Synthesis, characterization, spectral and catalytic activity of tetradentate (NNNO) azo-imine Schiff base copper(II) complexes

Article abstract of DOI:10.1016/j.ica.2018.04.042

The hexadentate ligand, 2,2′-bis(salicylideneamino)azobenzene, 1 has been synthesized from 2,2′-diaminoazobenzene and salicylaldehyde in refluxing diethyl ether. Reaction of ligand 1 with Cu(II) acetate and Cu(II) perchlorate separately in methanol afforded tetradentate (N,N,N,O) Cu(II) complexes, Cu(L) & [Cu(HL)]ClO₄ respectively [where H₂L represent the one imine moiety cleavage product of ligand 1 (H represents the dissociable amino and phenolic protons)]. These were characterized by microanalytical data and spectroscopic studies. In addition, the crystal structures of the ligand 1 and complexes Cu(L) & [Cu(HL)]ClO₄ were determined by X-ray diffraction analysis. The diffraction analysis revealed that the ligand (H₂L) binds Cu(II) centers in (N,N,N,O) tetra dentate fashion in distorted square planer geometry. In complex [Cu(HL)]ClO₄ the apical position of copper center is weakly coordinated with one perchlorate ion. The dimeric structure of the molecule [Cu(HL)]ClO₄ is stabilized through NH₂···O hydrogen bonds. The fluorescence and redox property of ligand 1 and complexes Cu(L) & [Cu(HL)]ClO₄ were studied. Preliminary DFT calculations were carried out using crystallographic coordinates to understand the electronic spectra and redox properties of the ligand and complexes. The complex Cu(L) shows very good catalytic activities towards oxidation of benzyl alcohol to benzaldeyhde (under solvent-free condition) and organic thioethers to sulfoxide and sulfones using H₂O₂ as the oxidant.

Full text of DOI:10.1016/j.ica.2018.04.042

Accepted Manuscript
Research paper
Synthesis, characterization, spectral and catalytic activity of tetradentate
(NNNO) azo-imine Schiff base copper(II) complexes
Jahar Lal Pratihar, Paritosh Mandal, Paula Brandão, Dasarath Mal, Vitor Felix
PII:
S0020-1693(17)31880-7
https://doi.org/10.1016/j.ica.2018.04.042
ICA 18233
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
12 December 2017
17 April 2018
20 April 2018
Please cite this article as: J.L. Pratihar, P. Mandal, P. Brandão, D. Mal, V. Felix, Synthesis, characterization, spectral
and catalytic activity of tetradentate (NNNO) azo-imine Schiff base copper(II) complexes, Inorganica Chimica
Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.04.042
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Revised Manuscript
Ms. Ref. No.: ICA-2017-1641R1
Synthesis, characterization, spectral and catalytic activity of tetradentate
(NNNO) azo-imine Schiff base copper(II) complexes
Jahar Lal Pratihara*, Paritosh Mandala, Paula Brandãob, Dasarath Malb, Vitor Felixb
aDepartment of Chemistry, Kandi Raj College, Murshidabad 742137, India
bDepartamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal
________________________________________________________________________________
Abstract
The hexadentate ligand, 2,2′-bis(salicylideneamino)azobenzene, 1 has been synthesized from 2,2'-
diaminoazobenzene and salicylaldehyde in refluxing diethyl ether. Reaction of ligand 1 with Cu(II)
acetate and Cu(II) perchlorate separately in methanol afforded tetradentate (N,N,N,O) Cu(II)
complexes, Cu(L) & [Cu(HL)]ClO4 respectively [where H2L represent the one imine moiety
cleavage product of ligand 1 (H represents the dissociable amino and phenolic protons)]. These
were characterized by microanalytical data and spectroscopic studies. In addition, the crystal
structures of the ligand 1 and complexes Cu(L) & [Cu(HL)]ClO4 were determined by X-ray
diffraction analysis. The diffraction analysis revealed that the ligand (H2L) binds Cu(II) centers in
(N,N,N,O) tetra dentate fashion in distorted square planer geometry. In complex [Cu(HL)]ClO4 the
apical position of copper center is weakly coordinated with one perchlorate ion. The dimeric
structure of the molecule [Cu(HL)]ClO4 is stabilized through NH2···O hydrogen bonds. The
fluorescence and redox property of ligand 1 and complexes Cu(L) & [Cu(HL)]ClO4 were studied.
Preliminary DFT calculations were carried out using crystallographic coordinates to understand the
electronic spectra and redox properties of the ligand and complexes. The complex Cu(L) shows
very good catalytic activities towards oxidation of benzyl alcohol to benzaldeyhde (under solvent-
free condition) and organic thioethers to sulfoxide and sulfones using H2O2 as the oxidant.
_______________________________________________________________________________
Keywords: Copper(II); C=N bond cleavage; Crystal structures; Redox; Emission; Oxidation
reactions
_______________________________________________________________________________
Corresponding author. Fax.: +91 3484 255230; E-mail address: jlpr@rediffmail.com (J. L. Pratihar)
1

