The tris-urea motif and its incorporation into polydimethylsiloxane-based supramolecular materials presenting self-healing features

Article abstract of DOI:10.1002/chem.201203518

Materials of supramolecular nature have attracted much attention owing to their interesting features, such as self-reparability and material robustness, that are imparted by noncovalent interactions to synthetic materials. Among the various structures and synthetic methodologies that may be considered for this purpose, the introduction of extensive arrays of multiple hydrogen bonds allows for the formation of supramolecular materials that may, in principle, present self-healing behavior. Hydrogen bonded networks implement dynamic noncovalent interactions. Suitable selection of structural units gives access to novel dynamic self-repairing materials by incrementing the number of hydrogen-bonding sites present within a molecular framework. Herein, we describe the formation of a tris-urea based motif giving access to six hydrogen-bonding sites, easily accessible through reaction of carbohydrazide with an isocyanate derivative. Extension towards the synthesis of multiply hydrogen-bonded supramolecular materials has been achieved by polycondensation of carbohydrazide with a bis-isocyanate component derived from poly-dimethylsiloxane chains. Such materials underwent self-repair at a mechanically cut surface. This approach gives access to a broad spectrum of materials of varying flexibility by appropriate selection of the bis-isocyanate component that forms the polymer backbone. Healing power: Easy access to a tris-urea motif, with the possibility of forming six H-bonds, has been demonstrated. This concept was further extended to the formation of soft functional materials based on polydimethylsiloxane moieties amenable to self-repair (see figure). Copyright

Full text of DOI:10.1002/chem.201203518

DOI: 10.1002/chem.201203518
The Tris-Urea Motif and Its Incorporation into Polydimethylsiloxane-Based
Supramolecular Materials Presenting Self-Healing Features
Nabarun Roy,[a] Eric Buhler,[a, b] and Jean-Marie Lehn*[a]
Abstract: Materials of supramolecular
nature have attracted much attention
owing to their interesting features, such
as self-reparability and material robust-
ness, that are imparted by noncovalent
interactions to synthetic materials.
Among the various structures and syn-
thetic methodologies that may be con-
sidered for this purpose, the introduc-
tion of extensive arrays of multiple hy-
drogen bonds allows for the formation
of supramolecular materials that may,
in principle, present self-healing behav-
ior. Hydrogen bonded networks imple-
ment dynamic noncovalent interac-
tions. Suitable selection of structural
units gives access to novel dynamic
self-repairing materials by increment-
ing the number of hydrogen-bonding
sites present within a molecular frame-
work. Herein, we describe the forma-
tion of a tris-urea based motif giving
access to six hydrogen-bonding sites,
easily accessible through reaction of
carbohydrazide with an isocyanate de-
rivative. Extension towards the synthe-
sis of multiply hydrogen-bonded supra-
molecular materials has been achieved
by polycondensation of carbohydrazide
with a bis-isocyanate component de-
rived
from
poly-dimethylsiloxane
chains. Such materials underwent self-
repair at a mechanically cut surface.
This approach gives access to a broad
spectrum of materials of varying flexi-
bility by appropriate selection of the
bis-isocyanate component that forms
the polymer backbone.
