Synthesis, spectral and RAHB studies on some arylhydrazones of β-diketones: Crystal and molecular structures of 2-(2-(3-pyridyl)hydrazono) -5,5-dimethylcyclohexane-1,3-dione and 2-(2-(2-methoxyphenyl)hydrazono)-5,5- dimethylcyclohexane-1,3-dione

Article abstract of DOI:10.1007/s11224-010-9677-8

The present study describes the synthesis and spectral analysis of a series of arylhydrazones of β-diketones (1-6) along with the single crystal X-ray structural studies of 2-(2-(3-pyridyl)hydrazono)-5,5-dimethylcyclohexane- 1,3-dione (3) and 2-(2-(2-meth

Full text of DOI:10.1007/s11224-010-9677-8

Struct Chem (2011) 22:23–33
DOI 10.1007/s11224-010-9677-8
ORIGINAL RESEARCH
Synthesis, spectral and RAHB studies on some arylhydrazones
of b-diketones: crystal and molecular structures of 2-(2-(3-
pyridyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione and 2-(2-
(2-methoxyphenyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione
•
Annamalai Sethukumar Balasubramaniyam Arul
•
Prakasam Sabeta Kohli Rajnikant Verma
•
Received: 2 September 2010 / Accepted: 21 October 2010 / Published online: 17 November 2010
Ó Springer Science+Business Media, LLC 2010
Abstract The present study describes the synthesis and
spectral analysis of a series of arylhydrazones of b-dike-
tones (1–6) along with the single crystal X-ray structural
studies of 2-(2-(3-pyridyl)hydrazono)-5,5-dimethylcyclo-
hexane-1,3-dione (3) and 2-(2-(2-methoxyphenyl)hydraz-
ono)-5,5-dimethylcyclohexane-1,3-dione (6). Spectral and
structural studies clearly exemplify the effective intramo-
lecular hydrogen bonding and as a result the NH proton
signals and carbonyl carbon signals are shifted to higher
frequency region in NMR spectra. Structural implications
imposed by the formation of pseudo six-membered ring
after hydrogen bonding is well observed in XRD analysis.
XRD analysis also evidences the envelop conformation of
the dimedone ring in both the cases and resonance assisted
hydrogen bonding (RAHB) studies were also performed.
and they have been widely utilized in the dye industry over
the years. Recently, it was foundthat they can be employed as
precursors of potential antidiabetic drugs [8–10] and inter-
estingly, 2-(2-(2-methoxyphenyl)hydrazono)-5,5-dimethyl-
cyclohexane-1,3-dione (6) has been screened for its ability to
alter the lifespan of eukaryotic organisms [11].
In recent years, much attention has given to structural
studies of these compounds, because they are characterized
by the formation of various intramolecular and intermo-
lecular hydrogen bonds. Long range, anisotropic interac-
tions and those of electrostatic origin like hydrogen bonds
[12] received special interest due to its significance from
biological [13] perspective and determination of molecular
conformation. Studies of how hydrogen bonding can be
used to control molecular association continue to yield
exciting discoveries in supramolecular chemistry [14].
Hydrogen bonded networks can also be purposefully
designed to create novel liquid materials, including liquid
crystals, gels and fluids.
Keywords Arylhydrazones ꢀ Intramolecular hydrogen
bonding ꢀ NMR ꢀ XRD ꢀ RAHB
Azoaromatic compounds are of interest in recent times
due to their emerging applications. It is known that nitro-
gen substitution to an aromatic hydrocarbon molecule is an
effective way to modify its crystal structure [15]. For
instance, aza substituted derivatives of benzene show dif-
ferent crystal structures depending on the position of
nitrogen substitution; pyrimidine and pyrazine crystallize
to noncentrosymmetric and centrosymmetric crystal struc-
tures, respectively [16, 17]. This is because the steric effect
and the charge distribution at the substituted nitrogen atom
are different from those in the unsubstituted molecule. The
evolution of the resonance assisted hydrogen bonding
(RAHB) concept [18] results in the detailed spectral and
structural investigations of numerous b-diketone deriva-
tives in recent years. By considering the significance of
arylhydrazone derivatives in medicinal and structural
Introduction
Arylhydrazones of b-diketones are important intermediates
in organic synthesis as they can be converted into a diverse
array of molecules such as poly functional heteroaromatics.
