Spectral and cyclic voltammetric studies on some intramolecularly hydrogen bonded arylhydrazones: Crystal and molecular structure of 2-(2-(3-nitrophenyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione

Article abstract of DOI:10.1016/j.molstruc.2009.11.001

A series of arylhydrazone derivatives (1-7) were prepared by the coupling of acetylacetone/dimedone with respective aromatic diazonium salts and characterized by IR, ¹H and ¹³C NMR spectra. The IR and NMR spectral data clearly manife

Full text of DOI:10.1016/j.molstruc.2009.11.001

Journal of Molecular Structure 963 (2010) 250–257
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Spectral and cyclic voltammetric studies on some intramolecularly hydrogen
bonded arylhydrazones: Crystal and molecular structure of
2-(2-(3-nitrophenyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione
*
A. Sethukumar, B. Arul Prakasam
Department of Chemistry, Annamalai University, Annamalainagar, Chidambaram 608 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 July 2009
Received in revised form 1 November 2009
Accepted 1 November 2009
Available online 4 November 2009
A series of arylhydrazone derivatives (1–7) were prepared by the coupling of acetylacetone/dimedone
with respective aromatic diazonium salts and characterized by IR, 1H and 13C NMR spectra. The IR and
NMR spectral data clearly manifests the effective intramolecular hydrogen bonding in all the cases. Cyclic
voltammetric studies certainly indicate that in all the cases the reduced center is C@N bond of hydrazonic
moiety. The single crystal X-ray structural analysis of 2-(2-(3-nitrophenyl)hydrazono)-5,5-dimethylcy-
clohexane-1,3-dione (6) is also reported. Single crystal X-ray analysis of 6 evidences the intramolecular
hydrogen bonding with the N(2)ꢀ ꢀ ꢀO(4) distance of 2.642(15) Å, which can be designated as S(6) accord-
ing to Etter’s graph nomenclature. The cyclohexane ring conformation in the molecule (6) can be
described as an envelope. RAHB studies suggest that the resonance assistance for hydrogen bonding is
significantly reduced for the compound (6) due to the non-planarity of the six atoms which are involved
in resonant cycle S(6) of Etter’s graph. The planarity of the resonant cycle S(6) is very much disturbed by
the conformational requirement of the cyclohexane ring and hence RAHB concept is less operative in this
case.
Keywords:
Hydrogen bonding
Cyclic voltammetry
NMR
XRD
RAHB
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
precursors of potential antidiabetic drugs [12–14]. The arylhydraz-
one derivatives of b-diketones are important intermediate in
Hydrogen bonds are the most important intermolecular interac-
tions that determine molecular recognition and aggregation in
biology and chemistry are implicated in the structures of many no-
vel materials. Long range, anisotropic interactions and those of
electrostatic origin like hydrogen bonds [1] received special inter-
est due to its significance from biological [2] perspective and deter-
mination of molecular conformation. In some cases hydrogen
bonding acts as an active site for initiation of chemical reaction.
Studies of how hydrogen bonding can be used to control molecular
association continue to yield exciting discoveries in supramolecu-
lar chemistry [3]. Hydrogen bonded networks can also be purpose-
fully designed to create novel liquid materials, including liquid
crystals, gels and fluids. The types of hydrogen bonding and their
effects in molecular conformation and in packing patterns of or-
ganic compounds have been extensively studied by analysing the
numerous crystal structure data available from Cambridge Struc-
tural Database [4].
organic synthesis because they can be converted into a diverse ar-
ray of molecules such as poly functional heteroaromatics. Recently,
the single crystal X-ray structural studies of two para-substituted
phenylhydrazone derivatives of acetylacetone have been reported
[15].
Several reports are available in the literature [5–11,15] regard-
ing the synthetic and structural aspects of arylhydrazones of mono,
di and triketones. The implications of hydrogen bonding on the
spectral, electrochemical and structural properties of the b-dike-
tone derivative have received much attention in recent years after
the evolution of resonance assisted hydrogen bonding (RAHB) con-
cept. However, such collective studies are very few in the literature
and in view of the importance of arylhydrazone derivatives in
medicinal and structural chemistry it was thought worthwhile to
synthesize some arylhydrazone derivatives of diketones like ace-
tylacetone and dimedone. In the present study a series of arylhyd-
razone derivatives of acetylacetone and dimedone has been
prepared and analyzed by electronic, IR and NMR spectroscopy.
