Synthesis, characterization and molecular structures of homologated analogs of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone

Article abstract of DOI:10.1016/j.molstruc.2013.10.016

Four analogues of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone (where n-alkyl is methyl in L-1Br, ethyl in L-2Br, propyl in L-3Br and butyl in L-4Br) are synthesized and characterized. A reaction mechanism is proposed for the formation of L-1Br to L-4Br from the starting material 2,3-dibromo-1,4- naphthoquinone. The ν_NH frequency in the FT-IR spectra is affected by the intramolecular hydrogen bonding in L-1Br to L-4Br and is observed 3267 cm^-1 in L-2Br. A shift of 25 cm^-1 is observed in the ν_CBr frequency in all the compounds as compared to 2,3-dibromo-1,4-naphthoquinone (627 cm^-1). A broad charge transfer band is observed between 400 and 600 nm in the UV-Vis spectra, which imparts red colour to all the compounds. Molecular structures of L-2Br and L-3Br were studied by single crystal X-ray diffraction studies. Molecules of L-2Br crystallize in Pca2₁, whereas the molecule L-3Br crystallizes in the P-1 space group. Molecules of L-2Br forms a polymeric chain through NHa?O interaction and forms beautiful butterfly like arrangement of molecules when viewed down the 'a' axis. Ladder like polymeric chain of molecules is observed in L-3Br via CHa?O and NHa?O interactions. Every alternating neighbouring chains of L-3Br, show stacking interactions between the quinonoid rings of the molecules, however this interaction is not observed in L-2Br.

Full text of DOI:10.1016/j.molstruc.2013.10.016

Journal of Molecular Structure 1056–1057 (2014) 97–103
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, characterization and molecular structures of homologated
analogs of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone
a
a
a
b
Sunita Salunke-Gawali a, , Omkar Pawar , Milind Nikalje , Rishikesh Patil , Thomas Weyhermüller ,
⇑
Vedavati G. Puranik c, V. Badireenath Konkimalla d
a Department of Chemistry, University of Pune, Pune 411007, India
b MPI für Chemische Energiekonversion, Stiftstr. 34-36, 45470 Müllheim an der Ruhr, Germany
c Center for Material Characterization, National Chemical Laboratory, Pune 411008, India
d School of Biological Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar 751005, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Synthesis and characterization of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone are discussed.
X-ray structures of ethyl and propyl derivatives are presented.
Propyl derivative show CAHꢁ ꢁ ꢁO, NAHꢁ ꢁ ꢁO and
Ethyl derivative show only NAHꢁ ꢁ ꢁO interaction
Ethyl derivative shows, butterfly like arrangement of molecules down ‘a’ axis.
p
–
p
stacking interaction.
p–p stacking interaction are absent.
a r t i c l e i n f o
a b s t r a c t
Article history:
Four analogues of 2-bromo-3-(n-alkylamino)-1,4-napthoquinone (where n-alkyl is methyl in L-1Br, ethyl
in L-2Br, propyl in L-3Br and butyl in L-4Br) are synthesized and characterized. A reaction mechanism is
proposed for the formation of L-1Br to L-4Br from the starting material 2,3-dibromo-1,4-naphthoqui-
none. The mNAH frequency in the FT-IR spectra is affected by the intramolecular hydrogen bonding in
L-1Br to L-4Br and is observed ꢂ3267 cmꢃ1 in L-2Br. A shift of ꢂ25 cmꢃ1 is observed in the mCABr fre-
quency in all the compounds as compared to 2,3-dibromo-1,4-naphthoquinone (627 cmꢃ1). A broad
charge transfer band is observed between 400 and 600 nm in the UV–Vis spectra, which imparts red col-
our to all the compounds. Molecular structures of L-2Br and L-3Br were studied by single crystal X-ray
diffraction studies. Molecules of L-2Br crystallize in Pca21, whereas the molecule L-3Br crystallizes in
the P-1 space group. Molecules of L-2Br forms a polymeric chain through NAHꢁ ꢁ ꢁO interaction and forms
beautiful butterfly like arrangement of molecules when viewed down the ‘a’ axis. Ladder like polymeric
chain of molecules is observed in L-3Br via CAHꢁ ꢁ ꢁO and NAHꢁ ꢁ ꢁO interactions. Every alternating neigh-
Received 10 August 2013
Received in revised form 12 September
2013
Accepted 7 October 2013
Available online 16 October 2013
Keywords:
p–p Stacking
Aminonaphthoquinone
Hydrogen bonding
Michael addition
2,3-Dibromo-1,4-naphthoquinone
bouring chains of L-3Br, show
p–p stacking interactions between the quinonoid rings of the molecules,
however this interaction is not observed in L-2Br.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
SOD-catalyzed reaction and the hydroxyl radical (HOꢀ) would be
formed by Fenton reaction on reduction of peroxide [3–5]. All these
highly ROS (reactive oxygen species) may react directly with DNA
or other cellular macromolecules, such as lipids and proteins, lead-
ing to cell damage.
