A Robust Nanocontainer Based on a Pure Organic Free Radical

Article abstract of DOI:10.1021/ja038988v

A new pure organic paramagnetic porous molecular material, which combines hydrophilic windows with large hydrophobic nanocontainers, is shown to be stable even in absence of solvent guest molecules. Copyright

Full text of DOI:10.1021/ja038988v

Published on Web 01/03/2004
A Robust Nanocontainer Based on a Pure Organic Free Radical
Daniel Maspoch,† Neus Domingo,‡ Daniel Ruiz-Molina,† Klaus Wurst,§ Javier Tejada,‡
Concepcio´ Rovira,† and Jaume Veciana*,†
Institut de Cie`ncia dels Materials de Barcelona (CSIC), Campus UAB, 08193, Cerdanyola, Catalonia, Spain,
Facultat de F´ısica, UniVersitat de Barcelona, Diagonal 647, 08028-Barcelona, Spain, and Institut fu¨r Allgemeine,
Anorganische und Theoretische Chemie, UniVersita¨t Innsbruck, Innrain 52a, A-6020, Innsbruck, Austria
Received October 10, 2003; E-mail: vecianaj@icmab.es
Scheme 1
Proper supramolecular self-assembly of organic molecules in the
solid state is one of the most important challenges for the devel-
opment of multifunctional molecular materials.1 Among them, re-
cently much more effort has been focalized toward the obtaining
of magnetic nanoporous materials. Indeed, thus far several metal-
organic open-framework structures that combine the inherent char-
acteristics of nanoporous materials and the occurrence of magnetic
exchange interactions between transition metal ions have been ob-
tained.2,3 However, even though several examples of pure organic
nanoporous systems have already been described,4,5 to the best of
our knowledge, none of them exhibit magnetic properties due to
the lack of transition-metal ions and the diamagnetic character of
the organic molecular bricks used thus far. Herein, we present the
first example of a purely organic magnetic and robust nanoporous
two complementary hydrogen bonds between two carboxylic groups
[R2 (8)], with a bond distance of 1.865 Å and bond angle of
2
lattice based on the supramolecular arrangement of an open-shell
163.2°.11 In this way, each hexamer is linked with six more identical
molecule, the dicarboxylic perchlorinated triphenylmethyl (PTM)
units in an hexagonal topology, extending the infinitely hydrogen-
radical 1. The use of this free radical as a supramolecular synthon
bonded net along the ab plane (Figure 1). Further supramolecular
has several advantages:6 (1) radical 1 exhibits a high thermal and
interactions, in particular four H-bonds between chlorine atoms and
chemical stability, (2) its trigonal symmetry provides a typical tem-
carboxylic groups and six Cl‚‚‚Cl contacts per molecule,12 contribute
plate for getting channels held together by hydrogen bonds through
to an additional stabilization of the hydrogen-bonded network.
the two carboxylic groups, (3) the molecular bulkiness and rigidity
A view down the c axis of the crystal structure of POROF-1
of PTM radicals is expected to prevent close packing of molecular
reveals a stacking of laterally shifted layers held together through
units, and (4) besides their structural control, hydrogen bonds have
several Cl‚‚‚Cl contacts (Figure 2), yielding an ABCABC arrange-
ment. The packing of these pillared sheets leads a pure organic
open-framework structure with one-dimensional nanochannels
also been shown to favor magnetic exchange interactions between
bound organic radical molecules.7 Crystallization of the dicarboxylic
radical 18 generates the pure organic radical open-framework
formed by narrowed polar windows and larger hydrophobic cavities.
(POROF-1) material, combining large hydrophobic nanocavities
Indeed, as illustrated in Figure 2, these channels contain large
hydrophobic nanocavities where a sphere 10 Å in diameter can fit
inside them and smaller windows with a highly hydrophilic
environment due to the presence of the carboxylic groups at the
with hydrophilic windows along with magnetic characteristics.
