Efficient oxidation of benzene catalyzed by Cu(II) tetrazolato complexes under mild conditions

Article abstract of DOI:10.1016/j.inoche.2019.05.006

Two new coordination complexes containing 5-(4-chlorophenyl)tetrazolato (4-Clptz) and polypyridyl ligands, [Cu₂(bipy)₂(4-Clptz)₄] [1, bipy = 2,2′-bipyridine] and [Cu₂(phen)₂(4-Clptz)₄] [2, phen = 1, 10 phenanthroline] have been synthesized following metal ligated [2 + 3] cycloaddition reaction between copper bound azide polymer containing polypyridyl and 4-chloro benzonitrile. Both the complexes have been fully characterized by different spectroscopic techniques. Single crystal X-ray diffraction study reveals that the complexes are dimeric in nature with penta-coordinated geometry around the Cu(II) centre through N-donor sites. These complexes have been explored as homogeneous catalysts for the selective oxidation (in presence of hydrogen peroxide) of benzene to phenol and benzoquinone under mild conditions in an open atmosphere condition.

Full text of DOI:10.1016/j.inoche.2019.05.006

Accepted Manuscript
Efficient oxidation of benzene catalyzed by Cu(II) tetrazolato
complexes under mild conditions
Jyoti Kumari, Shaikh M. Mobin, Suman Mukhopadhyay, Komal
M. Vyas
PII:
S1387-7003(19)30410-1
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2019.05.006
INOCHE 7395
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
24 April 2019
5 May 2019
5 May 2019
Please cite this article as: J. Kumari, S.M. Mobin, S. Mukhopadhyay, et al., Efficient
oxidation of benzene catalyzed by Cu(II) tetrazolato complexes under mild conditions,
Inorganic Chemistry Communications, https://doi.org/10.1016/j.inoche.2019.05.006
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ACCEPTED MANUSCRIPT
Efficient Oxidation of Benzene Catalyzed by Cu(II) Tetrazolato
Complexes Under Mild Conditions
Jyoti Kumari,a Shaikh M. Mobin,a Suman Mukhopadhyaya*, Komal M. Vyas,a,b**
aDiscipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Simrol,
Indore-453552, India
bDepartment of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388120, India
*Corresponding authors: *suman@iiti.ac.in, **komal_vyas@spuvvn.edu
Abstract
Two new coordination complexes containing 5-(4-chlorophenyl)tetrazolato (4-Clptz) and
polypyridyl ligands, [Cu2(bipy)2(4-Clptz)4] [1, bipy = 2,2'-bipyridine] and [Cu2(phen)2(4-Clptz)4]
[2, phen = 1, 10 phenanthroline] have been synthesized following metal ligated [2+3]
cycloaddition reaction between copper bound azide polymer containing polypyridyl and 4-chloro
benzonitrile. Both the complexes have been fully characterized by different spectroscopic
techniques. Single crystal X-ray diffraction study reveals that the complexes are dimeric in
nature with penta-coordinated geometry around the Cu(II) centre through N-donor sites. These
complexes have been explored as homogeneous catalysts for the selective oxidation (in presence
of hydrogen peroxide) of benzene to phenol and benzoquinone under mild conditions in an open
atmosphere condition.
Keywords: Cu(II) tetrazolato complexes, [2+3] cycloaddition, oxidation of benzene
A great deal of attention has been given towards the synthesis of functional compounds
containing polyazole rings, mainly tetrazoles and their derivatives, due to their huge range of
applications in diverse fields. The metal complexes of tetrazoles have been a centre of attraction
in recent years because of its regnant applications in coordination chemistry, materials science,
organic synthesis [1], gas generating composition [2], biochemical and pharmaceutical, catalysis
[3], medicinal chemistry [4] and anti-corrosion species [5]. In recent years many metal
tetrazolate complexes have been prepared using [2+3] cycloadditions between metal mediated
azides and nitriles [6-13]. Metal complexes of variety of tetrazoles with copper ion have been
extensively studied by different researchers in the past few years. [3, 14-26] However, catalytic
activity of only few copper tetrazolato complexes has been explored till date [9, 27-34]. Till date
no reports are available on oxidation of benzene catalyzed by Cu(II) tetrazolato complexes. The

