Investigation of steric influences on hydrogen-bonding motifs in cyclic ureas by using X-ray, neutron, and computational methods

Article abstract of DOI:10.1002/asia.201300530

A series of urea-derived heterocycles, 5N-substituted hexahydro-1,3,5- triazin-2-ones, has been prepared and their structures have been determined for the first time. This family of compounds only differ in their substituent at the 5-position (which is derived from the corresponding primary amine), that is, methyl (1), ethyl (2), isopropyl (3), tert-butyl (4), benzyl (5), N,N-(diethyl)ethylamine (6), and 2-hydroxyethyl (7). The common heterocyclic core of these molecules is a cyclic urea, which has the potential to form a hydrogen-bonding tape motif that consists of self-associative R₂ ²(8) dimers. The results from X-ray crystallography and, where possible, Laue neutron crystallography show that the hydrogen-bonding motifs that are observed and the planarity of the hydrogen bonds appear to depend on the steric hindrance at the α-carbon atom of the N substituent. With the less-hindered substituents, methyl and ethyl, the anticipated tape motif is observed. When additional methyl groups are added onto the α-carbon atom, as in the isopropyl and tert-butyl derivatives, a different 2D hydrogen-bonding motif is observed. Despite the bulkiness of the substituents, the benzyl and N,N-(diethyl)ethylamine derivatives have methylene units at the α-carbon atom and, therefore, display the tape motif. The introduction of a competing hydrogen-bond donor/acceptor in the 2-hydroxyethyl derivative disrupts the tape motif, with a hydroxy group interrupting the N-H×××O-C interactions. The geometry around the hydrogen-bearing nitrogen atoms, whether planar or non-planar, has been confirmed for compounds 2 and 5 by using Laue neutron diffraction and rationalized by using computational methods, thus demonstrating that distortion of O-C-N-H torsion angles occurs to maintain almost-linear hydrogen-bonding interactions. The incredible bulk: A series of cyclic ureas has been studied to examine the influence of bulky substituents on the hydrogen-bonding motifs that form and the degree to which the urea group can be distorted to maintain strong intermolecular contacts. Copyright

Full text of DOI:10.1002/asia.201300530

FULL PAPER
DOI: 10.1002/asia.201300530
Investigation of Steric Influences on Hydrogen-Bonding Motifs in Cyclic
Ureas by Using X-Ray, Neutron, and Computational Methods
Laura J. McCormick,[a] Ciaran McDonnell-Worth,[a] James A. Platts,[b]
Alison J. Edwards,[c] and David R. Turner*[a]
Abstract: A series of urea-derived het-
erocycles, 5N-substituted hexahydro-
1,3,5-triazin-2-ones, has been prepared
and their structures have been deter-
mined for the first time. This family of
compounds only differ in their substitu-
ent at the 5-position (which is derived
from the corresponding primary
amine), that is, methyl (1), ethyl (2),
isopropyl (3), tert-butyl (4), benzyl (5),
N,N-(diethyl)ethylamine (6), and 2-hy-
droxyethyl (7). The common heterocy-
clic core of these molecules is a cyclic
urea, which has the potential to form
a hydrogen-bonding tape motif that
consists of self-associative R22(8)
dimers. The results from X-ray crystal-
lography and, where possible, Laue
neutron crystallography show that the
hydrogen-bonding motifs that are ob-
served and the planarity of the hydro-
gen bonds appear to depend on the
steric hindrance at the a-carbon atom
of the N substituent. With the less-hin-
dered substituents, methyl and ethyl,
the anticipated tape motif is observed.
When additional methyl groups are
added onto the a-carbon atom, as in
the isopropyl and tert-butyl derivatives,
a different 2D hydrogen-bonding motif
is observed. Despite the bulkiness of
the substituents, the benzyl and N,N-
(diethyl)ethylamine derivatives have
methylene units at the a-carbon atom
and, therefore, display the tape motif.
The introduction of a competing hydro-
gen-bond donor/acceptor in the 2-hy-
droxyethyl derivative disrupts the tape
motif, with a hydroxy group interrupt-
À
ing the N H···O=C interactions. The
geometry around the hydrogen-bearing
nitrogen atoms, whether planar or non-
planar, has been confirmed for com-
pounds 2 and 5 by using Laue neutron
diffraction and rationalized by using
computational methods, thus demon-
strating that distortion of O-C-N-H tor-
sion angles occurs to maintain almost-
linear hydrogen-bonding interactions.
Keywords: computational chemis-
try · hydrogen bonds · neutron
diffraction · self-assembly · supra-
molecular chemistry
Introduction
ating molecules typically require a high degree of rigidity;
thus, the key functional groups have a predictable geometri-
cal preference.[2] Urea and urea-based molecules have been
of particular interest, owing to the near-planar geometry of
Hydrogen-bonding interactions between self-complementary
molecules has been an area of significant interest within the
field of crystal engineering for many years.[1] The direction-
ality that is afforded by these relatively strong dipolar inter-
actions offers a degree of geometric predictability that is un-
rivalled by other classes of supramolecular interaction.
Strong and predictable hydrogen bonds between self-associ-
the HNACTHNURGTNE(NUG C=O)NH unit and its ability to form cyclic hydro-
gen-bonding motifs (Scheme 1), in particular the R12(6) and
R22(8) motifs (using Etter’s graph-set nomenclature).[3] For
this reason, it is a well-utilized functional group in crystal
engineering.[4] The utility of the R12(6) motif has been dis-
played in the formation of a “tape motif” between urea
(and, to a lesser extent, thiourea) groups,[5] which was first
reported in the 1980s by Etter et al.[6] Since these seminal
papers, there have been many reports on the limitations of
this pattern[7] and of examples of its utilization in other
supramolecular systems, such as in gels[8] or tethering to-
gether two halves of a molecular capsule by virtue of a hy-
drogen-bonding belt.[9] The R22(8) motif is most notably ob-
served in hexagonal urea-clathrate materials,[10] around kO-
urea complexes (in which the oxygen atoms cannot form the
tape motif),[11] and in a number of heterocycles. The pres-
ence of a disubstituted urea functionality within a heterocy-
clic molecule is widely observed in species such as the
highly polymorphic phenobarbital and other barbiturate de-
[a] Dr. L. J. McCormick, C. McDonnell-Worth, Dr. D. R. Turner
School of Chemistry
Monash University
Clayton, VIC 3800 (Australia)
Fax : (+61)(0)3-9905-4597
E-mail: david.turner@monash.edu
[b] Dr. J. A. Platts
School of Chemistry
Cardiff University
Park Place, Cardiff, CF10 3AT (UK)
[c] Dr. A. J. Edwards
Bragg Institute
Australian Nuclear Science and Technology Organisation
Locked Bag 2001, Kirrawee DC, NSW 2234 (Australia)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201300530.
