Molecular Tectonics. Dendritic Construction of Porous Hydrogen-Bonded Networks

Article abstract of DOI:10.1021/ol035712j

(Equation Presented) Molecules that associate to form porous networks can be made by attaching multiple hydrogen-bonding sites to suitable cores. Pentaerythrityl tetraphenyl ether, a four-armed core, is the progenitor of dendritic derivatives with more ar

Full text of DOI:10.1021/ol035712j

ORGANIC
LETTERS
2003
Vol. 5, No. 25
4787-4790
Molecular Tectonics. Dendritic
Construction of Porous
Hydrogen-Bonded Networks
Dominic Laliberte´,† Thierry Maris, Ariane Sirois, and James D. Wuest*
De´partement de Chimie, UniVersite´ de Montre´al, Montre´al, Que´bec H3C 3J7, Canada
wuest@chimie.umontreal.ca
Received September 7, 2003
ABSTRACT
Molecules that associate to form porous networks can be made by attaching multiple hydrogen-bonding sites to suitable cores. Pentaerythrityl
tetraphenyl ether, a four-armed core, is the progenitor of dendritic derivatives with more arms, including dipentaerythrityl hexaphenyl ether 7.
An advantage of such dendritic derivatives is that the resulting networks are held together by larger numbers of intermolecular hydrogen
bonds.
An established strategy for assembling materials by design
is to build them from molecules with well-defined structures
and multiple sticky sites that interact intermolecularly
according to reliable motifs. In favorable cases, this approach
places neighboring molecules in predetermined positions,
giving ordered networks with predictable properties. Mol-
ecules of this type have been called tectons from the Greek
word for builder,1 and molecular tectonics refers to supra-
molecular construction directed by tectonic subunits.1-5 This
strategy promises to be a fruitful source of new ordered
molecular materials with diverse architectures and properties,
ranging from highly crystalline solids to liquid crystals, gels,
and other less-ordered structures. Of special note is the
inherent difficulty of forming ordered arrays in which tectons
are closely packed at the same time that their programmed
interactions are optimized; instead, tectonic association
typically produces open networks that include guests.
Useful tectons can be devised by the simple expedient of
attaching a suitable core to multiple sticky sites. For example,
tecton 1 incorporates a four-armed core derived from
pentaerythrityl tetraphenyl ether (2), linked to diaminotriazine
groups that form multiple intermolecular hydrogen bonds
according to established motifs. As planned, crystallization
† Boehringer-Ingelheim Fellow, 2001-2002.
(1) Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc. 1991, 113, 4696.
(2) Fournier, J.-H.; Maris, T.; Wuest, J. D.; Guo, W.; Galoppini, E. J.
Am. Chem. Soc. 2003, 125, 1002.
(3) Sauriat-Dorizon, H.; Maris, T.; Wuest, J. D.; Enright, G. D. J. Org.
Chem. 2003, 68, 240.
(4) Laliberte´, D.; Maris, T.; Wuest, J. D. J. Org. Chem., accepted for
publication.
(5) For references to related work, see: Fournier, J.-H.; Maris, T.; Wuest,
J. D. J. Org. Chem., in press. Fournier, J.-H.; Maris, T.; Simard, M.; Wuest,
J. D. Cryst. Growth Des. 2003, 3, 535.
of tecton 1 (DMSO/dioxane) yielded an open hydrogen-
bonded network with significant space for the inclusion of
10.1021/ol035712j CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/08/2003

other molecules.4 In this structure, 66% of the volume is
accessible to guests,6,7 which occupy interconnected channels
with cross sections up to 9 × 5 Å2 in diameter.8 The high
porosity of the network is noteworthy because the pen-
taerythrityl tetraphenyl ether core is flexible and can adopt
various nontetrahedral geometries, yet a close-packed guest-
free structure is not favored.