Several reports are available in the literature [1–7] about
the numerous arylhydrazones of mono, di and triketones
A. Sethukumar ꢀ B. Arul Prakasam (&)
Department of Chemistry, Annamalai University,
Annamalainagar 608002, Tamil Nadu, India
e-mail: arul7777@yahoo.com
S. Kohli ꢀ R. Verma
Chemical Crystallography Laboratory, P.G. Department of
Physics, University of Jammu, Jammu Tawi 180006, India
123

24
Struct Chem (2011) 22:23–33
chemistry we prompted to synthesize some arylhydrazone
derivatives of diketones like acetylacetone and dimedone.
In this paper, we report a series of arylhydrazone deriva-
tives of acetylacetone and dimedone which have been
prepared and analyzed by electronic, IR and NMR spec-
troscopy. Their IR and NMR (1H and 13C) spectra are
discussed in terms of the extent of intramolecular hydrogen
bonding. Along with the spectral studies we also report
the single crystal X-ray structural analysis of 2-(2-(3-
pyridyl)hydrazono)-5,5-dimethylcyclohexane-1,3-dione (3)
and 2-(2-(2-methoxyphenyl)hydrazono)-5,5-dimethylcy-
clohexane-1,3-dione (6).
Experimental
Unless otherwise stated, the reagents and solvents
employed were commercially available analytical grade
materials used as supplied, without further purification.
Scheme 1 Preparation of compounds 1–4
UV–vis spectra were recorded on
a SHIMADZU
UV-1650PC digital spectrophotometer by dissolving the
sample in spectral grade methanol. IR spectra were recor-
ded on Avatar Nicholet FT-IR spectrophotometer (range
4000–400 cm-1) as a KBr pellet. Proton NMR spectra
were recorded at room temperature (298 K) on Bruker
AMX-400 spectrometer operating at 400.23 MHz. 13C
NMR spectra were recorded in proton decoupled mode on
Bruker AMX-400 spectrometer operating at 100.63 MHz.
X-ray crystallography
Details of the crystal data, data collection and refinement
parameters for 3 and 6 are summarized in Table 2. Selected
bond lengths and bond angles are given in Table 3. In both
the cases, crystals were grown by slow evaporation tech-
nique using ethanol as solvent. The data were collected on
X’calibur single crystal X-ray diffractometer having CCD
˚
camera. MoKa (k = 0.71073 A) radiation was used. The
Preparation of compounds 1–5
cell measurement and refinement was carried at 293(2) K.
The CCD was operated at -40 °C. The CrysAlisPro pro-
gram was used for data collection, reduction and space
group determination. The structure was solved by direct
methods and successive Fourier difference syntheses
(SHELXS-97 [20]) and refined by full matrix least square
procedure on F2 with anisotropic thermal parameters. All
non-hydrogen atoms were refined (SHELXL-97 [21]) and
placed at chemically acceptable position.
Compounds 1–4 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 aqueous 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 (25 cm3). Corresponding mixtures were further stir-
red for 5 h at room temperature (*25 °C). Solids thus
obtained were filtered and washed several times with water
followed by ethanol and then dried in vacuo.
Results and discussion
Compound 5 was prepared by step wise diazotization
followed by coupling as shown in Scheme 2. The prepa-
ration and the spectral analysis of 6 was reported already
[19] and in view of its important biological application [11]
the single crystal X-ray structural analysis alone is reported
in this paper. Compounds 3 and 5 were purified by column
chromatography using benzene as eluent. The crude
products were recrystallized in ethanol. Yield and melting
points of the derived compounds are summarized in
Table 1.
