Tautomeric equilibria of para-bromophenyl substituted arylhydrazones of β-diketones

Article abstract of DOI:10.1016/j.molstruc.2011.10.006

New arylhydrazone derivatives of β-diketones, 2-(4- bromophenylhydrazono)-5,5-dimethylcyclohexane-1,3-dione (1), 5-(4-bromophenylhydrazono)pyrimidine-2,4,6(1H,3H,5H)-trione (2) and (E)-2-(4-bromophenylhydrazono)-1-phenylbutane-1,3-dione (3), were prepared

Full text of DOI:10.1016/j.molstruc.2011.10.006

Journal of Molecular Structure 1006 (2011) 576–579
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Tautomeric equilibria of para-bromophenyl substituted arylhydrazones
of b-diketones
b
b
b
Kamran T. Mahmudov a,b, , Abel M. Maharramov , Rafiga A. Aliyeva , Famil M. Chyragov ,
⇑
Rizvan K. Askerov b, Parvin Q. Hasanov b, Maximilian N. Kopylovich b, Armando J.L. Pombeiro b,
⇑
a Baku State University, Department of Chemistry, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
b Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Technical University of Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
New arylhydrazone derivatives of b-diketones, 2-(4-bromophenylhydrazono)-5,5-dimethylcyclohexane-
1,3-dione (1), 5-(4-bromophenylhydrazono)pyrimidine-2,4,6(1H,3H,5H)-trione (2) and (E)-2-(4-bro-
mophenylhydrazono)-1-phenylbutane-1,3-dione (3), were prepared by coupling of b-diketone with the
respective para-bromophenyldiazonium chloride, and characterized by IR, 1H and 13C NMR spectra.
The IR and NMR spectral data disclose an effective intramolecular hydrogen bonding in all the cases.
The single crystal X-ray analysis of 3-(4-bromophenyl-hydrazono)pentane-2,4-dione (4) evidences the
intramolecular resonance-assisted hydrogen bonding (RAHB) which is significantly strengthened in view
of the planarity of the six atoms involved in the resonance cycle. Compound 3 derived from the unsym-
metric 1-phenylbutane-1,3-dione exists in solution as a mixture of the enol-azo and hydrazone tauto-
meric forms and an increase of the solvent polarity shifts the tautomeric balance to the enol-azo form,
while for 1, 2 and 4, derived from the symmetric b-diketones, the hydrazone form is dominating under
all the studied conditions.
Received 18 August 2011
Accepted 5 October 2011
Available online 25 October 2011
Keywords:
Arylhydrazones of b-diketones
Hydrogen bonding
Tautomers
Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
the other hand, recent interest in AHBD has been focused on their
coordination chemistry and in particular for the preparation of cat-
The arylhydrazones of b-diketones (AHBD) are important
intermediates in organic synthesis, e.g. they can be converted into
polyfunctional heteroaromatics [1]. The reactivity of AHBD is related
to their structure and in particular to the tautomeric balance. Hence,
it should be useful to study the tautomeric distribution at different
conditions. Generally, a tautomeric equilibrium in AHBD derived
from unsymmetric b-diketones was found between the (E,Z)-hydra-
zone and (E,Z)-enol-azo tautomeric forms [2–5], while special atten-
tion should be paid to the nature of the strong intramolecular
Oꢀ ꢀ ꢀHAN resonance-assisted hydrogen bond (RAHB) and its influ-
ence on the enol-azo ꢀ hydrazone transformation [6,7]. The RAHB
systems involve a synergistic reinforcement of hydrogen bonds by
alysts for oxidation and diasteroselective Henry reactions [2,9].
Thus, there is a justification for expanding the number of represen-
tatives of AHBD, as well as showing the generality of the found rela-
tions between the structure and properties of AHBD.
