Polymer-anchored mononuclear and binuclear CuII Schiff-base complexes: Impact of heterogenization on liquid phase catalytic oxidation of a series of alkenes

Article abstract of DOI:10.1002/aoc.5094

Liquid phase catalytic oxidation of a number of alkenes, for example, cyclohexene, cis-cyclooctene, styrene, 1-methyl cyclohexene and 1-hexene, was performed using polymer-anchored copper (II) complexes PS-[Cu (sal-sch)Cl] (5), PS-[Cu (sal-tch)Cl] (6), PS-[CH₂{Cu (sal-sch)Cl}₂] (7) and PS-[CH₂{Cu (sal-tch)Cl}₂] (8). Neat complexes [Cu (sal-sch)Cl] (1), [Cu (sal-tch)Cl] (2), [CH₂{Cu (sal-sch)Cl}₂] (3) and [CH₂{Cu (sal-tch)Cl}₂] (4) were isolated by reacting CuCl₂·2H₂O with [Hsal-sch] (I), [Hsal-tch] (II), [H₂bissal-sch] (III) and [H₂bissal-tch] (IV), respectively, in refluxing methanol. Complexes 1–4 have been covalently anchored in Merrifield resin through the amine nitrogen of the semicarbazide or thiosemicarbazide moiety. A number of analytical, spectroscopic and thermal techniques, such as CHNS analysis, Fourier transform-infrared, UV–Vis, PMR, ¹³C-NMR, electron paramagnetic resonance, scanning electron microscopy, energy-dispersive X-ray analysis, thermogravimetric analysis, atomic force microscopy, atomic absorption spectroscopy, and electrospray ionization-mass spectrometry, were used to analyze and establish the molecular structure of the ligands (I)–(IV) and complexes (1)–(8) in solid state as well as in solution state. Grafted complexes 5–8 were employed as active catalysts for the oxidation of a series of alkenes in the presence of hydrogen peroxide. Copper hydroperoxo species ([Cu^III (sal-sch)-O-O-H]), which is believed to be the active intermediate, generated during the catalytic oxidation of alkenes, are identified. It was found that supported catalysts are very economical, green and efficient in contrast to their neat complexes as well as most of the recently reported heterogeneous catalysts.

Full text of DOI:10.1002/aoc.5094

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Received: 24 April 2019
DOI: 10.1002/aoc.5094
Revised: 30 May 2019
Accepted: 13 June 2019
F U L L P A P E R
Polymer‐anchored mononuclear and binuclear CuII
Schiff‐base complexes: Impact of heterogenization on liquid
phase catalytic oxidation of a series of alkenes
Abhishek Maurya1 | Neha Kesharwani1 | Payal Kachhap1 | Vivek Kumar Mishra1 |
Nikita Chaudhary2 | Chanchal Haldar1
1 Department of Chemistry, Indian
Liquid phase catalytic oxidation of a number of alkenes, for example, cyclohex-
ene, cis‐cyclooctene, styrene, 1‐methyl cyclohexene and 1‐hexene, was per-
formed using polymer‐anchored copper (II) complexes PS‐[Cu (sal‐sch)Cl] (5),
PS‐[Cu (sal‐tch)Cl] (6), PS‐[CH2{Cu (sal‐sch)Cl}2] (7) and PS‐[CH2{Cu (sal‐tch)
Cl}2] (8). Neat complexes [Cu (sal‐sch)Cl] (1), [Cu (sal‐tch)Cl] (2), [CH2{Cu
(sal‐sch)Cl}2] (3) and [CH2{Cu (sal‐tch)Cl}2] (4) were isolated by reacting
CuCl2·2H2O with [Hsal‐sch] (I), [Hsal‐tch] (II), [H2bissal‐sch] (III) and
[H2bissal‐tch] (IV), respectively, in refluxing methanol. Complexes 1–4 have
been covalently anchored in Merrifield resin through the amine nitrogen of
the semicarbazide or thiosemicarbazide moiety. A number of analytical, spec-
troscopic and thermal techniques, such as CHNS analysis, Fourier transform‐
infrared, UV–Vis, PMR, 13C‐NMR, electron paramagnetic resonance, scanning
electron microscopy, energy‐dispersive X‐ray analysis, thermogravimetric anal-
ysis, atomic force microscopy, atomic absorption spectroscopy, and electrospray
ionization‐mass spectrometry, were used to analyze and establish the molecular
structure of the ligands (I)–(IV) and complexes (1)–(8) in solid state as well as in
solution state. Grafted complexes 5–8 were employed as active catalysts for the
oxidation of a series of alkenes in the presence of hydrogen peroxide. Copper
hydroperoxo species ([CuIII (sal‐sch)‐O‐O‐H]), which is believed to be the active
intermediate, generated during the catalytic oxidation of alkenes, are identified.