The amino-1,4-naphthoquinone moiety is the component of
molecular framework of many natural products and has been used
as synthetic key intermediate for synthesis of several compounds
with important biological applications [6]. The amino and
thioether derivatives of 1,4-naphthoquinones posses extremely
rich biological activities because of their redox potentials [6].
Single crystal X-ray analysis reveals the accurate carbonyl bond
distances, which may enable us to identify the oxidation state of
Mass screening programs of natural products by the National
Cancer Institute have identified the quinone moiety as an impor-
tant pharmacophore element for cytotoxic activity [1]. The one
electron reduction of quinones is catalyzed by NADPH–cytochrome
P450 reductase generating unstable semiquinone radicals [2]. They
transfer electrons to molecular oxygen and return to their original
quinone form, thus generating superoxide anion radical ðOꢀ2ꢃÞ.
Superoxide can be converted to hydrogen peroxide (H2O2) by a
⇑
Corresponding author. Tel.: +91 2025601397x531; fax: +91 2025693981.
E-mail address: sunitas@chem.unipune.ac.in (S. Salunke-Gawali).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.10.016

98
S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
O
O
(n-alkylamino)-1,4-naphthoquinone (n-alkyl: methyl; L-1Br, ethyl;
L-2Br, propyl; L-3Br and butyl; L-4Br) derivatives as shown in
Scheme 1. Molecular structures of L-2Br and L-3Br revealed by sin-
gle crystal X-ray diffraction studies show the formation of a supra-
molecular assembly.
NHR
Br
where,
L-1Br: R=CH3; L-2Br: R=C2H5;
L-3Br: R=C3H7; L-4Br: R=C4H9
2. Experimental section
Scheme 1. Molecular formula of L-1Br to L-4Br.
2.1. General materials and methods
The materials used viz. 2,3-dibromo-1,4-naphthoquinone, ethyl
amine solution (70%), propyl amine and butyl amine were pur-
chased from Sigma–Aldrich and used as received and methyl amine
solution (40%), was purchased from LOBA chemicals India. The sol-
vents used such as dichloromethane, toluene, methanol were of
analytical grade and purchased from Merck Chemicals, India. The
solvents were distilled in nitrogen atmosphere by standard meth-
ods [11] and dried wherever necessary. Melting points of L-1Br to
L-4Br were determined using melting point apparatus (METTLER)
and were corrected using DSC (Differential Scanning Calorimetry)
(METTLER, STAR SW 8.10) (Fig. S1 in ESI ). FT-IR spectra of the com-
pounds were recorded between 4000 and 400 cmꢃ1 as KBr pellets
on SHIMADZU FT 8400 spectrometer (Fig. S2 and Table S1 in ESI )
and 1H NMR of ligands are recorded (Fig. S3 in ESI ) in CDCl3, on
Varian Mercury 300 MHz with TMS (tetramethylsilane) used as
the reference. Elemental analysis were performed on Thermo
Finnigan EA 1112 Flash series Elemental Analyzer. The mass of
the derivatives, L-1Br to L-4Br were determined using LC–MS
1,4-naphthoquinone derivatives. Molecular structures of 1,4-naph-
thoquinone derivatives show associations of the ligand molecules
to form three dimensional polymeric network via CAHꢁ ꢁ ꢁO and
OAHꢁ ꢁ ꢁO hydrogen bonding and
p–p stacking interactions be-
tween the benzenoid and quinonoid rings [7]. These are modes
by which the substituted 1,4-naphthoquinone derivatives could
bind to biomolecules to perform the biological activities. The
molecular associations through hydrogen bonding interactions of
substituted 1,4-naphthoquinone derivatives may lead to stabiliza-
tion of radical species and their redox cycling will decide their bio-
logical activity.
