Self-assembly of alkyl N-acetylglyoxylic amides of varying chain lengths

Article abstract of DOI:10.1039/c2ce25994j

Alkyl N-acetylglyoxylic amides have been synthesised from N-acetylisatins and their self-assembly studied in the solid state by X-ray crystallography. Different molecular conformations were observed for molecules 5a, 5b, 5d and 6b, which have a strong influence on their supramolecular organization in the crystals. Both strong and weak interactions played key roles in the self-assembly process and the formation of well-defined supramolecular architectures. Crystal structures of 5b revealed novel C=O...C=O (carbonyl-carbonyl) dipolar interactions involving three carbonyl groups and intermolecular halogen bonding interactions, either C-H...Br contacts with inclusion of cyclohexane in 5b-1 or Br...Br contacts without the inclusion of solvent in 5b-2. This journal is

Full text of DOI:10.1039/c2ce25994j

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
CrystEngComm
Cite this: CrystEngComm, 2012, 14, 7345–7354
www.rsc.org/crystengcomm
PAPER
Self-assembly of alkyl N-acetylglyoxylic amides of varying chain lengths{
Venty Suryanti,{*ab Mohan Bhadbhade,b Roger Bishop,a David StC Blacka and Naresh Kumar*a
Received 21st June 2012, Accepted 27th July 2012
DOI: 10.1039/c2ce25994j
Alkyl N-acetylglyoxylic amides have been synthesised from N-acetylisatins and their self-assembly
studied in the solid state by X-ray crystallography. Different molecular conformations were observed
for molecules 5a, 5b, 5d and 6b, which have a strong influence on their supramolecular organization
in the crystals. Both strong and weak interactions played key roles in the self-assembly process and
the formation of well-defined supramolecular architectures. Crystal structures of 5b revealed novel
…
CLO CLO (carbonyl–carbonyl) dipolar interactions involving three carbonyl groups and
…
intermolecular halogen bonding interactions, either C–H Br contacts with inclusion of cyclohexane
…
in 5b-1 or Br Br contacts without the inclusion of solvent in 5b-2.
between the single free amide NH proton and the a-carbonyl
Introduction
oxygen atom. This motif was also evident in the secondary N-[2-(2-
Inter- and intramolecular interactions are recognised to be of
oxobutanoyl)phenyl]benzamide, which exists as a dimer through
fundamental importance to the self-assembly of supramolecules,
additional hydrogen bonding between the benzamide carbonyl
crystal engineering and molecular recognition.1 Particular
oxygen atom and the glyoxylamide NH proton.12
emphasis has always been placed on strong hydrogen bond
We are interested in the use of the glyoxylamide moiety in the
… …
interactions (e.g. O–H O and O–H N), but there is now also
development of peptidomimetics and have reported a facile ring
opening reaction of N-acylisatins with amino acids and peptides
to generate novel mono-, bis- and dendrimeric glyoxylamide
derivatives that show antibacterial activity.13–15 The non-
proteinogenic amino acid 2-aminophenylglyoxylic acid unit
allows for unique molecular conformations not shown by
natural amino acids. During these studies we found the simple
glycine peptidomimetic derivative 3 self-assembled into a dimer
in a similar manner to the reported 29-benzamidophenylglyox-
ylamides (Fig. 2). Strong intermolecular hydrogen bonding was
increasing interest in the role played by weak interactions (e.g.
… …
p stacking, X–H A (X = C, N, O; A = O, Cl, p) and
p
…
halogen halogen) and in elucidating how the cooperative action
of these weak interactions influences molecular packing, crystal
structure stability, molecular recognition and self-assembly
processes.2–4
Carboxylic acids and amides are widely exploited in crystal
engineering because they generate reliable dimeric and catemer
supramolecular synthons via hydrogen bonding.5–9 Conversely,
the supramolecular synthons produced by glyoxylamides have
received less attention. Greater versatility is offered by the use of
glyoxylamides in self-assembly owing to the presence of the
additional hydrogen bond accepting keto groups and the
variable glyoxylamide torsional angle. For example, the self-
assembly of indol-2-ylglyoxylamides was found to give dimeric
structures exhibiting new hydrogen bonded motifs 1 and 2
(Fig. 1).10 Unique intermolecular hydrogen bonding was also
apparent in a related tertiary indol-7-ylglyoxylamide.11
Simple 29-benzamidophenylglyoxylamides also self assemble by
hydrogen bonding. The tertiary N-[2-(oxo-1-piperidinylacetyl)phe-
nyl]acetamide exists as a monomer with strong hydrogen bonding
aSchool of Chemistry, The University of New South Wales, Sydney, NSW,
2052, Australia. E-mail: n.kumar@unsw.edu.au
bMark Wainwright Analytical Centre, The University of New South Wales,
Sydney, NSW, 2052, Australia
{ Electronic supplementary information (ESI) available. CCDC refer-
ence numbers 887275–887279. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2ce25994j
{ On leave from Department of Chemistry, The University of Sebelas
Maret, Surakarta, Jawa Tengah, 57126, Indonesia. E-mail: venty@mi-
pa.uns.ac.id
Fig. 1 Supramolecular synthons of indol-2-ylglyoxylamides.
CrystEngComm, 2012, 14, 7345–7354 | 7345
This journal is ß The Royal Society of Chemistry 2012

View Article Online
Scheme 2 Reagents and conditions: amino acid alkyl ester hydrochloride
salt, NaHCO3, CH2Cl2/H2O (1 : 1 v/v), 0 uC to r.t., 24 h.
The second synthetic targets of interest were the related alkyl
substituted 29-acetylamidophenylglyoxylamide peptide deriva-
tives 6. The introduction of a hydrophilic amino acid to the side
chain of these structures was anticipated to enhance the
hydrogen bonding interactions and hence aggregation of these
compounds. In accordance with our reported methodology,9
N-acetylisatin 4 was stirred in dichloromethane for 24 h with an
amino acid alkyl ester and sodium hydrogen carbonate, initially
at 5 uC and then gradually warmed to room temperature.
Glycine butyl ester hydrochloride, glycine hexyl ester hydro-
chloride, L-alanine butyl ester hydrochloride and L-alanine hexyl
ester hydrochloride were selected as representative amino acid
alkyl esters. The resulting peptidomimetics 6a–d were isolated in
45–67% yield upon workup (Scheme 2, Table 2).
