Synthesis, characterization, and catalytic activity of a water soluble copper(II) and nickel(II) heterobimetallic complex [CuNi(μ-OH)(μ-OH2)(μ-OAc)(bpy)2](ClO4)2 in aqueous medium in the absence of a base and co-catalyst

Article abstract of DOI:10.1080/00958972.2017.1358812

A copper(II)–nickel(II)-based catalyst system has been synthesized and characterized by elemental analysis, molar conductance, mass spectra, magnetic moment, EPR, UV-Vis, IR spectroscopy, and cyclic voltammetry. The structure of the complex was established by X-ray crystallography. The complex is an efficient catalyst, which oxidizes primary and secondary alcohols to the corresponding aldehydes and ketones at 70?°C employing 15% H₂O₂ as the oxidant in the absence of a base and co-catalyst.

Full text of DOI:10.1080/00958972.2017.1358812

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, characterization, and catalytic activity
of a water soluble copper(II) and nickel(II)
heterobimetallic complex [CuNi(µ-OH)(µ-OH2)
(µ-OAc)(bpy)2](ClO4)2 in aqueous medium in the
absence of a base and co-catalyst
Ram A. Lal, Arvind Kumar, Ibanphylla Syiemlieh & Sunshine D. Kurbah
To cite this article: Ram A. Lal, Arvind Kumar, Ibanphylla Syiemlieh & Sunshine D. Kurbah
(2017): Synthesis, characterization, and catalytic activity of a water soluble copper(II) and
nickel(II) heterobimetallic complex [CuNi(µ-OH)(µ-OH2)(µ-OAc)(bpy)2](ClO4)2 in aqueous
medium in the absence of a base and co-catalyst, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2017.1358812
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Date: 01 August 2017, At: 05:15

Journal of Coordination Chemistry, 2017
https://doi.org/10.1080/00958972.2017.1358812
Synthesis, characterization, and catalytic activity of a water
soluble copper(II) and nickel(II) heterobimetallic complex
[CuNi(μ-OH)(μ-OH2)(μ-OAc)(bpy)2](ClO4)2 in aqueous medium
in the absence of a base and co-catalyst
Ram A. Lala, Arvind Kumarb, Ibanphylla Syiemlieha and Sunshine D. Kurbaha
aCentre for advanced studies, department of Chemistry, north-eastern hill university, shillong, india;
bdepartment of Chemistry, faculty of science and technology, the university of West-indies, st. augustine,
trinidad and tobago
ABSTRACT
ARTICLE HISTORY
received 2 June 2016
accepted 3 July 2017
A copper(II)–nickel(II)-based catalyst system has been synthesized
and characterized by elemental analysis, molar conductance, mass
spectra, magnetic moment, EPR, UV-Vis, IR spectroscopy, and cyclic
voltammetry. The structure of the complex was established by X-ray
crystallography. The complex is an efficient catalyst, which oxidizes
primary and secondary alcohols to the corresponding aldehydes and
ketones at 70 °C employing 15% H2O2 as the oxidant in the absence
of a base and co-catalyst.
KEYWORDS
heterobimetallic; [Cuni(μ-
oh)(μ-oh2)(μ-oac)(bpy)2]
(Clo4)2; selective oxidation
1. Introduction
Applications of heterobimetallic complexes are very important in diverse areas of research
such as magnetochemistry [1], bioinorganic chemistry [2], synthesis of mixed metal oxides
[3], and catalysis [4]. Although the field of magnetochemistry, bioinorganic chemistry, and
synthesis of mixed metal oxides have been extensively investigated, that of catalysis has
begun to draw attention in recent years only [4].
CONTACT sunshine d. Kurbah
sunshinekurbah@yahoo.com
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2017.1358812.
