Synthesis, crystal structure of copper(II) complexes comprising 2-(biphenylazo)phenol and 1-(biphenylazo)naphthol ligands and their catalytic activity in nitroaldol reaction

Article abstract of DOI:10.1016/j.inoche.2017.06.027

New series of copper(II) complexes of the type [Cu(L)₂] (L = L₁–L₅) comprising bidentate 2-(biphenylazo)phenol (HL₁–HL₄) and 1-(biphenylazo)naphthol (HL₅) ligands have been synthesized. The composition of complexes and ligands (HL₁–HL₄) has been established by elemental analysis and spectral (FT–IR, UV–Vis, ¹H NMR and EPR) methods. Molecular structures of copper complexes [Cu(L₃)₂] (3) and [Cu(L₅)₂] (5) were established by X-ray crystallography. These Copper(II) biphenylazo complexes exhibit a very good catalytic activity towards nitroaldol reaction of various aldehydes with nitromethane.

Full text of DOI:10.1016/j.inoche.2017.06.027

Accepted Manuscript
Synthesis, crystal structure of copper(II) complexes comprising
2-(biphenylazo)phenol and 1-(biphenylazo)naphthol ligands and
their catalytic activity in nitroaldol reaction
Raja Nandhini, Madhan Ramesh, Ganesan Prabusankar, Galmari
Venkatachalam
PII:
DOI:
Reference:
S1387-7003(17)30365-9
doi: 10.1016/j.inoche.2017.06.027
INOCHE 6694
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
28 April 2017
16 June 2017
28 June 2017
Please cite this article as: Raja Nandhini, Madhan Ramesh, Ganesan Prabusankar, Galmari
Venkatachalam , Synthesis, crystal structure of copper(II) complexes comprising
2-(biphenylazo)phenol and 1-(biphenylazo)naphthol ligands and their catalytic activity
in nitroaldol reaction, Inorganic Chemistry Communications (2017), doi: 10.1016/
j.inoche.2017.06.027
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ACCEPTED MANUSCRIPT
Synthesis, crystal structure of copper(II) complexes comprising
2–(biphenylazo)phenol and 1–(biphenylazo)naphthol ligands and their
catalytic activity in nitroaldol reaction
Raja Nandhinia, Madhan Ramesha, Ganesan Prabusankarb, Galmari Venkatachalam*a
aPG & Research Department of Chemistry, Government Arts College, Dharmapuri- 636 705,
Tamilnadu, India.
bDepartment of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Sangareddy,
TS 502 285, India.
Corresponding Author: *E-mail address: gvchem@gmail.com [G. Venkatachalam]
Abstract
New series of copper(II) complexes of the type [Cu(L)2] (L = L1–L5) comprising bidentate
2–(biphenylazo)phenol (HL1–HL4) and 1–(biphenylazo)naphthol (HL5) ligands have been
synthesized. The composition of complexes and ligands (HL1–HL4) has been established by
elemental analysis and spectral (FT‒IR, UV‒Vis, 1H‒NMR and EPR) methods. Molecular structures
of copper complexes [Cu(L3)2] (3) and [Cu(L5)2] (5) were established by X-ray crystallography.
These Copper(II) biphenylazo complexes exhibit a very good catalytic activity towards nitroaldol
reaction of various aldehydes with nitromethane.
Keywords: Copper; Bidentate biphenylazo ligands; Crystal structure; Nitroaldol reaction

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Copper complexes are of considerable interest mainly due to their variety in coordination
geometry, spectroscopic properties and their biochemical significance [1–3]. Transition metal
complexes incorporating azo ligands have displayed several interesting properties related to
electron-transfer reactions [4], liquid crystals [5], photochromism [6] and organic transformations
[7,8]. The π-acidity and metal binding ability of azo nitrogen have drawn attention to the exploration
of the chemistry of metal complexes incorporating azo-ligands [9–14].
The Henry(nitroaldol) reaction is one of the classical C‒C bond forming reactions in
synthetic chemistry. The resulting products of β–nitroalcohols can be conveniently transformed into
various valuable building blocks, such 1,2–aminoalcohols, 2–hydroxy carboxylic acids and
conjugated nitroalkanes. The copper catalyzed Henry reaction performed at room temperature has
captured much focus in recent years [15]. Since the pioneering work of Shibasaki in 1992 [16] the
catalytic Henry reaction has received considerable attention and various types of metal-based
catalysts have been developed [17,18]. Although many catalytic systems have been reported, to the
best of our knowledge, only a very few studies on copper(II) Schiff base complexes in the catalytic
nitroaldol reaction [19] and there is no report for these copper(II) biphenylazo complexes to
catalyzed nitroaldol Henry reaction.
Hence, this work is focused on the synthesis and structure of copper(II) complexes
comprising 2–(biphenylazo)phenol and 1–(biphenylazo)naphthol ligands and their potential to
catalyze nitroaldol reaction of various aldehyde.