It was found that supported catalysts are very economical, green and efficient in
contrast to their neat complexes as well as most of the recently reported hetero-
geneous catalysts.
Institute of Technology (Indian School of
Mines), Dhanbad 826004Jharkhand, India
2 Department of Chemistry and Polymer
Science, Stellenbosch University,
Matieland 7602Stellenbosch, South Africa
Correspondence
Chanchal Haldar, Department of
Chemistry, Indian Institute of Technology
(Indian School of Mines), Dhanbad
826004, Jharkhand, India.
Email: chanchal@iitism.ac.in
Funding information
Science and Engineering Research
Board(SERB), Grant/Award Numbers: SB/
EMEQ‐055/2014 and SB/FT/CS‐027/2014
KEYWORDS
chloromethylated polystyrene, copper (II) complexes, electron paramagnetic resonance,
heterogeneous catalysis, oxidation of alkenes
1 | INTRODUCTION
There are mainly three processes, namely epoxidation,
oxidative cleavage and allylic/benzylic oxidation,[1] by
Conversion of alkenes into various oxygenated products
is academically challenging and industrially important.
which unproductive alkenes can be transformed into syn-
thetically and commercially important chemicals. Alkene
Appl Organometal Chem. 2019;e5094.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.5094

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MAURYA ET AL.
epoxidation contributes important and vital epoxide
intermediates for the synthesis of many pharmaceutical
and fine chemicals.[2] Numerous chemicals, like alcohols,
carbonyl compounds, glycols, alkanolamines and poly-
mers, can be prepared from epoxides. Also, the epoxide
ring can easily interact with a number of nucleophiles,
hence epoxides are commonly used for the production
of additives, anticorrosives, plasticizers, perfumes, epoxy
resins and surfactants, etc.[3]
CuCl2·2H2O with [Hsal‐sch] (I), [Hsal‐tch] (II),
[H2bissal‐sch] (III) and [H2bissal‐tch] (IV). Ligands I
and II were prepared by reacting salicylaldehyde with
semicarbazide hydrochloride and thiosemicarbazide,
respectively, while ligands III and IV were synthesized
by reacting 5,5′‐methylenebis (2‐hydroxybenzaldehyde)
with semicarbazide hydrochloride and thiosemicarbazide,
respectively. All the synthesized copper complexes were
heterogenized by immobilizing on to the surface of
chloromethylated polystyrene. The behavior of synthetic
catalysts towards catalytic oxidation of various alkenes
was closely monitored. The effect of solid support on
alkenes oxidation was also examined. Based on UV–Vis
spectroscopy, liquid chromatography‐mass spectrometry
(LC–MS) and electron paramagnetic resonance (EPR)
analyses, catalytic cycle for the oxidation of alkenes by
synthesized copper complexes in the presence of hydro-
gen peroxide was proposed.