Electronic spectra
The kmax values of 1–5 are listed in Table 4, together with
the IR and NMR spectral data. Generally, carbonyl groups
show characteristic absorption due to nonbonding electrons
on the oxygen atom and the band usually appears in the
near UV region. Compounds in which carbonyl bond is
conjugated with C–N double bond have their absorption at
longer wavelengths. UV spectra of the compounds 1–5,
showed three absorption maxima at 350–454, 242–245
123

Struct Chem (2011) 22:23–33
25
Scheme 2 Preparation of
compound 5
The observed shift in N–H stretching frequency values
towards the longer wave number region can also be accounted
again by the aforementioned intramolecular hydrogen bond-
ing. The mC=N stretching occurs around 1620 cm-1 for all the
compounds. Compound 4 has the diazo (N=N) stretching
frequency at 1464 cm-1 which is characteristic of p-substi-
tuted azobenzene. The signal is weak because of the non polar
nature of the bond. The band corresponding to N–H stretching
frequency is less intense and it is observed around
*3320 cm-1 for all the compounds. The C–H stretching
vibration bands are observed in the region of
2849–2879 cm-1. For compounds 3, 4 and 5 the asymmetric
stretching (mas CH2) and symmetric stretching (ms CH2) for the
Table 1 Yield and melting points of the compounds
Entry
X
Y
-
Z
Ar
m.p. (° C) Yield (%)
1
CH3
CH3
82-84
70
75
70
65
N
N
2
3
4
CH3
-
CH3
105
H3C
CH3
CH2 C(CH3)2 CH2
CH2 C(CH3)2 CH2
182-184
170-174
N
N
5
Refer Scheme 2
202
50
All the compounds are recrystallized from ethanol
methylene groups occur in the region of 2869–3000 cm-1
.
The C–C bending vibrations occur at very low frequencies
(below 500 cm-1) and therefore they do not appear in the
spectra. The bands assigned to C–C stretching vibrations are
and 205–226 nm values. The observed bands at around
350–454 nm indicate that compounds are in hydrazone
form [10]. For compounds 4 and 5, the kmax values are
shifted to longer wavelength region (420 nm) and this can
be ascribed to the extended conjugation.
weak and appear in the broad region of 1200–800 cm-1
.
NMR spectra
NMR spectral data of all the synthesized compounds are
given in Table 4 along with the splitting patterns. 13C
NMR spectrum of 5 is shown in Fig. 1.
IR spectra
Important absorptions in the arylhydrazones of diketones are
due to mC=O and mC=N stretching modes. The observed maxima
in the region of 1670–1681 cm-1 are characteristic for car-
bonyl (C=O) stretching frequencies of a,b-unsaturated
ketone. In general, intra-molecular hydrogen bonding lowers
1H NMR spectra
PMR spectrum of 1 and 2 showed the less intense signal
corresponding to [NH proton in the higher frequency
the carbonyl stretching frequency by about 50 cm-1
.
123

26
Struct Chem (2011) 22:23–33
Table 2 Crystal data, data collection and refinement parameters for 3 and 6
Compound
2-(2-(3-Pyridyl)hydrazono)-5,5-dimethylcyclohexane-
1,3-dione (3)
2-(2-(2-Methoxyphenyl)hydrazono)-5,5-
dimethylcyclohexane-1,3-dione (6)
Empirical formula
Formula weight
C13H15N3O2
245.3
C15H18N2O3
274.31
Temperature
293(2) K
293(2) K
˚
0.71073 A
˚
0.71073 A
Wavelength
Crystal system, space group
Unit cell dimensions
Triclinic, P1
Monoclinic, P2(1)/n
˚
˚
a = 5.9099(3) A a = 96.711(5)°
a = 11.4428(12) A a = 90°
˚
˚
b = 10.3545(7) A b = 103.046(5)°
b = 11.5155(7) A b = 116.314(14)°
˚
˚
c = 10.5587(7) A c = 98.509(4)°
˚
614.91(7) A
c = 12.3135(16) A c = 90°
˚
1454.4(3) A
3
3
Volume
Z
2
4
Calculated density
Absorption coefficient
F(000)
1.325 mg/m3
0.092 mm-1
1.253 mg/m3
0.088 mm-1
260
584
h range
3.06–32.21°
-8 B h B 8, -14 B k B 15, -15 B l B 12
3.26–32.33°
