Molecular tectonics - Use of urethanes and ureas derived from tetraphenylmethane and tetraphenylsilane to build porous chiral hydrogen-bonded networks

Article abstract of DOI:10.1139/v03-208

Tetraphenylmethane, tetraphenylsilane, and simple derivatives with substituents that do not engage in hydrogen bonding typically crystallize as close-packed structures with essentially no space available for the inclusion of guests. In contrast, derivatives with hydrogen-bonding groups are known to favor the formation of open networks that include significant amounts of guests. To explore this phenomenon, we synthesized six new derivatives 5a-5e and 6a of tetraphenylmethane and tetraphenylsilane with urethane and urea groups at the para positions, crystallized the compounds, and determined their structures by X-ray crystallography. As expected, all six compounds crystallize to form porous three-dimensional hydrogen-bonded networks. In the case of tetraurea 5e, 66% of the volume of the crystals is accessible to guests, and guests can be exchanged in single crystals without loss of crystallinity. Of special note are: (j) the use of tetrakis(4-isocyanatophenyl)methane (1f) as a precursor for making enantiomerically pure tetraurethanes and tetraureas, including compounds 5b, 5c; and (ii) their subsequent crystallization to give porous chiral hydrogen-bonded networks. Such materials promise to include chiral guests enantioselectively and to be useful in the separation of racemates, asymmetric catalysis, and other applications.

Full text of DOI:10.1139/v03-208

386
Molecular tectonics — Use of urethanes and
ureas derived from tetraphenylmethane and
tetraphenylsilane to build porous chiral hydrogen-
bonded networks1
Dominic Laliberté, Thierry Maris, and James D. Wuest
Abstract: Tetraphenylmethane, tetraphenylsilane, and simple derivatives with substituents that do not engage in hydro-
gen bonding typically crystallize as close-packed structures with essentially no space available for the inclusion of
guests. In contrast, derivatives with hydrogen-bonding groups are known to favor the formation of open networks that
include significant amounts of guests. To explore this phenomenon, we synthesized six new derivatives 5a–5e and 6a
of tetraphenylmethane and tetraphenylsilane with urethane and urea groups at the para positions, crystallized the com-
pounds, and determined their structures by X-ray crystallography. As expected, all six compounds crystallize to form
porous three-dimensional hydrogen-bonded networks. In the case of tetraurea 5e, 66% of the volume of the crystals is
accessible to guests, and guests can be exchanged in single crystals without loss of crystallinity. Of special note are:
(i) the use of tetrakis(4-isocyanatophenyl)methane (1f) as a precursor for making enantiomerically pure tetraurethanes
and tetraureas, including compounds 5b, 5c; and (ii) their subsequent crystallization to give porous chiral hydrogen-
bonded networks. Such materials promise to include chiral guests enantioselectively and to be useful in the separation
of racemates, asymmetric catalysis, and other applications.
Key words: crystal engineering, molecular tectonics, hydrogen bonding, networks, porosity, urethanes, ureas,
tetraphenylmethane, tetraphenylsilane.
Résumé : Le tétraphénylméthane, le tétraphénylsilane et leurs dérivés simples ne portant pas de substituants pouvant
participer à des liaisons hydrogène cristallisent généralement sous la forme de structures compactes dans lesquelles il
n’y a pas de place disponible pour l’inclusion de molécules invitées. Par contraste, il est bien connu que les dérivés
comportant des groupes pouvant donner lieu à des liaisons hydrogène favorisent la formation de réseaux ouverts qui
peuvent inclure des quantités importantes de molécules invitées. Dans le but d’étudier ce phénomène, on a réalisé la
synthèse de six nouveaux dérivés, 5a–5e et 6a, du tétraphénylméthane et du tétraphénylsilane portant des groupes uré-
thane et urée en position para qui ont été cristallisés et dont on a déterminé la structure par diffraction des rayons X.
Tel que prévu, les six composés cristallisent en formant des réseaux poreux tridimensionnels reliés par des ponts hy-
drogène. Dans le cas de la tétraurée 5e, 66 % du volume des cristaux est disponible pour des molécules invitées et les
invités peuvent être échangés dans les cristaux uniques sans perte de cristallinité. Il est particulièrement important de
noter: (i) qu’il est possible d’utiliser le tétrakis(4-isocyanatophényl)-méthane (1f) comme précurseur dans la synthèse de
tétrauréthanes et de tétraurées énantiomèrement pures, y compris les composés 5b, 5c et (ii) qu’il est possible de les
cristalliser subséquemment pour donner des réseaux chiraux poreux reliés par des ponts hydrogène. Des tels produits
pourraient inclure énantiosélectivement des molécules invitées chirales et ainsi être utilisés dans la séparation de racé-
mates, en catalyse asymétrique ou dans d’autres applications.
Mots clés : ingénierie cristalline, tectonique moléculaire, liaison hydrogène, réseaux, porosité, uréthanes, urées, tétra-
phénylméthane, tétraphénylsilane.
[Traduit par la Rédaction] Laliberté et al. 398
Introduction
between the structure of crystals and their physical proper-
ties remains poorly understood. For these reasons, attempts
to control molecular crystallization and to engineer crystals
for specific applications remain largely empirical activities.
No reliable method yet exists for predicting the detailed
structure of molecular crystals (1–4), and the relationship
Received 10 October 2003. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 4 March 2004.
Dedicated with respect and affection to Professor Ed Piers.
