2,2′-Bipyridyl-acetylphenolato mixed ligand copper(II) complexes: Syntheses, characterizations and catalytic activity in styrene epoxidation

Article abstract of DOI:10.1080/00958972.2012.747088

Two copper(II) complexes, [Cu(bipy)(5-Br-2-hap)(ClO₄)]2 (1) and [Cu(bipy)(2-hap)(ClO₄)] (2) (where bipy = 2,2′-bipyridine, 5-Br-2-hap = 5-bromo-2-hydroxyacetophenone, 2-hap = 2-hydroxyacetophenone), were synthesized and characterized. The crystal structure of 1 was determined by single crystal X-ray diffraction while 2 was reported earlier. Structural characterization reveals that the presence of bromine in 5-Br-2-hap plays a structure-determining role in dimeric 1 in comparison with the mononuclear 2 where 2-hap was used. Studies of the catalytic potential toward styrene epoxidation in homogeneous system using H₂O₂ as oxidant reveal that 1 is more efficient than 2 with respect to epoxide selectivity.

Full text of DOI:10.1080/00958972.2012.747088

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2,2′-Bipyridyl-acetylphenolato mixed
ligand copper(II) complexes: syntheses,
characterizations and catalytic activity
in styrene epoxidation
Shyamapada Shit a , Umesh Yadava b , Debraj Saha c & Roland
Fröhlich d
a Department of Chemistry, Krishnagar Government College,
Nadia, India
b Department of Physics, DDU Gorakhpur University, Gorakhpur,
India
c Department of Chemistry, Jadavpur University, Kolkata, India
d Organisch-Chemisches Institut, Universität Münster, Münster,
Germany
Accepted author version posted online: 07 Nov 2012.
To cite this article: Shyamapada Shit , Umesh Yadava , Debraj Saha & Roland Fröhlich (2013): 2,2′-
Bipyridyl-acetylphenolato mixed ligand copper(II) complexes: syntheses, characterizations and
catalytic activity in styrene epoxidation, Journal of Coordination Chemistry, 66:1, 66-76
To link to this article: http://dx.doi.org/10.1080/00958972.2012.747088
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Journal of Coordination Chemistry
Vol. 66, No. 1, 10 January 2013, 66–76
2,2′-Bipyridyl-acetylphenolato mixed ligand copper(II)
complexes: syntheses, characterizations and catalytic activity
in styrene epoxidation
SHYAMAPADA SHIT†*, UMESH YADAVA‡, DEBRAJ SAHA§ and ROLAND
{
FRÖHLICH
†Department of Chemistry, Krishnagar Government College, Nadia, India
‡Department of Physics, DDU Gorakhpur University, Gorakhpur, India
§Department of Chemistry, Jadavpur University, Kolkata, India
{Organisch-Chemisches Institut, Universität Münster, Münster, Germany
(Received 7 May 2012; in final form 20 September 2012)
Two copper(II) complexes, [Cu(bipy)(5-Br-2-hap)(ClO4)]2 (1) and [Cu(bipy)(2-hap)(ClO4)] (2)
(where bipy = 2,2′-bipyridine, 5-Br-2-hap = 5-bromo-2-hydroxyacetophenone, 2-hap = 2-hydroxyace-
tophenone), were synthesized and characterized. The crystal structure of 1 was determined by
single crystal X-ray diffraction while 2 was reported earlier. Structural characterization reveals that
the presence of bromine in 5-Br-2-hap plays a structure-determining role in dimeric 1 in compari-
son with the mononuclear 2 where 2-hap was used. Studies of the catalytic potential toward styrene
epoxidation in homogeneous system using H2O2 as oxidant reveal that 1 is more efficient than 2
with respect to epoxide selectivity.
Keywords: Mixed ligand copper(II) complexes; Spectral and structural characterizations;
Homogeneous catalysis; Hydrogen peroxide; Styrene epoxidation
1. Introduction
Mixed ligand copper complexes of 2,2′-bipyridine (bipy) or similar ligands (rigid or semi-
rigid) give a variety of complexes depending on the oxidation state of copper and the
nature of other coordinating ligands [1, 2]. Such complexes have been extensively studied
over several decades due to their magnetic, optical and catalytic properties [1, 3]. Mono-/
binuclear copper(II) complexes of this type are also important as biological model systems
in understanding the role of bimetallic centers in several metalloproteins and metalloen-
zymes [4–7] as the active sites of such proteins/enzymes usually involve coordination of
copper by at least two nitrogen and oxygen donors from other ligands [8]. Thus, synthesis
and characterization of mixed ligand complexes with diamine and other oxygen donors are
of continuous interest. Some copper(II) complexes with diamines and other ligands exhibit
unique biological activities against various bacterial and fungal strains [1] as well as strong
antitumor activities [8–10].
*Corresponding author. Email: shyamakgcchem@gmail.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online Ó 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.747088