Their IR and NMR (1H and 13C) spectra are discussed in terms of
the extent of intramolecular hydrogen bonding. Cyclic voltammet-
ric studies is also carried out along with the single crystal X-ray
structural analysis of 2-(2-(3-nitrophenyl)hydrazono)-5,5-dimeth-
ylcyclohexane-1,3-dione (6).
Arylhydrazones of mono, di and triketones as well as cyclic
1,3-dione viz., cyclohexane-1,3-dione have been reported in the lit-
erature [5–11]. Such compounds have been extensively used as
* Corresponding author. Tel.: +91 9443583619.
E-mail address: arul7777@yahoo.com (B. Arul Prakasam).
0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2009.11.001

A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
251
Table 1
2. Experimental
Yield and melting points of the compounds.
2.1. General
Entry
X
Y
Z
R
Mp (°C)
Yield (%)
1
2
3
4
5
6
7
CH3
CH3
CH3
CH3
CH2
CH2
CH2
–
–
–
–
CH3
CH3
CH3
CH3
CH2
CH2
CH2
2-OCH3
3-Cl
142
92
140
92
146
126
110
75
75
78
75
73
75
70
All the reagents and solvents employed were commercially
available analytical grade materials used as supplied, without fur-
ther purification. UV–vis spectra were recorded on a SHIMADZU
UV-1650PC digital spectrophotometer by dissolving the sample
in spectral grade methanol. IR spectra were recorded on Avatar
Nicholet FT-IR spectrophotometer (range 4000–400 cmꢁ1) as a
KBr pellet. Proton NMR spectra were recorded at room tempera-
ture (298 K) on Bruker AMX-400 spectrometer operating at
400.23 MHz (SWH, 8223.685 Hz; FIDRES, 0.125483 Hz) using
TMS as internal reference. 13C NMR spectra were recorded in pro-
ton decoupled mode on Bruker AMX-400 spectrometer operating
at 100.63 MHz (SWH, 24038.461 Hz; FIDRES, 0.366798 Hz). Cyclic
voltammetric studies were carried out using a computerized HCH
CHI604C Electrochemical analyzer. A three-electrode configuration
was used, comprising of glassy carbon as the working electrode, a
platinum wire as the counter electrode and Ag|AgCl as the refer-
ence electrode. Experiments were carried out at room temperature
in DMF using 0.01 M tetrabutylammonium perchlorate as the sup-
porting electrolyte. The solutions were deaerated by purging with
a stream of nitrogen gas for 10 min before recording the current–
voltage curve and then a blanket of nitrogen was maintained
throughout the experiment. In order to perform CV studies, the
GC was polished with fine grade emery powder followed by polish-
3-NO2
4-OCH3
2-OCH3
3-NO2
3-Cl
C(CH3)2
C(CH3)2
C(CH3)2
All the compounds are recrystallized by using ethanol.
2.3. X-ray crystallography
Details of the crystal data, data collection and refinement
parameters for 6 are summarized in Table 2. Crystal was grown
by slow evaporation technique using ethanol as solvent. Determi-
nation of the unit cell and data collection were performed on a
Bruker, 2004 APEX 2 diffractometer using graphite-monochromat-
ed Mo K radiation (k = 0.71073 Å) at 293 K with crystal size of
a
0.25 ꢃ 0.20 ꢃ 0.20 mm. Multi-scan absorption corrections were
applied using the SADABS program. The structure was solved by di-
rect methods and successive Fourier difference syntheses (SHELXS-
97 [16]) and refined by full matrix least square procedure on F2
with anisotropic thermal parameters. All non-hydrogen atoms
were refined (SHELXL-97 [17]) and placed at chemically acceptable
position. Selected bond lengths and bond angles are given in Table
3. Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre as Supplementary Publications No.
CCDC-720911 for 6. Copies of the 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,
ing alumina (0.5 lm).
