Tetradentate amido azo Schiff base Cu(II), Ni(II) and Pd(II) complexes: Synthesis, characterization, spectral properties, and applications to catalysis in C–C coupling and oxidation reaction

Article abstract of DOI:10.1016/j.poly.2019.01.002

New Schiff base ligands (H₂L¹ & H₂L²) were synthesized by the reaction of salicylaldehyde with (2-((2-aminophenyl)diazenyl)-N-alkylaniline), and were used for the preparation of complexes with Cu(II), Ni(II) and Pd(II) metal ions. The structural features of the synthesized compounds were examined by UV–Vis, IR and ¹H NMR spectroscopy. The crystal structures of Schiff base (H₂L¹) and two metal complexes Cu(L¹) and Ni(L²) were determined by single crystal X-ray diffraction. The studies revealed that the synthesized Schiff bases existed as tetradentate (N,N,N,O) ligands and bonded to the metal ions through the donor atoms of the amido nitrogen, azo nitrogen, azomethine nitrogen and phenolic oxygen atoms. The redox property of Cu(L¹) and Ni(L¹) and emission behavior of ligand H₂L¹ and complexes Cu(L¹) and Ni(L¹) were examined. The complex Cu(L¹) shows excellent catalytic activity towards oxidation of benzyl alcohol to benzyldehyde (under solvent-free condition) using H₂O₂ as the oxidant. Complex Pd(L¹) acts as highly efficient catalyst in the Suzuki–Miyaura cross-coupling reaction of various aryl halides with phenyl boronic acid to produce the corresponding biaryls with high yields under mild reaction conditions. Both the catalysts Cu(L¹) and Pd(L¹) were easily recovered by simple chromatographic separation and reused for next catalytic cycle.

Full text of DOI:10.1016/j.poly.2019.01.002

Accepted Manuscript
Tetradentate amido azo Schiff base Cu(II), Ni(II) and Pd(II) complexes: Syn-
thesis, characterization, spectral properties, and applications to catalysis in C-C
coupling and oxidation reaction
Jahar Lal Pratihar, Paritosh Mandal, Cheng Kung Lai, Surajit Chattopadhyay
PII:
S0277-5387(19)30017-8
https://doi.org/10.1016/j.poly.2019.01.002
POLY 13675
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 October 2018
26 December 2018
3 January 2019
Please cite this article as: J.L. Pratihar, P. Mandal, C.K. Lai, S. Chattopadhyay, Tetradentate amido azo Schiff base
Cu(II), Ni(II) and Pd(II) complexes: Synthesis, characterization, spectral properties, and applications to catalysis in
C-C coupling and oxidation reaction, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.01.002
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Revised Ms. Ref. No.: POLY-D-18-01000
Tetradentate amido azo Schiff base Cu(II), Ni(II) and Pd(II) complexes:
Synthesis, characterization, spectral properties, and applications to catalysis in
C-C coupling and oxidation reaction
Jahar Lal Pratihara*, Paritosh Mandala,c, Cheng Kung Laib, Surajit Chattopadhyayc*
aDepartment of Chemistry, Kandi Raj College, Murshidabad 742137, India
bDepartment of Chemistry, National Central University, Jhong-Li 32001, Taiwan
cDepartment of Chemistry, University of Kalyani, Kalyani 741235, India
________________________________________________________________________________
Abstract
New Schiff base ligands (H2L1 & H2L2) were synthesized by the reaction of salicylaldehyde
with (2-((2-aminophenyl)diazenyl)-N-alkylaniline), and were used for the preparation of complexes
with Cu(II), Ni(II) and Pd(II) metal ions. The structural features of the synthesized compounds
1
were examined by UV-Vis, IR and H-NMR spectroscopy. The crystal structures of Schiff base
(H2L1) and two metal complexes Cu(L1) and Ni(L2) were determined by single crystal X-ray
diffraction. The studies revealed that the synthesized Schiff bases existed as tetradentate (N,N,N,O)
ligands and bonded to the metal ions through the donor atoms of the amido nitrogen, azo nitrogen,
azomethine nitrogen and phenolic oxygen atoms. The redox property of Cu(L1) and Ni(L1) and
emission behavior of ligand H2L1 and complexes Cu(L1) and Ni(L1) were examined. The complex
Cu(L1) shows excellent catalytic activity towards oxidation of benzyl alcohol to benzyldehyde
(under solvent-free condition) using H2O2 as the oxidant. Complex Pd(L1) acts as highly efficient
catalyst in the Suzuki–Miyaura cross–coupling reaction of various aryl halides with phenyl boronic
acid to produce the corresponding biaryls with high yields under mild reaction conditions. Both the
catalysts Cu(L1) and Pd(L1) were easily recovered by simple chromatographic separation and
reused for next catalytic cycle.
_______________________________________________________________________________
Keywords: Copper(II), Nickel(II) and Palladium(II); Crystal structures; Redox; Emission;
Oxidation and C-C coupling reactions
_______________________________________________________________________________
Corresponding author. Fax.: +91 3484 255230; E-mail address: jlpr@rediffmail.com (J. L. Pratihar)
1