Keywords: hydrogen bonds · neu-
tron scattering · noncovalent inter-
actions · self-healing materials · su-
pramolecular chemistry
Introduction
molecular[6,7g,k] types. One may distinguish materials capable
of undergoing either supramolecular/noncovalent self-heal-
ing,[10] implementing noncovalent interactions, or molecular/
covalent self-healing,[11] based on reversible chemical reac-
tions, or both combined within the same structure. On a
general note, self-healing materials described so far can be
categorized into three broad classes, namely: vascular,[8] cap-
sule based,[9] and intrinsic self-healing materials.[3,10,11]
Dynamic materials capable of component exchange and re-
combination offer great potential for the generation of
novel properties (mechanical, optical, electrical etc.) on
both the molecular and supramolecular levels. One of the
salient features that distinguish animate entities from inani-
mate ones is their ability to spontaneously heal against
damage or injury inflicted upon them. With the advent of in-
terdisciplinary research incorporating biological aspects into
synthetic materials, much research interest has been vested
in the search for materials presenting, in particular, self-
healing features.[1–3] Dynamic covalent chemistry[4] and su-
pramolecular chemistry[5] are powerful tools for the develop-
ment of adaptive functional materials, in particular dynamic
polymers (dynamers)[6a,7g,k] of either supramolecular[7] or
Intrinsic self-healing materials that make use of reversible
chemical reactions to triggering and perform the healing
pro
stimulus or a catalyst may be necessary to initiate the pro-
AHCTUNGTREGcNNNU ess. Recently, several methodologies have been explored
ACHTUNGTRENcNUGN ess are especially coveted. In specific cases, an external
towards the formation of materials conducive to spontane-
ous healing.[3,10,11] Covalent dynamics have been implement-
ed based on reversible Diels–Alder reactions for the forma-
tion of self-healing covalent polymers.[11] Noncovalent inter-
actions such as hydrogen-bonding have been widely imple-
mented in the investigation of supramolecular dynamers
based on various H-bonding motifs.[7a–b,g,k,12] They offer a
direct pathway for introducing intrinsic self-healing features
into materials, so that an increase in the number of H bonds
within a molecular framework would enhance the chance of
autonomic healing. One of the major advantages of systems
based on noncovalent interactions is their ability to undergo
self-repairing without the addition of any external agent.
Herein we describe a simple and versatile approach to-
wards the formation of a multiply H-bonded molecular net-
work. It builds upon the very attractive features of the urea
group[7h,13,14] as a supramolecular H-bonding synthon, in par-
[a] Dr. N. Roy, Prof. E. Buhler, Prof. J.-M. Lehn
Laboratoire de Chimie Supramolꢀculaire, ISIS
Universitꢀ de Strasbourg
8, allꢀe Gaspard Monge
67000, Strasbourg (France)
Fax : (+33)368-855140
E-mail: lehn@unistra.fr
[b] Prof. E. Buhler
Matiꢁre et Systꢁmes Complexes
Universitꢀ Paris Diderot-Paris 7
10, rue Alice Domon et Lꢀonie Duquet
75205 Paris Cedex 13 (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203518.
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Chem. Eur. J. 2013, 19, 8814 – 8820

FULL PAPER
ticular for the development of multiply hydrogen-bonded
supramolecular materials. Although the implementation of
several separate urea groups has been previously
reported,[7h,13a–g,15] this approach markedly extends its poten-
tial through the use of a triple unit, as embodied in a con-
densed tris-urea based supramolecular motif. A hydrazino-
bis-urea group[13i] is present in the commercial textile poly-
mer Lycra.[13j] The synthesis of the present tris-urea motif is
easily realized by the reaction of carbohydrazide with an
isocyanate derivative. Extension towards the generation of
polymers presenting multiple hydrogen-bonding has then be
achieved by polycondensation of carbohydrazide with bis-
isocyanate components derived from polydimethylsiloxane
(PDMS) chains. We herein describe our investigation of the
basic tris-urea motif, its introduction into polymers of type
PDMS,[14] as well as the self-healing behavior of these dy-
namic materials.
Scheme 1. Synthesis of the tris-urea motif 2,2’-carbonyl-bis-[N-(3,5-di-tert-
butylphenyl)]hydrazine carboximide.
bohydrazide to isolate compound 1 in good yield as shown
in Scheme 1.
Compound 1 could indeed be recrystallized from DMSO,
yielding crystals that were suitable for single-crystal X-ray
diffraction, and the crystal structure was determined to sub-
stantiate the conformation of the unit and the extensive H-
bonding network. The solid-state molecular structure of 1 is
shown in Figure 2; selected bond lengths and angles are
Results and Discussion
The tris-urea motif
The condensed tris-urea unit derived from the reaction of
carbohydrazide with an isocyanate would have, in principle,
two main features: 1) a multiply H-bonding motif having
the potential to establish six hydrogen bonds, in particular
with one another, and 2) an orthogonal orientation of the
three successive urea units owing to the fact that they are
ꢀ
linked through the N N bonds of the hydrazine residues
(Figure 1). The conformation about such bonds is known to
Figure 1. Structural representation of the tris-urea compound 1 depicting
ꢀ
the twisted orientation about the N N bonds.