The single crystal X-ray structures of bromine derivatives of
naphthoquinones are scarce [8] as compared to other naphthoqui-
nones derivatives. Owing to the excellent pharmacological
potential of aminonaphthoquinones [9,10] and our growing
interest in biological activities of such derivatives, the present
work elaborates the synthesis and characterization of 2-bromo-3-
Table 1
Crystallographic data for L-2Br and L-3Br.
Compound
L-2Br
12H10BrNO2
280.12
296(2) K
0.71073 Å
Orthorhombic
Pca21
L-3Br
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C
C13H12BrNO2
294.15
100(2) K
0.71073 Å
Triclinic
P-1, No. 2
Unit cell dimensions
14.846(1)
a = 7.186(1) Å,
a = 110.878(12)°
4.5979(4)
b = 7.844(1) Å, b = 91.369(4)°
16.643(2)
c = 10.94(1) Å,
573.86(12) Å3
2
c = 94.663(8)°
Volume
Z
1136.08(18) Å3
4
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
Max. and min. Transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
1.638 g/cc
1.702 g/cc
3.569 mmꢃ1
296
3.601 mmꢃ1
560
0.50 ꢄ 0.09 ꢄ 0.05 mm3
0.38 ꢄ 0.37 ꢄ 0.17 mm3
2.45–24.99°
3.57–29.22°
ꢃ9 6 h 6 9, ꢃ9 6 k 6 10, ꢃ14 6 l 6 14
7265
7265/3060 [R(int) = 0.0398]
=29.12°, 99.3%
ꢃ17 6 h 6 17, ꢃ5 6 k 6 5, ꢃ18 6 l 6 19
6687
6687/1886 [R(int) = 0.0280]
=24.99°, 99.9%
Semi-empirical from equivalents
0.8518 and 0.2651
Full-matrix least-squares on F2
1886/1/146
Gaussian
0.56904 and 0.33438
Full-matrix least-squares on F2
3060/0/158
1.055
1.057
Final R indices [I > 2
R indices (all data)
Largest diff. peak and hole
r
(I)]
R1 = 0.0354, wR2 = 0.0818
R1 = 0.0539, wR2 = 0.0898
0.554 and ꢃ0.512 e Åꢃ3
R1 = 0.0233, wR2 = 0.0573
R1 = 0.0251, wR2 = 0.0581
0.727 and ꢃ0.657 e Åꢃ3
Table 2
Details of concentration used in synthesis of L-1Br to L-4Br.
Comp.
NQ⁄
Weight g (mM)
Amine (% soln)
Density (g/cm3)
Volume (mL)
Yield g (%)
L-1Br
L-2Br
L-3Br
L-4Br
DBrNQ
DBrNQ
DBrNQ
DBrNQ
0.316(1.0)
0.316(1.0)
0.50(1.6)
1.00(3.2)
Methyl (40%)
Ethyl (70%)
Propyl (99%)
Butyl (99%)
0.893
0.800
0.717
0.740
0.130
0.120
0.200
0.470
0.198 (74.5)
0.203 (72.3)
0.363 (78.1)
0.734 (75.3)

S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
99
O
O
O
O
O
Br
H
Br
Br
H
N
H
N
Br
N+
H
R
R
R
H2N
R
H
Br
2
Br
Br
Br
OR
R = Methyl
= Ethyl
4
4
1
O-
O
3
OH
- HBr PATH-B
PATH-A - HBr
= Propyl
= Butyl
R
O
R
O
N
NH
Br
Br
O
O
5b
5a
H
Scheme 2. Probable reaction mechanism involved in synthesis of L-1Br to L-4Br.