Radical 1 was prepared following a new three-step procedure,
starting from compound 2.9 Dichloromethyl groups were introduced
by reacting 2 with chloroform following a Friedel-Crafts reaction
inner rims. The diameter of such hydrophilic windows is 5 Å,
using AlCl3 as catalyst (78%) and subsequently were converted to
considering van der Waals radius. The unusual combination of
carboxylic groups by a hydrolysis-oxidation reaction using oleum
connected nanocavities and windows gives way to solvent-
20% (31%). Finally, the subsequent treatment of the hydrocarbon
accessible voids in the crystal structure that amount to 31% (5031
Å3 per unit cell) of the total volume (16 158 Å3).13
To evaluate the structural robustness of POROF-1, we have
precursor 4 with excess of NaOH, I2, and HCl gave radical 1 (90%).
Single crystals of POROF-1, suitable for X-ray diffraction, were
obtained by a diffusion of n-hexane into a solution of 1 in dichlo-
examined, by a combination of thermal gravimetric analysis and
romethane.9
X-ray powder diffraction (XRPD) experiments, an as-synthesized
crystalline sample of POROF-1.14 In the as-synthesized material,
Radical 1 crystallizes in a trigonal R-3 space group with 18
molecules of radical 1 packed in the unit cell. Molecular arrange-
the large hydrophobic cavities are occupied by six n-hexane solvate
ment of 1 creates a primary structure consisting of two-dimensional
molecules (one molecule of n-hexane per one molecule of radical
hydrogen-bonded sheets along the ab plane, where each molecule
1). Accordingly, when an as-synthesized sample of POROF-1 was
heated to 100 °C, a weight loss of 9%, ascribed to the complete
loss of all guest n-hexanes molecules, was observed. Such a loss
was confirmed by complementary elemental analysis. Beyond this
temperature, thermal analysis does not exhibit any significant
change up to 275 °C, whereupon an abrupt weight loss attributed
to the decomposition of 1 was observed. Simultaneously, XRPD
studies confirmed that POROF-1 remains crystalline and stable up
of 1 participates in the construction of two different hydrogen-
bonded motifs. The repetitive unit consists of an unusual hexamer
6
of radicals hydrogen-bonded through one carboxylic group [R6
(24)], with bond distances of 1.837 Å and bond angles of 168.5°
(Figure 1). The second carboxylic group of each radical acts as a
connecting element between hexameric units by the formation of
† Institut de Cie`ncia dels Materials de Barcelona.
‡ Universitat de Barcelona.
§ Universita¨t Innsbruck.
to 275 °C. Indeed, the XRPD pattern of a sample that was heated
9
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J. AM. CHEM. SOC. 2004, 126, 730-731
10.1021/ja038988v CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
of polycarboxylic organic radicals as building blocks for obtaining
pure organic magnetic nanoporous molecular materials are currently
underway.
Acknowledgment. We thank the Programa Nacional de Ma-
teriales (MAT2003-04699) and DGR, Catalunya (2001SGR00362).
D.M. thanks the Generalitat de Catalunya for a predoctoral grant.
Supporting Information Available: Crystallographic data, thermal
gravimetric analysis, and XRPD patterns and magnetic properties of
POROF-1 (PDF, CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
References
(1) (a) Supramolecular Chemistry, Lehn, J. M., Ed., VCH: Weinheim,
Germany, 1995. (b) Aakero¨y, C. B.; Beatty, A. M.; Helfrich, B. A. Angew.
Chem., Int. Ed. 2001, 40, 3240.
(2) For recent examples on metal-organic open frameworks, see: (a) Chen,
B.; Eddaoudi, M.; Hyde, S. T.; O’Keefe, M.; Yaghi, O. M. Science 2001,
291, 1021. (b) Noro, S. I.; Kitagawa, S.; Kondo, M.; Seki, K. Angew.
Chem., Int. Ed. 2000, 12, 2081.