2.2. Preparation of compounds 1–7
Compounds were prepared by the general procedure (Scheme
1) in which anilines (0.5 mmol) were dissolved separately in 1 N
HCl (25 cm3) at 0–5 °C temperature and in each case cooled aque-
ous solution (10 cm3) of NaNO2 (0.40 g) was added drop wise with
stirring followed by the addition of acetylacetone (0.50 g,
0.5 mmol)/dimedone (0.70 g, 0.5 mmol) and sodium acetate
(5.0 g) dissolved in water (30 cm3). Corresponding mixtures were
further stirred for 4 h at room temperature (ꢂ25 °C). Solids thus
obtained were filtered and washed several times with water fol-
lowed by ethanol and then dried in vacuo. The crude products
were recrystallized in ethanol. Compounds 5 and 7 were purified
by column chromatography by using benzene as eluent. Yield and
melting points of the derived compounds are summarized in
Table 1.
Table 2
Crystal data, data collection and refinement parameters for 6.
Compound
2-(2-(3-Nitrophenyl)hydrazono)-5,5-
dimethylcyclohexane-1,3-dione
C14H15N3O4
289.29
0.25 ꢃ 0.20 ꢃ 0.20
Empirical formula
Formula weight
Crystal dimensions
(mm)
Temperature
Wavelength
Crystal system, space
group
293(2) K
0.71073 Å
ꢀ
Triclinic, P1
Unit cell dimensions
a
b
5.8348(10) Å
10.2105(3) Å
12.1654(3) Å
c
a
98.353(10)°
b
95.166(10)°
c
105.45(10)°
Volume
684.84(3) Å3
Z
2
Calculated density
Absorption coefficient
F(0 0 0)
1.403 mg/m3
0.105 mmꢁ1
304
h range
1.71–30.87°
Index ranges
Reflections collected
Observed reflections
Completeness to
theta = 30.87
Absorption correction
Max. and min.
transmission
Refinement method
Data/restraints/
parameters
ꢁ8 6 h 6 8, ꢁ14 6 k 6 14, ꢁ17 6 l 6 17
17,807
4320
99.8%
Semi-empirical from equivalents
0.9793 and 0.9743
Full-matrix least-squares on F2
4320/0/196
Final R, RW (obs., data) 0.0446, 0.1336
GOOF 1.044
Scheme 1.

Table 4
UV, IR and NMR spectral data of the compounds.
a
Compound UV (kmax
)
IR (cmꢁ1
)
1H NMR (ppm)b
13C NMR (ppm)
mCAH
mNAH mC@O mC@N
2.50 (s, CH3), 2:60ðs;CH03Þ, 3.96 (s, OCH3), 6.94–7.74
26.6 (CH3), 31:5ðCH03Þ, 55.9 (OCH3), 133.7 (C-3), 111.1–148.5 (aromatic carbons), 197.2 (C-2
1
2
3
4
5
6
7
396, 257, 3006, 2932,
214 2825
3366 1668 1627
(aromatic protons), 14.77 (s, NH)
and C-4)
2.49 (s, CH3), 2:60ðs;CH03Þ, 7.14–7.45 (aromatic protons),
26.5 (CH3), 31:6ðCH30 Þ, 133.7 (C-3), 114.4–142.7 (aromatic carbons), 196.8 (C-4), 198.2 (C-2)
359, 248, 3079, 3003,
215 2925, 2854
3300 1672 1630
3326 1673 1633
3304 1672 1616
14.59 (s, NH)
2.53 (s, CH3), 2:63ðs;CH03Þ, 7.56–8.26(aromatic protons),
26.6 (CH3), 31:7ðCH30 Þ, 134.3 (C-3), 110.7–149.3 (aromatic carbons), 196.7 (C-4), 198.5 (C-2)
375, 237, 3094, 3008,
215 2923, 2853
14.65 (s, NH)
2.48 (s, CH3), 2:59ðs;CH03Þ, 3.83 (s, OCH3), 6.92–7.38
26.5 (CH3), 31:4ðCH03Þ, 55.5 (OCH3), 132.7 (C-3), 114.9–158.0 (aromatic carbons), 196.8 (C-
393, 251, 3062, 2992,
215 2923, 2830
(aromatic protons), 14.95 (s, NH)
4), 197.5 (C-2)
400, 256, 3021, 2943,
210 2926, 2855
–
–
–
1673 1619 1.13 (s, CH3), 2.60 (s, CH2), 3.97 (s, OCH3), 6.93–7.98
(aromatic protons), 15.48 (s, NH)
28.4 (CH3), 30.7 (C-10), 52.3 (C-9), 52.5 (C-11), 55.9 (OCH3), 130.6 (C-7), 110.9–149.0
(aromatic carbons), 193.4 (C-8), 196.2 (C-12)
376, 247, 2975, 2932,
212 2888
1681 1633 1.15 (s, CH3), 2.64 and 2.65 (CH2), 7.57–8.30 (aromatic
protons), 15.24 (s, NH)
28.5 (CH3), 30.6 (C-10), 52.6 (C-9 and C-11), 131.2 (C-7), 112.1–149. 1 (aromatic carbons),
193.1 (C-8), 197.9 (C-12)
380, 251, 3076, 2955,
210 2927
1675 1624 1.13 (s, CH3), 2.61 (s, CH2), 7.19–7.61 (aromatic protons),
15.24 (s, NH)
28.4 (CH3), 30.6 (C-10), 52.6 (C-9 and C-11), 130.6 (C-7), 115.5–142. 2 (aromatic carbons),
193.1 (C-8), 197.4 (C-12)
a
For 5–7, the NH stretching bands were masked by the signals due to residual water.