DBrNQ
L-1Br
L-2Br
L-3Br
L-4Br
100
80
60
40
20
(a)
(b)
70
60
50
DBrNQ
40
L-1Br
L-2Br
L-3Br
L-4Br
30
20
1700
1650
1600
1550
3500
3400
3300
3200
3100
Wavenumber (cm-1)
Wavenumber (cm-1)
DBrNQ
L-1Br
L-2Br
L-3Br
L-4Br
(c)
(d)
60
50
40
30
20
80
60
40
20
DBrNQ
L-1Br
L-2Br
L-3Br
L-4Br
1320
1300
1280
1260
660
640
620
Wavenumber (cm-1)
Wavenumber (cm-1)
Fig. 1. Characteristic FTIR frequencies of L-1Br to L-4Br and DBrNQ, (a) m mC@O and m
NAH band (3500–3050 cmꢃ1); (b) CAN frequency (1720–1550 cmꢃ1); (c) bands corresponds
to p-NQ vibration (1320–1245 cmꢃ1); and (d) mCABr frequency (675–610 cmꢃ1).
2010-eV (Make SHIMADZU). UV–Vis spectra of L-1Br
(9.7 ꢄ 10ꢃ5 M), L-2Br (8.8 ꢄ 10ꢃ5 M), L-3Br (8.4 ꢄ 10ꢃ5 M) and L-
4Br (8.5 ꢄ 10ꢃ5 M) were recorded on UV-1800 (Make SHIMADZU)
in methanol.
used for structure solution and full matrix least squares refinement
on F2. Hydrogen atoms were included in the refinement as per the
riding model. Data collection and refinement parameters are listed
in Table 1.
Single crystals of complexes L-3Br were coated with perfluoro-
polyether, picked up with nylon loops and were mounted in the
nitrogen cold stream of the diffractometer. Monochromated Mo
2.2. X-ray crystallographic data collection and refinement of the
structures
Ka radiation (Bruker-AXS Kappa Mach3 with Incoatec Helios mir-
ror, k = 0.71073 Å) from a Mo-target rotating-anode X-ray source
was used for L-3Br. Final cell constants were obtained from least
squares fits of strong reflections. Intensity data were corrected
for absorption using intensities of redundant reflections with the
program SADABS [13]. The structures were readily solved by direct
methods and subsequent difference Fourier techniques. The Sie-
mens ShelXTL [12] software package was used for solution and art-
work of the structures, ShelXL97 [14] was used for the refinement.
All non-hydrogen atoms were anisotropically refined and hydro-
gen atoms were placed at calculated positions and refined as riding
atoms with isotropic displacement parameters. Crystallographic
data of the compounds are listed in Table 1.
Single crystals of the compound L-2Br were grown by slow evap-
oration of toluene. Red coloured needle crystal of approximate size
0.50 ꢄ 0.09 ꢄ 0.05 mm3 was used for data collection at 296(2) K, on
Bruker SMART APEX CCD diffractometer using Mo Ka radiation with
fine focus tube with 50 kV and 30 mA. Crystal to detector distance
6.05 cm, 512 ꢄ 512 pixels/frame, multirun data acquisition. Total
frames = 1618, Oscillation/frame ꢃ0.3°, exposure/frame = 20.0 s/
frame, maximum detector swing angle = ꢃ30.0°, beam cen-
ter = (260.2, 252.5), in plane spot width = 1.24, SAINT integration,
h range = 2.44–24.99°, completeness to h of 24.99° is 99.9%. SADABS
correction applied. All the data were corrected for Lorentzian,
polarisation and absorption effects. SHELX-97 (ShelxTL) [12] was

100
S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
3
3
3
(t, 3H, J = 7.2 Hz), 3.93 (m, 2H, J = 6.5 Hz), 6.04 (br. s, 1H, D2O
3x10
2x10
L-1Br
L-2Br
L-3Br
L-4Br
ex.), 7.62 (t, 1H, J = 7.5 Hz), 7.72 (t, 1H, J = 7.5 Hz), 8.04 (d, 1H,
J = 7.5 Hz), 8.16 (d, 1H, J = 7.8 Hz). UV–Vis; (kmax/nm, MeOH):
281, 335, 471. LC–MS (EI); m/z:279.80 (M++H).