Following this line, in the present work we have studied effects
of b-diketone substituents and solvents on the equilibria among
different tautomeric forms in the new 2-(4-bromophenylhydrazono)-
5,5-dimethylcyclohexane-1,3-dione (1), 5-(4-bromophenylhydra-
zono)pyrimidine-2,4,6(1H,3H,5H)-trione (2) and (E)-2-(4-bromo-
phenylhydrazono)-1-phenylbutane-1,3-dione (3), as well as in the
known [5] 3-(4-bromophenylhydrazono)pentane-2,4-dione (4),
Scheme 1. NMR measurements and single crystal X-ray analysis
have been combined to explore the tautomer prevalence in the solid
state and in solutions with different solvents.
delocalization of a
tor atoms; they have been applied for the activation of a carbon in
-position to a carbonyl, induced enolization in keto-enol tautomer-
p-conjugated chain connecting donor and accep-
a
ism, controlled crystal packing, formation of bistable H-bonds in
functional molecular materials, activation of dinitriles towards for-
mation of amidines, carboxamides and iminoesters, etc. [6–8]. On
2. Experimental
2.1. General
⇑
Corresponding authors. Address: Centro de Química Estrutural, Complexo I,
Instituto Superior Técnico, Technical University of Lisboa, Av. Rovisco Pais, 1049-
001 Lisbon, Portugal (K.T. Mahmudov).
All the chemicals were obtained from commercial sources (Al-
drich) and used as received. Infrared spectra (4000–400 cmꢁ1) were
recorded on a BIO-RAD FTS 3000MX instrument in KBr pellets. 1H
and 13C{1H} NMR spectra were recorded on Bruker Avance II + 300
E-mail addresses: kamran_chem@yahoo.com (K.T. Mahmudov), pombeiro@is-
t.utl.pt (A.J.L. Pombeiro).
0022-2860/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.10.006

K.T. Mahmudov et al. / Journal of Molecular Structure 1006 (2011) 576–579
577
...
...
...
O
C
H
N
O
C
H
N
O
C
H
N
N
Br
HN
N
Br
H3C
O
C
C
C
C
N
Br
H2C
C
C
H3C
H3C
O
C
HN
C
H2C
CH3
O
O
1
2
4
...
O
C
H..
O
C
H
N
.
N
Br
N
Br
H3C
O
C
C
H3C
O
C
N
C
enol-azo
hydrazone
C6H5
C6H5
3
Scheme 1. Arylhydrazones of b-diketones studied in this work and tautomeric equlibria in 3.
and 400 MHz (UltraShieldTM Magnet) spectrometers at ambient
temperature. Chemical shifts (d) are relativeto internal TMS. Carbon,
hydrogen, and nitrogen elemental analyses were carried out by the
Microanalytical Service of the Instituto Superior Técnico.
145.3 (ArANHAN), 159.0 (C@O), 161.8 (C@O), 163.3 (C@O),
196.23 (C@O).
3: yield 78% (based on 1-phenylbutane-1,3-dione), yellow pow-
der soluble in DMSO, methanol, ethanol and acetone, and insoluble
in water. Elemental analysis: C16H13BrN2O2 (M = 345.2); C 55.32
(calc. 55.67)%; H 3.73 (3.80)%; N 8.03 (8.12)%. IR (KBr): 3367
2.2. Preparation of compounds 1–3
m(NH), 1663
m(C@O), 1619
m
(C@Oꢀ ꢀ ꢀH), 1590
m .