In all the cases the residual solvent signal at 1.5 ppm corresponding to water dissolved in CDCl3 [21] is ignored.
b

A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
253
Fig. 2. Intramolecular hydrogen bonding in dimedone derivatives. (with spectro-
scopic numbering).
Fig. 3. UV spectrum of 2 in methanol.
The lack of absorption bands between 330 and 355 nm [18] and
above 400 nm [19] could be attributed to an azo function, together
with observation of bands at around 370–400 nm, indicated that
compounds are in hydrazone form [14].
3.3.1. 1H NMR spectra
PMR spectra of compounds 1–4, show less intense signal cor-
responding to >NH proton in the downfield region of 14.59–
14.95 ppm. The observed deshielding can be accounted for by
considering the intramolecular hydrogen bonding of >NH proton
with the electronegative oxygen. The intramolecular hydrogen
bonding results in creation of six-membered pseudo-ring as
shown in Fig. 1. The observed two different singlets for two
methyl groups with an significant difference of about 1 ppm
indicates the restricted rotation about C@N bond and the elec-
tronic charge asymmetry due to intramolecular hydrogen bond-
ing. Aromatic protons resonate in the region of 6.92–8.26 ppm
with the expected splitting patterns depending on the position
of the substituents.
For the dimedone derivatives 5–7, a low field singlet, dNAH at
around 15 ppm confirm strong intramolecular NAHꢀ ꢀ ꢀO@C
hydrogen bonding (Fig. 2) and the maximum dehielding of
NAH proton is observed for 5, due to the presence of AOCH3
at ortho position. Methyl substituents associated with the dime-
done ring appear as a singlet at around 1.13 ppm as an average
from rapidly inter-converting conformers. The methylene signals
(2.60–2.65 ppm) also reflect rapid mobility of the dimedone ring
at room temperature. For 6, a set of signals were observed for
the methylene protons of dimedone ring and it indicates that
the chemical shift difference is large enough to appear as a sep-
arate signal. However, this difference is not observed for the
other two compounds. In all the cases aromatic proton signals
appear in the downfield region of 6.93–8.30 ppm with the ex-
pected splitting patterns. If a comparison is made between the
arylhydrazone derivatives of acetylacetone and dimedone, the
NH protons of dimedone derivatives are deshielded to the extent
of about 0.5–0.7 ppm. It exemplifies the effective intramolecular
hydrogen bonding in dimedone derivatives (in solution) which is
well supported by 13C NMR spectral studies presented
subsequently.
3.2. IR spectra
IR spectrum of 6 is shown in Fig. 4. In the IR spectra of the com-
pounds, maxima in the region of 1668–1681 cmꢁ1 are observed,
which are characteristic carbonyl (C@O) stretching frequencies of
a,b-unsaturated ketone. Intramolecular hydrogen bonding is one
of the factors which lower the carbonyl stretching frequency by
about 50 cmꢁ1 [20]. This lowering is, however, dependent on the
strength of the hydrogen bond and hence for the present set of
compounds the shift of mC@O to lower wave number can be attrib-
uted to the intramolecular hydrogen bonding (NAHꢀ ꢀ ꢀO@C). For all
the compounds a sharp and less intense band corresponding to
NAH stretching frequency are observed at 3304–3366 cmꢁ1 region
and the observed shift in values towards the longer wave number
region can also be accounted again by the aforementioned possible
intramolecular hydrogen bonding.