2.3.4. Characterization of 2-bromo-3-propylamino-1,4-
naphthoquinone
L-3Br: Red crystals, 0.363 (78.1%), m.p.: 109–110 °C. Anal. data
calcd. for C13H12NO2Br: C, 53.08; H, 4.11; N, 4.76. Found C, 52.35;
H, 3.44; N, 6.04. FT-IR (KBr, v
max/cmꢃ1): 3300, 3195, 3076, 3026,
1x10
2949, 2874, 1682, 1630, 1595, 1568, 1520, 1443, 1364, 1333,
1298, 1267, 1167, 1126, 1071, 1024, 970, 895, 860, 777, 723,
679, 654, 592, 559, 517, 453, 432. 1H NMR (300 MHz, CDCl3) d:
1.03 (t, 3H J = 7.4 Hz), 1.70 (m, 2H, J = 7.3 Hz), 3.85 (q, 2H,
J = 6.9 Hz), 6.14 (br.s, 1H, D2O ex.), 7.62 (t, 1H, J = 7.5 Hz), 7.71 (t,
1H, J = 6.8 Hz), 8.03 (d, 1H, J = 7.5 Hz), 8.15 (d,1H, J = 7.8 Hz). UV–
Vis; kmax/nm, MeOH): 281, 335, 471. LC–MS (EI); m/z: 295.90
(M++H).
0
350
400
450
500
550
600
Wavelength (nm)
Fig. 2. UV–Vis spectra of L-1Br to L-4Br in methanol with concentration ꢂ1
lM.
2.3.5. Characterization of 2-bromo-3-butylamino-1,4-naphthoquinone
L-4Br: Red crystals, 0.734 g (75.3%), m.p.: 103 °C. Anal. data
calcd. for C14H14NO2Br: C, 54.56; H, 4.58; N, 4.54. Found C, 54.36;
2.3. Synthesis
H, 4.53; N, 4.59. FT-IR (KBr, v
max/cmꢃ1): 3275, 3178, 3068, 2932,
2.3.1. General procedure for synthesis L-1Br to L-4Br
2868, 1676, 1640, 1601, 1572, 1506, 1439, 1331, 1290, 1258,
1223, 1165, 1225, 1068, 1003, 866, 802, 779, 741, 719, 675, 652,
596, 554, 507. 1H NMR (300 MHz, CDCl3) d: 0.98 (t, 3H,
J = 7.2 Hz), 1.45 (m, 2H, J = 7.5 Hz), 1.68 (q, 2H, J = 7.2 Hz), 3.88
(q, 2H, J = 7.2 Hz), 6.09 (br.s, 1H, D2O ex.), 7.63 (t, 1H, J = 7.5 Hz),
7.72 (t, 1H, J = 7.4 Hz), 8.04 (d, 1H, J = 7.5 Hz), 8.16 (d, 1H,
J = 7.5 Hz). UV–Vis; (kmax/nm, MeOH): 281, 335, 471. LC–MS (EI);
m/z: 307.90 (M++H).
Calculated amount of 2,3-dibromo-1,4-naphthoquinone
(DBrNQ) (Table 2) were dissolved in about 30 ml of dichlorometh-
ane and stirred for about 15 min. To this solution required amount
of amine (methyl, ethyl, propyl and butyl) were added drop wise.
The reaction mixture was stirred for 24 h and then kept in a refrig-
erator at 4 °C for overnight. The crude product was purified by col-
umn chromatography using toluene: methanol (9:1) as the eluent.
2.3.2. Characterization of 2-bromo-3-methylamino-1,4-
naphthoquinone
3. Results and discussion
L-1Br: Dark red crystals, 0.198 g (74.5%), m.p.: 161 °C. Anal. data
calcd. for C11H8NO2Br: C, 49.65; H, 3.03; N, 5.26. Found C, 49.28; H,
3.1. Reaction mechanism involved in synthesis of L-1Br to L-4Br
3.07; N, 5.16. FT-IR (KBr, v
max/cmꢃ1) 3314, 3194, 3075, 3013, 2945,
2884, 1682, 1645, 1628, 1599, 1570, 1516, 1452, 1414, 1331, 1298,
1256, 1235, 1175, 1123, 1071, 1028, 955, 899, 783, 726, 681, 650,
594, 546, 451. 1H NMR (300 MHz, CDCl3) d: 3.45 (3H, d, J = 8.4 Hz),
6.17 (br, s, 1H, D2O ex.), 7.63 (t, 1H, J = 7.5 Hz), 7.72 (t, 1H,
J = 7.5 Hz), 8.03 (d, 1H, J = 7.2 Hz), 8.03 (d, 1H, J = 7.5 Hz). UV–Vis;
(kmax/nm, MeOH): 281, 335, 471. LC–MS (EI); m/z: 265.85 (M++H).
