A family of ligand and anion dependent structurally diverse Cu(II) Schiff-base complexes and their catalytic efficacy in an O-arylation reaction in ethanolic media

Article abstract of DOI:10.1039/c5ra14758a

Two nitrato bridged dinuclear systems [Cu₂(L1)₂(NO₃)₃]NO₃·H₂O (1) and [Cu₂(L2)₂(NO₃)₃]NO₃·MeOH (2), five monomeric complexes viz. [Cu(L3)(NO₃)]NO₃ (3), [Cu(L4)(NO₃)]NO₃ (4), [Cu(L5)(NO₃)]NO₃ (5), [Cu(L6)(NO₃)NO₃] (7), [Cu(L7)(NO₃)]NO₃ (8) and one hetero bi-bridged (phenoxido and water) dinuclear complex [Cu₂(L2)₂(H₂O)₂](ClO₄)₄·4H₂O (6) have been synthesized and characterized using several physicochemical methods (L1 = 1-(N-3-methoxysalicylideneimino)-ethane-2-piperazine, L2 = 1-(N-3-ethoxysalicylideneimino)-ethane-2-piperazine, L3 = 1-(N-4′-ethoxy-α-methylasalicylideneimino)-ethane-2-piperazine, L4 = 1-(N-5′-chloro-α-methylasalicylideneimino)-ethane-2-piperazine, L5 = 1-(N-5-chlorosalicylideneimino)-ethane-2-piperazine, L6 = 1-(N-4-methoxysalicylideneimino)-ethane-2-piperazine and L7 = 1-(N-4′-methoxy-α-methylasalicylideneimino)-ethane-2-piperazine). X-ray structural analysis showed that complexes 1 and 2 are discrete dinuclear species where the pentacoordinated metal centers are bridged through a nitrate ion. In 3, 4, 5 and 8 the monomeric copper center displays a square pyramidal geometry with a weak axial Cu-O bond. In 7, the monomeric copper center shows a distorted octahedral geometry with two coordinated nitrate anions. However, in 6 the two copper centers coordinate in different manners (one is square-pyramidal and the other is distorted octahedral) and are bridged through a phenoxido group and a water molecule. All complexes efficiently catalyze the C-O coupling reaction under homogeneous conditions at 80 °C to afford unsymmetrical diaryl ethers using nitroarenes to act as an excellent electrophile. Notably, the reaction is carried out in ethanol media which facilitates the avoidance of toxic wastes. Structurally diverse copper(ii) Schiff-base complexes have rarely been used systematically in catalytic C-O coupling reactions.

Full text of DOI:10.1039/c5ra14758a

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A family of ligand and anion dependent structurally diverse Cu(II)
Schiff–base complexes and their catalytic efficacy in O–arylation
reaction in ethanolic medium†
a
a
a
b
a
Tanmoy Maity, Debraj Saha, Susmita Bhunia, Paula Brandão, Soma Das and
a
Subratanath Koner*
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
(E–mail: snkoner@chemistry.jdvu.ac.in)
b
Department of Chemistry, University of Aveiro, 3810–193 Aveiro, Portugal
Abstract
Two nitrato
Cu (L2) (NO ) ]NO .MeOH (2); five monomeric complexes viz. [Cu(L3)(NO )]NO (3),
bridged
dinuclear
systems
[Cu (L1) (NO ) ]NO .H O
(1)
and
2
2
3 3
3
2
[
2
2
3 3
3
3
3
[
Cu(L4)(NO )]NO (4), [Cu(L5)(NO )]NO (5), [Cu(L6)(NO )NO ] (7), [Cu(L7)(NO )]NO (8)
3
3
3
3
3
3
3
3
and
one
hetero
bi–bridged
(phenoxido
and
water)
dinuclear
complex
[
Cu (L2) (H O) ](ClO ) .4H O (6) have been synthesized and characterized by several
2 2 2 2 4 4 2
physicochemical methods (L1 = 1–(N–3–methoxysalicylideneimino)–ethane–2–piperazine, L2 =
1
–
(N–3–ethoxysalicylideneimino)–ethane–2–piperazine,
L3
=
1–(N–4ˊ–ethoxy–α–
1–(N–5ˊ–chloro–α–
methylasalicylideneimino)–ethane–2–piperazine,
methylasalicylideneimino)–ethane–2–piperazine, L5
ethane–2–
L4
=
=
1–(N–5–chlorosalicylideneimino)–
†
Electronic supplementary information (ESI) available. CCDC data 1010924–1010929 for
complex (1–6) and 1401268 for complex 7 can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam. ac.uk/data_request/cif.