© 2017 informa uK limited, trading as taylor & francis Group

2
R. A. LAL ET AL.
The oxidation of alcohols to carbonyl compounds is one of the most important and chal-
lenging transformations in the synthesis of fine chemicals and intermediates [5]. There is an
urgent need for inexpensive and waste free oxidants and recyclable catalysts that may be
used to perform these transformations. Therefore, using molecular oxygen or air as the
terminal oxidant has attracted research interest, and many highly efficient catalyst
systems have been developed for selective aerobic alcohol oxidation using transition metal
catalysts alone or in combination with stable nitroxyl free radicals. The introduction of
N-oxy radicals such as TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) and ABNO
(ABNO = 9-azabicyclo[3.3.1]nonane-N-oxyl) as co-catalyst in transition metal-based catalytic
systems was effective in improving the reaction selectivity under mild conditions; they have
proven to be excellent reagents for selective oxidation of primary alcohols [6]. The catalytic
aerobic oxidation of primary alcohols using the Cu/TEMPO system initiated by Stahl and
co-workers [7] has been the focus of considerable interest. This protocol exhibits excellent
functional group tolerance and displays high chemoselectivity. A number of other workers
have also investigated the copper-TEMPO systems in relation to oxidation of alcohols [8,9].
Similar procedures have been developed that utilize other metals including V [10], Co [11],
Ni [12], Ru [13], Ir [14], Mo [15], W [16], Mn [17], Fe [18], Pd [19], Au [20], Ag [21], Pt [22]
expanding the substrate scope, especially toward secondary alcohols. But few of these reac-
tions approach the synthetic utility and scope of the traditional stoichiometric oxidants such
as chromium oxides, Dess-Martin periodinane, or activated DMSO methods. Tandem reac-
tions employing Cu/radical catalysts in aerobic oxidation of alcohols have also been studied
[23]. Hetero-metal systems such as Cu–Al [23], Cu-Ni [24], Cu–Co [25], Cu–Mn [25], and Cu–Zn
[26] have been developed. Mechanisms of Cu/TEMPO systems have been investigated in
detail [27]. Hayton and co-workers [28] reported the use of Lewis acids FeCl3 and AlCl3 to
activate TEMPO toward the oxidation of alcohols. The resulting MCl3 (η1-TEMPO) (M = Fe1,
Al, 2) adducts were observed to oxidize both primary and secondary alcohols within minutes
at room temperature. However, deactivation of the transition metal catalyst in these aerobic
systems has been a recurring problem due to water competing with the substrate for vacant
coordination sites on the active metal catalyst [29].
Metal-based aerobic oxidation may possibly leave toxic traces of heavy metals in the
products. The development of an efficient non-metallic catalyst for the aerobic oxidation of
alcohols under mild conditions has long been desired. In this context, attention is drawn to
the work of Liang et al. [30] who used the system based on TEMPO and NaNO2 employing
O2 or air as the terminal oxidant utilizing Br2/HCl/1,3-dibromo-5,5-dimethylhydantoin
(PBDMH)/N-bromosuccimide. They disclosed a process for highly efficient aerobic oxidation
of a wide range of primary and secondary alcohols utilizing a TEMPO-based catalyst system
that was free from any transition metal co-catalysts [29]. However, two limitations of the
reported metal-free catalysis are that an autoclave is required to achieve the required partial
pressure of oxygen and that primary aliphatic alcohols are oxidized with only modest selec-
tivity in favor of the formation of aldehydes [31].