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direct oxidation of benzene to phenol [35, 36] has attracted much attention and has been
investigated extensively as a promising route for phenol production. This is due to the inertness
of C-H bonds especially, in saturated hydrocarbons, which is one of the difficulties towards
oxidation of hydrocarbons, where the bond energy amount is 400kJ mol-1 [37]. Secondly, it’s a
one-step process for the production of phenol. Generally, phenol is produced mainly by a three-
step reaction process in industry with low phenol yields, low atomic efficiency and with the
formation of explosive intermediates [38-41]. Hence, our interest in tetrazoles concerns their use
as precursors for the formation of new functionalized tetrazolato complexes which have potential
applications as catalysts for oxidation of benzene. Herein, we report synthesis of two Cu(II)
tetazolato complexes containing 4-chloro tetrazole and phen/bipy as auxiliary ligand. The
catalytic efficiency of these complexes on the direct oxidation of benzene to phenol in presence
of H2O2 has also been investigated.
Cu(II) tetrazolato complexes have been prepared using metal ligated [2+3] cycloaddition
reaction between copper bound azide polymer containing polypyridyl and 4-chloro benzonitrile
under refluxing condition for 12 hrs at 130°C (Scheme 1). In the presence of 4-
chlorobenzonitrile, the reaction of [Cu(bipy)(N3)2]n and [Cu(phen)(N3)2]n affords blue crystals of
complexes 1 and 2 (Fig.1). Both the complexes have been fully characterized using various
techniques. The IR spectra of complexes display bands in the range of 1610-1678 cm-1 indicating
the formation of tetrazolato moiety and there is no azide band observable in these complexes
(Figs. S1 & S2). The ESI-MS spectrometry analysis of 1 reveals molecular ion peak at 577.1
(Fig. S3); and a molecular ion peak at 677.3 for 2 (Fig. S4). The nature of the complexes has
been unambiguously determined by single–crystal X-ray crystallography. The title complexes
have bridging tetrazolato ligand to generate a dimer and crystallize in the triclinic system and
space group P-1. The crystallographic data of the complexes are summarized in Table S1.
Important bond lengths and bond angles are depicted in Tables S2 & S3. As shown in Fig.1, two
copper complexes are found to be dimeric in nature with five-coordinated geometry around the
metal centre through N donor sites. There are two types of tetrazolato ligand: one acting as a
terminal ligand and the other exhibiting a bridging nature, adopting simultaneously asymmetric
end-on and end-to-end bridging modes. Single crystal X-ray diffraction reveals that complex 1
has square pyramidal and complex 2 has distorted square pyramidal geometry around the metal
centre. The crystal structure of 1 shows that all the bond lengths and bond angles are similar for

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regular square pyramidal geometry, while 2 shows that Cu(II) ion displays Jahn–Teller distortion
with higher axial Cu1–N3 bond distance of 2.251(3) Å than equatorial bond distances in the
range of 1.999(3)-2.063(2) Å. The structure is strongly perturbed from the normal observed
tetragonally elongated Jahn–Teller distortion.
Scheme 1. Schematic representation for the formation of Cu(II)-tetrazolato complexes.
(A)
(B)
Fig.1. ORTEP views of the (A) complex 1 and (B) complex 2 with displacement ellipsoids
drawn at 30%. H atoms have been removed for clarity.
Catalytic oxidation of hydrocarbons is an important class of reaction from the viewpoint
of the environments as well as for the production of the specialty chemicals. Oxidation of