Chem. Asian J. 2013, 8, 2642 – 2651
2642
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Laue neutron diffraction were obtained by the slow diffu-
sion of Et2O into a solution of the compound in EtOH. The
substituent at the 5-position was varied to determine the
effect of steric bulk on the ability of the target ribbon motif
to form (Scheme 2). Methyl (1), ethyl (2), isopropyl (3), and
Scheme 1. The urea tape motif, which contains R12 (6) rings (left) and the
self-complementary R22 (8) motif, which can exist as a dimer between syn/
anti urea groups (top right) or as an infinite tape between anti/anti or
heterocyclic urea groups (bottom right).
rivatives,[12] benzimidazolones,[13] cyanuric acid (and deriva-
tives thereof),[14] and others.[15]
Recently, our attention has turned to the 5N-substituted
1,3,5-triazin-2-one family, which, to date, have not been
structurally characterized despite their first synthesis in the
late 1940s.[16] These heterocyclic compounds can be viewed
as cyclic ureas and, therefore, they have the potential to
form hydrogen-bonding ribbons through self-complementary
interactions. 4,6-Disubstituted triazinones, which are depro-
tonated at the 5N position, have recently been reported in
the literature as the basis for anionic ligands in metalloma-
crocycles.[17] There are obvious parallels between triazinone
species and compounds such as cyanuric acid and barbitu-
rates. The potential advantages of triazinones are that
a wide variety of their derivatives can be readily prepared
(starting from primary amines), which leads to the possibili-
ty of forming bifunctional molecules with self-recognition
abilities.
Scheme 2. The 5N-substituted 1,3,5-triazin-2-one derivatives in this study.
tert-butyl (4) derivatives were examined to test the effect of
the steric bulk on the a-carbon atom of the substituent. The
benzyl (5) and N,N-(diethyl)ethylamine (6) derivatives were
synthesized to explore the use of larger substituents, al-
though neither had significant steric bulk around the a-
carbon atom itself. The structure of the 2-hydroxylethyl de-
rivative (7) was also determined to test the robustness of the
ribbon motif in the presence of a competing hydrogen-
bond-donor/acceptor group within the same molecule. The
structures of all seven derivatives were determined free
from solvent, thereby allowing unperturbed comparisons to
be made. X-ray powder diffraction (XRPD) was performed
on all of the samples to demonstrate the phase purity (see
the Supporting Information). The only exception was the
benzyl derivative, for which two phases were crystallograph-
ically characterized, with both phases present in the XRPD
data.
Herein, we report the synthesis and structural characteri-
zation of a family of seven triazinone derivatives and the
effect that the steric bulk of the 5N substituent has on the
supramolecular networks that are formed.
The triazinone ring features a planar urea group, which
includes the two sp3 carbon atoms that are bound to the ni-
trogen atoms. A common feature of the molecular structures
of the triazinone molecules is the geometry at the sp3-hy-
bridized nitrogen atom at the 5-position of the heterocyclic
ring. There are potentially two conformations that can occur
at this position, one in which the substituent is almost per-
pendicular to the C-N-C-N-C plane of the heterocycle, that
is, an axial conformation, and a second, more planar equato-
rial conformation (Figure 1). In all of the structures that
have been obtained, with various 5N substituents, only the
Results and Discussion
The 5N-substituted 1,3,5-triazin-2-one derivatives (hence-
forth abbreviated as “triazinone”) were synthesized by
modification of previously published procedures, starting
from dimethylolurea and a primary amine. In all cases,
single crystals suitable for X-ray diffraction were obtained
by recrystallization from EtOH. In the case of compound 2
and polymorph 5a, crystals of suitable size and quality for
Chem. Asian J. 2013, 8, 2642 – 2651
2643
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Figure 1. The two potential conformations of 5N-substituted 1,3,5-triazin-
2-ones (structure of compound 1 is shown), of which only the left-hand
conformation is observed.
Figure 3. Views along the hydrogen-bonding ribbons of compounds
1 (left) and 2 (right), which show the different relative arrangements of
the substituent groups with respect to the ribbon (alternating “up–down”
for compound 1 and all of the alkyl groups on the same side of the
ribbon for compound 2).
axial conformation is observed. Density functional theory
(DFT) calculations predict that the energy of the axial con-
formation is 7.6 kJmolÀ1 lower than that of the equatorial
one (calculations based on methyl-substituted compound 1).
This result suggests that the conformation is an intrinsic
property of the molecules and not a consequence of packing
effects in the observed structures.
À
tances within the range 2.86–2.88 ꢁ and almost-linear N
H···O angles (177–1788, Table 1). From the X-ray refine-
ments, we observe that the urea group, including the hydro-
Table 1. Hydrogen-bond lengths and angles in compounds 1, 2, 5a, 5b,
and 6, taken from the X-ray diffraction data.[a]
À
À
D H [ꢁ]
D···A [ꢁ]
H···A [ꢁ]
D H···A [8]
The Ribbon Motif
1
The first structures to be determined were those of the
methyl- and ethyl-triazinone derivatives, which crystallized
in different space groups but both displayed the expected
self-complementary ribbon motif, with R22(8) hydrogen-
bonding motifs between the molecules (Figure 2). The
N1···O1
0.870(17)
2.863(1)
1.994(17)
177.3(14)
2
N1···O1
N2···O1
0.908(16)
0.885(15)
2.876(1)
2.888(1)
1.969(16)
2.003(15)
176.7(14)
178.2(12)
5a
N1···O2
N2···O2
N4···O1
N5···O1
0.894(16)
0.906(16)
0.881(17)
0.863(17)
2.847(2)
2.851(2)
2.862(2)
2.927(2)
1.953(16)
1.952(16)
1.989(17)
2.067(17)
176.4(15)
171.1(14)
170.5(15)
175.5(14)
5b
N1···O1
N2···O1
N4···O2
N5···O2
0.91(3)
0.90(3)
0.90(3)
0.85(3)
2.913(2)
2.844(2)
2.889(2)
2.822(2)
2.00(3)
1.96(3)
1.98(3)
1.99(3)
178(2)
169(2)
178(2)
166(2)
6
Figure 2. The ribbon motif, as observed in the structure of compound 2.
The same motif is observed in the methyl-substituted analogue (1).
N1···O1
N2···O1
0.891(15)
0.885(15)
2.919(1)
2.848(1)
2.032(15)
1.967(15)
173.2(13)
174.1(13)
À
[a] The positional parameters of the N H hydrogen atom were freely re-
fined (see the Experimental Section).
methyl derivative (1) crystallized in the space group P21/m,
with a mirror plane bisecting the molecule (i.e., through the
plane of the carbonyl group and the methyl substituent).
The ethyl analogue (2) crystallized in the Pbca space group,
with one molecule per asymmetric unit. The two structures
primarily differ in the manner in which the molecules within
the hydrogen-bond chain are orientated with respect to each
other (Figure 3). In compound 1, the substituents are on al-
ternate sides of the ribbon, whereas, in compound 2, the
molecules are oriented such that the substituents at the 5-
position are on the same side with respect to the mean
plane of the ribbon motif.