Figure 2. Molecular structure of tris-urea compound 1 determined by
single-crystal X-ray diffraction analysis. The tert-butyl groups have been
truncated for clarity. Top) Numbering of the atoms. Bottom) Intermolec-
ular hydrogen-bonding arrays displayed by the tris-urea compound 1
adopt a more or less perpendicular orientation with a rather
pronounced rotational barrier.[16] As a result, the plane of
the second urea group would be at an angle with respect to
the plane containing the first and third urea groups. To con-
firm these features, we have synthesized and structurally
characterized a simple monomeric unit based on this com-
pact tris-urea motif.
ꢀ
within the unit cell: H-bonds between the C=O and N H sites of the ter-
minal urea groups in adjacent molecules (left); H-bonding between the
ꢀ
N H sites of the central urea group and the O site of the solvent of crys-
tallization DMSO (right).
Simple condensation of phenyl isocyanate with carbohy-
drazide yielded the desired compound as an amorphous
powder, which was soluble only in DMSO. Attempts to crys-
tallize it from DMSO resulted in the formation of micro-
crystalline hair-like thin crystals that were unsuitable for
single-crystal X-ray diffraction analysis. With the aim of alle-
viating this problem, tert-butyl groups were introduced at
the 3,5-positions of the phenyl ring. Thus, the 3,5-di-tert-
butyl analogue was synthesized in two steps starting from
3,5-di-tert butyl benzoic acid and then condensed with car-
given in Table 1. The carbonyl C,O bond lengths of the urea
motifs were typically in the range of C=O double bond
ꢀ
lengths (1.22–1.23 ꢃ). The C N bond of the urea motif
(C17-N6) adjacent to the phenyl ring is somewhat shorter
(1.349 ꢃ) than that adjacent to the second urea motif (C17-
N5, 1.386 ꢃ). This shortening could be accounted for by the
ꢀ
ꢀ
different vicinal bonds C N and N N, respectively.
Bond lengths and bond angles do not display any particu-
larly remarkable characteristics. Of special interest are, how-
ever, the dihedral angles between the planes of the three
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J.-M. Lehn et al.
Table 1. Selected bond lengths and bond angles.
Atoms
Bond length [ꢃ]
Atoms considered for
bond angle
Bond angle [8]
ꢀ
C15 O1
1.225(5)
1.220(5)
1.233(5)
1.349(5)
1.386(5)
1.354(5)
1.381(5)
aN1-C15-N2
aN1-C15-O1
aN2-C15-O1
aN4-C16-N3
aN4-C16-O2
aN3-C16-O2
aN5-C17-N6
113.9(3)
124.7(4)
121.4(3)
113.3(3)
123.6(4)
123.1(4)
114.3(3)
ꢀ
C16 O2
ꢀ
C17 O3
ꢀ
C17 N6
ꢀ
C17 N5
ꢀ
C15 N1
ꢀ
C15 N2
Scheme 2. Synthesis of polymers P1 and P2.
successive urea units of the tris-urea motif. Thus, they
amount respectively to 67.718 and 66.258 between the planes
N1,C15,O1,N2 (plane 1) and N3,C16,O2,N4 (plane 2) on
one hand, and between the planes N3,C16,O2,N4 (plane 2)
and N5,C17,O3,N6 (plane 3) on the other hand, confirming
the twisted orientation of the individual urea motifs with re-
spect to one another, the first and third urea groups being
oriented in opposite directions (see the Supporting Informa-
tion).
From the stacking of the molecules within the unit cell, it
was observed that, in accordance to our expectations, they
exhibited two arrays of intermolecular H bonds, the first
and third urea group participating in four hydrogen bonds,
more or less in the same plane but oriented in opposite ori-
entations. The two terminal carbonyl groups of a given tris-
properties to the polymeric entities, which otherwise would
probably have been hard and insoluble, in view of the many
hydrogen bonds, as exemplified by the parent tris-urea
entity 1.