(C@N) cmꢁ1 1H
NMR of a mixture of enol-azo and hydrazo tautomers in DMSO-
d6, internal TMS, enol-azo, d (ppm): 2.38 (s, 3H, CH3), 6.63–7.92
(5H, C6H5 and 4H, C6H4), 13.11 (s, 1H, HO-enol). Hydrazone, d:
2.22 (s, 3H, CH3), 6.63–7.92 (5H, C6H5 and 4H, C6H4), 14.19 (s,
1H, NH). 13C{1H} NMR in DMSO-d6, internal TMS, enol-azo, d
(ppm): 25.7 (CH3), 110.2 (CAN), 113.4 (ArABr), 116.5 (ArAH),
128.0 (ArAH), 128.6 (ArAH), 130.2 (ArAH), 132.1 (ArAH), 133.5
(ArACO), 145.4 (ArANHAN), 191.3 (CAO), 192.5 (C@O). Hydrazone,
d: 30.3 (CH3), 113.2 (ArABr), 115.9 (ArAH), 127.6 (ArAH), 128.2
(ArAH), 129.8 (ArAH), 131.7 (ArAH), 132.6 (ArACO), 133.7
(C@N), 144.9 (ArANHAN), 192.0 (C@O), 196.4 (C@O). 1H NMR of
a mixture of enol-azo and hydrazone tautomers in CD3OD, internal
TMS, enol-azo, d (ppm): 2.34 (s, 3H, CH3), 6.72–7.97 (5H, C6H5 and
4H, C6H4). Hydrazone, d: 2.17 (s, 3H, CH3), 6.72–7.97 (5H, C6H5 and
4H, C6H4). 13C{1H} NMR in CD3OD, internal TMS, enol-azo, d (ppm):
24.3 (CH3), 109.8 (CAN), 113.1 (ArꢁBr), 116.3 (ArAH), 127.9
(ArAH), 128.4 (ArAH), 130.1 (ArAH), 132.0 (ArAH), 133.3 (ArACO),
145.2 (ArANHAN), 191.2 (CAO), 192.4 (C@O). Hydrazone, d: 27.7
(CH3), 113.1 (ArABr), 115.8 (ArAH), 127.4 (ArAH), 128.1 (ArAH),
129.6 (ArAH), 131.5 (ArAH), 132.5 (ArACO), 133.4 (C@N), 144.7
(ArANHAN), 192.0 (C@O), 196.2 (C@O).
Compounds 1–3 were synthesized according to the
JappꢁKlingemann reaction [10–12] between the diazonium salt
of 4-bromoaniline and b-diketones.
2.2.1. Diazotization
0.0250 mol of 4-bromoaniline were dissolved in 50.00 mL of
water upon addition of 1.000 g of crystalline NaOH. The solution
was cooled in an ice bath to 0 °C, and 0.025 mol of NaNO2 were
added with subsequent addition of 5.00 mL HCl (33% w/w) in por-
tions of 0.20 mL for 1 h, under vigorous stirring. During the reac-
tion the temperature of the mixture must not exceed +5 °C.
2.2.2. Azocoupling
1.000 g of NaOH was added to a mixture of 0.0250 mol of b-
diketone with 50.00 mL of water. The solution was cooled in an
ice bath, and a suspension of 4-bromoaniline diazonium (prepared
according to the procedure of Section 2.2.1) was added in two
equal portions, under vigorous stirring for 1 h. The formed precip-
itate of 1–3 was filtered off. The crude product was recrystallized
in ethanol. Compounds 1–3 were purified by column chromatogra-
phy by using methanol as eluent.
1: yield 82% (based on 5,5-dimethylcyclohexane-1,3-dione), yel-
low powder soluble in DMSO, methanol, ethanol and acetone, and
insoluble in water. Elemental analysis: C14H15BrN2O2 (M = 323.2);
C 52.03 (calc. 51.94)%; H 4.68 (4.42)%; N 8.67 (8.59)%. IR (KBr):
2.3. X-ray structure determination
Details of the crystal data, data collection and refinement param-
eters for 4 are summarized in Table 1. The crystals were grown by
slow evaporation of a methanol solution. Determination of the unit
cellanddatacollectionwereperformedonaBruker, 2004APEX2 dif-
3423
m
(NH), 1665
m
(C@O), 1634
m
(C@Oꢀ ꢀ ꢀH), 1595
m .