D. Laliberté, T. Maris, and J.D. Wuest.2 Département de Chimie, Université de Montréal, Montréal, QC H3C 3J7, Canada.
1This article is part of a Special Issue dedicated to Professor Ed Piers.
2Corresponding author (e-mail: wuest@chimie.umontreal.ca).
Can. J. Chem. 82: 386–398 (2004)
doi: 10.1139/V03-208
© 2004 NRC Canada

Laliberté et al.
387
As a result, crystal engineering offers exceptional challenges
and opportunities, and it has become one of the most excit-
ing areas in contemporary science (5).
Tetraphenylmethane and tetraphenylsilane crystallize to
form isomorphous close-packed structures held together by
multiple phenyl–phenyl embraces (17, 18), and simple deriv-
atives with substituents that do not engage in hydrogen
bonding also typically form close-packed structures (19–29).
In contrast, 4-substituted derivatives with simple hydrogen-
bonding groups, including tetraphenols 1b and 2b (13),
tetraboronic acids 1c and 2c (8), and other compounds (15,
16), are known to favor open networks that include signifi-
cant amounts of guests. Even derivatives with substituents
that form relatively weak C-H···O hydrogen bonds, such as
tetrakis(4-nitrophenyl)methane (1d) (14), can form open net-
works.
To explore the scope of this phenomenon, help identify its
origins, and test its potential for creating ordered materials
with new properties, we have extended our study of tectons
derived from tetraphenylmethane and tetraphenylsilane to in-
clude new 4-substituted derivatives bearing urethane and
urea groups, which have been used previously in crystal en-
gineering because they self-associate reliably in the solid
state to form hydrogen-bonded chains 3 and 4 (30–37). In
this paper, we show how such compounds can be made and
used to generate networks that are both porous and chiral.
In a recent review (1), Dunitz identifies a strategy for di-
recting molecular crystallization that has played a key role in
bringing crystal engineering to the forefront of science:
“…computational methods for predicting crystal structures
of organic compounds cannot yet be regarded as reliable.
From a more qualitative and descriptive viewpoint has come
the notion that certain groupings in organic molecules exer-
cise attractive intermolecular interactions and so guide the
molecules into distinctive patterns in their crystal struc-
tures…This indeed has become one of the tenets of crystal
engineering. The prime example of a structure directing in-
teraction is, of course, the hydrogen bond.”
This strategy can be used to design molecules to serve as
subunits for the programmed construction of particular crys-
talline networks. This can be achieved by incorporating
carefully oriented arrays of sticky sites that interact inter-
molecularly according to reliable motifs. Such molecules
have been called tectons (6), a term derived from the Greek
word for builder, and molecular tectonics refers to the strat-
egy of building supramolecular structures from tectonic sub-
units (6, 7).
Tectons have arrays of sticky sites that direct inter-
molecular association, linked to cores that orient the sticky
sites and introduce other desirable molecular features. In fa-
vorable cases, the oriented sticky sites play a dominant role
in association and place each tecton in a predetermined posi-
tion relative to its neighbors. Normally, tectons cannot form
structures that pack efficiently and simultaneously obey the
imperatives of strong directional interactions, so open mo-
lecular networks are favored, with significant space for the
inclusion of guests. Many different sticky sites and cores can
be used, giving molecular tectonics wide scope as a strategy
for creating ordered materials with unique properties. In par-
ticular, we have recently made hydrogen-bonded molecular
crystals that (i) can be engineered with sub-nanometric pre-
cision (8); (ii) use at least 75% of their volume to include
guests, which can be exchanged without loss of crystallinity
(9); (iii) can be deformed by at least 30% without loss of
crystallinity (10); and (iv) react with external agents to give
single crystals of new compounds with retention of the origi-
nal crystalline architecture (11, 12).3
Invaluable basic information about molecular tectonics
can be acquired through systematic comparison of structures
built from tectons with a fixed core and variable sticky sites,
or with fixed sticky sites and variable cores. In such studies,
relatively rigid cores are beneficial because they facilitate
comparison by minimizing polymorphism due to multiple
conformations. The tetraphenylmethyl core (1a) and its
tetraphenylsilyl analogue 2a are particularly attractive
choices, and various tectons with simple hydrogen-bonding
sites attached to these cores have been used in crystal engi-
neering (8, 13–16).
Results and discussion
Synthesis of urethanes and ureas derived from
tetraphenylmethane and tetraphenylsilane
Tetrakis(4-nitrophenyl)methane (1d) (38) was reduced by
an improved procedure (H2/Pd/C, THF) to provide a 99%
yield of the known tetrakis(4-aminophenyl)methane (1e) (16,
38, 39). Subsequent reaction of tetramine 1e with triphosgene
and triethylamine gave tetrakis(4-isocyanatophenyl)methane
(1f) in 97% yield. The analogous isothiocyanate 1g was ob-
tained in 89% yield by the reaction of tetramine 1e with
thiophosgene and triethylamine. Heating tetra(isocyanate) 1f
with alcohols and amines then gave the corresponding ure-
thanes and ureas. In this way, urethanes 5a, 5b and ureas 5c,
5d were prepared from ethanol, (R)-2-butanol, (S)-2-butyl-
amine, and (S)-α-methylbenzylamine in yields of 83%, 78%,
99%, and 99%, respectively. Treatment of tetra(isocyanate)
1f with aqueous ammonia at 25 °C gave simple urea 5e in
92% yield. For comparison with tetraurethane 5a, analogue
6a with a tetraphenylsilyl core was prepared in 18% yield by
Pd-catalyzed coupling of tetrakis(4-bromophenyl)silane (2d)
(40) with ethyl carbamate (41).