Based on the hydrogen-bonding features of the parent
tris-urea motif analyzed above (see also Figure 2), one may
expect the present polymers to be of the class of lateral su-
pramolecular materials,[7a,d,f,17] in which covalent main chains
undergo multiple noncovalent lateral interactions, in this
case hydrogen-bonding, between polymeric chains. Further-
more, the hydrogen-bonding network is expected to present
a two-dimensional array involving the terminal urea groups
of each tris-urea motif. In addition, in principle it may
extend in the third dimension through H-bonding between
the central urea groups of different chains, as illustrated by
the interactions with the DMSO solvent molecules
(Figure 2). The polymers obtained have been characterized
in greater detail using Small Angle Neutron Scattering
(SANS).
ꢀ
urea motif were H-bonded each to two N H sites of the
tris-urea motif in the parallel neighboring strand, as depict-
ed in Figure 2.
A single molecule of solvent of crystallization, DMSO, oc-
cupied the third, perpendicular array of H bonding sites.
ꢀ
This was H-bonded through its oxygen atom to the two N
Characterization of polymers P1 and P2 by SANS: More de-
tailed insight into the structural properties of polymers P1
and P2 in solution was obtained by Small Angle Neutron
Scattering (SANS) studies. A major advantage of SANS is
its ability to provide information across a broad range of
length and scales ranging from 1 to 100 nm.[18] Figure 3
shows variations in scattered neutron intensity as a function
of the scattered wave-vector, q, for each of the two poly-
mers, P1 and P2. The two spectra are markedly similar, sug-
gesting that morphologically related structures are present
in the solution. Each scattering profile exhibits a Guinier
regime at low q, associated with the finite size of scattering
objects, an intermediate regime in which the q dependence
of the scattered intensity can be described by a power law
with an exponent close to ꢀ5/3, followed by another power
law regime with an exponent close to ꢀ1.
H protons of the central urea group, originating from the
carbohydrazide component. The DMSO molecule was re-
moved in silico to display how the third array of hydrogen-
bonding could look like (see the Supporting Information).
This further illustrates that through simple condensation of
an isocyanate derivative with carbohydrazide, it is possible
to gain easy access to a multiple hydrogen-bonding network
presenting six hydrogen-bonding sites located in two differ-
ent planes, four in one plane and two in the second
(Figure 2).
Tris-urea-based polymeric supramolecular materials
Synthesis and nature of tris-urea polymeric supramolecular
materials: The tris-urea concept was extended to gain access
to supramolecular materials of polymeric nature through
polycondensation of carbohydrazide with suitably selected
bis-isocyanates. Such polymers, owing to the presence of
multiple H bonding interactions, can be envisaged to display
interesting functional material properties, in particular self-
repairing features. With this aim, we synthesized two differ-
ent polydimethylsiloxane based diisocyanate derivatives and
condensed them with carbohydrazide to form polymers P1
and P2, respectively, as shown in Scheme 2. The polydime-
thylsiloxane fragment was expected to convey soft material
The data obtained at low q correspond to relatively large
spatial scales and can be fitted by the Guinier law as:
ꢀ
ꢁ
1
1
RG2
3
ð1Þ
¼
1 þ q2
IðqÞ Ið0Þ
in which RG is the radius of gyration of the chain in dilute
regime. The extrapolation to zero q of the scattered intensi-
ty, I(0), provides a measure of the weight-average molecular
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Self-Healing Materials
FULL PAPER
ents a larger number of possible interchain H-bonds). This
can also explain why the self-repairing of polymer P1 re-
quires less time compared with P2.
The crossover value, q*, between the intermediate exclud-
ed volume regime qꢀ5/3, and the high q rod regime qꢀ1 per-
mits the persistence length to be measured by using the fol-
lowing expression:
1:91
ð3Þ
Lp ¼
q
*
We obtained Lp ꢁ14.7 ꢃ for both polymers. This ensem-
ble of results is characteristic of flexible polymer chains in
good solvent.
