Full text of DOI:10.1007/BF02900545

ü
ü
in urea[1 10]/thiourea[11 20]/selenourea[21] adducts stems
from an attempt to generate different inclusion topologies
by incorporating additional components into the construc-
tion of the hydrogen-bonded host lattice[22]. In our re-
search program more than 70 novel inclusion compounds
have been prepared. The versatility of urea or thiourea as
a key component in the construction of novel anionic host
lattices is clearly demonstrated by the occurrence of many
new types of linkage modes. The results show that
co-molecular aggregates of urea or thiourea with other
neutral molecules or anionic moieties can be considered
as fundamental building blocks for the constructions of
various types of novel host lattices.
Novel inclusion compounds
with urea/thiourea/seleno-
urea-anion host lattices
LI Qi1, SHI Mei1 & Thomas C. W. Mak2
1. Department of Chemistry, Beijing Normal University, Beijing 100875,
China;
2. Department of Chemistry, the Chinese University of Hong Kong, Sha-
tin, New Territories, Hong Kong
Correspondence should be addressed to Li Qi (e-mail: qili@bnu.edu.cn)
The chemistry of inclusion compounds has a long
history and is nowadays a subject of wide-ranging and
intense study. With the awarding of the 1987 Nobel Prize
in Chemistry to Donald J. Cram, Jean-Marie Lehn and
Charles J. Pedersen for their fundamental work on
“host-guest” or “supramolecular” systems, inclusion
chemistry has come to the fore front in contemporary re-
searches. Increasing varieties of novel inclusion com-
pounds and new host molecules have been synthesized
recently. The term “crystal engineering” was coined by
Schmidt to describe the rational design and control of
molecular packing arrangements in the solid state, and the
structural study of clathrates has contributed substantially
to our understanding of the problem. Generalizations
concerning the preferred structural motifs generated by
hydrogen bonding and weaker non-covalent interactions
between specific functional groups or molecular frag-
ments have led to the realization of some impressive pre-
dictions about the construction of supramolecular net-
works.
Urea, thiourea and selenourea are good host mole-
cules because they have a well-defined trigonal planar
geometry and can form at least six hydrogen bonds. The
co-crystallization behavior of urea and thiourea with
straight- and branched-chain aliphatic compounds, respec-
tively, was first reported over half a century ago. New
concepts about various structural aspects of urea inclusion
compounds, such as temperature-dependent structural
properties, phase transition and structure change, disor-
dered crystal structure, and migration of molecules into
the channel structure, as well as unusual conformational
behavior in thiourea inclusion compounds, have been re-
ported. More and more research work about them has
shown that the inclusion phenomenon is much more com-
plex, varied, and interesting than that had been realized all
along.
1 General structural features and relationships of
urea/thiourea/selenourea-anion inclusion compounds
The results of our studies on urea/thiourea/seleno-
urea-anion inclusion compounds have demonstrated that
the classical urea or thiourea hydrogen-bonded host lattice
can be modified in interesting ways by the incorporation
of various anionic moieties, with or without co-crystal-
lized water or other uncharged molecules, and that novel
host frameworks bearing different urea, thiourea or se-
lenourea/guest molar ratios are generated by variation in
size of the hydrophobic, pseudo-spherical R4N+ guest spe-
cies. As envisaged, the urea/thiourea/selenourea-anion
host lattices exhibit a rich variety of inclusion topologies
besides the typical uni-directional channel structure, such
s a channel with widened cross-section accommodating
two parallel columns of guest species, corrugated-layer or
double-layer sandwich structure, and a three-dimensional
network containing isolated cages, intersecting tunnels, or
dual channel systems. It is noted that the types of host
lattices formed are dependent on the stoichiometric ratio
of urea derivatives to anions and co-crystallized solvent
molecules (water in most cases) or neutral molecules as
additional host components.
Only van der Waals contacts are made between the
included guest molecules and the molecules or anions of
the host lattices in almost all inclusion compounds. The
fundamental role of hydrogen bonding in stabilizing the
host lattices is so outstandingly important that regiospecti-
fic hydrogen bonds control the way in which small mo-
lecular units, such as the present urea derivatives and
various anions, form aggregate host lattices. It can be seen
that the requirement for strong host-host hydrogen bond-
ing in the maintenance of a host lattice need not be re-
strictive on the shape or dimensions of the lattice. Hydro-
gen bonding dimensions, particularly angles, are variable,
and host-host hydrogen bonding networks can possess a
substantial flexibility.
The strategy for the generation of inclusion com-
pounds by use of various organic host materials with
appropriate guest molecules as templates has been shown
to be an important approach in the structural design or
modification of the host lattice, and has the potential of
yielding fruitful results in crystal engineering. Our interest
We now present our results demonstrating that the N
üHĂO and NüHĂS hydrogen bonds play an instru-
mental role in the structures of a novel class of
urea/thiourea inclusion compounds. There have been the
Chinese Science Bulletin Vol. 46 No. 21 November 2001
1761

VIEW
contrasted with the classical urea or thiourea inclusion
compounds formed by self-assembly, as well as inclusion
compounds built of urea or thiourea together with various
anions, in which the hydrophobic guest species are re-
tained by steric barriers formed by the hydrogen-bonded
host lattices.
average lengths of 103 measured NüHĂO hydrogen
bonds in 23 urea-anion inclusion complexes and 148
measured NüHĂS hydrogen bonds in 28 thiourea-anion
host lattices. Although these bond lengths vary over a
wide range of values, the average values of NüHĂO and
NüHĂS are in good agreement with those found for
several types of hydrogen bonds, NĂO = 2.93 0.11 and
NĂS = 3.40 0.20 Å[23]. Generally, the hydrogen bond
length is related to the environment and configuration of
both donor and acceptor atoms, following the order of
head-to-tail mode < shoulder-to-shoulder fashion < singly
H-bond.
2
Linkage modes of urea and thiourea molecules
In an idealized scheme of host design, the well-knit
hexagonal (P6122 or P6522) urea or pseudo-hexagonal
(R3) thiourea channel framework is first divided into hy-
C
drogen-bonded chains or ribbons, and hydrogen bonding
between these units with suitable anionic and neutral spe-
cies would then lead to new varieties of host systems. The
versatility of urea, thiourea or selenourea as a key compo-
nent in the construction of novel host lattices is clearly
demonstrated by the occurrence of many new types of
linkage modes, which include various discrete units,
chains or ribbons and composite ribbons with corre-
sponding anions. The hydrogen-bonded layers can be
generated by replication of the isostructural and inde-
pendent discrete units or by other bridging moieties. The
hydrogen-bonded layers can also be built from a parallel
arrangement of ribbons or composite ribbons, ribbons
bridged by some anions, or an alternate arrangement of
two different types of ribbons. The three dimensional
channel-type network may be formed by cross-linkage of
two types of layers or by two crisscross series of ribbons
(fig. 1).
The CüHĂO hydrogen bond exists in only one
case, in which the formyl atom forms a CüHĂO hydro-
gen bond with the carboxy O atom belonging to the adja-
cent molecule, with HĂO = 2.303(6), CĂO = 3.213(6) Å
and CüHĂO = 157.9(4)e. The HĂO distance is shorter
than the sum of the relevant van der Waals radii (WH + WO
= 2.4 Å) and lies closer to the lower end of the HĂO
range of 1.5ü3.0 Å for CüHĂO hydrogen bonding es-
tablished by accurate X-ray and neutron diffraction
analyses, indicating that the formyl CH group has a rela-
tively strong donor capability.
ꢀ
+
(CH3)3N (CH2)2OHgNH2CONHCO2 g(NH2)2CO
(fig. 1) is an inclusion compound consolidated by
host-host and host-guest hydrogen bonding, in which the
allophanate O atoms of the resulting hydrogen-bonded
hydrophilic host lattice form acceptor hydrogen bonds
with the hydroxyl groups of choline guests. This may be
3 Co-molecular aggregates of urea/thiourea and
other host components
As mentioned above, in urea/thiourea-anion inclu-
sion compounds both urea and thiourea molecules adopt
various linkage modes not only of self-assembled hydro-
gen-bonded chains or ribbons but also of composite rib-
bons with the corresponding anions. The results show that
co-molecular aggregates of urea or thiourea with other
neutral molecules or anionic moieties can be considered
as fundamental building blocks for the constructions of
various types of novel host lattices. As compared with
urea, the thiourea molecule has a lesser tendency to com-
bine with other host components to form a dimer or com-
posited ribbon. In most thiourea-anion inclusion com-
pounds the host lattices are built from connections of
thiourea ribbons and the anionic dimers or just the ribbons
only.
