Molecular tectonics. Use of the hydrogen bonding of boronic acids to direct supramolecular construction

Article abstract of DOI:10.1021/ja0276772

Tetraboronic acids 1 and 2 have four -B(OH)₂ groups oriented tetrahedrally by cores derived from tetraphenylmethane and tetraphenylsilane. Crystallization produces isostructural diamondoid networks held together by hydrogen bonding of the -B(OH)₂ groups, in accord with the tendency of simple arylboronic acids to form cyclic hydrogen-bonded dimers in the solid state. Five-fold interpenetration of the networks is observed, but 60% and 64% of the volumes of crystals of tetraboronic acids 1 and 2, respectively, remain available for the inclusion of disordered guests. Guests occupy two types of interconnected channels aligned with the a and b axes; those in crystals of tetraphenylmethane 1 measure approximately 5.9 × 5.9 A² and 5.2 × 8.6 A² in cross section at the narrowest points, whereas those in crystals of tetraphenylsilane 2 are approximately 6.4 × 6.4 A² and 6.4 × 9.0 A². These channels provide access to the interior and permit guests to be exchanged quantitatively without loss of crystallinity. Because the Si-C bonds at the core of tetraboronic acid 2 are longer (1.889(3) A) than the C-C bonds at the core of tetraboronic acid 1 (1.519(6) A), the resulting network is expanded rationally. By associating to form robust isostructural networks with predictable architectures and properties of porosity, compounds 1 and 2 underscore the usefulness of molecular tectonics as a strategy for making ordered materials.

Full text of DOI:10.1021/ja0276772

Published on Web 12/28/2002
Molecular Tectonics. Use of the Hydrogen Bonding of Boronic
Acids To Direct Supramolecular Construction
Jean-Hugues Fournier,1,† Thierry Maris,† James D. Wuest,*,† Wenzhuo Guo,‡ and
Elena Galoppini*,‡
Contribution from the De´partement de Chimie, UniVersite´ de Montre´al,
Montre´al, Que´bec H3C 3J7 Canada, and Department of Chemistry, Rutgers UniVersity,
Newark, New Jersey 07102
Received July 12, 2002 ; E-mail: wuest@chimie.umontreal.ca; galoppin@andromeda.rutgers.edu
Abstract: Tetraboronic acids 1 and 2 have four -B(OH)2 groups oriented tetrahedrally by cores derived
from tetraphenylmethane and tetraphenylsilane. Crystallization produces isostructural diamondoid networks
held together by hydrogen bonding of the -B(OH)2 groups, in accord with the tendency of simple arylboronic
acids to form cyclic hydrogen-bonded dimers in the solid state. Five-fold interpenetration of the networks
is observed, but 60% and 64% of the volumes of crystals of tetraboronic acids 1 and 2, respectively, remain
available for the inclusion of disordered guests. Guests occupy two types of interconnected channels aligned
with the a and b axes; those in crystals of tetraphenylmethane 1 measure approximately 5.9 × 5.9 Å2 and
5.2 × 8.6 Å2 in cross section at the narrowest points, whereas those in crystals of tetraphenylsilane 2 are
approximately 6.4 × 6.4 Å2 and 6.4 × 9.0 Å2. These channels provide access to the interior and permit
guests to be exchanged quantitatively without loss of crystallinity. Because the Si-C bonds at the core of
tetraboronic acid 2 are longer (1.889(3) Å) than the C-C bonds at the core of tetraboronic acid 1 (1.519(6)
Å), the resulting network is expanded rationally. By associating to form robust isostructural networks with
predictable architectures and properties of porosity, compounds 1 and 2 underscore the usefulness of
molecular tectonics as a strategy for making ordered materials.