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R
OH
N
OH
N
N N
HL5
R = CH3 (HL1), OCH3 (HL2),
C(CH3)3 (HL3), Cl (HL4)
Fig 1. Structure of 2–(biphenylazo)phenol (HL1–HL4) and 1–(biphenylazo)naphthol
ligands (HL5).
Four 2–(biphenylazo)phenol (HL1‒HL4) [20] and 1–(biphenylazo)naphthol (HL5) [21]
ligands have been prepared and used in the present study. The brown colored copper(II) biphenylazo
complexes of the type [Cu(L)2] [L = L1‒L5] were achieved [22] by reacting 1:2 ratio of
Cu(OAc)2.H2O and biphenylazo ligands HL1‒HL5 (Fig 1) refluxed in ethanol for 3 h and simple
work up procedure by filtration (Scheme 1). These copper(II) complexes were obtained as crystalline
solid of yield up to 80%. The 1H-NMR spectra of the ligands have been recorded in CDCl3 solution.
All the ligands show multiplets around 6.7‒7.9 ppm for the presence of aromatic protons of
biphenylazo ligands. A sharp singlet appeared for OH proton of ligands in the region at 11.7‒11.9
ppm. The methoxy protons of the ligand (HL2) appeared as singlet at 3.8 ppm. The presence of the
methyl group and t-butyl group for the ligands HL1 and HL3 is appeared as singlet at 2.3 and 1.4
ppm respectively. Preliminary characterization by elemental analysis of the synthesized complexes
supports the expected composition. The structure of the copper complexes shows that the
2–(biphenylazo)phenol and 1–(biphenylazo)naphthol is coordinated to copper atom, via loss of
phenolic proton and azo-nitrogen, an a bidentate O,N-donor forming a six-membered chelate rings.

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R
O
N N
O
Cu
EtOH
Cu(OAc) H O
+
HL1-HL4
2.
2
N N
R
reflux/ 3h
R = CH3 (L1) (1), OCH3 (L2) (2),
C(CH3)3 (L3) (3), Cl(L4) (4)
N N
O
O
EtOH
Cu
HL
Cu(OAc)2.H2O
+
5
reflux/ 3h
N N
[Cu(L5)2] (5)
Scheme 1. Synthesis of Cu(II) complexes containing 2–(biphenylazo)phenol and
1–(biphenylazo)naphthol ligands
Infrared spectra of all the ligands displayed strong bands around 1490–1450 and
1275–1315 cm‒1 corresponding to ν(N=N) and phenolic ν(C–O) stretching, respectively. After
complexation, ν(N=N) bond is decreases and appears at 1360–1390 cm‒1 indicating the coordination
of azo nitrogen of copper metal ion. The coordination through phenolic oxygen is confirmed by the
increase of ν(C–O) bond at higher frequencies in the region 1290–1325 cm‒1 in all the complexes.
This is further supported by the disappearance of ν(OH) band in the range 3440–3457 cm‒1 in all the
complexes [23‒25]. The IR spectra of the complexes are provided in Fig S1–S13 (supporting
information). Electronic spectra of all the complexes recorded in chloroform solution, show several
intense absorptions in the ultraviolet and visible regions. The absorption in the ultraviolet region are

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attributable to transitions 330–250 nm which ligand orbitals and those in the visible region 400–475
nm are probably due to ligand-to-metal charge transfer transitions [26]. Representation spectra are
given in Fig S14–S18 (see supporting information). The solution EPR spectrum of the complex (5)
in the liquid nitrogen temperature and the ‘g’ value is around g = 2.106. These values indicate that
the unpaired electron lies predominantly in the dx2-y2 orbital of Cu(II) and the spectral features are
characteristics of axial symmetry. The representation EPR spectrum is shown in Fig S19.