1. Introduction
Transition metal complexes incorporating Schiff base ligands work as active catalysts in
homogeneous and heterogeneous reactions and their reactivity varies from metal to metal, nature of
ligands and coordination sites [1-10]. Copper(II) complexes with tetradentate ligands are of
considerable interest due to their structural, catalytic and magnetic properties, in addition to being
potential models for several important biological systems [1-4, 11-14]. The copper complexes of
tetradentate Schiff bases have been used as catalysts during the last decades in many reactions such
as; epoxidation of olefins [15-18], alkylation [19], arylation [20], asymmetric Henry reaction [21-
22], alcohol oxidation [23-26], nitroaldol reaction [27], asymmetric aziridination [28-29],
asymmetric sulfoxidation [30-32], enantioselective ring opening polymerization [33-34] and
oxidation of organic compound that has attracted a vast attention in the field of academic and
industrial study [1-4, 10, 35-36].
The selective oxidation of organic molecules catalyzed by transition metal complex is one
of the most successful and smooth processes for the oxo functionalization of organic substrates [10,
15-18, 23-26, 30-32, 35-36]. The oxidation of alcohols to aldehydes or ketones and organic
thioethers to sulfoxides and sulfones are the important reactions in synthetic organic chemistry and
fine chemical synthesis [23-26, 30-32]. Conventional methods for performing such transformations
generally involve the use of stoichiometric amounts of inorganic oxidants which generate
environmentally hazardous or toxic byproducts [37-38]. From environmental standpoints there is a
strong need for the establishment of efficient and selective catalytic systems using green oxidant.
Hydrogen peroxide is found to be a superior oxidizing agent for the oxidation of alcohols and
thioethers as it is efficient and yield water only the byproduct [39-40]. Many catalytic systems for
the hydrogen peroxide oxidation of alcohols and thioethers have been developed [23-26, 30-32], the
interest is growing continuously in the search for new efficient metal catalysts in this purpose.
2

Herein, we describe the synthesis and characterization of hexadentate ligand, 2,2′-
bis(salicylideneamino)azobenzene, 1. The reactions of ligand 1 with Cu(OAc)2.H2O and
Cu(ClO4)2.6H2O (1:1) separately in methanol afforded Cu(II) complexes (Cu(L) & [Cu(HL)]ClO4),
have been described. The complexes have been characterized by the spectroscopic data. The crystal
structures of the complexes (Cu(L) & [Cu(HL)]ClO4) were determined by X-ray diffraction
analysis. The fluorescence and redox property of ligand 1 and complexes (Cu(L) & [Cu(HL)]ClO4)
were studied. Probable explanations of redox orbitals and electronic spectra have been co-related on
the basis of single point DFT calculations. The oxidation of benzyl alcohol to benzaldeyhde and
organic thioethers to sulfoxide and sulphone has been examined in the presence of catalytic
amounts of Cu(L) and using H2O2 as oxidant.
2. Experimental
2.1. Materials
The solvents used in the reactions were of reagent grade (E. Marck, Kolkata, India) and
were purified and dried by general procedure [41]. Methyl iodide, ethyl bromide, allyl chloride,
benzyl chloride, salicylaldehyde were purchased from E. mark, India. 4-methylbenzyl alcohol, 2-
methylbenzyl alcohol, 4-methoxybenzyl alcohol, thiophenol, copper acetate monohydrate, copper
perchlorate hexahydrate were purchased from Sigma-Aldrich. 2,2'-diaminoazobenzene and alkyl
thiophenol were prepared by following reported procedures [42-43].
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-
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
3

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. Synthesis of ligand 1
To a solution of 2,2'-diaminoazobenzene (0.4 g, 1.88 mmol), in 50 mL diethyl ether was
added salicylaldehyde (0.465 g, 3.81 mmol). The resulting mixture was then heated to reflux for 5
h. It was kept undisturbed for 1 h to obtain solid crystals of 1. The solid product was then filtered
and washed with diethyl ether. Yield: 1, 95%. Anal. Calc. for C26H20N4O2: C, 74.27; H, 4.79; N,
13.33. Found: C, 74.03; H, 4.52; N, 13.57%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane):
1
290 (28740), 235 (28970). IR (KBr, cm−1): 1615 ν(C=N), 1473 ν(N=N), 1283 ν(O-C). H NMR
(400 MHz, CDCl3): 6.87 (ArH, t, 1H, J = 8 Hz), 7.00 (ArH, d, 1H, J = 8 Hz), 7.19-7.35 (ArH, m,
5H), 7.47 (ArH, t, 1H, J = 8 Hz), 7.75 (ArH, d, 1H, J = 8 Hz), 8.60 (CH, s, 1H), 13.58 (OH, s, 1H).
2.3.2. Synthesis of complex Cu(L)
A solution of 1 (0.15 g, 0.36 mmol) in 10 mL methanol was added to a solution of
Cu(OAc)2.H2O (0.071 g, 0.36 mmol) in 10 mL methanol. The mixture was stirred for 1 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(L) was
obtained. Yield: 75%. Anal. Calc. for C19H14CuN4O: C, 60.39; H, 3.73; N, 14.83. Found: C, 60.65;
H, 3.93; N, 14.61%. UV/VIS (λmax/nm (ε/ dm2 mol-1), dichloromethane): 560 (11560), 530 (14100),
420 (18250), 380 (21480), 315 (28530), 240 (44960). IR (KBr, cm−1): 3328 ν(NH), 1609 ν(C=N),
1439 ν(N=N), 1313 ν(O-C).
2.3.3. Synthesis of complex [Cu(HL)]ClO4
4