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piperazine, L6 = 1–(N–4–methoxysalicylideneimino)–ethane–2–piperazine and L7 = 1–(N–4ˊ–
methoxy–α–methylasalicylideneimino)–ethane–2–piperazine). X–ray structural analysis showed
that complexes 1 and 2 are discrete dinuclear species where the pentacoordinated metal centers
are bridged through nitrate ion. In 3, 4, 5 and 8 monomeric copper center experience a square
pyramidal geometry with a weak axial Cu–O bond. In 7 monomeric copper center experience
distorted octahedral geometry with two coordinated nitrate anions. However, in 6 two copper
center coordinates in different manner (one square–pyramidal and other is distorted octahedral)
and bridged through a phenoxido group and a water molecule. All complexes efficiently catalyze
C–O coupling reaction under homogeneous condition at 80 °C to afford unsymmetrical diaryl
ethers using nitroarenes act as an excellent electrophile. Notably, the reaction is carried out in
ethanol medium which facilitates to avoid toxic wastes. Structurally diverse copper(II) Schiff–
base complexes have rarely been used systematically in catalytic C–O coupling reaction.
1. Introduction
Diaryl ethers are ubiquitous structural motifs that represent a large number of natural
biologically active compounds (K13, perrottetin, teicoplanin and vancomycin) (Scheme 1) and
1
–8
they are also important in polymer industries.
Classically, these compounds have been
synthesized via copper–catalyzed Ullmann type cross–coupling reactions of aryl halides and
9
–11
phenols at very high temperature (125‒220ºC) that requires stoichiometric amount of catalyst.
Low to moderate yields are often obtained following these classic method. Recently attempts
have been made to develop of new synthesis route of diaryl ethers under milder reaction
6
,10–22
condition.
cross–coupling reactions under mild condition.
being highly active their prospect in application in large scale reaction for industrial production
Buchwald, Hartwig and Beller developed palladium catalyzed processes for C–O
2
3–28
In spite of palladium catalyzed methods are

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is limited owing to high expense of palladium and its toxicity as well as need of using
2
9,30
phosphorated ligands in the process.
In 1998, Chen, Lam and Evans developed a new route to
build carbon–heteroatom bonds, using arylboronic acid instead of aryl halides which produces
3
1–33
undesirable by–products.
This problem is difficult to mitigate as phenylboronic acids release
water through triphenylboroxime formation and enhances phenol formation from phenylboronic
3
4–38
acid that leads to competition between phenol and phenolic derivatives for O–arylation.
Copper–based catalysts have been developed for O–arylation reaction of nitroarenes to achieve
2
,7
diaryl ethers. In this study we demonstrated that cheap nitroarenes resolve the problem of
unwanted by–product formation.
Selection of suitable metal salts and multidentate ligands are very important as they play
39–42
dominant role in the control and adjustment of the architecture of coordination complexes.
Coordination chemistry of Schiff–base ligand is particularly interesting as selection of suitable
amine and aldehyde/ketone, which can afford steric and electronic influence originated from
4
3–46
different substituents groups, may create structural and functional variations.
These types of
4
7–48
49–50
structural diversity have significant impact in the field of catalysis,
magnetism,
and
5
1
DNA cleavage study. Ramadan et al. studied oxidase catalytic activity of mononuclear and
4
7
dinuclear Cu(II) complexes towards the aerobic oxidation of vitamin C. Enantioselective
cyclopropanation of styrene is efficiently catalyzed by monomeric and dimeric Cu(II) chiral
4
8
Schiff–base complexes. Lin et al. used structurally diverse copper complexes as catalyst for
52
copolymerization of carbon dioxide and cyclohexene oxide. Copper(II) Schiff–base complexes
containing Cu O moiety have also been investigated exhaustively with reference to their
2
2
5
3–58
catecholase activity.
We have employed copper(II) Schiff–base complexes in catalytic olefin
5
9–62
epoxidation reaction in homogeneous and heterogeneous conditions.
Nevertheless, catalytic

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efficacy of Cu(II) Schiff–base complexes vis–à–vis their structural diversities has rarely been
explored. We are particularly interested to explore the catalytic efficacy of structurally diverse
Schiff–base copper complex in coupling reaction.
We report here the syntheses and crystal structures of a family of structurally diverse
copper(II) Schiff–base complexes and their application in catalytic O–arylation reaction. All the
complexes efficiently catalyze O–arylation reaction of phenolic derivatives with nitroarenes in
ethanol medium under milder condition. For practical applications, copper(II)–based catalysts
are promising alternative by virtue of their low cost and insensitivity to air as well as their ability
to cut down undesirable byproducts in O–arylation reaction. To our knowledge, there have been
only very few reports in which copper(II) Schiff–base complex being used to catalyze C–O
6
3–64
coupling reaction.
2
. Experimental section
.1. Materials
2
N–(2–ethylamino)piperazine, o–vanilline, 3–ethoxysalicylaldehyde, 5–chlorosalicylaldehyde,
ˊ–ethoxy–2ˊ–hydroxyacetophenone, 5ˊ–chloro–2ˊ–hydroxyacetophenone, 4–
4
methoxysalicylaldehyde and 4ˊ–methoxy–2ˊ–hydroxyacetophenone were purchased from
Aldrich. Solvents (analytical grade), cesium carbonate, copper(II) nitrate trihydrate, copper(II)
perchlorate hexahydrate, substituted phenols and other chemicals were purchased from Merck
(
India) Pvt. Ltd. Solvents were dried before use.
.2. Physical measurements
2
Elemental analysis was performed using the Perkin–Elmer 240C elemental analyzer. Fourier
transform–infrared spectra of KBr pellets were measured on a Perkin–Elmer SPECTRUM II