Structure of tetrakis(4-isothiocyanatophenyl)methane
(1g)
Tetra(isothiocyanate) 1g was crystallized from CHCl3–
hexane, and its structure was determined by X-ray diffrac-
tion (Table 1, Fig. 1).4 Like most other simple derivatives of
tetraphenylmethane and tetraphenylsilane (17–29), com-
pound 1g forms a close-packed structure without included
guests. This observation confirms that molecules with a
3 For citations of other recent studies of the engineering of hydrogen-bonded molecular crystals, see refs. 8–12.
4 Supplementary data may be purchased from the Directory of Unpublished Data, Document Delivery, CISTI, National Research Council
Canada, Ottawa, ON K1A 0S2, Canada (http://www/nrc.ca/cisti/irm/unpub_e.shtml for information on ordering electronically). CCDC
221549–221554 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax: +44 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk).
© 2004 NRC Canada

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Can. J. Chem. Vol. 82, 2004
close structural relationship to tectons 5a–5e, but without an
ability to self-associate by hydrogen bonding, are not predis-
posed to form inclusion compounds.
shown in Figs. 2 and 3. The tectons can be divided into two
equal populations (Fig. 2), one that donates single hydrogen
bonds to four symmetrically oriented neighbors, and the
other that accepts hydrogen bonds from four symmetrically
oriented neighbors and simultaneously donates hydrogen
bonds to four molecules of THF. Connecting the tetrahedral
centers of hydrogen-bonded tectons defines a diamondoid
network (42) with an alternating hydrogen bond donor–
acceptor arrangement, in which each node is surrounded by
four belonging to the other population. The network is held
Analysis of hydrogen-bonded networks formed by
crystallization of tetraurethane 5a and the analogous
silane 6a
Tecton 5a crystallized from THF–pentane in the tetra-
gonal space group I-4 as an inclusion compound of well-
defined composition 5a·2THF (Table 1).4 The structure is
© 2004 NRC Canada

Laliberté et al.
389
Fig. 1. ORTEP drawing of the crystal structure of tetrakis(4-
isothiocyanatophenyl)methane (1g). Ellipsoids are drawn at the
30% probability level, and hydrogen atoms are represented by a
sphere of arbitrary size. One of the arms (N46) is disordered
over two positions.
Table 1. Crystallographic data for compounds 1g, 5a·2THF, and
6a·2dioxane.4
Compound
1g
5a·2THF
6a·2dioxane
Formula
Fw
F(000)
C29H16N4S4 C45H56N4O10 C44H56N4O12Si
548.74
1128
812.94
1736
861.02
916
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/n
11.6490(1)
11.4547(1)
20.5132(1)
90
95.890(1)
90
2722.75(4)
4
Tetragonal
I-4
24.9135(8)
24.9135(8)
7.1597(2)
90
90
90
4443.9(2)
4
1.215
293(2)
0.0688
0.1795
1.008
29
Tetragonal
P42/n
17.7781(2)
17.7781(2)
7.1904(1)
90
α (°)
β (°)
90
90
γ (°)
Volume (Å3)
Z
2272.60(5)
2
1.258
293(2)
0.0724
0.2160
1.024
Density (g/cm3)
1.339
Temperature (K) 293(2)
R1
wR2
GoF
0.0609
0.1810
1.056
0
Porosity (%)
0
Fig. 2. Representation of the structure of crystals of tetraurethane 5a grown from THF–pentane, showing a central tecton (dark gray)
that donates single hydrogen bonds to four neighboring tectons (light gray). Each neighbor accepts single hydrogen bonds from three
other tectons and donates hydrogen bonds to the guests (THF). Hydrogen bonds appear as broken lines.
© 2004 NRC Canada

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Can. J. Chem. Vol. 82, 2004
Fig. 3. View along the c axis of the network constructed from
tetraurethane 5a showing a 2 × 2 × 4 array of unit cells. Guests
are omitted, and atoms are shown as spheres of van der Waals
radii to reveal the cross sections of the channels. Atoms of hy-
drogen appear in white, atoms of carbon in light gray, atoms of
oxygen in gray, and atoms of nitrogen in dark gray.
Fig. 4. Stereoscopic representation of the parallel helical chan-
nels defined by the network constructed from tetraurethane 5a.
The image shows a 1 × 1 × 4 array of unit cells viewed with the
c axis horizontal. 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 centre of a sphere of diameter
3 Å as it rolls over the surface of the ordered network.7
(Table 1).4 The structure is shown in Figs. 5 and 6. Each
tecton donates four hydrogen bonds to four neighboring
molecules of dioxane, and each molecule of dioxane accepts
two hydrogen bonds from different tectons, thereby creating
a diamondoid network (42) in which dioxane is an essential
structural component (Fig. 5). The average Si···Si distance
between closest pairs of tectons within the network is 16.6
Å, which is large enough to permit sixfold interpenetration
(Fig. 6) (43, 44). This fills the available volume completely,
giving a close-packed structure containing only tecton 6a
and molecules of dioxane that are an integral part of the net-
work.