Although molecular oxygen is an ideal oxidant and considerable advancements have
been seen in the development of catalytic methods using either pure oxygen or air as oxidant
in alcohol oxidation, it cannot be used on a large scale because contamination of O2 and
organic solvents and reagents [32] make an explosive mixture. Further, halogenated solvents
[13] are undesirable for the environment. These safety concerns offer limited uses of molec-
ular oxygen in the oxidation process. Hydrogen peroxide is another ecofriendly oxidant

JOURNAL OF COORDINATION CHEMISTRY
3
(instead of pure oxygen) which can be safely used in oxidation reactions, and which can
reduce the safety hazards occurring when an organic solvent is heated under elevated O2
pressures. The expected side product in H2O2 mediated oxidation reactions is water. H2O2
in dilute aqueous solution (concentration less than 60%) is quite safe, non-toxic, and inex-
pensive. It is a moderate inorganic oxidant and it does not form a homogeneous solution
with most organic solvents. Hence, it gives a homo-heterogeneous oxidant and is easy to
separate from the reaction mixture after completion of the reaction [33]. Although some
work has been done on alcohol oxidation using homogeneous catalyst employing aqueous
H2O2 as a terminal oxidant [34], oxidation of alcohols with H2O2 using heterogeneous systems
as a catalyst still remains a challenge [35]. Hence, it was thought to develop a homo-heter-
ogeneous catalyst system [36] with high activity that would cause oxidation of a large num-
ber of alcohols.
Marx and co-workers [37] recently synthesized and characterized hetero-transition metal
complexes which mimic the purple acid phosphatase activity. Heterometal systems of Ni(II)–
Cu(II) or Mn(III)–Cu(II) nitrates were found to catalyze the oxidation of various primary and
secondary alcohols (benzylic and aliphatic) to aldehydes and ketones using molecular oxy-
gen or air as oxidizing agents and in combination with TEMPO in acetic acid solvent in high
yield (95%) [25]. A bipyridine copper(II)–zinc(II) heterometallic complex was found to catalyze
the oxidation of primary benzylic and aliphatic alcohols to corresponding aldehydes in excel-
lent yield using 15% H2O2 as terminal oxidant and in solvent-free medium with co-catalyst
benzoic acid in the absence of a base [38]. However, the catalyst did not catalyze the oxida-
tion of secondary alcohols to ketones. The method is very well suitable for oxidation of base
sensitive alcohols. Cheaper copper(II)–cobalt(II)-based heterometal complexes bearing
asymmetric tetranuclear cores catalyzed the oxidation of cyclohexane into cyclohexanol
and cyclohexanone under mild conditions using aqueous hydrogen peroxide as a terminal
oxidant. The reactions were carried out in liquid (CH3CN/H2O2) medium in the presence of
nitric acid (co-catalyst) at room temperature and atmospheric pressure. A high overall selec-
tivity toward the formation of aldehydes and ketones was achieved [1a]. Garcia-Cabeza and
co-workers [23] synthesized and characterized new copper-aluminum mixed oxide that
showed good activity and yielded the corresponding γ- or ϵ-hydroxylated enones starting
from different α, β- or α, β, γ-unsaturated ketones [23]. The same authors used the same
catalyst for allylic oxidation of cyclic alkenes. The treatment of an alkene with a carboxylic
acid employing tert-butyl hydroperoxide as the oxidant yielded the corresponding allylic
esters. When L-proline is employed, again allylic esters are obtained [23]. Ray and co-workers
[24] studied the oxidation of phenols by Cu(III) – (μ-O)2 – Ni(III)-complexes containing
nucleophilic oxo groups. The oxidation occurs by both proton coupled electron (PCET) and
hydrogen atom transfer mechanism. Minisci and co-workers [25] used catalytic amounts of
Mn(II)-Co(II) nitrates in combination with TEMPO in the selective oxidation of primary and
secondary alcohols to aldehydes and ketones by oxygen under very mild condition in excel-
lent yield. They studied the mechanism of the reaction also. Lal and co-workers [26] synthe-
sized water-soluble heterobimetallic complexes of copper and zinc, studying the oxidation
of primary alcohols catalyzed by these complexes in the absence of base and co-catalyst.
