Spontaneous assembly of a hydrogen-bonded tetrahedron

Article abstract of DOI:10.1039/b207026j

Triphenylamine ortho-tricarboxylic acid (1) has been synthesized and the crystal structure reported. This molecule is shown to spontaneously self-assemble into a hydrogen-bonded tetrahedron. Furthermore, Electrospray Ionization Mass Spectroscopy shows evi

Full text of DOI:10.1039/b207026j

Spontaneous assembly of a hydrogen-bonded tetrahedron
a
a
a
b
Jason E. Field, Marianny Y. Combariza, Richard W. Vachet and D. Venkataraman*
a
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts, USA 01003.
E-mail: rwvachet@chem.umass.edu; Fax: +1-413-545-4490; Tel: +1-413-545-2733
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts, USA 01003.
E-mail: dv@chem.umass.edu; Fax: +1-413-545-4490; Tel: +1-413-545-2028
b
Received (in Columbia, MO, USA) 17th July 2002, Accepted 21st August 2002
First published as an Advance Article on the web 10th September 2002
Triphenylamine ortho-tricarboxylic acid (1) has been syn-
thesized and the crystal structure reported. This molecule is
shown to spontaneously self-assemble into a hydrogen-
bonded tetrahedron. Furthermore, Electrospray Ionization
Mass Spectroscopy shows evidence for the stability of such
aggregates from an ethanol/water solution.
Single crystal X-ray diffraction† shows that 1‡ assembles
into tetrahedral clusters (Fig. 2). These clusters are held together
by hydrogen-bonding interactions, however, it is not the usual
dimeric interaction commonly observed for carboxylic acid
moieties.12 Instead, the acid groups are partially bonded to each
other and partially bonded to a 3-fold disordered ethanol
molecule that is present in the center of each face of the
tetrahedral cluster. The secondary carbon of each ethanol
molecule lies on the 3-fold symmetry axis. When connected,
these carbons form a second tetrahedron within the larger
The assembly of molecular components into discrete regular
(
Platonic) and semiregular (Archimedean) polyhedra via supra-
molecular interactions has received substantial attention in
recent years. The interest in this area stems mainly from the
1
13
tetrahedron, but with opposite geometry. Both of the tetra-
desire to rationalize and mimic various naturally occurring
discrete high-symmetry structures such as those found in
viruses and in proteins. Furthermore, since these structures
hedral clusters have the typical dimensions of a perfect
tetrahedron, namely 60° inner angles. However, the angles
between the carbon–nitrogen bonds of the triarylamines
occupying the vertices of the outer cluster are 118°, which
deviates from the ideal tetrahedral angles of 109.5° but are
consistent with the carbon–nitrogen–carbon bond angles of
120° observed for triphenylamine. The sides of the outer and
inner clusters are 9.7 Å and 4.4 Å, respectively.
Although there are no hydrogen-bond interactions between
these tetrahedral clusters, they do self-organize to form a
secondary structure, namely a cubic packing motif. Fig. 3 shows
that the tetrahedra pack in a motif that is geometrically body-
centered cubic (bcc) with each side of the cube being 15.8 Å in
length. Symmetrically, the packing would be more accurately
described as two interpenetrated simple cubes. Although
common for inorganic systems, cubic space groups are
relatively unusual for purely organic systems. In fact, a search
of the Cambridge Structural Database shows that less than 0.1%
of all the reported organic compounds there are cubic. This
crystal structure was reproducible over multiple recrystalliza-
tions. Attempts to incorporate larger alcohols, like isopropanol,
resulted in open structures.
2
have cavities that can encapsulate guest molecules, applications
such as catalysis and drug delivery have also been envisaged.3
A review of supramolecular systems based on polyhedral
structures has recently been published by Atwood.4 The
tetrahedron, with four vertices and six edges, belongs to a
family of five regular polyhedra collectively called the Platonic
solids. In 1975, Lehn and Graf reported a macrotricyclic host
that was the first covalent rendition of a tetrahedron.5
Subsequently, several examples have appeared in the literature
that use metal–ligand coordination bonds to assemble mole-
6
cules into a tetrahedron. However, we found that the clathrate
7
8
compounds of quinol and triphenylmethanol were the only
examples that exist for the assembly of a tetrahedron through
hydrogen bonds. Our research group is interested in the directed
assembly of electroactive solids into targeted topologies. In this
regard, we recently reported the assembly of di- and triar-
ylamine ortho-dicarboxylic acids into zig-zag and helical
9
topologies. We report here the spontaneous assembly of the
triphenylamine ortho-tricarboxylic acid 1 (Fig. 1) into a
tetrahedron through hydrogen bonding.