A solution of 1 (0.15 g, 0.36 mmol) in 10 mL methanol was added to a solution of
Cu(ClO4)2.6H2O (0.132 g, 0.36 mmol) in 10 mL methanol. The mixture was stirred for 1 h. The
colour of the solution immediately changed to pink-violet. The resulting solution was then filtered
and allowed to stand at room temperature. After slow evaporation of the solvent at room
temperature, a pink violet coloured needle shape crystals suitable for x-ray analysis were obtained.
Yield: 70%. Anal. Calc. for C19H15ClCuN4O5: C, 47.71; H, 3.16; N, 11.71. Found: C, 47.46; H,
3.05; N, 11.79%. UV/VIS (λmax / nm (ε / dm2 mol-1), dichloromethane): 560 (9360), 530 (11340),
420 (14230), 380 (15590), 320 (19970), 240 (34500). IR (KBr, cm−1): 3421, 3328 ν(NH2), 1608
ν(C=N), 1429 ν(N=N), 1313 ν(O-C), 1091 ν(Cl-O).
2.4. General procedure for catalytic oxygenation of thioethers
To the catalyst Cu(L) (0.014 mmol) dissolved in 10 mL dichloromethane-methanol (8:2)
mixed solvent, 3 mmol of sulfide and 1.5 mL of 50% H2O2 were all inserted in a 25 mL round-
bottom flask at 00C. The mixture was stirred for 6 h. After the completion of the reaction the
mixture was evaporated to dryness, water was added and the product was extracted with
dichloromethane. The organic layer was washed with brine, dried over Na2SO4, filtered. The
products were separated and purified by preparative TLC using hexane–ethyl acetate (2:1) mixed
solvent. The yields of the products obtained from all the reactions were determined after isolation,
and characterized by 1H NMR and IR spectra.
2.5. Procedure for catalytic oxidation of benzyl alcohol
To a solution of benzyl alcohol (5 mmol), catalyst Cu(L) (0.012 mmol), and 50 % H2O2 5
mL was added and the mixture was vigorously stirred at 750C for 2 h. After that the mixture was
poured into water and the product was extracted with dichloromethane and dried over Na2SO4. The
product was passed through silica column (60-120 mesh) and the complex remains trapped. Upon
removal of the solvent, pure product was obtained. The yields of the products obtained from all the
reactions were determined after isolation, and characterized by 1H NMR and IR spectra.
5

2.6. Crystallography
Single crystals of 1, [Cu(HL)]ClO4 were grown by slow evaporation of ether and methanol
solution at 250C and Cu(L) 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 [44-45]. Full matrix least square refinements on
F2 were performed using SHELXL–97 program [44-45]. 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.
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Please insert Table 1 here
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3. Results and discussion
3.1. Synthesis
The ligand 1 used in this work, synthesized by the condensation of 2,2'-diaminoazobenzene
and salicylaldehyde (1:2) in diethyl ether. The ligand was isolated as brown-red solid crystals on
cooling. The preformed ligand 1 upon reaction with copper(II) acetate monohydrate and copper(II)
perchlorate hexahydrate separately in methanol afforded pink violet azo aldimine Cu(II) complexes
of composition (Cu(L) and [Cu(HL)]ClO4) by the reductive cleavage of one N=CHC6H4OH bond
of ligand 1, where the ligand (H2L) offers (N,N,N,O) coordination mode (Scheme 1). Presumably
methanol is the source of electrons and protons for the cleavage of imine function which is
supported by the observation that no reaction is observed when ligand 1 with copper(II) acetate
monohydrate and copper(II) perchlorate hexahydrate are separately in refluxing CH2Cl2 in presence
of triethyl amine base. In complex Cu(L) the ligand binds in tetradentate (N,N,N,O) bianionic
fashion through deprotonated amino nitrogen, azo nitrogen, imino nitrogen and deprotonated
6