An excellent yield of corresponding aldehyde was obtained. The heterometal copper(II)–
nickel(II) complex in the presence of pyridine-carboxylic acids allowed the selective oxidation
of styrenes to acetal with H2O2 as oxidant and to aldehydes using PhIO [39]. In spite of the
availability of numerous heterometal [40] complexes in the literature, only a few applications

4
R. A. LAL ET AL.
of such complexes in catalysis are known [1a, 24–27, 39, 40]. With growing interest in the
development of environmentally benign oxidation protocols [39] and our interest in devel-
oping catalysts from biologically relevant metal ions [26, 38] capable of catalyzing the oxi-
dation of a variety of alcohols to carbonyl compounds by aqueous hydrogen peroxide, we
have synthesized a heterobimetallic copper(II)–nickel(II) complex [CuNi(μ-OH)(μ-OH2)(μ-OAc)
(bpy)2](ClO4)2 and established its structure and explored its catalytic properties toward oxi-
dation of several alcohols using aqueous hydrogen peroxide as the terminal oxidant.
Here, we report the use of a readily prepared heterobimetallic copper(II)–nickel(II) complex
along with its structure as a catalyst in oxidation of various alcohols to carbonyl compounds.
In order to construct a high performance catalytic system, we have attempted to prepare a
heterobimetallic complex from inexpensive and readily available commercial materials, and
its use as a catalyst for hydrogen peroxide mediated oxidation of alcohols.
2. Experimental
2.1. Physical measurements
Solvents were reagent grade and other chemicals were E-Merck, Himedia, or equivalent
grades, and all solvents were used as received. All operations were performed under
aerobic conditions. Copper and nickel were determined by standard literature procedure
[41]. C, H and N were determined by microanalytical method using a Perkin-Elmer 2400
CHNS/O Analyzer 11. Infrared spectra from 4000–200 cm−1 were recorded as KBr disks
using a BX-III/ FTIR Perkin Elmer spectrophotometer. Electronic spectra were recorded on
a Perkin-Elmer Lambda-25 spectrophotometer. Cyclic voltammograms were recorded on
a CH Electrochemical Analyzer using a standard three electrode assembly (glassy-carbon
working, Pt wire auxiliary, SCE reference) and 0.1 M NBu4ClO4 as supporting electrolyte.
Room temperature magnetic data of the powdered sample were measured using a
Sherwood Scientific Magnetic Susceptibility Balance. The magnetic data were corrected
for diamagnetism using Pascal’s constants [42]. Conductance measurements were done
using a Wayne Kerr B 95 Automatic Precision Bridge with a dip-type conductivity cell
having a platinized platinum electrode. The cell constant was determined using a standard
KCl solution. Mass spectrum of the complex was recorded on a Water ZQ-4000 Micromass
Spectrometer. The X-band EPR spectrum at room temperature for the complex was
recorded from DMSO solution on a Varian E-112 ESR Spectrometer using TCNE (g = 2.0027)
as an internal standard. Gas Chromatographic (GC) analysis was performed on a Bruker
430-GC Gas Chromatograph equipped with a 30 m × 0.32 mm × 0.5 μm HP-Innowax cap-
illary column and flame ionization detector (FID).
2.2. Procedure for preparation of [CuNi(μ-OAc)(μ-OH)(μ-OH2)(bpy)2](ClO4)2
2.2.1. Warning
Perchlorate salts are potentially explosive. Although during the preparation of the complex
no detonation tendencies of the reaction mixture were observed, caution is advised and it
is recommended that small quantities of the reactants be handled.
A mixture of Ni(OAc)2·4H2O (0.125 g, 0.5 mmol), Cu(OAc)2·H2O (0.1 g, 0.5 mmol), 2,2′-bipyri-
dine (0.197 g, 1.26 mmol), and NaClO4 (0.165 g, 1.5 mmol) was powdered in a pestle mortar
and heated in an oven for 5 h at 150 °C. The reaction mixture was then cooled to room

JOURNAL OF COORDINATION CHEMISTRY
5
temperature, followed by dissolution in hot methanol. The resulting hot violet solution was
filtered, while hot to remove small quantities of undissolved residue and then cooled to
room temperature. Violet single crystals suitable for X-ray analysis were obtained from
the resultant solution after 1 day. Yield: 67.5%. Color: light blue. Anal (%), Calcd for
C22H22N4O12ClCuNi: C, 36.31; H, 3.03; N, 7.70. Found: C, 36.72; H, 3.00; N, 7.81. Mass (m/z): expt.