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Fig 2. (i) The ORTEP picture of asymmetric unit of 3 with displacement ellopsolids drawn at
50% probability. (ii) The molecular structure of 3. (iii) The polyhedral view of copper core in 3.
Selected bond lengths [Å]: Cu–O = 1.904(3), Cu–N = 1.972(3), N–N = 1.298(3), N–C = 1.442(5),
N–C = 1.362(6) and selected bond angles [o]: O(1)–Cu(1) –O(1) = 88.74 (19), N(1)-Cu(1)–O(1) =
89.88(13), N(1)–Cu(1)–O(1) = 158.75(12), N(1)–Cu(1)–N(1) = 98.9(2), C(13)–O(1)–Cu(1) =
124.4(3), N(2)–N(1)–Cu(1) = 127.5(3), C(1)–N(1)–Cu(1) = 119.43(19), C(1)–N(1)–N(2) = 112.8(3),
C(14–N(2)–N(1) = 121.2(3).

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Fig 3. (i) The ORTEP picture of asymmetric unit of 5 with displacement ellopsolids drawn at
50% probability. (ii) The molecular structure of 5. (iii) The polyhedral view of copper core in 5.
Selected bond lengths [Å]: Cu–O = 1.904(3), Cu–N = 1.972(3), N–N = 1.298(3), N–C = 1.442(5),
N–C = 1.362(6) and selected bond angles [o]: O(1)–Cu(1) –O(1) = 88.74 (19), N(1)-Cu(1)–O(1) =
89.88(13), N(1)–Cu(1)–O(1) = 158.75(12), N(1)–Cu(1)–N(1) = 98.9(2), C(13)–O(1)–Cu(1) =
124.4(3), N(2)–N(1)–Cu(1) = 127.5(3), C(1)–N(1)–Cu(1) = 119.43(19), C(1)–N(1)–N(2) = 112.8(3),
C(14–N(2)–N(1) = 121.2(3).
Suitable X‒ray quality crystals of complexes [Cu(L3)2] (3) and [Cu(L5)2] (5) was grown by
slow evaporation of CH2Cl2 and EtOH solution. The molecular structure and important structure
parameters of 3 and 5 are depicted in Fig 2 and 3. Crystal data and structure refinement for molecule
3 and 5 is shown (Table S20 see supporting information). The molecule 3 and 5 crystalized in
triclinic space group, Pῑ monoclinic space group C2/c, respectively. 3 and 5 is a mononuclear Cu(II)
complex. The copper center in (5) is tetra coordinated. The geometry of copper can be described as
distorted square planar geometry. The sum of bond angles around copper is 360o. The coordination
sphere of copper is satisfied by two phenolic oxygen and two azo nitrogen atoms.
2–(biphenylazo)naphthol act as a bidentate and mono anionic ligand, which results in a
neutral copper complexes. As shown in Fig 2 & 3 (iii), the central copper atom and the ligands form
two six-membered rings, which increase the stability of the complex. These two rings are not in the
same plane. The CuO bond distances are comparable. Similarly the CuN bond distances are
comparable. The CuO and CuN bond distances found in 3 and 5 is comparable with that of
copper(II) Schiff base square planar complexes (1.902 Å and 1.964 Å) [27]. As shown in molecular
packing, week interactions between molecules are absent in Fig S21(a) and S21(b) (see supporting
information).