Fig. 2 Hydrogen bonding motifs between molecules of peptidomimetic
3.
evident between the gloxylamide NH proton and the acetamide
oxygen atom. Intramolecular hydrogen bonding was also evident
between the acetamide NH proton and the a-carbonyl oxygen
atom.
As an extension of this work we now report our initial findings
into the self-assembly behaviour of these 2-aminophenyl-
glyoxylic acid moieties. Alkyl side chains have been reported to
promote hydrophobic aggregation through van der Waals
forces,16–17 so our particular interest was in the development of
more amphiphilic analogues of 3, bearing longer alkyl and ester
alkyl chains, which could potentially display enhanced gelation
and self-assembly properties.
Glyoxylamides 5a–i and 6a–d were subsequently recrystallized
from
a range of solvents including hexane, cyclohexane,
methanol, ethanol, dichloromethane/hexane and acetonitrile via
slow evaporation of the solvent at room temperature. Crystals
suitable for X-ray structure determination were successfully
obtained for compounds 5a, 5b, 5d and 6b. The other
compounds afforded only amorphous solids.
Results and discussion
The first synthetic targets prepared were the simple alkyl
substituted 29-acetylamidophenylglyoxylamides 5. Accordingly,
N-acetylisatins 4 were heated at reflux for 4 h in dichloro-
methane with hexylamine, dodecylamine or octadecylamine to
give analogues 5a–i in 42–84% yield after column chromato-
graphy (Scheme 1, Table 1).
Crystal structures of 5a, 5b, 5d and 6b
Single crystal X-ray structure determinations were carried out on
compounds 5a, 5b 5d and 6b and a summary of crystallographic
data is provided in Table 3. The R factors for structures 5a and 5d
from synchrotron data are somewhat higher as compared to the rest
of the structures, but structural parameters are all normal and
therefore allow us to include them for our discussion here.
Conformationally flexible side chains exhibited disorder in struc-
tures 5b-1, 5b-2 and 5d (details in the ESI{). These crystal structures
are grouped in two categories based on their intramolecular strong
and weak interactions. The crystal structures are shown in Fig. 3
depicting these intramolecular interactions. The molecular con-
formations are different in these two groups of molecules and have a
strong influence on their supramolecular organization.
Scheme 1 Reagents and conditions: amine, CH2Cl2, reflux, 4 h.
Table 1 Synthesis of 29-acetylamidophenylglyoxylamides 5
The first group, 5a, 5d and 6b, contains an intramolecular
…
N–H O hydrogen bond that is formed by O2A as an acceptor
Entry
Product
R1
R2
Yielda (%)
Table 2 Synthesis of 29-acetylamidophenylglyoxylamide peptidomi-
metics 6
1
2
3
4
5
6
7
8
9
a
5a
5b
5c
5d
5e
5f
5g
5h
5i
H
Br
CH3
H
Br
CH3
H
(CH2)5CH3
(CH2)5CH3
(CH2)5CH3
(CH2)11CH3
(CH2)11CH3
(CH2)11CH3
(CH2)17CH3
(CH2)17CH3
(CH2)17CH3
48
55
68
72
84
76
42
70
60
Entry
Amino acida
Product
R1
R2
Yieldb(%)
1
2
3
Glycine
Glycine
L-Alanine
6a
6b
6c
H
H
CH3
(CH2)3CH3
(CH2)5CH3
(CH2)3CH3
52
45
63
4
6d
CH3
(CH2)5CH3
67
L-Alanine
Br
CH3
a
b
As the alkyl ester hydrochloric salt. Isolated yield (reaction was not
optimized).
Isolated yield (reaction was not optimized).
7346 | CrystEngComm, 2012, 14, 7345–7354
This journal is ß The Royal Society of Chemistry 2012

View Article Online
Table 3 Crystal data collection and structure refinement parameters of 5a, 5d, 6b, 5b-1, and 5b-2
Compound
5a
5d
6b
5b-1
5b-2
Chemical formula
Mr
Crystal system
Space group
T/K
C
16H22N2O3
C22H34N2O3
374.51
Monoclinic
P21/c
100
C18H24N2O5
348.39
Monoclinic
P21
155
C16H21BrN2O3?(C6H12)0.5
411.34
Triclinic
¯
P1
150
C16H21BrN2O3
369.26
Triclinic
290.36
Monoclinic
P21/c
100
¯
P1
150
˚
a, b, c/A
36.447 (7), 5.012 (1), 24.240 (5), 5.021 (1), 5.0331 (3), 33.007 (2), 8.8992 (12), 9.9071 (17), 8.5713 (11), 9.9041 (11),
17.294 (4)
90, 103.65 (3), 90
17.387 (3)
90, 91.28 (3), 90
5.5043 (3)
90, 99.240 (4), 90
12.299 (2)
75.265 (7), 81.633 (7),
76.227 (7)
1014.4 (3)
2
11.2296 (14)
97.717 (4), 108.250 (4),
101.874 (4)
865.48 (18)
2
2.39
0.14 6 0.06 6 0.04
0.726, 0.902
a, b, c (u)
3
˚
V/A
3069.8 (11)
8
0.09
2115.6 (7)
4
0.10
902.55 (9)
2
0.09
Z
m/mm21
2.05
Crystal size/mm
Tmin, Tmax
0.10 6 0.04 6 0.03 0.10 6 0.02 6 0.02 0.34 6 0.23 6 0.04 0.37 6 0.08 6 0.08
Absorption
corrections
Absorption
corrections
0.969, 0.996
0.522, 0.849
are not applied are not applied
31 592, 5360, 4467 24 994, 3480, 3413 6437, 3076, 2899
No. of measured,
independent and observed
[I . 2s(I)] reflections
Rint
12 828, 3548, 2328
11 690, 3012, 2430
0.102
0.034
0.130, 0.410, 1.05
0.022
0.030, 0.069, 1.35
0.079
0.061, 0.189, 1.10
0.049
0.040, 0.129, 0.89
R[F2 . 2s(F2)], wR(F2), S 0.200, 0.452, 1.16
No. of reflections
No. of parameters
No. of restraints
5360
383
0
1.00, 20.74
3480
300
356
0.74,20.83
3076
228
1
0.13, 20.16
3548
264
83
0.51, 20.65
3012
228
30
0.62, 20.56
23
˚
DTmax, DTmin/e A
and N1A–H1A and N2A–H2A as the two donor groups.