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FTIR spectrometer. The UV−Vis spectral measurements were carried out using a Shimadzu
UV−Vis 1700 spectrophotometer. Thermogravimetric differential thermal analysis (TG–DTA)
measurements were performed using a PerkinElmer (Singapore) Pyris Diamond TG/DTA unit.
−1
The heating rate was programmed at 4 °C min with a protecting stream of N flowing at a rate
2
−
1
of 150 mL min . The powder X–ray diffraction (XRD) patterns of samples were recorded on a
1
13
Bruker D8 Advance diffractometer using CuKα radiation (λ = 1.54Å). H and C NMR spectra
were measured on a Bruker Avance DPX 300 NMR (300 MHz) and 600 MHz spectrometer.
Mass spectra were measured on a Waters XEVO–G2QTOF#YCA351 high resolution Mass
Spectrometer.
Caution! Perchlorate salts of metal complexes coordinated with organic ligands are potentially
explosive. It should be handled with care and only a small amount of material should be
prepared.
2.3. Synthesis of the Schiff–base ligands L1, L2, L3, L4, L5, L6 and L7
The Schiff–base L1 was prepared by mixing 20 mL ethanolic solution of N–(2–
ethylamino)piperazine (0.491 g, 4 mmol) and o–vaniline (0.608 g, 4 mmol) together in a flat
bottom flask. The mixture was then refluxed for 30 min. The resulting yellow colored solution
was then cooled to room temperature. Ethanol was then separated almost completely from the
mixture using a rotary evaporator. The amine N–(2–ethylamino)piperazine was mixed with 3–
ethoxysalicylaldehyde,
4ˊ–ethoxy–2ˊ–hydroxyacetophenone,
5ˊ–chloro–2ˊ–
hydroxyacetophenone, 5–chlorosalicylaldehyde, 4–methoxysalicylaldehyde and 4ˊ–methoxy–2ˊ–
hydroxyacetophenone to synthesize ligands L2, L3, L4, L5, L6 and L7 respectively. All ligands
1
13
were characterized by H and C NMR, HRMS and elemental analysis (see ESI).
.4. Synthesis of the complex 1
2

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A 10 mL ethanolic solution of Cu(NO ) .3H O (0.24 g, 1 mmol) was added dropwise to a 10 mL
3
2
2
ethanolic solution of L1 (1 mmol). The resulting deep green mixture was kept undisturbed
without stirring at room temperature. On slow evaporation of the filtrate, dark green cube shaped
crystals appeared in a day. They were collected by filtration and washed first with mother liquor
then with few drops of diethylether. Yield ca. 81% based on the metal. The phase purity of bulk
1
was confirmed by powder XRD (Fig. S14, ESI) and elemental analysis. Elemental analysis
Calcd. C 35.94, H 4.74, N 14.97; found C 35.9, H 4.7, N 14.9.
.5. Synthesis of the complexes 2–8
2
All complexes were synthesized by following the similar procedure to that of 1, using L2, L3,
L4, L5, L6 and L7, respectively, for complexes 2, 3, 4, 5, 7 and 8 instead of L1. The crystals
which separated were deep green in color. Complex 6 was prepared by same procedure only
selecting L2 as ligand and Cu(ClO ) .6H O instead of Cu(NO ) .3H O. The single crystals of 6
4
2
2
3 2
2
were light green in color. Yield of these complexes are ca. 86, 73, 76, 57, 79 and 83%
respectively based on the metal. The phase purities of the complexes were confirmed by powder
XRD (Fig. S15–21, ESI) and elemental analysis. Elemental analysis for 2 Calcd. C 38.71, H
5.24, N 14.56; found C 38.7, H 5.2, N 14.5; for 3 Calcd. C 40.21, H 5.26, N 14.62; found C 40.2,
H 5.2, N 14.6; for 4 Calcd. C 35.83, H 4.30, N 14.92; found C 35.8, H 4.2, N, 14.9; for 5 Calcd.
C 34.29, H 3.98, N 15.38; found C 34.3, H 3.9, N 15.4; for 6 Calcd C 30.80, H 4.83, N 7.18;
found C 30.8, H 4.8, N 7.1; for 7 Calcd C 37.31, H 5.22, N 14.50; found C 37.3, H 5.2, N 14.4).
2.6. X–ray crystallography
X–ray diffraction data for all compounds were collected on a Bruker Smart Apex CCD X–ray
diffractometer using graphite–monochromated Mo–Kα radiation (λ = 0.71073 Å). Integrated
6
5
intensities and cell refinement were determined with the SAINT software package using a

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6
6
narrow–frame integration algorithm. An empirical absorption correction (SADABS) was
applied. The structure was solved by direct methods and refined using a full–matrix least–
squares technique against F2 with the anisotropic displacement parameters for non–hydrogen
6
7
atoms, with the programs SHELXS97 and SHELXL97. Hydrogen atoms were placed at
calculated positions using suitable riding models with isotropic displacement parameters derived
from their carrier atoms. In the final difference Fourier maps there were no noteworthy peaks
other than the ghost peaks surrounding the metal centers. A summary of the crystal data and
relevant refinement parameters are given in Table 1.
2.7. Catalytic reactions
Fixed amount of Cu(II) complex (1 mol%) was added to a round–bottom flask that contained a
solution of 4–nitrobenzaldehyde (0.151 g, 1 mmol), phenol (0.094 g, 1 mmol) and K CO (0.326
2
3
g, 1 mmol) in ethanol (4 mL). The reaction mixture was refluxed at 80 °C for 8 h. The reaction
conversion was monitored by TLC (thin layer chromatography) method. After 8 h the reaction
mixture was cooled to room temperature and the mixture was extracted with water and diethyl
ether (2 × 15 mL). The organic layers thus collected were combined and washed with brine,
dried over anhydrous Na SO , and concentrated in vacuo. The residue was purified by column
2
4
chromatography on silica gel (mesh 60−120) using an n–hexane/ethyl acetate mixture as the
1
13
eluent to give the desired product. The product was characterized by H, C–NMR, HRMS and
elemental analysis then compared with literature data. To study progress of the reaction mixtures
were collected at desired interval and products have been isolated following the above procedure
(Fig. 3 and 4).
3
. Results and discussion
.1. Synthesis
3