-14 B h B 16, -13 B k B 16, -18 B l B 13
Index ranges
Reflections collected
Observed reflections
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Final R, RW (obs, data)
GOOF
6638
8849
4844
4658
90.2%
89.5%
Semi-empirical from equivalents
0.9818 and 0.9801
Full-matrix least-squares on F2
4844/3/329
Semi-empirical from equivalents
0.9826 and 0.9809
Full-matrix least-squares on F2
4658/0/189
0.0451, 0.1246
0.0491, 0.1080
1.021
0.794
Table 3 Selected bond distances and angles for 3 and 6
Bond length
Bond angle
3
C(1)–N(1)
C(6)–N(2)
C(6)–C(7)
C(6)–C(11)
C(7)–O(1)
C(7)–C(8)
C(8)–C(9)
C(9)–C(10)
C(9)–C(13)
C(9)–C(12)
C(11)–O(2)
N(1)–N(2)
H(1A)ꢀꢀꢀO(2)
N(1)ꢀꢀꢀO(2)
C(14)–N(4)
C(19)–N(5)
C(19)–C(24)
1.419(5)
1.311(6)
1.442(6)
1.510(5)
1.251(5)
1.473(6)
1.565(6)
1.521(7)
1.530(7)
1.537(6)
1.211(5)
1.285(5)
1.91
C(5)–C(1)–N(1)
C(2)–C(1)–N(1)
N(2)–C(6)–C(7)
N(2)–C(6)–C(11)
C(7)–C(6)–C(11)
O(1)–C(7)–C(6)
O(1)–C(7)–C(8)
C(10)–C(9)–C(8)
C(13)–C(9)–C(8)
C(12)–C(9)–C(8)
C(6)–C(11)–C(10)
N(2)–N(1)–C(1)
N(1)–N(2)–C(6)
N(1)–H(1A)ꢀꢀꢀO(2)
C(15)–C(14)–N(4)
C(15)–C(14)–C(18)
N(4)–C(14)–C(18)
116.5(4)
121.9(4)
118.2(4)
121.2(4)
120.5(4)
120.6(4)
119.8(4)
105.1(3)
111.4(4)
108.5(4)
115.5(4)
120.8(4)
122.6(4)
133.7
2.582 (5)
1.399(6)
1.341(6)
1.416(6)
125.1(4)
117.3(4)
117.6(4)
123

Struct Chem (2011) 22:23–33
27
Table 3 continued
Bond length
Bond angle
C(19)–C(20)
C(20)–O(3)
C(20)–C(21)
C(21)–C(22)
C(22)–C(26)
C(22)–C(23)
C(23)–C(24)
C(24)–O(4)
N(1)–N(2)
N(4)–N(5)
H(4A)ꢀꢀꢀO(4)
N(4)ꢀꢀꢀO(4)
6
1.519(6)
1.187(5)
1.541(6)
1.483(7)
1.520(7)
1.538(6)
1.479(6)
1.252(5)
1.285(5)
1.319(5)
1.93
N(5)–C(19)–C(24)
N(5)–C(19)–C(20)
C(24)–C(19)–C(20)
O(3)–C(20)–C(19)
O(3)–C(20)–C(21)
C(19)–C(20)–C(21)
C(22)–C(21)–C(20)
O(4)–C(24)–C(19)
O(4)–C(24)–C(23)
C(19)–C(24)–C(23)
N(5)–N(4)–C(14)
N(4)–H(4A)ꢀꢀꢀO(4)
127.0(4)
111.2(4)
121.6(4)
123.4(4)
121.9(4)
114.6(4)
116.4(4)
120.6(4)
119.1(4)
120.2(4)
119.0(4)
132.9
2.594(5)
C(1)–N(1)
C(1)–C(6)
C(1)–C(2)
C(2)–O(1)
C(2)–C(3)
C(3)–C(4)
C(4)–C(13)
C(4)–C(5)
C(4)–C(14)
C(6)–O(2)
C(7)–N(2)
N(1)–N(2)
N(2)–H(2)
H(2)ꢀꢀꢀO(2)
N(2)ꢀꢀꢀO(2)
1.3277(17)
1.4612(19)
1.4799(18)
1.2158(16)
1.504(2)
N(1)–C(1)–C(6)
N(1)–C(1)–C(2)
C(6)–C(1)–C(2)
O(1)–C(2)–C(1)
O(1)–C(2)–C(3)
C(1)–C(2)–C(3)
C(2)–C(3)–C(4)
C(3)–C(4)–C(13)
C(12)–C(7)–N(2)
C(8)–C(7)–N(2)
N(2)–N(1)–C(1)
N(1)–N(2)–C(7)
N(1)–N(2)–H(2)
C(7)–N(2)–H(2)
N(2)–H(2)ꢀꢀꢀO(2)
124.24(12)
114.71(12)
120.99(13)
122.54(13)
120.84(13)
116.61(13)
115.15(12)
110.74(13)
122.33(13)
117.54(13)
119.72(12)
120.17(13)
118.6(10)
1.523(2)
1.523(2)
1.525(2)
1.535(2)
1.2359(16)
1.4078(17)
1.3053(15)
0.902(16)
1.860(15)
2.5788(17)
121.1(10)
135.1(13)
region [14.56 ppm (1) and 14.97 ppm (2)]. Similarly, two
singlets are observed in the lower frequency region corre-
sponding to methyl protons (C-5). The observed difference
in chemical shifts between two methyl groups and the very
high magnitude in deshielding of NH protons clearly evi-
dences the electronic charge asymmetry due to intramo-
lecular hydrogen bonding of NH proton with the
electronegative oxygen as shown in Fig. 2. There are four
signals of aromatic protons in the region of 7.31–8.67 ppm
for 1. Among the aromatic proton signals, the peak at
8.67 ppm can be conveniently assigned to the proton which
is present in the carbon adjacent to electronegative nitrogen
and the ipso carbon. Similarly for 2, the three aromatic
protons appeared at three different regions between 6.93
and 7.55 ppm evidences the presence of 1,2,5-trisubstitued
benzene and the observed two singlets at 2.35 and
2.38 ppm are due to two methyl groups in the aromatic
ring, respectively, at 2nd and 5th positions.