phenolato oxygen atom forming two six membered and one five membered ring. Whereas, in
complex [Cu(HL)]ClO4 the ligand binds in tetradentate (N,N,N,O) monoanionic fashion through
amino nitrogen, azo nitrogen, imino nitrogen and deprotonated phenolato oxygen and one
perchlorate ion satisfy the charge of Cu(II) ion.
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Please insert Scheme 1 here
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3.2. UV-Vis and Fluorescence spectra
Ligand 1 and complexes Cu(L) & [Cu(HL)]ClO4 are soluble in common organic solvents,
such as methanol, acetone, chloroform, dichloromethane, etc., producing intense orange-red and
pink-violet colours respectively. Electronic spectra of the ligand 1 and complexes were recorded in
dichloromethane solutions (Fig. 1). Spectral data are collected in experimental section. Ligand 1
display an intense broad band at 290 nm characteristic of the overlapping n→π* transitions of azo
(N=N) and aldimine (N=CH) function [46-48]. Complexes Cu(L) & [Cu(HL)]ClO4 shows several
intense absorptions within the 240-560 nm range. The high energy absorptions in ultraviolet region
are majorly due to intra ligand π−π* charge transfer transitions [49-50]. A Characteristic low
energy absorption band for Cu(L) & [Cu(HL)]ClO4 at 415 nm is assigned as mixed metal ligand
charge transfer [46-47]. To gain insight into the nature of the absorptions in the visible region, DFT
calculations were performed on both the complexes. Surface plots of the molecular orbitals
(HOMO & LUMO) of 1, Cu(L) and [Cu(HL)]ClO4 are shown in Fig S5 (Supplementary Materials).
From the orbital diagrams it appears that the HOMO and LUMO of complexes Cu(L) and
[Cu(HL)]ClO4 both are metal ligand mixed orbitals.
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Please insert Fig. 1 here
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7

The fluorescence properties of ligand 1 and complexes Cu(L) and [Cu(HL)]ClO4 were
investigated in dilute chloroform solution at room temperature (Fig. 2). The ligand 1 exhibited a
low intensity emissions bands at 450 nm upon excitation at 380 nm. Whereas complexes Cu(L) and
[Cu(HL)]ClO4 exhibited very high intensity emissions bands at 440 nm upon excitation at 385 nm
[though coordinated ligand (H2L) is little different from ligand 1]. Quenching of fluorescence of a
ligand by particular transition metal ions during complex formation is a common fact and it
explained by photoinduced electron transfer process. But in this case the photoinduced electron
transfer is prevented by stable complex formation after Cu(II) and ligand (H2L) interaction,
fluorescence intensity greatly enhanced due to metal ligand chelation effect and the overall process
is referred as “CHEF” [51].
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Please insert Fig. 2 here
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3.3. IR and 1H-NMR
The characteristic stretching frequency of the azomethine ν(HC=N) group in the free ligand
1615 cm-1 shifted to 1609 & 1608 cm-1 in the complexes due to coordination to the metal ion
[37,46,52-53]. The phenolic C–O band of the ligand 1283 cm−1 have been shifted to higher
frequency in the complex 1313 cm−1 indicating the coordination through the phenolic oxygen atom
[46,53]. Similarly the νN=N band (1473 cm-1) of the ligand shifted to lower frequency (1439 & 1429
cm-1) in the complexes, consistent with coordination of the azo nitrogen [42,47-48]. New bands, at
3328 cm-1 and 3421 & 3328 cm-1, in the IR spectra of the complexes Cu(L) & [Cu(HL)]ClO4 unlike
the ligand 1, has been attributed to NH and NH2 groups which is formed after cleavage of one of
the aldimine function from the ligand 1 upon complexation. A very strong and split broad band near
1091 cm-1 and a split strong band near 622 cm-1 are observed in the IR spectrum of the
[Cu(HL)]ClO4 complex, which could be due to the antisymmetric stretching and bending of
perchlorate ions, respectively [46,53]. Spectral data are collected in experimental section.
8