It is important to note the onset of a scattered intensity
increase at very low q, which is probably associated with the
presence of a minority second population in solution (few
aggregates or larger chains), is in line with dynamic light
scattering measurements showing the presence of ca. 100 nm
objects in very small proportion.
Figure 3. SANS profiles collected at T=208C for polymer solutions of P1
and P2 formed in deuterated chloroform at an initial concentration of
10 mm [thick line: fit with Eq. (2)]. For clarity, the spectra have been
shifted by one log unit along the y axis with respect to each other. The
inset shows plots of 1/I versus q2 and the best linear fits to the data
[Eq. (1)].
Polymer films and self-healing properties: Polymers P1 and
P2 were used for the preparation of polymer films, with both
forming inelastic films by evaporation from a solution in
chloroform. Remarkably, these polymeric materials, when
cut or damaged mechanically, were found to self-repair at
the damaged place in the course of a few hours.
In the case of polymer P1, when the brittle polymer frag-
ments were brought together inside a glass vial, self-repair
took place by formation of a smooth and completely healed
bulk polymer in the course of an hour (Figure 4a and b).
weight of the particles, MW, for dilute systems (or of the part
of the chain forming the mesh of the transient network in
semidilute regime, in which polymeric chains are entangled).
The inset of Figure 3 shows the plots of 1/I versus q2 for
both polymers. From the best linear fits to the data, we ob-
tained: RG =5.5 nm, MW =18480 gmolꢀ1 and RG =7.8 nm,
MW =16432 gmolꢀ1 for samples P1 and P2, respectively.
In the intermediate regime we would expect a power law
behavior with an exponent m characteristic of the chain con-
formation. The results of Figure 3 clearly indicate that, in
suitable solvent, both polymer samples consist of expanded
chains with m=ꢀ5/3. At larger q values, I(q) is controlled by
smaller distances than the persistence length Lp, over which
the polymers are rod-like, and we observe a crossover to an
asymptotic qꢀ1 regime. In this high-q regime, in which I(q)/
P(q), the scattering curves can be fitted satisfactorily using
the form factor derived for rigid rod particles given by
Figure 4. a) Native-damaged polymer P1. b) Self-repaired polymer P1.
c) Mechanically cut polymer P2. d) Self-healed polymer P2.
p
Polymer P2 was less brittle and more elastic than P1 and
was found to undergo self-repairing in the course of two
hours at the mechanically cut surface. When two cut frag-
ments of about 1 mm thickness, were brought together
within 5 min and kept in contact with one another (as
shown above in Figure 4c and d), they underwent self-heal-
ing at the interface of the polymer fragments. However, if
they were kept separated for a longer period of time (more
than ten minutes) without any face-to-face contact, self-re-
pairing no longer occurred. This result is reminiscent of re-
ported observations on the self-repair of supramolecular
polymers,[10] and can be explained by the fact that the free
hydrogen-bonding groups exposed at the freshly cut surface
will, in the course of time, undergo noncovalent interaction
with and be occupied by the available partners on the same
face, thus leading to a progressive reduction in self-healing
PðqÞ ¼
ð2Þ
qLc
in which Lc represents the contour length of the scattered
objects. By fitting the above equation to the high-q data, we
can determine the linear mass density, ML, of the polymers.
From the fits, we obtained ML =23.2 gmolꢀ1 ꢃꢀ1 for P2 (i.e.,
Lc =708 ꢃ), a value that is in excellent agreement with the
linear mass density of a single strand polymer chain, which
can be estimated by using the monomer molecular parame-
ters m/a=5141/254=20.2 gmolꢀ1 ꢃꢀ1, in which m and a rep-
resent the mass and the length of the dimer repeat unit, re-
spectively. For polymer P1, the fit gave ML =
40.8 gmolꢀ1 ꢃꢀ1, a value suggesting a double strand poly-
mer-like structure in solution and in agreement with the rel-
atively low value of RG =55 ꢃ for this polymer (which pres-
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J.-M. Lehn et al.
capability between pieces. Unfortunately, the present poly-
mers are not elastic enough to carry out a detailed rheologi-
cal stress-strain analysis so that the self-healing features
could only be ascribed qualitatively.
zine units, four H-bonds are located in one plane and the
remaining two lie in a plane more or less orthogonal to
the first one.