The hydrogen-bonding ribbon is very similar in the two
structures, although, in compound 1, the hydrogen-bonded
rings are constrained to planarity by crystallographic sym-
metry, whereas, in compound 2, the ribbon has a very shal-
low “V-shaped” character. The hydrogen-bond lengths and
angles are very similar in the two structures, with N···O dis-
gen atoms, is effectively planar (average deviation from the
mean plane: 0.008 ꢁ). The tilt of the ligands into the “V-
shaped” chain motif ensures that linear hydrogen bonds can
be formed whilst maintaining the planarity of the urea func-
tionality.
The benzyl derivative (5) was obtained as two poly-
morphs, which form at the same time (see the Experimental
Section). Each polymorph displays the expected hydrogen-
bonding tape motif. The difference between the two poly-
morphs arises from the relative orientations of the benzyl
substituents; one phase (5a) is centrosymmetric, P21/n,
whereas the other (5b) is chiral, P21 (Figure 4). Both struc-
tures contain two crystallographically independent mole-
cules in the asymmetric unit, with those of polymorph 5a
belonging to a single hydrogen-bonded chain, whilst those in
Chem. Asian J. 2013, 8, 2642 – 2651
2644
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Figure 5. a) The hydrogen-bonding chain in polymorph 5a is “staggered”,
with one independent R22(8) ring almost planar and the other between
the non-coplanar urea groups. b) One of the two unique chains in the
structure of polymorph 5b, which shows the non-planar nature of the
urea groups, which maximize the hydrogen-bonding interactions in the
staggered chain. CH hydrogen atoms are omitted for clarity.
Figure 4. The different ligand alignments of the tape motifs in the struc-
tures of centrosymmetric polymorph 5a (top) and chiral polymorph 5b
(bottom;
a single representative crystallographically unique chain is
shown for clarity). Symmetry equivalents: #1: x, 1+y, z; #2: x, 1Ày, z; #3:
2Àx, y+1/2, 1Àz; #4: 2Àx, 1/2+y, 1Àz.
polymorph 5b each constitute unique (yet very similar)
chains.
The hydrogen-bonding chains in both polymorphs 5a and
5b do not have the same planarity that is observed in the
structures of compounds 1 and 2, in which adjacent mole-
cules on one side of the chain are coplanar (on both sides
on the chain in the case of compound 1). In the case of poly-
morph 5a, there are two unique R22(8) motifs in the struc-
ture (Figure 5a). One of these rings is almost planar, with
an average non-hydrogen-atom displacement from the mean
plane of 0.056 ꢁ. The second ring is between two molecules
that are offset from each other, with an angle of 7.88 be-
tween the mean planes of the urea functionalities. Within
Figure 6. Difference Fourier plot of one unique molecule from the struc-
À
ture of polymorph 5a, which shows that the N H bond that is associated
with the offset ring motif (left) is significantly out-of-plane. All of the
molecules in polymorphs 5a and 5b show similar behavior (see the Sup-
porting Information).
À
this “offset” hydrogen-bond ring, the two N H bonds devi-
ate significantly from coplanarity with the remainder of the
urea group, as evidenced from the difference Fourier map,
pound 6. Whilst this species has the largest substituent at
the 5N position, it should be noted that the steric bulk is not
on the a-carbon atom, which has two hydrogen atoms at-
tached to it, and, therefore, the bulk is further removed
from the hydrogen-bonding sites. The structure is similar to
that of compound 2, that is, it crystallizes in the same space
group (Pbca) with a similar unit cell (Figure 7). The a and
b axes in these two structures are almost the same, with the
ab plane parallel to the chain motif. As with compound 2,
the hydrogen-bonding chain is not planar itself, adopting
a shallow “trough” shape, but molecules on the same side of
the chain are coplanar with each other and the urea groups
themselves are also planar.
À
whilst the N H bonds that are associated with the planar
ring motif are in the plane (Figure 6). The hydrogen-bond
lengths and angles are similar to those observed in com-
pounds 1 and 2, with the angles in the “offset” ring slightly
less linear at about 1718 (Table 1). The hydrogen-bonding
chains in polymorph 5b have all of the adjacent molecules
offset from each other, with no planar R22(8) motifs, which
À
results in the distortion of all of the N H bonds away from
planarity with the non-hydrogen urea atoms (Figure 5b).
The final structure in this study that displays the chain
motif is that of N,N-(diethyl)ethylamine-substituted com-
Chem. Asian J. 2013, 8, 2642 – 2651
2645
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Figure 8. Molecular structures of the two independent molecules in the
structure of polymorph 5a, taken from the neutron-diffraction data,
viewed along the N-C-O-N plane (top, CH2 hydrogen atoms of the het-
erocycle are omitted for clarity) in comparison to the difference Fourier
plots taken from the X-ray data (difference nets located around the N
À
and O atoms, as well as the electron density that is associated with N H
hydrogen atoms). In each instance, there is one planar and one non-
planar urea nitrogen atom, with the two techniques in good agreement.
Figure 7. The structures of ethyl derivative 2 (left) and N,N-(diethyl)e-
thylamine derivative 6 (right) are remarkably similar to one another, de-
spite the increased bulk of the 5N substituent. Structures viewed along
the b axis.
lent agreement with those from the X-ray data, with longer
À
N H distances, which corresponded to the definition of the
Neutron Diffraction and Computational Studies of
“true” nuclear position from the neutron data.[20] On aver-
Compound 2 and Polymorph 5a
À
age, the N H distances as observed from the neutron data
The influence of intermolecular hydrogen-bonding interac-
tions on the planarity of the amide and urea groups has re-
cently been studied by using ab initio and DFT calcula-
tions.[18] This work concluded that the deformations in pla-
narity could be adequately offset by the formation of hydro-
gen-bonding interactions. The X-ray diffraction results for
the two polymorphs of compound 5 strongly suggest that
the NH hydrogen atoms lie out of the plane so that they
may form almost-linear hydrogen bonds, in contrast to the
planar urea functionalities in the structures of compounds
1 and 2. Whilst the X-ray data is of high quality, accurate
determination of the nuclear positions of the hydrogen
atoms is difficult, owing to the low electron density and
bond polarization. Crystals of compound 2 and polymorph
5a were obtained with a suitable size for analysis by Laue
neutron diffraction. The re-emergence of the Laue tech-
nique allows for the neutron-diffraction data to be obtained
by using modestly sized samples (about 0.1 mm3) within
a relatively short time.[19] Data from these experiments are
able to unequivocally determine the positions of the hydro-
gen atoms to assign the geometry of the hydrogen-bonding
interactions.
compound of 2 and polymorph 5a are about 0.1 ꢁ longer
than those observed from the X-ray data (Table 1 and
Table 2). The agreement between the apparent non-planar
À
N H bonds in the X-ray data of compound 2 and poly-
morph 5a with the experimentally determined positions of
the hydrogen nuclei from the neutron data give confidence
in the assignment of non-planar urea functionalities in the
other structures that form part of this study (see the Sup-
porting Information).