Scheme 2 indicates that 2,3-dibromonapthoquinone (1), the
starting material used in the synthesis is a symmetrical Michael
acceptor molecule, which could undergo the Michael addition
reaction with variety of amines (2) to give the nucleophilic substi-
tution reaction at the sp2 carbon atom. As a result, the formation of
compound 5a could be the major product in this reaction.
Mechanistically, it is predicted that Michael addition of primary
amine 2 will lead to the generation of an intermediate compound
3. It is then believed to reorganize to key intermediate 4, which
actually gives an idea about elimination of the hydrogen bromide
by two possible pathways (A & B) as shown in Scheme 2. PATH-A
in Scheme 2 represents the enol form 4 that is directly converted
to compound 5a by keto ꢁ enol tautomerization with loss of
the ‘Br’ moiety.
2.3.3. Characterization of 2-bromo-3-ethylamino-1,4-naphthoquinone
L-2Br: Red crystals, 0.203 g (72.3%). m.p.:112–113 °C. Anal. data
calcd. for C12H10NO2Br: C, 51.45; H, 3.59; N, 5.00. Found C, 50.89;
H, 3.71; N, 4.43. FT-IR (KBr, v
max/cmꢃ1): 3266, 3187, 3073, 2999,
2970, 2930, 1678, 1643, 1605, 1574, 1555, 1512, 1447, 1331,
1292, 1256, 1227, 1165, 1123, 1086, 984, 907, 804, 775, 745,
723, 675, 650, 596, 552, 474. 1H NMR (300 MHz, CDCl3) d: 1.35
Fig. 3. ORTEP diagrams of (a) L-2Br and (b) L-3Br thermal ellipsoids drawn at 50% probability.

S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
101
in mN-H frequency suggest that the frequency is influenced by intra
and intermolecular hydrogen bonding. The carbonyl stretching fre-
quency is observed at ꢂ1676 cmꢃ1 for reactant DBrNQ. The mC@O
frequency remained the same in L-4Br and it shifted to higher
wavenumber in L-1Br to L-3Br. It increased by ꢂ6 cmꢃ1 in L-1Br
and L-3Br (1682 cmꢃ1) and by ꢂ2 cmꢃ1 in L-2Br (1678 cmꢃ1
)
(Fig. 1b). The mCAN frequency is observed at ꢂ1599 4 cmꢃ1 in L-
1Br to L-4Br (Fig. 1b). The characteristic paranaphthoquinone (p-
NQ) frequency is observed at ꢂ1260 cmꢃ1 in L-1Br to L-4Br
(Fig. 1c). The mCABr vibrations is shifted ꢂ25 cmꢃ1 in L-1Br to L-
4Br which is at higher frequency as compared to DBrNQ
(627 cmꢃ1) (Fig. 1d).
3.3. 1H NMR spectroscopy
1H NMR spectra of the L-1Br to L-4Br were recorded in CDCl3
solvent (Fig. S3 in ESI ). The peaks due to aliphatic hydrogens are
observed between d = 1–2 ppm (Table S2 in ESI ). A multiplet is ob-
served for protons of C(12) and C(13). Aromatic protons appeared
between 7.6 and 8.1 ppm. Since the H(5) and H(8) protons are
more deshielded than H(6) and H(7), their chemical shifts were
shifted further down field. H(6) and H(7) protons showed triplet
while a doublet was observed for H(5) and H(8) protons. A broad
peak is observed at ꢂ6 ppm, which is assigned to the NAH proton
in L-1Br to L-4Br. The chemical shifts for the aromatic protons in
L-1Br to L-4Br differed by 0.02 ppm. The C(11) protons
(ꢂ3.45 ppm) of L-1Br showed upfield shift probably its participa-
tion in the intermolecular hydrogen bonding. The signal of C(11)
proton for L-1Br to L-4Br were observed between 3.81 and
3.91 ppm.
3.4. UV–Vis spectra of L-1Br to L-4Br
The electronic spectra of compounds L-1Br to L-4Br are shown
in Fig. 2, ꢂ1
lM concentrations were used to record the electronic
spectra of L-1Br to L-4Br and the starting materials DBrNQ. The
absorption in the visible region was observed in the starting mate-
rials viz. DBrNQ as well as in L-1Br to L-4Br owing to a fully conju-
gated cyclic dione system [15], wherein the
p ?
pꢅ transitions [16]
are well pronounced in the UV region and the nonbonding elec-
trons of the amino group allows the possibility of n ? pꢅ transition
in the visible region [16].