Besides epoxidation, oxidative cleavage of alkenes to the
corresponding carbonyl compounds is another important
reaction for the functionalization of olefins. In the prepara-
tion of important intermediates or stock chemicals, oxida-
tive cleavage of alkenes appears to be a key process. For
this oxidative cleavage, a large number of processes are
documented in the literature. Classically, ozonolysis was
a method for C=C bond cleavage in olefins.[1c] Because of
the safety issues, usability of ozonolysis is often
restricted.[4] A number of bibliographic evidence can be
found where first‐, second‐ or even third‐row transition
metals (e.g. Mn, Fe, Ru, Os, Au, Pd, W, Re, etc.)[5] were
used to catalyze the oxidative cleavage of olefins in the
absence of ozone. Despite the significant advancement in
the field of oxidative cleavage of olefins, there is still a
deficiency in economic and green catalytic processes.
Lastly, allylic oxidation of olefins and alkylarenes into
corresponding carbonyl compounds and α,β‐unsaturated
enones through C‐H bond activation is one of the impor-
tant conversions with synthetic as well as industrial
utility. Often allylic or benzylic oxidation processes of
olefins and alkylarenes are connected with the pre-
paration of drug precursors and are involved in the build-
ing blocks of many organic syntheses.[6] A number of
organocatalysts have emerged in the field of allylic or
benzylic oxidation, but the metal‐based catalysts for this
oxidation remain exceptionally efficient.[7]
2 | EXPERIMENTAL
2.1 | Materials
1,3,5‐Trioxane (Sigma‐Aldrich, USA), CuCl2·2H2O
(Loba‐Chemie, India), salicylaldehyde (SRL, India),
thiosemicarbazide (Avra, India), semicarbazide hydro-
chloride (Avra, India), Merrifield's peptide resin (3.5–
4.5 mmol/g Cl−, 1% crosslinked; Sigma‐Aldrich, USA),
potassium iodide (Rankem, India), cyclohexene (Alfa‐
Aesar, India), styrene (Alfa‐Aesar, India), 1‐methyl cyclo-
hexene (Alfa‐Aesar, India), cis‐cyclooctene (Alfa‐Aesar,
India), 1‐hexene (Alfa‐Aesar, India), 30% H2O2 (Merck,
India), 70% TBHP (Alfa‐Aesar, India), Nujol (SRL, India),
H2SO4 (Merck, India) and AR grade solvent (Merck &
Rankem, India) were used as received. HPLC grade meth-
anol (Spectrochem, India) was used for gas chroma-
tography (GC) and GC–MS analysis. 5,5′‐Methylenebis
(2‐hydroxybenzaldehyde) (H2bissal) was prepared by fol-
lowing the reported method.[8]
Easy separation, recyclability, high thermal stability
and the eco‐friendly nature of heterogeneous catalysts
make them an attractive alternative of homogeneous
catalysts. Hence, heterogeneous catalysts are extensively
used in the diverse field of catalysis. To make a sus-
tainable and non‐polluting catalytic cycle, covalently
anchored homogeneous catalysts on to the surface of
chloromethylated polystyrene crosslinked with divinyl
benzene appear to be an exotic field of research in the
area of oxidation of various organic substrates. Easy avail-
ability, simple functionalization process, elevated thermal
stability and low price makes chloromethylated poly-
styrene an irresistible choice among various organic, inor-
ganic and hybrid solid supports.
2.2 | Physical methods and analysis
Fourier transform‐infrared (FT‐IR) spectra (4000–400 cm
−1) were recorded on Agilent Cary 600 Series FT‐IR
Spectrometer by ATR method. CHNS elemental analysis
was done in CHNS elemental model electron probe
1
microanalyser SX, CAMECA (France). The H‐ and 13C‐
NMR spectra of the ligands were recorded from Bruker
AC‐400 NMR spectrometer in dimethylsulfoxide (DMSO)
solution using TMS as an internal standard. Electronic
spectra of ligands and metal complexes were recorded
in a SHIMADZU UV‐1800 spectrophotometer using
Herein we have reported four CuII complexes [Cu (sal‐
sch)Cl] (1), [Cu (sal‐tch)Cl] (2), [CH2{Cu (sal‐sch)Cl}2] (3)
and [CH2{Cu (sal‐tch)Cl}2] (4) synthesized by reacting

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MAURYA ET AL.