1
Free Schiff base ligand 1 has been characterized by H-NMR spectrum. The ligand 1 show
highly resolved 1H-NMR spectrum, which is in complete agreement with the X-ray structure of the
ligand. The centrosymmetric ligand 1 contains eight aromatic protons which show five distinct set
of aromatic signals in the range 6.86-7.75 ppm. Two triplets 6.86 & 7.46 ppm and two doublets
7.00 & 7.75 ppm are assigned for those of aromatic proton of rings adjacent to azo function. One
multiplate in the range 7.19-7.35 ppm is assigned for four protons of aldehydic ring. The phenolic –
OH and azomithine (―C=N―) protons appeared as a sharp singlet at 13.58 ppm and 8.60 ppm
respectively [37,46,52].
3.4. X-ray crystallography
The crystal structure of ligand 1 and complex Cu(L) & [Cu(HL)]ClO4 were determined by
single crystal X-ray diffraction method. The crystallographic and measurement data are shown in
Table 1. Ligand 1 and complex Cu(L) are orthorhombic, complex [Cu(HL)]ClO4 is triclinic and are
crystallized in the space group Pbca, P212121 and P-1 with Z = 8, 4 and 2 respectively.
The crystal structure of ligand 1 is shown in Fig. 3. Selected bond lengths and bond angles
are given in Tables 2. The centrosymmetric structure of 1 reveals the planar geometry of the ligand
with intra molecular hydrogen bonding between phenolic proton and imine nitrogen [O(1)-
H….N(1) and O(2)-H….N(4)]. All the non-hydrogen atoms of ligand 1 make a good plane (mean
deviation 0.135Å). The O(1)-C(1), N(1)-C(7), N(1)-C(8), and N(2)-C(13) bond lengths 1.349(2),
1.273(2), 1.406(2) and 1.426(2) Å are in agreement with the reported values, whereas the N(2)-N(3)
azo distance (1.246(19) Å) is comparatively shorter [16,42,48].
------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 3 and Table 2 here
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The crystal structure of Cu(L) 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. The
9

------------------------------------------------------------------------------------------------------------------------
Please insert Fig. 4 and Table 3 here
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Ligand (H2L) is coordinated with Cu(II) in a dianionic (L2-) tetra dentate (N,N,N,O) mode, forming
two six membered chelate ring and one five membered ring. The X-Ray crystal structure of Cu(L)
exhibits a copper(II) ion within a nearly square planar environment. All the non-hydrogen atoms of
Cu(L) make a good plane (mean deviation 0.241Å). The metal centre is coordinated to imine
nitrogen N1, azo nitrogen N2, amido nitrogen N4 and phenolate oxygen O1. The Cu-O1, Cu-N1,
Cu-N4 and Cu-N2 bond distances are 1.844(3), 1.848(3), 1.862(3) and 1.850(3) Å respectively.
They are in agreement with previously reported structures [16,18,24-25,32,54-55]. The N2-N3 azo
distance in Cu(L) is longer (1.302(6) Å) than that of ligand 1. The O1-Cu-N1, N1-Cu-N2, N2- Cu-
N4 and O1-Cu-N4 angles are (differ only slightly from 90°) 90.89(16), 84.31(15), 93.54(15) and
91.35(16)° respectively, and the O1-Cu- N2 (174.160), N1-Cu- N4 (177.350) angles are close to
180°.
The crystal structure of [Cu(HL)]ClO4 is shown in Fig. 5 along with the atom labels.
Hydrogen atoms, perchlorate ion are omitted for the clarity. Selected bond lengths and bond angles
are listed in Table 4. The molecular structure of the Cu(II) complex consists of one Cu(II) atom,
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Please insert Fig. 5 and Table 4 here
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one monoanionic ligand unit, and one weakly coordinated perchlorate molecule. The ligand (H2L)
is coordinated with Cu(II) in a monoanionic (HL-) tetra dentate (N,N,N,O) mode, forming two six
membered chelate ring and one five membered ring. The environment of the Cu(II) atom could be
best described as a slightly distorted square-pyramidal (τ = 0.038) geometry with the metal atom
being penta-coordinated [56]. One phenolic oxygen atom (O1), one amine nitrogen, one imine
nitrogen atom (N1) and one azo nitrogen of the monoanionic ligand unit constitute the basal plane
(Cu–O(1), 1.903(4); Cu–N(4), 1.983(6); Cu–N(1), 1.930(5) and Cu–N(2), 1.985(4) Å), and one O
atom (O(2)) of the coordinated perchlorate molecule is occupying the axial position (Cu–O(2),
10

2.580(5) Å). It is clear that the axial bond length Cu–O(2) is elongated (due to the Jahn-Teller effect
of copper(II)) [43,57]. The bond lengths of Cu–N and Cu–O are similar as those of analogous
Cu(II) complexes of salen ligands [16,18,24-25,32]. The dimeric packing structure of
[Cu(HL)]ClO4 is stabilized through three-dimensional NH2···O hydrogen bonds to form chair like
arrangement (Fig. 6) and crystal packing contains very weak pi-pi interaction (Fig S6).
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Please insert Fig. 6 here
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3.5. Electrochemistry
The electrochemical behavior of ligand 1 and complexes Cu(L) and [Cu(HL)]ClO4 were
investigated in dichloromethane acetonitrile mixed solvent (0.1 M TBAP) by cyclic voltammetry
with a scan rate of 0.1 V s−1 (Fig. 7) vs. SCE. The cyclic voltammogram of the ligand 1 and
complexes (Cu(L) & [Cu(HL)]ClO4) [though coordinated ligand (H2L) is little different from ligand
1] displayed nearly identical one electron quasi reversible oxidative response at 1.26 V, 0.90 and
0.89 V, respectively. The identical nature of the cyclic voltamogram indicates that oxidation may be
ligand centered which is well in agreement with the DFT result.
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Please insert Fig. 7 here
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According to the DFT results the frontier orbitals of HOMO-1, HOMO, LUMO and
LUMO+1 (Fig. S5) of complexes Cu(L) & [Cu(HL)]ClO4 are mostly ligand character signifying
the oxidation to be ligand centric.
3.6. Oxidation of thioethers
Catalytic oxidation of sulfides into corresponding sulfoxides and sulfones is an important
area in current chemical research [30, 58]. These are generally used as synthetic intermediates for
the production of various chemically and biologically significant molecules [59-60], as well as for
the synthesis of drugs and natural products [59, 61-62]. Initial investigation of aerobic oxidation
11