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benzene has been widely studied with different transition metal complexes under relatively mild
condition. Benzene tends to produce phenol upon oxidation which tends to get over oxidized to
form catechol and quinones as shown in reaction scheme 2. In this particular study oxidation of
benzene has been carried out in presence of complexes 1 and 2 as oxidation catalysts. Different
reaction conditions have been explored for an optimum set of reaction condition for most
efficient activity of the metal complexes under relatively mild condition.
Scheme 2. Oxidation of benzene catalysed by Cu(II) complexes using hydrogen peroxide.
Under optimized (70ºC, 3 h of refluxing and 1 mole% of Cu-catalyst) conditions, in
acetonitrile with hydrogen peroxide as an oxidant led to excellent yields of the oxygenated
products with almost 100% conversion for complex 1. The reaction mixture has shown
unprecedented selectivity towards formation of phenol as production of up to 91% of phenol has
been detected in GC. The selectivity for the phenol is relatively lower for complex 2 as 75.97 %
of phenol has been detected. Catechol and benzoquinone were the two other products detected by
GC-MS analysis under the above conditions, however formation of these products is found to be
much lower in terms of yield. All the results of oxidation study are combined in Table 1. A blank
reaction, in the absence of any catalyst, was carried out under the studied reaction conditions and
no significant conversion and selectivity for product was observed. If compared with reported
oxidation of benzene to phenol [40, 42], the present system shows encouraging conversion and
selectivity under mild reaction conditions. p-chloro substitution with electron withdrawing effect
makes the ligand weaker donor. So, it may tend to dissociate easily making the metal center open
for substrate binding. This may be the probable reason for observing better catalytic activity for
the synthesized complexes.

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Table 1. Catalytic details for the oxidation of benzene by Cu(II) complexes
Catalyst
Phenol
Benzoquinone Catechol %Conversion TON
1
2
91.60
75.97
-
-
8.40
21.67
100
97.64
37
24
In summary two new dinuclear Cu(II)-tetrazolato complexes 1-2 have been synthesized
by means of cycloaddition between copper ligated azide and 4-chlorobenzonitrile. The structural
characterization of the complexes was established by various physicochemical techniques like
single crystal X-ray diffraction, IR spectroscopy, elemental analyses and mass spectrometric
studies. Single crystal X-ray diffraction revealed that 1 has square pyramidal and 2 has distorted
square pyramidal geometry around the metal centre. Also, 2 exhibits Jahn-Teller distortion.
These complexes exhibited promising catalytic activity concomitant with high selectivity for the
oxidation of benzene to phenol using hydrogen peroxide as an oxidant. The selectivity for the
phenol is relatively higher for complex 1. The obtained yields are quite high with respect to
previously reported results. This catalytic system presents several advantages in terms of
sustainability: they are fast, facile work-up process and use a non-toxic and cheap metal as
catalyst. A more detailed study is currently under way to understand the effect of reactant
concentration and different conditions on reaction rate and plausible mechanistic pathway.
Acknowledgement
The authors are thankful to SIC, IIT Indore for analytical studies has been gratefully
acknowledged.
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Efficient Oxidation of Benzene Catalyzed by Cu(II) Tetrazolato
Complexes Under Mild Conditions
Jyoti Kumari,a Shaikh M. Mobin,a Suman Mukhopadhyaya*, Komal M. Vyas,a,b**
aDiscipline of Chemistry, School of Basic Sciences, Indian Institute of Technology Indore, Khandwa Road, Simrol,
Indore-453552, India
bDepartment of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388120, India
*Corresponding authors: *suman@iiti.ac.in, **komal_vyas@spuvvn.edu
Graphical Abstract

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Highlights
 Two new coordination complexes [Cu2(bipy)2(4-Clptz)4] and [Cu2(phen)2(4-Clptz)4]
have been synthesized following metal ligated [2+3] cycloaddition reaction.
 Single crystal X-ray diffraction study reveals that the complexes are dimeric in nature
with penta-coordinated geometry around the Cu(II) centre through N-donor sites.
 These complexes have been explored as homogeneous catalysts for the selective
oxidation of benzene.