1. Introduction
Transition metal complexes with Schiff base ligands have explored continuously due to
their interesting coordination chemistry and vast application in catalysis and bioinorganic chemistry
[1-12]. Schiff bases play important roles in coordination chemistry due to their synthetic flexibility
and the very good binding nature of azomethine nitrogen participating in chelation process and also
they play an important role in inorganic chemistry due to their strong binding ability to form stable
complexes with most of the transition metals [13-17]. These ligands may contain a variable of
substituents having different electron-donating and / or withdrawing groups and therefore may have
interesting chemical properties [18-22]. In the last few years the unsymmetrically substituted Schiff
base metal complex systems have been reported with many new features. In particular
unsymmetrically-substituted metal Schiff base complexes are more efficient catalysts than
symmetrically-substituted complexes for olefin polymerization [1, 22]. Therefore the design and
synthesis of a new Schiff base ligand is most important step in the development of metal
complexes. Azo Schiff base ligands which contain both azo (–N=N–) and aldimine (–C=N–)
function in their own structures. The molecules that have a large conjugated π bonds system are
strongly form stable complexes with transition metals. Moreover, the coordination chemistry of
transition metals with azo Schiff base ligands have been studied due to their interesting properties
like C-H bond activation, oxygen insertion into the metal-carbon bond, oxidative addition, redox,
catalytic and biological activity [23-29]. In our previous communication focus on the coordination
chemistry of copper with amido azo aldimine ligand which was formed via reductive cleavage of
aldimine function upon complexation and their spectroelectrochemical and catalytic activity [30].
From this background and in continuation of our ongoing interest directed towards the design,
synthesis and development of new catalytically active materials [30-32], the present paper describes
the coordination behaviour of a amido azo Schiff base derived from the condensation of
2

salicylaldehyde with (2-((2-aminophenyl)diazenyl)-N-alkylaniline) towards Cu(II), Ni(II), and
Pd(II) transition metals.
Herein, we describe the synthesis and characterization of tetradentate azo-imine ligands
H2L1 & H2L2. Reactions of ligands H2L with Cu(OAc)2·H2O, Ni(OAc)2·4H2O and [Na2(PdCl4)]
separately in methanol afforded amido azo Schiff base complexes Cu(L), Ni(L) and Pd(L), have
been described. All the ligands and complexes have been characterized by the elemental analysis,
1
UV-Vis, IR and H NMR spectra. The crystal structures of ligand H2L1 and complexes Cu(L1) &
Ni(L2) were determined by X-ray diffraction analysis. The redox property and emission behavior of
the ligand and complexes have been studied. The catalytic efficacy of the Cu(L1) complex towards
oxidation of benzyl alcohol using H2O2 as the oxidant has been studied. The catalytic activity of the
Pd(L1) complex toward Suzuki–Miyaura cross–coupling reaction of several aryl halides with
phenyl boronic acid under mild conditions have been examined.
2. Experimental
2.1. Materials
The solvents used in the reactions were of reagent grade (Merck, Kolkata, India) and were
purified and dried by general procedure [33]. Methyl iodide, ethyl bromide, benzyl alcohol,
salicylaldehyde were purchased from E. mark, India. Copper(II) acetate monohydrate, Nickel(II)
acetate tetrahydrate, Iodobenzene, Bromobenzene, 1-iodo-2-nitrobenzene, 4-iodotoluene, 4-
bromotoluene, 1-bromo-2-nitrobenzene, Phenyl boronic acid were purchased from Sigma-Aldrich.
2,2'-diaminoazobenzene and Sodium tetrachloropalladate were prepared following reported
procedure [31,34].
2.2. Physical measurements
Microanalysis (C, H, N) was performed using a Perkin-Elmer 2400 C, H, N, S / O series II
elemental analyzer. Infrared spectra were recorded on a Perkin Elmer BX-1 FT-IR spectrometer
with the samples prepared as KBr pellets. Electronic spectra were recorded on a Shimadzu UV-
3

1800 PC spectrophotometer. 1H NMR spectra were obtained on Bruker 400 NMR spectrometers in
CDCl3 using TMS as the internal standard. Emission spectra were recorded with a Perkin Elmer
LS-55 Luminescence Spectrometer. Electrochemical measurements were made under dinitrogen
atmosphere using a CH instruments model 600D potentiostat. A platinum disc working electrode, a
platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were
used in a three-electrode configuration. All electrochemical data were collected at 298 K and are
uncorrected for junction potentials.
2.3. Syntheses
2.3.1. Syntheses of ligands
Compound 3 (2-((2-aminophenyl)diazenyl)-N-alkylaniline) (Scheme 1) was prepared by
following reported procedure using alkyl halides (methyl iodide and ethyl bromide) with 2,2'-
diaminoazobenzene in (1:1) molar ratio [32].
1
Yield: 3a, 75%. H NMR (400 MHz, CDCl3): 2.89 (CH3, s, 3H), 5.43 (NH2, s, 2H), 6.64-
6.73 (ArH, m, 4H), 7.06 (ArH, t, 1H), 7.19 (ArH, t, 1H), 7.56 (ArH, d, 1H), 7.61 (ArH, d, 1H), 7.88
(NH, s, 1H).
1
Yield: 3b, 70%. H NMR (400 MHz, CDCl3): 1.28 (CH3, s, 3H) 3.23 (CH2, d, 2H), 5.41
(NH2, s, 2H), 6.64-6.74 (ArH, m, 4H), 7.08 (ArH, t, 1H), 7.17 (ArH, t, 1H), 7.56 (ArH, d, 1H), 7.62
(ArH, d, 1H), 7.99 (NH, s, 1H).
Ligands, H2L1 and H2L2, were prepared following similar procedures. A representative
procedure for H2L1 is given below.
To a solution of 3a (0.3 g, 1.33 mmol), in 25 mL diethyl ether was added salicylaldehyde
(0.165 g, 1.33 mmol). The resulting mixture was then heated to reflux for 4 h. After slow
evaporation of the solvents, a red crystal of pure H2L1 was obtained. The solid product was then
washed with diethyl ether.
4