The observed structures of tectons 5a and 6a are largely
consistent with the emerging principles of molecular tecton-
ics. In particular, both compounds crystallize as expected to
form open hydrogen-bonded diamondoid networks that in-
corporate solvent. Unexpectedly, the networks are not
isostructural, but it is important to note that tecton 5a was
crystallized from THF–pentane and tecton 6a from dioxane–
hexane. We subsequently found that tecton 5a can also be
crystallized from dioxane and tecton 6a from THF. However,
we elected not to study these new structures further because
those already solved did not show exceptional porosity.
together by only four hydrogen bonds per tecton. The aver-
age distance between the tetrahedral centers of hydrogen-
bonded neighbors defines an intertectonic separation of
12.6 Å, which is too small to permit interpenetration (43,
44). Approximately 29% of the volume of crystals of tecton
5a is available for including guests,5 which occupy parallel
channels that lie along the c axis and have cross sections
measuring nearly 11 × 3 Å2 (Fig. 3).6 Close examination of
the channels reveals that each is helical and chiral, with
equal numbers of helices of opposite handedness (Fig. 4).7
Crystallization of tetraboronic acid 1c and the analogous
silane 2c under similar conditions is known to produce iso-
structural hydrogen-bonded networks (8). Replacing the cen-
tral C—C bonds in compound 1c with longer Si—C bonds
in analogue 2c results in a rational expansion of the network,
with predictable control of geometry on the sub-nanometric
scale. To further test the ability of molecular tectonics to
control crystallization with this degree of precision, we com-
pared the structure of tetraurethane 5a with that formed by
the corresponding silane 6a. Tecton 6a crystallized from
dioxane–pentane in the tetragonal space group P42/n as an
inclusion compound with the composition 6a·2 dioxane
Porous chiral hydrogen-bonded networks formed by
crystallization of enantiomerically pure tetraurethane
5b and tetraurea 5c
Exciting opportunities for constructing new types of po-
rous molecular networks are created by using enantio-
merically pure tectons such as analogues 5b–5d of
tetraurethane 5a. The principles of molecular tectonics sug-
gest that crystallization of these analogues should produce
porous hydrogen-bonded networks, as observed in the case
of tecton 5a. However, these new networks will be chiral
and may therefore be able to include chiral guests enantio-
5 The percentage of volume accessible to guests was estimated by the PLATON program (45, 46), using standard parameters (9, 13).
6 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 be-
cause (i) they measure the cross section at the most narrow constriction, and (ii) they systematically underestimate the sizes of channels that
are not uniform and linear.
7 Representations of channels were generated by the Cavities option in the programs ATOMS Version 5.1 (Shape Software, 521 Hidden Val-
ley 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.
© 2004 NRC Canada

Laliberté et al.
391
Fig. 5. View of part of the structure of crystals of tetraurethane 6a grown from dioxane–pentane, showing two tectons linked by hydro-
gen bonds donated to an intervening molecule of dioxane. Hydrogen bonds appear as broken lines.
drogen-bonded networks and then to use the resulting mate-
rials for the enantioselective inclusion and exchange of
guests. However, few successes have been reported so far
(50–58).
Fig. 6. Representation of the sixfold interpenetrated diamondoid net-
works generated by association of tetraurethane 6a with dioxane. In
this drawing, the central silicon atom of each tetraurethane lies at
the intersection of solid lines that represent hydrogen bonding to
four neighbors via intervening molecules of dioxane. Independent
networks are shown in different shades of gray.
Enantiomerically pure tetraurethane 5b crystallized from
THF–pentane in the tetragonal space group I41 as an inclu-
sion compound of approximate composition 5b·2 pentane
(Table 2).4,8 As shown in Fig. 7, each tecton forms a total of
eight hydrogen bonds with four symmetrically oriented
neighbors, and each urethane group serves simultaneously as
a donor and acceptor of hydrogen bonds. This defines a
diamondoid network (42) in which the average intertectonic
distance between the tetrahedral centers of neighbors is 9.73
Å. No interpenetration is observed (43, 44), and 16% of the
volume remains available for including guests.5 Despite in-
tensive effort, we were unable to grow crystals of tecton 5b
suitable for X-ray diffraction when we used racemic solvents
such as ( )-2-bromobutane, ( )-2-chloropentane, and ( )-2-
methyltetrahydrofuran, so we were unable to examine the
enantioselectivity of inclusion.
Crystals of the analogous enantiomerically pure tetraurea
5c were obtained by allowing dioxane to diffuse into solu-
tions in HCOOH. X-ray crystallography established that
tecton 5c crystallized under these conditions in the tetra-
gonal space group I41 as an inclusion compound of well-
defined composition 5c·2HCOOH (Table 2).4 Self-
association of tetraurea 5c produces a hydrogen-bonded
diamondoid network (42) isostructural to the one generated
by the analogous tetraurethane 5b (Fig. 8). Each tecton
forms a total of 16 hydrogen bonds with four symmetrically
oriented neighbors, and each urea group serves simulta-
neously as a double donor and double acceptor of hydrogen
bonds. No interpenetration is observed (43, 44), and 17% of
the volume remains available for including guests.5 Unfortu-
nately, we were unable to grow crystals of tecton 5c suitable
for X-ray diffraction from racemic solvents, so we were not
able to confirm that inclusion is enantioselective.
selectively. Such behavior would illustrate a key advantage
of using molecules to make porous materials for applications
in separation and catalysis, rather than using inorganic ana-
logues such as zeolites. Specifically, creating asymmetric
environments within supramolecular materials can be
achieved by simple modifications of the individual molecu-
lar components, whereas very few zeolites are inherently
chiral (47–49). As result, an important goal in crystal engi-
neering is to devise molecules that form porous chiral hy-
1
8 The composition was estimated by X-ray crystallography and by H NMR spectroscopy of dissolved samples. The amount of any H2O
included could not be determined accurately.