(527.19 and 531.93) theo. (528.06, 530.06). Molar conductance (Ω−1 mol−1 cm2): 230. Magnetic
Moment (μB): 2.92 B.M, Electronic spectrum [λmax, nm (εmax, dm3 mol−1 cm−1)]: in MeCN, 670
(252). IR data (cm−1, KBr): 1560 s, vas(COO); 1444s (vsCOO), 1142 s, 1116 s, 1091s v(ClO4), 771 s
(out-of-plane ring deformation mode of 2,2′-bipyridine), 683 m (in-plane ring deformation
mode. EPR: g|| = 2.140, g⊥ = 2.064, gav = 2.089; Cyclic voltammogram (Epc mV s−1): −0.29, −0.54
and -0.70 V: (Epa mV s−1): −0.23 and −0.40 V.
2.3. Procedure for oxidation of benzylic alcohols by [CuNi(μ-OAc)(μ-OH)(μ-OH2)
(bpy)2](ClO4)2
To a mixture of benzyl alcohol (0.5 mL, 4.64 mmol) and 15% aqueous hydrogen peroxide
(1.1 mL, 4.85 mmol), the catalyst [CuNi(μ-OAc)(μ-OH)(μ-OH2)(bpy)2](BF4)2 (8 mg, 0.01 mmol)
was added and stirred at 70 °C for 5 h. The organic and aqueous phase were separated by a
separating funnel and then purified by column chromatography to afford benzaldehyde.
The identity of the benzaldehyde was ascertained by comparison with authentic sample
using IR, 1H NMR, and 13C NMR.
2.3.1. Characterization data for benzaldehyde
1
Colorless liquid; Yield: 93%; H (400 MHz, CDCl3): δ 9.96 (s, 1H), 7.87–7.41 (m, 5H); 13C
(100 MHz, CDCl3): δ 192.5, 136.2, 134.4, 129.8, 128.9; IR (KBr): 1702 cm−1.
2.4. Oxidation of benzyl alcohol under anaerobic condition
Benzyl alcohol (0.5 mL, 4.64 mmol) and the catalyst ([CuNi(μ-OAc)(μ-OH)(μ-OH2)(bpy)2](BF4)2
(8 mg, 0.01 mmol) were mixed together in a round bottom flask. To this reaction mixture
15% H2O2 (1.1 mL, 4.85 mmol) was added and the resulting reaction mixture was stirred for
5 h under dinitrogen. The organic and aqueous phases were separated in the usual way. The
identity of the product was ascertained with authentic sample by usual spectroscopic meth-
ods. Benzaldehyde, yield: (97%, GC).
3. Results and discussion
The heterobimetallic complex [CuNi(μ-OH)(μ-OH2)(μ-OAc)(bpy)2](ClO4)2 was prepared by
heating a mixture of Cu(OAc)2·H2O, Ni(OAc)2·4H2O, 2,2′-bipyridine and NaClO4 in air in an
electronic oven at 150 °C for 5 h followed by dissolution in methanol, and crystallizing over-
night at room temperature. The complex is violet, clearly different from the color of constit-
uent metal ions.
All relevant data and figures regarding the characterization of the crystal are found in
Tables 1, S1 and S2 and Figures 1, S1 and S2. The complex consists of dinuclear [CuNi(μ-OH)
(μ-OH2)(μ-OAc)(bpy)2]2+ cation and two well separated perchlorate anions (Figure 1). The
molecular formula and packing diagram of the complex is shown in Figures S1 and S2. In

6
R. A. LAL ET AL.
Table 1. oxidation of alcohols catalyzed by [Cuni(μ-oac)(μ-oh)(μ-oh2)(bpy)2](Clo4)2 under solvent-free
condition in the absence of base.a
R = aryl, alkyl, heteroaryl, aliphatic.
