius
KappaCCD. Structure solutions were done using SHELXS86 and
refinements with SHELXL97. CCDC 276958. See http://dx.doi.org/
10.1039/b508952b for crystallographic data in CIF or other electronic
format.
Variable temperature magnetic susceptibility data for as-
synthesized crystalline samples of both solvates were obtained on
a SQUID susceptometer, under a temperature range of 2–300 K,
in the presence of their mother liquors. In both cases, a
paramagnetic behavior was observed in the 20–300 K temperature
range, with x.T product values that fully agree with the
theoretically value of 0.375 emu.K.mol21, expected for uncorre-
lated spins (S 5 K) with g 5 2.0. Main differences were observed
below 20 K. For [PTMHC?(THF)6], the x.T value smoothly
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decreases upon decreasing temperature until
a value of
0.358 emu.K.mol21 at 2 K (Fig. 4)., consistenly with the presence
of very weak intermolecular antiferromagnetic interactions
(through space), in agreement with the lack of direct hydrogen
bonds between PTMHC radicals in that solvate. For
[PTMHC?(Et2O)3], the x.T value smoothly increases upon
decreasing temperature to reach a value of 0.395 emu.K.mol21
at 1.8 K (Fig. 4). This result is in agreement with the presence of
very weak intermolecular ferromagnetic interactions, which are
ascribed to the presence of direct hydrogen bonds between radical
molecules in this crystal structure.9
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In summary, we have described the synthesis of a new PTM
radical derivative that is the first example of hexacarboxylic acid
with an electronic open-shell characteristic. Depending on the
solvent used, the self assembly of PTMHC molecules yield to
isolated radicals surrounded by THF molecules or to radical
hydrogen-bonded layers which are isolated by diethyl ether layers.
While weak antiferromagnetic interactions are obtained in the first
case, the presence of hydrogen bonds in the second one yields very
weak intermolecular ferromagnetic interactions in the solid state.
Taking into account the structural resemblance between the
[PTMHC?(Et2O)3] solvate and POROF-2,4b we can also expect a
8 A structure presenting comparable cell parameters and without disorder
for THF molecules has been obtained for compound 3 ([3?(THF)6]. See
Electronic Supplementary Information.
9 A similar behaviour was observed for crystalline a and b-phases of
PTMA. See Ref. 4.
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