Table 1. Reactions of Dipentaerythrityl Hexatosylate (8) with
Phenols
Tecton 1 can be considered to be the progenitor of a family
of self-associating dendrimers derived from the pentaeryth-
rityl tetraphenyl ether core.9,10 Further dendritic growth offers
promising new multiarmed cores for supramolecular con-
struction, including hexaphenyl ether 3 (derived from di-
pentaerythritol), octaphenyl ether 4 (derived from tripen-
taerythritol), decaphenyl ether 5 (derived from tetrapenta-
erythritol), and dodecaphenyl ether 6 (derived from pentapenta-
erythritol).11-13 Dendritic tectons with such cores are exciting
targets for synthesis for the following reasons: (1) The
number of sticky sites per tecton will increase with each
generation, thereby increasing the number of intermolecular
interactions and strengthening the resulting networks. (2) The
density of sticky sites on the periphery of each tecton is
expected to increase with each generation,9,10 thereby reduc-
ing the number of neighboring tectons not held in positions
imposed by directional forces. (3) The porosity of materials
built from dendritic tectons should be enhanced because it
will have dual origins, one corresponding to intertectonic
spaces (as in normal tectonic networks) and the other
corresponding to intratectonic spaces (due to the character-
entry
phenol
product
yield (%)
1
2
3
4
5
6
7
X ) 4-Br
X ) 3-Br
X ) 2-Br
X ) 4-NO2
X ) 4-CN
X ) 3-CN
X ) 4-CHO
9
10
11
12
13
14
15
79
72
68
85
86
76
75
istically low density of the dendritic cores of the tectons
themselves).9,10,14 (4) The crystallinity of progenitor 1 sug-
gests that dendritic derivatives of higher generations may
also yield single crystals suitable for X-ray diffraction,
whereas conventional dendrimers without strong intermo-
lecular interactions normally resist crystallization because
they are conformationally flexible and globular.15,16
To permit an initial test of these hypotheses, we synthe-
sized diverse hexasubstituted derivatives of dipentaerythrityl
hexaphenyl ether (3), including tecton 7, in which diami-
(6) The percentage of volume accessible to guests was estimated by the
PLATON program,7 using standard parameters.2-5
(7) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2001. van der Sluis, P.; Spek, A. L.
Acta Crystallogr. 1990, A46, 194.
(8) The dimensions of a channel in a particular direction correspond to
the cross section of an imaginary cylinder that could be passed through the
hypothetical open network in the given direction in contact with the van
der Waals surface. Such values are inherently conservative because (1) they
measure the cross section at the most narrow constriction, and (2) they
systematically underestimate the sizes of channels that are not uniform and
linear.
(9) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendrimers and
Dendrons: Concepts, Syntheses, Applications; VCH: Weinheim, Germany,
2001.
(10) For other recent reviews of the subject of dendrimers, see: Grayson,
S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819. Chow, H.-F.; Leung,
C.-F.; Wang, G.-X.; Zhang, J. Top. Curr. Chem. 2001, 217, 1. Zimmerman,
S. C.; Lawless, L. J. Top. Curr. Chem. 2001, 217, 95. Vo¨gtle, F.;
Gestermann, S.; Hesse, R.; Schwierz, H.; Windisch, B. Prog. Polym. Sci.
2000, 25, 987.
(11) For recent use of derivatives of dipentaerythritol in materials science,
see: Shukla, A. A.; Bae, S. S.; Moore, J. A.; Barnthouse, K. A.; Cramer,
S. M. Ind. Eng. Chem. Res. 1998, 37, 4090.
(12) For other recent uses of derivatives of dipentaerythritol in materials
science, see: Biela, T.; Duda, A.; Rode, K.; Pasch, H. Polymer 2003, 44,
1851. Gigant, K.; Posset, U.; Schottner, G.; Baia, L.; Kiefer, W.; Popp, J.
J. Sol-Gel Sci. Technol. 2003, 26, 369. Mayadunne, R. T. A.; Moad, G.;
Rizzardo, E. Tetrahedron Lett. 2002, 43, 6811. Rohr, T.; Knaus, S.; Gruber,
H.; Sherrington, D. C. Macromolecules 2002, 35, 97. Kader, M. A.;
Bhowmick, A. K.; Inoue, T.; Chiba, T. J. Mater. Sci. 2002, 37, 1503.