Similarly the oxygen O3A accepts protons from two C–H
groups, namely C3–H3 and C11-–H11. In molecule 5d, the
oxygen O3A accepts protons from three C–H groups, namely
C3–H3, C11–H11 and C12–H12B. In addition, there is an extra
a transoid conformation,22 as is the case for the five glyox-
ylamide crystal structures described here. However, the intra-
molecular triple orthogonal carbonyl dipolar interaction present
in the structures 5b-1 and 5b-2 is novel and has not previously
received any attention.
…
…
C–H O bond, C6–H6 O1 (Fig. 3 (a–c) and Tables 4–6).
In the second group, two crystal forms of 5b were produced:
5b-1 containing cyclohexane, and solvent free 5b-2 that was
crystallised from hexane. Both structures showed an absence of
Supramolecular organization
The glyoxylamides (5a, 5b, 5d and 6b) comprise an aromatic
head group and associated side chains. The group 1 structures
have some features in common (Fig. 4a–c), whereas the bromo
derivative 5b, because of halogen interactions predominating in
the molecular organization, differs sharply from these (Fig. 4d–
e). The shorter alkyl chain in 5a (with 6 carbon atoms) shows a
slightly bent conformation whereas the longer chain compounds
6b (glycine ester added to the 6 carbon chain) and 5d (12 carbon
atoms) have essentially extended chain conformations with
…
the intramolecular N1–H1 O2 hydrogen bond interactions
which was characteristic in the previous group. In both crystal
forms of 5b, the moieties C7LO1, N1–H1 and C9LO2, C10LO3
and N2–H2 are almost reversed by rotating y180u with respect
to the conformations in group 1. The overlap of the five
structures (Fig. 3f) shows the conformational differences among
them. This striking conformational difference in the structures
brings three carbonyl groups to form a cluster that shows dipolar
contacts among them. Thus, carbonyl group C7LO1 makes short
contacts with the carbons C9 and C10 of the two carbonyl
groups C9LO2 and C10LO3 respectively. As a result, the intra
˚
interchain separation of approximately 4 A. The longer chain
lengths in 5d and 6b resulted in highly packed structures due to
extensive van der Waals interactions between the alkyl chains.
Conversely, the shorter chain lengths in 5a showed less dense
supramolecular structures.
…
C6–H6 O1 bond in group 1 is absent here. Atom O2 acts as an
acceptor to groups C3–H3 and N2–H2N. In addition, O3 forms
…
Amongst the first group, the compound 6b shows a difference
in the supramolecular organization from the other two
…
a single C–H O type contact with C11–H11 of the side chain in
both 5b-1 and 5b-2 and with a possible weaker contact from
C12–H12 in 5b-2 (Fig. 3 (d) and (e)). Numerical details of the
molecular interactions present in the crystal forms are listed in
Tables 7–8.
structures; the head head association seen in 5a and 5d is
absent in 6b. The reasons for this are best understood from the
close up view of the interactions in these structures (Fig. 5). The
responsible factor is the addition of glycine in the alkyl chain
that restricts the free rotation of the side chain, possibly due to
Both the antiparallel and orthogonal carbonyl–carbonyl
dipolar interactions are known to be able to compete effectively
with strong hydrogen bonds in crystal packing.18 The antipar-
allel interaction is more subject to molecular steric effects and
therefore the orthogonal motif is encountered more frequently,19
especially in crystal structures of proteins and other amide-
containing compounds.20,21 1,2-Dicarbonyl groups usually adopt
…
the presence of CLO CLO (carbonyl–carbonyl) dipolar inter-
actions. Interdigitating alkyl chains were observed in 5a and 5d,
whereas parallel alkyl chains were seen in 6b.
The crystal structure of 5a contains two crystallographically
independent molecules (A and B, shown in blue and green,
respectively). The association between these molecules occurs
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 7345–7354 | 7347

View Article Online
Fig. 3 Intramolecular interactions present in molecule: (a) 5a, (b) 5d, (c) 6b, (d) 5b-1, (e) 5b-2, and (f) structure overlay of all five structures showing
conformational differences (colour codes: 5a: blue; 5d: red; 6b: purple; 5b-1: dark green and 5b-2: yellow). Disordered atoms of the side chains are
omitted for clarity reasons.
… …
through CH O contacts, namely C16B–H16F O2A and
…
C16A–H16B O2B. Molecules A (or molecules B) are connected
Table 4 Numerical details of the intramolecular interactions present in
5a
to each other by strong intermolecular hydrogen bonds, N2–
…
…
…
˚
H2N O3LC10 with d(N O) of 2.764 A and the angle N–H
O
…
A
D–H
˚
H
…
Donor (D)–H Acceptor (A) (A)
…
D A (A) Angle (u)
˚
(A)
˚
Table 5 Numerical details of the intramolecular interactions present in
5d
…
N1A–H1NA O2AL[C9A]
0.86
0.86
0.93
0.97