The UV–Vis spectra of L-1Br and L-4Br showed the naphthoqui-
none bands in the UV region around 281–287 nm (k1) and at
337–330 nm (k2) (p ?
pꢅ electronic transition). In addition, a third
lower energy transition appeared in the visible region of ꢂ471 nm
(k3). This absorption is typical of amino-substituted quinones [17].
The lower energy absorption ꢂ471 nm is assigned, to the charge
transfer (CT) transition and weak n ? pꢅ transition. CT band results
from the electron donating effect of the substituted amine in L-1Br
to L-4Br. It is known that full delocalization of the nitrogen lone
pair requires its orthogonality to be in the plane of naphthoqui-
none. Thus, the shape of the amines influences their basicity,
which in turn influences the extent of bathochromic shift. L-1Br
to L-4Br differed in the length of the alkyl chain length present
on amino nitrogen and therefore does not differ greatly in their
absorption [16,18,19].
Fig. 4. (a and b) Butterfly like structures of L-2Br down a-axis, (c) hydrogen bonding
interactions in L-2Br molecules.
Alternatively, PATH-B is indicative of the loss of bromine by
neighbouring group participation of the basic amino functionality
2, with the bromine being pushed out and generating the imino-
ketone 5b as an unstable form of compound 5a, which could be
transformed to the most stable form 5a by tautomerization.
3.5. Single crystal X-ray structures of L-2Br and L-3Br
3.2. FT-IR studies
Fig. 3 represents ORTEP diagram of L-2Br and L-3Br and the
crystallographic data is represented in Table 1. The crystal
structure data of L-2Br to L-3Br reveals that the quinonoid and aro-
matic bond distances are similar to those observed in several
1,4-naphthoquinone derivatives [20]. The carbonyl C(1)AO(1)
and C(4)AO(4) bond distances are 1.229 and 1.217 Å in L-2Br and
The band observed ꢂ3300 cmꢃ1 in FT-IR spectra of L-1Br to L-
4Br (Table S1 in ESI ) is assigned for mN-H frequency, it is observed
at ꢂ3314 cmꢃ1 for L-1Br, ꢂ3267 cmꢃ1 for L-2Br, ꢂ3300 cmꢃ1 for
L-3Br and ꢂ3275 cmꢃ1 in L-4Br as shown in Fig. 1. The variation

102
S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
Table 3
Hydrogen bonding geometries in L-2Br and L-3Br.
Compound
DAH (Å)
Hꢁ ꢁ ꢁA (Å)
2.159(3)
2.115(3)
Dꢁ ꢁ ꢁA (Å)
2.838(4)
2.581(5)
\DAHꢁ ꢁ ꢁA (°)
135.6(2)
113.5(2)
L-2Br
L-3Br
NAHꢁ ꢁ ꢁO(1)a
0.860(3)
0.860(3)
NAHꢁ ꢁ ꢁO(4)intra
NAHꢁ ꢁ ꢁO(1)b
0.83(2)
2.430(2)
2.615(1)
2.590(2)
2.718(1)
2.110(2)
3.048(2)
3.309(2)
3.153(2)
3.389(2)
2.344(2)
132.0(2)
130.3(1)
127.0(2)
125.4(1)
115.0(2)
C(5)AH(5)ꢁ ꢁ ꢁO(4)b
NAHꢁ ꢁ ꢁO(4)c
0.950(1)
0.830(2)
0.990(1)
0.830(2)
C(11)AH(11A)ꢁ ꢁ ꢁO(4)c
NAHꢁ ꢁ ꢁO(4)intra
a
b
c
½ + x, 2 ꢃ y, z.
ꢃ1 + x, y, z.
1 ꢃ x, 2 ꢃ y, ꢃz.
Fig. 6.
p–p stacking interaction between dimeric chains of L-3Br between
quinonoid rings down b-axis.
L-2Br
L-3Br
Fig. 5. (a) Ladder like polymeric chain formed by CAHꢁ ꢁ ꢁO and NAHꢁ ꢁ ꢁO interac-
tions. (b) Dimeric chains through p–p stacking, CAHꢁ ꢁ ꢁO interaction of alkyl group
and NAHꢁ ꢁ ꢁO interaction.