For 3 and 4, methyl substituents (at C-5) associated with
the dimedone ring (Fig. 3) appears as a singlet at 1.04 and
1.13 ppm, respectively. The integral value for this singlet
corresponds to six protons and the signal appears as an
average from rapidly inter-converting conformers. The
methylene signals also reflect rapid mobility of the dime-
done ring at room temperature. A set of signal is being
observed for the methylene protons of dimedone ring in 3.
However, this difference is not observed for 4. Aromatic
protons in 3 appear in the region of 7.26–8.60 ppm and the
integral values evidence the presence of four protons in the
aryl side chain. Similarly for 4, the signal due to aromatic
protons occurred in the region of 7.38–8.00 ppm with the
very small difference between the chemical shifts corre-
sponding to nine protons.
In an effort to compare the effectiveness of hydrogen
bonding and there by making an straightforward RAHB
correlation between the two moieties (acetylacetone and
123

28
Struct Chem (2011) 22:23–33
dimedone) in a same molecule, we made an attempt to
synthesize and crystallize the compound 5, however, our
attempts to obtain the suitable single crystals for structural
analysis is not successful so for. In 5, the observed PMR
signals confirm the presence of both acetylacetone and
dimedone moieties. As expected two methyl group proton
signals are observed at 2.50 and 2.61 ppm, respectively
(attached to C-5 and C-1 carbons as in Fig. 3). The methyl
protons associated with the dimedone fragment resonate at
1.15 ppm as a singlet with the six protons integral. It also
evidences the rapid mobility of the dimedone ring at room
temperature (which denies the conformational preference
in solution) and it was further supported by the observed
methylene protons signals at 2.62 and 2.63 ppm, respec-
tively. There are two low intense signals in the higher
frequency region of 14.79 and 15.53 ppm, respectively,
due to intramolecularly hydrogen bonded NH protons.
Among the two the one which is highly deshielded is
assigned to dimedone fragment as reported earlier [19].
The aromatic protons appeared in the region of
7.45–7.62 ppm. A low field singlet, dN–H at around 15 ppm
confirm strong intramolecular N–HꢀꢀꢀO=C hydrogen
bonding in all the dimedone derivatives (3–5). The
observed effective deshielding (0.5–0.7 ppm) of NH pro-
tons in dimedone derivatives (3–5) compared to arylhyd-
razone derivatives of acetylacetone (1 and 2) point out the
healthy hydrogen bonding in dimedone derivatives com-
pared to later.
13C NMR spectra
For 1 and 2, the aromatic carbons resonate in the region of
115.4–145.6 ppm. The C-5 carbon appears at 25.5 ppm
and 26.7 ppm for 1 and 2, respectively. The C-1 carbon is
slightly deshielded and it resonates at 30.6 ppm for 1 and
31.6 ppm for 2. Carbonyl carbons are absorbed at higher
frequency region [195.8 (C-4) and 197.3 ppm (C-2) for 1
and 197.2 (C-4) and 197.8 ppm (C-2) for 2, respectively].