the principal sites of association,4 but the field remains rich in
unexplored potential. For example, the crystallization of aryl-
Introduction
Molecular tectonics is a strategy for building predictably
ordered networks from subunits called tectons, which are
molecules designed to interact strongly with their neighbors in
ways that are specific and directional.2,3 The strategy provides
chemists with a useful tool for achieving the spontaneous
assembly of new ordered materials with predetermined structures
and properties. Of particular interest is the observation that
tectons cannot typically form normal close-packed structures
in which their ability to take part in specific intermolecular
interactions is fully exploited at the same time; instead, they
tend to form open networks in which significant volumes are
available for the inclusion of guests.
boronic acids typically produces cyclic hydrogen-bonded dimers
(eq 1),5,6 but this interaction has not yet been exploited in
supramolecular assembly,7,8 despite the extensive use of boronic
acids in other areas of molecular recognition.9 In this paper,
we describe the crystal structures of tetraboronic acids 1 and 2,
and we confirm our expectation that they are predisposed to
associate by hydrogen bonding according to eq 1 and to thereby
form open three-dimensional, four-connected networks with
significant internal volumes for the inclusion of guests.
In a conceptually simple method for making tectons, multiple
functional groups that promote strong intermolecular interactions
can be attached to geometrically suitable cores. Many sticky
functional groups have now been tested and found useful as
* To whom correspondence may be addressed.
† Universite´ de Montre´al.
‡ Rutgers University.
(1) Fellow of the Ministe`re de l’EÄ ducation du Que´bec, 2000-2002. Fellow of
the Natural Sciences and Engineering Research Council of Canada, 1998-
2000.
(2) Brunet, P.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1997, 119, 2737.
Su, D.; Wang, X.; Simard, M.; Wuest, J. D. Supramol. Chem. 1995, 6,
171. Wuest, J. D. In Mesomolecules: From Molecules to Materials;
Mendenhall, G. D., Greenberg, A., Liebman, J. F., Eds.; Chapman & Hall:
New York, 1995. Wang, X.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc.
1994, 116, 12119. Simard, M.; Su, D.; Wuest, J. D. J. Am. Chem. Soc.
1991, 113, 4696.
Results and Discussion
Synthesis. Tetraboronic acid 1 was prepared by minor
modifications of the known method.10 The analogous silane 2
was prepared in 84% overall yield by tetralithiation of tetrakis-
(4-bromophenyl)silane (BuLi),11 followed by the addition of
B(O-i-Pr)3 and subsequent hydrolysis. Tetraboronic acids 1 and
9
1002
J. AM. CHEM. SOC. 2003, 125, 1002-1006
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Molecular Tectonics
A R T I C L E S
2 are both of intrinsic interest as substrates for Suzuki couplings
and as precursors for the synthesis of a wide range of tetrahedral
molecular building blocks derived from tetraphenylmethane and
tetraphenylsilane.11
General Description of Structures. Crystallization of com-
pounds 1 and 2 was achieved by partial evaporation of solutions
in wet ethyl acetate or by diffusion of hexane into solutions in
wet ethyl acetate,12 and the structures were determined by X-ray
crystallography. Both compounds crystallize in the tetragonal
space group I41/a to give isostructural networks held together
by hydrogen bonding of -B(OH)2 groups, in accord with the
Figure 1. ORTEP view of the structure of tetraboronic acid 2. Disordered
guests are omitted, non-hydrogen atoms are represented by ellipsoids
corresponding to 30% probability, and hydrogen atoms are shown as spheres
of arbitrary size.
(3) For recent studies of hydrogen-bonded supramolecular networks by other
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Figure 2. Representation of the system of 5-fold interpenetrated diamondoid
networks generated by tetraboronic acid 2. In this drawing, the tectons lie
at the intersections of solid lines that represent their interactions with four
neighbors by hydrogen bonding according to the motif shown in eq 1. The
independent networks are shown in different shades of gray.
characteristic motif shown in eq 1.13 Because the two structures
are closely similar, only views of the network derived from
tetraphenylsilane 2 are shown (Figures 1-4). The -B(OH)2
groups are oriented tetrahedrally by the relatively rigid tet-
raphenylmethyl and tetraphenylsilyl cores to which they are
attached, so both networks are predisposed to favor diamondoid
connectivity,14 which is observed, or a related three-dimensional,
four-connected arrangement. The average intertectonic distances
between the tetrahedral centers of neighboring hydrogen-bonded
tetraboronic acids are 15.79 and 16.63 Å in the structures of
compounds 1 and 2, respectively. As a result, each diamondoid
network is open enough to permit interpenetration by four
independent diamondoid networks (Figure 2).15
(4) Nangia, A.; Desiraju, G. R. Top. Curr. Chem. 1998, 198, 57.