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The Schiff–base ligand L1 and L2 were prepared by the 1:1 condensation of 3–
methoxysalicylaldehyde and 3–ethoxysalicylaldehyde with N–(2–ethylamino)piperazine in
ethanol solution respectively. Ethanolic solution of the ligand was allowed to react with
copper(II) nitrate salt to prepare bis–complexes, [Cu (L1) (NO ) ]NO .MeOH (1) and
2
2
3 3
3
[
Cu (L2) (NO ) ]NO .MeOH (2). Absence of any electron donating functional group at 3–
2 2 3 3 3
position of salicylaldehyde and/or 2–hydroxyacetophenone produced monomer complexes, e.g.
[
(
Cu(L3)(NO )]NO (3), [Cu(L4)(NO )]NO (4), [Cu(L5)(NO )]NO (5), [Cu(L6)(NO ) ]MeOH
3 3 3 3 3 3 3 2
7) and [Cu(L7)(NO )]NO (8). Here nitrate anions also have some contribution for structure
3
3
direction. It shows three different modes in 1 and 2, as a bridging agent, terminal capping and
also as a charge balancing ion. In 3–5, 7 and 8 it only acts as a capping and charge balancing
agent. In all the above seven complexes Schiff–base ligand showed tridentate (NNO donor)
coordination. When copper(II) perchlorate salt was used instead of copper(II) nitrate L2 affords a
hetero–bridged dinuclear complex, [Cu (L2) (H O) ](ClO ) .4H O (6), where L2 acts as both
2
2
2
2
4 4
2
tri– and tetra–dentate (NNO and N O donor, respectively) chelating ligand. Besides, phenoxide
2
2
and water oxygen bridged two different copper centers. In 6 water molecules are in three
different environments, as bridging or simple coordinating ligand and as water of crystallization.
In all Schiff–base ligands proton liberated from phenolic OH group bonded to nitrogen atom of
piperazine ring to form Zwitter ionic neutral molecule. Synthetic routes of complexes (1–8) are
given in Scheme 2.
3
.2. Thermal analysis
Thermogravimetric analysis confirmed that complexes 1–5 and 7 were thermally stable up to ~
85 °C as shown in Fig. S1 and S2 (see ESI). The TG curve indicated that 1 and 2 starts to lose
water molecules from the beginning and mass loss of approximately 2.02 and 1.91% have shown
1

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upto a temperature 140 °C. This mass loss was well in agreement with the theoretical values 1.93
and 1.87 % for 1 and 2 respectively, corresponds to the loss of one molecule of crystalline water.
Corresponding DTA (differential thermal analysis) curve showed endothermic peak for both
compounds at ~ 75 °C. Compounds 3–5 and 7 experienced no such loss for deaquation, however,
after 180 °C (230 °C for 7) all the compounds showed continuous mass loss up to 800 °C due to
decomposition showing two exothermic peaks at ~ 230 °C and ~ 410 °C in DTA curves.
Thermogravimetric analysis of complex 6 was performed upto a temperature 200 °C, further
heating was avoided as it contains explosive perchlorate anions (Fig. S2, see ESI). TG curve of 6
showed mass loss from room temperature which continued up to 200 °C. This mass loss
corresponds to the liberation of water of crystallization and thereafter coordinated water
molecules. Corresponding DTA curve showed two broad endothermic peaks at ~ 62 and 148 °C.
3.3. Spectroscopic analysis
In the IR spectra of complexes 1–8 the characteristic vibration band of azomethine ʋ(C=N) group
–1
were observed at around 1600 cm (Fig. S3 and S4). Peaks observed in the range 3060–2610
–
1
cm were due to the stretching vibration of methyl (–CH ), methylene (–CH –) and aromatic C–
3
2
–
1
H bonds. The appearance of strong band for the phenolic C–O group near 1210 cm in IR
spectrum of the compounds indicates the presence of the phenolic oxygen atom. The presence of
–
1
–1
a broad band at ca. 3450 cm for 1–2 and 3500 cm for 6 indicates the presence of water
molecules. In complexes 1–5, 7 and 8 the characteristic, strong peaks for stretching vibrations of
–
1
–1
–1
nitrate molecules appeared at ~ 1440 cm (ʋ ), 1380 cm (ʋ ) and 1020 cm (ʋ ). Two strong
5
1
2
–
1
–1
peaks appearing at 1100 cm (ʋ ) and 915 cm (ʋ ) for 6 were ascribed to the characteristic
3
4
vibration bands of uncoordinated perchlorate anions.