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DMF/methanol as a solvent, whereas electronic spectra
of polymer‐bound metal complexes were recorded by dis-
persing in Nujol (heavy paraffin wax). The presence of
copper content and surface morphology of polymer‐
anchored copper complexes were analyzed by energy‐
dispersive X‐ray analysis (EDX) and scanning electron
microscopy (SEM) on a HITACHI S‐3400 N instrument
after coating the polymer bead surface with a thin film
of gold to block the surface charging and thermal damage
by the electron beam. A scanning probe microscope from
DIMENSION iCON with ScanAsyst was used for atomic
force microscopy (AFM) imaging. Thermogravimetric
analysis (TGA) and differential thermal analysis (DTA)
were performed by using Perkin Elmer, Diamond TG/
DTA instrument. Copper content was confirmed by
atomic absorption spectrometer (Model No. Lab India,
AA 8000) after decomposing the polymer‐anchored metal
complexes by concentrate HNO3 and subsequent dilution
of the filtrate. The electrospray ionization (ESI)+ mass of
the metal complexes was estimated on Waters Q‐Tof
Micromass instrument. EPR spectra of the polymer‐
anchored metal complexes were recorded in a Bruker
EMX X‐band spectrometer operating at 100‐kHz field
modulation at room temperature. LC–MS analysis of
the reactive intermediates was done on an Agilent
(Model No. G1978 B) LC–MS instrument. The catalytic
activity of various alkenes was monitored by an Agilent
7890 B GC fitted with an HP‐5 capillary column
(30 m × 0.25 mm × 0.25 μm), and an FID detector
was used to analyze the reaction products. Oxidation prod-
ucts of various alkenes were identified by thermoscientific
GC–MS (Model no. Trace 1300, ISQ QD) fitted with a
TG‐5MS capillary column (30 m × 0.25 mm × 0.25 μm)
and an EI+ mass detector.
calculated at the same level of theory. The absence of
imaginary frequency signifies that optimized geometry
appears for local minima. All the geometry optimizations
were performed without any symmetry constrain.
2.4 | Synthesis of [Hsal‐sch] (I) and [Hsal‐
tch] (II)
Ligand [Hsal‐sch] (I) was prepared by the following
method, which is different from the earlier reported
method.[14] A methanolic solution (20 ml) of semi-
carbazide hydrochloride (1.1153 g, 10 mmol) was reacted
with a methanolic solution (20 ml) of salicylaldehyde
(1.2204 g, 10 mmol), and the resulting mixture was
refluxed for ~4 hr (Scheme 1). The volume of the solution
was reduced to ~10 ml and kept in a refrigerator where a
white color solid separated out, which was filtered, washed
with methanol and dried in vacuum over silica gel.
Data for [Hsal‐sch] (I): yield: 70.45% (1.33 g); anal.
calcd for C8H9N3O2 (MW 179.18); C, 53.63%; H, 5.06%;
N, 23.45%. Found: C, 53.58%; H, 5.13%; N, 23.32%. FT‐
IR (ATR, cm−1): 3479(νO‐H), 3274(νN‐H), 1684(νC=O),
1581(νC=N); UV–Vis [λmax (nm), ε (lmol−1 cm−1)]: 216
(2.07 × 103), 224 (sh), 277 (2.41 × 103), 286 (sh), 317
(1.48 × 103). 1H‐NMR (DMSO‐d6, δ in ppm): 6.24 (s,
2H), 6.75–6.79 (t, 1H), 6.80–6.82 (d, 1H), 7.09–7.13 (t,
1H), 7.52–7.53 (d, 1H), 8.10 (s, 1H), 9.89 (br, 1H), 10.20
(s, 1H); 13C‐NMR (DMSO‐d6, δ in ppm): 115.86, 118.97,
119.98, 127.13, 129.89, 138.97, 155.93, 156.50.