Yield: H2L1, 95%. Anal. Calc. for C20H18N4O: C, 72.71; H, 5.49; N, 16.96. Found: C,
72.53; H, 5.58; N, 16.47%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane): 475 (25330), 315
(46920), 260 (51720), 210 (75950). IR (KBr, cm−1): 3060 ν(N-H/O-H), 1607 ν(C=N), 1378
1
ν(N=N), 1279 ν(O-C). H NMR (400 MHz, CDCl3): 2.81 (CH3, t, 3H), 6.70-6.80 (ArH, m, 2H),
6.95 (ArH, t, 1H), 7.06 (ArH, d, 1H), 7.20-7.29 (ArH, m, 2H), 7.33-7.45 (ArH, m, 4H), 7.87-7.90
(ArH, m, 2H), 8.63 (CH, s, 1H), 9.27 (NH, s, 1H), 13.46 (OH, s, 1H).
Yield: H2L2, 93%. Anal. Calc. for C21H20N4O: C, 73.23; H, 5.85; N, 16.27. Found: C,
73.45; H, 5.55; N, 16.51%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane): 480 (25450), 315
(22050), 265 (32600), 230 (45490). IR (KBr, cm-1): 3064 ν(N-H/O-H), 1608 ν(C=N), 1379
1
ν(N=N), 1279 ν(O-C). H NMR (400 MHz, CDCl3): 0.98 (CH3, t, 3H), 3.11-3.16 (CH2, m, 2H),
6.68 (ArH, t, 2H), 6.87 (ArH, t, 1H), 6.96 (ArH, d, 1H), 7.15-7.20 (ArH, m, 2H), 7.25-7.38 (ArH,
m, 2H), 7.72 (ArH, d, 1H), 7.80 (ArH, d, 1H), 8.55 (CH, s, 1H), 9.04 (NH, s, 1H), 13.29 (OH, s,
1H).
2.3.2. Syntheses of complexes Cu(L1) and Cu(L2)
Complexes, Cu(L1) and Cu(L2), were prepared following similar procedures. A
representative procedure for Cu(L1) is given below.
A solution of H2L1 (0.1 g, 0.3 mmol) in 10 mL methanol was added to a solution of
Cu(OAc)2·H2O (0.06 g, 0.3 mmol) in 10 mL methanol. The mixture was stirred for 2 h. The pink
violet solid precipitate was separated by filtration and the solid mass was dissolved in
dichloromethane. After the evaporation of the solvent, a pink-violet solid of pure Cu(L1) was
obtained.
Yield: Cu(L1), 70%. Anal. Calc. for C20H16CuN4O: C, 61.29; H, 4.12; N, 14.30. Found: C,
61.03; H, 4.42; N, 13.97%. UV/VIS (λmax/nm (ε/ dm2 mol-1), dichloromethane): 550 (11520), 430
(13700), 385 (15700), 310 (20030), 245 (39800). IR (KBr, cm−1): 1614 ν(C=N), 1425 ν(N=N),
1330 ν(O-C).
5

Yield: Cu(L2), 65%. Anal. Calc. for C21H18CuN4O: C, 62.13; H, 4.47; N, 13.80. Found: C,
62.05; H, 4.64; N, 13.63%. UV/VIS (λmax/nm (ε/ dm2 mol-1), dichloromethane): 550 (11830), 430
(13800), 385 (15300), 310 (18730), 250 (33050). IR (KBr, cm−1): 1614 ν(C=N), 1417 ν(N=N),
1326 ν(O-C).
2.3.3. Syntheses of complexes Ni(L1) and Ni(L2)
Complexes, Ni(L1) and Ni(L2), were prepared following similar procedures. A
representative procedure for Ni(L1) is given below.
A solution of H2L1 (0.1 g, 0.3 mmol) in 10 mL methanol was added to a solution of
Ni(OAc)2·4H2O (0.075 g, 0.3 mmol) in 10 mL methanol. The mixture was stirred for 1 h. The
colour of the solution immediately changed to deep green. The resulting solution was then filtered
and allowed to stand at room temperature. After slow evaporation of the solvent at room
temperature, dark green coloured crystals suitable for X-ray analysis were obtained.
Yield: Ni(L1), 75%. Anal. Calc. for C20H16N4NiO: C, 62.06; H, 4.17; N, 14.47. Found: C,
61.76; H, 4.42; N, 14.64%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane): 680 (6750), 410
(68630), 335 (38900), 265 (166200). IR (KBr, cm−1): 1610 ν(C=N), 1438 ν(N=N), 1324 ν(O-C). 1H
NMR (400 MHz, CDCl3): 3.15 (CH3, t, 3H), 6.60 (ArH, t, 1H), 6.66 (ArH, t, 1H), 6.96 (ArH, d,
1H), 7.08 (ArH, d, 2H), 7.17 (ArH, t, 3H), 7.29-7.35 (ArH, m, 2H), 7.53 (ArH, d, 1H), 7.63 (ArH,
d, 1H), 8.05 (ArH, d, 1H), 8.45 (CH, s, 1H).
Yield: Ni(L2), 72%. Anal. Calc. for C21H18N4NiO: C, 62.89; H, 4.52; N, 13.97. Found: C,
63.14; H, 4.65; N, 13.76%. UV/VIS (λmax/nm (ε/ dm2 mol-1), dichloromethane): 680 (6940), 410
1
(21520), 335 (13880), 270 (42500). IR (KBr, cm-1): 1611 ν(C=N), 1440 ν(N=N), 1321 ν(O-C). H
NMR (400 MHz, CDCl3): 1.33 (CH3, t, 3H), 3.68 (CH2, s, 2H), 6.59-6.61 (ArH, m, 1H), 6.68
(ArH, t, 1H), 6.95 (ArH, d, 1H), 7.15-7.23 (ArH, m, 4H), 7.32 (ArH, t, 1H), 7.40 (ArH, d, 1H), 7.58
(ArH, d, 1H), 7.70 (ArH, d, 1H), 8.13 (ArH, d, 1H), 8.54 (CH, s, 1H).
2.3.4. Syntheses of complexes Pd(L1) and Pd(L2)
6