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Can. J. Chem. Vol. 82, 2004
Table 2. Crystallographic data for compounds 5b·2pentane, 5c·2HCO2H, 5e·2DMSO·5dioxane, and 5e after replacement of
dioxane by THF.4
5e·2DMSO·5dioxane after
exchange with THF
5c·2HCO2H
Compound
5b·2Pentane
5e·2DMSO·5 dioxane
Formula
Fw
F(000)
C55H80N4O8
925.23
2008
C47H62N8O12
931.05
1864
C53H80N8O16S2
1149.37
2456
—
—
Crystal system
Space group
a = b (Å)
c (Å)
Tetragonal
I41
17.7201(6)
16.0845(7)
5050.6(3)
4
Tetragonal
I41
17.2392(3)
17.3495(3)
5156.1(2)
4
Tetragonal
P-421c
17.281(2)
20.591(2)
6149(1)
4
Tetragonal
P-421c
17.184(8)
20.838(2)
6152(87)
—
—
223(2)
—
Volume (Å3)
Z
Density (g/cm3)
Temperature (K)
R1
wR2
GoF
1.217
1.12
1.242
223(2)
293(2)
0,1022
0,2888
1,038
0.1(18)
16
293(2)
0,1089
0.3059
1.265
–0.2(3)
17
0.0989
0.2721
1.249
0.14(10)
66
—
—
—
Flack Parameter
Porosity (%)
Fig. 7. Representation of the structure of crystals of tetraurethane 5b grown from THF–pentane, showing a tecton (dark gray) hydro-
gen-bonded to one of four neighbors (light gray). The illustrated neighbor is equivalent by symmetry to three others, which are omitted
for clarity. Hydrogen bonds appear as broken lines, and guests are omitted for clarity. Each urethane group serves simultaneously as a
donor and acceptor of hydrogen bonds.
guests. As shown in Fig. 9, each tecton forms hydrogen
bonds with six neighboring tectons (Fig. 9a). Two neighbors
that lie along the c axis on opposite sides of the central
tecton are equivalent by symmetry, and each accepts four
hydrogen bonds from the central tecton (Fig. 9b). The other
four neighbors form a second symmetry-equivalent set, and
each donates two hydrogen bonds to the central tecton
Porous hydrogen-bonded network formed by
crystallization of tetraurea 5e
Tetraurea 5e crystallized from DMSO–dioxane in the
tetragonal space group P-421c as an inclusion compound
with the approximate composition 5e·2DMSO·5dioxane (Ta-
ble 2).4,8 The composition of the crystals indicated immedi-
ately that a large fraction of the volume is accessible to
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Laliberté et al.
393
Fig. 8. Representation of the structure of crystals of tetraurea 5c grown from HCOOH–dioxane, showing a tecton (dark gray) hydro-
gen-bonded to one of four neighbors (light gray). The illustrated neighbor is equivalent by symmetry to three others, which are not
shown. Hydrogen bonds appear as broken lines, and guests are omitted for clarity. Each urea group serves simultaneously as a double
donor and double acceptor of hydrogen bonds.
1
(Fig. 9c). In addition, part of the N—H bonds not used in
intertectonic hydrogen bonding remain available for interact-
ing with included DMSO. Each tecton therefore participates
in a total of 16 hydrogen bonds with neighboring tectons.
Joining the tetrahedral centers of the tectons defines the ap-
proximately octahedral 6-connected network shown in
Fig. 9d. The average intertectonic distances are 10.30 Å in
the c direction and 12.22 Å in the a and b directions.
24 h. Subsequent analysis of recovered samples by H NMR
spectroscopy in solution showed complete replacement of
dioxane by THF. The recovered samples remained transpar-
ent, showed no morphological changes visible by optical
microscopy, and continued to diffract. Study of single ex-
changed crystals by X-ray diffraction showed little variation
in the unit cell parameters (Table 2), suggesting that the net-
work undergoes little distortion.
No interpenetration is observed (43, 44), and approxi-
mately 66% of the volume of crystals of tecton 5e remains
available for the inclusion of guests,5 which occupy inter-
connected channels. The porosity is unusually high, and it
far exceeds the fraction of volume (normally about 30%)
that is considered to remain unfilled in normal molecular
crystals because of inefficient packing (59). The most signif-
icant channels are aligned with the ab diagonal and have
cross sections measuring approximately 5.7 × 6.3 Å2 at the
narrowest points (Fig. 10).6 The surface shown in Fig. 11
further defines the shape and complex interconnectivity of
the channels.7
The network built from tecton 5e is very porous, the inte-
rior of crystals is made accessible by significant intercon-
nected channels, and high structural integrity is ensured by
the formation of 16 hydrogen bonds per tecton. As a result,
guests can be exchanged in single crystals without loss of
crystallinity. For example, single crystals of estimated com-
position 5e·2DMSO·5dioxane and approximate dimensions
1 mm × 1 mm × 5 mm were suspended in THF at 25 °C for
Conclusions
Our studies of tetraurethanes and tetraureas 5a–5e and 6a
confirm that (i) tetraphenylmethane and tetraphenylsilane are
effective cores for constructing molecules of value in crystal
engineering, and (ii) substituting these cores with hydrogen-
bonding groups produces tectons that crystallize predictably
to give open three-dimensional networks that can include
significant amounts of guests. Of special note is the observa-
tion that enantiomerically pure tectons such as tetraurethane
5b and tetraurea 5c can be made conveniently from tetra-
(isocyanate) 1f and crystallize to give porous chiral net-
works. Such molecular materials have unique properties that
promise to make them useful in enantioselective catalysis,
the resolution of racemates, and other applications.