3) A simple polycondensation between carbohydrazide and
suitably substituted diisocyanate derivatives generates
lateral supramolecular materials, presenting dynamic hy-
drogen-bonding interactions between flexible polymer
chains, as characterized by small angle neutron scattering
experiment.
ATR FTIR spectroscopy: To further substantiate the role of
H-bonding in polymers P1 and P2, ATR FTIR studies were
performed on solid samples of 1, a model di-tert-butyl
model amide derivative (M1) (see the Supporting Informa-
ꢀ
tion, Figure S5, for N H stretching frequency comparison),
4) This versatile synthetic route can provide easy access to
novel functional materials incorporating various compo-
nents of interest for mechanical, optical, and electronic
properties.
ꢀ
P1 and P2. In M1, the N H stretching vibration was observed
around 3338 cmꢀ1, which is the stretching frequency vibra-
tion for a free amide.[19a] In contrast, in the case of 1, the hy-
ꢀ
drogen bonded N H band is shifted to a much lower fre-
5) The use of polydimethylsiloxane monomers confers soft-
matter properties to the polymers obtained. Further-
more, these polymers may also be considered as a type
of supramolecular silicone, a blend between silicone type
materials and H-bonding supramolecular materials.[14]
6) The materials obtained yield self-repairing polymer
films.
quency at 3285 cmꢀ1 (see Figure S4). Such a shift of 53 cmꢀ1
ꢀ
results from weakening of the (N H) bond due to H bond-
ing with the (C=O) group. This observation further con-
forms to the crystal structure data for 1. Additionally, a
C=O stretching vibration was also observed around
1600 cmꢀ1, indicative of H-bonding, in addition to the free
amide C=O stretch around 1680 cmꢀ1 in both M1 and 1. In
7) These materials can find potential applications in a
number of fields, such as coatings and paint industry.
the case of polymers P1 and P2, extremely broad bands were
observed around 3300 cm , indicative of strong N H···O=C
ꢀ1
ꢀ
H-bonds (see Figure S6 and S7). Moreover, P1 and P2 also
exhibit broad peaks around 1670 cmꢀ1, corresponding to the
H-bonded C=O stretching vibration (see Figure S8 and S9).
The stretching frequency values observed here are in good
agreement with the reported values.[13h,19] Attempts to meas-
ure solution-state IR and its dilution effects on the H-bond-
ing motifs were unsuccessful, owing to the extremely broad
The self-repairing ability of the polymers should also give
access to robust polymeric materials. The monomeric com-
ponents, being relatively inexpensive, would also make them
attractive contenders for larger scale industrial applications
that are economically viable. Moreover, the physical at
ACHTUNGTRENNUNGtri-
AHCTUNGTREGbNNNU utes of the polymers formed can also be easily tuned by al-
tering the polymer backbone using different substituents, for
instance for the preparation of novel dynamic materials of
tuneable mechanical or optical properties.
ꢀ
nature of the peaks observed corresponding to the N H and
C=O stretching vibrations.
The self-healing features observed here for the soft P1
and P2 materials can be attributed to the extensive H-bond-
ing network that may be established between the polymer
chains, in line with the multiple intermolecular H-bonding
displayed by the model monomer analogue 1 discussed
above. More generally, a major question is that of the inter-
play between the mechanical properties of a polymeric ma-
terial and its self-healing behavior, in particular whether
such materials are necessarily soft or can be hard.[20] Self-
healing features have been attributed to supramolecular
gels,[21] but have also been achieved in a relatively hard elas-
tomer.[22] The present supramolecular materials may be con-
sidered to be of intermediate type.