(C@N) cmꢁ1
1H NMR in DMSO-d6, internal TMS, d (ppm): 1.08 (s, 3H, CH3), 1.12
(s, 3H, CH3), 2.72 (2H, CH2), 2.78 (2H, CH2), 6.75–6.83 (m, 2H, ArAH),
7.38–7.47 (m, 2H, ArAH), 14.95 (s, 1H, NAH). 13C{1H} NMR in DMSO-
d6, internal TMS, d (ppm): 28.2 (2CH3), 30.2 (Cipso), 51.8 (2CH2), 116.8
(ArABr), 120.7 (ArAH), 131.1 (ArAH), 133.9 (C@N), 141.5
(ArANHAN), 200.1 (C@O), 202.2 (C@O).
fractometer using graphite-monochromated MoK
a radiation (k,
0.71073 Å) at 296 K. Multi-scan absorption corrections were applied
usingtheSADABSprogram. Thestructurewassolved bydirect meth-
ods with successive Fourier difference syntheses (SHELXS-97 [13])
and refined by full matrix least square procedure on F2 with aniso-
tropic thermal parameters. All non-hydrogen atoms were refined
(SHELXL-97 [14]) and placed at chemically acceptable positions.
2: yield 85% (based on pyrimidine-2,4,6(1H,3H,5H)-trione), or-
ange powder soluble in DMSO, methanol, ethanol and acetone,
and insoluble in water. Elemental analysis:
(M = 311.1); C 38.58 (calc. 38.61)%; H 2.30 (2.27)%; N 17.87
(18.01)%. IR (KBr): 3426 (NH), 3185 (NH), 3075 (NH), 1705
(C@O), 1685 (C@O), 1654
(C@Oꢀ ꢀ ꢀH), 1534
(C@N) cmꢁ1 1H
C10H7BrN4O3
m
m
m
3. Results and discussion
m
m
m
m
.
NMR in DMSO-d6, internal TMS, d (ppm): 6.92–7.24 (m, 2H, ArAH),
7.35–7.66 (m, 2H, ArAH), 11.21 (s, 1H, NAH), 11.34 (s, 1H, NAH),
14.48 (s, 1H, NAH). 13C-{1H} NMR in DMSO-d6, internal TMS, d
(ppm): 115.4 (ArABr), 118.9 (ArAH), 129.2 (ArAH), 130.5 (C@N),
3.1. Spectroscopic investigation of 1–3
The new AHBD compounds 1–3 and the known [5] analog 4
were prepared by reaction of b-diketones with 4-bromoaniline

578
K.T. Mahmudov et al. / Journal of Molecular Structure 1006 (2011) 576–579
Table 1
Crystallographic data and structure refinement details for 4.
However, the N(2)AH(N2)ꢀ ꢀ ꢀO(1) hydrogen bonding is disclosed
with the distance of 2.554(3) Å which is shorter than the sum of
the covalent radii (2.9 Å). The O(1)ꢀ ꢀ ꢀH(N2) distance is 1.83 Å and
the N(2)AH(N2)ꢀ ꢀ ꢀO(1) angle is 134°, what manifests the effective-
ness of the hydrogen bonding. The character of the C@Oꢀ ꢀ ꢀHAN
intramolecular hydrogen bonding reflects the bond parameters
of the pseudo six-membered ring. A relative increase in the
Empirical formula
fw
C11H11BrN2O2
283.13
k (Å)
0.71073
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
Triclinic
P1
ꢀ
7.472(4)
7.798(4)
10.880(5)
88.671(10)
80.745(10)
67.389(9)
577.0(5)
2
N(1)AN(2) bond length corresponds to
a decrease in the
C(3)AN(1) bond length. The observed variation in bond angles
between C(7)AC(6)AN(2) [122.84(17)°] and C(11)AC(6)AN(2)
[117.56(18)°] is very small. However, the C(2)AC(3)AN(1) angle
[123.46(17)°] is greater than that of N(1)AC(3)AC(4)
[113.25(17)°] to the extent of ca. 10°. It is consistent with an
acceptable geometric distortion for maximal hydrogen bond for-
mation between O(1) and N(2). Also there is a six-membered pseu-
do-ring closed by the NAHꢀ ꢀ ꢀO intramolecular hydrogen bonding.