It is of interest to note that the allophanate ion, which
is known in the form of its derivatives, and the dihydrogen
ꢀ
borate anion BO(OH)2 , the fugitive conjugate base of
boric acid, can be generated in situ and stabilized in the
crystalline state through hydrogen-bonding interactions
with its nearest neighbors. Notably the bulky and hydro-
phobic tetraalkylammonium ion plays a dual and com-
plementary role, i.e. it serves as a template for the
self-assembly of the anionic host lattice and, abetted by
the presence of urea, provides a suitably alkaline aqueous
medium that induces boric acid to function as a Brønsted
Fig. 1. Packing diagram of crystal of (n-C4H9)4N+[B5O6(OH)4]ꢀ
ꢁg2(NH2)2COgB(OH)3 viewed down the b axis, showing the puckered
ꢀ
layers parallel to (202) and planar B3O3(OH)2 ribbons extending in the
[010] direction. Broken lines represent hydrogen bonds, and atom types
are distinguished by size and shading; ƻ, oxygen;ꢀƻ, boron; ƻ, carbon;
ƻꢁꢀ, nitrogen. For clarity the enclosed (n-C4H9)4N+ ions are represented by
large shaded circles and hydrogen atoms are omitted.
1762
Chinese Science Bulletin Vol. 46 No. 21 November 2001

structures in inclusion compounds of urea, tetraalkylammonium
terephthalate/trimesate and water, Acta Crystallogr., Sect. B, 2000,
56: 142.
acid.
In summary, we have shown that novel anionic host
lattices can be constructed from urea, thiourea or se-
lenourea molecules and various anions as building blocks,
which readily adopt different topologies for the accom-
modation of tetraalkylammonium ions of various sizes.
By employing organic cations as templates and suitable
counter anions as an ancillary host material, the ‘lattice
engineering’ of new urea, thiourea and selenourea inclu-
sion compounds by self-assembly may be further ex-
plored.
10. Xue, F., Mak, T. C. W., Hydrogen-bonded anionic layer structures
constructed from 4,4Ą-biphenyldicarboxylate and water molecules,
and also with urea as an additional component, J. Phys. Org.
Chem., 2000, 13: 405.
11. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (Ċ)üüHydrogen-bonded host lattices
built of thiourea and cyclic dimeric bicarbonate moieties, J. Incl.
Phenom., 1995, 20: 73.
12. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (ċ)üüHydrogen-bonded host lattices
built of thiourea and nitrate ions, Acta Crystallogr., Sect. B, 1996,
52: 989.
13. Li, Q., Mak, T. C. W., Tetra-n-butylammonium chloride-thiourea
(1/2), a layer-type inclusion compound, Acta Crystallogr., Sect. C,
1996, 52: 2830.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (Grant No. 29973005) and we are grateful
to Hong Kong Research Grants Council Earmarked Grant (CUHK
456/95P) for supporting this research work.
14. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (Č)üüHydrogen-bonded host lattices
built of thiourea and formate ions, J. Incl. Phenom., 1997, 27: 319.
15. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (č)üüHydrogen-bonded host lattices
built of thiourea and acetate ions, J. Incl. Phenom., 1997, 28: 183.
16. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (Ď)üüHydrogen-bonded host lattices
constructed from thiourea and anions of oxalic acid and fumaric
acid, Acta Crystallogr., Sect. B, 1997, 53: 252.
17. Li, Q., Mak, T. C. W., Unsymmetrical quaternary ammonium
cations R3RN+ as guest templates for the generation of novel host
lattices constructed from thiourea and oxocarbon anions,
Supramol. Chem., 1998, 10: 49.
18. Li, Q., Mak, T. C. W., Novel inclusion compounds consolidated
by host-host and host-guest hydrogen bonding: Complexes of
thiourea with (2-hydroxyethyl)trimethylammonium carbonate and
oxalate, Cryst. Eng., 1998, 2: 169.
19. Li, Q., Mak, T. C. W., Inclusion compounds of thiourea and per-
alkylated ammonium salts (ď)üüHydrogen-bonded host lattices
constructed from thiourea and anionic species derived from ben-
zoic acid, J. Incl. Phenom., 1999, 35: 621.
20. Lam, C. K., Mak, T. C. W., Novel anionic host lattices built of
squarate and thiourea molecules, Tetrahedron, 2000, 56: 6657.
21. Li, Q., Mak, T. C. W., New inclusion compounds of selenourea
with tetraethyl- and tetra-n-propylammonium halides, Acta Cryst-
allogr., Sect. B, 1997, 53: 262.
22. Mak, T. C. W., Li, Q., Novel inclusion compounds with
urea/thiourea/selenourea-anion host lattices, in Advances in Mo-
lecular Structure Research (eds. Hargittai, M., Hargittai, I.),
Greenwich, Connecticut: JAI Press, 1998, Vol. 4, pp. 151ü225.
23. Wallwork, S. C., Hydrogen-bond radii, Acta Crystallogr., 1962, 15:
758.
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(Received May 28, 2001)
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