In addition to the
the cases the weak aromatic CAH bands were observed in the re-
m
C@N bands in the region of 1630 cmꢁ1, for all
gion of 3000–3030 cmꢁ1. Similarly aliphatic
mCAH bands appear be-
low 3000 cmꢁ1 with moderate intensity. The methylene groups in
cyclohexyl ring causes the significant increase in intensity of ali-
phatic mCAH bands for dimedone derivatives (5–7) compared to
the arylhydrazones of acetylacetone.
3.3. NMR spectral studies
NMR spectral data of all the synthesized compounds are given
in Table 4 along with the splitting patterns. 1H NMR spectrum of
5 and 13C NMR spectrum of 7 are shown in Figs. 5 and 6,
respectively.

254
A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
Fig. 4. IR spectrum of 6.
Fig. 5. 1H NMR spectrum of 5 in CDCl3.
3.3.2. 13C NMR spectra
for C-9 and C-11 carbons are observed only for 5 and not for other
compounds. This is perhaps, of some subtle geometry or electronic
charge asymmetry with in dimedone ring brought about by hydro-
gen bonding. Coalescence into one signal for C-9 and C-11 for com-
pounds 6 and 7 merely suggests that the chemical shift differences
are too small to be detected under the experimental conditions.
Carbonyl sp2 carbon atoms appear as separate signals in the low
field region of about 197 and 193 ppm for C-12 and C-8, respectively.
Strong intramolecular NAHꢀ ꢀ ꢀO@C hydrogen bonding deshields
C-12 with respect to C-8 (Fig. 2) to the extent of ca. 4 ppm. Polariza-
For compounds 1–4, the two methyl carbons are observed in
two different regions [26.5 and 31.5 ppm] with the chemical shift
difference of about 5 ppm as a consequence of restricted rotation
due to C@N bond as well as the intramolecular H-bonding. The
drift of electron density from carbonyl carbon as a result of hydro0-
gen bonding leads to the decrease in electron density on A13CH3
(Fig. 1) and by considering this, the slightly deshielded signal in
the range of 31.5 ppm is assigned to A13CH30 . The aromatic carbon
signals appear in the range of 110.7–158.0 ppm and the observed
variations are in line with the substituents at ortho, meta and para
positions. C-3 carbon resonates in the region of 133 ppm with con-
siderably low intensity associated with the sp2 carbons. As ex-
pected, the 13C signals for carbonyl carbons are observed in
different regions with the small variation in chemical shift values
as a consequence of H-bonding.
tion changes of the carbonyl bond through
p-conjugation with
restricted rotation about NAN bond may well be accompanying fac-
tors for the observed deshielding. In general 13C@N and ipso carbons
are identified by their relative low intensities due to longer relaxa-
tion times and lower nuclear Overhauser effects.
For compounds 5–7, retention of a single signal for methyl
groups at around 28 ppm denies any conformational preference.
C-10, the carbon furthest from carbonyl groups, gives a single line
at around 30.6 ppm and the upfield shift of around 20–22 ppm
with respect to C-9 and C-11 is observed. Shielding differences
3.4. Cyclic voltammetric studies
Typical cyclic votammogram of 1 in DMF at 100 mV sꢁ1 scan
rate is depicted in Fig. 7 and the reduction potentials for the de-
rived compounds (1–7) are listed in Table 5. The redox behaviour

A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
255
Fig. 6. 13C NMR spectrum of 7 CDCl3.
of similar compounds has been reported already [22,23]. From CV
Table 5
Reduction potentials for the compounds 1–7 in DMF.