[Hsal‐tch] (II) was prepared by adopting the method
used to synthesize [Hsal‐sch] (I).
Data for [Hsal‐tch] (II): yield: 67.91% (1.32 g); anal.
calcd for C8H9N3OS (MW 195.24); C, 49.21%; H, 4.65%;
N, 21.52%. Found: C, 49.10%; H, 4.71%; N, 21.45%. FT‐
IR (ATR, cm−1): 3430(νO‐H), 3283(νN‐H), 1604(νC=N),
1037(νC=S); UV–Vis [λmax (nm), ε (lmol−1 cm−1)]: 230
(1.13 × 103), 294 (sh), 304 (1.88 × 103), 331 (2.35 × 103).
1H‐NMR (DMSO‐d6, δ in ppm): 6.76–6.80 (t, 1H), 6.83–
6.85 (d, 1H), 7.15–7.19 (t, 1H), 7.84 (s, 2H), 8.07 (s, 1H),
8.37 (s, 1H), 9.79 (s, 1H), 11.37 (s, 1H); 13C‐NMR
(DMSO‐d6, δ in ppm): 115.91, 120.08, 126.55, 130.22,
132.35, 137.97, 154.08, 156.55.
2.3 | Density functional theory
calculations
Gaussian ‘09 rev. D.01[9] was used to perform all the the-
oretical calculations. For the preparation of input files
and visualization of the output files, GaussView 5.0.8[10]
was used. Gas phase optimization of all ground state
molecular structures was executed by density functional
theory (DFT) in its unrestricted form using LANL2DZ
with an effective core potential for Cu atoms and 6–
311 + G(d, p) for the rest of the atoms (C, H, N, O, S
and Cl), employing a HP‐Z440 workstation. Becke3–
Lee–Yang–Parr (B3LYP) procedure[11–13] was used to
incorporate electron correlation into the DFT calculation.
Initial geometry for the optimization was taken from
single‐crystal X‐ray refinement data of the mononuclear
[Cu (sal‐sch)Cl](1).[14] Vibrational frequencies were
2.5 | Synthesis of [H2bissal‐sch] (III) and
[H2bissal‐tch] (IV)
A hot methanolic solution (25 ml) of 5,5′‐methylenebis
(2‐hydroxybenzaldehyde) (H2bissal) (2.5625 g, 10 mmol)
was reacted with the methanolic solution (25 ml) of
semicarbazide hydrochloride or thiosemicarbazide
(20 mmol), and this reaction mixture was refluxed for
1 hr (Scheme 1). During refluxing, the white colored solid

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MAURYA ET AL.
SCHEME 1 Proposed synthetic scheme for the preparation of ligands and their corresponding copper (II) complexes reported in this paper
product precipitated out, which was filtered, washed with
ether, and dried in vacuum over silica gel.
1:1 ratio, respectively. Likewise, [CH2{Cu (sal‐sch)Cl}2]
(3) and [CH2{Cu (sal‐tch)Cl}2] (4) were isolated by
reacting [H2bissal‐sch] (III) and [H2bissal‐tch] (IV) with
CuCl2·2H2O in 1:2 ratio, respectively. During the synthe-
sis of 1–4, all the complexes separated out from the
refluxing reaction mixture, which were filtered, washed
with methanol (3 × 10 ml) and dried in vacuum over
silica gel (Scheme 1).