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2,2′-Bipyridyl-acetylphenolato Mixed Ligand Copper(II) Complexes
67
Focus has been made on synthesis and characterization of mono-/binuclear copper(II)
complexes with bipy and salicylaldiminato ligands [2, 11]; however, the catalytic potential
of such copper(II) complexes is yet to be explored. Olefin epoxidation catalyzed by transi-
tion metal complexes has widespread applications in chemical and pharmaceutical indus-
tries [12, 13]. Attempts to use Schiff-base copper(II) complexes in homogeneous catalytic
epoxidation have been reported [14–17]. Several oxidants such as 30% hydrogen peroxide,
tert-butyl hydroperoxide (tert-BuOOH), sodium hypochlorite and even molecular oxygen
are used for the production of epoxides from alkenes. Among them, 30% hydrogen perox-
ide attracts increased attention in industrial use because it is environmentally friendly. As
recently outlined by Beller [18], H2O2 is characterized by unique features and advantages
[19] such as high atom efficiency [20], moderate cost, safe handling and storage and pro-
duction of water as the only byproduct [21, 22], making it the most interesting oxidant
after molecular oxygen and stimulating its use in liquid-phase oxidations, especially for
fine chemicals production [23].
In this article, we describe the synthesis of a mixed ligand copper(II) complex, [Cu(bipy)
(5-Br-2-hap)(ClO4)]2 (1). The use of 5-Br-2-hap having electron-withdrawing bromine on
the acetylphenolato moiety leads to dimeric 1 in comparison with monomeric 2 [2]. Com-
plex 1 was characterized by elemental analyzes, Fourier transform infrared spectra (FTIR),
UV–vis, cyclic voltammetry and single crystal X-ray diffraction, and compared with 2. The
catalytic potential in homogeneous medium of the complexes has been investigated and
compared, which reveal that 1 is more efficient than 2 with respect to epoxide selectivity.
2. Experimental
2.1. Materials and physical measurements
Caution! Perchlorate salts of metals in the presence of organic ligands are potentially
explosive. Even though we did not encounter any problems, they should be prepared in
small amounts and handled with caution.
Copper(II) perchlorate hexahydrate, 2,2′-bipyridine, 2-hydroxyacetophenone, 5-bromo-2-
hydroxyacetophenone, styrene, 3-Me styrene and 4-Me styrene were purchased from
Aldrich Chemical Co. and used as received. Hydrogen peroxide (30 wt.% aqueous) and
solvents were purchased from E. Merck (India). The solvents were distilled and dried
before use. Complex 2 was synthesized according to the literature procedure [2]. Elemental
analyzes (carbon, nitrogen and hydrogen) were carried out using a Perkin-Elmer 2400 II
elemental analyzer. The copper content in 1 has been estimated by standard iodometric
method. Infrared spectra were recorded on a Perkin-Elmer Spectrum RX FTIR instrument
from 4000–400 cmꢀ1 as KBr pellets. UV–vis spectra in solution were recorded at room
temperature on a Perkin-Elmer Lambda 40 UV–vis spectrophotometer using acetonitrile in
1 cm quartz cuvettes. Electrochemical measurements were performed using a PAR VersaS-
tat-potentiostat/Galvanostat II electrochemical analysis system under dry argon using con-
ventional three-electrode configurations in acetonitrile with tetrabutylammonium
perchlorate as the supporting electrolyte. Platinized platinum millielectrode and saturated
calomel electrode (SCE) were used as working and reference electrodes, respectively,
along with a platinum counter electrode in cyclic voltammetry performed at a scan rate of
ν = 50 mV secꢀ1. The products of the catalytic reactions were identified and quantified by
an Agilent HP 6890 series gas chromatograph using a HP-5 GC column.