C-3 carbon resonates [133.3 ppm (1) and 133.7 ppm (2)]
with considerably low intensity associated with the sp2
carbons. The chemical shift difference of about 5 ppm in
the case of methyl carbons and the observed difference in
carbonyl carbon chemical shifts are due to the restricted
rotation about C–N double bond as well as the intramo-
lecular H-bonding.
The methyl carbons resonate at 28.4 and 28.5 ppm for 3
and 4, respectively. The C-5 carbon absorbs at 30.6 and
30.7 ppm for 3 and 4, respectively. Carbonyl sp2 carbon
atoms appear as separate signals in the higher frequency
region of about 197 and 193 ppm for C-1 and C-3,
respectively. Strong intramolecular N–HꢀꢀꢀO=C hydrogen
bonding deshields C-1 with respect to C-3 to the extent of
ca. 4 ppm. For 4, the aromatic carbons appear in the range
123

Struct Chem (2011) 22:23–33
29
Fig. 1 13C NMR spectrum of 5
in CDCl3
in 3, the signal for 13C=N carbon is merged with the aro-
matic carbon signals and in the remaining five signals, the
one at 147.6 ppm is assigned to the ipso carbon. In general
13C=N and ipso carbons are identified by their relative low
intensities due to longer relaxation times and lower nuclear
Overhauser effects. Retention of a single signal for methyl
groups at around 28 ppm denies any conformational pref-
erence as evidenced from PMR spectral analysis. Shielding
differences for C-4 and C-6 carbons are observed for both
the compounds 3 and 4. This is perhaps of some subtle
geometry or electronic charge asymmetry with in dime-
done ring brought about by hydrogen bonding.
Fig. 2 Spectroscopic
numbering for 1 and 2
Fig. 3 Spectroscopic
numbering for 3 and 4
For 5, beginning from low frequency end of the spectrum
(Fig. 1), the 13C resonances at 26.6, 28.5 and 31.6 ppm are
conveniently assigned to methyl carbons located at C-5,
C-50(A&B) and C-1 positions, respectively. There are two
closely spaced 13C resonances at 52.51 and 52.58 ppm
which are assigned to methylene carbons of the dimedone
ring (C-40 and C-60). The quaternary carbon which bears
two methyl groups (C-50) absorbs at 29.6 ppm. Aromatic
carbons occurred in the region of 117.3–119.1 ppm. In the
higher frequency region [193–198 ppm], four less intense
peaks are observed and they are suitably assigned to car-
bonyl carbons (individual assignments are given in
Table 4). The observed shielding differences between the
carbonyl carbons are due to the strong intramolecular
hydrogen bonding as shown in Fig. 4. The observed 13C
of 117.5–152.6 ppm. The three signals in the higher fre-
quency region of 152.6, 150.9 and 142.94 ppm can be
readily assigned to ipso carbons (in 4-substituted azoben-
zene) based on their relatively low intensity. As expected
123

30
Struct Chem (2011) 22:23–33
puckering at C(9). From the geometric calculations it is
observed that the mean plane displacement parameters for
˚
C(9) and C(6) are 0.71 and 0.023 A, respectively, from the
plane defined by C(7), C(8), C(10) and C(11). The bond
parameters of the pyridyl ring are normal. The nitrogen in
˚
the pyridine ring is too away (*2.9 A) from the methyl
hydrogen of the adjacent molecule to consider any possible
long range anisotropic interactions. However, the mole-
cules are packed in such a way that their tails are in
opposite direction.
Fig. 4 Spectroscopic numbering for 5
chemical shifts of 5 are in line with the proposed structure
of the compound.
Compound 6 crystallizes into a monoclinic lattice with
P21/n symmetry. Unit cell comprises of four discrete
molecules with insignificant intermolecular interactions.
The ORTEP of 6 along with the atom numbering scheme is
given in Fig. 6. Packing diagram of 6 is given in Fig. 7.
The diketone condenses with diazonium chloride produc-
ing a new azomethine bond (C(1)–N(1)) whose bond
Single crystal X-ray structural analysis of 3 and 6
The structure and geometry of 3 and 6 in the solid state
were established by single crystal X-ray structural analysis.