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0
Electronic absorption spectra of all the complexes and ligands were recorded in ethanol
medium (Fig. S5–S7, see ESI). The free ligand L1 showed three intra–ligand charge transfer
bands at 416, 292 and 220 nm, which were significantly shifted to lower wavelength in complex
1
(Fig. S6, see ESI). Ligand L2 showed intra–ligand charge transfer bands at 370, 293 and 218
nm, amongst them first band were shifted to lower wavelength and other bands to higher
wavelength upon complexation both in case of 2 and 6 (Fig. S6, see ESI). Rest of the ligands L3,
L4, L5, L6 and L7 also showed three intra–ligand charge transfer bands; amongst them first two
bands were shifted to lower wavelength and other band remain in the same position in complexes
3
, 4, 5, 7 and 8. The spectra showed a single absorption band at 639, 638, 620, 624, 641, 632,
641 and 622 nm for complexes 1–8, respectively. The position of these bands were typical of d–d
68
transition for Cu(II) complex. For all the complexes bands appearing in the region 290–310 nm
6
9
were attributed to a π–π* transition within the ligand and the other bands appeared around 390
nm was due to L → M charge transfer transition.
3.4. Crystal structure of complexes (1–7)
Complexes 1 and 2 containing 3–methoxy– and 3–ethoxy– derivatives of Schiff–base are
isostructural. The crystal structures confirm the chair conformation of the piperazine ring in both
complexes. N O donor Schiff–base binds Cu(II) centers in a tridentate manner (Fig. 1). Both
2
complexes crystallize in the monoclinic space group C2/c and consist of a centrosymmetric
+
dinuclear cationic [Cu (Lx) (NO ) ] (x = 1 or 2) moiety as shown in Fig. 1a and 1b respectively.
2
2
3 3
Selected bond distance and angles are collated in Table S1 (see ESI). Copper centers feature a 5
coordinated square pyramidal geometry in both compounds as Addison parameter of (τ = |b –
a|/60° where b and a are the two largest angles around the central atom; τ = 0 and 1 for perfect
square pyramidal and trigonal bipyramidal geometries, respectively) 1 and 2 are 0.022 and 0.038

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1
7
0
respectively. The axial position is occupied by oxygen atom (O(6)) of bridged nitrato group
with the long distance (Table S1). Three donor atoms (O(2), N(1) and N(2) in 1 and O(1), N(1)
and N(3) in 2) from chelating ligand occupy three equatorial positions and other one is occupied
by oxygen atom (O(3)) from terminal nitrate group. Apart from that a weak interaction is found
from oxygen atom (O(4)) of terminal nitrato group at the distances 2.734(7) and 2.651(4) for 1
and 2, respectively. Intermolecular H–bonding gives further stability to both 1 and 2 (Table S4,
see ESI). Fig. S8 and S9 shows both 1 and 2 afford a 2D supramolecular network.
Structure determination reveals that complex 3 consists of cationic monomer (Fig. 2a)
and it crystallizes in a monoclinic space group P2 /c. The selected bond lengths and angles are
1
summarized in Table S2. The copper center is five–coordinated with an elongated square–
pyramidal (4+1) geometry. A oxygen atom (O(23)) from the nitrato group coordinates axially at
a very long distance 2.5440(15) Å. The Addison parameter of the copper center is 0.069,
indicating its square pyramidal geometry. Copper(II) complex with similar structure was
observed in case of Schiff–base derived from 4ˊ–methoxy–2ˊ–hydroxyacetophenone (complex 8
7
1
in our case) instead of 4ˊ–ethoxy–2ˊ–hydroxyacetophenone. Complex 3 gains further
stabilization through intermolecular H–bonding and a 1D supramolecular chain has been found
(
Table S4, see ESI) where, uncoordinated anionic nitrate molecule plays important role to form
this zigzag chain (Fig. S10).
Single crystal X–ray structure determination reveals that complexes 4 and 5 are also
cationic monomer like 3 and crystallizes in a monoclinic space group P2 /c (Fig. 2b and 2c
1
respectively). In both of them Cu(II) ion features a pentacoordinated square pyramidal geometry
where a oxygen atom (O(20), O(19) for 4 and 5 respectively) of the coordinated nitrato group
occupies the axial position with a long distance of 2.550(3) and 2.5440(15) Å (Table S2). The

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2
Addison parameters of the copper centers are 0.067 and 0.108 for 4 and 5 respectively, indicating
their square pyramidal geometry. Both 4 and 5 forms 1D zigzag supramolecular structure
through intermolecular H–bonding like complex 3 (Table S4, Fig. S11 and S12 respectively; see
ESI).
Complex 6 consist discreate dinuclear unit of copper(II) (Fig. 2d). Selected bond
parameters and angles are given in Table S3. The complex contains two different [CuL2] units,
two coordinated water molecules and four uncoordinated perchlorate anions with four crystalline
water molecules. Cu(1) atom features a pentacoordinated coordination sphere with square–
pyramidal geometry. The equatorial plane is formed by the two nitrogen atoms N(12) and N(15),
and the bridged phenoxido oxygen atom (O(1)) of one Schiff–base (tetradentate). An oxygen
atom O(41) from the bridged water molecule completes the basal plane. Another terminal water
molecule (O42) occupies the axial position at a distance 2.233(3) Å (Fig. 2d). The Addison
parameter of Cu(1) is 0.052, indicating its square pyramidal geometry. The basal positions of
octahedral Cu(2) atom is occupied by two nitrogen atoms N(32) and N(35), and a phenoxido
oxygen atom O(21) of neighboring Schiff–base ligand. The fourth position of the equatorial
plane and one elongated epical position are occupied by the bridged phenoxido oxygen atom
(O(1)) and ethoxide oxygen atom (O(8)) of the tetradentate Schiff–base ligand. The bridged
oxygen atoms of water molecule (O(41)) coordinates to the elongated side to complete the
distorted octahedral geometry around Cu(2) with the bond distance 2.493(2) Å. The CuꢀꢀꢀꢀCu
distance in 6 is much closer (3.227) than other two dimers 1 and 2 (6.08 and 6.18, respectively).
In 1 and 2 nitrates act as ꢁ1,3 bridging ligand but in 6 phenoxide and water has no other option
than to bind in ꢁ1,1 bridging mode. This probably is the cause for two copper centers to come
closer. Crystalline water and perchlorate anions give the complex further stability through inter–

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3
molecular H–bonding (Table S4, see ESI). Numerous examples of mixed bi–bridged complexes
containing phenoxido, alkoxido or hydroxido ligand featuring Cu O or Cu O moieties have
2
2
2
3
been reported so far, however, there is no example of Cu(II) Schiff–base complex in which two
copper centers are bridged through mixed phenoxido/water bridges. In this count complex (6) is
a unique example.
Last member of the family, compound 7 crystallizes in a monoclinic space group P2 /c.
1
Copper center is five–coordinated with square–pyramidal geometry where three donor atoms of
the Schiff–base ligand (O(4), N(2) and N(3)) occupy the basal plane. An oxygen atom (O(1)) of
coordinated nitrato group binds copper center to complete the equatorial plane. Second oxygen
atom (O(8)) from second coordinated nitrato group coordinates axially at a long distance
2.484(3) Å. The structure is quite similar with compounds 3, 4 and 5, but the difference is in
weak interaction originates from oxygen atom (O(3)) of one nitrate, at a distance 2.734(7),
coordinates axially to copper center. This minute structural change in 7 generates 2D
supramolecular structure through H–bonding (Table S4, Fig. S13 see ESI).
3.5. Catalytic activity study
Initially, optimization studies for C–O coupling reaction was undertaken using p–methylphenol
and p–nitrobenzaldehyde as substrates and complex 1 as catalyst under various reaction
conditions as given in Table 2. To begin with we explored the effect of solvents since solvent
plays important role in transition–metal catalyzed transformation. Among different solvents,
highest yield of product was obtained in DMF (entry 1). Though, ethanol was selected (entry 14)
for environmental concern. It was generally found that O–arylation reaction was much faster
72
with Cs CO than with K CO or Na CO due to higher solubility of Cs CO . However, use of
2
3
2
3
2
3
2
3
K CO will be more economic than Cs CO for large scale production. Other bases such as KOH,
2
3
2
3

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4
Na CO , CH COONa, tert–BuOK, and DABCO showed slower formation of 4–formayl–4ˊ–
2
3
3
methyldiphenylether after 8 hour (entries 8–12). Cheap K CO showed satisfactory yield and has
2
3
been chosen for the O–arylation reaction (entry 14). Isolated yield increased sharply with the
increase of reaction temperature (entries 14–16). The copper concentration in the reaction
mixture was another important factor to investigate. Thus, the O–arylation reaction was carried
out in the presence of 0.1, 0.2, 0.5, 1 and 2 mol% of catalyst (entries 17–21). A maximum
conversion was achieved with 1 mol% of catalyst in 8 hours as shown in Fig. 3 and 4. Notably
isolated yields were not up to the mark in lower concentration of catalyst. Besides, increasing in
catalyst concentration beyond 1 mol% the reaction rate was not enhanced significantly. No
reaction occurred in the absence of catalyst in 8 hours (entry 13) as well as no induction period
was observed in all the reactions (Fig. 3 and 4). A mixture of copper nitrate salt and ligand L1 as
well as copper nitrate salt itself has been used as catalyst which afforded a moderate to low
conversion in coupling reaction (Table 2; entry 21 and 22). Thus the optimum condition of the
catalytic reaction was as follows: K CO (base), ethanol (solvent), 1 mol% catalyst and reaction
2
3
temperature 80 °C.
Under the optimized reaction conditions, the scope and applicability of the coupling
reaction using p–nitrobenzaldehyde with substituted phenol to unsymmetrical diaryl ethers were
investigated (Table 3). At first, impact of electronic properties of the aryl moiety of phenols on
the yield of catalytic reactions was evaluated using various substitutions on phenol moiety.
Results obtained in test reactions demonstrate that electronic properties of phenols affect the
product yield (Table 3, entries 1–5). Substituted phenols possessing electron donating methyl
group accelerates the reaction to yield product in higher amount in comparison to simple phenol.
Though, p–methylphenol showed higher yield than o–methylphenol indicating yield being