Data for [H2bissal‐sch] (III): yield: 65.21% (1.82 g);
anal. calcd for C17H18N6O4 (MW 370.36); C, 55.13%; H,
4.90%; N, 22.69%. Found: C, 55.08%; H, 5.01%; N,
22.55%. FT‐IR (ATR, cm−1): 3482(νO‐H), 3273(νN‐H),
1690(νC=O), 1583(νC=N); UV–Vis [λmax (nm), ε (lmol
−1
cm−1)]: 221 (2.65 × 103), 281 (3.26 × 103), 289 (sh),
325 (1.63 × 103); H‐NMR (DMSO‐d6, δ in ppm): 3.72 (s,
1
2H), 6.27 (br, 4H), 6.71–6.73 (d, 2H), 6.93–6.96 (d, 2H),
7.50 (s, 2H), 8.06 (s, 2H), 9.67 (br, 2H), 10.12 (s, 2H);
13C‐NMR (DMSO‐d6, δ in ppm): 37.05, 116.04, 119.20,
120.06, 127.02, 130.94, 140.49, 156.42, 177.62.
2.6.1 | Data for [Cu (sal‐sch)Cl] (1)
Data for [H2bissal‐tch] (IV): yield; 81.25% (2.53 g);
anal. calcd for C17H18N6O2S2 (MW 402.49); C, 50.73%;
H, 4.51%; N, 20.88%. Found: C, 50.69%; H, 4.71%; N,
20.75%. FT‐IR (ATR, cm−1): 3437(νO‐H), 3278(νN‐H),
1608(νC=N), 1039(νC=S); UV–Vis [λmax (nm), ε (lmol
Yield: 78.34% (2.17 g); anal. calcd for C8H8ClCuN3O2
(MW 277.17): C, 34.67%; H, 2.91%; N, 15.16%. Found: C,
34.56%; H, 3.05%; N, 15.01%. FT‐IR (ATR, cm−1):
3344(νO‐H), 3155(νN‐H), 1648(νC=O), 1536(νC=N). UV–Vis
[λmax (nm), ε (lmol−1 cm−1)]: 227 (4.32 × 103), 252
(3.43 × 103), 270 (2.94 × 103), 295 (2.49 × 103), 309
(1.76 × 103), 367 (2.19 × 103), 724 (91). ESI+‐MS: m/z
243.04 ([[Cu (sal‐Hsch)] + H]+).
−1
cm−1)]: 235 (4.57 × 103), 298 (sh), 309 (6.20 × 103),
1
339 (6.44 × 103). H‐NMR (DMSO‐d6, δ in ppm): 3.69 (s,
2H), 6.70–6.73 (d, 2H), 7.00–7.03 (d, 2H), 7.92 (s, 4H),
8.04 (s, 2H), 8.30 (s, 2H), 9.53 (s, 2H), 11.33 (s, 2H). 13C‐
NMR (DMSO‐d6, δ in ppm): 37.10, 116.10, 120.44,
126.42, 130.52, 132.71, 137.51, 154.15, 156.86.
2.6.2 | Data for [Cu (sal‐tch)Cl] (2)
Yield: 82.04% (2.40 g); anal. calcd for C8H8ClCuN3OS
(MW 293.23): C, 32.77%; H, 2.75%; N, 14.33%. Found: C,
32.66%; H, 2.91%; N, 14.17%. FT‐IR (ATR, cm−1):
3378(νO‐H), 3250(νN‐H), 1586(νC=N), 1026(νC=S). UV–Vis
[λmax (nm), ε (lmol−1 cm−1)]: 207 (3.33 × 103), 230
(2.76 × 103), 270 (4.02 × 103), 321 (2.79 × 103), 353
(1.71 × 103), 385 (1.78 × 103), 641 (113). ESI+‐MS: m/z
258.98 ([Cu (sal‐Htch)]+).
2.6 | Synthesis of [Cu (sal‐sch)Cl] (1), [Cu
(sal‐tch)Cl] (2), [CH2{Cu (sal‐sch)Cl}2] (3)
and [CH2{Cu (sal‐tch)Cl}2] (4)
Complexes 1–4 were prepared by following a common
synthetic route. [Cu (sal‐sch)Cl] (1) and [Cu (sal‐tch)Cl]
(2) were prepared by reacting the methanolic solution of
CuCl2·2H2O with [Hsal‐sch] (I) and [Hsal‐tch] (II) in