was carried out using alkyl phenyl sulfide as substrates with (0.014 mmol) catalyst Cu(L) and H2O2
under 0°C in dichloromethane methanol (8:2) mixed solvent. Two oxidation products sulfoxide and
sulfone are given in scheme 2. The oxidation of phenyl methyl sulfide, as a standard substrate, by
aqueous 50% H2O2 has been carried out using Cu(L) as catalyst to achieve the optimized reaction
conditions. In order to get optimum reaction conditions, amount of catalyst and oxidant were varied
while carrying the reaction at 00C. Three different amount of catalyst (0.005, 0.014, 0.02 mmol)
were used for 3 mmol of phenyl methyl sulfide under above mentioned reaction conditions have
been studied. When increasing the catalyst amount from 0.005 to 0.014 mmol the conversion
increased (yield 45% to 80%) and nearly the same results were obtained with catalyst 0.02 mmol. It
is clear from the above observation 0.014 mmol catalyst is enough to give ~80% conversion in 6 h
of reaction time. Similarly, the effect of different H2O2 concentration (2, 4, 6 eqv.) on the oxidation
of phenyl methyl sulfide have been studied and 4 eqv 50% H2O2 is the most appropriate for this
oxidation reactions. The products were isolated after separation and characterized by IR spectra.
The results of the oxidation products were shown in Table 5. The conversions were calculated on
the basis of isolated yields.
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Please insert Table 5 & Eq. 1 here
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3.7. Oxidation of benzyl alcohol
Oxidation of benzyl alcohol to benzaldehyde is an important organic transformation both at
a laboratory and industrial scale [63]. Benzaldehyde is a very essential chemical which has vast
applications in perfumery, dyestuff and agro chemical industries [64-65]. In the present study we
report the oxidation of four benzyl alcohols having different substituents in the aromatic ring under
atmospheric pressure and in absence of any additive other than the substrate, benzyl alcohol, H2O2
(50%) and catalyst Cu(L) (Scheme 3). Before detailed study on the catalytic oxidation of
substituted benzyl alcohols, initially we first optimized the reaction conditions by choosing benzyl
12

alcohol as the model substrate. It was observed that nearly the same yields of benzaldehyde were
obtained with catalyst amounts varying from 0.012 - 0.02 mmol. The influence of temperature was
studied from 250C-1000C, to obtain the optimum temperature for the oxidation reaction. The
oxidation of benzyl alcohol by H2O2 at room temperature was very slow (yield ~20%) for 5h. The
conversion of benzaldehyde increased above 500C. Above 900C, there was no significant change in
the conversion of benzaldehyde due to the decomposition of H2O2 at high temperature. Hence, 750C
was selected as the optimum temperature. The influence of H2O2 concentration on the oxidation
reaction was analysed by keeping the other parameters constant. The best yield of benzaldehyde
was obtained during the oxidation of benzyl alcohol, with (5 eqv.) 50% H2O2 for 2 h. The products
were isolated after separation and characterized by IR spectra. The results of the oxidation studies
were shown in Table 6. The conversions were calculated on the basis of isolated yields.
------------------------------------------------------------------------------------------------------------------------
Please insert Scheme 3 and Table 6 here
------------------------------------------------------------------------------------------------------------------------
4. Conclusion
Newly designed multidentate azo ligand has been synthesized by the condensation between
2,2'-diaminoazobenzene and salicylaldehyde. Tetradentate (NNNO) Cu(II) complexes were formed
after cleavage of one imine function of the ligand 1. The characterization of the ligand and
complexes were accomplished by analytical and spectral data. X-ray crystallographic studies
confirmed the coordination mode of the ligand to the metal through NNNO donor and reveal the
square planar and square-pyramidal geometry around the copper centers. The redox property and
emission behavior of both the ligand and complexes were examined. Plausible descriptions of redox
orbitals and electronic spectra have been ascribed on the basis of DFT calculations. Fluorescence
intensity of Cu(L) & [Cu(HL)]ClO4 is greatly enhanced due to the formation of stable complexes of
Cu(II) ion with ligand. The prepared catalyst gave excellent catalytic activity towards the oxidation
13

of benzyl alcohol to benzaldeyhde (under solvent-free condition) and organic thioether to
sulfoxides and sulfones in presence of H2O2 as oxidant under atmospheric condition.
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.
Appendix A. Supplementary data
Figures S1–S3 shows the IR spectra of the ligand and complexes and Figure S4 1H NMR spectrum
of the ligand. CCDC 1572763, 1572729 and 1572762 contains the supplementary crystallographic
data for compounds 1, Cu(L) & [Cu(HL)]ClO4. 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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18