(3) For recent examples on magnetic metal-organic open frameworks, see:
(a) Maspoch, D.; Ruiz-Molina, D.; Domingo, N.; Wurst, K.; Cavallini,
M.; Biscarini, F.; Tejada, J.; Rovira, C.; Veciana, J. Nat. Mater. 2003, 2,
195. (b) Beauvais, L. G.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 12096.
(c) Barthelet, K.; Marrot, J.; Riou, D.; Fe´rey, G. Angew. Chem., Int. Ed.
2002, 41, 281. (d) Moulton, B.; Lu, J.; Hajndl, R.; Hariharan, S.;
Zaworotko, M. J. Angew. Chem., Int. Ed. 2002, 41, 2821. (e) Price, D. J.;
Tripp, S.; Powell, A. K.; Wood, P. T. Chem.-Eur. J. 2001, 7, 200. (f)
Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams,
I. D. Science 1999, 283, 1148.
Figure 1. Crystal structure of POROF-1. Two-dimensional hydrogen-
bonded layer. Within one layer, the repetitive R6 (24) H-bonded hexamer
originates a polar window due to the presence of six carboxylic groups,
whereas linking of each hexamer with six more identical units in an
hexagonal topology originates six trigonal-shaped hydrophobic voids.
6
(4) For recent reviews on organic nanoporous open-framework materials,
see: (a) Nangia, A. Curr. Opin. Solid State Mater. Sci. 2001, 5, 115. (b)
Zaworotko, M. J. Angew. Chem., Int. Ed. 2000, 39, 3052. (c) Langley, P.
J.; Hulliger, J. Chem. Soc. ReV. 1999, 28, 279. (d) Desiraju, G. R. Curr.
Opin. Solid State Mater. Sci. 1997, 2, 451.
(5) For recent examples on organic open-framework materials, see: (a)
Miyahara, Y.; Abe, K.; Inazu, T. Angew. Chem., Int. Ed. 2002, 41, 3020.
(b) Bong, D. T.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001, 40, 2163.
(c) Sada, K.; Sugahara, M.; Kato, K.; Miyata, M. J. Am. Chem. Soc. 2001,
123, 4386. (d) Mu¨ller, T.; Hulliger, J.; Seichter, W.; Weber, E.; Weber,
T.; Wu¨bbenhorst, M. Chem.-Eur. J. 2000, 6, 54. (e) Kobayashi, K.;
Shirasaka, T.; Sato, A.; Horst, E.; Furukawa, N. Angew. Chem., Int. Ed.
1999, 38, 3483. (f) Biradha, K.; Dennis, D.; MacKinnon, V. A.; Sharma,
C. V. K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 11894. (g)
Russel, V. C.; Evans, C. C.; Li, W.; Ward, M. D. Science 1997, 276, 575.
(6) For recent reviews on magnetism of organic materials, see: (a) Magnetic
Properties of Organic Materials, Lahti, P. M., Ed.; Marcel Dekker: New
York, 1999. (b) Veciana, J.; Iwamura, H. MRS Bull. 2000, 25, 41. (c)
Magnetism: Molecules to Materials; Miller, J. S.; Drillon, M., Eds.; Wiley-
VCS: Weinheim, Germany, 2001; Vols 1-3.
Figure 2. Space-filling view along the b axis of the large nanocontainers
formed along the one-dimensional channel.
(7) Domingo, V. M.; Castan˜er, J.; Riera, J.; Labarta, A. J. Org. Chem. 1994,
59, 2604.
at 265 °C shows that the positions and intensities of all lines remain
unchanged when compared with the XRPD pattern of an as-syn-
thesized sample. Such a thermal stability is highly remarkable and
comparable to those observed for other stable supramolecular
systems, such as multi H-bonded aggregates derived from the
cyanuric acid and melamine.15
Variable temperature magnetic susceptibility data for an as-syn-
thesized crystalline sample of POROF-1 were obtained on a SQUID
magnetometer, under a temperature range of 1.8-300 K. POROF-1
exhibits a paramagnetic behavior in the 50-300 K temperature
range, with a ø‚T product value of 0.385 emu K mol-1 at 300 K,
that fully agrees with the theoretical value of 0.375 emu K mol-1
expected for an uncorrelated spin (S ) 1/2). Below 50 K, the ø‚T
value decreases upon decreasing temperature, consistently with the
presence of weak intermolecular antiferromagnetic interactions. This
magnetic behavior remains constant in the absence of guest solvent
molecules.