The structures of compound 2 and polymorph 5a as ob-
tained by neutron diffraction were used as the starting
points for computational studies of the hydrogen-bonding
interactions and the difference in planarity of the urea
Table 2. Hydrogen-bond lengths and angles in compounds 2 and 5a,
taken from the neutron-diffraction data (c.f. Table 1 for parameters
taken from the X-ray data).
À
À
D H [ꢁ]
D···A [ꢁ]
H···A [ꢁ]
D H···A [8]
2
N1···O1
N2···O1
1.008(9)
1.010(10)
2.873(5)
2.893(5)
1.866(9)
1.883(10)
176.9(9)
178.1(8)
The neutron data show that the molecules in the structure
of compound 2 do contain planar urea functionalities, whilst
those in polymorph 5a contain one out-of-plane NH bond
per molecule (Figure 8). The angles in the structures that
were determined by using the neutron data were in excel-
5a
N1···O2
N2···O2
N4···O1
N5···O1
1.015(18)
1.027(15)
1.008(16)
1.007(17)
2.847(7)
2.919(8)
2.855(8)
2.823(7)
1.839(17)
1.893(16)
1.854(16)
1.817(16)
171.4(12)
176.0(12)
171.5(12)
176.9(9)
Chem. Asian J. 2013, 8, 2642 – 2651
2646
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Table 4. H-N-C-O torsion angles from the neutron structures of com-
pounds 2 and 5a and the angles in calculated structures, in which the hy-
drogen atoms were allowed to relax (with the heavy atoms held at calcu-
lated positions).
groups between these two compounds. Initial DFT studies
differentiated between the energetics of the conformations
of the molecules as determined from the neutron-diffraction
studies, in which the idealized geometries contained planar
urea groups (i.e., geometries in which the H-N-C-O torsion
angles are exactly zero). For compound 2, the difference
was found to be negligible, 0.03 kJmolÀ1, because the experi-
mentally determined structure was very close to planar (tor-
sion angles of 0.3 and 0.98). For the two independent
benzyl-substituted molecules in polymorph 5a, in which one
of the NH hydrogen atoms was significantly out of the plane
in both molecules, the observed structure was 1–2 kJmolÀ1
less stable than the idealized planar geometry (Table 3).
Starting angle [8]
Final angle [8]
2
0.4, 1.1
2.2, 17.8
2.1, 11.9
3.5, 3.8
2.5, 19.4
0.4, 11.0
5[a]
[a] Two crystallographically unique molecules were present in the asym-
metric unit.
to “adjust” to form almost-linear intermolecular interac-
tions.
2D Networks of the iPr and tBu Derivatives
Table 3. H-N-C-O torsion angles from the neutron structures of com-
pounds 2 and 5a and the calculated energy difference from their ideal-
ized planar structures.
The structures of the isopropyl- (3) and tert-butyl-triazinone
derivatives (4) do not display the ribbon motif that is ob-
served in the previously discussed structures; rather, they
form 2D hydrogen-bonding networks. In these two com-
pounds, there is increased steric bulk at the a-carbon atom
of the 5N substituent. In the previously discussed structures
(1, 2, 5, and 6) the a-carbon atom has a CH2R environment,
that is, a methylene bridge. In compounds 3 and 4, the
number of hydrogen atoms on the a-carbon atom decreases
to one and zero, respectively, which corresponds to the large
observed changes in the hydrogen-bonding motifs in the
solid state.
Both compounds crystallize in the monoclinic space group
P21/c, with similar (but not identical) unit cells. In both in-
stances, the same 2D hydrogen-bonding network is observed
(Figure 9). The hydrogen-bonding sheets contain two dis-
tinct ring motifs, that is the anticipated self-associative R22(8)
motif, as observed in the above-described structures, as well
as a much-larger R46(20) motif that involves six separate mol-
ecules.
Torsion angle [8]
DE [kJmolÀ1
]
2
0.4, 1.0
2.2, 17.8
2.1, 11.9
0.03
1.11
1.81
5[a]
[a] Two crystallographically unique molecules were present in the asym-
metric unit.
DFT calculations also gave a stabilization energy for the
hydrogen-bonding ring motif of À63.37 kJmolÀ1 in the struc-
ture of compound 2 (which only contained one crystallo-
graphically unique synthon). There were two R22(8) motifs in
the structure of polymorph 5a, one of which contained two
À
N H bonds that were in-plane with their respective urea
À
groups, whilst the other contained the two out-of-plane N
H bonds. These hydrogen-bonding rings have calculated en-
ergies of À68.2 and À62.7 kJmolÀ1, respectively. The
strengths of these interactions more than account for the
small energy costs that are associated with distortion of the
The 2D sheets extend parallel to the ab plane, with the
bulky alkyl substituents approximately normal to the plane
(Figure 9). There are four unique NH···O hydrogen bonds in
each structure. The D···A distances within the two structures
are very similar (Table 5), but significant deviations are ob-
À
N H hydrogen atoms from the plane of the urea functional-
À
ity. It is also clear that distortion of the N H bonds out of
the urea plane has no impact on the overall energy of the
motif.
À
À
To further study whether the distortion of the N H bonds
served in the D H···A angles, both within the individual
is due to local hydrogen-bonding effects, DFT calculations
were conducted by using a dimer, that is, two molecules
with a single R22(8) motif, in which the non-hydrogen atoms
were fixed at coordinates that were determined from the
neutron structures and the hydrogen atoms were allowed to
relax. Table 4 summarizes the starting and final positions of
the hydrogen atoms, based on the H-N-C-O torsion angles.