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Table 1. Different catalyst loadinga
OH
NO2
CHO
Cu-complex(1)
CH3NO2/3h
Entry
catalyst loading
(mol%)
Yield(%)b
TONc
5600
3400
1700
1
2
3
0.01
56
68
85
0.02
0.05
aSubstrate (0.5mmol), catalyst (0.05mol%) and nitromethane
(2.5mmol) stirred in CH2Cl2 at room temperature.
bIsolated yield after column chromatography.
cTON = moles of product per mole of catalyst.
Benzaldehyde has been chosen an a model substrate to explore the catalytic performance of
these copper(II) complexes in Henry nitroaldol reaction. This may be due to the poor Lewis acidity
of the catalytic system, which is insufficient for the formation of nitronate ion. The best results
observed when substrates were stirred for 3 h in presence of 0.0025mmol (0.05 mol%) of 1–5 at
room temperature. The reaction uses relatively mild reaction conditions and catalyst loading. In the
absence of catalyst no product was observed. Higher catalyst loading did help to drive the reaction to
minimum yield when 0.05 mol% catalyst was used (Table 1). As expected, desired reaction did not
proceeds at all in the absence of the catalyst.
Table 2. Nitroaldol reaction of nitromethane with various aldehydes by [Cu(L1-5)2] complexesa.
OH
CHO
NO2
[Cu(L1-5)2](0.05 mol %)
CH2Cl2/3h
CH3NO2
+

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Yield(%)b
__________________
Entry
Substrate
Product
Complex
TONc
____________________________
1
2
3 4 5
OH
CHO
NO2
NO2
1
85 75 76 76 78
170 150 152 152 156
OH
CHO
CHO
87 85 80 83 85
174 170 160 166 170
2
3
H3CO
H3CO
NO2
72 65 68 70 70
78 75 76 75 74
65 60 62 60 62
156 130 136 140 140
130 150 152 150 148
130 120 124 120 124
O
O
OH
NO2
4
N
CHO
CHO
N
OH
OH
H3CO
H3CO
NO2
5
HO
HO
aSubstrate (0.5mmol), catalyst (0.05mol%) and nitromethane (2.5mmol) stirred in CH2Cl2 at room
b
c
temperature. Isolated yield after column chromatography. TON = moles of product per mole of
catalyst.
Reactions were performed with various solvents like CHCl3, CH2Cl2, EtOH etc. Among the
solvents studied, CH2Cl2 was found to be the choice for these reactions. Under the optimized
reaction (CH2Cl2, room temperature) condition, different aldehydes were tested and results are
summarized in Table 2. It was found that these catalysts worked well for various aromatic aldehydes

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with generally good yields. The catalytic reaction was performed using Cu(OAc)2.H2O as catalyst
for nitroaldol reaction and obtained only 30% yield. The catalyst design required a weakly acidic
metal complex bearing moderately basic charged ligands that would facilitate deprotonation of
nitroalkanes and these copper(II) complexes were found to fulfill the requirements best. The product
nitroalcohol was formed after a work up procedure [28] characterization of representative isolated
products are given in Fig S22–S24 (see supporting information) and these copper(II) complexes
have been proved to be a good catalytic system for the Henry reaction. It was possible to re-use the
catalyst, which was recovered by the addition of ethanol to cool the reaction mixture followed by
filtration to isolate the catalyst. The isolated catalyst (1) was used for the nitroaldol reaction of
benzaldehyde under reaction conditions similar to that of fresh catalyst and gave same conversion
(85%).

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O
O
O
N
O
R
Cu
Cu
O
N
O
R
H
O
N
N N
H
N
O
CH2
CH2
b
a
O
N
O
CH2
H
O
O
R
O
Cu
Cu
N N
O
H
N N
N
CH2
O
c
O
N
OH
O
R
d
Scheme 2. Possible mechanism for [Cu(L)2] catalyzed Henry reaction
The possible mechanism for the Henry reaction catalyzed by [Cu(L)2] complex is explained
by the mechanistic pathway proposed by J.R. Pedro et al [29]. The reaction of copper(II) complexes
with nitromethane (a) and aldehyde can generate an intermediate complex (c) via intramolecular
addition of nitronate to aldehyde (b) followed by reaction with fresh nitroalkane. The intermediate
(c) can complete the catalytic cycle by reaction with fresh aldehyde (Scheme 2). The procedure
nitroalcohol (d) was formed after work up procedure.