This type of hydrogen bonding may be designated as S(6) according
to Gilli’s graph nomenclature [17], where S denotes intramolecular
hydrogen bond and the size or degree of the motif is 6.
a
(°)
b (°)
c
(°)
V (Å3)
Z
qcalc (mg/m3)
1.630
GOOF
1.044
Rint
R1 (I P 2
wR2 (I P 2
0.0187
0.0293
0.0712
a
r
)
r
b
)
a
R1
wR2 ¼ ½
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
2
2
1=2
½wðF2o ꢁ F2c Þ ꢂ=
R
½wðF2oÞ ꢂꢂ
.
b
R
Extensive studies carried out on homonuclear and heteronu-
clear OAHꢀ ꢀ ꢀO and NAHꢀ ꢀ ꢀO bonds [6,7,17–19] have shown that
hydrogen bonds of strength comparable with those presently con-
sidered can occur in neutral molecules only when the donor and
diazonium chloride in the presence of sodium hydroxide. 1H NMR
spectra of compounds 1–3 display the signal of the NH proton in
the region of 14.19–14.95 ppm. The observed downfield shift can
be accounted for by considering the intramolecular hydrogen
bonding of the NH proton with the electronegative oxygen. The
intramolecular hydrogen bonding results in the formation of a
six-membered pseudo-ring (Scheme 1). For the dimedone deriva-
tive 1, a low field singlet, dN–H ca. 15, confirms a strong intramolec-
ular NAHꢀ ꢀ ꢀO@C hydrogen bonding. Aromatic protons resonate in
the region of 6.63–7.97 ppm with the expected splitting patterns
depending on the b-diketone fragment of the molecules. The 1H
NMR spectra of 3 in CD3OD and DMSO-d6 at 20–25 °C consist of
two sets of signals, indicating its existence as a mixture of two tau-
tomeric forms (Scheme 1). The mole fraction of each tautomeric
form has been determined by the relative integration of the methyl
group of each tautomer as a reported methodology [3,15]. With an
increase in the polarity of solvents (CD3OD < DMSO) the tauto-
meric balance for 3 shifts to the enol-azo form (37% hydrazone,
63% enol-azo in CD3OD and 22% hydrazone, 78% enol-azo in
DMSO). However, the IR spectrum of 3 shows that in the solid state
acceptor atoms are connected by a
a way as to establish a synergistic interplay between the
ization of the conjugated fragment and hydrogen-bond strength-
ening [17]. -electron delocalisation occurs for RAHBs and in
p
-conjugated system, in such
p-delocal-
p
extreme cases one could observe the equalization of bonds within
the pseudo-ring with a nearly central position of the hydrogen
atom. To quantify the involvement of the resonant cycle in hydro-
gen bonding (Scheme 2), it is necessary to consider some relevant
geometrical features.
There are several ways to quantify a
p-system delocalization,
and, for a comparison, the formalism used by Gilli et al. [17] 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, normally positive, decreases towards zero or a negative
value with increasing hydrogen bond strength. The difference in
the C(2)AC(3) [d3] and the C(3)@N(1) [d2] bond lengths, q2 = d3–
d2, is also positive and decreases with increasing hydrogen bond
strength. In the case of a complete 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. The
it is stabilized in the hydrazone form (
m
(NH),
m
(C@O),
m
(C@Oꢀ ꢀ ꢀH)
and
m
(C@N) vibrations at 3367, 1663, 1619 and 1590 cmꢁ1, respec-
p-delocalisation must increase with reducing hydrogen bond dis-
tance, so that q1, q2 and their sum Q = q1 + q2 must correlate with
the Oꢀ ꢀ ꢀN (or Hꢀ ꢀ ꢀO) bond distance. To check out this concept, we
have calculated the Q value for 4 and for the known 2-(4-bro-
mophenylhydrazono)-1,3-diphenylpropane-1,3-dione (I) [20] and
the trend was observed (Scheme 2). The co-planarity of all the
tively), similarly to reported analogs [15]. The intramolecular
hydrogen bonding decreases the carbonyl stretching frequency
by about 90 cmꢁ1 in 3 [16]. This lowering is, however, dependent
on the strength of the hydrogen bond and hence for the present
set of compounds the shift of m(C@O) to lower wavenumbers can
be attributed to the intramolecular hydrogen bonding
(NAHꢀ ꢀ ꢀO@C). For all the compounds a sharp and less intense band
assigned to the NAH stretching frequency is observed in the 3367–
3426 cmꢁ1 region and the observed shift can also be accounted for
by the aforementioned possible intramolecular hydrogen bonding.