Experimental Section
General: All reagents were purchased from commercial suppliers (Acros,
Sigma–Aldrich ABCR and Fluka). Compound 3,5-di-tert-butyl benzoic
acid and the polydimethyl siloxane substituted diamines with different
chain lengths employed for the synthesis of polymers P1 and P2 were
available commercially and used without further purification. 1H NMR
spectra were recorded with a Bruker Avance 400 MHz spectrometer,
using the residual solvent peaks as reference (see the Supporting Infor-
mation for synthetic schemes and details of the SANS experiments). In-
frared (IR) studies were performed with a Nicolet 6700 FTIR spectrome-
ter equipped with an ATR accessory.
Synthesis of 3,5-di-tert-butyl-aniline (2): NaN3 (0.75 g, 11.5 mmol) was
added very slowly in portions in the course of an hour to a preheated sol-
ution of 3,5-di-tert-butyl benzoic acid (2.5 g, 10.6 mmol) in concentrated
H2SO4 (15 mL) and CHCl3 (15 mL) at 458C. The reaction mixture was
stirred for 5 h at 458C, then CHCl3 was removed under reduced pressure,
the residue was cooled with ice (08C), and ice-water (100 mL) was
added. The precipitate of 3,5-di-tert-butyl aniline sulfonate was dissolved
in ethanol (20 mL) and an aqueous solution of KOH (2.5 g in 50 mL
water) was added. The white precipitate formed was filtered and washed
with a large amount of water. The product was dried in air. Yield: 2.1 g
(90%); 1H NMR (400 MHz, CDCl3): d=6.89 (s, 1H), 6.61 (s, 2H), 3.51
(br. s, 2H), 1.33 ppm (s, 18H); 13C NMR (100.6 MHz, CDCl3): d=151.9,
145.7, 113.2, 109.8, 34.7, 31.4 ppm; MS (ESI): m/z: 206.197 [M+H]+.
Conclusion
The tris-urea motif investigated here presents several nota-
ble features.
1) It is easily generated by the condensation of carbohydra-
zide with an isocyanate.
2) It represents a sextuple H-bonding supramolecular syn-
thon; in view of the conformational properties associated
with the sequence of three urea groups linked by hydra-
Synthesis of 1: Triethylamine (1.6 mL, 11.4 mmol) and triphosgene
(0.72 g, 2.43 mmol) were added to a solution of 3,5-di-tert-butyl aniline
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Self-Healing Materials
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(1 g, 4.87 mmol) in benzene (25 mL) kept under an N2 atmosphere, and
the reaction mixture was stirred under nitrogen for 6 h. The solution was
then filtered off and the filtrate was evaporated to dryness to isolate the
unstable 3,5-di-tert-butyl isocyanate as an off white solid. This was redis-
solved in EtOH (50 mL) and stirred at RT. Carbohydrazide (0.17 g,
1.94 mmol) was added and the reaction mixture was stirred for 16 h at
RT under N2. After 16 h, the precipitate formed was filtered off and the
off-white solid was isolated, washed with a large excess of ethanol and re-
crystallized from DMSO. Single crystals suitable for X-ray diffraction
were obtained by slow diffusion of CH3CN into DMSO solution. Yield:
0.6 g (56%). 1H NMR (400 MHz, [D6]DMSO): d=8.50 (br. s, 4H), 7.87
(br. s, 2H), 7.34 (s, 4H), 7.02 (s, 2H), 1.25 ppm (s, 36H); 13C NMR
(100.6 MHz, [D6]DMSO): d=159.2, 156.5, 150.9, 139.4, 116.1, 113.7, 34.9,
31.7 ppm; MS (ESI): m/z: 575.37 [M+Na]+; elemental analysis calcd (%)
for C31H48N6O3: C 67.36, H 8.75, N 15.2; found: C 67.6, H 8.8, N 15.4.
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Acknowledgements
N.R. thanks UDS for a postdoctoral fellowship. This work was supported
by a post-doctoral fellowship (N.R.) and financial aid from CNRS and
the ANR 2010 BLAN-717-1. We also thank Dr. Lydia Brelot and Dr.
Corinne Bailly of Service de Radiocristallographie, University of Stras-
bourg for the single-crystal X-ray diffraction analysis.
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