Experimental
Tetrahydrofuran (THF) was dried by distillation from the
© 2004 NRC Canada

394
Can. J. Chem. Vol. 82, 2004
Fig. 9. (a) View of the structure of crystals of tetraurea 5e grown from DMSO–dioxane showing a central tecton (dark gray) and its
six hydrogen-bonded neighbors. Four of the neighbors (light gray) define a set equivalent by symmetry, and two others (gray) define a
second set. All atoms are shown as spheres of van der Waals radii, and guests are omitted for clarity. (b) Hydrogen bonding between
the central tecton (dark gray) and the set of two symmetry-equivalent neighbors (gray). Hydrogen bonds appear as broken lines.
(c) Hydrogen bonding between the central tecton (dark gray) and the set of four symmetry-equivalent neighbors (light gray). Hydrogen
bonds appear as broken lines. (d) Representation of the 6-connected network defined by joining the central carbon atom of each tecton
with the centres of the set of six neighbors shown in Fig. 1a.
sodium ketyl of benzophenone. Xantphos (60) and tetrakis-
(4-bromophenyl)silane (2d) (40) were synthesized according
to known procedures. All other reagents were commercial
products that were used without further purification.
der H2 (1200 kPa). The resulting mixture was filtered
through Celite®, and volatiles were removed by evaporation
under reduced pressure. The residue was triturated with
CHCl3 and washed with CH3OH to afford tetrakis(4-
aminophenyl)methane (1e, 3.09 g, 8.12 mmol, 99%) (16, 38,
39) as a colorless solid that was used without further purifi-
cation. A sample of analytical purity was obtained by crys-
tallization from ethanol–water to afford small colorless
crystals; mp 319 °C (lit. (16) mp 331 °C, lit. (38) mp 319 to
Tetrakis(4-aminophenyl)methane (1e) (16, 38, 39)
A
suspension of tetrakis(4-nitrophenyl)methane (1d,
4.10 g, 8.19 mmol) (38) and 10% Pd/C (410 mg) in THF
(300 mL) was stirred for 70 h in a Parr reactor at 25 °C un-
© 2004 NRC Canada

Laliberté et al.
395
Fig. 10. View along the ab diagonal of the network constructed
from tetraurea 5e showing a 2 × 2 × 2 array of unit cells. Guests
are omitted, and atoms are shown as spheres of van der Waals
radii to reveal the cross sections of the channels. Atoms of hy-
drogen appear in white, atoms of carbon in light gray, atoms of
oxygen in gray, and atoms of nitrogen in dark gray.
Hg (1 mm Hg = 133.322 Pa)), mp 53 to 54 °C (softening).
IR (KBr) (cm–1): 2260, 1604, 1574, 1519, 1109, 1016, 822,
563. H NMR (400 MHz, CDCl3) δ: 7.10 (d, J = 8.9 Hz,
8H), 7.00 (d, 3J = 8.9 Hz, 8H). 13C NMR (100 MHz, CDCl3)
δ: 143.6, 132.0, 131.8, 125.1, 124.4, 63.7. FAB-HR-MS (3-
nitrobenzyl alcohol) calcd. for C29H16N4O4 m/e:
484.117155; found: 484.116508.
1
3
Tetrakis(4-isothiocyanatophenyl)methane (1g)
Thiophosgene (0.264 mL, 3.46 mmol) was added
dropwise to a stirred solution of tetrakis(4-aminophenyl)
methane (1e, 300 mg, 0.788 mmol) and triethylamine
(0.880 mL, 6.31 mmol) in dry THF (25 mL) at –10 °C under
N2. The resulting mixture was then warmed to 25 °C during
1 h. Water was added, and the mixture was extracted twice
with CH2Cl2. The combined organic extracts were dried over
Na2SO4, and volatiles were removed by evaporation under
reduced pressure. The residual solid was purified by flash
chromatography (silica, CH2Cl2 (30%) – hexane (70%), Rf
0.59) to provide tetrakis(4-isothiocyanatophenyl) methane
(1g, 386 mg, 0.703 mmol, 89%) as a colorless solid, mp 203
to 204 °C. IR (KBr) (cm–1): 2200–2000 (bs), 1496, 932,
1
3
817. H NMR (400 MHz, CDCl3) δ: 7.15 (d, J = 8.9 Hz,
8H), 7.10 (d, 3J = 8.9 Hz, 8H). 13C NMR (100 MHz, CDCl3)
δ: 144.4, 136.4, 131.8, 130.1, 125.6, 64.2. FAB-MS (3-
nitrobenzyl alcohol) m/e: 548. Anal. calcd. for C29H16N4S4:
C 63.48, H 2.94, N 10.21; found: C 63.33, H 2.81, N 10.05.