The methyl ester derivative of 1 (i.e. R = CO Me) can be
2
synthesized by the copper-catalyzed Ullmann coupling of
methyl anthranilate and methyl 2-iodobenzoate.10 Alterna-
tively, this compound can be synthesized by a new methodology
for amination reactions using a well-defined copper catalyst,
previously reported by our group.11 Once synthesized, the tri-
ortho-phenyl ester can be easily hydrolyzed with sodium
hydroxide in a water/ethanol solution to give 1 in almost
quantitative yield. Recrystallization from a solution of ethanol/
water gave small, but well-defined, colorless cubic crystals in
6
2% yield. Concentration of the filtrate and subsequent
recrystallization gave a second crop of crystals for an overall
yield of 93%.
Fig. 2 Illustration of the supramolecular structure of 1 from single crystal X-
ray diffraction. Nitrogen atoms (blue) and the secondary carbons of the
ethanol molecules (green) are connected to show the tetrahedral cluster and
solvent ordering. The closest carbonyl oxygens have been connected (red
dashed) however, the disordered hydrogen-bonded atoms have been
removed for clarity.
Fig. 1 Schematic representation of 1 as a tetrahedral synthon.
2
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CHEM. COMMUN., 2002, 2260–2261
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possible to exploit the structural characteristics of this system
along with the well-documented electronic properties of
triarylamines,15 in order to assemble new materials with
interesting solid-state properties. Research towards this end is
underway and will be reported in due course.
Financial support for this research was provided by the
University of Massachusetts, Amherst start-up funds and the
National Science Foundation CAREER grant CHE-0134287.
We thank the X-ray Structural Characterization Laboratory
supported by National Science Foundation grant CHE-9974648
for assistance with the crystallographic analyses.
Notes and references
†
X-Ray data were collected using a Nonius kappa-CCD diffractometer
with MoKa (l = 0.71073 Å) as the incident radiation. Structures were
2
solved using SIR97 and refined by full-matrix least-squares on F
o
using
SHELXL97.
¯
6 2 5
H15NO ·C H OH, M = 377.35, cubic, P43n (no.
‡
2
Crystal data for 1: C21
18), a = 15.8558(3) Å, V = 3986.25(13) Å , Z = 8, m = 0.105 mm , T
= 0.0752, wR
= 0.0843, wR = 0.2255 (all
3
21
Fig. 3 Illustration of the bcc-like packing motif of the supramolecular
tetrahedra of 1 from single crystal X-ray diffraction. The center tetrahedron
= 90 K, data/parameters = 1518/91, converging to R
0.2101 (on 1327, I > 2s(I) observed data); R
1
2
=
1
2
23
(green) is surrounded by 8 other tetrahedra (blue) that occupy the corners of
data), residual electron density: 0.43 e Å . CCDC 186794. See http://
www.rsc.org/suppdata/cc/b2/b207026j/ for crystallographic data in CIF or
other electronic format.
the body-centered cube (orange).
§
The crystals remained stable in the solid state even after heating at 80 °C
for 30 min.
ESI-MS was carried on a Bruker Esquire-LC quadrupole ion trap mass
Having established the tetrahedral hydrogen-bonded struc-
ture in the solid state, we decided to investigate the stability of
this species in solution.§ Electrospray ionization mass spec-
trometry (ESI-MS) has been previously used to characterize
similar stable non-covalent aggregates in solution.14 Fig. 4
shows the ESI mass spectrum of 1¶ in a solution of 20% water
¶
spectrometer in positive ion mode. The capillary temperature was held at
150 °C, the capillary voltage was set at 50 V, skimmer 1 was held at 30 V,
and skimmer 2 was held at 5.0 V.
in ethanol. As can be seen, the peak at m/z 1531.6 is
1
(a) L. R. MacGillivray and J. L. Atwood, J. Solid State. Chem., 2000,
+
predominant and corresponds to an [M
4
+ Na] aggregate,
152, 199; (b) P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30,
502; (c) D. L. Caulder and K. N. Raymond, Acc. Chem. Res., 1999, 32,
975; (d) D. L. Caulder and K. N. Raymond, J. Chem. Soc., Dalton
presumably the tetrahedral cluster. This is an indication of the
intrinsic stability of the tetrahedral structure. Also present in
small amounts are peaks at m/z 399.9 and 1155.1 corresponding
Trans., 1999, 1185; (e) M. Fujita, K. Umemoto, M. Yoshizawa, N.