ORTEP view of 3 is shown in Fig. 5. In 3, the unit cell
contains two molecules which are asymmetric. Compound
3 is a typical representative of b-ketohydrazones and the
characteristic features of b-ketohydrazones are the
N–HꢀꢀꢀO intramolecular hydrogen bonds and the delocal-
ization of the electron density over the N–HꢀꢀꢀO=C central
fragment. From the crystal structure it is observed that
there is a effective hydrogen bonding between N(1)ꢀꢀꢀO(2)
˚
length, 1.3277 (17) A, is similar to that of a C=N bond
˚
whereas, the C(7)–N(2) with the distance of 1.4078 (17) A
is single bond in nature. However, the C(1)–N(1) [1.3277
˚
(17) A] and N(1)–N(2) [1.3053 (15)] distances are almost
similar and this equalization is as a result of effective p
electron delocalization over the C(1)–N(1)–N(2) fragment.
As observed for the previous compound (3) there is a pseudo
six membered ring formation as a consequence of intramo-
lecular hydrogen bonding [N(2)–H(2)ꢀꢀꢀO(2)] with the fol-
˚
[N(1)–H(1A)ꢀꢀꢀO(2)] having the distance of 2.582 (5) A.
˚
The O(2)ꢀꢀꢀH(1A) distance is 1.91 A and the N(1)–
˚
H(1A)ꢀꢀꢀO(2) angle is 133.7°, which exemplifies the
effectiveness of hydrogen bonding. The formation of
pseudo six-membered ring after hydrogen bonding results
lowing bond parameters: N(2)–O(2) = 2.5788 (17) A;
˚
O(2)ꢀꢀꢀH(2) = 1.860 (15) A; N(2)–H(2)ꢀꢀꢀO(2) = 135.1
(13)°. The observed increase of about 9.5° in N(1)–C(1)–
C(6) [124.24(12)°] bond angle over N(1)–C(1)–C(2) bond
[114.71 (12)°] exemplifies the adjustment in geometry for
the maximal hydrogen bonding interactions. There is a slight
increase in C(6)–O(2) distance as a outcome of hydrogen
˚
in the near equalization of the N(2)–C(6) [1.311(6) A] and
˚
N(1)–N(2) [1.285(5) A] bond lengths. It is also evidenced
from the considerable increase in N(2)–C(6)–C(11)
[121.2(4)°] bond angle compared to N(2)–C(6)–C(7)
[118.2(4)°] which will facilitate the maximal hydrogen
bonding interaction between O(2) and N(1). The dimedone
ring adopts envelope conformation with the apparent
˚
bonding [C(6)–O(2) = 1.2359 (16) A and C(2)–O(1) =
˚
1.2158 (16) A]. The dimedone ring is in envelope confor-
mation. The C(3) and C(5) which are bonded to sp2
Fig. 5 OTRP of 3
123

Struct Chem (2011) 22:23–33
31
Fig. 6 ORTEP of 6
Fig. 7 Packing diagram of 6
hybridized carbons C(2) and C(6), respectively, are forced to
be in the same plane [C(3)–C(2)–C(1)–C(6)–C(5) is almost
planar] whereas, C(4) is free from such an geometric
requirement and it results in the puckering at C-4 (with the
methoxy group is bent away to reduce the steric interactions
with the aryl ring.
From the crystal structures of 3 and 6, it is observed that
there is a six-membered pseudo ring closed by N–HꢀꢀꢀO
intramolecular hydrogen bonding is created in both com-
pounds and the pseudo rings are planar with no appreciable
deviations from planarity of any atoms. It was also supported
by the obtained torsion angles. When a comparison is made
between 3 and 6 the hydrogen bond distance is short in 6
˚
displacement of 0.664 A from C(3)–C(2)–C(1)–C(6)–C(5)
plane). It is well supported by the bond angles centered at
C(4) which are closer to the tetrahedral angle of 109°. The
molecules are packed with their tails in opposite directions.
The phenyl, pseudo six membered ketohydrazone [H-bon-
ded] and dimedone rings are almost in the same plane and the
˚
˚
[N(2)–O(2) = 2.5788 (17) A; O(2)ꢀꢀꢀH(2) = 1.860 (15) A
123

32
Struct Chem (2011) 22:23–33
˚
in 6 versus N(1)ꢀꢀꢀO(2) = 2.582 (5) A; O(2)ꢀꢀꢀH(1A) =
˚
1.91 A in 3] which might be due to better geometry adjust-
ment in 6 over 3 as indicated by the increase of about 9.5° in
N(1)–C(1)–C(6) bond angle for 6. In both the crystal struc-
tures the dimedone ring tends to adopt envelope conforma-
tion with the slightly more pronounced puckering of the
carbon which bears the methyl groups.