Scheme 1
CHO
OH
N
HO
H2N
N
N
N
OH
N
N
Et2O
NH2
Reflux
O
H
.
1
)
A
c
4
A
2
H
O
(
O
u
e
C
M
2
H2
N
O
NH
Cu
O
N
Cu
N
N N
N N
ClO4
Cu(L)
[Cu(HL)]ClO4
NH2
N
HO
After losing one imine
moiety from ligand
N
1
H2L
upon complexation (H
represent the dissociable
amino and phenolic
protons)
N
Scheme 1. Synthesis of ligand and copper(II) complexes
----------------------------------------------------------------------------------------------------------------------
Scheme 2
O
S
O
O
S
H2O2
Cu(L)
S
R
+
R
R
Cat.
Scheme 2. Catalytic conversion of sulfide to sulfoxide and sulfone
------------------------------------------------------------------------------------------------------------------------
Scheme 3
CHO
H2O2
OH
R
R
Cat. Cu(L)
Scheme 3. Catalytic conversion of benzyl alcohol to benzaldehyde
------------------------------------------------------------------------------------------------------------------------
19

Table 1
Crystallographic data for1, 2a and 2b
Parameter
1
Cu(L)
[Cu(HL)]ClO4
C19H15N4OCu, ClO4
478.34
Chemical formula
Formula weight
C26H20N4O2
420.46
C19H14N4OCu
377.88
Crystal system
space group
a/Å
Orthorhombic
P b c a
Orthorhombic
P 21 21 21
5.0203(3)
Triclinic
P -1
16.5940(4)
8.5878(10)
b/Å
11.4449(3)
21.2748(5)
90
16.7586(9)
18.4571(12)
90
9.6911(11)
12.3511(13)
91.935(6)
95.994(5)
110.762(5)
0.71073
c/Å
α/deg
90
90
β/deg
90
90
γ/deg
0.71073
4040.44(17)
1760
0.71073
1552.85(16)
772
λ /Å
V /Å3
953.14(19)
486
F(000)
Crystal Size [mm]
0.08x0.30x0.44
8
0.60x0.020x0.010 0.50x0.28x0.040
Z
4
2
T/K
D/mg/m-3
180(2)
180(2)
1.616
180(2)
1.667
1.382
µ /mm-1
0.090
1.421
1.328
Theta Min-Max [deg]
2.888, 27.898
2.431, 27.968
2.254, 25.966
Reflections measured
Unique reflections (Rint)
No. of reflections used [I> 2σ(I)]
R1,wR2(all data)
28548
33911
19487
4816
3738
3604
3307
3056
2450
0.0785, 0.1166
0.0493, 0.1027
1.019
0.0592, 0.1123
0.0424, 0.1053
1.054
0.0945, 0.1559
0.0543, 0.1306
1.070
R1, wR2[I> 2σ(I)]
GOF
20

Table 2
Selected Bond Distances (Å) and Angles (deg) for Compound 1.
Distances
O(1)-C(1)
1.349(2)
N(2)-N(3)
1.2460(19)
1.426(2)
O(2)-C(22)
1.351(2)
1.406(2)
1.273(2)
1.426(2)
N(3)-C(14)
N(4)-C(20)
N(4)-C(19)
C(6)-C(7)
N(1)-C(8)
N(1)-C(7)
1.276(2)
1.410(2)
1.444(2)
N(2)-C(13)
Angles
N(3)-C(14)-C(15)
N(3)-N(2)-C(13)
N(2)-N(3)-C(14)
N(4)-C(19)-C(18)
N(4)-C(20)-C(21)
N(1)-C(8)-C(13)
123.54(14)
114.33(13)
C(7)-N(1)-C(8)
C(19)-N(4)-C(20)
O(1)-C(1)-C(6)
N(1)-C(7)-C(6)
123.35(14)
123.77(14)
121.71(15)
120.74(15)
121.55(15)
116.27(13)
114.28(13)
124.82(14)
120.94(15)
116.50(13)
O(2)-C(22)-C(21)
N(3)-C(14)-C(19)
21