Complexes, Pd(L1) and Pd(L2), were prepared following similar procedures. A
representative procedure for Pd(L1) is given below.
A solution of H2L1 (0.05 g, 0.15 mmol) in 10 mL methanol was added to a solution of
Na2PdCl4 (0.045 g, 0.15 mmol) in 5 mL methanol. The mixture was stirred for 4 h. The colour of
the solution slowly changed to blue. The resulting solution was then filtered and allowed to stand at
room temperature. After slow evaporation of the solvent at room temperature, blue coloured
crystalline products were obtained.
Yield: Pd(L1), 80%. Anal. Calc. for C20H16N4OPd: C, 55.25; H, 3.71; N, 12.89. Found: C,
55.12; H, 3.79; N, 13.02%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane): 575 (8620), 365
1
(7110), 235 (33890). IR (KBr, cm-1): 1615 ν(C=N), 1418 ν(N=N), 1335 ν(O-C). H NMR (400
MHz, CDCl3): 3.17 (CH3, s, 3H), 6.50-6.53 (ArH, m, 2H), 6.99 (ArH, d, 1H), 7.15-7.21 (ArH and
CH, m, 4H), 7.24-7.30 (ArH, m, 3H), 7.39 (ArH, d, 1H), 7.54 (ArH, d, 1H), 8.11 (ArH, d, 1H).
Yield: Pd(L2), 75%. Anal. Calc. for C21H18N4OPd: C, 56.20; H, 4.04; N, 12.48. Found: C,
56.13; H, 4.15; N, 12.76%. UV/VIS (λmax/nm (ε/ dm2 mol-1), dichloromethane): 580 (8740), 365
1
(7620), 235 (35780). IR (KBr, cm-1): 1617 ν(C=N), 1403 ν(N=N), 1346 ν(O-C). H NMR (400
MHz, CDCl3): 1.65 (CH3, s, 3H), 3.27 (CH2, b, 1H), 3.82 (CH2, b, 1H), 6.18 (ArH, d, 1H), 6.26
(ArH, s, 1H), 6.40 (ArH, t, 1H), 6.74 (ArH, s, 1H), 6.96 (ArH, t, 1H), 7.12-7.32 (ArH & CH, m,
6H), 7.39 (ArH, d, 1H), 7.70 (ArH, d, 1H).
2.3.5. General procedures for the Suzuki–Miyaura reaction
Aryl halides (1.0 mmol) and phenyl boronic acid (1.2 mmol), were dissolved in THF (10
mL) in a 25 mL round bottomed flask and stirred for 10 min. K2CO3 (2.0 mmol) and catalyst (1.0
0
mol %) were added to the reaction flask. The reaction mixture was stirred at 80 C for 5h. After
completion, the reaction mixture was cooled to room temperature and the mixture was filtered and
then extracted with dichloromethane. The organic layer was washed with brine, dried over Na2SO4,
filtered, passed through silica column (60-120 mesh), the complex remain trapped. Upon removal
7

of the solvent, solid of pure products were obtained. The yields of the products obtained (based on
1
the aryl halides) from all the reactions were determined after isolation, and characterized by H
NMR spectra and melting points with the literature data.
2.3.6. Procedure for catalytic oxidation of benzyl alcohol
Benzyl alcohol (5 mmol), catalyst Cu(L1) (0.05 mmol), and 50 % H2O2 5 mL were taken in
a 25 mL round bottomed flask. The mixture was vigorously stirred at 800C for 3 h. After
completion, reaction mixture was cooled to room temperature then mixture was poured into water
and the product was extracted with dichloromethane. The organic layer was washed with brine,
dried over Na2SO4, filtered, passed through silica column (60-120 mesh), the complex remain
1
trapped. Upon removal of the solvent, pure products were obtained and characterized by H NMR
spectra.
2.6. Crystallography
Single crystals of ligand H2L1, Ni(L2) were grown by slow evaporation of ether and
methanolic mother solutions at 25 0C and Cu(L1) was grown by slow evaporation of
dichloromethane-methanol mixed solvent at room temperature. Data were collected by ω- scan
technique on a Bruker Smart CCD diffractometer with Mo-Kα radiation monochromated by
graphite crystal. Structure solution was done by direct method with SHELXS–97 program [35-36].
Full matrix least square refinements on F2 were performed using SHELXL–97 program [35-36]. All
non-hydrogen atoms were refined anisotropically using reflections I > 2σ (I). The C-bound
hydrogen atoms were included in calculated positions and refined as riding atoms and O, N-bound
hydrogen atoms were assigned and refined. Data collection parameters and relevant crystal data are
collected in Table 1.
------------------------------------------------------------------------------------------------------------------------
Please insert Table 1 here
------------------------------------------------------------------------------------------------------------------------
3. Results and discussion
8

3.1. Synthesis
The ligand H2L used in this work, synthesized by two step reaction (i) mono alkylation of
2,2'-diaminoazobenzen with alkyl halides (ii) the condensation of mono alkyl product 3 with
salicylaldehyde in diethyl ether. The ligand was isolated as red-brown solid crystals on cooling. The
preformed ligand H2L upon reaction with copper(II), nickel(II) and palladium(II) metal salts
separately in methanol afforded pink-violet, green and blue (respectively) amido azo aldimine
complexes of compositions (Cu(L), Ni(L) and Pd(L)) where the ligand (H2L) offers (N,N,N,O)
coordination mode (Scheme 1). In all the complexes ligand H2L binds in tetradentate (N,N,N,O)
bianionic fashion through deprotonated amino nitrogen, azo nitrogen, imine nitrogen and
deprotonated phenolato oxygen atom forming two six membered and one five membered ring.
------------------------------------------------------------------------------------------------------------------------
Please insert Scheme 1 here
------------------------------------------------------------------------------------------------------------------------
3.2. UV-Vis and Fluorescence spectra
UV–visible spectra of ligands H2L1 & H2L2 and their complexes Cu(L), Ni(L) and Pd(L)
were recorded at room temperature, using dichloromethane as a solvent. Representative for H2L2
and its corresponding complexes Cu(L2), Ni(L2) and Pd(L2) have been shown in Fig. 1. Spectral
data are collected in experimental section. The absorption spectrum of the free ligands H2L consists
of four relatively intense absorption peaks centered at ~230, ~260, 315 and ~480 nm, the first three
high energy can be assigned to the π–π* transitions of the phenyl rings and last one characteristic of
the overlapping n–π* transitions of azo (–N=N–) and aldimine (–N=CH–) function [37-39]. Upon
coordination of the ligands H2L, the π–π* transitions of the phenyl rings in the complexes Cu(L),
Ni(L) and Pd(L) were shifted to different wavelengths which indicating the coordination of Cu(II),
Ni(II) and Pd(II) atoms with the (L)2- units. A characteristic broad absorption peak is observed at
550 nm [Cu(L)], 680 nm [Ni(L)] and 580 nm [Pd(L)] this new absorption peak can be assigned to
mixed metal ligand charge transfer transition [38-40].
9

------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 1 here
------------------------------------------------------------------------------------------------------------------------
The fluorescence properties of ligand H2L1 and complexes Cu(L1) and Ni(L1) were
examined in dilute chloroform solution at room temperature (Fig. 2). The ligand H2L1 show low
intensity emissions bands at 450 nm upon excitation at 315 nm whereas Ni(L1) do not show
noticeable fluorescence upon excitation at 335 & 410 nm but complex Cu(L1) show very high
intensity emissions bands at 465 nm upon excitation at 385 nm. High fluorescent intensity of
Cu(L1) due to the strong chelate formation leading to reduction of photoinduced electron transfer
process (PET) [41-42]. In the copper complex both five and six-membered chelate ring increases
rigidity in comparison to H2L1, which in turn, reduces the loss of energy by vibrational decay and
enhances the fluorescence intensity [42-44].
------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 2 here
------------------------------------------------------------------------------------------------------------------------
3.3. IR and 1H-NMR
The azomethine vibration at 1608 cm-1 in the free ligands (H2L1 & H2L2) undergoes positive
shift and appears in the range 1610-1617 cm-1 in the complexes (Cu(II), Ni(II) and Pd(II))
indicating coordination to the metal ions [23-26,30]. The characteristic band due to ν(Ar–OH / or
N-H) at 3367 cm-1 and 3064 in free ligands disappears due to deprotonation in all the complexes
[30]. Furthermore, the phenolic ν(C–O) vibrations at 1279 cm-1 in the free ligands shows shift in
higher energy upon complexation with metal ions (Cu(II), Ni(II) and Pd(II)), and appears at ~1328
cm-1 (Cu(II)), ~1323 cm-1 (Ni(II)) and ~1340 cm-1 (Pd(II)), which is further established by the
presence of ν(Ar-O-M) vibration at ~1375 cm-1, ~1378 cm-1 and ~1384 cm-1 respectively, signifying
its coordination to metal ions via the phenoxy hydroxyl group [37,45]. Similarly the N=N band
(~1378 cm-1) of the ligands shifted to higher energy (1425 (Cu), 1438 (Ni) and 1418 (Pd) in the
10

complexes, consistent with coordination of the azo nitrogen [23-27, 30]. Spectral data are collected
in experimental section.
The –OH resonances at 13.46 ppm (H2L1) & 13.29 ppm (H2L2) and –NH resonances at 9.27
ppm (H2L1) & 9.04 ppm (H2L2) in the ligands disappears in the complexes (Ni & Pd), suggesting
the formation of Ni–O and Pd–O bond due to deprotonation [23,24]. A sharp singlet at 8.63 ppm
(H2L1) & 8.55 ppm (H2L2) due to azomethine proton in free ligands is shifted upfield in Ni(II)
complexes and appears at 8.45 ppm & 8.54 ppm, confirms that azomethine nitrogen is linked to the
nickel ion whereas in palladium complexes the azomethine protons are more shielded and appears
in the range 7.11-7.21 ppm. The phenyl ring protons observed at 6.68–7.90 ppm in both the ligands
undergoes small shifts and appears in the range 6.22-8.05 in all the complexes. Furthermore,
resonances due to –CH3 of H2L1 (2.81 ppm) and –C2H5 protons of H2L2 at 0.98 & 3.11-3.16 ppm, in
the ligands show large shift upon coordination to Ni(II) & Pd(II) ion, suggesting ligand is
coordinated to metal ions. Spectral data are collected in experimental section.
3.4. X-ray crystallography
Crystals suitable for X-ray diffraction analysis of H2L1 were obtained by slow evaporation
of diethyl ether solution at room temperature, which crystallizes in the space group Pbca. The
crystallographic and measurement data are shown in Table 1. The crystal structure of ligand H2L is
shown in Fig. 3. Selected bond lengths and bond angles are given in Table 2. The non-planar
structure of ligand H2L stabilized by two intra molecular hydrogen bonding between phenolic
proton and imine nitrogen [O(1)-H….N(3)] and amine proton and azo nitrogen N(4)-H….N(1)].
The O(1)-C(20), N(3)-C(14), N(1)-C(8), N(2)-N(3) and N(2)-C(13) bond lengths 1.346(2),
1.281(9), 1.416(9), 1.246(19) and 1.426(2) Å are in agreement with the reported values [30-32].
---------------------------------------------------------------------------------------------------------------------
Please insert Fig. 3 and Table 2 here
---------------------------------------------------------------------------------------------------------------------
11

The crystal structure of Cu(L1) is shown in Fig. 4 along with the atom labels. Hydrogen
atoms are omitted for the clarity. Selected bond lengths and bond angles are listed in Table 3.
---------------------------------------------------------------------------------------------------------------------
Please insert Fig. 4 and Table 3 here
---------------------------------------------------------------------------------------------------------------------
Cu(L1) crystallizes in the P212121 space group and also has a square planar geometry. All the non-
hydrogen atoms of Cu(L1) make a good plane (mean deviation 0.222Å). The dianionic ligand (L1)2-
is coordinated through (N,N,N,O) with Cu(II) ion, forming two six membered chelate ring and one
five membered ring. The copper(II) centre is coordinated to imine nitrogen N1, azo nitrogen N2,
amido nitrogen N4 and phenolate oxygen O1 atoms to constitute the equatorial plane. The
deviations of the four coordinating atoms from their mean plane are within ± 2.645 Å. The shorter
bond lengths of O1-C1 (~0.05 Å), N3-C14 (~0.04 Å), C6-C7 (~0.03 Å) and one longer bond N1-C8
(~0.02 Å) in the Cu(L1) complex relative to the ligand H2L1 shows the effect of metal ion
coordination. The Cu1-O1, Cu1-N2, Cu1-N4 and Cu1-N1 bond distances are 1.902(6), 1.924(6),
1.927(6) and 1.982(6) Å respectively are in typical range as found in related Cu(II) complexes [30,
46-48]. The O1-Cu1-N1, N2-Cu1-N4, N2-Cu1-N1 and O1-Cu1-N4 angles are 91.2(3), 91.1(3),
84.3(3) and 93.6(3)° respectively are very close to 90° and O1-Cu1- N2 (174.2(3), N4-Cu1- N1
(173.9(3)0 angles are close to 180° are similar to other reported Cu(II) complexes [30, 46-48].
The crystal structure of Ni(L2) is shown in Fig. 5 along with the atom labels. Hydrogen
atoms are omitted for the clarity. Selected bond lengths and bond angles are listed in Table 4.
------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 5 and Table 4 here
------------------------------------------------------------------------------------------------------------------------
Ni(L2) crystallizes in the P212121 space group, similarly to the complex Cu(L1), and also has a
square planar geometry (mean deviation 0.126Å). The dianionic ligand (L2)2- is coordinated
through imine nitrogen N1, azo nitrogen N2, amido nitrogen N4 and phenolate oxygen O1 atoms to
constitute the equatorial plane with Ni(II) ion and forming two six membered chelate ring and one
five membered ring like Cu(L1). The deviations of the four coordinating atoms from their mean
12

plane are within ± 2.62 Å. The shorter bond lengths of O1-C1 (~0.04 Å), N4-C15 (~0.02 Å), N3-
C14 (~0.06 Å) and C6-C7 (~0.04 Å) in the Ni(L2) complex relative to the ligand H2L1 (though the
H2L1 is little differ from coordinated ligand in Ni(L2)) shows the effect of metal ion coordination.
The N2-N3 azo and N1-C7 imine distances in Ni(L2) are longer ((~0.02 Å) than that of ligand H2L1.
The Ni1-O1, Ni1-N2, Ni1-N4 and Ni1-N1 bond distances are 1.853(3), 1.832(3), 1.893(3) and
1.887(3) Å and O1-Ni1-N1, N2-Ni1-N4, N2-Ni1-N1 and O1-Ni1-N4 angles are 92.33(14),
92.55(14), 85.33(14), 89.83(13) respectively are in typical range as found in related Ni(II)
complexes [48-50].
3.5. Electrochemistry
The electrochemical behavior of two complexes Ni(L1) and Cu(L1) were examined in
dichloromethane acetonitrile mixed solvent (0.1 M TBAP) by cyclic voltammetry with a scan rate
of 0.1 V s−1 (Fig. 6) vs. SCE. The cyclic voltammogram of Ni(L1) displayed one electron reversible
oxidative and reductive responses at 1.1 V and -1.04 V whereas Cu(L1) displayed only one electron
quasi reversible oxidative response at 1.03 V (Fig.S21). The reduction and oxidation have been
assigned to the couple of equations (1) & (2). The report of our previous work of copper(II)
complex with similar ligand system which support the oxidative responses of Cu(L1) and Ni(L1)
both are ligand centric [30]. The reductive response of Ni(L1) due to the metal centred Ni(II)/Ni(I)
reduction [37].
------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 6 here
------------------------------------------------------------------------------------------------------------------------
3.4. The Suzuki–Miyaura coupling reactions
The catalytic activity of Pd(L1) was studied in C–C bond formation by means of Suzuki–
Miyaura cross–coupling of aryl halides with phenyl boronic acid (Eq. 3) and results were shown in
Table 5. In the present study we report the coupling reactions of several aromatic halides having
different substituents in the aromatic ring under atmospheric pressure with changing amount of
13

catalyst and reaction temperature. Before detailed study on the catalytic reactions, initially we first
optimized the reaction conditions by choosing iodobenzene with phenyl boronic acid as the model
substrate. The important two factors for the reaction, catalyst amount and temperature, were
compared for the purpose of attaining the optimal conditions. The conclusion of the optimization of
------------------------------------------------------------------------------------------------------------------------
Please insert Eq. 3 and Table 5 here
------------------------------------------------------------------------------------------------------------------------
the conditions is shown in Table S1 (SI). The reaction was carried out when various amounts of the
catalyst Pd(L1) 1 mol % - 4 mol % based on the Pd element were present at 80 0C. It was observed
that nearly the same yields were obtained with catalyst amounts varying from 1-4 mol %. The
influence of temperature was studied from 250C-800C, to obtain the optimum temperature for the
reaction. Hence, the boiling temperature (800C) was selected as the optimum temperature. After the
completion of the catalytic reaction the palladium complex was intact and isolated from the reaction
mixture easily by column chromatography. The isolated catalyst Pd(L1) was reused and its
reusability was examined up to three times for a reaction without sufficient loss of its catalytic
activity keeping other condition intact.
3.7. Oxidation of benzyl alcohol
As the complex Cu(L1) was soluble in benzyl alcohol so our interest to examine this
compound as homogeneous catalyst for organic transformations in organic media. The
homogeneous oxidation reaction was carried out under atmospheric pressure and in absence of any
additive other than the substrate, benzyl alcohol, H2O2 (50%) and catalyst Cu(L1) (Eq.4) and results
were shown in Table 6. The oxidation of alcohols was carried out in the presence of catalytic
amounts (1 mol %) of Cu(L1) and using as oxidant H2O2 (5 eqv.) in an aqueous / organic biphasic
system or solvent-free condition. Before detailed study on the catalytic oxidation of substituted
benzyl alcohols, initially we chose benzyl alcohol as the model substrate for optimization of the
reaction. The oxidation of benzyl alcohol by H2O2 at room temperature was very low yield ~20%
14

0
for 10 h. The conversion of benzaldehyde significantly increased when temperature above 70 C.
The influence of temperature was studied to obtain the optimal condition for the oxidation reactions
(Table S2 in SI). The best yield of benzaldehyde was obtained during the oxidation of benzyl
0
alcohol, with (5 eqv.) 50% H2O2 for 3 h at 80 C. The products were isolated after separation and
characterized by IR spectra. The conversions were calculated on the basis of isolated yields.
------------------------------------------------------------------------------------------------------------------------
Please insert Eq.4 here
------------------------------------------------------------------------------------------------------------------------
4. Conclusion
In conclusion new Cu(II), Ni(II) and Pd(II) complexes Cu(L), Ni(L) and Pd(L) have been
synthesized using newly developed Schiff based ligand H2L and characterized by different
physicochemical and spectroscopic techniques. The solid-state structures of H2L1, Cu(L1) and
Ni(L2) were determined by single crystal X-ray crystallography, which revealed square planar
geometry. The newly synthesized ligand (H2L), bound with metal ions in a bi-negatively tetra-
dentate manner, with N, N, N, O donor atoms of the amido nitrogen, azo nitrogen, azomethine
nitrogen and phenolic oxygen atoms. The redox property of complexes Cu(L1) and Ni(L1) and
emission behavior of both the ligand and complexes were examined. Newly synthesized Pd(L1)
complex was investigated as highly active and efficient catalyst in the Suzuki–Miyaura cross–
coupling reaction of various aryl halides with phenyl boronic acid to produce the corresponding
biaryls with high yields under mild reaction conditions. Also we examined the catalytic activity of
Cu(L1) for the oxidation of benzyl alcohol using H2O2 as an oxidant under mild conditions.
Acknowledgments
The authors are grateful to the University Grants Commission of India (Minor Research
Project Sanction No.F.PSW-164/15-16(ERO)) for financial support. We are thankful to Professor
K. Ghosh and Dr. S. Chattopadhyay, Department of Chemistry, University of Kalyani, for their
spectral help.
15

Appendix A. Supplementary data
Figures S1–S12 shows the UV-Vis, IR spectra of the ligand and complexes and Figure S13-
1
S18 H NMR spectrum of the ligands, nickel and palladium complexes and Figure S19 CV of
Cu(L1). CCDC 1874624, 1874625 and 1874626 contains the supplementary crystallographic data
for compounds H2L1, Ni(L2) & Cu(L1). These data 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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19

Scheme 1
Scheme 1. Synthesis of ligands and metal complexes
----------------------------------------------------------------------------------------------------------------------
Eq. 1 & 2
------------------------------------------------------------------------------------------------------------------------
Eq. 3 & 4
------------------------------------------------------------------------------------------------------------------------
20

Table 1
Crystallographic data for1, 2a and 2b
Parameter
H2L1
Cu(L1)
Ni(L2)
Chemical formula
Formula weight
C20H18N4O
330.38
C20H16CuN4O
C21H18N4NiO
401.10
391.91
Crystal system
Orthorhombic
Pbca
Orthorhombic
Orthorhombic
P212121
space group
P212121
a/Å
11.5377(12)
14.3666(12)
20.920(3)
90
4.9090(3)
5.4146(2)
15.5889(7)
20.5140(8)
90
b/Å
10.6808(7)
31.746(2)
90
c/Å
== /deg
0.71073
0.71073
1664.50(19
800
0.71073
1731.54(12)
832
 /Å
V /Å3
3467.7(6)
F(000)
1392
Crystal Size [mm]
Z
0.3 x 0.2 x 0.15
0.30 x 0.04 x 0.02 0.25 x 0.08 x 0.08
8
4
4
T/K
D/mg/m-3
296(2)
296(2)
296(2)
1.539
1.266
1.560
 /mm-1
0.081
1.329
1.140
Theta Min-Max [deg]
Reflections measured
Unique reflections (Rint)
No. of reflections used [I> 2σ(I)]
R1,wR2(all data)
R1, wR2[I> 2(I)]
GOF
1.947-28.306
31992
2.012-28.456
15895
1.641-28.341
16875
4307
4144
4303
3426
2389
2424
0.1024, 0.1425
0.0503, 0.1169
1.011
0.1268, 0.1477
0.0532, 0.1021
1.038
0.0570, 0.0994
0.0370, 0.0804
0.979
21

Table 2
Selected Bond Distances (Å) and Angles (deg) for Compound H2L1.
Distances
N(1)-N(2)
N(1)-C(6)
1.2706(17)
N(3)-C(5)
1.4152(19)
1.347(2)
1.4214(19)
1.3990(19)
1.2809(19)
1.423(2)
N(4)-C(12)
N(4)-C(13)
O(1)-C(20)
C(14)-C(15)
C(15)-C(20)
N(2)-C(7)
1.447(2)
1.346(2)
1.452(2)
1.388(2)
N(3)-C(14)
C(7)-C(12)
C(5)-C(6)
1.399(2)
Angles
N(2)-N(1)-C(6)
N(1)-N(2)-C(7)
C(14)-N(3)-C(5)
114.24(12)
116.62(13)
C(6)-C(5)-N(3)
C(5)-C(6)-N(1)
118.65(13)
116.52(13)
120.13(14)
124.52(15)
N(2)-C(7)-C(12)
N(4)-C(12)-C(7)
O(1)-C(20)-C(15)
126.51(14)
121.80(14)
121.74(17)
C(12)-N(4)-C(13)
N(3)-C(14)-C(15)
121.88(16)
22

Table 3
Selected Bond Distances (Å) and Angles (deg) for Compound Cu(L1).
Distances
Cu(1)-O(1)
Cu(1)-N(2)
Cu(1)-N(4)
N(2)-C(13)
N(3)-C(14)
N(4)-C(19)
1.427(9)
1.358(9)
1.339(10)
1.902(6)
1.924(6)
1.927(6)
1.982(6)
1.271(10)
1.432(9)
1.279(9)
Cu(1)-N(1)
N(1)-C(7)
N(1)-C(8)
O(1)-C(1)
C(1)-C(6)
C(6)-C(7)
C(14)-C(19)
1.295(9)
1.445(11)
1.422(12)
1.437(10)
N(2)-N(3)
C(8)-C(13)
1.391(11)
Angles
O(1)-Cu(1)-N(2)
174.2(3)
O(1)-Cu(1)-N(1) 91.2(3)
O(1)-Cu(1)-N(4)
N(2)-Cu(1)-N(4)
93.6(3)
91.1(3)
N(2)-Cu(1)-N(1) 84.3(3)
N(4)-Cu(1)-N(1) 173.9(3)
23

Table 4
Selected Bond Distances (Å) and Angles (deg) for Compound Ni(L2).
Distances
Ni(1)-N(2)
Ni(1)-O(1)
Ni(1)-N(1)
1.832(3)
1.853(3)
1.887(3)
N(3)-C(14)
N(4)-C(15)
N(2)-C(9)
1.338(5)
1.329(5)
1.425(5)
Ni(1)-N(4)
O(1)-C(1)
N(1)-C(7)
N(1)-C(8)
N(2)-N(3)
1.893(3)
1.302(5)
1.301(5)
1.413(5)
1.295(4)
C(1)-C(6)
C(8)-C(9)
C(6)-C(7)
C(14)-C(15)
1.408(6)
1.394(6)
1.412(5)
1.444(5)
Angles
N(2)-Ni(1)-O(1)
N(2)-Ni(1)-N(1)
O(1)-Ni(1)-N(1)
177.14(16)
85.33(14)
92.33(14)
O(1)-Ni(1)-N(4)
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(4)
89.83(13)
177.48(15)
92.55(14)
24

Table 5
Suzuki–Miyaura cross-coupling reaction of various aryl halides with phenyl boronic acid catalyzed
by Pd(L1)
Solvent = THF, Base = K2CO3, Time = 5h, Catalyst = 4 mol%,
Substrate = 1:1.2 mmol (Aryl halide : Phenyl boronic acid)
25

Table 6
Oxidation of benzyl alcohol derivatives catalyzed by Cu(L1)
Solvent = CH3CN or solvent free, Catalyst = 1 mol%, Substrate = 5 mmol
26

Fig. 1 UV-Vis spectra of H2(L2) (―), Cu(L2) (―), Ni(L2) (―) and Pd(L2) (―) in
dichloromethane
27

Fig. 2 Emission spectra of H2(L1) (―), Cu(L1) (―) and Ni(L1) (―) in chloroform.
28

Fig. 3 Molecular structure of H2(L1) (40% probability ellipsoids) with atom numbering scheme.
Hydrogen atoms are omitted for clarity except N4, O1 and C14.
29