In contrast to 3, the symmetrical compounds 1 and 2 are stabilized
in DMSO exclusively in the hydrazone form.
3.2. Single crystal X-ray structural analysis of 4
The single crystal X-ray structural analysis of 4 (Fig. 1) reveals
that in the solid phase it exists in the hydrazone form (Schemes
1 and 2). The unit cell contains two discrete molecules. There is a
double bond between C(3) and N(1) with the length of
1.315(2) Å. The two carbonyl bond distances are nearly identical
Fig. 1. Molecular structure of 4. Selected bond lengths (Å) and angles (°): O1AC2
1.223(3); O2AC4 1.211(3); N1AN2 1.302(2); N1AC3 1.315(2); N2AC6 1.399(2);
C2AC3 1.474(3); C7AC6AN2 122.84(17); C11AC6AN2 117.56(18); N1AC3AC2
123.46(17); N1AC3AC4 113.25(17); O2AC4AC3 121.7(2); O2AC4AC5 119.8(2).
Intramolecular hydrogen bond (dotted line) N2AH(N2)ꢀ ꢀ ꢀO(1) [d(Nꢀ ꢀ ꢀO) Å;
\(NHO)°]: 2.554(3) Å, 134°.
[1.223(3) and 1.211(3) Å], the difference being within
3r.

K.T. Mahmudov et al. / Journal of Molecular Structure 1006 (2011) 576–579
579
Registered Charity No. 800579 for compound 4. Copies of this
information may be obtained free of charge from The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223
336 033. E-mail: data_request@ccdc.cam.ac.uk. Web page: http://
www.ccdc.cam.ac.uk.
C(2)
C(2)
d3
d3
d4
d4
(1)
C(3)
O
(1)
C(3)
O
q1 = d1-d4 = 0.079
q2 = d3-d2 = 0.159
q1 = d1-d4 = 0.085
d2
d2
q2 = d3-d2
=
0.170
°
N(1)
H
Q = q1+q2 = 0.238 A
°
N(1)
H
Q = q1+q2 = 0.255 A
d1
d1
N(2)
N(2)
Acknowledgements
4
I [20]
This work has been partially supported by the Foundation for
Science and Technology (FCT), Portugal, and its PEst-OE/QUI/
UI0100/2011 and ‘‘Science 2007’’ programs, as well as by the Baku
State University, Azerbaijan. K.T.M. and M.N.K. express gratitude to
the FCT for a post-doc fellowship and a working contract.
Scheme 2. RAHB in S(6) of Gilli’s graph.
six atoms (in the pseudo-ring) of S(6) Gilli’s group should lead to a
stronger RAHB. In fact, for compound I, the planarity of the six
atoms which are involved in the resonant cycle (S(6) of Gilli’s
graph – Scheme 2) is very much disturbed by the conformational
requirement of the two phenyl rings (b-diketone fragment).
References
Accordingly, the Oꢀ ꢀ ꢀN bond length and the
Q value for I
[1] E.V. Shchegol’kov, Y.V. Burgart, O.G. Khudina, V.I. Saloutin, O.N. Chupakhin,
Russ. Chem. Rev. 79 (2010) 31.
[2] M.N. Kopylovich, K.T. Mahmudov, A.J.L. Pombeiro, J. Hazard. Mater. 186 (2011)
1154.
[3] M.N. Kopylovich, K.T. Mahmudov, M.F.C. Guedes da Silva, M.L. Kuznetsov, P.J.
Figiel, Y.Y. Karabach, K.V. Luzyanin, A.J.L. Pombeiro, Inorg. Chem. 50 (2011)
918.