These results indicate that the hydrogen atoms, as deter-
mined from the neutron-diffraction data, are close to their
relaxed positions in a discrete dimer. Given the small ener-
structures and between them. The NH bonds in the struc-
tures of compounds 3 and 4 also deviate considerably from
coplanarity with the urea group (for the difference Fourier
maps, see the Supporting Information). The arrangement of
neighboring molecules in the structures of compounds 3 and
4 differs from that observed in the other compounds and ap-
pears to be influenced by the steric bulk of the 5N substitu-
ents (Figure 10). The presence of bulkier groups with more
than a single methyl substituent on the a-carbon atom dis-
rupts the packing that would otherwise occur between, not
within, hydrogen-bonded chains and, therefore, the forma-
tion of the chain motif itself is prevented. Figure 10b shows
a space-filling representation of the packing between the
ethyl substituents on adjacent chains in the structure of
compound 2. This packing allows for the molecules within
À
gies that are involved in the distortion of the N H bonds
from planarity, coupled with the strength of the hydrogen-
bonding interactions, it appears that there is a significant
degree of tolerance within the ribbon motif as to the relative
orientations of the molecules, with the hydrogen atoms able
Chem. Asian J. 2013, 8, 2642 – 2651
2647
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
Figure 10. a) A comparison between the relative orientations of three
neighboring molecules in the structures of compounds 2 (top) and 3
(bottom). (b) Packing in the structure of compound 2, which shows the
close fit between the ethyl groups of adjacent chains (one shown in white
for clarity, c.f. Figure 7).
placing the ethyl group with isopropyl or tert-butyl groups
would not allow the same packing, as observed experimen-
tally. Whilst the increased bulk of these groups is, hypotheti-
cally, not sufficient to disrupt the packing within a chain, it
appears that the inter-chain packing is disrupted, thus lead-
ing to the formation of an alternate hydrogen-bonded net-
work. Those structures that display the tape motif are all
able to orient the 5N substituent such that efficient packing
occurs.
Figure 9. One hydrogen-bonded sheet in the structures of compounds 3
(top) and 4 (middle), with only the a-carbon atom of the substituents
shown for clarity, and a “side-on” view of one sheet of compound 4
(bottom). The two distinct hydrogen-bonded ring motifs are highlighted
in green, R22(8), and blue, R46(20). The hydrogen-bond parameters are
listed in Table 5.
Table 5. Hydrogen-bond lengths and angles in compounds 3 and 4, taken
from the X-ray diffraction data.[a]
À
À
D H [ꢁ]
D···A [ꢁ]
H···A [ꢁ]
D H···A [8]
Structure of the 2-Hydroxyethyl Derivative
3
The 2-hydroxyethyl-triazinone derivative (7) was synthe-
sized to test whether the presence of a further potential hy-
drogen-bond-donor/acceptor group, that is, a hydroxy func-
tionality, would disrupt the formation of the ribbon motif.
As expected, this expectation was the case, with the OH
group both accepting a hydrogen bond from an NH donor
and acting as a donor towards a carbonyl group.
Compound 7 crystallizes in the space group P21, with
a complete molecule contained within the asymmetric unit.
The presence of the hydroxy group disrupts the self-comple-
mentary ring motif, which is not observed in this structure
(compared with the observations that it is the only motif in
structures 1, 2, 5a/b, and 6 and one of two motifs in struc-
tures 3 and 4). Instead, two hydrogen-bonding ring motifs
N1···O2
N2···O1
N4···O2
N5···O1
0.875(18)
0.887(18)
0.886(18)
0.843(18)
2.889(1)
2.896(1)
2.888(1)
2.895(2)
2.021(18)
2.014(18)
2.008(18)
2.057(18)
172.2(16)
172.5(15)
172.0(15)
172.0(16)
4
N1···O2
N2···O2
N4···O1
N5···O1
0.871(13)
0.882(14)
0.900(14)
0.849(13)
2.910(1)
2.947(1)
2.917(1)
2.900(1)
2.040(13)
2.071(14)
2.021(14)
2.053(14)
176.3(12)
172.6(12)
172.9(12)
175.1(12)
À
[a] The positional parameters of the N H hydrogen atom were freely re-
fined (see the Experimental Section).
each hydrogen-bonded chain to be ideally spaced for the
formation of hydrogen-bonding motifs. It is clear that re-
Chem. Asian J. 2013, 8, 2642 – 2651
2648
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
compensated for by stabilization owing to hydrogen-bonding
interactions. Work to incorporate the triazinone tape motif
into functionalized systems is underway.
Experimental Section
Synthesis
All of the starting materials and solvents were purchased from standard
commercial sources and used without further purification. The syntheses
were conducted by using modifications to the general strategy reported
by Burke.[16] NMR spectra were recorded on a Bruker Avance 400 spec-
trometer and the signals are reported in ppm measured relative to the re-
sidual solvent peaks. Elemental analysis was conducted at the Campbell
Analytical Laboratories, the University of Otago (New Zealand).
Figure 11. The hydrogen-bonded sheet in the structure of compound 7,
viewed along the a axis (all CH hydrogen atoms are omitted for clarity).
Hydrogen-bond parameters are listed in Table 6.
5-Methylhexahydro-1,3,5-triazin-2-one (1)
An aqueous solution of methylamine (40 wt.%, 2 mL, 0.023 mol) was
added to a suspension of dimethylolurea (2.40 g, 0.020 mol) in water
(1.4 mL). The solution was heated at reflux for 3 h and allowed to cool to
RT overnight. The water was removed by rotary evaporation and the
white residue was recrystallized from hot EtOH to afford 5-methylhexa-
hydro-1,3,5-triazin-2-one as a colorless solid (350 mg, 3.04 mmol, 15%
yield). 1H NMR (400 MHz, [D4]MeOH): d=2.55 (s, 3H; CH3), 4.14 ppm
(s, 4H; CH2); 13C NMR (100 MHz, [D4]MeOH): d=38.78 (CH3), 63.97
(CH2), 157.93 ppm (CO); elemental analysis calcd (%) for C4H9N3O:
C 41.73, H 7.88, N 3.65; found: C 41.58, H 8.04, N 3.46.
Table 6. Hydrogen-bond lengths and angles in compound 7, taken from
the X-ray diffraction data.[a]
À
À
D H [ꢁ]
D···A [ꢁ]
H···A [ꢁ]
D H···A [8]
7
N1···O1
N2···O2
O2···O1
0.92(3)
0.93(3)
0.89(3)
2.930(2)
2.841(2)
2.648(2)
2.08(3)
1.92(3)
1.76(3)
152(2)
173(2)
174(3)
5-Ethylhexahydro-1,3,5-triazin-2-one (2)
À
À
[a] The positional parameters of all of the N H and O H hydrogen
atoms were freely refined (see the Experimental Section).
A solution of dimethylolurea (2.40 g, 0.02 mol) and ethylamine (68 wt.%,
1.89 mL, 0.02 mol) in water (8 mL) was heated at reflux for 90 min. The
solution was allowed to cool to RT overnight and the solvent was re-
moved under reduced pressure. The colorless residue was recrystallized
from EtOH to yield 5-ethylhexahydro-1,3,5-triazin-2-one as colorless
crystals (830 mg, 6.4 mmol, 32% yield). 1H NMR (400 MHz,
[D4]MeOH): d=1.15 (t, J=7.2 Hz, 3H; CH3), 2.76 (quartet, J=7.2 Hz,
2H; CH2CH3), 4.19 ppm (s, 4H; CH2); 13C NMR (100 MHz, [D4]MeOH):
d=13.3 (CH3), 45.1 (CH2CH3), 61.7 (CH2), 158.3 ppm (CO); elemental
analysis calcd (%) for C5H11N3O: C 46.50, H 8.58, N 32.53; found:
C 46.25, H 8.68, N 32.76.
are observed, both of which incorporate all of the unique
donor/acceptor sites (Figure 11).