Sl. No.
Alcohol
Product
Time (h)
Yield (%)b
Yield (%) (GC %)c
1
5
94
97.0
2
3
4
2
2
2
93
95
95
97
98.2
99.7
5
6
2
3
93
95
99.5
99.6
7
8
3
2
96
94
99.2
98.5
9
2
2
95
96
99.7
99.3
10
11
12
2
1
95
96
99.5
99.8
13
14
2
1
92
96
98.7
98.9
15
1
94
99.2
16
17
1
2
93
97
98.9
99.5
18
19
24
24
no reaction
not isolated
–
2
areaction conditions: catalyst (0.010 mmol); 15% h2o2 (4.85 mmol); alcohol (4.84 mmol).
byield.
cGC yield.

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 1. orteP plot (50% probability) of [Cuni(μ-oh)(μ-oh2)(μ-oac)(bpy)2](Clo4)2.
the cation, CuII and Ni II are doubly bridged by the endogenous hydroxido oxygen and aquo
oxygen, respectively, with the Cu … Ni separation being 3.0289(2) Å. The CuII and NiII are
also bridged by an acetato group in the bidentate η1, η1, μ-bridging mode. The arrangement
of ligand atoms around the metal ions provides distorted five-coordinate geometry for both
metal ions.
The CuII center at the N2O2 site is made of two nitrogens from one bipyridine and two
oxygens, one from bridged hydroxo oxygen and the other from acetato oxygen. The fifth
position around CuII is occupied by a bridging oxygen of an H2O molecule giving five-coor-
dinate coordination geometry for copper. The CuII site is close to square pyramidal geometry
(the geometric parameter Ґ is 0.23) [43]. The assignment of the metal ions to different posi-
tions are based on the relative metal-N(0) bond distances and the geometric features of the
chromophores. The Cu-N and Cu-O bond lengths range from 1.922(4) to 1.999(4) Å, normal
for planar CuII. N1, N2, O2, and O3 form an approximate plane about CuII. The O2–Cu–O3
bond angle (94.67(16)) is slightly larger than 90°, while the N1–Cu–N2 bond angle (81.03(16))
is appreciably smaller than 90°.
The NiII is also situated at another N2O2 site with a second bipyridine and two oxygens,
one from a bridged acetato group (O1) and the other from a bridged hydroxo group. The
NiII is square pyramidal with oxygen from a bridging H2O molecule. The Ni–N and Ni–O bonds
are 1.926(4)–1.998(5) Å. The Ni–N and Ni–O bonds are significantly longer than the Cu–N
and Cu–O bonds, moreover, the axial Ni–O4 bond is longer (2.406(4)) than the Cu–O4 bond
length (2.393(4)) [44]. These facts indicate high-spin NiII [45]. The geometric parameter Ґ
around NiII is 0.14, revealing that the geometry around NiII is more square pyramidal [43]
than around CuII.
The molar conductance for the complex in acetonitrile solution is 230 Ω−1 M−1 cm2 at 10−3
dilution, which suggests that the complex is a 1:2 electrolyte [46]. The μeff value for Cu–Ni
complex is 2.92 B.M (theo: 3.49 B.M for non-bonded coupled ions of Scu = ½ and SNi = 1) at
room temperature, indicating antiferromagnetic exchange interaction between Cu(II) and
Ni(II) in the complex [47]. The magnetic moment is consistent with antiferromagnetic cou-
pling of intermediate strength in a high-spin regime. However, due to unavailability of a