1.230 Å and 1.216 Å in L-3Br (Tables S3 and S4 in ESI ). These dis-
tances are similar and typically represent the 1,4-naphthoquinone
carbonyl [20–22]. The C(3)AN bond distance is 1.338 Å in L-2Br,
1.321 Å in L-3Br and it is similar to that observed for amino substi-
tuted derivatives of 1,4-naphthoquinone [20a–e,22]. The
C(3)ANAC(11) bond angle, observed in L-2Br and L-3Br are
132.1(4)° and 131.5(1)° respectively (Tables S3 and S4 in ESI )
however these bond angles are more than the classical value for
Fig. 7. Overlay structures of L-2Br and L-3Br with respect to naphthoquinone ring.
bromine atoms looking like the eyes of the butterfly. The angle be-
tween the two planes of the benzenoid and the quinonoid rings in
the butterfly like structure is 82.35°. The molecules of L-2Br forms
a
polymeric chain via intermolecular NAHꢁ ꢁ ꢁO(NAHꢁ ꢁ ꢁO(1))
hydrogen bonding (Table 3, Fig. 4b). The molecules have twisted
orientation in the neighbouring chains and this is observed in
alternating chains.
sp3 hybridized nitrogen atom. L-3Br showed strong
tions through C(1), C(2) and C(4) of the adjacent chains of the qui-
nonoid moiety, such interactions is not observed in L-2Br.
p–p interac-
p–p
The derivative L-3Br crystallizes in the P-1 space group (Fig. 3b).
Oxygen O(4) of L-3Br showed multifurcated hydrogen bonding
[23] (Table 3). These molecules form a ladder like polymeric chain
through CAHꢁ ꢁ ꢁO and NAHꢁ ꢁ ꢁO interactions (Fig. 5a) via
C(5)AH(5)ꢁ ꢁ ꢁO(4) and NAHꢁ ꢁ ꢁO(1) respectively where the alkyl
groups are on the same side of every chain. The neighbouring
chains running in opposite direction to one another and are
The derivative L-2Br crystallizes in orthorhombic Pca21 space
group. An ORTEP view of L-2Br is shown in Fig. 3 a. The packing
of the molecules down ‘a’ axis is shown in Fig. 4a. The molecules
of L-2Br are pack in such way that it forms a beautiful butterfly like
structure down ‘a’ axis (Fig. 4a). The naphthoquinone moiety forms
the wings and the n-alkyl chain forms the tentacles with the

S. Salunke-Gawali et al. / Journal of Molecular Structure 1056–1057 (2014) 97–103
103
connected through
p
–
p
stacking as well as CAHꢁ ꢁ ꢁO interaction of
Appendix A. Supplementary material
the alkyl group and NAHꢁ ꢁ ꢁO interaction, thus forming a ladder like
dimeric chain (Fig. 5b). Interchain interactions are through
C(11)AH(11A)ꢁ ꢁ ꢁO(4) and NAHꢁ ꢁ ꢁO(4) (1 ꢃ x, 2 ꢃ y, ꢃz). When
viewed down the b-axis, quinonoid rings of every alternating
Crystallographic data have been deposited with the Cambridge
Crystallographic Data Centre and may be obtained on request
quoting the deposition number 953429 for L-2Br and 949769 for
L-3Br from the CCDC, 12 Union Road, Cambridge CB21EZ, UK
(Fax: +44 1223 336 033; E-mail address: deposit@ccdc.cam.ac.uk).
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2013.
10.016.
neighbouring chains are
p–p
stacked through C(4)ꢁ ꢁ ꢁC(1)
[3.375(2) Å] and C(4)ꢁ ꢁ ꢁC(2) [3.237(2) Å, ꢃx, 2 ꢃ y, ꢃz] of each
chain (Fig. 6).
Although the space group P-1 in L-3Br is same to that of single
crystal structures of chloro derivatives [24] viz 3-chloro-2-butyla-
mino-1,4-naphthoquinone and 3-chloro-2-octylamino-1,4-naph-
thoquinone, their
p–p stacking interaction differs. In L-3Br the
References
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The first four analogue of the homologated series 2-bromo-
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2012).