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In conclusion, the present study shows that the biphenylazo ligands derived from different
para- substituted phenols and β–naphthol with 2–aminobiphenyl can readily coordinate with copper
as bi-dentate O, N donor mode. These new copper(II) complexes were characterized by UV‒Vis,
ESR and X–ray crystallography. The copper(II) complexes were used as catalysts for nitroaldol
reaction of various aldehydes with nitromethane and proved as potential catalysts.
Acknowledgment
The authors are thankful to Prof. G. Prabusankar IIT‒Hyderabad for allowing us to use single
crystal X–ray diffraction facility established through an outstanding investigator award.
Appendix A. Supplementary data
Single crystal X-ray study, crystal data and structure refinement parameters for the molecule
3 and 5, FT–IR, UV–Vis, and EPR spectra of the complexes and FT–IR and 1H–NMR spectra of the
ligand are associated with this article. CCDC 1553019 (3) and 1544955 (5) contains the
supplementary crystallographic data for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Center via www.ccdc.cam. ac.uk/data_request/cif or from the
Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223336, 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found in the online version,

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2-aminobiphenyl (2.32 g, 13.745 mmol) was dissolved in 5 mL conc. HCl and 10 mL distilled
water and cooled to 0–5 °C. Sodium nitrite (2.276 g, 32.99 mmol) was dissolved in minimum
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sodium hydroxide (3.0 g, 28.30 mmol). Red precipitate of 2–(biphenylazo)phenols was
separated out and then recrystallized from hot methanol.

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Spectral data for the ligands (HL1–HL4): (HL1) M.p. 122oC, FT‒IR data (cm-1): 3424 ν(O-H),
1
1439 ν(N=N); 1267 ν(C‒O). H NMR (CDCl3, 300 MHz): δ (ppm) = 11.9 (s, OH), 6.8‒7.9
(m, Ar-H), 2.3 (s. CH3); (HL2). M.p. 90oC, FT‒IR data (cm-1): 3410 ν(OH), 1426 ν(N=N);
1253 ν(C‒O). 1H NMR (CDCl3, 300 MHz): δ (ppm) = 11.7 (s, OH), 6.7‒7.9 (m, Ar-H), 3.8 (s,
1
OCH3); (HL3). M.p. 98oC, FT‒IR data (cm-1): 3425 ν(O-H), 1432 ν(N=N) 1267 ν(C‒O); H
NMR (CDCl3, 300 MHz): δ (ppm) = 11.9 (s, OH), 6.8‒7.9 (m, Ar-H), 1.4 (s, (CH3)3C); (HL4).
1
M.p. 92oC, FT‒IR data (cm-1): 3478 ν(O-H), 1434 ν(N=N); 1270 ν(C‒O). H NMR (CDCl3,
300 MHz): δ (ppm) = 11.7 (s, OH), 6.7‒7.9 (m, Ar-H).
[21] M. Ramesh, G. Prabusankar, G.Venkatachalam, Inorg. Chem. Commun, 79 (2017) 89.
[22] General procedure for the synthesis of complexes: The metal precursor Cu(OAc)2.H2O (0.05g)
and biphenylazo ligands (HL1‒HL5) (0.165g) were taken in 20ml of ethanol. The reaction
mixture was heated under reflux for 3h, and then cooled to room temperature and solids, if any
were filtered off. The precipitated solid was filtered and washed with methanol (5 ml) then
dried in air and collected. The copper(II) complexes were obtained as crystalline solid of yield
up to 80%.
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[27] C. Dominik, K. Branko, Cryst. Eng. Comm, 13 (2011) 4351.
[28] To a solution of copper(II) complexes (0.0025mmol) in dichloromethane was added
nitromethane (2.5mmol) and the mixture was stirred. Aldehyde (0.5mmol) was added and

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refluxed for 3h. The volatile components were removed in vaccuo and the crude products were
purified by the column chromatography.
[29] G. Blay, E. Climent, I. Ferandez, V.H. Olmos, J.R. Pedro, Tetrahedron: Asymmetry 18 (2007)
1603.

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GRAPHICAL ABSTRACT

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Highlights
 New Cu(II) 2‒(biphenylazo)phenolate and 1‒(biphenylazo)naphtholate complexes have been
synthesized.
 Characterization of ligands and complexes were performed by analytical and spectral
methods.
 Structure of the complexes (3 & 5) was confirmed by single crystal XRD.

Synthesized complexes proved as efficient catalyst (0.05 mol%) for nitroaldol reaction.