8
R. A. LAL ET AL.
squid magnetometry, we are unable to confirm whether the interaction between the metal
ions is of ferromagnetic or antiferromagnetic nature and hence, are unable to confirm the
oxidation state and spin-state assignments of the complex. The mass spectrum (ESI–MS) of
a dilute acetonitrile solution of the complex (Figure S3) displays molecular ion peak for the
dication [CuNi(μ-OH)(μ-OH2) (μ-OAc)(bpy)2]2+ at m/z = 527.19 (theo. 528.06 (M-2ClO4)2+] and
(m/z = 531.93) (theo. 530.06) corresponding to two isotopes of copper and Ni. The UV-Visible
spectrum (Figure S4) of the complex in CH3CN solution shows only single asymmetrical band
in the region 400–1100 nm with maximum at 670 nm and molar extinction coefficient of
252 mol-1 dm3 cm−1, which reveals that its origin is due to both the metal centers copper
and nickel in five-coordinate square pyramidal structure. The IR spectroscopic study of the
above heterobimetallic compound from 4000–200 cm−1 shows all of the characteristic ligand
bands (Figure S5). The broad band at 3421 cm−1 can be assigned to both bridged hydroxido
group and water, while the medium intensity band as a shoulder at 1623 cm−1 to bridged
water. The strong νas (COO) and νs (COO) bands at 1560 and 1444 cm−1 arise due to antisym-
metric (vas) and symmetric (vs) of the bridged acetate. The significant difference between
these two bands suggests bridging between different metals. The strong band at 770 cm−1
and medium intensity band [48] at 633 cm−1 are attributed to out-of-plane of the hydrogens
−
and in-plane deformation mode of coordinated 2,2′-bipyridine, respectively. The v (ClO4 ) is
a split band at 1116 cm−1 characteristic of ionic perchlorate. The EPR spectrum of the complex
in DMSO solution at RT is anisotropic devoid of any metal-hyperfine structure with g||, g⊥,
and gav values equal to 2.140, 2.064, and 2.089 (Figure S6), respectively. The spectrum of
CuNi complex has Hamiltonian parameters in the order g|| > g⊥ > 2.0023, which is similar to
that of mononuclear copper(II) complex with one unpaired electron in thedx2−y2 orbital [47].
The DMSO solution spectrum corresponds to that of a simple doublet ground state. In the
present case, S = ½ state is lowest in energy for the pair, antisymmetric interaction is expected
between copper(II) and nickel(II) ions. When the metal ions d8 – d9 are bridged by two ligand
atoms, exchange coupling between them occurs, giving rise to antiferromagnetic interaction
of intermediate strength. This occurs by the overlap of dxy – orbitals containing one unpaired
electron on each metal atom [49]. The Cu–O3–Ni bond angle in the present case is 103.84(17),
not 90°, an angle required for ferromagnetic interaction of intermediate strength. Hence,
there is antiferromagnetic interaction of intermediate strength between metal ions in the
complex [49]. The essential features of the EPR spectrum in DMSO solution at RT reveals that
the unpaired electron resides on the molecular orbital of dx2−y2 character which is made up
of dx2−y2 (Cu) and dx2−y2 (Ni) showing that the unpaired electron is delocalized over the CuNi
core in the complex [50]. The complex shows three reduction waves (Figure 2) in the forward
scan in cyclic voltammetry at −0.29, −0.54, and -0.70 V, while two oxidative waves at −0.23
and −0.40 at a scan rate of 0.06 v/s in acetonitrile solution at 2 mM dilution with 0.1 M tet-
ra-n-butyl ammonium perchlorate (TBAP) as a supporting electrolyte. The first two couples
are attributed to reduction occurring at Cu(II) and Ni(II), respectively, while the third irrevers-
ible reductive wave at −0.70 V may be attributed to electron transfer centered on bipyridine
[51].
3.1. Catalysis
Copper and nickel are the catalytic centers in several metalloenzymes [52]; thus, it was worth-
while to investigate the catalysis of the present heterobimetallic complex toward hydrogen

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 2. Cyclic voltammetric diagram of [Cuni(μ-oh)(μ-oh2)(μ-oac)(bpy)2](Clo4)2 in Ch3Cn solution at rt.