Conclusions
A series of 5-substituted 1,3,5-triazin-2-ones have been syn-
thesized and structurally characterized to determine the ro-
bustness of the postulated self-complementary ribbon motif
between the cyclic-urea functionalities. From their struc-
tures, it appears that the steric bulk at the a-carbon atom of
the substituent at the 5N position of the ring plays a major
role in determining the crystal packing and, hence, the hy-
drogen-bonding motifs that occur. With a primary or secon-
dary a-carbon atom at this position, the expected tape motif
is observed, regardless of the bulk that is present further
along the carbon chain of the substituent (1, 2, 5, and 6).
The use of iPr or tBu derivatives (i.e. tertiary or quaternary
a-carbon atoms) promotes the formation of an alternative
2D network (3 and 4). This difference appears to be caused
by packing effects of the bulkier groups, rather than by an
inherent inability of the tape motif to form between these
molecules. Introducing a competing hydrogen-bond donor/
acceptor by virtue of a hydroxyethyl derivative (7) disrupts
the tape motif. A combination of neutron diffraction and
computational studies indicate that the positions of the hy-
drogen atoms can be perturbed from their ideal planar ge-
ometry to satisfy hydrogen-bonding requirements, with an
energy penalty of less than 2 kJmolÀ1, which is more than
5-Isopropylhexahydro-1,3,5-triazin-2-one (3)
To a cooled suspension of dimethylolurea (2.40 g, 0.02 mol) in water
(10 mL) was added isopropyl amine (1.71 mL, 0.02 mol) and the mixture
was heated at reflux for 90 min. The cooled solution was allowed to
stand overnight. The solvent was removed by rotary evaporation and the
colorless residual sludge was recrystallized from hot EtOH to yield 5-iso-
propylhexahydro-1,3,5-triazin-2-one as colorless needle-shaped crystals
(0.49 g, 3.4 mmol, 17% yield). 1H NMR (400 MHz, [D4]MeOH): d=1.16
(d, J=6.4 Hz, 6H; CH3), 3.10 (sept, J=6.4 Hz, 1H; CH), 4.28 (s, 4H;
CH2), 4.82 ppm (s, 2H; NH); 13C NMR (100 MHz, [D4]MeOH): d=21.42
(CH3), 48.42 (CH), 59.82 (CH2), 158.80 ppm (CO); elemental analysis
calcd (%) for C6H13N3O: C 50.33, H 9.15, N 29.35; found: C 50.58,
H 9.27, N 29.47.
5-tert-Butylhexahydro-1,3,5-triazin-2-one (4)
tert-Butylamine (2.1 mL, 0.02 mol) was added to a suspension of dime-
thylolurea (2.4 g, 0.02 mol) in water (3.4 mL) and the mixture was heated
at reflux for 2 h. The solution was allowed to cool to RT overnight,
during which time a crystalline solid was formed. This solid was recrystal-
lized from hot EtOH to yield 5-tert-butylhexahydro-1,3,5-triazin-2-one as
colorless plate-shaped crystals (0.71 g, 4.5 mmol, 23% yield). 1H NMR
(400 MHz, [D4]MeOH): d=1.23 (s, 9H; CH3), 4.32 (s, 4H; CH2),
4.83 ppm (s, 2H; NH); 13C NMR (100 MHz, [D4]MeOH): d=28.79
(CH3), 55.03 (CH), 58.03 (CH2), 159.10 ppm (CO); elemental analysis
calcd (%) for C7H15N3O: C 53.48, H 9.62, N 26.73; found: C 53.74,
H 9.79, N 26.80.
Chem. Asian J. 2013, 8, 2642 – 2651
2649
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
5-Benzylhexahydro-1,3,5-triazin-2-one (5)
158.39 ppm (CO); elemental analysis calcd (%) for C5H11N3O2: C 41.37,
H 7.63, N 28.95; found: C 41.19, H 7.73, N 28.96.
Benzylamine (2 mL, 0.018 mol) was added to a suspension of dimethylo-
lurea (2.19 g, 0.018 mol) in water (10 mL). The mixture was heated at
reflux for approximately 4.5 h and was then allowed to cool to RT over-
night, during which time the solution separated into two layers. The mix-
ture was concentrated under reduced pressure and the colorless oil was
recrystallized from hot EtOH to yield a tacky colorless solid. This solid
was recrystallized from hot EtOH a second time to give 5-benzylhexahy-
dro-1,3,5-triazin-2-one as colorless needle-shaped crystals (184 mg,
1.26 mmol, 7% yield). 1H NMR (400 MHz, [D4]MeOH): d=3.88 (s, 2H;
CH2), 4.16 (s, 4H; CH2), 7.0 ppm (m, 5H; CH); 13C NMR (100 MHz,
[D4]MeOH): d=55.22 (CH2), 61.76 (CH2), 128.63 (CH), 129.50 (CH),
130.23 (CH), 139.02 (CH2), 158.36 ppm (CO); elemental analysis calcd
(%) for C10H13N3O: C 62.81, H 6.85, N 21.97; found: C 63.09, H 6.93,
N 21.64.
X-ray Crystallography
Data were collected on the MX1 beamline at the Australian Synchrotron
operating at 17.44 keV (l=0.71090 ꢁ) with collection temperatures
maintained at 200 K by using an open-flow N2 cryostream. Data was col-
lected by using the BluIce software package and initial data processing
was conducted by using the XDS package.[21] Structure solutions were ob-
tained by using direct methods on SHELXS-97.[22] Structures were re-
fined against F2 by least-squares methods on SHELXL-97 with X-Seed as
a graphical interface.[23] All non-hydrogen atoms were refined by using
an anisotropic model. Hydrogen atoms that were attached to carbon
atoms were placed at idealized geometries and refined by using a riding
model. Hydrogen atoms that were attached to oxygen or nitrogen atoms
were located from the difference Fourier map. The positions of these hy-
drogen atoms were allowed to refine freely in all cases, with the Uiso
value fixed to be 1.2 times that of the isotropic equivalent of the atom to
which the hydrogen was attached. Full X-ray crystallographic details for
compounds 1–7 are provided in Table 7. XRPD data were collected on
a Bruker D8 Focus diffractometer that was equipped with CuKa radiation
(l=1.54 ꢁ) and operated at RT. Experimental patterns were compared
with calculated patterns as determined from the single-crystal data,
which were prepared by using the Mercury program (see the Supporting
Information).[24]
5-(2-(Diethylamino)ethyl)-hexahydro-1,3,5-triazin-2-one (6)
To a suspension of dimethylol urea (1.21 g, 0.010 mmol) in water (5 mL)
was added N,N-diethylethylenediamine (1.38 mL, 0.0098 mol) and the re-
sulting mixture was heated at reflux for 2.5 h. The solution was allowed
to cool overnight and the solvent was removed by rotary evaporation to
yield a viscous yellow-orange oil. This oil was allowed to stand for 2 days,
during which time pale-yellow crystals formed. The crystals were collect-
ed by vacuum filtration, washed with copious amounts of Et2O, and dried
at the pump for several hours to afford colorless crystals of 5-(2-diethyla-
mino)ethyl-1,3,5-triazin-2-one (350 mg, 1.75 mmol, 17% yield). 1H NMR
(400 MHz, [D4]MeOH): d=1.24 (t, J=7.2 Hz, 6H), 2.78 (quartet, J=
7.2 Hz, 4H), 2.84 (dd, J=8.3 Hz, 6.1 Hz, 2H), 3.03 (dd, J=8.3 Hz,
6.1 Hz, 2H), 4.37 ppm (s, 4H); 13C NMR (100.5 MHz): d=11.38, 48.21,
48.56, 52.06, 62.72, 158.31 ppm.