(5) Rettig, S. J.; Trotter, J. Can. J. Chem. 1977, 55, 3071.
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Tetrahedron Lett. 1999, 40, 6705. Schilling, B.; Kaiser, V.; Kaufmann, D.
E. Chem. Ber. 1997, 130, 923. Akita, T.; Kobayashi, K. AdV. Mater. 1997,
9, 346. Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli, M.; Zheng,
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J.; Meinhold, R. H.; Woolhouse, A. D. Acta Crystallogr., Sect. C 1995,
C51, 2694. Scouten, W. H.; Liu, X.-C.; Khangin, N.; Mullica, D. F.;
Sappenfield, E. L. J. Chem. Cryst. 1994, 24, 621. Soundararajan, S.; Duesler,
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H.; Linti, G.; Noth, H. Chem. Ber. 1990, 123, 1841. Zvonkova, Z. V.;
Gluskova, V. P. Kristallografija, SSSR 1958, 3, 559.
Inclusion of Guests. Despite this 5-fold interpenetration, the
structures of compounds 1 and 2 both retain very significant
volume for the inclusion of guests. Specifically, crystallization
produces inclusion compounds of approximate compositions 1‚
(7) The dimers of boronic acids have been overlooked in previous systematic
studies of cyclic hydrogen-bonded motifs. Davis, R. E.; Bernstein, J. Trans.
Am. Crystallogr. Assoc. 1999, 33, 7. Allen, F. H.; Raithby, P. R.; Shields,
G. P.; Taylor, R. J. Chem. Soc., Chem. Commun. 1998, 1043.
(8) Hydrogen bonding of a monoester of a phenylboronic acid has recently
been observed in a molecular network. Davis, C. J.; Lewis, P. T.;
Billodeaux, D. R.; Fronczek, F. R.; Escobedo, J. O.; Strongin, R. M. Org.
Lett. 2001, 3, 2443.
(11) Fournier, J.-H.; Wang, X.; Wuest, J. D. Submitted for publication.
(12) Wet ethyl acetate was used to disfavor the formation of boroxines by
dehydration of tetraboronic acids 1 and 2.
(13) Crystallization of tetraboronic acids 1 and 2 from primarily aqueous mixtures
appears to yield a different structure that is currently being studied.
(14) For a review of diamondoid hydrogen-bonded networks, see: Zaworotko,
M. J. Chem. Soc. ReV. 1994, 23, 283.
(15) For discussions of interpenetration in networks, see: Batten, S. R. Cryst.
Eng. Commun. 2001, 18, 1; Batten, S. R.; Robson, R. Angew. Chem., Int.
Ed. 1998, 37, 1460.
(9) For example, see: Takeuchi, M.; Ikeda, M.; Sugasaki, A.; Shinkai, S. Acc.
Chem. Res. 2001, 34, 865.
(10) Wilson, L. M.; Griffin, A. C. J. Mater. Chem. 1993, 3, 991.
9
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A R T I C L E S
Fournier et al.
assembled from tecton 2 are clearly visible in the view along
the a axis shown in Figure 3.
The channels themselves and their connectivity are repre-
sented by the surface shown in Figure 4.18 This establishes that
(1) the channels are highly interconnected, (2) their detailed
shapes are much more complex than those suggested by the
simple cross sections shown in Figure 3, and (3) guests that
diffuse within the network can reach any point within the
channels by multiple redundant pathways.19 In principle, the
high connectivity should help ensure that diffusion is not
prevented by occasional obstacles or defects in the system of
channels.
Overall, nearly 60% and 64% of the volume of crystals of
tectons 1 and 2, respectively, are available for including
guests.20,21 These values are notably high and far exceed the
unused space in normal molecular crystals, typically close to
30%, that is created by inefficient packing.22 In fact, the
available volume in crystals of tectons 1 and 2 even exceeds
that used to include water in many protein crystals, despite the
characteristically large, complex, and irregular shapes of
proteins.23 In principle, the very simple shapes of tectons 1 and
2 would allow them to be packed with normal efficiency to
create relatively compact periodic structures; indeed, tetraphen-
ylmethane, tetraphenylsilane, and simple substituted derivatives
do not typically form inclusion compounds.24 By not acting like
normal molecules, compounds 1 and 2 show special behavior
that fully supports the emerging principles of molecular tecton-
ics. In particular, the inherent tendency of tectons to form strong
directional interactions disfavors efficient molecular packing and
promotes the formation of structures in which significant volume
is available for the inclusion of guests.
Figure 3. View along the a axis of the network constructed from
tetraboronic acid 2 showing a 3 × 3 × 1 array of unit cells with disordered
guests omitted.
Exchange of Guests. In the structures derived from tetra-
boronic acids 1 and 2, each tecton participates in a total of eight
hydrogen bonds with four neighbors to form a diamondoid
network. Moreover, each diamondoid network is linked to the
two adjacent interpenetrating diamondoid networks by supple-
mentary hydrogen bonds between cyclic dimers, as shown in
structure 3. Similar interdimeric hydrogen bonding has also been
Figure 4. Stereoscopic representation of the interconnected channels
defined by the network constructed from tetraboronic acid 2. The image
shows a 2 × 2 × 1 array of unit cells viewed with the c axis vertical. The
outsides of the channels appear in light gray, and dark gray is used to show
where the channels are cut by the boundaries of the array. The surface of
the channels is defined by the possible loci of the center of a sphere of
diameter 3 Å as it rolls over the surface of the ordered tectonic network.18
5CH3COOC2H5‚xH2O and 2‚5CH3COOC2H5‚yH2O, as esti-
mated by 1H NMR spectroscopy of dissolved samples.16 In both
cases, the guests are highly disordered, and their location within
the networks cannot be determined precisely. However, the
volume available for inclusion defines two types of intercon-
nected channels aligned with the crystallographic a and b axes.
In the network derived from tetraphenylmethane 1, these
channels have cross sections of approximately 5.9 × 5.9 Å2
and 5.2 × 8.6 Å2 at the narrowest points, whereas those in the
isostructural network derived from tetraphenylsilane 2 are 6.4
× 6.4 Å2 and 6.4 × 9.0 Å2.17 Both cross sections in the structure
observed in the structures of simpler arylboronic acids.5,6 As a
result, each tecton participates in a total of 16 hydrogen bonds,
(18) Representations of channels were generated by the Cavities option in the
program ATOMS Version 5.1 (Shape Software, 521 Hidden Valley Road,
Kingsport, TN 37663; www.shapesoftware.com). We are grateful to Eric
Dowty of Shape Software for integrating this capacity in ATOMS at our
suggestion.
(16) Small amounts of H2O may be included, but the quantity could not be
determined accurately by 1H NMR spectroscopy. In addition, rapid loss of
guests from crystals removed from their mother liquors prevented us from
using thermogravimetric analysis to estimate the composition.
(17) 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 empty 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.
(19) For a discussion of pathways for diffusion in microporous materials, see:
Venuto, P. B. Microporous Mater. 1994, 2, 297.
(20) The percentage of volume accessible to guests was estimated by the
PLATON program.21
(21) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2001. van der Sluis, P.; Spek, A. L.
Acta Crystallogr., Sect. A 1990, A46, 194.
(22) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants
Bureau: New York, 1961.