Resonance assisted hydrogen bonding (RAHB) studies
Generally, six-membered ring intramolecular 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 [22] where
S denotes intramolecular hydrogen bond and the size or
degree of the motif is 6. The concept of resonance assisted
hydrogen bonding (RAHB) was introduced by Gilli et al.,
in 1989 [18]. The idea of RAHB systems is based on the
statement that owing the existence of conjugated system
where the p electron delocalisation exists which causes the
enhancement of the strength of hydrogen bonds. The pro-
posed extra stabilization due to partial delocalization of p
electrons with in the HB motif (RAHB) was criticized
recently by Sanz et al. [23–25] and according to them, the
characteristics of the r-skeleton are responsible for the
extra stability of the H-bonds and not the RAHB phe-
nomenon. Despite the difference in view over this concept
[23–29], 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 centre position of hydrogen atom.
Fig. 8 RAHB in S(6) of Etter’s graph
of resonance for the strength of the hydrogen bonding in 3
and 6 are not in line with the results obtained for similar
compounds [30]. It is also very clear that the OꢀꢀꢀN bond
˚
˚
length is [2.582 (5) A in 3 and 2.5788 (17) A in 6] slightly
˚
longer than that of the mean distance of 2.552 A (derived
from about 40 structurally similar compounds) [30]. This is
convincing confirmation that the mechanism of resonance
assisted hydrogen bonding is notably reduced for the
present set of compounds (3 and 6) due to the geometric
constraints imposed by the dimedone ring.
Conclusions
We conclude that a series of arylhydrazones of b-diketones
have been prepared by the coupling of diketones (acetyl-
acetone/dimedone) with respective aromatic diazonium
salts. The prepared compounds were characterized by UV,
IR, 1H and 13C NMR spectra. Intramolecular hydrogen
bonding results in the shift of mC=O to lower wave number
and mN–H to longer wave number in IR spectra. The PMR
spectra of compounds show less intense signals corre-
sponding to NH proton in the higher frequency region of
15 ppm. The observed deshielding can be accounted for by
considering the intramolecular hydrogen bonding of NH
proton with the electronegative oxygen. In 13C NMR
spectra, as expected, the 13C signals for carbonyl carbons
are observed in different regions due to hydrogen bonding
and restricted rotation about C=N bond. Strong intramo-
As a result of hydrogen bonding charge delocalization
prevails over the pseudo six-membered ring (Fig. 8) and
hence, the N–N [d1] and C–C [d3] bonds gain some double
bond character and are shortened, whereas the C=O [d4]
and C=N [d2] bonds loose part of their double bond char-
acter and are lengthened. To quantify the involvement of
resonant cycle (Fig. 8) in hydrogen bonding, the formalism
used by Gilli et al. 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
becomes zero or negative with increasing hydrogen bond
strength. Similarly, the difference in the d3 and d2 bond
lengths, q2 = d3 - d2, also decreases with increasing
hydrogen bond strength. In the case of total p-delocalisa-
tion at –N=C–C–, 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 distance, so that q1, q2 and their sum Q = q1 ? q2
must correlate with OꢀꢀꢀN (or HꢀꢀꢀO) bond distance. How-
ever, the observed results of correlation are not satisfactory
for the present set of molecules and hence the contribution
~
lecular NH ꢀ ꢀ ꢀ O=C hydrogen bonding deshields one of the
carbonyl carbons to the extent of ca. 4 ppm in the case of
dimedone derivatives. XRD analysis of 3 and 6 confirm the
envelop conformation of the dimedone ring and also the
formation of pseudo six-membered ring after hydrogen
bonding in both the compounds. Resonance assistance for
hydrogen bonding is notably reduced for the present set of
compounds (3 and 6) due to the geometric constraints
imposed by the dimedone ring.
123

Struct Chem (2011) 22:23–33
33
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Supplementary materials
Crystallographic data for the structure reported in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publications num-
bers CCDC 760122 for 3 and 764698 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,
Cambridge CB2 1EZ, UK; fax: (?44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments The authors are extremely thankful to Prof.
Dr. K. Ramalingam of Annamalai University for his valuable sug-
gestions regarding the X-ray structural analysis.
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