Kaczmarek, H.; Ołdak, D.; Szalla, A. J. Appl. Polym. Sci. 2002, 86, 3725.
Huang, H.; Zhang, J.-Z.; Shi, W.-F. J. Appl. Polym. Sci. 2001, 80, 499.
Joziasse, C. A. P.; Grablowitz, H.; Pennings, A. J. Macromol. Chem. Phys.
2000, 201, 107. Bunning, T. J.; Kirkpatrick, S. M.; Natarajan, L. V.;
Tondiglia, V. P.; Tomlin, D. W. Chem. Mater. 2000, 12, 2842. Menger, F.
M.; Migulin, V. A. J. Org. Chem. 1999, 64, 8916.
notriazine groups are attached to all six arms of the core.
Our syntheses are similar to those used previously to make
analogous derivatives of pentaerythrityl tetraphenyl ether (2).4
Base-induced reactions of phenols with the known dipen-
taerythrityl hexatosylate (8)11,17 (K2CO3/DMF/∆) provided
previously unknown derivatives 9-15 in good yields (Table
1). In this way, various substituted six-armed cores of
potential utility in molecular and supramolecular construction
can be obtained conveniently in a single step.
(14) For recent reviews of dendritic encapsulation, see: Gorman, C. B.;
Smith, J. C. Acc. Chem. Res. 2001, 34, 60. Hecht, S.; Fre´chet, J. M. J.
Angew. Chem., Int. Ed. 2001, 40, 74.
(15) For structural studies of derivatives of dipentaerythritol by X-ray
crystallography, see: Na¨ttinen, K. I.; Rissanen, K. Cryst. Growth Des. 2003,
3, 339.
(16) For other recent structural studies of dendritic molecules by single-
crystal X-ray diffraction, see: Bauer, R. E.; Enkelmann, V.; Wiesler, U.
M.; Berresheim, A. J.; Mu¨llen, K. Chem. Eur. J. 2002, 8, 3858. Ranganathan,
D.; Kurur, S.; Gilardi, R.; Karle, I. L. Biopolymers 2000, 54, 289. Friedmann,
G.; Guilbert, Y.; Wittmann, J. C. Eur. Polym. J. 1999, 35, 1097.
(17) For the structure of hexatosylate 8, see the Supporting Information.
(13) Dipentaerythritol and tripentaerythritol are commercially available
and inexpensive. For preparations of tetrapentaerythritol and pentapen-
taerythritol, see: Padias, A. B.; Hall, H. K., Jr.; Tomalia, D. A.; McConnell,
J. R. J. Org. Chem. 1987, 52, 5305. Suchanec, R. R. Anal. Chem. 1965,
37, 1361.
4788
Org. Lett., Vol. 5, No. 25, 2003

Table 2. Further Reactions of Hexasubstituted Derivatives of
Dipentaerythrityl Hexaphenyl Ether (3)
entry starting compd conditionsa
product
yield (%)
1
2
3
4
12 (X ) NO2)
15 (X ) CHO)
9 (X ) Br)
a
b
c
d
16 (Y ) NH2)
99
82
87
79
17 (Y ) CH2OH)
18 (Y ) B(OH)2)
7 (Y ) DATb)
13 (X ) CN)
a (a) H2/Pd/C, THF; (b) NaBH4, CH3OH, -10 to 25 °C; (c) (1) BuLi,
THF, -78 °C; (2) B(O-iPr)3, -78 to 25 °C; (3) HCl; (d) dicyandiamide,
KOH, 2-methoxyethanol, reflux. b DAT ) 4,6-diamino-1,3,5-diaminotriazin-
2-yl.
Figure 1. View of the structure of crystals of tecton 7 grown from
DMSO/dioxane, showing a central tecton (red) and its six sym-
metry-equivalent hydrogen-bonded neighbors (colored blue, green,
and yellow according to their interaction with the central tecton).
All tectons adopt a characteristic +-shaped conformation in which
two pairs of arms interact by intramolecular π-stacking.
Dipentaerythrityl hexaphenyl ethers 9-15 can then be
subjected to further reactions, such as those summarized in
Table 2, to produce an even broader range of substituted
derivatives. In particular, reduction of hexanitro derivative
12 (Table 1, entry 4) provided the corresponding hexamine
16 (Table 2, entry 1), and reduction of hexaaldehyde 15
(Table 1, entry 7) gave hexol 17 (Table 2, entry 2). Lithiation
of hexabromide 9 (Table 1, entry 1), followed by addition
of B(i-OPr)3 and then HCl, produced hexaboronic acid 18
(Table 2, entry 3). This compound can be considered to be
a tecton because it incorporates multiple B(OH)2 groups,
which are known to self-associate by reliable patterns of
hydrogen bonding.2 Tecton 18 also promises to be a valuable
intermediate for making more complex derivatives of di-
pentaerythrityl hexaphenyl ether by Suzuki coupling.
open three-dimensional network with significant volume for
the inclusion of guests. Each tecton adopts a characteristic
+-shaped conformation in which two pairs of the six arms
lie parallel and interact intramolecularly by π-stacking
(Figure 1).21 Each of the four diaminotriazine groups attached
to these arms forms four hydrogen bonds according to
established motifs with diaminotriazine groups on equivalent
π-stacked arms of four neighboring tectons (Figure 2a).3,4
Each of the two remaining diaminotriazine groups forms two
additional hydrogen bonds with two other neighbors (Figure
2b), giving a network in which each tecton forms a total of
20 hydrogen bonds with six neighboring tectons.22 Additional
Tecton 7 was prepared in 79% yield by the reaction of
hexanitrile 13 (Table 1, entry 5) with dicyandiamide under
standard conditions.18 Single crystals suitable for X-ray
diffraction could be grown by slow diffusion of dioxane into
solutions of tecton 7 in DMSO. Tecton 7 crystallized in the
monoclinic space group P2/c as an inclusion compound of
approximate composition 7‚4 DMSO‚16 dioxane.19,20 Views
of the structure appear in Figures 1-3. Like four-armed
progenitor 1, six-armed tecton 7 self-associates by extensive
hydrogen bonding of its diaminotriazine groups to form an
(18) Simons, J. K.; Saxton, M. R Organic Syntheses; Wiley: New York,
1963; Collect. Vol. IV, p 78.
Figure 2. Hydrogen bonding in the network constructed from
tecton 7. (a) View of one of the four π-stacked arms of the central
tecton (red), showing how it forms four hydrogen bonds (broken
lines) with equivalent arms of the blue and green neighbors, as
well as an additional hydrogen bond with a molecule of DMSO.
(b) View of one of the other two arms of the central tecton (red),
showing how it forms two hydrogen bonds with an equivalent arm
of the yellow neighbor.
(19) Crystal data for tecton 7‚4 DMSO‚16 dioxane: T ) 223 K, crystal
size 0.22 × 0.12 × 0.10 mm3, monoclinic, space group P2/c, a ) 19.758
(1) Å, b ) 10.865(1) Å, c ) 36.649(2) Å, â ) 90.759(3)°, V ) 7866.5(8)
Å3, Z ) 2, θ max ) 62.36°, 63526 reflections measured, 11176 unique
(Rint ) 0.052). Final residual for 581 parameters and 6356 reflections with
I > 2(I): R1 ) 0.0970, wR2 ) 0.2195, and GoF ) 1.041. Details are
provided as Supporting Information.
(20) The composition was estimated by X-ray crystallography and by
1H NMR spectroscopy of dissolved samples. The amount of any H2O
included could not be determined accurately.