0.93
2.02
2.36
2.44
2.48
2.32
2.653(11)
2.691(11)
2.938(13)
3.236(17)
2.891(12)
130
103
113
98
…
N2A–H2NA O2AL[C9A]
…
…
A
˚
(A)
…
D–H
˚
(A)
H
A
D
Angle
(u)
C3A–H3A O3AL[C10A]
…
…
˚
(A)
Donor (D)–H Acceptor (A)
C11A–H11A O3AL[C10A]
…
C6A–H6A O1AL[C7A]
119
…
N1–H1N O2L[C9]
0.86
0.86
0.93
0.97
0.97
0.93
1.96
2.38
2.45
2.69
2.62
2.29
2.660(6)
2.684(6)
2.932(6)
2.781(6)
3.139(6)
2.888(6)
138
101
112
85
114
122
…
N2–H2N O2L[C9]
…
N1B–H1NB O2BL[C9B]
0.86
0.86
0.93
0.97
0.93
2.04
2.41
2.45
2.55
2.30
2.656(13)
2.699(11)
2.909(14)
3.251(16)
2.863(12)
128
100
111
95
…
C3–H O3L[C10]
…
N2B–H2NB O2BL[C9B]
…
C11H11A O3L[C10]
…
C3B–H3B O3BL[C10B]
…
…
C12AH12B O3L[C10]
C11B–H11C O3BL[C10B]
…
…
C6–H O1L[C7]
C6B–H6B O1BL[C7B]
119
7348 | CrystEngComm, 2012, 14, 7345–7354
This journal is ß The Royal Society of Chemistry 2012

View Article Online
Table 6 Numerical details of the intramolecular interactions present in
6b
Table 8 Numerical details of the intramolecular interactions present in
5b-2
…
…
…
…
…
…
˚
˚
˚
˚
˚
˚
Donor (D)–H Acceptor (A) D–H (A) H A (A) D A (A) Angle (u)
Donor (D)–H Acceptor (A) D–H (A) H A (A) D A (A) Angle (u)
…
N1–H O2L [C9]
…
N2–H O2L [C9]
0.88
0.88
0.95
0.99
0.95
0.99
0.99
1.98
2.53
2.48
2.65
2.27
2.56
2.64
2.683(2) 137
2.770(2) 97
2.954(2) 111
2.700(2) 82
2.873(2) 121
0.88
0.95
0.99
0.99
2.34
2.55
2.67
2.64
2.700(4) 104
…
N2G–H2N O2L [C9]
…
C3–H O2L [C9]
2.829(4)
2.805(4)
97
86
…
…
…
C3–H O3L [C10]
C11A–H11A O3L [C10]
…
C1GH1G O3L [C10]
C12A–H12A O3L [C10]
3.169(4) 114
…
C6–H6 O1L [C7]
…
…
C1H11A O1GLC2G
2.639(2)
2.639(2)
83
79
…
CLO CLO
…
…
CLO C (u)
…
O CLO (u)
˚
C (A)
O
C1H11B O1GLC2G
…
C7LO1 C9LO2
2.548(4)
2.832(4)
3.133(4)
104.7(1)
89.2(1)
75.7(1)
105.9(1)
104.9(1)
90.1(1)
…
CLO CLO
…
…
CLO C (u)
…
…
C7LO1 C10LO3
˚
C (A)
O
O
CLO (u)
…
C10LO3 C7LO1
…
C10LO3 C2GLO1G
2.895(2)
81.67(12)
101.45(12)
despite their differences in the flexible side chain conformations.
…
of ca. 145u. In addition, weak intermolecular hydrogen bonding
was also observed; O1 accepts protons from two C–H groups,
The intermolecular N–H O hydrogen bonding across a centre of
inversion doubly bridges two neighbouring molecules along the
b-axis of the monoclinic space group in both structures (Fig. 6b,
Tables 12 and 13). Although there is one dimensional isostructur-
ality in these two crystal forms, these 1D chains are associated
differently in the other dimensions due to different interactions
made by the halogen, Br (Fig. 6c). Crystal structure 5b-2 shows
…
˚
namely C5–H5 and C8–H8 with d(C O) of 3.267 and 3.494 A,
respectively (Fig. 5a and Table 9).
In the crystal structure of 5d, O3 acts as an acceptor for two
protons, which are N2–H2 forming strong intermolecular
…
hydrogen bonds with d(N O) of 2.788(6) A and C11–H11B
˚
…
…
linear chain Br Br short contacts with Br Br contact distance of
…
forming a weak intermolecular hydrogen bonds with d(C O) of
˚
3.226(6) A. The same weak intermolecular hydrogen bonding
…
3.531(1) A and slipped p p contacts between the aromatic groups
˚
present in 5a was also observed in 5d, where oxygen O1 accepts
(Fig. 5). In contrast, structure 5b-1 forms a pseudo-macrocycle
…
using two identical C–H Br interactions with d(H Br) of 3.09 A,
…
protons from two C–H groups, namely C5–H5 and C8–H8, with
…
˚
˚
…
…
d(C O) of 3.278(6) and 3.315(6) A, respectively (Fig. 5b and
Table 10).
˚
d(C Br) of 4.063 A and angle C–H Br ca. of 173u. The
cyclohexane guest is trapped at the centre of this macrocycle using
…
two identical bifurcated C–H p contacts with the aromatic ring
In the crystal structure of 6b, O3 acts as an acceptor for two
protons, with N2G–H2N forming strong intermolecular hydro-
and the carbonyl group (Fig. 6c). It is noteworthy that the
…
…
gen bonds with d(N O) of 2.801(1) A and CG–H21B forming
˚
cyclohexane guest makes weaker C–H p contacts with the host,
…
…
…
rather than using options of C–H Br or even C–H O hydrogen
bonding to form a stable inclusion crystal.
weak intermolecular hydrogen bonds with d(C O) of 3.192(2)
˚
A. A different type of weak head to tail intermolecular hydrogen
bonding was observed in 6b, where oxygen O1 accepts protons
from two C–H groups, namely C8–H8 and C16–H16A, from the
end of the alkyl side chain of the molecules in the adjacent row
Conclusions
…
˚
A number of alkyl N-acetylglyoxylamides were successfully
prepared by ring-opening of N-acetylisatins with a range of
amines. X-ray crystallography studies of the synthesised
molecules revealed different types of crystal packing for
compounds 5a, 5b, 5d and 6b. A cooperative effect between
strong and weak interactions played an important role in
directing the packing of these molecules during crystallization.
Structures 5a, 5d, 6b and the parent glycine peptidomimetic
with d(C O) of 3.352 (2) and 3.365(7) A, respectively (Fig. 5c
and Table 11).