studies it was observed that all the compounds undergo two elec-
tron reduction processes as reported already [22,23] and the elec-
trode reaction involves the transfer of two electrons per molecule
in two successive steps. Keeping in view the feasibility of the dif-
ferent sites of reduction i.e., C@N or C@O it was concluded that
the possible reduction site is C@N, since C@O require much higher
potential for reduction. Generally, the dimedone derivatives (5–7)
have less negative reduction potentials than the acetylacetone
derivatives (1–4) which shows a reluctance to add more electron
density to the already electron rich C@N bond in acetylacetone
derivatives. Less negative reduction potentials observed for com-
pounds 5–7, indicate the ease of electron addition in the dimedone
derivatives (5–7) and this is because of the weak C@N bond. The
ꢁEcp ðVÞ
ꢁEap ðVÞ
Compound
1
2
3
4
5
6
7
0.900
1.438
0.750
1.355
0.835
1.290
0.453
1.157
0.833
1.407
0.479
1.273
0.834
1.442
0.535
1.280
0.839
1.247
0.365
1.084
1.005
1.368
0.400
1.095
0.809
1.068
0.342
0.950
observed reduction peak potentials in CV clearly indicate that in
all the cases the reduced center is the C@N bond of hydrazonic
moiety.
3.5. Single crystal X-ray structural analysis of 6
ORTEP of 6 is given in Fig. 8 and packing diagram is shown in
Fig. 9. The unit cell contains two discrete molecules. From the bond
lengths it is observed that there is a double bond between C(7) and
N(3) with the distance of 1.3096(13) Å. The N(3)AN(2) bond
[1.3066(13) Å] is single bond in nature and the conjugation breaks
at this point. These values clearly suggest that the molecule is in
‘‘enazo” form. There is immediate confirmation of hydrogen bond-
ing between N(2)ꢀ ꢀ ꢀO(4) [N(2)AHꢀ ꢀ ꢀO(4)] with the distance of
2.642(15) Å which is shorter than the sum of their covalent radii
(2.9 Å). The O(4)ꢀ ꢀ ꢀH distance is 1.993(2) Å and the
N(2)AH(2A)ꢀ ꢀ ꢀO(4) angle is 128(15)°, which manifests the effec-
tiveness of hydrogen bonding. The character of the C@Oꢀ ꢀ ꢀHAN
intramolecular hydrogen bonding will reflect in the bond parame-
Fig. 7. Cyclic votammogram of 1 in DMF at 100 mV sꢁ1 scan rate.

256
A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
the dimedone molecule can be best described as an envelope with
the carbon which bears the methyl groups is 0.62 Å out of the
plane.
In 6, when compared to C(7)AC(8) length [1.4894(15) Å]
C(12)AC(7) length [1.4665(15) Å] is slightly decreased. Other
structural parameters of dimedone ring are observed as expected.
The cyclohexane ring conformation in the present molecule (6)
can be described as an envelope with the slightly more pronounced
puckering of the cyclohexane ring at C(10). The geometric calcula-
tions are done by PARST95 [26]. The envelope conformation has
been retained with the displacement of C(10) and C(7) to 0.69
and ꢁ0.048 Å from the plane defined by C(8), C(9), C(11) and
C(12). Phenyl ring shows normal bond parameters. Phenyl rings
of the adjacent molecules form a pair and are stacked with their
tails in opposite directions.
3.6. Resonance assisted hydrogen bonding (RAHB) studies
Fig. 8. ORTEP of compound 6.
Strong hydrogen bonding is possible if a suitably polarizable
system of
p-bonds allows charge flow through covalent bonds
from the donor to the acceptor. Such systems are provided, for
example, by conjugated double bonds. This cooperativity mecha-
nism is well established for several cases with NAH donors [27].
The concept of RAHB systems is based on the statement that owing
ters of the pseudo six-membered ring. A relative increase in
N(3)AN(2) bond length gives a corresponding decrease in the
C(7)AN(3) bond length. The observed variation in bond angles be-
tween C(4)AC(5)AN(2) [120.45(10)°] and C(6)AC(5)AN(2)
[118.79(10)°] is very small. However, C(12)AC(7)AN(3) angle
[124.97(10)°] is greater than that of N(3)AC(7)AC(8) angle
[113.90(9)°] to the extent of ca. 11°. It is consistent with the real-
ization of an acceptable geometry for maximal hydrogen bond for-
mation between O(4) and N(2).
the existence of conjugated system where the
p electron delocali-
sation exists which causes the enhancement of the strength of
hydrogen bonds. Despite the drawbacks of this concept [28], the
p
electron delocalisation is observed for RAHBs and that in extreme
cases one can observe the equalization of bonds within the pseudo-
ring with the nearly center position of hydrogen atom.