In summary, self-assembly of radical 1 generates a paramagnetic
open-framework structure with one-dimensional nanochannels
formed by narrowed polar windows and large hydrophobic nanocon-
tainers. Moreover, POROF-1 is one of the scarce examples of pure
organic porous materials that remains stable up to 275 °C, even
after removal of internal solvate guest molecules.16 Such structural
features, joined to its solubility in polar organic solvents, give to
POROF-1 the possibility to be a good candidate for future ship-
in-bottle synthesis applications.17 Further studies to validate the use
(8) Maspoch, D.; Gerbier, P.; Catala, L.; Vidal-Gancedo, J.; Wurst, K.; Rovira,
C.; Veciana, J. Chem.-Eur. J. 2002, 8, 3635.
(9) Ballester,M.;Riera,J.;Castan˜er,J.;Rovira,C.;Armet,O.Synthesis1986,64.
(10) X-ray structure analysis. Crystal data: C21H2Cl13O4‚C6H14, trigonal, space
group R-3, a ) 31.4651(6) Å, b ) 31.4651(7) Å, c ) 18.8447(7) Å, V )
16157.6(8) Å3, Z ) 18, T ) 233(2) K, λ-Mo KR ) 0.7173 Å, F(000) )
7758, µ ) 1.032 mm-1, Fcalcd ) 1.601 g/cm3, red prism 0.15 × 0.08 ×
0.08 mm3. Data collection: Nonius Kappa CCD, 19 275 measured
reflections were corrected with the program SCALEPACK. Structure
solution and refinement: anisotropic refinement on F2 (SHELXL 97),
hydrogens of the acid groups were refined with isotropic parameters;
R-values for 421 parameters and 2592 observed reflections [I > 2σ(I)]
R1 ) 0.0461 and wR2 ) 0.1183.
(11) Similar hydrogen-bonded dimers were observed in the monocarboxylic
perchlorotriphenylmethyl radical; see ref 7. In this case, bond distance
and angle are 1.955 Å and 144°, respectively.
(12) (a) Sarma, J. A. R. P.; Desiraju, G. R. Acc. Chem. Res. 1986, 19, 222. (b)
Desiraju, G. R.; Parthasarathy, R. J. Am. Chem. Soc. 1989, 111, 8725.
(13) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1998.
(14) Thermal gravimetric analyses were done with as-synthesized microcrystals
of radical 1 which were heated to 350 °C at a constant rate of 3 °C/min.
(15) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D.
N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37.
(16) There are only four examples of pure organic open-framework materials
remaining structurally ordered when the guest molecules are removed:
(a) Sozzani, P.; Comotti, A.; Simonutti, R.; Meersman, T.; Logan, J. W.
Angew. Chem., Int. Ed. 2002, 39, 2695. (b) Brunet, P.; Simard, M.; Wuest,
J. D. J. Am. Chem. Soc. 1997, 119, 2737. (c) Ung, A. T.; Gizachew, D.;
Bishop, R.; Scudder, M. L.; Dance, I. G.; Craig, D. C. J. Am. Chem. Soc.
1995, 117, 8745. (d) Ibragimov, B. T.; Talipov, S. A. J. Inclusion Phenom.
Mol. Recognit. 1994, 17, 317.
(17) There is a previous example of a metal-organic open-framework material
suitable for ship-in-bottle synthesis: Pan, L.; Liu, H.; Lei, X.; Huang,
X.; Olson, D. H.; Turro, N. J.; Li, J. Angew. Chem., Int. Ed. 2003, 42, 542.
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