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affected by the steric effects of substitution also (Table 3, entries 2 and 3). Besides, catalysts
were capable of activating less nucleophilic phenols (2–naphthol and 1–naphthol) to react with
p–nitrobenzaldehyde and afforded good yields (Table 3, entries 6 and 7 respectively). We have
investigated reactions of p–nitrobenzonitrile, and o– or p–dinitrobenzene with different phenols
and collected in Table 3. Notably they also exhibit almost same yields with same trends in
catalytic reaction. Progress of O–arylation reaction for p–nitrobenzaldehyde and p–
nitrobenzonitrile with p–methylphenol catalyzed by complex 1 was given in Fig. 3. The coupling
reactions of electron–deficient phenols with aryl halides have been challenging as the
7
3
corresponding phenolates are weak nucleophile. Recently, Li et al. reported the O–arylation of
p–hydroxyacetophenone with p–fluorobenzonitrile in DMF medium at 90 °C temperature under
7
4
N atmosphere. Buchwald and his group also studied the reactivity of some electron–deficient
2
phenols in C–O coupling at 100–110 °C. However, in all the above reported processes aryl
halides were used as electrophile. For the first time, the results presented here demonstrate that
p–nitrobenzonitrile can behave as an electrophile in O–arylation of p–hydroxyacetophenone and
can afford very good yield in ethanolic medium.
All the copper(II) Schiff–base complexes (1–8) efficiently catalyzed the O–arylation
reactions. The turn over frequency calculation demonstrates catalytic efficacy of complex 6 is
better than other monomeric and dimeric complexes. Two nitrato–bridged dimers 1 and 2 were
less efficient than others may be due to higher coordination number. Five monomers (3, 4, 5, 7
and 8) were little bit less efficient than 6. Amongst the monomers 7 shows lowest efficiency
which may be due to the steric–crowding around metal center. To test the stability of the
complexes in reaction medium HRMS spectra of all compounds have been recorded in ethanol
medium. Mass spectra confirm that both monomeric and dimeric complexes are stable in boiling

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ethanol (see ESI). We compared efficacy of our catalyst with some Cu(II) salts, Cu(II) salts with
61
amine and a previously reported nitrato–bridged 1D Cu(II) Schiff–base complex under the
optimized reaction condition and results were summarized in Table S5 (see ESI). Even a cursory
look at Table S5 it will be clear that Cu(II) salts act as poor catalyst but their activity increases in
presence of amine may be due to in situ formation of metal–amine chelate. Catalytic efficiency
(TOF) of the previously reported nitrato–bridged complex showed close efficiency to that of the
monomer catalysts.
7
5
A plausible mechanism has been suggested based on the previous reports. According to
recent studies on the mechanism of the copper–catalyzed N–arylation reactions, it assumes that
the reaction proceeds with the initial nucleoplilic substitution (NuH) over the copper complex
(Scheme 3). The base used here abstracts the N–H proton from the heterocycle, thereby
generating a nucleophile. Then oxidative addition of the nitroarene (ArNO ) occurs through
2
coordination with the Cu atom followed by reductive elimination to give the desired N–arylated
heterocycles (Scheme 3).
4. Conclusion
Seven structurally diverse copper complexes containing mono– and dimeric structure, depending
on the nature of Schiff–base ligands or copper salts, have been synthesized and fully
characterized by X–ray single–crystal structural analyses. Their molecular structures differ in the
solid state under the influence of varied stereo–electronic character of Schiff–base ligands with
copper salt used. Dinuclear complex 6 exhibits a hitherto unknown structure of Cu(II) Schiff–
base complex in which two copper centers are bridged through mixed phenoxido/water bridges.
All complexes are highly active to catalyze the C–O coupling reactions of phenols with p–
nitrobenzaldehyde/p–nitrobenzonitrile in environmentally benign solvent, ethanol. The result

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presented here is the rare example of C–O coupling reactions which are being catalyzed by
copper(II) Schiff–base complex. Notably, catalysts are efficient enough to activate electron–
deficient phenols towards O–arylation reaction to afford diarylethers. This catalytic procedure
also resolves the problem of unwanted byꢀproduct formation by using cheaper variety of
substrate, nitroarenes. Further investigations on the synthesis of structurally diverse Schiff–base
complexes and their application as catalysts in other organic reactions are currently on progress
in our laboratory.
Acknowledgement
Financial support from Council of Scientific and Industrial Research (CSIR), New Delhi by a
grant (Grant No. 01(2542)/11/EMR–II) (to S.K.) is gratefully acknowledged. T.M. and D.S.
thanks the CSIR, New Delhi, for awarding them fellowship. Authors also thank Department of
Science and Technology, Govt. of India to fund Department of Chemistry, Jadavpur University
for procuring single–crystal X–ray diffractometer, 300 MHz NMR spectrometer, HRMS and
powder XRD system. T. M. thanks Mr. Debabrata Ganguly, University of Calcutta for his help in
this work.
■
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Captions:
Scheme 1 Selected bioactive compounds featuring arylether moiety
Scheme 2 Synthesis route of complexes 1–8
Scheme 3 Plausible mechanism of the reaction
Fig. 1 ORTEP diagram of complexes 1 (a) and 2 (b)
Fig. 2 ORTEP diagram of complexes 3 (a), 4 (b), 5 (c), 6 (d) and 7 (e)
Fig. 3 Plot showing progress of O–arylation reaction for p–nitrobenzaldehyde (■) and p–
nitrobenzonitrile (●) with p–methylphenol catalyzed by complex 1
Fig. 4 Plot showing progress of O–arylation reaction catalyzed by 1, 3 and 6 using p–
nitrobenzaldehyde and p–methylphenol as reactant

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Table 1 Crystal data and refinement details of compounds 1–7
Complex
1
2
3
4
5
6
7
Formula
C H Cu N C H Cu N C H CuN
C H ClCu C H ClC C H Cl Cu C H CuN
14 20 13 18 30 56 4 5
28
42
2
31 47
2
16 25
5
15 25
0O
18
10O
16.50
O8
N O
5
7
uN O
5
7
2
N O
6 25
O9
1
Formula
Weight
Crystal
System
Space
Group
a (Å)
933.82
950.86
478.95
469.34
455.31
1169.68
482.95
Monoclinic
Monoclinic
Monoclinic
Monoclinic
Monoclinic Triclinic
Monoclinic
C2/c
C2/c
P2 /c
P2 /c
P2 /c
1
P2 /n
1
1
1
P1
15.3483(9)
12.7358(8)
15.2074(4)
12.6983(3)
23.3712(6)
6.5538(2)
12.8896(3)
90
22.6374(13) 22.0294(6) 9.1817(4)
12.0035(5)
9.5610(4)
b (Å)
6.6010(4)
12.4456(7)
90
6.6638(2)
11.7822(5)
c (Å)
19.0760(11) 20.2333(5)
12.1691(3) 22.0042(10) 18.1151(8)
α (°)
90
90
90
86.723(2)
89.312(2)
83.934(2)
90
β (°)
92.446(4)
90
91.6930(10) 90.7490(10) 97.759(2)
90 90 90
3905.51(17) 1974.13(9) 1842.71(19) 1766.06(8) 2363.19(18) 2001.28(15)
98.658(2)
105.715(3)
γ (°)
90
90
3
V (Å )
3725.4(4)
4
Z
4
4
4
4
2
4
Dcalc
1.665
1.617
1.611
1.692
1.712
1.644
1.603
‒
3
(
gcm )
‒
1
ꢀ
(mm ) 1.234
1.176
0.0322
4348
1.163
0.039
5305
1.380
0.049
3871
1.437
0.025
5396
1.217
0.032
12608
1.151
0.059
4521
Rint
0.0811
4116
Unique
data
Data with 2247
I > 2(I)
3525
4114
3487
4483
8436
2859
R1 (I > 0.0756
0.0620
0.1570
1.056
0.0346
0.0877
1.037
0.0397
0.0839
1.202
0.0329
0.0843
1.038
0.0492
0.1329
1.028
0.0480
0.1307
0.965
2
(I))
wR (I > 0.2470
2
2
(I))
(
F
G2 OF) on 1.032

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2
4
a
Table 2 Optimization of reaction conditions
b
Entry
Catalyst
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
–
Base
Solvent
DMF
DMSO
Yield (%)
1
2
3
4
5
6
7
8
9
K CO
97
87
18
36
73
70
42
62
64
0
2
3
3
3
3
3
3
3
K CO
2
K CO
Toluene
Acetonitrile
Methanol
Ethyleneglycol
Dioxane
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
2
K CO
2
K CO
2
K CO
2
K CO
2
KOH
Na CO
2
3
10
11
12
13
14
15
16
17
18
19
20
21
22
CH COONa
3
tBuOK
DABCO
K CO
18
0
0
2
3
3
3
3
3
3
3
3
3
3
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Cu(NO ) .3H O + L1
K CO
88
2
c
K CO
21
57
12
2
d
K CO
2
e
K CO
2
f
K CO
28
2
g
K CO
62
88
2
h
K CO
2
K CO
41
8
3
2
2
2
Cu(NO ) .3H O
K CO
2
3
2
2
a
Reaction condition: p–nitrobenzaldehyde (1.1 mmol), p–methylphenol (1.0 mmol), base (1.2
b
c
mmol), catalyst (1 mol%), solvent (3 mL) at 80 °C for 8 h. Isolated yield. Temperature was 30
°
d
e
f
g
h
C and 50 °C. 0.1 Mol% compound 1. 0.2 Mol% compound 1. 0.5 Mol% compound 1. 2
Mol% compound 1.

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5
a
Table 3 O–arylation of p–nitrobenzaldehyde with phenols
c
d
Entry
1
Aryl alcohol
Product
Catalyst Yield
TOF
–
1
(
wt%) (h )
1
2
3
4
5
81
81
88
84
84
94
78
87
88
88
93
91
92
97
83
89
5
5
11
11
11
16
10
11
6
b
6
7
8
1
2
3
4
5
6
7
8
2
6
12
12
12
16
10
11
3
4
5
6
92
15
6
94
16
6
95
16

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2
6
6
7
6
84
14
6
6
89
98
15
16
8
9
6
6
90
96
15
16
10
11
6
6
6
98
43
93
16
5
e
1
2
1
3
15

Page 27 of 33
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2
7
1
1
4
5
6
6
85
97
14
16
15
16
–
1
1
1
6
7
8
6
6
90
98
–
1
2
3
4
5
6
7
8
a
Reaction condition: p–nitrobenzaldehyde (1.1 mmol), aryl alcohol (1.0 mmol), K CO (1.2
2
3
b
c
mmol), catalyst (1 mol%), EtOH (3 mL) at 80 °C for 8 h. except for catalyst 6 (3 hours).
Isolated yield. Mol. diaryl ether/mol. copper/ h. 10 hours.
d
e

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8
Scheme 1 Selected bioactive compounds featuring arylether moiety
Scheme 2 Synthesis route of complexes 1–8

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Scheme 3 Plausible mechanism of the reaction
Fig. 1 ORTEP diagram of complexes 1 a) and 2 b)