From the crystal structure it is observed that there is a six-
membered pseudo-ring closed by NAHꢀ ꢀ ꢀO intramolecular hydro-
gen bonding is created. Generally, six-membered ring intramolec-
ular hydrogen bonds form in preference to intermolecular
hydrogen bonds. This type of hydrogen bonding may be designated
as S(6) according to Etter’s graph nomenclature [24] where, S de-
notes intramolecular hydrogen bond and the size or degree of
the motif is 6. Before considering the conformation of the dime-
done ring fragment in compound 6, it is worthwhile to discuss
the structural features of dimedone molecule. Single crystal
X-ray structural analysis of dimedone [5,5-dimethylcyclohexane-
1,3-dione] has been reported already [25]. It was found that the
structure of dimedone is enolic nature in the solid state. Also, it
is very clear that the effective intermolecular hydrogen bonding
(between the hydrogen atom of the enolic AOH and oxygen atom
of the carbonyl group) to form infinite chains. The conformation of
Because of the hydrogen bonding, the CAC [C(12)AC(7)] and the
NAN [N(2)AN(3)] bonds gain some double bond character and are
shortened, whereas the C@O [C(12)@O(4)] and C@N bonds loose
part of their double bond character and are lengthened. To confirm
this suggestion, and to quantify the involvement of resonant cycle
(Fig. 10) in hydrogen bonding, it is necessary to perform geometri-
cal tests. There are several ways to quantify
p-system delocalisa-
tion; to make comparison easy, the formalism used by Gilli et al.
[27] is used here. It is obvious that in RAHB of increasing strengths,
the bond length d1 reduces and d4 increases; this means that the
difference q1 = d1 ꢁ d4 is positive and becomes zero or negative
with increasing hydrogen bond strength. The difference in the
C(12)AC(7) [d3] and the C@N [d2] bond lengths, q2 = d3 ꢁ d2, is also
positive and decreases with increasing hydrogen bond strength. In
the case of total p-delocalisation at AN@CACA, q2 becomes zero. If
the arrangement as a whole is a resonant cycle, there must be a
correlation between the parameters q1 and q2. Furthermore, the
p-delocalisation must increase with reducing hydrogen bond dis-
Fig. 9. Packing diagram of compound 6.
Fig. 10. RAHB in S(6) of Etter’s graph.

A. Sethukumar, B. Arul Prakasam / Journal of Molecular Structure 963 (2010) 250–257
257
tance, so that q1, q2 and their sum Q = q1 + q2 must correlate with
Oꢀ ꢀ ꢀN (or Hꢀ ꢀ ꢀO) bond distance.
clear that the planarity of the six atoms which are involved in res-
onant cycle S(6) of Etter’s graph is very much disturbed by the con-
formational requirement of the cyclohexane ring and hence RAHB
concept is less operative in this case. The derived compounds could
be used as building blocks for the diverse array of organic com-
pounds with potential biological properties.
To check out this concept, we have collected the data of two
similar compounds for which the structures have been published
recently (compounds (3-(2-(4-methylphenyl)hydrazono)pentane-
2,4-dione and 3-(2-(4-chlorophenyl)hydrazono)pentane-2,4-dione
[15]) from CCDC and the correlations were made. The results of
correlation are not satisfactory and hence the contribution of reso-
nance for the strength of the hydrogen bonding in 6 is not in line
with the results obtained for similar compounds [15]. It is also very
clear that the Oꢀ ꢀ ꢀN bond length is [2.642(15) Å] slightly higher
than that of the mean distance of 2.552 Å (derived from about 40
structurally similar compounds [15]). This is a conclusive evidence
that the mechanism of resonance assisted hydrogen bonding is sig-
nificantly reduced for the present compound (6). To have a better
RAHB, all the six atoms (in pseudo-ring) of S(6) Etter’s group
should be in a same plane. Whereas, for compound 6, the planarity
of the six atoms which are involved in resonant cycle (S(6) of
Etter’s graph – Fig. 10) is very much disturbed by the conforma-
tional requirement of the cyclohexane ring (in dimedone fragment)
and hence RAHB concept is less operative in this case.
Acknowledgement
The authors are extremely thankful to Prof. Dr. K. Ramalingam
of Annamalai University for his valuable suggestions regarding
the X-ray structural analysis.
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Compounds 1–7, were prepared by the coupling of acetylace-
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