Fujita, T. Kusukawa and K. Biradha, Chem. Commun., 2001, 509; (f) J.
Lu, A. Mondal, B. Moulton and M. J. Zaworotko, Angew. Chem., Int.
Ed., 2001, 40, 2113; (g) B. Moulton, J. Lu, A. Mondal and M. J.
Zaworotko, Chem. Commun., 2001, 863; (h) L. R. MacGillivray and J.
L. Atwood, Nature, 1997, 389, 469.
to [M + Na]+ and the doubly charged [M
2+
6
+ 2Na] ,
respectively. It is also interesting to note that solutions of 1 in
pure ethanol show predominately the molecular ion peak, even
though the solid-state structure shows no evidence of water
participating in the hydrogen-bonded structure. Water concen-
trations above 20% gave cluttered spectra with additional peaks
2
3
K. Namba and G. Stubbs, Science, 1986, 231, 1401.
F. Hof, S. L. Craig, C. Nuckolls and J. Rebek,, Angew. Chem., Int. Ed.,
2 3 4
corresponding to M and M aggregates as well as M aggregate
2
002, 41, 1488.
peaks with incorporated water molecules.
In summary, we have reported a purely organic, hydrogen-
bonded, supramolecular tetrahedral structure based on molecule
4
L. R. MacGillivray and J. L. Atwood, Angew. Chem., Int. Ed., 1999, 38,
1018.
5 E. Graf and J.-M. Lehn, J. Am. Chem. Soc., 1975, 97, 5022.
1
. Moreover, the ethanol solvent molecules are intrinsically
6 (a) O. D. Fox, M. G. B. Drew and P. D. Beer, Angew. Chem., Int. Ed.,
2000, 39, 136; (b) A. J. Terpin, M. Ziegler, D. W. Johnson and K. N.
Raymond, Angew. Chem., Int. Ed., 2001, 40, 157; (c) C. Brückner, R. E.
Powers and K. N. Raymond, Angew. Chem., Int. Ed., 1998, 37, 1837; (d)
R. L. Paul, S. M. Couchman, J. C. Jeffery, J. A. McCleverty, Z. R.
Reeves and M. D. Ward, J. Chem. Soc., Dalton Trans., 2000, 845.
ordered in a tetrahedral fashion within this structure. It may be
7
8
O. Ermer and C. Röbke, J. Am. Chem. Soc., 1993, 115, 10077.
(a) G. Ferguson, J. F. Gallagher, C. Glidewell, J. N. Low and S. N.
Scrimgeour, Acta Crystallogr., Sect. C, 1992, 48, 1272; (b) A. E. Aliev,
E. J. MacLean, K. D. M. Harris, B. M. Kariuki and C. Glidewell, J.
Phys. Chem. B, 1998, 102, 2165.
9
J. E. Field and D. Venkataraman, Chem. Commun., 2002, 306.
1
1
1
0 (a) D. Hellwinkel and M. Melan, Chem. Ber., 1971, 104, 1001; (b) J. E.
Field and D. Venkataraman, Chem. Mater., 2002, 14, 962.
1 R. Gujadhur, D. Venkataraman and J. T. Kintigh, Tetrahedron Lett.,
2
001, 42, 4791.
2 (a) L. Leiserowitz, Acta Crystallogr., Sect. B, 1976, 32, 775; (b) M. C.
Etter, Acc. Chem. Res., 1990, 23, 120; (c) G. R. Desiraju, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 2311.
1
3 This sort of solvent ordering has been observed in other hydrogen-
bonded clathrate structures, like those reported by Atwood: J. L.
Atwood, L. J. Barbour, T. J. Ness, C. L. Raston and P. L. Raston, J. Am.
Chem. Soc., 2001, 123, 7192 and references cited therein.
4 (a) X. Cheng, Q. Gao, R. D. Smith, E. E. Simanek, M. Mammen and G.
M. Whitesides, J. Org. Chem., 1996, 61, 2204 and references therein (b)
C. J. Kuehl, T. Yamamoto, S. R. Seidel and P. J. Stang, Org. Lett., 2002,
4, 913.
1
Fig. 4 The electrospray ionization (ESI) mass spectrum of 1 in a 1+5 water/
+
4
ethanol solution. The peak at m/z 1531.6 corresponds to the [M + Na]
aggregate.
15 Y. Shirota, J. Mater. Chem., 2000, 10, 1.
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