The bromo derivative 5b produced two crystal forms, one
without the solvent (5b-2) and one with the solvent (5b-1). The
identical H-bonded one dimensional molecular rows are packed
differently in the two unit cells; closer packing of these rows in
structure 5b-2 does not leave any voids, whereas in 5b-1, larger
separation of these rows creates channels that are occupied by
the solvent molecule cyclohexane (Fig. 6a). There is one-
dimensional isostructurality23–26 between the two structures,
…
derivative 3 displayed characteristic N1–H1 O2 hydrogen bond
interactions which may represent a potential supramolecular
synthon in the crystallization of 2-aminophenylglyoxylic acid
based peptidomimetics. Conversely, novel intramolecular triple
orthogonal carbonyl dipolar interactions were observed in
structures 5b-1 and 5b-2. In many molecules, halogen atoms
often make short contacts with a variety of other atoms,
Table 7 Numerical details of the intramolecular interactions present in
5b-1
…
…
…
˚
˚
˚
Donor (D)–H Acceptor (A) D–H (A) H A (A) D A (A) Angle (u)
…
N2–H O2L [C9]
…
0.88
0.95
0.99
2.33
2.54
2.50
2.690(6) 104
including halogen–halogen (X1 X2) interactions, which play a
…
C3–H O2L [C9]
2.814(6)
2.805(6)
97
97
significant role in determining the crystal structures.27-30 This is
…
C11A–H11A O3L [C10]
certainly the case for molecule 5b, where the halogen atom either
…
…
uses a C–H Br type interaction (5b-1) or a Br Br type
interaction (5b-2). Introduction of the alkyl side chains was
found to enhance the self-assembly properties of these amino-
phenylglyoxylic acids, which no longer favoured the dimeric
conformation displayed by the parent compound 3.
…
CLO CLO
…
…
CLO C (u)
…
O
˚
C (A)
O
CLO (u)
…
…
C7LO1 C9LO2
2.558(6)
2.841(6)
3.148(6)
103.7(1)
88.0(1)
74.3(1)
105.71(1)
106.5(1)
91.1(1)
C7LO1 C10LO3
…
C10LO3 C7LO1
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 7345–7354 | 7349

View Article Online
Fig. 4 Crystal packing of (a) 5a, (b) 5d, (c) 6a, (d) 5b-1, and (e) 5b-2.
The self-assembly of substituted glyoxylamides is a complex
area which remains relatively unexplored. Further studies in this
area are ongoing in order to fully eludicate the factors which
control this process in the solid state.
corrected for Lorentz and polarisation effects using Bruker
APEX2 software.32 All structures were solved by direct
methods and the full-matrix least-squares refinements were
carried out using SHELXL.33 The non-hydrogen atoms were
refined anisotropically. The molecular graphics were generated
using Mercury.34
Experimental
The X-ray diffraction measurements for 5a and 5d were
carried out at MX1 and MX2 beamlines at the Australian
Synchrotron Facility, Melbourne. The procedure for diffraction
intensity measurements on both beam lines was similar. The
crystal was mounted on the goniometer using a cryo loop for
diffraction measurements, coated with paraffin oil and then
quickly transferred to the cold stream using Cryo stream
attachment. Data were collected using Si,111> monochromated
Physical measurements
NMR data were recorded using a Bruker DPX300 instrument
(1H 300 MHz, 13C 75.4 MHz) at 25 uC and reported as chemical
shift (d) relative to SiMe4. Carbon substitution information was
determined using the DEPT procedure, and the IR spectra were
recorded using a Perkin–Elmer PE298 spectrophotometer with
the sample prepared as a KBr pellet, in the range 4000–400 cm21
.
˚
synchrotron X-ray radiation (l = 0.71023 A) at 100(2) K and
were corrected for Lorentz and polarization effects using the
XDS software.35 The structure was solved by direct methods and
the full-matrix least-squares refinements were carried out using
SHELXL.33
Structure determination
Suitable single crystals of 5b and 6b, selected under the
polarizing microscope (Leica M165Z), were picked up on a
MicroMount (MiTeGen, USA) consisting of a thin polymer
tip with a wicking aperture. The X-ray diffraction measure-
ments were carried out on a Bruker KAPPA APEX II CCD
diffractometer at 150 K by using graphite-monochromated
Synthesis
GP 1: General procedure for the synthesis of N-acetyl
glyoxylicamides 5a–i. Alkylamine (1.0 mmol) was added to a
stirred solution of N-acetylisatin (1.0 mmol) in CH2Cl2 (10 mL)
and the mixture was heated at reflux for 4 h. After cooling to
room temperature, the solvent was evaporated under reduced
pressure and the crude product was purified by column
chromatography using silica gel and a mixture of hexane and
˚
Mo-Ka radiation (l = 0.710723 A). The single crystals,
mounted on the goniometer using cryo loops for intensity
measurements, were coated with paraffin oil and then quickly
transferred to the cold stream using an Oxford Cryo stream
attachment. Symmetry related absorption corrections using
the program SADABS31 were applied and the data were
7350 | CrystEngComm, 2012, 14, 7345–7354
This journal is ß The Royal Society of Chemistry 2012

View Article Online
Fig. 5 Intermolecular interactions present in (a) 5a, (b) 5d, and (c) 6b.
dichloromethane as eluent. The product was recrystallized from
n-hexane and dried in vacuo.
2-(2-Acetamidophenyl)-N-hexyl-2-oxoacetamide, 5a.
N-Acetylglyoxylic amide 3a was prepared from 1-acetylindo-
line-2,3-dione (0.38 g, 2 mmol) and hexylamine (0.21 g, 2 mmol)
according to GP 1. The title compound 5a was afforded as white
flakes after purification by column chromatography (0.28 g,
48%). The solid, recrystallized from CH3CN via slow evapora-
tion of the solvent at room temperature, yielded crystals suitable
for X-ray crystal structure determination. 5a: mp 114–116 uC;
UV (MeOH): lmax 200 nm (e 18 845 cm21 M21), 233 (21 882),
267 (7495), 339 (2826); IR (KBr): nmax 3265, 3120, 3054, 2951,
2931, 2853, 1665, 1606, 1536, 1484, 1466, 1432, 1367, 1321, 1300,
1279, 1259, 1234, 1210, 1160, 1114, 1013, 969, 896, 814, 764, 707,
GP 2: General procedure for the synthesis of N-acetyl
glyoxylamide peptide mimics 6a–d. A solution of amino acid ester
hydrochloride (2.5 mmol) containing saturated NaHCO3 was
added to a stirred solution of the N-acetylisatin (1.0 mmol) in
CH2Cl2 (10 mL) at 5 uC. The reaction mixture was warmed to
room temperature and stirred for 24–28 h. The organic layer was
diluted with CH2Cl2 (20 mL) and extracted with aqueous HCl
(0.5 M, 15 mL) and water (20 mL). The organic extract was dried
over anhydrous Na2SO4, filtered and concentrated under vacuum.