Table 3
Selected Bond Distances (Å) and Angles (deg) for Compound Cu(L).
Distances
Cu-O(1)
Cu-N(1)
Cu-N(2)
N(2)-N(3)
N(2)-C(13)
N(3)-C(14)
1.302(6)
1.416(6)
1.429(7)
1.898(3)
1.939(4)
1.938(3)
1.910(4)
1.316(7)
1.285(6)
1.420(6)
Cu-N(4)
N(4)-C(19)
C(1)-C(6)
C(6)-C(7)
C(8)-C(13)
1.292(6)
1.446(7)
1.349(7)
1.399(7)
O(1)-C(1)
N(1)-C(7)
N(1)-C(8)
C(14)-C(19)
1.433(7)
Angles
O(1)-Cu-N(1)
90.89(16)
N(1)-Cu-N(4)
N(2)-Cu-N(4)
177.35(14)
93.54(15)
O(1)-Cu-N(2)
O(1)-Cu-N(4)
N(1)-Cu-N(2)
Cu-N(1)-C(7)
Cu-N(1)-C(8)
Cu-N(2)-C(13)
N(1)-C(7)-C(6)
174.16(18)
91.35(16)
84.31(15)
130.8(3)
112.6(3)
112.8(3)
121.3(4)
N(1)-C(8)-C(13) 115.1(5)
Cu-O(1)-C(1) 127.9(3)
N(2)-C(13)-C(8) 115.1(4)
Cu-N(2)-N(3)
Cu-N(4)-C(19)
126.0(3)
127.8(3)
N(2)-N(3)-C(14) 124.0(5)
22

Table 4
Selected Bond Distances (Å) and Angles (deg) for Compound [Cu(HL)]ClO4.
Distances
Cu-O(1)
1.903(4)
1.407(7)
Cu-N(1)
1.930(5)
1.321(7)
N(3)-C(14)
O(1)-C(1)
Cu-O(2)
2.580(5)
1.259(7)
1.983(6)
1.433(5)
1.420(6)
1.286(7)
1.400(7)
1.419(7)
Cu-N(2)
1.985(4)
1.438(8)
1.418(5)
1.421(4)
1.396(5)
1.426(7)
1.408(8)
1.402(8)
N(2)-N(3)
Cu-N(4)
N(4)-C(15)
Cl-O(3)
Cl-O(2)
Cl-O(4)
N(1)-C(8)
N(1)-C(7)
C(14)-C(15)
C(6)-C(7)
Cl-O(5)
N(2)-C(13)
C(1)-C(6)
C(8)-C(13)
Angles
O(1)-Cu-N(1)
O(1)-Cu-N(2)
O(1)-Cu-N(4)
O(1)-C(1)-C(6)
N(1)-Cu-N(2)
N(1)-Cu-N(4)
N(1)-C(7)-C(6)
N(1)-C(8)-C(13)
Cu-N(1)-C(7)
Cu-N(2)-N(3)
N(2)-N(3)-C(14)
95.21(16)
176.81(18)
90.26(18)
124.2(4)
83.01(18)
174.52(19)
125.6(5)
115.3(5)
124.7(3)
132.0(4)
119.6(5)
O(1)-Cu-O(2)
O(2)-Cu-N(1)
O(2)-Cu-N(2)
O(2)-Cu-N(4)
N(2)-Cu-N(4)
O(2)-Cl-O(3)
Cu-O(1)-C(1)
Cu-O(2)-Cl
84.53(16)
92.68(18)
98.17(17)
87.4(2)
91.5(2)
107.5(3)
125.4(3)
118.3(3)
114.2(3)
112.7(3)
120.9(4)
Cu-N(1)-C(8)
Cu-N(2)-C(13)
Cu-N(4)-C(15)
23

Table 5
Oxidation of sulfides catalyzed by Cu(L) in dichloromethane methanol (8:2) mixed solvent
Yield (%)
Equiv
H2O2 (50%)
Time
(h)
Entry
Sulfoxide
Sulfone
S
40
36
38
35
6
6
6
4
4
4
4
Ph
S
42
42
45
Ph
S
S
Ph
Ph
6
32
24

Table 6
Oxidation of alcohol catalyzed by Cu(L)
Product
Equiv
Entry
Time (h) Yield (%)
H2O2 (50%)
CHO
OH
2
2
2
90
85
87
5
5
5
5
CHO
CHO
CHO
OH
OH
OH
78
2
MeO
MeO
25

Fig. 1 UV-Vis spectra of 1 (―), Cu(L) (―) and [Cu(HL)]ClO4 (―) in dichloromethane
26

Fig. 2 Emission spectra of 1 (―), Cu(L) (―) and [Cu(HL)]ClO4 (―) in chloroform.
27

Fig. 3 Molecular structure of 1 (40% probability ellipsoids) with atom numbering scheme.
Hydrogen atoms are omitted for clarity.
28

Fig. 4 Molecular structure of Cu(L) (40% probability ellipsoids) with atom numbering scheme.
Hydrogen atoms are omitted for clarity.
29