[4] A.M. Maharramov, R.A. Aliyeva, I.A. Aliyev, F.G. Pashaev, A.G. Gasanov, S.I.
Azimova, R.K. Askerov, A.V. Kurbanov, K.T. Mahmudov, Dyes Pigm. 85 (2010) 1.
[5] K.T. Mahmudov, A.M. Maharramov, R.A. Aliyeva, I.A. Aliyev, R.K. Askerov, R.
Batmaz, M.N. Kopylovich, A.J.L. Pombeiro, J. Photochem. Photobiol. A: Chem.
219 (2011) 159.
[6] G. Gilli, P. Gilli, The Nature of the Hydrogen Bond: Outline of a Comprehensive
Hydrogen Bond Theory, Oxford University Press, Oxford, 2009.
[7] P. Gilli, G. Gilli, J. Mol. Struct. 972 (2010) 2.
[8] M.N. Kopylovich, K.T. Mahmudov, M. Archana, A.J.L. Pombeiro, Chem.
Commun. 47 (2011) 7248.
[9] M.N. Kopylovich, T.C.O. Mac Leod, K.T. Mahmudov, M.F.C.G. Guedes, da Silva,
A.J.L. Pombeiro, Dalton Trans. 40 (2011) 5352.
[10] F.R. Japp, F. Klingemann, Liebigs Ann. Chem. 247 (1888) 190.
[11] H.C. Yao, J. Org. Chem. 29 (1964) 2959.
[2.6205(18) [20] and 0.255 Å, respectively] are slightly higher than
those in 4 [(2.554(3) and 0.238 Å, respectively]. This provides an
evidence that the RAHB is significantly improved in compound 4.
4. Conclusions
The new compounds 1–3 were prepared by coupling of the
respective b-diketones with 4-bromoaniline diazonium chloride
and characterized by elemental analysis, IR, 1H and 13C NMR spec-
tra. For the known compound 4, the single crystal X-ray structure
analysis was performed for the first time. 3 exists in solution as a
mixture of the enol-azo and hydrazone tautomeric forms, and an
increase of the solvent polarity shifts the tautomeric balance to
the enol-azo form. Single crystal X-ray analysis of 4 indicates that
the molecule is in the hydrazone form with an effective intramo-
lecular [NAHꢀ ꢀ ꢀO] hydrogen bonding, which can be designated
by the S(6) type according to Gilli’s graph nomenclature. RAHB
features suggest that the resonance assistance for hydrogen
bonding is appreciably strengthened for compound 4.
[12] M.N. Kopylovich, K.T. Mahmudov, M.F.C. Guedes da Silva, L.M.D.R.S. Martins,
M.L. Kuznetsov, T.F.S. Silva, J.J.R.F. Silva, A.J.L. Pombeiro, J. Phys. Org. Chem. 24
(2011) 764.
[13] G.M. Sheldrick, Acta Crystallogr. 46 (1990) 467.
[14] G.M. Sheldrick, SHELXL 97, University of Gottingen, Gottingen, Germany, 1997.
[15] M.N. Kopylovich, K.T. Mahmudov, M. Haukka, K.V. Luzyanin, A.J.L. Pombeiro,
Inorg. Chim. Acta 374 (2011) 175.
[16] P.S. Kalsi, Spectroscopy of Organic Compounds, third ed., New Age
International Limited, 1998.
Supplementary material
[17] G. Gilli, F. Bellucci, V. Ferretti, V. Bertolasi, J. Am. Chem. Soc. 111 (1989) 1023.
[18] P. Gilli, V. Bertolasi, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 116 (1994) 909.
[19] P. Gilli, V. Bertolasi, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 122 (2000) 10405.
[20] C. Bustos, L. Alvarez-Thon, J.-G. Carcamo, M.T. Garland, C. Sanchez, Acta
Crystallogr. E67 (2011) o1426.
Crystallographic data (excluding structure factors) for the struc-
ture reported in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication, CCDC 763944 Registered in England No. 2155347,