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 7345–7354 | 7351

View Article Online
Table 9 Numerical details of the intermolecular interactions present in
5a
Table 11 Numerical details of the intermolecular interactions present in
6b
…
…
…
…
D A
˚
(A)
D–H
˚
(A)
H
A
D
A
Angle
(u)
D–H
˚
(A)
H
A
Angle
(u)
…
Donor (D)–H Acceptor (A)
…
Donor (D)–H Acceptor (A)
˚
(A)
˚
(A)
˚
(A)
a
a
…
…
…
…
N2A–H2NA O3A L[C10A]
0.86
0.93
0.96
0.96
2.06
2.43
2.36
2.65
2.805(13)
3.267(13)
3.221(17)
3.494(16)
144
113
148
150
N2G–H O3 L[C10]
0.88
0.99
0.98
0.98
2.07
2.65
2.39
2.77
2.801(1)
3.192(2)
3.352(3)
3.365(3)
139
115
166
120
c
a
…
C5A–H5A O1A L[C7A]
C1G–H2G O3 L[C10]
a
a
…
C8A–H8A O1A L[C7A]
C8–H82 O1 L[C7]
e
…
f
a
…
C16A–H16B O2B L[C9B]
C16–H16A O1 L[C7]
Equivalent position indicators:a 21 + x, y, z.
b
…
N2B–H2NB O3B L[C10B]
0.86
0.93
0.96
0.96
2.01
2.43
2.30
2.65
2.764(14)
3.270(15)
3.246(13)
3.519(15)
145
150
170
150
d
…
…
C5B–H5B O1B L[C7B]
b
C8B–H8B O1B L[C7B]
g
h
…
(d, J = 7.04 Hz, 1H, H5), 8.56 (s, 1H, H3), 10.84 (s, 1H, NHCO);
13C NMR (75.6 MHz, CDCl3): d 190.66, 169.10, 161.91 (3 6
CLO), 140.76 (ArC), 138.91 (ArCH), 136.40 (ArCH), 122.30
(ArCH), 120.29 (ArCH), 114.94 (ArC), 39.66 (NHCH2),
31.29, 29.09, 26.44, 22.43 (CH2(CH2)4CH3), 25.33 (COCH3),
13.91 (CH2(CH2)4CH3); HRMS (ESI) m/z calculated for
C16H21BrN2O3 (M + 1)+ 369.0736. Found 369.0791; Anal.
Calcd. for C16H21BrN2O3: C, 52.04; H, 5.73; N, 7.59. Found: C,
52.26; H, 5.94; N, 7.53%.
C16B–H16F O2A L[C9A]
Equivalent position indicators:a x, 21 + y, z. b x, 1 + y, z. c 2x, 2 2 y,
d
e
f
g
h
2z. 1 2 x, 2y, 2z. x, 1 + y, z. x, 2.5 2 y, K + z. x, y, z. x,
1 + y, z.
669, 648, 603, 561, 518 cm21 1H NMR (300 MHz, CDCl3):
;
d 0.90 (t, J = 6.84 Hz, 3H, CH2(CH2)4CH3), 1.29–1.44
(m, 6H, CH2CH2(CH2)3CH3), 1.56–1.66 (m, 2H, NHCH2CH2
(CH2)3CH3), 2.24 (s, 3H, COCH3), 3.38–3.45 (m, 2H,
NHCH2CH2(CH2)3CH3), 6.91 (s, 1H, CONH), 7.11–7.17 (m,
1H, H4), 7.59–7.65 (m, 1H, H5), 8.41 (dd, J = 1.51, 8,11 Hz, 1H,
H3), 8.67 (dd, J = 8.60, 0.88, Hz, 1H, H6), 10.96 (s, 1H, NHCO);
13C NMR (75.6 MHz, CDCl3): d 192.21, 169.19, 162.84 (3 6
CLO), 141.93 (ArC), 136.38 (ArCH), 134.30 (ArCH), 122.47
(ArCH), 120.57 (ArCH), 118.65 (ArC), 39.55 (NHCH2), 31.29,
29.14, 26.44, 22.43 (CH2(CH2)4CH3), 25.43 (COCH3), 13.90
(CH2(CH2)4CH3); HRMS (ESI) m/z calculated for C16H22N2O3
2-(2-Acetamidophenyl)-N-dodecyl-2-oxoacetamide, 5d.
N-Acetylglyoxylic amide 5d was prepared from 1-acetylindo-
line-2,3-dione (0.28 g, 1.5 mmol) and dodecylamine (0.28 g,
1.5 mmol) according to GP 1. The title compound 5d was
afforded as off-white flakes (0.41 g, 72%). The solid, recrystal-
lized from hexane via slow evaporation of the solvent at room
temperature, yielded crystals suitable for X-ray crystal structure
determination. 5d: mp 116–118 uC; UV (MeOH): lmax 207 nm (e
20 584 cm21M21), 233 (33 683), 266 (14 783), 333 (7547); IR
(KBr): nmax 3284, 3068, 2915, 2848, 1696, 1651, 1604, 1581, 1522,
1467, 1449, 1371, 1324, 1295, 1247, 1219, 1232, 1164, 1129, 1040,
(M
+
1)+ 291.1630. Found 291.1696; Anal. Calcd. for
C16H22N2O3: C, 66.18; H, 7.64; N, 9.65. Found: C, 66.29; H,
7.76; N, 9.59%.
1006, 892, 854, 813, 769, 721, 690, 628, 599, 552, 512 cm21; H
1
2-(2-Acetamido-5-bromophenyl)-N-hexyl-2-oxoacetamide, 5b.