Table 2. recycling of catalyst.
Rn
Recovery (%)
Product (%)
1
2
3
>98
>97
>96
97
96
95
peroxide mediated oxidation of alcohols. Initially, the catalytic oxidation of benzyl alcohol
using the CuNi heterobimetallic complex as catalyst was performed and typical results are
listed in Table S3. The combination of catalyst (0.010 mmol), 15% H2O2 (1.1 mL, 4.85 mmol)
at 70 °C under solvent-free conditions was the ideal condition yielding 94% (97% GC yield)
of benzaldehyde from 0.5 mL (4.64 mmol) of benzyl alcohol. With the optimum conditions
available, the oxidation of the primary, secondary, allylic, and aliphatic alcohols were inves-
tigated and the results are listed in Table 2. All the reactions were performed in air for alde-
hydes and ketones and without any other product. This revealed that in the oxidation process,
air is not a significant co-oxidant and the heterobimetallic complex is air stable. The above
reaction was also carried under nitrogen. On analysis of organic phase, the yield of benzal-
dehyde was 97%. This proved that air was not a significant co-oxidant in the oxidation process.
The catalytic system was applied for oxidation of other benzylic alcohols. For benzylic alcohols
bearing electron-donating and electron-withdrawing substituents, the substrates were oxi-
dized into corresponding aldehydes in excellent yield (entries 2, 9, 11, 17, 3, and 8). Secondary
alcohols gave excellent isolated yields (99.6 and 99.2%, GC yield, entries 6 and 7). When the
reactions of aliphatic secondary cyclic alcohols were investigated, the ketones again were
obtained in excellent yields (entries 5 and 13). Conjugated allylic alcohol was smoothly con-
verted into its α, β-unsaturated aldehyde (entry 4) in excellent yield. This transformation did
not affect the carbon–carbon double bond. For secondary benzylic alcohols bearing

10
R. A. LAL ET AL.
electron-withdrawing group, such as benzoin, again excellent yield of corresponding ketone
was obtained (entry 12). Heterocyclic alcohols such as furfuryl alcohol, pyridine 2-methanol
and thiophene-2-methanol were also oxidized to the corresponding aldehydes in good to
excellent yield (entries 14–16). The O, N, and S atoms remained unaffected in this oxidation
process. The aliphatic alcohol i.e. butanol did not oxidize at all to the corresponding propanal
even in 24 h, while 2-hexyne-1-ol was oxidized to the extent of only 2% (entry 19) in 24 h.
Although the present catalyst is homogenous in a mixture of H2O2 and alcohols, it can
be recycled (Table 2). The complex catalyst becomes soluble in aqueous hydrogen peroxide
when added to the reaction medium. A miscible phase with alcohol is formed. A separate
organic phase is obtained as a result of oxidation of alcohols to aldehydes or ketones. The
organic phase (aldehyde) is easily separated using a separating funnel after completion of
oxidation. The aqueous layer was evaporated and the catalyst reused for the oxidation of
alcohol up to three runs. The yield of the reaction product and selectivity was almost the
same.
4. Conclusion
We have synthesized an efficient catalytic, heterobimetallic complex for oxidation of alcohols
to corresponding aldehydes and ketones. The method employs inexpensive and easily avail-
able starting materials which allow the oxidation of a wide variety of primary and secondary
alcohols to corresponding aldehydes or ketones in high yield, mild oxidation conditions and
selectivity, making this catalyst system highly practical.
Acknowledgment
Authors would like to thank to the Head, SAIF, North-Eastern Hill University, Shillong-793022, India
for recording 1H and 13C NMR spectra. Sunshine D. Kurbah would like to thank UGC-NFST, New Delhi,
India for award of University fellowship.
Disclosure statement
The authors confirm that this article content has no conflict of interest.
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