9
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A R T I C L E S
eight involving neighbors in the same diamondoid network and
eight involving tectons in the adjacent interpenetrating networks.
This extensive hydrogen bonding helps ensure that the structural
integrity of the ordered assembly is high.
be useful in heterogeneous acidic catalysis or in the separation
of bases according to size, shape, and other characteristics.27
Detailed Analysis and Comparison of Structures. The bond
lengths and angles in the structures of tectons 1 and 2 are similar
to those of simpler arylboronic acids. For example, the O‚‚‚O
distances in the dimeric units defined by eq 1 are 2.693(5) and
2.713(3) Å in the networks derived from compounds 1 and 2,
respectively, whereas they are 2.743(2) Å in the dimer of
phenylboronic acid itself.5 Similar values are also found for the
O-B-O angles (118.7(8)° and 119.5(4)° in the structures
derived from tectons 1 and 2, respectively, and 116.2(2)° in
the model dimer5), the average values for the angle between
the planes defined by the aryl groups and the -B(OH)2 groups
(13.2° and 1.6° in the structures derived from compounds 1 and
2, respectively, and 14.2° in the model dimer5), and other
parameters.
In contrast, the guests are disordered, potentially mobile, and
located in channels that in principle provide multiple routes of
escape from inside the crystal. As observed in other tectonic
materials,2 the exchange of guests can take place without
destroying the ordered network. For example, single crystals
of estimated composition 1‚5CH3COOC2H5‚xH2O and ap-
proximate dimensions 1 mm × 1 mm × 1 mm were suspended
in tetrahydrofuran (THF)/hexane at 25 °C for 24 h. The
recovered sample remained transparent and morphologically
unchanged, continued to diffract and to exhibit uniform extinc-
tion between crossed polarizers, and showed closely similar unit
cell parameters when studied by single-crystal X-ray diffraction.
1
However, analysis of dissolved samples by H NMR spectro-
In both structures derived from tetraboronic acids 1 and 2,
adjacent interpenetrating networks are related by a displacement
along the a or b axis equal to the unit cell parameters (a ) b )
10.627(11) Å for tetraphenylmethane 1 and a ) b ) 10.8370(15)
Å for tetraphenylsilane 2). The O‚‚‚O distances for the inter-
dimeric hydrogen bonds defined by structure 3 are 2.700(8) and
2.729(5) Å, respectively. These distances are slightly longer than
those corresponding to the intradimeric hydrogen bonds, so it
is appropriate to consider the cyclic arrangement shown in eq
1 as the primary hydrogen-bonding motif present in the
structures of tectons 1 and 2. Nevertheless, the interdimeric
hydrogen bonds also appear to make an important contribution.
Because the average Si-C distance at the core of tetraphen-
ylsilane 1 (1.889(3) Å) is significantly greater than the average
C-C distance at the core of tetraphenylmethane 2 (1.519(6)
Å), the average intertectonic distances between the centers of
hydrogen-bonded neighbors are correspondingly larger (15.79
vs 16.63 Å). As a result, the dimensions of the channels, the
percentage of volume available for inclusion, and various unit
cell parameters are increased predictably. Specifically, the values
of a ) b ) 10.627(11) Å, c ) 41.608(6) Å, and V ) 4699(7)
Å3 in the structure of tetraphenylmethane 1 are increased to a
) b ) 10.8370(15) Å, c ) 45.602(9) Å, and V ) 5355.6(15)
Å3 in the structure of tetraphenylsilane 2.
scopy established that the initial guest, CH3COOC2H5, had been
replaced quantitatively by THF to give new crystals of ap-
proximate composition 1‚5THF‚zH2O.16 In similar single-crystal
to single-crystal transformations, CH3COOC2H5 was also re-
placed quantitatively by two guests approximately twice as large,
diethyl malonate and diethyl methylmalonate. The exchange of
guests is feasible, at least within geometric limits imposed by
the channels, but the removal of guests by exposure of crystals
to air or vacuum leads to a loss of crystallinity.
The unit cell parameters before replacement of CH3COOC2H5
by THF were a ) b ) 10.627(11) Å, c ) 41.608(6) Å, R ) â
) γ ) 90°, and V ) 4699(7) Å3, whereas those after exchange
were a ) b ) 10.794(17) Å, c ) 42.08(7) Å, R ) â ) γ )
90°, and V ) 4902(5) Å3 at the same temperature (226 K). This
observation shows that the replacement of CH3COOC2H5 by
THF induces a small expansion of the network, possibly
associated with subtle changes in the conformation of tecton 1,
differences in internal solvation of the network by the included
guests, or an increase in the number of guests.
Together, our studies of exchange demonstrate that (1) the
crystalline network is robust enough to remain insoluble and to
retain its original ordered architecture during the exchange of
guests, yet (2) the network is deformable enough to accom-
modate small structural changes. In general, tectonic networks
may prove to offer a unique balance between structural integrity
and deformability that makes them distinctly different from both
normal soft organic materials and hard inorganic materials.25
Channels in the structures derived from tectons 1 and 2
provide access to -OH groups involved in intertectonic
hydrogen bonding. Because arylboronic acids are moderately
acidic (pKa 8.83 for benzeneboronic acid itself),26 porous crystals
constructed by the association of boronic acids may prove to
Although the networks constructed from tectons 1 and 2 are
predictably similar, several minor differences are noteworthy.
In particular, the C-C-C angles at the core of tetraphenyl-
methane 1 have values of either 102.1(4)° or 113.3(2)°, whereas
the C-Si-C angles at the core of tetraphenylsilane 2 vary less
widely (106.0(2)° or 111.2(1)°), despite the presumably greater
flexibility of the tetraphenylsilyl core. In neither case, however,
do deviations from ideal tetrahedral geometry give rise to
nondiamondoid architectures.
Detailed prediction of the structures of molecular crystals
remains impossible.28 However, our study of the behavior of
tectons 1 and 2 shows that the strategy of molecular tectonics
gives chemists an effective tool for engineering new crystalline
structures with important elements of predictability. As their
design intends, tectons 1 and 2 self-associate by hydrogen
bonding of -B(OH)2 groups according to the standard motif
(23) Quillin, M. L.; Matthews, B. W. Acta Crystallogr., Sect. D 2000, D56,
791. Andersson, K. M.; Hovmo¨ller, S. Acta Crystallogr., Sect. D 2000,
D56, 789.
(24) Galoppini, E.; Gilardi, R. J. Chem. Soc., Chem. Commun. 1999, 173.
Oldham, W. J., Jr.; Lachicotte, R.; Bazan, G. C. J. Am. Chem. Soc. 1998,
120, 2987. Thaimattam, R.; Reddy, D. S.; Xue, F.; Mak, T. C. W.; Nangia,
A.; Desiraju, G. R. New J. Chem. 1998, 143. Lloyd, M. A.; Brock, C. P.
Acta Crystallogr., Sect. B 1997, B53, 780. Reddy, D. S.; Craig, D. C.;
Desiraju, G. R. J. Am. Chem. Soc. 1996, 118, 4090. Charisse´, M.; Gauthey,
V.; Dra¨ger, M. J. Organomet. Chem. 1993, 448, 47. Charisse´, M.; Roller,
S.; Dra¨ger, M. J. Organomet. Chem. 1992, 427, 23. Gruhnert, V.; Kirfel,
A.; Will, G.; Wallrafen, F.; Recker, K. Z. Kristallogr. 1983, 163, 53.
Robbins, A.; Jeffrey, G. A.; Chesick, J. P.; Donohue, J.; Cotton, F. A.;
Frenz, B. A.; Murillo, C. A. Acta Crystallogr., Sect. B 1975, B31, 2395.
(25) For related work, see: Thaimattam, R.; Xue, F.; Sarma, J. A. R. P.; Mak,
T. C. W.; Desiraju, G. R. J. Am. Chem. Soc. 2001, 123, 4432.
(26) Nakatani, H.; Morita, T.; Hiromi, K. Biochim. Biophys. Acta 1978, 525,
423. Juillard, J.; Geugue, N. C. R. Acad. Paris C 1967, 264, 259.
(27) For a review of the ability of arylboronic acids to serve as acid catalysts,
see: Ishihara, K.; Yamamoto, H. Eur. J. Org. Chem. 1999, 527.
(28) For a discussion, see: Gavezzotti, A. Acc. Chem. Res. 1994, 27, 309.
9
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A R T I C L E S
Fournier et al.
suitable crystals of tetraboronic acid 1 or 2 could also be obtained by
slow evaporation of solutions in wet CH3COOC2H5.
(eq 1), thereby forming isostructural three-dimensional, four-
connected networks with significant degrees of openness that
depend predictably on the size of the individual subunits.
X-ray Crystallographic Studies. The structures were solved by
direct methods using SHELXS-97 and refined with SHELXL-97.31 All
non-hydrogen atoms were refined anisotropically, whereas hydrogen
atoms were placed in ideal positions and refined as riding atoms. In
both structures, the included guests were found to be highly disordered
and could not be resolved. The SQUEEZE option of the PLATON
program was used to eliminate the contribution of the guests and to
give final models based only on the ordered part of the structures.21
Conclusions
Our studies of tetraboronic acids 1 and 2 are of broad
chemical interest for the following reasons: (1) both compounds
are promising precursors for synthesizing a range of tetrahedral
molecular building blocks by Suzuki couplings; (2) their
association establishes for the first time the usefulness of boronic
acids in the self-assembly of supramolecular structures; and (3)
their predictable behavior confirms that molecular tectonics is
an effective strategy for making ordered supramolecular materi-
als by design.
Structure of Tetraboronic Acid 1. Data were collected using a
Bruker SMART 2000 CCD diffractometer with Cu KR radiation at
226 K. Crystals of compound 1 belong to the tetragonal space group
I41/a (No. 88) with a ) b ) 10.627(11) Å, c ) 41.608(6) Å, V )
4699(7) Å3, Dcalcd (without solvent) ) 0.701 g cm-3, and Z ) 4. Full-
matrix least-squares refinements on F2 led to final residuals Rf ) 0.0938,
Rw ) 0.2759, and GOF ) 0.699 for 358 reflections with I > 2σ(I).
Experimental Section
Tetrahydrofuran (THF) was dried by distillation from the sodium
ketyl of benzophenone. All other reagents were commercial products
that were used without further purification.
Structure of Tetraboronic Acid 2. Data were collected using an
Enraf-Nonius CAD-4 diffractometer with Cu KR radiation at 210 K.
Crystals of compound 2 belong to the tetragonal space group I41/a (No.
88) with a ) b ) 10.8370(15) Å, c ) 45.602(9) Å, V ) 5355.6(15)
Å3, Dcalcd (without solvent) ) 0.634 g cm-3, and Z ) 4. Full-matrix
least-squares refinements on F2 led to final residuals Rf ) 0.0701, Rw
) 0.1717, and GOF ) 0.666 for 804 reflections with I > 2σ(I).
Exchange of Guests in Crystals of Tetraboronic Acid 1. Single
crystals of estimated composition 1‚5CH3COOC2H5‚xH2O and ap-
proximate dimensions 1 mm × 1 mm × 1 mm were grown in the
normal manner. The mother liquors were removed by pipet, and the
crystals were immediately covered with a 1:9 mixture of THF/hexane
and kept unstirred at 25 °C for 24 h. The liquid was removed by pipet,
the recovered crystals were washed three times with hexane, and their
content was determined by 1H NMR spectroscopy. Replacement of CH3-
COOC2H5 by diethyl malonate and methyl diethylmalonate was carried
out under similar conditions.
(Methanetetrayltetra-4,1-phenylene)tetrakisboronic Acid (1).10
A
solution of tetrakis(4-bromophenyl)methane (1.27 g, 2.00 mmol)29,30
in THF (125 mL) was stirred at -78 °C under dry N2 and treated
dropwise with a solution of butyllithium (6.4 mL, 2.5 M in hexane, 16
mmol). The resulting mixture was kept at -78 °C for 30 min, and
then B(O-i-Pr)3 (5.50 mL, 23.8 mmol) was added dropwise. The mixture
was stirred at -78 °C for 20 min and then at 25 °C for 1 h. After
acidification with 1 M aqueous HCl (25 mL), the mixture was
concentrated by partial evaporation of volatiles under reduced pressure.
The concentrate was treated with aqueous 1 M NaOH and filtered, and
the filtrate was acidified with 1 M aqueous HCl. This yielded a
precipitate, which was separated by filtration and dried in air to afford
(methanetetrayltetra-4,1-phenylene)tetrakisboronic acid (1; 0.914 g, 1.84
mmol, 92%) as a colorless solid: mp >300 °C; IR (KBr) 3369 cm-1
;
3
1H NMR (400 MHz, DMSO-d6) δ 8.03 (s, 8H), 7.66 (d, 8H, J ) 8.4
3
Hz), 7.12 (d, 8H, J ) 8.4 Hz); 13C NMR (100 MHz, DMSO-d6) δ
Acknowledgment. We thank Dr. Richard Gilardi for pre-
liminary crystallographic studies and helpful discussions. We
are grateful to the Natural Sciences and Engineering Research
Council of Canada, the Ministe`re de l’EÄ ducation du Que´bec,
Merck Frosst, the US National Science Foundation, and the
Rutgers University Research Council for financial support. In
addition, acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for support of this research.
149.3, 134.5, 132.3, 130.6, 65.9; MS (FAB, glycerol) m/e 721. Anal.
Calcd for C25H24B4O8‚4H2O: C, 52.89; H, 5.68. Found: C, 53.51; H,
5.93.
(Silanetetrayltetra-4,1-phenylene)tetrakisboronic Acid (2). An
analogous procedure converted tetrakis(4-bromophenyl)silane (1.30 g,
1.99 mmol)11 into (silanetetrayltetra-4,1-phenylene)tetrakisboronic acid
(2; 0.860 g, 1.68 mmol, 84%), which was isolated as a colorless solid:
mp >300 °C; IR (KBr) 3369 cm-1; 1H NMR (400 MHz, DMSO-d6) δ
8.14 (s, 8H), 7.79 (d, 8H, 3J ) 7.8 Hz), 7.42 (d, 8H, 3J ) 7.8 Hz); 13
C
NMR (100 MHz, DMSO-d6) δ 136.9, 136.4, 135.9, 134.5; MS (FAB,
glycerol) m/e 737. Anal. Calcd for C24H24B4O8Si‚1H2O: C, 54.41; H,
4.95. Found: C, 53.66; H, 4.79.
Crystallization of Tetraboronic Acids 1 and 2. In typical crystal-
lizations, tetraboronic acid 1 or 2 (0.2 mmol) was treated with CH3-
COOC2H5 (15 mL), and the mixture was shaken with H2O.12 The
organic phase was separated and exposed to vapors of hexane. Crystals
suitable for X-ray diffraction were obtained after 2 days. Alternatively,
Supporting Information Available: ORTEP drawings and
tables of crystallographic data, atomic coordinates, anisotropic
thermal parameters, and bond lengths and angles for tetraboronic
acids 1 and 2. This information is available free of charge via
the Internet at http://pubs.acs.org.
JA0276772
(29) Grimm, M.; Kirste, B.; Kurreck, H. Angew. Chem., Int. Ed. Engl. 1986,
25, 1097. Other methods for the synthesis of tetrakis(4-bromophenyl)-
methane have been published subsequently.10,30
(31) Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal
Structures; Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997. Sheldrick,
G. M. SHELXL-97, Program for the Refinement of Crystal Structures;
Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1997.
(30) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546.
9
1006 J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003