The N-acetylglyoxylic amide 5b was prepared from 1-acetyl-5-
bromoindoline-2,3-dione (0.40 g, 1.5 mmol) and hexylamine
(0.15 g, 1.5 mmol). according to GP 1. The title compound 5b
was afforded as a light brown solid after purification by column
chromatography (0.61 g, 55%). The solid, recrystallized from
hexane or cyclohexane via slow evaporation of the solvent at
room temperature, yielded crystals suitable for X-ray crystal
structure determination. 5b: mp 116–118 uC; UV (MeOH): lmax
203 nm (e 31 023 cm21 M21), 238 (32 886), 345 (3000); IR (KBr):
nmax 3277, 2955, 2927, 2850, 1664, 1599, 1521, 1482, 1369, 1293,
1258, 1203, 1137, 822, 693 cm21; 1H NMR (300 MHz, CDCl3): d
0.91 (t, J = 6.76 Hz, 3H, CH2CH2(CH2)3CH3), 1.25–1.44 (m, 6H,
CH2CH2(CH2)3CH3), 1.56–1.66 (m, 2H, CH2CH2(CH2)3CH3),
2.21 (s, 3H, COCH3), 3.36–3.43 (m, 2H, NHCH2(CH2)4CH3),
6.95 (s, 1H, CONH), 7.66 (dd, J = 2.41, 9.08 Hz, 2H, H6), 8.53
NMR (300 MHz, CDCl3): d 0.87 (t, J = 6.65 Hz, 3H,
CH2CH2(CH2)9CH3), 1.25–1.39 (m, 18H, CH2CH2(CH2)9CH3),
1.56–1.66 (m, 2H, NHCH2CH2(CH2)9CH3), 2.21 (s, 3H,
COCH3), 3.36–3.43 (m, 2H, NHCH2(CH2)10CH3), 6.87 (s, 1H,
CONH), 7.14–7.09 (m, 1H, H4), 7.61–7.56 (m, 1H, H5), 8.35 (d,
J = 7.84 Hz, 1H, H3), 8.64 (d, J = 8.39 Hz, 1H, H6), 10.96 (s, 1H,
NHCO); 13C NMR (75.6 MHz, CDCl3): d 192.14, 169.16, 162.77
(3 6 CLO), 141.95 (ArC), 136.40 (ArCH), 134.32 (ArCH),
122.46 (ArCH), 120.59 (ArCH), 118.66 (ArC), 39.57 (NHCH2),
31.81, 29.54, 29.52, 29.47, 29.42, 29.24, 29.18, 29.13, 26.78, 22.59
(CH2(CH2)10CH3), 25.35 (COCH3), 14.02 (CH2(CH2)10CH3);
HRMS (ESI) m/z calculated for C22H34N2O3 (M + 1)+ 375.5170.
Found 375.2622; Anal. Calcd. for C22H34N2O3: C, 70.55; H,
9.15; N, 7.48. Found: C, 70.84; H, 9.21; N, 7.53%
Hexyl 2-(2-(2-acetamidophenyl)-2-oxoacetamido)acetate, 6b.
N-Acetyl glyoxylamide peptide mimic 6b was prepared from
1-acetylindoline-2,3-dione (0.37 g, 2 mmol) and glycine hexyl
ester hydrochloride (1 g, 5 mmol) according to GP 2. Purification
by gravity column chromatography over silica with CH2Cl2 gave
the title compound 6b as off-white flakes (0.31 g, 45%). The
solid, recrystallized from hexane via slow evaporation of the
solvent at room temperature, yielded crystals suitable for X-ray
crystal structure determination. 6b: mp 56–58 uC; UV (MeOH):
Table 10 Numerical details of the intermolecular interactions present in
5d
…
…
A
˚
(A)
D–H
˚
(A)
H
A
D
Angle
(u)
…
Donor (D)–H Acceptor (A)
˚
(A)
a
…
N2–H O3 L[C10]
0.86
0.97
0.93
0.96
2.04
2.70
2.46
2.50
b
2.788(6)
3.226(6)
3.278(6)
3.315(6)
144
115
147
133
a
…
C11–H11 O3 L[C10]
b
…
…
C5–H O1 L[C7]
b
C8–H O1 L[C7]
l
max 191 nm (e 30 116 cm21M21), 233 (29 362), 273 (12 824), 341
Equivalent position indicators:a x,21 + y, z. 1 2 x, 2 2 y, 1 2 z.
(6847); IR (KBr): nmax 3319, 2959, 2929, 2856, 1754, 1701, 1644,
7352 | CrystEngComm, 2012, 14, 7345–7354
This journal is ß The Royal Society of Chemistry 2012

View Article Online
Fig. 6 Crystal packing (a), intermolecular interactions (b) and Br interactions (c) present in 5b-1 (left) and 5b-2 (right).
Table 12 Numerical details of the intermolecular interactions present in
5b-1
Table 13 Numerical details of the intermolecular interactions present in
5b-2
…
…
A
˚
(A)
D–H
˚
(A)
H
A
D
Angle
(u)
…
…
A
˚
(A)
…
Donor (D)–H Acceptor (A)
D–H
˚
(A)
H
A
D
Angle
(u)
˚
(A)
…
Donor (D)–H Acceptor (A)
˚
(A)
a
…
…
N1–H O3 L[C10]
0.88
0.88
0.99
0.99
1.94
2.06
2.48
2.59
2.805(6)
2.814 (6)
3.169(12)
3.280(14)
169
143
126
127
a
…
…
b
N1–H O3 L[C10]
0.88
0.88
0.99
1.94
2.05
2.68
2.799(4)
2.802(4)
3.224(5)
166
142
115
N2–H O1 L[C7]
b
c
…
…
N2–H O1 L[C7]
C13A–H O2 L[C9]
b
…
c
C11–H11B O1 L[C7]
C14A–H O2 L[C9]
Equivalent position indicators:a 2 2 x, 2 2 y, 1 2 z. b 2 2 x, 1 2 y, 1 2 z.
Equivalent position indicators:a 1 2 x , 1 2 y, 2z. 1 2 x, 2 2 y, 2z.
21 + x, y, z.
b
c
(CH2)3CH3), 2.22 (s, 3H, COCH3), 4.16 (d, J = 5.48 Hz, 2H,
NHCH2COO), 4.19 (t, J = 6.75 Hz, 2H, COOCH2(CH2)4CH3),
7.09–7.14 (m, 1H, Ar-H4), 7.30 (s, 1H, NHCH2COO), 7.57–7.63
(m, 1H, Ar-H5), 8.35 (dd, J = 1.62, 8.10 Hz, 1H, Ar-H3), 8.66
1605, 1586, 1533, 1451, 1360, 1322, 1297, 1196, 1096, 1035, 961,
940, 816, 767, 754, 685, 661, 630, 522 cm21; 1H NMR (300 MHz,
CDCl3): d 0.88 (t, J = 6.75 Hz, 3H, CH2(CH2)4CH3), 1.26–1.39
(m, 6H, CH2CH2(CH2)3CH3), 1.63–1.71 (m, 2H, CH2CH2
This journal is ß The Royal Society of Chemistry 2012
CrystEngComm, 2012, 14, 7345–7354 | 7353

View Article Online
(dd, J = 0.94, 8.55 Hz, 1H, Ar-H6), 10.93 (s, 1H, NHCO); 13C
NMR (75.6 MHz, CDCl3): d 191.02, 169.31, 168.97, 162.84
(4 x CLO), 142.29 (ArC), 136.79 (ArCH), 134.45 (ArCH),
122.57 (ArCH), 120.71 (ArCH), 118.42 (ArC), 66.12 (COOCH2
(CH2)4CH3), 41.33 (NHCH2COO), 31.36, 28.47, 25.50, 22.51
(CH2(CH2)4CH3), 25.47 (COCH3), 13.97 (CH2(CH2)4CH3); HRMS
(ESI) m/z calculated for C18H24N2O5 (M + 1)+ 349.1685. Found
349.1180; Anal. Calcd. for C18H24N2O5: C, 62.05; H, 6.94; N, 8.04.
Found: C, 62.05; H, 7.16; N, 8.03%.
12 D. StC. Black, G. C. Condie, D. C. Craig and D. B. McConnell, Mol.
Cryst. Liq. Cryst., 2005, 440, 141–146.
13 W. C. Cheah, D. S. Black, W.K. Goh and N. Kumar, Tetrahedron
Lett., 2008, 49, 2965–2968.
14 W. C. Cheah, K. Wood, D. S. Black and N. Kumar, Tetrahedron,
2011, 67, 7603–7610.
15 T. Le, W. C. Cheah, K. Wood, D. S. Black, M. D. Willcox and N.
Kumar, Tetrahedron Lett., 2011, 52, 3645–3647.
16 L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104, 1201–1217.
17 J. H. Jung, G. John, M. Masuda, K. Yoshida, S. Shinkai and T.
Shimizu, Langmuir, 2001, 17, 7229–7232.
18 F. H. Allen, C. A. Baalham, J. P. M. Lommerse and P. R. Raithby,
Acta Crystallogr., Sect. B, 1998, 54, 320–329.
19 R. Paulini, K. Mu¨ller and F. Diederich, Angew. Chem., Int. Ed., 2005,
44, 1788–1805.
Acknowledgements
20 F. R. Fischer, P. A. Wood, F. H. Allen and F. Diederich, Proc. Natl.
Acad. Sci. U. S. A., 2008, 105, 17290–17294.
21 C. Fa¨h, L. A. Hardegger, M.-O. Ebert, W. B. Schweizer and F.
Diederich, Chem. Commun., 2010, 46, 67–69.
22 R. Bishop and N. K. Hamer, J. Chem. Soc. C, 1970, 1197–1202.
23 L. Fa´bia´n, A. Argay, A. Ka´lma´n and M. Ba´thori, Acta Crystallogr.,
Sect. B, 2002, 58, 710–720.
We thank the Directorate General of Higher Education, Ministry
of National Education, Indonesia through the University of
Sebelas Maret, Indonesia for a PhD scholarship to Venty Suryanti.
We also thank Dr Tom Caradoc-Davies, a Principal Scientist
(Australian Synchrotron), for his help in data acquisition.
24 L. Fa´bia´n and A. Ka´lma´n, Acta Crystallogr., Sect. B, 2004, 60,
547–558.
25 G. V. Bhosekar, C. Murali, R. G. Gonnade, M. S. Shashidar and
M. M. Bhadbhade, Cryst. Growth Des., 2005, 5, 1977–1982.
26 A. Anthony, A. Nangia, G. R. Desiraju and M. Jasko´lski, Chem.
Commun., 1998, 2537–2538.
27 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311–2327.
28 G. R. Desiraju and R. Parthasarathy, J. Am. Chem. Soc., 1989, 111,
8725–8726.
29 T. Sakurai, M. Sundaralingam and G. A. Jeffrey, Acta Crystallogr.,
1963, 16, 354–363.
30 N. Ramasubbu, R. Parthasarathy and P. Murray-Rust, J. Am. Chem.
Soc., 1986, 108, 4308–4314.
31 Bruker, SADABS, Bruker AXS Inc., Madison, Wisconsin, USA,
2001.
32 Bruker, APEX2 and SAINT, Bruker AXS Inc., Madison, Wisconsin,
USA, 2001.
References
1 J. W. Steed and J. L. Atwood, Supramolecular Chemistry, John Wiley
& Sons, Ltd., UK, 2nd edn, 2009.
2 C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885–3896.
3 C. Janiak and T.G. Scharmann, Polyhedron, 2003, 22, 1123–1133.
4 C. M. Reddy, M.T. Kirchner, R. C. Gundakaram, K. A. Padmanabhan
and G. R. Desiraju, Chem.–Eur. J., 2006, 12, 2222–2234.
5 R. E. Melendez, A. D. Hamilton, Design of Organic Solids:
Hydrogen-Bonded Ribbons, Tapes and Sheets as Motifs, Springer-
Verlag, Berlin, 1998.
6 G. R. Desiraju, Angew. Chem., Int. Ed. Engl., 1995, 34, 2311–2327.
7 G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in Structural
Chemistry and Biology, Oxford Science Publication, Oxford, 1999.
8 S. S. Kuduva, D. Blaser, R. Boese and G. R. Desiraju, J. Org. Chem.,
2001, 66, 1621–1626.
9 C. Greenberg, M. Breneman and J. F. Liebman, The Amide Linkage:
Structural Significance in Chemistry, Biochemistry and Materials
Science, Wiley-IEEE, 2002.
10 D. StC. Black, D. C. Craig and D. B. McConnell, J. Am. Chem. Soc.,
1996, 118, 8148–8149.
33 G. M. Sheldrick, Acta Cryst., 2008, A64, 112–122.
34 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P.
McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. van de
Streek and P. A. Wood, Mercury CSD 2.0 - new features for the
visualization, investigation of crystal structures, J. Appl. Crystallogr.,
2008, 41, 466–470.
11 D. StC. Black, D. C. Craig and D. B. McConnell, Tetrahedron Lett.,
1997, 38, 4287–4290.
35 W. Kabschi, J. Appl. Crystallogr., 1993, 26, 795–800.
7354 | CrystEngComm, 2012, 14, 7345–7354
This journal is ß The Royal Society of Chemistry 2012