CCDC 934004 (1), CCDC 934005 (2), CCDC 934006 (3), CCDC 934007
(4), CCDC 934008 (5a), CCDC 934009 (5b), CCDC 934010 (6),
CCDC 934011 (7), CCDC 934012 (2), and CCDC 934013 (5) contain the
supplementary crystallographic data (X-ray and neutron) for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
5-(2-Hydroxyethyl)-hexahydro-1,3,5-triazin-2-one (7)
Laue Neutron Crystallography
To a suspension of dimethylolurea (2.42 g, 0.020 mol) in water (3.4 mL)
was added 2-ethanolamine (1.21 mL, 0.02 mol) and the mixture was
heated at reflux for 2 h. The solution was allowed to cool overnight
before being concentrated under reduced pressure. The resulting yellow
solid was recrystallized from hot EtOH to give colorless crystals of 5-(2-
hydroxyethyl)-hexahydro-1,3,5-triazin-2-one (269 mg, 1.9 mmol, 9%
yield). 1H NMR (400 MHz, [D4]MeOH): d=2.86 (t, J=5.6 Hz, 2H;
CH2), 3.69 (t, J=5.6 Hz, 2H; CH2), 4.21 ppm (s, 4H; CH2); 13C NMR
(100 MHz, [D4]MeOH): d=53.54 (CH2), 61.18 (CH2), 62.91 (CH2),
In all cases, crystals of suitable size and quality for neutron diffraction
were grown by the slow diffusion of Et2O into a solution of the triazi-
none in MeOH. Data were collected on the KOALA instrument at the
Bragg Institute, ANSTO, Australia. The data-collection temperature was
maintained at 200 K by using an open-flow N2 cryostream. Suitable crys-
tals were mounted onto a quartz pin by using viscous perfluorinated oil.
Laue diffraction images were measured from the stationary crystal at set
rotation intervals. Datasets were reduced by using the unit cells as deter-
mined from the X-ray data with the in-house-developed Laue1234 pro-
Table 7. X-ray crystallographic data for compounds 1–7 at 200 K.
1
2[a]
3
4
5a[a]
5b
6
7
N substituent
formula
Mw
Me
C4H9N3O
115.14
Et
iPr
C6H13N3O
143.19
tBu
C7H15N3O
157.22
PhCH2
C10H13N3O
191.23
PhCH2
C10H13N3O
191.23
Et2NCH2CH2
C9H20N4O
200.29
HOCH2CH2
C5H11N3O2
145.17
C5H11N3O
129.17
crystal system
space group
monoclinic
P21/m
orthorhombic
Pbca
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21
orthorhombic
Pbca
monoclinic
P21
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
5.039(1)
7.165(1)
7.635(1)
90
9.589(2)
7.185(1)
19.634(1)
90
10.059(1)
5.720(1)
27.581(2)
90
5.912(1)
10.271(1)
27.328(1)
90
16.777(4)
7.104(4)
17.413(5)
90
10.112(1)
7.071(1)
14.596(1)
90
9.444(1)
7.215(1)
32.751(2)
90
5.049(1)
8.303(1)
8.196(1)
90
b [8]
97.479(4)
90
273.31(8)
2
90
90
98.125(1)
90
1571.0(3)
8
91.796(2)
90
1658.6(3)
8
108.318(6)
90
1970.2(13)
8
106.720(2)
90
999.52(19)
4
90
90
2231.6(4)
8
97.448(4)
90
340.69(9)
2
g [8]
V [ꢁ3]
1352.7(5)
8
Z
unique reflns/I>2s(I)
Rint
719/673
0.0346
1705/1474
0.0369
3918/3516
0.0453
4132/3698
0.0554
4979/4008
0.0845
2673/2578
0.0540
2849/2553
0.1051
890/878
0.0490
no. of parameters
R1 (all data)/I>2s(I)
wR2 (all data)/I>2s(I)
GoF
47
89
197
217
266
265
135
100
0.0407/0.0375
0.1004/0.0930
1.104
0.0460/0.0431
0.1166/0.1139
1.066
0.0544/0.0503
0.1362/0.1331
1.058
0.0468/0.0434
0.1240/0.1208
1.054
0.0597/0.0467
0.1290/0.1195
1.030
0.0388/0.0374
0.0999/0.0968
1.071
0.0517/0.0475
0.1325/0.1286
1.082
0.0313/0.0304
0.0836/0.0787
1.222
residual max./min.
0.263/À0.194
0.255/À0.179
0.233/À0.249
0.324/À0.215
0.336/À0.213
0.202/À0.154
0.219/À0.237
0.197/À0.169
[a] Neutron data were also collected.
Chem. Asian J. 2013, 8, 2642 – 2651
2650
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemasianj.org
David R. Turner et al.
gram suite.[25] Refinements were carried out by using SHELXL on the
structures that were obtained from the X-ray data as the starting
points.[22] All atoms were fully refined by using an anisotropic model with
no restraints. Data-collection and -refinement parameters are given
below.
F. W. Fowler, J. W. Lauher, J. Am. Chem. Soc. 2001, 123, 11057–
11064; d) L. S. Reddy, S. K. Chandran, S. George, N. J. Babu, A.
Nangia, Cryst. Growth Des. 2007, 7, 2675–2690.
[8] J. W. Steed, Chem. Soc. Rev. 2010, 39, 3686–3699.
[9] a) A. Rajbanshi, R. Custelcean, Supramol. Chem. 2012, 24, 65–71;
b) D. R. Turner, M. J. Paterson, J. W. Steed, Chem. Commun. 2008,
1395–1397.
5-Ethylhexahydro-1,3,5-triazin-2-one (2)
[10] K. D. M. Harris, Chem. Soc. Rev. 1997, 26, 279–289.
[11] S. Smeets, M. Lutz, Acta Crystallogr. Sect. C 2011, 67, m50–m55.