Tetraurethane 5a
A solution of tetrakis(4-isocyanatophenyl)methane (1f,
315 mg, 0.650 mmol) in absolute ethanol (15 mL) was
heated at reflux for 1 h. Volatiles were then removed by
evaporation under reduced pressure, and the residual solid
was purified by flash chromatography (silica, CH3COOC2H5
(50%) – hexane (50%), Rf 0.50) to afford tetraurethane 5a
(360 mg, 0.538 mmol, 83%) as a colorless solid, mp 223 to
224 °C. IR (KBr) (cm–1): 3292, 2980, 1729, 1597, 1535,
1
1412, 1317, 1297, 1221, 1113, 1063, 872, 811. H NMR
3
(300 MHz, DMSO-d6) δ: 9.59 (s, 4H), 7.33 (d, J = 8.4 Hz,
3
3
8H), 6.98 (d, J = 8.4 Hz, 8H), 4.09 (q, J = 6.5 Hz, 8H)
1.22 (t, 3J = 6.5 Hz, 12H). 13C NMR (100 MHz, DMSO-d6)
δ: 153.5, 140.7, 136.8, 130.5, 117.4, 62.3, 60.0, 14.4. FAB-
HR-MS (3-nitrobenzyl alcohol) calcd. for C37H40N4O8 m/e:
668.28461; found: 668.28410. Anal. calcd. for
C37H40N4O8·H2O: C 64.71, H 6.16, N 8.16; found: C 65.11,
H 6.16, N 8.00.
320 °C, lit. (39) mp 318 °C). IR (KBr) (cm–1): 3396, 3171,
1
3026, 1610, 1507, 1271, 1183, 860, 829, 813, 578. H NMR
3
(300 MHz, DMSO-d6) δ: 6.69 (d, J = 8.5 Hz, 8H), 6.40 (d,
3J = 8.5 Hz, 8H), 4.86 (s, 8H). 13C NMR (75 MHz, DMSO-
d6) δ: 145.7, 135.9, 131.4, 112.6, 61.1. MAB-HR-MS (N2)
calcd. for C25H24N4 m/e: 380.200097; found: 380.200966.
Anal. calcd. for C25H24N4: C 78.92, H 6.36, N 14.73; found:
C 78.65, H 6.33, N 14.23.
Tetraurethane 5b
(R)-2-Butanol (0.66 mL, 7.2 mmol) was added to a stirred
solution of tetrakis(4-isocyanatophenyl)methane (1f, 440 mg,
0.908 mmol) in dry THF (5 mL), and the mixture was
heated at reflux for 1 h. Volatiles were then removed by
evaporation under reduced pressure, and the residual solid
was purified by flash chromatography (silica, CH3COOC2H5
(30%) – hexane (70%), Rf 0.43) to provide tetraurethane 5b
(553 mg, 0.708 mmol, 78%) as a colorless solid, mp 273 °C.
[α]D = –21° (c 0.30 in ethanol). IR (KBr) (cm–1): 3311,
2974, 2936, 2879, 1705, 1600, 1528, 1409, 1379, 1321,
1296, 1228, 1127, 1054, 826, 771, 530. 1H NMR (400 MHz,
Tetrakis(4-isocyanatophenyl)methane (1f)
A solution of triphosgene (1.39 g, 4.68 mmol) in dry THF
(10 mL) was added dropwise to a stirred solution of tetrakis-
(4-aminophenyl)methane (1e, 1.19 g, 3.13 mmol) and
triethylamine (3.49 mL, 25.0 mmol) in dry THF (60 mL) at
–78 °C under N2. The mixture was then warmed to 0 °C dur-
ing 2 h, and insoluble solids were removed by filtration
through Celite®. Volatiles were removed by evaporation un-
der reduced pressure to give tetrakis(4-isocyanatophenyl)
methane (1f, 1.48 g, 3.05 mmol, 97%) as a colorless solid
that was used without further purification. A sample of ana-
lytical purity was obtained by sublimation (130 °C/0.1 mm
3
3
CDCl3) δ: 7.25 (d, J = 8.7 Hz, 8H), 7.10 (d, J = 8.7 Hz,
3
8H), 6.54 (s, 4H), 4.84 (m, 4H), 1.61 (m, 8H), 1.26 (d, J =
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Can. J. Chem. Vol. 82, 2004
Fig. 11. Stereoscopic representation of the interconnected channels within the network constructed from tetraurea 5e. The image shows
a 2 × 2 × 1 array of unit cells viewed along the c axis. 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
centre of a sphere of diameter 3.5 Å as it rolls over the surface of the ordered network.7
6.3 Hz, 12H), 0.94 (t, 3J = 7.4 Hz, 12H). 13C NMR
(100 MHz, CDCl3) δ: 153.7, 141.9, 136.2, 131.7, 117.7,
73.4, 63.1, 29.2, 19.9, 9.9. FAB-MS (3-nitrobenzyl alcohol)
m/e: 780.1. Anal. calcd. for C45H56N4O8: C 69.21, H 7.23, N
7.17; found: C 69.21, H 7.31, N 7.18.
(75 MHz, DMSO-d6) δ: 154.4, 145.2, 139.7, 137.9, 130.7,
128.3, 126.6, 125.8, 116.6, 62.1, 48.6, 23.1. FAB-MS (3-
nitrobenzyl alcohol) m/e: 969. Anal. calcd. for C61H60N8O4:
C 75.60, H 6.24, N 11.56; found: C 75.26, H 6.37, N 11.39.
Tetraurea 5e
Tetraurea 5c
Aqueous NH3 (28%, 5 mL) was added to a stirred solu-
tion of tetrakis(4-isocyanatophenyl)methane (1f, 480 mg,
0.991 mmol) in dry THF (20 mL), and the resulting mixture
was stirred for 1 h. Volatiles were then removed by evapora-
tion under reduced pressure, and the residual solid was
washed with water and crystallized from 2-methoxyethanol–
THF to provide tetraurea 5e (501 mg, 0.907 mmol, 92%) as
a colorless solid, mp > 330 °C. IR (KBr) (cm–1): 3500–3000
(S)-2-Butylamine (261 mg, 3.57 mmol) was added to a
stirred solution of tetrakis(4-isocyanatophenyl)methane (1f,
392 mg, 0.809 mmol) in dry THF (20 mL), and the mixture
was heated at reflux for 1 h. Hexane was then added, and the
resulting precipitate was separated by filtration and washed
with CHCl3 and then hexane to afford tetraurea 5c (622 mg,
0.800 mmol, 99%) as a beige solid. A sample of analytical
purity was obtained by crystallization from HCOOH–
dioxane to afford pale orange crystals, mp > 330 °C. [α]D =
+22° (c 1.00 in DMF). IR (KBr) (cm–1): 3330, 2965, 2931,
1
(bs), 1666, 1588, 1532, 1509, 1410, 1344, 816. H NMR
3
(400 MHz, DMSO-d6) δ: 8.49 (s, 4H), 7.26 (d, J = 8.7 Hz,
3
8H), 6.93 (d, J = 8.7 Hz, 8H), 5.80 (s, 8H). 13C NMR
1
2875, 1653, 1595, 1541, 1508, 1316, 1228, 816. H NMR
(100 MHz, DMSO-d6) δ: 156.1, 139.9, 138.1, 130.6, 116.9,
62.2. FAB-HR-MS (3-nitrobenzyl alcohol) calcd. for
C29H28N8O4 m/e: 552.223352; found: 552.225072. Anal.
calcd. for C29H28N8O4·1.5H2O: C 60.09, H 5.39; found: C
59.91, H 5.96.
3
(400 MHz, DMSO-d6) δ: 8.27 (s, 4H), 7.23 (d, J = 8.8 Hz,
3
3
8H), 6.90 (d, J = 8.8 Hz, 8H), 5.92 (d, J = 8.1 Hz, 4H),
3
3.57 (m, 4H), 1.39 (m, 8H), 1.04 (d, J = 6.6 Hz, 12H), 0.85
(t, J = 7.4 Hz, 12H). 13C NMR (100 MHz, DMSO-d6) δ:
3
154.8, 139.6, 138.1, 130.6, 116.5, 62.0, 46.0, 29.3, 20.7,
10.2. FAB-HR-MS (3-nitrobenzyl alcohol) calcd. for
C45H60N8O4 + H m/e: 777.481578; found: 777.484089.
Anal. calcd. for C45H60 N8O4·3.5H2O: C 64.34, H 8.04, N
13.34; found: C 64.22, H 7.84, N 13.73.
Tetraurethane 6a
Deoxygenated dioxane (10 mL) was added to a de-
oxygenated mixture of tetrakis(4-bromophenyl)silane (2d,
652 mg, 1.00 mmol) (40), Pd(OOCCH3)2 (45.0 mg,
0.200 mmol), Xantphos (180 mg, 0.311 mmol) (60), Cs2CO3
(1.70 g, 5.22 mmol), and ethyl carbamate (392 mg,
4.40 mmol). The resulting mixture was stirred at reflux for
48 h under N2 and then filtered through Celite®. Volatiles
were removed by evaporation under reduced pressure, and
the residual solid was purified by flash chromatography (sil-
ica, CH3COOC2H5 (40%) – hexane (60%), Rf 0.25) to give
tetraurethane 6a (125 mg, 0.183 mmol, 18%) as a colorless
solid, mp 138 to 139 oC. IR (KBr) (cm–1): 3500–3300, 2980,
2928, 1707, 1596, 1519, 1396, 1319, 1222, 1062, 824, 685,
Tetraurea 5d
A procedure similar to the one used to synthesize
tetraurea 5c converted tetrakis(4-isocyanatophenyl)methane
(1f, 125 mg, 0.258 mmol) and (S)-α-methylbenzylamine
(0.166 mL, 1.30 mmol) into tetraurea 5d (248 mg,
0.256 mmol, 99%), which was isolated as a colorless solid.
A sample of analytical purity was obtained by crystallization
from CH3OH, mp 262 to 263 °C. [α]D = –116° (c 0.30 in
ethanol). IR (KBr) (cm–1): 3400, 1677, 1596, 1506, 1407,
1
1
1319, 1292, 1238, 1115, 821. H NMR (400 MHz, DMSO-
536. H NMR (300 MHz, DMSO-d6) δ: 9.74 (s, 4H), 7.49
3
3
3
d6) δ: 8.36 (s, 4H), 7.31 (m, 16H), 7.22 (m, 12H), 6.88 (d,
(d, J = 8.3 Hz, 8H), 7.33 (d, J = 8.3 Hz, 8H), 4.12 (q, J =
3J = 8.7 Hz, 8H), 6.56 (d, J = 7.6 Hz, 4H), 4.79 (dq, J =
7.1 Hz, 8H), 1.24 (t, J = 7.1 Hz, 12H). 13C NMR (75 MHz,
3
3
3
7.6, 6.9 Hz, 4H), 1.36 (d, J = 6.9 Hz, 12H). 13C NMR
DMSO-d6) δ: 153.5, 140.6, 136.4, 127.2, 117.7, 60.2, 14.5.
3
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Laliberté et al.
397
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(3-nitrobenzyl
alcohol)
calcd.
for
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We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Ministère de
l’Éducation du Québec, the Canada Foundation for Innova-
tion, the Canada Research Chairs Program, and Merck
Frosst for financial support. In addition, acknowledgment is
made to the donors of the Petroleum Research Fund, admin-
istered by the American Chemical Society, for support of
this research. Dominic Laliberté is grateful to Boehringer
Ingelheim for a fellowship.
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