Org. Lett., Vol. 5, No. 25, 2003
4789

adopts a divergent conformation. In general, dendritic growth
of the core may give networks with more hydrogen bonds
per tecton, but porosity will not necessarily increase.
Our observations show that (1) the network built from
tecton 7 is held together by an unusually large number of
hydrogen bonds per tecton; (2) a major part of the volume
is accessible to guests; and (3) large interconnected channels
provide multiple routes of entry and escape. For these
reasons, we expected single crystals of tecton 7 to be robust
and to undergo rapid exchanges of guests without loss of
crystallinity, as observed in networks built from many other
tectons.2-5 In fact, placing crystals of estimated composition
7‚4 DMSO‚16 dioxane20 in tetrahydrofuran (25 °C, 24 h)
led to loss of crystallinity, whereas exchange with retention
of crystallinity occurred under similar conditions in crystals
of four-armed progenitor 1. We suggest that increased
intermolecular contacts in the network built from six-armed
tecton 7 enhance robustness as planned but not enough to
compensate for increased flexibility of the dendritic core.
As a result, the network derived from six-armed tecton 7 is
less able to endure stresses induced by exchange.
Figure 3. Views along the b axis of the network constructed from
tecton 7, showing a 4 × 4 × 2 array of unit cells (left) and an
enlarged view of the cross section of a single channel (right). Guests
are omitted, and atoms are shown as spheres of van der Waals radii.
Our study of dendritic tectons is noteworthy because it
creates new links between crystal engineering and the science
of dendrimers and shows that concepts drawn from each of
the two fields can be applied fruitfully to the other. In
particular, dendritic growth of suitable tectonic cores can
increase the number of intermolecular interactions per tecton
and yield porous crystalline networks, as planned. However,
dendritic cores that are too flexible will not necessarily give
networks with abnormally high stability and porosity.
Continued efforts to exploit the advantages of dendritic
construction and minimize the disadvantages are likely to
be productive.
hydrogen bonds join molecules of DMSO to the network
(Figure 2a).
As planned, the network constructed from six-armed tecton
7 is held together by more intertectonic hydrogen bonds per
tecton (20) than is the network built from four-armed
progenitor 1 (12 per tecton).4 This significant difference
underscores the advantage of building tectons from dendritic
cores.
The network constructed from six-armed tecton 7 is highly
porous, and approximately 66% of the volume of the crystals
is accessible to guests.6 The guests occupy interconnected
channels, which are illustrated in Figure 3. Channels lying
along the b axis are S-shaped and have a cross section of
approximately 13 × 12 Å2, and those lying along the c axis
have a cross section of approximately 11 × 2 Å2.8
Although the porosity of the network constructed from
six-armed tecton 7 is impressive, it is not greater than that
of the network built from four-armed progenitor 1, possibly
because the dipentaerythrityl hexaphenyl ether core assumes
a compact conformation with π-stacked arms, whereas the
pentaerythrityl tetraphenyl ether core is less flexible and
Acknowledgment. We are grateful to the Natural Sci-
ences and Engineering Research Council of Canada, the
Ministe`re de l’EÄ ducation du Que´bec, the Canada Foundation
for Innovation, the Canada Research Chairs Program, and
Merck Frosst for financial support.
Supporting Information Available: Experimental pro-
cedures for making compounds 7, 8, and 9-18; characteriza-
tion of compounds 7, 8, and 9-18; ORTEP drawings and
tables of crystallographic data, atomic coordinates, aniso-
tropic thermal parameters, and bond lengths and angles for
compounds 7 and 8 in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
(21) Similar π-stacked conformations have been observed in other aryl-
substituted derivatives of dipentaerythritol.15
(22) Joining the central oxygen atom of each tecton with the centers of
the six neighboring tectons defines a complex noninterpenetrated six-
connected network (see Supporting Information).
OL035712J
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Org. Lett., Vol. 5, No. 25, 2003