[12] a) N. Zencirci, T. Gelbrich, V. Kahlenberg, U. J. Griesser, Cryst.
Growth Des. 2009, 9, 3444–3456; b) J. Han, L. Zhao, C.-W. Yau,
T. C. W. Mak, Cryst. Growth Des. 2009, 9, 308–319.
Crystal dimensions: 2.0ꢂ0.8ꢂ0.8 mm; 8 frames; frame spacing: 138; ex-
posure time/frame: 5400 s; 1055 unique reflections; 786 reflections with
I>2s(I); 181 parameters; R1 =0.0734 (I>2s(I)).
5-Benzylhexahydro-1,3,5-triazin-2-one (5a)
[13] K. E. Schwiebert, D. N. Chin, J. C. MacDonald, G. M. Whitesides, J.
Am. Chem. Soc. 1996, 118, 4018–4029.
[14] S. Marivel, E. Suresh, V. R. Pedireddi, Tetrahedron Lett. 2008, 49,
3666–3671.
Crystal dimensions: 1.3ꢂ1.0ꢂ0.8 mm; 11 frames; frame spacing: 178; ex-
posure time/frame: 10000 s; 1658 unique reflections; 1034 reflections
with I>2s(I); 487 parameters; R1 =0.0498 (I>2s(I)).
[15] a) D. Gherase, J.-V. Naubron, C. Roussel, M. Giorgi, Acta Crystal-
logr. Sect. C 2012, 68, o247–o252; b) S. Graus, S. Uriel, J. L. Serrano,
CrystEngComm 2012, 14, 3759–3766.
[16] W. J. Burke, J. Am. Chem. Soc. 1947, 69, 2136–2137.
[17] A. Poethig, T. Strassner, Organometallics 2012, 31, 3431–3434.
[18] J. A. Platts, H. Maarof, K. D. M. Harris, G. K. Lim, D. J. Willock,
Phys. Chem. Chem. Phys. 2012, 14, 11944–11952.
Computational Studies
All of the computational studies employed the B3LYP DFT method[26]
with the 6–31+GACHTUNGTRENNUNG
(d,p) basis set,[27] as implemented within the Gaussi-
an 09 package.[28] The coordinates of single molecules and hydrogen-
bonded dimers of compounds 2 and 5a were extracted from their corre-
sponding neutron structures and used either without modification or with
altered hydrogen positions (for the forced planar geometries), whilst
keeping all of the heavy-atom coordinates fixed.
[19] A. J. Edwards, Aust. J. Chem. 2011, 64, 869–872.
[20] a) F. H. Allen, Acta Crystallogr. Sect. B 1986, 42, 515–522; b) F. H.
Allen, I. J. Bruno, Acta Crystallogr. Sect. B 2010, 66, 380–386.
[21] a) W. Kabsch, J. Appl. Crystallogr. 1993, 26, 795–800; b) T. M.
McPhillips, S. E. McPhillips, H. J. Chiu, A. E. Cohen, A. M. Deacon,
P. J. Ellis, E. Garman, A. Gonzalez, N. K. Sauter, R. P. Phizackerley,
S. M. Soltis, P. Kuhn, J. Synchrotron Radiat. 2002, 9, 401–406.
[22] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[23] L. J. Barbour, J. Supramol. Chem. 2001, 1, 189–191.
[24] C. F. Macrae, P. R. Edgington, P. McCabe, E. Pidcock, G. P. Shields,
R. Taylor, M. Towler, J. van de Streek, J. Appl. Crystallogr. 2006, 39,
453–457.
[25] R. O. Piltz, Acta Crystallogr. 2011, A67, C155–C156.
[26] a) C. T. Lee, W. T. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789;
b) A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
[27] a) T. Clark, J. Chandrasekhar, G. W. Spitznagel, P. V. Schleyer, J.
Comput. Chem. 1983, 4, 294–301; b) R. Ditchfield, W. J. Hehre,
J. A. Pople, J. Chem. Phys. 1971, 54, 724–728; c) M. S. Gordon,
Chem. Phys. Lett. 1980, 76, 163–168.
Acknowledgements
D.R.T. gratefully acknowledges the Australian Institute of Nuclear Sci-
ence and Engineering (AINSE) for a research fellowship and funding.
Part of this work was undertaken on the MX1 beamline of the Australian
Synchrotron, Victoria, Australia. Part of this work was conducted at the
OPAL, ANSTO, under Bragg Institute proposal no. PPR2215.
[1] a) G. Desiraju, T. Steiner, The Weak Hydrogen Bond In Structural
Chemistry and Biology, Oxford University Press, Oxford, 2001;
b) J. W. Steed, J. L. Atwood, Supramolecular Chemistry, 2nd ed.,
John Wiley & Sons Ltd, Chichester, 2009; c) J. W. Steed, D. R.
Turner, K. J. Wallace, Core Concepts in Supramolecular Chemistry
[28] Gaussian 09, Revision C.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant,
S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gom-
perts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Po-
melli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzew-
ski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J.
Fox, Gaussian, Inc., Wallingford, CT, 2010.
and Nanochemistry, John Wiley
d) G. A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford Uni-
versity Press, New York, 1997.
& Sons Ltd, Chichester, 2007;
[2] a) L. Infantes, W. D. S. Motherwell, Chem. Commun. 2004, 1166–
1167; b) M. C. Etter, J. Phys. Chem. 1991, 95, 4601–4610; c) A. D.
Burrows, Struc. Bonding 2004, 108, 55–95.
[3] a) M. C. Etter, Acc. Chem. Res. 1990, 23, 120–126; b) J. Bernstein,
R. E. Davis, L. Shimoni, N. L. Chang, Angew. Chem. 1995, 107,
1689–1708; Angew. Chem. Int. Ed. Engl. 1995, 34, 1555–1573.
[4] R. Custelcean, Chem. Commun. 2008, 295–307.
[5] R. Custelcean, M. G. Gorbunova, P. V. Bonnesen, Chem. Eur. J.
2005, 11, 1459–1466.
[6] a) M. C. Etter, T. W. Panunto, J. Am. Chem. Soc. 1988, 110, 5896–
5897; b) M. C. Etter, Z. Urbanczyklipkowska, M. Ziaebrahimi, T. W.
Panunto, J. Am. Chem. Soc. 1990, 112, 8415–8426.
[7] a) R. W. Troff, R. Hovorka, T. Weilandt, A. Luetzen, M. Cetina, M.
Nieger, D. Lentz, K. Rissanen, C. A. Schalley, Dalton Trans. 2012,
41, 8410–8420; b) P. Byrne, D. R. Turner, G. O. Lloyd, N. Clarke,
J. W. Steed, Cryst. Growth Des. 2008, 8, 3335–3344; c) T. L. Nguyen,
Received: April 17, 2013
Published online: August 8, 2013
Chem. Asian J. 2013, 8, 2642 – 2651
2651
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim