Supramolecular architectures constructed through self-assembly of a chalcone and substituted diazo-β-diketones

Article abstract of DOI:10.1016/j.molstruc.2007.08.011

Organic compounds namely pyridyl chalcone viz. 3-[4-(3-oxo-3-pyridin-2-yl-propenyl)-phenyl]-1-pyridin-2-yl-propenone (L¹), p-cholorophenyldiazopentane-2,4-dione (L²) and p-methyl phenyldiazopentane-2,4-dione (L³) have been

Full text of DOI:10.1016/j.molstruc.2007.08.011

Available online at www.sciencedirect.com
Journal of Molecular Structure 879 (2008) 1–6
www.elsevier.com/locate/molstruc
Supramolecular architectures constructed through self-assembly
of a chalcone and substituted diazo-b-diketones
a
a,
b
c
a
*
R. Prajapati , L. Mishra , S.J. Grabowski , G. Govil , S.K. Dubey
a
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
Department of Physics and Chemistry, University of Ł o´ d z´ , Pomorska 149/153, 90 236 Ł o´ d z´ , Poland
b
c
Raja Ramanna Fellow, Tata Institute of Fundamental Research, Mumbai 400 005, India
Received 30 June 2007; received in revised form 8 August 2007; accepted 8 August 2007
Available online 24 August 2007
Abstract
Organic compounds namely pyridyl chalcone viz. 3-[4-(3-oxo-3-pyridin-2-yl-propenyl)-phenyl]-1-pyridin-2-yl-propenone (L ),
1
2
3
p-cholorophenyldiazopentane-2,4-dione (L ) and p-methyl phenyldiazopentane-2,4-dione (L ) have been characterized by their single-
crystal X-ray crystallographic studies. Several structural motifs resulting upon their self-association through probable non-covalent inter-
actions have been discussed. The studies of related motifs found in Cambridge Structural Database are performed and the results are
related to the structural data obtained for crystal structures reported here in.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Crystal structure; Hydrogen bonding; Self-association
1
. Introduction
face and edge to face p–p interactions between aromatic
rings [6]. Additionally, supramolecular approach has also
been used to control reactivity in the organic solid that
employs molecules in the form of linear templates to direct
the [2+2] photo dimerization through network of hydrogen
bonds [7]. Among various molecular interactions, the
H-bond has received special attention due to its signifi-
cance from biological view-point. Special importance is
the creation of higher order structures of peptides and
nucleic acids and the biochemical processes particularly
those in enzyme-catalyzed reactions [8]. Hydrogen-bond
also plays role in biological systems arising from a stronger
directional interaction and to dynamic feature of the pro-
ton. It therefore acts as an active site for initiation of chem-
ical reaction.
It is well known that H-bond in crystals of organic com-
pounds often acts as a decision factor governing packing
and it helps in the design of novel and interesting crystal
structures especially in the area of crystal engineering [9].
Additionally, if one goes by the Pauling [10] definition of
hydrogen bonding that: ‘‘A hydrogen-bond is an interac-
tion that directs the association of a covalently bound
Porous structures or open frameworks including remov-
able solvents or exchangeable ions that provide new sizes,
shapes and chemical environments have been of great inter-
est in recent years, due to their potential applications [1,2]
like mimicking zeolites etc. In recent years, efforts have
been devoted to the synthesis and characterization of
supramolecular complexes displaying a large variety of
topologies [3]. Such structures can find diverse applications
in the field of catalysis, host–guest chemistry, electrical and
magnetic materials [4]. In this context, particular attention
has been focused on metal assisted self-assembly of helical
and box like metallic complexes [5]. Structural studies of
complexes specially derived from Schiff base ligands have
shown that CH-p and p–p interactions lead to the di or
multi-helical topology of the complexes. Such structures
originate from weak intermolecular (inter-strand) face to
*
Corresponding author. Tel.: +91 542 2307321; fax: +91 542 2368174.
E-mail address: lmishrabhu@yahoo.co.in (L. Mishra).
0
022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.08.011

2
R. Prajapati et al. / Journal of Molecular Structure 879 (2008) 1–6
1
hydrogen atom with one or more other atoms, groups of
atoms, or molecules into an aggregate structure that is suf-
ficiently stable to make it convenient for the chemist to
consider it as an independent chemical species’’ thus it
nicely corresponds to the patterns which are often observed
in crystal structures. The cooperative activity of hydrogen-
bond is also involved in the molecules with multiple
p-bonds as p-electron delocalization and is related with
intermolecular and intramolecular H-bonds [11].
Theoretical studies on several Schiff base derivatives;
phenols and their crystal structure have been investigated.
The crystal structure of 2-hydroxyacetophenone indicates
that the proton is at the hydroxylic oxygen where as in
ortho-hydroxyacetonapthoimine, proton resides at the
nitrogen atom [12]. Additionally, fabrication of artificial
receptors through H-bonding, CH-p interaction, p–p
stacking along with hydrophobic interaction put their
effects as consolidated factors in effective and specific rec-
ognition at molecular level [13]. Participation of both H-
bonding and p–p stacking interactions has offered rational
approach for the understanding of higher order association
especially between host and guests [14].
t(C@O), 1600 t(phenyl); H NMR (DMSO-d , 300 MHz)
6
d (ppm) 8.4 (d, 2H; HC@CH), 8.2 (d, 2H; HC@CH),
7.7–8.0 (m, 8H; pyridyl protons), 7.5 (s, 2H; phenyl), 7.4
(s, 2H; phenyl).
2
L
prepared by the general procedure [14] in which
p-chloroaniline (purchased from Sigma–Aldrich), 0.64 g;
were dissolved in 1 N HCl (25 cm ) at 0–5 °C temperature
and cooled aqueous solution (10 cm ) of NaNO (0.40 g)
was added drop wise with stirring followed by the addition
of pentane-2,4-dione (0.50 g) and sodium acetate (5.0 g)
dissolved in water (30 ml). Corresponding mixture was fur-
ther stirred for 4 h at room temperature (ꢁ25 °C). Solids
thus obtained were filtered and washed several times with
distilled water followed by ethanol and then dried in vacuo.
The crude product was then re-crystallized in ethanol,
which melted at 78–80 °C and showed satisfactory elemen-
tal analysis. IR (KBr pellet, cm ): 1668 t(C@O), 3070
t(NH), 1626 t(C@N); H NMR (CDCl , 300 MHz) d
(ppm) 14.69 (s, 1H; NH), 7.35 (m, 4H; phenyl), 2.60 (s,
3H; CH ), 2.48 (s, 3H; CH ).
L was also prepared using the same procedure as used
for L , mp. 80–82 °C, and shows satisfactory elemental
analysis. IR (KBr pellet, cm ) 1668 t(C@O); 1626
3
3
2
ꢀ
1
1
3
3
3
3
2
ꢀ
1
Based on these precedence and also our interest in the
design of several conjugated organic compounds for their
spectral and bioactivities by complexation especially with
1
t(C@N); 1587 t(phenyl); H NMR (300 MHz, CDCl3,
ppm) d 14.8 (s, 1H, ANH); 7.4 (m, 4H, aromatic), 1.8–
2.2 (m, 9H, ArACH3 and OCACH3);
(75 MHz, CDCl ) d (ppm) 197.4, 197.0 (C@O); 139.2,
II
III
13
Ru /Ru ions, we have carried out single-crystal X-ray
crystallographic studies of organic compounds like
C NMR
3
3
-[4-(3-oxo-3-pyridin-2-yl-propenyl)-phenyl]-1-pyridin-2-yl-
135.9, 132.9, 130.2 (phenyl); 116.2 (C@N), 31.6, 26.6
(COACH ); 20.9 (phACH ).
1
propenone (L ), p-cholorophenyldiazopentane-2,4-dione
3
3
2
3
(
L ) and p-methylphenyldiazopentane-2,4-dione (L ). The
details of the possible structural motifs resulting from their
self-association through non-covalent interaction have
been reported.
2.3. X-ray crystallography
The X-ray data were collected at 297 K using Cu-Ka
radiation (k = 1.54184) and graphite monochromator.
The structure was solved by direct methods. All non-
hydrogen atoms were refined anisotropically by full-matrix
least squares. Computations were carried out on a PC
using SHELXL-97 [16] program package.
2
. Experimental
2
.1. Materials and methods
All the chemicals are purchased from Sigma–Aldrich
and are used without further purification. Elemental anal-
ysis and FAB mass data are recorded on Carbo-Erba ele-
mental analyzer 1108 and JEOL SX-102 mass
spectrometer, respectively. IR spectra in the 400–
3. Results and discussion
To look into the structural pattern obtained after asso-
ciation of conjugated organic compounds, the chalcone a
class of organic compound used as precursor in the synthe-
sis of bioactive flavone derivatives [17] was targeted first in
the present study. The compound was crystallized from
ꢀ
1
4
000 cm range were recorded as KBr pellets on a JASCO
1
FT-IR 5300 spectrometer where as H NMR spectra were
recorded on a JEOL AL 300 MHz spectrometer using
CDCl and DMSO-d as solvents and TMS as internal
ethanol:chloroform (1:1) with space group P2 /c. Major
3
6
1
reference.
crystallographic data are summarized in Table 1. The
ORTEP view of the compound is shown in Fig. 1. The
bond length between C1AC2 = 1.373(3) A is quite close
˚
2
.2. Synthesis
to that obtained for aromatic ring system. Other bond
1
L was synthesized using reported procedure [15] by the
length data are in normal length. However, bond between
˚
condensation of 2-acetyl pyridine (0.02 mol, 2.2 ml) with
terepthalaldehyde (0.01 mol, 2.2 ml) in alkaline medium.
The resulting yellow microcrystalline solid (ꢁ80%) melted
at 148–150 °C and gave satisfactory elemental analysis
C5AC6 = 1.504(3) A is slightly longer than the single bond
˚
between C6AC7 = 1.474(3), C7AC8 = 1.342(3) A, is
longer than typical double C@C bond (as for example for
ethene). Thus these, observations are indicative of partial
delocalization in this region of the structure. However,
ꢀ1
and mass (m/z = 340). IR (KBr pellet, cm ) 1670

R. Prajapati et al. / Journal of Molecular Structure 879 (2008) 1–6
3
Table 1
Summary of crystallographic data for L , L and L
due to highly conjugated nature of the compound, no sig-
nificant difference in bond radii as compared to the normal
one could be observed.
1
2
3
1
2
3
Parameters
L
L
L
Formula
M
C
22
H
16
N
O
2 2
C
11
H
11ClN
238.67
Monoclinic
/c
2
O
2
C
12
H
14
N
2
O
2
From the X-ray data it is found that the pyridine plane
deviates by ± 5.0° from the plane of central aromatic ring.
The plane of pyridyl ring deviates from the plane contain-
ing AC@O group by 0.3°. Thus this structure indicates par-
tial delocalization of electrons in this region in support
with the variation of bond lengths between C5AC6,
C6AC7 and C7AC8. It can lead to the formation of inter-
molecular bonds with either complementary charge on the
partners or with the complementary functionality present
on the partners through non-covalent interactions as
reported for several Schiff bases and for benzene-dimer
[17]. The intermolecular association between similar mole-
cules via CAHꢂ ꢂ ꢂN (pyridyl) that is CH group lying closer
340.37
Monoclinic
P2 /c
3.8670(2)
12.5840(6)
17.0560(8)
96.356(3)
824.88(7)
2
1.370
7127
1519
0.033
218.27
Monoclinic
P2 /c
13.151(3)
4.7288(10)
17.895(4)
103.568(3)
1081.8(4)
2
1.012
6601
2633
0.046
Crystal system
Space group
1
C
2
1
˚
a (A)
18.9160(6)
4.3200(2)
28.0660(8)
99.835(2)
2259.77(14)
8
1.403
10,090
2108
0.031
˚
b (A)
˚
c (A)
b (°)
V (A )
˚
3
Z
ꢀ3
D
c
(mg m
)
Reflns. collected
Reflns. unique
R
int
Index ranges
ꢀ4 6 h 6 4
ꢀ23 6 h 6 23
ꢀ17 6 h 6 12
ꢀ
12 6 k 6 15
21 6 l 6 21
ꢀ5 6 k 6 5
ꢀ6 6 k 6 6
1
to pyridyl N in L molecule is allowed to interact with N-
ꢀ
ꢀ34 6 l 6 34
ꢀ22 6 l 6 23
pyridyl of other molecule and CH of other molecule with
N-pyridyl of earlier molecule results into a single helical
motif, shown in Fig. 2. Two equivalent CAHꢂ ꢂ ꢂN hydro-
gen bonds exist since pyridyl rings are related through
2
Refinement method
wR
Full-matrix, least squares on F
2
0.1566
0.0622
1.107
0.1603
0.0567
1.071
0.0829
0.0520
1.006
R
1
GoF
inversion centre. According to the Etter’s graphs’ terminol-
2
ogy such a ring may be designated as R ð6Þ [18].
2
2
These R ð6Þ motifs are observable in crystal structures.
2
The Cambridge Structural Database (CSD) [19] was
searched through such patterns existing between pyridyl
rings with the reservations that no errors for the crystal
structures, not polymeric, no ions and no powder struc-
tures exist. Additionally, the conditions for the accuracy
of the crystal structures were included, R-factor less than
˚
8% and e.s.d.’s for CC bond lengths less-equal 0.005 A;
XAH bonds were normalized to the mean neutron diffrac-
tion corresponding bond lengths according to the rules
applied in CSD. Two hundred motifs mentioned above
were found, thus centro symmetric dimer with two equiva-
lent CAHꢂ ꢂ ꢂN bonds between pyridyl rings exist sometimes
in crystal structures of organic compounds. The histogram
of Hꢂ ꢂ ꢂN distances within CAHꢂ ꢂ ꢂN hydrogen bonds is
presented in Fig. 3. The range of Hꢂ ꢂ ꢂN distances amounts
˚
˚
to 0.822 A, the shortest Hꢂ ꢂ ꢂN distance is equal to 2.401 A,
˚
˚
the longest one 3.223 A, mean Hꢂ ꢂ ꢂN value – 2.669 A. For
1
the L structure analyzed here Hꢂ ꢂ ꢂN distance is equal to
˚
2
.635 A.
1
Such associations observed for L are nicely in line with
the rules given early on by Etter [18] who stated that:
-
-
all good proton donors and acceptors are used in hydro-
gen bonding,
six-membered-ring intramolecular hydrogen bonds form
in preference to intermolecular hydrogen bonds,
1
2
3
1
Fig. 1. ORTEP diagram of L (a), L (b) and L (c). Thermal ellipsoids are
at 50% probability.
Fig. 2. Helical assembly (wire form) in L through N(1)ꢂ ꢂ ꢂHAC(2) and
N(2)ꢂ ꢂ ꢂHAC(1) (1 and 2 refer to two molecules).

4
R. Prajapati et al. / Journal of Molecular Structure 879 (2008) 1–6
˚
Fig. 4. Double helical structure through CAHꢂ ꢂ ꢂp (2.948 A).
molecular as depicted in Fig. 4, it results into a double heli-
˚
cal structure with r[CHꢂ ꢂ ꢂp] 2.948 A, providing continuous
formation of the cavity.
It is evident that O@CACH@CH groups do not lie in
the same plane as the aromatic rings are hence, show asym-
metric distribution of the electron density in this conju-
gated system. However, as it was indicated earlier, one
can observe the p-electron delocalization of CAC bonds
(C5AC6, C6AC7 and C7AC8).
The creation of slight positive and negative charges on
the whole skeleton of the molecule is evident. This behav-
iour can provide an opportunity for similar molecules to
get associated via complementary charges viz. +ve part of
one molecule can be attached to the ꢀve end of the other
molecule or vice versa. Based on this simple electrostatic
model concept, when the association of this molecule
through CAHꢂ ꢂ ꢂN interactions was examined, it results in
a single helical motif. On the other hand CAH interaction
˚
Fig. 3. The histogram of Hꢂ ꢂ ꢂN distance (in A) based on the CSD search
and concerning CAHꢂ ꢂ ꢂN bonds in centrosymmetric dimers of pyrroles.
-
the best proton donors and acceptors remaining after
intramolecular hydrogen-bond formation form intermo-
lecular hydrogen bonds to one another.
˚
Best proton donors and acceptors were defined early on
with p-electrons (2.948 A) of aromatic rings results into a
by Donohue [20] and good proton donors are those in car-
boxylic acids, amides, ureas, anilines, imides and phenols,
less acidic protons in acetylenes, aldehydes, activated aro-
matic and aliphatic compounds while good proton accep-
tors in acid and amide carbonyl groups, sulfoxides,
phosphoryls, nitroxides and amine nitrogens.
double helical motif with the extension of continuous for-
˚
mation of uniform cavity of dimension 20.97 A in length
˚
and 3.37 A in width. This is more evident in its space-filling
model Fig. 5. Additionally, face-to-face association through
˚
p–p interaction separated at (3.37 A) provides nice parallel-
unlimited onward arrangements of pyridyl groups.
2
One can see that there is the preference in crystal struc-
tures to form six-membered rings closed through intramo-
lecular hydrogen bonds. This is nicely shown in the next
L crystallizes in monoclinic crystal system with space
group C /c. The molecular structure and crystallographic
2
numbering scheme are illustrated in Fig. 1. The unit cell
contains eight discrete molecules arranged in the form of
two helical chains, which extend all along the lattice. Evi-
dently, there are p-stacking among the molecules and there
are two different arrangements; two molecules are paired
(analogues to parallel displaced case again like benzene
2
3
sections of this study where L and L structures are
analyzed.
Etter also stated [18] that if all best proton donors and
acceptors are involved in hydrogen bonds, the less acidic
and less basic centers are consecutively involved in such
1
˚
interactions. This is visible for the crystal structure of L
dimer) with an average intermolecular distance of 3.57 A
where there are two typical proton acceptors: oxygen atom
of carbonyl group and nitrogen of pyridyl ring. However,
there are not any good proton donors. Thus CAH bonds
are involved in hydrogen bonds, the motifs containing
CAHꢂ ꢂ ꢂN H-bonds were described above and there are
followed by an assembly of three molecules. Phenyl rings
of the adjacent molecules from a pair and trio are stacked
also C3AH3ꢂ ꢂ ꢂO1 contacts with Hꢂ ꢂ ꢂO distance of
˚
2
.551 A which may be hardly classified as hydrogen bonds;
for the latter interaction the O-atom of carbonyl group acts
as a proton acceptor.
Hence consequently, because of the lack of typical pro-
ton donors and only two proton acceptors the other types
1
of interactions are realized for the L crystal structure.
When pyridyl CH of one molecule was allowed to interact
1
Fig. 5. Space-filled model of double helical structure in L showing
with p-electron sphere of pyridyl ring of another similar
formation of continuous cavity.

R. Prajapati et al. / Journal of Molecular Structure 879 (2008) 1–6
5
perpendicularly (the closest CAC intermolecular distance is
Etter graphs’ nomenclature they may be designated as
S(6). As it was mentioned earlier there is the preference
of the formation of such motifs in crystal structures.
Numerous studies were performed on S(6) connections
where homonuclear OAHꢂ ꢂ ꢂO hydrogen bonds exist
but also NAHꢂ ꢂ ꢂO, NAHꢂ ꢂ ꢂN and OAHꢂ ꢂ ꢂN interac-
tions of S(6)’s type were analyzed. They are usually
attributed to so-called resonance assisted hydrogen bonds
(RAHBs) [21]. Briefly speaking the concept of RAHBs’
systems is based on the statement that owing the exis-
tence of conjugated system of single and double bonds
the p-electron delocalization exists which causes the
enhancement of the strength of hydrogen bonds. Such
concept is criticized recently [22] but there is no doubt
˚
ꢁ
3.484 A) with their tails in opposite directions.
Each molecule has an almost planar structure as can be
3
expected in any such conjugated system. However, the sp
hybridized N8 atom prevails perfect planarity. The inter
planar angle between the planes constituted by the phenyl
ring and the quasi N C OH ring (HN N C C O ) is
2
2
8
9 10 11 13
3
.7°. Interestingly, both the oxygen atoms are in keto form.
There is a double bond between C10 and N9 (bond length
is 1.315(3) A). N9AN8 and N8AC5 are thus single bonds
˚
and the conjugation breaks at this point. There is an intra-
molecular hydrogen bond (N8AHꢂ ꢂ ꢂO13) with N8AO13 of
˚
2
.585 A, which is shorter than the sum of their covalent
˚
radii (2.9 A).
3
L crystallizes in monoclinic crystal system with space
group P2 /c. The molecular structure and crystallographic
1
Table 2
numbering scheme are illustrated in ORTEP. The unit cell
contains two discrete molecules. There is substantial
arrangement of molecules through CHꢂ ꢂ ꢂp interaction in
˚
The geometrical parameters (in A) of six-membered ketohydrazone rings
taken from Cambridge Structural Database, mean, minimum and
maximum values are also included
one-dimensional forming uniform cavity with length of
˚
Refcode
N@N
C@C
CAC
C@O
Hꢂ ꢂ ꢂN
Nꢂ ꢂ ꢂO
7
.632 and 3.908 A width. The distances between phenyl
ADEGAS
BARPAM
CIVYIP01
CIVZEM10
CIVZEM10
DIRQEA
DIRQIE
DIRQUQ
HENJOZ
1.292
1.314
1.309
1.315
1.315
1.300
1.299
1.309
1.301
1.304
1.307
1.309
1.305
1.311
1.295
1.341
1.308
1.307
1.305
1.303
1.301
1.306
1.305
1.304
1.339
1.306
1.298
1.313
1.313
1.311
1.300
1.301
1.290
1.303
1.313
1.311
1.311
1.293
1.298
1.299
1.307
1.290
1.341
1.319
1.309
1.320
1.329
1.315
1.330
1.319
1.299
1.327
1.327
1.348
1.339
1.333
1.317
1.341
1.329
1.337
1.336
1.340
1.344
1.346
1.342
1.346
1.316
1.311
1.330
1.309
1.310
1.308
1.306
1.329
1.341
1.340
1.338
1.332
1.324
1.325
1.336
1.338
1.342
1.328
1.299
1.348
1.464
1.492
1.493
1.487
1.501
1.453
1.466
1.474
1.470
1.472
1.454
1.455
1.458
1.478
1.456
1.497
1.457
1.459
1.454
1.447
1.458
1.437
1.440
1.501
1.475
1.464
1.490
1.500
1.515
1.500
1.466
1.458
1.461
1.460
1.474
1.475
1.477
1.464
1.442
1.433
1.469
1.433
1.515
1.225
1.233
1.236
1.241
1.241
1.229
1.224
1.211
1.234
1.232
1.284
1.262
1.257
1.226
1.273
1.259
1.262
1.265
1.268
1.276
1.268
1.276
1.276
1.231
1.239
1.235
1.205
1.236
1.233
1.237
1.260
1.265
1.270
1.270
1.282
1.232
1.239
1.234
1.264
1.281
1.249
1.205
1.284
1.754
1.789
1.709
1.784
1.769
1.747
1.772
1.795
1.795
1.790
1.691
1.746
1.655
1.755
1.627
1.786
1.705
1.707
1.679
1.669
1.595
1.721
1.749
1.781
1.762
1.768
1.793
1.733
1.754
1.717
1.767
1.702
1.649
1.629
1.688
1.744
1.790
1.771
1.662
1.705
1.730
1.595
1.795
2.575
2.570
2.538
2.565
2.601
2.596
2.560
2.615
2.559
2.561
2.556
2.553
2.502
2.542
2.489
2.569
2.548
2.548
2.545
2.541
2.485
2.539
2.548
2.579
2.517
2.586
2.599
2.562
2.560
2.571
2.546
2.519
2.500
2.530
2.542
2.575
2.579
2.565
2.514
2.540
2.552
2.485
2.615
ring and terminal methyl hydrogen lie between 2.780 and
˚
2
.866 A.
Additionally, there are intermolecular as well as intra-
molecular Nꢂ ꢂ ꢂH interactions. Intramolecular Nꢂ ꢂ ꢂH
˚
methyl) distances amount to 2.739 A whereas intermolec-
(
˚
ular distances are equal H(phenyl)ꢂ ꢂ ꢂN 2.460 A,
˚
˚
H(methyl)ꢂ ꢂ ꢂN 2.579 A, H(methyl)ꢂ ꢂ ꢂN 2.638 A.
2
3
One can see that for both L and L crystal structures,
the six-membered rings closed by NAHꢂ ꢂ ꢂO intramolecu-
lar hydrogen bonds are created (Fig. 6). According to the
HENJUF
JARPEX
JARPEX01
KAQSAW
LALQIZ
LEZHON
LIKFUG
NAMMEU
NAMMEU01
NAMMEU02
NAMMEU03
OLOCAT
PAMBOO01
PAMBOO01
SANZEM
SCNPHO
TAPFEV
TEJJUO
UNEWUF
UNEWUF01
UNEWUF01
VEHXOV
WIKWOC
WIKWUI
WIKXAP
YAPDID
YAWVUN
YAWVUN01
YAWWIC
ZUCQOD
ZUCRAQ
Mean value
Minimum
2
Fig. 6. Creation of six-membered ketohydrazone pseudo-ring in L (a)
3
Maximum
and L (b).

6
R. Prajapati et al. / Journal of Molecular Structure 879 (2008) 1–6
1
2
3
that the p-electron delocalization is observed for RAHBs
and that in extreme cases one can observe the equaliza-
tion of bonds within the pseudo-ring with the nearly cen-
tre position of hydrogen atom. The crystal structure of
benzylacetone where S(6) motif with intramolecular
OAHꢂ ꢂ ꢂO hydrogen bond is an example of the latter sit-
uation [23].
For L and L structures analyzed here the ketohydraz-
one pseudo-ring with NAHꢂ ꢂ ꢂO bond is observed. Such
ketohydrazone species were analyzed in detail previously
experimentally since numerous crystal structures were
reported and also theoretically since DFT calculations on
the model species were carried out [24]. We have performed
the search through Cambridge Structural Database to find
ketohydrazone rings with intramolecular NAHꢂ ꢂ ꢂO hydro-
gen bonds. The same reservations and conditions concern-
ing the accuracy of the determination of crystal structures’
measurements were applied here as for the CAHꢂ ꢂ ꢂN
bonds between the pyridyl rings analyzed in the previous
section.
L , L and L , respectively. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK, fax: +44 1223 336 033, or e-mail: deposit@ccdc.ca-
m.ac.uk. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.molstruc.2007.08.011.
2
3
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[
[
[
[
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1
[
[
3
˚
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˚
[8] (a) G.A. Jeffrey, W. Saenger, Hydrogen Bonding in Biological
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(
b) G.A. Jeffrey, An Introduction to Hydrogen Bonding, Oxford
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University Press, New York, 1997.
2
3
of L and L investigated here are well fixed into the ranges
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[
[
[
[
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1
667.
[
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2
[
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these are for example R ð8Þ between pyridyl rings and
2
S(6) of intramolecular hydrogen bond.
[
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Appendix A. Supplementary data
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